A method for preparing a diamond coating on a cemented carbide surface by means of a texture pretreatment

Abstract

The present application relates to a kind of preparation of cemented carbide surface diamond coating crystal arrangement pretreatment method, using the tungsten cobalt type cemented carbide pretreated by acid and alkali as coating substrate material, application electrophoresis assisted static self-assembly crystal arrangement method, a layer of dense uniform diamond seed is coated on the surface of substrate material: first, prepare the nanodiamond suspension containing stabilizer, with specific pH value, then the substrate is placed in the electrophoresis device containing the suspension and is planted crystal, then it is washed and dried.The present application uses electrophoresis assisted static self-assembly crystal arrangement technology as one of the pretreatment process flow of chemical vapor deposition diamond coating growth, can realize high-density uniform crystal arrangement on the surface of any complex shape substrate material, shorten the subsequent diamond nucleation time, realize the deposition thickness and quality uniform, with excellent adhesion diamond coating, greatly improve the service life of original coating tool, it has great significance to ensure the efficient use of coating tool and improve product quality.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of diamond manufacturing, and particularly relates to a crystal arrangement pretreatment method for preparing a diamond coating on a surface of a hard alloy. BACKGROUND

[0002] Hard alloy tools are widely used as cutting tool materials (such as turning tools, milling tools, drill bits, etc.) and wear-resistant device materials (such as wire drawing dies and bearings, etc.). In the above application fields, the working environment of hard alloy tools is extremely harsh, which leads to very serious material wear, short service life, and large loss, and seriously affects the processing efficiency and processing quality. Traditional hard alloy tools are difficult to meet the needs of long-term stable work in these fields, and a coating is usually used to further increase the mechanical properties of the tool. The chemical vapor deposition (CVD) diamond coating has very close hardness to natural diamond, high elastic modulus, high thermal conductivity, good chemical stability and self-lubricating properties, and is often used as a hard alloy tool coating material to improve the working efficiency and service life of the tool.

[0003] At present, one of the key factors affecting the service life of diamond-coated hard alloy tools is the poor film-substrate adhesion performance, which is mainly affected by the following aspects: (1) Co element in the hard alloy material continuously diffuses to the surface under the high temperature trend in the deposition process, which easily leads to interface graphitization of diamond and thus reduces the film-substrate adhesion; (2) the difference between the thermal expansion coefficients of the hard alloy and the diamond coating is large, which leads to a large thermal stress in the cooling process after deposition, causing harm to the film-substrate adhesion; (3) the acid-base pretreatment of the hard alloy substrate before diamond coating deposition causes the surface to be roughened, and a large number of loose pores are generated, and the subsequent non-uniformity of diamond nucleation leads to the existence of pores at the interface, which further reduces the film-substrate adhesion performance. Some researchers have tried transition layer technology or boronizing and carburizing to convert free cobalt into cobalt compounds to block the diffusion of cobalt and reduce the influence of aspect (1) on the adhesion of the coating, but these process flows are relatively complex and are not conducive to the large-scale application of diamond coatings. Aspect (2) is a problem that inevitably exists in the process of growing a diamond coating on a hard alloy substrate, and can only be alleviated by slow cooling. For aspect (3), only a further suitable pretreatment process needs to be added after the acid-base pretreatment, so as to alleviate the problem of loose pores and improve the adhesion of the diamond coating.

[0004] Through literature retrieval of the prior art, it is found that the Chinese patent "Preparation method of diamond coating high temperature and high pressure spray nozzle" (CN201410005575.2) records a grinding process based on acid and alkali pretreated hard alloy substrate. The specific operation is to use fine-grained sandpaper dipped with diamond grinding paste to manually grind the surface of the hard alloy substrate, remove the loose layer formed after acid and alkali pretreatment, and preliminarily arrange the crystals on the substrate surface. However, this method has high labor cost and low grinding efficiency, and the grinding and crystal arrangement effect and stability are difficult to guarantee. The Chinese patent "Ultra-dispersed nanodiamond and preparation method thereof" (CN201710104715.5) records a preparation method of nanodiamond suspension which can be used for crystal arrangement pretreatment before diamond film growth. This method can prevent the clustering of nanodiamonds and also can regulate the surface potential of nanodiamonds, so that they are selectively adsorbed on the surface of substrate materials with different potentials. However, the preparation method of the suspension mentioned in the record is universal, and no specific scheme for hard alloy substrate is mentioned.

