Influence of sol-gel derived alumina coatings on oxide scale growth of nickel-base superalloy Inconel-718

Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base superalloy substrates using sol-gel processing. Mass gain measurements served for studying the oxidation behavior of coated metal surfaces if exposed to 800 °C and 900 °C for up to 4000 h in static air. It was found, that alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900 °C for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal substrate (IN-718) into the alumina coatings and - to a much lower degree - from the coating into the substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially by the presence of alpha-alumina.