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

Model thermal-barrier coating (TBC) systems consisting of cast NiCrAlY subs trates and electron-beam, physically vapor-deposited (EB-PVD) partially ytt ria-stabilized zirconia (PYSZ) coatings with three different microstructure s were tested in I-hr cycles at 9500 degreesC under hot-corrosion condition s (sodium and sodium/potassium sulfates) likely to occur in biomass-derived fuel-fired gas turbines. In contrast to conditions initiating Type I hot-c orrosion attack, a modified test procedure was used in this study where per iodically salt-coated specimens were subjected to an oxygen atmosphere whil e SO, was omitted, thus taking into account the essentially sulfur-free bio mass fuel-combustion atmosphere. For comparison, similar tests were conduct ed on bare MCrAlY-type alloys. TBC failure by spallation of the PYSZ coatin g was observed between 300 and 500 1-hr cycles. Irrespective of PYSZ micsos tructure and deposit chemistry, failure was primarily induced by crack form ation and propagation within the voluminous oxide scale formed as a result of hot-corrosion attack of the metal, rather than degradation of the cerami c layer. Since the major attack mode of the TBC seemed to be hot corrosion of the bond coat, this paper highlights degradation mechanisms and microstr uctures of uncoated bond-coat compositions. On the basis of the present res ults, implications of the failure mode of EB-PVD PPSZ on in-service compone nts are discussed.