SYNTHESIS OF FINE MoSi₂–Si₃N₄ COMPOSITE POWDERS

Abstract

The features peculiar to the solid-state synthesis of MoSi₂ through vacuum heat treatment of a powder mixture of molybdenum and silicon nitride, as a precursor, in the temperature range 1000–1400 °C were examined. X-ray diffraction established that the solid-state interaction began at 1100 °С and progressed through the reaction diffusion of highly active silicon, resulting from the decomposition of Si₃N₄, into molybdenum to form lower Mo₃Si and Mo₅Si₃ silicide phases. In the temperature range 1100–1300 °С, the redistribution of phases occurred: the contents of the starting molybdenum and β-Si₃N₄ components in the reaction mixtures gradually decreased, while the contents of lower molybdenum silicides increased. Molybdenum disilicide formed in situ at 1400 °С via successive development of lower silicide phases. The final product contained Mo₅Si₃. This was attributed to a deficiency of silicon as it evaporated at a temperature above 1200 °С.  This led to the conclusion that the addition of an excess of 20 wt.% silicon nitride was necessary to produce a homogeneous MoSi₂ phase and an excess of up to 40 wt.% silicon nitride to produce a two-phase MoSi₂–Si₃N₄ composite powder. The elevated temperature in the synthesis of MoSi₂ compared to conventional synthesis from simple elements was explained by the slow formation of active silicon in the Si₃N₄ dissociation process. Based on the features observed in the solid-state vacuum interaction within the powder mixture of molybdenum and silicon nitride, as a precursor, a method was proposed for producing MoSi₂–Si₃N₄ composite powders, involving the introduction of excess amounts of 30 and 40 wt.% Si₃N₄ powder. The synthesis resulted in agglomerated composite powders with a homogeneous distribution of the MoSi₂ and β-Si₃N₄ phases. The MoSi₂ phase exhibited a capsular structure with a smooth surface. The synthesized composite powders are intended for the fabrication of components and products with high oxidation resistance and corrosion resistance at elevated temperatures.