[0005] Electrostatic self-assembly is one of the most potential crystal arrangement technologies for diamond film growth at present. It utilizes the electrostatic interaction between well-dispersed and charged nanodiamond particles in the liquid and the charged substrate surface, and can realize high-density adsorption of diamond on the substrate surface, i.e. uniform crystal arrangement. This method has wide application range and can develop corresponding suspensions for different substrate materials, significantly improving the density of diamond particles on the substrate surface. For acid and alkali pretreated hard alloy, high-density diamond seeds can fill the surface pores, providing favorable and uniform nucleation sites for subsequent diamond deposition, which is beneficial to the growth of high-quality diamond film with uniform thickness and density.

[0006] The Chinese patent "Preparation method of diamond coating and diamond coating and cutting tool prepared thereby" (CN201811044751.8) discloses a diamond coating preparation method. This method utilizes the principle of electrostatic self-assembly, and requires that the charge of nanodiamond particles in the nanodiamond suspension is opposite to the charge on the substrate surface. Therefore, special treatment such as hydrogen plasma is required to make the substrate have a specific charge. However, the process is complicated and the equipment is expensive, which is not conducive to industrialization. In addition, the preparation process requires ultrasonic vibration assistance, which has poor controllability and the possibility of uncharged diamond particles being adsorbed on the substrate surface through collision, which is not conducive to the adhesion of the coating.

[0007] The present application directly utilizes the negative electricity characteristic of the cemented carbide substrate after acid-base pretreatment, and produces adsorption with the positive electricity ammonium ion on the nanoparticles in the suspension, thereby promoting high-quality combination of the two, so that the substrate does not need to be specially pretreated, and the preparation process is simple and economical; and the substrate is completely placed in a liquid environment full of charged nanoparticles by the electrophoresis auxiliary technology, thereby increasing the probability of electrostatic adsorption of the two, reducing collision adsorption of the uncharged particles and the substrate, and being beneficial to improving the adhesion performance of the diamond film. SUMMARY

[0008] The present application provides a crystal arrangement pretreatment method for preparing a diamond coating on a cemented carbide surface, and the method is used for the outer surface of a complex-shaped tool, the inner hole surface and the entrance conical surface of a wire drawing die, and realizes high-density and uniform crystal arrangement on the substrate surface by using an electrophoresis auxiliary electrostatic self-assembly technology, and then a diamond film is deposited on the substrate surface by a CVD method, thereby greatly improving the quality uniformity and adhesion performance of the original diamond film, and being of great significance to prolonging the tool life and guaranteeing the workpiece machining quality.

[0009] The object of the present application can be achieved by the following scheme:

[0010] The present application provides a crystal arrangement pretreatment method for preparing a diamond coating on a cemented carbide surface, and the method is used for the outer surface of a complex-shaped tool, the inner hole surface and the entrance conical surface of a wire drawing die, and realizes high-density and uniform crystal arrangement on the substrate surface by using an electrophoresis auxiliary electrostatic self-assembly technology, and then a diamond film is deposited on the substrate surface by a CVD method, thereby greatly improving the quality uniformity and adhesion performance of the original diamond film, and being of great significance to prolonging the tool life and guaranteeing the workpiece machining quality.

[0011] S1, a to-be-coated surface of a tungsten-cobalt cemented carbide is taken as a coating substrate and subjected to acid-base pretreatment;

[0012] S2, the coating substrate after the acid-base pretreatment is subjected to crystal arrangement pretreatment by using an electrophoresis auxiliary electrostatic self-assembly crystal arrangement method;

[0013] S3, the coating substrate after the crystal arrangement pretreatment is subjected to diamond coating by using a CVD method.

[0014] As an embodiment of the present application, in step S1, the acid-base pretreatment refers to that the coating substrate material is immersed in a Murakami solution, and then is immersed in a Caro mixed acid solution. The cemented carbide substrate material is immersed in the Murakami solution for 30-60 min, so as to expose the cobalt element (Co) on the surface, and then is immersed in the Caro mixed acid solution for 0.5-2 min, so as to remove the Co element on the surface.

[0015] The Murakami solution is composed of three main components: KOH (potassium hydroxide), K3(Fe(CN)6) (potassium ferricyanide) and H2O (water), and the mass ratio of the components is KOH:K3(Fe(CN)6):H2O=1:1:10.

