Machining method for inner surface and outer surface of polycrystalline diamond

Abstract

A polycrystalline diamond inner and outer surface processing method belongs to the technical field of optical element manufacturing, and comprises the following steps: processing a silicon-based substrate with the outer surface having a predetermined spherical curvature, the outer surface precision and surface roughness meeting the requirements of the inner surface of the processed polycrystalline diamond; growing a polycrystalline diamond layer on the outer surface of the silicon-based substrate to form a composite structure body, and enabling the inner surface of the polycrystalline diamond layer to directly copy the spherical morphology of the outer surface of the silicon-based substrate; multi-stage step-by-step machining is conducted, rough grinding, laser and accurate grinding combined machining or laser and accurate grinding combined machining is conducted on the outer surface of the polycrystalline diamond layer, and therefore the quality of the outer surface is improved; removing the silicon-based substrate to obtain a complete polycrystalline diamond; and cleaning the inner and outer surfaces of the polycrystalline diamond, and detecting the roughness, surface shape precision and wall thickness uniformity of the inner and outer surfaces to meet the process requirements, thereby finishing the machining of the inner and outer surfaces of the polycrystalline diamond. According to the device, efficient removal, low damage and high-precision inner and outer surface forming are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical element manufacturing, and in particular to a method for processing the inner and outer surfaces of polycrystalline diamond. Background Art

[0002] Polycrystalline diamond (PCD) is a material composed of numerous tiny nano- or micron-sized diamond particles bonded together by unsaturated bonds. Its crystal structure is uneven, with numerous grain boundaries. As a result, its hardness and thermal conductivity are lower than those of single-crystal diamond, but it possesses higher toughness and wear resistance. PCD possesses excellent physical and optical properties, which give it broad potential for industrial and optical applications. In the field of optics, the main advantages of polycrystalline diamond are reflected in the following aspects: (1) High transparency and low absorption: Polycrystalline diamond has high transparency in a wide band, with good transmittance from the LWIR (long-wave infrared, 8-12μm) and millimeter wave (radar wave) regions, making it an ideal material for optical windows, lenses and filters; (2) Excellent thermal conductivity and extremely low thermal expansion coefficient: Diamond has extremely high thermal conductivity (20 W / cm·K at room temperature, almost 5 times that of Cu); (3) Adaptability to extreme environments: Its hardness (Mohs hardness 10) and chemical stability make it resistant to high temperature, high pressure, strong corrosion and particle impact environments. Therefore, optical-grade diamond is considered to be an irreplaceable material in the field of missile head covers and optical windows under extreme conditions.

[0003] PCD offers significant advantages over single-crystal diamond: First, its manufacturing cost is relatively low, making it more economical. Second, it can be fabricated in large, complex shapes using techniques such as chemical vapor deposition (CVD), allowing for controllable fabrication and good dimensional flexibility. Third, its polycrystalline structure imparts isotropy to the material, eliminating the cleavage planes characteristic of single-crystal diamond. This makes it less susceptible to catastrophic fracture along specific directions when subjected to stress, and its mechanical behavior is more stable and controllable. However, these very advantages also present extremely challenging ultra-precision machining challenges. The extreme hardness (approaching 100 GPa) and inherent brittleness of PCD are key challenges in its machining.

[0004] Currently, the main limitations of precision / ultra-precision machining of polycrystalline diamond (PCD) spherical optics are as follows:

[0005] 1. While mechanical lapping / grinding / polishing methods can achieve a certain degree of precision, they are limited by the extremely high hardness of PCD, resulting in low material removal efficiency and severe wear of machining tools (such as diamond grinding wheels). When machining curved surfaces (especially complex spherical surfaces), precise control of contour accuracy is difficult, and microcracks and deep damage in the subsurface layer of the material are inevitably introduced, directly affecting the fatigue strength and ultimate optical performance of the optical component.

