A FRICTIONAL-WEAR MECHANISM FOR FATIGUE-CRACK GROWTH IN GRAIN BRIDGING CERAMICS

A frictional-wear mechanism to account for cyclic fatigue degradation in polycrystalline ceramics exhibiting grain bridging as the principal toughening mechanism is contemplated. The mechanism is based on repetitive sliding wear degradation of frictional grain bridges which reduces the toughening capacity from bridging under cyclic loads. A micromechanical model is developed to relate the reduction in the frictional pullout stress of bridging grains to material removed by wear processes at the grain/matrix interface. The model provides an insight into the effects of salient mechanical and microstructural variables such as wear rate at the grain/matrix interface, initial residual stress of bridging grains, and the grain size, on fatigue-crack growth behavior. Results of the analysis are compared to experimentally observed fatigue-crack growth rates in coarse grained alumina and silicon nitride and implications of the predicted crack-growth behavior discussed.