Life prediction for surface crack initiated contact fatigue of silicon nitride bearing balls

Technical ceramics as materials for rolling contact bearing components show some practical advantages over traditional bearing steels. The properties of ceramics, specifically low density and high stiffness, are of most interest to gas turbine and machine tool manufactures. High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Silicon nitride has been found to have the optimum combination of properties suitable for this application. Much research over the past two decades on its structure, quality control and manufacturing techniques has produced material which can seriously be considered for rolling contact bearing design. However, the difficulties of both sintering and machining the material may result in surface crack defects. It is difficult to detect surface cracks during high volume production processes and, hence there is an important need to understand their influence and fundamental mechanism of the failures they cause. In the present study, a quantitative three-dimensional boundary element model has been developed which can be used to determine the geometry of acceptable surface defects and to predict rolling contact fatigue life. The concentrated contact analysis focuses on a hybrid ceramic/steel combination. A modified four-ball machine is employed to validate the predictions. Results from the experimental test are in good agreement with the results from the numerical analysis.