Microstructure of NbSi2/SiC nanocomposite coating formed on Nb substrate

The microstructure of a NbSi2/α-SiC nanocomposite coating formed by a pack siliconizing process of Nb-carbide layers at 1100 °C, which had already been formed on the surface of a Nb substrate by a carburizing process, was investigated. The Nb-carbide layers formed by the carburizing process at 1400 °C using a gas mixture of CH4-H2 consisted of two layers, i.e., an inner layer of Nb2C and an outer layer of NbC. While the monolithic NbSi2 coating showed a typical columnar microstructure perpendicular to the Nb substrate, the NbSi2/α-SiC nanocomposite coating formed by the solid-state displacement reaction of the NbC layer by Si was composed of equiaxed NbSi2 grains with an average size of 67-134 nm and the α-SiC particles with an average size of 45-60 nm. The morphology of the α-SiC particles exhibited an oblate-spheroidal shape and were mostly located at the grain boundaries of NbSi2. The nanocomposite coating formed by the solid-state displacement reaction of the Nb2C layer by Si showed a lamella microstructure. The average lamella diameters of the NbSi2 grains and the α-SiC particles were about 250 and 60 nm, respectively. The volume percentage of the α-SiC particles ranged from 17.3 to 31.1% with respect to the carbon concentration in Nb-carbide layers. No cracks were observed in the NbSi2/α-SiC nanocomposite coating, indicating that its thermal expansion coefficient was close to that of the Nb substrate.