Thermal barrier coatings (TBCs) are used in turbines to protect the metallic engine components against high temperatures and corrosive atmospheres. This is preferable due to the increase of the turbine efficiency by increasing the operating temperature. Since the 1980s, yttria stabilized zirconia (YSZ) is used in combination with an airflow to protect the engine.^[1] But this system is limited to a surface temperature of 1200 °C. Above, a phase separation of YSZ occurs.^[1] To increase the efficiency and therefore reduce the impact of a turbine on the environment, an alternative TBC system is needed to allow an increase of the temperature.

High-entropy oxides (HEOs) are a promising class of material to overcome the limitations of YSZ. They are oxides with five or more cations in equimolar ratio that are statistically distributed in the sublattice. Rost et al. synthesised the first HEO in 2015 with the formular (Mg_0.2Ni_0.2Co_0.2Cu_0.2Zn_0.2)O.^[2] Through the number of different cations, the configurational entropy of the system is higher compared to similar materials with less cations. The high entropy results in a temperature stability in a wide range and the disorder leads additionally to an increased phonon scattering and therefore to a low thermal conductivity.^[3] Another positive aspect of HEOs are their designable properties due to the different cations that can be used.

To use a material as TBC their coefficient of thermal expansion must fit to that of the substrate, and it must be also chemically inactive towards the other materials of the layer system. To investigate the usability of zirconate-based HEOs as TBCs, the compatibility of different single-phase high-entropy zirconates with alumina which forms as thermally grown oxide  is investigated as well as their thermal conductivity. Furthermore, the thermocyclic behaviour of a layer system of HEO with alumina will be investigated with laser-rig in the future to determine the lifetime of such systems.

Acknowledgements:
The work was supported by the German Federal Ministry of Education and Research (03XP00301).

References:
[1]    R. Vaßen, M. O. Jarligo, T. Steinke, D. E. Mack, D. Stöver, Surf. Coat. Technol. 2010, 205, 938–942.
[2]    C. M. Rost, E. Sachet, T. Borman, A. Moballegh, E. C. Dickey, D. Hou, J. L. Jones, S. Curtarolo, J.-P. Maria, Nat. Commun. 2015, 6, 8485.
[3]    J. Gild, M. Samiee, J. L. Braun, T. Harrington, H. Vega, P. E. Hopkins, K. Vecchio, J. Luo, J. Am. Ceram. Soc. 2018, 38, 3578–3584.