Time-dependent strength degradation of a siliconized silicon carbide determined by dynamic fatigue

Both fast-fracture strength and strength as a function of stressing rate at room temperature, 1100°, and 1400°C were measured for a siliconized SiC. The fast-fracture strength increased slightly from 386 MPa at room temperature to 424 MPa at 1100°C and then dropped to 308 MPa at 1400°C. The Weibull moduli at room temperature and 1100°C were 10.8 and 7.8, respectively, whereas, at 1400°C, the Weibull modulus was 2.8. The very low Weibull modulus at 1400°C was due to the existence of two exclusive flaw populations with very different characteristic strengths. The data were reanalyzed using two exclusive flaw populations. The ceramic showed no slow crack growth (SCG), as measured by dynamic fatigue at 1100°C, but, at 1400°C, an SCG parameter, n, of 15.5 was measured. Fractography showed SCG zones consisting of cracks grown out from silicon-rich areas. Time-to-failure predictions at given levels of failure probabilities were performed.