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Proceedings of the

VII Iberian Conference on Tribology

IBERTRIB 2013

Porto, FEUP, June 20-21, 2013

Editors

António Monteiro Baptista, Luís Andrade Ferreira, Jorge Seabra

www.fe.up.pt/ibertrib2013

ISBN 978-972-752-156-2

http://www.fe.up.pt/ibertrib2013

IBERTRIB 2013

2 Editors: António Monteiro Baptista, Luís Andrade Ferreira, Jorge Seabra

TECHNICAL INFORMATION Title: IBERTRIB 2013 - Proceedings of the VII Iberian Conference on Tribology Editors: Baptista, António Monteiro; Ferreira, Luís Andrade; Seabra, Jorge Date: June 2013 Printing: FEUP Edições ISBN: 978-972-752-156-2


Porto, Portugal, 20-21 June, 2013 3

Index

Introduction and Objectives 4

Conference Topics 4

Scientific Committee 5

Organizing Committee 6

Conference Office 6

Sponsor 6

Keynote Speakers 7

Publication in International Scientific Journals 7

Conference Languages 7

Venue 8

Scientific Program 11

Plenary Session A1 – OPENING and KEYNOTE LECTURE 1 17

Session A2 – ROLLING BEARINGS 1 21

Session B2 – BIOTRIBOLOGY 31

Session A3 – ROLLING BEARINGS 2 41

Session B3 – SURFACE MODIFICATION 51

Session A4 – GREASE LUBRICATION 61

Session B4 – GEARS 71

Plenary Session A5 – KEYNOTE LECTURE 2 and INVITED PAPER 81

Session A6 – BEARINGS AND SEALS 87

Session B6 – SURFACE COATINGS 97

Session A7 – AEROSPACE LUBRICANTS AND IONIC LIQUIDS 107

Session B7 – FRICTION AND WEAR 1 117

Session A8 – LUBRICANT ADDITIVES 127

Session B8 – FRICTION AND WEAR 2 135

ADDITIONAL ABSTRACTS 143

POSTERS 151

IBERTRIB 2013


Introduction and Objectives

After four editions of the Portuguese Conference on Tribology (Guimarães 1994, Aveiro 1996,

Coimbra 1998, Porto 2000), the first edition of the Iberian Conference on Tribology took place in

Gijon in 2001, followed by Valencia 2003, Guimarães 2005, Bilbao 2007, Coimbra 2009 and

Madrid (Móstoles) 2011.

The VII Iberian Conference on Tribology - IBERTRIB 2013 - is organized by the Tribology

Group of Porto University (CETRIB), at the Faculdade de Engenharia of Universidade do Porto on

June 20-21, 2013.

The VII Iberian Conference on Tribology, IBERTRIB 2013, focus on all aspects of Tribology:

Surface engineering, surface treatments and coatings, lubricants, additives and special fluids,

wear and lubrication, condition monitoring and tribology cantered maintenance, tribology case

studies.

Two keynote lectures by reputed tribologists integrate the conference program.

Apart from the conference, training courses, lectured by experienced Tribologists, will be

organized the day before the conference (June 19th, 2013), with an independent registration. An

exhibition will be organized during the conference for interested companies (June 19-21, 2013).

Conference Topics

Contact mechanics: Surface roughness, surface tractions, surface deformation, sub-

surface stresses, surface and sub-surface damage, contact fatigue models.

Friction and wear: Erosion, abrasion, tribofilms.

Surface engineering, surface treatment, coatings, ceramics and composites.

Lubricants, additives and special additives – Industrial and automotive lubricants, food

lubricants, dielectric fluids, ionic fluids, metal working fluids, greases, environmentally

friendly lubricants.

Lubrication: Hydrodynamic, elastohydrodynamic, mixed film and boundary film.

Tribology of machine elements – cam-follower systems, engines, gears & transmissions,

hydrodynamic and hydrostatic bearings, rolling bearings, seals …

Biotribology.

Micro and nanotribology.

Characterization techniques and testing methods in tribology.

Tribology based maintenance - Lubrication maintenance practices, condition monitoring,

on-line sensors, machinery and lubricant diagnosis.

Other topics in Tribology.



Scientific Committee

Marc Anglada Universidade de Politécnica de Cataluña, Spain

António Monteiro Baptista FEUP, Portugal

Manuel Belmonte I. de Cerámica y Vidrio-CSIC, Spain

Mª Dolores Bermúdez Universidade de Politécnica de Cartagena, Spain

Armando Vilaça Campos ISEP-IPP, Portugal

Albano Cavaleiro Universidade de Coimbra, Portugal

Manuel Jorge Castro ISEP-IPP, Portugal

José Pimenta Claro Universidade do Minho, Portugal

Rogério Colaço IST, Portugal

Juan José Damborenea CENIM-CSIC, Spain

Paulo Davim Universidade de Aveiro, Portugal

Esteban Fernández Universidade de Oviedo, Spain

Luís Andrade Ferreira FEUP, Portugal

Carlos Ferrer Universidade de Politécnica de Valencia, Spain

José Gomes Universidade de Minho, Portugal

Amaia Igartua Tekniker, Spain

Luís Leite Magalhães ISEP-IPP, Portugal

Ramiro Carneiro Martins INEGI, Portugal

António Sousa Miranda Universidade de Minho, Portugal

Amílcar Ramalho Universidade de Coimbra, Portugal

Jesús Rodríguez Universidade Rey Juan Carlos, Spain

J. Carlos Sánchez I. C. Materiales de Sevilla-CSIC, Spain

Jorge Oliveira Seabra FEUP, Portugal

Francisco Silva ISEP-IPP, Portugal

IBERTRIB 2013


Organizing Committee

CETRIB - Unidade de Tribologia, Vibrações e Manutenção Industrial of FEUP:

António Monteiro Baptista, FEUP

Armando Vilaça Campos, ISEP-IPP

Manuel Jorge Castro, ISEP-IPP

Luís Andrade Ferreira, FEUP

Fernanda Fonseca, FEUP

Beatriz Maria Graça, INEGI-FEUP

Luís Leite Magalhães, ISEP-IPP

Ramiro C. Martins, INEGI-FEUP

Arnaldo Guedes Pinto, ISEP-IPP

Jorge Oliveira Seabra, FEUP

Francisco Silva, ISEP-IPP

Conference Office

Mrs. Fernanda Fonseca

FEUP - DEMec

Rua Dr. Roberto Frias s/n, 4200-465 PORTO, Portugal

Tel: +351 22 508 1716

Email: [email protected]

Website: www.fe.up.pt/ibertrib2013

Sponsor

Unidade de Tribologia, Vibrações

e Manutenção Industrial

CETRIB

mailto:[email protected]

http://www.fe.up.pt/ibertrib2013



Keynote Speakers

The conference program included two Keynote Plenary Lectures by distinguish scientists in the

field of Tribology, to provide thematic presentations on their most recent findings:

Prof. Guillermo Morales-Espejel

Senior Scientist, SKF Engineering & Research Centre B.V. ,The Netherlands,

Professor, LaMCoS, INSA de Lyon, France.

and

Prof. Albano Cavaleiro de Carvalho

Full Professor, Depart. de Engenharia Mecânica, Universidade de Coimbra, Portugal.

Publication in International Scientific Journals

Extended versions of all papers presented in IBERTRIB 2013 will be considered for publication

in several Scientific Journals of international reputation.

An agreement has already been established with the:

International Journal of Surface Science and Engineering,

Editors in Chief: Prof. J. Paulo Davim, Prof. Liangchi Zhang

http://www.inderscience.com/browse/index.php?journalID=195

and

Tribology – Materials, Surfaces & Interfaces

Editors in Chief: Prof. Anne Neville, University of Leeds, UK

http://www.maney.co.uk/journals/notes/trb

Conference Languages

The official language of the conference is English.

Abstracts, Papers, Posters and PPT presentations were written in English.

Authors that do not feel comfortable with English language may present their papers either in

Castellan or Portuguese.

http://www.inderscience.com/browse/index.php?journalID=195

http://www.maney.co.uk/journals/notes/trb

IBERTRIB 2013


Venue

FEUP Campus

The conference will be held in the beautiful city of

Porto, Portugal, at the new campus of the Faculty of

Engineering, University of Porto, which is very

convenient and well equipped for conferences in terms

of facilities, transportation and proximity to hotels.

Situated on the banks of the Douro river, Porto is one of the oldest tourist destinations in

Europe. Sloping steeply down towards the river with its remarkable bridges - one of them

designed by Gustave Eiffel - it has been an inspiration to a variety of artists. Porto made its name

over two centuries ago because of its connection with the port wine industry. Many areas in

Porto, well planned and landscaped, have been dedicated to leisure, public gardens,

sophisticated and attractive shopping areas and excellent Museums and Theaters, which justifies

the city’s wish to be granted “World Heritage Status” by UNESCO.

Porto World Heritage (Ribeira area)

Porto, the country´s second largest city, is in itself full of interest, but the district it heads, though

largely industrialized, offers the visitor plenty to see. Along the coast, holiday resorts like the

cosmopolitan beach of Espinho, busy ports like Matosinhos, with splendid seafood, or traditional

fishing towns like Póvoa de Varzim, but where there is also an animated casino. Inland, for



example, the quaint charm of Amarante, with 17th century mansions overlooking the river and

famous for a kind of sweet egg pastries. Right next to Porto you can visit, in Vila Nova de Gaia,

the lodges where the famous Port wine is blended and aged and where you can taste the

different varieties, or you may choose to take a river cruise along the Douro. The whole district

is filled with prosperous towns, as commerce and industry thrive there, but you can drive along

many calm roads with wonderful views over the river or enjoy a rugged and still unspoilt

coastline.

Douro river and vineyards

Finally, about the weather in

Portugal, we would like to

inform all participants in VII

Iberian Conference on

Tribology, IBERTRIB 2013,

that we have ordered warm

and dry weather for you, with

temperatures ranging from

30ºC day time to 20ºC night

time. So please be prepared

and hopefully enjoy your stay

in this country.

IBERTRIB 2013




SCIENTIFIC PROGRAM

IBERTRIB 2013


JUNE 20th



JUNE 20th

IBERTRIB 2013


JUNE 21th



JUNE 21th

IBERTRIB 2013




Plenary Session A1

Room B002

Chair:

Prof. António Monteiro Baptista and Prof. Jorge Seabra

Opening

Keynote Lecture 1

SURFACE LIFE: THE NEXT CHALLENGE IN ROLLING BEARINGS AND

OTHER HEAVILY LOADED LUBRICATED CONTACTS

Prof. Guillermo Morales-Espejel

Senior Scientist, SKF Engineering & Research Centre B.V. ,The Netherlands,

Professor, LaMCoS, INSA de Lyon, France.

IBERTRIB 2013




Keynote Lecture 1

SURFACE LIFE: THE NEXT CHALLENGE IN ROLLING BEARINGS AND OTHER HEAVILY LOADED LUBRICATED CONTACTS Prof. Guillermo Morales-Espejel

Senior Scientist, SKF Engineering & Research Centre B.V. ,The Netherlands, Professor, LaMCoS, INSA de Lyon, France.

ABSTRACT

The talk will cover the motivation for the understanding of the surface life of heavily-loaded lubricated contacts and its industrial context in relation to rolling bearings and SKF.

It would then review the different global challenges for engineers in this field (system approach, competitive failure modes, uncertain operating conditions, etc). Next, it would review different surface failure modes in these contacts and would discuss calculation schemes for them (e.g. surface distress, indentations, smearing, additive interaction, vibrations, friction, etc.).

Finally a review of the remaining challenges and possible treatment schemes will be outlined. The main idea is to leave the audience with a clear message of the importance in the understanding of surface life in lubricated contacts, the current knowledge and the remaining gaps for future research.

IBERTRIB 2013




Session A2

Room B001

ROLLING BEARINGS 1

Chair:

Prof. Guillermo Morales-Espejel and Prof. Armando Campos

IBERTRIB 2013


A2.1 - A010

TESTING AVIATION OILS AND COMPONENT MATERIALS FOR TRIBOLOGY DESIGN AND PERFORMANCE Lavern Wedeven, William Black, Graham Wedeven, Steven Kratz

Wedeven Associates, Inc., Edgmont, PA, USA Email: [email protected]

ABSTRACT

A testing strategy is described that characterizes aviation oils, along with bearing and gear materials, for scuffing, wear and contact fatigue performance in jet engines and gearbox hardware. Testing is conducted in sequence starting with the most critical tribology performance attribute of scuffing. Traction coefficient is used as a measure of anti-wear (AW) offering a means to map anti-wear and extreme pressure (EP) performance properties in a single screening test. Subsequent testing is conducted for micro-pitting.

INTRODUCTION

Meaningful service performance tribology testing of aviation oils, along with bearing and gear materials, is challenging. A thoughtful strategy can be based on simulation of application contact conditions and fundamental performance mechanisms. Performance limits are defined by three categories of failure mechanisms: scuffing, wear and contact fatigue. Catastrophic scuffing is a design limit for transmitted power. Wear and fatigue mechanisms control the life limit of lubricated contacts. Wear affects gear tooth profile and the performance deterioration associated with loss of tooth profile. Wear across a bearing raceway, including unequal wear, affects operating clearances and stresses. Surface fatigue mechanisms are associated with local stresses at roughness features resulting in loss of material on a fine scale (micro-pitting) and subsequent propagation to spalling on a gross scale.

The connection between failure mechanisms and lubrication mechanisms and how they play out in hardware and testing methods are covered with five key tribology parameters: (1) the entraining velocity, Ue = ½(U1 + U2), which is the vector average of the surface velocities entraining fluid into a lubricated contact, (2) the degree of asperity penetration (h/), which is the ratio between the elastohydrodynamic (EHD) film thickness (h) and the combined surface roughness, = (12 + 22)0.5 of the contacting surfaces, (3) sliding velocity vector, Us = U1 – U2, which causes wear and heat generation, (4) the contact temperature (T), which is the sum of the bulk temperature (Tb ) and the “flash” or friction generated temperature (Tf) within the contact and (5) contact stress, both Hertzian stress and local asperity stress.

To satisfy the most critical performance attribute of scuffing, gear simulation tests can be

less than 1.0 allows the traction (friction) coefficient to be used as a means to measure wear. Wear of roughness features lowers the traction coefficient (see Fig. 1).



RESULTS AND CONCLUSIONS

A performance test for scuffing is conducted using Us values equivalent to critical spiral bevel gears, which are vulnerable to scuffing. The scuffing test utilizes a step loading (load stage) protocol covering normal and overload gear stresses (see Fig, 1). The entraining velocity (Ue) is selected to control h/ to a value equal to or less than 1.0. As load stages and temperature increase the polishing of roughness features and events of micro-scuffing or macro-scuffing are reflected in the traction (friction) coefficient. Wear of roughness features lowers the traction coefficient (see Fig. 1). Anti-wear (AW) and scuffing (or EP) performance attributes can then be mapped, as illustrated in Fig. 2. This screening test method is useful to the oil formulator and the design engineer who desires to understand the trade-offs between wear resistance and scuffing resistance [1].

Fig. 1 Wear and scuffing test for oils Fig. 2 Performance map for AW and EP

The results of the scuffing test method are usually quantified using the maximum Hertzian contact stress or load stage. In some cases, the limited consistency of gear or bearing materials for scuffing requires the scuffing tests to be normalized with a reference oil (see Fig. 2). Surface fatigue (micro-pitting) is evaluated subsequent to wear/scuffing tests. Micro-pitting is mostly a stress based failure mechanism associated with roughness features. Its connection with oil chemistry is primarily through AW attributes which control asperity stresses. Meaningful test protocols for micro-pitting are derived from the five key tribology parameters identified above. Micro-pitting results are correlated with scuffing/wear tests through anti-wear performance involving both oil and material attributes, as well as surface roughness specifications and finishing processes.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of the U.S. Air Force, U.S. Army, U.S. Navy, Pratt & Whitney and Wedeven Associates, Inc. .

REFERENCES

WAM Scuffing and Wear Test Method, SAE ARP6156, Draft Oct 11, 2012

WAM High Speed Load Capacity Test Method

Load Stage

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Tra

ction C

oeff

icie

nt

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Lower bound reference,polished surfaces, STD oil

Failure criteria(avg. of all tests)

Test suspended

= combined roughness

h

Base stock

Good AW

additive

Good EP,

Poor AW

Good EP,

Mild AW

Good AW

Mild EP

EP

behavior

Anti-wear

behavior

Anti-Wear (AW) and Extreme Pressure (EP) Performance

Scuffing (EP) Performance(Percent of 5 cSt STD Ref. Oil)

0 20 40 60 80 100 120 140

An

ti-W

ear

Perf

orm

an

ce

(Ma

x.

tra

ctio

n c

oe

ffic

ien

t)

0.02

0.04

0.06

0.08

9-355a

Mild AW

Good EP

Poor AW

Good EP

Good AW

Mild EP

Good AW

Poor EP

Base

Stock

Poor AW

Poor EP

EP

behavior

Anti-wear

behavior

5 cSt STD

Ref OilTest

Oil C

IBERTRIB 2013


A2.2 - A037

APPLICATION OF A STRUCTURED FAILURE ANALYSIS TO THE TRIBOLOGY OF WEAR: BEARING OIL-OUT EVENT Herbert A. Chin1, David A. Haluck1, Lavern Wedeven2

1UTC-Pratt & Withney, Materials & Processes Engineering, East Hartford, CT06108, USA 2Wedeven Associates, Inc., Edgemont, PA 19028, USA

ABSTRACT

In pursuance of the desired state for Wear and Tribology “Wear properties of interacting parts are known (stress, force, relative motion, environment, friction coefficient, … , etc) & the variation is understood, therefore the wear can be accurately predicted and proactively managed, the authors have successfully utilized a Structured Failure Analysis Approach for expeditiously and cost effectively determining Root Cause for Wear. This paper presents details of that work.

INTRODUCTION

Tribology of Wear has evolved over the centuries from tribal knowledge, to lessons learned, best practices, and evolved to the fundamental science of interacting surfaces1. Yet the current state lay somewhere between best practices and a basic Tribology Understanding for aerospace propulsion systems2. As such, root cause and corrective actions are either point solutions or false directions with over engineered compromises. In contrast, the desired state for Wear and Tribology is Wear properties of interacting parts are known (stress, force, relative motion, environment, friction coefficient, … , etc) & the variation is understood, therefore the wear can be accurately predicted and proactively managed. To get to the desired state, the authors have been pursuing a global synergistic systems approach involving the Engineering Disciplines, Design, Materials, Lubrication and Tribology. Key to that approach has been the fundamental tribological science of interacting material pair/surfaces (surface, near surface, sub-surface and core regions). Yet understanding of wear has been elusive as a result of (a) numerous input variables, some of which are not fully understood, and may involve confounding effects of those variables – stress, force, relative motion, etc. and (b) the evolving events from Root Cause, event 1, event 2, event 3, etc., to Final Outcome of Premature Failure, especially when there is a gap in knowledge of the events. To sort these critical factors into meaningful effects and ultimate Root Cause, the authors have successfully utilized a Structured Failure Analysis Approach for expeditiously and cost effectively determining Root Cause for Wear.


Structured Failure Analysis utilized two key elements. (a) Collaboration of an Interdisciplinary Team (Customers, Field Representatives, Integrated Product Teams, Engineering, Suppliers, Quality, Manufacturing & Regulatory Agencies), (b) Root Cause extracted from a Logical Sequence of Events as shown in figure 1. This sequence of events is then interrogated thoroughly via tools like a formalized Fault Tree to identify contributors and confounding effects of input variables. This has been successfully utilized by the authors in determining the effect of oil starvation on premature failure of rolling element bearings as illustrated in figure 2. From that effort, key tribological events were identified that led backwards to the root cause of failure. Once understood, then corrective actions involving, materials type and processing, lubricants



and additive packages, surface finishes etc. were optimized for a robust solution with boundary operating conditions clearly identified.

Figure 1. Schematic illustration of Root Cause extraction from a logical sequence of events.

Figure 2. Schematic illustration of the sequence of events that occur in an oil starved rolling element bearing.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the technical contributions by technical specialists from both Pratt & Whitney and Wedeven Associates that made this global understanding of the Tribology of Wear possible.

REFERENCES

1. Dowson, Duncan, “History of Tribology” Publisher, John Wiley & Sons , Limited, 2012. ISBN, 0470057823, 9780470057827.

2. Wedeven, L., Haluck, D. A., and Chin, H. A., Internal Wedeven Associates & Pratt & Whitney Tribology Research from 1975 to 2012.

WARNING -- This document contains technical data the export of which is or may be restricted by the Export Administration Act and the Export Administration Regulations (EAR), 15 C.F.R. parts 730-774. Diversion contrary to U.S. law is prohibited. The export, reexport, transfer or re-transfer of this technical data to any other company, entity, person, or destination, or for any use or purpose other than that for which the technical data was originally provided by P&W, is prohibited without prior written approval from P&W and authorization under applicable export control laws.

EAR Export Classification: ECCN 9E991.

IBERTRIB 2013


A2.3 - A008

FRICTION ANALYSIS IN BALL BEARINGS Mihai – Tiberiu Lates1(*), Radu Velicu2

1, 2Department of Product Design, Mechatronics and Environment, Transilvania University of Brasov, Brasov, Romania (*)Email: [email protected]

ABSTRACT

The paper presents the friction phenomenon analysis in ball bearings. On a tribometer which is equipped with a block on ring test module, there are achieved tests which indicate the dependencies of the main functional parameters of radial and angular ball bearings (radial force, rotational speed, type of motion) on the friction which appears in them, and which is quantified as the global friction coefficient and the friction torque. Finally, the paper offers recommendations regarding the radial and radial-axial ball bearings exploitation in order to reduce the usage of it with high friction.

INTRODUCTION

The friction in the ball bearings is depending mainly on the friction between the balls and the ball grooves, the friction between the balls and the balls cage and the friction with the lubricant. Analytically is hard to identify with accuracy the influence of each of these frictions on the ball bearings behavior, due to the exploitation conditions which are in a wide range of values and conditions (Jula, 1992), (Williams, 2011).

In the paper are achieved tests, with different exploitation conditions, for a radial ball bearing and for a radial-axial ball bearing; all the results are presented comparatively for the two bearings and the friction differences between them are identified. The tests are performed on a UMT tribometer which is equipped with a block on ring testing module (UMT, 2009). The rotational speed, as input data for the motion, is imposed as values between 10 rpm and 3000 rpm and is defined a one-way and two-way type of motion. The radial load used in the tests has values between 0 and 800 N. The results present, comparatively for the two tested ball bearings, the functioning conditions which assures high and small values for the global friction coefficient.


One of the tests was made for a radial ball bearing 6204 with a rotational speed of the inner ring which is increased forward from 10 rpm to 3000 rpm (during 1 min. for each step) and then the rotational speed is decreased backward from 3000 rpm to 10 rpm. On the outer ring is acting a constant radial load of 800 N.

The results from the tensile tests are shown in Fig. 1. The values of the global friction coefficient are obtained by averaging the values from the forward increasing of the rotational speed and for the backward decreasing of the average speed.

The most significant difference of the global friction coefficient in the ball bearing is between the small values of the rotational speed (between 200 rpm and 1000 rpm) and the highest values (2400 rpm and 2800); in these cases the efficiency of the radial ball bearing is low, so, the exploitation of it is recommended to be avoided for this values of the rotation.




Fig.1 The global friction coefficient variation

The paper will present the variation of the global friction coefficient for ball bearings like: radial none sealed, radial sealed, radial – two rows and angular contact bearings according to the input parameters (the radial force between 0 and 800 N and the rotational speed between 10 and 3000 rpm). The conclusions offered by the studies will be useful as recommendations to the ball bearing exploitation conditions.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Schaeffler Technologies AG & Co. KG, Germany, under the grant CDS Dynamic Tribology 4029/26.03.2008, additional 3/01.02.2012.

REFERENCES

*** UMT Multi-Specimen Test System. Hardware Installation & Application Manual. Center for Tribology, Dell Ave, Campbell, USA, 2009, p.36 - 39.

Jula a.o. Machine elements. Applications. (Organe de masini. Indrumar pentru lucrari de laborator). Transilvania University of Brasov, Romania, 1992, p.156 - 163.

Williams, J. Engineering tribology. Cambridge University Press, USA, 2011, p.56 - 68.

IBERTRIB 2013


A2.4 - A058

TORQUE LOSS ON ROLLING BEARINGS LUBRICATED WITH WIND TURBINE GEAR OILS AT CONSTANT TEMPERATURE Carlos M. C. G. Fernandes1(*), Pedro M. P. Amaro2, Ramiro C. Martins1, Jorge H. O. Seabra2

1Instituto Nacional de Engenharia Mecânica e Gestão Industrial (INEGI), Universidade do Porto, Porto, Portugal 2Departamento de Engenharia Mecânica, Universidade do Porto, Porto, Portugal (*)Email: [email protected]

ABSTRACT

Six fully formulated wind turbine gear oils with the same viscosity grade and different formulations were selected and their physical properties were determined.

Both thrust ball and cylindrical thrust roller bearing friction torque tests were performed on a modified Four-Ball Machine at a constant temperature of 80° C under the following operating conditions: speed between 75 and 1200 rpm and two loads (700 N and 7000 N).

The experimental results showed that wind turbine gear oil formulation has a significant influence on rolling bearing friction torque.

INTRODUCTION

Previous works were done [Fernandes, 2013] whose friction torque generated in cylindrical roller thrust bearings and thrust ball bearings lubricated with wind turbine gear oils was measured in auto-induced temperature conditions. In the present work the operating temperature was kept constant in order to maintain the same viscosity characteristics of the lubricants for different speeds and loads. For that purpose, rolling bearing tests were performed, using thrust ball bearings (TBB – 51107) and cylindrical roller thrust bearings (RTB – 81107) lubricated with wind turbine gear oils (ISOVG 320) at constant temperature, during which the internal friction torque was measured for low and high load, 700 and 7000 N respectively, and varying the rotational speed between 75 and 1200 rpm. The results obtained aim to clarify the influence of gear oil formulation on rolling bearing friction torque for different loads and speeds.

Table 1 – Contact parameters for the different tests

Axial Load (N) TBB RTB

Bearing Type 700 7000 700 7000

RX [m] 6.00x10-3 5.00x10-3

RY [m] 5.34x10-2 -

Ac [µm2] 57.109 123.87 13.76 43.50

p0 [GPa] 1.15 2.48 0.32 1.00


For the same speed and the same load, the specific lubricant film thickness show the same increasing trend when the speed increases in both rolling bearings, whatever the wind turbine gear oils considered. However, RTB always generated higher values than TBB. As an example, at 600 rpm and using PAOR oil, the RTB had a value 33% higher than the TBB.




Figure 1.a) and 1.b) show the total friction torque (MT) inside thrust ball bearings (TBB – 51107) and cylindrical roller thrust bearings (RTB – 81107 TN), respectively. As expected, whatever the gear oil and operating speed, RTB always generated higher total friction torques than the TBB. As an example, at 900 rpm and using ESTR oil, the RTB generated a MT of 295 N.mm, while the TBB generated a MT of 180 N.mm (39% lower). Furthermore, in the case of TBB, MT increased when the speed increased, while RTB showed the opposite trend.

a)

b)

Fig.1 – Total friction torque of (a) Thrust Ball Bearing and (b) Thrust Roller Bearing.

ACKNOWLEDGMENTS

The authors acknowledge to “Fundação para a Ciência e Tecnologia” for the financial support given through the project “High efficiency lubricants and gears for windmill planetary gearboxes" with research contract PTDC/EMEPME/100808/2008.

REFERENCES

[1] C. M. Fernandes, R. C. Martins, J. H. Seabra, Friction torque of cylindrical roller thrust bearings lubricated with wind turbine gear oils, Tribology International (0) (2012) {.doi:10.1016/j.triboint.2012.05.030.

[2] C. M. Fernandes, R. C. Martins, J. H. Seabra, Friction torque of thrust ball bearings lubricated with wind turbine gear oils, Tribology International 58 (0) (2013) 47 {54.doi:10.1016/j.triboint.2012.09.005

.

IBERTRIB 2013




Session B2

Room B002

BIOTRIBOLOGY

Chair:

Dr. Alberto Higuera-Garrido and Prof. Luís Magalhães

IBERTRIB 2013


B2.1 - A022

TRIBOLOGICAL PROPERTIES OF POLYVINYL ALCOHOL HYDROGEL AGAINST ARTICULAR CARTILAGE V.M. Sardinha1, L.L. Lima2,3,6, W.D. Belangero3,4, C.A. Zavaglia3,6, V.P. Bavaresco3,5, J.R. Gomes1,2(*)

1Centre for Mechanical and Materials Technologies (CT2M) Guimarães, Portugal 2Department of Mechanical Engineering, University of Minho, Guimarães, Portugal 3National Institute of S&T in Biofabrication (INCT/BIOFABRIS), Campinas - SP, Brazil 4Orthopaedic Biomaterials Laboratory, School of Medical Sciences, State University of Campinas (UNICAMP), Campinas - SP, Brazil 5Department of Plastics, CTC, State University of Campinas (UNICAMP), Campinas - SP, Brazil 6Department of Materials Engineering, College of Mechanical Engineering, State University of Campinas (UNICAMP), Campinas - SP, Brazil (*)Email: [email protected]

ABSTRACT

This study aims to demonstrate that under tribological loading mimicking in vivo conditions polyvinyl alcohol hydrogel (PVA) against natural articular cartilage consists in a particular combination of mating surfaces with friction and wear characteristics compatible with the application as articular joints. PVA hydrogel membranes were processed to be used as tribological samples against bovine articular cartilage in the presence of distilled water and phosphate buffered saline solution (PBS). The tribological tests demonstrated that PVA hydrogel present excellent tribological performance against natural articular cartilage with very low friction coefficient values (≈0.02 to 0.06) combined with the preservation of both mating surfaces.

INTRODUCTION

The limited self-repair ability of articular cartilage has been leading to the investigation on polymeric hydrogels as potential materials to be used in cartilage tissue repair or replacement. PVA possesses excellent properties as biocompatibility, low coefficient of friction in lubricated conditions against different kind of surfaces, low interfacial tension and high permeability to fluids, emerging as an eligible material to be studied for biomedical applications.

PVA hydrogel membranes with approximately 1 mm thickness were processed from PVA aqueous solutions (Aldrich Mw 89000-98000 g/mol, 99% hydrolized), with 10% (w/w) concentration of polymer in solution, to be used as tribological samples against bovine articular cartilage. In order to ensure the natural properties and resistance of the tissue, articular cartilage samples were collected from condyles and prepared with ~4 mm subchondral bone. Tribological tests were performed on a pin-on-plate tribometer with a linear reciprocating geometry in the presence of distilled water and PBS at 37ºC. The reciprocating sliding frequency and stroke length were kept constant at 1 Hz and 8 mm, respectively. Contact pressures ranging from 1 MPa to 5 MPa were applied. For each experiment the friction coefficient evolution was analyzed throughout the test duration. The surface degradation by wear was characterized by Scanning Electron Microscopy (SEM) and the topography of hydrogel membranes in the hydrated state was evaluated with Atomic Force Microscopy (AFM) for quantitative and qualitative analysis of worn surfaces.





Results show that when tribological tests begin and sliding movement initiates a start-up friction occurs with relatively high values of friction coefficient, typically around 0.10. Than the tendency is the friction coefficient to decrease to a steady-state regime during the immediate seconds after starting the test, characterized by very low friction values, in the range of 0.02 to 0.06. This typical friction coefficient evolution was observed for both lubricant media. Independently of the lubricant fluids there is a tendency for steady-state friction coefficient to decrease with the increase of contact pressure, with PBS showing lower friction coefficient values (Fig. 1). This observation reinforces the advantage taken from using PBS on biotribological tests instead of distilled water.

Fig.1 Steady-state friction coefficient for the different values of contact pressure for PVA hydrogels tested against articular cartilage in distilled water (dH2O) and PBS.

A great resemblance was observed between worn surfaces resulting from sliding in both fluids presenting smooth and homogeneous surfaces. For the maximum contact pressure (5.0 MPa) the level of superficial preservation was relatively kept for both lubricants being visible some slightly marks aligned parallel to the sliding direction. Besides this particularity, the PVA hydrogel contact surfaces seemed to remain well preserved independently of the increase on contact pressure or lubricant used, corroborating the very low friction coefficient values presented in Fig.1. On the other hand, the AFM topographic evaluation of hydrogel worn surfaces (3-D scans and RMS values) was in accordance with SEM observations and friction results.

For all test conditions no signs of surface damage were evidenced on the opposing articular cartilage contact surfaces. This fact, together with the reported friction and wear response of PVA under tribological loading mimicking in vivo conditions, demonstrates that this hydrogel has high potential to be used as substitute of articular cartilage.

ACKNOWLEDGEMENTS

L.L. Lima acknowledges CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior, Brazil, under grant 5858-11-9.

