Morphologies adopted by Al2O3 single-crystal surfaces in contact with Cu droplets

The morphologies of alpha-Al2O3 single-crystal substrates, annealed in contact with Cu droplets ranging in size from I to 10 mum, 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.