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

(1)在氮化硅中裂纹生长期间，在弱晶界上发生微裂纹，随后发生桥接。与静态疲劳相比，循环疲劳下裂纹扩展速率的加快是由于桥梁在循环变形下断裂和移除的结果。(2)氮化硅中的裂纹扩展行为强烈地受到由于桥的形成而导致的裂纹扩展历史的影响。因此，裂纹扩展速率不能由所应用的K值唯一地给出。(3)裂纹尖端应力强度因子K-tip是裂纹尖端实际应力场对桥连应力屏蔽效应的反映，通过对实测裂纹尖端张开位移进行K折减，可以得到裂纹尖端应力强度因子K-tip。K-尖端和裂纹扩展速率之间的独特的关系被发现独立的裂纹扩展的历史。(4)当温度超过1200 ℃时，在裂纹尖端的高应变区观察到氮化硅晶粒在拉伸方向上的取向，这对应于晶界玻璃相的软化点。断裂机制由脆性沿晶断裂向韧性拔出断裂转变，并伴有边界玻璃相的剪切变形。

(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.