Yanshan University Key Laboratory of Metastable Materials Preparation Technology and Science, Han Ameng, Institute of Science and Technology, Wei Lintao
[News Center] Recently, the team of Academician Tian Yongjun from the Key Laboratory of Metastable Materials Preparation Technology and Science at Yanshan University, in collaboration with researchers from Nanjing University of Science and Technology and Ningbo University, has achieved a significant breakthrough in the field of superhard materials. They successfully synthesized ultrafine nanocrystalline twinned diamond blocks with a hardness of 276 GPa, setting a new world record for material hardness. The related research, titled "Enhancing the hardness of diamond through twin refinement and interlocked twins," was published online in Nature Synthesis on January 3, 2025.
Diamond is the hardest natural material, widely used in industries such as machining, oil and gas exploration, and geological surveys. The hardness of single-crystal diamonds varies between 60 and 120 GPa, depending on crystal orientation. Researchers have long been exploring how to synthesize diamonds with higher hardness. Grain refinement, a classic method to improve diamond hardness, has encountered a bottleneck at the nanoscale: excessively high grain boundary energy promotes grain growth, hindering further refinement and limiting hardness enhancement.
To overcome this challenge, Academician Tian Yongjun's team took a different approach and pioneered the refinement of twinned crystals. Compared to conventional grain boundaries, twin boundaries have lower interface energy, offering new possibilities for material refinement. Previous studies by the team confirmed the feasibility of this method—they successfully synthesized diamond blocks with an average twin thickness of only 5 nanometers, achieving a hardness of 200 GPa, twice that of natural diamonds.
In their latest study, the research team precisely controlled the size of the onion-like carbon precursor and conducted high-pressure phase transformation, successfully synthesizing ultrafine nanocrystalline twinned diamond blocks with groundbreaking performance. The average grain size of the blocks was 18 nanometers, and the average twin thickness was only 2.3 nanometers, with a hardness reaching 276 GPa. Electron microscopy observations revealed both through-type and interlocked-type twins in the samples, with interlocked-type twins being dominant. Comparative experiments further confirmed the critical role of this structure. Nanocrystalline diamonds made from diamond powder, though with similarly small grains (24 nanometers), had a hardness of only 125 GPa due to the absence of dense interlocked-type twin structures.
This breakthrough not only sets a new hardness record but also opens up new avenues for surpassing the hardness limits of covalent materials. By refining and controlling twin structures, the research team has pioneered a new method for enhancing material properties, which holds significant guiding implications for future research and development of superhard materials.
This research was funded by the National Natural Science Foundation (52288102, 52325203, 52090020), the National Key R&D Program (2018YFA0703400, 2023YFA1406200), the Hebei Provincial Natural Science Foundation (E2022203109, E2023203256, E2023203126), and other projects. Yanshan University’s Ying Pan, Li Baozhong, and Ma Mengdong are co-first authors, with Tong Ke, Zhao Zhisheng, and Xu Bo as corresponding authors; Pan Yilong (Ningbo University) and Tang Guodong (Nanjing University of Science and Technology) are co-corresponding authors.
Figure Caption: Microstructure and Vickers hardness of ultrafine nanocrystalline twinned diamonds. a) STEM bright field image showing the overall microstructure of the sample, with the inset displaying the statistical grain size distribution, with an average grain size of 18 nm. b) High-resolution TEM images showing the characteristic twin structures of the sample: the main image shows the dominant interlocked-type twins, and the inset shows through-type twins observed in a local region. c) Hardness as a function of twin thickness based on the dislocation model and experimental hardness data, with the inset showing an optical image of the ultrafine nanocrystalline twinned diamond sample.
Source: Yanshan University