Transient-Alumina Transformations During the Oxidation of Magnetron-Sputtered CoCrAl Nanocrystalline Coatings

The thermally-grown alumina formed at 1000, 1100, and 1200°C on magnetron-sputtered, nanocrystalline CoCrAl coatings, with and without yttrium, has been characterized using photostimulated-luminescence spectroscopy. The measurements enable the evolution of initially-formed transient alumina to its stable, phase to be followed, and in particular, the effect of yttrium on the transformation. Yttrium retards the transformation from gamma to theta alumina and also its subsequent transformation to alumina. The retardation of the transformation decreases with increasing oxidation temperature until at 1200°C the transformation is complete within minutes. The presence of yttrium in the coatings also affects the residual stress in the thermally-grown oxide. For samples oxidized at 1100 and 1200°C the residual stress is 0.3GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.