Superhard crb2c2 polymorphic material and preparation method thereof

Abstract

The application belongs to the technical field of superhard materials, and particularly relates to a series of CrB2C2 polymorphic materials with superhard properties. Through crystal structure prediction and first principle calculation, a series of novel polymorphic compounds with a stoichiometric ratio of CrB2C2 are disclosed. The theoretical Vickers hardness of multiple phases exceeds 40 GPa, meeting the superhard material standard. In particular, phases and phases are dynamically stable at normal pressure and can be obtained by pressure release after high-pressure synthesis. The hardening mechanism of phases and phases at the atomic scale is revealed, and a "double bond synergistic enhancement mechanism" is proposed, providing a new idea for the rational design of superhard materials. The superhard material has good mechanical stability, dynamic stability and excellent hardness, and can be widely used in cutting tools, wear-resistant coatings, super-high-temperature ceramics and other fields.

Description

Technical Field

[0001] This invention belongs to the field of superhard materials technology, specifically relating to a series of polycrystalline chromium boron carbon compound CrB2C2 materials with superhard properties, as well as their crystal structure, mechanical properties and preparation methods. Background Technology

[0002] Transition metal borides are widely used as key materials in cutting tools, wear-resistant coatings, and ultra-high temperature ceramics due to their excellent mechanical strength, high hardness, and extremely high melting point. Among them, chromium borides have been extensively studied, but the experimental and theoretical Vickers hardness values ​​of binary Cr-B compounds (such as CrB, CrB2, CrB4, etc.) are usually between 15-30 GPa, and have not yet broken through the 40 GPa threshold for superhard materials, which greatly limits their application in harsh working conditions.

[0003] To obtain a superior three-dimensional covalent network structure, carbon, capable of forming extremely strong covalent bonds, has been introduced into the Cr-B system to form ternary chromium-boron-carbon (Cr-BC) compounds, which hold promise for achieving breakthroughs in hardness. However, systematic research on the new phases of the ternary Cr-BC system and its atomic-scale hardening mechanism is currently limited, especially regarding the superhard polycrystalline CrB₂C₂ material, which remains a technological gap. Therefore, developing novel superhard Cr-BC compounds and elucidating their intrinsic hardening mechanisms is of great significance for advancing the field of superhard materials. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a novel type of superhard CrB2C2 polycrystalline material, which possesses high hardness, good kinetic stability, and potential feasibility for high-pressure synthesis. Furthermore, this invention reveals its atomic-scale hardening mechanism, providing theoretical guidance for the rational design of superhard materials.

[0005] To achieve the above objectives, this invention employs a combination of crystal structure prediction and first-principles calculations to systematically study the CrB2C2 system, discovering several new phases that are kinetically stable and possess superhard properties. Among them, nine crystal forms exhibit a Vickers hardness H... V Exceeding 40 GPa, it meets the standards for superhard materials. In particular, two thermodynamically stable phases exhibit superhard properties: Harmony The phases were analyzed, and their strengthening mechanism was revealed. These materials are kinetically stable at atmospheric pressure and can be obtained by high-pressure synthesis followed by quenching to atmospheric pressure.

[0006] On the one hand, an ultrahard chromium boron carbon compound with the chemical formula CrB2C2 is disclosed, exhibiting three thermodynamically stable phases in different pressure ranges: the crystal structure S1 exists below 25 GPa. Phase (H) V= 32.3 GPa) Stable; Crystal structure S2 is stable in the pressure range of 25-76 GPa. Phase (H) V Stable at 41.0 GPa; the S3 crystal structure is stable above 76 GPa. (H) V = 48.3 GPa) phase stable. Its crystal structures are as follows:

[0007] Crystal structure S1: Space group is , recorded as The reference phase has the following unit cell parameters: a = b = 2.8456 Å, c = 9.2959 Å, α = β = γ = 90°; the atomic positions are: Cr atom occupies the 2a site with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 4e site with coordinates (0.00000, 0.00000, 0.40279); C atom occupies the 4f site with coordinates (0.00000, 0.50000, 0.17004).

