Fine polishing process for large-scale polycrystalline diamond block material synthesized by direct conversion method, polycrystalline diamond product and application of polycrystalline diamond product

Abstract

The invention belongs to the technical field of precision machining of superhard materials, and particularly relates to a fine polishing process of a large-scale polycrystalline diamond block synthesized by a direct conversion method, a polycrystalline diamond product and application of the polycrystalline diamond product. The fine polishing process comprises the steps of material drawing through laser cutting, cooperative cooling rough polishing, diamond micro powder fine polishing and low-temperature heat treatment destressing. According to the process, mechanical vibration and local overheating can be restrained through three-in-one treatment of a ceramic bond polishing disc, forced water cooling and polishing solution collaborative supply and equipment dynamic balance, and stress introduction is reduced from the source; accumulated microscopic internal stress can be released through low-temperature heat treatment, and the structural integrity and long-term service reliability of the product are guaranteed. The considerable material removal rate can be guaranteed on the premise of low stress through graded polishing, and the total machining time is shortened; the yield is high, and the material loss and the comprehensive cost are low. The polycrystalline diamond product is high in dimensional precision, surface quality and internal reliability, and can be stably applied to the frontier fields of high-performance numerical control tools, ultra-precise optical elements and the like.

Description

A direct conversion method for fine polishing large-scale polycrystalline diamond blocks, polycrystalline diamond products and their applications Technical Field

[0001] This invention belongs to the field of precision machining technology of superhard materials, specifically relating to a fine polishing process for large-scale polycrystalline diamond blocks synthesized by a direct conversion method, as well as high-surface-quality polycrystalline diamond products prepared by this process and their applications. Background Technology

[0002] Superhard materials are the cornerstone of modern industrial development, playing an irreplaceable role in precision machining, oil and gas drilling, optical devices, and defense technology. Among many superhard materials, diamond stands out for its extremely high hardness, excellent thermal conductivity, wide optical transmission band, and outstanding chemical stability. However, natural diamond single crystals are expensive, limited in size, and exhibit significant cleavage planes and anisotropy, making them prone to brittle fracture along cleavage planes during processing and use, thus limiting their application range. To overcome these shortcomings, synthetic polycrystalline diamond sintered bodies (PCD) have emerged, formed by sintering diamond micropowder under high temperature and pressure using a binder (such as cobalt). Although isotropic and with improved toughness, the presence of a metallic binder results in poor thermal stability (typically below 700℃), making it prone to softening and oxidation at high temperatures. Furthermore, its hardness and wear resistance are lower than those of single-crystal diamond.

[0003] In recent years, a breakthrough has been achieved in the synthesis of a novel polycrystalline diamond material via a direct conversion method. This material is a bulk material composed of nanoscale diamond grains, synthesized under ultra-high pressure (14–18 GPa) and ultra-high temperature (2300–2600 K) conditions, using non-diamond carbon (such as glassy carbon, highly ordered pyrolytic graphite, etc.) as a precursor, without any catalysts or binders, through direct phase transformation. Its grain size is typically between 10 and 100 nanometers, exhibiting a single phase and uniform structure. This unique nanocrystalline structure endows it with a series of outstanding properties: First, its Knoop hardness is as high as 120~140 GPa, exceeding the hardest (100) and (001) faces of single-crystal diamond (about 115 GPa), making it the hardest material known to mankind; Second, since it is composed entirely of covalently bonded nanodiamond grains and there is no grain boundary phase, it has extremely high thermal stability and can withstand temperatures above 1000℃ in atmospheric conditions without graphitization or oxidation; Third, its isotropic properties completely avoid the cleavage cracking problem of single-crystal diamond, and its toughness is significantly improved.

[0004] Crucially, this nanocrystalline polycrystalline diamond exhibits a unique "self-sharpening" property during cutting or grinding. Because the grains are connected by unsaturated bonds, under processing stress, the tiny nanocrystals can automatically peel off, continuously exposing new, sharp microscopic cutting edges. This self-sharpening effect ensures consistently high grinding rates and low cutting forces, while minimizing microscopic damage to the working surface—ideal characteristics for ultra-precision machining. Based on these advantages, polycrystalline diamond synthesized via the direct conversion method is considered a next-generation superhard material to replace traditional PCD, showing enormous application potential in high-end CNC machine tool cutting tools, ultra-precision optical windows, high-wear-resistant cutting tools, and precision instrument probes.

