Low temperature pressureless sintering of silicon carbide ceramics with alumina–yttria–magnesia-calcia

For cost-effective production of dense silicon carbide (SiC) ceramic parts, pressureless sintering of SiC ceramics at lower temperatures is preferred. This study suggests a new additive composition based on Al2O3–Y2O3–MgO–CaO that leads to successful densification of SiC without applied pressure at a temperature as low as 1800°C. The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with the new quaternary additive was investigated. Fracture toughness of the SiC ceramics continuously increased with increasing sintering temperature because of the growth of platelet SiC grains at temperatures ≥1750°C. In contrast, the flexural strength and hardness showed maxima at 1800°C due to the decreased density and increased grain size at higher sintering temperatures. The thermal conductivity of SiC ceramics increased with increasing sintering temperature from 1700 to 1900°C as a result of the decrease in lattice oxygen content in the SiC lattice. Typical sintered density, fracture toughness, hardness, flexural strength, and thermal conductivity of the 1800°C-sintered SiC ceramics were 98.8%, 5.2 MPa·m1/2, 29.3 GPa, 347 MPa, and 83 Wm−1 K−1 at room temperature, respectively.