Influence of Ar + ion bombardment on the initial interaction of water vapour with polycrystalline magnesium surfaces

Abstract

The room temperature interaction of water vapour with magnesium surfaces irradiated with Ar + ions in the dose range θ = 10 to 2000 ions/surface atom and ion energy range 1 to 5 keV has been systematically studied by Auger electron spectroscopy (AES). The character of the kinetics of water interaction with irradiated surfaces has been found to be dependent upon the total ion bombardment dose and the ion energy and to change with the level of water exposure. The effect of ion bombardment was found to be most pronounced in the oxide nucleation and growth stage of the oxidation process. The dissociative chemisorption and final bulk thickening regimes were only weakly affected by prior irradiation. The results have been interpreted based on the assumption of competition between the effects of radiation defects (vacancies, vacancy clusters, dislocation loops) and implanted argon atoms on the oxidation process. The effect of vacancy-type defects was speculated to be the provision of adsorption sites of high sticking probability and nucleation sites of reduced activation energy for place exchange and subsequent island growth. The effect of implanted argon atoms was speculated to be the blocking of adsorption and nucleation sites and interference with oxide island ordering. At relatively high water exposures (20 L) there was enhanced penetration of oxygen into the magnesium lattice postulated to occur along dislocation emergence points. No such enhanced penetration was observed for shorter water exposures (0.3 L). The limiting thickness of the oxide layer formed on magnesium at room temperature was not found to be affected by the level of prior ion bombardment.