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Fretting generated by the vibrations in ball bearings in dry and oil-bath lubricated contact

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Fretting generated by the vibrations in ball bearings in dry and oil-bath lubricated contact 초록

볼 베아링의 진동은 여러 mechanism에 의하여 발생한다. 일부는 베아링의 간극, 하중의 변동, 오염에 따른 Over

rolling과 관련이 있다. 구르는 요소와 Ring의 접촉은 탄성접촉이다. 따라서 베아링의 강성은 시간에 의존하게

된다.작은 진동과 낮은 속도를 가지는 진동으로서의 Fretting은 Fretting as an oscillatory motion with small

amplitudes and a very low velocity leads to the situation when dynamic fluid lubrication during sliding is intractable.

베아링이 구르지 않는 경우 구르는 요소와 raceway 사이에는 윤활막이 생성되지 않는다. 따라서 금속과 금속의

직접적인 접촉과 외부로 부터의 진동(주로 운반 시)은 구름요소와 Rig 사이에 매우 작은 움직임을 유발하게된다.

일반적으로 많이 사용하는 AISI 52100 Cr steel ball의 표면상태에 대한 연구를 건식 및 윤활 조건에서 수행하였다.

Fretting 시험은 왕복동 미끄럼 ball on flat 시험으로 10 Hz속도로서 상온 및 40% 상대습도에서 행하였으며 최대

Hertian 압력은 1~2Gpa, 진폭은 10~150um 사이의 다양한 진폭을 사용하였다.

Manuscript received January 31, 2007. This work was supported by European Committee as a part of Artificial Intelligence for Industrial Applications (AI4IA) Marie Curie FP6 Research Training Programme, Contract No. 514510. T. Kolodziejczyk is a PhD at Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systemes, CNRS UMR 5513, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France S. Fouvry is a direct supervisor of the PhD at Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systemes, CNRS UMR 5513, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France USA (corresponding author to provide phone: +33 (0)4 72 18 65 62; e-mail: [email protected]). G. Morales-Espejel is industrial supervisor working at SKF Engineering & Research Centre (ERC), Nieuwegein, The Netherlands and Laboratoire de Mecanique des Contacts et des Solides, UMR 5514, INSA de Lyon, 18,20 rue des Sciences, 69621 Villeurbanne, France

서론

FRETTING은 왕복동 미끄러짐의 특별한 경우이다. 이는 주위 분위기에 절대 노출되지 않는 마모Track의

숨어있는 중앙부에서 발생하게된다. 이상황은 정상적인 왕복동 미끄럼에 비교하여 특별한 접촉열화를

유발하게된다[1]. 열 성장[2, 3], 구조변화에 따른 소성변형 (즉 white layer) [4, 5, 6] 및 tribofilm 형성 [7, 8] 과 같은

추가적인 효과는 마모 및 마찰거동에 지대한 영향을 초래한다.

다양한 조건에서의 정적 및 동적 마찰의 전이에 대한 표면 거칠기의 효과는 참고문헌 9에 정리되었다.

Local adhesive junctions‟ formation and micro-grooving can strongly affect the value of coefficient of friction, real area of contact,

pressure distribution which leads to higher concentration of the frictional energy at the asperity contacts. The main point in

analysis of surface contact concerning roughness is Archard‟s sentence saying: if primary result of increasing the load is to cause

existing contact areas (high asperities are plastically deformed) to grow, then area and load will not be proportional. If the primary

result is to form new areas of contact (contact of lower asperities is initiated), then area and load will be proportional. Same

considerations should be taken into account, when it comes to sliding, because the fundamentals of friction do not change; only

the stresses on the interface differ.

The nominal geometry used in present study is a Hertzian contact between a ball and a plane so we can study such contacts

approximately by assuming there are so many micro-contacts within the contact, that there are many even within annulus so small

that the separation between the two surfaces can be taken as constant. We can treat the roughness as a compliant layer with a

highly non-linear pressure/compliance relation separating the two elastic bodies and the nominal pressure will be composed of

individual contact loads.

