Improving Heat Resistance of Silicon Nitride Ceramics with Rare-Earth Silicon Oxynitride

Abstract

Heat resistant silicon nitride ceramics sintered with RE₂O₃ (RE: Sc, Y, and lanthanide) and those with RE₂Si₂O₇ as grain boundary phase were briefly reviewed. High-temperature strength and internal friction of silicon nitride ceramics with sintering additive of RE₄Si₂O₇N₂ composition (RE: La, Lu, Sc, and Yb) were measured to select candidates for evaluation of creep resistance. Following the results of high-temperature strength test and internal friction measurement, Yb and Lu were selected for sintering additive systems. Si₃N₄ powder with 2.4 mol% Yb₄Si₂O₇N₂ were gas-pressure hot-pressed at 1800°C for 1 h under 20 MPa in 1 MPa N₂ (YB). Si₃N₄ powder with 1.2 mol% (LU12) and 4.8 mol% (LU48) of Lu₂O₃ were gas-pressure hot-pressed at 1950°C for 1 h under 20 MPa in 1 MPa N₂. High temperature strength of LU12 was at its maximum among the three ceramics. Stress-strain curve of YB was linear at 1200°C, but at 1400°C it was non-linear. The Stress-strain curve of LU48 was non-linear at 1500°C. For LU12 the stress-strain curve was linear, and it was linear even at 1600°C, indicating that LU12 broke as a brittle fracture at 1600°C. LU12 had excellent oxidation resistance, and weight gain during the oxidation at 1500°C for 1000 h was 4 g/m². Creep resistance of LU48 was higher than that of YB. Creep life time of LU12 at 1500°C under tensile stress of 137 MPa exceeded 1678.5 h. Time-to-failure of LU12 was strongly stress sensitive.