SI3N4/SIC-WHISKER COMPOSITES WITHOUT SINTERING AIDS .3. HIGH-TEMPERATURE BEHAVIOR

The fracture behavior at high temperature of a Si3N4-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-degrees-C and relating this resistance to the microfracture behavior of Si3N4 grains, SiC whiskers, and the intergranular, glassy SiO2 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.