Molecular Dynamics Simulation Of Thermal Stability And Mechanical Properties Of Nanocrystalline Diamond

Nanocrystalline Diamond(NCD)film is a new type of diamond material which inherited the excellent properties of diamond.Moreover,it has unique microstructure,and can satisfy the large-area preparation of complex shapes.Therefore,it has broad application prospects in wear-resistant devices,micro-electromechanical systems and flat panel displays.It is very important to study the thermal stability and mechanical properties of nanocrystalline diamond,since there are a large number of grain boundary structures in NCD films.In this paper,the molecular dynamics method was used to simulate the thermal treatment and uniaxial tensile tests of nanocrystalline diamond,while the atomic trajectories,radial distribution functions,atomic coordination numbers,bond lengths,and bond angles were used to analyze the changes in the grain boundary structure.This work is helpful to explain the cause of the film failure and the damage mechanism in harsh environments,and promote the wide application of NCD film in the high-tech manufacturing field.Research indicates:(1)The axial internal stress changes significantly before and after thermal treatment,which is mainly manifested by the decrease of tensile stress and the increase of compressive stress.The higher the temperature is,the more obvious the change of stress is.The thermal treatment has less effect on the nanocrystalline diamond grains,but the grain boundary changes greatly.Hybridization transformation occurs at the grain boundary,and the proportion of sp~3 decreases significantly.Due to the larger volume of the sp~2 atom,the volume of the grain boundary structure expands,which leads to the axial stress change of the nanocrystalline diamond.When the temperature is lower than 700 K,the grain boundary structure only experiences local sp~3structural stress release with a small amount of sp~2 formation.As temperature rises further,it will lead to a large number of hybridization transformations,indicating that there is the critical temperature for grain boundary hybrid transition.(2)Tensile strain rate,thermal treatment and Fe atoms have little effect on the elastic modulus,but have significant effects on the yield strength and fracture strain.Large strain rate will leave the atoms without full relaxation,resulting in higher yield strength and fracture strain.The effects of thermal treatment and Fe atoms on the mechanical properties is more complicated,which may be attributed to the influence of the atomic structure at the grain boundaries.Thus,the optimum mechanical properties can be obtained by appropriate thermal treatment temperature and Fe doping.The fracture and failure of the nanocrystalline diamond model was firstly generated at the grain boundary and showed a clear grain boundary failure mechanism.The grain boundary structure determined the yield strength and fracture strain of the nanocrystalline diamond.During the stretching process,the grain boundary structure undergoes complex changes in the atomic chemical bond and hybridization structure under the external load.And a large number of sp~2 hybridization networks are formed at the grain boundary.When the strain exceeds its bearing limit,the grain boundary structure will break and destroy,and eventually lead to the damage of nanocrystalline diamond.