Dense SiC ceramics were obtained by pressureless sintering of β-SiC and α-SiC powder using $Al_2O_3$-$Y_2O_3$ additives. Sintering mechanism was attributed to liquid phase sintering via the formation of a eutectic liquid between $Al_2O_3$ and $Y_2O_3$. From an evidence of core/rim grain structure, solution and reprecipitation were considered to control the mass transport for densification and grain growth. Second phase in sintered SiC was identified as a $Y_3Al_5O_{12}$ (YAG, yttrium aluminum garnet) phase.
The resulting microstructure depended highly on the crystalline form of the starting SiC powder, α or β. In the case of α-SiC powder as a starting material, the sintered microstructure was composed of the equiaxed grains of mostly 6H polytype. As the sintering time was increased the grain size was increased without appreciable change in grain shape. In the case of β-SiC powder, the β->α phase transformation occurred during sintering and the major phase of transformed SiC was 4H polytype. When the sintering time was increased the microstructure was changed from equiaxed to plate like grains. The plate like grains were considered to be formed due to the grain growth associated with β->α phase transformation during sintering.
The fracture mode of sintered SiC was a mixture of intergranular and transgranular. The fracture mode was attributed to a weak grain boundary and/or a residual stress field resulted from the difference of the coefficient of thermal expansion between SiC and $Y_3Al_5O_{12}$ phase.
The fracture toughness for the sintered SiC using α-SiC powder was increased slightly from 4.5 to 5.7 MPa $m^\frac{1}{2}$ with sintering time. In the case of the sintered SiC using β-SiC powder, fracture toughness was increased significantly from 4.4 to 8.3 MPa $m^\frac{1}{2}$ with sintering time. This improved fracture toughness was attributed to crack deflection and crack bridging by the plate like grains.
Flaw tolerance and R-curve behavior of Bl-SiC with equiaxed grains and B3-SiC with plate like grains were evaluated by the indentation-strength method. For comparison, the R-curve behavior of the conventional sintered, boron- and carbon-doped SiC(SS-SiC) was evaluated. The B1-SiC and the B3-SiC exhibited rising R-curve behavior with toughening exponents of m = 0.042 and m = 0.135, respectively. The B3-SiC exhibited a better flaw-tolerance property and more sharply rising R-curve behavior than the B1-SiC. The more sharply rising R-curve and the better flaw-tolerance property of B3-SiC were attributed mainly to gain bridging of the crack faces by plate like grains. Due to high degree of transgranular fracture the SS-SiC exhibited a flat R-curve behavior despite the microstructural feature with plate like grains.
