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

摘要

Heat resistant silicon nitride ceramics sintered with RE_2O_3 (RE: Sc, Y, and lanthanide) and those with RE_2Si_2O_7 as grain boundary phase were briefly reviewed. High-temperature strength and internal friction of silicon nitride ceramics with sintering additive of RE_4Si_2O_7N_2 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. Si_3N_4 powder with 2.4 mol percent Yb_4Si_2O_7N_2 were gas-pressure hot-pressed at 1800 deg C for 1 h under 20 MPa in 1 MPa N_2 (YB). Si_3N_4 powder with 1.2 mol percent (LU12) and 4.8 mol percent (LU48) of Lu_2O_3 were gas-pressure hot-pressed at 1950 deg C for 1 h under 20 MPa in 1 MPa N_2. High temperature strength of LU12 was at its maximum among the three ceramics. Stress-strain curve of YB was linear at 1200 deg C, but at 1400 deg C it was non-linear. The Stress-strain curve of LU48 was non-linear at 1500 deg C. For LU12 the stress-strain curve was linear, and it was linear even at 1600 deg C, indicating that LU12 broke as a brittle fracture at 1600 deg C. LU12-had excellent oxidation resistance, and weight gain during the oxidation at 1500 deg C for 1000 h was 4 g/m~2. Creep resistance of LU48 was higher than that of YB. Creep lifetime of LU12 at 1500 deg C under tensile stress of 137 MPa exceeded 1678.5 h. Time-to-failure of LU12 was strongly stress sensitive.