Synthesis of refractory silicon nitride using liquid phase with high nitrogen content

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Silicon nitride sintered material in which the grain boundary phase crystallized as Lu-containing oxynitride found to have the excellent high-temperature properties. By reduction and crystallization of the intergranular glassy phase, further improvement is expected. As a result of examining the phase relationships in detail, we found that the grain boundary composition was controlled as Lu_4Si_2O_7N_2 using the liquid phase with high nitrogen content. New silicon nitride was fabricated using high nitrogen content liquid phase in the Si_3N_4-LuN-Lu_2O_3 system.Pure lutetium nitride LuN), indispensable for high nitrogen content liquid phases, was synthesized. Lu metal was directly nitrided by gas-pressure sintering. Using this LuN, Si_3N_4-Lu_2O_3-LuN reaction root was designed for a new high-nitrogen content phase in Lu-Si-O-N quaternary system, and the phase relationship and the crystal structure was refined by X-ray analysis. Finally, we fabricate the silicon nitride ceramics with Lu-oxynitride grain boundary phase by the addition of small amount of Lu_2O_3 and by gas-pressure hot-pressing. This material showed excellent strength, oxidation resistance, high-fracture toughness and creep resistance at 1500 ℃ during 1400 h under 137 MPa tensile stress.