Atmospheric Effects on Compressive Creep of SiC‐Whisker‐Reinforced Alumina

Abstract

A SiC-whisker-reinforced alumina composite was crept in compression at 1200" to 1400°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°C and n = 3 at 1300" and 1400°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. [

Creep Mechanisms in Multiphase Ceramic Materials

“…9). The stress exponent increases sharply above a critical stress, however, especially at higher temperatures 30,34,36 (see Fig. 9).…”

“…Most of the creep studies of particulate-reinforced ceramics to date involve the use of whiskers, most commonly incorporating SiC whiskers into an Al 2 O 3 , [24][25][26][28][29][30][31][32][33][34][35][36][37][38][40][41][42][43][44][45]147,164,165 Al 2 O 3 -ZrO 2 , 51,52 or mullite 46 matrix. The shape of the creep curves show more curvature than for pure Al 2 O 3 with a moreextensive primary creep regime and some tertiary creep.…”

Comparison of short and longer term loading on the creep behaviour of alumina-silicon carbide whisker composites

“…This is believed to be due to the more extensive shape changes required for the alumina grains in order to accommodate the grain boundary sliding in the presence of the rigid Sic whiskers. This belief has become widely accepted in recent years [8][9][10][11], however the details of the actual deformation process, e.g. how the stresses are transferred from the matrix to the whiskers, etc., is still unknown.…”

“…The different specimen geometry and apparatus should not cause any difference in measured creep rate, but the environmental difference has been shown to affect the composite creep rate [9,10]. The creep rate has been observed to increase by a factor of -3 to 5 [lo] in air as compared to an inert environment.…”

Properties and Applications of Ceramic Composites Containing Silicon Carbide Whiskers

“…The cavitation and cracking is believed to be result of whiskers acting as stress concentrators and the presence of glass pockets is from the thermal oxidation of whiskers (de Arellano-López et al, 1993). High stress results in less grain-boundary sliding and promotes cracking, separation of matrix grains from whiskers, and the formation of cavities within glass pockets at whisker/whisker and whisker/matrix interfaces (Lipetzky et al, 1991). Whisker clusters containing cavitation and crack growth have also been reported for higher (25 vol%) loadings of SiC w (O'Meara et al, 1996).…”