L15 Friction

8/3/2019 L15 Friction

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Lecture 15Tribological Characterization



Tribology

The science and technology of interacting surfaces in relative motion: The

study of lubrication, adhesion, friction, and wear between contacting

surfaces

New materials and coatings

Can lower friction and reduce

wear, and thus can have

a positive impact on futuretribological systems

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



Economic Impact of Tribology

• Economic Losses in U.S. dueto inadequate control of

friction and wear

• Worldwide, it is estimated that

1/3 to 1/2 of world’s energy

production 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 energyefficiency (friction) and durability

(wear) can save billions of dollars.

• Friction has a direct impact on

environmental cleanliness as

well.

Loss Cost(b$)

Material 100

Wear 100

Friction 70

When lost-labor, down-time, cost

of replacement parts added, these

figures may double.

Latest OverallEstimates: $500B

P. Cummins/ORNL



single asperity

or nano-contact

engineering

surfaces

microsystem domainAtomic ScaleContacts

Molecular

Debris

Tribological Characterization:Scale of Test Methods

1Å cm-m

MostlySimulations

J. Che, et al., CalTech

AFM, FFM MicrotribologyMachines

d f

F R

M. Dugger

Pin-on-disk



ATOMIC/NANOSCALE TEST METHODS



Examples of Atomic Scale Studies/Simulations

Work by Motohisa Hirano and others both theoretically simulated andexperimentally 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 Regimeof Ultralow Friction”, PRL, 92(2004)134301.

single asperity or nanotribology

engineering

surfaces

Multiple-asperity contact:microsystem domainAtomic Scale

Studies

Molecular

Debris

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 byAFM vs SEM

AFM SEM

SEM

AFM



AFM/FFM/SFM

PositionSensitivedetector



FCA

N: 0 at%

FCA

N: 8 at%

FCA

N: 16 at%

Sputtering pCVD

NanoNano Wear Tests with Carbon OvercoatsWear Tests with Carbon Overcoats

Load: 10μN ×12 scan

X: 0.5μm/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 20

Carbon Thickness (nm)

R o t a

t i o n a l p a

s s n u m b e

r

FCA

pCVD

Sputtering



Observation of stick-slip on gold

A 5x5 nm2 atomic scale friction

measurement on Au(111) at 4x10-10 Torr at room temperature. The atomic latticeof gold causes stick-slip friction tooccur with the periodicity of the lattice.The inset line trace shows the clearlyresolved 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 due

to 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)

f r i c t i o n

c o e f f i c i e n t

Courtesy of G. Sawyer



TRIBOLOGICAL

CHARACTERIZATION AT

MESO/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 thecontact geometry.

•Some of them are standardized and require the certain

procedures to follow.

Pin-on-disk Machines



Pin-on-disk Machines

Load

Sapphire

Ball

Disk

Load: 1 - 20 N

Speed: 0.3 - 1 m/s

Environment: Dry Nitrogen

Ball 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 Machines

Four Ball Machine

High-temperature

Foil bearing test

machine

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

E t h a n o l

E 8 5

M 8 5

G a s o l i n

e

D r y

NFC-2

NFC-6

Diamonex STD

Balzers

Uncoated

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

W e

a r R a t e ( m m ^ 3 / N - m )

Coating

Fuel

Injector Wear

Courtesy of J. Hershberger



Images of Rubbing Surfaces

3D-Pin Surface 3D-Disk Surface

2D ImagesOf Pin Surfaces



lon9706

THE RANGE OF TRIBOLOGICAL PROCESSES TO CONSIDERWHILE TESTING COATED SURFACES

MATERIAL INPUTGEOMETRY:MacrogeometryTopography

Loose particlesFluids, environmentPROPERTIES:Chemical composite.Microstructure

Shear strengthElasticityViscosity

ENERGY INPUTVelocityTemperatureNormal LoadTangential force

Macromechanicalchanges

Tribochemicalchanges

Micromechanicalchanges

Material transfer

MATERIAL OUTPUTGEOMETRY:MacrogeometryTopography

Loose particlesFluids, environmentPROPERTIES:Chemical compositionMicrostructure

Shear strengthElasticityViscosity

ENERGY OUTPUT

FrictionWear VelocityTemperatureDynamics

Courtesy of K. Holmberg, VTT/Finland

T ib i d d f il d



Tribo-induced failure modesHogmark 01

Initial state Coating detachment Cracking & spalling

Coating & substratedeformation

Coating & substratedeformation + fracture

Transfer from thecounterface

Gradual coating wear Initial gradual wear + premature detachment

Coating detachment+ substrate wear

Premature failure Failure due to gradual wear Courtesy of C. Donnet

Friction and Wear Mechanisms



Friction and Wear Mechanisms

Macro mechanisms

Holmberg 01

Micro mechanisms

Transfer

Tribochemistry

Nano mechanisms

Courtesy of C. Donnet

Macro mechanisms



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


Micro mechanisms



Micro-mechanisms

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

Material response at the µm scale Electroless Ni coating / gear

Hogmark 01

Holmberg 01 Energy accommodation modes

TiN / HSS

Hogmark 01


Micro Stress Distribution on a



Micro Stress Distribution on aCoated Surface

Hogmark et al.

