Examination of Oxide and Hydroxide Films Grown on Aluminium and some Aluminium Alloys

Abstract

Aluminium oxide and hydroxide films grown on aluminium sheet and evaporated aluminium films were studied using a number of techniques. Electron Spectroscopy for Chemical Analysis (ESCA), Ellipsometry and a combination of Transmission Electron Microscopy TEM, Scanning Transmission Electron Microscopy STEM and Scanning Electron microscopy SEM, known as (TEMSCAN). For a radiation wavelength of 549 nm the refractive index for aluminium hydroxide films less than 80 nm was found to be n = 1,58 +- 0.03. For thicker films it was necessary to assume a k value of 0.0009 rather than zero. There were discrepancies between aluminium oxide and hydroxide film thicknesses as measured by ESCA, using the ratio of chemically shifted and unshifted peaks. This discrepancy is explained by assuming that the oxide and hydroxide film grows in islands and the height of these islands is greater than the escape depth of the electron. The ESCA technique therefore was not detecting oxide or hydroxide at the base of the islands. Examination of electron transparent specimens after exposure to water vapour saturated air (WVSA) at 70° C in the SEM mode, revealed reaction sites in the forms of blisters. Examination of the same area in STEM, only showed the underlying microstructure due to the fact that blistering is a surface phenomenon. Electron transparent specimens prepared from sheets of aluminium alloy (A15%/Ng) and (A15%/Zn), exposed to WVSA for 10 minutes were examined in TEM, STEM and SEM specimens prepared from sheets heated at 350° C and 450° C showed reaction sites along the grain boundary surface interface whereas specimens prepared from 550° C heated sheets showed randomly distributed reaction sites. Closer examination (in TEM mode) of the grain boundary surface interface revealed hydrogen bubbles which could be viewed by their stress field contrast.