Carbon nanotube-nano polycrystalline diamond composite material and preparation method thereof

Abstract

The invention discloses a carbon nanotube-nano polycrystalline diamond composite material and a preparation method thereof. The raw materials comprise carbon nanotubes and carbon nano scallions, the OLC is prepared from detonation method nano-diamond with the average particle size of 5 nm through vacuum annealing at the temperature of 800-1800 DEG C, and the OLC contains a diamond crystal core, a diamond structure core and a complete structure; and the CNT is of a graphite curled tubular structure with the tube diameter of 4-8nm and the length of 0.5-2mu m. The preparation method comprises the following steps: ultrasonically dispersing and drying CNT with ethanol, mixing with OLC, pre-pressing for 30-60 seconds under 400-600 MPa, filling a mold according to a specific mold assembly process, preserving heat for 5-60 minutes under the conditions of 7-25 GPa and 1800-2200 DEG C, sintering at high temperature and high pressure, and grinding and deburring a blank to obtain a finished product. The closed spherical structure formed by CNT fracture is used for balancing the internal pressure loss of the sintered body, the OLC sintering condition is remarkably reduced, the Vickers hardness of the product reaches 17-202 GPa, and the method has high hardness and preparation feasibility and can be widely applied to the fields of aerospace, gem processing, petroleum drilling and the like.

Description

Technical Field

[0001] This invention belongs to the field of superhard composite materials technology, specifically, it relates to a carbon nanotube-nanopolycrystalline diamond composite material and its preparation method. Background Technology

[0002] Polycrystalline diamond (PCD), as a high-performance superhard material, overcomes the defects of anisotropy and easy dissociation and damage of the {111} crystal plane in single-crystal diamond due to its extremely high hardness, excellent wear resistance and stability, and has been widely used in many high-end fields.

[0003] In existing technologies, researchers have conducted various studies on the preparation of polycrystalline diamond based on carbon nanotubes (OLC). For example, Wang Mingzhi et al. used nano-onion-carbon as raw materials and synthesized polycrystalline diamond using a six-sided press under conditions of 2~6 GPa, 1000~1600℃, and holding time of 1~6 min. The Vickers hardness reached HV45~61 GPa, and the sintered body grain size was less than 20 nm, which to some extent overcame the performance defects caused by weak phases in existing processes. Subsequently, the team used carbon nanotubes to combine with micron-sized diamond and prepared polycrystalline diamond with a smooth surface and dense bulk under conditions of 4~6.5 GPa, 1000~1600℃, and holding time of 1~15 min, with a Vickers hardness of HV41~70 GPa. Tian Yongjun et al. synthesized nanotwinned diamond using carbon nanotubes with high-density defects at 18–25 GPa and 1850–2000 °C. The Vickers hardness reached 155–350 GPa, and the Knoop hardness was 140–240 GPa, significantly superior to traditional single-crystal diamond and superhard polycrystalline diamond. Tang Hu et al.'s research confirmed that the transformation of OLC to diamond is a martensitic phase transformation process. During the phase transformation, the sliding of the OLC(002) plane is restricted by the closed carbon shell, generating stress. The formation of twinned diamond is the result of this stress release.

[0004] However, existing technologies for synthesizing polycrystalline diamond using OLC as a precursor still have significant drawbacks: on the one hand, sintering requires extremely high pressure and temperature conditions (typically requiring pressure P≥18GPa and temperature T≥2300℃), which places stringent requirements on the pressure and heat resistance of production equipment, resulting in high production costs; on the other hand, due to the extreme sintering conditions, the maximum size of synthesized polycrystalline diamond is less than 3mm, which is difficult to meet the application needs of large-size superhard materials in industrial production, and seriously limits the large-scale promotion of this type of material.

[0005] Therefore, developing a novel polycrystalline diamond composite material and its preparation method that can reduce the sintering conditions of OLC while taking into account both high hardness performance and preparation feasibility has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] To address the problems of harsh sintering conditions (high pressure, high temperature), high production costs, and limited product size in the preparation of polycrystalline diamond using carbon nanotubes (OLC) as raw materials in existing technologies, this invention provides a carbon nanotube-nanopolycrystalline diamond composite material and its preparation method. By introducing carbon nanotubes (CNTs) into the composite with OLC, the structural characteristics of CNTs are used to balance the internal pressure loss of the sintered body. This reduces the sintering conditions while obtaining a composite material with high hardness and stable performance, thus broadening its industrial application scenarios.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A carbon nanotube-polycrystalline diamond composite material is prepared by high-temperature and high-pressure sintering of carbon nanotubes (OLC) and carbon nanotubes (CNTs). The raw material mass percentage is: carbon nanotubes account for 10~30 wt.%, and the balance is carbon nanotubes. The closed spherical structure formed after the carbon nanotubes break can balance the internal pressure loss of the sintered body, so that the sintering conditions are reduced to 7~25 GPa pressure and 1800~2200℃ temperature. The Vickers hardness of the composite material is 17~202 GPa.

