Micro-nano polycrystalline diamond composite material and preparation method thereof

Abstract

The invention discloses a micro-nano polycrystalline diamond composite material and a preparation method thereof, the micro-nano polycrystalline diamond composite material comprises the following raw materials: carbon nano onion (OLC) and micron diamond (MD), and the mass ratio of the MD to the OLC is 10: 90-75: 25. The preparation method comprises the following steps: mixing the two raw materials MD and OLC according to a mass ratio; and a hard alloy mold is filled with the mixed MD and OLC mixture to be pre-pressed, the pre-pressing pressure ranges from 400 MPa to 600 MPa, and pre-pressing is conducted for 30 s to 60 s. And then, loading the pre-pressed sample into an assembly module for high-temperature and high-pressure sintering. The sintering pressure ranges from 5 GPa to 40 GPa, the sintering temperature ranges from 1200 DEG C to 3000 DEG C, the heat preservation time ranges from 0 min to 60 min, then cooling and pressure relief are conducted, and the micro-nano polycrystalline diamond composite material is prepared. The MD and nano polycrystalline diamond synergistically enhanced microarchitecture is constructed at high temperature and high pressure, the sintering condition of OLC is reduced, the problem that the sintering condition of the OLC is high is solved, and hardness and toughness are synchronously improved through heterogeneous interface design.

Description

Technical Field

[0001] This invention belongs to the field of composite material technology, specifically, it relates to a micro-nano polycrystalline diamond composite material and its preparation method. Background Technology

[0002] Polycrystalline diamond (PCD), as a high-performance superhard composite material, has extremely high hardness and wear resistance, effectively overcoming the defects of anisotropy and easy dissociation and damage of the {111} crystal plane in single-crystal diamond, and occupies an irreplaceable position in many high-end manufacturing and engineering fields. In the existing technology, many studies have been carried out on the preparation and performance optimization of PCD, but significant technical bottlenecks still exist: Wang Mingzhi et al. synthesized polycrystalline diamond using nano-onion-carbon as raw material and a six-sided press under conditions of 2~6GPa, 1000~1600℃, and holding time of 1~6min. The Vickers hardness reached HV45~61GPa, and the grain size of the sintered body was less than 20nm. Although it overcame the negative impact of the weak relative on PCD performance in the traditional process, there is still room for improvement in hardness (Patent No.: CN101723358A, Publication Date: June 9, 2010). The team subsequently used OLC and micron-sized diamond as raw materials to synthesize polycrystalline diamond under conditions of 4~6.5GPa, 1000~1600℃, and holding time of 1~15min. The product has a smooth surface and dense bulk, but the Vickers hardness is only HV41~70GPa, which is difficult to meet the requirements of high-end applications for ultra-high hardness (Patent No.: CN103274398A, Publication Date: September 4, 2013). Lian Min et al. synthesized a cubic diamond-cubic carbon two-phase composite structure using graphene as a raw material at 22 GPa and 1800-2000℃, achieving a Vickers hardness of 120-150 GPa and a fracture toughness of 12-15 MPa·m. 1 / 2 Although it improves thermal stability and toughness, the sintering pressure is still high, and the raw material (graphene) is expensive (Patent No.: CN119455806A, Publication Date: February 18, 2025). Ma Shuailing et al. used 50nm nano-onion carbon and 40~60μm micro-graphite as raw materials, and after mechanical grinding and high-temperature and high-pressure sintering at 1800℃ and 15GPa, formed a bimodal grain distribution with a Vickers hardness of 153~169GPa and a toughness of 14~15MPa·m. 1 / 2 It achieves a balance between high hardness and high toughness, but the phase transformation efficiency of micron-sized graphite in the raw material is low, requiring higher sintering energy input (Patent No.: CN119390066A, Publication Date: February 7, 2025). Tang Hu's research confirms that the transformation of OLC to diamond is a martensitic phase transformation process. The closed and continuous carbon shell restricts the sliding of the OLC (002) surface and generates stress. The formation of twinned diamond is the result of stress release (Tang Hu. Research on the synthesis of nano-polycrystalline diamond and the high-temperature and high-pressure phase transformation mechanism of carbon nanoparticles [D]. Qinhuangdao: Yanshan University, 2018). However, existing techniques for synthesizing PCD using OLC as a precursor generally have the following drawbacks: The sintering conditions are harsh: extremely high pressure (P≥18GPa) and temperature (T≥2300℃) are required, which places extremely high demands on the pressure resistance and temperature control accuracy of the equipment, resulting in high production costs; Product size limitations: The maximum size of the synthesized PCD is less than 3mm, which makes it difficult to meet the needs of large-size components in engineering applications; Poor performance uniformity: Although the low-pressure synthesis technology reduces sintering conditions, the product hardness is low and the performance stability is poor due to uneven mixing of raw materials and fluctuations in process parameters. The purity of raw materials has a significant impact: existing technologies do not clearly define the purity and impurity content of OLC and MD, which makes it easy to generate impurity phases during the phase transformation process, reducing the hardness and toughness of the material.

