CYCLIC FATIGUE IN SILICON-NITRIDE CERAMICS

Static and cyclic fatigue behavior was investigated for two kinds of silicon nitride with different microstructures. These materials hardly show rising R-curve behavior. Nevertheless the considerable cyclic loading effect is obviously recognized in them. There has been observed an insignificant difference in the pure cyclic loading effect between coarse grained silicon nitride (SSN-1) and fine grained one (SSN-2). In addition, crack growth rate shows a transient acceleration or retardation when variable-amplitude cyclic loading is applied to SSN-1. Such a result implies the existence of a process zone near a crack tip which has a similar function to plastic zone in metals. Since the above cyclic crack behavior is inexplicable by the grain bridging degradation mechanism, possible cyclic fatigue mechanisms are, thus, discussed.