l15-final tribological...
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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.
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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.