Improving Heat Resistance of Silicon Nitride Ceramics with Rare-Earth Silicon Oxynitride(<Special Issue>Guest Editors Dedicated to Prof. Gunter Petzow: Modern Trends in Advanced Ceramics)

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Heat resistant silicon nitride ceramics sintered with RE_2O_3 (RE: Sc, Y, and lanthanide) and those with RE_2Si_2O_7 as gram 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 2.4mol% Yb_4Si_2O_7N_2 were gas-pressure hot-pressed at 1800℃ for 1h under 20MPa in 1MPa N_2 (YB). Si_3N_4 powder with 1.2mol% (LU12) and 4.8mol% (LU48) of Lu_2O_3 were gas-pressure hot-pressed at 1950℃ for 1h under 20MPa in 1MPa N_2. High temperature strength of LU12 was at its maximum among the three ceramics. Stress-strain curve of YB was linear at 1200℃, but at 1400℃ it was non-linear. The Stress-strain curve of LU48 was non-linear at 1500℃. For LU12 the stress-strain curve was linear, and it was linear even at 1600℃, indicating that LU12 broke as a brittle fracture at 1600℃. LU12 had excellent oxidation resistance, and weight gain during the oxidation at 1500℃ for 1000h was 4g/m^2. Creep resistance of LU48 was higher than that of YB. Creep life time of LU12 at 1500℃ under tensile stress of 137MPa exceeded 1678.5h. Time-to-failure of LU12 was strongly stress sensitive.
2006-11-01