CYCLIC FATIGUE OF REACTION-BONDED SILICON-NITRIDE AT ELEVATED-TEMPERATURES

Cyclic and static loading tests were performed on reaction-bonded sili con nitride from 1000-1400 degrees C in air. This porous, fine-grained material contained no glassy grain-boundary phase and exhibited no sl ow crack growth at room temperature. Under cyclic loading, the crack-g rowth behaviour at 1000 degrees C was similar to room-temperature resu lts; however, at 1200 and 1400 degrees C crack-growth rates increased significantly. Under static loading, significant crack growth was dete cted at 1000 degrees C and increased with temperature. Most of the cra ck growth under cyclic loading was attributed to slow crack-growth mec hanisms, but evidence of cyclic crack-growth mechanisms were also obse rved. Oxidation played a major role in crack-growth velocity at high t emperature.