Interfacial Structure And Mechanical Propertiesof Superhard Nano-multilayers:a First-principle Study

Superhard materials are a class of materials,which have the Vickers hardness(Hv)of over 40 GPa and has been used in many fields,such as oil exploitation,cutting tool in machining,electronics technology,and nuclear safety.Superhard nano-multilayer,as a kind of superhard materials,has become a promising coating material of cutting tool due to its high hardness,high oxidation resistance,high corrosion resistance,and high thermal stability.Superhard nano-multilayer could greatly make up for the deficiency of the existing cutting tools and has become a hot research spot in science and technology.In this paper,we studied the atomic structure,and electronic and mechanical properties of some typical superhard nano-multilayers with first-principle calculations,which includes the following parts:(1)We analyzed the atomic structure and electronic properties of the interface of metal-ceramic nano-multlayers,including the interfaces between Al and a one of the ceramics of TiC,TiN,VC,and VN on the(001),(110),and(111)planes,respectively.We found that the stable interfaces are those with the bonds between Al atom and metalloid C(or N)atom.The interfacial bonding is a mixture of covalent and ionic properties,of which the covalent bond is attributed to the hybridization of s and p states of Al and d states of metalloid atoms.Among the interfaces with the three typical orientations,the(111)interfaces,which possess the largest adhesion energy,are the most stable.The interfaces between Al and metal carbides(TiC and VC)are more stable than those between Al and metal nitrides(TiN and VN).We also analyzed the atomic structure and electronic properties of the interface of diamond and c-BN multilayers on the(111)plane.It shows that there are two kinds of stable interfacial structures with the stacking sequences near the interfaces identical with that in bulk diamond,implying a smooth electronic transition across the interface.The interfacial bonds are primarily of mixed covalent-ionic type,of which the covalence of interfacial bonds stems from the sp3 hybridization between the sp states of C and B atoms.The effects of the interface are localized.(2)The generalized stacking fault energy(GSFE)is usually used to describe the slip property of slip system in crystals.We calculated the GSFEs of the interfaces of the diamond(111)/c-BN(111)stable structures.The results show that the trends of the GSFE curves for the interfaces are similar to that in bulk monocrystalline diamond and c-BN,but the unstable GSFE for the diamond(111)/c-BN(111)surface are smaller than that for Bulk c-BN.For the B1 pattern interface,the instable GSFE for the {111}<110> shuffle-set slip system has the lowest unstable stacking fault energy,suggesting that this slip system is the easiest to form a perfect dislocation.For the B2 pattern interface,there are two different values of unstable GSFEs for the {111}<112> glide-set slip system,indicating the possibility for Shockley partial dislocation(a0/6<112>)to take place.The GSFEs of the slip planes near the interface alters remarkably due to the interfacial effect.The GSFEs of different slip systems have different changes.Compared with the GSFEs of the constituents,the GSFEs for some slip systems are enhanced,that for some other slip systems are suppressed,while that for the rest may remain unchanged.(3)The ideal strengths of perfect crystals are analyzed and the changes of the atomic structure under deformation are explored with the simulations of uniaxial tensile and pure shear deformation on bulk diamond,bulk c-BN,and the multilayers composed of diamond(111)/c-BN(111)stable interfacial structures,respectively.The results show that the bulk diamond and c-BN in the <111> direction possess the lowest ideal tensile strength,suggesting that the chemical bond in this direction is easiest to be fractured under tensile deformaion.In the {001}<110> and {110}<110> directions of bulk diamond and c-BN perfect dislocation are easy to form,while in the {111}<112> direction partial dislocation are easy to form.The ideal tensile strength of diamond/c-BN nano-multilayer is higher than that of bulk c-BN,but lower than that of bulk diamond.When the diamond/c-BN nano-multilayer is subjected to tensile deformation fracture first takes place in c-BN layer.When subjected to shear deformation,the shear stress-strain curve of the multilayer develops in a zigzag way,attributed to the continuous break and recombination of atomic bonds under shear deformation.The nano-multilayer in the(7)111(8)[1(?)1] direction possess the lowest ideal shear strength.