HIGH-TEMPERATURE FATIGUE OF A GAS-PRESSURE-SINTERED SILICON-NITRIDE

The fatigue behaviour of a gas-pressure-sintered silicon nitride has b een investigated at 1000 degrees C. The growth of long (>100 mu m) sub critical fatigue cracks was observed directly and the time to failure of uniaxially loaded specimens caused by the growth small (<100 mu m), machining flaws was measured Oxidation in the crack-tip region reduce d the fatigue resistance of the material. Cracks grew under the co-ope rative effect of stress, oxidation and flow of the viscous oxide phase formed This mechanism would explain why crack growth rates were great er under static loading than cyclic loading because of the mote damagi ng effect of sustained crack opening. Because of the relatively small size of the oxidized crack-tip region and its slow advancement, the hi gh temperature fast fracture behaviour was similar to that at room tem perature and a similar value of K-IC was estimated. (C) 1996 Elsevier Science Limited.