Development of high-entropy oxides as new thermal barrier coatings
Research Associate: Giulia Bianchi
To reduce the environmental impact of aircrafts, the improvement of the efficiency of their propulsors – namely, gas turbines – is of the utmost importance. The efficiency of a gas turbine highly depends on the operating temperature: the higher the temperature, the better the engine works. However, operating temperatures are now reaching the limits posed by the materials used.[1] Up to date, the blades of a gas turbine are made of a nickel-based superalloy, covered with a protective layer, called thermal barrier coating (TBC). The TBC has the purpose of thermally insulate the Ni superalloy, thus prolonging its lifetime. In order to be used as TBC, a material needs to have a coefficient of thermal expansion (CTE) matching that of the substrate, low thermal conductivity (κ), no phase transitions in the temperature range of use, good mechanical properties and high corrosion resistance.[2] At the moment, the TBC in gas turbines is usually made of yttria stabilized zirconia (YSZ) in the metastable tetragonal t’ phase. The choice of applying Ni-based superalloys and YSZ in gas turbines relies on their properties: while the single-crystal nickel-based superalloy ensures the necessary mechanical strength, the YSZ allows a good thermal insulation thanks to its low thermal conductivity (~2 Wm-1K-1 at 1000 °C, lowerable with different deposition methods). At the same time, YSZ is characterized by a quite high coefficient of thermal expansion (~11∙10-6 K-1), similar to that of the substrate, therefore reducing stresses in the junction. However, at temperatures higher than 1200°C, the t’ phase decomposes to the thermodynamically stable YO1.5 rich cubic phase, and YO1.5 poor tetragonal phase. Therefore, the operating temperature of the gas turbine is limited to ~1200 °C.[2,3]
To overcome the temperature limitations, to improve the efficiency, and to lower the environmental impact of gas turbines, this project has the goal to develop new substrate-coating systems. To achieve this objective, a SiCr alloy will be developed (at the partner University of Bayreuth) to substitute the Ni-based superalloy and high-entropy oxides (HEOs) will be investigated as TBC materials (Justus Liebig University).
The choice of using HEOs as TBCs relies in their outstanding tuneable properties such as coefficient of thermal expansion and thermal stability at high temperatures. HEOs are complex, single-phase oxides with five or more cations in equimolar amount. Hence, the cations are equally and randomly distributed in the crystal structure, increasing its configurational entropy and stabilizing it.[2] However, the best performing HEOs as TBC are usually made with rare earths, therefore increasing the cost of the final materials and lowering their industrial appeal.[4,5] Consequently, another goal of this work is to decrease the cost of HEOs for TBC applications by using cheaper and more widely available elements, such as transition metals.
References:
[1] J. H. Perepezko, Science (New York, N.Y.) 2009, 326, 1068.
[2] E. Bakan, D. E. Mack, G. Mauer, R. Vaßen, J. Lamon, N. P. Padture in Advanced Ceramics for Energy Conversion and Storage, Elsevier, 2020, pp. 3–62.
[3] L. Cong, W. Li, J. Wang, S. Gu, S. Zhang, Journal of Materials Science & Technology 2022, 101, 199.
[4] D. R. Clarke, M. Oechsner, N. P. Padture, MRS Bull. 2012, 37, 891.
[5] K. Ren, Q. Wang, G. Shao, X. Zhao, Y. Wang, Scripta Materialia 2020, 178, 382.
This projects is funded by Bundesministerium für Wirtschaft und Klimaschutz (FKZ: 20E2222A).
