Effect of Grain-Boundary Phase on Dynamic Compression Fatigue in Hot-Pressed Silicon Nitride

Two types of hot-pressed silicon nitride, one having an amorphous grain-boundary phase (6 wt % yttria, 3 wt% alumina) and the other having a predominantly crystalline grain-boundary phase (8 wt % yttria, 1 wt % alumina), were tested on a split Hopkinson pressure bar with a momentum trap, such that, in each test, the sample was subjected to a single predefined stress pulse and then recovered without being subjected to any other loads. The specimens were loaded repeatedly with a triangular pulse of 3.2 GPa am-plitude at a strain rate of approximately 400/s. The dy-namic fatigue life of amorphous grain-boundary-phase sili-con nitride was observed to be higher than that of the corresponding fatigue life of crystalline grain-boundary-phase silicon nitride. The difference in fatigue lives is cor-related to the microstructural damage occurring in both materials. I