TY  - THES
AU  - Ebert, Svenja Maria
TI  - Versagensverhalten plasmagespritzter Mg-Al-Spinell-Schichten unter Thermozyklierung
VL  - 166
PB  - Ruhr-Universität Bochum
VL  - Dissertation
CY  - Jülich
M1  - FZJ-2013-01778
SN  - 978-3-89336-853-2
T2  - Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment
SP  - X, 173 S.
PY  - 2013
N1  - Dissertation, Ruhr-Universität Bochum, 2013
AB  - One possibility to increase the efficiency in gas turbines is the application of abradable coatings. The housing from the inside is coated with special, high temperature resistant ceramics. Thermal loadings and the centrifugal force causes the turbine to expand. Theblade tips drive into the liners. This minimizes the clearance distance between the rotating blade tips and the stationary components, which leads to increases in efficiency and reductions in fuel consumption. The presented work grew out of the collaboration with Rolls-Royce Deutschland. The cycling behaviour of a multilayer system was tested. Disc-shaped Inconel738-substrates were coated with a vacuum plasma-sprayed bondcoat and a doublelayer ceramic system. The ceramic system was atmospheric plasma-sprayed and consisted of a yttria-stabilisedzirconia (YSZ) layer and a Mg-Al-spinel layer. The cycling was carried out with a gas burner and thermal gradient from surface to bottom. Two different failure mechanisms were observed for the Mg-Al-spinel layer. On the one hand the coating spalled of at the interface between YSZ and Mg-Al-spinel. On the other hand only a spalling on top of the Mg-Al-spinel layer was found. A dependence on the spraying parameters was observed. Goal of the presented work was the clarification of these failure mechanisms. First of all the microstructure after spraying and cycling was analysed. For an alumina rich spinel the thermal cycling and thermal energy leads to the formation of three different phases in thermal equilibrium: Mg-Al-spinel, $\alpha$ -Al$_{2}$O$_{3}$ und CaO*6Al$_{2}$O$_{3}$ (CaO as contamination in the powder). During the heating time of the cycling a dense layer at the surface of the Mg-Al-spinel coating is formed. This layer consists of Mg-Al-spinel and CaO*6Al$_{2}$O$_{3}$ needles. Stress calculationswere carried out, which show that the surface of the Mg-Al-spinel coating is under compression stress during heating. Hot pressing experiments confirmed the compression stress as a cause for the densification. Under this dense layer cracks are formed during cycling, which are the reason for the spalling. Crack formation under the dense layer takes place, when the energy release rate stored in the coating system reaches the critical value. The transfer of the spraying in the industry required new spraying parameters for coating of real engine parts. In the context of this work it was found out, that a coating temperature under 200°C seems to be too low for a good connection between the Mg-Al-spinel splats inside the coating. The coating spalls close to the interface between YSZ and Mg-Al-spinel. 
KW  - Dissertation (GND)
LB  - PUB:(DE-HGF)3 ; PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/133240
ER  -