Influence of annealing treatments on microstructure and toughness of liquid-phase-sintered silicon carbide

The possibility of an in situ toughening through the β → α phase transition was evaluated on liquid-phase-sintered SiC (with Al2O3 and Y2O3 as additives). Dense hot-pressed materials with an equiaxed morphology were annealed at 1850, 1900 and 1950 °C for 1 to 4 h so the effect of time, temperature, composition, and amount of second phase could be investigated. Microstructure features revealed that 1900 °C was the best temperature in producing a high percentage of elongated grains with limited grain coarsening and reduction of second-phase pockets. Mean grain size and aspect ratios increased from 0.5 to 1.5 μm and to 5, respectively, during the first 2 h of treatment at 1900 °C and then maintained a constant value. The amount and composition of second phase influenced the rate of transformation from equiaxed to elongated grain morphology. Toughness increased from 3.3 to 5.5 Mpa · m1/2 in 1900 °C annealed samples due to a crack-deflection mechanism.