In situ formation of MoSi₂–SiC through reaction of SiO₂ or Si₃N₄ with Mo and carbon

Abstract

Composite powders of molybdenum silicide–SiC were synthesized by reacting mixtures of (Mo–SiO₂ –C), (Mo–Si₃N₄ –C), and (Mo–SiO₂ –Si₃N₄ –C) powders at 1300 °C. In the (Mo–SiO₂ –C) system Mo₅Si₃ and Mo₃Si formed predominantly. MoSi₂ formed the major constituent of the reaction product from powder mixtures containing Si₃N₄. Vapor-solid SiC whiskers formed in the (Mo–SiO₂ –C) system. Vapor-liquid-solid whiskers of SiC and Mo₅Si₃C formed in (Mo–SiO₂ –Si₃N₄–C) and (Mo–Si₃N₄ –C) systems, respectively. The mechanism of formation of the VLS whiskers and molybdenum silicides was identified as follows: initially a thin layer of Mo₂C forms on Mo particle; the Si vapor from thermal decomposition of Si₃N₄ deposits on the Mo₂C surface and forms a droplet of ternary “Nowotny phase” Mo_<5Si₃C_<1; an SiC/Mo₅Si₃C whisker forms by nucleation and growth from the supersaturated ternary phase; after reaction with the Mo₂C layer, the SiO/Si vapor further reacts with Mo particle to form bulk silicides.