Study On Strengthening/Toughening Mechanisms Of Nanostructure Superhard Carbon Materials

Superhard carbon materials are widely concerned because of their excellent mechanical properties,such as high hardness,high strength and high wear resistance.Diamond,as a typical superhard carbon material,is the hardest material known in the world and plays an irreplaceable role in superhard materials.With the rapid development of modern manufacturing industry,in the fields of aerospace,medical devices,ultra-high precision processing and artificial intelligence sensors,the demand for ultra-high mechanical properties,ultra-high stability,ultra long service life and ultra-high precision parts becomes more and more urgent.In order to obtain these kinds of high performance material,researchers have tried continuously to improve the mechanical properties of the material by introducing nanotwinned structure,or multi-phase or multi-layer.It has been found that a proper design of the internal structure of a material could enhance sufficiently the mechanical propertiy and performance of the material.However,the strengthening/toughening mechanism caused by structural design is still unclear,which limits the further development and application of superhard carbon nanomaterials.In this dissertation,several kinds of superhard materials with diamond structure are selected as the research objects,of which,the stress-strain responses,thermal stabilities,deformation mechanisms under extreme service conditions were studied using first principles calculation(FPC)and molecular dynamics(MD)simulations.The following main results were achieved:(1)The thermal stability and mechanical properties of the experimentally synthesized Nt-diamond with extremely high hardness were studied by FPC and MD simulations.FPC studies the energy change,atomic motion,chemical bond break and recombination of a system during deformation.It showed that continuous partial slip in a grain under large strain could delay the graphitization of the structure,resulting in the increase of strength and toughness.MD simulations showed that the high-density twin boundaries in a material could inhibit the nucleation and movement of dislocations,and contribute to the improvement of mechanical properties of material.(2)It was found in experiment that the diamond/lonsdaleite biphases have excellent mechanical properties.The responses of diamond/lonsdaleite biphases subjected to compressive-shear strain as well as the stability of each phase were analyzed using FPC.The results show that under large strain loading,partial slip occurs in the hexagonal phase,which leads to phase transformation and improves the strength and toughness of the material.The deformation of polycrystalline diamond/lonsdaleite biphases under compression and indentation ware also simulated with MD simulations,and the results show that the derformation resistance of the biphases is lower than that of the polycrystalline nanotwinned diamond,attributed to that the phase boundary is not so stable as the twin boundary,thereore,the ability for the phase boundary to inhibit the nucleation and movement of dislocations is weaker than that of the twin boundary.(3)The mechanical behaviors and thermal stability of diamond/c BN multilayers under shear strain were studied with FPC,and the effects of B-C atomic interface and twin boundary on the deformation mode and stability of the multilayers were analyzed.The simulation results indicate that the alternate “exchange” of the positions of C and B atoms occurs at the interface under extreme strain conditions,releasing the stress in the model,which could be beneficial to structural stability.The introduction of twin boundaries may change the stacking sequence of multilayer,resulting in the appearance of weak atom plane with low energy barrier.During loading,the cleavage and recombination of chemical bonds occurs preferentially in the c BN region,followed by that in the diamond region,and finally occurs at the interface.This multi-level deformation model can provide available idea for the design of high-performance carbon materials.The thermal stability of diamond/c BN multilayers lies between those of diamond and c BN,but the introduction of twin boundaries results in better result than any of its constituent single phases.(4)The effects of the substitution of B/N impurities on the ideal strengths of diamond were studied,and the strength of the chemical bonds and the distribution of the charge at the impurity area were analyzed.A strain toughening mechanism,i.e.,continuous atomic reconstruction through partial slip increases the toughness while maintain the ideal strength of diamond.It is more surprising that the B-impurity atom could change the charge distribution in diamond and makes the diamond semiconductor,which is consistent with the viewpoint that the B-doped diamond is not only a promising semiconductor material,but also has excellent mechanical properties.The results obtained in this dissertation are helpful to understand the strengthening/toughening mechanisms of nanostructured carbon materials at atomic scale,and provide support for the design of such kind of superhard materials with excellent mechanical properties,thermal stability and electrical characteristics.