Effects of additive amount on microstructure and mechanical properties of silicon carbide-silicon nitride composites

Powder mixtures of β-SiC-α-Si₃N₄ in a weight ratio of 1:1 containing 5-20 wt% Y-Mg-Si-Al-O-N oxynitride as a sintering additive were liquid-phase sintered at 1800 °C for 3 h by hot-pressing. These materials had a microstructure of `in situ-toughened composites' as a result of the α→β phase transformation of Si₃N₄ during sintering. The introduction of larger amount of additives accelerated the grain growth of elongated β-Si₃N₄ grains with higher aspect ratio, resulting in the improved fracture toughness and strength. Typical flexural strength and fracture toughness of SiC-Si₃N₄ composites containing 15 wt% oxynitride glass were 860 MPa and 5.7 MPa·m^1/2, respectively.