Nanoscale enrichments of substrate elements in the growth of thin oxide films

The enrichment of alloying elements as a consequence of formation of amorphous anodic films on relatively dilute, metastable binary aluminium alloys, prepared by magnetron sputtering, has been determined quantitatively 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 the alloy/film interface to proceed. The enrichments, occurring in a thin layer of alloy of 1-5 nm thickness immediately beneath the anodic film, can be correlated with the Gibbs free energy per equivalent for formation of the alloying element oxides relative to that of alumina. The enrichment increases progressively, approximately linearly, for alloying elements associated with oxides of increasingly higher Gibbs free energy per equivalent, with no enrichment for alloying elements associated with oxides of lower Gibbs free energy per equivalent. A thermodynamic approach per se is insufficient to explain the enrichment phenomenon completely. (C) 1997 Elsevier Science Ltd.