Mechanical properties of barrier-type anodic alumina films using nanoindentation

This work uses nanoindentation to determine the mechanical properties of barrier-type anodic alumina films formed on aluminium in ammonium pentaborate electrolyte. Using indents to a depth of ∼45 nm and a maximum load of 0.5 mN, with films of thicknesses 480 and 600 nm, values of ∼140 and ∼8.5 GPa were indicated for the elastic modulus and the hardness, respectively. Lower values were obtained for deeper indents, at higher loads, and for specimens with thinner films. The reduction of hardness was consistent with the commonly applied rule that the indent depth should not exceed 10% of the film thickness in order to avoid influences of the substrate. Although substrate properties can affect modulus measurement to shallower depths, the similarity of results for the two thickest films suggests a reasonable estimate of the film property.