Dynamic-compression fatigue of hot-pressed silicon-nitride

This paper presents a technique to study dynamic compression fatigue of hard ceramics. In this technique, a split-Hopkinson-compression-bar apparatus is modified to allow recovery tests on hard ceramic samples. The exact number of fatigue cycles along with the stress level and strain rate can be controlled, and strain rates as high as 400s^–1 at a peak stress of 3.2 GPa were obtained for each fatigue cycle. Two types of hotpressed silicon-nitrides, one having crystalline-grain-boundary phase, and the other having amorphous-grain-boundary phase, were repeatedly tested on this apparatus. Fatigue lives of both materials were found to be very sensitive to sample-surface roughness, and for equivalent sample-surface condition, the dynamic fatigue life of amorphous-grain-boundary phase silicon-nitride was observed to be higher than that of crystalline-grain-boundary phase silicon-nitride.