Ways to Improve Load Carrying



Ways to Improve Load CarryingCapacity of Coatings

Hogmark et al.

C O C S S

Summary of Wear Mechanisms



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FRICTION MECHANISMS

PARTICLE CRUSHING

ASPERITY FATIGUE

PARTICLE

PLOUGHING

PENETRATION REDUCED CONTACT

AREA & INTERLOCKING

SHEARING SUBSTRATE

DEFORMATION

SCRATCHING

PLOUGHING

HARDNESSOF COATING

THICKNESSOF COATING

SURFACEROUGHNESS

DEBRIS

COATEDCONTACT

SOFTHARD

E

T M -

- K G H \ T C B \ F R I C T M 9 7 . d s f .

PARTICLE

EMBEDDING

LOAD CARRIED BY

COATING STRENGTH

PARTICLE

HIDING

a b c d

f g h

i j k l

e

HARD SLIDER

HARDSOFT

Courtesy of K. Holmberg/VTT-Finland

Summary of Wear Mechanismsin Coated Surfaces

MAJOR SOURCES OF FRICTION



MAJOR SOURCES OF FRICTION

Roughness

Real Contact Areas

Elastic/plastic

Deformation

Major Causes of

Friction

Capillary

Forces

H2O OH O

Physisorption/chemisorption

Deformation

Adhesion

Adh i M h i f F i ti



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

A1 A2

N

F

Ar = A1 + A2 + . . .

Ff =σ

.Ar

Transfer Films vs Friction



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 90

TiN, CrN, (Ti,Al)N Huang 94, Wilson 98

MoS2 Fayeulle 90, Wahl 95

DLC Ronkainen 93, Donnet 95, Grill 97

Effect of Transfer Film Forming



0

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600

ZirconiaSteelSapphireDLC-Coated Steel

Distance (m)

g

Tendency on Friction

Sapphire

Uncoated

Steel

Ball

Zirconia

DLC Coated

Steel Ball

FcoCoce

Dry N2

Transfer Film

Coated

Steel

Ball

DLC-coated Steel Disk Against Various Counterface Balls

Tribochemistry vs Friction



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=0.003

0.001

0.01

0.1

1

0 100 200 300 400 500Number of cycles

µ=0.007

µ=0.710 hPa H2 UHVor 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, HBNMäkelä 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,

Sture Hogmark

Tribochemical film Formation in Lubricated Contacts



Steel /DLCEP

Stee/DLCEP

30 µm 30 µm

300 µm 300 µm

C

C

S

WO

W

W

Fe

O

C

S

W

Fe

WO NiAfter 8000 cycles at 700 N

Roughness vs Friction



Roughness vs Friction

W = W1 + W2 + . . .

F = W tanθ

F1 = W1 tan θ

Tribology of Diamond Films



Tribology of Diamond Films

Roughness Effect

Erdemir, et al., Surface and Coatings Technology,

121(1999) 565-572

Roughness Effect on Friction



Roughness Effect on Friction

Rough

Polished

MCD

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

MCDDiamond Films

Environment vs Friction



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

In water In air In argon

F r i c t i o n c o e f f i c i e n t

# Revolutions

Initial friction is 0.1-0.2

Due to higher degree of

covalent bond interactions

Diamond Coated Disk

Courtesy of J. Andersson

Diamond Coated Ball

H2O OH O

Physisorption/chemisorption

Effect of Water Partial Pressure on



0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100 120

Time (s)

F r i c t i o n

c o e f f i c i e n t

At 2000 Pa

At 460 Pa

At 0.4 Pa

Smoother and

lower frictionat lower water

vapor pressures

Vacuum Experiments

Frictional Behavior of DLC Film

J. Anderson and R. Erck/ANL

?

Environmental Sensitivity of MoS2 Type Solid Lubricant Coating



The performance and durability of these solids are strongly affected by the presenceof 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 MoS 2 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 frictionwhen present on sliding surfaces.

Thickness of the film is very important After Bowden and Tabor

Most desired case

S l t d R f



Selected References

• K. Holmberg and A. Matthews, CoatingsTribology: Properties, Techniques, and

Applications in Surface Engineering, Elsevier,1994.

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

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

Documents

L15 Friction