FAILURE MODES OF CERAMICS IN ROLLING-CONTACT

A comprehensive experimental study has been carried out to ascertain t he prevailing mode of failure of concentrated rolling ceramic contacts subjected to cyclic loading conditions. The findings of the studies p resented 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 roll ing contact bearings for gas turbines. The results of extensive testin g 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 enabl ed the delamination failures to be classified in terms of location of crack initiation and subsequent crack propagation. By using X-ray diff raction, residual stresses on delaminated surfaces were measured and r esults 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, nitrog en and oxygen may take place on delaminated surfaces. Experiments with artificially pre-cracked balls revealed the role played by a lubricat ing medium in the process of fatigue failure. All findings are illustr ated with SEM micrographs and the physical aspects of failure mechanic s are discussed.