Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si3N4 ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si3N4 ceramics containing SiO2–MgO–Y2O3-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO2 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% SiO2, 3 mass% MgO and 1 mass% Y2O3-added Si3N4 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 m1/2) 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 Y2O3–SiO2-added and Y2O3–Al2O3-added Si3N4 ceramics.