In-situ synthesis and mechanical properties of TiN-Si₂N ₂O-Si₃N₄ composites

Si₃N₄-Si₂N₂O-TiN composite ceramics were in-situ fabricated by using following reactions of (1) 3TiO ₂ + Si₃N₄ → 3TiN + 3SiO₂ + N₂ and (2) Si₃N₄+ SiO₂ → 2Si₂N₂O. The mixed powder of α-Si₃N ₄, Al₂O₃, Y₂O₃ and TiO₂ was hot-pressed at 24 MPa and 1800°-1900°C for 1-4 h in N₂. Sintered composite ceramics were characterized by XRD, SEM, TEM, four-point bending test and Vickers indentation method. XRD results and TEM observation showed that TiN and amorphous SiO₂ were formed at 1250°C by the reaction (1), and the Si₂N₂O phase formed by reaction (2) above 1800°C. Si₃N₄-Si ₂N₂O-TiN composites consisted of ≥2 μm sized Si ₂N₂O grains with TiN and Si₃N₄ grains. Hardness and fracture strength of the composites were comparable to those of Si₂N₂O ceramics, with fracture toughness being improved at 5vol% TiN containing composites.