A Comparative Study of the Isothermal Oxidation Behaviour of APS, VPS and HVOF CoNiCrAlY Coatings

Cobalt-Nickel based superalloy, such as CoNiCrAlY coatings are commonly used as bond coats for ceramic thermal barrier coatings in industrial gas turbines. During high temperature service CoNiCrAlY coatings form aluminium based surface oxide scales and protect the substrate. The oxidation behaviour and the oxidation resistance depend not only on the composition and on the thickness of the coating, but also on the coating manufacturing process which can influence microstructure, composition, porosity and surface morphology. In the present work, by means of vacuum plasma spraying (VPS), air plasma spraying (APS) and high velocity oxy-fuel (HVOF) techniques, coatings of comparable thickness were deposited onto a superalloy substrate by using a CoNiCrAlY powder of the same composition. Samples were isothermally oxidized at 1273 K for different periods up to 3000 hours. Before ad after the furnace tests samples were extensively characterized. The different properties and characteristics of the coatings were analysed and discussed.