Interface thermodynamics of ultra-thin, amorphous oxide overgrowths on AlMg alloys

A thermodynamic model has been presented for the prediction of the type of initial, amorphous oxide overgrowth (i.e. am-Al2O3, am-MgO or am-MgAl2O4) on bare AlMg substrates as a function of the Mg alloying element content at the substrate/oxide interface, the growth temperature and the oxide-film thickness (up to 5 nm). On the basis of the macroscopic atom approach, expressions have been derived for the estimation of the energies of the interfaces between the AlMg substrate and the competing am-Al2O3, am-MgO and am-MgAl2O4 overgrowths. For all cases a strong driving force has been revealed for the interfacial (chemical) segregation of Mg. am-Al2O3 was found to be the most stable amorphous oxide phase on the AlMg substrate for T < 610 K; its relatively high stability is governed by a relatively low interface energy. The model predictions are in good agreement with corresponding experimental results on the chemical constitution of ultra-thin amorphous oxide films grown on Al- and Mg-based alloy substrates in the temperature range of 300–400 K.