Comparative study of mechanical and tribological properties of alumina coatings formed on aluminium in various conditions

Influence of oxalic acid addition to sulphuric acid bath on the mechanical properties of the oxide layers formed on aluminium has been examined. For this purpose two Doehlert experimental designs with three variables (temperature, current density, sulphuric acid concentration) and four variables (oxalic acid concentration, temperature, current density, sulphuric acid concentration) were realized. Four responses were studied namely: growth rate (Ve), Vickers microhardness (D), weight loss after abrasion (Wa) and deflection at failure (Df) of the anodic oxide layer. A comparative study based on surface responses was achieved. Compared with sulphuric acid bath, it was found that the addition of oxalic acid permits high growth rates, high abrasion resistance and high microhardness but less ductile layers. The observed mechanical properties of the oxide layers can be related to their morphology revealed by SEM observations and their chemical composition determined by GDOES.