Equilibrium contact angle in the magnesium oxide aluminium system

The change in the contact angle between two kinds of MgO [sintered sample and single crystal (100) sample] and molten aluminium over time was 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 atmosphere was 1.2 arm (1.2 x 10(5) Pa) to minimise the vaporisation of aluminium. Using a logarithmic time scale, three distinct wetting phases (I, II, III) were easily observed in both systems, as is typical in reactive systems. Phase IV (equilibrium-wetting phase), however, was not observed in the present systems even when the experiment was conducted over a long time. Instead, when investigating the change in interfacial area (interfacial diameter) with the change in contact angle, four detailed stages were observed in phase III (interfacial-reaction-wetting phase). These stages in phase III were caused by a decrease in the volume of the molten aluminium due to the interfacial reaction and the vaporisation of the drop. The progress through the stages represented a transition in contact angle from an advancing contact angle to a receding contact angle. In systems where the decrease in volume is too great, the equilibrium contact angle cannot be obtained directly. Here, a method is proposed for the estimation of the equilibrium contact angle in such a system. Copyright (C) 1996 Acta Metallurgica Inc.