[0003] Therefore, developing a preparation technology that can reduce sintering conditions and control costs while ensuring high hardness, high toughness and performance stability of PCD, and clarifying key process parameters and raw material indicators to expand the scope of protection, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] To address the shortcomings of existing technologies in the synthesis of PCD using OLC as a precursor, such as stringent sintering conditions, high costs, and size limitations, insufficient hardness and poor performance stability of PCD synthesized under low pressure, and vague definitions of raw material indicators and process parameters, this invention provides a micro-nano polycrystalline diamond composite material and its preparation method. By clarifying the purity of raw materials, optimizing the mixing process, and precisely controlling sintering and cooling parameters, a synergistic breakthrough is achieved in "low sintering cost, large-size preparation, high hardness and toughness, and stable performance".

[0005] The core idea of ​​this invention is "structural compatibility regulation + precise control of full-process parameters". By selecting high-purity OLC and MD as composite raw materials, clarifying the key indicators of raw materials, optimizing the entire process of mixing, pre-pressing, sintering, cooling and post-treatment, a "barrier-free diffusion system" is constructed to prepare high-performance micro-nano polycrystalline diamond composite materials under mild high temperature and high pressure conditions.

[0006] The specific technical solution is as follows: A micro-nano polycrystalline diamond composite material, the raw materials of which include two different structures of carbon nanotubes (OLC) and micron diamond (MD), wherein the mass ratio of MD to OLC is 10:90~75:25.

[0007] OLC composition and purity: Carbon content ≥99.5%, free of metal impurities or metal impurity content ≤0.1wt% (metal impurities can hinder phase transition and form weak phases); Structural characteristics: The crystal structure is a nano-onion structure with an average grain size of 5nm±0.5nm, crystal structure integrity ≥90%, and no obvious lattice distortion; based on the core structure, it is divided into OLC with a diamond crystal core (the outermost layer is graphite sheet, single layer of graphene or several layers of graphene, and the core is a diamond core) and OLC without a diamond core (composed entirely of graphite sheets). Preparation methods: It can be prepared by vacuum annealing, arc discharge (current 50~200A, voltage 20~50V), pyrolysis (precursor is methane, acetylene or benzene series), laser ablation (laser power 100~500W) and catalytic chemical vapor deposition (catalyst is one or more of Fe, Co, Ni). Vacuum annealing (using detonation nanodiamond as raw material) is preferred.

[0008] MD structural characteristics: It has a similar cubic diamond structure, with B and N atoms arranged alternately in plane in each layer, and the interlayer packing mode is ABC type. The grain size is 0.1~100μm, the average grain size is 0.5~100μm, and the grain morphology is spherical or polyhedral. Purity and Composition: Purity ≥ 99%, with a molar ratio of B to N atoms of 1:1 ± 0.05 (to ensure structural stability), and impurity element content (excluding O and C) ≤ 0.5 wt%; Dispersibility: The particles are well dispersed, with agglomerate size ≤500μm and agglomerate percentage ≤5wt% (to avoid performance fluctuations caused by uneven mixing).

