Processing and mechanical properties of Si₃N₄/SiC nanocomposites using Si nitrided Si₃N₄ powder

The pressureless sintering was investigated to fabricate dense Si₃N₄SiC nanocomposites from commercially available Si nitrided Si₃N₄ powder. MgAl₂O₄+ZrO₂ (totally 10 and 15 wt%) or Y₂O₃+Al₂O₃ (totally 10.5 and 14 wt%) and 5 to approximately 30 vol% SiC were used as sintering additives and second phase dispersions, respectively. With increasing SiC content, the density of the composite containing less amount of sintering additives decreased linearly, while the large amount of sintering additives decreased gradually the density or kept it constant. The composites with higher SiC content presented a relatively smaller Si₃N₄ grain size, because the Si₃N₄ grain growth was inhibited by SiC particles that pinned the grain boundary movement. While decrease in strength was observed for the composites with less amount of sintering additives, an improvement of the strength was found for the composites with high amount of sintering additives. The strength change of the composites was consistent with their densification behavior and microstructural observation. Composite with 20 vol% SiC exhibited high strength up to 1 GPa. With increasing SiC content, hardness increased but toughness decreased. It is concluded that the results of this study provide an economic way to fabricate the Si₃N₄/SiC nanocomposite with high performance.