Lecture 15Tribological Characterization

TribologyThe science and technology of interacting surfaces in relative motion: The study of lubrication, adhesion, friction, and wear between contacting surfaces

New materials and coatingsCan lower friction and reduce wear, and thus can havea positive impact on future tribological systems

It impacts national economy of all nations and lifestyles of most people

Economic Impact of Tribology

• Economic Losses in U.S. due to inadequate control of friction and wear

• Worldwide, it is estimated that 1/3 to 1/2 of world’s energyproduction is used to combat friction and wear (A. Z. Szeri, Tribology: Friction, Lubrication, and Wear; Hemisphere Publishing, 1980, p.2)

• Therefore, even very small improvements in energy efficiency (friction) and durability (wear) can save billions of dollars.

• Friction has a direct impact on environmental cleanliness as well.

Loss Cost(b$)

Material 100Wear 100Friction 70

When lost-labor, down-time, cost of replacement parts added, these figures may double.

Latest Overall Estimates: $500B

P. Cummins/ORNL

single asperity or nano-contact

engineering surfaces

microsystem domainAtomic ScaleContacts

MolecularDebris

Tribological Characterization:Scale of Test Methods

1Å cm-m

MostlySimulations

J. Che, et al., CalTech

AFM, FFM MicrotribologyMachines

df

FR

M. Dugger

Pin-on-disk

ATOMIC/NANOSCALE TEST METHODS

Examples of Atomic Scale Studies/Simulations

Work by Motohisa Hirano and others both theoretically simulated and experimentally demonstrated superlubricity (or frictionless sliding) between sliding pairs of Si(001) and a W (011) tip in ultra-high vacuum, (PRL, 78(1997)1448)). Also see, Socoliuc, et al., “Entering a new Regime of Ultralow Friction”, PRL, 92(2004)134301.

single asperity or nanotribology

engineering surfaces

Multiple-asperity contact: microsystem domainAtomic Scale

Studies

MolecularDebris

Commensurate Incommensurate

Tribolever

µ ~ 0.001STM of one layer of

graphite

2D/2D

Dry N2

Dienwiebel et.al.,

PRL, 92(2004)126101

Tribological Characterization at Nanoscales

AFM Tips

Surface Characterization of Diamond Films by AFM vs SEM

AFM SEM

SEM

AFM

AFM/FFM/SFM

PositionSensitivedetector

FCAN: 0 at%

FCA N: 8 at%

FCA N: 16 at%

Sputtering pCVD

NanoNano Wear Tests with Carbon OvercoatsWear Tests with Carbon Overcoats

Load: 10 μN ×12 scan

X: 0.5 μm/div.Z: 20 nm/div.

FCA: Filtered Cathodic Arc

DurabilityDurability・ Pin: Al2O3-TiC ball (2 mmφ)・ Applied load: 10 gf・ Sliding velocity: 0.2 m/s

0

2000

4000

6000

8000

10000

0 5 10 15 20Carbon Thickness (nm)

Rot

atio

nal p

ass n

umbe

r

FCA

pCVD

Sputtering

Observation of stick-slip on gold

A 5x5 nm2 atomic scale friction measurement on Au(111) at 4x10-10 Torrat room temperature. The atomic lattice of gold causes stick-slip friction to occur with the periodicity of the lattice. The inset line trace shows the clearly resolved stick-slip features for the forward and backward traces.

From R. Carpick/U. Wisconsin

Friction Force Maps

700nm x 700nm image of a few nanometer flat carbon islands on a magnetite single crystal. "Material dependend friction contrast" in the right image is dueto more or less adsorbates between carbon islands (lower friction) and magnetite (higher friction).

(Images taken by Stefan Müller)

Nano-to-micro Scale Test Machines

Courtesy of G. Sawyer

Contact Geometry

Nano/Macrotribology of DLC Films

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0 50 100 150 200 250 300 350

time (seconds)

fric

tion

coef

ficie

nt

Courtesy of G. Sawyer

TRIBOLOGICALCHARACTERIZATION ATMESO/MACRO-SCALES

Tribological Characterization:Typical contact Geometries for Macroscale Experiments

•There are so many contact configurations to chose from.•Each geometry is very unique and designed to simulate an application.•Test conditions may vary a great deal, depending on the contact geometry.•Some of them are standardized and require the certainprocedures to follow.

