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Moisture-induced instability at the Al2O3/Ni3Al(110) interface: interfacial chemistry

Abstract STM, XPS, AES and LEED have been used to characterize the growth of ~ 1 nm thick Al2O3 films on Ni3Al(110), and on Ni3Al(111), and their reactivity towards H2O pressures (PH2O) >10−6 Torr, 300 K. As-grown films on Ni3Al(110) exhibit a true oxide layer with an incommensurate interface involving both aluminum and oxygen atoms in intermediate oxidation states. Upon exposure to PH2O > 10−5 Torr, this oxide film undergoes severe reorganization and reconstruction, and the eventual loss of all long-range order. This effect is specific to H2O; similar exposures to O2 have no effect. STM measurements indicate that this effect is pressure- rather than exposure- dependent, indicating that oxide reconstruction is due to a cooperative surface reaction involving multiple H2O molecules at a single surface site. This effect is not correlated with surface hydroxylation, observed only for PH2O > 1 Torr, in common with other alumina surfaces. The H2O-induced instability is also observed for the Al2O3/Ni3Al(111) films of similar thickness and composition but at much longer exposure time at a given water vapor pressure. The results suggest that the incommensurate nature of the Al2O3/Ni3Al(110) interface is important in the moisture-induced spallation of alumina scales.