[0005] However, its extremely high hardness, wear resistance and isotropy, while giving it excellent performance, also make its own finishing - especially the precision polishing of large-scale blocks - a huge bottleneck restricting its industrial application. Traditional mechanical polishing methods are inadequate when facing this material: (1) extremely low polishing efficiency: using conventional diamond grinding discs, the material removal rate is very low, and it may take tens or even hundreds of hours to process a plane with a thickness of millimeters, which is very costly.

[0006] (2) Extremely easy to introduce and accumulate stress: During the polishing process, the huge mechanical pressure and local high temperature generated by friction will generate significant mechanical and thermal stress inside the material. If these stresses cannot be effectively dissipated, microcracks will be generated inside the material, and even macroscopic fracture (fracture) will occur during the processing. These microcracks and residual stresses are not only potential fracture sources, which seriously affect the reliability of the tool, but also degrade its optical performance and reduce its light transmittance.

[0007] (3) Difficulty in obtaining high surface finish: Due to its nanocrystalline structure, if the polishing process is not proper, the grains are easily pulled out as a whole, forming “pits” or “scratches” on the surface, rather than achieving a smooth mirror surface, making it difficult to achieve the ultra-precision requirement of Ra<5nm for surface roughness.

[0008] (4) Limited application: The above-mentioned processing problems directly lead to the instability of the final product performance and the limitation of its application. For example, tools with excessive internal stress are at risk of chipping during high-speed intermittent cutting; optical windows with substandard surface finish will introduce light scattering, affecting the imaging or laser transmission quality.

[0009] While existing technologies have attempted to use diamond abrasives of different grit sizes for progressive polishing, they generally neglect the core challenges of stress control and thermal management during the polishing process. Conventional polishing equipment often lacks effective synergistic cooling systems and precise dynamic balance control, leading to instability, severe localized overheating, and vibration impacts, failing to meet the processing requirements of this top-tier material. Therefore, developing a fine polishing process that balances high efficiency, low stress, high surface finish, and low wear is the key technology to unlocking the full application potential of direct conversion polycrystalline diamond. Summary of the Invention

[0010] Therefore, the purpose of this invention is to address the technical problems encountered in the fine polishing process of large-scale polycrystalline diamond blocks synthesized by the direct conversion method, such as low efficiency, high stress, difficulty in ensuring surface finish, and susceptibility to cracking, by providing a novel and systematic fine polishing process. This process, through specific equipment combinations and coordinated process parameters, aims to achieve a polishing effect with high efficiency, low stress, and high surface quality.

[0011] To achieve the above objectives, the present invention adopts the following technical solution: The present invention provides a fine polishing process for large-scale polycrystalline diamond blocks synthesized by direct conversion method, comprising the following steps: S1, cutting treatment: cutting the large-scale polycrystalline diamond blocks synthesized by direct conversion method into diamond sheets; S2, rough polishing treatment: using a ceramic binder polishing pad and a synergistic polishing fluid to perform rough polishing treatment on the diamond sheets described in S1 to obtain rough polished diamond sheets; S3, fine polishing treatment: using a non-woven fabric or polyurethane polishing pad and diamond micro powder abrasive fluid to perform fine polishing treatment on the rough polished diamond sheets described in S2 to obtain fine polished diamond sheets; S4, heat treatment: heat treating the fine polished diamond sheets described in S3 under an inert atmosphere, and obtaining polycrystalline diamond products after cooling.

[0012] Furthermore, a picosecond or femtosecond laser is used to complete the cutting process described in S1.

[0013] Furthermore, the ceramic-bonded polishing disc in S2 has a mesh size of 350-450 mesh, and the diamond concentration in the ceramic-bonded polishing disc is 75-85 wt%.

[0014] Furthermore, the rough polishing process described in S2 is performed on a polishing machine equipped with a forced water cooling system and a polishing slurry supply system; the spindle of the polishing machine undergoes dynamic balancing correction; the coolant provided by the forced water cooling system works synergistically with the polishing slurry provided by the polishing slurry system.

[0015] Further, the synergistic polishing fluid in S2 is a mixture of coolant and polishing fluid, wherein the volume ratio of coolant to polishing fluid in the mixture is 1~3:1; the coolant includes cooling water; the polishing fluid is an aqueous solution containing a dispersing lubricant, a corrosion inhibitor, and a pH adjuster; the dispersing lubricant includes sodium polyacrylate, and the corrosion inhibitor includes benzotriazole; the amount of the dispersing lubricant added to the polishing fluid is 0.5~2.0 wt%; the amount of the corrosion inhibitor added is 0.1~0.5 wt%; the pH adjuster includes potassium hydroxide, and the pH of the polishing fluid is adjusted to 8~10.

