The effect of reactively-sputtered alumina films on the oxidation resistance of CoCrAlY coatings

Thin alumina films have found applications in electronic circuits and cutting tools and have been recently explored as protective coatings against high temperature oxidation, but these studies have been relatively rare. In this study, amorphous alumina films 3.5 µm thick were produced by reactive sputtering on Co-30Cr-6Al-0.5Y coatings, using a pure aluminum target in Ar + O2 atmosphere. Isothermal oxidation tests were carried out at 1000-1100 °C in static air. The results showed that the alumina films improved the oxidation resistance at all testing temperatures. At 1000°C, the Co-30Cr-6Al-0.5Y coating could form pure alumina scale on its surface, but the reactively-sputtered alumina films reduced the oxidation rate. At 1100 °C, the Co-30Cr-6Al-0.5Y coating could not form pure alumina scale, as a result of a severe internal oxidation of aluminum along the columnar defects. The scales were composed of Al2O3, Cr2O3 and TiO2, which were not so protective as pure alumina scales. On the contrary, the Co-30Cr- 6Al-0.5Y coatings with 3.5 µm preformed alumina films exhibited excellent oxidation resistance at 1100 °C. Only minor amounts of TiO2 were detected on the surface. Moreover, the reactively-sputtered alumina films were very adherent to the CoCrAlY coatings. At 1000 °C no spallation was found, while at 1100 °C only a little spallation produced during cooling was observed