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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

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