Wear is the damage to a solid surface, generally involving progressive loss of material, that is due to relative motion between that surface and a contacting substance or substances. This definition would not necessarily exclude scuffing or galling , in which surface material may be displaced but not removed. Sometimes surface damage , which may not involve material removal, is differentiated from wear.
Sliding and Adhesive Wear:
Sliding and adhesive wear refer to a type of wear generated by the sliding of one solid surface along another surface. Erosion, cavitation, rolling contact, abrasion, oxidative wear, fretting, and corrosion are traditionally excluded from the class of “sliding” wear problems even though some sliding (slip) may occur in some of these types of wear. Apparently, sliding wear is a category of wear that is “left over” when all other types of wear are identified under separate headings.
Adhesive wear is as ambiguously defined as is sliding wear, though the two are not strictly synonymous. Adhesive wear take place by transference of material from one surface to another during relative motion due to a process of solid-phase welding. Note: Particles that are removed from one surface are either permanently or temporarily attached to the other surface.
Sliding Surface:
The focus of this section is the case of long-distance sliding of nominally clean and dry surfaces in the general atmosphere. Such surfaces are far from perfectly clean, and even if they were at one time very clean, inevitably they become “contaminated” by the atmosphere around them.
The standard conditions for sliding metals will usually be covered with a film of oxide, which is covered by a second film of adsorbed gases and hydrocarbons (oils). The thickness of the adsorbed layers is of the order of 10 nm. These films are very important because they intervene in the bonding or adhesion between the substrate materials.
Adhesive Wear Mechanisms:
When two such surfaces slide against each other:
There will be some squeezing out of the adsorbed films, producing some contact between solid (oxide, sulfide, and so on) layers.
If the contact pressure is high, a solid layer may fracture and expose substrate material, particularly if the substrate beneath the solid layer deforms plastically.
Small regions of both substrate materials may finally come into contact and bond together.
Material-Dependent Bond Strength:
At the small points of substrate-to-substrate contact, the bond strength will depend on the materials. Identical metals (for example, iron against iron) bond together most completely when the bond strength is very near the strength of the metals themselves. Dissimilar metals will usually bond with less strength in the first instant, but these “bonds” increase in strength with time, particularly between metal pairs that will mix or alloy via diffusion.
Conformity of Contacting Surfaces:
A second consideration in bonding is the conformity of the contacting surfaces. Ductile materials will conform somewhat via plastic flow, but the brittle materials will not conform.
Sliding adds shear stress to any normal stresses already imposed, increasing the probability of plastic flow and fracture of materials in the contact region. After some sliding, there will be a layer of turbulently mixed substances:
Some of which will fall out as wear debris.
Most of which will remain as a “transfer film.”
The wear rate will depend on:
The properties of all of the substances.
The properties of the mixed composite material.
The tendency for bits of that turbulently mixed material to leave the system (depend on the shape and vibration modes of the contacting bodies, the temperature, possible chemical reactivity with the surrounding environment).
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