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IBERTRIB 2013


B2.2 - A004

BIOTRIBOLOGY ANALYSIS OF A SURFACE MODIFIED CoCr ALLOY FOR USE IN METAL-on-METAL HIP PROSTHESIS. Cristina Díaz1(*), Stephan Mändl2, Jose Antonio García1, Rafael Rodríguez1

1Asociación de la Industria Navarra (AIN), Ingeniería de Superficies, Pamplona, Spain 2Leibniz-Institut für Oberflächenmodifizierung (IOM), Leipzig, Germany (*)Email: [email protected]

ABSTRACT

Recently renewed interest in metal-on-metal (MoM) hip prosthesis is observed due to persistent problems of loosening and osteolysis in UHMWPE artificial joints (MoP). However, MoM contacts may present important problems of wear, which can lead to a shortening of the prosthesis’ lifetime. In order to reduce the wear rate between both metal parts, some modifications on the surface of CoCr alloys were carried out in this work.

Using the plasma immersion ion implantation technique (PI3), CoCr alloys samples were modified for reducing friction coefficient and wear rate by using either nitrogen, oxygen or a combination of nitrogen and oxygen ion implantation in the temperature range between 300 and 450ºC, allowing a thermally assisted diffusion process in the near surface region. The subsequent analysis of the wear rate in a biotribometer using bovine serum at 37ºC as lubricant. Additionally, roughness and hardness were studied, in order to analyze the influence of these parameters on wear rate.

The results showed a strong reduction of the abrasive wear rate, especially after nitrogen or nitrogen and oxygen implantation into the surface region. A hard and wear resistant zone consisting of an expanded austenitic CoCr:N layer was characterized as the mechanism underlying the improvement. However, no significant decrease of the friction coefficient was observed.

INTRODUCTION

From 2006, releasing of hip resurfacing systems introduced a better solution for hip prosthesis: higher stability and a reduction on the number of dislocations. In this context MoM joints are the contact which offer a lower wear rate (after ceramic-on-ceramic (CoC) contacts which show a reduction between 40 to 100 times lower than MoP joints (Tipper, 2005)). MoM joints present a superior mechanical properties and present longer lifetime than CoC or MoP joints. Therefore, the fracture risk is reduced in comparison to the CoC or MoP joints (Cawley, 2003). At this respect, these days MoM systems for hip resurfacing systems are showing encouraging clinical results (Tipper, 2005; Moraes, 2008).

However, the combination of MoM and resurfacing system is not a full solution. Main fails of MoM joints are coming from residuals of wear processes and also, from corrosive effects which joints are suffering during their lifetime. These processes introduce in the body some harmful ion releasing.

CoCr alloys are used in MoM joints for articulate prosthesis. In this work plasma immersion ion implantation of nitrogen, oxygen and a combination of both of gases was used in order to reduce wear rate in the areas of higher contact of these joints.





Wear tests were carried out using a ball-on-disk equipment, using a treated and untreated CoCr ball. Wear rate of nitriding CoCr alloy were reduced around 80% in comparison to untreated sample (Table 1). However, oxygen treated CoCr alloys showed a wear rate close to untreated sample against untreated CoCr ball. In the case of modified CoCr alloys using nitrogen and oxygen at 400ºC, a reduction in wear rate of 90% approximately were observed in relation to untreated par (Figure 1).

Table 2 Wear rate for treated with nitrogen CoCr against CoCr unimplanted.

Samples Friction

coefficient Wer rate

(10-14m3/Nm)

Reference CoCr alloy 0,26±0,01 7,20±0,80 N (350ºC) 0,16±0,01 1,70±0,20 N (400ºC) 0,26±0,01 1,10±0,10 N (450ºC) 0,26±0,01 1,40±0,10

a). b).

Fig.1 Wear tracks after wear rate tests: a). Reference CoCr alloys against untreated

ball of CoCr, b). N/O(400ºC) treated CoCr alloys against treated ball of CoCr.

ACKNOWLEDGMENTS

The authors would like to thank to the Ministerio de Ciencia e Innovación & Gobierno de Navarra for FUNCOAT project and to the Ministerio de Economía y Competitividad for MoMLIFE (MAT2011-29698-C03-02) and for providing financial support to this projects”.

REFERENCES

Tipper JL, Ingham E, Jin ZM, Fisher J, Current Orthopaedics (2005) 19, 280–287.

Cawley J, Metcalf JEP, Jones AH, Bandb TJ, Skupien DS, Wear 255 (2003) 999–1006.

Moraes M, Rodrigues R, Barr R, Ono NK, Fujiki EN, Milani C, Acta Ortop Bras (2008), 16(1), 19-22.

IBERTRIB 2013


B2.3 - A029

TRIBOLOGICAL BEHAVIOR OF THERMAL-SPRAYED COATINGS WITH BIOMEDICAL PURPOSES H. Melero1*, M. Torrell1, J. Fernández1, J.R. Gomes2, J.M. Guilemany1

1 Thermal Spray Centre, Universitat de Barcelona, Martí i Franqués, 1, 08028 Barcelona, Spain 2 Centre for Mechanical and Materials Technologies (CT2M) and Department of Mechanical Engineering, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal * Corresponding autor: Hortensia Melero. Thermal Spray Centre, Universitat de Barcelona, Martí i Franqués, 1, 08028 Barcelona, Spain. Phone: (+34) 93 402 13 25, Fax: (+34) 93 402 16 38, E-mail: [email protected]

ABSTRACT

This work tries to answer a question not very frequently posed, i.e. which is the behaviour of a biomedical coating in the time of implantation. Some XRD analysis, Ball-on-Flat tests and SEM observation have been done in order to conclude, among four studied cases, that the case with a lower percentage of amorphous phase is the one with better tribological behaviour.

INTRODUCTION

Bioceramic coatings have been employed for many years to improve the biological and mechanical properties of bone implants. Among this kind of materials, the most commonly evaluated mechanical properties are adherence (using the bond strength test or scratch test) (De Groot et al., 1987), shear (Whitehead et al., 1993) or fracture toughness (Filiaggi et al., 1991). However, other properties, such as tribologic ones, have received less attention. Only a few studies have focused on wear behavior, which has been considered of secondary importance compared to other properties. Furthermore, the majority of these studies focus on fretting (Fu et al., 1998). But there is another juncture when the coating is subjected to considerable wear, which is from the time the implant is placed into the bone until it is completely stabilized (Gross et al., 2002). The present study demonstrates that contrary to this assumption, it is important to guarantee the integrity of the coatings during and after implantation, a process that subjects the surface to high wear.

EXPERIMENTAL METHOD

In this study, two approaches have been employed to obtain biomedical coatings: mixing hydroxyapatite powder with titania powder (80%HAp-20%TiO2), and substituting plasma-spraying by High Velocity Oxy-Fuel (HVOF) spraying. These two options have been shown to have a positive influence on mechanical behavior in general (Melero, H. et al., 2011); however, the goal of this study has been to evaluate the influence of these two approaches on tribological behavior. Four spraying conditions have been considered. The crystalline phases of the coating have been analyzed by XRD and Rietveld calculations. Ball-on-Flat tests have been performed at 37ºC and immersed in Hanks solution to characterize the wear properties of HAp-TiO2 coatings, which have previously been shown to present other good mechanical and biological properties, at physiological conditions. Three loads have been employed, 5, 10 and 15 N. The parameters studied have been the friction coefficient and the wear coefficient, calculated with the lost volume analyzed by confocal microscopy. Scanning Electron microscopy has been employed in order to evaluate the coating surface after the tests.





The results show a clear advantage of coatings with a lower percentage of amorphous phase, since these present a higher friction coefficient indicating better implant fixation and a lower wear rate, thus ensuring integrity of the coating. Surface studies reveal a fatigue mechanism in certain zones of the track (presence of cracks), but an abrasion mechanism is also present that intensifies its role with higher loads.

Figure 1. Zone with cracks (left) and delamination (right)

Further research is required to complete wear studies of these coatings, using highly accurate simulations of implantation and subsequent quasi-static conditions.

REFERENCES

De Groot K, Geesink R, Klein CP, Serekian P. Plasma-sprayed coatings of hydroxylapatite. Journal of Biomedical Materials Research, 1987, 21, p. 1375-1381.

Filiaggi MJ, Coombs NA, Pilliar RM. Characterization of the interface in the plasma-sprayed HA coating/Ti-6Al-4V implant system. Journal of Biomedical Materials Research, 1991, 25, p. 1211-1229.

Fu Y, Batchelor AW, Wang Y, Khor KA. Fretting wear behaviors of thermal sprayed hydroxyapatite (HA) coating under unlubricated conditions. Wear, 1998, 217, p. 132-139.

Gross KA, Babovic M. Influence of abrasion on the surface characteristics of thermally sprayed hydroxyapatite coatings. Biomaterials, 2002, 23, p. 4731-4737.

Melero H, Fernández J, Dosta S, Guilemany JM. Caracterización de nuevos recubrimientos biocompatibles de hidroxiapatita-TiO2 obtenidos mediante Proyección Térmica de Alta Velocidad. Boletín de la Sociedad Española de Cerámica y Vidrio, 2011, 50, p. 59-64.

Whitehead RY, Lacefield WR, Lucas LC. Structure and integrity of a plasma sprayed hydroxylapatite coating on titanium. Journal of Biomedical Materials Research, 1993, 27, p. 1501-1507.

IBERTRIB 2013


B2.4 - A016

WEAR ANALYSIS ON DENTAL IMPLANT MATERIALS UNDER PARTIAL SLIP CONDITIONS Rolf Wäsche1(*), Manfred Hartelt2, Simon Zabler3, Katja Nelson4

1, 2 BAM Federal Institute for Materials Research and Testing, Berlin, Germany 3Faculty of Physica and Astronomy, Julius-MaximilianUniversity, Würzburg, Germany 4Department of Oral and Maxillofacial Surgery, University of Freiburg, Freiburg, Germany (*)Email: [email protected]

ABSTRACT

This work investigates the tribological response of titanium materials used for dental implants under partial slip fretting conditions. The tribological tests were carried out on an self built tribometer using the crossed-cylinder contact geometry. The cylindrical samples were made of grade 4 titanium and of titanium alloy Ti6Al4V. Some of these Ti-based samples were surface hardened by oxygen diffusion at high temperature. The results obtained so far show that the surface-treated hardened samples tend to increase the contact area under load. This is an indication for the presence of an effect known as "junction growth" and points at a potentially increased mobility of dislocations in surface hardened samples. An influence of the interfacial medium on the wear behavior could not be detected.

INTRODUCTION

For dental implants as basis for dental prosthesis’s titanium is one of the most used materials mainly due to its good bioactive and reasonably good mechanical properties. However, about 10 to 15% of actual implants cause problems due to loosening of the implant within the bone with a need of a new surgery and replacement of the implant. One reason for this phenomenon could be seen in the materials response under the stress situation developing during the chewing process. Chewing can cause a multi-axial stress in the contact surfaces of the screw connection, which leads to partial slip in the interface of this contact. This can initiate a wear process, which can promote the loosening of the implant. A good wear resistance postpones or even prevents a premature loosening. A better wear resistance might be obtained by surface hardening.

Surface hardening was carried out by an oxygen diffusion process at high temperatures leading to a controlled formation of TiO at the surface. In order to characterize the tribological behavior of the surface hardened titanium material fretting tests under partial slip conditions have been performed. In the tribological tests the crossed cylinder contact geometry was used. The normal force was 10 N resulting in an average Hertzian contact pressure of 500 MPa and a maximum Hertzian contact pressure of 750 MPa, resp. The oscillating tangential force was 5.9 N leading to a tangential stress of 460 MPa. The frequency was 2 Hz. The tests were done at a temperature of 37°C in dry condition, in distilled water and in artificial saliva after Fusayama, resp. The running time was 1000, 30000 and 200 000 cycles.


27 tests have been carried out mating Ti grade 4 and Ti6Al4V materials. Ti grade 4 was used in 3 different modifications: not heat treated (hardened), heat treated for 30 minutes and 120 minutes, resp. The heat treatment time corresponds to different diffusion depths and oxygen profiles in the titanium surface [1]. The wear scars and the obtained profiles do not depend on



the interface condition and are similar for all three cases (distilled water, artificial saliva, dry). However, an increased contact area was observed in all cases, when Ti grade 4 was heat treated for 120 minutes and when the number of fretting cycles was 200 000. Fig. 1 shows optical microscopic images of obtained wear scars on surface hardened material Ti grade 4 after 1000 (left), 30 000 (middle) and 200 000 cycles (right) in distilled water. As may be seen, after 1000 and 30000 cycles no contact area enlargement has developed, only after 200 000 cycles the enlargement of contact area occurs. In the left as well as in the middle picture, the contact area corresponds to the diameter of the Hertzian contact area of about 160 μm in diameter. The profiles in these cases show only a slight increase of surface roughness without measurable wear in depth. For 200 000 cycles the wear scar shows a significant enlargement of 320 μm in diameter. The core area of the Hertzian contact pressure zone is also visible and similar to the areas in the two other images. The calculation of the relevant yield stress out of normal and tangential force shows that it was in all cases below the yield value of the mated materials. Therefore the observed results may point at the effect known as junction growth and described by Tabor [2] and is practically the material response for stress relaxation under the applied experimental conditions [3].

Fig. 1. Optical microscopy images of the wear scars and the corresponding profiles for surface hardened material Ti grade 4 (thermal treatment 120 min at 850°C).

ACKNOWLEDGMENTS

The help of Mrs. A. Krause is gratefully acknowledged.

REFERENCES

[1] Zabler S. Interstitial oxygen diffusion hardening – A practical route for the surface protection of titanium. Materials Characterisation 62, (2011)1205-1213

[2] Tabor, D. Junction growth in Metallic Friction: The role of combined Stresses and Surface Contamination. Proc.R.Soc.Lond.A9 vol251no.1266, (1959) 378-393

[3]Etsion E. Revisiting the Cattaneo-Mindlin Concept of Interfacial Slip in Tangentially Loaded Compliant bodies. Journal Trib. 132,(April 2010) 020801-1-9

IBERTRIB 2013




Session A3

Room B001

ROLLING BEARINGS 2

Chair:

Prof. Erik Kuhn and Eng. Beatriz Graça

IBERTRIB 2013


A3.1 - A039

EVOLUTION OF HARDENED AND SOFTENED REGIONS IN RCF AFFECTED ZONES IN M50NIL RODS Abir Bhattacharyya, Ghatu Subhash, Nagaraj Arakere

Mechanical and Aerospace Engineering, University of Florida, Gainesville FL, USA 32607

INTRODUCTION

Advanced aerospace applications require the rolling element bearings to endure billions of rolling contact fatigue (RCF) cycles at very high stress levels. This requirement led to the development of new generation steels such as M-50 Nil and P-675. Unlike through hardened steels (M-50), these steels are graded such that the plastic mechanical properties, such as hardness vary along the depth from case to the core. The hard case along with tough core is expected to increase the fatigue resistance of these new generation steels.

EXPERIMETNTAL PROCEDURE

Our current study intends to investigate the evolution of plastic deformation during rolling contact fatigue of M-50 NiL rods of 9.5 mm diameter using a 3-ball rod tester (Glover, 1982). In this tester, shown schematically in Fig.1, the rod is supported by three silicon nitride balls of 12.7 mm diameter and the rod is rotated at a speed of 3600 rpm resulting in 8600 contacts per minute on the rod. A Hertzian contact stress of 5.5 GPa (800 Ksi) was used to accelerate the fatigue induced microstructural changes. This test was performed at a desired location along the axis of the rod for set number of cycles and then the rod is advanced to repeat the same process for a different number of RCF cycles. At each location RCF tracks are formed as a result of accumulated damage in the rod. The damaged zone was revealed by sectioning the rods longitudinally and radially and then by etching.

Figure 1: Schematic of 3-ball-rod test performed on M-50 NiL rod

EXPERIMENTAL RESULTS AND DISCUSSION

The RCF affected zones were probed using Vickers micro-indentation technique at 200 g to characterize the influence of accumulated plastic strain. The micro indentation hardness measurements revealed that the material beneath each track behaves elastically up to several



thousand cycles and then starts to accumulate plastic strains over several hundred million of cycles. This RCF affected plastic zone grows in size with increasing number of cycles. The highest hardness is measured at the center of the plastic zone directly beneath the contact point and the hardness reduces with increasing radial distance until it finally reaches virgin hardness. The change in hardness value from the virgin hardness was plotted as a function of depth from the surface to generate hardness contour plots. A typical plot at 27.5 million RCF cycles is shown in Fig. 2. These contour plots represent the evolution of plastic strains in the RCF affected zones. Such plots are generated at different RCF cycles from few thousand cycles to several hundred million cycles to investigate the evolution plastic zone size as well as the level of hardening and softening zones. Maximum hardening is observed near the surface along the centerline of the plastic zone and the hardening level reduces with the depth and width inside the plastic zone. Surrounding this plastic zone, regions of lower hardness (compared to virgin material) were noted which represent softened regions. These regions also grow in size with increasing RCF cycles. Utilizing Hertzian stress field solution we have been able to rationalize the origins of these hardening and softening regions inside the plastic zone.

Figure 2: Contours of hardness change from virgin hardness for M-50 Nil steel rod subjected to 27.5 million RCF cycles. The numbers are in HV scale.

REFERENCES

Douglas Glover, 1982, “ A Ball–Rod Rolling Contact Fatigue Tester”, Rolling Contact Fatigue Testing of Bearing Steels, ASTM STP 771, J.J.C Hoo, Ed., American Society for Testing and Materials, pp. 107-124.

IBERTRIB 2013


A3.2 - A040

DETERMINATION OF MECHANICAL PROPERTIES OF ROLLING CONTACT FATIGUE AFFECTED ZONES IN M50 BEARING STEEL BALLS Nagaraj Arakere1(*), Ghatu Subhash2

1, 2 Mechanical & Aerospace Engineering, University of Florida, Gainesville, Florida-32611 (*)Email: [email protected]

ABSTRACT

The subsurface material of bearing raceways subjected to rolling contact fatigue (RCF) experience localized cyclic micro plastic deformation. Current life prediction methods based on extensions of the Lundberg and Palmgren (LP) model do not account for the gradual subsurface material degradation due to RCF as a function of cycles. In this research, we investigate the evolution of the constitutive response of bearing steel as a function of number of cycles. M50 steel balls were subjected to RCF loading under controlled temperature, Hertzian stress level and cycle count, to characterize the evolution of local mechanical properties within the RCF-affected zone as a function of cycles. A novel approach involving extraction of micro-compression specimens inside the RCF-affected zone is used to determine localized stress-strain curves. Micro-indentation mapping is also used to measure micro hardness. We present changes in micro-hardness, yield strength, and strain hardening behavior of the RCF affected material.

INTRODUCTION

Rolling contact fatigue (RCF) in bearings subjects a small volume of material in the vicinity of contacting surfaces to a complex multi-axial state of stress with non-proportional loading [1], resulting in microstructural evolution as a function of cycles, eventually leading to subsurface crack nucleation and spall generation. Current approaches to bearing fatigue life estimation [2, 3] do not account for the evolution of elasto-plastic material properties within the RCF-affected zone, as a function of cycles. We have performed RCF tests on 2.25 in diameter M50 balls under carefully controlled temperature and Hertzian stress conditions. The balls were then sectioned (Fig. 1b), polished and nital-etched to reveal the RCF-affected microplastic zones (Fig. 1a). Micro-indentation mapping was performed to measure the changes in micro hardness and hence yield strength of the RCF-affected material. We have also used a novel approach to measure local stress-strain properties by extracting micro-compression specimens via EDM (Fig. 1c).


Results from the indentation study show that there is a clear increase of over 10% in hardness of over virgin material within the RCF affected zone, from work hardening. Micro-compression testing and resulting flow curves confirmed the increase in yield stress, as shown in Fig. 1(d). The plastic modulus of the RCF-affected material was also measured. This is the first meaningful characterization of local material properties within the RCF-affected zone and will be valuable for a better understanding of the complex fatigue damage evolution process of ball bearings as a function of cycles and applied stress.



ACKNOWLEDGMENTS

This work was partially supported by the National Science Foundation Award CMMI-0927849 under program officer Dr. Clark V. Cooper. The authors thank Don Anthony of SKF, Falconer, NY for supplying the RCF-tested M50 balls.

REFERENCES

Voskamp, A.P., 1998, “Fatigue and Material Response in Rolling Contact,” Bearing Steels: Into the 20th Century, ASTM STP 1328, pp. 152-166.

ISO, 1989, “Rolling Bearings – Dynamic Load Ratings and Rating Life,” Draft International Standard ISO/DIS 281, 1989, ISO, Geneva, Switzerland.

E. Ioannides, T.A. Harris, A New Fatigue Life Model for Rolling Bearings, Journal of Tribology, 107 (1985) 367-377.

Zaretsky, E.V. (2010). Rolling bearing life prediction, theory, and application. in Recent developments in wear prevention, friction and lubrications, G.K. Nikas, Ed., 45-136.

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IBERTRIB 2013


A3.3 - A038

EFFECTS OF SURFACE/SUB-SURFACE COMPRESSIVE RESIDUAL STRESS ON AEROSPACE BEARING BALL PERFORMANCE

David A. Haluck1, William P. Ogden2, Jefferi J. Covington3

1Pratt and Whitney Aircraft, West Palm Beach, FL, USA 2Pratt and Whitney Aircraft, East Hartford, CT, USA 3United Technologies Research Center, East Hartford, CT, USA

ABSTRACT

This work compares the performance of aerospace quality balls made from AISI 6491 steel (M50) processes to produce various levels of compressive residual stress. Testing was performed in single ball tester apparatus under various stress and temperature ranges and finally in operational field service conditions.

Keywords: aerospace balls, M50 steel, M50NiL steel, compressive residual stress.

INTRODUCTION

The chronology of improvements in aerospace bearing materials have been thoroughly documented (Zaretsky, 1989), illustrating the importance of steel cleanliness, improvements in non destructive inspections and with the introduction of case carburized materials such as M50NiL, and the resultant deep compressive residual impact on significantly improving rolling contact fatigue capability. Similar results were achieved on case carburized AISI 9310 via shot peening (Zaretsky, 1982). Resistance to stress corrosion cracking (SCC) was documented when comparing the through hardened AISI 440C to case carburized AISI 9310 and M50NiL (Torres, 1991). The results clearly showed a significant benefit in arresting SCC if the deep compressive residual stresses were greater than the mounted hoop stress of the bearing inner race. Finally, work performed on different processes on M50 balls (Spitzer, et al, 2004), again showed similar results, i.e. the magnitude and depth of compressive residual stresses improve the fatigue life of balls. The purpose of this study was to evaluate the metallurgical performance (Voskamp, 1996) of various processes on M50 balls while operating at various temperatures and stresses.


Single ball testing was performed in a rig developed by Pratt & Whitney by SKF Aerospace. The variation in load and temperature was specifically designed to provide the maximum amount data with minimum number of tests. As such, loads often exceeded actual operating conditions, while temperatures were rig limited.

The various conditions tested included conventional M50; duplex hardened M50 and tumbled M50. Previous testing (Spitzer et al, 2004) showed duplex hardened ball has significantly long fatigue lives when compared to M50 and tumbled M50, but at unrealistic Hertzian contact stress. Under these conditions the harder duplex hardened surface proved to have longer lives. However, when more realistic stresses were tested the depth of the Von Mises stress were significantly below the compressive residual stress of the duplex hardened surface, as such, the metallurgical change was more similar to the results of conventional M50. However the surface and significantly higher subsurface compressive residual stress profile of tumble M50 showed less metallurgical change when tested under similar conditions to duplex hardened M50.



Tumbled balls processed with different levels of compressive residual stress at a common depth beneath the surface showed again the more compressive residual stress process out performed the shallower processed balls. These results confirm previous industry test results and provide insight into achieving longer life balls for aerospace applications.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Ronald F. Spitzer whose in-depth understanding and experience in testing aerospace bearing components while at SKF Aerospace provided significant credibility to our findings

REFERENCES

Zaretsky, Erwin V., Selection of Rolling Element Bearing Steels for Long Life Applications, NASA Special Technical Publication 987, 1989

Townsend, Dennis P, Zaretsky, Erwin V., Effects of Shot Peening on Surface Fatigue Life of Case Hardened AISI 9310 Gears, NASA Technical Paper 2047, 1982

Harris, T.A, Skiller, John, Spitzer, Ronald F., On the Fatigue Life of M50NiL Rolling Bearings, Tribology Transactions, Volume 35 1992, p731-737

Torres, P.D., Stress Corrosion Study of Carburized AISI 9310 and Carburized M50NiL Steels, NASA Technical Paper 3148, Sept. 1991

Voskamp, Arend Pieter, Microstructural Changes during Rolling Contact Fatigue, ISBN 90-90 10 187, 1996

WARNING -- This document contains technical data the export of which is or may be restricted by the Export Administration Act and the Export Administration Regulations (EAR), 15 C.F.R. parts 730-774. Diversion contrary to U.S. law is prohibited. The export, reexport, transfer or re-transfer of this technical data to any other company, entity, person, or destination, or for any use or purpose other than that for which the technical data was originally provided by P&W, is prohibited without prior written approval from P&W and authorization under applicable export control laws. EAR Export Classification: ECCN 9E991.

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IBERTRIB 2013


A3.4 - A025

THEORETICAL AND EXPERIMENTAL STUDY OF FRICTION IN BEARING MOUNTINGS Radu Velicu1(*), Mihai – Tiberiu Lates2, Radu Saulescu3, Alina Todi-Eftimie4

1, 2,3,4 Department of Product Design, Mechatronics and Environment, Transilvania University of Brasov, Brasov, Romania (*)Email: [email protected]

ABSTRACT

The paper presents a study of friction on the existing bearing mountings of a testing rig for chain or belt drives. Theoretical study is based on separation of friction depending on their origin and establish of analytical dependencies between friction torque, load, rotational speed, oil temperature and other parameters involved. Experimental tests show an image of the influence of load, rotational speed and oil temperature on the global friction torque. A study of the dynamic friction during heating and cooling is also performed and conclusions on influences on global friction are drawn.

INTRODUCTION

The subjects of the study are the two bearing mountings of a testing rig for transmissions with parallel axes (chain, belt). Figure 1 presents one of the bearing mountings. The other is similar but not identical.

Fig.1 Bearing mounting

Both bearing mountings are consisted of: radial ball bearing (1 – see Fig.1), which takes radial force and possible axial forces on both directions; radial cylindrical roller bearing (2), taking the most important radial force; sealing rings (3) at both ends; lubrication with oil circuit. The rig gives the possibility to measure friction torque, by controlling and measuring radial loading at one end of the shaft, rotational speed, oil temperature and pressure.




It is of maximal importance in the correct evaluation of experimental measurements on the rig, to have accurate data on the friction on bearing mountings. The global friction on these bearing mountings is a sum of friction in bearings and sealing elements, with important influence of the lubricating circuit. It is difficult to separate and identify with accuracy each friction and consider all the influences (Williams, 2011). Anyway, the analytical relations show that the global friction depends on rotational speed, radial load (axial load will not be considered), bearings and sealing types (geometry and materials), type of oil, temperature and pressure of oil.

EXPERIMENTAL TESTING AND CONCLUSIONS

Tests determining friction torque have been developed for radial forces in the range 0...3kN, rotational speed in the range 500...5000 rpm, oil temperature in the range 20...50ºC. Figure 2 shows the variation of the ratio between friction torques measured at 35ºC and 50ºC, for two values of the radial load F (see Fig. 1), for several values of rotations.

Fig.2 The influence of oil temperature and load on friction torque depending on

rotation

There have also been investigated the effects of dynamic change of temperature (heating and cooling) on the global friction, for constant speed rotations n = {1000, 2000, 3000, 4000, 5000rpm} and for constant loads F = {1kN, 3kN}. Conclusions on the influence of different parameters on global friction torque can be usefull.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Schaeffler Technologies AG & Co. KG, Germany, under the grant CDS Dynamic Tribology 4029/26.03.2008, AA 3/01.02.2012.

REFERENCES

Williams, J. Engineering tribology. Cambridge University Press, USA, 2011.

Stachowiak, G.W., Batchelor, A.W. Engineering tribology, Elsevier, 3rd ed. 2005

Tfr

35

/Tfr

50

.

Rotation [rpm]

Load 1kN

Load 3kN

IBERTRIB 2013




Session B3

Room B002

SURFACE MODIFICATION

Chair:

Prof. Amílcar Ramalho and Dr. Ramiro Martins

IBERTRIB 2013


B3.1 - A048

MULTI-BODY DYNAMIC INVESTIGATIONS OF DRY ARTIFICIAL HIP JOINTS WITH A CLEARANCE Ehsan Askari1(*), Paulo Flores2, Dane Turner1, Richard Appleyard1

1 Australian School of Advanced Medicine, Macquarie University, Sydney 2Department of Mechanical Engineering, School of Engineering, University of Minho, Portugal (*)Email: [email protected]

ABSTRACT

An assessment of the applicability of modified Hertzian contact and friction-induced vibration to prosthetic hip bearings as clearance joints is investigated throughout this work. Moreover, a simple and robust multibody dynamic model is proposed by considering a specific cross-section. A FFT frequency analysis of the audible sounds from CoC hip acceleration is carried out to examine hip squeaking. In addition, the ball center trajectory associated with a normal walking cycle and the frequecnies in which hip implants squeak are studied.

INTRODUCTION

Ceramic-on-ceramic (COC) bearings for hip arthroplasty have shown high wear resistance and low biological reactivity of wear particles. However, the occurrence of squeaking has been discussed recently as a cause for concern in total hip arthroplasty (THA). Hip squeaking of the hip implants has been reported over the last few years with a prevalence rate from under 1% to 20% (Restrepo et al. 2008).

In the present work, a multibody dynamic model is formulated to evaluate the trajectory of the ball center with respect to the cup, which is assumed to be clamped. The friction-induced vibration and contact-impact events occurring between the head and cup surfaces are taken into consideration as external generalized forces in the governing equation of the ball motion. In order to solve nonlinear and non-smooth dynamic governing equations, an explicit numerical approach, adaptive Runge-Kutta-Fehlberg method, is used to discretize the time interval of interest. In-house software is written to handle the problem. After obtaining the acceleration of the ball center versus the time, a FFT frequency analysis of the audible sounds from CoC hip acceleration is carried out to assess the occurrence of hip squeaking. The effect of hip implant sizes and friction coefficient are also studied.


In the contact of this study, the angular velocity around the horizontal axis, that representing the flexion/extension motion, together with the load in the vertical direction is considered. In the sequel of this process, vertical load and angular velocity are utilized from ISO14’242-1.

The trajectory of the head centre is shown in Fig. 1 during a normal walking cycle. The corresponding clearance joint is 5×10-5 m and the initial position of the ball center is r=0i+1×10-

5j, as it can be observed in Fig.1.

FFT frequency analysis of the audible sounds from CoC hips are represented in the plots of Fig. 2. The frequencies are in the range of hip squeaking frequencies 400-7500 Hz, which are corroborated by the work by Walter et al. (2008).



Fig. 1. Head center trajectory relative to cup center, which rx and ry are defined as r=rxi+ryj.

Fig. 2. FFT frequency analysis of the squeaking sounds from COC hips.

In short, this study proposed a multibody dynamic model to consider the relative motion between the head and cup of artificial hip joints, including both friction-induced vibration and contact-impact mechanics. Moreover, it was shown that the model was successful to address squeaking of hip implants as the friction coefficient increases. The size of artificial hip joint’s components affected the magnitude of hip squeaking frequencies.

REFERENCES

Restrepo C, Parvizi J, Kurtz SM, Sharkey PF, Hozack WJ, Roth- man RH. The noisy ceramic hip: is component malpositioning the cause?, Journal of Arthroplasty, 2008, 23, p. 643-649.

Walter WL, Waters TS, Gillies M, Donohoo S, Kurtz SM, Ranawat AS, Hozack WJ, Tuke MA. Squeaking hips Journal of Bone and Joint Surgery-American, 2008, 90(4), p. 102-111.

IBERTRIB 2013


B3.2 - A047

CHARACTERIZATION OF MODIFIED STEEL SURFACES WITH SCANNING AUGER AND SCANNING MICROPROBE XPS J. S. Hammond, D. F. Paul, S. Alnabulsi, W. Betz

Physical Electronics USA, 18725 Lake Drive East, Chanhassen, MN. 55317 USA

ABSTRACT

The surface analysis techniques of scanning Auger and X-ray photoelectron spectroscopy

(XPS) have become powerful tools for the characterization of tribology phenomena on steel

surfaces. Recently, the improvements in field emission electron based scanning Auger

instrumentation allow the elemental and chemical analysis of features with spatial resolution

down to 10 nm. The use of a Cylindrical Mirror Analyzer (CMA) with the field emission

electron gun column placed in a coaxial geometry within the CMA facilitates the acquisition

of Auger images without topographically induced shadowing for features from the micron to

nanometer scale. High energy resolution acquisition of Auger images and Auger depth also

facilitates chemical state characterization of inorganic species on complex modified steel

surfaces. These nanometer scale surface analysis capabilities will be illustrated with studies

of steel surfaces modified under different annealing conditions as well as steel surfaces used

in tribological research studies.