[0008] Furthermore, the carbon nanotubes are tubular structures formed by rolling up layers of graphite carbon atoms, with a diameter of 4~8nm and a length of 0.5~2μm. Carbon nanotubes within this size range can be precisely broken during sintering to form closed spherical structures, achieving efficient balance of pressure loss.

[0009] Furthermore, the carbon nanoparticles have an average particle size of 5 nm and are prepared by vacuum annealing of nanodiamonds produced by detonation. They are composed of carbon and have a nano-onion structure. The carbon nanoparticles are selected from one of the following: carbon nanoparticles with diamond crystal cores, carbon nanoparticles with diamond structure cores, and carbon nanoparticles with complete structure. The three types of carbon nanoparticles correspond to different sintering pressure adaptation ranges.

[0010] Furthermore, when the sintering pressure is ≤14GPa, carbon nanotubes with diamond cores are selected, and the Vickers hardness of the composite material is 31~151GPa; when the sintering pressure is >14GPa, intact carbon nanotubes are selected, and the Vickers hardness of the composite material is 17~202GPa, and the hardness increase rate is ≥5GPa / GPa when the pressure is >14GPa; carbon nanotubes with diamond crystal cores are suitable for the entire pressure range, and the corresponding composite material has a Vickers hardness of 21~94GPa.

[0011] Furthermore, the carbon nanotubes containing diamond crystal cores are prepared by vacuum annealing at 800~1000℃, the carbon nanotubes containing diamond structural cores are prepared by vacuum annealing at 1050~1350℃, and the carbon nanotubes with complete structure are prepared by vacuum annealing at 1400~1800℃. The vacuum degree of the vacuum annealing is 1 Pa and the holding time is 0~2 h.

[0012] This invention also discloses a method for preparing a carbon nanotube-nanopolycrystalline diamond composite material, comprising the following synergistic steps: S1. Directional preparation of carbon nanotubes: Using detonation-processed nanodiamonds with an average grain size of 5 nm as raw material, under a vacuum of 1 Pa, annealing temperatures of 800–1800 °C and holding times of 0–2 h are selected according to the target sintering pressure range to directionally prepare carbon nanotubes containing diamond crystal cores, diamond structural cores, or complete structures. Among them, annealing temperatures of 800–1000 °C correspond to carbon nanotubes containing diamond crystal cores, 1050–1350 °C correspond to carbon nanotubes containing diamond structural cores, and 1400–1800 °C correspond to carbon nanotubes with complete structures.

[0013] S2. Highly efficient dispersion of carbon nanotubes: Carbon nanotube powder is placed in ethanol and dispersed without agglomeration by ultrasonic vibration for 3-10 minutes. Then it is dried at 60-120℃ for 30 minutes to ensure that the carbon nanotubes are evenly distributed in the mixture.

[0014] S3. Gradient molding and sintering: The carbon nanotubes dispersed in step S2 are mixed with the carbon nanotubes obtained in step S1 at a ratio of 10-30 wt.% of the total raw materials to obtain a mixture. The mixture is loaded into a cemented carbide mold and pre-pressed at 400-600 MPa for 30-60 s to obtain a pre-pressed sample. The pre-pressed sample is loaded into a graphite mold according to a specific mold assembly process, and a gradient sintering process is adopted, which involves "slowly increasing the pressure to 7-25 GPa for 10-24 h + heating to 1800-2200℃ at 10-30℃ / min + holding for 5-60 min". After cooling in the furnace, the surface is ground and deburred to obtain the target composite material.

[0015] Furthermore, the specific mold assembly process described in step S3 is as follows: the pre-pressed sample is placed into a BN crucible and the crucible is brought into close contact with the rhenium sheet. Then, the BN crucible is embedded into a LaCrO3 insulator. After the entire MgO octahedron is assembled, an Al2O3 plug is added. A four-hole tube with electrodes is placed on the plug and sealed with Al2O3 glue. This assembly structure can achieve temperature uniformity and pressure stability during the sintering process, and the pressure balancing effect of carbon nanotubes synergistically improves the density of the composite material.

[0016] Furthermore, in step S3, the mass percentage of carbon nanotubes is matched with the type of carbon nanotubes. When carbon nanotubes containing diamond crystal cores or diamond structural cores are used, the composite material has the best hardness when the carbon nanotube percentage is 20 wt.%. When carbon nanotubes with complete structure are used and the sintering pressure is >14 GPa, the carbon nanotube percentage can be increased to 30 wt.% as the pressure increases, so as to further enhance the pressure balance effect.

[0017] Compared with the prior art, the present invention can achieve the following technical effects: This invention utilizes the closed spherical structure formed when carbon nanotubes fracture to balance the internal pressure loss of the sintered body, reducing the sintering conditions of OLC and thus solving the problem of high sintering requirements for OLC. It provides a novel carbon nanotube-nanoscale polycrystalline diamond composite material and its preparation method. Furthermore, the nanodiamond prepared by the detonation method can generate three different OLC structures during annealing. All three OLC structures can be sintered with carbon nanotubes to form a carbon nanotube-nanoscale polycrystalline diamond composite material, and the resulting composite material exhibits higher hardness and greater selectivity.

