Method for realizing uniform cobalt plating of submicron diamond in high-temperature molten salt environment

Abstract

The invention provides a method for realizing uniform cobalt plating of submicron diamond in a high-temperature molten salt environment, and belongs to the technical field of diamond surface metallization treatment. On the basis of the redox reaction principle, by controlling parameters such as the type of fused salt, the content of a reducing agent, the content of cobalt salt and the experimental temperature, vacuumizing is performed in a tubular furnace, reaction diffusion is performed under the protection of argon atmosphere, and a uniform cobalt coating is formed on the surface of diamond. The reaction temperature is controlled to be 800-1000 DEG C, the heat preservation time is 1-2 h, a high-purity cobalt plating layer can be obtained on the surface of the diamond, the local cobalt plating deposition mass ratio can reach 20.17%, the total deposition mass ratio can reach 1.78%, and the plating layer mainly comprises carbon and cobalt elements and is low in impurity content. Compared with traditional methods such as chemical cobalt plating, the method has the advantages of being high in safety, good in deposition efficiency, easy and convenient to operate and the like, and is suitable for surface metallization treatment of the submicron-scale diamond powder.

Description

Technical Field

[0001] This invention belongs to the field of diamond surface metallization technology, specifically relating to a method for achieving uniform cobalt plating of submicron diamonds in a high-temperature molten salt environment. Background Technology

[0002] Diamond, the hardest natural substance, is widely used in various cutting and grinding tools such as diamond drill bits, saw blades, thin-walled drills, and abrasives due to its excellent wear resistance and superhardness. Among them, polycrystalline diamond composite (PDC composite) sheets, with their high hardness, excellent wear resistance, and good impact toughness, have become a key material for drill bit cutting teeth in fields such as oil drilling and geological exploration, undertaking the important task of high-speed cutting of rock formations. The performance advantages of PDC composite sheets mainly stem from two aspects: first, the DD bonds formed between the particles in its polycrystalline diamond structure achieve a highly dense bond, which not only ensures the material's hardness but also significantly improves its wear resistance; second, the effective bonding between the diamond layer and the cemented carbide substrate further enhances the overall structure's weldability and impact toughness.

[0003] To promote the formation of DD bonds between diamond particles and improve the poor wettability and high interfacial energy between diamond and cemented carbide substrates, cobalt (Co) is commonly used as a binder. Cobalt has good wettability and suitable diffusion properties, which can effectively promote the bonding between diamond particles, act as an interfacial bridge, and enhance the stability of the bonding interface. However, cobalt can catalyze the graphitization of diamond at high temperatures, reducing the hardness and wear resistance of PDC and affecting the overall performance of the composite sheet. Therefore, the method of cobalt addition and content control become key factors determining the material properties.

[0004] While the commonly used method of adding cobalt powder externally can achieve targeted and quantitative doping, it suffers from problems such as uneven mixing, easy oxidation of cobalt powder, and poor dispersibility. In contrast, using surface treatment technology to coat the surface of diamond with a cobalt layer to achieve diamond metallization can form a continuous and uniform cobalt coating layer on its surface, achieving good interfacial metallurgical bonding. This significantly improves the bonding strength, sintering density, and high-temperature stability between diamond and the metal substrate, thereby effectively improving the mechanical properties and service life of tools.

[0005] Among various cobalt surface plating methods, the redox reaction method based on a molten salt environment has significant advantages. Its reaction principle is: Zn + Co 2+ →Co+Zn 2+ Due to the standard reduction potential of zinc (E° (Zn) 2+ / Zn) = -0.76 V) is significantly lower than that of cobalt (E°(Co) 2+ / Co) = -0.28 V), meaning that zinc has a stronger reducing power, which makes the free energy change ΔG < 0 for the reaction, allowing it to proceed spontaneously thermodynamically.

