CHEMICAL VAPOR-DEPOSITION OF NIOBIUM ON GRAPHITE

Chemical vapor deposition of niobium on a graphite rod has been carried out at temperatures exceeding 1200 °C using NbBr5 as a feed material. The properties of the deposits have been studied by X-ray diffraction analysis, surface hardness measurements, metallographic and scanning electron microscopy observations. It was found that coatings deposited at a low temperature (below 1300 °C) have a smooth surface and a fine grained structure with layers consisting of Nb + Nb2C or Nb2C + NbC depending on their deposition temperature. In contrast, the coatings deposited at a higher temperature were found to have decreased smoothness of the surface and a coarse grained structure with a single layer consisting of NbC. Annealing experiments on the coatings with Nb and Nb2C layers showed that these layers readily transformed to a single layer of NbC at 900 °C within 10 h. It was found that the layer growth rates of carbides in the deposits are much faster than the layer growth rates estimated from ordinary diffusion data. © 1979.