[0009] Composite material properties and phases: Mechanical properties: Vickers hardness is 50~245 GPa, fracture toughness is 4~20 MPa·m 1 / 2 ; Physical properties: Density ≥ 3.45 g / cm³ 3 Porosity ≤2% (ensuring density and improving wear resistance); Phase composition: mainly composed of MD and cubic diamond, with a small amount of hexagonal diamond (content ≤5wt%), and no obvious graphite impurities (graphite content ≤1wt%).

[0010] (2) Preparation method The preparation method of the above-mentioned micro-nano polycrystalline diamond composite material includes the following steps: Preparation of S1 and OLC: Nanodiamond powder (purity ≥99%) with an average grain size of 5 nm ± 0.5 nm, prepared by detonation method, was placed under a vacuum of 1.0 × 10⁻⁶. -1 Pa±5×10 -2 In an environment of Pa, OLC was annealed at 900-1600℃ with a heating rate of 5-20℃ / min (without holding) to obtain OLC with an average grain size of 5nm±0.5nm; among which, the product annealed at 900-1350℃ was OLC containing a diamond core, and the product annealed at 1400-1600℃ was OLC without a diamond core.

[0011] Reason for controlling the heating rate: to avoid excessively rapid heating that could cause nanodiamond particles to agglomerate and affect the integrity of the onion structure of OLC.

[0012] S2, Mixing and Pre-compression: Mix the MD powder and the OLC obtained in step S1 at a mass ratio of 10:90~75:25 evenly, put them into a cemented carbide mold, apply a pressure of 400~600MPa, hold the pressure for 30~60s, and complete the pre-compression molding.

[0013] Mixing method: Use one or more of mechanical grinding, planetary ball milling or high-energy ball milling, and the mixing time is 10~60min; Mechanical grinding: rotation speed 300~800 r / min, suitable for small batch preparation; Planetary ball mill: ball-to-material ratio of 5:1 to 20:1, rotation speed of 200 to 600 r / min, suitable for mass production; Dispersion treatment: Anhydrous ethanol can be used as a dispersant during the mixing process (the mass ratio of dispersant to mixture is 1:1~3:1) to improve the mixing uniformity; dry mixing can also be performed without a dispersant. In the case of dry mixing, an inert gas (N2 or Ar) should be introduced for protection to avoid oxidation of the raw materials. Pre-compression parameter control: Too low pressure will result in insufficient density of the green body, while too high pressure will easily cause stress concentration; the holding time ensures stable green body forming and avoids cracking during sintering.

[0014] S3, High-temperature and high-pressure sintering: Select the corresponding assembly module process according to the target sintering pressure to complete the sample assembly. Apply pressure slowly to 5~40GPa within 0.1~20h, then raise the temperature to 1200~3000℃ at a heating rate of 10℃ / min±2℃ / min, hold for 0~60min, then cool down to room temperature (25±5℃) at a cooling rate of 5~50℃ / min, and then depressurize at a rate of 0.1~2GPa / h to obtain the blank.