[0006] 2. While laser processing (e.g., ablation) significantly increases material removal rates, its thermal effects are significant, resulting in a pronounced heat-affected zone (HAZ), a melt recast layer, and microcracks in the processed area. This results in high surface roughness and poor quality, making it difficult to directly meet the stringent requirements of optical-grade surfaces. Repairs are often still required through time-consuming subsequent precision processes.

[0007] 3. Although precision methods such as focused ion beam (FIB) and plasma etching can achieve extremely high local processing accuracy, their material removal rate is extremely low and the equipment and operating costs are extremely high. They are essentially not suitable for large-scale workpieces or mass industrial production needs.

[0008] Crucially, for optical components with inner and outer spherical surfaces, existing processes present fundamental challenges in achieving high-precision coordinated machining of these surfaces. Traditional methods typically require independent machining of the inner and outer surfaces. This process inevitably involves multiple datum conversions, resulting in significant cumulative errors. Consequently, ensuring critical geometric parameters such as concentricity between the inner and outer spherical surfaces and wall thickness uniformity is extremely difficult, severely limiting the reliability and optical performance consistency of high-performance PCD optical windows. Summary of the Invention

[0009] The present invention overcomes the shortcomings of the prior art and provides a method for processing the inner and outer surfaces of polycrystalline diamond. The method achieves efficient removal, low damage, and high-precision inner and outer surface shaping.

[0010] A method for processing the inner and outer surfaces of polycrystalline diamond comprises the following steps: S1. Processing a silicon-based substrate with a predetermined spherical curvature on its outer surface, wherein the outer surface accuracy and surface roughness meet the requirements of the inner surface of the polycrystalline diamond after processing;

[0011] S2. growing a polycrystalline diamond layer on the outer surface of the silicon-based substrate to form a composite structure, so that the inner surface of the polycrystalline diamond layer directly replicates the spherical morphology of the outer surface of the silicon-based substrate;

[0012] S3, multi-step processing, using rough grinding, laser and fine grinding combination processing or laser and fine grinding combination processing of the outer surface of the polycrystalline diamond layer obtained in step S2 to improve the outer surface quality;

[0013] S4, removing the silicon-based substrate to obtain a complete polycrystalline diamond;

[0014] S5, cleaning the inner and outer surfaces of the polycrystalline diamond in step S4, and testing the inner and outer surface roughness, surface accuracy and wall thickness uniformity thereof to meet the process requirements and complete the inner and outer surface processing of the polycrystalline diamond.

[0015] Furthermore, if there are no defects or cracks on the outer surface of the polycrystalline diamond layer before processing, the following step-by-step processing method is adopted, and the specific process of step S3 is: a. Rough grinding: Use a coarse-grained diamond grinding wheel to remove the particles so that the outer surface of the polycrystalline diamond approaches the outer spherical profile required by the process;

[0016] b. Laser processing: Use pulsed laser to scan the outer surface of the polycrystalline diamond after rough grinding to eliminate or reduce the subsurface damage layer introduced by rough grinding;

[0017] c. Fine grinding: Use fine-grained diamond grinding wheels to grind the outer surface of polycrystalline diamond, eliminate the subsurface damage layer, and refine the outer spherical surface profile to achieve nanometer-level surface roughness and submicron-level surface accuracy.

[0018] Furthermore, if there are defects or cracks on the outer surface of the polycrystalline diamond layer before processing, the following step-by-step processing method is adopted, and the specific process of step S3 is:

[0019] b. Laser processing: Use pulsed laser to scan and process the outer surface of polycrystalline diamond to quickly remove excess and reduce new damage or expansion of existing microcracks induced by the fine grinding process;

[0020] c. Fine grinding: Use a fine-grained diamond grinding wheel to grind the outer surface of the polycrystalline diamond, eliminate the damaged layer, and refine the outer spherical surface contour to achieve nanometer-level surface roughness and submicron-level surface accuracy.

