Effect of initial particle size on microstructure of liquid-phase sintered α-silicon carbide

Three α-SiC powders of different particle sizes (∼0.42, ∼0.50 and ∼0.71 μm), containing 7.2 wt% Y3Al5O12 (YAG) and 4.8 wt% SiO2 as sintering aids, were hot-pressed at 1850°C and subsequently annealed at 1950°C to initiate grain growth. All the hot-pressed specimens consisted of equiaxed grains and showed unimodal grain size distribution. The smaller the starting powder the finer the microstructure was developed. After annealing, the fine (∼0.7 μm) and the medium (∼1.1 μm) grain-sized specimens showed self-reinforced microstructures whereas the large (∼1.5 μm) grain-sized specimen maintained an unimodal grain size distribution. These results suggest that the abnormal grain growth of α-SiC grains during annealing is critically dependent on the average grain size of sintered materials, which in turn depends on initial particle size. The fracture toughnesses (5.6 and 6.1 MPa m12) of the annealed specimens with self-reinforced microstructures were higher than for the specimen with an unimodal grain size distribution (5.0 MPa m12).