FRICTION. Friction: FRICARE = to rub (Latin)

Transcription

1 FRICTION 1 Friction: FRICARE = to rub (Latin) Resisting force (F) tangential to the interface between two bodies when, under the action of an external force, one body moves or tends to move relative to the other * Coefficient of friction: Ratio of the force (F) resisting tangential motion between two bodies, to the normal force (N) pressing those bodies together * µ = F N * Peter J. Blau, Friction Science and Technology, Marcel Dekker, Inc., 1996

2 CHARACTERISTICS OF DRY FRICTION 2 Kinetic friction generally nonconservative (power loss) Independent of APPARENT contact area Dependent on REAL contact area Dependent on normal force N Strongly dependent on surface geometry, material & environment µ s µ µ k v o = µ s + (µ k - µ s ) (1-exp{- v/v o } ) v Friction coefficient µ weakly dependent on speed v exponential model µ

3 Some causes of friction force 3 surface deformations surface rupture surface welding & rupture surface plowing Effects of friction power loss heating high temperatures material changes stresses cracking fatigue Harshness, noise and vibration slip-stick instabilities brake squeal, groan & judder Friction & Wear interrelated often resulting from same physical events

4 Friction & Phenomena Caused by Interfacial Conditions between Bodies 4 Materials of slider (1 st body) & counter-surface (2 nd body) Deformations of slider & counter-surface Deposited films Lubricants: solid, liquid & gas Presence of Third bodies: accumulated material, separate from 1 st & 2 nd bodies O Films on surfaces O Transferred material from one body to another O Entrapped wear debris O Deposits Environment O Thermodynamic states: temperature, pressure, etc. O Chemistry SHOW NRL

5 APPLICATIONS OF FRICTION IN DESIGN 5 Reduce (eliminate): electrical brushes, rolling or sliding bearings damage power loss friction force Increase (make large): brakes, clutches, traction drives friction force power removal

6 FRICTION MODELS 6 Friction force from multiple effects: F = F i effects Friction coefficient: µ = F N = F i N = µ i effects effects Adhesion coefficient of friction: µ a Plowing coefficient of friction: µ p Composite coefficient of friction: µ = µ a + µ p Energetics model of friction Frictional heating model

7 ADHESION FRICTION COMPONENT 7 N F F N Sliding: asperities contact & plastically deform Contact area δa N Η Shear failure of asperities Shear force F S δa S: material strength for shear failure Coefficient of friction, adhesion µ a = F N = S H 0. 2, most metals

8 PLOWING FRICTION COMPONENT 8 Asperities of harder surface plow softer surface Rigid cone plows groove into rigid plastic body Compressive strength Y rigid cone N r θ Normal force: N = Y (π r 2 ) F plowed groove r tan θ unplowed material Cross sectional area, triangular groove A p = 1 2 (2r) r tan θ Plowing force: F = Y A p = Y r 2 tan θ

9 Coefficient of friction, plowing: 9 µ p = F N = tan θ π Estimate: asperity slopes: θ 10, µ p 0.056

10 ENERGETICS & DISSIPATION 10 sliding direction N F slider metal matrix h SDR region of plastic deformations countersurface w sliding track width Metals Tsuya: copper on steel Rigney & Hirth: Severely Deformed Region (SDR) thin surface "skin": h 10 to 50 µm very intense deformations strain hardened very intense heating "surface" heating Plastically deformed region

11 Power loss mainly in SDR, via plastic work 11 W = F s E w h s E: average deformation energy per volume s: distance slid h: thickness of SDR w: width of sliding track E τ ε τ,ε: average shear stress & strain in SDR µ = F N wh( τε) N = µ (load, geometry, material parameters)

12 FRICTIONAL DISSIPATION 12 Power dissipated in SDR: P F v Concentrated heat sources in SDR: Q = power volume = P V SDR F v h Ar V SDR : volume of Severely Deformed Region F: friction force v: sliding speed h: thickness of SDR Ar: real contact area Order estimate F = 50 N, v = 1 m/s, h 50 µm, Ar 10% Aa, Aa 1 cm 2 Q W/m W/mm 3 Equivalent to light bulb heat concentrated within 1 mm 3 VERY high temperatures (1000's C ) at surface! Often treated as surface heat source (h 10 to 50 µm)

13 OTHER FRICTION EFFECTS 13 Tribo-electricity rubbing energy excites surface electrons higher energy state electric potential electric field On insulator, electrons immobile static charge (rub balloon against hairs, static charge sticks balloon to wall) large voltages (many kilovolts) possible On sled rails against ice, Soviets investigated as possible winter weapon

14 ME 383S Bryant January 24, Tribo-plasma around contact high tribo-induced electric field ionizes air molecules (N2, He2) plasma glow discharge in ultra-violet range