9 - Surface modification for wear resistance

Abstract

Wear is damage involving progressive loss of material because of the relative motion between a surface and a contacting substance. There are four tribological systems : smooth solids sliding on smooth solids; hard, sharp substances sliding on softer surfaces; surface fatigue caused by repeated stressing; and relative motion between fluids, possibly with suspended solids and a solid surface. The techniques to reduce the wear rate caused by these tribological systems can be briefly summarized as separating the contacting surfaces with a lubricant film, choosing material combinations that are resistant to sliding interaction, hardening the wearing surface, improving the resistance to fracture of a wearing surface, improving the corrosion resistance of an eroding surface, and improving the fatigue resistance of a wearing surface. One of the techniques widely used to achieve this objective is the application of heat treatments to modify the microstructure and consequently properties. Virtually all alloys are affected by heat treatment to some degree. Most heat treatments change the properties of the workpiece throughout. However, it is sometimes desirable to modify only the surface of the workpiece to improve wear resistance. Often an advantage of modifying only the surface is that a tough, fracture-resistant microstructure can be retained in the bulk of the workpiece, while a hard wear resistant surface is obtained.