A novel two-step sintering strategy for highly transparent AlON ceramics by combination of ultra-fast high-temperature sintering followed by conventional sintering

AlON is an advanced transparent ceramic material with excellent optical transparency, high strength and hardness, and outstanding thermal stability, making it an ideal candidate for next-generation transparent armor systems and high-performance infrared windows in extreme environments. Apparently, pressureless sintering, which does not rely on HIP, is the most favored method due to its low cost and practice feasibility. Specifically, pressureless sintering has the additional significant advantage in the fabrication of large-scale and/or complex-shaped components. Although AlON ceramics with high transmittance (≥80%) have been successfully prepared via pressureless sintering, a holding time of 6-30 h is still generally required to fabricate of AlON ceramics with high optical transparency across the visible to infrared range. This undoubtedly increases the energy consumption, cost and risk in the fabrication of transparent AlON ceramics. As well as, the long holding durations at high temperature usually result in massive grain growth. This may not be conducive to achieving the high mechanical properties expected from AlON products.

Recently, a team of material scientists led by Yingchun Shan from Dalian Maritime University, China first reported AlON ceramics with excellent optical transmittance (≥80% over a wide wavelength range of 350-4900 nm) were successfully fabricated by employing ultra-fast high-temperature sintering followed by conventional sintering (UHS+CS) method, with 2 min of UHS holding at 1850 ℃ followed by 2 h of CS at 1880 ℃. This work not only explains the underlying mechanisms of the rapid densification of UHS and UHS+CS, but also provides a reference for the rapid pressureless sintering preparation of high-density fine-grained AlON ceramics.

The team published their work on September 29, 2025 in Journal of Advanced Ceramics.

“In this report, we fabricated AlON pre-sintered samples via UHS by heating at a rate of 6000 ℃/min to 1850 ℃ and holding for 2 min. Then, the pre-sintered samples were heated to 1880 ℃ and held for 2 h via CS to fabricate highly transparent AlON ceramics.” said Yingchun Shan, professor in Department of Materials Science and Engineering at Dalian Maritime University (China), a senior expert whose research interests focus on the field of structure function integrated ceramics, including SiAlON, AlON and AlN.

After UHS processing at 1850 ℃ for just 2 min, the AlON pre-sintered body achieves a remarkably high relative density of 97.43%, characterized by fine grains and non-equilibrium grain boundaries. This microstructure forms the ideal foundation for subsequent densification. “The UHS step is found to play a critical role in achieving a high-density pre-sintered body, which is crucial for the final excellent transparency,” said Yingchun Shan.

In terms of optical performance, the final ceramics demonstrate high transmittance of ≥80% across an exceptionally broad wavelength range from 350 to 4900 nm, reaching a maximum transmittance of 84.73% at 3750 nm. The combination of high relative density (99.71%) and a significant reduction in both the number and size of pores, which range from just 0.11 to 0.37 µm, are the key factors enabling the excellent transparency. “The ultra-fast heating rate of UHS effectively inhibits decomposition of AlON and microstructural coarsening in the early stages of sintering, thereby preserving the potential for rapid densification and obtaining high optical quality,” said Yingchun Shan.

The AlON ceramics fabricated by the UHS+CS process exhibit outstanding mechanical properties, with a Vickers hardness of 19.57±0.23 GPa. The superior hardness of the UHS+CS sample can be attributed to its higher relative density, more uniform grain size, and finer grain size compared to the CS-only sample. “This hardness value is significantly higher than that of many AlON ceramics prepared by pressureless sintering reported in the literature, demonstrating the effectiveness of the two-step method in enhancing mechanical strength,” said Yingchun Shan.

The drastic reduction in the total high-temperature holding time to 122 min (about 2 h), compared to the 6-30 hours required by conventional methods, positions UHS+CS as an efficient technique. “From the perspective of achieving both high optical transparency and superior mechanical properties efficiently, the UHS+CS method establishes itself as a viable and promising strategy for the rapid fabrication of high-performance AlON ceramics,” said Yingchun Shan.

However, more delicate research works are still needed to explore the suitability of UHS+CS as a new AlON sintering method. In this regard, Shan also put forward three major development directions may be pursued in future works including the optimization of the UHS process parameters, suitability and effectiveness of other sintering additives systems and in-depth mechanistic understanding of sintering acceleration.

Other contributors include Haoran Guo, Hang Zhang, Xiuling Zhang, Hui Zhang and Jiujun Xu from the Department of Materials Science and Engineering at Dalian Maritime University, China; Jiangtao Li from Technical Institute of Physics and Chemistry at Chinese Academy of Sciences, China.

This work was financially supported by the National Natural Science Foundation of China (52372058), the Fundamental Research Funds for the Central Universities (3132023515).

About Author

Yingchun Shan holds a M.S. and Ph.D. in materials science from Harbin Institute of Technology. She was a visiting scientist at the Powder Technology Laboratory, San Diego State University in the 2016-2017. She is now professor in Department of Materials Science and Engineering at Dalian Maritime University, China.

Shan has led or participated in 15 major research projects, including grants from the National Natural Science Foundation of China, Key R&D Programs of the Ministry of Science and Technology, etc. She has published over 80 academic papers and holds 26 authorized invention patents.

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