Superior thermal and oxygen barrier properties of high-entropy ferroelastic rare earth tantalate (8RE1/8)TaO4

Thermal/Environmental barrier coatings (T/EBCs) are widely used in aircraft engines and power-generation gas turbines to protect hot-section superalloys and/or ceramic matrix composite components from hot corrosion and oxidation. To address this issue, researchers discovered that multilayer structural coating materials exhibit better anti-oxidation and thermal insulation properties than single-layer coating. However, the introduction of different materials will exacerbate the thermal mismatch problem and have limited improvement on oxygen barrier properties. Additionally, Y₂O₃-ZrO₂ (YSZ) is a common T/EBCs material used in turbine blades. However, YSZ cannot provide sufficient protection for substrate material due to the high ionic conductivity, thermal conductivity and sintering rate.

Recently, a team of material scientists led by Jun Wang and Jing Feng from Kunming University of Science and Technology, China first reported the all-in-one rare-earth tantalate (8RE_1/8)TaO₄ materials with superior thermal-and oxygen-barrier properties. This work reported the (8RE_1/8)TaO₄ has ultra-low thermal conductivity, oxygen ionic conductivity and excellent mechanical properties, and is expected to be a candidate for a new generation of thermal barrier coating materials.

The team published their work in Journal of Advanced Ceramics on November 14, 2024.

“In this report, we obtain a novel rare-earth tantalate with excellent oxygen and thermal insulation via a high-entropy strategy. The high-entropy components (8RE_1/8)TaO₄ designed by large size disorder and mass disorder have been reassembled into a stabilized monoclinic structure, and XRD results demonstrate the increase of conformational entropy contributes to the enhancement of lattice distortion, the in-situ XRD results demonstrate that the (Y_1/8Dy_1/8Lu_1/8Yb_1/8Er_1/8Sm_1/8Nd_1/8Tm_1/8)TaO₄ has no phase transition and excellent phase stability below 1500 °C.” said Jun Wang, research assistant at Faculty of Material Science and Engineering at Kunming University of Science and Technology (China), a young scientist whose research interests focus on the field of thermal protection and stealth function ceramic materials.

“We found that grain size and density can be increased by heat treatment and investigated the effects of grain size and thermal conductivity, to achieve more dense samples, (8RE_1/8)TaO₄ were heat treated at 1600 °C for 5 h. meanwhile, we found that the domain structure is formed by changing the distance between atomic planes, and reported for the first time the existence of dislocation structures in high-entropy rare-earth tantalates” said Jun Wang.

“The lower the thermal conductivity and oxygen ionic conductivity of the thermal barrier ceramic material, the superior the thermal insulation and oxygen barrier properties. The eight-component rare-earth tantalate (8RE_1/8)TaO₄ exhibits superior thermodynamic properties compared with the single-RE RETaO₄ and (Y_1/5Dy_1/5Sm_1/5Yb_1/5Er_1/5)TaO₄, among them, (Y_1/8Dy_1/8Lu_1/8Yb_1/8Er_1/8Sm_1/8Nd_1/8Tm_1/8)TaO₄ (8HEC-3) possesses an extremely low oxygen-ionic conductivity (1.17 × 10⁻⁶ S·cm⁻¹@900 °C), thermal conductivity (1.15 W·m⁻¹·K⁻¹@1200 °C), we have also analyzed the thermal and oxygen transport mechanisms of high-entropy rare-earth tantalates, the results indicate that the all-in-one rare-earth tantalate (8RE_1/8)TaO₄ materials with superior thermal-and oxygen-barrier properties ” said Jun Wang.

“(Y_1/8Dy_1/8Lu_1/8Yb_1/8Er_1/8Sm_1/8Nd_1/8Tm_1/8)TaO₄ has an high thermal expansion coefficient (11.5 × 10^-6 K^-1@1200 °C) benefiting from the low ionic bond strength . In addition, the high-entropy rare earth tantalates have excellent mechanical properties, with Young's modulus of 60 GPa, 89 GPa and 91 GPa for 8HEC-1, 8HEC-2 and 8HEC-3, respectively, and hardness values of 5.9 GPa, 6.3 GPa, and 5.1 GPa.” said Jun Wang.

However, more delicate research works are still needed to explore the suitability of rare earth tantalates as a new thermal barrier material. In this regard, Wang also put forward five major development directions may be pursued in future works including the CMAS resistance, water vapor resistance, coating deposition methods, stability during plasma deposition, and thermal/structural stability of the coatings during performance.

Other contributors include Xiaoyu Chong, Manyu Zhang, Peng Wu, Jing Feng from the Faculty of Materials Science and Engineering at Kunming University of Science and Technology in Yunnan, China; Yongpan Zeng, Yanjun Sun, Xiangwei Tang from the Guangdong Midea Kitchen Appliances Manufacturing Co., Ltd in Foshan, China; Qianqian Jin from the Materials Science and Engineering Research Center at Guangxi University of Science and Technology in Liuzhou, China.

This work was supported by the National Natural Science Foundation of China (52402077), Open Project of Shaanxi Laboratory (2021SXSYS-01-05), Open Project of Yunnan Precious Metals Laboratory (YPML-2023050240), Yunnan Fundamental Research Projects (202201BE070001-008, 202201AT070192, 202101BE070001-011).

About Author

Jun Wang, Distinguished Professor, Kunming University of Science and Technology, School of Materials Science and Engineering, Master's tutor. Research direction: ultra-high temperature thermal protective coating materials, antioxidant coating materials, composite materials. He won the first prize of China Nonferrous Metals Industry Science and Technology, Geneva International Invention Gold Medal, Excellent Advanced Individual of “Academic Science and Technology Festival”, Second Prize of Academic Science and Technology Achievement Innovation, Third Prize of Heat Treatment Innovation and Entrepreneurship Competition, Scientific and Technological Achievement Evaluation Certificate, Member of China Society of Rare Earths, Silver Prize of China Innovation Challenge and Zhongguancun Emerging Fields Competition, Silver Prize of “Internet+” University Students' Innovation and Entrepreneurship Competition, etc. He is also a member of the review panel of J Adv Ceram and Surf Coat Tech, etc. He has published more than 30 papers as the first author in Acta Mater, J Adv Ceram, J Mater Sci Technol, Surf Coat Tech, Prog Org Coat, J Mater Res Technol, Scripta Materialia, Ceram Int, J Rare Earth, J Am Ceram Soc, etc., with more than 1,000 citations. He has obtained 25 authorized invention patents and accepted more than 20 invention patents, presided over one project of the National Youth Fund, and presided over or participated in eight other projects.

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