Abstract
To improve the thermal conductivity of Si 3N 4 ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si 3N 4 ceramics containing SiO 2–MgO–Y 2O 3-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO 2 and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO 2, 3 mass% MgO and 1 mass% Y 2O 3-added Si 3N 4 ceramics increases from 44 to 89 Wm −1 K −1 by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m 1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y 2O 3–SiO 2-added and Y 2O 3–Al 2O 3-added Si 3N 4 ceramics.
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