Exploring Novel Superhard Materials From The First-principles Calculations

Superhard materials, defined as Vickers hardness exceeding40GPa, usuallyhave excellent physical and chemical properties including corrosion resistance, highmelting point, large strength, high hardness and wear resistance. Superhard materialshave numerous industrial applications, such as abrasives, cutting tools, polishing tools,protective coatings, and so on. Therefore, They play very important roles in modernadvanced science fields such as mechanics, metallurgies, electronics and aerospace etal. Of all known superhard materials, diamond with the Vickers hardness of96GPa isthe hardest material in the world. However, diamond has itself weakness. Diamond isnot very stable under high temperature and is easy to react with iron, which havelimited its applications in many fields. Although cubic boron nitride, whose hardnessis ranked second, can make up the shortage of diamond, the hardness of cubic boronnitride is much lower than that of diamond. Therefore, searching for other outstandingsuperhard materials is considered to be quite necessary and challenging.The first-principles calculations were performed to study superhard materials inthis paper. Boron carbide (BxCy) compounds and rhenium carbonitride ternary alloysare the major frames of research work. Based on theoretical models and calculatedresults, by analysing the formation enthalpies, elastic constants and moduli, bandstructures, density of states, stress-strain relationships, Mulliken bond populations,Vickers hardness and charge density differences et al, we studied the stabilities,electrical and mechanical properties of materials, and several novel superhardmaterials are predicted as well. This paper’s major research contents were as follows:1. BxCycompounds are potential superhard materials. We constructed structuralmodels and studied the structural stabilities, electrical and mechanical properties of the BxCycompounds based on three kinds of phases including diamond-like, C20-likeand B15-like phases. In low boron concentration region, diamond-like BxCycompounds are more stable and potential conductive superhard materials. TheC20-like structure B8C12, whose Vickers hardness is58.6GPa, is found to be a newstable potential conductive superhard structure with relatively low formation enthalpyin middle boron concentration and is expected to be synthesized experimentally. Inhigh boron concentration region, B15-like BxCycompounds are more stable and B14Cis predicted to be a new superhard material. Combining a microscopic model withstructural and electrical parameters from the first-principles calculations, the Vickershardness of the different configurations of BxCycompounds is analyzed with thevariation of boron concentration. It is found that the hardness of the B–C system has adecreasing trend with the increase of boron concentration. In addition, all thestructures have metallic properties, except B12C3and B14C. With the analysis ofdensity of states, Mulliken bond populations and charge density differences, thebonds with high electron density，short bond length and highly bond directionalcharacteristics have an important contribution to the hardness in the B–C system,while the effect of metallicity to hardness can be ignored.2. In the system containing rhenium, we found that the hardness of rheniumdinitride (ReN2) is not as much high as expected result, rhenium dicarbide (ReC2) isdifficult to be synthesized experimentally. However, the new predicted rheniumcarbonitride (ReCN) is found to be possibly synthesized easily and possess very highVickers hardness. Based on the first-principles calculations, two hexagonal P63mc (HI)and trigonal P3m1(HII) phases of ReCN without any C-C or N-N bonds existing inare thermodynamically and mechanically stable. The two phases of ReCN withlayered arrangement have very close formation enthalpies and mechanical propertiesbecause of their similar structures. By comparing the elastic constants and moduli, volume compressible ability under pressure, hardness, ideal tensile and shearstrengths of ReCN with the same structures of ReN2and ReC2, it is found that HI andHII ReCN phases may have the largest bulk，shear and young’s moduli，the highestVickers hardness and the most excellent ideal tensile and shear strengths. Besides,ReCN also are the most ultra-incompressible compounds. These results suggest thatReCN may have outstanding mechanical properties and are potentialultra-incompressible materials. Moreover, by analyzing density of states and bandstructures, it suggests that HI and HII of ReCN belong to indirect and directsemiconductors with the energy band gaps of0.67eV and0.73eV, respectively. Withthe Vickers hardness exceeding40GPa, HI and HII ReCN phases compounds areconsidered to be potential novel functional ultra-incompressible superhard materialand may be extremely useful in practical applications.