ME 383S Bryant February 10, 2005

Transcription

1 WEAR 1 Damage to a surface from relative motion Loss of material from surface during sliding Movement of material (plastic deformation) Cracking of surfaces Applications: reduce: brakes, clutches increase: chalk on blackboard, cutting, grinding Wear & Friction related. Changing one alters the other.

2 CONTROLLING FRICTION & WEAR 2 Lubrication: Lubricant: liquid, solid, gas Pressures/forces separate surfaces Reduced surface contact reduces friction & wear Lubricant sacrificed Mechanically compliant Moderate friction: µ 10-2 to 10-1 Surface Coatings: Coating protects substrate Hard or soft Example: tungsten carbide coating on saw blade Surface vibrations: Vibrations alter geometric clearance & alignments Interfacial events disrupted Interfacial physics changed

3 WEAR 3 Loss of material from surface during sliding Complicated: many mechanisms operating at same time Adhesive: mild Abrasive: moderate to severe Corrosive Delamination/peeling Pitting Thermal effects Thermomechanical effects: severe Measuring wear weight loss volume reduction surface recession: constant cross sectional area radioactive implants collect/sample wear debris

4 Surface Damage 4 Surfaces of machine parts damaged Part malfunctions slowdown machine inaccuracies further damage part/ machine replacement shutdown Types (cost in 1978) corrosion (70 billion) wear (20 billion) surface fatigue thermal fatigue arcing (~ 5 billion) ~ $100 billion 5% GNP COSTLY!

5 WEAR 5 Relationships to other variables Geometry Force Sliding speed Distance slid Power dissipated Material combinations Environment Sensitive to environment temperature humidity vibrations chemistry: atmosphere, contaminants, surface deposits Same test, but different location and/or time Variations of 2 to 10 common

6 WEAR MECHANISMS 6 Total wear sum over competing mechanisms W = Σ W i all mechanisms W k proper conditions (Dominates)

7 ADHESIVE WEAR 7 slider countersurface 10 1 µm asperities interfere asperity breaks wear particle generated fundamental (always present), but low speed mechanism sliding asperities approach each other asperities interefere: bond/weld further sliding breaks 10 to 100 µm wear particle Example:

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9 ARCHARD/HOLM WEAR LAW 8 asperities interfere contact area δa δp δp δa asperity breaks wear particle generated hemispherical models asperities contact, maximum area of contact δa = δp/h wear particle detaches, volume δv (δa)3/2 sliding distance between interactions δl (δa)1/2

10 9 assemble: δv δl = K δa = K δp H sum over all asperities between sliders: δp P sum over sliding path:δl L V = K H P L V: total volume of material lost from slider [m3] P: total normal force [N] H: material hardness [N/m2] L: total distance slid [m] K: wear coefficient

11 10 VALUES OF WEAR COEFFICIENT K Counter Slider H Friction 10 8 N/m 2 surface Coeff mild steel 18 7 x 10-3 mild steel /40 leaded brass x 10-4 tool steel 0.24 stainless steel x 10-5 tool steel 0.53 tungsten carbide x 10-6 tungsten 0.35 carbide polyethylene x 10-7 tool steel 0.53 electrographite 1 to 2 8 x 10-8 copper 0.2 Environmental conditions: dry/air

12 Example Aluminum impregnated graphite block (area A o = 1 cm 2, length l = 10 cm, hardness H = 100 MPa) 11 slides against steel (H 3000 MPa). Surface speed U = 1 m/s, normal force F = 100 N. Estimate total wear over 1 hour. Solution H graphite < H steel graphite wears Total wear volume V = W V t = K G/St F U t H g A o = (0.26 x Pa -1 ) 100N (1 m/s) (3600s) 10-4 m 2 = m = 9.36 cm Block almost gone!

13 WEAR LAW/OTHER FORMS 12 Volumetric V = Kv P L Kv = K H Surface recession δ = V/A = Kδ P L Kδ = Kv /A A: cross sectional area of slider [m2] Volumetric wear rate V. = Kv P v v: relative surface sliding speed [m/s] Mass wear rate WR = Km P v Km = ρ Kv ρ: mass density of slider [g/m3]

14 ABRASIVE WEAR 13 hard ab rasive partic le, trapped b etween surfaces plowing of surfac e wear particle generated Hard abrasive particles from environment generated by wear processes become entrapped within sliding interface Sliding particles become plows surfaces gouged/scratched large wear particles High wear coefficient K abrasive 10 to 100 K adhesive

15 OTHER WEAR MECHANISMS 14 Corrosive wear Adhesive or abrasive wear, but with corrosion affecting wear surface. K can be higher or lower. Delamination/peeling Stress from repeated application of load causes subsurface cracking parallel to the contact surface. Strips deliminate and peel off. Pitting Stress from repeated application of load causes subsurface cracking at angle ( 20 ) to contact surface. Cracks eventually meet surface, resulting in wear particle and "pit".

16 THERMAL EFFECTS ON WEAR 15 Heat generated by friction Q = µ P v [W] µ: friction coefficient P: total normal force [N] v: relative surface sliding speed [m/s] Material properties altered metals softened: yield strength & hardness reduced carbon mixtures hardened thermal & electrical properties altered thermal, electrical conductivities reduced heat capacity Concentrated contact (hot spots, thermal mounds) much higher wear

17 CONCENTRATED CONTACT 16 slider countersurface contact between rough surfaces discrete spots (islands) of contact between surfaces slider e xpa nded expa nded disc onn ected countersurface sliding higher sliding speeds friction contact spots spots expand & grow some spots separate loads transferred to still-connected spots

18 17 slider expanded expanded disconnected disconnected countersurface loads transferred to still-connected spots more intense conditions process continues until slider runs on few spots

19 References Peter J. Blau, Friction Science and Technology, Marcel Dekker, Inc., N.K Myshkin, Chung Kyun Kim, Mark I Petrokovets, Introduction to Tribology, Cheong Moon Gak, Raymond C. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, Inc., Ragnar Holm, Electric Contacts: Theory and Application, 4th edition, Springer Verlag, M.B. Peterson & W.O. Winer, Wear Control Handbook, ASME, 1980.