Unravelling the evolution of oxides at the aluminum liquid/vapor interface

The present study characterizes the evolution of oxides at the Al liquid/vapor interface by combining high-temperature wetting, droplet-pushing experiments, in-situ observation of oxide clusters, analysis of the quenched droplets and thermodynamic calculations. The droplet shape of liquid Al on alumina substrates is analyzed and the magnitude of its deviation from a spherical cap is considered as an indicator of the level of oxides present at the surface. Characterizing whether the sessile droplet recovers its spherical shape or not when pushing on its top part reveals that the oxides at the liquid/vapor interfaces become dense at low temperatures, but are almost completely removed above 1100 °C. In-situ observation of the oxides and the subsequent analysis of the quenched droplets confirm this evolution. Wetting experiments combined with 3D-rebuilted structures of quenched samples reveal that the presence of dense oxides prevents the spreading of liquid Al droplets. By thermodynamic calculations and measuring evaporation rates of droplets, it is confirmed that the Al ₂ O gas produced by reaction between oxides and liquid Al contribute significantly to the oxides elimination at high temperatures. Our results are relevant for improving the control of the spreading of liquid metals among many high-temperature processes, including welding, coating and composites fabricating.