Novel structured anodic oxide films containing surface layers and porous sublayers showing excellent wear resistance performance

Abstract

The fabrication of porous structures on aluminum surfaces can be performed readily and routinely using anodic oxidation technology. However, the design and preparation of smooth and compact films on the porous structures is difficult to achieve. In this work, by replacing Al cathode with graphite cathode and also by tailoring the Al³⁺ concentration, anodic oxide films (AOFs) possessing novel multilayer structures which contain compact surface layers, porous sublayers and barrier layers and which show excellent wear resistance were fabricated via common anodic oxidation technology. The surface morphologies and chemical compositions of the as-prepared films were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The results showed that the surface layer was amorphous aluminum oxide. The surface microhardness of the as-prepared AOFs was 2 to 7 times that of the untreated Al alloys. The thicknesses of the surface layer and porous sublayer were manipulated by the anodic current density and the concentration of sulfuric acid. The wear resistance properties of the anodic oxide film were characterized using a UMT-3 tribometer. When the sample was anodized in a 90 mg mL⁻¹ sulfuric acid bath and the current density was 5 A dm⁻² (AOF2-A), the wear rate significantly decreased to 1.81 × 10⁻⁵ mm³ N⁻¹ m⁻¹ under dry sliding and 7.54 × 10⁻⁶ mm³ N⁻¹ m⁻¹ under seawater sliding. The wear rate of the AOF2-A sample was 3 orders of magnitude lower than that of 2024 Al alloys. It was found that the surface layer of the AOF2-A sample, which showed high hardness and excellent toughness, presented the lowest wear rate owing to columnar wear debris in the wear track, which acted as a rolling log in the sliding process. The influences of the anodic oxidation current density and the concentration of sulfuric acid on the wear resistance and the self-lubrication mechanism of desquamated wear debris in the sliding process were discussed in detail.