Si 3 N 4 /SiC‐Whisker Composites without Sintering Aids: III, High‐Temperature Behavior

The fracture behavior at high temperature of a Si 3 N 4 ‐based SiC‐whisker composite fabricated by hot isostatic pressing without sintering aids is compared with that of other highly refractory materials. Particular attention is directed toward evaluating the slow‐crack‐growth resistance of the composite up to 1440°C and relating this resistance to the microfracture behavior of Si 3 N 4 grains, SiC whiskers, and the intergranular, glassy SiO 2 phase. Only thick whiskers operate to bridge the wake of the crack; these whiskers may make a positive contribution to the slow‐crack‐growth resistance. Impurities detected by EDX microanalysis at the grain boundary, however, apparently degrade the high‐temperature properties, a finding supported by internal‐friction measurements. Nevertheless, the high potential of the system without sintering aids for high‐temperature structural applications has been demonstrated by the time to failure estimated from the measured slow‐crack‐growth resistance for a fixed flaw size.