[0018] Of course, any product implementing this invention does not necessarily need to achieve all of the technical effects described above at the same time. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a schematic diagram of the preparation method of the carbon nanotube-nanopolycrystalline diamond composite material of the present invention.

[0020] Figure 2 This is a schematic diagram of a specific mold assembly structure for this invention.

[0021] Figure labels: 1 - Four-hole tube, 2 - Electrode, 3 - Al2O3 plug, 4 - MgO octahedron, 5 - LaCrO3 insulator, 6 - BN crucible, 7 - Rhenium sheet, 8 - Pre-pressed sample, 9 - Al2O3 adhesive Detailed Implementation

[0022] The following will describe in detail the implementation of the present invention with reference to the accompanying drawings and embodiments, so that the process of how the present invention uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

[0023] This invention provides a carbon nanotube-polycrystalline diamond composite material, which is prepared by high-temperature and high-pressure sintering of carbon nanotubes (OLC) and carbon nanotubes (CNTs). The raw material mass percentage is: carbon nanotubes account for 10~30 wt.%, and the balance is carbon nanotubes. The closed spherical structure formed after the carbon nanotubes break can balance the internal pressure loss of the sintered body, so that the sintering conditions are reduced to 7~25 GPa pressure and 1800~2200℃ temperature. The Vickers hardness of the composite material is 17~202 GPa.

[0024] Carbon nanotubes are tubular structures formed by rolling up layers of carbon atoms in graphite. Their diameter is 4~8nm and their length is 0.5~2μm. Carbon nanotubes in this size range can be precisely broken during sintering to form closed spherical structures, achieving efficient balance of pressure loss.

[0025] The carbon nanoparticles have an average particle size of 5 nm and are prepared by vacuum annealing of nanodiamond produced by detonation. They are composed of carbon and have a nano-onion structure. The carbon nanoparticles are selected from one of the following: carbon nanoparticles with diamond crystal cores, carbon nanoparticles with diamond structure cores, and carbon nanoparticles with complete structure. The three types of carbon nanoparticles correspond to different sintering pressure adaptation ranges.

[0026] When the sintering pressure is ≤14GPa, carbon nanotubes with diamond cores are selected, and the Vickers hardness of the composite material is 31~151GPa; when the sintering pressure is >14GPa, intact carbon nanotubes are selected, and the Vickers hardness of the composite material is 17~202GPa, and the hardness increase rate is ≥5GPa / GPa when the pressure is >14GPa; carbon nanotubes with diamond crystal cores are suitable for the entire pressure range, and the corresponding composite material has a Vickers hardness of 21~94GPa.

[0027] Carbon nanotubes with diamond crystal cores are prepared by vacuum annealing at 800-1000℃, carbon nanotubes with diamond structural cores are prepared by vacuum annealing at 1050-1350℃, and carbon nanotubes with complete structure are prepared by vacuum annealing at 1400-1800℃. The vacuum degree of vacuum annealing is 1 Pa, and the holding time is 0-2 h. This invention also discloses a method for preparing carbon nanotube-nanopolycrystalline diamond composite material, as shown in Figure 1, including the following synergistic steps: S1. Directional preparation of carbon nanotubes: Using detonation-processed nanodiamonds with an average grain size of 5 nm as raw material, under a vacuum of 1 Pa, annealing temperatures of 800–1800 °C and holding times of 0–2 h are selected according to the target sintering pressure range to directionally prepare carbon nanotubes containing diamond crystal cores, diamond structural cores, or complete structures. Among them, annealing temperatures of 800–1000 °C correspond to carbon nanotubes containing diamond crystal cores, 1050–1350 °C correspond to carbon nanotubes containing diamond structural cores, and 1400–1800 °C correspond to carbon nanotubes with complete structures.

[0028] S2. Highly efficient dispersion of carbon nanotubes: Carbon nanotube powder with a diameter of 4~8nm and a length of 0.5~2μm is placed in ethanol and dispersed without agglomeration by ultrasonic vibration for 3~10min. Then, it is dried at 60~120℃ for 30min to ensure that the carbon nanotubes are uniformly distributed in the mixture.

[0029] S3. Gradient molding and sintering: The carbon nanotubes dispersed in step S2 are mixed with the carbon nanotubes obtained in step S1 at a ratio of 10-30 wt.% of the total raw materials to obtain a mixture. The mixture is loaded into a cemented carbide mold and pre-pressed at 400-600 MPa for 30-60 s to obtain a pre-pressed sample. The pre-pressed sample is loaded into a graphite mold according to a specific mold assembly process (as shown in Figure 2), and a gradient sintering process is adopted, which involves "slowly increasing the pressure to 7-25 GPa for 10-24 h + heating to 1800-2200℃ at 10-30℃ / min + holding for 5-60 min". After cooling in the furnace, the surface is ground and deburred to obtain the target composite material.