[0015] Assembly module process: Sintering pressure 5~7GPa: Place the pre-pressed sample into a BN crucible with a purity ≥99%, ensuring good contact between the crucible and a salt tube made of sodium chloride or potassium chloride (the salt tube serves as insulation and pressure transmission). Then place the crucible into a graphite column with a density ≥1.8g / cm³, and add graphite gaskets and conductive steel caps (purity ≥99.5%) in sequence. Finally, embed the entire sample into pyrophyllite (pyrophyllite is the pressure transmission medium, with a water content ≤0.5wt% to avoid moisture affecting sintering). Sintering pressure 8~40GPa: Place the pre-pressed sample into a BN crucible with a purity ≥99%, ensuring good contact between the crucible and a rhenium sheet with a thickness of 0.1~0.5mm (rhenium sheet is heat-resistant and conductive). Then place the crucible into a LaCrO3 insulator with a porosity ≤5% (to reduce heat loss). The entire sample is then encased in an MgO octahedron with a purity ≥99.5%, and an Al2O3 plug with a density ≥95% is added. An Al2O3 four-hole tube with electrodes is placed on the plug and sealed with Al2O3 adhesive. After sealing, the module's airtightness is ≤1×10⁻⁶. -3 Pa・s (to ensure a stable sintering environment); Sintering parameter control: Pressure application: Apply pressure slowly to avoid uneven stress on the billet and cracking; the pressure fluctuation should not exceed ±0.5 GPa. Heating process: Temperature fluctuation range not exceeding ±50℃ to ensure phase transition synchronization between OLC and MD; During the heat preservation stage: the pressure and temperature fluctuations shall not exceed ±0.3GPa and ±30℃ respectively, to ensure sufficient phase transformation; Cooling and depressurization: Control the cooling rate (5~50℃ / min) to avoid rapid cooling and internal stress. Depressurize after cooling to room temperature (depressurization rate 0.1~2GPa / h) to prevent the billet from cracking due to sudden pressure drop.

[0016] S4. Post-processing: The blank is subjected to surface grinding and deburring to obtain micro-nano polycrystalline diamond composite material; wherein, surface grinding is performed by diamond grinding wheel grinding with a grinding accuracy of ±0.01mm (meeting dimensional tolerance requirements), and deburring is performed by ultrasonic cleaning (power 100~300W, time 5~20min) or mechanical polishing (suitable for high precision surface requirements).

[0017] Compared with the prior art, this application can achieve the following technical effects: (1) Sintering conditions are significantly optimized: MD is used to balance the internal pressure loss of the sintered body and to promote diamond growth. The minimum sintering pressure of OLC to synthesize PCD is reduced from 18GPa in the existing technology to 5GPa, and the minimum sintering temperature is reduced from 2300℃ to 1200℃. This reduces the pressure and temperature control requirements of the equipment and saves a lot of preparation costs. At the same time, there is no limitation on product size, and large-size composite materials can be prepared, which broadens the application scenarios.

[0018] (2) Synergistic and stable performance enhancement: By constructing a microstructure that synergistically enhances MD and nano-polycrystalline diamond through "structural compatibility regulation", combined with raw material purity control (OLC carbon content ≥99.5%, MD purity ≥99%) and hybridization process optimization, the product's Vickers hardness reaches 50~245GPa (the lowest being more than twice that of single crystal cBN(111) crystal plane, and the highest reaching the level of nano-twinned diamond), and the fracture toughness reaches 4~20MPa·m. 1 / 2 (Up to 6 times that of single-crystal cBN and 3 times that of single-crystal diamond), and with a density ≥ 3.45 g / cm³. 3 With a porosity of ≤2%, it possesses excellent wear resistance, chemical inertness, and dimensional stability, and its performance fluctuation range is ≤5% (far superior to ±10% of existing technologies).

[0019] (3) Comprehensive coverage of protection scope: The key indicators such as the purity, structure and dispersibility of raw materials are clearly defined, the process parameters of the entire process such as mixing, sintering, cooling and post-processing are limited, and new claims such as the purity of raw materials, mixing method, cooling and depressurization rate and phase composition are added. The protection system is constructed from multiple dimensions such as raw materials, process, product performance and phase structure, which effectively prevents others from circumventing patent protection by adjusting the impurity content of raw materials, mixing method and process parameters.

[0020] (4) The preparation process is industrializable: all process parameters are clear and easy to control, the mixing method is suitable for small-batch and large-batch production, and the sintering and post-processing processes are compatible with existing superhard material production equipment. No special equipment modification is required, which provides reliable support for industrial production.

[0021] 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

[0022] 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 The image shows the XRD phase analysis diagram of the micro-nano polycrystalline diamond composite material in Example 4.

[0023] Figure 2 The image shows the XRD phase analysis diagram of the micro-nano polycrystalline diamond composite material in Example 6.

[0024] Figure 3 The image shows the XRD phase analysis diagram of the micro-nano polycrystalline diamond composite material in Example 9. Detailed Implementation

[0025] 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. Example 1

[0026] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC without diamond crystal cores was prepared by annealing at 1500 ℃ without heat treatment.

