Thick and macroporous anodic alumina membranes for self-lubricating surface composites

Aluminum-base self-lubricating surface composites require thick and macroporous alumina membranes to add lubricants easily. The thick anodic alunima films obtained in sulfuric acid and oxalic acid are always microporous, while the macroporous films in general phosphoric acid are very thin. In this paper, thick and macroporous alumina layers were fabricated successfully with a phosphate-base solution. The anodizing process in this compound solution and the effects on the thickness of the oxide films were investigated in detail. The results indicate that the growth of porous layers in this solution undergo three stages during anodizing, same with in the general solution. This electrolyte is sensitive to anodizing temperature which affects current density in great degree. The additive (including organic carboxylic acid and Ce salt) in the phosphate-base electrolyte plays an important role on the increase of film thickness. In this compound electrolyte, the pore diameter of the anodic alumina membranes is about 100 nm in average, pore density is 18 pores mm 2 and porosity 14%. The thickness can exceed 20 mm with appropriate anodizing time and temperature.