Effect of Grain Size on Fatigue Crack Growth in Silicon Nitride and Alumina

Abstract

Fatigue crack growth tests of alumina with four different grain sizes and silicon nitride with two different grain sizes were carried out at room temperature to study the effect of grain size on the fatigue properties. Also, the effect of grain size on high temperature fatigue of the silicon nitride was studied by conducting fatigue tests at 1300°C. The room temperature tests showed that the fatigue fracture was intergranular and that threshold values and crack growth resistance increased with increasing grain size for both materials. Measurements of the crack mouth opening showed that grain bridging alone could account for most of the observed difference in crack growth resistance. The high temperature tests showed that the crack growth resistance was lower than at room temperature and that the effect of grain size was much smaller. The lower crack growth resistance at high temperature was the result of weakening of the amorphous phase present at grain boundaries. Also at high temperatures bridging was an important phenomena, especially at high K-levels, but the nature of bridging was different compared to at room temperature. At room temperature the bridges consisted of one or a few grains while at high temperature they consisted of clusters of many grains and therefore the influence of grain size was less significant.