Microstructure of in situ MOSi2/SiC nanocomposite coating formed on mo substrate by displacement reaction

Abstract

The microstructure of an in-situ MoSi2/beta-SiC nanocomposite coating formed by the solid-state displacement reactions of Si deposited by chemical vapor deposition (CVD) with Mo-carbide layers at 1100 degrees C, which had previously been formed on the surface of a Mo substrate by a CVD process, was investigated. The Mo-carbide layers formed by the simultaneous CVD of Mo and carbon at 900 degrees C for 5 h using a gas mixture Of C2H4-MoCl5-Ar consisted of two layers, an inner layer Of Mo2C and an outer layer of MoC. While the monolithic MoSi2 coating showed a typical columnar microstructure perpendicular to the Mo substrate, the MoSi2/beta-SiC nanocomposite coating formed by the solid-state displacement reactions between the Mo-carbide layers and Si was composed of equiaxed MoSi2 grains with an average size of 150-500 nm and beta-SiC particles with an average size of 80-105 nm. The beta-SiC particles exhibited an oblate-spheroidal shape and were located mostly at the grain boundaries of MoSi2. The volume percentage of beta-SiC particles ranged from 18.5 to 29.2 % with respect to the carbon concentration in Mo-carbide layers.