Determination of Anodized Aluminum Material Characteristics by Means of Nano-Indentation Measurements

An aluminium AD-3 has been anodized under four different conditions, namely at low temperature (−5˚C), room temperature (25˚C), with and without sealing the anodized coating in boiling distilled water. The solution used for formation of alumina layer in all cases was an electrolyte containing 180 g/l sulphuric acid at a constant forming voltage (voltastatic anodizing). In order to assess the mechanical properties of the obtained anodic alumina layers a series of nanoindentation tests was performed employing different indentation procedures. The two mechanical characteristics of the alumina films, the indentation hardness (HIT) and the indentation modulus (EIT), were determined by means of the instrumented indentation and the Oliver & Pharr approximation method. All measurements were done on Agilent G200 Nanoindenter fitted with a diamond Berkovich type tip. Time dependent effects were investigated by tests with different peak hold time and different loading rate. The change of the mechanical properties with indentation depth was also examined. The effect of the working temperature during the growth of the alumina layers and the influence of the pore sealing on the mechanical properties are evaluated via comparison of the average load-displacement curves. The role of the temperature of the electrolyte and the sealing process during the formation of the alumina films, with respect to possible changes of their chemical composition and structure, are discussed in order to explain the observed differences in the measured load-displacement curves and the determined HIT and EIT.