Superhard Mechanism And Microscopic Design Of Transition Metal Borides

Superhard materials which have higher hardness,higher fusion point,resistance wearly,corrosion resistance and other comprehensive physical and chemical properties are widely used in special devices,PV industry,electrical or mechanical implements,and petrochemical industries.The design of new superhard material has developed rapidly in recent years,while it has become increasingly important to overcome the inherent limitations of traditional superhard materials?e.g.unstable high-temperature high-speed diamond processing,extreme cBN synthesis conditions?.The superhard mechanism and microscopic design of the transition-metal borides?TMBs?is an important research field for a synthetic superhard material of a novel superhard material.The research of new superhard materials focuses on solid compounds with strong properties.The current design criteria for superhard materials include high bulk modulus,high electron concentration,strong covalent bonds,and thermodynamic stability.Among them,valence electron concentration?VEC?and strong covalent bond are key factors for the design of TMBs.Recently,the main research methods are the formation of transition metal and boron compounds,combined experimental synthesis of the phases.We adopted the first-principles calculations?ab initio?to clarify the design mechanism and mechanical performances of new superhard materials.It is traditionally believed that the hardness of borides is positively related to the VEC,should gradually increase.Laboratory synthesized W_0.5Ta_0.5B hardness test up to 42Gpa,more than its ancestral WB.We have found that the hardness of the transition metal monoboride?TMB?exhibits a breakpoint as the value of the VEC rises the hardness will decreases.This phenomenon of abnormal mechanical behavior is different from the previous positive correlation between VEC and hardness.Nine candidates?Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,W?of transition-metal monoboride(TM_1-xB_x)are selected to observe as research objects.The main features of the study are TMB structural properties,mechanical characteristics,and superhard mechanisms.The low borides has an unexpectedly high hardness.On the contrary,the high borides exhibits an abnormal hardness reduction.Firstly,we analysis structural characteristics mainly focuses on the space group CmCm.The metal coordinates and Boron are lattice structure characteristics occupying the 4c position of Wyckoff,and it is found that it has the structural characteristics of the bimetallic layers and the Zigzag boron chains.Secondly,mechanical analysis:Calculation of the stable phases of the TMB structure indicates that the formation energy of these phases is negative and have highly thermodynamically stable phases.We also study the relationship between mechanical properties,and VEC.Thirdly,superhard mechanism and the origin of electronic structures:analysis of the density of states and band structures of the WB and VB phases,we analyze the induction of the bond on the hardness mechanism of the electronic s,p,d binding states?dd,pd bonds?of metals and boron,analysis of dd and pd bonds.This unconventional phenomenon stems from the unique bonding mechanism between boron atoms.Further studies have shown that monoborides have a closer composition,similar structure and energy reduction.The enhanced hardness of an atomic and nanosale mechanism was found by identifying higher thermodynamically stable phases and strengthening weak slip planes.A large number of interfacial interfaces between different boron atoms can easily synthesize a multiphase material,and the strong pressure created by the nanoscale interlocking at these interfaces causes a slip dislocation of the bimetallic atomic layers,thus greatly improving the intrinsic properties.After studying the abnormal mechanical behavior and the origin of superhardness mechanism of TMB,low borides V-B,Ta-B,and Nb-B have been selected to analyze low borides.The structural characteristics,mechanical performances,and superhard mechanism origin have been investigated.Researchers show that their compositions is cognate,while their crystal structures are similarly.In addition,the common structural feature is the presence of a bimetallic layers limits its hardness.In order to further increase the overall hardness,relatively weak two-layers sliding dislocations under relatively shear must be limited.The hardness of V-B?VB,V₅B₆,V₃B₄,V₂B₃?are nearly close to the superhard threshold.So,V-B is a strong competitive phase for novel superhard phases.Our study aimed to analyze the intrinsic hardness of TMB and low borides.There are also some methods for increasing the extrinsic hardness.For example,there is a large number of interfaces between different borides to create a heterogeneous solid solution material.These interfaces form a nanoscale interlock that strongly impedes the sliding position of the two metal layers of each boride phase,thereby enhancing the external hardness and obtaining a true superhard material.This method is a reinforcing effect for our theoretical analysis.This abnormal hardness phenomenon is a special bonding mechanism and band filling due to the increase of boron content.Our work not only revealed the unique mechanism of abnormal hardness trends such as TM1-xBx.But it also illustrates an atomic and nanoscale method to create new superhard materials with multiple functions.