First-principles Study Of BCN And T_d-C₂₈-based Superhard Materials

The material with Vickers hardness(H?)higher than 40 GPa is considered to be superhard materials.Superhard materials have a unique structure and show excellent characteristics:incompressibility,wear resistance.Such materials are essential for a variety of industrial applications,such as turning,cutting,drilling and grinding.In recent years,the global demand for superhard materials are constantly rising.Carbon is an important element.Because it can form a variety of hybrid types,and can be combined with other elements to produce a variety of compounds.Elemental carbon has three famous allotropes:graphite,diamond and amorphous carbon.Since the eighties of the twentieth century,researchers have had a great interest in the new allotrope of carbon,including fullerene,carbon nanotubes and graphene.In particular,various structures can be obtained by compressing graphite,and researchers have a great interest in the new allotrope of elemental carbon.Among the most successful examples are fullerenes,carbon,nanotubes and graphene.These synthetic allotropes have had a tremendous impact on the science and technology in the fields of chemistry,physics,materials and information sciences,resulting in the creation of many derivatives,equipments and products.Now ternary B-C-N compounds also attracted a lot of interest to the researchers,it is possible to find new stable superhard materials with some performance better than diamond or c-BN.Some of the superhard B-C-N compounds have been synthesized by high-temperature and high-pressure.In this paper,the properties of m-BCN are systematically studied by using the first-principles calculations.The system was built from a superhard carbon phase by substituting some of carbon atoms with B and N atoms to form ternary materials to improve its thermodynamic stability.The results show that the mechanical parameters of the system are high and meet the requirements of superhard materials,the Vickers hardness is 65.9GPa,which is higher than the threshold value(40 GPa)of superhard material.The hydrostatic pressure calculations show that it has elastic anisotropy and its incompressibility is very close to c-BN.The shear stress-strain curve shows that the system has a higher ability to resist deformation.The electronic structure shows that the system is a semiconductor;there is no imaginary frequency in the Brillouin zone,suggesting that the structure is in the state of mechanical stability.So the system is a potential superhard material.We also systematically studied the id-C28 system and the hl-C28 system with the first-principles calculations.The id-C28 system and the hl-C28 system were constructed from the Td-C28-based cubic diamond structure and hexagonal diamond structure.Since Td-C28 is a tetrahedral symmetrical structure.The four unpaired electrons in each set of triplets of pentagons cause the high reactivity of the C28 cage.The high reactivity,combined with Td symmetry and ready to donate or to accept four electrons creates the possibility to form solids based on the tetrahedral C28.The results show that the Vickers hardness of id-C28 is 47.9 GPa and the Vickers hardness of hl-C28 is 51.4 GPa,which are higher than the threshold value(40 GPa)of superhard material,and have some ability to resist deformation.Elastic anisotropy calculation shows that both systems have elastic anisotropy.Electronic structure calculations show that the energy gap of the id-C28 system is 1.63eV;the energy gap of the hl-C28 system is 1.47 eV,both of which have semiconductor characteristics.In a word,both systems are potentially superhard materials.