Nanocrystalline cubic boron nitride (nt-cBN) with extremely high hardness, toughness and stability has attracted much attention in the field of materials. In 2014, the research team led by Prof. Tian Yongjun, State Key Laboratory of Metastable Materials Preparation Technology and Science, Yanshan University, cooperated with a number of scientific research institutions to successfully synthesize nano-antimony with ultra-high hardness (hardness twice that of natural diamond) using high temperature and high pressure technology. Crystal structure cubic boron nitride block. Related research results were published in Nature.
Recently, Professor Tian Yongjun's team published a research paper on Science China Materials. They found that the structure and mechanical properties of nt-cBN are closely related to the particle size change of the onion boron nitride (oBN) used in the synthesis process. As the particle size of the precursor oBN decreased from ~320 nm to ~90 nm, the hardness of the synthesized nanostructured block gradually increased from 61 GPa to 108 GPa. It shows that the large-diameter oBN has a flat, ordered outer layer structure similar to ordinary hexagonal boron nitride. This has a large number of curved, pleated boron nitride atomic layers and high-density layer faults in the small-diameter oBN nanoparticles. The structural characteristics are in sharp contrast; these ordered structures in the large particle size oBN precursors significantly reduce the content of ultrafine twin substructures in the final product, resulting in a decrease in the hardness of the corresponding product.
This study shows that only the oBN precursor with a sufficiently small particle size can synthesize nt-cBN with high density and ultrafine nano twin structure and excellent performance. It provides a practical solution to further improve the performance of such nanocrystalline materials.
Figure 1 shows the microstructure of cBN.
Figure 2 Vickers hardness of cBN at different pressures.
The research results were recently published in Science China Materials, 2019, doi.org/10.1007/s40843-019-9409-1
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