In addition to scanning Auger, the unique scanning x-ray microprobe XPS technique can

provide even more complex chemical surface characterization with a 10 micron spatial

resolution. In this study, a wear track of MoS2 produced by a pin on a rotating steel disc

surface was characterized by a monochromated scanning x-ray XPS. The x-ray source

utilizes a focused electron beam that can be electronically raster scanned upon an Al anode for

x-ray generation and an ellipsoidally shaped quartz crystal monochromator that refocuses the

x-ray beam onto the sample surface. Therefore, when the electron beam is scanned on the

anode surface the refocused x-ray beam is scanned on the sample surface. This scanning x-ray

source provides high performance large-area spectroscopy, x-ray beam induced secondary

electron imaging for rapid and confident location of small sample features, unique multi-point

micro-area spectroscopy performance, chemical state XPS imaging, and multi-point high

performance sputter depth profiling with x-ray beams from less than 10 µm to 300 µm in

diameter.

The scanning x-ray induced secondary electron image (SXI) identified parallel 15 micron and

25 micron tracks separated by 10 microns. XPS survey spectra, XPS chemical state imaging

and XPS chemical state line scans quantified the presence of MoS2 tracks on the surface of

the steel. Sulfate species were also observed on the surface of the MoS2 tracks as well as

adjacent to the MoS2 tracks.

Multi-point sputter depth profiles on and off the track confirm the presence of the chemistry

of MoS2 mixed with iron oxide and iron hydroxide in the wear track. Underneath the wear

track, the depth profile confirmed the presence of elemental iron. The depth profile at the

point adjacent to the wear track confirmed the presence of an iron oxide and iron hydroxide

layer covering the elemental iron. These micro-XPS results elucidate the two and three

dimensional chemistry of the MoS2 wear tracks.



The synergistic capabilities of these two surface analysis techniques provide a powerful

capability for the characterization of micro- and nano-scale features for tribology research.

Scanning Auger provides nano-scale quantitative two dimensional surface characterization

with the additional capability for sputter ion induced depth profiling from the surface through

films up to one micron in thickness. Scanning x-ray microprobe XPS can provide micro-scale

quantitative chemical state characterization with two dimensional imaging as well as chemical

state depth profiling from the surface through films up to one micron in thickness.

IBERTRIB 2013


B3.3 - A035

COVERING AND HARDENING STRUCTURED SURFACES BY LASERS ACTIONS FOR TRIBOLOGICAL APPLICATIONS G. Vasconcelos1, D. C. Chagas1, K.C. Jorge1, R. Riva1, C.L. Barbosa1, J.G.A.B. Simões1, Martins V. G.2

1Institute of Advanced Studies - IEAv, EFO-L, S. J. dos Campos, SP, Brazil Instituto Nacional de Pesquisas Espaciais2 - INPE, S. J. dos Campos, SP, Brazil

ABSTRACT

In the present work, a laser beam was used to structure and cover, with a solid lubricant material, surfaces samples of AISI 4340 steel. The rate of holes ranged from to 10% to 55% to the original surface. The pits were made with a pulsed Nd:YAG laser beam; the covering and the thermal treatment process, with a CO2

laser beam of 50 W. The reduced friction of the mechanical part was confirmed by a tribo-tester in a reciprocal mode. Friction near to 0.08 were obtained with hardened enhanced to 900 Hv. Keywords: lasers, structured, coefficient, friction, wear, carbon black, CO2 laser, coating.

INTRODUCTION

The application of photo-absorbing coatings is a common practice, especially when lasers of low power density are used. These materials, normally MoS2, graphite and carbon black, further the coupling of incident radiation, reducing the losses by reflection, common to the process, when CO2 lasers are used as radiation source. One surface pre-coated with graphite when irradiated with a controlled laser beam parameters remains, after irradiation, part of the carbon coating. In pin on disc tests, it was observed a reduction in the coefficient of friction in surface of these coatings. Reis [1] improvement on the surface hardness, even using laser with low density of energy power. This hardenning process was attributed to better coupling in the region of beam interaction with the metal surface. The laser hardening consists in heating and rapid cooling the steel surface. If the power density is enough, a layer on the steel surface will reach the austenitizing temperature (during heating) and then with rapid cooling, place the formation of martensites Ganeev [2]. The depth of the surface treated is determined by the law of thermal conductivity, where the propagation of heat occurs in a region of higher temperature to a region of lower temperature Benedeck [3]. The laser hardening allows the hardening of specific areas with controlled depth and with minimal surface deformation when compared to others methods. It also improves the mechanical properties and fatigue resistance, attraction, wear (reducing the friction factor) and increase resistance to corrosion Dohotre [4]. In this work it will be evaluate the use of carbon black to replace the graphite used in the work of Reis [1], to eliminate the stage of solution preparation and the performance of a structured surface with a Nd:YAG laser. The carbon black will be lodge in the micropits generated by a Nd-YAG laser. These pits can reduce the frictional forces and wear rate, acts as a trap for abrasive particles and act as reservoirs for the carbon black lubricant Schreck [5].

METHODOLOGY

The steel used in this work was AISI 4340. Its chemical composition was assessed by the optical spectrometer Thermo Scientific, Model ARL 3460 OES Metals Analyzer, presented in Table 1. Carbon black is formed by fine particles obtained by the process of pyrolysis or partial combustion of hydrocarbon gases or liquids. These nano-particulate structure, allows the coating with thin layers Sector Report [6]. The shape of the particles was observed by scanning electron microscopy (SEM), Zeiss / EVO MA10, as shown in Figure 1.

Table 1: Chemical composition of steel AISI 4340 -% mass

Fe C Mn Si Cr Ni Mo P S Steel 4340 95.8 0.361 0.638 0.261 0.794 1.702 0.221 0.024 0.008



Fig. 1: SEM of particles of carbon black (1320X) Fig. 2: Histogram of particle size distribution of carbon Black.

The particle size of the lubricant can influence the thickness of the coating deposited after irradiation with the laser beam. In order to determine the size distribution of particles, carbon black was subjected to particle size analysis through testing by laser diffraction (CILAS 1064L, range from 0.04 to 500 μm). The results of this analysis are presented in Figure 2.

Samples of AISI 4340 steel with a thickness of 3 mm and 20 mm diameter, previously sanded (SIC paper 600), were used. A ND:YAG laser with beam parameters described according to the To generate the pits, the laser beam was focused on the steel surface.Three rates of structuring were used 10, 28 and 55% of the total area. The pits diameter and depth for a sample 55% structured can be seen in the Fig. 3.

The samples of AISI 4340 steel after laser structured were coated with a solution prepared with 10 g of carbon black and 0.1g of carboxilmetilcelulose in 100 ml of ethanol.

This solution was mechanically mixed for 20 minutes in a plastic container with metal balls for the homogenization of the solution.

Subsequently, the solution was sprayed with a pneumatic pistol on the surface of steel samples previously heated to 60°C. Then, the samples were irradiated with a CO2 laser beam (50W) with diameter of 300 μm. In the region of action beam on the sample surfaces, we used a gas flow of nitrogen to prevent oxidation.

Figure 3: Size and depth of the surface structured with rate of 55% of the total area. Depth near to 5 m and diameter near to 70 m.

Figures 4 and 5 show the diagram of experimental set-up of the treatment process.

CONCLUSIONS

The experiments conducted, indicate that the use of carbon black is feasible, presenting results as microhardness improvements, reduction of friction coefficient and extent of the treated layer, similar to results reported by Reis [1]. An addiction, it eliminates the grinding step, which is required when using graphite. Laser coatings with carbon black can promote reduction of friction coefficients even in structured mechanical parts.

REFERENCES

1- REIS, J. L., Thermal treatment of AISI M2 Steel by CO2 Laser, 104f. Master Dissertation in Physics and Chemistry, Aeronautics Institute of Technology, ITA, Brazil, 2009.

2- Ganeev, R. A., Low-power laser hardening of steels, Journal of Materials Processing Tech., 121, 414-419, 2002.

3- Benedeck, J.; Shachrai, A.; Levin, L., Case hardening of steel by a CO2 laser beam, Optics and Laser Technology, 1980.

4- Dohotre, N. B, Lasers in Surface Engineering: Surface Engineering Series, Volume 1, ASM International – The Materials Information Society, Chapter 1 and 3, 1998.

5- S. Schreck, K.H. Zum Gahr, Laser-assisted structuring of ceramics and steel surfaces for improving tribological properties, Applied Surface Science, 247 616-622, 2005.

IBERTRIB 2013


B3.4 - A036

SURFACE TEXTURING DISTRIBUTION FOR CIRCULAR RELATIVE MOTION Alberto Higuera1(*), Andy Cross2, José Luis Viesca3, Rubén González4, Farshid Sadeghi5.

1, 3, 4 Department of Mechanical Engineering, University of Oviedo, Oviedo, Spain. 2, 5 Mechanical Engineering Tribology Laboratory, University of Purdue, IN, USA. (*)Email: [email protected]

ABSTRACT

This work studies the annular distribution of surface texturing that leads to achieve more accurate results when pin-on-disk tests or circular relative motion are carried out. Different types of distributions were analysed according to the ones that are commonly used by several authors. More accurate and efficient surface texturing distributions were proposed and studied, showing a better layout and a more constant density, even at the farthest track of dimples, than the ones used so far.

INTRODUCTION

When pin on disk tests are carried out to study the behavior of surface texturing, authors use to choose between two types of texturing global distribution (Kovalchenko, 2011; Cho, 2011): random or according to a pattern. A random distribution guarantees a constant macro-density, not so if micro-density is taken into account. Furthermore, lubrication flow cannot be controlled as it responds to the distribution of the pockets on the surface. On the other hand, if the texturing distribution follows a pattern the flow can be easily controlled and its behavior can be known. However, current used patterns don’t fit well with disk specimens and pin-on-disk tests as authors use to texture the surface according to a fixed angle or a rows-columns pattern. Consequently, the farthest tracks to the center of the disk, where linear speed is also higher, show longer distance between dimples than the closest ones. It has been proved that the texturing distribution affects the tribological behavior of surfaces in contact (Higuera, 2011; Amanov, 2013), that so, an inefficient distribution, even with the same density, can show worse wear and friction behavior than a surface with an efficient texturing distribution.

The aim of this study is to find a texturing distribution that minimizes the effect of the track radius on the density/distance between pockets. As a secondary goal, the way to carry out an offset distribution of the dimples is also studied. The offset distribution has been proved that improves the wear behavior of the surface as increases the area covered by the lubricant film, preventing the surface-surface contact (Higuera, 2012).

On the other hand, the viability of the texturing process was also considered a paramount factor, as well as the ability to cut different pocket shapes. That so, a software application was developed and four types of surface distribution were studied: same angle, same angle with offset, same “linear density”, and same “linear density” with offset. The software application can create whatever distribution is required, for this presentation, an annular distribution with an inner diameter of 40 mm, outer diameter of 168 mm, 9 tracks (17 when offset is chosen) and an angle of 20º has been used. The dimples’ diameter was set at 3600 µm only for viewing purposes. The same density distribution was achieved by adding two times the number of dimples in the previous track when the distance between pockets was long enough to keep the same minimum distance between them.





The above mentioned distributions were carried out, as shown in Fig. 1.

Same Angle Same Angle with Offset Same Density Same Density with Offset

Fig. 1. Texturing Distributions.

The results were studied representing two pins located at different distances from the center of the disk, as shown in Fig. 2.

Same Angle Same Angle with Offset

Same Density Same Density with Offset

Fig. 2. Pins on Disk.

The results show that when the proposed same density distribution is used the contact area under the pin shows a more homogeneous number of dimples in that area. Consequently the expected tribological behavior will be comparable.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by the Ministry of Education and Science and the Ministry of Science and Innovation, Spain, within the framework of the research project DPI2010-18166. The authors also wish to express their thanks to the Mechanical Engineering Tribology Laboratory (University of Purdue, IN) and to the Campus of International Excellence, University of Oviedo, for their contribution to this work.

REFERENCES

Kovalchenko A, Ajayi O, Erdemir A, Fenske G. Friction and wear behavior of laser textured surface under lubricated initial point contact. Wear, 2011, 271 (9-10), pp. 1719-1725.

Cho MH, Park S. Micro CNC surface texturing on polyoxymethylene and its tribological performance in lubricated sliding. Tribology International, 2011, 44 (7-8), pp. 859-867.

Amanov A, Tsuboi R, Oe H, Sasaki S. The influence of bulges produced by laser surface texturing on the sliding friction and wear behaviour. Tribology Int., 2013, 60, pp. 216-223.

Higuera A, González R, Cadenas M, Battez AH. Tribological behavior of laser-textured NiCrBSi coatings. Wear, 2011, 271 (5-6), pp. 925-933.

Higuera A, Wang C, González R, Battez AH, Sadeghi F. Non-uniform behavior of lubricant flow according to surface texturing distribution. Proceedings of the ASME/STLE 2012 International Joint Tribology Conference.

IBERTRIB 2013




Session A4

Room B001

GREASE LUBRICATION

Chair:

Dr. Lavern Wedeven and Prof. Jorge Castro

IBERTRIB 2013


A4.1 - A001

ENERGETICS OF STRUCTURAL DEGRADATION OF LUBRICATING GREASES E.Kuhn, Department of Mechanical Engineeing and Production, Hamburg University of Appl. Sc.

(*)Email: [email protected]

ABSTRACT

The tribological behaviour of lubricating greases is investigated from an energy point of view.

Some experimental work is done with the help of rheometer experiments. Finally an analysis is

done to describe the energetic situation of the observed system.

INTRODUCTION

Lubricating greases are viscoelastic lubricants with a complex rheological and tribological

behaviour. An applied shear stress leads to a strong dependence on time of the liquid friction

and to a degradation process.

To get information about the tribological process inside the grease film oscillating rheometer

measurements were done and interpreted from an energetic point of view [1].

The degradation of the thickener network is understood as a process of transition of a critical

energy level. This mechanism is caused by the liquid friction. Some ideas are presented to

describe the structural degradation with the help of energy densities obtained from oscillating

measurements [2, 3].

Finally a thermodynamic analysis is made to describe the energetic situation for the observed

grease volume element. For this purpose an idea of entropy production and entropy flow within

the grease film is developed.

INTERPRETATION OF OSCILLATING MEASUREMENTS

Oscillating rheometer measurements were used to characterise the tribological process inside

the grease film. Amplitude sweeps were made to observe the lubricant behaviour during an

increasing frictional stress.

To investigate the structural degradation, an interpretation of Fig.1 delivers an energy density

for the elastic deformation inside the linear-viscoelastic range. We obtain a critical energy

density for the transition to irreversible effects of friction for the - point and we

interpret the crossing point as an apparent rheological frictional energy density.

erheoelast G ' 2

cos (1)

erheocritical G ' critical

2

cos (2)

eRrheox cross cross (3)



With model samples with different soap content were investigated to present the friction

behaviour and to describe the structural degradation.

Fig.1: Illustration of the evolution of storage and loss modulus for an amplitude sweep

CORRELATION BETWEEN CHANGE OF SYSTEM ENTROPY AND STRUCTURAL

DEGRADATION

A volume element of the grease film is modelled as an open thermodynamic system. The entropy

change is defined with

(4)

The effects of friction (entropy production) lead to a deformation ( ), an accumulation ( )

and a transition process ( ). We observe

- a relevant energetic release,

- no energetic release,

- an additional entropy production term take place.

CONCLUSIONS

The possibility to get information about the friction and wear process with the help of oscillating

measurements is explained. An investigation of the structural degradation process of the grease

is made from a thermodynamic point of view. A correlation between energetic release (entropy

leaving the system) and degradation process is presented.

REFERENCES

[1] Kuhn, E, Investigation of the Structural Degradation of Lubricating Greases due to Tribological Stress. 18th International Colloquium Tribology, TAE, Esslingen 2012.

[2] Kuhn, E, Tribological Stress and Structural Behaviour of Lubricating Greases. ECOTrib, Vienna 2011.

[3] Kuhn, E, Behaviour of Lubricating Greases in a Tribological Contact. 15th ICEM, Porto, 2012.

IBERTRIB 2013


A4.2 - A059

ANALYSIS OF GREASE LUBRICATION MECHANISMS ON A ROLLER-ON-DISC CONTACT Please leave this line blank Tiago Cousseau1(*), André Gama2,Beatriz Graça1,Armando Campus3, Jorge Seabra2

1Institute of Mechanical Engineering and Industrial Management (INEGI), University of Porto, Porto, Portugal 2Faculty of Engineering (FEUP), University of Porto, Porto, Portugal 3School of Engineering (ISEP), Polytechnique of Porto, Portugal


ABSTRACT

A pin-on-disc machine was modified in order to simulate rolling bearings lubricated by grease or oil under controlled slide-to-roll ratio, transversal speed, temperature, contact pressure and entrainment speed. The traction coefficient and the contact potential difference might be measured during the whole test. This new arrangement proved to be efficient in the characterization of lubricating grease and oils it terms of Stribeck curves, film separation, generation of tribofilms and in the analysis of grease capacity to replenish the contact under boundary operating conditions.

INTRODUCTION

The supply of grease to the contact region is very important for long life and maintenance-free operation in machine elements with rolling contacts. Some key factors influencing the grease replenishment into the rolling contacts are grease rheology, bleed rate, bleed-oil properties, slide-to-toll ratio (SRR), transversal speed, contact geometry and cage effects [Cann, 2007]. However, there is a consensual agreement that after the early stages of grease lubrication, grease lubricated rolling contacts generally operate under starved condition. The key factors influencing grease performance on this stage (after churning and bleeding phase) are related to boundary lubrication and thickener and additives performance on generating a low-shear protective tribolayer [Jiménez, 2011].

In order to simulate bearing operating conditions during the different lubrication stages of grease lubricated rolling contacts, a pin-on-disc machine (Cameron Plint - TE67 / 7771) was modified. The standard arrangement (pin – used in pure sliding tests) was replaced by a spherical roller and the disc was replaced by a thrust roller bearing raceway. The spin and SRR effects are imposed by rotating the roller in a certain angle θ, i.e., shifting the roller circumferential speed direction (see Figure 1). The roller/raceway configuration allows controlling the entrainment speed of the disc, the operating temperature and the load applied, and monitoring traction coefficient and the contact potential difference. Spherical roller angular speed depends on θ and is easily calculated. Once roller and disc tangential speed direction and modulus are known, the SRR can be determined.

Few experiments were carried out with a lithium mineral lubricating grease, its base oil and bleed-oil. The early stages of grease lubrication in rolling contacts were studied in fully flooded tests, while grease replenishment and tribofilm formation were studied in starved conditions.





The friction behaviour of fully flooded rolling contacts lubricated with grease, base oil and bleed-oil is compared in Figure 2. It was observed that lubricants present the same friction behaviour but different levels, and that grease friction values are closer to the bleed-oil values than the base oil ones. The starved tests showed that tribofilms can be monitored through friction and contact potential difference variations. The use of a video recorder allowed attesting the tribofilm formation with such variations (see Figure 3).

Fig.1 Roller-on-disc test schematic Fig.2 Friction behavior of lubricating grease LiM1 and

their respective base and bleed-oil

Fig.3 Tribofilm influence on friction and contact potential difference

ACKNOWLEDGMENTS

The authors would like to thank the Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, for support given to this study through the project PTDC/EME-PME/122271/2010, to Dr. Harold Bock from ROWE Mineralölwerk Gmbh, in Bubenheim, Germany, for supplying the LiCaE grease and Dr. Michael Kruse from AXEL Christiernsson AB, Sweden, for providing the polymer thickened grease PPAO.

REFERENCES

A.E. Jiménez and M.D. Bermúdez. Surface interactions and tribological behaviour of aluminium and titanium alloys with ionic liquids. 6th Iberian Congress on Tribology, 2011.

P.M. Cann and A.A. Lubrecht. Bearing performance limits with grease lubrication: the interaction of bearing design, operating conditions and grease properties. Journal of Physics D: Applied Physics, 2007.

O

2x

1y

2y

1x

10

R

IBERTRIB 2013


A4.3 - A062

RHEOLOGY, AGING AND FRICTION TORQUE OF POLYMER GREASES; INFLUENCE OF ELASTOMER CONTENT David Gonçalves1(*), Beatriz M. Graça2, Armando V. Campos3, Jorge H.O. Seabra4, Johan Lecker5, René Westbroek5

1, 2Instituto de Engenharia Mecânica e Gestão Industrial – INEGI, Portugal 3 Instituto Superior de Engenharia do Instituto Politécnico do Porto – ISEP-IPP, Portugal 4Faculdade de Engenharia da Universidade do Porto – FEUP, Portugal 5Axel Christiernsson AB, Sweden (*)Email: [email protected]

ABSTRACT

Rolling bearings are often grease lubricated. New and innovative polymer based greases of very high “lifetime” have been slowly introduced in the market. This study intends to evaluate the rheological properties, the friction torque in thrust ball bearings and the thermal stability of polypropylene (PP) thickened greases with different elastomer content. This is part of a work that intends to develop an aging method to simulate the grease degradation in rolling bearings and its effects on rheological and tribological properties.

INTRODUCTION

Lubricating greases consist in a colloidal suspension of a thickener agent which is dispersed in mineral or synthetic oil. Although lithium is the most common metal soap used as thickener, polymers have been being introduced not only as thickeners but also, and primarily, as viscosity index improvers and as dispersants [1,2]. While used as thickener, the polymers form short and thick elements connected to each other and randomly distributed, trapping the oil and modifying the rheology of the grease and thus, the flow and hydrodynamic lubricating properties [1,2]. On the other hand, when used as additives, the polymers are added to increase the viscosity index, promote the adhesion and cohesion characteristics of lubricating greases and also to improve mechanical and chemical stability at high and low temperatures [1,2].

The tested greases are formulated with PAO base oil (ν@40º C=48.0 cSt, ν@100º C=8.0 cSt), 13 % of PP thickener, no additives and different concentration of an elastomer.

The rheological evaluation was performed with a rheometer using a plate-plate geometry at constant temperatures of 80 and 110º C. Flow, oscillatory and thixotropy tests were performed to evaluate the mechanical stability of the greases and its rheological behavior.

The greases were thermally aged in the oven at 120ºC for 5 consecutive days, and the grease molecular alterations were evaluated through Infrared Spectroscopy (FTIR). The FTIR spectra were obtained with controlled window size of 50µm.

The friction torque was measured with a piezoelectric torque cell, in a modified Four-ball machine, using a rolling bearing assembly in the place of the typical four ball arrangement. The tests were performed in a thrust ball bearing lubricated with 2ml of grease at constant temperature (80 and 110ºC), contact pressure P0=2.34GPa and at different rotational speeds of 250, 500, 1000, 2000 and 3000rpm. For each speed, 8 measurements were performed and the mean value was registered to reduce the scatter associated with the measurement.





Figure 1 shows the apparent viscosity (η*) of the greases M2 (without elastomer), M5 (2.6% elastomer content) and M6 (1.3% elastomer content) at 80ºC. From the results it seems that the addition of elastomer leads to lower viscosities at low shear rate when comparing to the sample without elastomer, but also to higher viscosity when the shear rate is high. On the other hand, comparing only the samples with elastomer, the viscosity and yield stress (from the oscillatory tests) tend to increase with the elastomer content.

Figure 2 presents the results from the FTIR analysis of all the three greases for the same aging time. It was observed that the higher the elastomer content, the lower the oxidation peaks observed.

Figure 3 shows the friction torque measurements for different speeds after a run-in period of 4h at 500rpm and 80ºC. The grease with higher elastomer content (M5) presented an increased friction torque, while the other greases seem to lead to the same friction torque values.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, under grants PTDC/EME-PME/122271/2010. The authors also wish to express their gratitude towards Dr. Johan Lecker and Dr. René Westbroek from AXEL Christiernsson AB, Sweden for providing the polymer thickened greases and for sharing their toughts regarding the greases rheology.

REFERENCES

[1] - J.E. Martín-Alfonso, C. Valencia, M.C. Sánchez, J.M.Franco, C.Gallegos, Evaluation of Different polyolefins as rheology modifiers additives in lubricating grease formulations. Materials Chemistry and Physics 128(2011) 530-538.

[2] - S. Bair, F. Qureshi, The high pressure rheology of polymer-oil solutions. Tribology International 36 (2003) 637-645.

Figure 3 – Flow curves.

Figure 2 – FTIR analysis of the aged greases.

Figure 3 – Measured friction torque.

IBERTRIB 2013


A4.4 - A060

FILM THICKNESS ANALYSIS THROUGH FRICTION TORQUE MEASUREMENTS Tiago Cousseau1(*),Beatriz Graça1,Armando Campos3, Jorge H. O. Seabra2

1Institute of Mechanical Engineering and Industrial Management (INEGI), University of Porto, Porto, Portugal 2Faculty of Engineering (FEUP), University of Porto, Porto, Portugal 3School of Engineering (ISEP), Polytechnique of Porto, Portugal


ABSTRACT

Friction torque measurements of three different lubricating greases were performed over time in a wide range of operating conditions. The results were correlated with the SKF friction torque model and an evaluation of the parameters related to the lubrication mechanisms (hoc, K, Krs and φbl) was performed. It was verified that all the parameters performed reasonably well up to the moment that grease degradation and boundary layers formation takes place. These phenomena play a major role in the friction torque values, hence in the lubrication mechanisms.

INTRODUCTION

Film thickness in greased lubricated rolling bearings has being investigated over the last 50 years and it is still not well known. The major challenge on grease film thickness prediction is the determination of the properties of the separating film over time. It is known that greases properties can change drastically during operation. Many of these changes are permanent and affects significantly the grease mechanisms of lubrication, hence the friction torque. The physical and chemical changes over time depend on the operating conditions and grease formulation, and up to the moment are not well predictable. Besides that, the properties of the contacting surfaces are altered due to the tribolayers generated by the lubricants [Lugt, 2009].

A modified Four-Ball machine was used to measure the friction torque in greased lubricated thrust ball bearings. Two sets of tests were performed. First, a series of friction torque measurements were performed over time under severe conditions - constant operating temperature T=80 ºC, contact pressure P0=2.35 GPa and volume of grease vg=0.25 ml. These tests intend to analyse the influence of grease and surface changes over time in the friction torque values. After that, the friction torque was measured over speed and temperature at the same pressure and with the same amount of grease - operating temperature of T = 80, 100, 120 and 140 ºC, rotational speed of n = 500, 1500, 2500 and 4000 rpm. This set of tests was carried out in order to set the operating conditions in which the rolling bearing works without excessive friction losses. The operating conditions defined based on the friction torque measurements are compared to some theoretical predictions.


Figure 1a) shows the friction torque measurements at 400 and 2000 rpm over 20 hours. While the film thickness predictions (hoc and K), as well as the lubricant parameters (Krs and φbl) remains constant over time and indicates a situation of full film lubrication, an increase of ≈ 60 % of the friction torque is observed. Such increase is related with grease degradation and surface changes, as observed in Figure 1b).




Figure 2 shows the friction torque values at 4 different speeds and rotational speeds. For this lubricating grease and operating conditions, it is observed a reduction of the friction torque with increasing speed and temperature. It does not happen only at 350 rpm and 140 ºC, which indicates a transition to starved lubrication. Here the friction torque increase is not related with tribofilm formation because at higher speeds the friction torque behave ‘normally’ again, and due to the absence of boundary layers in the bearing raceway, see Figure 2b). The predictions for this particular situation indicate light mix lubrication.

Fig.1a) Fig.1b) Friction Torque changes over time at 80 ºC Degraded greases in a bearing surface

Fig.2 Friction Torque measurements vs rotational speed at 4 different temperatures.

ACKNOWLEDGMENTS

The authors would like to thank the Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, for support given to this study through the project PTDC/EME-PME/122271/2010, to Dr. Harold Bock from ROWE Mineralölwerk Gmbh, in Bubenheim, Germany, for supplying the LiCaE grease and Dr. Michael Kruse from AXEL Christiernsson AB, Sweden, for providing the polymer thickened grease PPAO.

REFERENCES

P.M. Lugt. A Review on Grease Lubrication in Rolling Bearings. Tribology Transactions, 2009, 52, 470–480.

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IBERTRIB 2013




Session B4

Room B002

GEARS

Chair:

Prof. José Pérez-Diaz and Prof. Francisco Silva

IBERTRIB 2013


B4.1 - A049

PREDICTIVE MAINTENANCE OF A REDUCTOR WITH CONTAMINANTS AND UNBALANCED LOAD Aparecido Carlos Gonçalves1(*), Maria da Consolação Fonseca de Albuquerque2, Murilo Parra Cuerva3

1, 2,3 Department of Mechanical Engineering (DEM) , Faculty of Engineering of Ilha Solteira (FEIS), University of the State of Sao Paulo (UNESP). (*)[email protected]

ABSTRACT

Among all Predictive Maintenance techniques the oil analysis and vibration analysis are the most important for monitoring some systems. The integration of these techniques has the potential to revolutionize industrial practices and provide a large economic gain for industries. To study the integration of both techniques a bench test was set up and put to work to the extreme limit of use. Tests were carried out with the lubricant recommended by the manufacturer of the equipment, using lubricants supplemented with various percentages of liquid contaminant and lubricants supplemented with several percentages of solid contaminant. This paper presents the results of the first test, that is, with the oil recommended by the manufacturer in extreme conditions. From the results it was observed that if there is an abnormal instantaneous load in a system, the subsequent vibration analysis may not perceive what occurred if there was no permanent damage, which is not the case with the lubricant analysis.

INTRODUCTION

Among the techniques for Predicitive Maintenance of Reducers, the most used are the oil analysis and vibration analysis, and the simultaneous use of both of these provides accurate results about the condition of a system under test.

The principle of the vibration analysis is based on the idea that the structures of the machines excited by the dynamic efforts (power action) give vibrational signs, whose frequency is equal to the frequency of the exciting agents. An imbalance in a machine component will cause increased vibration, once it causes an imbalance in the system and consequent increase in the power. Thus, observing the progression of the level of vibration, it is possible to obtain information on the state of the machine.

The oil analysis enables identifying the first signs of wear of a component. The identification begins with studying the amount of particles, size, shape and composition, which provide accurate information on the conditions of the moving surfaces without having to disassemble the entire set that these parties belong to. These solid particles are generated by the dynamic friction between the parts in contact. According to the study of these particles the wear situations of the set can be related and attributed to physical and chemical conditions. The oil analysis is achieved through laboratory techniques involving, reagents, instruments and equipment.


Initially, the inner elements of the reducer were photographed for a subsequent comparison and verification of wearing as in Figure 1.




Figure 1 – Inner elements of the reducer. On the left is the worn screw and on the right the crown gear.

Figure 2 shows the photographs after the first week of the experiment. In Figure 2 it can be seen severe wear particles (1A), bronze particles (2A), laminar particles (3A) and cutting particles (4A).

Figure 2 – Wear particles generated in the first week of the first experiment with transmitted and reflected light. First picture: inner ring of the RPD. Second photo: outer ring. Other photos: intermediary ring.

ACKNOWLEDGMENTS

The authors gratefully acknowledge FAPESP - State of Sao Paulo Research Foundation for continuous support.

REFERENCES

Gonçalves A C, Lago D F, Cunha R C. (2007). Vibration and wear Particles Analysis in a test Stand. Industrial Lubrication and Tribology. Vol 59, issue 5, 2007, ISSN: 0036-8792.

Gonçalves, A. C. ; Campos-Silva, J.B.(2011). Predictive Maintenance of a Reducer with Contaminated oil Under an Excentrical Load Throug Vibration and Oil Analysis. Journal of the Brazilian Society of Mechanical Sciences and Engineering (Impresso), v. XXXIII, p. 1/1-7, 2011.

IBERTRIB 2013


B4.2 - A061

ROUGH WEAR SIMULATION OF SPUR GEAR MESHING José A. Brandão1(*), Ramiro Martins2, Jorge H.O. Seabra3, Manuel J.D. Castro4

1, 2Institute of Mechanical Engineering (INEGI), University of Porto, Porto, Portugal 3Department of Mechanical Engineering (DEMEC), University of Porto, Portugal 4Department of Mechanical Engineering (DEMEC), University of Porto, Portugal (*)Email: [email protected]

ABSTRACT

The present work consists in a numerical simulation of mild wear in an actual gear test conducted on spur gears in a FZG gear testing machine. The simulation includes a mixed lubrication model able to account for overpressure effects due to gear roughness and a mild wear model based on a differential version of Archard’s wear law. The results show good agreement between simulated and measured wear distribution profiles.

INTRODUCTION

Faure (Faure, 1990) listed several classes of surface damage: wear, corrosion, overheating or burning, erosion by cavitation, electric erosion, plastic deformation and contact fatigue. In gear transmissions, contact fatigue has gained importance in recent years. Particularly, a form of contact fatigue damage called micropitting, which consists in the formation of a multitude of micron-sized pits on the surface of a gear tooth flank, has been distinguished by Höhn et al. (Höhn, 1996) as the single most limiting factor for gear performance.

