A high temperature oxidation resistant gradient coating and its preparation method

Abstract

The present invention discloses a high-temperature oxidation resistant gradient coating and a preparation method thereof, and belongs to the field of coating preparation. The coating comprises a nickel plating layer, a nano adhesive layer and a working layer arranged in sequence from the inside to the outside. The method comprises the following steps: 1. preparing a titanium alloy substrate and performing substrate pretreatment; 2. electroplating a nickel layer on the titanium alloy substrate; 3. spraying a nano adhesive layer on the nickel plating layer; 4. spraying a high-entropy alloy powder on the nano adhesive layer to form a base layer; 5. performing a debonding and semi-melting-solidification treatment on the base layer; 6. cold-spraying nano adhesive and high-entropy alloy powder on the base layer layer by layer until a working layer is formed; 7. vacuum sintering; 8. after the vacuum sintering is completed, slowly cooling to room temperature to complete the coating preparation. The present invention can prepare a gradient coating with excellent high-temperature oxidation resistance and adhesion by redesigning the structure of the coating and coordinating with a unique preparation process.

Description

Technical Field

[0001] The present invention belongs to the technical field of coating preparation, and more specifically, relates to a high-temperature oxidation resistant gradient coating and a preparation method thereof. Background Art

[0002] As a key structural material in the aerospace field, titanium alloy has the advantages of low density, high specific strength, and excellent corrosion resistance. Among them, heat-resistant titanium alloy can form a dense oxide film at high temperature, blocking oxygen penetration and improving oxidation resistance. It is widely used in high-temperature components such as aircraft engines, turbine blades, and combustion chambers. However, when the temperature exceeds 500°C, titanium alloy is very prone to "oxygen embrittlement" damage, resulting in a serious decline in mechanical properties and even the occurrence of "titanium fire", which has become the main bottleneck restricting the high-temperature application of titanium alloys. Since high-temperature oxidation mainly occurs on the surface of titanium alloys, preparing an excellent high-temperature oxidation-resistant coating on its surface is considered to be the most effective way to improve the high-temperature service performance of titanium alloys. In response to this problem, existing technologies have also made some research on improving the performance of coatings.

[0003] As a new type of coating material, high entropy alloy has become a research hotspot due to its excellent high-temperature oxidation resistance, wear resistance and chemical stability. Zhang Xiaoxian et al. [Zhang Xiaoxian, Jie Fang, Zhai Changsheng, et al. Effect of induction coating temperature on the high-temperature oxidation resistance of FeCoCrNiMoBSi high entropy alloy coatings [J]. Heat Treatment of Metals, 2023, 48(6): 52-58] prepared three types of FeCoCrNiMoBSi high entropy alloy coatings on 15CrMo steel substrates by flame thermal spraying combined with induction coating. The results showed that after oxidation at 900℃ for 120h, the oxidation mass increases of the above three coatings were 0.58, 0.50 and 0.54 mg / cm, respectively. 2 , and the oxidation mass increase of the matrix is ​​73.28 mg / cm 2 , approximately 146 times the increase in coating mass due to oxidation. This indicates that high-entropy alloy coatings have significant oxidation resistance at high temperatures. However, flame thermal spraying technology faces challenges during application, such as heat-affected zones, high-temperature oxidation, coating quality, and thickness control, which limit its use in certain situations.

[0004] Another example is the Chinese patent application number CN202111250907.X, published on January 28, 2022, which discloses a Cr2O3 / Al2O3 gradient anti-oxidation coating for titanium alloys and a preparation method thereof. The method uses a magnetron sputtering method to sputter-plate a Cr metal layer on the surface of the titanium alloy, then covers it with aluminum foil and melts the aluminum foil through high-temperature heat treatment, ultimately forming a Cr2O3 / Al2O3 coating, thereby improving the oxidation resistance of the titanium alloy. However, although this coating is also a gradient coating for titanium alloys, its bonding depends solely on the high-temperature melting and covering of the aluminum foil. The adhesion between the gradient coatings during actual preparation is not high, and the coating strength is also affected.

[0005] In summary, although existing technologies have made some progress in improving the high-temperature oxidation resistance of titanium alloys, certain problems still exist in practical applications. Therefore, researching and developing new coating materials and preparation processes to further improve the high-temperature service performance of titanium alloys is of great practical significance. Summary of the Invention

[0006] 1. Problems to be solved

[0007] In view of the fact that the existing high-temperature oxidation resistant gradient coatings for titanium alloys still have certain problems in practical applications, the present invention provides a high-temperature oxidation resistant gradient coating and a preparation method thereof. By redesigning the structure of the coating and combining it with a unique preparation process, a gradient coating with excellent high-temperature oxidation resistance and adhesion can be prepared.

[0008] 2. Technical solution

[0009] To solve the above problems, the present invention adopts the following technical solutions.

