Dual-phase medium-entropy diboride-carbide ceramics with metal element exchange during sintering

Diborides and carbides of transition metal elements, including zirconium (Zr), hafnium (Hf), and tantalum (Ta), which are important members of the UHTCs family, exhibit good ablation resistance under the extreme environments of ultra-high temperatures and high heat flux density, and are expected to be the optimal choice of material for the thermal protection structures in hypersonic vehicles. In general, diboride UHTCs show better high-temperature oxidation resistance and good thermal conductivity, while carbide UHTCs show higher melting temperatures as well as high hardness and lower thermal expansion coefficients. The replacement of phase interfaces with grain boundaries has been demonstrated to be an effective method of inhibiting grain growth. The preparation of dual-phase diboride-carbide UHTCs through the multiphase composite design is expected to improve complementary properties. Hence, we focus on ZrB₂, HfB₂, TaB₂, ZrC, HfC, and TaC in the family of UHTCs and hope that the properties of UHTCs will be further improved through multiphase composition design and entropy engineering control. In addition, various single-phase ternary medium-entropy diboride and carbide ceramics have been reported, but investigations of the phase composition and material properties of dual-phase medium-entropy diboride-carbide ceramics, especially on the chemical compatibility and the phase equilibrium relationship between the medium-entropy diboride and carbide, are scarce till now. We synthesized the medium-entropy diboride and carbide powders and then prepared the dense dual-phase (Zr, Hf, Ta)B₂-(Zr, Hf, Ta)C UHTCs. The densification, the possible existing metal element exchange between the diboride and carbide phases during the sintering process, microstructure, mechanical properties, and thermal conductivities will be investigated.

The team published their work in Journal of Advanced Ceramics on November 25, 2024.

Dual-phase medium-entropy diboride-carbide UHTCs were prepared from self-synthesized medium-entropy diboride and carbide powders for the first time. EDS analysis, Rietveld refinement and thermodynamic calculations revealed the metal element exchange in the (Zr, Hf, Ta)B₂-(Zr, Hf, Ta)C system. The metal element exchange promoted the sintering densification of dual-phase UHTCs. Due to the pinning effect, the refined grain sizes of diboride and carbide phases were obtained in the dual-phase UHTCs. The dual-phase UHTCs exhibited higher hardness owing to the refined microstructure. The dual-phase UHTCs demonstrated higher Young’s modulus due to the metal element exchange between the medium-entropy diboride and carbide phases.

With the increasing demand for UHTCs' high-temperature mechanical properties and antioxidant and ablation-resistant properties, the development of new UHTCs systems is of great significance. ²The multiphase composite design and entropy engineering are expected to improve further the high-temperature mechanical properties, oxidation resistance and ablation resistance of UHTCs.² said Guo-Jun Zhang.

Other contributors include Pai Peng and Ji-Xuan Liu from the Institute of Functional Materials at Donghua University in Shanghai, China; Yongcheng Liang from the College of Physics at Donghua University in Shanghai, China; Xiaoting Xin, Weichao Bao, and Fangfang Xu from Shanghai Institute of Ceramics, Chinese Academy of Sciences, China.

The present work was financially supported by the National Natural Science Foundation of China (No. 52032001, 52371023, 52332003, 52211540004) and the Fundamental Research Funds for the Central Universities (2232024G-07).

About Author

Mr. Pai Peng (first author) is a Ph.D. student at the College of Materials Science and Engineering, Donghua University. His research focuses on the preparation and properties of dual-phase diboride-carbide UHTCs.

Dr. Ji-Xuan Liu (corresponding author) is an associate professor at the Institute of Functional Materials, Donghua University. He has published over 80 papers in peer-reviewed international journals with about 2700 citations and an H-index 32. He mainly engages in the phase equilibrium relationship, microstructure design, processing and properties of advanced non-oxide ceramics and their composites.

Dr. Guo-Jun Zhang (corresponding author) is a professor at the Institute of Functional Materials, Donghua University. He was elected Academician of the World Academy of Ceramics in 2021. He is listed in the 'Highly Cited Chinese Researchers' in 2021, 2022, and 2023. His research interests include preparation, microstructure control, and mechanical properties of non-oxide ceramics.

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