[0030] In step S3, the specific mold assembly process is as follows: the pre-pressed sample is placed into a BN crucible and the crucible is in close contact with the rhenium sheet. Then, the BN crucible is embedded into a LaCrO3 insulator. After the whole is packed into an MgO octahedron, an Al2O3 plug is added. A four-hole tube with electrodes is placed on the plug and sealed with Al2O3 glue. This assembly structure can achieve temperature uniformity and pressure stability during the sintering process. The pressure balancing effect of carbon nanotubes synergistically improves the density of the composite material.

[0031] In step S3, the mass percentage of carbon nanotubes is matched with the type of carbon nanotubes. When carbon nanotubes containing diamond crystal cores or diamond structural cores are used, the composite material has the best hardness when the carbon nanotube percentage is 20 wt.%. When carbon nanotubes with complete structure are used and the sintering pressure is >14 GPa, the carbon nanotube percentage can be increased to 30 wt.% with increasing pressure to further enhance the pressure balance effect. This pressure balance effect is verified by the increase in the density of the composite material (density ≥98%).

[0032] Please refer to Figure 2 The specific mold assembly structure of the present invention solves the key technical problems of temperature uniformity, pressure stability and sealing performance during high temperature and high pressure sintering through the synergistic cooperation of multiple functional components. It complements the pressure balancing effect of the closed spherical structure formed by the fracture of carbon nanotubes, and finally achieves the excellent effect of composite material density ≥98%.

[0033] The mold assembly structure uses MgO octahedron 4 as the outer support core. As a regular octahedral high-purity magnesium oxide crystal structure, it has an outer circumscribed circle diameter of 60-65mm and an internal cylindrical cavity with a diameter of 42-47mm. With its high-temperature resistance of ≥2800℃ and excellent compressive strength, it provides structural support for the entire assembly under a high-pressure environment of 7-25GPa, preventing internal components from deforming under pressure. A LaCrO3 insulator 5 is embedded inside the MgO octahedron 4. This insulator is a cylindrical sleeve structure with a wall thickness of 5-7.5mm and a thermal conductivity ≤1.5W / (m・K) (at 2000℃). Its extremely low thermal conductivity blocks heat transfer, ensuring that the temperature difference within the sintering zone is controlled within ±20℃. This avoids both localized overheating of the sample leading to abnormal grain growth and insufficient temperature affecting the complete phase transformation reaction.

[0034] Inside the LaCrO3 insulator 5, a BN crucible 6 is centrally located. This crucible is made of hot-pressed sintered boron nitride ceramic with a purity ≥99.5%, with an inner diameter of 20~25mm and a height of 35~40mm. It exhibits excellent chemical stability and does not react with carbon materials. As the direct carrier for the pre-pressed sample 8, it avoids material contamination under high temperature and pressure, ensuring product purity. Inside the BN crucible 6, two layers of high-purity rhenium sheets 7 are laid, with a diameter of 19~24mm and a thickness of 0.1~0.2mm, placed at the bottom of the crucible and the top of the sample, respectively. Utilizing the high-temperature resistance of rhenium metal (≥3180℃) and its excellent thermal conductivity, heat is uniformly conducted across the upper and lower surfaces of the sample, solving the problem of uneven performance caused by excessive temperature differences between the upper and lower surfaces of the sample in traditional molds. The pre-pressed sample 8 is placed in the center between two rhenium sheets, with its center coinciding with the central axis of the BN crucible 6, LaCrO3 insulator 5, and MgO octahedron 4. This ensures that the pressure is applied evenly to all surfaces of the sample during sintering, avoiding structural defects caused by local pressure concentration.

[0035] An Al₂O₃ plug 3 is embedded at the opening of the MgO octahedron 4. The plug has a diameter of 40-45 mm, a height of 10-12 mm, and a density ≥3.9 g / cm³. 3The first sealing barrier is formed by the tight fit with the inner wall of the octahedron, preventing internal heat loss and external air from entering. A four-hole tube 1 is placed at the center of the top of the Al2O3 plug 3. This cylindrical structure made of ceramic material has four axial through holes for inserting tungsten or molybdenum wire electrodes 2. The electrodes have a diameter of 2.8~3.8mm and a length of 30~35mm. The lower part extends to the area near the top of the BN crucible 6, and the upper part is exposed for connecting to external power supply equipment, providing a high temperature environment of 1800~2200℃ for sintering through the Joule effect. To further enhance the sealing and fixing effect, Al2O3 adhesive 9 with a thickness of 0.2~0.5mm is evenly applied to the joint between Al2O3 plug 3 and MgO octahedron 4, the contact edge between four-hole tube 1 and plug, and the gap between electrode 2 and the through hole of four-hole tube. After curing, this high-temperature resistant inorganic adhesive forms a dense sealing layer, which not only prevents heat from leaking from the gap, but also enhances the stability of the overall structure under high pressure and avoids component displacement.

