Superalloys are widely applied as materials for components in the hot section of gas turbines. As superalloys have a limited oxidation life, the application of a coating is vital. The most commonly applied coatings in stationary gas turbines are MCrAlY coatings. Since the turbine components are exposed to high cyclic thermal stresses, MCrAlY coatings must also show a high thermal fatigue resistance. In this thesis, the effect of Cobalt and Rhenium on microstructure, oxidation and thermal fatigue of NiCoCrAlY coatings is presented. Additionally the condition of the coatings after testing in an industrial gas turbine is shown. The influence of Cobalt and Rhenium on coating microstructure was investigated by thermodynamic modelling and by metallography. It could be shown that both elements reduce the γ`-phase fraction and increase the β-phase fraction owing to an expansion of the γ+β field in the phase diagram. Modelling showed that Rhenium promotes the formation of α-Cr, which could be explained by a shift of the α-Cr solvus to higher temperatures and lower Cr concentrations. In the real coatings Re causes the precipitation of TCP-phase. The oxide scale growth rate is increased by Cobalt and Rhenium and it appears that Yttrium plays a significant role for that effect. Coating consumption due to simultaneous oxidation and interdiffusion could be decreased by the application of Cobalt and Rhenium. In thermal fatigue testing Rhenium reduces the time to crack initiation and increases crack propagation rate, although it could be shown that Rhenium increases the creep resistance of the coating. The effect could be explained by the influence of Rhenium on the microstructure, which increases creep resistance, but also reduces the ductility of the coating.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:22488 |
| Date | 09 December 2004 |
| Creators | Täck, Ulrike |
| Contributors | Oettel, Heinrich, Martin, Ulrich, Biermann, H., Bürgel, Ralf, TU Bergakademie Freiberg |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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