Ultrasonic-assisted hot pressing (UAHP): A novel strategy to enhance densification and improve mechanical properties of B4C

Difficult-to-sinter ceramics are advanced materials characterized by strong covalent bonding and extremely low diffusion rates, which make densification particularly challenging. Achieving high density typically requires very high temperatures, longer processing times, or sintering additives, often at the expense of microstructural control and mechanical reliability. Recently, ultrasonic-assisted hot pressing (UAHP) has shown significant potential in enhancing both densification and mechanical performance of metallic materials. However, the poor high-temperature stability of the ultrasonic system severely limits its application in the fabrication of high-melting-point materials, such as monolithic boron carbide (B₄C). Filling this research gap is imperative since UAHP opens a new avenue for the preparation of difficult-to-sinter ceramics.

Recently, teams of material scientists led by Yanchun Zhou from Suzhou National Laboratory and Jinpeng Song from Taiyuan University of Technology, China, first reported the operating temperature of UAHP exceeding 2000 ℃ and employed it in the preparation of monolithic boron carbide (B₄C) ceramics. This work not only demonstrates that UAHP achieves efficient sintering for the difficult-to-sinter ceramics but also enhances the mechanical properties (hardness, flexural strength, and fracture toughness) of B₄C ceramics.

The team published their work in Journal of Advanced Ceramics on December 28, 2025.

“In this report, we demonstrate the leap of the operating temperature from the fabrication of metals to the difficult sintering of covalently bonded ceramics. In UAHP, the heating element of the furnace is graphite, which allows the realization of the furnace temperature over 2000 ℃ and the ultrasonic transducer and the horn are cooled by water to enable stable operation of the ultrasonic system at high operating temperatures,” said Yanchun Zhou, professor at Suzhou National Laboratory (China), a senior expert whose research interests focus on the field of high-temperature ceramics.

“Ultrasonic vibration effectively accelerates the densification process, increases the densification rate, and lowers the sintering temperature. Compared to conventional HP, UAHP demonstrates a higher sintering efficiency, achieving near fully density at a sintering temperature approximately 50 ℃ lower and requiring shorter holding time to reach equivalent density.” said Yanchun Zhou.

“The enhanced densification under UAHP is primarily attributed to three mechanisms: (i) particle rearrangement facilitated by friction reduction, (ii) thermal softening induced by local friction heating, and (iii) acoustic softening coupled with stress superposition, which promotes plastic deformation and accelerates pore shrinkage and elimination.” said Yanchun Zhou.

“UAHP can not only increase the sintering efficiency but also improve the mechanical properties. After optimizing the holding time, the B₄C ceramics sintered by UAHP at 1950 ℃ for 20 min and at 2050 ℃ for 5 min exhibit excellent mechanical properties compared to those sintered for 30 min at the same temperatures. Specifically, the flexural strength increases to 669.3±19.4 MPa (by 12%) and 668.3±32.5 MPa (by 21%), and the fracture toughness elevates to 4.37±0.13 MPa·m^1/2 (by 4%) and 4.22±0.29 MPa·m^1/2 (by 11%), respectively.” said Yanchun Zhou.

Despite the leap in operating temperature achieved in this work, the potential and advantages of UAHP still need to be explored, especially in the fields of difficult sintering ceramics and refractory metals. In this regard, Zhou et al also put forward further research to clarify the densification and grain growth kinetics, to optimize processing parameters, and to tailor the properties of diverse materials. Moreover, the current understanding of ultrasonic effects—especially the surface and volume effects—has been largely derived from metallic systems. Their applicability and underlying mechanisms in covalently bonded ceramics, such as B₄C, require systematic experimental and theoretical investigation in future work.

Other contributors include Zhuo Wang, Jinpeng Song, Ming Lv from the College of Mechanical Engineering at Taiyuan University of Technology in Taiyuan, China; Jiaojiao Gao from the College of Aeronautics and Astronautics at Taiyuan University of Technology in Taiyuan, China; Kuang Sun, Xiong Niu and Wei Xu from Shanghai Chenhua Science Technology Co., Ltd., in Shanghai, China.

This work was supported by National Natural Science Foundation of China (Grant No. 52205492), Fundamental Research Program of Shanxi Province, China (Grant No. 202303021221037), and Scientific and Technological Achievements Transformation Guidance Project of Shanxi Province, China (Grant No. 202204021301039).

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. He is now a professor at Suzhou National Laboratory in Suzhou, 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 31000 times with H-index of 95.

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-in-chief of Extreme Materials, editor-in-chief J Adv. Ceram., vice editor-in-Chief of JMST, and editor of JACerS.

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 2024 IF is 16.6, ranking in Top 1 (1/33, 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