Hot corrosion of an EB-PVD thermal-barrier coating system at 950 °C

Model thermal-barrier coating (TBC) systems consisting of cast NiCrAlY substrates and electron-beam, physically vapor-deposited (EB-PVD) partially yttria-stabilized zirconia (PYSZ) coatings with three different microstructures were tested in 1-hr cycles at 950 °C under hot-corrosion conditions (sodium and sodium/potassium sulfates) likely to occur in biomass-derived fuel-fired gas turbines. In contrast to conditions initiating Type I hot-corrosion attack, a modified test procedure was used in this study where periodically salt-coated specimens were subjected to an oxygen atmosphere while SO_x was omitted, thus taking into account the essentially sulfur-free biomass fuel-combustion atmosphere. For comparison, similar tests were conducted on bare MCrAlY-type alloys. TBC failure by spallation of the PYSZ coating was observed between 300 and 500 1-hr cycles. Irrespective of PYSZ microstructure and deposit chemistry, failure was primarily induced by crack formation and propagation within the voluminous oxide scale formed as a result of hot-corrosion attack of the metal, rather than degradation of the ceramic layer. Since the major attack mode of the TBC seemed to be hot corrosion of the bond coat, this paper highlights degradation mechanisms and microstructures of uncoated bond-coat compositions. On the basis of the present results, implications of the failure mode of EB-PVD PYSZ on in-service components are discussed.