Mechanical properties of anodic oxides on aluminium from nanoindentation

The elastic modulus and the hardness of porous anodic alumina films, of thickness between 5 - 40 mm, formed on 1050 aluminium alloy have been determined using nanoindentation. The results are compared with previous findings for barrier-type anodic films, of thickness to 500 nm, with indent depths about 50 nm. For porous films, measurements were made in section and in plan. Nanoindentation of the porous films, formed in sulphuric acid solution at temperatures from 0 to 35°C, revealed property variations through the film thickness, due to chemical degradation of the film by the electrolyte. Films formed for constant times revealed an increase in hardness with reducing temperature, while films formed for various times to achieve a constant thickness revealed approximately no variation of mechanical properties with temperature. Typically the elastic modulus and hardness of the films were about 90 GPa and 7 GPa respectively in the inner film regions least affected by chemical degradation. These results compare with about 130 and 7-8 GPa respectively determined by nanoindentation for barrier-type films.