Effect of LaB6 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Si3N4 ceramics was hot-pressed with LaB6–MgO as sintering additives in the temperature range from 1550 °C to 1750 °C for 2 h under the external mechanical pressure of 60 MPa, followed by post-sintering treatment at 1900 °C for 2 h under the N2 pressure of 2 MPa. The effect of sintering temperature on the densification behavior, mechanical properties, thermal conductivity, phase composition and microstructure evolution were investigated in detail. Compared to the Si3N4 ceramics sintered with La2O3–MgO, the addition of LaB6–MgO was beneficial for the improvement of fracture toughness, Vickers' hardness and thermal conductivity of Si3N4 ceramics. The maximum Vickers’ hardness of 18.98 ± 0.63 GPa was achieved at 1550 °C. The bending strength of 1039.1 ± 70.1 MPa and fracture toughness of 8.46 ± 0.23 MPa m1/2 could be obtained at 1750 °C. After post-sintering treatment, the Si3N4 ceramics exhibited improved thermal conductivity of 75.63 W m−1 K−1 and fracture toughness of 8.72 ± 0.11 MPa m1/2 due to the repeated dissolution-precipitation process.