Method for preparing Ta-B-C coating through fused salt electrophoretic deposition

Abstract

The invention relates to the technical field of surface coating preparation, in particular to a method for preparing a Ta-B-C coating through fused salt electrophoretic deposition, which comprises the following steps: firstly, putting raw materials of a boron source, a carbon source, tantalum pentoxide and a fluoride inorganic salt system into a conductive crucible, heating to 950-1200 DEG C, and carrying out in-situ chemical reaction; suspended molten salt containing the boron tantalum carbide nano-particles is directly generated in the molten salt. Then, the conductive crucible serves as a positive electrode, a substrate to be coated serves as a negative electrode, the ratio of the surface area of the positive electrode and the surface area of the negative electrode immersed in the fused salt is controlled to be 10-30, electrophoretic deposition is conducted under the voltage of 0.15-0.3 V, and therefore the Ta-B-C coating is formed on the surface of the substrate. According to the method, the in-situ synthesis process and the electrophoretic deposition process of the nano-particles are integrated, the dense Ta-B-C coating with high hardness, excellent fracture toughness and controllable thickness is achieved under the extremely low voltage by optimizing electrode arrangement, and the method has the advantages of being simple in process, low in cost, low in equipment requirement, suitable for various base materials such as graphite and titanium alloy and the like.

Description

Technical Field

[0001] This invention relates to the field of surface coating preparation technology, and specifically to a method for preparing Ta-BC coatings by molten salt electrophoretic deposition. Background Technology

[0002] In demanding environments such as machining, aerospace, and mold manufacturing, hard protective coatings are typically applied to the surfaces of critical components to enhance their performance and lifespan. Current technologies primarily utilize binary ceramic materials such as carbides and borides for these hard coatings. While these coatings offer advantages like high hardness and wear resistance, their inherent brittleness is a fatal weakness. During service, once microcracks develop within the coating, they propagate rapidly, ultimately leading to catastrophic failures such as peeling and cracking, jeopardizing the safety of the base component. Although elemental doping, the construction of nanocomposite or multilayer film structures can improve the coating's toughness to some extent, it remains difficult to fundamentally resolve the inherent contradiction between hardness and toughness, failing to meet the increasingly stringent demands of industrial applications.

[0003] To achieve a better balance between hardness and toughness, researchers have turned their attention to ternary or multi-component ceramic systems. Among them, tantalum-boron-carbon (Ta-BC) ternary ceramic coatings have shown great application potential. This system combines the high hardness (up to 22-40 GPa) and excellent high-temperature oxidation resistance of binary Ta-B coatings with the high hardness (greater than 20 GPa), thermal shock resistance, and chemical corrosion resistance of binary Ta-C coatings. Existing technologies have demonstrated that Ta-BC coatings prepared through specific methods can achieve an excellent combination of high hardness, high elastic modulus, and high fracture toughness, and are considered highly promising next-generation ultra-hard protective coating materials.

[0004] However, significant technical bottlenecks still exist in the preparation of Ta-BC coatings. Currently, the mainstream technology for preparing this coating is physical vapor deposition, especially magnetron sputtering. Although this method is technically mature, its inherent defects severely restrict the realization and widespread application of the performance advantages of Ta-BC coatings, specifically: 1) Complex and costly process: Magnetron sputtering relies on a high vacuum environment and sophisticated equipment such as vacuum chambers, targets, and power supplies. This not only leads to extremely high initial investment and maintenance costs but also a cumbersome process with stringent operating conditions. 2) Low deposition efficiency, making it difficult to prepare thick coatings: The deposition rate of magnetron sputtering is typically low (e.g., 100-250 nm / min), resulting in low production efficiency. For applications requiring coatings several micrometers or even tens of micrometers thick to withstand heavy loads or severe wear, this method is difficult to achieve economically and efficiently; currently, the coating thickness prepared is typically no more than ten micrometers. 3) Poor adaptability to complex morphology: For workpieces with geometric features such as deep holes, narrow grooves, internal cavities or complex curved surfaces, this method is difficult to achieve uniform and dead-angle-free coating coverage. It is easy to produce phenomena such as missed coating or excessively thin coating in the shielded area, forming a protection weakness.

