Chemical composition of thin oxide films on aluminium studied with infrared and visible spectroscopic ellipsometry

The knowledge of the composition and structure of hydrated films on aluminium is important for the bonding between aluminium and polymers. Native surface layers on aluminium consist of an anhydrous inner layer of amorphous A1203 and a hydrated outer layer, where the outer layer determines the adhesion properties. On the one hand the pseudoboehmite oxyhydroxide surface can be seen as a model for the chemistry found at the surface of the air-formed film after ambient exposure of the aluminium metal. On the other hand pseudoboehmite layers are promising for improving the adhesion between polymers and aluminium. Spectroscopic Ellipsometry in the visible (VISSE) and IR (IRSE) are powerful tools for the morphological (thickness and refractive index) and chemical characterisation of oxides formed on aluminium. Both optical techniques are used to study anodizing and hydration under boiling water conditions of aluminium. The growth in thickness and the chemical composition of the oxide formed is shown. The porosity of the layer together with the chemical composition is obtained and the changes as a function of boiling time are presented. In addition the above type of hydration is applied on the anodically formed barrier oxide film. IRSE shows that the hydration of this type of film by immersion in boiling water results in the conversion of aluminium oxide to pseudoboehmite. Finally the further oxidation of a native oxide film on aluminium is measured; the results are compared with the growth of the hydrated layers.