FAILURE MODES OF CERAMICS IN ROLLING-CONTACT

A comprehensive experimental study has been carried out to ascertain the prevailing mode of failure of concentrated rolling ceramic contacts subjected to cyclic loading conditions. The findings of the studies presented in this paper confirm that good quality silicon nitride balls in non-conforming contact do not fail in a catastrophic mode and thus can be considered for important engineering applications such as rolling contact bearings for gas turbines. The results of extensive testing carried out on perfect and artificially pre-cracked balls under high compressive cyclic stress showed that the prevailing mode of failure is delamination. Scanning electron microscope (SEM) observations enabled the delamination failures to be classified in terms of location of crack initiation and subsequent crack propagation. By using X-ray diffraction, residual stresses on delaminated surfaces were measured and results showed that under certain conditions plastic deformation of the ceramic can take place. Results of supplementary chemical analysis of failure surfaces suggest that changes in the level of silicon, nitrogen and oxygen may take place on delaminated surfaces. Experiments with artificially pre-cracked balls revealed the role played by a lubricating medium in the process of fatigue failure. All findings are illustrated with SEM micrographs and the physical aspects of failure mechanics are discussed.