First-principles Study On The Coherent Structure And Its Formation Mechanism Of TiC/VC Nano-multilayers

Nano-multilayer have the advantages of ultra-high hardness and wear resistance,high temperature resistance and oxidation resistance,good chemical stability and material tailoring,etc.,which are widely used in mechanical processing,mining,building decoration,crustal composition investigation and other fields.TiC and VC,as materials whose hardness is close to diamond,have the same crystal structure and similar lattice constants.Therefore,TiC and VC can form nano-multilayers with coherent structure and high hardness.Although a variety of nano-multilayers have been successfully prepared experimentally,there are relatively few theoretical studies on the formation cause and formation process of their coherent structure.At present,experimental observation and macroscopic analysis are still the main research methods,but they still cannot accurately describe the atomic behavior of multilayers at the micro level.Therefore,in this paper,model simulation method is adopted to study the coherent structure and formation mechanism of TiC/VC nano-multilayers from the perspective of atoms,which is of great significance to analyze the microstructure of nano-multilayers and broaden the selection range of materials for nano-multilayers.At present,most of the researches on TiC/VC multilayers focus on analyzing the effects of modulation period,stoichiometric ratio of elements and other preparation parameters on the hardness of multilayers.However,the researches on the surface and interface structure of TiC/VC multilayers are relatively lacking,and the formation mechanism and structural characteristics of unique coherent multilayers are lacking.In this paper,the first-principles calculation method based on density functional theory is used to study the formation mechanism of TiC/VC nano-multilayer surface and interface,the generation and characteristics of coherent structure and the mechanical properties of multilayer,which provides valuable theoretical reference for explaining the formation process and characteristics of coherent structure of nano-multilayer.The main research results of this paper are as follows:（1）The most stable surfaces of the two types of components were determined as TiC（111）_-Tiplane and VC（111）_-Vplane respectively,which were used as the base plane of subsequent atomic adsorption and migration and the bonding surface of nano-multilayers.The calculation of single atom adsorption migration determines that the most stable adsorption positions of atoms on the surface are located at lattice points,which is conducive to the formation of coherent structures.The adsorption of poly atoms and the migration of Dimer atoms indicate that C atoms are more easily adsorbed by metal atoms,and C atoms can promote the migration of metal atoms.By calculating the evolution of the surface atom island,it is found that if there is C atom above the Ti atom island,the C atom will eventually move to the bottom of the atom island and adsorb on the Ti terminal lattice,which is easy to form a stable coherent structure.（2）It is found that the local aggregation of C atoms at the interface will destroy the lattice integrity.If a metal atom is adsorped near the interface,the local aggregation of C atoms will be redistributed at the lattice point.By calculating the evolution of the atomic islands,it is found that another metal,which is opposite to the type of the base metal atoms,can promote the migration of the C prodaughter islands from the local aggregation position to the lattice position,which makes the crystal structure more complete.The distance between atoms perpendicular to the growth direction of the thin film is affected by the template effect.By calculating the change of atomic layer spacing along the growth direction of the film,it is found that the change of TiC and VC lattice constants is contrary to the experimental value.Therefore,the influence of roughness on the TiC/VC multilayer spacing is analyzed,and it is found that the multilayer with"brick-like wall"structure has the phenomenon of convergence of lattice constants,which is consistent with the template effect.The microscopic characteristics of the coherent structure"three-direction cooperative strain"are confirmed.（3）The calculation shows that the"brick-like wall"structure has a larger generalized faulting energy,which is 24.298m J/m²,indicating that the structure is more difficult to occur faulting and has stability in the direction parallel to the surface growth.By analyzing the evolution law of the proto-sub-island of interface C,we believe that the"brick-like wall"interface structure has better lattice integrity than other interface structures.We can approximately believe that this structure has fewer or no vacancies and has greater tensile strength than other structures with more vacancies.Therefore,we believe that the special structure of"brick-like wall"interface and the lattice integrity of nano-multilayers are important factors to improve the mechanical properties of multilayers.