It is no surprise that considerable effort has been expended in the study of micropitting. A micropitting model that makes use of a mixed lubrication model and of a multi-axial fatigue criterion was presented (Brandao, 2010). It was later used (Brandao, 2012) to simulate contact fatigue surface damage on a pinion gear during a FZG gear micropitting test, where it was found that the alterations in tooth flank roughness profile could not be accounted for solely by surface contact fatigue damage. A hypothesis was put forward: because wear and fatigue are competing damage mechanisms, some mass loss on the tooth surface, and consequently some of the roughness profile variation, could be due to wear.

A fatigue model was constructed to attempt to simulate wear during the very same micropitting test. The model uses the same mixed lubrication model to determine contact pressure and tangential tractions, as well as a differential form of Archard’s wear law (Archard, 1953).


The wear model was applied to each load stage of the micropitting test and the distribution of wear depth along the pinion tooth profile was determined. Having obtained this wear depth distribution, it is possible to predict the final tooth flank profile by subtracting the wear depth profile from the initial, measured profile.

For load stage K9, Figure 1 shows the initial tooth flank profile, the predicted profile and the actual, measured final profile. The abscissa represents the position on the tooth flank (x=0 is intersection of the pitch circle with the tooth flank, positive values of x corresponds to points inside the pitch circle), and the ordinate represents the profile height, positive outward from the




tooth. It can be seen that the similarity between the predicted and final profile is good. It can also be seen that the wear simulation does not lead to the prediction of micropits.

Fig.1 Comparison of the initial pinion tooth flank profile (in black) with the final

profile (in blue) and the predicted profile (in red).

It can be concluded that micropitting cannot be studied without the simultaneous consideration of mild wear, and that proper prediction of profile evolution demands the integration of both phenomena into a numerical model.

ACKNOWLEDGMENTS

The present work is funded by the European Regional Development Fund (ERDF) through the ‘COMPETE – Competitive Factors Operational Program’ and by Portuguese Government Funds through ‘FCT – Fundacao para a Ciencia e Tecnologia’ as part of project ‘Projecto Estrategico – LA 22 – 2011–2012’, reference number ‘Pest- OE/EME/LA0022/2011’.

REFERENCES

Archard J. Contact and rubbing of flat surfaces, Journal of Applied Physics, 1953, 24 (8), p. 981–988.

Brandão J.A., Seabra J.H.O., Castro J. Surface initiated tooth flank damage. Part I: numerical model. Wear, 2010, 268 (1–2), p.1–12.

Brandão J.A., Seabra J.H.O., Castro J. Surface initiated tooth flank damage. Part II: prediction of micropitting initiation and mass loss. Wear, 2010, 268 (1–2), p.13–22.

Brandão J.A., Martins R., Seabra J.H.O., Castro J. Surface damage prediction during an FZG gear micropitting test. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2012, 226 (12), p. 1051–1073.

Faure L. Aspect des dentures d’engrenage après fonctionnement. Centre Technique des Industries Mécaniques, 1990.

Höhn B., Oster P., Emmert S. Micropitting in case-carburized gears – FZG micro-pitting test. VDI BERICHTE, 1996, 1230, p. 331–344.

IBERTRIB 2013


B4.3 - A013

POWER-LOSS EVALUATION OF LOW-LOSS GEARS OPERATING WITH ESTER AND PAO OILS Luis Magalhaes 1(*), Ramiro Martins 2, Jorge Seabra 3

1 Instituto Superior de Engenharia do Porto, Portugal ([email protected]) 2 Instituto de Engenharia e Gestão Industrial, Porto, Portugal. ([email protected]) 3 Faculdade de Engenharia da Universidade do Porto, Portugal. ([email protected]) (*) corresponding author

ABSTRACT

Advanced low-loss gears were tested in a FZG machine in order to evaluate power dissipation when operating under different speed and torque, lubricated by ester and polyalphaolephine-based oils. Three novel 20º pressure-angle gears were produced with low-loss type geometries, all developed aiming progressive levels of efficiency. Although identical in terms of mechanical resistance, these gears have different ability to reduce friction between mating teeth. The gearbox operated in churning conditions (no lubricant circulation or cooling was done), and comparing the oil bath temperature among tests allowed an evaluation of the relative efficiency of each gear model operating under different conditions. All tests were repeated for two lubricants, a biodegradable ester oil and a full-formulated PAO-based gear oil. The tests were also repeated for two immersion depths, one corresponding to the usual FZG lubricant level and the other to a lower oil level, in order to evaluate the influence of the churning losses in the total gearbox efficiency.

OVERVIEW

Gearboxes are usually designed to meet power transmission requirements, providing acceptable operating temperatures and reasonable operating life for their parts. Gears are usually dimensioned according to teeth bending fatigue (preventing ruptures on tooth root) and surface contact pressure (preserving surfaces from failure). Such design results on gears that consume a considerable amount of energy, frequently requiring strongly additivated viscous oils. Gearboxes are also intended to provide long-lasting reliable service, thus are usually equipped with over-dimensioned gears and bearings and, when helical gears are used, the axial efforts are frequently supported by taper bearings. The geometry of common gearboxes is not usually optimized for the circulation of the lubricant, mainly restrained by mounting dimensions and other construction issues. All these options can be considered aiming better efficiency. Friction sources in gearboxes are well studied and characterized [1, 2]. According to recent results [3] it is possible to reduce up to 50% of the power lost by a common gearbox adopting a careful selection of the bearings and their arrangements, using efficient gears and correctly selecting the lubricant, which may alone contribute to about 20 % of the energy savings. Efficient 20º pressure angle gears were designed according to the low-loss gear concept [4], granting safe load-carrying capacity. The tooth profiles of the tested gears were modified by



positive profile shifts, enlarging their bases. Although manufactured using current 20º tools, these low-module gears operate with higher working pressure angle, reaching almost 30º. The use of standard tools is an advantage from the point of view of simple and low cost manufacture.


Comparing the gears tested, a clear decrease in the Stabilization Temperature was measured, meaning that friction was significantly reduced by the more advanced geometries (Figure 1 shows a comparison of results from two test sets of gear models 311 and 611).

Fig. 1 - Ts 311-Ts 611 using low and high level of PAO oil.

The oil level revealed to be very influent in the lost power, namely at higher speed, showing that considerable energy savings can be obtained avoiding excessive churning losses due to unnecessary amount of lubricant. The new gears also revealed to be very effective reducing wear, preserving tooth surface condition and lowering oil contamination, and in many tests the best efficiency was achieved using the ester biodegradable oil, an advantage concerning environmental issues.

ACKNOWLEDGMENTS

The authors thank the Portuguese Foundation for Science and Technology (FCT) for the support of the project Low Power Loss ADI Gears – PTDC/EME-PME/73389/2006.

REFERENCES

1. Höhn B, Michaelis K, Vollmer T. Thermal Rating of Gear Drives-Balance Between Power Loss and Heat Dissipation, AGMA, ISBN: 1-55589-675-8, 1996.

2. Changenet C, Velex P. Housing Influence on Churning Losses in Geared Transmissions

Journal of Mechanical Design Vol. 130, June 2008.

3. Höhn B, Michaelis K, Hinterstoißer M. Influence Factors on Gearbox Power Loss.

3rd International Conference on Integrity, Reliability and Failure, Porto, 20-24 July 2009.

4. Höhn B, Michaelis K, Wimmer A. Low Loss Gears. Gear Technology, AGMA June 2007.

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IBERTRIB 2013


B4.4 - A063

POWER LOSS IN A MULTIPLIER GEARBOX LUBRICATED WITH WIND TURBINE GEAR OILS Pedro M. T. Marques2(*), Carlos M. C. G. Fernandes2, Ramiro C. Martins2, Jorge H. O. Seabra1

1 FEUP, Universidade do Porto, Rua Dr. Roberto Farias s/n, 4200-465 Porto, Portugal 2 INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Farias 400, 4200-465 Porto, Portugal (*)Email: [email protected]

ABSTRACT

In this work four fully formulated ISO VG 320 wind turbine gear oils were tested on a back-to-back gearbox test rig with recirculating power in order to investigate the influence of the lubricant oil on gearbox efficiency. A model simulating all the relevant power loss components was implemented aiming to understand the influence of the various power loss mechanisms.

INTRODUCTION

As proven source of clean and affordable energy, wind resources clearly have a vital role to play in energetic sustainability (E. E. Agency, 2009). Wind turbines have multiple stages gearboxes that operate in order to convert the high input torque coming from the low speed wind propelled turbine, to an adequate higher speed for the generator. It is of great interest to have the maximum possible efficiency on this speed multiplying process.

Accurate power loss predictions at the design stage of a gearbox will allow for the development of more efficient and reliable designs in less time, ultimately saving resources not only at the design stage, but also during operation.

A large number of tests totaling over 300h of experimental work were performed. The test gearbox was installed on a multiplying configuration and the oil sump temperature was set free. The tests were conducted at low speeds, (100, 200, 400 rpm), and high torques, (500, 750, 1000Nm). The operating temperatures and torques were recorded and their averages at the thermal equilibrium state were considered.

Between the selected gear oils three different base oils can be found. A Mineral, a Polyalkylene Glycol, a Polyalphaolefin and a Mineral+PAMA mixture were considered.


Figure 1 shows the thermal balance temperatures promoted by each gear oil during each test.

The Polyalkylene Glycol oil (PAGD) has promoted up to 20ºC lower operating temperatures compared to the Mineral based oil (MINR). The Polyalphaolefin (PAOR) shown thermal balance temperatures up to 10ºC higher than those promoted by PAGD for the most severe operating conditions. The Mineral+PAMA mixture performed close to PAOR.

In order to account for the different oil formulations the implemented coefficient of friction formulation (Höhn, 1996) had to be corrected through the use of a lubricant factor that accounted for the speed and load influences (Martins, 2004).

Figure 2 shows the power loss distribution obtained with the model for the tested operating conditions.




Figure 4 - Thermal equilibrium temperatures for the various operating conditions.

Figure 5 - Power loss distribution obtained with the model for PAGD oil.

The model allowed understanding of the influence of each one of the power loss components on the test gearbox. At low speed and high torque conditions the model has shown that the power loss generated by the meshing gears accounts for the majority of the power loss at such conditions.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, under grants PTDC/EME-PME/100808/2008.

REFERENCES

B.-R. Höhn, K. Michaelis, T. Vollmer, Thermal Rating of Gear Drives: Balance between Power Loss and Heat Dissipation, AGMA technical Paper, 1996.

E. E. Agency, Europe’s onshore and offshore wind energy potential – an assessment of environmental and economic constraints, EEA Technical report N0 6 (2009) 91.

R. Martins, J. Seabra, C. Seyfert, R. Luther, A. Igartua, A. Brito, Power loss in FZG gears lubricated with industrial gear oils: biodegradable ester vs. mineral oil, in: Proceedings of the 31th “Leeds-Lyon Symposium” on Tribology, Leeds, UK, 2004.

IBERTRIB 2013




Plenary Session A5

Room B002

Chair:

Prof. António Monteiro Baptista and Prof. Jorge Seabra

Keynote Lecture 2

SELF-LUBRICATING COATINGS –

SOLUTION FOR NEAR FRICTIONLESS CONTACTS?

Prof. Albano Cavaleiro de Carvalho

Full Professor, Departamento de Engenharia Mecânica, Universidade de Coimbra, Portugal.

Invited Paper - A009

THE WORK ON HYDRODYNAMIC JOURNAL BEARINGS

CARRIED OUT AT MINHO UNIVERSITY IN THE LAST 30 YEARS

Claro, J C P ; Miranda, A S ; Costa, L ; Brito, F P ; Fillon, M

IBERTRIB 2013


Keynote Lecture 2

SELF-LUBRICATING COATINGS – SOLUTION FOR NEAR FRICTIONLESS CONTACTS A. Cavaleiro1(*), M. Evaristo2, T. Polcar3,4

1, 2CEMUC, Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal 3National Centre for Advanced Tribology at Southampton (nCATS), School of Engineering Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, Hampshire, UK 4Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic (*)Email: [email protected]

ABSTRACT

The development of coatings with self-lubricating properties can be a solution to either avoid the use of harmful liquid lubricants, reducing the environmental impact, or to contribute to significantly reduction and control friction and wear in rolling and sliding contacts, in order to saving energy, extend operational life and reduce maintenance requirements. In this talk, we will transmit our experience with transition metal dichalcogenides (TMD) coatings and the main achievements in reducing friction and improving wear resistance through TMD alloying with different elements, particularly carbon and nitrogen. This solution allowed reaching friction coefficients as low as 0.03 and 0.005 in humid and dry atmospheres, respectively, under contact pressures higher than 1 GPa.

INTRODUCTION

Solid lubricant coatings can be an excellent solution for applications where liquid lubrication is inappropriate, due to restrictions caused by either contamination or impossibility to be used. Furthermore, this solution has also a great potential for decreasing the environmental impact caused by lubricants disposal, leading to important savings. Side by side with Diamond Like Carbon (DLC), transition metal dichalcogenides (TMDs) hve been the most studied materials as coatings for self-lubrication. The weak interactions between basal planes of TMD structure need only very low shear strengths, which gives rise to excellent frictional properties. However, external oxidation during sliding, in particular when humidity is present in the environment, makes friction coefficient increases leaving the competitiveness of the coatings in relation to liquid lubrication, i.e. ultra low friction, only interesting in vacuum or dry atmospheres. Besides frictional problems, TMD coatings have also poor adhesion to the substrates, high porosity and very low hardness. Therefore, most of the research studies performed in last decades in relation to these coatings have been dedicated to either the reduction of the sensitivity to humidity or the improvement of the mechanical properties, through their alloying with different elements.

Our contribution to this field has been mainly based on the alloying of TMDs with C and N. Sulfides and selenides have been studied. Important improvements in the density, increases in the hardness, in the adhesion to the substrate and in the wear resistance have been achieved. Ultra-low friction was reached in dry atmosphere, but these low values in humid air are still a challenge.


The coatings were deposited by sputtering in different modes, DC and RF, in reactive (CH4 and N2) and non reactive modes. In this last case, co-sputtering using C and TMD targets were used.




A strong increase in the mechanical properties was achieved with the hardness increasing from 0.5 GPa up to 10 GPa, corresponding to a great improvement in the density of the coatings (Figure 1). In particular deposition conditions, nanocomposite microstructures consisting in TMD platelets embedded in a C-based matrix could be deposited (figure 1c)). In these conditions excellent tribological behaviour could be achieved with friction coefficients lower than 0.05 in both dry and humid conditions, under extreme loading conditions (see figure 2a)). The very low friction could be attributed to a progressive re-orientation of the TMD platelets in the C-matrix, creating a top surface layer with the TMD basal planes oriented parallel to the sliding direction (inset of figure 2a)).

Ultra-low friction could only be achieved in N2 dry environment with amorphous deposited W-S-N coating. Figure 2b)) shows that the friction could decrease down to almost unmeasurable values. These coatings lasted for more than 1 million cycles without complete wearing out. Again, a well crystallized and oriented TMD crystal layer could be observed in the top sliding surface, in both the disc and the counterbody ball. Unfortunately, the tribological behaviour was very bad in room testing, with RH ratio higher than 30%.

Fig.1 SEM cross section morphologies of W-S-C films without a) and with b) addition of C. c) HRTEM cross section of b)

a) b)

Fig.2 Friction coefficient measured in pin-on-disk testing of a) TMD-C films tested in dry and humid environments and b) W-S-N film tested in dry nitrogen

CONCLUSIONS

Although significant improvements could be achieved already for almost frictionless sliding contact in dry environments with TMD-based sputtered coatings, the dream to eliminate liquid lubrication is still very far. The research activity in this field should be focused now on the achievement of super-low friction when sliding in humidity containing atmospheres.

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IBERTRIB 2013


Invited Paper - A009

THE WORK ON HYDRODYNAMIC JOURNAL BEARINGS CARRIED OUT AT MINHO UNIVERSITY IN THE LAST 30 YEARS Claro, J C P 1 (*); Miranda, A S 2; Costa, L 3; Brito, F P 4,5; Fillon, M 6

1, 2, 4 Mechanical and Materials Technologies Centre (CT2M), University of Minho, Guimarães, Portugal 3 Ministry of Energy and Water/DNAAS, Luanda, Angola 5 ISEP, Porto, Portugal 6 Institut Pprime, CNRS - Université de Poitiers-ENSMA, Dpt GMSC, Futuroscope Chasseneuil, France (*)Email: [email protected]

ABSTRACT

1. A brief review on the advances of hydrodynamic journal bearing design

From the primordial Osborne Reynolds’ work (1886) to the 1st half of the XX century, the assumption of a proper analytical definition of the cavitation phenomena, as a basic principle for fluid’s hydrodynamic pressure generation (and, consequently, of bearing’s load carrying capacity) lead to an intensive experimental and theoretical research on feeding conditions and their influence on the overall performance predictions. Also pioneer studies on heat generation and flow were carried on. However, design procedures kept based on simplified solutions, scarcely reflecting the real geometrical and working conditions and, so, with poor advantage for the designer. Along the 2nd half of the XX century, advances in iterative mathematical and computational methods allowing the solution of the full Reynolds equation, brought to a ‘second generation’ of calculating methods, based on an isothermal approach, although by that time backed up on an hybrid (analytical and semi-empiric) approach – a drawback due to the simplicity of the original analytical simulation. Later on, the advent of implicit methods to deal with the cavitation phenomena, allied to the burst of data processing capacity, allowed the incorporation of much more complexity into the geometry of the model. This not only took the analyses to a higher degree of detail on fluid’s flow simulation, but also to the introduction of other associated phenomena – such as the global thermal analysis and the pressure and/or thermal elastic deformation influences, among other – still in development and refinement. So, nowadays, the question for the design engineer is to realize what may be the best choice – between a simple and rapid method, not so precise or reliable, or a dedicated but much complex analysis, with more guaranties of an efficient solution – that ensures a given level of accuracy.

2. The evolution of hydrodynamic journal bearing’s studies at UMinho

The implementation of Elrod’s algorithm to the isothermal analysis of the plain journal bearing with a finite axial groove at (hmax), made by Miranda, A S (1983), brought a new highlight on the importance of feeding conditions, namely the strong influence of groove’s geometry on film extent and shape, in what concerns bearing’s performance prediction. Subsequent extension of the isothermal analysis to different, and most commonly found, feeding conditions by Claro, J C P (1994) – namely one or two axial grooves at right angle to the load line and the ‘crown’ bearing – also conveyed the attention to new problems, such as the possibility of existence of a back flow on the 2nd groove of a +/-90º arrangement, actually promoting a negative feeding effect. Later on, the introduction of the thermal analysis applied to the bearing with one axial groove (at +90º and the ‘crown’ bearing) by Costa, L (2000), promoted a detailed analysis of the



recirculating flow phenomena (reverse and back flow) on groove’s up and downstream edges, and its influence on lubricant feeding conditions. Finally, with the extension of the thermal analysis to the bearing with two axial grooves at +/90º, by Brito, FP (2009), a modeling of lubricant mixing at the groove – coupling recirculation, reverse and backflow phenomena – was incorporated in the global heat exchange analysis.

3. A comparison of results

A basic geometry (d=100 mm, b=80 mm) with axial feed groove dimensions of (a=80 mm, w=18 mm), a fixed shaft rotating speed (N=3000 rpm) and a lubricant supply by mineral oil with (29.3/5.5 mPa.s at 40º/100ºC), a feeding pressure of (pf=0.1 MPa) at 40ºC and two different feeding arrangements (one groove at +90º and two grooves at +/-90º to the load line) where considered. In order to cover a reasonable spectrum of conditions, eccentricity ratio () was varied from 0.3 to 0.9, corresponding to load carrying capacities from over 1 kN to around 7 kN. The study was carried out to compare the performance predictions of the isothermal and the thermohydrodynamic analytical methods, developed at UMinho since 1983 till 2009. Additionally, the predictions of ESDU 84031, Amendment A (1991) were also plotted, in order to evaluate the response of a (yet) widely used commercial design method. This exercise had as prime objective to highlight the improvements – and, for specific cases, the substantial differences – obtained in the prediction of the main working parameters, with the progressively sophistication of the method. But it also intends to be a first approach on the identification of possible conditions where a lighter approach – being that a quick and straightforward commercial calculation method, or a simpler and less computational demanding procedure – may be able to get reasonable solutions for engineering design. Or, on the other hand, when a specifically developed analysis – a ‘heavy’ thermo-hydrodynamic, or even a more complex thermo-elasto-hydrodynamic methodology – has to be used in order to ensure reliability and efficiency to the designed component. ACKNOWLEDGMENTS

The members of the Tribology Group of the University of Minho would like to express their gratitude to C M Taylor (The Institute of Tribology, University of Leeds, UK) and M Fillon (Laboratoire de Mécanique des Solides, Université de Poitiers, France) for their invaluable support in the last 30 years.

REFERENCES

- Miranda, A S – Oil Flow, Cavitation and Film Reformation in Journal Bearings, Including an Interactive Computer-Aided Design Study. Ph.D. Thesis, Dep. Mechanical Engineering, University of Leeds, UK, 1983

- Claro, J C P – Reformulação do Método de Cálculo de Chumaceiras Radiais Hidrodinâmicas. Ph.D. Thesis, Departamento de Engenharia Mecânica, Universidade do Minho, Portugal, 1994

- Costa, L A M – Análise do Desempenho de Chumaceiras Radiais Hidrodinâmicas Considerando Efeitos Térmicos. Ph.D. Thesis, Departamento de Engenharia Mecânica, Universidade do Minho, Portugal, 2000

- Brito, F C P – Thermohydrodynamic Performance of Twin Groove Journal Bearings Considering Realistic Lubricant Supply Conditions: a Theoretical and Experimental Study. Ph.D. Thesis, Departamento de Engenharia Mecânica, Universidade do Minho, Portugal, 2009

IBERTRIB 2013




Session A6

Room B001

BEARINGS AND SEALS

Chair:

Prof. António Sousa Miranda and Prof. Armando Campos

IBERTRIB 2013


A6.1 - A012

LEVITATION FORCE RELAXATION AND HYSTERESIS IN A FRICTIONLESS SUPERCONDUCTING MAGNETIC BEARING Cristian Cristache, Efren Diez-Jimenez*, Ignacio Valiente-Blanco*, Marco A. Alvarez-Valenzuela, Juan Sanchez-Garcia-Casarrubios, Jose L. Perez-Diaz1)

1 Departamento de Ingeniería Mecánica, Universidad Carlos III de Madrid (*)Email: [email protected]

ABSTRACT

In this work, the mechanical behaviour of a frictionless superconducting magnetic bearing is presented. The force relaxation and the hysteresis in the forces are experimentally measured. The bearing is a thrust kind bearing composed of a YBaCuO superconducting disk and a high quality NdFeB ring permanent magnet. A clear hysteretic behaviour of the radial and axial forces is observed. Finally, a relaxation in the levitation force about a 20-30% of the initial levitation force is measured.

INTRODUCTION

Conventional mechanisms such as gears or bearings require lubrication to operate. However, when mechanisms operates at cryogenic temperatures present severe tribological problems like backlash and cold spots, fatigue or wearing . At cryogenic temperatures, liquid lubricants of grease loss their properties and only solid lubrication is available with a reasonable good quality (Fleischer et al. 2003; Ostrovskaya et al. 2001; Trautmann et al. 2005). High temperature superconductors can provide stable levitation without direct contact between them and a magnetic source (typically a permanent magnet). In this context, superconducting magnetic levitation provides a new tool for mechanical engineers to design non-contact mechanisms solving all the tribological problems associated with contact at very low temperature (Moon and Chang 1994). In the last years, different mechanisms have been proposed taking advantage of superconducting magnetic levitation. Flywheels (Mulcahy et al. 1999), conveyors (Iizuka et al. 1994) or mechanisms for high-precision positioning (Pérez-Díaz et al. 2011). In this paper, the results of the experimental characterization of a typical thrust superconducting bearing are presented. The measurements are focused on the hysteretic behavior of the radial and axial forces and the relaxation of the levitation force along time.


Hysteretic behaviour

Levitation force vs. axial distance (Z axis) from the initial cooling position (HFC) and the radial force vs. radial position (X axis) from the initial position are plotted in Fig 1. A clear hysteretic behaviour is observed in both forces.

Relaxation of the levitation force

Furthermore, there has been observed a relaxation of the levitation force with time, when the gap between the permanent magnet and the superconductors is fixed and different to the HFC. The reduction of the levitation force can even achieve a 20-30 % of the initial force measured.




Fig.1 Levitation force vs distance from the HFC (left) and radial force vs. radial position (right) at a temperature of 77 K.

Fig.2 Levitation force vs time. Z=7 mm, X=0 mm and T=77 K.

ACKNOWLEDGMENTS

The research leading to these results has received funding from the European Community's Seventh Framework Programme ([FP7/2007-2013]) under grant agreement n° 263014.

REFERENCES

Fleischer, N. et al., "New nanotechnology solid lubricants for superior dry lubrication", Proceedings of the 10th European Space Mechanisms and Tribology Symposium, 2003, pp.65 - 66.

Ostrovskaya, Y.L. et al., "Low temperature tribology at the B. Verkin Institute for Low Temperature Physics & Engineering (historical review)", Tribology International. 2001, v. 31 pp. 265–276.

Trautmann, A. et al., "Lubrication of polycarbonate at cryogenic temperatures in the split Hopkinson pressure bar", International Journal Of Impact Engineering. 2005, V.31, pp. 523-544.

F.C. Moon and PZ. Chan, Superconducting Levitation: Applications to bearings and magnetic transportation, Wiley-VCH, Germany, 1994.

Iizuka, T. et al.. “A Micro X-Y-θ Conveyor by using Superconducting Magnetic Levitation” IEEE Symposium on Emerging Technologies and Factory Automation. 1994 pp. 62–67.

Pérez-Díaz, J.L. et al., “Non-contact linear slider for cryogenic environment.” Mechanism and Machine Theory. 2012 V.49, pp.308-314.

IBERTRIB 2013


A6.2 - A020

A NUMERICAL ANALYSIS ON CONSTANT OR VARIABLE VISCOSITY ON OIL BEARINGS Luigi C. Greco*, Roberto M. Souza, Rodrigo L. Stoeterau

Surface Phenomena Laboratory, Polytechnic School of the University of São Paulo, São Paulo, Brazil (*)Email:[email protected]

ABSTRACT

This work presents a numerical study on the calculation of pressure and temperature distributions in short bearing, comparing one case considering constant viscosity and another in which the viscosity is adjusted according to the model of Roelands (Höglund, 1999). This work also analyses the variation of the density across the film and in the whole bearing surface. In order to provide input to the model, the temperature field was determined, as well as the region at which film rupture occurs. The two calculation procedures provide a significant difference on the maximum and average pressures and a large difference in the heat exchange along the film thickness.

INTRODUCTION

The need to understand phenomena related to lubrication has raised several models, including sets of analyses dedicated to specific cases (Dowson, 1962). However, in order to include ever more cases, other considerations began to be taken into account. In particular, the temperature became a variable to be considered in the calculation of lubrication (Boncampain, 1987), especially when one realizes that temperature affects properties of the fluid, such as density and viscosity.

Several correction models for viscosity and density are designed to better adjust the properties of the fluid to a given application. However, each model has its limitations and the experimental results suggest the need for additional formulations. Some experiments were designed in order to obtain fundamental constants of fluids and to produce parameters that could be used in different representations (Höglund, 1999).

Similarly to a previous study (Fatu, 2006), this work used a combination of models presented in other studies, as an attempt to obtain better predictions and comparisons with other data available in the literature. The main difference from the previous study (Fatu, 2006) lies in the thermal model adopted in this work and the use of a viscosity model that renders more accurate calculation. The model described in this paper consists in the coupling of the calculation of the short bearing pressure by FEM (Finite Element Method), and its velocity field, with the thermal model (Boncampain, 1987), with the addition of viscosity correction and pressure models for a generic configuration for the housing bearing.


Fig. 1 presents the results of pressure fields calculated with constant (Fig. 1a) or variable (Fig. 1b) viscosity, using water as a lubricant. The graph related to variable viscosity (Fig. 1b) was created from the calculation of heat dissipation that would be generated by the fluid and the resulting viscosity at each grid point. The two graphs have similar distribution shapes, but it can be noted that the maximum range and the distribution of the contour lines under the graph is different. It is noticed that there are more regions with higher pressures when the viscosity is



variable, which can be explained by the decrease in viscosity of the film, either by pressure or temperature. The main values of each field are shown in Table 1. Both in maximum and average pressures, the difference were on the order of 24%.

Table 3 Pressure results for different viscosities

Viscosity Max Pressure (105 Pa)

Average Presure (103 Pa)

Constant 5.2287 12.828 Variable 3.9331 9.7341

Difference 1.2956 3.0939

Fig.1 Pressure field with constant viscosity (a) and with a variable viscosity (b).

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Departamento de Engenharia Mecânica da Escola Politécnica, USP, Brazil, and the sponsorship provided by CAPES.

REFERENCES

Dowson, D. A generalized Reynolds equation for fluid-film lubrication. International Journal of Mechanical Science. Vol. 4 p. 159-170. 1962.

Boncampain R, Fillon M, Frene J. Effets thermiques dans les paliers hydrodynamiques. Aspects théoriques et expérimentaux. Journal de Mécanique théorique et Appliquée, vol. 6, nº2, 1987, p. 253-293.

Höglund E. Influence of lubricant properties on elastohydrodynamic lubrication. Wear 232, 1999, p. 176-184.

Fatu, A, Hajjam, M, Bonneau, D. A new model of thermoelastohydrodynamic lubrication in dynamically loaded journal bearings. Journal of Tribology, January 2006, vol. 128, p.85-95.

IBERTRIB 2013


A6.3 - A041

MODELING ELASTOHYDRODYNAMIC MECHANICAL JOINTS UNDER THE FRAMEWORK OF MULTIBODY SYSTEMS Qiang Tian1(*), Yanlei Sun1, Cheng Liu1, Haiyan Hu1, Paulo Flores2(*)

1Ministry of Education, School of Aerospace Engineering, Beijing Institute of Technology, Beijing, China 2Department of Mechanical Engineering, School of Engineering, University of Minho, Portugal (*)Email: [email protected]

ABSTRACT

In this work, a methodology for dynamic analysis of rigid-flexible multibody systems with elastohydrodynamic (EHD) lubricated joints is presented. The EHD lubricated cylindrical joint is formulated by the Natural Coordinate Formulation (NCF) and the twenty-node hexahedral element of Absolute Nodal Coordinate Formulation (ANCF), being the lubricant pressure determined through the resolution of the Reynolds’ equation employing the finite difference method. The outcomes are validated with those obtained by using the commercial software ADINA. It is shown that the bearing flexibility plays a significant role in the system responses, extends the lubricant distribution space and reduces the lubricant pressure.

INTRODUCTION

A mechanical system usually consists of two major kinds of components, bodies and joints. The bodies can be modeled as rigid or flexible elements, while the joints are represented by a set of kinematic constraints. The functionality of a mechanical joint relies upon the relative motion allowed between the connected components. This fact implies the existence of a clearance between the mating parts, and thus joint surfaces can contact each other or may be separated with a lubricant. It is of paramount importance to quantify the effects of both clearance joints and bodies flexibility on the global system response in order to define the minimum level of suitable tolerances that allow systems to achieve required performances.

In practice, lubricant is often utilized in mechanical joints to avoid the body-to-body (typically metal-to-metal) contact. This measure can reduce the level of impact and vibrations, and extends the joints lifetime. It is quite important to develop appropriate computational models that can account for the lubricant action in mechanical joints in the context of multibody system dynamics. For this purpose, two kinds of approaches can be found in the scientific domain of tribology, namely the hydrodynamic (HD) theory and the elastohydrodynamic (EHD) formulation. According to the HD theory, in the presence of dynamics of journal bearings, the hydrodynamic forces, which include both squeeze and wedge effects, generated by the lubricant fluid, oppose the journal motion. The hydrodynamic forces can be obtained by integrating the pressure distribution evaluated with the aid of Reynolds’ equation established for the dynamic regime. Liu et al. (2010) are among the very few authors who performed the EHD analysis for lubricated high-speed rotor-bearing systems by using the Fluid-Structure Interaction (FSI) analysis. These studies clearly demonstrated that the bearing deformations affect the pressure field in the clearance and increase the minimal film thickness. Yet, these works were performed only for isolated journal bearing systems. Thus, the issue of integrating the EHD theory in the flexible multibody system formulations is still an open and challenging problem, being the motivation for this study.




