INFLUENCE OF AR-BOMBARDMENT ON THE INITIAL INTERACTION OF WATER-VAPORWITH POLYCRYSTALLINE MAGNESIUM SURFACES( ION)

The room temperature interaction of water vapour with magnesium surfac es irradiated with Arf ions in the dose range theta = 10 to 2000 ions/ surface atom and ion energy range 1 to 5 keV has been systematically s tudied by Auger electron spectroscopy (AES). The character of the kine tics of water interaction with irradiated surfaces has been found to b e dependent upon the total ion bombardment dose and the ion energy and to change with the level of water exposure. The effect of ion bombard ment was found to be most pronounced in the oxide nucleation and growt h stage of the oxidation process. The dissociative chemisorption and f inal bulk thickening regimes were only weakly affected by prior irradi ation. The results have been interpreted based on the assumption of co mpetition between the effects of radiation defects (vacancies, vacancy clusters, dislocation loops) and implanted argon atoms on the oxidati on process. The effect of vacancy-type defects was speculated to be th e provision of adsorption sites of high sticking probability and nucle ation sites of reduced activation energy for place exchange and subseq uent island growth. The effect of implanted argon atoms was speculated to be the blocking of adsorption and nucleation sites and interferenc e 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 enhanc ed penetration was observed for shorter water exposures (0.3 L). The l imiting thickness of the oxide layer formed on magnesium at room tempe rature was not found to be affected by the level of prior ion bombardm ent.