Abstract
Pressureless sintering of Si3N4 with Y2O3 and Al2O3 as additives was carried out at 1750°C in N2 atmosphere. Si3N4 materials which had more than 92% relative density were obtained with 20wt% addition of additives. The flexural strength of as-sintered materials containing 5 to 8.6wt% Al2O3 and 15 to 11.4wt% Y2O3 was in the range of 480 to 560 MPa at room temperature. The glassy grain-boundary phase of as-sintered materials crystallized to 3Y2O3 · 5Al2O3 (YAG), Y2O3 · SiO2 (YS), Y2O3 · 2SiO2 (Y2S) and 10Y2O3 · 9SiO2 sd Si3N4 (NA) by heat-treatment at 1250° C for 3 days. A specimen containing 15wt% Y2O3 and 5wt% Al2O3 sintered at 1750° C for 4 h was heat-treated at 1250° C for 3 days to precipitate YAG and YS. The nitrogen concentration of the grain-boundary glassy phase of the specimen was found to be very high, and therefore the flexural strength of the crystallized specimen scarcely decreased at elevated temperatures (the flexural strength of this specimen is 390 MPa at room temperature and 360 MPa at 1300° C). Resistance to oxidation at 1200° C of the specimen was good as well as the flexural strength, compared with that of as-sintered materials.
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