[0016] Furthermore, in S2, the total flow rate of the synergistic polishing fluid is 100~500mL / min; the pressure of the rough polishing treatment is 0.05~0.2MPa, the polishing disc speed is 200~600rpm, and the polishing time is 30±5 minutes.

[0017] Furthermore, in S3, the diamond micron powder in the diamond micron powder polishing slurry has a particle size of 1~9μm and a concentration of 5~15wt%; the supply flow rate of the diamond micron powder polishing slurry is 50~200 mL / min.

[0018] Furthermore, the pressure of the fine polishing process described in S3 is 0.02~0.1MPa, the polishing disc speed is 100~300rpm, and the polishing time is 60±5 minutes.

[0019] Furthermore, the surface roughness Ra of the polished diamond sheet described in S3 is < 5 nm.

[0020] Furthermore, the heating rate of the heat treatment in S4 is 2~5℃ / min, the temperature is 250~400℃, and the time is 1~4 hours; the cooling temperature is ≤60℃; the inert gas used in the inert atmosphere includes argon or nitrogen.

[0021] The present invention further provides a polycrystalline diamond product prepared by a fine polishing process of a large-scale polycrystalline diamond block synthesized by the above-mentioned direct conversion method.

[0022] Furthermore, the polycrystalline diamond product has a surface roughness Ra < 5 nm and a Vickers hardness > 130 GPa; it has a smooth surface, good light transmittance, low internal residual stress, and no macroscopic or microscopic cracks.

[0023] The present invention further provides an application of the above-mentioned polycrystalline diamond products in high-end CNC machine tool special tools, ultra-precision optical windows, high wear-resistant cutting tools or precision detection instrument probes.

[0024] The beneficial effects of this invention are as follows: 1. The fine polishing process provided by this invention can effectively avoid chipping and reduce internal stress: Through the integrated technical solution of "ceramic binder polishing disc + forced water cooling and polishing fluid synergistic supply + equipment dynamic balancing", mechanical vibration and local overheating are greatly suppressed, reducing stress introduction from the source. Subsequent low-temperature heat treatment can safely and effectively release the accumulated microscopic internal stress, ensuring the structural integrity and long-term service reliability of the product.

[0025] 2. The fine polishing process provided by this invention can achieve ultra-high surface smoothness: adopting a graded polishing strategy, the coarse polishing is efficient in shaping, and the fine polishing uses micron-level diamond powder to achieve nano-level material removal and surface smoothing, and finally obtains an ultra-smooth mirror surface with Ra<5nm, which meets the most demanding optical and application requirements.

[0026] 3. The fine polishing process provided by this invention can achieve high polishing efficiency and low wear: the optimized coarse polishing parameters ensure a considerable material removal rate under low stress conditions, shortening the total processing time; the overall process has a high yield, low material loss, and significantly better overall cost than traditional methods.

[0027] 4. The polycrystalline diamond products provided by this invention have greatly improved dimensional accuracy, surface quality and intrinsic reliability, and can be stably applied in cutting-edge fields such as high-performance CNC cutting tools and ultra-precision optical components. Attached Figure Description

[0028] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 is a flowchart of the fine polishing process used in Example 1 of the present invention; Figure 2 is a microscopic image of the surface morphology of the product after fine polishing in Example 1 of the present invention; Figure 3 is a microscopic image of the microcracks appearing on the surface of the product after polishing in Comparative Example 1 of the present invention; Figure 4 is a comparison image of the products of Example 1 and Comparative Example 1 under a polarizing stress meter (left is the example, right is the comparative example). Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments.

[0031] The following embodiments provide detailed implementation procedures for the technical solutions of the present invention. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0032] Example 1: A polycrystalline diamond block with dimensions of [50mm×50mm×2mm] synthesized by direct conversion method was taken and processed according to the process shown in Figure 1, specifically including the following steps: (1) Laser cutting of material: The plate was precisely cut into test pieces of [10mm×10mm×2mm] using a femtosecond laser cutting system. The laser processing parameters were optimized to minimize the heat-affected zone.

