A major goal in the development of new materials in gas turbines is a reduction in both fuel consumption and pollution emissions. In spit of improved cooling and single crystalline material rotating turbine blades are coming close to their application limits. Thermal barrier coatings can substantially improve the application potential of these blades. Due to the extremely low thermal conductivity of these ceramic layers, with only 200µm thickness, an increase in the turbine inlet temperature between 100 and 150°C can be achieved.
Thermal barrier coating systems architecture
Zirconia provides a favourable combination of low thermal conductivity and virtually matching thermal expansion behaviour. It must be stabilized with other oxides such as Y2O3, MgO, or CeO2 to avoid phase transformation that is accompanied by volume changes. Within recent years the Gadoliniumzirconate (Gd2Zr2O7) gained interest in R&D as alternative thermal barrier coating material since it provide improved resistance against the detrimental effect of volcanic ash attacks.
A TBC system consists of the columnar ceramic layer made by EB-PVD, the metallic bond coat that provides oxidation protection and adhesion of the ceramic topcoat, and the substrate (typically a Ni-based superalloy).
In the institute plasma sprayed, EB-PVD deposited, optimized MCrAlY’s (z.B. refractory- or rare earth containing), B2-NiAl+X (X = Cr, reactive elements such as Y, Zr, Hf,… or platinum group metals such as Pt, Pd, …) as well as (Ni, Pt)-Al-coatings are under investigation. The lifetime of thermal barrier coatings is mainly restricted by the growth of metal oxides (so called thermally grown oxide). To improve TBC systems understanding of the interplay between processing parameter, coating properties, failure mechanisms, and lifetime is of uppermost importance. These aspects are within the major focus of our research activities. more…
Besides the manufacturing of coatings by EB-PVD, the major focus of our work is on characterization of the coatings, assessment of lifetime and failure mechanisms, and development of life prediction methods based on close to reality tests. Development and manufacture of new coating systems aims at increased application temperature and lifetime, and at reduced thermal conductivity. Composition, distribution and stability of the involved phases, microstructure, defect distribution e.g. in the TGO, and oxidation protection of the bond coat are some key areas. Some examples for reduced thermal conductivity are multilayer and alternative ceramic compositions such as zirconates.
Testing of thermal barrier coatings
Exact knowledge of the failure mechanisms is necessary for further improvement in the coatings and for development of life prediction methods. Characterization of the microstructure is one aspect, but close to reality tests are very important, too. We operate standardized thermocyclic test rigs, oxidation experiments, CMAS-tests, complex thermomechanical tests including internally cooled hollow samples in combination with fracture mechanics experiments as well as close to reality tests in a small gas turbine that blades can be coated with a huge variety of thermal barrier coating systems.