INTERACTION BETWEEN THE TIC(TICN)-NI-MO HARDMETALS AND CHROMIUM VAPORS

The interaction between TiC or TiCN-based hard metals with a Ni-Mo binder, or cermets, and chromium vapour in a vacuum was investigated over a wide temperature range acceptable for depositing wear-resistant coatings without the formation of a liquid phase in the cermets. Computer modelling in the Ti-C-Cr system showed that a direct interaction of TiC with chromium, leading to the formation of chromium carbides, is not possible because of the high thermodynamic stability of titanium carbide. It was established experimentally that as a result of the interaction between the cermets and chromium vapours, a coating characterized by a two-layer structure was deposited on the cermet surface. The coating consists of an inner layer adjacent to the substrate, which is composed of the chromium and carbon solid solution in nickel, and an outer layer composed of a mixture of (Cr, Ni)(7)C-3 and (Cr, Ni)(23)C-6. The activation energy of the deposition process is 387 kJ mol(-1) which is close to the value of the chromium heat of evaporation. The coating deposition process is supposed to be limited by the rate of the external supply of chromium from the vapour phase. The results of the investigation of the structure, composition and morphology of the coating are presented. A mechanism responsible for the interaction of the cermets with chromium vapour leading to the formation of the two-layer coating, is proposed.