Wear Mechanisms

Abstract

Wear is a damage of a surface in contact with another one, which results in the formation of fragments (or debris) that leave the tribological system. Wear may cause direct failure, may reduce tolerances and surface finish, or induce a surface damage that is responsible for the subsequent failure of the component (most often by fatigue).The characteristics of the relative motion between two bodies in contact define the wear processes. Some examples are schematically shown in Fig. 4.1. If the bodies' slide one over the other, the resulting wear process is sliding wear. If they roll one over the other, the resulting wear process is rolling wear. A rolling-sliding wear is obtained if the two types of motion are superimposed. When a reciprocating sliding is present with very small displacement, the resulting wear process is called fretting. When one of the two bodies consists in one or more hard particles that abrade a softer surface, wear is called abrasion by hard, granular material. If a fluid carries such abrading particles, wear is called erosion.Despite the high number of wear processes encountered in practice, the investigation of wear damage is facilitated by the observation that each wear process is determined by the action of a predominant wear mechanisms, and the wear mechanisms are only four [1, 2]:(1) adhesive wear; (2) tribo-oxidative wear; (3) abrasive wear; (4) wear by contact fatigue.An understanding of the four wear mechanisms is crucial to properly control every wear process. This control can be done in the designing stage, when it is possible to recognize in advance the acting wear mechanism. This control can be also done subsequently, when there is the need to re-design a tribological system after a wear induced failure. To achieve this task, a proper failure analysis is required and this can be carried out only if the main wear mechanisms are correctly understood. In this chapter the four wear mechanisms will be described, while in the next chapter the salient features of the main wear processes will be outlined.