First-principles Investigation On The Mechanical Properties Of Superhard Materals In The BC_XN(X=1,4,6) System

In this dissertation,the electronic structure,mechanical properties,dynamics stability,thermodynamic stability,ideal strength and theoretical hardness of novel superhard BC_XN?X=1,4,6?compouds with low enthalpy were systematically investigated by means of first-principles calculations.It will be useful to designing and synthesising new materials or understanding the properties of materials.The synthesis conditions of stable structure with unique and excellent mechanical properties were clarified,and the structure-property relationship of boron-carbon-nitrogen multi-component complex structure and performance changes affected by different carbon content and chemical bond arrangement was explored.Several novel low-enthalpy superhard phases,such as t-BCN,o-BC₄N and Cm-BC₆N,were predicted on the basis of particle swarm optimization?PSO?algorithm and first-principles calculations.The structure,electronic and mechanical properties of these new superhard phases were studied by using the first-principles calculation software based on DFT.The atoms in the above structure exhibit near tetrahedral sp³ hybrid electronic state and form covalent bonds with the surrounding atoms.The calculation results of elastic constants show that the new B-C-N structures are all mechanically stable phases.It was revealed that t-BCN is more energetically favorable than graphite-like g-BCN above the pressure of 22.2 GPa.The ideal strength of t-BCN is 48.6 GPa under shear loading,which is higher than that of?-BC₂N.In the[001]direction of t-BCN,the N-C,C-B and B-N covalent bonds show obvious interlaced and sequential structural characteristics,which helps to redistribute charges uniformly under large strain,to reduce differences of the kinds of bond strengths,and to defend the tensile loading-resulting in a high peak stress reaches 146.4 Pa.The calculated hardness of t-BCN is 61.8 GPa,which is close to the hardness of cubic boron nitride.The calculations show that the o-BC₄N is more thermodynamically stable than the layered g-BC₄N when the pressure is above 25GPa.The ideal strength of o-BC₄N is the peak stress 58.5 GPa in?100?[001]shear direction,and mechanical failure mode of o-BC₄N is dominated by the shear type.The hardness of o-BC₄N reached 78.7 GPa,which was significantly higher than that of?-BC₂N.The results show that the lowest peak stress and theoretical hardness of Cm-BC₆N are significantly higher than those of?-BC₂N.The ideal strength of Cm-BC₆N is 53.9 GPa and its hardness is as high as 80.6 GPa.For the high strength and high hardness of these structures,the origin of these structures is explored by calculating the electronic structure and charge density.The results show that several superhard phases in BC_XN?X=1,4,6?possess excellent mechanical properties.In these structures,Cm-BC₆N was inferior only to diamond on hardness.The investigation above can provide the fundamental knowledge to predict novel BC_XN?X=1,4,6?ternary superhard materials with potential applications and to guide the experimental synthesis.