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A stagnation point cold-wall reactor was used for the CVD of corundum alumina (a-Al 2 O 3 ) on metallic substrates. Depositions were carried out under low pressure using the thermally induced pyrolytic oxidation of aluminum tri-isopropoxide (ATI). The effects of the substrate temperature (300±1080 C) and the total pressure (50±250 mbar) on the growth rate and morphology of the deposits were investigated. An excess of oxygen facilitates the formation of dense alumina films. Precursor depletion was prevented using high gas velocity, low ATI concentration, and a high temperature gradient. X-ray diffraction (XRD) analysis provided evidence of corundum alumina deposition at substrate temperatures above 1000 C.

Coated nickel-based superalloys are widely used as superior material systems in gas turbines, where the overlay coatings help withstand hot corrosion and serve as bond coats for the thermal barrier layer. However, interdiffusion may limit the coatings' lifetime, a potential remedy being a diffusion barrier layer. In this context, the chemical vapor deposition ͑CVD͒ process was investigated to deposit ␣-Al 2 O 3 on superalloy substrates. Deposited films using an AlCl 3 /CO 2 /H 2 gas mixture at low and atmospheric pressure were characterized. Difficulties including whisker growth at low pressure and the growth of noncompact large crystals at atmospheric pressure are discussed jointly with strategies for suppressing them. In addition, the potential of alternative precursors to produce ␣-Al 2 O 3 films was investigated using aluminum tri-isopropoxide in an adequate reactor geometry.

The present study concerns the deposition of α–Al2O3 for diffusion barrier applications on superalloy substrates. The growth of α–Al2O3 has been achieved by chemical vapor deposition (CVD) using an AlCl3/CO2/H2 gas mixture at 1080 °C. Among several growth-controlling parameters with potential importance for the whisker growth process, the reactor pressure during deposition seems to be highly influential on the resulting film structure. Deposited films at low pressure presented solely a fine whisker structure. This non-closed structure is not suitable as diffusion barrier; however, the observed high porosity makes the deposit a potential candidate as a catalysis support. An increase of the deposition pressure led to a competitive growth of whiskers and grains. A suitable microstructure was attained at relatively high pressure (100 mbar) where the surface was fully covered by 2 μm large alumina crystals that formed a closed structure. Further increase of the pressure led to an irregular and rough surface microstructure.

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