予き裂を導入したセラミックスの常温における静疲労と繰返し疲労

抄録

The static and cyclic fatigue behaviors of alumina, silicon carbide, silicon nitride and partially stabilized zirconia were investigated at room temperature. Flexural specimens, with an indentation induced flaw at the center of each specimen, were tested under cyclic and static loads applied by four-point bending. All ceramics showed static and cyclic fatigue susceptibility. The fatigue degradation compared to its flexural strength increased in this order; silicon carbide, silicon nitride, alumina and partially stabilized zirconia. The cyclic fatigue strength of zirconia became as small as one-third of its flexural strength. Moreover, the fracture mode for the static and cyclic fatigue in zirconia was the mixed type of transgranular and intergranular cracking, although the intergranular cracking was dominant in the fast fractured portion. Between the static and cyclic lifetimes, little difference was observed in silicon carbide. However, a small acceleration by cyclic loading was observed in alumina at a relatively low applied stress, and a large acceleration was obtained in silicon nitride and in partially stabilized zirconia. The conventional fatigue theory, where the crack growth rate is time-dependent only, was not applicable in these materials. This acceleration by the cyclic loading is considered to be a result of the interaction between fatigue crack and microstructure, such as an interlocking effect against intergranular cracking in elongated columnar polycrystals of silicon nitride, and a transformation and microcracking in the process zone for partially stabilized zirconia. These interactions improve fracture toughness but cause a non-linear fracture behavior, and thus cyclic fatigue damage is accumulated due to stress hysteresis in cyclic loading.