Three wetting phases in the chemically reactive MgO/Al system.

The wetting characteristics, by molten aluminum, of three types of MgO, two sintered and one single crystal (100), were examined by means of the improved sessile drop method to make clear the effects of interfacial reactions and the difference of surface state on wetting.The reaction zone products vary with the initial characteristics of the ceramics such as impurity content and grain boundaries. The contact angle in each system progresses through three phases (I, II, III). Similar contact angles were produced in phase II, regardless of the type of MgO, although the single crystal showed a slightly higher angle than the polycrystalline types. In phase III, however, contact angles become significantly different.The phase III contact angle of the single crystal decreases more slowly than that of the sintered types since interfacial reactions progress relatively slowly. Different reaction products were formed at the interface depending on the type of MgO. These observations suggest that the chemical reactions at the interface must be clarified to understand the phase III behavior of wetting systems.