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 degrees C to 1750 degrees C for 2 h under the external mechanical pressure of 60 MPa, followed by post-sintering treatment at 1900 degrees C for 2 h under the N-2 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 degrees C. The bending strength of 1039.1 +/- 70.1 MPa and fracture toughness of 8.46 +/- 0.23 MPa m(1/2) could be obtained at 1750 degrees 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 m(1/2) due to the repeated dissolution-precipitation process.