Wetting of alumina and spinel single crystals by molten aluminum

The wetting of sapphire (alpha-Al2O3) and spinel (MgAl2O4) single crystals by molten aluminum was investigated by the sessile drop technique from 1073K to 1473K. The measurements of the contact angle, the surface tension and the work of adhesion were performed by the optimization of the description of experimental aluminum sessile droplets using the Laplace Equation of Capillarity and an objective function. Different factors affecting the aluminum wetting of these single crystals that were studied include: (i) the roughness of the ceramic surface, (ii) the exposure time, (iii) the temperature, (iv) the crystal structure and (v) the inert/reactive character of the interface.; The results of the contact angles indicated that the inert alpha-Al 2O3 (0001)/Al couple increases the contact angles towards wetting with increasing time and temperature. The contact angles are significantly reduced towards non-wetting by increasing the roughness of the alpha-Al 2O3 (0001) substrate. The reactive MgAl2O 4 (111)/Al couple increases the contact angles towards wetting with time, temperature and the roughness of the MgAl2O4 (111) substrate. Relative to the roughness effect the inert sapphire and reactive spinel exhibit opposite trends.; Aluminum wetting of sapphire single crystal indicates that (101¯0) and (112¯0) planes are wetted more by molten aluminum than the (0001) plane. This is because of the structure of the interface. The planes (101¯0) and (112¯0) are oxygen terminated structures, while the (0001) plane is an aluminum terminated structure.; The effect of the crystal structure of MgAl2O4 illustrates that the (100) plane is more wetted than the (111) plane. This is because of the proximity of the Mg2+ ions to the surface and their ability to react with molten aluminum.; The sapphire (0001) and spinel (111) planes show similar wetting characteristics because they have similar O2- ion structural arrangements. This is especially true at lower temperatures, T < 1273K. Above 1273K a structural transformation of the alpha-Al2O3 (0001) plane occurs. It creates a lack of O2- ions in the (0001) plane at high temperatures.; Further analysis of the surface tensions validates the results found in the contact angles. It was observed that any change in the contact angle is reflected into the surface tension. This is because there is a droplet curvature change with the different experimental conditions used in this study. The surface tension factor, S, and the roughness factor, R, are helpful to understand the wetting trends of these systems through the surface roughness. Results of the surface tensions and the contact angles suggest new criteria for engineering aspects of reducing or increasing wetting at elevated temperatures through surface roughness modifications.