Improving the wear resistance of aluminum by a nickel-filled anodized porous alumina layer

Abstract

Anodizing is a widely used method for increasing the wear and corrosion resistance of aluminum alloys. However, it is known that the porous brittle alumina layer can easily crack during tribological stresses, which results in the detachment of large particles from the worn surface. Electroplating, on the other hand, can produce a compact ductile metallic coating with enough wear resistance, though a specific pretreatment like the zincate process is needed for an aluminum substrate. In this paper, a combination of the two methods, anodizing and electroplating, is applied on the surface of pure aluminum intending to improve the wear resistance. Anodizing was performed in oxalic acid by applying a constant current density of 2 A/dm2. The anodized samples were then immersed in phosphoric acid for up to 10 h to increase the diameter of the pores. Nickel electroplating was carried out on the anodized samples at a current density of 0.5 A/dm2 in a Watts solution. Cross-sectional studies show that there is an optimum pore widening time that establishes a good adhesion between nickel and anodic oxide layers, while the structure of the oxide film is largely preserved. Sliding wear tests were performed on the samples using a ball-on-disk reciprocating tribometer. The results show much higher wear life for all Ni/Al2O3 composite coatings as compared with the alumina layer. The maximum wear life was obtained for the sample etched for an optimum pore widening time of 4 h.