[0008] Crystal structure S2: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6962 Å, c = 11.4458 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.18062); B2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.85492); C1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.32193); C2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.11982).

[0009] Crystal structure S3: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6685 Å, c = 11.5742 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.34335); B1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.52915); B2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.16451); C1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.77728); C2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.02095).

[0010] CrB2C2 crystal structure The Vickers hardness is 32.3 GPa; The Vickers hardness is 41.0 GPa; The Vickers hardness of the three phases is 48.3 GPa. The elastic constants of these three phases satisfy the Born-Huang mechanical stability criterion, indicating mechanical stability. Furthermore, phonon spectrum calculations show no imaginary frequencies, demonstrating both mechanical and dynamic stability.

[0011] By analyzing the change in enthalpy of formation with pressure, it was determined that... The phase is thermodynamically stable within a pressure range below 25.1 GPa. The phase is thermodynamically stable in the pressure range of 25.1–76.2 GPa. The phases are thermodynamically stable within a pressure range greater than 76.2 GPa, and both exhibit no imaginary frequency phonons at ambient pressure, indicating atmospheric pressure kinetic stability. The crystal structures S2 and S3 of the superhard CrB2C2 material were prepared by high-pressure synthesis under the following pressure conditions: synthesis at 25–76 GPa. Phase, synthesized under conditions above 76 GPa. After preparation, the pressure is released to atmospheric pressure, and the material still maintains kinetic stability, demonstrating its potential for high-pressure synthesis.

[0012] Through analysis Harmony The stress-strain relationship of the phase reveals the key microscopic mechanism that determines its macroscopic hardness:

[0013] The two aforementioned ultrahard structures share common structural features, consisting of stacked double-folded honeycomb layers, with the BC layer serving as the primary load-bearing framework. Their Vickers shear deformation mechanism is controlled by the breaking or synergistic reinforcement of the BC covalent bonds. Through stress-strain response under Vickers shear deformation, combined with electronic localization function analysis, the atomic-scale origin of their ultrahardness is revealed: The ideal strength of the phase is controlled by the fracture of a single key BC bond, while The phase achieves higher hardness through the synergistic enhancement mechanism of two BC bonds within the layer.

[0014] On the other hand, besides the two superhard structures mentioned above, the other seven CrB2C2 superhard chromium boron carbide compounds possess thermodynamic metastable phases with Vickers hardness exceeding 40 GPa. They exhibit mechanical and kinetic stability within the 0–100 GPa range and have the potential to be obtained through non-equilibrium synthesis methods. Their crystal structures are one of the following:

[0015] Crystal structure S4: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6859 Å, c = 11.5905 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.18010); B2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.35519); C1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.32253); C2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.12031).

[0016] Crystal structure S5: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6673 Å, c = 5.9163 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 1a site with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 2c site with coordinates (0.00000, 0.00000, 0.34917); C atom occupies the 2d site with coordinates (0.33333, 0.66667, 0.25258).

[0017] Crystal structure S6: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6828 Å, c = 5.7806 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.36079); B2 atom occupies position 1b, with coordinates (0.33333, 0.66667, 0.71831); C1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.64569); C2 atom occupies position 1b, with coordinates (0.33333, 0.66667, 0.24031).

[0018] Crystal structure S7: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6900 Å, c = 5.7684 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.36073); B2 atom occupies position 1c, with coordinates (0.66667, 0.33333, 0.70340); C1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.64397); C2 atom occupies position 1b, with coordinates (0.33333, 0.66667, 0.24154).

[0019] Crystal structure S8: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6697 Å, c = 11.7580 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2b site, with coordinates (0.00000, 0.00000, 0.25000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.07463); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12665).

