EQUILIBRIUM CONTACT-ANGLE IN THE MAGNESIUM-OXIDE ALUMINUM SYSTEM

The change in the contact angle between two kinds of MgO [sintered sam ple and single crystal (100) sample] and molten aluminium over time wa s measured at 1373 K. The atmosphere was a purified He-3vol%H-2 where the formation of the alumina is prevented, and the pressure of the atm osphere was 1.2 arm (1.2 x 10(5) Pa) to minimise the vaporisation of a luminium. Using a logarithmic time scale, three distinct wetting phase s (I, II, III) were easily observed in both systems, as is typical in reactive systems. Phase IV (equilibrium-wetting phase), however, was n ot observed in the present systems even when the experiment was conduc ted over a long time. Instead, when investigating the change in interf acial area (interfacial diameter) with the change in contact angle, fo ur detailed stages were observed in phase III (interfacial-reaction-we tting phase). These stages in phase III were caused by a decrease in t he volume of the molten aluminium due to the interfacial reaction and the vaporisation of the drop. The progress through the stages represen ted a transition in contact angle from an advancing contact angle to a receding contact angle. In systems where the decrease in volume is to o great, the equilibrium contact angle cannot be obtained directly. He re, a method is proposed for the estimation of the equilibrium contact angle in such a system. Copyright (C) 1996 Acta Metallurgica Inc.