The formation of a homogeneous α-alumina coating on a Ni-based superalloy from a layer stack deposited by cathodic arc evaporation

Cathodic arc evaporation was applied to form a layer stack on a Ni-based superalloy single crystalline substrate in a single in-situ vacuum deposition process. The initial layer was deposited using a target with the same nominal composition as the superalloy substrate. Subsequently, a second layer was deposited using a target with a composition of 70 at.% Al and 30 at.% Cr, and the deposition was conducted in flowing oxygen to form Al-Cr-O. The thermal stability of the layer stack was investigated by means of a heat treatment at 1100 degrees C in air. The substrate-like coating recrystallized at elevated temperature and showed epitaxial growth on the superalloy single crystal, according to transmission Kikuchi diffraction measurements. A thin layer of pure alpha-alumina formed on top of the substrate-like coating and a compact Al-Cr-O phase with corundum structure developed towards the top of the coating. Microstructural and chemical analyses of the coating architectures in the as deposited and annealed states were performed by means of transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Rutherford backscattering spectrometry, and utilized to explain the recrystallization and diffusion processes in the layer stack.