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

Dense SiC ceramics were obtained by hot pressing of beta-SiC powders using Al2O3-Y2O3 and La2O3-Y2O3 additive systems. The effect of the addition of a n amount of ultrafine SiC to commercial silicon carbide powder was evaluate d. Sintering behaviour and microstructure depended on type and amount of li quid phase, as densification proceeded via a classical solution-reprecipita tion mechanism. A core/rim structure of SiC grains indicated that reprecipi tation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorpho us silicates. Values of flexural strength up to similar to 750 MPa at RT an d up to similar to 550 MPa at 1000 degrees C were measured. At 1300 degrees C a strong degradation of strength was attributed to softening of the amor phous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa.m(1/2) and hardness from 21 to 23 GPa. (C) 200 0 Kluwer Academic Publishers.