[0027] S2. Mix 5 mg of MD powder (average grain size 0.5 μm) and 45 mg of OLC powder evenly in a mortar. Fill the mixture of MD and OLC powder into a cemented carbide mold for pre-compression. The pre-compression pressure is 400 MPa, and the pressure is held for 30 s.

[0028] S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 25 GPa; then, the temperature is increased from room temperature to 2500 ℃ at a rate of 10 ℃ / min and held for 25 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 160 GPa and a toughness of 9 MPa·m. 1 / 2 Example 2

[0029] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1OLC containing a diamond crystal core was prepared by annealing at 900 ℃ without heat treatment.

[0030] S2. Mix 17.5 mg of MD powder (average grain size 5 μm) and 32.5 mg of OLC powder evenly in a mortar. Fill the MD and OLC powder mixture into a cemented carbide mold and pre-press it at a pressure of 400 MPa for 40 seconds. S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 5 GPa; then, the temperature is increased from room temperature to 1200 ℃ at a rate of 10 ℃ / min and held for 60 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 50 GPa and a toughness of 4 MPa·m. 1 / 2 Example 3

[0031] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC without diamond crystal cores was obtained by annealing at 1250 °C without holding at Pa.

[0032] S2. Mix 10 mg of dispersed MD powder (average grain size 20 μm) with 40 mg of OLC powder in a mortar until homogeneous. Fill the MD and OLC powder mixture into a cemented carbide mold and pre-press it at a pressure of 400 MPa for 50 seconds. S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 20 GPa; then, the temperature is increased from room temperature to 2000 ℃ at a rate of 10 ℃ / min and held for 30 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 154 GPa and a toughness of 8 MPa·m. 1 / 2 . Example 4

[0033] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC containing a diamond crystal core was prepared by annealing at 1100 ℃ without heat treatment.

[0034] S2. Mix 17.5 mg of MD powder (average grain size 50 μm) and 32.5 mg of OLC powder evenly in a mortar. Fill the mixture of MD and OLC powder into a cemented carbide mold for pre-compression. The pre-compression pressure is 400 MPa and the pressure is held for 60 s.

[0035] S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 10 GPa; then, the temperature is increased from room temperature to 1800 ℃ at a rate of 10 ℃ / min and held for 40 min. The pressure is then released by cooling to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain the micro-nano polycrystalline diamond composite material. Figure 1 It can be observed that the main phases of the material are MD and cubic diamond, with a small amount of hexagonal diamond. The resulting sintered bulk has a Vickers hardness of 120 GPa and a toughness of 18.8 MPa·m. 1 / 2 . Example 5

[0036] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC without diamond crystal cores was prepared by annealing at 1400 ℃ without holding at Pa.

[0037] S2. Mix 37.5 mg of dispersed MD powder (average grain size 100 μm) with 12.5 mg of OLC powder in a mortar until homogeneous. Then, fill the MD and OLC powder mixture into a cemented carbide mold and pre-press it at a pressure of 400 MPa for 35 seconds. S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 8 GPa; then, the temperature is increased from room temperature to 1400 ℃ at a rate of 10 ℃ / min and held for 15 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 109 GPa and a toughness of 7 MPa·m. 1 / 2 . Example 6

[0038] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC containing a diamond crystal core was prepared by annealing at 1200 ℃ without heat treatment.

[0039] S2. Mix 25 mg of dispersed MD powder (average grain size 100 μm) and 25 mg of OLC powder evenly in a mortar. Fill the MD and OLC powder mixture into a cemented carbide mold and pre-press it at a pressure of 400 MPa for 40 seconds. S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 16 GPa; then, the temperature is increased from room temperature to 1800 ℃ at a rate of 10 ℃ / min and held for 10 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain the micro-nano polycrystalline diamond composite material. Figure 2 It can be observed that the main phases of the material are MD and cubic diamond. The resulting sintered bulk has a Vickers hardness of 192 GPa and a toughness of 13 MPa·m. 1 / 2 . Example 7

[0040] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC containing a diamond crystal core was prepared by annealing at 1600 ℃ without heat treatment.

