| 摘要 |
A silicon nitride sintered material includes silicon nitride and rare earth element compounds. Y and Yb are contained as the rare earth elements in a total amount of 5-7 mole % in terms of oxides, Yb/Y is 4/6 to 9/1 in terms of molar ratio of oxides, and the crystal phases of grain boundary contain a H phase and a J phase with the proportion of the H phase being larger than that of the J phase. A process for producing a silicon nitride sintered material includes the steps of mixing a Si3N4 powder with Y2O3 and Yb2O3 both as a sintering aid, the total amount of Y2O3 and Yb2O3 being 5-7 mole % and the molar ratio of Yb2O3/Y2O3 being 4/6 to 9/1, molding the resulting mixture, sintering the molded material in a nitrogen atmosphere at 1,850-1,950 DEG C., and heat-treating the sintered material in air at 1,000-1,500 DEG C. for 0.5-10 hours. The silicon nitride sintered material has, in a small or large thickness, oxidation resistance and strength at low temperatures while retaining high strength at high temperatures.
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