Microstructures and mechanical properties of liquid-phase sintered seeded silicon carbide

Three matrix/seeds SiC systems (β–α, β–β, α–α platelets) were elaborated with similar experimental conditions (sintering aid: 3 wt.% eutectic YAG–Al2O3, temperatures: 1950, 1780, 1930°C, respectively). The effect of seeding on the microstructure of these liquid-phase sintered materials was investigated. Samples with and without seeds presented very close microstructural parameters (d50 and R95). So the adding of seeds did not seem to control the matrix microstructure. The image analysis of microstructures allowed to show that the growth of equiaxed grains was continuous, whereas that of elongated grains occurred in two steps. The influence of microstructure on mechanical properties was also examined. The hardness of materials was inversely proportional to the square root of mean grain diameter and their fracture toughness was proportional to this square root. Such results suggested that the mechanical properties could be controlled by the microstructure. A material with a high fracture toughness of 6 MPa m and a flexural strength comparable to that of solid-state sintered SiC (450 MPa), constituted by elongated α-SiC grains, was obtained.