Study On High-temperature And High-pressure Synthesis Of Nano-polycrystalline Superhard Materials

Superhard materials represented by diamond and cubic boron nitride show excellent properties such as high hardness and good wear resistance due to the strong covalent bond structure formed by the internal atoms through sp~3hybridization,and have broad application prospects in geological drilling,national defense,industrial manufacturing and other fields.The formation of nano-polycrystalline by structural nanosizing can regulate the performance of superhard materials and obtain superhard materials with higher hardness and toughness and other excellent properties,and the development and preparation of nano-polycrystalline superhard materials is a hot topic of much attention in the field of superhard materials.It is well known that high temperature and high pressure can induce the transformation of sp~2carbon/boron nitride to sp~3structure to form diamond/cubic boron nitride structure,so it is a means to prepare nano-polycrystalline superhard materials with excellent properties.At present,finding cost-effective carbon precursors,regulating the structure of nano-polycrystalline,and revealing the mechanism of nano-polycrystalline formation are still urgent scientific problems to be solved in this research direction.It is shown that the conformation of precursors is closely related to the synthesis conditions,microstructure and properties of the corresponding polycrystalline superhard materials.In-depth study of the transformation process of different precursors to nano-polycrystalline superhard materials under high temperature and pressure,and exploration of the influence of precursor structure on the structure and properties of polycrystals,are important guidelines for the design and optimization of the preparation of nano-polycrystalline superhard materials with excellent properties.In this thesis,the high-temperature and high-pressure synthesis of nano-polycrystalline diamond and diamond/BN nano-polycrystalline composites,nano-curly graphite and amorphous fullerene/nano-boron nitride were selected as precursors,and their structural transformation and the preparation of corresponding nano-polycrystalline materials under high temperature and pressure were investigated by using the large-volume high-pressure technique,and the following research results were obtained.1.The structural transformation of nano-curly graphite under high temperature and pressure conditions and the preparation of nano-polycrystalline diamond were investigated using a large-volume press.First,the structural transformation of nano-curly graphite at 25 GPa and relatively low temperature(800-1200℃)was explored.Combining Raman spectroscopy,X-ray diffraction and electron microscopy characterization,it was found that at 800℃and 1000℃,the uncompressed samples still maintained the sp~2hybridized form,but interestingly,their disorder was significantly increased.When the temperature was further increased to 1200℃,the sample started to transform into nanodiamond structure by diffusion phase transformation inside the sample,forming numerous disordered carbon-coated diamond nanograins.The anomalous phenomenon of amorphization of nano-curly graphite under high temperature and pressure is attributed to the highly distorted graphite layer structure inside,which may lead to the formation of sub-stable sp~3structure in situ at high pressure,but the presence of large internal stress makes the stress release during the unloading process and transforms into highly disordered sp~2amorphous carbon structure.To prepare high-quality nano-polycrystalline diamond,the experimental temperature was increased,and nano-polycrystalline diamond with average grain size of 21 nm,25 nm,and 40 nm was synthesized at 14 GPa 2100℃,20 GPa 1800℃,and 20 GPa 2100℃,respectively.Combined with the mechanical property characterization,it was found that the grain size of nano-polycrystalline diamond kept getting larger with increasing pressure and temperature,and its Vickers hardness increased to 133.2 GPa(higher than the hardness of 120 GPa of single-crystal diamond).It is shown that nano-curly graphite can undergo interesting amorphous transformation at high temperature and pressure,and the regulation of grain size of synthetic nano-polycrystalline diamond can be achieved by changing the temperature and pressure,which provides a new way to prepare superhard nano-polycrystalline materials.2.Diamond/BN nano-polycrystalline composites were obtained by high-temperature and high-pressure preparation of amorphous fullerenes/nanoboron nitride precursors by mechanical ball milling using large-volume press technology.Using Raman spectroscopy,X-ray diffraction,electron microscopy and other characterization tools,it was found that amorphous fullerenes/nano-boron nitride transformed into composites composed of diamond,c-BN,and w-BN nanocrystals with small size grains(6-9 nm)at 25 GPa 1000℃and 1200℃.X-ray photoelectron spectroscopy confirmed the existence of C-B-N,B-C,and C-N bonds inside the samples,indicating the internal formation of certain amount of B-C-N ternary structure.In addition,electron energy loss spectroscopy and Vickers hardness measurements revealed that the sp~3hybridization content inside the sample increased with the increase of temperature and showed better crystallinity,and the hardness of the sample increased from 30.5 GPa to 36.2 GPa.The experimental results indicated that the amorphous fullerene/BN nano-polycrystalline composites with small grain size configuration could be prepared under high temperature and pressure,and showed excellent mechanical properties.The experimental results show that diamond/BN nanopoly-crystalline composites constructed with small grain size can be prepared at high temperature and high pressure and exhibit excellent mechanical properties,which provides a new idea for the design and synthesis of diamond/BN nano-polycrystalline superhard composites.