[0021] Furthermore, the specific process of cleaning and testing in step S5 is as follows:

[0022] After removing the silicon substrate, the inner and outer surfaces of the polycrystalline diamond are initially rinsed, and then the inner and outer surfaces are respectively soaked and cleaned with dilute hydrochloric acid solution and diluted ammonia and hydrogen peroxide mixture for acidic and alkaline cleaning. Rinsing is required before and after cleaning with different solutions. The chemical inertness of diamond is used to remove chemical residues and particles or sediments that may be attached to the surface. After cleaning, the inner and outer surface roughness, surface accuracy and wall thickness uniformity of the polycrystalline diamond are tested.

[0023] Furthermore, the surface accuracy of the outer surface of the silicon-based substrate is PV<0.5μm, and the surface roughness is Ra<5nm.

[0024] Furthermore, the grit size range of the diamond grinding wheel for rough grinding is #100-#170, and the grit size range of the diamond grinding wheel for fine grinding is #500-#2000.

[0025] The beneficial effects of the present invention compared to the prior art are:

[0026] 1. This application is a method for integrally processing inner and outer surfaces, and the processing method is efficient and controllable.

[0027] 2. The method of this application achieves a balance between efficient removal of outer surface material and final ultra-precision forming (high surface accuracy, nanometer-level roughness); and realizes coordinated and precise control of the geometric accuracy of the inner and outer surfaces (such as concentricity and wall thickness uniformity).

[0028] 3. Effectively solve the problems of low efficiency in pure mechanical processing and poor surface quality in pure laser processing, and give full play to their respective advantages through combination;

[0029] 4. Using a high-precision silicon substrate as a "mold," a high-precision inner surface is directly obtained through CVD growth, eliminating the most difficult step of machining the inner surface of the diamond sphere. Using the inner surface as a reference for machining the outer surface makes it easier to ensure the concentricity and wall thickness uniformity of the inner and outer spheres.

[0030] 5. Targeted removal of subsurface damage layers through laser processing steps, and finally combined with fine grinding to eliminate damage and obtain optical-grade subsurface quality.

[0031] The following is a further description of the scheme of the application in conjunction with the accompanying drawings and embodiments: BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a process flow chart of the polycrystalline diamond internal and external surface processing method of the present application;

[0033] Figure 2 Schematic diagram of multi-stage step-by-step processing. DETAILED DESCRIPTION

[0034] The embodiments of the technical solution of the present invention will be described in detail below with reference to the accompanying drawings. Unless otherwise specified, the technical terms or scientific terms used in this application have the common meanings understood by those skilled in the art.

[0035] Example 1. This embodiment provides a polycrystalline diamond spherical surface precision / ultra-precision machining method based on multi-stage combined machining of preformed high-precision inner and outer surfaces. It is a process method that combines the preformed inner surface reference with the outer surface multi-stage removal process to achieve overall high-precision forming, realizing efficient removal, low damage, and high-precision inner and outer surface forming.

[0036] Step 1: Processing a silicon-based substrate with a predetermined spherical curvature, wherein the outer surface of the substrate is a high-precision optical spherical surface, and its accuracy and surface roughness meet the requirements of the final diamond inner surface;

[0037] Step 2: Growing a polycrystalline diamond layer on the outer surface of the silicon-based substrate. A polycrystalline diamond layer is grown on the high-precision silicon spherical substrate by chemical vapor deposition to form a composite structure. The inner surface of the polycrystalline diamond layer (the surface in contact with the silicon substrate) directly replicates the spherical morphology of the silicon substrate, thus meeting the inner surface dimensional accuracy requirements without the need for additional processing.

[0038] Step 3: Multi-level processing of the outer surface, such as Figure 2 As shown in the figure, Vms represents the swing rate of the silicon-based substrate. The diamond outer surface obtained in step 2 is processed by a combination of the following three processes. The process sequence can be adjusted according to the initial margin and processing accuracy. The silicon-based substrate is retained as the clamping support for processing the outer surface of the polycrystalline diamond spherical surface to improve the support rigidity of the polycrystalline diamond during the processing of the outer surface;

[0039] a. Rough grinding: Use a coarse-grained diamond grinding wheel for efficient material removal, so that the outer surface of the diamond quickly approaches the target outer spherical contour and removes most of the excess, but it will introduce a deeper subsurface damage layer and more surface defects;