[0041] S2. Mix 25 mg of dispersed MD powder (average grain size 95 μm) and 25 mg of OLC powder evenly in a mortar. Fill the mixture of MD and OLC powder into a cemented carbide mold for pre-compression. The pre-compression pressure is 400 MPa and the pressure is held for 50 seconds.

[0042] S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 20 GPa; then, the temperature is increased from room temperature to 2000 ℃ at a rate of 10 ℃ / min and held for 10 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 245 GPa and a toughness of 20 MPa·m. 1 / 2 . Example 8

[0043] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC containing a diamond crystal core was prepared by annealing at 1300 ℃ without heat treatment.

[0044] S2. Mix 22.5 mg of dispersed MD powder (average grain size 85 μm) and 27.5 mg of OLC powder evenly in a mortar. Fill the MD and OLC powder mixture into a cemented carbide mold and pre-press it at a pressure of 400 MPa for 55 s. S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 40 GPa; then, the temperature is increased from room temperature to 3000 ℃ at a rate of 10 ℃ / min and held for 30 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain a micro-nano polycrystalline diamond composite material. The resulting sintered block has a Vickers hardness of 122 GPa and a toughness of 10 MPa·m. 1 / 2 . Example 9

[0045] S1. Anneal the detonation nanodiamond powder (average grain size 5 nm) under a vacuum of 1.0 × 10⁻⁶. -1 OLC containing a diamond crystal core was prepared by annealing at 1200 ℃ without heat treatment.

[0046] S2. Mix 25 mg of dispersed MD powder and 25 mg of OLC powder evenly in a mortar. Fill the mixture of MD and OLC powders into a cemented carbide mold for pre-compression. The pre-compression pressure is 400 MPa, and the pressure is held for 45 s.

[0047] S3. The pre-compressed sample is loaded into the assembly module for high-temperature and high-pressure sintering. First, the sample is slowly pressurized to 5 GPa; then, the temperature is increased from room temperature to 1200 ℃ at a rate of 10 ℃ / min and held for 10 min. The pressure is then released to obtain a blank. The prepared blank is then subjected to surface grinding and deburring to obtain the micro-nano polycrystalline diamond composite material. Figure 3 It can be observed that the main phases of the material are MD and cubic diamond. The resulting sintered bulk has a Vickers hardness of 173 GPa and a toughness of 8 MPa·m. 1 / 2 .

[0048] A longitudinal comparison of Examples 3 and 7 revealed that the structure of carbon nanotubes significantly impacts the performance of the composite material. The composite material prepared using OLC without a diamond core exhibits the best hardness and toughness. This is because OLC without a diamond core has more graphite structure compared to OLC with a diamond core, allowing it to withstand greater pressure, and the cumulative stress release can generate a large number of twins and stacking faults. A lateral comparison of Examples 3-4, 4-5, and 1-2 showed that sintering pressure, sintering temperature, and holding time have a significant impact on the performance of the composite material. Examples 1-2 demonstrate that the hardness of the composite material increases with increasing sintering pressure; comparing Examples 4-5 shows that the hardness increases with increasing sintering temperature; comparing Examples 1-2 shows that extending the holding time is beneficial for improving the hardness of the composite material; comparing Examples 1 and 7 shows that further increasing the pressure after exceeding 20 GPa does not increase the hardness of the composite material. This is because excessively high pressure alters the plastic deformation mode of diamond, resulting in increased twin thickness. Comparing Examples 1 and 8, it was found that the hardness of the composite material did not increase further after the sintering temperature exceeded 2500 ℃. This is because the excessively high temperature caused abnormal growth of diamond grains. Regarding hardness: the lowest hardness of the composite material prepared in this invention (Example 4) is more than twice that of the single-crystal cBN (111) crystal plane (44 GPa), exceeding the hardness of WC cemented carbide, BC2N, and polycrystalline cubic boron nitride, and approaching that of nanotwinned cBN; the hardness of Examples 6 and 7 exceeds 188 GPa, reaching the hardness of nanotwinned diamond. Regarding fracture toughness: the composite material prepared in this invention achieves a toughness of 17.8 MPa·m. 1 / 2 It is a single-crystal cBN (2.8 MPa·m 1 / 2 Six times that of single-crystal diamond (5 MPa·m) 1 / 2 3 times that of BC2N (4.5 MPa·m 1 / 2 It is 4 times stronger than that of polycrystalline cubic boron nitride (15 MPa·m). It even exceeds the highest fracture toughness of polycrystalline cubic boron nitride (15 MPa·m). 1 / 2 ).

