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| Home > Publications database > Untersuchungen zu suspensionsplasmagespritzten Wärmedämmschichtsystemen |
| Book/Dissertation / PhD Thesis | FZJ-2016-01727 |
2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-118-7
Abstract: A higher gas inlet temperature can increase the efficiency of gas turbines, that are applied in stationary power plants or in planes. Therewith, the thermal stress of the turbine components is increased as well, which means that all turbine components need to be protected against high temperature at the same time. Therefore, thermal barrier coatings (TBCs) are used by means of thermal protection. TBCs reduce the heat conduction to the components and thus ensure that the increasing heat caused by the increased gas inlet temperature does not destroy any turbine components. In general, top coats are applied by the atmospheric plasma spraying (APS, lamellar coatings) or by the electron beam-physical vapor deposition process (EB-PVD, columnar coatings). To date, the top coats still consist of yttria partially-stabilized zirconia (YSZ). $\underline{In}$ $\underline{the}$ $\underline{first}$ $\underline{part}$ $\underline{of}$ $\underline{this}$ $\underline{work}$, YSZ layers have been applied by suspension plasma spraying (SPS). The spray parameters have been optimized, thus coatings consisting of a columnar microstructure were produced. Hence, the SPS process combines the advantages of the APS and the EB-PVD process: high deposition rates at atmospheric pressure. The behavior of the coatings under cyclic temperature load has been tested. In this work, it was noticed that the deposition temperature has a major impact on the lifetime of the coatings in thermal cycling tests. Cold deposition parameters have led to better results in thermal cycling lifetime tests. However, the results have been worse than those of the YSZ coatings deposited with the APS process. Since the coating thicknesses of the SPS YSZ coatings have been thinner than the thicknesses of the APS YSZ coatings, a thin APS YSZ interlayer has been applied between bond coat (BC) and SPS coating. As consequence, the thermal cycling lifetime is increased. It is similar to the lifetime of the APS YSZ standard coatings. However, the failure mechanism of the SPS/APS double layers is different to that of the APS coatings. SPS/APS double layers show a significant depletion of alumina in the BC. This depletion leads to the formation of ductile, fast, and bulky growing oxides at the interface between BC and APS YSZ coating, which lead to the failure of the coating. $\underline{In}$ $\underline{the}$ $\underline{second}$ $\underline{part}$ $\underline{of}$ $\underline{this}$ $\underline{work}$, alternative materials for the application as TBC materials have been tested, due to the fact that the operation temperature of YSZ is limited to 1200 °C. The alternative materials, which are magnesia alumina spinel, Gd$_{2}$Zr$_{2}$O$_{7}$ and La(Al$_{0.25}$Mg$_{0.5}$Ta$_{0.25}$)O$_{3}$ have been applied by the SPS process as well. An APS YSZ interlayer has been applied to all three materials. The produced magnesia alumina spinel coatings show a columnar microstructure. In comparison to the APS YSZ coatings, they have a longer lifetime in the thermal cycling test. The Gd$_{2}$Zr$_{2}$O$_{7}$ layers are segmented. A first test has shown that the lifetime of these coatings are on the same level as those of the APS YSZ standard coatings. SPS/APS Gd$_{2}$Zr$_{2}$O$_{7}$ double layers fail in the same way as the SPS/APS YSZ double layers. La(Al$_{0.25}$Mg$_{0.5}$Ta$_{0.25}$)O$_{3}$ coatings show a columnar microstructure. They have been tested under cyclic temperature load and in the furnace test. Due to the high durability of these coatings, their resistance against calcium-magnesium-alumino-silicates (CMAS) has been tested at elevated temperatures. The La(Al$_{0.25}$Mg$_{0.5}$Ta$_{0.25}$)O$_{3}$ coatings show excellent thermal cycling lifetimes under CMAS-attack, too.
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