[0016] The Caro mixed acid solution is composed of H2SO4 and H2O2, and the volume ratio of H2SO4 to H2O2 is 1:10.

[0017] As an embodiment of the present application, in step S2, the electrophoretic auxiliary electrostatic self-assembly method is as follows: pouring the nanodiamond micro-powder suspension into an electrophoresis device, inserting a graphite electrode into the solution, placing the substrate near the anode, and performing the pre-treatment of the assembly by electrophoresis.

[0018] The nanodiamond suspension comprises a stabilizer, deionized water and nanodiamond particles. In the nanodiamond suspension, the concentration of the stabilizer is 4-6×10 -6 mol / L, the mass fraction of the nanodiamond particles is 0.005-0.1 wt.%, and the particle size is 3-30 nm. The stabilizer is [2-(methacryloyloxy)ethyl]trimethylammonium chloride (TMAEMC). The pH value of the nanodiamond suspension is 4, which is adjusted by adding an appropriate amount of dilute hydrochloric acid and then detected by pH test paper.

[0019] The electrophoresis time is 30-60 min, and the assembly is then simply rinsed and dried. The zeta potential of the nanodiamond suspension is -30 to -20 mV. During the electrophoresis, the graphite electrode is inserted into the solution to a depth of 2-3 cm, and the substrate is placed near the anode for 30-60 min, so that the negatively charged nanodiamond particles with the specific group adsorbed on the surface are electrostatically adsorbed on the surface of the substrate material. The specific group adsorbed on the surface of the nanodiamond particles refers to the ammonium group in TMAEMC, which is electrostatically adsorbed on the surface of the oxidized hard alloy substrate as an anchor of the nanodiamond particles.

[0020] As an embodiment of the present application, in step S2, the coating substrate after the pre-treatment of the assembly is further cleaned, specifically by placing the coating substrate under clean water for 3-5 s to remove the excess nanodiamond particles that are not firmly adsorbed, and then drying the substrate.

[0021] As an embodiment of the present application, in step S2, the density of the diamond seeds on the surface of the coating substrate after the pre-treatment of the assembly can reach 10 10 cm -2 , and the distribution is uniform and no agglomeration occurs.

[0022] As an embodiment of the present application, in step S3, the CVD method comprises a hot-wire CVD method, a plasma CVD method, a microwave plasma CVD method, etc.

[0023] The present application directly utilizes the substrate pretreated by acid and alkali to carry out subsequent crystal planting, and utilizes the electrostatic interaction between the positively charged ammonium ions and the substrate to realize, without surface modification, a simple and convenient process.In addition, the conventional ultrasonic assisted crystal planting, including the crystal planting effect generated by electrostatic interaction, also contains a large amount of collision adsorption generated by external force, specifically, some uncharged nanoparticles are embedded into the substrate surface groove, and therefore the adsorption is not conducive to the film adhesion; the present application utilizes the electrophoresis principle, places the substrate in an environment full of charged nanoparticles, greatly eliminates the adsorption generated only by collision, and is conducive to the adhesion force improvement of the coating.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] (1) The present application adopts the tungsten-cobalt hard alloy pretreated by acid and alkali as the coating substrate material, applies the electrophoresis assisted electrostatic self-assembly crystal planting method to coat a layer of dense and uniform diamond seeds on the surface of the substrate material, so as to improve the growth efficiency and quality of the subsequent CVD diamond coating.

[0026] (2) The electrophoresis assisted technology adopted in the present application ensures that the substrate is surrounded by a large number of charged nanodiamond particles, greatly improves the electrostatic adsorption probability of the two, and significantly enhances the adhesion performance of the diamond coating.

[0027] (3) The present application directly utilizes the substrate pretreated by acid and alkali to carry out subsequent crystal planting, without surface modification and other complicated operations on the substrate, and has a simple and economical process and strong executability. BRIEF DESCRIPTION OF DRAWINGS

[0028] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments with reference to the following drawings:

[0029] Figure 1 It is the electrophoresis assisted electrostatic self-assembly crystal planting principle diagram of the present application.