Two contradictory effects of roughness on adhesion exist: for two surfaces stick together the roughness increases the

force needed to peel them apart, but for two solids placed in contact, roughness reduces a pull-off force. The topic of energy

dissipation in both static and sliding contact and associated with them hysteresis, involving mechanical, bulk and chemical effects,

is a result of avalanching; if adhesion energy is released then enlarged asperity junctions are formed.

Temperature growth (negligible for small Peclet numbers) and severe plastic deformation of asperities are decisive for formation

of wear particles, their further oxidation and tribolayer formation [10]. When it comes to comparison of oxide film growth rate

with friction and with its absence it can be concluded that the growth law changes from logarithmic for static state to parabolic for

sliding respectively. It happens because the mechanism of oxidation and kinetics changes. Diffusion through the oxide layer,

which is decisive concurrent process, becomes faster than electromigration due to generation of structural defects, which are the

places with higher mean energy per elementary cell. Formation of the defects is one of the mechanisms by which friction increases

chemical reactivity on the interface.

The role of oxide wear debris depends on the dominant wear mechanism of fretting wear [12]. With dominant adhesive

wear mechanism the oxide wear particles act like a solid lubricant to reduce the damage caused by fretting. When abrasive

mechanism occurs hard particles facilitate the wear rather than protect against it. Consequently, it can be said that the significant

factor governing fretting wear is the behaviour of oxide particles [11, 20]. The trapping action of wear debris depends on the

fretting conditions such as normal load, slip amplitude, frequency and contact geometry.




Protective effect is caused by the formation of the compacted oxide layer that is why the transition from severe to mild wear

hardly depends on the rate of the oxide formation and the size of the particles that determines the possibility of tribolayer

formation. The roughness influences as well the critical length needed for the tribo-layer to build [13]. It is absolutely clear that

shorter the severe wear period exists, smaller is the wear. Moreover, the mild wear rate is stabilized by the destruction rate

(connected with normal load) and repair rate (connected with the surface roughness). When the compacted oxide layer gets broken

the first body contact occurs less for the surface with sharp asperities than for the flattened ones. The most crucial is the existence

of more oxide particles trapped in the grooves that leads to easier reconstruction of compacted oxide layer.

In this paper the influence of the surface roughness and oil viscosity under different mechanical conditions on the coefficient of

friction and wear is presented. The tribofilm and tribolayer formation phenomenon are described as key factors of understanding

the behaviour of the system under both, dry and lubricated fretting conditions. More likely the author has described the results of

his first year PhD studies in qualitative way.

I. 실험

A. 기기구조

The experiments have been conducted under dry and oil lubricated conditions in ball-on-flat configuration on the fretting rig

presented on Fig. 1. The chamber permits to control the environmental conditions during the test (temperature and relative

humidity). Electrodynamic shaker induces reciprocating horizontal movement of the upper sample with frequency and

displacement (measured by an optical sensor) set by the user. The tangential force is measured by piezoelectric sensor.

B. 시험과정

The samples (ball and cylinder) are cleaned in acetone and mounted to the specially designed holders. The ball is mounted in

upper holder and the cylinder depending from the type of test is mounted in bottom holder for dry test or empty oil-bath for

lubricated one. For dry conditions clean samples are introduced into the point-contact and normal force is set to be zero at this

stage. Required normal force is imposed and the test starts. For lubricated contact, after the zero point is found the samples are

separated and the oil is introduced into the bath until it creates a thin film on the bottom flat sample. Then the samples are

introduced into the point-contact and required force is imposed. For the whole duration of test the bottom (flat) specimen is

submerged in oil. During the test the environmental and mechanical conditions are measured simultaneously in the time intervals

chosen by a user.

C. 시험재료의 특성

The counter-bodies are made from the heat treated AISI 52100 chromium steel (100Cr6), typical composition can be found in [14].

The Vickers microhardness of the steel (HV0.1) is about 830 HV. The microstructure of the steel is presented on Fig. 2. and the

structure of chromium steel after quenching and tempering contains of tempered martensite, retained austenite and chromium

carbides. For the flat surface the 20 mm in diameter cylinder‟s base was used. It was grinded on sandpaper and subsequently

polished using the diamond pastes to reach an arithmetical mean surface roughness (Ra) of 0.050 μm.