[0020] Crystal structure S9: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6622 Å, c = 11.8744 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2a site, with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.17449); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12474).

[0021] Crystal structure S10: space group is Its unit cell parameters are: a = b = 2.6626 Å, c = 5.9008 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies 1a site, with coordinates (0.00000, 0.00000, 0.00000); B atom occupies 2g site, with coordinates (0.00000, 0.00000, 0.35064); C atom occupies 2i site, with coordinates (0.66667, 0.33333, 0.24386).

[0022] The ten crystal structure design methods mentioned above are:

[0023] Step 1: Using a particle swarm optimization algorithm and the CALYPSO software package, an unbiased crystal structure search was performed on the CrB2C2 system within a pressure range of 0–100 GPa. First-principles calculations were performed based on density functional theory using the VASP software package, employing the projected fused wave method and the Perdew-Burke-Ernzerhof generalized gradient approximation to handle the exchange correlation energy. (The last sentence appears to be incomplete and requires further context.) 5 4s1, 2s 2 2p 1 and 2s 2 2p 2 The valence electrons of Cr, B, and C are used; in each prediction, the population size in one generation is set to 40, and each search is performed for 50 generations; the proportion of structures generated by discrete particle swarm optimization is 60%, and the structure search produces several crystal structures, among which crystal structures S1, S2, S3, S4, S5, S6, S7, S8, S9, and S10 are the target structures.

[0024] Step 2: Perform structural relaxation on the target structure from Step 1, setting the plane wave cutoff energy to 700 eV and the k-point grid spacing to 2π × 0.025 Å. -1 The total energy converges to within 1 meV / atom; the force converges to 1×10⁻⁶. -3 eV / Å, residual stress less than 0.1 GPa; atomic positions and unit cell parameters were obtained after the structure was fully relaxed.

[0025] The beneficial effects of this invention are as follows: It provides a novel type of superhard CrB2C2 polycrystalline material, which possesses high hardness, good kinetic stability, and potential feasibility for high-pressure synthesis. The main advantages are as follows:

[0026] (1) Filling the gap: For the first time, a series of novel ternary CrB2C2 superhard polymorphic materials were systematically predicted and disclosed, adding new members to the family of superhard materials.

[0027] (2) Excellent performance: The theoretical Vickers hardness of the disclosed phases all exceed the superhard threshold of 40 GPa, and the performance is significantly better than that of known binary chromium borides.

[0028] (3) High syntheticability: the key and Although the phase requires high pressure for synthesis, theoretical calculations show that it is kinetically stable at atmospheric pressure, possessing great potential for experimental synthesis and preservation up to atmospheric pressure; and the synthesis pressure range of thermodynamically stable ultrahard phases has been clearly defined, providing clear guidance for experimental synthesis.

[0029] (4) Revealing the microscopic mechanism: The “double bond synergistic enhancement mechanism” is revealed to be the key to improving the hardness of layered covalent materials, providing a new atomic-scale perspective and operable structural model for the design of superhard materials, and opening up a new direction for the study of transition metal boron carbide systems. Attached Figure Description

[0030] Figure 1 A schematic diagram comparing ten crystal structures of CrB2C2 polymorphic materials.

[0031] Figure 2 The graph shows the enthalpy of formation of CrB2C2 with pressure for different crystal structures.

[0032] Figure 3 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 25 GPa.

[0033] Figure 4 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0034] Figure 5 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0035] Figure 6 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0036] Figure 7 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0037] Figure 8 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0038] Figure 9 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0039] Figure 10 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0040] Figure 11 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0041] Figure 12 for Phonon dispersion relation of the CrB2C2 compound at 0 GPa and 100 GPa.

[0042] Figure 13 for Harmony Comparison of ideal strength of phases under Vickers shear deformation in different crystal orientations.

[0043] Figure 14 for Following Stress-strain curves in the direction of stress.

[0044] Figure 15 for Structural snapshots and isosurface plots of electronic localization functions under key strain.