[0036] Mold assembly must be carried out in a cleanroom with a cleanliness level of ≥1000, with the ambient temperature controlled at 20~25℃ and relative humidity ≤60%. All components must be ultrasonically cleaned with anhydrous ethanol for 10~15 minutes and dried before use to avoid impurities affecting sealing performance and product quality. The assembly process is carried out in the following steps: First, use ceramic tweezers to pick up the lower rhenium sheet 7 and lay it flat on the bottom of the BN crucible 6, ensuring that it completely covers the bottom of the crucible without wrinkles, and the gap between the edge and the inner wall of the crucible is ≤0.2mm; then, slowly place the pre-pressed sample 8 into the center of the crucible, and use calipers to measure the distance from the edge of the sample to the inner wall of the crucible, ensuring that it is uniform all around and that the top of the sample is kept horizontal; then cover it with the upper rhenium sheet 7, making it completely adhere to the sample surface without air bubbles or gaps.

[0037] The BN crucible 6 containing the sample and rhenium sheet is slowly inserted into the internal cavity of the LaCrO3 insulator 5 along the axial direction. The top of the crucible is gently pressed to ensure that the bottom of the crucible is in close contact with the insulator, ensuring that the gap between the outer wall and the inner wall of the insulator is uniform and ≤0.3mm. Then, the entire assembly is placed into the internal cavity of the MgO octahedron 4, and the position is adjusted so that the top of the assembly leaves a 10~12mm installation space between the top of the assembly and the opening of the octahedron to ensure the embedding depth of the Al2O3 plug 3.

[0038] Slowly insert the Al2O3 plug 3 into the top opening of the MgO octahedron 4, and gently press the top surface of the plug to ensure there are no gaps that allow light to pass through. Place the four-hole tube 1 at the center of the top of the plug, and use calipers to calibrate it so that the deviation between its center and the center of the octahedron is ≤0.5mm. Then insert the four electrodes 2 into the through holes of the four-hole tubes respectively, and adjust the length of the electrodes so that the lower part is 5~8mm away from the top of the BN crucible 6, and the exposed length of the upper part is consistent.

[0039] Finally, using a PTFE scraper, apply Al2O3 adhesive 8 evenly to the gaps between the components, ensuring a continuous, uninterrupted adhesive layer. After application, place the mold in an 80℃ oven for 2 hours or at room temperature for 24 hours. After assembly, multiple checks are required: visually confirm that all components are intact and secure, electrodes are not bent, and rhenium sheets are undamaged; measure key dimensions with calipers to ensure they meet design requirements; test the sealing performance by blowing with a dry airflow, ensuring no airflow leakage is acceptable; use a multimeter to measure the resistance between adjacent electrodes to ensure insulation performance meets standards and prevents short circuits during sintering.

[0040] In step S3, the mass percentage of carbon nanotubes is matched with the type of carbon nanotubes. When carbon nanotubes containing diamond crystal cores or diamond structural cores are used, the composite material has the best hardness when the carbon nanotube percentage is 20 wt.%. When carbon nanotubes with complete structure are used and the sintering pressure is >14 GPa, the carbon nanotube percentage can be increased to 30 wt.% with increasing pressure to further enhance the pressure balance effect. This pressure balance effect is verified by the increase in the density of the composite material (density ≥98%). Example 1

[0041] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 800℃ without heat preservation to obtain OLC containing diamond crystal core.

[0042] S2. 1 mg of CNT powder (tube diameter 4 nm, length 0.5 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0043] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 7 GPa over 10 hours; then, the temperature was increased from room temperature to 1800℃ at a rate of 10℃ / min and held for 5 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0044] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 21 GPa and the density was 98.2%. Example 2

[0045] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 850℃ for 0.5h to obtain OLC containing a diamond crystal core.

[0046] S2. 2 mg of CNT powder (tube diameter 5 nm, length 1 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 6 min. Then it was placed in a drying oven at 90 °C for 30 min to complete the dispersion of CNT powder.

[0047] S3. 2 mg of dispersed CNT powder and 8 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 500 MPa for 45 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 10 GPa over 14 hours; then, the temperature was increased from room temperature to 2200℃ at a rate of 30℃ / min and held for 60 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0048] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 94 GPa and the density was 98.5%. Example 3

[0049] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 900℃ for 1h to obtain OLC containing a diamond crystal core.

[0050] S2. 1 mg of CNT powder (tube diameter 6 nm, length 1.2 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0051] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 600 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature, high-pressure sintering. First, the sample was slowly pressurized to 14 GPa over 19 hours; then, the temperature was increased from room temperature to 2000℃ at a rate of 20℃ / min and held for 30 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0052] After high-pressure sintering, the carbon nanotube-nanoscale polycrystalline diamond composite material sample was polished and tested for its microstructure and properties. The Vickers hardness of the sintered block was 90 GPa and the density was 98.3%. Example 4

[0053] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 1000℃ for 1.5h to obtain OLC containing a diamond crystal core.

