PROPERTIES OF ALUMINA FILMS PREPARED BY LOW-PRESSURE METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION

Thin amorphous alumina films were deposited on AISI 304 by thermal decomposition at low pressure (0.17 kPa (1.25 Torr)) of aluminium-tri-sec-butoxide (ATSB). The effect of the deposition temperature (within the range 200-380 degrees C) has been studied in relation to film properties including the protection of the underlying substrate against high-temperature corrosion, the chemical composition of the film, the microstructure and its refractive index. The activation energy for the heterogeneous reaction was 65 +/- 2 kJ mol(-1). Corrosion experiments, performed at 450 degrees C in a hydrogen sulphide gas, showed that the alumina coatings giving the best protection were obtained at a deposition temperature of 280 degrees C. The alumina films showed an amorphous structure, in agreement with the index of refraction. Transmission electron microscopy analysis revealed extremely line grains of gamma-Al2O3/AlO(OH). Only OH groups were found as an ''impurity'' in the oxide film. Carbon was not detected.