[0045] Figure 16 for Following Stress-strain curves in the direction of stress.

[0046] Figure 17 for Structural snapshots and isosurface plots of electronic localization functions under key strain. Detailed Implementation

[0047] Example 1

[0048] Crystal structure design

[0049] Step 1: Using a particle swarm optimization algorithm and the CALYPSO software package, an unbiased crystal structure search was performed on the CrB2C2 system within a pressure range of 0–100 GPa. First-principles calculations were performed based on density functional theory using the VASP software package, employing the projected fused wave method and the Perdew-Burke-Ernzerhof generalized gradient approximation to handle the exchange correlation energy. (The last sentence appears to be incomplete and requires further context.) 5 4s1, 2s 2 2p 1 and 2s 2 2p 2 The valence electrons of Cr, B, and C are used; in each prediction, the population size in one generation is set to 40, and each search is performed for 50 generations; the proportion of structures generated by discrete particle swarm optimization is 60%, and the structure search produces several crystal structures, among which crystal structures S1, S2, S3, S4, S5, S6, S7, S8, S9, and S10 are the target structures.

[0050] Step 2: Perform structural relaxation on the target structure from Step 1, setting the plane wave cutoff energy to 700 eV and the k-point grid spacing to 2π × 0.025 Å. -1 The total energy converges to within 1 meV / atom; the force converges to 1×10⁻⁶. -3 eV / Å, residual stress less than 0.1 GPa; atomic positions and unit cell parameters were obtained after the structure was fully relaxed.

[0051] The crystal structures screened are as follows Figure 1 As shown, where The phase is the reference phase.

[0052] Example 2

[0053] The crystal structure S1 was obtained using the design method of Example 1.

[0054] Crystal structure S1: Space group is , recorded as The reference phase has the following unit cell parameters: a = b = 2.8456 Å, c = 9.2959 Å, α = β = γ = 90°; the atomic positions are: Cr atom occupies the 2a site with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 4e site with coordinates (0.00000, 0.00000, 0.40279); C atom occupies the 4f site with coordinates (0.00000, 0.50000, 0.17004). The phase compound was synthesized under pressure conditions ranging from 0 GPa to 25 GPa. After synthesis, the pressure was released to atmospheric pressure to obtain a CrB2C2 compound with a Vickers hardness of 32.3 GPa and stable kinetics.

[0055] Example 3

[0056] The crystal structure S2 was obtained using the design method of Example 1.

[0057] Crystal structure S2: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6962 Å, c = 11.4458 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.18062); B2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.85492); C1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.32193); C2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.11982). The phase compound was synthesized under pressure conditions of 25 GPa-76 GPa, and then depressurized to atmospheric pressure to obtain a superhard CrB2C2 compound with a Vickers hardness of 41.0 GPa and stable kinetics.

[0058] Example 4

[0059] The crystal structure S3 was obtained using the design method of Example 1.

[0060] Crystal structure S3: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6685 Å, c = 11.5742 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.34335); B1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.52915); B2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.16451); C1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.77728); C2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.02095). The phase compound was synthesized under a pressure of not less than 76 GPa, and then depressurized to atmospheric pressure to obtain a superhard CrB2C2 compound with a Vickers hardness of 48.3 GPa and stable kinetics.

[0061] Example 5

[0062] The crystal structure S4 was obtained using the design method of Example 1.

[0063] Crystal structure S4: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6859 Å, c = 11.5905 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.18010); B2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.35519); C1 atom occupies position 2a, with coordinates (0.00000, 0.00000, 0.32253); C2 atom occupies position 2b, with coordinates (0.33333, 0.66667, 0.12031).

[0064] Example 6

[0065] Crystal structure S5 was obtained using the design method of Example 1.

[0066] Crystal structure S5: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6673 Å, c = 5.9163 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 1a site with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 2c site with coordinates (0.00000, 0.00000, 0.34917); C atom occupies the 2d site with coordinates (0.33333, 0.66667, 0.25258).

