FLAW-TOLERANCE AND R-CURVE BEHAVIOR OF LIQUID-PHASE-SINTERED SILICON CARBIDES WITH DIFFERENT MICROSTRUCTURES

beta-SiC powder containing 6 wt% Al2O3 and 4 wt% Y2O3 as sintering additives was pressureless sintered at 2000 degrees C for 1 h (AYE-SiC) and 3 h (AYP-SiC). AYE-SiC consisted of an equiaxed grain structure and AYP-SiC exhibited a microstructure with platelike grains as a result of grain growth related to beta --> alpha phase transformation during sintering. R-curve behavior and flaw tolerance for these silicon carbides were evaluated by the indentation-strength technique. For comparison, the R-curve behavior of conventional sintered, boron- and carbon-doped SiC (SS-SiC) was evaluated. AYE-SiC and AYP-SiC exhibited rising R-curve behavior with toughening exponents of m = 0.042 and m = 0.135, respectively. AYP-SiC exhibited better flaw tolerance and more sharply rising R-curve behavior than AYE-SiC. The more sharply rising R-curve behavior and the better flaw tolerance of AYP-SiC were attributed mainly to grain bridging of crack faces by platelike grains. Because of the high degree of transgranular fracture, SS-SiC exhibited a flat R-curve despite a microstructural feature with platelike grains.