SiC-whisker-reinforced Al[sub 2]O[sub 3] composites by free sintering of coated powders

SiC whiskers were coated with a thick cladding of fine-grained Al[sub 2]O[sub 3] powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5 C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40--45% of the theoretical, composites containing up to [approx]20 vol% whiskers (aspect ratio [approx]15) were sintered freely to nearly theoretical density below 1800 C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al[sub 2]O[sub 3] powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.