[0040] b. Laser processing: Pulsed laser is used to scan and process the outer surface after rough grinding. Laser processing can eliminate or reduce the subsurface damage layer introduced by rough grinding. The high energy density of pulsed laser can accurately and layer by layer ablate the damaged layer containing high residual stress left by rough grinding, greatly reducing the risk of inducing new serious damage or causing the expansion of existing microcracks during the fine grinding process, and obtaining a more stable and controllable fine grinding process. That is, laser processing effectively cuts off the transmission path of the damaged layer to the final workpiece.

[0041] c. Fine Grinding: A fine-grained diamond grinding wheel is used to grind the polycrystalline diamond to remove the recast layer, microcracks, and heat-affected zone that may be generated by laser processing, eliminate the subsurface damage layer, and refine the outer spherical profile to achieve nanometer-level surface roughness and submicron-level surface accuracy. The diamond grinding wheel used in this embodiment is existing technology.

[0042] Step 4: Remove the silicon substrate to obtain a complete polycrystalline diamond;

[0043] Step 5: Cleaning and testing: Clean the inner and outer surfaces (spherical surface) of the processed polycrystalline diamond and test key parameters such as inner and outer surface roughness, surface accuracy, and wall thickness uniformity.

[0044] The polycrystalline diamond inner surface of this embodiment is free of machining: a high-precision silicon substrate is used as a "mold" to directly obtain a high-precision inner surface through CVD growth, eliminating the most difficult step of machining the inner surface of the diamond spherical surface;

[0045] This embodiment of the multi-stage collaborative processing chain for the polycrystalline diamond outer surface combines three processes: rough grinding (high removal), laser processing (damage removal / reshaping), and fine grinding (surface quality / surface accuracy). The core principle is to use laser processing as a bridge, effectively cutting the path from rough grinding to fine grinding, effectively severing the damage layer's path to the final workpiece. This creates favorable conditions for final fine grinding, improving overall efficiency and final quality. Furthermore, the order of rough grinding and laser processing can be reversed, increasing process adaptability (for example, rough grinding can be performed first when the stock is large, while laser roughing can be performed first when the stock is medium or damage sensitivity is high).

[0046] Example 2: The precision / ultra-precision machining process of the inner and outer surfaces of the polycrystalline diamond curved surface in this example is as follows:

[0047] Step 1: Silicon substrate preparation

[0048] Using the outer surface of the silicon substrate as a "mold", the inner surface of the polycrystalline diamond is directly replicated by CVD growth to replicate the surface morphology of silicon. Therefore, the accuracy (surface accuracy and roughness) of the outer surface of the silicon-based substrate directly determines the initial accuracy of the inner surface of the final PCD surface.

[0049] Single-crystal silicon (Si) has excellent machinability and is a cornerstone material for the semiconductor and optical industries. Its ultra-precision machining technology is highly mature and developed. Through methods such as ultra-precision turning / fly cutting, ultra-precision grinding, and magnetorheological polishing, reliable and economical high-precision optical spherical surfaces can be obtained. At the same time, the chemical properties of single-crystal silicon and diamond (carbon) differ significantly. Therefore, after PCD is grown, the Si substrate can be completely removed without damaging or minimizing damage to the polycrystalline diamond layer grown on it, demonstrating excellent removability.

[0050] The spherical silicon-based substrate is obtained by ultra-precision grinding. Its outer surface shape accuracy (PV<0.5μm) and surface roughness (Ra<5nm) meet the design requirements of the final diamond window inner surface.

[0051] Step 2: Growth of PCD on silicon substrate

[0052] The microwave plasma chemical vapor deposition (MPCVD) method is selected. Compared with the hot wire chemical vapor deposition (HFCVD) method, MPCVD can obtain a smoother surface, good film thickness and property uniformity, and can be used for the preparation of diamond films over a larger area.

