Chemical constitution and microstructure of TiCx coatings chemically vapour deposited on Fe-C substrates; effects of iron and chromium

TiC_x coatings were chemically vapour deposited in an industrial reactor on Fe-C substrates with carbon contents between 0.06 and 1.20 wt % C. Electron probe microanalyses showed that significant amounts of chromium and iron were present in the coatings and that chromium was also present in the substrate region adjacent to the coatings. By comparing calculated and measured lattice parameters (corrected for the internal stresses present) it became evident that the chromium was in solid solution in TiC_x, whereas the iron was not. This was confirmed by micro Auger electron spectroscopy and X-ray diffraction phase analyses. The carbon to metal ratio,x, of the TiC_x coatings decreased with increasing distance to the coating/substrate interface. The effect of iron on the X-ray diffraction line broadening and hardness of the coatings was large (in contrast with the effect of chromium) and increased with increasing distance to the coating/substrate interface because of a decreasing iron particle size. The TiC_x crystallite size was small and constant throughout the thickness of the coatings. The chromium present in the substrate region adjacent to the TiC_x coatings influenced the microstructure of the substrate by formation of iron, chromium-carbides and reduced the growth rate of the coatings.