Pin-on-disk Machines

LoadSapphireBall

DiskLoad: 1 - 20 NSpeed: 0.3 - 1 m/sEnvironment: Dry NitrogenBall Radius:3.175 - 5 mm

Contact geometry Operating principles

Coating

Operating Principles• In most cases, friction and wear data. • Friction coefficient, µ = Ff / Fn (where, Fn is the normal force)

Friction coefficient

Wear rate in the ball and in the flat

Wear Volume on ball: Wb=πd4/64r (d:wear scar diameter, r: ball radius)Wear Rate=Wb/LN (N: Normal force; L:Sliding distance)

Other Popular MachinesFour Ball Machine

High-temperatureFoil bearing testmachine

Twin-disk rolling/sliding machine

Block-on-ring test machine

Reciprocating Test Machine

• Major Test Variables– Time, Speed (rpm), Track Radius– Load / Stress– Material Composition (Pin/Ball &

Flat)– Coating Composition– Test Environment (Dry, Inert, RH),

Lubricant (& Additive) Composition and RheologicalProperties

• Test Output– Continuous Friction &

Temperature Data Typical Contact Geometries

Courtesy of G. Fenske

Low-Amplitude Reciprocating (Fretting) Test Machine

• Issue - performance of SIDI components at higher pressures with low-lubricity fuels

Ethanol

E85

M85

Gasoline

Dry

NFC-2

NFC-6

Diamonex STD

BalzersUncoated

Diamonex-HT

0.E+00

5.E-09

1.E-08

2.E-08

2.E-08

3.E-08

3.E-08

4.E-08

4.E-08

5.E-08

5.E-08

Wea

r Rat

e (m

m^3

/N-m

)

Coating

Fuel

Injector Wear

Courtesy of J. Hershberger

Images of Rubbing Surfaces3D-Pin Surface 3D-Disk Surface

2D Images Of Pin Surfaces

lon9706

THE RANGE OF TRIBOLOGICAL PROCESSES TO CONSIDERWHILE TESTING COATED SURFACES

MATERIAL INPUTGEOMETRY:MacrogeometryTopographyLoose particlesFluids, environmentPROPERTIES:Chemical composite.MicrostructureShear strengthElasticityViscosity

ENERGY INPUTVelocityTemperatureNormal LoadTangential force

Macromechanicalchanges

Tribochemicalchanges

Micromechanicalchanges

Material transfer

MATERIAL OUTPUTGEOMETRY:MacrogeometryTopographyLoose particlesFluids, environmentPROPERTIES:Chemical compositionMicrostructureShear strengthElasticityViscosity

ENERGY OUTPUTFrictionWearVelocityTemperatureDynamics

Courtesy of K. Holmberg, VTT/Finland

Tribo-induced failure modesHogmark 01

Initial state Coating detachment Cracking & spalling

Coating & substratedeformation

Coating & substratedeformation + fracture

Transfer from the counterface

Gradual coating wear Initial gradual wear+ premature detachment

Coating detachment+ substrate wear

Premature failure Failure due to gradual wearCourtesy of C. Donnet

Friction and Wear Mechanisms

Macro mechanisms

Holmberg 01

Micro mechanisms

Transfer

Tribochemistry

Nano mechanisms

Courtesy of C. Donnet

Macro-mechanisms

• Mechanical properties (H, E, stress)• Thickness of the coating• Surface roughness• Debris

Main parameters

Quantification by scratch testLee 98

TiN/Steel

Principle of load-carrying capacity

Hogmark 01

Courtesy of C. Donnet

Micro-mechanisms

• Stress and strain at the asperity level• Crack generation and propagation• Material release & Particle formation

Material response at the µm scaleElectroless Ni coating / gear

Hogmark 01

Holmberg 01 Energy accommodation modes

TiN / HSS

Hogmark 01

Courtesy of C. Donnet

Micro Stress Distribution on a Coated Surface

Hogmark et al.

Ways to Improve Load Carrying Capacity of Coatings

Hogmark et al.

lon9708

FRICTION MECHANISMS

PARTICLE CRUSHING

ASPERITY FATIGUE

PARTICLE PLOUGHING

PENETRATION REDUCED CONTACT AREA & INTERLOCKING

SHEARING SUBSTRATEDEFORMATION

SCRATCHING

PLOUGHING

HARDNESS OF COATING

THICKNESS OF COATING

SURFACE ROUGHNESS

DEBRIS

COATEDCONTACT

SOFTHARD

ETM

- - K

GH

\TC

B\F

RIC

TM97

.dsf

.

PARTICLEEMBEDDING

LOAD CARRIED BY COATING STRENGTH

PARTICLEHIDING

a b c d

f g h

i j k l

e

HARD SLIDER

HARDSOFT

Courtesy of K. Holmberg/VTT-Finland

Summary of Wear Mechanisms in Coated Surfaces

MAJOR SOURCES OF FRICTION

Roughness

Real Contact Areas

Elastic/plastic Deformation

Major Causes ofFriction

CapillaryForces

H2O OH O

Physisorption/chemisorption

Deformation

Adhesion

Adhesion Mechanisms of Friction

- Covalent sigma (the strongest)- Ionic- Metallic- Magnetic-π-π* Attraction (in the case of graphite)- van der Waals-Electrostatic-Capillary

Capillary

Electrostatic

van der Waals

The Case of Carbon Films

Not applicable to carbon

A1A2

N

F

Ar = A1 + A2 + . . .