[0005] In summary, existing Ta-BC coating preparation technologies (represented by magnetron sputtering) suffer from one or more technical problems, including high production costs, low deposition efficiency, inability to prepare thick coatings, and difficulty in uniformly depositing on workpieces with complex morphologies. These problems have become key obstacles hindering the large-scale industrial application of Ta-BC, a high-performance coating material.

[0006] In view of the above-mentioned defects, the inventors of this invention have finally obtained this invention after a long period of research and practice. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of complex process, high equipment cost and low production efficiency in existing Ta-BC coating preparation methods, especially magnetron sputtering, and to provide a method for preparing Ta-BC coatings by molten salt electrophoretic deposition.

[0008] To achieve the above objectives, this invention discloses a method for preparing Ta-BC coatings by molten salt electrophoretic deposition, comprising the following steps:

[0009] S1, a boron source, a carbon source, tantalum pentoxide and fluoride inorganic salt are mixed and placed in a conductive crucible, and heated to 950-1200℃ to carry out an in-situ reaction to prepare a suspended molten salt containing boron tantalum carbide nanoparticles.

[0010] S2, using a conductive crucible as the positive electrode and the substrate to be coated as the negative electrode, the negative electrode is immersed in the suspended molten salt obtained in step S1 for electrophoretic deposition to obtain the Ta-BC coating.

[0011] In step S1, the boron source is boron powder or boron carbide powder with a particle size of 100nm-10μm; the carbon source is graphite powder or acetylene black with a particle size of 20-100nm; and the tantalum pentoxide has a particle size of 50nm-100nm.

[0012] In step S1, the molar ratio of boron source, carbon source and tantalum pentoxide is 10~20:2~5:1; the total mass of boron source, carbon source and tantalum pentoxide is 2-5% of the mass of fluoride solid salt.

[0013] In step S1, the fluoride inorganic salt is any one of NaF-AlF3 mixed salt, KF-AlF3 mixed salt, or NaF-KF-AlF3 mixed salt.

[0014] The molar ratio of NaF to AlF3 in the NaF-AlF3 mixed salt is 0.6-2.5, and the molar ratio of KF to AlF3 in the KF-AlF3 mixed salt is 0.7-2.2.

[0015] In step S1, the material of the conductive crucible is any one of graphite, silicon carbide, and ZrB2.

[0016] In step S1, the particle size of the boron tantalum carbide nanoparticles is 5-30 nm.

[0017] In step S2, the substrate material is any one of graphite, carbon / carbon composite material, cemented carbide, metallic titanium, and titanium alloy.

[0018] In step S2, the electrophoresis voltage is 0.15-0.3V, and the ratio of the surface area of ​​the positive electrode to the surface area of ​​the negative electrode immersed in the suspended molten salt is 10-30.

[0019] In step S2, the thickness of the Ta-BC coating is 5~200µm.

[0020] The theoretical basis of the molten salt electrophoretic deposition method for preparing Ta–B–C coatings is the electrophoretic migration phenomenon of tantalum borocarbide nanoparticles in molten salt. The core principle of this method is that, driven by an applied electric field, positively charged nanoparticles migrate directionally and deposit on the substrate surface, which acts as the negative electrode, forming an initial coating. Subsequently, during the deposition process, the electroosmotic outflow effect of the molten salt and the high-temperature environment of the system work synergistically to drive the deposited nanoparticles to rearrange and tightly assemble through non-classical crystallization methods such as oriented attachment, thereby achieving in-situ densification of the coating and ultimately forming a structurally complete and highly dense Ta–B–C coating.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. Simplified process and low cost: This invention integrates the in-situ synthesis and electrophoretic deposition of tantalum boron carbide nanoparticles into the same molten salt system, eliminating the complex steps of pre-preparing and dispersing nanoparticles, thus significantly shortening the process flow. Furthermore, the preparation can be completed using only a conventional high-temperature resistance furnace, greatly reducing equipment investment and operating costs compared to expensive magnetron sputtering deposition equipment.

