Abstract
Several double anodic oxidation coatings were formed on aluminum under different anodizing conditions (such as bath temperature and current density) and the effiects of anodizing conditions on surface properties and structure of coatings were studied. A hard anodized coating of excellent surface properties was obtained under a constant current density of 4Amp/dm2, in which the 1st layer (upper layer) was formed by the ordinary anodizing method (20°C, 30min.) and the 2nd layer (lower layer) was formed by the hard anodizing method (-5°C, 30min.). In particular, the coating formed by the 1st anodizing at 10°C for 30min. and the 2nd anodizing at -5°C for 30min. showed few cracks, a very plain surface, and a high wear resistance. When the bath temperature was constant at -5°C, the coating formed under a low current density in the 1st anodizing and under a high current density in the 2nd anodizing showed a wear resistance higher than that formed by the reverse order of treaments. However, the double anodic oxidation coatings under differrent current densities showed little differences in wear resistances according to the order of treatments for the 1st and 2nd processes; in contrary to the same conditions (with respect to the order of treatments) in different bath temperatures. The effect of structure of the cell on wear resistance was pursued. The coating formed under the condition for a large cell size in the 1st layer and a small cell size in the 2nd layer showed a remarkable decrease of wear resistance near the interface of the two layers. On the contrary, a high wear resistance was obtained by the reverse condition; that is, the condition for a small cell size in the 1st layer and a large cell size in the 2nd layer.