[0033] (2) Co-cooling rough polishing: Equipment: A high-precision surface polishing machine (model: Logitech PM5 or similar) is used. The equipment parameters are adjusted so that the radial runout of the spindle is ≤ 1μm, and it undergoes precision dynamic balancing correction of G0.4 grade or above to suppress mechanical vibration to the greatest extent. The equipment is equipped with an independent Teflon or stainless steel forced water cooling circulation system (temperature control accuracy ±1°C), and an independently adjustable dual-path (coolant and polishing fluid) polishing fluid delivery and mixing system driven by a high-precision peristaltic pump or metering pump (flow control accuracy ±2%).

[0034] Tools and parameters: Install a 400-mesh (approximately 35μm) ceramic-bonded diamond polishing disc with an 80% diamond concentration. Set the polishing pressure to 0.1MPa and the disc rotation speed to 400rpm.

[0035] Cooling and Lubrication: Turn on the water cooling system (water temperature 20℃) and the polishing slurry supply. The polishing slurry is an aqueous solution, its components are: deionized water as the base liquid, containing 1.5 wt% sodium polyacrylate (as a dispersant and lubricant), 0.3 wt% benzotriazole (as a corrosion inhibitor), and potassium hydroxide to adjust the pH value to 9.0~10.0. The two are mixed at a volume ratio of 2:1 and continuously supplied to the polishing interface at a total flow rate of 300 mL / min.

[0036] Process: The rough polishing lasts for 30 minutes, with the aim of quickly removing the surface damage layer caused by laser cutting and initially smoothing the surface.

[0037] (3) Diamond micro-powder fine polishing: Replace the polishing disc: Replace the ceramic diamond disc with a porous polyurethane polishing disc.

[0038] Polishing agent: A polishing slurry prepared with diamond micro powder with a particle size of 3μm and deionized water at a concentration of 10% (by weight).

[0039] Parameters: Set the polishing pressure to a low level of 0.05 MPa and the polishing disc speed to 150 rpm.

[0040] Process: Polishing time is 60 minutes until the surface achieves a mirror effect.

[0041] (4) Low-temperature heat treatment to relieve stress: Equipment: Place the polished sample into a tube furnace filled with high-purity argon gas.

[0042] Process: Heat from room temperature to 300℃ at a slow heating rate of 3℃ / min, hold at this temperature for 2 hours, then turn off the power and allow the sample to cool naturally with the furnace to below 60℃ before taking it out.

[0043] Implementation effect test of Example 1: (1) Surface morphology: The surface roughness Ra value was 3.2 nm as measured by white light interferometer. Under scanning electron microscope (SEM), the surface was uniform and flat, without scratches, pits or other defects (Figure 2).

[0044] (2) Internal stress: When observed with a polarizing stress meter, the color was uniform throughout the entire field of view of the sample, and there were no obvious stress stripes, indicating that the internal residual stress was extremely low (Figure 4, left).

[0045] (3) Integrity: No macro or micro cracks were found under optical microscope and SEM.

[0046] (4) Polishing efficiency: The material removal rate in the rough polishing stage is approximately 1.5 μm / min.

[0047] Comparative Example 1 (using conventional equipment without effective cooling and lubrication for comparison): Polycrystalline diamond blocks of the same batch and size [10mm×10mm×2mm] were used.

[0048] (1) Laser cutting material acquisition: Same as in Example 1.

[0049] (2) Rough polishing: An old-fashioned, simple surface grinding and polishing machine (model: MTI Corporation PG-1A basic model or similar early model) was used. This model did not have a built-in forced circulation cooling system, the spindle had a simple bearing structure, and the dynamic balance accuracy was not marked. The measured radial runout was usually >10μm. The polishing slurry had to be manually and intermittently dripped onto the polishing disc by the operator using a dropper bottle, which made it impossible to achieve precise control of the flow rate and mixing ratio.

[0050] Tools and parameters: Use a 400-mesh, 80% ceramic-bonded diamond polishing disc. Set the polishing pressure to 0.1 MPa and the rotation speed to 400 rpm.

[0051] Cooling and lubrication: Relying solely on the intermittent addition of small amounts of water-based polishing fluid by the operator is insufficient to form a stable cooling and lubrication film.

[0052] Procedure: Approximately 5 minutes after polishing begins, noticeable friction noise and vibration can be heard. At 15 minutes of polishing, the sample emits a crisp sound and macroscopically cracks (breaks in two). Replace with a new sample and shorten the polishing time to 10 minutes, then stop immediately.

[0053] (3) Test samples that are not broken but have been improperly polished as described above.