[0030] Figure 2 It is the substrate surface morphology diagram prepared by different methods; wherein a is the substrate surface morphology diagram after manual grinding by dipping diamond grinding liquid, b is the substrate surface morphology diagram after ultrasonic crystal planting by using the diamond and methanol mixed solution of Comparative Example 2, and c is the substrate surface morphology diagram after electrophoresis assisted electrostatic self-assembly crystal planting of Example 1.

[0031] Figure 3 It is the coating morphology diagram prepared by different methods; wherein a is the substrate surface morphology diagram after nucleation for 30 minutes after manual grinding by dipping diamond grinding liquid, b is the substrate surface morphology diagram after nucleation for 30 minutes after ultrasonic crystal planting by using the diamond and methanol mixed solution of Comparative Example 2, and c is the substrate surface morphology diagram after nucleation for 30 minutes after electrophoresis assisted electrostatic self-assembly crystal planting of Example 1.

[0032] Figure 4 Figure 1 is a surface morphology diagram of the diamond coating layer after the electrophoretic assisted electrostatic self-assembly crystal arrangement of Example 1.

[0033] Figure 5 Figure 2 is a diagram of the flank wear of the diamond coating milling cutter after milling graphite for the diamond coating prepared by different pretreatment methods; wherein a is the manual grinding method; b is the ultrasonic crystal arrangement method of the diamond and methanol mixed solution of Comparative Example 2; c is the electrophoretic assisted electrostatic self-assembly crystal arrangement method of Example 1.

[0034] Figure 6 Figure 3 is a Rockwell indentation morphology diagram of the diamond coating layer, wherein the diamond coating layer is prepared based on different pretreatment methods: a is manual grinding; b is ultrasonic crystal arrangement of the diamond and methanol mixed solution of Comparative Example 2; c is the electrophoretic assisted electrostatic self-assembly crystal arrangement of Example 1.

[0035] Figure 7 Figure 4 is a seed density diagram and indentation morphology diagram of the substrate surface after the electrophoretic assisted crystal arrangement of Example 1 and Comparative Example 1; wherein a is the seed density diagram of the substrate surface after the crystal arrangement without the electrophoretic assisted technology of Comparative Example 1, b is the seed density diagram of the substrate surface after the crystal arrangement with the electrophoretic assisted technology of Example 1, c is the Rockwell indentation morphology diagram of the diamond coating layer of the sample without the electrophoretic assisted technology of Comparative Example 1; d is the Rockwell indentation morphology diagram of the diamond coating layer of the sample with the electrophoretic assisted technology of Example 1. DETAILED DESCRIPTION

[0036] The present application will be described in detail below with reference to the accompanying drawings and specific examples. The following examples are implemented on the premise of the technical solutions of the present application, and detailed implementation methods and specific operation processes are provided, which will help those skilled in the art to further understand the present application. It should be pointed out that the protection scope of the present application is not limited to the following examples, and several adjustments and improvements made on the premise of the concept of the present application all belong to the protection scope of the present application.

[0037] Example 1

[0038] The substrate in this embodiment is a four-blade spiral milling cutter for CNC milling processing application, and the cutter material is YG6 tungsten carbide hard alloy. The outer size is Φ4mmx75mm, and the blade length is 12mm. As shown in Figure 1, the preparation method is as follows: Figure 1

[0039] First step, acid-base pretreatment

[0040] ​The milling cutter was inverted, and the 12 mm long cutting edge portion was immersed in a Murakami solution for 30 min ultrasonic cleaning to expose the surface cobalt element, wherein the Murakami solution was composed of potassium hydroxide (KOH), potassium ferricyanide (K3(Fe(CN)6)) and water (H2O) with a mass ratio of KOH:K3(Fe(CN)6):H2O = 1:1:10. After rinsing the residual alkaline solution on the cutter surface, the 12 mm long cutting edge portion was immersed in a Caro mixed acid solution for 1 min etching to remove the cobalt (Co) on the substrate surface layer, wherein the Caro mixed acid solution was composed of concentrated sulfuric acid (H2SO4) and (H2O2) with a volume ratio of H2SO4:H2O2 = 1:10.

[0041] Second step, electrophoretic assisted electrostatic self-assembly of crystal pre-treatment

[0042] The prepared nanodiamond suspension was poured into an electrophoresis device and started, wherein the nanodiamond particle size was 3 nm, the mass fraction was 0.1 wt.%, and the main components of the liquid were TMAEMC and deionized water, and the TMAEMC concentration was 5×10 - 6 mol / L, and the pH of the suspension was adjusted to 4 by dilute hydrochloric acid. Then the milling cutter was inverted, and the 12 mm long cutting edge portion was immersed near the anode of the electrophoresis device for 30 min, and the zeta potential was -20 mV, and then it was simply rinsed with water and dried.

