Abstract

Reaction bonded MgSiN2 (RBMSN) with CaF2 and YbF3, as sintering additives was prepared by direct nitridation of Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range 1350-1550 degreesC. The influence of the ratio of constituents on the final phase composition, the amount of sintering additives and the influence of sintering temperature are discussed. It was shown that the starting powders should be in a ratio Mg2Si/Mg greater than or equal to 3 and Si3N4/Si-tot greater than or equal to 0. 5 respectively, to obtain single-phase MgSiN2. The oxygen content of MgSiN2 was in the range of 0.4-0.7 wt.%. The final density of pressureless sintered RBMSN with 4 vol.% fluorine additives was 85% of theoretical density. Higher density (similar to92%) was obtained for the reaction-bonded composite consisting of 48 vol.% MgSiN2 and 52 vol.% Si3N4, due to the volume increase after the nitridation of silicon. The mechanical properties of fluorine-doped RBMSN were further improved by hot-pressing at 1600 degreesC. The 4-point bending strength and Vickers hardness (HVl) of hot-pressed materials are 427 MPa and 20.8 GPa, respectively. The indentation fracture toughness is 5.3 MPa m(1/2), due to the presence of elongated beta-Si3N4 grains. The thermal conductivity of the dense reaction bonded MgSiN2 is 22 W m(-1)K(-1).