AES and ELS characterization of surface oxides on Al-Li alloys

Abstract

Auger electron spectroscopy (AES) and electron loss spectroscopy (ELS) have been performed in order to investigate the thin oxide layers formed on the surface of an aluminium (Al-Li) alloy including the alloying elements: lithium, copper, magnesium, zirconium. As on pure aluminium, the naturally formed surface layers on the (Al-Li) sample consists of an Al 2O 3 oxide film contaminated by carbon in the graphite form. This impurity is readily removed by argon ion bombardment. On the contrary, no carbon contamination is detected on (Al-Li) alloy after air oxidation at 700 K, which mainly contains magnesium and lithium oxides in the top layers. Heat treatments in vacuum induce the diffusion of Mg and Li through the Al 2O 3 grain boundaries and the initial Al 2O 3/alloy system is then changed to a MgO, Li 2O/Al 2O 3/alloy one. These oxides are characterized by typical Auger spectra and loss structures; they were compared to those recorded from pure MgO and Al 2O 3 samples. Changes of the O KLL emission (quasi-atomic character, width of the main O KL 2,3L 2,3 structure, energy differences and relative intensities between the Auger lines) are interpreted in terms of oxygen ionicity. The ELS spectra of the multi-oxide system appears as a superimposition of the loss structures of the different phases. A careful examination of the thresholds for individual excitations, excitonic states, plasmon structures and interband transitions is reported. The relative content of lithium and magnesium oxides in the top layers is affected by the primary electron beam because lithium oxide is not stable in vacuum under electron irradiation.