Wear is progressive material loss or surface damage caused by relative motion and mechanical interaction. It is not one mechanism.

Common modes include:

• adhesive wear: microscopic junctions form between contacting asperities and fragments are transferred or torn away during sliding;
• abrasive wear: a harder asperity or particle ploughs or cuts a softer surface;
• surface-fatigue wear: repeated contact stresses initiate cracks that eventually detach material;
• erosive wear: impacting particles or droplets remove material;
• tribochemical wear: sliding and environmental reactions create or remove surface films.

A simple empirical model for sliding wear is Archard's relation

$$V=k\frac{Ws}{H},$$

where $V$ is worn volume, $W$ normal load, $s$ sliding distance, $H$ hardness and $k$ a dimensionless wear coefficient representing the contact system.

The equation captures useful trends but $k$ can change by orders of magnitude with lubrication, roughness, counterface, debris and wear mechanism.

Wear resistance is therefore a system property, not a single intrinsic number. Material choice, hardness, toughness, coatings, lubrication, contact geometry and environment must be considered together.