[0006] Currently, commonly used methods for metallizing diamond surfaces include electroless plating, electroplating, vacuum physical / chemical vapor deposition, and vacuum plating. Electroless plating requires multiple processes such as surface activation and sensitization, and the preparation of chemical reagents is complex and poses safety risks. In particular, it results in slow deposition rates and poor plating effects on submicron diamond powders. Cobalt coatings prepared by electroplating mainly rely on physical adsorption and bonding, with weak interfacial diffusion and limited bonding strength, and are also costly. Ordinary vacuum plating is carried out at temperatures below 400 °C, resulting in thin metal powder deposits with low bonding strength. In chemical / physical vapor deposition, the coating and diamond are mostly physically attached, and no chemical reaction usually occurs, making the coating prone to peeling off.

[0007] Based on this, this invention proposes a molten salt cobalt plating process. Molten salt provides an excellent fluid contact environment for diamond and metallic cobalt, allowing for continuous exchange and movement between the two, promoting the uniform deposition of cobalt particles on the diamond surface. Its high-temperature ionic environment facilitates strong metallurgical bonding between the plating layer and the diamond substrate. Through interdiffusion and interfacial chemical reactions (such as carbide formation) at high temperatures, this method achieves adhesion far exceeding that of electroplating and electroless plating, while also possessing excellent wraparound properties, coating density, and high-temperature stability, providing a reliable technical path for metallization of diamond surfaces. Summary of the Invention

[0008] The purpose of this invention is to provide a method for achieving uniform cobalt plating on submicron diamonds in a high-temperature molten salt environment. This method effectively solves the technical problems of uneven distribution, easy oxidation, and poor dispersibility of added cobalt powder in existing technologies, as well as the slow deposition rate, insufficient coating adhesion, and complex processes of traditional chemical plating and electroplating methods on submicron-scale diamond surfaces. By optimizing the molten salt cobalt plating process, a continuous, uniform, and densely bonded cobalt coating layer is formed on the surface of submicron diamonds, achieving a strong metallurgical bond between diamond and the metal matrix, significantly improving the overall performance and service life of the composite material.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for achieving uniform cobalt deposition on submicron diamond in a high-temperature molten salt environment involves mixing submicron diamond powder, cobalt salt, zinc powder, and a molten salt medium under a protective atmosphere. Through a redox reaction in a high-temperature molten salt environment, a uniform cobalt metal layer is deposited on the surface of the submicron diamond. The method specifically includes the following steps:

[0011] (1) Pretreatment: Mix submicron diamond powder with dilute hydrochloric acid and stir to ensure that the diamond powder is fully in contact with the acid solution to remove surface impurities and oil stains, while roughening the diamond surface to provide more active sites for subsequent cobalt plating.

[0012] As a preferred embodiment of the present invention, the volume of dilute hydrochloric acid is 10%, and the submicron diamond powder is mixed with dilute hydrochloric acid at a volume ratio of 1:1~3. The mixture is then stirred at room temperature for 15~30 min. This optimized condition ensures the cleaning effect while minimizing excessive corrosion of the diamond surface. After treatment, the diamond is repeatedly rinsed with deionized water until the pH is neutral to ensure complete removal of acidic substances. Finally, it is dried at 80~100 °C for later use.

[0013] (2) Removal of water of crystallization: Since the original cobalt salt (CoCl2·6H2O) contains a large amount of water of crystallization, this water of crystallization will undergo hydrolysis during the high-temperature reaction, introducing oxygen impurities and affecting the coating quality. Therefore, CoCl2·6H2O needs to be heat-treated under an argon protective atmosphere for a period of time to completely remove the water of crystallization and obtain anhydrous CoCl2 powder.

[0014] As a preferred embodiment of the present invention, the temperature for removing water of crystallization is 200~400 ℃, more preferably 300~400 ℃; the holding time is 1~2 h, more preferably 1.5~2 h. This temperature range and time control can effectively remove water of crystallization while avoiding the decomposition or oxidation of cobalt salt.