[0053] During the growth process, it is necessary to ensure good adhesion between the PCD layer and the silicon substrate so that the inner surface can accurately replicate the morphology of the silicon substrate. The growth thickness needs to take into account the wall thickness of the final window and the outer surface processing allowance.

[0054] Step 3: Multi-step processing of PCD outer surface

[0055] For rough grinding, a vitrified / resin bonded diamond grinding wheel with a grit range of #100-#170 (sand grade) is used. By controlling the grinding depth, feed rate, grinding wheel and workpiece rotation speed, and cooling method, efficient removal of grinding allowance is achieved, quickly approaching the target contour. However, this also introduces a significant subsurface damage layer.

[0056] Laser processing uses a femtosecond laser and a spiral scanning strategy. By optimizing the scanning speed and line spacing, it can achieve efficient and uniform removal of the PCD outer surface and minimize the heat-affected zone and recast layer. Laser ablation can accurately remove the top area of ​​the subsurface damage layer introduced by rough grinding, or reduce the strength of the damage layer by ablation, reducing the difficulty and time of subsequent fine grinding to remove the damage layer. Through precise scanning path control, the accuracy of the outer spherical surface contour can be further improved.

[0057] The finishing process aims to achieve the final optical-grade surface quality and remove sub-surface damage. It adopts diamond grinding technology with a grinding wheel grit size of #500-#2000 (sand number) and is processed with extremely small grinding depth and feed rate. The need for further polishing is determined based on the processing results. Through extremely low pressure, speed control, polishing liquid concentration control, polishing time control and process monitoring methods, the final surface shape is refined and an ultra-smooth surface is obtained.

[0058] The process sequence can be adjusted based on the actual machining situation: If the initial stock is large, a rough grinding-laser-fine grinding approach is used (rough grinding for efficient stock removal, laser for damage removal / reshaping, and fine grinding for convergence accuracy). If the initial stock is moderate or sensitive to rough grinding damage, a laser (roughing / potential damage removal)-fine grinding approach is used (skipping rough grinding, using the laser for efficient stock removal and preliminary fine finishing in one step, and fine grinding for convergence accuracy) or a laser-small-scale fine grinding correction-fine grinding approach is used. If the stock after rough grinding is extremely small and uniform, a rough grinding-fine grinding approach is used, skipping the laser step, but ensuring that fine grinding effectively removes the rough grinding damage layer. Regardless of the order, fine grinding is a necessary step to ultimately achieve an optical surface. The primary value of the laser step lies in its efficient removal / weakening of the damage layer and reshaping, which reduces the burden on fine grinding. The dynamic adjustment logic for multi-stage, step-by-step processing of the external surface is shown in Table 1.

[0059] Table 1. Dynamic adjustment logic of multi-level step-by-step processing of the outer surface

[0060]

[0061] Step 4: Remove the Silicone Base

[0062] The wet chemical etching method is selected, using concentrated KOH or concentrated NaOH solution, heated to 80-100 ° C, and by controlling the etching time, the silicon-based substrate is completely removed without corroding the diamond.

[0063] Step 5: Cleaning and testing

[0064] Immediately after the corrosion reaction is completed, the inner and outer surfaces of the polycrystalline diamond are preliminarily rinsed, and then acidic and alkaline immersion cleaning are carried out with dilute hydrochloric acid solution and diluted ammonia hydrogen peroxide mixture respectively. Rinsing is required before and after cleaning with different solutions to utilize the chemical inertness of diamond to remove chemical residues and particles or sediments that may be attached to the surface.

[0065] After cleaning, the surface roughness, surface accuracy and optical properties (such as wall thickness uniformity) of the polycrystalline diamond need to be tested. If sub-surface damage needs to be detected, interface TEM and chemical etching methods can be used.

[0066] This application has been disclosed as above with preferred implementation cases, but it is not intended to limit this application. Any technician familiar with this profession can make slight changes or modifications to the above-disclosed structures and technical contents without departing from the scope of the technical solution of this application, and the equivalent implementation cases with equivalent changes are all within the scope of the technical solution of this application.

