The Effect Of Fullerene Additive On The Synthesis And Properties Of Polycrystalline Diamond Under High Temperature And High Pressure

Diamond is the hardest material in nature.In nature,diamond always exists in the form of single crystal,which exhibits anisotropic properties and limits its processing and applications.In terms of application,the bulk single crystal diamond is costly and difficult to produce because its growth requires long time.In contrast,polycrystalline diamond has received extensive attention due to its advantages of high toughness,isotropy,no cleavage surface,bulk preparation and ease of processing.Industrial polycrystalline diamond is usually prepared by adding sintering aids into the diamond powder during high temperature and high pressure sintering process,but the hardness of sintering diamond is significantly lower than single crystal diamond.The hardness of nanopolycrystalline diamond could be similar to single crystal diamond,and its isotropy suggests a wide range of applications in industry.However,the synthesis of nanopolycrystalline diamond requires an extremely high temperature and high pressure environment,which hinders the industrial mass production of NPD.The industrial high temperature and high pressure conditions generally relies on the six-sided top press that made in China,which cannot reach the temperature and pressure conditions for the synthesis of nanopolycrystalline diamond.The production of nanopolycrystalline diamond materials is currently still in the lab.The generation of high-temperature and high-pressure conditions in the lab generally relies on the large volume presses(LVP).In this work,we succeed in the generation of super-high pressure that surpasses pressure limit of the commercial Walker-type large volume press firstly.We analyzed the physical properties of the materials from different second stage anvils in the multi-stage pressurization device of the large volume press,and selected the second stage anvil with small grain size,less binder,and higher hardness.We also upgraded the selected one on the basis of its structure by adding a 1°inclined plane at the truncated angle of the second stage anvil.Through the improvement of the second stage anvil,we successfully broke the pressure limit of 25-27GPa reached by the common commercial Walker type large volume press,and realized the generation of an ultra-high pressure of 34.5GPa.By using this type of the optimized second stage anvils,we synthesize nanopolycrystalline diamond without impurity elements by sintering the precursors containing fullerene C60 additives and nanodiamond powder under high temperature and high pressure conditions.We also systematically studied the influence of different amount of C₆₀ addition on the mechanical properties of the synthesized nanopolycrystalline diamond.Through transmission electron microscopy(TEM),Vickers hardness measurement and other techniques,we found that adding an appropriate amount of fullerene into the nanodiamond precursor can significantly increase the hardness of the obtained NPD,and the highest Vickers hardness of the obtained NPDcan reach 145 GPa,which exceeds the hardest type IIa single crystal diamond.It is slightly higher than the nopolycrystalline diamond synthesized by graphite.Due to the fact that fullerene can improve the stress and structural transformation of nanodiamonds under high temperature and high pressure condition,and thus improves the interaction of the nanocrystalline interface,the hardness of the obtained sample with appropriate amount of addition is higher than other samples.The new idea of realizing grain boundary enhancement by adding soft fullerene into the precursors during the sintering provides a new method to synthesize other covalent bond compounds.In addition,the mechanical properties of nano-polycrystalline diamond synthesized by using our method is comparable to the nanopolycrystalline diamond synthesized from graphite,but can be prepared at lower temperature and pressure conditions,which indicates it is possible to further decrease the synthesis conditions for the industrial production and application of nano-polycrystalline diamond in future.