[0015] (3) Salt bath mixing: The pretreated diamond powder is initially mixed with the molten salt medium to ensure that the diamond powder is evenly dispersed in the molten salt. Then, the CoCl2 after removing the water of crystallization is mixed with zinc powder in proportion. Finally, all samples are poured into a mortar and ground to ensure that all components are fully and evenly mixed.

[0016] As a preferred embodiment of the present invention, the molten salt medium is composed of CaCl2 and KCl in a mass ratio of 1:1 to 2, more preferably 1:1. This preferred ratio of molten salt system has a low eutectic melting point, enabling the formation of a uniform liquid phase environment at the reaction temperature, thus promoting the diffusion and mass transfer of reactants.

[0017] As a preferred embodiment of the present invention, the mass ratio of diamond, CoCl2, zinc powder, and molten salt medium is 3~6:1~1.5:0.1~1:20. This ratio ensures that the reaction proceeds fully while avoiding the introduction of impurities by excessive zinc powder.

[0018] As a preferred technical solution of the present invention, all samples are poured into a mortar and ground with a grinding rod for 10 to 30 minutes, more preferably 15 to 25 minutes, to ensure that all components are fully mixed and uniform.

[0019] (4) Molten Salt Reaction: The uniformly mixed materials are loaded into a graphite crucible, which is then placed in a vacuum tube furnace. After sealing, a vacuum is drawn and high-purity argon is introduced as a protective atmosphere. The temperature is raised to the reaction temperature, causing the zinc powder and cobalt ions to undergo a redox reaction (Zn + Co). 2+ →Co+Zn 2+ The generated cobalt atoms diffuse in the molten salt medium and deposit on the diamond surface, forming a uniform cobalt coating.

[0020] As a preferred embodiment of the present invention, the reaction temperature is set at a heating rate of 5-10 °C / min to 800-1000 °C, more preferably 900-950 °C; the holding time is 1-2 h, more preferably 1.5-2 h. This temperature range and holding time ensure that the redox reaction proceeds fully, while avoiding excessively high temperatures that could lead to diamond graphitization or excessive aggregation of the cobalt coating.

[0021] (5) Post-processing: After the reaction is completed, the furnace is cooled to room temperature and the reaction product is taken out. The product is washed and dried to obtain cobalt-plated diamond powder.

[0022] As a preferred embodiment of the present invention, the product is added to deionized water, and the soluble molten salt is dissolved by stirring at 60-80 °C. The washing process is repeated 3-5 times with water changes until the molten salt is completely removed. Finally, the product is dried at 80-100 °C for 2-4 h to obtain cobalt-plated diamond powder.

[0023] This invention, through systematic process optimization and parameter control, successfully prepared a high-quality cobalt coating on the surface of submicron diamond, providing an effective technical solution to problems such as uneven cobalt powder distribution and weak interfacial bonding in diamond tools. Compared with existing technologies, the beneficial effects of this invention are as follows:

[0024] (1) This invention achieves uniform deposition of a cobalt metal layer on the surface of submicron diamond through a redox reaction in a molten salt environment. The molten salt medium provides a good fluid environment for the reaction, enabling cobalt atoms to be uniformly deposited on the diamond surface. The resulting coating is continuous and dense, forming a strong metallurgical bond with the substrate, which significantly improves the interfacial bonding strength and service stability of the diamond composite sheet.

[0025] (2) Compared with traditional chemical plating and electroplating, the process of the present invention does not require complex pretreatment steps such as surface activation and sensitization, avoids the use of hazardous chemicals, is safer to operate, and has lower costs. At the same time, this method is particularly suitable for submicron-sized fine particles, effectively solving the problems of slow deposition rate and uneven coating distribution in traditional methods at the submicron scale.

