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Friction
Adnan Akay
NSF
Carnegie Mellon University
Friction: Dual role
• Friction is a force that resist motion
• Friction provides dissipation
V
Friction
Force
Energy Dissipation
Friction: Dissipation of Energy
Ordered state of energy
KINETIC ENERGY
Vibration
Excitation
Vibration
Sound
Plastic
Deformation
Anelastic
Deformation
Heat
Deformation
Energy
THERMAL ENERGY
Disordered state of energy
Akay, 1999
Viscous
Dissipation
Adhesion
Fracture
Surface
Energy
Chemical
Reactions
Tribo-Chemical
Energy
POTENTIAL ENERGY
Photoemission
TriboEmissions
Mechanisms Contributing Friction Force
Elastic Contact of
Asperities
Deformation Force
f = Ka3/2 + ηε
Adhesive Force
r = τ a + βf
Adhesion of Asperities
F = (C1 + C2 P) A
Fracture of Asperities
V
V
V
V
V
P
F
F
P
Plastic Contact of
Asperities
Stress P = k
1 + π + 2 γ - 2 α + sin2
2
F = k cos2
γ
Elasto-Viscous Surface
Interaction
γ
Vs
V
γ
α
F
P
Green, 1954
Protasov & Kragel’skii, 1981
Hierarchy of Friction Scales
Geometry
Dynamics
Flatness
Roughness
Material properties
Chemistry
Surface Physics
Akay, 1998
Engineering
Scale
0.1 - 1.0 m
0.001 -1 s
Continuum
Mechanics
10-6 - 10-3 m
10-4 - 100 s
Microstructural Models
100 A - 1mm
1ms - 1ms
Flatness
Parallelism
Thickness
Dynamic geometry
Hot spots, wear
Heat generation
Wear
Elastic deformation
Plastic deformation
Dislocations
Slip lines
Molecular/Quantum Scale
Dissipation
10 A - 1000 A / 1 A - 10 A
1 ps - 1ms / 1 fs - 1 ps
Dynamometer
Force
hysteresis
measurements
Surface force
apparatus
Friction force
microscopy
Quartz crystal
microbalance
Atomic force
microscopy