THE MECHANISM OF INTERACTION BETWEEN CL-CONTAINING GASEOUS-PHASE AND CEMENTED CARBIDES IN THE CHEMICAL-VAPOR-DEPOSITION PROCESS

The mechanism of interaction between the gaseous phase of TiCl4-H-2 and cemented carbides of the WC-Co system was investigated experimentally and by means of thermodynamic calculations. The value of the activation energy of the process shows that the limiting stage of the process is a chemical reaction. Although the TiC layer obtained in such a way is formed for the substrate carbon only, significant decarbonization of the substrate is absent. The thermodynamic calculations show that tungsten and cobalt are present in the form of chlorides in W-C-Ti-Co-Cl-H system, whereas titanium is present in the form of the TiC phase. The mechanism of interaction between the Cl-containing gaseous phase and the WC-Co cemented carbide in the initial stage of the chemical vapour deposition process can be explained in the following way. Chlorination of the WC and Co phases by Cl-containing products of TiCl4 reduction, and evaporation of their chlorides with subsequent partial redeposition in the TiC coating volume, occur during the coating process. Carbon contained in the WC and Co phases does not interact with chlorine and is used up in the TiC coating formation. Although carbon diffusion from the substrate to the coating also takes place, it is possible to deposit relatively thick TiC coatings without formation of much eta phase, when an external source of carbon is absent.