Reaction sintering of fluorine-doped MgSiN2

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
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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).