Chemical aspects of ceramic tribology

Chemical interactions with ambient gases and with lubricant liquids have been found to influence friction forces and wear rates of ceramics and to determine the mechanisms by which these materials wear. In the presence of water vapor, for example, tribochemical reactions (which are accelerated by simultaneous friction) produce smooth surfaces and decrease wear by several orders of magnitude in silicon nitride and chemisorption embrittlement increases wear rates of zirconia. Hydrocarbon lubricants produce similar effects, decreasing wear by surface reaction in some cases and increasing it by grain boundary attack in others. These interactions can be understood in terms of the surface chemistry of ceramics, which is dominated by charge transfer with ambients through acid-base reactions. The surface chemistry of ceramics is reviewed and related to their electronic structure. Tribochemical maps are proposed for the interaction with water and selected hydrocarbons by comparing the electronic structure of the ceramics and fluids. The ability to generalize the behavior of lubricants on ceramics by the use of these maps indicates an improved understanding of the lubricant-surface interaction and provides a tool for the development of new lubricant systems.