Anodic film growth in the Al–Ta alloy system

The ionic transport numbers, relative migration rates of cation species and formation ratios are reported for barrier anodic films formed on metastable, solid solution Al–Ta alloys, with compositions extending from the aluminium-rich to the tantalum-rich ends of the system. The data were obtained by marker experiments, using ion implanted xenon, transmission electron microscopy, Rutherford backscattering spectroscopy and medium energy ion scattering. The films are amorphous and form by migration of metal and oxygen species. The ionic transport numbers and the formation ratios depend approximately linearly upon the composition of the alloy, between the values for anodic alumina and anodic tantala. The migration rate of Al3+ ions exceeds that of Ta5+ ions, but reduces in relative magnitude as the tantalum content of film increases. The faster migration of Al3+ ions is consistent with the higher energy of the Ta5+–O bond compared with that of the Al3+–O bond. Due to the difference in mobilities of the cation species, the films comprise an outer layer of alumina and an inner layer containing units of both alumina and tantala. The two-layered films can develop fingers of inner layer oxide that penetrate the outer alumina layer due to the higher ionic resistivity of the alumina. Such channelling of current can lead to mixing of inner layer oxide and alumina and thereby hinder formation of an alumina layer, particularly in films on the more tantalum-rich alloys.