Plasma-sprayed Al2O3-ZrO2 composite coatings with nano-eutectic structures for wear protection: a novel strategy based on feedstock powder design

In critical fields such as aerospace, military, and industrial manufacturing, surface protection of components has been a longstanding challenge. Ceramic materials with high melting points, hardness, chemical inertness, and electrical insulating properties have garnered increasing attention for their potential in wear, thermal, and corrosion protection. Thermally sprayed nanostructured ceramic coatings have demonstrated superior performance compared to traditional micrometer-structured coatings, owing to their uniform composition, fine structure, and enhanced cohesive strength. However, the fabrication of nanostructured coatings has persistently depended on reconstituted feedstocks, which are typically produced through complex reconstitution methods, such as slurry-spray-drying. Due to the high demand for nanoparticles, issues such as high cost, agglomeration, and potential harm to human health are difficult to avoid. More importantly, the microstructure of coatings is only limitedly adjustable, as it is strongly dependent on the size and distribution of the nanoparticles in the feedstock. These issues have significantly hindered the development and large-scale application of nanostructured ceramic coatings. Therefore, the development of new feedstock preparation technologies for thermal-sprayed coatings is of paramount importance.

Recently, a team of scientists led by Yongting Zheng from Harbin Institute of Technology (HIT) in China reported the preparation of a novel nano-eutectic structured Al₂O₃-ZrO₂ composite powder using a unique combustion synthesis-air atomization (CS-AA) technique. The powder was used as the feedstock for atmospheric plasma spraying (APS) to construct a ceramic composite coating with a nano-eutectic structure. The team also explored the potential applications of the coating in wear protection.

The team published their findings in the Journal of Advanced Ceramics on 19 August 2025.

"Herein, a novel nano-eutectic powder prepared by combustion synthesis-air atomization (CS-AA) is used in atmospheric plasma spraying (APS) to fabricate a nanostructured Al₂O₃-ZrO₂ coating for wear protection. The composite coating has a bimodal microstructure featuring the coexistence of the fully melted (FM) region and the partially melted (PM) region. Significantly, the PM region exhibits a multi-colony structure composed of fibrous or lamellar nano-eutectic phases with an average spacing of 65 nm." said Yongting Zheng, professor at the School of Astronautics, HIT (China), a senior expert dedicated to research in combustion synthesis, structural ceramics, and nanoceramics.

"The microstructure of coatings is fundamentally determined by the characteristics of the feedstock," emphasized Yongting Zheng. "However, current research on nanostructured coatings focuses on compositional design, spray parameter optimization, and coating post-treatment. The exploration of the preparation techniques and microstructure design of feedstock powder remains limited." In response, the team proposed this new feedstock powder based on their previously developed original technology: the CS-AA process, and achieved a unique nano-eutectic structure in the coating.

"Compared with common reconstitution methods for preparing nanostructured powders, the CS-AA technique offers notable advantages in cost-effectiveness and environmental sustainability by eliminating the need for expensive nano-sized powders and simplifying processing steps. Crucially, the fine microstructure forms spontaneously in the powder through the coupled growth of eutectic phases from the melt during rapid cooling," said Yuchen Yuan, the first author and Ph.D. candidate at HIT (China), a member of the team.

The team conducted a detailed analysis of the microstructure of the Al₂O₃-ZrO₂ composite coating and its formation mechanism, as well as systematically studying its mechanical and wear properties. "The PM region in these coatings plays a crucial role in improving mechanical properties and wear resistance," Zheng noted. Compared to the FM region, the PM region exhibited significantly higher hardness and stronger crack resistance. After optimizing spray power, the coating achieved a microhardness of 1008.39 ± 308.54 HV_0.2, a toughness of 4.22 ± 0.58 MPa·m^1/2, and a wear rate of 5 × 10^-5 mm³·N^-1·m^-1 under a 6 N load at 500 rpm. The CS-AA method offers a promising approach for producing nanostructured feedstocks for thermal spraying, and the findings of this work provide valuable references for the development and optimization of nanostructured ceramic coatings in wear protection applications.

Despite these advances, more extensive and in-depth studies are still required to explore the application potential of CS-AA as a new feedstock preparation technique in the field of thermal spraying. In this regard, Zheng has outlined four major directions for future investigation, including exploring the preparation of other system ceramic composite powders, regulating the atomization cooling rate and atomization process, optimizing thermal spraying parameters, and expanding the potential applications of coatings in thermal and corrosion protection.

Other contributors include Yuchen Yuan, Liqin Wang, Zhen Li, Yuelei Bai, and Hang Yin from HIT (China); Ming Liu from the Army Academy of Armored Forces (China); as well as Yongdong Yu and Qian Yang from the University of Jinan (China).

This work was supported by the Industry-University-Research Program of Aero-Engine Corporation of China (Grant No. HFZL2022CXY016), National Natural Science Foundation of China (Grant No. 91016014), and the Natural Science Foundation of Shandong Province (Grant No. ZR2024QE153).

About Author

Yongting Zheng was awarded PhD degree of Materials Science from Harbin Institute of Technology (China) in February 1996, prior to joining Harbin Institute of technology as a full professor in 2000. His main research interests include combustion synthesis, structural ceramics, high temperature resistant materials, microstructure and mechanical properties of nanocomposite ceramics, phase transformation kinetics, and synthesis of nano-structure powders. So far, he has published more than 100 academic articles, authored 2 monographs, authorized 29 national invention patents and 1 international patent. Prof. Yongting Zheng was invited to serve as the editorial board of international journals such as Powder Metallurgy and Functional Coatings, and served as a director of organizations such as China Society of Mechanical Engineering (Professional Committee of Engineering Ceramics) and China Silicate Society (Special Ceramics Branch).

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