IN-SITU FORMATION OF MOSI2-SIC THROUGH REACTION OF SIO2 OR SI3N4 WITHMO AND CARBON

Composite powders of molybdenum silicide-SiC were synthesized by react ing mixtures of (Mo-SiO2-C), (Mo-Si3N4-C), and (Mo-SiO2-Si3N4-C) powde rs at 1300 degrees C. In the (Mo-SiO2-C) system Mo5Si3 and Mo3Si forme d predominantly. MoSi2 formed the major constituent of the reaction pr oduct from powder mixtures containing Si3N4 Vapor-solid SiC whiskers f ormed in the (Mo-SiO2-C) system. Vapor-liquid-solid whiskers of SiC an d Mo5Si3C formed in (Mo-SiO2-Si3N4-C) and (MO-Si3N4-C) systems, respec tively, The mechanism of formation of the VLS whiskers and molybdenum silicides was identified as follows: initially a thin layer of Mo2C fo rms on Mo particle the Si vapor from thermal decomposition of Si3N4 de posits on the Mo2C surface and forms a droplet of ternary ''Nowotny ph ase'' Mo<5Si3C<1; an SiC/Mo5Si3C whisker forms by nucleation and growt h from the supersaturated ternary phase; after reaction with the Mo2C layer, the SiO/Si vapor further reacts with Mo particle to form bulk s ilicides.