Microstructure and thermal oxidation behavior of yttria-stabilized hafnia nanostructured coatings deposited on alumina

Nanostructured yttria-stabilized hafnia (YSH) coatings were grown on α-Al2O3 substrates with variable coating thickness in a wide range of ~ 50 nm to 1 μm. Microstructure and thermal oxidation behavior of the grown YSH coatings were studied employing X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and isothermal furnace oxidation testing. The effect of coating thickness on the crystal structure, surface/interface morphology and thermal oxidation was investigated. X-ray diffraction analyses revealed the formation of monoclinic phase for relatively thin coatings (< 100 nm) indicating that the interfacial phenomena play a dominant role in phase stabilization. The evolution towards stabilized cubic phase with increasing coating thickness is observed. The SEM results indicate the dense, columnar structure of YSH coatings as a function of thickness. Thermal oxidation measurements indicate the enhanced high-temperature oxidation resistance of cubic YSH coatings.