Effect of additive content on the mechanical and thermal properties of pressureless liquid-phase sintered SiC

Pressureless liquid-phase sintered SiC (LPS-SiC) ceramics fabricated from β-SiC and 1 vol% α-SiC seeds with 3–15 vol% Al2O3-Y2O3-CaO additives were prepared to investigate the effects of additive content on the mechanical and thermal properties. All the specimens consisted primarily of elongated α-SiC grains that were predominantly formed via β to α phase transformation and via the growth of the initially added α-SiC seeds during the sintering. The grain growth and polytypic transformation exhibited diffusion-controlled kinetics. The highest value for fracture toughness (5.4 MPa·m1/2) was exhibited at an additive content level of 7 vol%; this was attributable to the optimal interfacial strength and well-interlocked elongated grains with active crack–microstructure interaction. The specimen with 7 vol% additive content exhibited the highest flexural strength (686 MPa) among the reported pressureless LPS-SiC ceramics, and this was attributable to its high density and the optimal content of soft intergranular phase (IGP). The hardness and thermal conductivity decreased gradually with the increase in additive content, owing to the increased volume fraction of the soft IGP with low thermal conductivity.