Study On Strengthening And Toughening Mechanisms And Heat Transfer Performance Of Nanotwined Diamond

Nanotwinned diamond is a typical representative of a new family of highperformance superhard materials.Its hardness is twice that of diamond,and its fracture toughness is up to 5 times that of diamond.Therefore,this material is expected to replace ordinary diamond in superhard material processing,high-pressure physics,petroleum exploration and other fields.However,the hardening and toughening mechanism of nanotwinned diamond is unclear,and the influence degree and mechanism of the existence of twin boundaries on the thermal conductivity of nanotwinned diamond are unclear,which limits the continuous improvement of the properties of this kind of superhard materials.In-depth study on the hardening,toughening and heat transfer mechanism of nanotwinned diamond,revealing the strong-weak synergy of nanotwinned diamond and the phonon transfer mechanism at atomic scale,is the core problem to be solved urgently in the research fields of mechanics,materials science and physics.In this paper,nanotwinned diamond is taken as the research object,and the plastic deformation mechanism of diamond materials under room temperature indentation is deeply explored,and the internal relationship between the atomic scale structure of nanotwinned diamond and its hardness,fracture toughness and thermal conductivity is explored,which provides a theoretical basis for the synthesis and application of superhard materials.Firstly,this thesis introduces the atomic plane arrangement of single crystal diamond and nanotwinned diamond,and analyzes the microstructure characteristics of nanotwinned diamond.This paper summarizes the research status of domestic and foreign researchers on the hardening,toughening mechanism and thermal conductivity simulation of nanotwinned materials,thus establishing the main research contents and research methods of this thesis.Secondly,the graphitization phase transformation behavior and plastic deformation mechanism of single crystal diamond under indentation are studied by means of nanoindentation test and molecular dynamics simulation.It is found that diamond will change from diamond phase to graphite phase under the action of indentation,and the results of molecular dynamics simulation are in good agreement with the experimental results.The change of indentation force and the number of graphitized transformation atoms with indentation depth indicates that the plastic deformation of diamond under indentation comes from graphitized transformation.Further analysis of the micro-phase transformation process in graphitized area shows that diamond will be transformed into graphite structure by four bond breaking modes under the action of indentation.The cleavage planes of the four phase transformation modes are {111} crystal plane,{(?)1(?)} crystal plane,{1(?)(?)} crystal plane,and {11(?)}crystal plane respectively.By changing the angle of indenter to control the ratio of normal stress to shear stress on diamond substrate,it is proved that shear stress plays a key role in graphitization phase transformation of diamond.Then,the graphitization phase transformation process of nanotwinned diamond under indentation is studied by molecular dynamics simulation,and it was revealed that the hardening mechanism of nanotwinned diamond is the barrier of twin boundary to graphitization phase transformation.The effect of twin thickness on graphitization transformation and hardness of nanotwinned diamond is studied.The results show that the smaller the twin thickness,the stronger the resistance of the material to graphitization transformation and the greater the hardness.The results of experiment and molecular dynamics simulation show that the existence of multiple grain boundaries will also hinder the graphitization phase transformation of diamond materials under indentation,among which the lattice mismatch angle of adjacent grain boundaries increases,and the resistance of grain boundaries to graphitization phase transformation increases.Based on the research results of graphitization transformation under twin boundaries and grain boundaries,the microstructure characteristics of ultra-high hardness diamond-like materials are predicted.Thirdly,using the method of molecular dynamics simulation,the effects of three interface forms,twin boundary,phase boundary and layer defects,on the fracture toughness of nanotwinned diamond are investigated.Twin boundary and phase boundary have the same toughening effect on diamond,and the fracture toughness value changes greatly with the angle between crack and interface.In the range of 40-80 included angles,the existence of twin boundaries and phase boundaries has obvious toughening effect on diamond materials,and the maximum fracture energy can reach 1.93 times of that of diamond,while the existence of twin grain boundaries and phase boundaries has little toughening effect on diamond at other included angles.The existence of layer defects will delay the time when the crack tip reaches the critical fracture stress,thus toughening the nano twin diamond.The fracture toughness increases with the increase of layer defect length,layer number and layer spacing,and the critical energy release rate of the model with layer defect is up to 5.6times that of the model without layer defect.Finally,molecular dynamics simulation and DFT calculation are used to explore the influence of twin boundaries on the thermal conductivity of diamond.Based on the DFT calculation results,COMPASS is selected as the potential function that can accurately calculate the harmonic and anharmonic characteristics of phonons of diamond materials.Using this potential function,the influence of twin thickness on the thermal conductivity of nanotwinned diamond is calculated.The results show that the thermal conductivity decreases at first and then increases with the increase of twin thickness.The existence of twin boundaries will affect the phonon lifetime and phonon group velocity of diamond.The phonon lifetime of nanotwinned diamond decreases with the decrease of twin thickness,while the phonon group velocity increases with the decrease of twin thickness.These two competitive mechanisms lead to the existence of twin thickness that minimizes the thermal conductivity of nanotwinned diamond.