[0054] S2 and 3 mg of CNT powder (tube diameter 7 nm, length 1.5 μm) were placed in a glass beaker containing ethanol and ultrasonically vibrated for 10 min. Then, the beaker was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0055] S3. Dispersed 3 mg of CNT powder and 7 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 60 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 18 GPa over 21 hours; then, the temperature was increased from room temperature to 1900℃ at a rate of 15℃ / min and held for 10 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0056] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 36 GPa and the density was 98.1%. Example 5

[0057] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 1000℃ for 2 hours to obtain OLC containing a diamond crystal core.

[0058] S2. 1 mg of CNT powder (tube diameter 8 nm, length 2 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0059] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 25 GPa over 24 hours; then, the temperature was increased from room temperature to 1950℃ at a rate of 15℃ / min and held for 15 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0060] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 85 GPa and the density was 98.4%. Example 6

[0061] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) is annealed at a vacuum of 1Pa and an annealing temperature of 1050℃ without heat preservation to obtain OLC containing a diamond structure core.

[0062] S2. 1 mg of CNT powder (tube diameter 4 nm, length 0.8 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0063] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 7 GPa over 10 hours; then, the temperature was increased from room temperature to 1800℃ at a rate of 10℃ / min and held for 5 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0064] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 31 GPa and the density was 98.0%. Example 7

[0065] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1200℃ for 0.5h to obtain OLC containing a diamond structure core.

[0066] S2. 2 mg of CNT powder (tube diameter 5 nm, length 1.1 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 6 min. Then it was placed in a drying oven at 90 °C for 30 min to complete the dispersion of CNT powder.

[0067] S3. 2 mg of dispersed CNT powder and 8 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 500 MPa for 45 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 10 GPa over 14 hours; then, the temperature was increased from room temperature to 2200℃ at a rate of 30℃ / min and held for 60 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0068] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 64 GPa and the density was 98.6%. Example 8

[0069] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1250℃ for 1h to obtain OLC containing a diamond structure core.

[0070] S2. 1 mg of CNT powder (tube diameter 6 nm, length 1.4 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0071] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 600 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature, high-pressure sintering. First, the sample was slowly pressurized to 14 GPa over 19 hours; then, the temperature was increased from room temperature to 2000℃ at a rate of 20℃ / min and held for 30 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0072] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 151 GPa and the density was 98.8%. Example 9

[0073] A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1300℃ for 1.5h to obtain OLC containing a diamond structure core.

[0074] S2 and 3 mg of CNT powder (tube diameter 7 nm, length 1.7 μm) were placed in a glass beaker containing ethanol and ultrasonically vibrated for 10 min. Then, the beaker was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0075] S3. Dispersed 3 mg of CNT powder and 7 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 60 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 18 GPa over 21 hours; then, the temperature was increased from room temperature to 1900℃ at a rate of 15℃ / min and held for 10 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0076] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 89 GPa and the density was 98.2%.

[0077] Example 10 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1350℃ for 2 hours to obtain OLC containing a diamond structure core.

[0078] S2. 1 mg of CNT powder (tube diameter 8 nm, length 2 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0079] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 25 GPa over 24 hours; then, the temperature was increased from room temperature to 1950℃ at a rate of 15℃ / min and held for 15 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0080] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 121 GPa and the density was 98.5%.

[0081] Example 11 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1400℃ without heat preservation to obtain a complete OLC structure.

[0082] S2. 1 mg of CNT powder (tube diameter 4 nm, length 0.6 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0083] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 7 GPa over 10 hours; then, the temperature was increased from room temperature to 1800℃ at a rate of 10℃ / min and held for 5 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0084] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 17 GPa and the density was 98.0%.

[0085] Example 12 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1500℃ for 0.5h to obtain a complete OLC structure.

[0086] S2. 2 mg of CNT powder (tube diameter 5 nm, length 0.9 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 6 min. Then it was placed in a drying oven at 90 °C for 30 min to complete the dispersion of CNT powder.

[0087] S3. 2 mg of dispersed CNT powder and 8 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 500 MPa for 45 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 10 GPa over 14 hours; then, the temperature was increased from room temperature to 2200℃ at a rate of 30℃ / min and held for 60 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0088] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 30 GPa and the density was 98.3%.

[0089] Example 13 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1600℃ for 1h to obtain a complete OLC structure.

[0090] S2. 1 mg of CNT powder (tube diameter 6 nm, length 1.3 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 60 °C for 30 min to complete the dispersion of CNT powder.

[0091] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 600 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature, high-pressure sintering. First, the sample was slowly pressurized to 14 GPa over 19 hours; then, the temperature was increased from room temperature to 2000℃ at a rate of 20℃ / min and held for 30 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0092] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 94 GPa and the density was 98.6%.

