Effect of additive content on liquid-phase sintering on silicon carbide ceramics

Submicron silicon carbide (SiC) was sintered to about 98% of its theoretical density by using alumina and yttria as sintering additives. This densification was attributed to the liquid-phase sintering of a eutectic liquid formed between Al2O3 and Y2O3 at sintering temperatures. Observation by scanning electron microscopy (SEM) revealed that SiC exhibited a uniformly distributed fine-grained microstructure and a highly intergranular fracture behavior. The maximum values of strength and toughness were as high as 650 MPa and 7.5 MPa·m1/2, respectively. The improved toughness is considered to be associated mainly with the deflection of cracks along interphase boundaries, due to a weak interface, as well as with the introduction of microcracks at the interface between SiC grains and the secondary phases, due to a residual tensile stress from thermal expansion mismatch.