[0043] Third step, deposition of diamond coating on the surface of the milling cutter

[0044] The crystal pre-treated milling cutter was placed in the HFCVD reaction chamber. The hot wire was a twisted wire made of two 0.4 mm tantalum wires, with the height of the hot wire being flush with the height of the cutter tip, and the distance between the two hot wires being 35 mm, and the hot wire temperature being 2200°C. A nucleation-growth two-step method was used to deposit a diamond film on the surface of the cemented carbide milling cutter, and the process parameters were as follows: during nucleation, CH4:H2 = 18:300, gas pressure was 1600 Pa, and time was 30 min; during growth, CH4:H2 = 16:300, gas pressure was 4000 Pa, and time was 4.5 h, and the obtained diamond coating thickness was about 5 μm.

[0045] The crystal density on the surface of the cemented carbide after crystal planting by the above method was higher than 10 10 cm -2 , and the surface morphology of the substrate after manual grinding with diamond grinding liquid (b) was compared with the surface morphology of the substrate after crystal planting (a), as shown in (c), the number of holes at the film-substrate interface after nucleation was greatly reduced, and the film was denser Figure 2 a、 Figure 3 a) compared with the surface morphology of the substrate after manual grinding with diamond grinding liquid (b), as shown in (c), the number of holes at the film-substrate interface after nucleation was greatly reduced, and the film was denser Figure 2 Figure 3 ​(c) shows that the final prepared diamond coating is more uniform in thickness and quality, as Figure 4 (c) shows that the final prepared diamond coating is more uniform in thickness and quality, as Figure 5 (c) shows that the final prepared diamond coating is more uniform in thickness and quality, as Figure 6 (c) shows that the final prepared diamond coating is more uniform in thickness and quality, as

[0046] Example 2

[0047] The substrate in this example is a wire drawing die for high-precision wire production, made of YG6 tungsten carbide hard alloy, with an outer dimension of Φ16mm x 14mm and an inner hole size range of Φ2.1-3.3mm. The acid-alkali pretreatment and crystal arrangement pretreatment steps are basically the same as in Example 1.

[0048] First step, wire drawing die pretreatment

[0049] First, immerse the wire drawing die in Murakami solution for ultrasonic treatment, then rinse off the residual alkali solution on its surface and immerse it in Caro mixed acid solution for etching to remove the cobalt (Co) on the surface layer of the substrate, then rinse and dry. Then, arrange the crystals near the anode of the electrophoresis device containing the nanodiamond suspension, and then rinse with clean water and dry.

[0050] Second step, deposition of diamond coating on the inner hole surface of the wire drawing die

[0051] Place the pretreated wire drawing die in the HFCVD reaction chamber. The hot wire is a twisted wire made of two 0.4mm tantalum wires, which passes through the inner hole of the wire drawing die and is fixed at one end of the hot wire, and the other end is pulled straight with a weight. After the reaction chamber is evacuated, the reaction gas (CH4, H2) is introduced, and after adjusting the pressure of the reaction chamber, the chemical vapor deposition of diamond coating is started. The process parameters for nucleation stage are: pressure 1600Pa, CH4:H2=18:30, time 30min, hot wire temperature 2200℃; the process parameters for growth stage are: pressure 4000Pa, CH4:H2=16:30, time 7h, hot wire temperature 2200℃, to obtain micron diamond coating, thickness about 8μm.

[0052] The diamond coating prepared by the method has more uniform thickness and quality. The diamond coating drawing die is used for drawing test of ER-50 gas shielded welding wire, the drawing yield is greatly improved, and the surface quality of the wire is improved. Moreover, the drawing die after acid-base pretreatment does not need to be polished and ground manually, thereby saving labor cost, and being economic and efficient.

