Tribology of Silicon Based Nitride Ceramics

As members of the Tribological Materials Group of the Joint Consortium on Synergy Ceramics until 2004, the authors have been involved in research in to the mechanical and tribological properties of silicon based nitride ceramics (silicon nitride and sialons) for a number of years. In this article we review the research carried out at AIST in to the wear and friction properties of these materials, focusing on the effects of rare earth additives, solid lubricant incorporation and friction under both dry and wet conditions. Both microstructure and additive composition have an effect on the tribological response of these materials, with the development of a fine elongated microstructure leading to improved wear resistance under severe loading conditions. The use of rare earth oxides with smaller cationic radius lead to a more refractory grain boundary phase and this provides improved wear resistance under tribochemical dominated wear as well as delaying the transition to microfracture dominated wear. Small additions of graphitic carbon fibers as a solid lubricant had a significant effect on the friction behaviour of these materials under both dry and lubricated sliding. Under dry sliding, the addition of even a small amount of fibers is sufficient to reduce the friction coefficient by over 75% due to the formation of a graphitic transfer layer on the counterbody. Under water lubrication these fibers cause a delay in the transiton to the boundary lubrication regime, reduce the magnitude of friction in this regime and allow for shorter running in periods. In a non self-mated thrust bearing application against steel, the carbon fiber additions help to maintain low friction during the instantaneous increases in load and prevent transfer of metal to the ceramic surface, resulting in superior performance when compared with a monolithic silicon nitride.