In this section, the EHD analysis of a lubricated cylindrical joint with rotating journal is presented (Tian et al., 2013). The length of the cylindrical joint is equal to 66 mm. The journal rotates around its axis at a constant angular speed of 3000 rpm. The dynamic lubricant viscosity is equal to mPas. The rigid journal is modeled by using the NCF, while the flexible bearing is modeled by the twenty-node hexahedral element of ANCF. The nodes on the outer bearing surface are assumed to be fixed in the space. The material density and the thickness of the bearing are set to be 7800 kg/m3 and 20 mm, respectively. As a comparison, the systems with and without journal misalignment are studied. Initially, the journal axis is assumed to be parallel with the bearing axis, that is, the rigid rotating journal is not misaligned. The clearance is equal to 0.03 mm. The Young’s modulus of the bearing material is set to be 2.1×1012 Pa.

Figure 1 depicts the lubricant pressure distributions for this journal bearing system. As expected, Fig. 1 shows that if the journal is aligned, the lubricant pressure exhibits a symmetrical distribution about the plane ξ-Cj-η.

Figure 2 shows a scaled view of the deformation of the inner bearing surface. The figure indicates that for the system with a rotating journal without misalignment, the distribution of the deformation of the inner bearing surface also exhibits a symmetrical distribution about the plane ξ-Cj-η, which is consistent with the lubricant pressure distribution shown in Fig. 1.

ζ

η

ξ

Cj

ζ

η

ξCj

Fig.1 Lubricant pressure distribution Fig.2 Elastic deformation of bearing surface

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Fundação para a Ciência e a Tecnologia (FCT) Portugal, under grants PTDC/EME-PME/099764/2008 and SFRH/BPD/77831/2011.

REFERENCES

Liu HP, Xu H, Ellision, PJ, Jin ZM. Application of computational fluid dynamics and Fluid–Structure Interaction method to the lubrication study of a rotor bearing System. Tribol Lett 2010, 38(3), p. 325-336.

Tian Q, Sun Y, Liu C, Hu H, Flores P. Elastohydrodynamic lubricated cylindrical joints for rigid-flexible multibody dynamics. Computers and Structures, 2013, 114-115, p. 106-120.

IBERTRIB 2013


A6.4 - A026

MEASUREMENT OF THE PUMP RATE OF RADIAL LIP SEALS DURING LONG TERM TESTS Marco Remppis1(*), Frank Bauer1, Werner Haas1 1Institute of Machine Components (IMA), University of Stuttgart, Stuttgart, Germany (*)Email: [email protected]

ABSTRACT

Elastomeric radial lip seals are complex tribological systems. The sealing mechanism is based on reverse pumping. The pump effect is essential for the seal’s reliability on preventing leakage in dynamic applications. In this paper some theoretical aspects of the sealing mechanism are reviewed and different methods to measure the pump rate are presented. The pump rate is measured during long term tests with a duration of 1000 hours in order to assess the degradation of the sealing system. The results are discussed and the wear of seal and shaft are examined.

INTRODUCTION

Radial lip seals are used to seal shafts and axles in a wide range of mechanical and automotive applications. Because of complex failure modes until now there is no possibility to predict lifetime and reliability of such sealing systems. They can fail due to wear of the seal or the shaft as well as due to thermal or chemical damages. According to a study by Klein (Klein, 2010) degradation models are the most promising lifetime models for radial lip seals. The pump rate can be used to describe the degradation of the sealing system. In this work a method of Kunstfeld (Kunstfeld, 2005) is applied to measure the pump rate during long term tests. A secondary chamber is installed on the air side of the seal. It remains empty during the main part of the test and is only flooded during the pump rate measurements. When it is flooded the seal begins to pump the oil from the secondary chamber (air side) into the main chamber (oil side). The filling level of the secondary chamber can be observed by a standpipe, see fig.1. It decreases as the seal pumps the oil into the main chamber. Measuring the oil volume which is pumped into the main chamber and the time of the pumping process a pump rate can be determined. The pump rate was measured about nine times within 1000 hours. Standard lip seals without sealing aids and two different materials, FKM and NBR, were used. They were tested at different shaft speeds in order to analyze the degradation of the pump rate at different operating conditions.


Depending on the shaft speed different results were achieved. The test runs with high rotational speeds (v = 25 m/s for FKM; v = 12.5 m/s for NBR) led to a degradation of the pump rates, as shown in fig.2. All the FKM seals remained leak tight during 1000 hours, whereas some NBR seals showed leakage because of thermal damage. In the tests with low rotational speeds (v = 1 m/s) the pump rates remained rather constant. Mixed lubrication because of the low rotational speed caused abrasive wear at the sealing edge and the shaft. This could be observed especially in sealing systems with an FKM seal, see fig.3, but did not affect the leak tightness of the systems.

In comparison to the results of Kunstfeld (Kunstfeld, 2005) the degradation of the pump rate was lower. The quality of the elastomeric materials has increased since 2005. To develop



degradation models for lip seal wear a testing time of 1000 hours is not enough. The duration of the tests should be increased until leakage occurs. Accelerated testing usually is not applicable because failure modes change when an increased load is applied.

Fig.1 Pump rate measurement with a secondary chamber

Fig.2 Pump rates during 1000 hours tests (FKM seal)

Fig.3 Wear at the sealing edge and the shaft surface after the tests

ACKNOWLEDGMENTS

The IGF project 16402 N of the Forschungsvereinigung Antriebstechnik e.V. (Research Association for Drive Technology) was funded by the AiF as a support of the Industrielle Gemeinschaftsforschung (IGF, Industrial Collective Research) by the Federal Ministry of Economics and Technology due to a decision of the German Federal Parliament.

REFERENCES

Klein, B.; Haas, W. Bertsche, B.: Methoden zur Verringerung des Versuchsaufwands bei Lebensdauertests am Beispiel des Radial-Wellendichtrings. 16th ISC, Stuttgart, Oct. 2010. Ebelsbach: Leithner Media Production, 2010, pp. 433-443.

Kunstfeld, T.: Einfluss der Wellenoberfläche auf das Dichtverhalten von Radial-Wellendichtungen. PhD thesis, Inst. of Machine Components, University of Stuttgart, 2005.

standpipe

secondary

chamber

(f looded)

lip seal breather

main

chamber

shaf t

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0 250 500 750 1000

pu

mp

rate

Q[µ

l/m

]

Time [h]

FKM #05

FKM #06

FKM #08

FKM #09

t = 1000 h

n = 6000 min-1

ϑoil = 80 C

0,56 mm 0,22 mm

-20

-15

-10

-5

0

5[µm]

[mm]

0 1

-20

-15

-10

-5

0

5[µm]

[mm]

0 1

Sealing edge (NBR)

Wear track on shaft Wear track on shaft

t = 1000 h

n = 250 min-1

ϑoil = 80 C

Sealing edge (FKM)

IBERTRIB 2013




Session B6

Room B002

SURFACE COATINGS

Chair:

Prof. Albano Cavaleiro and Prof. Francisco Silva

IBERTRIB 2013


B6.1 - A027

COMPARISON OF THE MICRO-ABRASION PERFORMANCE BETWEEN HVOF NI-BASED AND SPUTTERED TISI(V)N COATINGS F. Fernandes1,*, A. Loureiro1, A. Ramalho1, A. Cavaleiro1

1CEMUC - Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos, 3030-788 Coimbra, Portugal. *Email address: [email protected], tel. + (351) 239 790 745, fax. + (351) 239 790 701

ABSTRACT

The aim of this work was to compare the structure, mechanical properties oxidation resistance and micro-abrasion resistance of three different commercial nickel based hardfacing coatings currently used in high temperature applications, deposited by HVOF process with thin TiSiN hard coatings without and with vanadium additions deposited by sputtering. The abrasion tests allowed identifying significant differences in the specific wear rates of the different coatings.

INTRODUCTION

Molds and accessories used in the glass industry in direct contact with melted glass are subject to very severe conditions of wear, corrosion and oxidation during service (Fernandes, 2011). A wide diversity of coating materials have been successfully applied to protect the surface of these components by using hardfacing and/or thermal spraying processes (Fernandes, 2012; La Barbera-Sosa, 2010). In these cases, nickel-based alloys have been those which have been especially used in their protection due to their unique combination of properties (Vaßen, 2010). Other coatings with superior features, deposited by physical vapor deposition, presented promising potential application in glass industry. Thus the aim of this investigation was to compare the micro-abrasion resistance of three different commercial nickel based hardfacing coatings deposited by HVOF process with thin TiSi(V)N hard coatings deposited by sputtering.

The structure, mechanical properties and oxidation resistance were firstly characterized by microhardness or nanoidentation, X-ray diffraction, scanning electron microscopy (SEM) and thermo gravimetric analysis. Afterwards, their microabrasion resistance was evaluated by micro-scale abrasion equipment using silica slurry as abrasive. SEM analyses are conducted at the spherical cup depressions induced by the wear tests in order to study the different wear mechanisms. The micro-abrasion equipment was selected in order to try to reproduce the wear mechanism presented at the surface of the coatings on glass molds which have been in service for a long time. Ageing studies were performed to the Ni based coatings in order to study the effect of the temperature on their properties. As micro-scale abrasion equipment does not reproduce the real working conditions of molds, to consider the effect of temperature in abrasion resistance, the coatings were tested in a special developed equipment simulating the contact between the mould and the ceramic material.

RESULTS AND CONCLUSION

All coatings revealed to have good adhesion to the substrate. The hardness of TiSiN and TiSiVN coatings is 2-3 times higher than Ni coatings (20-36 against 8-9 GPa), as shown in Figure 1 b). TiSiN coatings display the greatest oxidation resistance. Increasing V content in TiSiN coatings decreases their onset of oxidation to a lower value than the Ni-based coatings. Abrasion resistance of Ni coatings is higher when WC particles are presented in their matrix. The thermal




exposure showed increasing abrasion resistance of Ni coatings. Substantial differences were found in the abrasion resistance of thicker and thin coatings.

Figure 1 – a) chemical composition of coatings, b) hardness of coatings, c) thermo gravimetric analysis of coatings, d) abrasion resistance of Ni coatings.

ACKNOWLEDGMENTS

The authors wish to express their sincere thanks to the Portuguese Foundation for the Science and Technology (FCT), through COMPETE program from QREN and to FEDER, for financial support in the aim of the projects: “13545” and “PTDC/EME-TME/122116/2010”, as well as for the grant (SFRH/BD/68740/2010).

REFERENCES

Fernandes F, Lopes B, Cavaleiro A, Ramalho A, Loureiro A. Effect of arc current on microstructure and wear characteristics of a Ni-based coating deposited by PTA on gray cast iron. Surf Coat Technol, 2011, 205, pp. 4094-4106.

Fernandes F, Cavaleiro A, Loureiro A. Oxidation behavior of Ni-based coatings deposited by PTA on gray cast iron. Surf Coat Technol. 2012,207, pp. 196-203.

La Barbera-Sosa JG, Santana YY, Moreno E, Cuadrado N, Caro J, Renault PO, et al. Effect of spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by HVOF thermal spraying. Surf Coat Technol. 2010, 205, pp.1799-1806.

Vaßen R, Jarligo MO, Steinke T, Mack DE, Stöver D. Overview on advanced thermal barrier coatings. Surf Coat Technol. 2010, 205:938-942.

B C Co Cr Fe Si W Ni

Ni-1 2.5 0.6 - 13.6 3.8 3.7 14.5 63.8

Ni-2 1.6 2.5 5.2 7.2 2.4 2.3 33.6 47

Ni-3 1.3 2.9 6.3 6 2 1.9 41 40

Si Ti N V

TiSiN 13.3 61.48 27,2 -

TiSiVN 10.7 46.39 26,3 13.1

Chemical composition of coatings (wt.%)a)

Thick coatings

Thin coatings

b) Hardness of coatings

0

5

10

15

20

25

30

Ni-1 Ni-2 Ni-3 TiSiN TiSiVN

Ha

rd

ness

of

Co

ati

ng

s

(GP

a)

c) Thermo gravimetric analysis of coatings

0

0,2

0,4

0,6

0,8

1

1,2

400 600 800 1000 1200

Ma

ss g

ain

(m

g/c

m2)

Temperature 0C

TiSiVNNi-1

Ni-2

Ni-3

TiSiN

d) Abrasion resistance of Ni coatings

0,0E+00

6,0E-04

1,2E-03

1,8E-03

2,4E-03

0 1 2 3

Vo

lum

e d

ue

to w

ear

Sliding distance Load

Ni-1

Ni-2

Ni-3

IBERTRIB 2013


B6.2 - A028

TRIBOLOGICAL BEHAVIOUR OF W–TI–N COATINGS AGAINTS AA5754 ALLOY AT ROOM AND WARM TEMPERATURE J.M. Figueiredo1, A. Cavaleiro2, M.C. Oliveira3(*), T. Polcar4, L.F. Menezes5

1, 2, 3, 5CEMUC, Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal 4Department of Control Engineering, Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic (*)Email: [email protected]

ABSTRACT

This work presents the results of pin-on-disks tests performed at room and warm temperature, between an AA5754 alloy and a steel (AK7) coated with a thin film (W-Ti-N). Two types of steel samples were coated: planar and spherical, which are tested against aluminium pins and 1.0 mm thickness sheets, respectively. Globally, the results show that the average friction coefficient increases with temperature. The evolution of the friction coefficient with time differs in the two types of tests, mainly as a result of the differences in the surfaces roughness.

INTRODUCTION

Nowadays, the European Union directives impose severe restrictions on the use (reduction or removal), disposal (cleaner environment) and manufacturing (without toxic products) of lubricants, including those for sheet metal forming industry. However, the high contact pressures between the sliding elements in applications, such as deep drawing processes, make the production very difficult without proper lubrication conditions, leading frequently to sticking problems that cannot be overcome without “critical” amounts of lubricant (Silva et al., 2008). In fact, the absence of lubricants leads invariably to adhesion problems and premature wear of the tools, particularly in deep drawing of ductile materials, namely aluminum alloys. One possible solution is the surface modification of the tools by applying hard coatings which can, simultaneously, reduce the necessary lubricant amount, by decreasing the contact friction coefficient, and improving their lifetime. Previous works have shown the potentialities of implementing coatings of the W–Ti–N system in industrial applications, by selecting the N content that renders the best compromise between a very high wear resistance and a reduced friction coefficient (Severo, 2009). On the other hand, the continuous demand for higher fuel efficiency and lower emission vehicles is driving the increasing use of aluminium alloys sheets in automotive industry. In that context and for economic reasons, 5000 series (Al-Mg) alloys could advantageously replace the more expensive 6000 (Al-Mg-Si) alloys but, most of the times, Portevin-Le Chatelier (PLC) effect limits its use, because the formability is less at room temperature and it generates non-aesthetic stretcher lines on the deformed sheets. Thus, Al-Mg alloys are applied to interior panels whereas the Al-Mg-Si alloys are used in outer door panel. Since the PLC effect is known to be temperature dependent these alloys could be formed at warmer temperature in order to suppress it. In fact, phenomena such as the springback and the PLC effect are known to be considerably lower for the AA5754-O alloy, at warm temperature (Coër et al., 2010; Laurent et al., 2011). However, since AA5754-O becomes softer and more ductile with the temperature increase (Coër et al., 2010), it is expected that the contact conditions become even more important.

This work presents the results of pin-on-disk tests, at room and warm temperature, between AA5754-O alloy and a steel (AK7) coated with a thin film (W-Ti-N). In order to reproduce the




different contact conditions associated to the sheet metal forming process, two types of steel samples were coated: planar and spherical. The first type of samples was tested using as counter-body aluminium pins. The second type of samples was the counter-body of the tests performed with aluminium sheets. The lubricant used was a JELT oil and all tests were performed considering a load of 1 N, a linear speed of 15cm/s and 3000 laps.


For the tests performed with the aluminium sheets as disk it was observed an alteration of the lubricant properties with the number of cycles, with an initial reduction of the friction coefficient followed by an increase. These alterations occur for a lower number of cycles and are sharper, with the temperature increase. For the tests performed with the aluminium pins these alteration does not occurs, as a result of the lower roughness of the surface. Fig. 1 presents the results for the friction coefficient, obtained considering the initial and the steady state values. The results show that, globally, there is an increase of the friction coefficient with temperature.

Fig.1 Minimum, maximum and average friction coefficient obtained from pin-on-disk tests

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT), under projects PTDC/EME-TME/103350/2008 and PEst-C/EME/UI0285/2011, and by FEDER through the program QREN (COMPETE: FCOMP-01-0124-FEDER-010301).

REFERENCES

Coër J, Bernard C, Laurent H, Andrade-Campos A, Thuillier S. The effect of temperature on anisotropy properties of an aluminium alloy. Experimental Mechanics, 2010, 51, 1185-1195.

Laurent H, Coër J, Grèze R, Manach PY, Andrade-Campos A, Oliveira MC, Menezes LF. Mechanical behaviour and springback study of an aluminium alloy in warm forming conditions. ISRN Mechanical Engineering, 2011, Article ID 381615.

Severo V, Vilhena L, Silva PN, Dias JP, Becker D, Wagner S, Cavaleiro A. Tribological behaviour of W–Ti–N coatings in semi-industrial strip-drawing tests. Journal of Materials Processing Technology, 2009, 209, 4662–4667.

Silva PN, Dias JP, Cavaleiro A. Performance of W–TI-(N) coated pins in lubricated pin-on-disk tests. Surface & Coatings Technology, 2008, 202, 2338–2343.

0

0.05

0.1

0.15

0.2

0.25

WTiN-Al WTiN-Al Al-WTiN Al-WTiN

Fric

ton

co

eff

icie

nt

Pin-Disk reference

TA

100ºC

150ºC

200ºC

1.5g/cm2 1.8g/cm2 1.4g/cm2 4.5g/cm2

IBERTRIB 2013


B6.3- A056

STRUCTURAL AND MECHANICAL CHARACTERIZATION OF W-S-N COATINGS DEPOSITED BY REACTIVE MAGNETRON SPUTTERING F. Ribeiro

1, M. Evaristo

1, T. Polcar

2, A. Cavaleiro

1

1 SEG – CEMUC, Departamento de Engenharia Mecânica, Universidade de Coimbra, Rua Luís Reis Santos, P-3030 788 Coimbra, Portugal 2 nCATS School of Engineering Sciences, University of Southampton, Highfield, Southampton, United Kingdom

ABSTRACT

Transition metal dichalcogenides (TMD) are well-known for low friction behaviour in vacuum or dry conditions due to weak van der Waals forces between basal planes. However the low load bearing capacity and the environmental conditions are detrimental for the tribological behaviour. In recent years, to improve the tribological performance, TMD have been alloyed with a metal or with C with the propose of having nanocomposite structures ensuring high load bearing capacity and more resistant to environmental attack. MoS2 and WS2 are the most studied TMD and it is known that WS2 has better oxidation resistance than MoS2.

W-S-N coatings were deposited by DC reactive magnetron sputtering in a Ar+N2 atmosphere with increasing N2 partial pressure to achieve different compositions. Also the discharge pressure was varied since it could influence the coatings composition and the mechanical properties and structure. The coatings nitrogen content varied from 10 to 25 at. % for all deposition pressures. The increase of the deposition pressure results in an increase of the S/W ratio from of approx. 0.8 when deposited at 6.0×10-3 mbar to 1.5 with a pressure of 2.0×10-2 mbar. The hardness of the coatings increases from 2.5 GPa to the pure WS2 to ±7 GPa when N is added to the films. Nevertheless no significant changes in the hardness are observed with increasing N contents. The adhesion of the coatings, evaluated by scratch testing, was found to be the lowest for the highest deposition pressure (2.0×10-2 mbar). X-ray diffraction analysis allowed concluding that the coatings have a nanocrystalline WS2 structure with the basal planes preferentially oriented parallel to the coating surface. Taking into account these results, it was concluded that the best compromise between the sulphur content and the adhesion, envisaging a good tribological behaviour, is achieved for the coatings deposited with a pressure of 1.2×10-2 mbar.

IBERTRIB 2013


B6.4 - A055

STUDY OF THE EFFECT OF FILLER CONTENT IN THE

TRIBOLOGICAL BEHAVIOUR OF PEEK POLYMER

Beatriz Fernandez-Diaz1)*

, Amaya Igartua1)

, Marcello Conte1)

, Fracesci Pagano1)

, Alois K. Schlarb2),

Patrick de Baets3)

1)

IK4-TEKNIKER, Tribology Unit, C/ Ignacio Goenaga, 5, 20600 Eibar, Spain. 2)

INM-Leibniz-Institute for New Materials, Campus D2 2, 66123 Saarbrucken, Germany 3)

University of Gent, Department of Mechanical Construction and Production, Technologiepark

Zwijnaarde 903, B-9052 Zwijnaarde, Belgium


ABSTRACT

The good mechanical and thermal properties of polyether-ether-ketone (PEEK) raised it as an

attractive engineering material. Further investigations in the field of Tribology have also highligted its

good performance, but this performance can be even improved by adding suitable fillers in its

formulation.

This work analyzes the synergy in the friction and wear behaviour of different combinations of Short

Carbon Fibers (SCFs) and micro-nanoparticles in a matrix of PEEK. Special interest is focused on the

role of the nanoparticles as solid nanolubricants in the contact of interacting surfaces.

MATERIALS AND FRICTION AND WEAR TESTS

Table 1 shows the four different PEEK-based materials investigated. The reference material is pure

PEEK (S-00), while the other 3 materials (S-01, S-02 and S-05) are PEEK composites with different

types and quantities of inorganic fillers (graphite, short carbon fibers and micro- and nanoparticles).

Table 4 Tested materials

S-

00

S-01 S-02 S-05

COMPOSITION (wr %)

PEEK 100 80 60 60

Graphite - 10 10 10

SCF (7 m, l=6

mm)

- 10 10 10

TiO2 (340 nm) - - 10 5

ZnS (300 nm) - - 10 5

SiO2 (12 nm) - - - 10

Friction and wear tests were performed using the UMT-3 tribometer (CETR) using the “pin on plate”

configuration. A pin of PEEK-based material (4x4x4 mm3) was rubbed in dry conditions against a

100Cr6 steel disc (24 mm, l=7.9 mm, Ra=2.2 m) following a linear reciprocating movement.

Maximum sliding velocities were set to 20 and 50 mm/s, respectively. The averaged apparent contact

pressure was fixed to 8 MPa. Initial test temperature was 25 ºC, and relative humidity 50%. Tests were

run for 67500 cycles (15 hours). Each test was carried out at least twice. In order to avoid dependence

of fiber orientation, sliding direction was in all the cases parallel to the injection direction of the

materials.




It was recorder the evolution of coefficient of friction along the time and a mean value was calculated.

Wear rate W (W=V/FL) was also calculated measuring the mass loss after the tests. It was also

measured the temperature increment close to the contact area using a thermocouple.


The addition of described fillers into the PEEK matrix clearly modifies the friction and wear

behaviour of the tested materials (Fig. 1). Regardless of the nature of the fillers, mean coefficient of

friction was reduced and wear resistance was improved. A noticeable increase in the wear resistance

was obtained, with a minimum decrease of the wear rate of 78 %. It is probably due to the important

increase of the hardness after the incorporation of the inorganic fillers as reported in a previous work

(Lin, 2012). The addition of microparticles in material S-02 increases the mean coefficient of friction

compared with the behaviour of S-01, but at the same time it moderately improves wear resistance.

Nanoparticles in material S-05 aids to reduce a little bit friction, although its effect on wear resistance

is not positive. Tests performed at 20 mm/s showed similar results so results are no reported.

Temperature variation in the contact was lower than 7 ºC in all the cases, so thermal effects were not

dominant.

Fig.1 Results of the tribological tests at 50 mm/s

According to the reported results it was observed that the incorporation of described inorganic fillers

in the PEEK formulation reduces the mean friction coefficient and improves wear resistance of the

material. Transfer layer on the steel surface contained fillers, so it is assumed that they assisted the

contact to reduce friction promoting the formation of this transfer layer and acting as solid lubricants.

Not noticeable role of micro-nanoparticles was observed. An optimization of the quantities of these

particles could improve results. Velocity effect on studied properties was negligible.

ACKNOWLEDGMENTS

The authors would like to acknowledge financing by the Spanish Ministry of Science and innovation

under Contract MAT2008-02999-E/MAT under the Framework of FANAS Initiative bellow

EUROCORES Programme of European Science Foundation (ESF).

REFERENCES

Lin LY, Tlatlik H, Gralla R; Igartua MA, de Baets P, Schlarb AK. “Mechanical and thermal

behaviours of polyetheretherketone-based multi-scale composites”, Journal of Composite Materials.

DOI:10.1177/0021998312454317, 2012

IBERTRIB 2013




Session A7

Room B001

AEROSPACE LUBRICANTS AND IONIC LIQUIDS

Chair:

Prof. Miguel Delgado and Prof. Jorge Castro

IBERTRIB 2013


A7.1 - A043

THE ROLE AND EVOLUTION OF THE AEROSPACE LUBRICANT IN THE DESIGN AND PERFORMANCE OF THE GAS TURBINE ENGINE

Susan C. Brown1, Lavern D. Wedeven2, Ronald E. Yungk3 1UTC-Pratt and Whitney Lubricant Fellow (retired), Binghamton, NY, USA 2Wedeven Associates, Inc., Edgemont, PA, USA 3Air BP Lubricants, Naperville, IL, USA

ABSTRACT

The gas turbine engine mechanical systems is the engine rotor support system that must transmit significant thrust loads under high stresses, high speeds and elevated temperature way beyond the automotive bearing system. Some of the primary components of the mechanical system include bearing systems, gears, seals, o-rings, and the lubricant system. All these components must function reliably per engine requirements with occasional operation under adverse conditions. The paper looks at the evolutionary performance of the aerospace lubricant properties on meeting the every increasing requirements of the jet engine.

INTRODUCTION

Unlike the automotive bearing system, aerospace mechanical systems must perform for thousands of hours under multi-dimensional operating conditions with a myriad of components, such as bearings, gears, and other rotating components. The aerospace bearing system operates at up to 3X contact stresses, temperatures way beyond the boiling point of water, and up to 5X the speed of an automotive bearing. It must lubricate, (reduce friction and wear), cool the system components, and transport debris from the system to the main oil filter. The lubricant must be compatible with all other system components from organic o-rings to non-metallic seals, and structural metals like steel, titanium and nickel alloys. Some of the critical lubricant properties include chemistry, temperature-pressure-viscosity characteristics density, vapor pressure, foaming characteristics, thermal-oxidative & hot spot stability, auto-ignition temperature, specific heat, thermal conductivity, and critical tribological performance under adverse operating conditions. These properties influence almost evry aspect of the mechanical system from size and weight of the system to the material choices, operating temperatures, speeds, design and life of bearings & gears. The very demanding thermal and operational environment of the mechanical systems of modern gas turbine engines, requires the use of high temperature capable liquid lubricants. The state of the art gas turbine engine lubricants are additive enhanced neo-pentyl polyol esters. How the industry got there is detailed in the paper.


Practical applications of the theory of gas turbine propulsion was demonstrated in the late 1930’s with the 27 August 1939 inaugural flight of the Heinkel He178 powered by the HeS 3b engine, developed by Dr. Hans von Ohain of Germany. In England, Sir Frank Whittle was also developing the Power Jet W.1 engine, which powered the British Gloster G.40 on 15 May 1941. By the end of World War II, both the van Ohain and Whittle engines were successfully powering fighter aircraft.



Prior to World War II, aircraft engine lubricants were predominantly mineral oils. The properties of those oils made them unacceptable for use in the much more demanding environment of gas turbine engines. During the late 1930’s and early 1940’s, research programs in Germany, England and the USA led to suitable lubricants. The most promising organic compounds were the dibasic acid esters (diesters). Initially, the diesters were blended with mineral oils for improved low temperature performance. With the continued advances in aircraft turbine engines and the increased lubrication and thermal demands placed on the lubricant, the diester/mineral oil blends were no longer adequate in the late 1940’s and led to fully synthetic engine lubricants.

The diester based lubricants offered some significant advantages over mineral oils. They included higher thermal-oxidative stability, improved lubricity, good low temperature pumpability, and a good viscosity-pressure-temperature profile. In 1952, the first fully synthetic ester based lubricant was developed by Esso-European Laboratory and designated EEL-3 and qualified to British DERD 2487 Specification. In 1953, the U.S. Military issued the MIL-L-7808 specification, the first for synthetic based gas turbine engine lubricants. Esso Turbo Oil 15, developed by Standard Oil Co., was the first of many diester based lubes qualified to MIL-L-7808 Specification.

Continued advances in the aircraft gas turbine engines, resulted in increased thrust-to-weight ratio engines, from higher compression ratios, higher combustion & turbine inlet temperatures, reduced cooling air, and higher rotor thrust and gear speeds. All those served to increase the temperature and stresses in the mechanical system. By the early 1950’s, it has become obvious that lubricants possessing higher thermal-oxidative stability was needed. The neopentyl polyol esters seemed the logical choice. Much like diesters, this class of compounds exhibited excellent lubricity, good low temperature pumpability, and good viscosity-pressure-temperature performance. In addition, they offered significant improvement in thermal-oxidative stability. The two most commonly used neopentyl polyol esters in modern gas turbine engine are trimethyolpropane (TMP) and pentaerythritol (PE).

The requirements for gas turbine engines continue to evolve. The desire for further performance, improved durability, lower costs, and increased survivability, will result in continuing design in a synergistic systems approach with material and lubricant advances uder the umbrella of Tribology.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the work of many aerospace engineering discipline engineers with whom we have worked with over 3 decades.

REFERENCES

Brown, Susan C., Chin, Herbert A., Haluck, David A., and Wedeven, Lavern D., “Linking Lubricants, Materials, Design and Tribology in Lubrication Systems: Cultural Changes for Development of Advanced Mechanical Systems, 2001.

IBERTRIB 2013


A7.2 - A003

COPPER-COPPER LUBRICATION AND SURFACE INTERACTIONS WITH PROTIC AND APROTIC IONIC LIQUIDS Tulia Espinosa, Carmen Palazón, José Sanes, Ana-Eva Jiménez, María-Dolores Bermúdez

Grupo de Ciencia de Materiales e Ingeniería Metalúrgica. Departamento de Ingeniería de Materiales y Fabricación. Universidad Politécnica de Cartagena. Campus de la Muralla del Mar. C/Doctor Fleming, s/n. 30202-Cartagena (Spain) (*)Email: [email protected]

ABSTRACT

Protic ammonium carboxylate and aprotic imidazolium ionic liquids (ILs) have been studied as

lubricants of oxygen-free high conductivity (OFHC) copper, and their results compared with

polyalphaolefin (PAO6). The results are discussed in terms of surface interactions and corrosion

processes. The best friction reducing and antiwear performance is obtained for the diprotic

ammonium lubricants, with friction coefficients as low as 0.01-0.03 and negligible wear. X-ray

photoelectron spectroscopy (XPS) show that ILs interact with the copper oxide surface layers.

The best lubricant forms adsorbed layers without corrosion.

INTRODUCTION

Some previous studies have used protic ILs in tribology (Qu, 2006; Kondo, 2008), but they

contain fluorine which can cause tribocorrosion of metal surfaces due to decomposition

products of the anions, such as HF. The most recent tendency in IL lubrication research is the use

of halogen-free ILs in order to prevent tribocorrosion. In the present study, the corrosion and

tribological performance of fluorine-containing aprotic imidazolium ILs have been compared

with that of new protic ammonium carboxylate salts, which only contain C, O, N and H in their

composition (Álvarez, 2010; Bermúdez, 2012). OFHC copper has been selected as it is a

reference material for corrosion tests and previous studies on copper alloys lubrication with ILs

are very scarce.


Table 1 shows the mass loss (∆m) and surface roughness (ΔSa) change of OFHC Cu in the

presence of the ILs.

The highest copper corrosion rate was obtained for the triportic MSu, in contrast with the

diprotic DSa and DAd. DAd gives the lowest friction coefficient and wear rate of all tested

lubricants (figure 1), with a one order of magnitude friction reduction with respect to PAO 6.

Table 1. Results of corrosion tests. Ionic Liquid ∆m(%) ΔSa(%)

Di-[bis(2-hydroxyethyl)ammonium] adipate (DAd) -0.04 -3.0 Bis(2-hydroxyethyl)ammonium salicilate (DSa) -0.05 2.5 Di-(2-hydroxyethylammonium) succinate (MSu) -1.40 225.0 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6) 0.12 -3.6 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([HMIM](CF3SO2)2N)

0.03 33.0



Among the aprotic imidazolium ILs, a lower corrosion rate also corresponds to a lower friction

coefficient and wear rate. XPS results show that the good lubricating performance of DAd is due

to the formation of adsorbed layers on the copper oxide surface. In contrast, MSu reacts with

copper oxide to give corrosion products which could be responsible for the friction increase. The

weight increase observed for [HMIM]PF6 is due to the formation of a P- and F-containing surface

layer. The results reported here open a new perspective for the development of efficient IL

lubricants.

Fig. 1. Friction coefficient records with sliding distance.

ACKNOWLEDGMENTS

We thank Dr. M. Iglesias (Universidad Federal de Bahia, Brazil) for the synthesis of the PILs, and

Ministerio de Economía y Competitividad (Spain) (MAT2011-23162) for financial support. T. Espinosa is

grateful to the MECD (Spain) for a research grant (AP2010-3485).