[0093] Example 14 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1700℃ for 1.5h to obtain a complete OLC structure.

[0094] S2 and 3 mg of CNT powder (tube diameter 7 nm, length 1.6 μm) were placed in a glass beaker containing ethanol and ultrasonically vibrated for 10 min. Then, the beaker was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0095] S3. Dispersed 3 mg of CNT powder and 7 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 60 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 18 GPa over 21 hours; then, the temperature was increased from room temperature to 1900℃ at a rate of 15℃ / min and held for 10 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0096] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 159 GPa and the density was 99.0%.

[0097] Example 15 A method for preparing a carbon nanotube-nanopolycrystalline diamond composite material includes the following steps: S1. Detonation nanodiamond powder (average grain size 5nm) was annealed at a vacuum of 1Pa and an annealing temperature of 1800℃ for 2 hours to obtain a complete OLC structure.

[0098] S2. 1 mg of CNT powder (tube diameter 8 nm, length 2 μm) was placed in a glass beaker containing ethanol and ultrasonically vibrated for 3 min. Then it was placed in a drying oven at 120 °C for 30 min to complete the dispersion of CNT powder.

[0099] S3. 1 mg of dispersed CNT powder and 9 mg of OLC powder were mixed evenly in a mortar. The CNT and OLC powder mixture was then filled into a cemented carbide mold for pre-compression at a pressure of 400 MPa for 30 seconds. The pre-compressed sample was then assembled into the mold according to a specific mold assembly process and subjected to high-temperature and high-pressure sintering. First, the sample was slowly pressurized to 25 GPa over 24 hours; then, the temperature was increased from room temperature to 1950℃ at a rate of 15℃ / min and held for 15 minutes. The sample was then cooled in the furnace to obtain a blank. The prepared blank was then surface-ground and deburred to obtain a carbon nanotube-nanopolycrystalline diamond composite material.

[0100] After high-pressure sintering, the carbon nanotube-nanopolycrystalline diamond composite material sample was polished and its microstructure and properties were tested. The Vickers hardness of the sintered block was 202 GPa and the density was 99.2%.

[0101] The carbon nanotube-nanoscale polycrystalline diamond composite material prepared by this invention has a Vickers hardness of 17-202 GPa. The closed spherical structure formed when the carbon nanotubes break balances the internal pressure loss of the sintered body, reducing the sintering conditions of OLC, thereby solving the problem of high sintering conditions of OLC. This invention provides a novel carbon nanotube-nanoscale polycrystalline diamond composite material and its preparation method.

[0102] A horizontal comparison of Examples 1-5, 6-10, and 11-15 revealed that the structure of carbon nanotubes significantly affects the performance of the composite materials. At pressures below 14 GPa, the composite material prepared with OLC containing a diamond core exhibits the best performance, followed by the composite material with an OLC containing a diamond crystal core, while the composite material with a fully-structured OLC exhibits the worst performance. At pressures above 14 GPa, the composite material prepared with a fully-structured OLC exhibits the best performance, followed by the composite material with an OLC containing a diamond core, while the composite material with an OLC containing a diamond crystal core exhibits the worst performance. A vertical comparison of Examples 1-5, 6-10, and 11-15 showed that sintering pressure, sintering temperature, holding time, and CNT content have a significant impact on the performance of the composite materials. Studies using Examples 1-5 show that when the sintering pressure is less than 10 GPa, the hardness of the composite material increases with increasing sintering pressure; when the sintering pressure is greater than 10 GPa, the hardness of the composite material increases with increasing sintering temperature. Studies of Examples 1, 2, and 4 reveal that the hardness of the composite material first increases and then decreases with increasing CNT content. Comparison of Examples 4 and 5 shows that the hardness of the composite material increases with prolonged holding time. Studies using Examples 6-10 show that when the sintering pressure is less than 14 GPa, the hardness of the composite material increases with increasing pressure; when the sintering pressure is greater than 14 GPa, the hardness of the composite material increases with increasing sintering temperature. Studies of Examples 6, 7, and 9 reveal that the hardness of the composite material first increases and then decreases with increasing CNT content. Comparison of Examples 9 and 10 shows that the hardness of the composite material increases with prolonged holding time. Studies in Examples 11-15 show that sintering pressure is the main determinant of the hardness of the composite material. The hardness of the composite material increases with increasing pressure, especially when the sintering pressure is greater than 14 GPa, the increase in hardness is very significant. Studies in Examples 11 and 12 show that the hardness of the composite material increases with increasing CNT content. Comparison with Examples 14 and 15 shows that the hardness of the composite material increases with prolonged holding time.