[0067] Example 7

[0068] The crystal structure S6 was obtained using the design method of Example 1.

[0069] Crystal structure S6: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6828 Å, c = 5.7806 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.36079); B2 atom occupies position 1b, with coordinates (0.33333, 0.66667, 0.71831).

[0070] Example 8

[0071] The crystal structure S7 was obtained using the design method of Example 1.

[0072] Crystal structure S7: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6900 Å, c = 5.7684 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.00000); B1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.36073); B2 atom occupies position 1c, with coordinates (0.66667, 0.33333, 0.70340); C1 atom occupies position 1a, with coordinates (0.00000, 0.00000, 0.64397); C2 atom occupies position 1b, with coordinates (0.33333, 0.66667, 0.24154).

[0073] Example 9

[0074] The crystal structure S8 was obtained using the design method of Example 1.

[0075] Crystal structure S8: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6697 Å, c = 11.7580 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2b site, with coordinates (0.00000, 0.00000, 0.25000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.07463); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12665).

[0076] Example 10

[0077] The crystal structure S9 was obtained using the design method of Example 1.

[0078] Crystal structure S9: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6622 Å, c = 11.8744 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2a site, with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.17449); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12474).

[0079] Example 11

[0080] The crystal structure S10 was obtained using the design method of Example 1.

[0081] Crystal structure S10: space group is Its unit cell parameters are: a = b = 2.6626 Å, c = 5.9008 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies 1a site, with coordinates (0.00000, 0.00000, 0.00000); B atom occupies 2g site, with coordinates (0.00000, 0.00000, 0.35064); C atom occupies 2i site, with coordinates (0.66667, 0.33333, 0.24386).

[0082] Example 12

[0083] Crystal structure stability analysis

[0084] (1) Mechanical stability: The screened crystal structures belong to the hexagonal, tetragonal, and rhombohedral crystal systems. For all predicted structures, the elastic constants satisfy the Born-Huang mechanical stability criterion. For hexagonal and tetragonal crystals: , , , For rhombohedral crystals: , , , All structures possess mechanical stability.

[0085] (2) Thermodynamic stability and pressure phase transition: The thermodynamic stability of each phase was determined by calculating the enthalpy of formation. The reference states of the elements were: body-centered cubic Cr, α-B (<19.7 GPa) or γ-B (>19.7 GPa), and diamond structure C. The enthalpy of formation was defined as: ,like Figure 2 As shown, the pressure ranges for the three thermodynamically stable phases are illustrated. Calculations indicate that below 25 GPa, The phase is thermodynamically stable; between 25–76 GPa. The phase is a stable phase; above 76 GPa, The first phase is a stable phase. The other superhard phases are all thermodynamically metastable phases. That is, the three materials in Examples 2, 3, and 4 are thermodynamically stable phases, while the other materials are thermodynamically metastable phases.

[0086] (3) Dynamic Stability: The dynamic stability of the structure was verified by calculating the phonon dispersion relation using phonopy code. The absence of imaginary frequencies in the phonon dispersion relation indicates dynamic stability. Phonon spectra for each phase were calculated under representative pressures: For The phonon spectra at pressures of 0 and 25 GPa were calculated; for The phase was calculated, and the phonon spectra at pressures of 0 and 76 GPa were calculated; for Phonon spectra of the three thermodynamically stable phases were calculated at pressures of 0 and 100 GPa. For the thermodynamically metastable phases, their phonon dispersion relations at 0 and 100 GPa were evaluated. No imaginary frequencies were observed across the entire Brillouin zone in all cases, confirming the kinetic stability of each phase studied. The phonon spectra of the three thermodynamically stable phases are shown below. Figure 3-5 As shown, the phonon spectra of other thermodynamically metastable phases are as follows: Figure 6-12 As shown in Figure 3-12 (the left figure is the dispersion relation curve at 0 GPa, and the right figure is the dispersion relation curve under high pressure). and After the two phases are synthesized under high pressure, they remain kinetically stable even after being depressurized to atmospheric pressure (see...). Figure 4 and Figure 5 This demonstrates the feasibility of experimental synthesis. Other ultrahard thermodynamically metastable phases possess kinetic stability and have the potential to be prepared via non-equilibrium synthesis methods.