[0054] Implementation effect test of Comparative Example 1: (1) Surface morphology (cracks and defects): More than 50% of the samples experienced macroscopic fracture during polishing. The unbroken samples showed dense surface microcracks under a microscope (Figure 3).

[0055] (2) Internal stress: When observed under a polarized stress meter, strong and disordered colored stress stripes can be seen, indicating that the internal residual stress is huge and unevenly distributed (Figure 4, right).

[0056] (3) Surface smoothness: The surface roughness Ra value measured by white light interferometer is as high as 45nm. There are a lot of deep scratches and pits formed by grain pull-out on the surface, and the smoothness is extremely poor.

[0057] (4) Polishing efficiency: Due to vibration and overheating, the actual effective material removal is not stable, and due to frequent cracking and defective products, the overall polishing efficiency is extremely low and the cost is high.

[0058] In summary, the product obtained in Comparative Example 1 is of poor quality and completely unsuitable for use in high-performance cutting tools (microcracks lead to an extremely high risk of chipping) or optical windows (high roughness and stress cause severe light scattering), thus losing its application value. The polycrystalline diamond product obtained in Example 1 has a surface roughness Ra < 5 nm and a Vickers hardness > 130 GPa; it has a smooth surface, good light transmittance, low internal residual stress, and no macroscopic or microscopic cracks. It can be widely used in high-end CNC machine tool cutting tools, ultra-precision optical windows, high wear-resistant cutting tools, or precision detection instrument probes.

[0059] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A fine polishing process for large-scale polycrystalline diamond blocks synthesized by direct conversion method, characterized in that, Includes the following steps: S1, Cutting process: Cutting the large-scale polycrystalline diamond block synthesized by the direct conversion method into diamond sheets; S2, Rough polishing process: Rough polishing the diamond sheets described in S1 using a ceramic binder polishing pad and a synergistic polishing slurry to obtain rough polished diamond sheets; S3, Fine polishing process: Fine polishing the rough polished diamond sheets described in S2 using a non-woven fabric or polyurethane polishing pad and diamond micro powder abrasive slurry to obtain fine polished diamond sheets. S4： Heat treatment: The polished diamond sheet described in S3 is heat-treated in an inert atmosphere and then cooled to obtain a polycrystalline diamond product.

2. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The cutting process described in S1 is performed using a picosecond or femtosecond laser.

3. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The ceramic-bonded polishing disc in S2 has a mesh size of 350-450 mesh, and the diamond concentration in the ceramic-bonded polishing disc is 75-85 wt%.

4. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The synergistic polishing fluid in S2 is a mixture of coolant and polishing fluid, wherein the volume ratio of coolant to polishing fluid in the mixture is 1~3:1; the coolant includes cooling water; the polishing fluid is an aqueous solution containing a dispersing lubricant, a corrosion inhibitor, and a pH adjuster; the dispersing lubricant includes sodium polyacrylate, and the corrosion inhibitor includes benzotriazole; the amount of the dispersing lubricant added to the polishing fluid is 0.5~2.0 wt%; the amount of the corrosion inhibitor added is 0.1~0.5 wt%; the pH adjuster includes potassium hydroxide, and the pH of the polishing fluid is adjusted to 8~10.

5. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The total flow rate of the synergistic polishing fluid in S2 is 100~500mL / min; the pressure of the rough polishing treatment is 0.05~0.2MPa, the polishing disc speed is 200~600rpm, and the polishing time is 30±5 minutes.

6. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The diamond micron powder in the diamond micron powder polishing slurry in S3 has a particle size of 1~9μm and a concentration of 5~15 wt%; the flow rate of the diamond micron powder polishing slurry is 50~200 mL / min.

7. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The fine polishing process described in S3 has a pressure of 0.02~0.1MPa, a polishing disc rotation speed of 100~300rpm, and a polishing time of 60±5 minutes.

8. The fine polishing process for large-scale polycrystalline diamond blocks synthesized by the direct conversion method as described in claim 1, characterized in that, The heating rate of the heat treatment in S4 is 2~5℃ / min, the temperature is 250~400℃, and the time is 1~4 hours; the cooling temperature is ≤60℃.

9. A polycrystalline diamond product prepared by a fine polishing process of large-scale polycrystalline diamond blocks synthesized using the direct conversion method described in any one of claims 1 to 8.

10. The application of the polycrystalline diamond product of claim 9 in high-end CNC machine tool cutting tools, ultra-precision optical windows, high wear-resistant cutting tools or precision detection instrument probes.

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