Failure Modes of SiC-Fiber/Si3N4-Matrix Composites at Elevated Temperatures

An investigation of composite failure modes as a function of temperature and fiber‐volume fraction was carried out in SiC‐fiber‐reinforced Si3N4‐matrix composites fabricated in our laboratories. Mechanical testing was carried out at temperatures from 25° to 1500°C. Matrix‐cracking stress and ultimate strength of the composites were measured from load‐displacement curves. They were both found to decrease with increasing temperature, but their temperature sensitivity decreased with increasing fiber‐volume fraction. The tendency for noncatastrophic failure increased with fiber‐volume fraction, while the tendency for catastrophic failure increased with temperature. The failure mode was demonstrated experimentally to be determined by the fiber bundle strength, Sfb, and the matrix cracking stress, σc. These two parameters, in turn, were shown to be controlled by the fiber‐volume fraction, f, and the temperature. Failure at various temperatures was noncatastrophic when Sfb > σc, and catastrophic when Sfb < σc. Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fibervolume fraction and testing temperature. Catastrophic failure at high temperatures was found to be mainly a result of fiber strength loss.