ATMOSPHERIC EFFECTS ON COMPRESSIVE CREEP OF SIC-WHISKER-REINFORCED ALUMINA

A SiC-whisker-reinforced alumina composite was crept in compression at 1200-degrees to 1400-degrees-C in an air ambient and in nitrogen. The data were described by a power-law-type constitutive relation. The measured value of the stress exponent was n = 1 at 1200-degrees-C and n = 3 at 1300-degrees and 1400-degrees-C in both ambients. TEM observations were correlated with the measured creep response to determine active deformation mechanisms. Values of n = 1 were associated with diffusional creep and unaccommodated grain-boundary sliding, while values of n = 3 were associated with increased microstructural damage in the form of cavities. Experiments conducted in circulated air resulted in higher creep rates than comparable experiments in nitrogen. The accelerated creep rates were caused by the thermal oxidation of SiC and the resultant formation of a vitreous phase along composite interfaces. The glassy phase facilitated cavitation, weakened interfaces, and enhanced boundary diffusion.