[0087] Example 13

[0088] Mechanical property calculation

[0089] The elastic constants C of each structure were obtained through first-principles calculations. ij The bulk modulus B, shear modulus G, Young's modulus E, and Poisson's ratio ν were calculated from the elastic constants using the Voigt-Reuss-Hill averaging method. The Vickers hardness H was estimated using the Chen model.V = 2(k 2 G) 0.585 - 3, where k = G / B. The bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν, and hardness values ​​of each structure are listed in Table 1. Among them, the hardness of nine polymorphs of CrB2C2 exceeds 40 GPa.

[0090] Table 1 shows the elastic constant Cij (GPa), bulk modulus B (GPa), shear modulus G (GPa), Young's modulus E (GPa), Poisson's ratio ν, and Vickers hardness H of the polymorphic CrB2C2 compound. V The (GPa) data table shows that all phases satisfy the Born-Huang stability criterion.

[0091] Table 1

[0092] Example 14

[0093] Atomic-scale hardening mechanism analysis—double bond synergistic enhancement mechanism

[0094] Simulation using first principles The Vickers shearing process along different crystal orientations yields stress-strain curves for each direction. The weakest direction is... Its peak stress (ideal strength) is 24.6 GPa, such as Figure 13 As shown. By tracking the bond length changes and electronic localization function (ELF) during deformation, it was observed that before reaching the critical stress, the B1-C1 and B2-C2 bonds were simultaneously elongated, the charge density decreased simultaneously, and they eventually fractured almost simultaneously near the stress peak, triggering structural instability, as shown. Figure 14 and Figure 15 As shown, the double bond synergistic enhancement mechanism is demonstrated, and this synergistic effect is the fundamental source of its high hardness.

[0095] Example 15

[0096] Comparison— Phase single-key control mode

[0097] simulation The Vickers shearing process along different crystal orientations yields stress-strain curves for each direction. The weakest direction is also... Its peak stress is 23.8 GPa (e.g. Figure 13 (As shown). During deformation, failure is controlled by only one critical B1-C1 bond, such as... Figure 16 and Figure 17As shown, this illustrates the fracture mechanism of a single BC bond. With increasing strain, the B1-C1 bond continues to stretch until fracture, while the deformation of other bonds within the layer is relatively small. This "single-point failure" mode limits its overall strength; its ideal shear strength and Vickers hardness are both lower than [previous values]. Mutually.

Claims

1. A superhard CrB2C2 polycrystalline material, characterized in that, The CrB2C2 compound has a crystal structure S2 or a crystal structure S3 that is a thermodynamically stable phase. Crystal structure S2: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6962 Å, c = 11.4458 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 2a site, with coordinates (0.00000, 0.00000, 0.00000); Atom B1 occupies position 2a, with coordinates (0.00000, 0.00000, 0.18062); Atom B2 occupies position 2b, with coordinates (0.33333, 0.66667, 0.85492); atom C1 occupies position 2a, with coordinates (0.00000, 0.00000, 0.32193); atom C2 occupies position 2b, with coordinates (0.33333, 0.66667, 0.11982). Crystal structure S3: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6685 Å, c = 11.5742 Å, α = β = 90°, γ = 120°; the atomic positions are as follows: Cr atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.34335); B1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.52915); B2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.16451); C1 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.77728); C2 atoms occupy 2d sites with coordinates (0.33333, 0.66667, 0.02095).

