Hot corrosion behaviour of plasma-sprayed functionally graded thermal barrier coatings in the presence of Na2SO4 + V2O5 molten salt

Abstract

Hot corrosion behaviour of plasma-sprayed functionally graded thermal barrier coatings (TBCs) was evaluated in this study. Two layer (duplex) and functionally graded ZrO2/NiCrAlY coatings (FGC) were deposited on the Inconel 738LC substrate by atmospheric plasma spraying (APS). The hot corrosion behaviour of coatings was assessed in the presence of a corrosive molten salt. The salt was composed of 45wt% Na2SO4+55wt% V2O5. The hot corrosion test was carried out at 800°C and samples were placed in an atmospheric air electrical furnace for 120h. The microstructure, phase composition and elemental analysis of the coatings were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and energy dispersive spectroscopy (EDS), respectively. Investigations revealed that the dominant hot corrosion mechanism of coatings in the presence of the molten sulphate-vanadate salt was destabilization of Yttria Stabilized Zirconia (YSZ), resulting in reactions between Y2O3 and the corrosive salt. YVO4, which contained the depleted Y2O3 of YSZ, was the main product of the reaction, motivating the destructive destabilization and transformation of YSZ. In this transformation, the structure of ZrO2 was changed from a tetragonal phase to a monoclinic one accompanied by volume change. Also, hot corrosion test results depicted the improved corrosion resistance of FGCs in comparison with the duplex coating due to the formation of AlV and CrVO4, which postponed the mentioned destructive transformation.