SPS sintering of silicon nitride with fluoride additive

Silicon nitride, one of the major structural ceramics, is sintered using additives such as Al2O3+MgO to provide conditions for liquid phase sintering to full density. The final microstructure contains high aspect ratio β-Si3N4 grains and intergranular glass. In this study, Spark Plasma Sintering (SPS) of silicon nitride, using Al2O3+MgO or MgF2 as sintering additives, has been investigated in the temperature range 1400–1600 °C for 3 min since MgF2 should produce liquid phases with lower melting temperatures than with MgO and therefore should sinter at lower temperature. Densification, microstructural development and mechanical behaviour have been compared. Maximum density (3.146 g cm−3) was obtained for Si3N4+3% Al2O3+9.3% MgF2 sintered by SPS at 1550 °C. In all samples, α- to β-Si3N4 transformation was incomplete but samples with MgF2 showed higher β:α ratio than with MgO. Fluoride doped samples also exhibited higher fracture toughness than equivalent samples sintered with MgO.