Effect of beta-to-alpha phase transformation on the microstructural development and mechanical properties of fine-grained silicon carbide ceramics

Ultrafine beta -SiC powders mixed with 7 wt% Al2O3, 2 wt% Y2O3, and 1.785 w t% CaCO3 were hot-pressed and subsequently annealed in either the absence o r the presence of applied pressure. Because the beta -SiC to (beta -SiC pha se transformation is dependent on annealing conditions, the novel processin g technique of annealing under pressure can control this phase transformati on, and, hence, the microstructures and mechanical properties of fine-grain ed liquid-phase-sintered SiC ceramics. In comparison to annealing without p ressure, the application of pressure during annealing greatly suppressed th e phase transformation from beta -SiC to alpha -SiC. Materials annealed wit h pressure exhibited a fine microstructure with equiaxed grains when the ph ase transformation from beta -Sic to alpha -SiC was < 30 vol%, whereas mate rials annealed without pressure developed microstructures with elongated gr ains when phase transformation was > 30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mecha nical properties could be achieved through annealing with or without pressu re.