As a counter-body the 13/16 inch (20.6375 mm) balls were used. They were bisected to groups with different surface finishing –

different grades. The arithmetical mean roughness of the balls with respect to the grades was 0.020 μm for G10 and 0.032 μm, for

G20 respectively. The oils used for examination of their efficiency under partial and gross slip fretting conditions were: Shell

Turbo Oil T68 and Shell T9. The viscosity of the oils at the temperature of 40°C is 68.2 cSt and 9 cSt.




D. 시험방법

Dry and lubricated tests have been performed at ambient temperature and relative humidity range from 35 – 45%. The

matrix of dry tests according to the parameters of Table 1 was built. All the tests were performed at 10Hz frequency of the

movement.

Oil-bath lubricated tests were conducted for two oils with different values of viscosity (presented in chapter I. C.). The

matrix of the tests is presented in Table II. After the tests the samples are cleaned up in acetone and further investigations are

taking place. The quantification of wear and the roughness is made by the means of stylus profilometry. The optical investigations

of the wear scars were done on Olympus microscope. The microhardness measurements were carried out on Vickers

microhardness tester Sopelem. SEM and EDX investigations have been conducted on TESCAN VEGA TS5136 XM scanning

electron microscope with ROENTEC energy dispersive system.




II. RESULTS

a) dry tests

Firstly, the results from the dry tests will be presented. The aim of the study under dry conditions was to reveal the influence of

different values of the roughness of the ball on coefficient of friction and wear during first 2500 cycles. Fig. 3. is showing

that the wear rate for the first 1500 cycles is higher for the surface that is smoother.

This fact is well confirmed by the analysis of the fretting loop and total energy dissipated during fretting test, as well as by

profilometric measurements of the roughness in fretting scar.




From roughness measurements it can be seen for both grades of the ball that roughness parameters firstly are increasing above

the initial values (Fig. 4. – presenting roughness changes in the scar on flat surface and Fig. 5 – presenting roughness

changes in the scar on ball surface).

Then due to truncation and rubbing off the surfaces of the ball and cylinder the surface becomes smoother (strongly visible for

the pair where Ra of the ball is higher). After 1500 cycles the roughness starts to grow as the reseat period comes to an end

and the decisive role has now the third body formation, that decides which kind of regime wear will bring on (severe –

caused by adhesive metal-to-metal contact or mild – caused by debris formation and its abrasive or protective behaviour).

Appearance of this effect is widely discussed in next chapter.




The formation of tribolayer after 1500 cycles is confirmed by SEM examination of the scar on the flat sample (Fig. 6.). The

oxygen map shows that the distribution is not uniform and that there are places, where the atomic content of the oxygen

reaches 50 % (marked with number 2 on images). From SEM observations it can be seen that the rich in oxygen compounds are:

the tribolayer formed on the steel surface and wear debris found outside the scar. The composition of element at the points

marked by 1 and 2 is showed in Table III and IV and was obtained by EDX technique. The chemical composition of the

material marked with 1 seems to be in agreement with the composition of examined steel.

b) oil-bath lubricated tests

The wear scar originated from 50k cycles test in lubricated fretting conditions presented on Fig. 7. is showing the signs of

scratches and many wholes created that can be a result of adhesive junctions in the running-in period and/or extraction of

hard carbides (typically 1 – 3 μm in diameter) from the surface during sliding. The existence of these carbides may

influence the tribofilm formation.

After examination of the fretting scar after oil-bath lubricated test on with exploitation of EDX technique increased oxygen

content has not been observed. It means that enduring tribofilm has not been formed.




The micro hardness tests performed on the scar are showing high rise of the Vickers hardness when approaching the middle of

the scar (Fig. 8.). Its is a result of plastic deformation, which as a result of high pressure, increases the density of

dislocations and results in strengthening of the material. The next results that are presented are showing the influence of

various parameters on the COF vs. number of cycles curve for gross slip fretting regime for oil-bath lubricated tests. Fig. 9.

shows the influence of the viscosity of an oil on frictional behavior during sliding. It can be seen that „more fluid like‟ oil

(with lower viscosity) is showing higher efficiency in decreasing the coefficient of friction. The running-in period is shorter

for T9 and no regular instabilities in COF have been seen. It means that oil with lower viscosity has better surface separation

capabilities and forms uniform lubricating layer that results in lower wear value.