[0053] Comparative Example 1

[0054] The comparative example provides a crystal arrangement pretreatment method for preparing a diamond coating on a cemented carbide surface, and the preparation steps are basically the same as those in Example 1, except that the crystal arrangement is not assisted by electrophoresis, but the cemented carbide substrate is placed in a nano-diamond suspension for ultrasonic adsorption. The seed density on the surface after treatment by the two methods is more than 10 10 cm -2 , and there is little difference, as shown in Figure 7 (a) and (b). However, after diamond coating deposition based on the two methods, the diamond coating is subjected to a Rockwell indentation test, and it is found that the diamond coating treated by electrophoresis-assisted crystal arrangement has no obvious peeling phenomenon, as shown in Figure 7 (d), while the coating prepared without electrophoresis-assisted pretreatment has small-area peeling, as shown in Figure 7 (c), which proves the enhancing effect of electrophoresis-assisted technology on the adhesion of the coating.

[0055] Comparative Example 2

[0056] The comparative example provides a crystal arrangement pretreatment method for preparing a diamond coating on a cemented carbide surface, and the preparation steps are basically the same as those in Example 1, except that methanol solution is used instead of deionized water solution with a specific pH value and [2-(methacryloyloxy)ethyl] trimethylammonium chloride (TMAEMC) as a stabilizer, and the solution is mixed with nano-diamond particles to form a suspension for ultrasonic crystal arrangement of the substrate.

[0057] The crystal arrangement sample prepared by the method has very low seed density, which is difficult to distinguish with the naked eye, as shown in Figure 2 (b), so that the nucleation on the surface is slow and the pores are obvious, as shown in Figure 3 (b), and finally the wear resistance and film-substrate adhesion of the diamond coating cutter prepared based on this method are far from those of the cutter prepared by the present application, as shown in Figures 5-6 (b).

[0058] The specific embodiments of the present application are described above. It should be understood that the present application is not limited to the above specific embodiments, and those skilled in the art can make various modifications or changes within the scope of the claims, which does not affect the essential content of the present application.

Claims

1. A method for preparing a cemented carbide surface diamond coating by means of a texture pretreatment, characterized in that The method comprises the following steps: S1, the surface to be coated of the tungsten-cobalt-based hard alloy is subjected to acid-alkali pretreatment as a coating substrate; S2, the coating substrate after the acid-alkali pretreatment is subjected to crystal arrangement pretreatment by an electrophoresis-assisted electrostatic self-assembly method; S3, the coating substrate after the crystal arrangement pretreatment is subjected to diamond coating by a CVD method; In step S2, the electrophoresis-assisted electrostatic self-assembly method is specifically as follows: a nano-diamond suspension is poured into an electrophoresis device, a graphite electrode is inserted into the solution, and the substrate is placed near the anode, and then the crystal arrangement pretreatment is performed by electrophoresis. The nanodiamond suspension comprises a stabilizer, deionized water and nanodiamond particles; the stabilizer is [2-(methacryloyloxy)ethyl] trimethylammonium chloride; the concentration of the stabilizer in the nanodiamond suspension is 4-6×10 -6 mol / L. The time of the electrophoresis is 30-60 min, and the substrate is simply rinsed and dried after the crystal arrangement.

2. The crystal pretreatment method according to claim 1, characterized in that, In step S1, the acid-alkali pretreatment refers to that the coating substrate is immersed in a Murakami solution, and then is immersed in a Caro mixed acid solution.

3. The crystal pretreatment method according to claim 2, characterized in that, The Murakami solution is composed of KOH, K3(Fe(CN)6) and H2O (water), and the mass ratio of the components is KOH:K3(Fe(CN)6):H2O = 1:1:

10.

4. The crystal pretreatment method according to claim 2, characterized in that, The Caro mixed acid solution is composed of H2SO4 and H2O2, and the volume ratio is H2SO4:H2O2 = 1:

10.

5. The crystal pretreatment method according to claim 1, characterized in that, In the nano-diamond suspension, the mass fraction of the nano-diamond particles is 0.005-0.1 wt.%, and the particle size is 3-30 nm.

6. The crystal pretreatment method according to claim 1, characterized in that, In step S2, the coating substrate after the crystal arrangement pretreatment is also subjected to cleaning treatment, specifically as follows: the coating substrate is rinsed under clean water for 3-5 s to remove the nano-diamond particles that are not firmly adsorbed, and then the substrate is dried.

7. The crystal pretreatment method according to claim 1, characterized in that, In step S3, the CVD method includes one of a hot-wire CVD method, a plasma CVD method and a microwave plasma CVD method.

Images