| As the most important superhard materials,diamond and cBN were successively synthesized in the middle of last century.Various machining tools made from them have been widely used in industry.In recent years,people have improved the mechanical properties of these materials by means of structural regulation such as grain reinforcement and nanotwin reinforcement.It was found that when the grain size is reduced to less than 10 nm,grain reinforcement is limited due to the anti-Hall-Petch effect.In contrast,nanotwin strengthening can provide continuous strengthening effect due to the joint contribution from Hall-Petch effect and quantum confinement effect.The synthetic nanotwinned cBN and diamond bulk materials achieved a comprehensive improvement in hardness,toughness and thermal stability.However,it remains a challenge to further refine the twin thickness to prepare diamond bulk materials with better performance.In addition,cBN and diamond nanopowders have attracted numerous research interest.How to effectively prepare cBN and diamond nanopowders with twin substructures is also a significant topic in the research of superhard nanopowder materials.In this dissertation,synthesis of nanotwinned cBN and diamond powders and twin refinement of diamond bulk material were studied.Onion carbon nanoparticles were put into methanol solution with KCl,and the precursors of KCl-coated onion carbon nanoparticles were prepared by ultrasonic and magnetic stirring treatment.The morphology of synthesized sample,including porous diamond bulk,submicron diamond powders and nanoscale diamond powders,can be regulated by adjusting the proportion of onion carbon and KCl in the precursors.Microstructure characterization of the as-prepared diamond powders revealed that all the diamond particles contained high-density nanotwin substructures,with an average thickness of approximately 5 nm.Pure-phase nanotwinned diamond bulks were synthesized at 25 GPa and 2100°C using centrifugally separated onion carbon of different sizes as precursor.Microstructure characterization revealed that average twin thicknesses in samples synthesized of onion carbon with average particle sizes of 46 nm,38 nm,28 nm and 22 nm were 5.2 nm,4.3 nm,2.3 nm and 2.4 nm,respectively.It means that smaller precursors can lead to thicker nanotwins in the synthesized nanotwinned diamond bulks.Hardness test showed that the samples with fined twin size had higher hardness,in which the hardness of the sample with twin thickness of about 2 nm reached 270 GPa under applied load of 4.9 N.The hardness values and twin sizes of the synthesized samples agreed well with the microscopic hardness model of covalent crystals.We also analyzed the mechanical properties of different diamond samples(single crystal,nanopolycrystalline and nanotwinned blocks).Vickers hardness of single crystal diamond(110)crystal plane,nanopolycrystalline diamond and nanotwinned diamond were tested,and the indentations(indentation size,crack length,etc.)were carefully characterized using optical microscope and scanning electron microscope respectively.The average hardness of them were 80 GPa,135 GPa and 212 GPa respectively under applied load of 4.9 N,in agreement with the reported values in the literature.The results showed that nanotwinned diamond had a more remarkable hardness than single crystal diamond and nanopolycrystalline diamond.Meanwhile,nanotwinned diamond and nanopolycrystalline diamond possess better toughness than single crystal diamond.Similar to the synthesis of twin-structured diamond nanopowders,twin-structured cBN nanopowders were synthesized at 15 GPa and 1800°C with oBN as raw material and KCl as medium.The KCl insulation layer can effectively prevent oBN nanoparticles from sintering into bulks at HPHT conditions.cBN nanoparticles were finally extracted after a water-washing process to remove KCl.Microstructure characterization revealed that all of the as-prepared cBN nanoparticles contained high-density nanotwin substructures.The average twin thickness was approximately 5 nm. |
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