Effects of α‐Sic versus β‐Sic Starting Powders on Microstructure and Fracture Toughness of Sic Sintered with Al ₂ O ₃ ‐Y ₂ O ₃ Additives

Dense Sic ceramics were obtained by pressureless sintering of β‐Sic and α‐Sic powders as starting materials using Al 2 O 3 ‐Y 2 O 3 additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α‐SiC powder was composed of equiaxed grains, whereas SiC obtained from α‐SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α‐SiC powder increased slightly from 4.4 to 5.7 MPa.m 1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β‐SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m 1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.