The behavior of the oil under different pressure is presented on Fig. 10. Higher value of pressure induces higher shear stress in

the film and results in decrease of the film thickness. It results in more unsettled behavior of coefficient of friction, which is

showed by higher disturbances.

Finally the influence of the surface roughness on the coefficient of friction is studied and depicted in Fig. 11. The number of

cycles in running-in period is higher for lower value of ball roughness. As well the disturbances of COF are higher for lower

Ra value, but they seem to display similar characteristic (shape and periodicity).




III. DISCUSSION

The wear results for dry sliding for different surface roughness of the ball are showing different behavior in running-in period

until about 1500 cycles (Fig. 3.). It is a result of changes of value of the coefficient of friction on the transition from static to

kinetic friction [9]. It was mentioned by D.-H. Hwang that this effect has more influence for higher values of pressure when

the abrasive wear is a consequence of micro-grooving that destructs the natural (normally 3mm thick) oxide layer as pull-

out of carbides from the mated surface takes place. Adhesion and plastic deformation of asperities in the contact area

seemed to be the dominant mechanisms which determined transition behavior of the self-mated steel pairs. Increase of

contact area contributed to substantial increase of the coefficient of friction after onset of first sliding. The real area of

contact between the ball and the polished surface was greater than that with the ground surface; hence in the absence of the

lubricant the kinetic friction was higher on the polished than on the ground surface.

Increased oxygen content showed on Fig. 6 b) is a result of tribo layer formation. Iwabuchi in [13] has examined the influence of

surface roughness on severe to mild wear transition. The conclusion is similar to the ones in this paper.

When the compacted oxide layer (third body or tribo layer) is fractured, metal-to-metal contact occurs. Because the first

body contact between the surfaces with sharp asperities occurs less than for the surface with flattened asperities, owing to the

existence of more oxide particles trapped in the microgrooves, therefore repair rate of rougher surface is greater. After

formation of tribo layer the wear of the first body depends on the abrasiveness of the debris and the

dynamic of tribo layer formation is a function of mechanical and environmental parameters of the test [10].

The results presented on Fig. 4. and Fig. 5. are showing that the changes in characteristic surface roughness parameters can be

useful in future study as a surface damage criteria. Decrease in arithmetic mean surface roughness, as well as other

roughness parameters (not presented in this paper), is a result of adhesive wear and plastic deformations of the asperities.

After the transition the roughness increases, which may be a result of oxide debris formation and its abrasiveness.

The understanding of transition from severe to mild wear for examined contact is presented on Fig. 12.




As it can be seen from Fig. 7. there is a vital role in the wear response of the system caused by the microstructure of contacting

bodies. Clearly visible round spots on SEM image could have been the places, where the pull-out of the hard

chromium carbides took place. It has been observed and presented by Schoefer in [8]. The mechanism of third body formation is

presented on Fig. 13. The influence of initial microstructure on the wear mechanisms and their transition has been widely

presented by Wang in [6]. The structure of material used (AISI 52100) contains tempered martensite and chromium carbides.

It was shown that this structure do not posses higher wear resistance than other structures. There are two reasons: this

structure does not have higher toughness and the ability for work hardening, as well as the low difference between the

hardness of martensite matrix and hard particles (chromium carbides). Hard particles pull-out can lead to easier crack

initiation and propagation. Further examination of the transfer film initially formed on the ball surface must be conducted to

confirm that hypothesis.

The structural changes of the material are unlikely to be results of the frictional heating in the contact but more likely the result

from mechanical deformations. It was calculated (using the hypothesis proposed by Kuhlmann-Wilsdorf in [2], the

equations presented in chapter VII of [21] and the material parameters form [22]) that for dry contact and the most severe

mechanical parameters (the highest maximal hertzian pressure) the temperature rise, called flash temperature, cannot exceed

the value of 30 K above the test temperature (in studies presented in this paper it was room temperature).

