Effect Of Boron Doping And Heat Treatment On NPD Preparation And Properties

Nano-polycrystalline diamond?NPD?has excellent hardness,toughness,thermal stability and cutting performance.At present,Carbon nano-onion with diamond core is used as precursor in this project,NPD with good performance was successfully synthesized under lower pressure and temperature,which made NPD industrialization possible.At the same time,doping boron into diamond can make it obtain more excellent properties,but the current research of boron doping mostly focuses on boron-containing diamond films,single crystal,less research on boron-doped NPD.In addition,foreign scholars through the nano-diamond heat treatment effectively reduce the internal graphite content.Therefore,this paper mainly studies how to synthesize boron-doped NPD with good performance under low synthesis conditions,and improve the quality of diamond by various means of heat treatment,so as to promote the application of NPD.In this paper,NPD doped with boron was synthesized by vacuum annealing at1100?,using carbon nano-onion as raw material,doped with Amorphous boron at different mole ratio,under low pressure and high temperature and high pressure,at the same time,the boron-doped NPD and nano-diamond were heat treated under different temperature,pressure and atmosphere.Through the analysis of the experimental results,it is shown that:?1?When the synthetic condition is 6 GPa,1300?and the boron content is4 mol.%,the boron-doped NPD has the best mechanical properties,the average vickers hardness is 45 GPa,the toughness is 8.5 MPa·m^1/2,the diamond content is 86 wt%.Compared with the common diamond,the initial oxidation temperature is 1215?,which is 430?higher.?2?In air atmosphere,when the heat treatment temperature is 490?and the heat preservation time is 5 h,the NPD with boron can improve the mechanical properties most obviously,especially when the heat treatment temperature is 490?,the vickers hardness of the sintered body is increased to 62 GPa,the results showed that the toughness of the composites increased to 17.1 MPa·m^1/2and 17 GPa respectively.?3?In the vacuum atmosphere,the products of nano-diamond annealed by SPS are similar to that of hexagonal graphite.The increase of pressure promotes the graphitization of nano-diamond.When the synthesis conditions are 50 MPa and 1300?,nano-diamond is oxidized violently and produces abundant graphene structure,and the graphitization is complete at 1400?.At a lower pressure of 20 MPa,the graphitization can be achieved at1400?.?4?The graphitization rate of nano-diamond is slow under high temperature and high pressure.When the pressure was 5.5 GPa,the nano-diamond changed obviously after1300?.when the temperature reached 1500?,the graphite degree of the sample was similar to that of SPS at 50 MPa and 1300?meanwhile,the graphitization could be promoted by prolonging the holding time to 5 min.