[0026] (3) By precisely controlling the composition of the molten salt, the reaction temperature, and the material ratio, the method of the present invention effectively suppresses the introduction of impurities and the graphitization of diamond. In particular, by removing the water of crystallization from the cobalt salt in advance, the introduction of oxygen impurities is avoided, ensuring the purity of the coating and the integrity of the diamond's properties.

[0027] (4) The cobalt layer formed by this method is mainly in the metallic state, with only a very thin Co-C transition layer on the surface, which ensures both good metallic properties and excellent interfacial compatibility. According to the EDS energy dispersive spectroscopy analysis results, the local deposition mass ratio of cobalt in cobalt-plated diamond can reach 20.17%, and the overall deposition mass ratio can reach 1.78%, providing a new technical approach for the preparation of high-performance diamond tools.

[0028] (5) The process parameters provided by this invention are clear, the operation is simple, the reproducibility is good, and it is easy to achieve large-scale production. By optimizing key parameters such as molten salt ratio, reaction temperature and time, it is possible to improve production efficiency while ensuring coating quality, and it has significant prospects for industrial application. Attached Figure Description

[0029] Figure 1 Comparison of macroscopic morphology before (left) and after (right) cobalt plating of diamond molten salt.

[0030] Figure 2 This is a schematic diagram of the vacuum tube furnace used in the molten salt cobalt plating equipment of this invention.

[0031] Figure 3 This is a schematic diagram of the structure of the cobalt-plated molten salt interface.

[0032] Figure 4 The image shows the microstructure of cobalt-coated diamond obtained in Example 1 at 700 °C using MgCl2, KCl, and CaCl2 as molten salt media.

[0033] Figure 5 The image shows the microstructure of cobalt-coated diamond obtained in Example 2 at 800 °C using MgCl2, KCl, and CaCl2 as molten salt media.

[0034] Figure 6 This is a microstructure image of cobalt-coated diamond obtained in Example 3 at 900 °C using KCl and CaCl2 as molten salt media.

[0035] Figure 7 The image shows the microstructure of cobalt-coated diamond obtained in Example 4 at 950 °C using KCl and CaCl2 as molten salt media.

[0036] Figure 8 This is a microstructure image of cobalt-coated diamond obtained in Example 5 at 1000 °C using KCl and CaCl2 as molten salt media. Detailed Implementation

[0037] The core of this invention lies in achieving uniform deposition of cobalt on the surface of submicron diamond through a precisely controlled redox reaction in a molten salt environment. This method fully utilizes the high-temperature fluidity and ion conduction properties of the molten salt medium to provide ample contact opportunities between diamond and cobalt atoms, while effectively suppressing impurity introduction and diamond graphitization by optimizing process parameters.

[0038] Key technical points of this invention include: selecting a suitable molten salt system to ensure the formation of a uniform liquid phase at the reaction temperature; pre-removing the cobalt salt crystal water to avoid the introduction of oxygen impurities; precisely controlling the reaction temperature and time to balance the reaction rate and the thermal stability of the material; and optimizing the raw material ratio to ensure a complete reaction.

[0039] The following specific embodiments further illustrate the implementation of the present invention, but the scope of protection of the present invention is not limited to these embodiments.

[0040] Example 1

[0041] First, 0.8 μm diamond powder was immersed in 10% dilute hydrochloric acid (volume ratio 1:1) and mechanically stirred continuously for 30 min to ensure full contact between the acid and the diamond surface, removing impurities and roughening the surface. Then, it was repeatedly rinsed with deionized water until the pH was neutral, and vacuum dried at 80 ℃ for 2 h.

[0042] Take 5g of the treated diamond powder and mix it initially with molten salt medium (10g MgCl2, 7.5g CaCl2, 7.5g KCl). Then add 1.1g CoCl2·6H2O and 0.3g Zn powder and mix for 5 hours using a V-type mixer to ensure uniform dispersion.