Microhardness rises from the border to the centre of the scar (Fig. 8.) is not an effect of oxide tribofilm formation since

higher oxygen content in the scar has not been found. High value of normal load results in response of the microstructure. The

internal factors, which affect the hardness of worn surface layers of steel, are: mobility of dislocations, strain hardening,

recovery and recrystallization [6, 19 and 23].




The formation of white-layer structures known as tribologically transformed structures (TTS) is presented in [19]. The hardness

rise of about 900 HV (in comparison with bulk material) is in a range presented in that paper for iron alloys. Chemical

composition is quite similar to bulk material, but the microstructure differs. Normally for iron alloys carbon-rich ferrite

phase is clearly visible and its formation does not depend on the initial microstructure of steel (the retained austenite

existence is not crucial for ferrite to form). As it was said before the temperature cannot have any influence on TTS

formation as for fretting conditions this process is linked to the presence of high plastic strains and transfer phenomena.

Mechanically mixed layer is present ad it is possible that after oil-bath lubricating tests, some additional carbon atoms to the

interface were introduced and formed TTS. In the hardened bearing steels high hertzian pressure field and chromium

carbides as inclusions inside the material can generate strong dislocation densities, which are responsible for

recrystallization and nucleation of nanograins of ferrite (Fig. 15.).

The early studies of fretting under oil lubricated contact were performed by Shima with cooperation with R.B. Waterhouse

group [8]. The graphs of coefficient of friction vs. number of cycles for different oil viscosities (Fig. 9.) are showing similar

behavior as presented by Shima. For gross slip conditions it is clear that lower viscosity leads to better contact penetration,

which results in more stable course of COF curve without regular instabilities (as it can be seen for oil T68 with 68.2 cSt

viscosity). The ability to infiltrate into and remain in the fretting contact zone and less effective formation of oxygen shield




are the main factors for more efficient restricting of metal-to-metal contact [16]. Sensitivity to the contact pressure (Fig. 10)

resulting in decreasing palliative effectiveness with increase of normal pressure was reported. The competition between self-

cleaning action and the self repair of fretting can be influenced by both contact pressure and oil viscosity [18]. In deep

studies of fretting under lubrication have been performed by Phillipon and Fouvry [17]. Same steel has been used in

experimental part, but as lubricants many types of grease have been examined. Opposite results has been presented for the

influence of contact roughness on coefficient of friction. The differences can be a result of different contact configuration

and the roughness (cylinder on- flat, higher values of roughness Ra>0.05 μm for shotpeened specimen). The existence of

higher asperities that

was beneficial in present study for oil accommodation in microgrooves could have been a disadvantage in homogenous lubricant

film formation. It is clear that depending from the morphology of surfaces that are examined, two contrary effects can take

place: easier formation of oxides for higher roughness can provide better lubrication, because even when there is a lack of

oil no metal to- metal contact can be established; in contrary rough surface induces high overstressing and lubricant

squeezing.

IV. CONCLUSIONS

Qualitative study of the coefficient of friction and wear response of the system under dry and oil-bath lubricated fretting

conditions has been done. Next step of the work will be a quantitative description of observed behavior that should lead to

more formalized understanding of the problem.

In-depth review of multidisciplinary literature from the field of: tribology, contact between solids, materials science, lubrication,

rheology and mechanics of fluids has been performed.

The fundamental achievement of work presented in this paper is finding the transition point and understanding the

mechanisms that are leading to different behavior of same materials with disparate surface finishing.

Crucial for further work is wear particle analysis as it is regarded as a proactive method for determining the present and future

wear state of the critical components. As the formation of third body is in great interest of both scientists and industry

understanding of the kinetics of formation and influence of mechanical and environmental conditions on that process will be

a key point of further studies.

The usefulness of artificial intelligence techniques (in principal neural networks, fuzzy logic and genetic algorithms) will be

quantified to solve this complicated problem after acquiring its physical understanding. Neural network and fuzzy inference

system are looking to be very useful tools for modelling the reliability of the system.

ACKNOWLEDGMENT

The author would like to thank the members of Fretting & Endommagements des Interfaces Group for their technical and

scientific help.

REFERENCES

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[16] Z. R. Zhou, L. Vincent, “Lubrication in fretting – a review,” Wear, vol. 225-229, pp. 962-967, 1999

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