FORMATION OF AMORPHOUS ANODIC OXIDE-FILMS OF CONTROLLED COMPOSITION ON ALUMINUM-ALLOYS

Binary, non-equilibrium Al-29at%Nb, Al-44at%Ta, Al-19at%Ti, Al-25at%Ti and Al-32at%Zr alloys were prepared by magnetron sputtering and subse quently anodized at high Faradaic efficiency to grow barrier-type anod ic films. Examination in the transmission electron microscope revealed amorphous anodic films of relatively uniform compositions across the film thicknesses, except for a layer of relatively pure alumina, of ab out 5% of the film thickness, present at the film/electrolyte interfac e of the AI-Ta alloy. The film compositions, from Rutherford backscatt ering spectroscopy, indicate that the alloy constituents are oxidized in their alloy proportions to form films comprising intimately mixed u nits of the various oxides, namely alumina, niobia, titania, tantala a nd zirconia. The films grow by co-operative transport of metal and oxy gen ions under the electric field with formation of film material by b oth migration of metal ions outwards and of oxygen, and possibly hydro xyl, ions inwards. The average migration rates of Al3+, Nb5+, Ti4+ and Zr4+ ions are similar, to within 10%, but Ta5+ ions migrate more slow ly than Al3+ ions. The results of the study show that a wide range of compositions of amorphous oxide films can be readily formed by anodic oxidation oi appropriate alloys, including compositions containing uni ts of normally crystalline anodic oxides, namely TiO2 and ZrO2. (C) 19 97 Elsevier Science S.A.