High Temperature Fatigue Crack Growth in Silicon Nitride and Role of SCC.

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(1) During crack growth in silicon nitride, microcracking on weak grain boundaries and subsequently bridging occur. The acceleration of crack growth rate in cyclic fatigue compared to the case in static fatigue will result from that the bridges are fractured and removed by cyclic deformation.(2) The crack growth behavior in silicon nitride is strongly affected by the history of crack growth due to formation of the bridges. So, the crack growth rate can not be given uniquely by the applied K-value.(3) The crack tip stress intensity factor K-tip, which is given the actual stress field at the crack tip country on the effect of stress shield by the bridging, can be estimated by reducing the K at crack tip from the measured crack mouth opening displacement. The unique relationship between K-tip and crack growth rate is found independent of the history of crack growth.(4) The orientation of silicon nitride grain in the tensile direction in the high strain region at the crack tip was observed at the temperatures over 1200ﾟC, which corresponds to the softening point of the grain boundary glass phase. The transition of fracture mechanism was observed from brittle intergranular to ductile pulling out of grain with shear deformation of boundary glass phase.