Microstructure, elastic/mechanical and thermal properties of CrTaO4: A new thermal barrier material?

Materials scientists have been enchanted by the search for new materials that exceed the temperature capability of Ni-based superalloys for many decades. High melting point and solidus temperatures and elevated temperature strength endow refractory metals (RM) and their alloys materials of choice for this purpose. However, poor oxidation resistance is the main drawback that impedes their applications in oxygen containing environments due to the formation of porous and fasting growing or volatile oxides. Recently introduced new design paradigm of high-entropy alloys has enabled the overcome of such a shortcoming possible due to the vast opened composition space. It is noteworthy that some RHEAs exhibit not only outstanding mechanical properties above beyond 1200 ℃ but also good oxidation resistance through the formation of either well-known protective scale such as Al₂O₃ or rarely encountered complex oxide CrTaO₄. Despite the vast reports on the oxidation behavior of RHEAs, the mechanical and thermal properties of the protective complex oxide CrTaO₄ have yet to be studied, and filling this research gap is imperative since this complex oxide has been overlooked for decades.

 

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 CrTaO₄. This work not only explains the effect of CrTaO₄ on improving the oxidation resistance of Cr and Ta containing high-entropy alloys, but also predicts that CrTaO₄ can be used as a new type of high-temperature thermal barrier material.

 

The team published their work in Journal of Advanced Ceramics on March 29, 2024

 

“In this report, we synthesized phase-pure CrTaO₄ powder by the solid-state reaction between Cr₂O₃ and Ta₂O₅. Then, near fully dense bulk CrTaO₄ was prepared by hot-press sintering the CrTaO₄ powders. The grains are well crystallized with average equiaxed grain size of 2.07±0.73 μm and well faceted shapes. Using HAADF and ABF-STEM techniques, the rutile crystal structure of CrTaO₄ 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 material.

 

“CrTaO₄ 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 CrTaO₄ have not been reported. For a new material, it is necessary to explore its properties. ” said Yanchun Zhou.

 

CrTaO₄ exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young’s, shear, and bulk modulus of 268, 107, and 181 GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 12.2±0.44 GPa, 142±14 MPa, and 1.87±0.074 MPa·m^1/2. “The analogous elastic/mechanical properties of CrTaO₄ to those of YSZ has spurred inquiries to lucrative leverage it as a new thermal barrier material,” said Yanchun Zhou.

 

The melting point of CrTaO₄ is 2103±20 K. The anisotropic thermal expansion coefficients (TECs) are α_a = (5.68±0.10)×10⁻⁶ K⁻¹, α_c = (7.81±0.11)×10⁻⁶ K⁻¹, with an average TEC of (6.39±0.11)×0⁻⁶ K⁻¹. The room temperature thermal conductivity of CrTaO4 is 1.31 W·m⁻¹·K⁻¹ and declines to 0.66 W·m⁻¹·K⁻¹ at 1473 K, which are lower than most of the currently well-known thermal barrier materials. “From the perspective of matched thermal expansion coefficient, CrTaO₄ pertains to an eligible thermal barrier material for refractory metals and ultrahigh temperature ceramics.” said Yanchun Zhou.

 

However, more delicate research works are still needed to explore the suitability of CrTaO₄ as a new thermal barrier material. In this regard, Zhou 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 Shuang Zhang, Huimin Xiang, Cheng Fang, Mingliang Li, Hailong Wang from the School of Materials Science and Engineering at Zhengzhou University in Henan, China; Xiaohui Wang, Chao Zhang from the Institute of Metal Research at Chinese Academy of Sciences in Shenyang, China; Yingwei Li from the School of Civil Engineering at Wuhan University in Wuhan, China.

 

This work was supported by the National Natural Science Foundation of China (Nos. U23A20562 and 52302074). The authors would like to acknowledge Bin Liu and Yiran Li at Shanghai University for helpful discussion and 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 25600 times with H-index of 85.

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 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 on behalf of the State Key Laboratory of New Ceramics and Fine Processing (Tsinghua University) and the Advanced Ceramics Division of the Chinese Ceramic Society, and exclusively available via SciOpen. JAC has been indexed in SCIE (IF = 16.9, top 1/28, Q1), Scopus, and Ei Compendex.

About SciOpen 

SciOpen is a professional open access resource for discovery of scientific and technical content published by the Tsinghua University Press and its publishing partners, providing the scholarly publishing community with innovative technology and market-leading capabilities. SciOpen provides end-to-end services across manuscript submission, peer review, content hosting, analytics, and identity management and expert advice to ensure each journal’s development by offering a range of options across all functions as Journal Layout, Production Services, Editorial Services, Marketing and Promotions, Online Functionality, etc. By digitalizing the publishing process, SciOpen widens the reach, deepens the impact, and accelerates the exchange of ideas.

---

---