Multi-scale Study On Fracture Behavior Of Hexagonal Boron Nitride And Polycrystalline Graphene

Nanotechnology has become an important part of the scientific and technological advancement in the new era,and has played an significant role in the development of national economy,national defense,science and technology,etc.Graphene and hexagonal boron nitride(h-BN),as representatives of nanomaterials,have good application prospects in optical,electrical and thermal fields due to their unique structures.The fracture property of material is an important factor affecting its application.However,the existing preparation methods graphene and h-BN have a variety of defects and grain boundaries,which inevitably affect the fracture properties of materials.In this paper,the fracture behavior of h-BN and polycrystalline graphene are studied by means of stick-spiral model theory,molecular dynamics simulation and finite element analysis.The effects of atomic defects and grain boundaries on their fracture behavior are discussed,and the influence of the interaction between cracks and defects on their fracture behavior is analyzed.The main researches of this paper are listed as follows:(1)Fracture behavior of h-BN was studied by molecular dynamics simulation and theoretical analysis.By establishing a stick-spiral model of chirality dependent h-BN which based on the Tersoff potential function between h-BN atoms,the nonlinear mechanical behavior of chirality-dependent h-BN under large deformation have been investigated.The findings indicate that the fracture modes of h-BN range from ductile fracture to brittle fracture when the chiral angle changes from armchair(AC,chiral angle is 0°)to zigzag(ZZ,chiral angle is 30°).That the change of bond angles plays a more important role in the stretching process than that of bond lengths is the main reason of the occurrence of ductile fracture,through the analysis of the change of bond lengths and bond angles in the stretching process.By analyzing the distribution of maximum stress and fracture stress of h-BN with chiral angle,the chiral angle of brittle-ductile transition is around 15°.The change of bond length dominates the fracture mode of graphene in the stretching process,while the change of both bond length and bond angle dominate the fracture mode of h-BN,by comparison the h-BN and graphene under the same conditions.Finally,checking against the molecular dynamics simulations shows that the theoretical results are reasonable.(2)The fracture behavior of chiral h-BN with different crack-tip shapes and the interactions of nanoscale crack-defects are researched with molecular dynamics simulations and finite element analysis.The current MD and FE results indicate that the fracture strength of h-BN with two crack tips(t=2)is significantly higher than that with one crack tip(t=1),in which the difference in ZZ direction is more obvious than that in AC direction,mainly due to the fact that the change of bond angles near the crack tips is more substantial in the ZZ direction than in the AC direction.The interaction between defects and the main crack is studied,combining with the study of crack-tip shape,and the fracture strength of h-BN depends on crack-tip shape,deflection angle,the defect-to-crack tip spacing and the chiral angle.Checking against the current MD simulations and FE analysis shows the present results are reasonable.(3)The single crystal and bimorph boundary graphene were established.The fracture behavior of polycrystalline graphene was studied by molecular dynamics simulation and finite element method,including the deflection of polycrystalline graphene at the grain boundary and the effect of the number and direction of the point defect on the crack propagation path.The results show that: firstly,when the loading direction is perpendicular to the grain boundary of graphene,the fracture strength is obviously lower than that when the loading direction is parallel to the grain boundary;secondly,at the grain boundary of polycrystalline graphene,the crack propagation path will deflect,and the deflection angle is related to the orientation angle of graphene grains on both sides of the grain boundary.Through this phenomenon,the artificially designed grain boundary can be realized,so as to control the crack propagation;finally,the number of point defects and the angle between the defect group and the crack growth path also affect the crack growth of polycrystalline graphene.When the angle is small,the crack can propagate to the direction of defect group,and the length of propagation distance can be controlled by the number of defects.This study not only highlights the significance of bond angle change in the fracture process of h-BN,provides direction for improving its fracture strength,but also has important significance for controlling crack path by using grain boundary and defects.