PHASE EQUILIBRIA AND INTERFACIAL PHENOMENA IN THE Mg-Al-O SYSTEM AT 800°C

Abstract
The wetting of ceramic surfaces by aluminum alloys has been studied using Al-Mg-O as a model system. This chemical system was chosen since it is technologically important and high-purity, well-characterized materials are readily available. Magnesium alloyed with the aluminum sessile drop and silicon picked up from the experimental apparatus cause an initial reduction in contact angle by altering the protective nature of the oxide film formed on the sessile drop. Evidence of spreading is observed as an intermediate process in the reactive sessile drop pairs. Reactions between the Al-Mg alloys and sapphire (Al₂O₃), spinel (MgAl₂O₄), and periclase (MgO) are interpreted with predicted phase stability and the change in magnesium activity of the alloy due to volatilization. Only the rate of the periclase-alloy interaction was rapid enough to result in a continuous product layer after 24 hours. Contact angles and surface energies reflecting a true Al-Al₂O₃ interface are reported.