The effect of thermal annealing on the properties of alumina films prepared by metal organic chemical vapour deposition at atmospheric pressure

Thin films deposited at 330°C by metal organic chemical vapour deposition on stainless steel, type AISI 304, were annealed in a nitrogen atmosphere for 1, 2 and 4 h at 600, 700 and 800°C. The film properties, including the protection of the underlying substrate against high temperature corrosion, the chemical composition of the film and the microstructure, were investigated. Corrosion experiments performed at 450°C in a hydrogen sulphide containing gas, showed that the cracks in the alumina films almost completely disappeared after a post-deposition heat treatment, probably as a result of stress relaxation. The porosity of the alumina films was not affected by this heat treatment. X-ray diffraction measurements of these films, deposited at 330°C, revealed an amorphous structure. Owing to the thermal annealing process, the amorphous alumina films were converted to λ-alumina, and OH-groups disappeared.