Oxidation behavior of Si₃N₄ prepared with different sintering additives

Si₃N₄ ceramics were sintered by hot pressing using α-Si₃N₄ as raw materials and Y₂O₃-La₂O₃ or Y₂O₃-CeO₂ as sintering additives. The oxidation behavior of ceramics in air at high temperature was investigated. The results show that α-Si₃N₄ converts into β-Si₃N₄ completely after sintering. The mass of Si₃N₄ ceramic prepared using Y₂O₃-La₂O₃ as sintering additive increases after oxidization at 1000-1350°C for 100 h, its mass gain is lower than 0.389 mg/cm², and the oxidization process satisfies the parabolic law. For the Si₃N₄ ceramic prepared using Y₂O₃-CeO₂ sintering additive, however, its mass decreases after oxidation at 1000°C for 100 h, reaching -0.248 mg/cm². The mass of the ceramic increases after oxidation at 1230 and 1350°C for 100 h. Its mass gain are 0.024 and 0.219 mg/cm², respectively, and its oxidization process did not satisfy the parabolic law. The oxidation process contains two diffusive mechanisms: the outward diffusion of the rare earth elements and the inward diffusion of oxygen.