Critical Factors Affecting the Wettability of α‐Alumina by Molten Aluminum

Abstract

The wetting behaviors of α‐Al2O3 single crystals with three different faces—R(0112), A(01120), and C(0001)—and polycrystals (PC) by molten aluminum were studied over a wide temperature range using both a conventional and an improved sessile‐drop method. The critical factors affecting the wettability, such as temperature, atmosphere, substrate surface roughness, and crystallographic orientation, and the influence from the experimental technique, were thoroughly investigated. The results show that the aluminum surface oxidation and the thickness of the oxide film have a pronounced effect on the wettability, especially at low temperatures. To eliminate this effect, the experimental temperature must be over a critical value. Vacuum favors lowering this value compared with atmosphere, and the improved sessile‐drop method, particularly using an impingement‐dropping mode (I‐mode), helps to weaken this effect by mechanical disruption and removal of the oxide film. However, the dropping distance and the dropping force must be controlled to prevent an overspreading of the drop. The effects of the substrate surface roughness and temperature are not significant in the case of a clean aluminum surface and a fine‐prepared alumina surface. On the other hand, the effect of the alumina surface crystallographic orientation is noticeable and the wettability is in the order of R > A > PC > C. The intrinsic contact angles of the Al/α‐Al2O3 system in the temperature range of 1000°–1500°C were estimated to be 76°–85° for the R and A faces, 88°–100° for the C face, and 77°–90° for the polycrystal, depending on the temperature.