Phase Transformations of Metallorganic Chemical Vapor Deposition Processed Alumina Coatings Investigated by In Situ Deflection

International audiencePhase transformations of Al2O3 films, deposited by metallorganic chemical vapor deposition from aluminium tri-isopropoxide on AISI 301 stainless steel, were investigated using an original technique of deflection associated with X-ray diffraction and electron microscopy. The samples were first oxidized at 1123 K in air to obtain a 0.9 m thick Cr2O3 protective oxide film on one side of the samples. Then, 1 m thick amorphous Al2O3 films were deposited on the opposite side at 823 K and 2 kPa. The deflection of such dissymmetrical samples was recorded during anisothermal treatments, consisting in slow heating to 1173 K in Ar atmosphere. The coefficient of thermal expansion of both the Cr2O3 and the amorphous Al2O3 films was determined to be 710−6 K−1 and 14.7 10−6 K−1, respectively. Crystallization kinetics of amorphous to mainly -Al2O3 become significant at temperatures equal or greater than 983 K. Transformation of metastable Al2O3 to -Al2O3 is initiated below 1173 K. It is demonstrated that deflection is a powerful tool for investigating the behavior of thin films deposited on a substrate and especially to reveal transformations occurring in these films during heat-treatments