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Wurtzite boron nitride, due to its superior performance and many potential practical and scientific applications

wallpapers News 2021-06-09
Wurtzite boron nitride (WBN), due to its superior performance and many potential practical and scientific applications, such as ideal machined/cut/milling iron and carbide materials, particularly as ideal dielectric substrates for optical, electronic and two-dimensional graphene-based devices, has recently received a lot of attention from academia and industry. Despite decades of research, the debate about whether single-phase WBN is harder than diamond continues due to the difficulty of producing pure WBN materials. This paper reports the synthesis of pure single-phase polycrystalline boron nitride (BN) bulk material from wurtzite powder by a controlled process under ultra-high pressure and high temperature. Cubic boron nitride (CBN) was successfully prepared from WBN for the first time and was compared and verified. X-ray diffraction (XRD) and transmission electron microscopy (TEM) clearly confirmed that the pure single-phase WBN compact was obtained. Firstly, the microstructure and mechanical properties of pure single-phase WBN, including Vickers hardness, fracture toughness and thermal stability, were evaluated.
Boron nitride is a kind of material with three crystal structures: hexagonal, cubic and fibrous. Hexagonal boron nitride (HBN) is a stable phase at atmospheric pressure. Cubic boron nitride (CBN) and fibrous boron nitride (WBN) can be synthesized at high temperatures and pressure. There are some literature reports on the mechanical properties of CBN, especially with a hardness of about 45-50 GPA17. We know almost nothing about the mechanical properties of WBN because wurtzite is a metastable phase of BN at all pressures and temperatures, and it is difficult to prepare pure phase 20. Previous studies have shown that the hardness of WBN varies significantly from 24 GPa to 54 GPA17,18. Some studies have shown that WBN can be as hard as or even harder than diamond, which is surprising because WBN and CBN have similar bond lengths, elastic modulus, and ideal tensile and shear strength. Therefore, the production of pure single-phase WBN bulk material is very important to accurately measure its hardness.
In previous studies, the synthesis of WBN was directly converted from HBN under high pressure and high temperature. During heat treatment and pressure treatment or the preparation of sintered billet, other phases coexist in the transformed sample, and the values obtained do not represent the values of pure WBN. In this paper, pure WBN and CBN densities were successfully synthesized from pure WBN powder under high temperature and high pressure under good control conditions for the first time. To clarify the long-standing debate in academia as to whether pure or not compared to diamonds, comparative studies of mechanical properties, in particular, pure hardness and fracture toughness, etc., and CBN, together with their microstructure and thermal stability have been investigated in detail.

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