Oxidation behavior of Ru-Al multilayer coatings

Ru0.63Al0.37 coatings were deposited through a cyclical gradient concentration deposition at 400 °C with a substrate-holder rotation speed of 1 rpm by direct current magnetron cosputtering. Scanning electron microscopy revealed that the as-deposited coatings exhibited a multilayer structure along with the columnar structure. The oxidation behavior of the Ru0.63Al0.37 coatings was examined through X-ray diffraction, Auger electron spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Oxidation kinetics was measured using a thermogravimetric analyzer. Internal oxidation was observed for Ru0.63Al0.37 coatings annealed in a 1% O2-99% Ar atmosphere at 400-600 °C accompanied with activation energies of 72-84 kJ/mol. By contrast, external oxidation was observed after annealing at 700-800 °C, resulting in the formation of a continuous alumina scale consisting of crystalline δ-Al2O3 domains, which can be attributable to the outward diffusion of Al.