SIC-WHISKER-REINFORCED AL2O3 COMPOSITES BY FREE SINTERING OF COATED POWDERS

SiC whiskers were coated with a thick cladding of fine-grained Al2O3 p owder by controlled heterogeneous precipitation in a concentrated susp ension of whiskers. After calcination, the coated whiskers were compac ted by cold isostatic pressing and sintered at a constant heating rate of 5-degrees-C/min in a helium atmosphere. The parameters which contr ol the coating process and the sintering characteristics of the consol idated powders are reported. Starting with an initial matrix density o f 40-45% of the theoretical, composites containing up to almost-equal- to 20 vol% whiskers (aspect ratio almost-equal-to 15) were sintered fr eely to nearly theoretical density below 1800-degrees-C. By comparison , a similar composite formed by mechanical mixing of the whiskers and the precipitated Al2O3 powder reached a density of only 68% of the the oretical after sintering under identical conditions. For a fixed whisk er content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio .