Study Of Mechanical Properties And Deformation Mechanisms Of Superhard Nano-structured CBN And Diamond

Ultrahard materials usually refer to the materials whose Vickers hardness exceeds40 GPa,of which cBN and diamond are two typical representatives.These materials usually have excellent mechanical properties such as high hardness,high strength and high wear resistance,which can often be used as surface coating materials of cutting and punching tools for precision/ultra-precision manufacturing.With the rapid development of modern manufacturing industry,more and more severe and even extremely harsh service conditions put higher and higher demands for such coating materials.In order to meet the demands,the concept of fine-grain strengthening in metals can be used to enhance the mechanical properties of superhard ceramics by introducing nanotwins,nano-multilayers and nanocrystalline structures.Therefore,the extrinsic microstructure design of the intrinsic superhard materials,such as diamond and cBN,should be an effective way to obtain coating materials with more excellent performance.However,the deformation mechanism,the optimal microstructures and their effects under different deformation conditions are not clear,which limits their design,development and applications.To solve these problems,we studied using molecular dynamics（MD）simulations the mechanical properties,deformation and damage mechanisms of nanostructured diamond and cBN,and achieved the following main progress:（1）Based on the results from first-principles calculations and experiments,the accuracy of the Tersoff interatomic potential for a diamond system was verified,and the Tersoff potential of a cBN system was modified,which can well describe the mechanical and physical properties of cBN.The generalized stacking fault energy was calculated with this potential,and the slip sequence of each slip system was predicted.（2）The mechanical behaviors of single crystal diamond and cBN under nanoindentation were studied,and their plastic deformation mechanisms and indentation anisotropy were analyzed.The simulation results indicate that plastic deformation occurs in both materials,which is dominated by shuffle-set{111}<110>full dislocations,accompanied by some glide-set partial dislocations,consistent with experimental results and theoretical predictions.The plastic deformation of cBN under indentation includes the processes of embryo dislocation loops,shear loops and prismatic loops.The cross-slip at the ends of the shear loops could account for the formation of the prismatic loops.However,only embryo dislocation loops and shear loops were observed in diamond under indentation,which could be attributed to the larger energy required to activate and drive dislocations in diamond.Both materials exhibit significant elastoplastic anisotropy due to the difference between the stress distributions caused by different stacking and arrangement of atoms on different surfaces.The elastoplastic responses and the corresponding deformation mechanisms of both the materials under nanoindentation are similar.（3）The nanoindentation responses of nanotwinned cBN with different twin thickness were studied and the effects of twin boundaries（TBs）and thickness were analyzed.The simulation results show that the introduction of nanotwins into cBN can significantly enhance the mechanical properties such as elastic modulus,hardness and plastic deformation energy,which increases with the decrease of twin thickness.The enhancement of elastic property is due to the higher elastic modulus of TBs than that of the twin interiors,and the increase of hardness and plastic deformation energy is due to the effects of TBs on the nucleation,propagation and evolution of nanoscale defects in the material during the indentation.The hardening can be attributed to slip transfer of dislocations,dislocations pile-up at the interface and suppression of dislocations nucleation,while the softening mechanisms are the dislocations-TBs interaction induced break of coherent TBs,the slip of dislocations parallel with TBs and the nucleation of dislocations from the reaction sites.The competition and coupling of the hardening and softening mechanisms determine the mechanical properties of the material.Based on the hardening and softening mechanisms,the relationship between hardness and twin thickness was summarized,which can well describe the dependence of hardness on twin thickness,and can be used to explain the superhard property of nanotwinned cBN in experiments.（4）The interface matching mode and indentation mechanical behavior of{111}semi-coherent cBN/diamond nano-multilayers（NMLs）was studied,and the different effects of coherent,nanotwinned and semi-coherent interface on the mechanical properties and plastic deformation of{111}diamond/cBN NMLs were discussed.It showed that the misfit dislocation network in the relaxed（111）semi-coherent interface exhibits as equilateral triangles,including coherent regions,stacking fault regions and nodes.The misfit dislocations are edge type a/6<112>Shockley partial dislocations.Stacking fault regions and misfit dislocations of the semi-coherent interface continue to grow until the nucleation of dislocations in the cBN or diamond layers.The dislocations in a coherent or twinned NMLs nucleate in the region beneath the indenter.while the dislocations in a semi-coherent NMLs nucleate preferentially at the interface and develop respectively into the cBN and diamond layers.The difference in the elastic modulus and the stacking fault energy between the two components can result in the hardening of the three samples.In addition,the coherent sample can also be hardened by coherency stress and softened by interface crossing.The semi-coherent sample can also be hardened by misfit dislocations and softened by interfacial defects activities and interface crossing.In the nanotwinned sample,coherency stress and dislocations slip transfer would also result in hardening,while interface crossing can hardly occur.Thus,the hardness of the nanotwinned sample is higher than that of the semi-coherent and coherent samples.（5）The uniaxial tensile mechanical behavior of nanocrystalline diamond was studied,and the effects of strain rate,annealing temperature and grain size were discussed.The simulation results indicate that the tensile failure mode is the nucleation and propagation of both transgranular and intergranular cracks,which less depends on strain rate,annealing temperature and grain size.The failure stress and strain increase with the increase of strain rate,and the tension can be regarded as quasi-static as strain rate is smaller than 5×10⁹ s^-1.The failure stress and strain increase with the increase of annealing temperature,attributed to that at higher annealing temperature the grain boundary atoms are more active and can move to a position with lower potential energy more easily,which makes the system more stable.With the increase of grain size,the fraction of grain boundaries decreases,resulting in the increase of Young’s modulus and fracture strain,while the more severe stress concentration at grain boundaries may induce smaller failure stress.