Influence of crystal orientation and surface topography of aluminum substrate on pore nucleation of anodic porous alumina

Pore nucleation and growth processes of anodic oxide films formed on aluminum substrate were investigated by atomic force microscopy with focusing on the crystal orientation and surface topography of aluminum substrate. Nanotopographies of electropolished aluminum were extremely affected by the crystal orientation of aluminum substrate. For as-received aluminum, regularly aligned striped structure appeared after electropolishing. On the other hand, aluminum substrate annealed at 300°C exhibited isotropic hexagonal cell structure. From X-ray diffraction patterns, it was confirmed that the preferential crystal orientation of aluminum was changed from (110) to (100) when annealing temperature was higher than 300°C. In the initial stage of anodization, specific nanotopography of electropolished aluminum surface was served as the initiation sites for pore generation, and accordingly it influenced cell arrangement. Using planarized aluminum with less than 0.3 nm asperities, however, a large number of fine pores initiated on the oxide film surface. Thus, the growth of anodic oxide films was seriously influenced by the surface topography of aluminum substrate.