Lecture 15
Tribological Characterization
Tribology
The science and technology of interacting surfaces in relative motion: The
study of lubrication, adhesion, friction, and wear between contacting
surfaces
It impacts national economy of all nations and lifestyles of most people
New materials and coatings
Can lower friction and reduce
wear, and thus can have
a positive impact on future
tribological systems
Economic Impact of Tribology
•
Economic Losses in U.S. due
to inadequate control of
friction and wear
Loss
Cost(b$)
Material
Wear
Friction
100
100
70
When lost-labor, down-time, cost
of replacement parts added, these
figures may double.
Latest Overall
Estimates: $500B
P. Cummins/ORNL
•
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 energy
efficiency (friction) and durability
(wear) can save billions of dollars.
Friction has a direct impact on
environmental cleanliness as
well.
Tribological Characterization:
Scale of Test Methods
Mostly
Simulations
Atomic Scale
Contacts
AFM, FFM
single asperity
or nano-contact
Microtribology
Machines
Pin-on-disk
microsystem domain
engineering
surfaces
Molecular
Debris
cm-m
1Å
FR
M. Dugger
J. Che, et al., CalTech
fd
ATOMIC/NANOSCALE TEST
METHODS
Examples of Atomic Scale Studies/Simulations
Atomic Scale
Studies
single asperity or
nanotribology
Multiple-asperity contact:
microsystem domain
engineering
surfaces
Molecular
Debris
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.
Commensurate
Incommensurate
Dry N2
STM of one layer of
µ ~ 0.001
graphite
Dienwiebel et.al.,
2D/2D
Tribolever
PRL, 92(2004)126101
Tribological Characterization at Nanoscales
AFM Tips
Surface Characterization of Diamond Films by
AFM vs SEM
AFM
SEM
AFM
SEM
AFM/FFM/SFM
Position
Sensitive
detector
Nano Wear Tests with Carbon Overcoats
FCA
N: 0 at%
FCA
N: 8 at%
Sputtering
pCVD
FCA: Filtered Cathodic Arc
FCA
N: 16 at%
Load: 10 μN ×12 scan
X: 0.5 μm/div.
Z: 20 nm/div.
Durability
Rotational pass number
・ Pin: Al2O3-TiC ball (2 mmφ)
・ Applied load: 10 gf
・ Sliding velocity: 0.2 m/s
10000
8000
FCA
6000
4000
pCVD
Sputtering
2000
0
0
5
10
15
20
Carbon Thickness (nm)
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 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 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
Contact Geometry
Courtesy of G. Sawyer
Nano/Macrotribology of DLC Films
0.8
0.6
friction coefficient
0.4
0.2
0
0
50
100
150
200
250
300
350
-0.2
-0.4
-0.6
-0.8
time (seconds)
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 the
contact geometry.
•Some of them are standardized and require the certain
procedures to follow.
Pin-on-disk Machines
Load
Coating
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
Operating Principles
•
•
In most cases, friction and wear data.
Friction coefficient, µ = Ff / Fn (where, Fn is the normal force)
Wear rate in the ball
and in the flat
Friction
coefficient
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
Block-on-ring test machine
Twin-disk rolling/sliding
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 Rheological
Properties
•
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
Injector Wear
5.E-08
5.E-08
4.E-08
3.E-08
3.E-08
2.E-08
2.E-08
Wear Rate (mm^3/N-m)
4.E-08
1.E-08
Diamonex-HT
Uncoated
5.E-09
Balzers
0.E+00
Diamonex STD
D
NFC-6
ry
as
G
e
E8
in
ol
NFC-2
85
M
Coating
5
ha
Et
l
no
Fuel
Courtesy of J. Hershberger
Images of Rubbing Surfaces
3D-Pin Surface
3D-Disk Surface
2D Images
Of Pin Surfaces
THE RANGE OF TRIBOLOGICAL PROCESSES TO CONSIDER
WHILE TESTING COATED SURFACES
MATERIAL INPUT
Macromechanical
GEOMETRY:
Micromechanical
changes
Macrogeometry
changes
Topography
Loose particles
Tribochemical
Fluids, environment
changes
PROPERTIES:
Chemical composite.
Microstructure
Shear strength
Elasticity
Viscosity
MATERIAL OUTPUT
GEOMETRY:
Macrogeometry
Topography
Loose particles
Fluids, environment
PROPERTIES:
Chemical composition
Microstructure
Shear strength
Elasticity
Viscosity
ENERGY INPUT
Velocity
Temperature
Normal Load
Tangential force
ENERGY OUTPUT
Friction
Wear
Velocity
Temperature
Dynamics
Material transfer
lon9706
Courtesy of K. Holmberg, VTT/Finland
Tribo-induced failure modes
Hogmark 01
Initial state
Coating detachment
Coating & substrate
deformation
Coating & substrate
deformation + fracture
Gradual coating wear
Initial gradual wear
+ premature detachment
Premature failure
Cracking & spalling
Transfer from the
counterface
Coating detachment
+ substrate wear
Failure due to gradual wear
Courtesy of C. Donnet
Friction and Wear Mechanisms
Macro mechanisms
Micro mechanisms
Transfer
Tribochemistry
Nano mechanisms
Holmberg 01
Courtesy of C. Donnet
Macro-mechanisms
Principle of load-carrying capacity
Main parameters
• Mechanical properties (H, E, stress)
• Thickness of the coating
• Surface roughness
• Debris
TiN/Steel
Lee 98
Hogmark 01
Quantification by scratch test
Courtesy of C. Donnet
Micro-mechanisms
Material response at the µm scale
Electroless Ni coating / gear
• Stress and strain at the asperity level
• Crack generation and propagation
• Material release & Particle formation
Hogmark 01
TiN / HSS
Hogmark 01
Courtesy of C. Donnet
Holmberg 01
Energy accommodation modes
Micro Stress Distribution on a
Coated Surface
Hogmark et al.
