Diamond Dislocation Nucleation And Nucleation Process For Graphite-to-Diamond Phase Transition Studied By Molecular Dynamics

Diamond is the hardest crystalline material in nature,and it is widely used in industrial production because of its excellent mechanical property,electrical property,etc.The direct transformation of graphite to diamond at high temperature and high pressure is the main method of synthetic diamond.Therefore,it is important to study the mechanical property of synthetic diamond and the phase transformation process of graphite to diamond for product quality improving.Although lots of related researches have been carried out until now,a deep understanding about the mechanical property of diamond and phase transformation process of graphite-to-diamond has not been obtained due to experimental difficulties and the lack parameters of simulation and theory,which seriously hinders the further development of diamond.Dislocation as the carrier of plastic deformation plays a pivotal factor in affecting the mechanical property of materials.On this base,diamond dislocation nucleation process and the influences of dislocation on nucleation process for graphite-to-diamond phase transition are studied theoretically in this paper.The basic dislocation structures in diamond and graphite are obtained by molecular dynamics method.The diamond dislocation core radiuses are all less than a magnitude of Burgers vector,and the energy of glide set partial dislocation is lower than that of shuffle set perfect dislocation.The core radiuses of graphite basal plane dislocation are dozens of times of Burgers vector,but dislocations energy is only around 0.1 eV/?.The activation parameters of diamond glide set partial and shuffle set perfect dislocation homogeneous and heterogeneous are calculated by molecular dynamics,the results show that diamond dislocation homogeneous and heterogeneous nucleation activation energy are all above 100 eV at 0 GPa condition.The critical nucleation stresses of both glide set partial and shuffle set perfect dislocations homogeneous nucleation close to diamond ideal shear strength,and these two types of dislocations have good probabilities to nucleate heterogeneously because of their lower critical nucleation stresses?28.9 GPa and 48.2 GPa?.Furthermore,there exists a deformation mode transformation for diamond deformation behavior between dislocation nucleation and mobility at strain rate of 10^-3/s.In the study about nucleation process for graphite-to-diamond phase transition by molecular dynamics calculation,it is found that when isostatic pressure is 90 GPa,the activation energy of homogeneous nucleation for graphite-to-diamond phase transition is still 61.1 eV.After dislocations are introduced into the graphite,the activation energy of heterogeneous nucleation for graphite-to-diamond phase transition is much lower than that of homogeneous nucleation at the same isostatic pressure condition.Moreover,the methods of reducing size of the graphite layer which has dislocations or increasing its number can reduce heterogeneous nucleation activation energy by half or more.