In this study, a new processing strategy based on presureless sintering was introduced for obtaining dense SiC with controlled microstructure consisting of elongate plate-like grains. Strategy entails the addition of partially β → α transformed seed particles to α-SiC and β-SiC starting powders. Partially transformed seed particles were obtained by the heat-treatment of β-SiC powders and consisted of 60α/40β.
α-SiC and β-SiC powders with addition of 0, 0.5, 1. 5 vol% of partially transformed seed particles were pressureless-sintered at 2000℃ using 3 wt% $Al_2O_3-17$ wt% $Y_2O_3$ additives. Relative density was above 98% and weight loss of sintered SiC was below 2 wt%.
The resulting microstructure depended on the addition of the partially transformed seed particles. The microstructure of α-SiC without addition of seed particles were composed of equiaxed grains, whereas α-SiC with seed particles were composed of equiaxed grains and plate-like grains resulting from in-situ growth of seed particles in α matrix. The microstructure of β-SiC annealed at 1950℃ after sintered at 2000℃ changed form equiaxed grain structure to elongated plate-like grain structure as annealing time increased. The microstructure of β-SiC without addition of seed particles were composed of equiaxed grains, whereas β-SiC with seed particles were composed of elongated plate-like grains 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 powders increased from 3.6 to $4.8 MPam^\frac{1}{2}$ with the amount of seed particles. In the case of the sintered SiC using β-SiC powders, fracture toughness increased significantly from 3.3 to $7 MPam^\frac{1}{2}$ with the amount of seed particles and the fracture mode was a mixture of intergranular and transgranular. This improved fracture toughness was attributed to crack deflection and crack bridging by the plate-like grains resulting from partially transformed seed particles.