2. The superhard CrB2C2 polycrystalline material according to claim 1, characterized in that, The crystal structure S2 is The phase compound was synthesized under pressure conditions of 25 GPa-76 GPa, and then depressurized to atmospheric pressure to obtain a kinetically stable superhard CrB2C2 compound with a Vickers hardness of 41 GPa.

3. The superhard CrB2C2 polycrystalline material according to claim 1, characterized in that, The crystal structure S3 is The phase compound was synthesized under a pressure of not less than 76 GPa, and then depressurized to atmospheric pressure to obtain a superhard CrB2C2 compound with a Vickers hardness of 48.3 GPa and stable kinetics.

4. A superhard CrB2C2 polycrystalline material, characterized in that, The CrB2C2 compound is any one of the following crystal structures having a thermodynamic metastable phase: S4, S5, S6, S7, S8, S9, or S10. Crystal structure S4: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6859 Å, c = 11.5905 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 2a site with coordinates (0.00000, 0.00000, 0.00000); Atom B1 occupies position 2a, with coordinates (0.00000, 0.00000, 0.18010); atom B2 occupies position 2b, with coordinates (0.33333, 0.66667, 0.35519); atom C1 occupies position 2a, with coordinates (0.00000, 0.00000, 0.32253); atom C2 occupies position 2b, with coordinates (0.33333, 0.66667, 0.12031). Crystal structure S5: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6673 Å, c = 5.9163 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 1a site with coordinates (0.00000, 0.00000, 0.00000); Atom B occupies position 2c, with coordinates (0.00000, 0.00000, 0.34917); atom C occupies position 2d, with coordinates (0.33333, 0.66667, 0.25258). Crystal structure S6: Space group is , recorded as The phase has the following cell parameters: a = b = 2.6828 Å, c = 5.7806 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 1a site with coordinates (0.00000, 0.00000, 0.00000); Atom B1 occupies position 1a, with coordinates (0.00000, 0.00000, 0.36079); atom B2 occupies position 1b, with coordinates (0.33333, 0.66667, 0.71831); atom C1 occupies position 1a, with coordinates (0.00000, 0.00000, 0.64569); atom C2 occupies position 1b, with coordinates (0.33333, 0.66667, 0.24031). Crystal structure S7: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6900 Å, c = 5.7684 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 1a site with coordinates (0.00000, 0.00000, 0.00000); Atom B1 occupies site 1a, with coordinates (0.00000, 0.00000, 0.36073); atom B2 occupies site 1c, with coordinates (0.66667, 0.33333, 0.70340); atom C1 occupies site 1a, with coordinates (0.00000, 0.00000, 0.64397); atom C2 occupies site 1b, with coordinates (0.33333, 0.66667, 0.24154). Crystal structure S8: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6697 Å, c = 11.7580 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2b site, with coordinates (0.00000, 0.00000, 0.25000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.07463); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12665). Crystal structure S9: Space group is , recorded as The phase has the following unit cell parameters: a = b = 2.6622 Å, c = 11.8744 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atom occupies the 2a site, with coordinates (0.00000, 0.00000, 0.00000); B atom occupies the 4e site, with coordinates (0.00000, 0.00000, 0.17449); C atom occupies the 4f site, with coordinates (0.33333, 0.66667, 0.12474). Crystal structure S10: space group is Its unit cell parameters are: a = b = 2.6626 Å, c = 5.9008 Å, α = β = 90°, γ = 120°; the atomic positions are: Cr atoms occupy the 1a site, with coordinates (0.00000, 0.00000, 0.00000); The B atom occupies the 2g site, with coordinates (0.00000, 0.00000, 0.35064); The C atom occupies the 2i site, with coordinates (0.66667, 0.33333, 0.24386).

5. The superhard CrB2C2 polycrystalline material according to claim 4, characterized in that, The Vickers hardness of the crystal structures S4, S5, S6, S7, S8, S9, or S10 is greater than 40 GPa.

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