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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.
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