Microstructure of NbSi2/SiC nanocomposite coating formed on Nb substrate

• Published in: Journal of alloys and compounds 2005-05, Vol.395 (1-2), p.185-191
• Main Authors: SON, Keun-Hyung, YOON, Jin-Kook, HAN, Jun-Hyun, KIM, Gyeung-Ho, DOH, Jung-Mann, LEE, Seong-Rae
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Physics
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2004.10.069

Nb and its alloys are candidates for high temperature structural materials, but the poor oxidation resistance has been a major barrier to such applications. The microstructure of a NbSi2/alpha-SiC nanocomposite coating formed by a pack siliconizing process of Nb-carbide layers at 1100 DGC, 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 1400DGC 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/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 urn. 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 Nb2C layer by Si showed a lamella microstructure. The average lamella diameters of the NbSi2 grains and the a-SiC particles were about 250 and 60 nm, respectively. The volume percentage of the a-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/a-SiC nanocomposite coating, indicating that its thermal expansion coefficient was close to that of the Nb substrate.