Ff = σ.Ar

Transfer Films vs Friction• Transfer formation : run-in phenomena + COF fluctuations• Transfer film (0.01 - 50 µm) “Repartition” of the lubricant reservoir• Interfilm sliding : general condition of steady-state• Wear not linear versus duration

Accommodation modes

Singer 92

Transfer formation Interfilm sliding

Donnet 01

PTFE & Polyimide Yamada 90TiN, CrN, (Ti,Al)N Huang 94, Wilson 98MoS2 Fayeulle 90, Wahl 95DLC Ronkainen 93, Donnet 95, Grill 97

0

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600

ZirconiaSteelSapphireDLC-Coated Steel

Distance (m)

Effect of Transfer Film Forming Tendency on Friction

Sapphire

UncoatedSteelBall

Zirconia

DLC CoatedSteel Ball

Fric

tion

Coe

ffici

ent

Dry N2

TransferFilm

CoatedSteelBall

DLC-coated Steel Disk Against Various Counterface Balls

Tribochemistry vs FrictionFriction-induced “fresh” surfaces

Temperature increaseEffect of the surrounding environment

Tribo-reactionsat the nm scale

0.001

0.01

0.1

1

0 100 200 300 400 500Number of cycles

µ=0.0030.001

0.01

0.1

1

0 100 200 300 400 500Number of cycles

µ=0.007

µ=0.710 hPa H2 UHV or Ar

Role of gaseous H2 on a-C:H films (H=34at%)

Donnet 01

Role of H2O on B2O3

Formation of lamellar boric acidErdemir 90-98

• Metal Jahanmir 89, Kuwano 90, Erdemir 91 • TiN, CrN, TiC, HBN Mäkelä 85, Gardos 89, Singer 91, Martin 92, Lin 96• Oxides Blomberg 93, Gee 95, Erdemir 95, Prasad 97 • Various (Ti, Al, Zr, Si)N, Rebouta 95• DLC Miyoshi 90, Ronkainen 90, Donnet 95, Erdemir 95, Voevodin 96, Grill 97, Fontaine 01• Diamond, Graphite Gardos 90, Hayward 90, Langlade 94, Blanchet 94• MoS2 Spalvins 80, Fleischauer 87, Singer 90, Martin 93, Wahl 95,

Steel/DLCEP

Stee/DLCEP

30 µm 30 µm

300 µm 300 µm

Sture Hogmark

C

CS

WO

W

W

Fe

O

C

S

W

Fe

WO Ni

Tribochemical film Formation in Lubricated Contacts

After 8000 cycles at 700 N

Roughness vs Friction

W = W1 + W2 + . . .

F = W tanθ

F1 = W1 tan θ

Tribology of Diamond FilmsRoughness Effect

Erdemir, et al., Surface and Coatings Technology,121(1999) 565-572

Roughness Effect on Friction

Rough

PolishedMCD

B. K. Gupta et al., J. Tribol., 116(1994)445.NCD

MCDDiamond Films

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

In waterIn airIn argon

Fric

tion

coef

ficie

nt

# Revolutions

Initial friction is 0.1-0.2

Environment vs Friction

Due to higher degree of covalent bond interactions

Diamond Coated Disk

Courtesy of J. Andersson

Diamond Coated Ball

H2O OH O

Physisorption/chemisorption

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100 120

Time (s)

Fric

tion

coef

ficie

nt

At 2000 Pa

At 460 Pa

At 0.4 Pa

Smoother andlower frictionat lower watervapor pressures

Vacuum Experiments

Effect of Water Partial Pressure on Frictional Behavior of DLC Film

J. Anderson and R. Erck/ANL

?

The performance and durability of these solids are strongly affected by the presence of moisture and oxygen in the environment. Aging may also pose a major problem. Doping with Ti, Ni, Au, and Pb may reduce environmental sensitivity.

Ti-Doped

Base MoS2

Multiarc, Inc.data

Environmental Sensitivity of MoS2 Type Solid Lubricant Coating

Work the best in dry, inert, or vacuum type environments

Friction Mechanisms of Soft Metals

Mainly because of their low shear strengths and rapid recovery as well as recrystallization, certain pure metals (e.g., In, Pb, Ag, Au, Pt, Sn, etc.) can provide low friction when present on sliding surfaces.

Thickness of the film is very importantAfter Bowden and Tabor

Most desired case

Selected References• K. Holmberg and A. Matthews, Coatings

Tribology: Properties, Techniques, and Applications in Surface Engineering, Elsevier, 1994.

• B. Bhushan and B. K. Gupta, Handbook of Tribology: Materials, Coatings and Surface Treatments, McGraw-Hill, 1991.

• B. Bhushan, Modern Tribology Handbook, Volumes I & II, CRC Press, 2000.