Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Dense SiC ceramics were obtained by hot pressing of β-SiC powders using Al2O3-Y2O3 and La2O3-Y2O3 additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to ∼750 MPa at RT and up to ∼550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m1/2 and hardness from 21 to 23 GPa.