[0023] 2. Excellent coating quality: In-situ reaction can directly generate boron tantalum carbide nanoparticles with a particle size of 5-30nm. These boron tantalum carbide nanoparticles form a stable suspension in molten salt, which is conducive to electrophoretic deposition to form a highly dense coating.

[0024] 3. Wide applicability to substrates and no need for pretreatment: This invention utilizes the purifying effect of fluoride molten salt on the substrate surface, which can clean the substrate surface in situ before coating deposition. It eliminates the need for additional complex pretreatment processes such as grinding and pickling for substrates such as graphite, carbon / carbon composites or titanium metal, simplifying the operation and improving the adhesion between the coating and the substrate. Attached Figure Description

[0025] Figure 1 XRD pattern of Ta-BC coating on graphite substrate surface;

[0026] Figure 2 XPS spectra of the Ta-BC coating on the graphite substrate surface: (a) Ta-B and Ta-O bonds; (b) C-C bonds; (c) Ta-O bonds; (d) BC bonds;

[0027] Figure 3 SEM image of the overall morphology of the cross-section of the Ta-BC coating on the graphite substrate surface;

[0028] Figure 4 SEM images (a) and mapping analysis (b~f) of the horizontal section of the Ta-BC coating on the graphite substrate surface.

[0029] Figure 5 SEM images (a) and mapping analysis (b~f) of the corner section of the Ta-BC coating on the graphite substrate surface.

[0030] Figure 6 High-magnification SEM images (a) and mapping analysis (b, c) of the interface between the graphite matrix and the Ta-BC coating;

[0031] Figure 7 The hardness-displacement curves (ac) and load-displacement curves (d) at different points were obtained by nanoindentation testing of the Ta-BC coating on the graphite substrate surface.

[0032] Figure 8SEM images (a) and mapping analysis (b~e) of the cross-section of the Ta-BC deposit obtained by molten salt electrophoretic deposition using two electrodes. Detailed Implementation

[0033] The above-mentioned and other technical features and advantages of the present invention will be described in more detail below with reference to the accompanying drawings.

[0034] The main phases of the Ta-BC coating prepared in this invention were analyzed by XRD and XPS (e.g., Figure 1 , Figure 2 As shown in the figure, the structure contains Ta-B bonds, BC bonds, Ta-C bonds, and CC bonds, collectively referred to as tantalum borocarbide.

[0035] Example 1

[0036] 10.8 g of boron powder (particle size approximately 1 μm) was used as the boron source, and 3.6 g of carbon powder (acetylene black, particle size approximately 30 nm) was used as the carbon source, along with 44.1 g of tantalum pentoxide (…). The powder (particle size approximately 80 nm) was mixed to achieve a molar ratio of boron source, carbon source, and tantalum pentoxide of approximately 10.5:3:1. Then, the above raw materials were mixed with 1053 g of sodium fluoride (NaF) and 947 g of aluminum fluoride (… The raw materials of the fluoride molten salt system are mechanically mixed (NaF and...) The molar ratio of the active materials (boron source, carbon source, and tantalum pentoxide) was approximately 1.1:1, with the total mass of the active materials accounting for 2.9% of the total mass of the fluoride salt. The mixture was then placed in a 3L graphite crucible and heated to 1000℃ under an argon atmosphere for 4 hours to conduct an in-situ reaction, generating boron-tantalum carbide nanoparticles with a particle size distribution of 5–15 nm in the molten salt, thus obtaining a stable suspension molten salt. Electrophoretic deposition was then performed, using the graphite crucible as the positive electrode and a 50 mm × 20 mm × 5 mm graphite plate as the negative electrode substrate, vertically immersed in the suspension molten salt. The electrode spacing was set to 2.5 cm, at which point the ratio of the surface area of ​​the positive and negative electrodes immersed in the molten salt was approximately 12. A constant DC voltage of 0.25 V was applied between the two electrodes for electrophoretic deposition for 60 minutes. After electrophoresis, the graphite substrate with the deposited coating is removed, cooled, and cleaned with deionized water, finally obtaining a uniform and dense Ta-BC coating on the substrate surface.