REFERENCES

Alvarez VH, Mattedi S, Martin-Pastor M, Aznar M, Iglesias M. Synthesis and thermophysical properties of

two new protic long-chain ionic liquids with the oleate anion. Fluid Phase Equil., 2010, 299, p. 42-50.

Bermúdez MD, Carrión FJ, Jiménez AE, Sanes J, Martínez-Nicolás G, Espejo C, Espinosa T, Arias J, Ojados G,

Gonzalez N, Jiménez M, Tribological performance and surface interactions of new ionic nanofluids and

nanomaterials. Chemistry and Physics in Tribology. ACS Tribology Symposium. 244st ACS National

Meeting and Exposition of the American Chemical Society (ACS). Philadelphia, PA (USA) 2012, 205-COLL.

Kondo H, Protic Ionic Liquids with Ammonium Salts as Lubricants for Magnetic Thin Film Media. Tribol.

Lett., 2008, 31, p. 211-218.

Qu J, Truhan JJ, Dai S, Luo H, Blau PJ, Ionic liquids with ammonium cations as lubricants or additives.

Tribol Lett., 2006, 22, p. 207-214.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 100 200 300 400 500

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on

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nt

PAO6 [HMIM](CF3SO2)2N MSu [HMIM]PF6 DSa

DAd

IBERTRIB 2013


A7.3 - A005

NEW STRATEGIES FOR THE REDUCTION OF FRICTION AND WEAR OF POLYMERS USING IONIC LIQUIDS AND NANOPHASES José Sanes, Francisco-José Carrión, Cayetano Espejo, Noelia Saurin, Gonzalo Ojados, María-Dolores Bermúdez

Grupo de Ciencia de Materiales e Ingeniería Metalúrgica. Departamento de Ingeniería de Materiales y Fabricación. Universidad Politécnica de Cartagena. Campus de la Muralla del Mar. C/Doctor Fleming, s/n. 30202-Cartagena (Spain) (*)Email: [email protected]

ABSTRACT

In the present work, two different methods have been studied for the improvement of the tribological performance of polymers. In the first place, ionic liquids (ILs) have been used as lubricants and lubricant additives in water. When the use of external lubricants is not advisable, new dispersions of polymers with carbon nanotubes or carbon nanotubes modified by ionic liquids have been obtained.

INTRODUCTION

Previous results by our research group have shown that neat imidazolium ILs can be good lubricants of polymer-steel systems, although the formation of corrosion products such as iron fluoride must be avoided. ILs have been called green lubricants, but a truly ecolubricant would be water. We are currently working (Bermúdez, 2011 and 2012) on some applications of protic ionic liquids (PILs) as lubricant additives in water for two high performance thermoplastics such as polymethylmethacrylate (PMMA) and polyetherimide (ULTEM)–steel contacts.


Fig 1.a shows the friction-distance records for PMMA disks sliding against stainless steel pins. Under dry conditions, PMMA shows high friction and relatively low wear. The use of water reduces friction but increases wear, probably due to the plasticizing effect of water on PMMA, which increases the mobility of the polymer chains, thus inducing a higher deformation and a wider wear track.

Fig. 1. Friction coefficient records with sliding distance for PMMA and ULTEM.

Distance (m)

0 100 200 300 400 500

CO

F

0.0

0.2

0.4

0.6

0.8

1.0Dry

H2O

H2O+1%PIL

PIL

ULTEM

Distance (m)

0 100 200 300 400 500

CO

F

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Dry

H2O

H2O+1%PIL

PIL

PMMA



The addition of a 1% PIL reduces the coefficient of friction but induces even a more severe wear. Only the use of neat PIL reduces friction in more than a 90%, and wear in more than one order of magnitude with respect to water. In the case of the high performance ULTEM, with a very high thermal stability and a higher resistance to the absorption of water, wear is always negligible under the testing conditions. Fig 1.b shows that for ULTEM the reductions in friction coefficient are similar to that found for PMMA. In the second strategy, the friction coefficients and the abrasive surface damage after viscoelastic recovery (residual depth) have been determined after 15 successive scratches under dry conditions, for neat PS and for three new nanocomposites containing single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT) or SWCNT modified by previous treatment with an IL (SWCNTm). The IL clearly improves the performance of SWCNT, to give a final result after 15 scratches close to that of MWCNT (Fig 2).

Fig. 2. Friction coefficients and residual depths vs number of scratches.

Similar dispersions have been developed for improving the tribological performance of other thermoplastic and thermoset polymers.

ACKNOWLEDGMENTS

We thank Dr. M. Iglesias (Universidad Federal de Bahia, Brazil) for the synthesis of the PILs, Ministerio de Economía y Competitividad (Spain) (MAT2011-23162 and BES2012-052210) for financial support and SABIC Innovative Plastics (Spain) for kindly providing Ultem.

REFERENCES

Bermúdez MD, Carrión FJ, Jiménez AE, Sanes J, Martínez-Nicolás G, Espejo C, Espinosa T, Arias J, Ojados G, Gonzalez N, Jiménez M, Tribological performance and surface interactions of new ionic nanofluids and nanomaterials. Chemistry and Physics in Tribology. ACS Tribology Symposium. 244st ACS National Meeting and Exposition of the American Chemical Society (ACS). Philadelphia, PA (USA) 2012, 205-COLL. Bermúdez MD, Jiménez AE, Sanes J, Líquidos iónicos próticos. Patent application P201131590. Spain, 2011.

Número de rayados

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IBERTRIB 2013


A7.4 - A019

FAP-ANION IONIC LIQUIDS AS OIL ADDITIVE IN STEEL-STEEL CONTACTS Alberto García1(*), Rubén González2, Antolin Hernández1, José L. Viesca1, Alfonso Fernández3

1 Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain 2 Department of Nautical Science and Technology, University of Oviedo, Asturias, Spain 3 Department of Physical and Analytical Chemistry, University of Oviedo, Asturias, Spain (*)Email: [email protected]

ABSTRACT

This work compares the tribological bevahior of two ionic liquids ([BMP][FAP] and [(NEMM)MOE][FAP]) used as oil additive in the lubrication of a steel-steel contact. The tribological results showed that the ionic liquids decrease friction and wear but the former ionic liquid had a better performance.

INTRODUCTION

In recent years many efforts have been made in order to study the advantages of ionic liquids (ILs) as lubricants or additives (Bermudez, 2009). Among the several combinations of anionic and cationic moiety that have been tested, the tris(pentafluoroethyl)trifluorophosphate (FAP) has begun to be used in the formulation of ionic liquids (Minami, 2008). The hydrolytic stability and hydrophobic character of FAP make this anion a good election to be used in lubrication.

In this work two ionic liquids, ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluorophosphate ([(NEMM)MOE][FAP]) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([BMP][FAP]), have been studied as 2 and 4 %wt. additives of a mineral hydrocraking oil (M2) for steel-steel contacts. Reciprocating ball-on-plate wear tests were performed on a HFRR machine using a load of 1000 g. (corresponding to a maximum contact pressure of 1.42 GPa), temperature of 100ºC, stroke length of 1 mm., a frequency of 30 Hz and testing time of 60 minutes. The electrical contact resistance (ECR) was measured during the tests and after them the wear volume was determined by using confocal microscopy.


The friction coefficient results from the tribological tests are shown in Fig. 1. In general, the addition of both ionic liquids to the base oil (M2) decrease friction in comparison with the friction results of the neat base oil. For both ionic liquids the highest friction reduction was obtained when the IL content was 4 %wt. However, the [BMP][FAP] ionic liquid had better tribological behaviour as oil additive than [(NEMM)MOE][FAP]. The better tribological behaviour of [BMP][FAP] is related to its quicker reaction (tribofilm formation) with the wear surfaces which can be observed in Fig. 2.

This study also showed the antiwear properties of these ionic liquids when they are used as additive. Further tests in XPS should be performed in order to analyze the chemical reaction of the oil samples (including ionic liquid) with the steel surfaces.




Table 1 Friction results of the base oil and the mixtures.

M2 M2 + 2%

[(NEMM)MOE][FAP] M2 + 4%

[(NEMM)MOE][FAP] M2 + 2%

[BMP][FAP] M2 + 4%

[BMP][FAP]

0.191 0.175 0.130 0.157 0.120

0

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60

70

80

90

100

0 10 20 30 40 50 60

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(%

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0

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Fig. 2 Electrical contact resistance (ECR) measured during the tribological tests.

ACKNOWLEDGMENTS

The authors wish to express their thanks to the Ministry of Education and Science and the Ministry of Science and Innovation, Spain, for supporting this work within the framework of the research project DPI2010-18166. They also thank the Government of the Principality of Asturias and the University of Oviedo for funding the pre-doc grant program “Severo Ochoa” and the research stay of Alberto Garcia at Imperial College London.

REFERENCES

Bermúdez MD, Jiménez AE, Sanes J, Carrión FJ. Ionic liquids as advanced lubricant fluids. Molecules, 2009, 14, p. 2888–2908.

Minami I, Kita M, Kubo T, Nanao H, Mori S. The tribological properties of trifluorotris (pentafluoroethyl) phosphate derived ionic liquids. Tribology Letters, 2008, 30, p. 215-223.

IBERTRIB 2013




Session B7

Room B002

FRICTION AND WEAR 1

Chair:

Prof. José Gomes and Prof. Luís Magalhães

IBERTRIB 2013


B7.1 - A018

LABORATORIAL AND INDUSTRIAL TRIBOLOGICAL CHARACTERIZATION OF PVD THIN FILMS SUITABLE FOR PLASTIC INJECTION MOULDING F. J. G. Silva1(*), R. P. Martinho2, A. P. M. Baptista3

1Depatment of Mechanical Engineering, ISEP – Instituto Superior de Engenharia do Porto (IPP), Porto, Portugal 2Department of Engineering, ESEIG – Escola Superior de Estudos Industriais e de Gestão (IPP), Porto, Portugal 3Department of Mechanical Engineering (DEMec), University of Porto, Portugal (*)Email: [email protected]

ABSTRACT

This work compares four different thin hard coatings obtained using PVD Magnetron Sputtering process: TiAlN, TiAlSiN, CrN/TiAlCrSiN and CrN/CrCN/DLC. The first two are monolayer coatings while the last ones are nanostructured and consist of multilayer systems. In order to carry out the corresponding tribological characterization, two different approaches were selected: a laboratorial method, using micro-abrasion wear tests based on ball-cratering configuration, and an industrial mode, analysing wear resistance of the coated samples when inserted in a plastic injection mould. As expected, the wear phenomena are not equivalent and the results between micro-abrasion and industrial tests are not similar. The best wear resistance performance in the laboratorial wear tests was attained by TiAlN monolayer coating when in the industrial wear tests was obtained by CrN/TiAlCrSiN nanostructured multilayer coating.

INTRODUCTION

Abrasion is a common phenomenon in the plastic injection moulding process and becomes even more important when plastics are reinforced with glass fibers. Indeed, the natural motion of the fiber tips during the injection cycle induces small scratches in the mould cavity leading to bright degradation of the mould cavity surface. In order to overcome this problem, some authors have investigated a number of surface treatments and tribological coatings (Bull, 2000). More recently, some authors have tested selected films submitting them to real injection conditions (Stock, 2008 and Martinho, 2011) obtaining results that yields thinking that coatings should be applied to moulds in a competitive way.

In this work, two monolayer films (TiAlN and TiAlSiN) and two multilayer nanostructured coatings (CrN/TiAlCrSiN and CrN/CrCN/DLC) were produced using CemeCon CC800/9 PVD unbalanced magnetron sputtering reactor. The deposition parameters were similar for every coatings here referred: Gas pressure 500 mPa, Temperature 500 oC, Target power density 16 A·cm−2, Bias in the range of −120 V and −50 V, and deposition time 4 hours. After deposition, films thickness was quantified by SEM and the morphology was also analysed by this way. Scratch-tests were featured evaluating the coatings’ adhesion. Micro-hardness tests were carried out leading to correlate this property with the tribological performance of each coating. Micro-abrasion wear tests were accomplished using PLINT TE-66 equipment using SiC F1200 as abrasive in a ratio of 35,4 g to 100 ml of distilled water, using as normal load 0,25 N. The industrial tests were carried out using inserts into the feed channel of the mould, performing 45.000 injection cycles of PP reinforced with 30% (wt) of glass fibers.





The thickness of the coatings was assessed by SEM, leading to the values pointed in table 1. The roughness was evaluated by profilemetry and yields to confirm that it is in the range of 0.033 µm for CrN/TiAlCrSiN and 0.061 µm for TiAlSiN coating. The micro-hardness was measured revealing that the hardest coating is the CrN/TiAlCrSiN and the softest is the CrN/CrCN/DLC coating. The morphology is similar among the four coatings, being assessed by SEM.

The micro-abrasion tests yield to observe that the wear coefficient (kc) is smaller for the TiAlN while is greater for the CrN/TiAlCrSiN coating, corresponding to better wear resistance (k-1) of the TiAlN, which also presents relatively low reduced Young’s Modulus. The H3/Er2 ratio is interesting, leading to better deformation resistance in the contact.

Regarding the industrial tests, after 45.000 injection cycles, the lowest volume of coating removed by abrasion and adhesion was performed by the hardest multilayer coating, CrN/TiAlCrSiN. The results in this test were obtained analysing the material removed from the surface due to the glass fibers abrasion during the injection cycle runs.

Table 5 Results report

Coating Thickness

Hardness Er H3/Er2 kc k-1 Life vs Steel

µm GPa GPa GPa mm3/N·m N·m/mm3 TiAlN 3.5 22.7 304 0.135 6.51E-5 15.37 30.8x

TiAlSiN 4.5 21.8 262 0.160 24.6E-5 4.068 25.0x CrN/TiAlCrSiN 4.3 30.9 325 0.293 37.3E-5 2.683 65.5x CrN/CrCN/DLC 3.6 19.2 288 0.085 31.8E-5 3.141 58.2x

Thus, the hardness seems to play an important role in the wear behaviour of the multilayer coating when submitted to injection of reinforced polypropylene. On the other hand, TiAlN presents the best wear behaviour when submitted to micro-abrasion tests, even presenting lower hardness than CrN/TiAlCrSiN coating and relatively low H3/Er2 ratio, showing low resistance to plastic deformation.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of CETRIB/INEGI.

REFERENCES

S.J. Bull, R.I. Davidson, E.H. Fisher, A.R. McCabe, A.M. Jones. A simulation test for the selection of coatings and surface treatments for plastics injection molding machines, Surface and Coatings Technology 130 (2000) 257–265.

H.-R. Stock, M. Diesselberg, H.-W. Zoch. Investigation of magnetron sputtered titanium–nickel–nitride thin films for use as mould coatings, Surface and Coatings Technology 203 (2008) 717-720.

R. P. Martinho, F. J. G. Silva, R. J. D. Alexandre and A. P. M. Baptista. TiB2 Nanostructured Coating for GFRP Injection Moulds, Journal of Nanoscience and Nanotechnology, Vol. 11, (2011) 5374–5382

IBERTRIB 2013


B7.2 - A024

WEAR BEHAVIOUR OF ALUMINUM ALLOYS AT VERY LOW SLIDING SPEEDS Meritxell Ruiz Andrés, Ana Conde del Campo, Juan de Damborenea Gonzalez, Ignacio García Diego(*)

Corrosion and Protection of Metallic Materials, Department of Surface Engineering, Corrosion and Durability, National Center for Metallurgical Research, Spanish National Research Council CENIM-CSIC, Madrid, Spain (*)Email: [email protected]

ABSTRACT

The aim of this work was to study the sliding wear behaviour of three aluminium alloys AA5754 (Al-Mg), AA6082 (Al-Mg-Si) and AA7075 (Al-Zn-Cu) at low sliding speeds, which have scarcely been investigated, in order to provide an overall framework for the wear mechanism map of aluminium alloys. Wear tests were performed using a block-on-ring configuration against AISI 52100 ring steel at a contact pressure range of 20-140 MPa and sliding velocity range of 0.001-1.5 m/s. At the sliding speed usually studied for other authors (> 0.1 m/s) the results confirmed the mild wear mechanism by mixing and oxidation of material coming from both aluminium block and steel ring. At the very low speed new range studied (< 0.1 m/s) an increment of wear rate and coefficient of friction for all the aluminium alloys seems to indicate that some alternative wear mechanism could be taking place. Therefore the investigated low sliding speed enables to complete the wear mechanism map of aluminium.

INTRODUCTION

Several experimental studies of dry sliding wear of aluminium alloys were carried out due to the interest of light aluminum alloys and composites in several engineering applications (Zhang 1997; Wilson 1999; Rao 2013)). Using the wear rate value and the aspect of the worn surfaces and loose debris, Zhang and Alpas (Zhang 1997) constructed an empirical wear mechanism map as a function of the applied load and the sliding speed showing that the transition from mild to severe wear regime is determined by changes of wear mechanism: mixing/oxidation, delamination, galling... The different transitions observed were correlated with a temperature rise in the aluminum surface due to the heat generated by friction. For example, seizure was related to the achievement of a critical temperature in the bulk of 125ºC approximately, meanwhile mixing/oxidation to delamination transition was related to the achievement of a local flash temperature that exceeds such critical temperature. It is also well known that sliding speed and load are the most relevant parameters which determine the surface temperature in dry sliding conditions (Ashby 1991). However, as most of the works on sliding wear of aluminum alloys and composites have been performed at sliding speeds above 0.1 m/s it is interesting to broaden the sliding speed range and study the wear behavior of aluminum alloys at lower velocities. It is assumed that at very low sliding speed the increment of surface temperature should be negligible and therefore is expected to find a limit to the oxidation/mixing wear mechanism.


Wear rate of the three aluminium allows shown a similar value and tendency with load and sliding speed, figure 1a. Wear rate shows a minimum value of around 2.16·10-3 mm3/m for



AA7075 at a sliding speed of 1 m/s that is in agreement with the mild wear mechanism described in the traditional wear maps for aluminium alloys (Zhang 1997; Wilson 1999; Rao 2013). Such mild wear regime corresponds to a mixing/oxidation mechanism. In the present work, SEM images, figure 1b, as well as EDX and XRD characterisation show that debris is actually a fine mixture of Al and Fe oxides as expected in such wear mechanism. At elevated sliding speeds, > 0.5-1 m/s, there are and increment in wear rate as well as a progressive presence of Al flakes in the debris. Both features points the transition to a delamination mechanism what is also consistent with the literature. Interestingly, at the lower speeds, < 0.1 m/s, an increment of the wear rate is also observed for the three alloys. This wear rate increment is associated with an increasing presence of large metallic-aspect flakes within the debris. These results suggest that, at speeds lower than 0.1 m/s, other wear mechanism prevails over mixing/oxidation mild wear in these aluminium alloys, leading to an increment of the wear rate at very low sliding speed. Additionally, the coefficient of friction (COF) for the three alloys also increases as sliding speed decreases, achieving high values of around 1.22 for AA5754 sliding at 0.01 m/s. This change of the COF values would corroborate the change produced in the tribo-contact between the aluminium alloy and the steel that influences on the wear mechanism at very low speed.

10-3

10-2

10-1

100

101

10-3

10-2

10-1

AA5754

AA6082

AA7075

Wear

rate

(m

m3/m

)

sliding speed (m/s)

Fig.1 Wear rate versus sliding speed for AA5754, AA6082, AA7075. SEM image of fine

debris (oxide compounds) produced at 0.001 m/s and 100 MPa on AA7075

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Ministerio de Economia y Competitividad under project Innpacto IPT-020000-2010-020 and project CONSOLIDER-INGENIO 2010 CSD 2008-0023 FUNCOAT.

REFERENCES

Ashby, M. F., J. Abulawi, et al., Temperature maps for frictional heating in dry sliding, Tribology Transactions, 1991, 34(4), p. 577-587.

Rao, R. N., S. Das, et al., Dry sliding wear maps for AA7010 (Al–Zn–Mg–Cu) aluminium matrix composite, 2013), Tribology International, 60 (0), p. 77-82.

Wilson, S. and A. T. Alpas, Thermal effects on mild wear transitions in dry sliding of an aluminium alloy, Wear, 1999, 225-229 (Part 1), p.440-449.

Zhang, J. and A. T. Alpas, Transition between mild and severe wear in aluminium alloys, Acta Materialia, 1997, 45(2), p. 513-528.

IBERTRIB 2013


B7.3 - A031

INFLUENCE OF THE MICROSTRUCTURE IN THE WEAR PREDICTION OF ALUMINA Álvaro Rico1(*), Felipe Orgaz2, Jesús Rodríguez3

1, 3 Departamento de Tecnología Mecánica. Universidad Rey Juan Carlos. Madrid (España). 2 Instituto de Cerámica y Vidrio. Consejo Superior de Investigaciones científicas. Madrid (España) (*)Email: [email protected]

ABSTRACT

Wear tests at different experimental conditions were carried out on alumina with different grain sizes. The effect of the microstructure was studied through a wear model based on an energetic balance previously developed. Results reveal that the grain size determines in a great extent the wear behaviour of materials, even when similar wear mechanisms are detected for the samples.

INTRODUCTION

Brittle fracture is one of the most common wear mechanisms reported for ceramics. It is well known that the fracture toughness and the operative conditions are the key parameters controlling the crack propagation throughout the material. In addition, several models predicting critical conditions for transitions from the mild to the severe wear regime have been previously developed. The transition is thought to be due to a change in the crack path propagation. Intergranular fracture is observed within the mild wear regime, while transgranular fracture is developed at the severe wear regime. These models seem to be in agreement with the influence of the defects distribution inside the ceramic material on the generation and propagation of cracks. However, the link between microstructure of the ceramic and the activation and stabilization of the fracture mechanism is not well established. In this work, an experimental program was carried out on similar aluminas which only differed in the grain size. Different nominal pressures were used to perform the wear tests. A new model explaining brittle fracture in wear is proposed for these materials. This model is based on energetic considerations and comprises mechanical and thermal properties of the material as well as the operating conditions in the test. The preliminary results point out that debris and grain size present a relation explaining the brittle fracture in wear using this new model.


3.1 Microstructure

Alumina samples showed a polycrystalline microstructure with grain size depending on the thermal treatment. A grain size ~273 nm was reached from the low temperature treatment, while an average grain of 176 µm in size was obtained from the high temperature treatment (Figure 1). 3.2 Tribological characterization

An energetic approach, developed through dimensional analysis, has been used to explain wear data. Wear rate, put in terms of the wear coefficient, kH, has been expressed as a function of a dimensionless parameter which comprises a dimensionless form of the input energy to the tribosystem and a dimensionless form of the energy dissipated by the material during the wear




process. The input energy can be expressed as the product between the friction coefficient, the normalized normal load and the normalized sliding velocity. Nonetheless, the key of this methodology is the fact that the dissipated energy is dependent on the operative wear mechanism. In this way, when the main wear mechanism is brittle crack propagation, the dissipated energy can be written as a dimensionless function of the energy release rate, G. If the wear coefficient is represented versus the dimensionless contact severity parameter, wear data corresponding to both low and high grain size materials can be described by the same function (Figure 2).

Figure 1. Microstructure of the ceramic samples. A) Low temperature treatment. B) high temperature treatment

Figure 2. Wear coefficient vs. the contact severity for both materials.

500 nm

t = 273,.2 nms = 37.2 nm

Grain size (nm)

200 µm

t = 176 µms = 34 µm

10-9

10-8

10-7

10-6

10-5

0,0001

0,001

0,01

0,1

10 100 1000 104

105

106

107

108

d = 0.276 nmd = 176 nm

kH = 1.14 10-10

(µPV G-1

)1.2

R = 0.99983

We

ar

co

eff

icie

nt,

kH

Dimensionless contact severity µPv G-1

IBERTRIB 2013


B7.4 - A044

INFLUENCE OF PLASTIC DEFORMATION ON SLIDING FRICTION OF ALUMINIUM ALLOY 5083. João Miranda1,2(*), A. Ramalho2

1 School of Technology and Management, Polytechnic Institute of Guarda, Guarda, Portugal 2 Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal (*)Email: [email protected]

ABSTRACT

This work discusses the influence of the effect of the strain hardening behaviour on the sliding friction. The change of mechanical properties near the surface promoted by plastic deformation and its influence in the friction was investigated using a unidirectional sliding tribometer with a sphere-plan contact. These experiments revealed that the effect of strain hardening made with small loads achieved values of the friction coefficient near the surface similar with those achieved with higher loads. The friction coefficient measures inside the tracks were quite similar for different applied loads and different number of passages.

INTRODUCTION

The friction coefficient is a conventional, but somewhat misunderstood, quantity in the field of engineering and science. It is a convenient and useful parameter for engineering when contact surfaces are subjected to heat and wear, but care should be exercised when assigning to it a fundamental significance. While friction coefficients are relatively easy to determine in laboratory experiments, the fundamental origins of sliding resistance are not as clear (Blau, 2001). Friction between a pair of materials depends on a lot of factors that could affect more or less this phenomenon. Some materials exhibit time-dependent tribological behaviour with an initial running-in period marked by changes in friction and influenced by contact and environmental conditions. This could be observed shortly after the start of sliding contact between fresh, unworn solid surfaces (Blau, 2005). These temporary variations are sometimes ignored or simply accepted as the normal course of operation and the tendency is to emphasize the knowledge in the steady state conditions. However, the engineers have found helpful the knowledge of such behaviour in the running-in period so that this phase could be conducted in an optimal approach leading to more efficient mechanical systems with longer life and less maintenance interventions.

This work aims to investigate the influence of the effect of the strain hardening behaviour on the sliding friction. The change of mechanical properties near the surface promoted by plastic deformation and its influence in the friction was studied using a unidirectional sliding tribometer with a sphere-plan contact. Materials tested were a commercial aluminium alloy - 5083 which was tested against an alumina sphere with a diameter of 10 mm and a steel DIN 100Cr6 sphere with a diameter of 1 mm. With this tribometer, two set of tests were made, those to produced plastic deformation in the sub-surface and those made to evaluate the modifications in the coefficient of friction as a result of plastic deformation near the surface.





Some of the results obtained from the unidirectional sliding tests are shown in Fig. 1 (a) and (b).

(a) (b)

Fig.1 (a) Friction coefficient inside tracks made with constant number of passages. (b) Hardness measure perpendicular to the track. (After 5 passes).

In figure 1-b, we can see that the higher the load applied, the higher the hardness obtained in the aluminium sample. However, in figure 1-a, we can observe that the friction coefficient for 50 N of applied load is quite small when compared to the other tested conditions. With this kind of load, the plastic deformation is high and the roughness higher peaks of the antagonist are deformed reducing the friction coefficient. When the number of passages increase with this load, the friction coefficient increase approaching the other test conditions. Increasing the number of passages the values of friction for lower applied loads, tend to the same value.

This work showed that the hardness measure in the samples with variable number of passages and variable applied loads was much higher than the matrix hardness and tends to the value of the matrix in an extension of approximately 20 μm, revealing a surface hardening of approximately that value. The contribution of the hardening effects and roughness was widely analysed.

REFERENCES

Blau PJ. The significance and use of the friction coefficient. Tribol Int, 2001, 34, p. 585–591.

Blau PJ. On the nature of running-in. Tribol Int, 2005, 38, p. 1007-1012.

IBERTRIB 2013




Session A8

Room B001

LUBRICANT ADDITIVES

Chair:

Dr. Francisco-José Carrion and Dr. José Brandão

IBERTRIB 2013


A8.1 - A011

RELAXATION OF THE WATER SURFACE TENSION BY HUMIDIFICATION OF THE ENVIRONMENT. Marco Antonio Álvarez-Valenzuela(*),Juan Sanchez-Garcia-Casarrubios, Ignacio Valiente-Blanco, Efren Diez-Jimenez, Cristian Cristache, José Luis Pérez-Díaz.

Department of Mechanical Engineering, Universidad Carlos III de Madrid, Madrid, Spain (*)Email: [email protected]

ABSTRACT

Precise measurements of the surface tension of water in air vs. humidity at 1, 6, 12 and 17ºC are shown. For constant temperature, surface tension decreases for increasing humidity in air and that this can be related to the dynamic effect of vaporization according to Newton's third law. A high humidity in the air reduces the net flow of evaporating water molecules lowering the effective surface tension on the drop. Therefore, just steam in air acts as an effective surfactant for the water-air interface. It can -partially- substitute chemical surfactants helping to reduce their environmental impact.

INTRODUCTION

Surface tension (γ) measurements is one the most important properties on fluids. The Surface tension, formally defined as energy per surface unit at the interface between two fluids (Raymond, 1934) makes the interface behaves as an elastic sheet (Erbil, 1997) and plays a key role in many physical, chemical and biological processes. It strongly conditions a broad number of natural and artificial processes like for example fog and mist formation, wetting, spraying, and foaming. In fact the main purpose of a lot of common chemical products is to lower the surface tension of some liquids.

Therefore, it is not surprising that many first-class scientists like Laplace, Young (Young, 1805), Thomson and Lord Rayleigh had contributed to describe different surface tension effects and properties. A clear dependence on the temperature is well known both theoretically (Erbil, 2006) and experimentally (Vargaftikl, 1983). However, discrepancies among values of surface tension measured by different authors, even for water-air, seem to be systematic and have not been explained till now (Vargaftikl, 1983).

Some authors pay careful attention to study the interface between a liquid and its saturated vapor (National Academy of Sciencesl, Vol. 4)(Kayserl, 1976), while others simply measure surface tension for a liquid in equilibrium with air (Noordmans, 1991), but it never before have studied the dependence of the surface tension with relative humidity. In previous studies, we demonstrated that there is a clear surface tension dependence to relative humidity (Pérez-Díaz, 2012) by measuring the surface tension of water in air vs. humidity at 5, 10, 15 and 20ºC. The use of an artificial vision technique with a fast B&W CCD makes it possible to see that dependence.

In the present paper we show new experimental data that demonstrate that the humidity in air acts as an effective surfactant for the water-air interface. Use the pendant drop method to measure the surface tension of water in air vs. humidity at 1, 6, 12 and 17ºC.




To test which describes the dependence of the effective or apparent surface tension on humidity, we have built a high precision climatic chamber for use with pending drop equipment. In every instance both the temperature of the liquid water drop and temperature of air were kept equal, to within a precision of 0.1 ºC. Relative humidity in air was stabilized to within a precision of 1%. Using this equipment we have measured (Pérez-Díaz, 2012) the surface tension in deionized water for different temperatures and humidities, as shown in Fig.1.

Fig.1 Effective surface tension vs. relative humidity in a 2.8 mm diameter pure water

drop. [1 Surface tension measured by different authors (Vargaftikl, 1983)]

It can be clearly seen how the effective surface tension decreases as humidity increases. In other words, the presence of steam in the air “relaxes” the surface tension on the interface.

REFERENCES

Erbil H.Y., Birdi K.S. (Ed.). "Handbook of Surface and Colloid Chemistry", Vol. 1. Chapter 9. CRC, Press, LCC, New York 1997.

Erbil H. Y., "Surface Chemistry Of Solid and Liquid Interfaces", Oxford : Blackwell 2006.

Kayser W.V., "Temperature dependence of the surface tension of water in contact with its saturated vapor", Journal of Colloid and Interface Science, 1976, 56, 622–627.

National Academy of Sciences (Washington), International critical tables of numerical data, physics, chemistry and technology", Vol 4. National Research Council by the McGraw- Hill Book Company.

Noordmans J., Busscher H. J., "The influence of droplet volume and contact angle on liquid surface tension measurements by axisymmetric drop shape analysis-profile (ADSA-P)", Colloids and Surfaces, 1991, 58, 239-249.

Pérez-Díaz J.L, Álvarez-Valenzuela M.A, García-Prada J.C., " The effect of the partial pressure of water vapor on the surface tension of the liquid water–air interface", Journal of Colloid and Interface Science, 2012, 381, 180–182.

Raymond de. F. "Étude Thermodynamique de la Tension Superficielle.", Vol. 3, Gauthier - Villars & C, Paris, 1934.

Vargaftik N. B., Volkov B. N., Voljak L. D., "International Tables of the Surface Tension of Water, Journal of Physical and Chemical Reference Data", 817, 12, 1983.

Young T., "An essay on the cohesion of fluids, Philosophical Transactions of the Royal Society of London", 1805, 95, 65–87.

IBERTRIB 2013


A8.2 - A007

INFLUENCE OF THE LUBRICATING ADDITIVE IN SLIDING WEAR SYSTEM ANALIZED BY RAMAN SPECTROSCOPY Ane Cheila Rovani1,2(*), Michel Mermeux2, Amilton Sinatora1

1 Polytechnic School of the University of Sao Paulo (EPUSP), Surface Phenomena Laboratory – LFS, Department of Mechanical Engineering, University of Sao Paulo, Brazil. 2 Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et Interfaces (LEPMI), UMR CNRS 5279, 1130 Rue de la Piscine, BP 75, 38402 Saint-Martin-d’Hères, France. (*) Email: [email protected]

ABSTRACT

Boundary lubricating films are critical in effective lubrication of engine components. This work presents the study of the lubricating additive influence in the friction coefficient as a function an applied load in sliding wear pin on disc system. The friction coefficient was determined in the presence of a lubricant made of mineral and ester base oil (22% v/v) and fatty acid additive (Stearic Acid C18H32O2). The phase formation after tests was investigated by Raman Spectroscopy. Results show evidences that when we used a low load, the friction coefficient was greater than for higher loads. Additionally, we observed the formation of and the magnetite oxide phase for lower loads and hematite and magnetite oxide phases for the highest loads. The increase of the additive concentration effectively leads to a decrease of the friction coefficient, for all applied loads, thus improving the efficiency of the sliding properties.

