Vickers microhardness of AlSi(Cu) anodic oxide layers formed in H2SO4 at low temperature

The effects of substrate composition and anodizing conditions (i.e. current density and anodizing time) on the Vickers microhardness of anodic oxide layers formed in 2.25 M H2SO4 at 0 °C on Al, AlSi10 and AlSi10Cu3 permanent mold cast substrates were assessed. The microhardness of the substrates and anodic oxide layers was determined on the polished cross-sections using a 25-g load. The results indicated that the hardest substrate, i.e. AlSi10Cu3 resulted in anodic oxide layers with lowest average microhardness (e.g. 222±78 HV0.025 at 4.2 A dm−2 and 50 min) followed by AlSi10 (489±63 HV0.025) and Al (541±28 HV0.025). Increasing the current density from 3.0 to 6.0 A dm−2 showed no improvement in average microhardness measured at half layer thickness. Anodizing times exceeding 25 min resulted in a decreased average microhardness for the binary and ternary compositions. Variations of microhardness across the layer thickness were encountered for all substrates anodized for 50 min. Their extent, however, depended on substrate composition and layer thickness. The main differences in layers’ microhardness could be associated with the defects generated by silicon and copper containing constituent particles during anodizing. Their presence hinders reliable microhardness measurements.