NANOSCALE ENRICHMENTS OF SUBSTRATE ELEMENTS IN THE GROWTH OF THIN OXIDE-FILMS

The enrichment of alloying elements as a consequence of formation of a morphous anodic films on relatively dilute, metastable binary aluminiu m alloys, prepared by magnetron sputtering, has been determined quanti tatively employing Rutherford backscattering spectroscopy for a range bf alloying elements. The measurements relate to the enrichments that must be developed in order for oxidation of the alloying element at th e alloy/film interface to proceed. The enrichments, occurring in a thi n layer of alloy of 1-5 nm thickness immediately beneath the anodic fi lm, can be correlated with the Gibbs free energy per equivalent for fo rmation of the alloying element oxides relative to that of alumina. Th e enrichment increases progressively, approximately linearly, for allo ying elements associated with oxides of increasingly higher Gibbs free energy per equivalent, with no enrichment for alloying elements assoc iated with oxides of lower Gibbs free energy per equivalent. A thermod ynamic approach per se is insufficient to explain the enrichment pheno menon completely. (C) 1997 Elsevier Science Ltd.