High temperature strength of silicon nitride ceramics with ytterbium silicon oxynitride

Silicon nitride ceramics with ytterbium silicon oxynitride (Yb₄Si₂O₇N₂) as secondary phase were fabricated by hot-pressing the powder mixtures, including 50.0 to 97.0 mol% of silicon nitride with a mixture of Yb₂O₃ and SiO₂ (Yb₂O₃/SiO₂ = 4). Sinterability of the materials with Yb₂O₃ was higher than that with Y₂O₃ in the same composition of raw powder mixtures. High density materials were obtained under the condition of 50.0 to 89.1 mol% of silicon nitride in raw powder mixtures. Mechanical properties of silicon nitride containing 97.6 mol% of Si₃N₄ and 2.4 mol% of Yb₄Si₂O₇N₂ were measured. Fracture toughness measured by the indentation technique was 5.9 MPam^1/2. Bending strength at room temperature and at 1500 °C was 977 MPa and 484 MPa, respectively. The silicon nitride grains consisted of highly elongated rod-like grains and thin needle-like grains. The Yb₄Si₂O₇N₂ grains were crystallized at multigrain junctions and bonded close to Si₃N₄ grains. High strength at high temperature is supposed to be based on the presence of crystalline Yb₄Si₂O₇N₂ having a high melting point.