Improvement in the Mechanical Properties of Silicon Nitride Ceramic in High-Temperature Deformation Processes

Studies have been made on the changes in structure and properties of sintered materials: Si₃N₄ - 5 mass% Y₂O₃ - 2 mass% Al₂O₃, Si₃N₄ - 5 mass% Y₂O₃ - 5 mass% Al₂O₃, and Si₃N₄ - 40 mass% TiN on deformation in a high-pressure chamber of toroid type (pressure 4–5 GPa, temperature 1000–1600 °C), and also by direct extrusion with degrees of reduction of 55 and 72% (temperature 1750–1850 °C, pressure on the plunger 20–30 MPa). After pressure-chamber treatment, the materials have elevated mechanical characteristics: HV₁₀ ≈ 16.7 GPa, K_Ic up to 8.4 MPa · m^1/2 for the system Si₃N₄ - Y₂O₃ - Al₂O₃; and HV₁₀ ≈ 16.9 GPa, K_Ic up to 9.4 MPa · m^1/2 for Si₃N₄ - TiN. A structure feature is the small size of the coherent-scattering regions: 51 nm for Si₃N₄ and 65 nm for TiN in the system Si₃N₄ - TiN, and 33 nm for specimens in the system Si₃N₄ - Y₂O₃ - Al₂O₃. High-temperature extrusion results in a structure with β-Si₃N₄ grains elongated along the deformation direction. The anisotropic structure has K_Ic values in directions perpendicular to and parallel to the direction of extrusion of 11.5–12.0 MPa · m^1/2 and 7.5–7.8 MPa · m^1/2, respectively. The hardness after extrusion becomes 16.0 GPa.