Damage processes in Si3N4 bearing material under contact loading

High quality silicon nitride ceramics have excellent high temperature strength, low density and excellent wear resistance due to their low friction. The combination of these properties makes silicon nitride an attractive material for rolling element bearing applications. The low density of the silicon nitride material has particular advantages when applied to the rolling elements of high-speed air turbine bearings. Much work has been done to improve Si3N4 mechanical properties. However, wear in Si3N4 bearing balls still is a major problem that limits the lifetime of hybrid Si3N4 bearings. The wear mechanisms of Si3N4 bearing balls in hybrid bearings have not been substantially characterized even though many experimental observations have been recorded. This paper describes Si3N4 bearing ball wear as a microcrack initiation and propagation process due to fatigue contact stresses. Experimental results agree with a proposed fracture model to describe the major cause of silicon nitride wear and the major factors affecting the lifetime of bearing balls.