MICROSTRUCTURAL DEVELOPMENT AND MECHANICAL-PROPERTIES OF PRESSURELESS-SINTERED SIC WITH PLATE-LIKE GRAINS USING AL2O3-Y2O3 ADDITIVES

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using beta-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the beta-SiC powder compact, the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the beta --> alpha phase transformation. With increasing holding time, the fraction of beta --> alpha phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m(1/2) to 8.3 MPa m(1/2) with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.