EFFECT OF MICROSTRUCTURE ON PASSIVE FILM FORMATION AND BREAKDOWN IN SPUTTER-DEPOSITED AL-TA ALLOY-FILMS

The role played by microstructural features in the passivation and bre akdown processes which lead to pitting has been examined for the first time by the new techniques of dynamic imaging microellipsometry (DIM) and local electrochemical impedance spectroscopy (LEIS). Contrary to expectations, an examination of the passivation of sputter-deposited A l-Ta alloy films found that the film on the Al3Ta precipitates formed a thicker passive layer than that on the solid solution matrix surroun ding the precipitates. The LEIS study of the microstructural effects u nder breakdown conditions found that the film on the Al3Ta precipitate had a higher capacitance than the dealloyed region surrounding it, in dicating that the film on the precipitate has different properties fro m the dealloyed region in its vicinity. This difference in film thickn ess and properties, found by both DIM and LEIS, may be responsible for the breakdown which leads to pitting initiated in the region adjacent to the precipitates.