Failure modes of ceramic rolling elements with surface crack defects

The properties of ceramics, specifically low density and high stiffness are of most interest to gas turbine and machine tool manufacturers. High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Much research over the past two decades on its structure, quality control and manufacturing techniques has produced ceramic materials which can seriously be considered for rolling contact bearing design. However, the difficulties of both sintering and machining the material may result in surface cracks. It is difficult to detect such 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, the mechanisms of fatigue failure from surface cracks subjected to rolling contact have been studied experimentally and numerically. A three-dimensional boundary element model has been developed to study the failure mechanisms in rolling contact. The calculated results show that surface crack initiated fatigue failure involves fatigue crack propagation from original surface cracks and secondary surface crack formation when the crack reaches a critical condition. The secondary surface cracks play a dominant role in the formation of spalling fatigue failure. A comprehensive experimental study has been carried out to verify the numerical prediction. A modified four-ball machine is employed to perform rolling contact fatigue tests. Results from the experimental test are in good agreement with the results from the numerical analysis.