Gas turbine aero engines employ the Brayton cycle in their operation. A critical parameter for high thermal efficiency is the turbine flow path temperature, which depends on the high-temperature capability of structural materials. Refractory metals and their alloys have been considered as candidate materials for hot section components due to their high melting point and elevated-temperature strength. Nevertheless, the poor oxidation resistance, which can be attributed to the rapidly growing of non-protective or volatile oxides upon oxidation, is the bottleneck that hinders their practical applications in engine environments for decades. Generally, the dense and protective scales formed on metals and ceramics, which can prevent metals and ceramics from further oxidation, are Al2O3, Cr2O3 or SiO2. These protective scales, however, are seldom found upon the oxidation of refractory metals and their alloys. Recent breakthrough in refractory high-entropy alloys (RHEAs) have demonstrated that although the traditional protective scales such as Al2O3, Cr2O3 or SiO2 are difficult to form upon the oxidation of RHEAs, the formation of rutile structured complex oxides like CrNbO4 and CrTaO4 are found to protect the RHEAs from further oxidation.
To understand the mechanism that underpin the oxidation resistance of Cr-Ta containing RHEAs, we have systematically studied CrTaO4, focusing on its microstructure, elastic properties, mechanical properties, and thermal properties in our previous work. We not only elucidated the reasons for its exceptional oxidation resistance but also provided clear guidance for its potential applications. It has demonstrated that CrTaO4 exhibits low thermal conductivity, measuring only 1.31 W·m-1·K-1 at room temperature. And its thermal expansion coefficient (TEC) is (6.39±0.11)×10−6K−1, which is very close to those of refractory metals and RHEAs. Furthermore, it exhibits outstanding resistance to CaO–MgO–Al2O3–SiO2 (CMAS) corrosion, making it suitable as a novel thermal barrier coating on Cr-Ta containing RHEAs. However, up to now little is known on the rutile structured CrNbO4, although its role on the protection of Cr-Nb containing RHEAs has been reported.
Recently, a team of material scientists led by Yanchun Zhou from Zhengzhou University, China first reported the synthesis, microstructure, elastic/mechanical properties (hardness, flexural strength and fracture toughness) and thermal properties (melting point, Debye temperature, anisotropic thermal expansion coefficients and thermal conductivity) of bulk CrNbO4. This work confirms that CrNbO4 can be regarded as a novel dual functional scale on top of RHEAs to protect them from oxidation and thermal attack.
The team published their work in Journal of Advanced Ceramics on May 23, 2025.
"In this report, we synthesized phase-pure CrNbO4 powder by the solid-state reaction between Cr2O3 and Nb2O5. Then, near fully dense bulk CrNbO4 was prepared by hot-press sintering the CrNbO4 powders. After hot-press sinering, approximately 3.7 vol% of Cr2O3 precipitated. However, a small amount of Cr2O3 can act as a strengthening agent, which is beneficial for improving the material's strength. The grains of CrNbO4 are well crystallized with average equiaxed grain size of 2.13±0.56 μm and well faceted shapes. Using HAADF and ABF-STEM techniques, the rutile crystal structure of CrNbO4 was confirmed and short-range order was directly observed，"said Yanchun Zhou, professor at School of Materials Science and Engineering at Zhengzhou University (China), a senior expert whose research interests focus on the field of high-temperature ceramics.
"The density of CrNbO4 is also lower than that of CrTaO4, providing additional benefit of Cr-Nb containing RHEAs over Cr-Ta containing RHEAs because of lightweight of both the RHEAs and the protective oxide scale. These combinations of fascinating properties of Cr-Nb containing RHEAs inspire us to further investigate the structure and properties of CrNbO4. CrNbO4 has been unexpectedly found to play a decisive role in improving the oxidation resistance of Cr and Ta-containing RHEAs. However, the mechanical and thermal properties of CrNbO4 have not been reported. For a new material, it is necessary to explore its properties." said Yanchun Zhou.
CrNbO4 exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young's, shear, and bulk modulus of 253, 100, and 180GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 10.2±0.58GPa, 205±8 MPa, and 1.54±0.124 MPa·m1/2. "The analogous elastic/mechanical properties of CrNbO4 to those of YSZ has spurred inquiries tolucrative leverage it as a new thermal barrier material," said Yanchun Zhou.
The melting point of CrNbO4 is 2053±20 K. The anisotropic thermal expansion coefficients (TECs) are αa = (5.38±0.09)×10−6 K−1, αc = (7.44±0.14)×10−6 K−1, with an average TEC of (6.07±0.12)×0−6 K−1. The room temperature thermal conductivity of CrNbO4 is 1.09W·m−1·K−1 and declines to 0.45W·m−1·K−1 at 1473 K, which are lower than most of the currently well-known thermal barrier materials. "CrNbO4 exhibits extremely low thermal conductivity. Therefore, CrNbO4 can be a novel dual functional scale to protect RHEAs from oxidation and thermal attack." said Yanchun Zhou.
Meanwhile, the researchers also studied its high-temperature stability, they found that Cr2O3 would precipitate when the temperature rose to 1500 K. "It is essential to research and determine the application scope of the material," said Yanchun Zhou.
Other contributors include Shuang Zhang, Jian Zhang, Huimin Xiang, Cheng Fang, Wei Xie from the School of Materials Science and Engineering at Zhengzhou University in Henan, China; Xiaohui Wang from the Institute of Metal Research at Chinese Academy of Sciences in Shenyang, China.
This work was financially supported by the National Natural Science Foundation of China (Nos. U23A20562 and 52302074). The authors would like to acknowledge Guogao Tang at Kaiple Company for TEM performance.
About Author
Yanchun Zhou holds a BSc in ceramics from Tsinghua University, and an M.S. in ceramics and Ph.D. in metals from Institute of Metal Research, Chinese Academy of Sciences. He was a visiting scientist at the Institute of Strength Physics and Materials, Russian Academy of Sciences, and a post doc at University of Missouri-Rolla in the 1990's. He was Professor and Director of High-performance Ceramic Division, Shenyang National Laboratory for Materials Science before moving to Aerospace Research Institute of Materials and Processing Technology in 2010. His now professor at School of Materials Science and Engineering at Zhengzhou University in Henan, China
Zhou has discovered more than 20 new ternary carbides, nitrides and borides. His current interests and fields of research are designing, understanding the structural-property relations of damage tolerant ceramics for high and ultrahigh temperature applications. He has published more than 500 papers in peer-reviewed international journals with citations ca 28900 times with H-index of 92.
He has received numerous prizes and awards, and was elected Academician of the World Academy of Ceramics in 2009, Fellow of ACerS in 2010 and Academician of Asian-Pacific Academy of Material in 2013. He served as a member of the Advisory Committee of WAC (2010-2014), and a member of the Nominating Committee of WAC (2010-2014), Chairman of the International Committee of the ECD-ACerS, Chair of Ross Coffin Purdy Award Committee of ACerS (2015). He also serves as editor of JACerS, vice editor-in-Chief of JMST, principle editor of JMR, and editor of J Adv. Ceram.
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC's 2023 IF is 18.6, ranking in Top 1 (1/31, Q1) among all journals in "Materials Science, Ceramics" category, and its 2024 CiteScore is 25.9 (5/130) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