Claims

1. A method for processing the inner and outer surfaces of polycrystalline diamond, characterized by: The method comprises the following steps: S1, processing a silicon-based substrate with a predetermined spherical curvature on its outer surface, wherein the outer surface precision and surface roughness meet the processing requirements. Requirements for the inner surface of polycrystalline diamond; S2. growing a polycrystalline diamond layer on the outer surface of the silicon-based substrate, so that the inner surface of the polycrystalline diamond layer directly replicates the spherical morphology of the outer surface of the silicon-based substrate; S3, multi-step processing, using rough grinding, laser and fine grinding combination processing or laser and fine grinding combination processing of the outer surface of the polycrystalline diamond layer obtained in step S2 to improve the outer surface quality; S4, removing the silicon-based substrate to obtain a complete polycrystalline diamond; S5, cleaning the inner and outer surfaces of the polycrystalline diamond in step S4, and testing the inner and outer surface roughness, surface accuracy and wall thickness uniformity thereof to meet the process requirements and complete the inner and outer surface processing of the polycrystalline diamond.

2. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: In step S2, a polycrystalline diamond layer is grown on the outer surface of the silicon-based substrate by chemical vapor deposition.

3. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: If there are no defects or cracks on the outer surface of the polycrystalline diamond layer before processing, the following step-by-step processing method is adopted, and the specific process of step S3 is: a. Rough grinding: using a coarse-grained diamond grinding wheel to remove the defects so that the outer surface of the polycrystalline diamond is close to the outer spherical profile required by the process; b. Laser processing: Use pulsed laser to scan the outer surface of the polycrystalline diamond after rough grinding to eliminate or reduce the subsurface damage layer introduced by rough grinding; c. Fine grinding: Use fine-grained diamond grinding wheels to grind the outer surface of polycrystalline diamond, eliminate the subsurface damage layer, and refine the outer spherical surface profile to achieve nanometer-level surface roughness and submicron-level surface accuracy.

4. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: If there are defects or cracks on the outer surface of the polycrystalline diamond layer before processing, the following step-by-step processing method is adopted, and the specific process of step S3 is: b. Laser processing: Use pulsed laser to scan and process the outer surface of polycrystalline diamond to quickly remove excess and reduce new damage or expansion of existing microcracks induced by the fine grinding process; c. Fine grinding: Use a fine-grained diamond grinding wheel to grind the outer surface of the polycrystalline diamond, eliminate the damaged layer, and refine the outer spherical surface contour to achieve nanometer-level surface roughness and submicron-level surface accuracy.

5. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: In step S4, the silicon-based substrate is removed by a wet chemical etching method.

6. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: The specific process of cleaning and testing in step S5 is as follows: After removing the silicon substrate, the inner and outer surfaces of the polycrystalline diamond are initially rinsed. Then, the inner and outer surfaces are cleaned by acidic and alkaline soaking with dilute hydrochloric acid solution and diluted ammonia and hydrogen peroxide mixture respectively. Rinsing is required before and after cleaning with different solutions to utilize the chemical inertness of diamond to remove chemical residues and particles or sediments that may be attached to the surface. After cleaning, the inner and outer surface roughness, surface accuracy and wall thickness uniformity of the polycrystalline diamond are tested.

7. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: The material of the silicon-based substrate in step S1 is single crystal silicon.

8. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 1, characterized in that: The surface accuracy of the silicon-based substrate outer surface is PV<0.5μm, and the surface roughness is Ra<5nm.

9. A method for processing the inner and outer surfaces of polycrystalline diamond according to claim 3 or 4, characterized in that: The grit range of the diamond grinding wheel for rough grinding is #100-#170, and the grit range of the diamond grinding wheel for fine grinding is #500-#2000.

10. The method for processing the inner and outer surfaces of polycrystalline diamond according to claim 5, characterized in that: Use concentrated potassium hydroxide solution or concentrated sodium hydroxide solution, heat to 80-100℃, control the etching time, and ensure that the silicon substrate is removed without corroding the diamond.

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