The reaction mechanisms of CVD-Al2O3 are briefly reviewed. Two independent reaction paths exist, one of which is rate limited by a volume reaction and the second one by a surface reaction. Al2O3 crystallizes in several different phases. The α-structure is the stable phase at normal deposition temperature (950-1050°C). The metastable ϰ-phase is the second most commonly occurring phase. It is shown that ϰ - and θ - Al2O3 nucleate on pure TiC (111) facets with well defined lattice orientation-relationships between TiC and Al2O3. In contrast, no epitaxial relationship between TiC and α-Al2O3 is found and the latter interface contains a considerable amount of porosity. The difference in the interface properties is reflected in the adhesion between TiC and Al2O3. It is concluded that the nucleation resulting in α-Al2O3 does not occur on TiC but on an intermediate thin layer of titanium oxide e.g. Ti2O3 or Ti3O5, which is dissolved during the subsequent Al2O3 growth

Aluminum oxide was deposited with a CVD technique onto polished normalTiC‐normalcoated and uncoated cemented carbide substrates. Experiments of short duration were performed with varying process parameters. The density of Al2O3 nuclei on the false(normalTi,Wfalse)C phase in cemented carbide and on the normalTiC coating was measured for each experiment. The corresponding chemical equilibria of the gas phase were calculated. The supersaturations of reactants false(pnormali,normalin/pnormali,normaleqfalse) were compared with the nuclei densities. It was found that the densities of nuclei vary with the supersaturation of the oxygen donors ( CO2 and H2O ). In these experiments there was a relative shortage of oxygen donors. The early stage of the Al2O3 deposition is supposed to influence the adhesion of the coating in two ways. First, voids that form in the cemented carbide substrate surface during heating up may cause pores, which decrease the strength of the interface. Second, too sparse a nucleation weakens the bond between the coating and the substrate. By using normalTiC‐normalcoated cemented carbide as the substrate it is possible to avoid the void formation and to obtain an evenly distributed nucleation.

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.

In spite of chemically vapor deposited (CVD) TiC-coated cemented tungsten carbide cutting tools being widely used, the influence of intermediate zones of eta phase (e.g., M12C

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