[0043] The mixture is loaded into a graphite crucible and placed in a vacuum tube furnace. Figure 2 As shown below, after evacuation, the sample was filled with high-purity argon for protection. The temperature was increased to 700 °C at a rate of 5 °C / min and held for 1.5 h for reaction, followed by furnace cooling to room temperature. After removing the sample, deionized water was added, and the soluble salts were dissolved by stirring in a 60 °C water bath. The washing process was repeated 5 times with water changes, and finally dried at 80 °C for 4 h to obtain cobalt-plated diamond powder.

[0044] like Figure 4 As shown, under these conditions, the coating coverage is incomplete, with obvious areas of missed coating. Since 700 ℃ is below the eutectic melting point of this molten salt system, the molten salt failed to form a uniform liquid phase, resulting in limited reactant transport, a low cobalt ion reduction deposition rate, and the possibility of local overheating causing graphitization on the diamond surface.

[0045] Example 2

[0046] The reaction temperature was increased to 800 °C, while all other conditions remained exactly the same as in Example 1. Figure 5 As shown, the increase in temperature accelerated the cobalt ion reduction kinetics, resulting in an improved deposition rate.

[0047] However, EDS analysis revealed significant Mg and O element contamination in the coating. This is due to the hydrolysis reaction between the water of crystallization in CoCl2·6H2O and MgCl2 (MgCl2 + H2O → MgO + 2HCl), producing solid impurities such as MgO. The results indicate that the magnesium-containing molten salt system is incompatible with hydrous cobalt salts, necessitating optimization of material selection.

[0048] Example 3

[0049] By changing the molten salt system and increasing the reaction temperature, cobalt salts are used to remove the water of crystallization, thus preparing cobalt-plated diamond.

[0050] The water of crystallization was completely removed by treating CoCl2·6H2O at 300 °C for 2 h under argon protection.

[0051] Take 4 g of the treated diamond powder and mix it initially with molten salt medium (10 g CaCl2, 10 g KCl). Then add 1.1 g CoCl2 and 0.3 g Zn powder and grind it in a mortar for 20 min. The reaction temperature is set at 900 ℃, and the other parameters are the same as in Example 1.

[0052] like Figure 6 As shown, the coating continuity was significantly improved, and no impurity elements such as Mg and O were detected by either EDS surface scanning or spot scanning, indicating that the removal of crystal water and optimization of the molten salt system effectively avoided the introduction of impurities. The KCl-CaCl2 system can form a homogeneous liquid phase at 900 °C, which promotes reactant transport and uniform cobalt deposition.

[0053] Example 4

[0054] By adjusting the mass ratio of cobalt salt to zinc powder and increasing the reaction temperature, cobalt-plated diamond can be prepared.

[0055] Take 4 g of the treated diamond powder and mix it initially with molten salt medium (10 g CaCl2, 10 g KCl). Then add 1.5 g CoCl2 and 0.75 g Zn powder and grind it in a mortar for 20 min. The reaction temperature is set at 950 ℃, and the other parameters are the same as in Example 3.

[0056] In this embodiment, the macroscopic morphology comparison images before (left image) and after (right image) cobalt plating with diamond molten salt are as follows: Figure 1 As shown. Figure 7 As shown, increasing the zinc powder content improved the reaction sufficiency and further enhanced the coating coverage. Under these conditions, the cobalt deposition amount increased significantly, and the coating thickness was quite ideal.

[0057] A schematic diagram of the structure of the molten salt cobalt plating interface is shown below. Figure 3 As shown, the outer side of the diamond is coated with a cobalt layer, while there is only a very thin Co-C transition layer in the middle, which ensures good metallic properties and provides excellent interfacial compatibility.

[0058] Example 5

[0059] The reaction temperature was increased to 1000 °C, while other parameters remained the same as in Example 4.

[0060] like Figure 8 As shown, although the coating can completely cover the diamond surface, EDS analysis clearly detected oxygen, indicating that excessively high temperatures triggered oxidation side reactions. Furthermore, 1000 °C is close to the graphitization critical point of diamond, which may cause thermal damage to the material and significantly increase energy consumption, resulting in poor economic efficiency.

