Zirconia based thermal barrier coatings for extended temperature ranges

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The research project will focus on two major aspects to increase the operation temperature of gas turbines. On the one hand, possibilities to increase the operation regime of the standard material 8YSZ to temperatures above 1250°C will be investigated. On the other hand, plasma-sprayed monolayers made out of alternative ceramics will be produced, which have compared to 8YSZ an improved phase stability at high temperatures and are suitable for an application at 1600°C. A first focus is a study of the transformation processes under changed process and operation conditions, which are necessary for an evaluation of lifetime in the extended operation regime of future thermal barrier coatings.For an extension of the operation regime of 8YSZ above 1250°C the influence of microstructure on the phase transformation and the sintering behavior of the standard material 8YSZ will be investigated at very high temperatures. As the high temperature stability and hence the lifetime of the thermal barrier coatings at very high temperatures are depending strongly on phase transformation and the sintering behavior, an understanding of the underlying mechanisms and of the development with time of these properties is of fundamental importance for the possible application fields. Both processes are temperature dependent and accelerated at high temperatures, however, there are hints, that with an especially fine-grained microstructure both phase transformation and sintering can be suppressed. In addition, very fast cooling rates can suppress the phase transformation. One objective is therefore, to identify underlying mechanisms to determine a suitable selection of process parameters or special coating processes for a production of coatings, which show due to their microstructural features improved phase stability and sintering behavior. The influence of the cooling rate on the phase transformation will be clarified by specific experiments and could additionally lead to an extension of the operation regime of the standard material 8YSZ.The second part of the project, which is dealing with new materials for an operation at 1600°C, is the development of a thermal barrier coating out of an alternative zirconium based ceramic. For that four representative candidates from the co-doped zirconium oxides were selected, which have compared to 8YSZ an extended temperature stability. The selection includes on the one hand simple systems, in which strategies for doping as stabilization of the tetragonal phase, increasing the fracture toughness, or improving the tetragonality are applied (Ta-Y, Ce/Sc-Y). In addition, the co-doping with alumina will be investigated. Finally, an already commercial available complex doped systems will be selected as a benchmark. Also here it has not been analyzed in depth how processing, microstructure and operation conditions influence the stability of the system.