Influence of water vapor on the useable lifetime of thermal barrier coatings (TBCs) on high temperature components in gas turbines

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Hydrogen combustion in gas turbines will play an important role in reducing CO2 emissions in the near future. An increasing hydrogen content in the fuel gas leads to a number of challenges in the materials used, e. g. also the thermal barrier coatings used in modern gas turbines. In this project, the influence of water vapor on the properties of the most commonly used thermal barrier coating material, yttria-stabilized zirconia (YSZ), will be investigated. Although this material has been used commercially in turbines for decades, knowledge about the influence of water vapor is limited and contradictory. Therefore, this project aims to investigate the influence of atmospheres containing water vapor on the properties of thermal barrier coatings. The commonly used atmospheric plasma sprayed (APS) and suspension plasma sprayed (SPS) systems are considered. Both systems have characteristic microstructural features (porosity, microcracks, columns) that are essential for the properties and especially the service life. On the one hand, the influence of water vapor on the sintering of free-standing coatings will be investigated and the underlying elementary processes will be analyzed using high-resolution characterization methods. Sintering leads to an increase in the modulus of elasticity and thus to higher stresses and earlier failure of thermal barrier coating systems. In addition to sintering, the influence on the undesired phase transformation of the YSZ will also be clarified. An influence of water vapor on phase transformations in the YSZ is clearly documented from the literature, but detailed investigations on thermal barrier coatings are lacking. In addition to these investigations, which are carried out on free-standing coatings, complete thermal barrier coating systems (substrate + bond coat + thermal barrier coating) will also be examined for their behavior in water vapor. The cooling conditions are also varied here, as it is known that they have an influence on the phase transformations. Ultimately, the knowledge gained should flow together and make it possible to make a clear statement on the influence of water vapor on the service life of plasma-sprayed YSZ-based thermal barrier coatings.