INTRODUCTION

Ultrathin organic monolayers deposited on solid substrates (fatty acid films) provide substantial chemical protection to the substrate, reduction of the friction coefficient and the war in the contact surfaces. The best boundary lubricant additives (e.g. the friction modifiers) are long chain molecules with a polar end group, typically fatty acids, alcohol or amide (Bowden & Tabor, 1950). The reduction of the friction coefficient leads to successive reductions in fuel usage in many mechanical components that operate under boundary lubrication regime. The influence of the carbon chain length of the fatty acid added to the base oil lubrication has been intensely studied in the literature and shows that increasing the amount of carbon in the molecule carries a considerable reduction in the coefficient of friction (Bowden, 1950, Studt, 1989). However, other studies have also shown that during a prolonged test time in very light loads, with fatty acid presence, the friction coefficient was greater than in high loads or on dry friction coefficient (Bowden & Tabor book (1950), Schofer, 2001). The effect of tribochemical film formation and the friction coefficient response, and their mechanism formation is not clear, and only a few studies were devoted this topic. Raman Spectroscopy is a well suited analysis method to identify of oxide-layers and organics phase formation (De Faria, 1997). In this work, mineral and the ester base oil (22% v/v) were used. The loads were varied from 10 to 240N and the independent variables were the sliding speed of 0.1m. s-1, room temperature and the proportion of the oil. AISI 4140 – 435 HV30 steel pin and AISI H13 – 530 HV30 steel discs were the materials used. The experimental conditions were a pin on disc test. The surfaces of samples after tests were characterized by Raman Spectroscopy with an excitation wavelength in the near-infrared region at 785 nm. The excitation power of the laser was chosen to avoid transformation of the expected iron oxides.





The extracted friction coefficients are given in Fig. 1A. With the presence of additives, the friction coefficient exhibited a different behavior than with dry friction conditions. The dry friction coefficient tends to increase with the applied load, while the addition of a lubricant film changes this behavior. In the presence of an additives and using Raman spectroscopy, we could observe the formation of an oxide layer on the sliding surface of the samples, see Fig, 1B. Different peaks in the 100-700 spectral range were observed. These peaks are attributed to the magnetite (Fe3O4) and hematite (Fe2O3) phases (De Faria, 1997, Thierry, 1990, Souza 2000), depending on the load and additive concentration conditions used. Without lubricant, we did not detect any oxide phase. Thus, the presence of an additive in sliding surface seems to contribute to the formation of an oxide layer. This may contribute to an improvement of the sliding wear and a decrease of the friction coefficient as a function of the applied load.

Figure 1: (A) Friction coefficient as a function of the load and stearic acid additive concentration, 0.001 and 0.1%. (B) Raman analysis of the pin surface (a) without lubrication sample, (b) 0.001% - 35N, (c) 0.1% - 35N (d) 0.001% - 240N and (e) 0.1% - 240N samples. Finally, for the highest contact pressures, we observed that the friction coefficient decreased. This behavior suggests that for very low loads, the oxide presence on the surface not formed the tribofilm with the lubricant film and the metal surface.

ACKNOWLEDGMENTS

The authors acknowledge the funding from the Sao Paulo Research Foundation (FAPESP) Brazil.

REFERENCES

Bowden, P.F., Tabor, D., Oxford, 1954.

De Faria, D.L.A., et al., Journal of Raman Spectroscopy, 1997, 28, p. 873-878.

Schofer, J., et.al., Wear, 2001, 248, p. 7 – 15.

Souza, H.M., et al., Journal of Raman Spectroscopy, 2000, 31, p. 185 – 191.

Studt, P., Tribology International, 1989, 22, p.111 – 119.

Thierry, D., et al., Corrosion Science, 1991, 32, p. 273 – 284.

0 50 100 150 200 250

0,3

0,6

0,9

1,2

1,5

0,001% of additive

0,1% of additive

Without lubrication

Fri

ctio

n C

oef

fici

ent

Load [N]

(A)1.5

1.2

0.9

0.6

0.3

500 1000 1500 2000 2500

Fe3O

4

Fe2O

3

(e)

(c)(b)

(d)

Inte

nsi

ty

Wavenumber [cm-1

]

(a)

(B)

IBERTRIB 2013


A8.3 - A014

Potential use of estolides as additive on vegetable oil-based lubricants L.A. García-Zapateiro1,2, J.M. Franco2,3, C. Valencia2,3, M.A. Delgado2,3(*), C. Gallegos2,3

1 Departamento de Operaciones Unitarias. Facultad de Ingeniería. Grupo de Investigación Ingeniería de Fluidos Complejos y Reología de Alimentos (IFCRA). Universidad de Cartagena. 130015 Cartagena de Indias. Colombia. 2 Departamento de Ingeniería Química. Campus de “El Carmen”. Universidad de Huelva. 21071 Huelva. Spain. Campus de Excelencia Internacional Agroalimentario, ceiA3. 3 Pro2TecS – Chemical Process and Product Technology Research Center. Universidad de Huelva. 21071 Huelva. Spain. (*)Email: [email protected]

ABSTRACT

This work deals tribological characterization of a variety of estolides, obtained from both oleic and ricinoleic acids, using different acid-catalysed synthesis protocols, and their blends with high-oleic sunflower (HOSO) and castor (CO) oils. Estolides were added at concentrations of 15% w/w. A homogenous blend was obtained in all cases. The mechanical tests performed in a tribosystems that allowed to identify the evolution of the friction coefficient, for selected estolide samples, in the form of Stribeck curves. As well, was studied the wear scars of the different estolides blends and the viscous behaviour.

INTRODUCTION

Recently, much effort has been focused on develop new types of lubricating oil additives to reduce wear and friction (Maleque, 2003).The use of vegetable oil-based lubricants present several advantages over the much more extended mineral bases. Estolides are a class of ester resulting from the reaction between both a carboxylic group and C=C bond of fatty acid molecules to form an ester linkage (Isbell, 1994, Isbell., 2000, Cermak, 2001). Their excellent cold temperature properties coupled with their good oxidative stability and high viscosity should see these derivatives in many industrial applications (Isbell, 2011).

Estolides were blended with HOSO and CO, under gentle agitation (250 rpm, 3h), at 60-80ºC. Afterwards, the samples were cooled down to room temperature. Estolides were added at concentrations of 15 % w/w, until homogenous blend. Tribological tests were performed using a tribological cell coupled with a Haake MARS-Rheostress rheometer (Thermo Scientific). In all cases, the ball was fixed to avoid rolling. This configuration of ball-on-plates was used to determine the friction coefficient, defined as the ratio between the applied normal force and the measured friction force. Stribeck curve was monitored by applying a rotational speed sweep (0-1000 rpm) and 3 N normal force, at room temperature (25ºC). Besides this, specific tests were also performed by applying a normal load of 20 N and a constant rotational speed of 400 rpm, during 2400 s in order to evaluate the wear.


Table 1 shows density and kinematic viscosity data as a function of vegetable oils used, temperature and estolide preparation method. The wide range of viscosity achieved allows these products to be proposed as promising formulations for different lubricant applications. Figure 1 shows the Stribeck curves for HOSO, CO and their blends with the different estolides studied, in a ratio 85:15 w/w. As can be observed, the overall evolution from the mixed to the hydrodynamic lubrication regime was always displayed for CO/estolide blends, whereas HOSO/estolide blends did not reach the hydrodynamic lubrication regime in the sliding speed range studied. It has worth pointing out, when friction curves are normalised with the Sommerfeld number, estolide




addition to HOSO or CO does not modify the frictional behaviour, at least at this concentration, barely resulting in just one single master curve.

Table 6 Densities (ρ) and kinematic viscosities (υ) for HOSO and castor oil (CO) containing 15% (w/w) of different estolides samples

Neat oil Oleic Acid -

Sulphuric method

Ricinoleic Acid - Sulphuric method

Ricinoleic Acid -Perchloric

method T (ºC) 40 100 40 100 40 100 40 100

HOSO ρ (g/cm3)a 0.8989 0.8700 0.9032 0.8999 0.9200 0.8772 0.9040 0.8643

(mm²/s)b 41.94 9.51 51.31 10.01 69.15 13.50 54.93 11.22

Castor oil

ρ (g/cm3)a 0.9458 0.9012 0.9433 0.9028 0.9493 0.9088 0.9453 0.9046

(mm²/s)b 222.04 20.94 219.81 22.42 354.49 26.61 269.35 23.42

a Standard deviation for density measurements is always lower than 10-4 b Standard deviation for kinematic viscosity measurements is always lower than 10-2

Fig.1 Stribeck curves for HOSO/estolide and CO/estolide blends (proportion: 85:15

w/w)

This study shows that estolide samples synthetized by using the sulphuric acid-catalysed method show the highest values of the friction coefficient in the mixed lubrication region, which suggests some influence of remaining traces of this acid, especially when the asperities contact becomes the predominant friction mechanism. The addition of 15% (w/w) of these estolides to both vegetable oils significantly reduces the wear scars, being this reduction much more effective for blends prepared with estolides obtained from ricinoleic acid than by using the sulphuric and perchloric acid-catalysed methods

ACKNOWLEDGMENTS

This work is part of a research project (CTQ2010–15338) sponsored by a MINECO-FEDER programme. The authors gratefully acknowledge its financial support.

REFERENCES

Cermak S.C, Isbell T.A. Synthesis of Estolides from Oleic and Saturated Fatty Acids. J. Am. Oil Chem. Soc. 2001. 78, 557-565. Isbell T.A. Chemistry and physical properties of estolides. Grasas y aceites 2011, 62. 8 -20. Isbell TA, Abbott TP, Asadauskas, S., Lohr JE . Biodegradable oleic estolide ester base stocks and lubricants. 2000 Patent no US 6,018,063. Isbell TA, Kleiman R, Plattner BA. Acid-catalyzed condensation of oleic acid into estolides and polyestolides. J. Am. Oil Chem. Soc. 1994, 71, p. 169-174. Maleque M., Masjuki H., Sapuan S. Vegetable-based biodegradable lubricating oil additives. Ind. Lubric. Tribol. 2003, 55, p. 137–143.

10-6

10-5

10-4

10-3

10-2

10-1

3,0x10-2

4,0x10-2

5,0x10-2

6,0x10-2

7,0x10-2

8,0x10-2

9,0x10-2

1,0x10-1

1,1x10-1

1,2x10-1

1,3x10-1

1,4x10-1

Sommerfeld number

Castor oil (CO)

CO 85% - Oleic/Estolide 15%

CO 85% - Ricinoleic/Estolide_1 15%

CO 85% - Ricinoleic/Estolide_2 15%

HOSO

HOSO 85% - Oleic/Estolide 15%

HOSO 85% - Ricinoleic/Estolide_1 15%

HOSO 85% - Ricinoleic/Estolide_2 15%

Frictio

n c

oe

ffic

ien

t

IBERTRIB 2013




Session B8

Room B002

FRICTION AND WEAR 2

Chair:

Prof. Alexandre Sottomayor and Dr. Ramiro Martins

IBERTRIB 2013


B8.1 - A042

EFFECT OF SINTERING TEMPERATURE OF TWO LITHIUM DISILICATE BASED GLASSES IN RECIPROCATING WEAR TESTS PERFORMANCE P. V. Antunes1,2(*), A. Ramalho2, J.M.F. Ferreira3, H.R. Fernandes3

1INEGI, FEUP, University of Porto, Portugal 2CEMUC, Department of Mechanical Engineering, University of Coimbra 3CICECO, University of Aveiro, Portugal (*)Email: [email protected]

ABSTRACT

This study aims to evaluate the tribological behaviour of lithium disilicate glasses in the system Li2O–SiO2–Al2O3–K2O. A glass with nominal composition 23Li2O–77SiO2 (mol%) (labelled as L23S77) and glasses containing Al2O3 and K2O with SiO2/Li2O molar ratios (3.13–4.88) were produced by conventional melt-quenching technique in bulk and frit forms. The glass-ceramics (GCs) were obtained from nucleation and crystallisation of monolithic bulk glasses as well as via sintering and crystallisation of glass powder compacts.

Produced from the controlled crystallization of vitreous materials through a thermal treatment at high temperatures, vitro-ceramic-based lithium disilicate are among the most used in dental applications, namely for dental crowns. These materials pretend to simulate the physic-mechanical and tribological properties of natural teeth. This study involves the assessment of the tribological performance of a commercial composition (ceramic A) and a novel formulation (ceramic B). The two batches of materials were sintered at 800ºC, 850ºC and 900ºC. In order to mimic the in vitro fiction-wear tests, specimens were tested under sliding reciprocating contact in a distilled water environment. A sphere/plane contact type, with constant: movement amplitude, normal load and duration tests (Ramalho and Antunes, 2005). In these tests was noticed a variation in the wear results, regarding the two compositions and for the three sintering temperatures wear volumes varied very much, up to four times in the material removed in the ceramic and in the soda lime glass sphere (antagonist body).

INTRODUCTION

The aim of the present study is wear behaviour of vitro-ceramic material for dental crowns application in sphere/plane under reciprocating motion and constant normal force and a distilled water bath. A load cell was used, mounted in the lower specimen carrier, to attain the tangential loads along the sliding wear tests. The objective of this test is to compare the different materials contact processed at different sintering temperatures; 800ºC, 850ºC and 900ºC. After the wear tests the wear scars were observed and the material removed from the ceramic and from the antagonist body was verified with 3D laser roughmeter. At the end of the tests systematic SEM observation of the morphology of the produced scar is made in order to identify the present failure modes.

A mechanical evaluation was also performed characterising the microhardness and dynamic elastic modulus of the materials in analysis.




The results from reciprocating wear tests, in distilled water bath are shown in Fig. 1, with wear volume of the two vitro-ceramic-based lithium disilicate materials and their opponents for the different sintering temperatures. A general increase in material removed from the ceramic and a decrease in the antagonists wear with the sintering temperature increase. Table 1 shows the results for the hardness (ASTM WK27978, 2010) and dynamic elastic modulus (ASTM E 111-04, 2004) tests.

a) b) Fig.1 Effect of sintering temperatures on material removed in reciprocating test; a)

ceramic’s wear, and a) antagonist’s wear.

Table 7 Microhardness and elastic modulus values for the ceramics and sintering temperatures in study.

Dynamic Elastic modulus [GPa] 800ºC 850ºC 900ºC Ceramic A 40 95 97 Ceramic B 42 57 92

Hardness [MPa] 800ºC 850ºC 900ºC Ceramic A 1201 5820 5250 Ceramic B 2068 3252 5254

This study shows that there are substantial differences on the mechanical properties of two materials in analysis. The novel material (ceramic B) presents less wear when compared with the commercial formulation, regarding the antagonist wear for the sintering temperatures of 850 and 900ºC this new formulation is more aggressive to its glass opponent. Regarding hardness and elastic modulus values the two compositions present similar values for 900ºC sintering temperature. This two physical properties show the same tendency for both ceramics as their wear when sintering temperature increases.

REFERENCES

A. Ramalho e P. V. Antunes, Reciprocating wear test of dental composites effect on the antagonist, Wear 259, Issues 7-12, July-August 2005, Pages 1005-1011.

ASTM E 111-04, Standard test method for Young’s modulus, tangent modulus, and chord modulus. In: Annual book of ASTM standards, ASTM International, vol. 03.01; (2004).

ASTM WK27978 - Revision of E384 - 10 Standard Test Method for Microindentation Hardness of Materials, (2010).

IBERTRIB 2013


B8.2 - A054

MECHANICAL AND TRIBOLOGICAL PERFORMANCES OF POLYPROPYLENE COMPOSITES CONTAINING MULTI-WALLED CARBON NANOTUBES A.Sinan Dike1, Harun Mindivan2(*)

1Department of Materials Engineering, Adana Science and Technology University, Adana, Turkey 2Department of Metallurgical and Materials Engineering, Ataturk University, Erzurum, Turkey (*)Email: [email protected]

ABSTRACT

Polypropylene (PP) based composites filled with different weight percentages of as-received carbon nanotubes (ASCNT) and purified carbon nanotubes (PCNT) were prepared by twin-screw extrusion and injection molding. Effects of CNT content and purification procedure on the mechanical properties and wear resistance were examined. The results showed that elastic modulus, microhardness and wear resistance values of the composites increased after purification when compared to the ASCNT based composites, due to the enhanced interactions between CNT and PP which improve the load transfer efficiency from PP to CNTs. For the composites prepared with both ASCNT and PCNT, as the loading of the CNTs increased, improved mechanical properties and wear resistance were observed. However, the mechanical properties and wear resistance decreased when the CNT content exceeded 0.8 wt.% due to agglomeration of CNTs.

INTRODUCTION

Due to their unique mechanical and physical properties, CNTs are an excellent candidate as nanofillers for polymer composites [1–3]. However, the potential benefits of incorporating CNTs in PP matrix have not been fully realized because of poor dispersion due to incompatibility between the matrix and CNTs. The dispersion of CNTs within the matrix and interfacial bonding between CNTs and the matrix can be improved by increasing the reactivity of CNTs. This can be achieved by increasing the number of defect sites and tailoring the structures and properties of CNTs via chemical modification of CNTs [2, 3]. Enhanced interactions result in better load transfer and homogeneous dispersion of agglomerated CNTs which improve the mechanical properties and wear resistance. General procedure for the CNT surface modification is to oxidize the CNTs with strong acids or bases which usually refers to the purification.

Considering the above factors, the objective of the research described in this paper was to study the effect of CNTs on the mechanical properties and wear resistance of PP composites. The CNTs were modified through oxidation for better dispersion in the composites. The CNTs filled PP composites were prepared by twin-screw extrusion and injection molding. The effects of CNT content and oxidation treatment on the elastic modulus, microhardness and wear properties of CNT reinforced PP composites were experimentally investigated.


The effects of CNT content variation on the elastic modulus, microhardness and wear resistance of the composites prepared with ASCNT and PCNT were shown in Fig. 1. Elastic modulus, microhardness and wear resistance values of the composites increased after purification when




compared to the ASCNT based composites. There was a similar variation trend of the elastic modulus, microhardness and wear resistance with the increase of CNT content.

Fig. 1. Elastic modulus and microhardness of PP composites as a function of CNT content.

This study shows that purification treatment of CNTs is useful to improve the interfacial strength of PP composites, which not only improves the CNTs dispersion in the polymer matrix but also increases the mechanical properties and wear resistance of the polymer matrix.

ACKNOWLEDGMENTS

The authors express sincere appreciation to Prof. Dr. Ulku Yilmazer and Prof. Dr. Goknur Bayram from the Chemical Engineering Department of Middle East Technical University for their valuable help.

REFERENCES

Saeed K, Park SY. Preparation and Properties of Multiwalled Carbon Nanotube/Polycaprolactone Nanocomposites. Journal of Applied Polymer Science, 2007, 104, p. 1957–1963.

Ma PC, Siddiqui NA., Marom G, Kim JK. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review. Composites: Part A, 2010, 41, p.1345–1367.

Roya N, Sengupta R, Bhowmick AK. Modifications of carbon for polymer composites and nanocomposites. Progress in Polymer Science, 2012, 37, p.781– 819.

IBERTRIB 2013


B8.3 - A015

EFFECT OF VARYING FRICTION DUST AND RESIN ON DISC BRAKE PAD: STABILITY AND SENSITIVITY OF µ TO PRESSURE AND SPEED. Sai Balaji.M.A1(*),Kalaichalvan.K1,Venkatramani.T.N2

1Department of Production Technology, Anna University Chennai, India 2R&D Division, Hindustan Composites Ltd., Aurangabad, India (*)Email: [email protected]

ABSTRACT

Inconsistent friction level during braking at different speed and pressure is a major concern to the brake pad/lining manufacturers as it is proven to be formulation dependent. The behavior of friction of a NAO brake pad is highly complex under a set of dynamically variable pressure and speeds. Normally, the organic components burn off at elevated temperature reducing the brake effectiveness. This paper discusses the effect of organic components namely resin and the cashew friction dust in a disc brake pad formulation. Three pads with varying resin (10.11, 11.11, 12.11 percentage by weight) and the cashew dust (9.33, 10.33, 11.33 percentage by weight) are fabricated and their effect in relation to stability of friction is studied by carrying out the test in the Inertia dynamometer following JASO C 406 schedule. . It is observed that composites with 10.11% resin and 9.33% cashew dust resulted in better performance with consistent friction level viz. least sensitivity of µ to operating variables namely speed and pressure. Regression analysis of µ was also carried out.

INTRODUCTION

Non-asbestos organic (NAO) based friction materials are essentially multi ingredient systems in order to achieve the desired combination of performance properties (Kim S.J,2007). According to the available literature, different combination of fibers, fillers and friction modifiers had been explored in Non Asbestos friction materials (Gopal.P,2004). The organic friction modifier, cashew friction dust is normally used as it stabilizes the friction level and suppresses noise. Also, the various ingredients in the brake pads are held together by the powder resin. An attempt is made here to find out the optimum level of the selected alkyl benzene modified resin and the organic friction modifier in the disc pad formulation. Three pads with varying resin (10.11, 11.11, 12.11 percentage by weight) and the cashew dust (9.33, 10.33, 11.33 percentage by weight) are fabricated and their effect in relation to frictional stability (fade and recovery performances) is studied by carrying out the test in the Inertia dynamometer following JASO C 406 schedule. It has been observed that higher the organic content, the higher is the occurrence of fade. But the sensitivity of µ towards the speed and pressure during the first, second and third effect could not be highlighted. Hence, in this paper an attempt is made to carry out the regression analysis by considering the experimental results in the second and third effect of the test design which is imperative to further validate the test results. The physical, chemical and mechanical properties are evaluated following the Indian Industrial IS 2742 standards.





Figure 8 Inertia Brake Dynamometer Setup for testing brake performance

Experimental arrangement of coefficient of friction during second effect in JASO test and results: Table 9 Experiment number

Speed (m/s) Z1

Pressure (m/s2) Z2

Z12=Z1*Z2 FCL Y1

FCM Y2

FCH Y3

1 180 3.0 540 0.36 0.36 0.35

2 180 6.0 1080 0.35 0.35 0.45

3 360 3.0 1080 0.47 0.46 0.42

4 360 6.0 2160 0.44 0.44 0.45

5 468 3.0 1404 0.43 0.42 0.41

6 468 6.0 2808 0.36 0.35 0.37

Regression analysis: (Second effect) µ1= 0.3903+ 0.0391X11 -0.0336X12 -0.0241X11*12 µ2=0.3801+ 0.0420 X11 -0.0286 X12 -0.0250 X11*12 µ3= 0.4046+ 0.0062 X11+ 0.0385 X12 -0.0609 X11*12 ( Third effect) µ1= 0.4770+ 0.0297 X11 -0.0028 X12+ 0.0018 X11*12 µ2= 0.4030+ 0.0048X11 -0.0270 X12 -0.0126 X11*12 µ3= 0.3564+ -0.0353X11+ 0.0355 X12 -0.0155 X11*12 From above equations, it was evident from the magnitude of coefficients that the most influencing parameter on the friction sensitivity was due to the material itself followed by the first order contribution from braking pressure and then the sliding speed. Influence of mutual interaction of the parameters, however, was negligible. Selected amount of resin and dust in the formulation had the most dominant influence on the µ rather than the applied pressure and speed.FCH composite showed maximum influence on µ-sensitivity followed by an FCM. Composite FCL is found to be least sensitive and hence the best performer.

REFERENCES

J.Bijwee: Polymers and Composites. 18(3) (1997) 378-396

Kim SJ, Jang H: (2000) Tribology International 33:477

Gopal P,Dharani L.P,Blum F.R: Wear 257(5-6),1-1,573-584 ( 2004)

IBERTRIB 2013




ADDICIONAL ABSTRACTS

IBERTRIB 2013


A034

TRIBOLOGICAL EVALUATION BLENDS LUBRICANTS OF VEGETABLE OILS AND POLYALKYLENE GLYCOLS Oliveira, C. D. M.1(*),Canale, L. C.F.2, Mello, V.S.3,Alves, S.M.4

1, 2,Department of Materials Engineering , University of São Paulo, Brazil 3 Program of Post Graduate in Mechanical Engineering, Federal University of Rio Grande do Norte,Natal, Brazil 4School of Science and Technology (ECT), Federal University of Rio Grande do Norte, Natal, Brazil (*)Email: [email protected]

ABSTRACT

This study evaluated the tribological performance of vegetable oils mixed with polyalkyleneglycols (PAG) and compared with mineral oils. The viscosities studied for both lubricants are150 and 220 cSt. Different lubricants were formulate involving two vegetable oils (castor oil and soybean) and PAG (ISO 68 and ISO 150) in different proportions 65/35, 80/20, 50/50, 38 / 62. The tribological performance of each formulation was verified on four ball tribometer ("Four Balls"), all in duplicate. The results of blends not show the expected performance such as mineral oils shows in terms of decrease wear. But for heating the results shows similar levels of mineral oils. However it should be considered that blends of vegetable oil and PAG is more suitable to environment than mineral oil.

INTRODUCTION

The lubricant industries have search products that minimize environmental impact and ensure their suitability environmental laws. However, the low oxidative and thermal stability of these oils leaves them susceptible to degradation (Ioan et al., 2002). An alternative to this problem is to add synthetic oil in vegetable oils, increasing its thermal properties. Thus, the present study aims to investigate the performance of mixtures of castor oil and soybean oil with polyalkyleneglycols in different proportions in four ball test. For comparison effect mineral oil was tested and both blends and mineral oils, in Grade ISO 150 and 220. For preparation of blends lubricants were used vegetable oils (castor oil and soybean) and polyalkylene glycols (OSP). The blends were prepared mixturing Castor oil + OSP68 (Grade ISO 150), castor oil + OSP150 (Grade ISO 150), soybean oil + OSP680 (Grade ISO 150), soybean oil + OSP680 (Grade ISO 220) in the proportions 65/35, 80/20, 50/50, 38/62, respectively. The kinematic viscosity was determined in Stabinger model SVM 3000 viscometer according to ASTM D445. The tribological performance was determined using the four ball test for 60 minutes with rotation speed of 1700 rpm and the applied normal load of 784 N. This lubrication regime was boundary lubrication and the temperature of contact was monitored during the test. After tests, the worn ball surfaces were analyzed by SEM (scanning electronic microscopy) and Wear Scar Diameter (WSD) were measured.


The results of wear scar diameter of balls were showed in the Tab. 1. It is possible observed the similar performance of the all blends, with difference approximately of 0,6 mm. The mineral oils showed the smaller (0,509 – 0,565 mm) of WSD. An answer for this may be the presence of EP additives in the oil (Carreteiro, 2006).



The results of system heating is showed in the Fig:1 (a) blends and mineral oils to grade ISO 150 and (b) blends and mineral oils to grade ISO 220.

Table 1 WSD of formulations

Test Formulation Wear scar diameter (mm) 01 OMa-80% / OSP150-20% 1,83 02 OMa-80% / OSP150-20% 2,10 03 OSo-38% / OSP680-62% 2,12 04 OSo-38% / OSP680-62% 1,67 05 Óleo Mineral 220 0,565 06 Óleo Mineral 220 0,542 07 OSo-50% / OSP680-50% 1,91 08 OSo-50% / OSP680-50% 2,03 09 OMa-65% / OSP150-35% 2,33 10 OMa-65% / OSP150-35% 1,86 11 Óleo Mineral 150 0,509 12 Óleo Mineral 150 0,530

a) b)

Fig.1 Results Heating of system

In the fig.1 (b), oils in grade 220, the blend 80% castor oil with 20%OSP 150, showed greater reduction in heating system about 20°C in relation to the mineral oils. Was not observed influence of viscosity on heating reduced because the curves presented the same level of heating.

ACKNOWLEDGMENTS

The authors wish their thanks to DOW Chemical of Brazil and Capes for their assistance in the present work.

REFERENCES

Ioan I. Ş. , Camelia C. , George C. On the future of biodegradable vegetable lubricants used for industrial trybosystems. The annals of university “dunărea de jos” of Galati, Romania; 2002. p. 94-98

ASTM D445: Standard Test Method for Kinematic Viscosity. Disponível em: < http://startechlube.com/new/pdf/Star-Tech_ASTM_D445.pdf >. Acess 30/01/2013.

ASTM D4172: Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four-Ball Method). (1994)

Carreteiro, R. P.; Belmiro, P. N.; Lubrificantes e lubrificação industrial. Interciência. (2006). 499p.

IBERTRIB 2013


A045

EVALUATION OF THE SURFACE OF A NEW AND AGED HDPE USED AS A COATING OF OIL PIPELINES J.R. de Souza

1(*); N.C.M.C. de Sá Leitão

1; M.S.P. de Azevedo

1; L.S.P. de Azevedo

1; P. M. Oliveira

1;; J.T.N. de

Medeiros1; João Marcos Sabino

1

1Group of Tribology Studies and Structural Integrity, Federal University of Rio Grande do Norte, Senador Salgado Filho Avenue, Campus Universitário Lagoa Nova, Postal Code 59072-970, Natal – RN,Brasil; (*)Email: [email protected]

ABSTRACT

The High Density Polyethylene (HDPE) is a material having high durability, corrosion resistance, and coefficient of friction on the order of 0.28, is nonstick and still has chemical resistance. Because it is a type of polymer used in pipelines where the material is constantly in contact with oil and water, it was necessary to study the wettability and surface energy of the HDPE, with liquid in different surface conditions (Aged HDPE and New HDPE). Among the physicochemical study of the liquid, there is the measurement of the wetting angle between a drop of liquid and the surface on which it rests.This study investigated the contact angle and surface energy of samples of HDPE, using as a polar liquids the distilled water.

INTRODUCTION

The HDPE has crystallinity above 90%, because it has a low content of branches. This polymer contains less than one side chain per 200 carbon atoms of the main chain, its crystalline melting temperature is approximately 132 ° C and its density is between 0.95 and 0.97 g / cm ³. The number average molecular weight is in the range of 50,000 to 250,000. The HDPE is used on different segments of the plastics processing industry, covering the processing of blow molding, extrusion and injection molding. Some Brazilian companies are already exploring a new niche market, a type (grade) specific HDPE for blow molding of fuel tanks and another for "containers" thousand liters.

The objective was to study the surface of a new and aged HDPE from the tests of wettability and surface energy, in order to observe and compare the results to liquid water, naphthenic and paraffinic oil. The contact angle is a quantitative measure of the wetting process. The difference between the energies of the species on the surface and within the material is what is called surface energy. The liquid will wet the surface completely when θ = 0 º, absent wetting occurs when θ = 180 °and partial wetting exists in the range of contact angles between 0 ° <θ<90 °.


The results from contact angle and surface energy are shown in Fig. 1 A) and B). It is noted that although the New HDPE present dispersal of the results of contact angle less when compared to Aged HDPE, the average of the two differs only about 2 degrees. The amounts found did not exceed the 90 degree angle, characterizing these as partially wettable surfaces.

Comparing the surface energy between the new and aged HDPE, it is identified that the first has a higher surface energy, because show naturally a homogeneous surface.



NEW HDPE AGED HDPE

50

55

60

65

70

75

80

85

90

95

100WATER

CO

NT

AC

T A

NG

LE

NEW HDPE AGED HDPE

20

25

30

35

40

45

50

SU

RF

AC

E E

NE

RG

Y

WATER

a) b)

Figure 1: A) Contact Angle of the New and Aged HDPE. B) Surface Energy of the New and Aged HDPE

ACKNOWLEDGMENTS

The authors thank to Professor Dr. Eduardo Lins de Barros Neto and Eng. André Ezequiel Gomes do Nascimento, Technical of the NUPEG- Center for Research on Oil and Natural Gas for the analysis of availability in wettability and surface energy.

REFERENCES

Hiper Metal. Polietileno PEAD. Disponível em: <http://www.hipermetal.com.br/site/produtos/plasticos_industriais/Polietileno.pdf>. Accessed on: 25 January in 2013

Billmeyer, F. W. J. – “Textbook of Polymer Science” – WileyInterscience, USA (1984).

Odian, G.– “Principles of Polymerization”, John WileyInterscience, New York (1991).

Reto, M.A.S. – Revista Plástico Moderno, p.22, Agosto (2000).

Silverstein, T. P., Polarity, miscibility and surface tension of liquids, J. Chem. Edu., 1993, 70, 253.

CETEC. Ciência dos Materiais Multimídia. Cap 12,Superfícies e Interfaces.