[0061] In addition, no oxygen element was detected in the product prepared in this embodiment by surface scanning, but trace oxygen signals were found in local areas by point scanning, which may be related to the sealing of the tube furnace or fluctuations in the argon flow rate.

[0062] Through a systematic comparison of the above embodiments, it can be seen that the present invention successfully solves the problems of discontinuous coating, impurity introduction, and thermal damage in the cobalt plating process on submicron diamond surfaces by optimizing the molten salt system (using KCl-CaCl2 to replace magnesium-containing salts), pretreatment for removing crystal water with cobalt salts, controlling the reaction temperature, and optimizing the raw material ratio. Under optimal process conditions, a uniform, dense, and well-bonded cobalt coating can be obtained, providing a reliable technical solution for the preparation of high-performance diamond tools.

Claims

1. A method for achieving uniform cobalt plating of submicron diamond in a high-temperature molten salt environment, characterized in that, Under a protective atmosphere, submicron diamond powder, cobalt salt, zinc powder and molten salt medium were mixed, and a cobalt metal layer was uniformly deposited on the surface of submicron diamond through a redox reaction in a high-temperature molten salt environment.

2. The method as described in claim 1, characterized in that, Includes the following steps: (1) Pretreatment: Mix submicron diamond powder with dilute hydrochloric acid and stir to ensure that the diamond powder is fully in contact with the acid solution to remove surface impurities and oil stains, while roughening the diamond surface to provide more active sites for subsequent cobalt plating. (2) Removal of water of crystallization: CoCl2·6H2O was heat-treated for a period of time under an argon protective atmosphere to completely remove the water of crystallization and obtain anhydrous CoCl2 powder; (3) Salt bath mixing: The pretreated diamond powder is initially mixed with the molten salt medium to make the diamond powder evenly dispersed in the molten salt; then the CoCl2 after removing the water of crystallization is mixed with zinc powder as a reducing agent in proportion, and the mixture is ground to ensure that the components are fully and evenly mixed. (4) Molten salt reaction: The uniformly mixed material is loaded into a graphite crucible, and then the crucible is placed in a vacuum tube furnace. After sealing, a vacuum is drawn and high-purity argon is filled in as a protective atmosphere. The temperature is raised to the reaction temperature so that the zinc powder and cobalt ions undergo an oxidation-reduction reaction. The generated cobalt atoms diffuse in the molten salt medium and are deposited on the diamond surface to form a uniform cobalt coating. (5) Post-processing: After the reaction is completed, the furnace is cooled to room temperature and the reaction product is taken out; the product is washed and dried to obtain diamond powder with cobalt coating.

3. The method as described in claim 2, characterized in that, In step (1), submicron diamond powder is mixed with dilute hydrochloric acid at a volume ratio of 1:1~3, and then stirred at room temperature for 15~30 min; after the treatment is completed, it is repeatedly rinsed with deionized water until the pH value is neutral, and finally dried at 80~100 ℃.

4. The method as described in claim 2, characterized in that, In step (2), the temperature for removing water of crystallization is 200~400℃, more preferably 300~400℃; the holding time is 1~2 h, more preferably 1.5~2 h.

5. The method as described in claim 2, characterized in that, In step (3), the mass ratio of diamond, CoCl2, zinc powder, and molten salt medium is 3~6:1~1.5:0.1~1:

20.

6. The method as described in claim 5, characterized in that, In step (3), the molten salt medium is composed of CaCl2 and KCl in a mass ratio of 1:1 to 2, more preferably 1:

1.

7. The method as described in claim 2, characterized in that, In step (4), the reaction temperature is 800~1000 ℃, more preferably 900~950 ℃; the holding time is 1~2 h, more preferably 1.5~2 h.

8. Submicron cobalt-plated diamond powder prepared by the method according to any one of claims 1 to 7.

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