Effects and Classification of Friction Wear

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Effects and Classification of Friction Wear

The effects of wear are commonly detected by visual inspection of surfaces. Surface damage can be classified as follows:

(a) Surface damage without exchange of material:

• Structural changes: aging, tempering, phase transformations, and recrystallization.

• Plastic deformation: residual deformation of the surface layer.

• Surface cracking: fractures caused by excessive contact strains or cyclic variations of thermally or mechanically induced strains.

(b) Surface damage with loss of material (wear):

• Characterized by wear scars of various shapes and sizes.

• Can be shear fracture, extrusion, chip formation, tearing, brittle fracture, fatigue fracture, chemical dissolution, and diffusion.

(c) Surface damage with gain of material:

• Can include pickup of loose particles and transfer of material from the opposing surface.

• Corrosion: Material degradation by chemical reactions with ambient elements or elements from the opposing surface.

Wear may also be classified as mild or severe. The distinguishing characteristics between mild and severe wear are as follows (Williams 1994):

(a) Mild

• Produces extremely smooth surfaces - sometimes smoother than the original.

• Debris is extremely small, typically in the range of 100 nanometers (nm) (3.28 10 ft) -13 in diameter.

• High electrical contact resistance, but little true metallic contact.

(b) Severe

• Rough, deeply torn surfaces - much rougher than the original.

• Large metallic wear debris, typically up to 0.01 mm (3.28 10 ft) in diameter. -5

• Low contact resistance, but true metallic junctions are formed.