Microstructure ofin situ MoSi2/SiC nanocomposite coating formed on Mo substrate by displacement reaction

The microstructure of an in-situ Mosi₂/β-SiC nanocomposite coating formed by the solid-state displacement reactions of Si deposited by chemical vapor deposition (CVD) with Mo-carbide layers at 1100°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°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 colummar microstructure perpendicular to the Mo substrate, the MoSi₂/β-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 β-SiC particles with an average size of 80–105 nm. The β-SiC particles exhibited an oblate-spheroidal shape and were located mostly at the grain boundaries of MoSi₂. The volume percentage of β-SiC particles ranged from 18.5 to 29.2% with respect to the carbon concentration in Mo-carbide layers.