Factors affecting the initial nucleation of alumina on cemented-carbide substrates in the CVD process

Abstract

Alumina nuclei were deposited, using chemical vapor deposition (CVD), under varying process conditions on polished cemented-carbide substrates (WC+TiC, TaC, NbC+Co) for 60 s in order to study the nucleation stage. During the heating up of the substrates the binder of the polished surfaces diffused onto the carbide surfaces. The surface of the WC was thereby converted to η-phase, while no attack on the (Ti,W)C surfaces could be detected. After the short deposition the alumina nuclei were counted. The nucleation was always most dense on the (Ti,W)C grains. The chemical equilibria of the gas for the various process conditions were calculated and supersaturations for the components of the gas and a total one were computed. It was found that the density of nuclei increased with increasing supersaturation of the aluminum chlorides and the total supersaturation. The supersaturation of the oxygen donors, however, did not correlate with the density of nuclei. A dense nucleation is believed to be important for the adherence of the final deposit.