[0049] A comparative analysis of the above embodiments shows that: Effect of raw material purity: When the carbon content of OLC is ≥99.5% and the purity of MD is ≥99%, the composite material has a low impurity phase content and better performance (as in Example 7). OLC structure influence: OLC containing diamond core is more conducive to improving the hardness and toughness of materials, because the diamond in the core can act as a phase transformation seed crystal to promote the growth of cubic diamond. Effect of sintering parameters: The performance is optimal when the sintering pressure is 20 GPa and the sintering temperature is around 2000℃ (Example 7); when the pressure exceeds 20 GPa or the temperature exceeds 2500℃, the material hardness no longer increases significantly, which is due to the change in the plastic deformation mode of diamond (increased twin thickness) and abnormal grain growth, respectively. Impact of mixing method: Planetary ball milling + ultrasonic dispersion or high-energy ball milling has the best mixing effect, which can reduce agglomeration and improve performance stability.

[0050] 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 micro-nano polycrystalline diamond composite material, characterized in that: The composite material is prepared by mixing, pre-pressing, and high-temperature and high-pressure sintering of two different structures of carbon nanotubes (OLC) and micron-sized diamond (MD); the mass ratio of MD to OLC is 10:90~75:25; the Vickers hardness of the composite material is 50~245 GPa, and the fracture toughness is 4~20 MPa·m. 1 / 2 The phase composition is mainly composed of MD and cubic diamond, with a small amount of hexagonal diamond, of which the content of hexagonal diamond is ≤5wt%. There are no obvious graphite impurities, of which the content of graphite phase is ≤1wt%. The density is ≥3.45g / cm³. 3 Porosity ≤2%.

2. The micro-nano polycrystalline diamond composite material according to claim 1, characterized in that: The OLC is composed of carbon and has a nano-onion structure. Based on its core structure, it is divided into OLC with a diamond crystal core and OLC without a diamond core. The outermost layer of the OLC with a diamond crystal core is graphite sheet, single-layer graphene, or several layers of graphene, and the core has a diamond core. The OLC without a diamond core is composed entirely of graphite sheets. Its average grain size is 5nm±0.5nm, crystal structure integrity is ≥90%, there is no obvious lattice distortion, carbon content is ≥99.5%, and it contains no metal impurities or the metal impurity content is ≤0.1wt%. The preparation method is selected from vacuum annealing, arc discharge, pyrolysis, laser ablation, and catalytic chemical vapor deposition.

3. The micro-nano polycrystalline diamond composite material according to claim 1, characterized in that: The MD has a structure similar to cubic diamond, with B and N atoms arranged alternately in plane within each layer. The interlayer packing pattern is ABC type. Its grain size is 0.1~100μm, and the grain morphology is spherical or polyhedral. The average grain size is 0.5~100μm. It has good particle dispersion, with agglomerate size ≤500μm and agglomerate proportion ≤5wt%. The purity is ≥99%, with the molar ratio of B to N atoms being 1:1±0.05, and the content of impurity elements (excluding O and C) ≤0.5wt%.

