High-temperature Fatigue of a Gas-Pressure-Sintered Silicon Nitride

Abstract

The fatigue behaviour of a gas-pressure-sintered silicon nitride has been investigated at 1000°C. The growth of long (>100 μm) subcritical fatigue cracks was observed directly and the time to failure of uniaxially loaded specimens caused by the growth small (<100 μm), machining flaws was measured. Oxidation in the crack-tip region reduced the fatigue resistance of the material. Cracks grew under the co-operative effect of stress, oxidation and flow of the viscous oxide phase formed. This mechanism would explain why crack growth rates were greater under static loading than cyclic loading because of the more damaging effect of sustained crack opening. Because of the relatively small size of the oxidized crack-tip region and its slow advancement, the high temperature fast fracture behaviour was similar to that at room temperature and a similar value of KIC was estimated. © 1996 Elsevier Science Limited.