Wetting of Polycrystalline α-Al₂O₃ by Molten Mg in Ar Atmosphere

The characteristic wetting behavior and modes of the evaporating Mg drops on the α-Al₂O₃ surfaces were evaluated by a combination of the variations in contact angle and drop geometry using both improved and modified sessile drop methods. The system is essentially partial wetting at 973 to 1173 K with initial contact angles of 94 to 67 deg, decreasing with increasing temperature. The wettability improves with interfacial reaction but deteriorates with Mg surface oxidation. The presence of the oxide film at the Mg drop surface strongly inhibits the movement of the triple line and thus makes the constant contact diameter mode dominate the entire wetting process. Removal of the oxide film greatly facilitates the depinning of the triple line. On the other hand, the interfacial reaction leads to the prior formation of MgO and later MgAl₂O₄ as a result of consumption of Mg, while it increases the amount of displaced Al. Nevertheless, the role of the interfacial reaction on the wetting is difficult to quantitatively evaluate because of concurrent diminishing in the drop volume under evaporation.