Microstructure of NbSi2/SiC nanocomposite coating formed on Nb substrate

Abstract

The microstructure of a NbSi2/alpha-SiC nanocomposite coating formed by a pack siliconizing process of Nb-carbide layers at 1100 degrees 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 degrees C using a gas mixture of CH4-H-2 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/alpha-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 alpha-SiC particles with an average size of 45-60 nm. The morphology of the a-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 NbC layer by Si showed a lamella microstructure. The average lamella diameters of the NbSi2 grains and the alpha-SiC particles were about 250 and 60 nm, respectively. The volume percentage of the alpha-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/alpha-SiC nanocomposite coating, indicating that its thermal expansion coefficient was close to that of the Nb substrate. (c) 2004 Elsevier B.V. All rights reserved.