Tribochemistry of Selected Ceramic Materials

Abstract

Ceramics, especially nitrides, oxides and carbides, offer advantages over conventional tribological mating elements. Such solids usually are very stable chemically. However, under friction conditions they become quite reactive. It is even possible to say that the friction and wear behavior of ceramics might be more sensitive to the environment than friction and wear behavior of metals. Considering the present trend towards the incorporation of these ceramics in tribological applications, the importance of a more comprehensive understanding of the ceramic surface physics and chemistry cannot be overestimated. The primary aim of this paper is to present some physicochemical factors that may influence the wear process of ceramics and account for some specific tribological findings related to lubrication of these materials from the view-point of a better understanding of their tribochemistry. Model of ceramic reaction with water, implying the effect of triboelectron emission process, leads to the generation of acidic and basic active sites. The model is mostly focused on silicon nitride tribochemistry. It accounts well for the formation of specific silicon compounds from silicon nitride lubricated with water and alcohols. The model is based on the emission process of low-energy electrons during friction with generation of positively charged sites (Si+) and free radicals (N●). Interaction of the emitted low-energy electrons with water or alcohol molecules (HOH, ROH) produces negative HO‾ or RO‾ ions, respectively and hydrogen radicals (H●). The described model provides a better understanding of silicon nitride tribochemistry than previous ones. The same is due to other ceramics.