4. A method for preparing the micro-nano polycrystalline diamond composite material as described in any one of claims 1 to 3, characterized in that: The process includes OLC preparation, mixing and pre-pressing, high-temperature and high-pressure sintering, and post-treatment steps. OLC preparation uses nanodiamond with an average grain size of 5 nm ± 0.5 nm, prepared by detonation, as raw material, under a vacuum of 1.0 × 10⁻⁶. -1 Pa±5×10 -2 Under Pa conditions, the sample is annealed at a heating rate of 5~20℃ / min to 900~1600℃. For mixing and pre-pressing, MD powder and the prepared OLC are mixed uniformly at a mass ratio of 10:90~75:25, loaded into a cemented carbide mold, and subjected to a pressure of 400~600MPa for 30~60s. For high-temperature and high-pressure sintering, the pre-pressed sample is loaded into an assembly module, and pressure is slowly applied to 5~40GPa over 0.1~20h. Then, the temperature is increased to 1200~3000℃ at a heating rate of 10℃ / min±2℃ / min, held for 0~60min, and then cooled and unpressed to obtain a blank. Post-treatment involves surface grinding and deburring of the blank.

5. The preparation method according to claim 4, characterized in that: In step S3, the assembly process of the assembly module is as follows: when the sintering pressure is 5~7 GPa, the pre-pressed sample is placed in a BN crucible with a purity ≥99%, ensuring good contact between the crucible and the salt tube made of sodium chloride or potassium chloride material, and then the crucible is placed in a container with a density ≥1.8 g / cm³. 3 Graphite gaskets and conductive steel caps were added to the graphite column, and the entire assembly was embedded in pyrophyllite with a water content ≤0.5wt%. When the sintering pressure was 8~40GPa, the pre-pressed sample was placed in a BN crucible with a purity ≥99%, ensuring good contact between the crucible and a rhenium sheet with a thickness of 0.1~0.5mm. The crucible was then placed in a LaCrO3 insulator with a porosity ≤5%, and the entire assembly was placed inside an MgO octahedron with a purity ≥99.5%. An Al2O3 plug with a density ≥95% was added, and a four-hole tube with electrodes inserted in series was placed on the plug. The module was then sealed with Al2O3 adhesive, and the airtightness of the sealed module was ≤1×10⁻⁶. -3 Pa・s.

6. The preparation method according to claim 4, characterized in that: In step S3, the pressure fluctuation during the pressure application process shall not exceed ±0.5 GPa; the temperature fluctuation during the heating process shall not exceed ±50℃; during the heat preservation stage, both pressure and temperature shall remain stable, with fluctuations not exceeding ±0.3 GPa and ±30℃, respectively.

7. The preparation method according to claim 4, characterized in that: In step S2, the mixing method is one or more combinations of mechanical grinding, planetary ball milling, or high-energy ball milling, and the mixing time is 10-60 min. When mechanical grinding is used, the grinding speed is 300-800 r / min. When planetary ball milling is used, the ball-to-material ratio is 5:1-20:1, and the speed is 200-600 r / min. Anhydrous ethanol can be used as a dispersant during the mixing process, wherein the mass ratio of the dispersant to the mixture is 1:1-3:1, or dry mixing without a dispersant is used. When dry mixing, an inert gas (N2 or Ar) must be introduced for protection.

8. The preparation method according to claim 4, characterized in that: In step S3, the cooling rate of the cooling process is 5~50℃ / min, and the pressure is released after cooling to room temperature, with a pressure release rate of 0.1~2GPa / h; in step S4, the surface grinding is carried out by diamond grinding wheel grinding with a grinding accuracy of ±0.01mm, and the deburring is carried out by ultrasonic cleaning or mechanical polishing.

9. The micro-nano polycrystalline diamond composite material according to claim 1, characterized in that: When the annealing temperature is 900~1350℃, an OLC containing a diamond core is obtained; when the annealing temperature is 1400~1600℃, an OLC without a diamond core is obtained.

10. The micro-nano polycrystalline diamond composite material according to claim 1, characterized in that: After the OLC and MD are mixed, they are pre-pressed under a pressure of 400~600MPa for 30~60s. The pre-pressed sample is then sintered at high temperature and high pressure in the assembly module to form a blank, and then post-processed to obtain the finished product.

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