Microscopic Design Of New Superhard Material

Superhard materials with comprehensive properties of high hardness,high melting point,corrosion resistance and chemical inertness,have been widely used in many mechanical areas as surface coating,drilling bits and cutting tools.It is the diamond could react with oxygen,iron,etc at high temperature that limits its extensive application.Looking for a more stable material,which is out of the limitation of existing superhard materials,has become an inevitable trend.At present,the mainstream approach is adding light element like boron,oxygen,carbon,nitrogen,etc into transition metal to form a short and strong covalent bond,which is like the bond in diamond.We use the first principles calculation under the framework of the density functional theory,with transition metal boride(WB_x,Mo B_x and Mn B₄)as the object,to study their mechanical behavior,and illustrate the micro mechanism,provide powerful basis for design of new superhard materials.（1）According to the structure characteristics of W-B and Mo-B system,we put forward the TMB₃ polytypes structure,which is composed of the same AH unit according to the different stacking sequence.We’ve listed 15 kinds of structures,and shown the detailed atomic coordinates,space group and stacking sequence.According to first principles calculation,we have calculated the formation of the 15 kinds of structures,our results show that the TMB₃ polytypes can be considered as a combination of 2H and 3R structure,and the TMB₃ polytypes is stable.Finally,we calculated the bulk modulus,shear modulus and theory hardness of the 15 kinds of structures,our results show that the mechanical properties are almost not affected by the number of metal atoms in the crystal.In addition,TMB₃ polytypes showed extremely low lattice thermal conductivity due to the structural disorderly and phonon folding.Thus,we can further put forward it can improve the external hardness by synthesis the solid solution of coexisting multiphase to achieve the goal of designing new superhard materials.（2）We have carried out a systematic investigation of the structural,energetic,and dynamical properties of the W-B and Mo-B systems using first principles methods.Our results show that the h P3-TMB₂ phases are energetically unfavorable and dynamically unstable,and their calculated structural parameters,mechanical properties and XRD deviate significantly from experiments.In contrast,the TMB₃ polytypes are energetically more favorable and dynamically stable,and their corresponding properties agree well with experiments.We thus conclude that the long perceived h P3-TMB₂phases are actually a family of more complex TMB₃ polytypes with a nanoscale ordering along the axial direction.More importantly,we demonstrate that such a structural and compositional modification is a consequence of relieving the strong antibonding interaction in the ideal h P3 structure.Therefore,the present work not only resolves several longstanding puzzles regarding structural and mechanical properties of the W-B and Mo-B systems,but also corroborates the existence of a family of polytypes.Our findings provide a new avenue for designing superhard nanocomposite materials with novel functionalities.（3）We have performed first principles calculations to systematically study the phase stabilities,mechanical properties and electronic structures of the three structures（i.e.m S10,o P10,and m P20）with the different magnetic states（i.e.NM,FM,and AFM）of Mn B₄.The m S10 structure is ruled out as a possible phase for Mn B₄,and the m P20and o P10 structures are viable according to the calculation.More importantly,our consequence show that Mn B₄ undergoes the concurring magnetostructural and electronic transitions from the low-symmetric NM m P20 insulator to the high-symmetric FM（AFM）o P10 metal at 438 K（824 K）.The Peierls distortion stabilizes the NM m P20 phase by breaking the structural degeneracy,while the Stoner magnetism stabilizes the FM（AFM）o P10 phase by lifting the spin degeneracy.Therefore,the unique insulator-metal transition,strong stiffness and high hardness may make Mn B₄ find promising technological applications as thermoelectric switches and field effect transistors at the extreme conditions.