A phenomenological study of the nucleation and growth of chemically vapour-deposited TiC coatings on cemented carbide

Abstract

The nucleation and growth process of chemically vapour-deposited TiC coatings on WC-Co substrates was studied for a wide range of process variables and of cobalt contents of the substrate. During the initial growth stage, we found the nucleation and growth to be strongly affected by the cobalt content of the substrate. This result was attributed to diffusion of carbon through a cobalt-rich phase. A correlation was found between the rate of surface coverage with TiC and the deposition time. During the secondary growth stage, both the growth rate and the activation energy for the substrate reaction increased with increasing cobalt content, but the growth rate and activation energy for the overall reaction did not change with the cobalt content. An empirical equation for the dependence of the growth rate on the deposition temperature and the total pressure in the system was formulated. Scanning electron micrographs revealed that the grain shape was equiaxed for all deposition conditions. They also showed that the grain size decreased when either the deposition temperature or the total pressure was decreased and when the carbon concentration in the reactant gas was increased. It is believed that the decrease in grain size is due to enhanced nucleation.