Wetting of the (0001) α-Al₂O₃ Sapphire Surface by Molten Aluminum: Effect of Surface Roughness

The wetting of molten aluminum on the “c”-plane (0001) of single-crystal α-Al₂O₃ (sapphire) was studied by the sessile drop technique from 800 °C (1073 K) to 1200 °C (1473 K). Systematically increasing the (0001) surface roughness by SiC abrasion increased the wetting contact angle, resulting in reduced wetting. The surface roughness factor R originally defined by Wenzel, was determined as a function of the abrasion, temperature, and time. The wetting decreases as the surface roughness increases. Rough surfaces also create time and temperature effects on wetting, changing those for a smoothly polished surface. The existence of a high-temperature surface structural transition for (0001) of α-Al₂O₃, which has been previously suggested, was confirmed. Increased roughness R accents the effect of the surface structural transition, increasing the wetting contact angle changes during the transition.