Fatigue Strength of Silicon Nitride Ceramics at High Temperatures.

Static and cyclic bending fatigue tests were carried out on two types of silicon nitride ceramics at high temperatures in air. The effect of the viscosity of grain-boundary phase on fatigue strength was investigated.The experimental results show that the effect of stress cycling on fatigue strength varies depending upon the following three temperature levels in relation to the viscosity of grain-boundary phase.(1) In the temperature region below the softening point of grain-boundary phase, the cyclic fatigue strength was lower than the static fatigue strength. This is attributed to the acceleration of crack propagation rate by stress cycling.(2) At temperatures just below the softening point of grain-boundary phase, the cyclic fatigue strength was higher than the static fatigue strength, and almost all the specimens of cyclic fatigue did not fracture at 107 cycles. The average flexural strength of these survival specimens was over 20% higher than the average static strength of virgin specimens at the same temperature. This may be due to the healing of defects such as machining-induced damages.(3) In the temperature region where the grain-boundary phase is softened, the cyclic fatigue strength was lower than the static fatigue strength. The cyclic fatigue life, which was predicted from the static fatigue strength with an assumption that the fatigue fracture depends upon only time, was shorter than that in the experimental data. The longer experimental life in cyclic fatigue is attributed to high loading rate which generates smaller strain.