Morphologies Adopted by Al ₂ O ₃ Single‐Crystal Surfaces in Contact with Cu Droplets

The morphologies of α‐Al 2 O 3 single‐crystal substrates, annealed in contact with Cu droplets ranging in size from 1 to 10 μm, are observed by high‐resolution scanning electron microscopy. After 45 min at 1373 K in a He‐5% H 2 atmosphere, the alumina substrates display different shapes depending on their orientation and on the addition of small amounts of titanium to the copper. The alumina surface may remain flat, display a hollow of faceted spherical shape at the solid/liquid interface, or develop anisotropic ridges at the triple line. The shape changes result from the diffusion of the components of alumina that takes place to achieve a minimum interfacial energy of the drop–substrate system. Explanations of these features are proposed with a discussion of the conditions under which the Young equation can be used for characterizing wetting at high temperature.