Mechanisms of wear in structural ceramic materials.

Abstract

Engineering ceramic materials were designed for structural applications under severe conditions of high temperature and chemically aggressive environments. They are, however, good prospects for tribological applications as well. In spite of the great amount of work being done in evaluating the friction and wear behavior of structural ceramics, no satisfactory guidelines exist for the selection of these materials for wear applications. In order to formulate a good predictive wear equation for these materials, the fundamental mechanism of material removal during wear must be adequately understood. In the present work, the wear mechanisms of four commercial structural ceramics (Si$\\sb3$N$\\sb4$,SiC,Al$\\sb2$O$\\sb3$,ZrO$\\sb2$) were investigated under different contact conditions spanning static indentation by a sharp indenter to smooth sliding of a ceramic cylinder on a ceramic flat. Material removal during the wear of ceramics occurs by fracture and fragmentation at the contact interface rather than by the widely assumed subsurface crack linkage. The contact interface damage is different for each of the materials studied. For SiC, wear occurs by grain boundary failure during both static and sliding contact. The other three materials showed some plastic deformation and some cracking at the contact interface during static contact, and wear in sliding occurs by surface fatigue. Some of the generated debris reattaches to the surface to form a transfer film. Thus the wear rate = loosening rate $-$ reattachment rate. The wear rate in ceramic materials is also influenced by the process damage incurred by the material during surface preparation and by the presence of a liquid environment. The processing damage effects the loosening rate, whereas the environment may influence the reattachment rate. The latter is to be distinguished for tribochemical processes whereby wear occurs by chemical reaction at the contact interface.