EFFECTS OF ALPHA-SIC VERSUS BETA-SIC STARTING POWDERS ON MICROSTRUCTURE AND FRACTURE-TOUGHNESS OF SIC SINTRED WITH AL2O3-Y2O3 ADDITIVES

Dense SiC ceramics were obtained by pressureless sintering of beta-SiC and alpha-SiC powders as starting materials using Al2O3-Y2O3 additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from alpha-SiC powder was composed of equiaxed grains, whereas SiC obtained from beta-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the beta --> alpha phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using alpha-SiC powder increased slightly from 4.4 to 5.7 MPa.m1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using beta-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.