IBERTRIB 2013


A046

ANALYSIS OF THE SURFACE OF A TABLET PTFE COMMERCIAL J.R. de Souza

1(*); N.C.M.C. de Sá Leitão

1; M.S.P. de Azevedo

1; L.S.P. de Azevedo

1; P. M. Oliveira

1; E. P.

Matamoros1; J.T.N. de Medeiros

1

1Group of Tribology Studies and Structural Integrity, Federal University of Rio Grande do Norte,

Senador Salgado Filho Avenue, Campus Universitário Lagoa Nova, Postal Code 59072-970, Natal – RN,Brasil;


ABSTRACT

With the advent of the petroleum industry, afloat is increasingly necessary to use technologies

that bring improvements in the exploitation of this raw material. The use of polymeric

materials as the inner lining of pipelines for the transport of liquids and gases derived from

petroleum has been an alternative to avoid wear of the same through corrosion, since such

materials are less chemically reactive. Amid other polymers, PTFE (polytetrafluoroethylene)

shows up an excellent candidate in this applicability, because among its features it has low

friction coefficients (0,01< <0,1), corrosion resistance, excellent thermal resistance and

electrical insulator. This study aimed to evaluate, by measuring the contact angle and surface

energy of the PTFE pads.

INTRODUCTION

Corrosion is a major cause of failures in equipment and piping oil production platforms.

These flaws undermine the process, delay the operational schedule of production, generate

high maintenance costs, and generate risks to health and the environment. To avoid this

problem, the use of polymer layers on the inner surface of the pipeline has been adopted in

order to avoid its direct contact with the substance carried.

PTFE is a substance practically inert, does not react with other chemicals except in very

special situations. This is basically the protection of fluorine atoms on the carbon chain. This

lack of reactivity allows its toxicity is virtually nil and it is also the material with the lowest

coefficient of friction known. Another quality feature is maintaining its impermeability

therefore their qualities in wet environments.

In order to evaluate the surface performance of the PTFE, the techniques of contact angle and

surface energy were performed, using distillated water.


The results from contact angle and surface energy are shown in Fig. 1 A) and B).

The results of contact angle were homogeneously included in a range 102-105 degrees, with

an average of 104 degrees and a standard deviation of only 1 degree.

The liquid will wet the surface completely when θ = 0 º, absent wetting occurs when θ = 180

°and partial wetting exists in the range of contact angles between 0 ° <θ<90 °. Thus, this

surface is characterized as being hydrophobic.



According to the surface energy of the standard deviation was only 1.16, with a mean of 31.

This result was expected, because surfaces that not wet are characterized by low surface

energy.

PTFE

95

100

105

110

115

CO

NT

AC

T A

NG

LE

WATER

PTFE

25

30

35

40

45

SU

RF

AC

E E

NE

RG

Y

WATER

Figure 1: A) Contact Angle of the PTFE. B) Surface Energy of the PTFE

ACKNOWLEDGMENTS

The authors thank to Professor Dr. Eduardo Lins de Barros Neto and Eng. André Ezequiel

Gomes do Nascimento, Technical of the NUPEG- Center for Research on Oil and Natural

Gas for the analysis of availability in wettability and surface energy.

REFERENCES

ASHBY, M. F. Materials Selection in Mechanical Engineering. Oxford, Elsevier, 2005. 3rd

Ed. 605 p.

TERZI, Rafael; MAINIER, Fernando B.. MONITORAMENTO DA CORROSÃO INTERNA

EM PLATAFORMAS OFFSHORE. Tecno-lógica, Santa Cruz do Sul, v. 1, n. 12, p.14-

21,jan/jun 2008.

Da origem às prateleiras. TEFLON (POLITETRAFLUORETILENO).Disponível em:

<http://blogdefisica302.blogspot.com.br/2011/07/teflon-politetrafluoretileno.html>. Acesso

em: 24 jan. 2013.

COUTINHO, Murilo Pereira. INFLUÊNCIA DA MORFOLOGIA DA SUPERFÍCIE NA

MOLHABILIDADE DO TITÂNIO COMERCIALMENTE PURO. 84 f. Dissertação

(Mestrado) - Curso de Ciências Dos Materiais, Instituto Militar de Engenharia, Rio de

Janeiro, 2007.

SILVA, Mônica Luis Vicente Júlio. TECNOLOGIA PARA PRODUÇÃO DE

SUPERFÍCIES HIDROFÓBICAS EM FILMES DE AMIDO DE MILHO

TERMOPLÁSTICOS POR PLASMA. 73 f. Projeto de graduação, Engenharia de Materiais

da Escola Politécnica, Universidade Federal do Rio de Janeiro, 2010.

A) B)

IBERTRIB 2013




POSTERS

IBERTRIB 2013


P001

TORQUE LOSS OF FZG SLAVE GEARBOXES LUBRICATED WITH WIND TURBINE GEAR OILS Carlos M. C. G. Fernandes1(*), Ramiro C. Martins1, Jorge H. O. Seabra2

1Instituto Nacional de Engenharia Mecânica e Gestão Industrial (INEGI), Universidade do Porto, Porto, Portugal 2Departamento de Engenharia Mecânica, Universidade do Porto, Porto, Portugal (*)Email: [email protected]

ABSTRACT

Eight fully formulated wind turbine gear oils were selected and their physical and chemical properties characterized. The gear oils have 320 ISO VG grade and different formulations: ester, mineral, PAO, PAG and mineral+PAMA. A power loss model was implemented to both test and slave gearboxes in order to predict the torque loss on the power loop. Friction generated between the meshing teeth, shaft seals and rolling bearing losses were predicted. The temperatures measured during each test were used in order to identify the oil temperature in each mechanical part.

INTRODUCTION

A back-on-back FZG test rig machine with re-circulating power was used and a torque-cell was included on the test rig in order to measure the torque loss. Eight thermocouples were included to monitor the temperatures in different locations of the test rig. The torque measurement was recorded for 3 hours of test. The torque value considered for analysis was taken considering stabilized temperature within the system. After thirty minutes of data acquisition with stabilized temperature the average torque was calculated. Tests at the conditions presented in Table 1. Both gearboxes were jet-lubricated with an oil flow of 3 dm3/min. The input oil flow temperature was kept almost constant (80° C).

Table 8 – Power loss test operating conditions for wind turbine gear oils.

FZG stage KFZG*

Wheel torque Nm

Wheel speed

rpm

Tangential speed

m/s

1 5

200 1.13

400 2.26

1200 6.79

5 105

200 1.13

400 2.26

1200 6.79

7 199

200 1.13

400 2.26

1200 6.79

9 323

200 1.13

400 2.26

1200 6.79

*(arm lever=0.35 m)





(a)

(b)

Fig.1 – Torque of a slave gearbox spur gear at: (a) 200 and (b) 1200 rpm.

This study shows that for low speed the PAGD lubricant is the most efficient due to its higher aptitude to generate a lubricant layer (higher Viscosity Index) (see Figure 1a). Also when the load and speed are very high the behavior is the same (see figure 1b). On the other hand the MINR has the worst efficiency; but for high speed and low load this behavior is not verified. Due to its lower Viscosity Index the power loss generated by the seals and rolling bearings is lower whose have a significant influence on the total torque loss on these conditions.

ACKNOWLEDGMENTS

The authors acknowledge to “Fundação para a Ciência e Tecnologia” for the financial support given through the project “High efficiency lubricants and gears for windmill planetary gearboxes" with research contract PTDC/EMEPME/100808/2008.

REFERENCES

IBERTRIB 2013


P002

PRELIMINARY RESULTS ON POWER LOSS MODELLING OF PLANETARY GEARBOXES Pedro M. T. Marques2(*), Carlos M. C. G. Fernandes2, Ramiro C. Martins2, Jorge H. O. Seabra1

1 FEUP, Universidade do Porto, Rua Dr. Roberto Farias s/n, 4200-465 Porto, Portugal 2 INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Farias 400, 4200-465 Porto, Portugal (*)Email: [email protected]

ABSTRACT

A numerical model simulating the power loss behaviour on a multiplier planetary gearbox was implemented. The aim of this model was to understand the influence of each one of the power loss components on the global power loss with special emphasis on the loss generated by the meshing gears.

INTRODUCTION

Planetary gear sets are used in a wide range of applications and situations, from the small pneumatic tool to wind turbines. The compact and coaxial design of planetary gearboxes provides to these units advantage over conventional gearboxes, since higher power to volume ratios can be achieved. Different speed ratios can be obtained using the same planetary gear set by simply changing the input, output and fixed members (internal gear, sun gear and planet carrier).

The first stages of speed multiplication on a wind turbine are usually done in planetary gear trains. Due to the high power involved and increasing number of wind turbine farms across the globe it is of great interest to have the maximum possible efficiency on this speed multiplication process. Accurate power loss predictions at the design stage allow for faster development of more efficient gearbox designs saving resources not only at the design stage but more importantly during operation.

A model aiming to simulate the relevant power loss sources on a planetary gearbox was implemented. The global power loss was calculated as the sum of the considered load and no-load loss generated in the planetary gearbox.

Load dependent losses occur in the contact of the power transmitting components. Load losses mainly depend on the transmitted torque, coefficient of friction and sliding velocity in the contact areas of the components (Höhn, 2009).

No load losses occur with the motion of mechanical components, even without torque transmission. These are mainly related to lubricant viscosity and density as well as immersion depth of the components on a sump lubricated gearbox, besides it heavily depends on operating conditions and internal housing design (Höhn, 2009).

The power loss due to the meshing gears was calculated taking into account the coefficient of friction variation along the path of contact (Hai Xu, 2005). The rolling bearings power loss was calculated according to SKF (SKF, 2005). The losses due to fluid circulation and splashing (oil sump lubrication) (Changenet, 2011) were calculated assuming that the planet-carrier assembly could be approximated by a disk rotating partly immersed in gear oil.





A series of numerical experiments simulating a one stage multiplier planetary gearbox were carried.

Figure 1 shows the influence of each one of the considered power loss sources on the overall power loss. At low input speeds and high input torques the power loss generated by the meshing gears has a dominant role.

Figure 6 - Power loss distribution. Input speed of 100rpm and torque of 1000Nm at the planet carrier.

Preliminary results also suggest that there is a significant difference between the power loss generated by the sun-planet and planet-ring gear meshes. These are related to some fundamental differences between the sun-planet and planet-ring contacts. The internal gearing allows for higher conduction ratios and dramatically lower sliding speeds compared to the external gearing between the planets and the sun.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Ministério da Ciência, Tecnologia e Ensino Superior, FCT, Portugal, under grants PTDC/EME-PME/100808/2008.

REFERENCES

B.-R Höhn, K. Michaelis, M. Hinterstoißer, Optimization of gearbox efficiency, goriva I maziva 48 (4) (2009) 462-480.

C. Changenet, G. Leprince, F. Ville, P. Velex, A Note on Flow Regimes and Churning Loss Modeling, Journal of Mechanical Design 133 (12) (2011) 121009.

[3] Hai Xu, Development of a generalized mechanical efficiency prediction methodology for gear pairs, Dissertation, The Ohio state university, 2005

[4] SKF General Catalogue 6000 EN, SKF, November 2005.

IBERTRIB 2013


P003

AXLE BEARING FAILURES, CAUSES AND MONITORING DATABASE André Gama1 (*), Beatriz Graça2, Jorge Castro3, Jorge Seabra1

1FEUP, Universidade do Porto, Rua Dr. Roberto Farias s/n, 4200-465 Porto, Portugal 2INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Farias 400, 4200-465 Porto, Portugal 3ISEP, Instituto Superior de Engenharia do Porto, Rua Dr. António Bernardino de Almeida 431, 4200-072 Porto, Portugal (*)Email: [email protected]

ABSTRACT

Rolling bearings are usually critical components and disposed to failure. The continuous monitoring thru vibration, acoustic, temperature and lubricant analysis techniques allows updating the lifespan analysis of the rolling bearing, and therefore provides valuable information for better prediction of re-lubrication intervals or bearing replacement. The main focus of this study is the correlation between current damage / failure matrix of axle bearings and the monitoring signals tested in laboratory under controlled conditions. It leads to a better understanding of the impact of the operating conditions as a root cause for bearing damage and failure.

INTRODUCTION

Actual bearing operation can be affected by friction, wear and lubrication mechanisms, fluid dynamics and lubricant rheology, material properties and contact mechanism [Halme, 2009], which consecutively affect rolling bearing life. Detecting premature bearing failure is essential to prevent multiple undesirable conditions such as railroad obstruction, repair time, axle damage and in worst case scenario railway line destruction, resulting in costly systems downtime [Kurfess, 2006]. Bearing health is usually monitored through vibration analysis, lubricant analysis and/or ultrasound techniques in the early stages of the bearing failure. Temperature analyses are considered a useful monitoring condition on remote and hazardous locations such as an axle of a train.

Vibration analysis

Rolling bearings often fail prematurely in service due to poor lubrication, contamination, temperature, poor fitting, unbalance and misalignment. All these potential conditions lead to an increase in bearing vibration which by condition monitoring (that has been use for many years) it is possible to detect degrading bearings before they catastrophically fail. Measuring the Root Mean Square (RMS) vibration of the bearing housing or some point near to the bearing over a wide frequency range, 10-1000 Hz or 10-10000 Hz is the example of one of the technique applied. The measurements then can be plotted over time and compare with known levels of vibration and alarm levels can be set to indicate a change in the bearing behavior. In the time domain due to machine vibration the specific contributions are difficult to identify so in order to make detection more sensitivity and possible at a much earlier stage Frequency Spectrum and Envelope Spectrum analysis are applied. In the frequency domain it become much easier to identify then link to individual sources to bearing characteristic frequencies making detection possible at much earlier stage.



Acoustic Emission analysis

Acoustic Emission (AE) is a method which is capable to identify minor changes outgoing from inside of the materials or propagation of micro cracks in surface. The AE analyze recognized the signal much earlier than vibration [Hort, 2010]. The combination of the measurements in time domain and frequency domain applied to AE is essential in order to obtained better analyze interpretation.

Temperature analysis

The temperature monitoring can be carried out by thermocouples in the bearing house surface. All thermal variables are filter except the increase in bearing temperature due to wear or lubrication failure, if one bearing temperature have a significant variation, the bearing health is in question and the cause of the increased bearing temperature must be determinate.

Lubricant analysis

Ferrographic analyze is a technique that offer important information about the wear evaluation in the machinery in general through the analysis of a representative lubricant sample. As a predictive maintenance strategy the ferrographic techniques allow to analyze "in-service" greases and detect potential rolling bearing damage in an earlier stage where other monitoring techniques are less accurate. Fourier Transform Infrared Spectroscopy (FTIR) is a non-destructive technique used to collect infrared wavelength data of materials for the purpose of identifying their composition. As a lubricant analyze technique FTIR offer "birth-to-dead" data of lubricant, i.e., the evolution of lubricant composition thru service time such as oxidation levels and monitoring additives.


Several techniques for a vibration, acoustic emission, temperature and lubricant analyze that can be used in order to develop the interpretation about the source of bearing damage and failure. It is important to know the limitations as well the correlation between techniques in order to achieve a more accurate analyze interpretation. Once the evaluations are gather it is possible to correlate with the existing damage/failure matrix of axle bearings and from there according with the historical data accurately upgrade the service life expectancy of the bearing.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by MAXBE project from European Commission Seventh Framework Programme.

REFERENCES

J. Halme, P. Andersson. Rolling contact fatigue and wear fundamentals for rolling bearing diagnostics – state of the art. Engineering Tribology J, 2009, p.377-393.

T. R. Kurfess, S. Billington, S. Y. Liang. Advance diagnostic and prognostic techniques for rolling element bearings. Springer Series in Advance Manufacturing, 2006, p.137-165.

F. Hort, P. Mazal, F. Vlasic. Monitoring of Acoustic Emission Signal of Loaded Axial Bearings. EWGAE 2010

IBERTRIB 2013


P004

CONTROLING LUBRICANT DEGRADATION TO AVOID WIND TURBINE GEARBOXES FAILURES Beatriz Graça1(*) Jorge Seabra2

1Institute of Mechanical Engineering and Industrial Management (INEGI), Unit of Tribology, Vibrations and Industrial Maintenance (CETRIB), Porto, Portugal 2Department of Mechanical Engineering (DEMEC), Engineering Faculty, University of Porto, Portugal (*)Email: [email protected]

ABSTRACT

Wind turbine gearbox failures represent dramatic consequences in terms of turbine availability to generate energy as well on maintenance and operating costs. While wind turbines have design lives of 20 or 30 years, gearbox warranties are often as short as two years (Galán, 2010). Because of that, the gearbox is considered a very critical component in wind turbines. Besides the extreme stresses imposed by the turbine blades which are directly connected, they have to handle a wide range of differences in wind velocity and direction, as well significant temperature variations. Maintaining an optimum protective film thickness of lubricant throughout this is fundamental to minimise wear and consequent failures in turbine gearboxes. So, the lubricant assumes this important charge and needs to be in good condition to extend and maximize the operational lives of the wind turbine gearbox components.

This paper will show the important information that can be withdrawn from some oil analysis techniques applied to wind turbine gearbox lubricants. The several changes taking place in the lubricant molecular structure, the loss of certain properties and presence of any type of contamination will reveal, not only the condition of the lubricant but also, exposes the abnormal occurrences that the lubricated components are experienced.

INTRODUCTION

Most gearbox failures do not begin as gear failures or gear-tooth design deficiencies. The observed failures appear to initiate at several specific bearing locations under certain applications, which may later advance into the gear teeth as bearing debris and excess clearances cause surface wear and misalignments (NREL, 2007). It is a fact that no gearbox can survive without operating in a clean lubricant. This is true since a clean lubricant acts as a gear protector against micropitting, prevents bearing failures and removes heat, air and debris generated in the gearbox. While the lubricant cleanness can be achieved by an efficient filtration system, the oil degradation (oxidation, contamination and additive depletion) is more difficult to control and can result in a serious problem when it reaches advance states. Not only is the lubricant’s performance diminished, the byproducts produced (mostly acids) will induce a corrosive component wear. It is important to understand the process and root causes of lubricant degradation and the warning signals generated by oil analysis. Typically, oil properties such as viscosity, additive concentration and the presence of metal particulates are good indictors of the oil’s (and by extension the gearbox’s) condition. Significant metal particles in the oil indicate that wear has occurred in the gearbox components. Oil analysis allows the maintenance operator to catch abnormal wear problems early and keep the gearbox working, avoiding the need for costly replacement.





As shown on Fig.1, the analytical ferrography results shown the presence of large fatigue wear ferrous particles (a), corrosive wear and non-ferrous particles (b) and the presence of some water by the evidences of orange ferrous oxides (c).

(a) (b) (c)

Fig.1 Microphotographs of wear particle through Analytical Ferrography

Fig.2 shows the viscosity differences (a) obtained for the new and used gearbox oil and the correspondent spectrum overlays (b) acquired through FTIR. There are clearly visible important alterations occurring in the lubricant which certainly affects the operational condition of the windmill gearbox.

(a) (b)

Fig.2 Viscosity differences with temperature (a) and FTIR Spectra in transmittance/wavenumber (cm-1) (b).

The combination of wear particles analysis, viscosity measurements and molecular evaluation through infrared spectroscopy seems to be a powerful tool to identify prematurely problems in turbine gearboxes and to understand the alterations occurred in the lubricant chemistry that could be related with the problem origin.

REFERENCES

Galán, Sergio, Web Tool for Predicting Maintenance of Gearbox, using oil Particle Analysis, EDP Renewables Europe. Sevilla, Spain, 2010.

Musial, W. et all, Improving wing Turbine Gearbox Reliability, National renewable Energy Laboratory, Conference Paper NREL/CP-500-41548. USA, 2007.

IBERTRIB 2013


P005

WEAR ANALYSIS AND SURFACE MORPHOLOGY OF Zn

PHOSPHATING ON STEEL

Belén Díaz, Lorena Freire, Mar Mojío*, X. Ramón Nóvoa

Encomat Group, EEI, University of Vigo, Vigo, Spain (*)Email: [email protected]

ABSTRACT

The performance of traditional Zn phosphate coatings was improved by modifications

introduced in the phosphating bath such as ultrasonic stirring and incorporation of carbon

microparticles. The obtained coatings show improved corrosion resistance. Moreover, a

significant modification in both the morphological appearance and the tribological behaviour

were obtained. The ultrasonic cavitation induces the growth of larger platelet-shaped crystals

with no significant influence in the friction behaviour. The incorporation of graphite particles

produces a lubricant effect since a reduction in the friction coefficient was measured.

INTRODUCTION

The traditional phosphate coatings are based on a chemical conversion reaction, widely used

as a corrosion protection pre-treatment. Good adherences as well as a great stability are

common characteristics found in these kinds of coverings (Rausch, 1990). It is an economical

process with numerous applications in the technological area, from the automobile industry,

as a primer coating, to the pre-treatment in the metals cold forming due to a suitable lubricant

effect (i.e. wiredrawing). Among the multiple phosphate technologies available, those

containing Zn, Fe or Mn, as well as their combinations, are the most commonly used for steel.

Several improvements have been introduced in the last 30 years, being the use of accelerators

or special additives the most promising advances. In this work a detailed electrochemical and

morphological analysis was performed after incorporating ultrasound stirring and graphite

particles to a traditional phosphate bath. Differences regarding the chemical composition and

the appearance (grain size or porosity) were observed. Not only modifications in the corrosion

performance were verified but also an enhanced wear behavior was proved.


The SEM pictures showing the final appearance of the phosphate coatings prepared under

different conditions are compiled in Fig. 1. Table 1 shows the corresponding atomic

percentages along with the corresponding coating weights. The crystals show a platelet shape

with larger size when the coating is prepared in a stirred bath. A less porous coating is formed

as verified from the higher charge transfer resistances measured by impedance (an increment

of 3 and 8 times was obtained when incorporating ultrasound and ultrasound and graphite,

respectively). The Zn/P ratios lie below 1.5, the expected value if the phosphate were

exclusively composed of Zn3(PO4)2.4H2O, hopeite, which points to the existence of a

significant percentage of the phosphophyllite phase (FeZn2(PO4)2.4H2O). The lower ratio for

the traditional phosphating agrees with a higher percentage of the Fe-Zn phosphate, with




reduced stability of the coating (Rausch, 1990). A higher coating weight was measured for the

traditional phosphating which should correspond to the formation of a thicker layer.

The evolution of the kinetic friction coefficients versus the sliding distance was analyzed for

the uncoated substrate and for the different phosphate coatings. The variation of the friction

coefficients as a function of the sliding distance was considered. The friction increment

pattern is different if one considers the effect of including graphite while keeping the

ultrasonic stirring. Thus for the coating developed in presence of ultrasounds, the coefficient

shows a sharp increment within a short stage and then reaches a steady state value. No

significant variation was observed in comparison to the conventional phosphate. Such an

increase is commonly associated with smoothening of the surfaces giving a larger component

of adhesive friction (Bushan, 2002). The average friction coefficient was 0.71, close to that

obtained for the conventional phosphate which was 0.69. Some modifications were found in

the phosphate wearing process when graphite particles are added to the phosphating bath.

Firstly the friction transitions were different during the sliding process. The initial “run-in”

period was longer. A low friction coefficient was measured in the first wearing process, about

0.17, showing some lubricating effect associated to the incorporation of the graphite particles.

After this initial period, the friction coefficient increases to a maximum and then falls again in

connection to the dropping in the ploughing component of the friction.

Table 9 Atomic percentages and coating weights of the phosphate layers obtained in the different conditions

analyzed

Coating Zn P Zn/P Coating weight

(g/m2)

Traditional 52.5 47.7 1.1 7.7

Ultrasound 54.3 45.7 1.2 4.8

Ultras. + graphite 54.4 45.6 1.2 4.9

Traditional

Ultrasound

Ultrasound + graphite

Fig.1 SEM images of the different phosphate coatings prepared on steel

ACKNOWLEDGMENTS

The authors gratefully acknowledge the funding by Ministerio de Educación of Spain under

project BIA2010-16950.

REFERENCES

Rausch W., The Phosphating of metals, ASM International, Finishing Publications LTD.,

1990.

Bushan B., Introduction to tribology, Wiley & sons, New York, 2002.

IBERTRIB 2013


P006

ISO 6336 REVISITED: REMARKS ON THE CALCULATION OF LOAD CAPACITY OF CYLINDRICAL GEARS

Sérgio M. O. Tavares1*

, Paulo M. S. T. de Castro2

1 LOME – Laboratório de Óptica e Mecânica Experimental, of INEGI – Instituto de Engenharia Mecânica e

Gestão Industrial, Porto, Portugal 2 Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto – FEUP, Porto,

Portugal


ABSTRACT

This work revisits the problem of the calculation of load capacity of spur and helical gears

according to ISO 6336 part 3. The implications of the evolution from the 1996 to the 2006

edition of this standard are discussed, particularly as regards teaching of load capacity of

cylindrical gears within Machine Design courses of Mechanical Engineering

INTRODUCTION

ISO 6336 standards provide an integrated framework for the calculation of load capacity of

gears. The procedures depend on many different factors, some of them clearly of an empirical

nature. An aspect where rigorous analysis is required is the determination of form and stress

correction factors in the calculation of bending strength (YF and YS respectively), covered by

Part 3 of the mentioned series of standards. The combination of Solid Mechanics and

Geometry principles involved makes this particular subtopic an excellent subject for deeper

discussion within a Machine Design course for a Mechanical Engineering degree. Interest for

bending strength starts with the Lewis approach where the tooth is considered loaded at the

tip and a parabola of equal bending strength is used to determine the thickness SFn (normal

cordal dimension). This evolved, within ISO, to the use of straight lines making a 30o angle to

the tooth bisecting line. After lengthy work, (IET, 1997), throughout which standards as

AFNOR E23-015, 1982 appeared, ISO published the 1996 version of 6336 Part 3 including

the so called B and C approaches for YF and YS calculation. These differ only as regards the

assumption for loading location: it is external gear tooth tip loading for approach C (YFa and

YSa), and loading at the outer point of single pair external gear tooth contact for method B (YF

and YS). The implication of the difference between B and C is, of course, the need for an

additional parameter, Y , when using approach C. Y , a value less than 1 that is a function of

the contact ratio , corrects the over-conservative calculation based on a load applied at the

tooth tip, leading to the relevant value for outer point of single pair external gear tooth

contact. Theoretically, it should be YFa . YSa . Y = YF . YS ; using the procedures of ISO 6336

Part 3, 1996, this is indeed the case, as shown eg by Dufailly, 1998.

DISCUSSION AND CONCLUDING REMARKS

The 2006 version of 6336 Part 3 no longer mentions method C. If method B gives the direct

answer to the problem, why bother ? The reason is simple and related to the use of standards

by practitioners and Mech. Engng. students. It just happens that while method C is based on

general graphs that anyone interested can use to obtain YFa and YSa for a particular gear

without difficulty, leaving for a later moment the consideration of the mating gear through the



very simple calculation of Y , method B is only accessible via a computer calculation taking

into account from the very beginning the calculation carried out for the gear pair. This means

that either the interested person has got calculating software (KISSSOFT, GWJ, Dontyne,

etc.) or the task of using method B can be daunting. By comparison, and since the relationship

YFa . YSa . Y = YF . YS is well proven, use of method C is rather straightforward, since it is

based on graphs provided for in standards (eg. AFNOR, 1982, ISO 6336 Part 3, 1996), or

textbooks (Dufailly, 1998, Haberhauer, Bodenstein, 2007). The graphs concern a particular

choice of basic rack geometry. Whereas IET, 1997 gives graphs for a variety of tools, more

easily found literature concentrates just on the basic values (n=20o, hfP=1,25m, fP=0,375m,

…). To take into consideration the pedagogical need for a general tool to deal with the

mentioned problems, within Machine Design courses, a MATLAB program was developed,

and Figure 1 illustrates, for the sake of an example, the calculation of the product YFa . YSa for

the previously mentioned geometrical parameters.

Fig.1 YFa . YSa, calculated according to ISO 6336 Part 3, 1996, using the MATLAB

software developed.

REFERENCES

AFNOR, E23-015, ‘Engrenages: détermination de la capacite de charge des engrenages cylindriques extérieurs

de mécanique générale’, 1982.

Dufailly, J, ‘Calcul de la capacité de charge des engrenages cylindriques de transmission de puissance :

présentation et analyse des méthodes ISO 6336’, Paris, Ellipses, 1998.

IET – Institut de l’Engrenage et des Transmissions, Cours special CACIA; VIII – Méthode de calcul de la

capacité de charge des engrenages à la rupture et à la pression superficielle, 13-17 Oct. 1997.

ISO 6336, Parts 1 to 6 (except 4, not yet published); various dates.

Haberhauer, H., Bodenstein, F., ‘Maschinenelemente: Gestaltung, Berechnung, Anwendung’, Springer, 2007.

IBERTRIB 2013


P007

TWIN DISC TESTS ON WHEEL AND RAIL STEELS

D.F.C. Peixoto1, D.M. Ramos

1, L.A.A. Ferreira

1, P.M.S.T. de Castro

1

1Mechanical Engineering Department, Faculty of Engineering of University of Porto, Rua Dr. Roberto Frias,

4200-465 Porto, Portugal (*)Email: [email protected]

ABSTRACT

This work aims at contributing with experimental results to the evaluation of the wear and the

rolling contact fatigue resistance of the wheel-rail contact materials, using a twin-disc

machine to simulate the most important dynamic conditions of a real contact.

Dry and lubricated tests were performed on specimens taken from an AVE train wheel and

from an UIC60 section rail in order to obtain an extended characterization of the contact

fatigue behaviour of these two materials.

The contact surface integrity was analysed during these tests.

INTRODUCTION

Rolling contact fatigue (RCF) is a kind of damage that appears in components subjected to

variable contact stress. In this type of fatigue loading, failure is caused by cracks that appear

not only on the surface but also under the surface of the bodies in contact, depending on

friction. Although most of the failures caused by RCF damage in railway transportation do

not cause casualties involving loss of life, they are a matter of concern since they cause

unplanned maintenance interventions, with decreased service availability and delays in train

traffic.

Fatigue failures due to RCF in railway wheels and rails are mainly caused by phenomena such

as heat generation, fatigue, wear and impacts (UIC, 2004) and can be categorized as surface

initiated defects or subsurface initiated defects.

Wear is the principal consequence of friction and is defined as the progressive material loss of

the active surface of a body in contact with another in relative movement.

It is known that there is a competition between RCF and wear, if the wear rate is too high it

can remove surface initiated defects.


In order to characterize these two materials, chemical composition, mechanical properties,

microstructure and hardness measurements, were carried out, (Peixoto, 2012). The materials

analysis revealed an ER7/ER8 grade wheel steel according to the EN 13262 standard and an

R260 MN grade rail steel according to the EN 13674 standard.

Fig. 1 shows some defects detected during lubricated tests.

Fig. 1 Some contact fatigue Surface defects detected during lubricated tests.



The high amount of mass loss during dry tests in comparison to lubricated tests is highlighted

by the contact surfaces state, after testing, shown in Fig. 2.

a) Contact surfaces after after 40x10

6 cycles

in lubricated conditions.

b) Contact surfaces after after 0.8x106 cycles

in dry conditions.

Fig. 2 Contact surfaces after testing.

During lubricated tests it was verified that the wheel material is more sensitive to defect

initiation, since the larger number of defects was observed in the wheel specimens.

However, during dry tests no defect was detected, probably due to the high wear rate that

removes any crack that appears at the contact surface. The wheel material presents a higher

wear rate than the rail material, a behavior that is more noticeable in dry tests. This difference

in wear rate is the cause of the corrugations observed in the contact surfaces of the discs tested

in dry conditions. The twin-disc machine used imposes the same rotating speed in the two

discs, and if one presents higher wear, the discs’ diameters will progressively become

different; consequently the tangential speed of the two discs in the contact will be different,

and sliding is verified leading to corrugations.

A methodology to calculate the mass loss from the variation of the roughness profile was

developed and good agreement was found with mass measurements, as shown in Fig 3.

Fig. 3 Roughness profiles and mass loss evaluation (M: 10

6).

ACKNOWLEDGMENTS

D.F.C. Peixoto acknowledges a Calouste Gulbenkian Foundation PhD grant, number 104047-

B.

The authors acknowledge the financial support of the Portuguese Government through "FCT -

Fundação para a Ciência e a Tecnologia" under the project PTDC/EME-PME/100204/2008

“Railways”. The wheel was kindly supplied by ALSTOM Spain. The rail was kindly supplied

by REFER through the Civil Engineering Department of the Faculty of Engineering of the

University of Porto.

References

Peixoto DFC, de Castro PMST, Ferreira LAA, Fatigue crack growth in railway steel, in J.

Pombo, (Editor), Proceedings of the First International Conference on Railway Technology:

Research, Development and Maintenance, Civil-Comp Press, Stirlingshire, UK, Paper 87,

2012.

UIC, Atlas of Wheel and Rail Defects, 2004.

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