Ways to Improve Load Carrying
Capacity of Coatings
Hogmark et al.
Summary of Wear Mechanisms
FRICTION MECHANISMS
in Coated Surfaces
COATED
CONTACT
HARD SLIDER
HARDNESS
OF COATING
SOFT
HARD
a
HARD
SOFT
b
c
d
THICKNESS
OF COATING
PLOUGHING
e
LOAD CARRIED BY
COATING STRENGTH
SHEARING
f
g
SUBSTRATE
DEFORMATION
h
SURFACE
ROUGHNESS
i
PENETRATION
j
REDUCED CONTACT
AREA & INTERLOCKING
k
ASPERITY FATIGUE
l
DEBRIS
PARTICLE
EMBEDDING
PARTICLE
PLOUGHING
PARTICLE
HIDING
PARTICLE CRUSHING
Courtesy of K. Holmberg/VTT-Finland
ETM - - KGH\TCB\FRICTM97.dsf.
SCRATCHING
lon9708
MAJOR SOURCES OF FRICTION
Physisorption/chemisorption
Roughness
H2O
OH
Major Causes of
Friction
O
Capillary
Forces
Adhesion
Elastic/plastic
Deformation
Real Contact Areas
Deformation
Adhesion Mechanisms of Friction
The Case of Carbon Films
- Covalent sigma (the strongest)
- Ionic
- Metallic
Not applicable to carbon
- Magnetic
N
-π-π* Attraction (in
F
the case of graphite)
- van der Waals
-Electrostatic
-Capillary
A2
A
van der Waals
Capillary
1
Ar = A1 + A2 + . . .
Ff = σ.Ar
Electrostatic
Transfer Films vs Friction
• Transfer formation : run-in phenomena + COF fluctuations
• Transfer film (0.01 - 50 µm) Î “Repartition” of the lubricant reservoir
• Interfilm sliding : general condition of steady-state
• Wear not linear versus duration
Accommodation modes
Transfer formation Î Interfilm sliding
Donnet 01
Singer 92
PTFE & Polyimide
TiN, CrN, (Ti,Al)N
MoS2
DLC
Yamada 90
Huang 94, Wilson 98
Fayeulle 90, Wahl 95
Ronkainen 93, Donnet 95, Grill 97
Effect of Transfer Film Forming
Tendency on Friction
DLC-coated Steel Disk Against Various Counterface Balls
0.25
Zirconia
Steel
Sapphire
DLC-Coated Steel
Friction Coefficient
Dry N2
0.2
0.15
Transfer
Film
Zirconia
0.1
0.05
0
Uncoated
Steel
Ball
Sapphire
DLC Coated
Steel Ball
0
100
200
300
400
Distance (m)
500
600
Coated
Steel
Ball
Tribochemistry vs Friction
Friction-induced “fresh” surfaces
Temperature increase
Effect of the surrounding environment
Tribo-reactions
at the nm scale
• 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,
Role of H2O on B2O3
Role of gaseous H2 on a-C:H films
1
1
10 hPa H2
0.1
µ=0.003
0.01
0.001
Formation of lamellar boric acid
Erdemir 90-98
(H=34at%)
0.1
100 200 300 400
Number of cycles
500
UHV
or Ar
0.01
0.001
0
µ=0.7
µ=0.007
0
Donnet 01
100 200 300 400
Number of cycles
500
Tribochemical film Formation in Lubricated Contacts
300 µm
300 µm
Steel/DLC
EP
Stee/DLC
EP
S
W
30 µm
Fe
Fe
S
C
W
W
After 8000 cycles
C
W
30 µm
O
O
C
Sture Hogmark
at 700 N
Ni
O
W
Roughness vs Friction
F1 = W1 tan θ
W = W1 + W2 + . . .
F = W tanθ
Tribology of Diamond Films
Roughness Effect
Erdemir, et al., Surface and Coatings Technology,
121(1999) 565-572
Roughness Effect on Friction
Diamond Films
MCD
Rough
Polished
MCD
NCD
B. K. Gupta et al., J. Tribol., 116(1994)445.
Environment vs Friction
Physisorption/chemisorption
O
OH
H2O
0.8
Due to higher degree of
covalent bond interactions
Friction coefficient
0.7
0.6
Diamond Coated Ball
0.5
In water
In air
In argon
0.4
0.3
Initial friction is 0.1-0.2
0.2
0.1
0
Diamond Coated Disk
0
50
100
150
# Revolutions
200
Courtesy of J. Andersson
Friction coefficient
Effect of Water Partial Pressure on
Frictional Behavior of DLC Film
0.14
At 2000 Pa
0.12
At 460 Pa
0.1
At 0.4 Pa
Vacuum Experiments
?
Smoother and
lower friction
at lower water
vapor pressures
0.08
0.06
0.04
0.02
0
0
20
40
60
80
100
120
Time (s)
J. Anderson and R. Erck/ANL
Environmental Sensitivity of MoS2 Type Solid Lubricant Coating
Work the best in dry, inert, or vacuum type environments
Base MoS2
Ti-Doped
Multiarc, Inc.
data
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.
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.
Most desired case
After Bowden and Tabor
Thickness of the film is very important
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.