ANODIZING OF MULTILAYER ALLOY-FILMS FORMED BY SPUTTER-DEPOSITION OF VALVE METALS

Nominally uniform and amorphous sputter-deposited binary alloys (Al-Ta , Al-Nb, Al-Zr, Al-Ti) prepared by conventional magnetron sputtering o f valve metals are shown in reality to comprise fine scale, multilayer ed alloy films for particular conditions of deposition. The layers, ea ch about 1 nm thick, are the result of a periodic modulation of the co mposition of the alloy him during the deposition process, leading to l ayers relatively enriched and depleted in the alloying element compare d with the mean composition of the alloy. During subsequent anodizing at high Faradaic efficiency, amorphous anodic films, that are mainly u niform in thickness, composition and structure, are formed on the sput ter-deposited alloys indicating that the heterogeneity of the substrat e is not imparted to the bulk of the anodic film material. This is app arently due to the smoothing effect of the ionic transport processes d uring anodic him growth. However, near to the alloy film/anodic him in terface, significant variations in the composition of the anodic film, associated with the heterogeneity of the substrate, are evident, sugg esting that the variation in substrate composition is of importance to the formation and subsequent composition of relatively thin anodic fi lms and the inner regions of thick ones. Consideration is given to the use of multilayered alloy films as investigative probes of anodic him growth, in particular of the migration of Al3+ ions and cations of th e alloying element under the electric held, and the formation of anodi c film material at the alloy film/anodic film interface. Of wider rele vance, the heterogeneity of the substrate would be of critical importa nce to the use of sputter-deposited alloys in studies of dealloying, p assivation and pitting as well as in the practical use of such sputter -deposited materials.