Atypical decomposition processes of Sm2Zr2O7 + 8YSZ dual-phase TBCs during hot corrosion

Abstract

The accelerated decomposition of an Sm2Zr2O7+8YSZ system in the form of air plasma spraying (APS) thermal barrier coatings of composites, double ceramic layer (DCL), and functional graded system (FGS) types under hot corrosion conditions were analyzed. Hot corrosion tests were performed using liquid sodium sulfate salt deposits and then X-Ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS) were performed to identify the reaction products. The obtained data indicated that negligible amounts of sulfates or oxysulfates were formed, but significant decomposition of pyrochlore Sm2Zr2O7 to the fluorite phase was detected. These phenomena were also observed in a much higher temperature range during pure oxidation tests, but the lowest decomposition temperature (920 °C) was observed in the sodium sulfate assisted process. The described observations were confirmed during similar tests and realized using model powder mixtures. Microstructural analysis of thermal barrier coatings (TBC) cross-sections revealed a complex decomposition process that was localized to areas near the Sm2Zr2O7 splats. The decomposition of the observed pyrochlore phase was mainly related to interactions between Sm2Zr2O7 and 8YSZ in accordance with the fluxing mechanism typical for the hot corrosion process of Ni-based superalloys. Additionally, the effect of eutectic Na-Sm-O was determined.