[0103] The foregoing description illustrates and describes several preferred embodiments of the present invention. However, as previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A carbon nanotube-nanopolycrystalline diamond composite material, characterized in that: The composite material is prepared by high-temperature and high-pressure sintering of carbon nanotubes (OLC) and carbon nanotubes (CNTs). The raw material mass percentage of the composite material is 10-30 wt.% carbon nanotubes, with the balance being carbon nanotubes. The closed spherical structure formed after the carbon nanotubes break can balance the internal pressure loss of the sintered body, thereby reducing the sintering conditions to 7-25 GPa pressure and 1800-2200℃ temperature. The Vickers hardness of the composite material is 17-202 GPa.

2. The carbon nanotube-nanopolycrystalline diamond composite material according to claim 1, characterized in that: The carbon nanotubes are tubular structures formed by rolling up layers of graphite carbon atoms. Their diameter is 4~8nm and their length is 0.5~2μm. Carbon nanotubes in this size range can be precisely broken during sintering to form closed spherical structures, achieving efficient balance of pressure loss.

3. The carbon nanotube-nanopolycrystalline diamond composite material according to claim 1, characterized in that: The carbon nanoparticles have an average particle size of 5 nm and are prepared by vacuum annealing of nanodiamonds produced by detonation. They are composed of carbon and have a nano-onion structure. The carbon nanoparticles are selected from one of the following: carbon nanoparticles with diamond crystal cores, carbon nanoparticles with diamond structure cores, and carbon nanoparticles with complete structure. The three types of carbon nanoparticles correspond to different sintering pressure adaptation ranges.

4. The carbon nanotube-nanopolycrystalline diamond composite material according to claim 3, characterized in that: When the sintering pressure is ≤14GPa, carbon nanotubes with a diamond structure core are selected, and the Vickers hardness of the composite material is 31~151GPa. When the sintering pressure is greater than 14 GPa, a complete carbon nanotube is selected. The Vickers hardness of the composite material is 17~202 GPa, and the hardness increase rate is ≥5 GPa / GPa when the pressure is greater than 14 GPa. Carbon nanotubes containing diamond crystal cores are suitable for the entire pressure range, and the corresponding composite materials have a Vickers hardness of 21~94 GPa.

5. The carbon nanotube-nanopolycrystalline diamond composite material according to claim 3, characterized in that: The carbon nanotubes containing diamond crystal cores are prepared by vacuum annealing at 800~1000℃, the carbon nanotubes containing diamond structure cores are prepared by vacuum annealing at 1050~1350℃, and the carbon nanotubes with complete structure are prepared by vacuum annealing at 1400~1800℃. The vacuum degree of the vacuum annealing is 1 Pa and the holding time is 0~2h.

6. A method for preparing the carbon nanotube-nanopolycrystalline diamond composite material as described in claim 1, characterized in that, Includes the following steps: S1. Directional preparation of carbon nanoparticles: Using detonation-methodized nanodiamonds with an average grain size of 5 nm as raw material, under a vacuum of 1 Pa, the annealing temperature of 800~1800℃ and the holding time of 0~2h are selected according to the target sintering pressure range to directionally prepare carbon nanoparticles containing diamond crystal cores, diamond structure cores or complete structures. S2. Highly efficient dispersion of carbon nanotubes: Carbon nanotube powder is placed in ethanol and dispersed without agglomeration by ultrasonic vibration for 3-10 minutes. Then it is dried at 60-120℃ for 30 minutes to ensure that the carbon nanotubes are evenly distributed in the mixture. S3. Gradient molding and sintering: The carbon nanotubes dispersed in step S2 are mixed with the carbon nanotubes obtained in step S1 at a ratio of 10~30wt.%, and the mixture is placed in a cemented carbide mold and pre-pressed at 400~600MPa for 30~60s. The pre-pressed sample is then placed into a graphite mold according to a specific mold assembly process. A gradient sintering process is adopted, in which the pressure is slowly increased to 7~25GPa for 10~24h, the temperature is increased to 1800~2200℃ at 10~30℃ / min, and the temperature is held for 5~60min. After cooling in the furnace, the surface is ground and deburred to obtain the target composite material.

7. The preparation method according to claim 6, characterized in that: The specific mold assembly process described in step S3 is as follows: the pre-pressed sample is placed into a BN crucible and the crucible is in close contact with the rhenium sheet. Then, the BN crucible is embedded into a LaCrO3 insulator. After the entire MgO octahedron is installed, an Al2O3 plug is added. A four-hole tube with electrodes is placed on the plug and sealed with Al2O3 glue. This assembly structure can achieve temperature uniformity and pressure stability during the sintering process. The pressure balancing effect of carbon nanotubes synergistically improves the density of the composite material.

8. The preparation method according to claim 6, characterized in that: In step S3, the mass percentage of carbon nanotubes is matched with the type of carbon nanotubes. When carbon nanotubes containing diamond crystal cores or diamond structural cores are used, the composite material has the best hardness when the carbon nanotube percentage is 20 wt.%. When carbon nanotubes with complete structure are used and the sintering pressure is >14 GPa, the carbon nanotube percentage can be increased to 30 wt.% as the pressure increases, so as to further enhance the pressure balance effect.

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