[0037] Analysis revealed that the coating exhibits typical Ta-BC characteristics. Figure 1 , Figure 2 The coating completely encapsulates the graphite substrate, with a uniform thickness of approximately 60µm. Figure 3 The coating is dense and uniform in thickness at both horizontal and corner positions. Figure 4 , Figure 5It seamlessly connects with the graphite matrix interface and even grows within the matrix pores, exhibiting excellent bonding performance. Figure 6 The coating was tested using nanoindentation, and the hardness-indentation depth curve was obtained. Figure 7 ac), determine the hardness value at which the material begins to fracture, and then use the load-displacement curve obtained from nanoindentation testing ( Figure 7 d) The hardness of the coating was measured to be 20.96 GPa, and its fracture toughness was calculated to be 40.93 MPa·m¹ / ² using the Oliver-Pharr method, showing excellent fracture toughness.

[0038] Example 2

[0039] This embodiment aims to verify the method of the present invention in KF- The molten salt system and its implementation effect on a titanium alloy matrix. 8.1 g of boron powder (particle size approximately 500 nm) was used as the boron source, 2.4 g of carbon powder (acetylene black, particle size approximately 30 nm) was used as the carbon source, and mixed with 22.05 g of tantalum pentoxide powder, resulting in a molar ratio of boron source, carbon source, and tantalum pentoxide of approximately 15:4:1. This mixture was then combined with 474 g of potassium fluoride (KF) and 456 g of aluminum fluoride (… A fluoride molten salt system consisting of raw materials (molar ratio approximately 1.5:1) was mixed. The mixture was then placed in a 3L graphite crucible and heated to 1100℃ under an argon atmosphere for 3 hours to generate boron tantalum carbide nanoparticles with a particle size distribution of 10–25 nm in situ, thus preparing a suspended molten salt. Electrophoretic deposition was then performed, using the graphite crucible as the positive electrode and a 40 mm × 15 mm × 3 mm Ti-6Al-4V titanium alloy plate as the negative electrode substrate, vertically immersed in the molten salt. The electrode spacing was set to 2.0 cm, at which point the ratio of the surface area of ​​the positive electrode to the surface area of ​​the negative electrode immersed in the molten salt was approximately 15. A constant DC voltage of 0.2 V was applied between the two electrodes for electrophoretic deposition for 90 minutes. After electrophoresis, the coated substrate was cleaned and dried, ultimately obtaining a uniform and dense Ta-BC coating with a thickness of approximately 105 µm on the substrate surface. Nanoindentation testing showed that the prepared coating had a hardness of 28.2 GPa and a fracture toughness of 35 MPa·m¹ / ². The coating bonded well to the titanium alloy substrate, with no cracks or peeling.

[0040] Example 3

[0041] This embodiment aims to verify the feasibility of preparing thick coatings at lower reaction temperatures and longer deposition times, using a carbon / carbon composite material as the matrix. 13.0 g of boron powder (approximately 5 μm particle size) was used as the boron source, 6.0 g of carbon powder (acetylene black, approximately 30 nm particle size) was used as the carbon source, and mixed with 44.1 g of tantalum pentoxide powder, resulting in a molar ratio of boron source, carbon source, and tantalum pentoxide of approximately 12:5:1. This mixture was then combined with 2868 g of NaF- A mixed salt (molar ratio approximately 1.9:1) was prepared. The material was then placed in a 5L graphite crucible and held at 950℃ for 5 hours under an argon atmosphere to generate boron tantalum carbide nanoparticles with a particle size distribution of 20–30 nm in situ, thus preparing a suspended molten salt. Electrophoretic deposition was then performed, using the graphite crucible as the positive electrode and a 30 mm × 30 mm × 10 mm carbon / carbon (C / C) composite material block as the negative electrode substrate immersed in the molten salt. The electrode spacing was set to 3.0 cm, resulting in a surface area ratio of approximately 25 between the positive and negative electrodes immersed in the molten salt. A constant DC voltage of 0.3 V was applied between the electrodes for electrophoretic deposition for 180 minutes. After electrophoresis, the material was cleaned and dried, ultimately obtaining an ultra-thick Ta-BC coating with a thickness of 190 µm on the surface of the C / C composite material substrate. The coating is dense and uniformly covers the complex surface of the C / C composite material, with a hardness of 32.8 GPa and a fracture toughness of 28 MPa·m¹ / ², exhibiting excellent comprehensive mechanical properties.

[0042] Comparative Example 1

[0043] This comparative example aims to verify the necessity of the technical feature defined in this invention: "the ratio of the surface area of ​​the positive electrode to the negative electrode is 10 to 30". Except for the electrode arrangement, all other steps and raw materials are exactly the same as in Example 1. In this comparative example, a conventional electrode arrangement is used, i.e., a graphite sheet is inserted into the graphite crucible as the positive electrode, so that the ratio of the surface area of ​​the positive electrode to the negative electrode (graphite matrix) immersed in the molten salt is approximately 1:1. After electrophoretic deposition, the graphite negative electrode is analyzed, and its cross-sectional SEM image is shown below. Figure 8 As shown. From Figure 8 (a) It is evident that the obtained deposition layer suffers from severe quality defects: 1) The thickness is extremely uneven, exhibiting significant differences at different locations, with the thickest point being approximately 17 μm and the thinnest only 2 μm, failing to form an effective overall protection; 2) The structure is loose and discontinuous, with numerous pores and unbonded particles present in the coating. Furthermore, as... Figure 8Energy dispersive spectroscopy (EDS) analysis showed that a large number of undeposited tantalum borocarbide nanoparticles agglomerated in the residual molten salt on and around the deposited layer. The results indicated that a 1:1 positive / negative electrode area ratio resulted in an extremely uneven electric field distribution between the electrodes and excessively high local field strength. This unstable electric field environment caused the electrophoresis process to become uncontrolled: in regions with high field strength, particles migrated too quickly and accumulated disorderly, forming a loose structure; while in regions with weak field strength or disturbed electric field, the driving force for particle migration was insufficient, making agglomeration and sedimentation in the molten salt more likely, ultimately preventing effective deposition.

[0044] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, all of which will fall within the protection scope of the present invention.

Claims

1. A method for preparing Ta-BC coatings by molten salt electrophoretic deposition, characterized in that, Includes the following steps: S1, a boron source, a carbon source, tantalum pentoxide and fluoride inorganic salt are mixed and placed in a conductive crucible, and heated to 950-1200℃ to carry out an in-situ reaction to prepare a suspended molten salt containing boron tantalum carbide nanoparticles. S2, using a conductive crucible as the positive electrode and the substrate to be coated as the negative electrode, the negative electrode is immersed in the suspended molten salt obtained in step S1 for electrophoretic deposition to obtain the Ta-BC coating.

2. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S1, the boron source is boron powder or boron carbide powder with a particle size of 100nm-10μm; the carbon source is graphite powder or acetylene black with a particle size of 20-100nm; and the tantalum pentoxide has a particle size of 50nm-100nm.

3. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S1, the molar ratio of boron source, carbon source and tantalum pentoxide is 10~20:2~5:1; the total mass of boron source, carbon source and tantalum pentoxide is 2-5% of the mass of fluoride solid salt.

4. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S1, the fluoride inorganic salt is any one of NaF-AlF3 mixed salt, KF-AlF3 mixed salt, or NaF-KF-AlF3 mixed salt.

5. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 4, characterized in that, The molar ratio of NaF to AlF3 in the NaF-AlF3 mixed salt is 0.6-2.5, and the molar ratio of KF to AlF3 in the KF-AlF3 mixed salt is 0.7-2.

2.

6. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S1, the material of the conductive crucible is any one of graphite, silicon carbide, and ZrB2.

7. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S1, the particle size of the boron tantalum carbide nanoparticles is 5-30 nm.

8. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S2, the substrate material is any one of graphite, carbon / carbon composite material, cemented carbide, metallic titanium, and titanium alloy.

9. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S2, the electrophoresis voltage is 0.15-0.3V, and the ratio of the surface area of ​​the positive electrode to the surface area of ​​the negative electrode immersed in the suspended molten salt is 10-30.

10. The method for preparing Ta-BC coating by molten salt electrophoretic deposition as described in claim 1, characterized in that, In step S2, the thickness of the Ta-BC coating is 5~200µm.

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