[0010] A high-temperature oxidation resistant gradient coating comprises a nickel plating layer, a nano-adhesive layer and a working layer arranged sequentially from the inside to the outside;

[0011] The chemical composition and mass percentage of the nickel plating layer are: nickel: 98-99%, sulfur: 0.01-0.05%, and the balance is other impurity elements;

[0012] The chemical composition and mass percentage of the nano adhesive layer are: polyvinyl alcohol: 50-70%, plasticizer: 5-10%, ceramic nano-Al2O3 particles: 10-20%, deionized water: 10-20%;

[0013] The working layer includes multiple high-entropy alloy layers, with a nano-adhesive between two adjacent high-entropy alloy layers. The chemical composition and mass percentage of the high-entropy alloy layers are: carbon: 0.1-0.5%, boron: 1-1.3%, chromium: 19-21%, nickel: 21-24%, silicon: 1-1.5%, molybdenum: 8-10%, cobalt: 22-24%, sulfur: 0.005-0.015%, phosphorus: 0.015-0.045%, and the remainder is iron and unavoidable impurities.

[0014] As a further improvement of the technical solution, the thickness of the nickel plating layer is 30 to 50 μm.

[0015] As a further improvement of the technical solution, the thickness of the nano adhesive layer is 10 to 20 μm.

[0016] As a further improvement of the technical solution, the thickness of the high entropy alloy layer is 100 to 200 μm, and the thickness of the working layer is 1 to 1.2 mm.

[0017] A method for preparing a high-temperature oxidation resistant gradient coating comprises the following steps:

[0018] 1. Prepare the titanium alloy substrate and perform ultrasonic cleaning and pickling activation treatment;

[0019] 2. electroplating a nickel layer on the titanium alloy substrate to form a nickel-plated layer;

[0020] 3. Spraying a nano adhesive layer on the nickel plating layer using a cold spraying process;

[0021] 4. Using a cold spraying process to spray high entropy alloy powder on the nano adhesive layer to form a base layer;

[0022] 5. Use oxyacetylene flame spray gun to remove adhesive and semi-melt-solidify the base layer;

[0023] 6. On the basis of the base layer, cold spray the nano adhesive and high entropy alloy powder layer by layer until a working layer is formed;

[0024] 7. Vacuum sintering the cold sprayed working layer;

[0025] 8. After vacuum sintering is completed, slowly cool to room temperature to complete the coating preparation.

[0026] As a further improvement of the technical solution, in step 2, the nickel plating layer is prepared by:

[0027] (1) Add nickel sulfate, nickel chloride and boric acid to the electroplating bath, adjust the pH value of the electroplating solution to 4-4.5, and heat the electroplating solution to 50-60°C;

[0028] (2) Use high-purity nickel as the anode material for electroplating, with a current density of 2 to 4 A / dm 2 , the electroplating time is 1 to 2 hours.

[0029] As a further improvement of the technical solution, in step three, the spraying pressure is 0.2-0.4 MPa, the nozzle diameter is 0.2-0.4 mm, the spraying distance is 10-20 cm, and the spraying speed is 100-200 mm / s.

[0030] As a further improvement of the technical solution, in step four, the spraying pressure is 1.5-3 MPa, the nozzle diameter is 0.8-1.2 mm, the spraying distance is 25-35 cm, and the spraying speed is 150-300 mm / s.

[0031] As a further improvement of the technical solution, in step five, when performing the debonding treatment, the flame temperature is 200-300°C, the nozzle diameter is 1.0-1.5 mm, the distance between the nozzle and the surface of the base layer is 10-15 cm, and the heating is performed for 5-10 seconds each time; when performing the semi-melting-solidifying treatment, the flame temperature is 600-800°C, the nozzle diameter is 1.0-1.5 mm, the distance between the nozzle and the surface of the base layer is 10-20 cm, and the heating is performed for 5-10 seconds each time.

[0032] As a further improvement of the technical solution, in step seven, a step-by-step heating method is adopted, specifically:

[0033] (1) Heating from room temperature to 500°C at a rate of 10°C / min and maintaining at 500°C for 30 minutes;

[0034] (2) heating from 500°C to 800°C at a rate of 10°C / min and maintaining at 800°C for 30 minutes;

[0035] (3) Raise the temperature from 800°C to 1200-1300°C at a rate of 5°C / min and keep it at 1200-1300°C for 1-2 hours.

[0036] 3. Beneficial effects

[0037] Compared to the prior art, the present invention provides a high-temperature oxidation-resistant gradient coating and its preparation method, which uses a CoCrFeNiBSi high-entropy alloy coating. The coating has excellent oxidation resistance in high-temperature environments and can effectively prevent oxygen from penetrating into the interior of the coating, making it suitable for high-temperature working environments. In addition, the ceramic nano-Al2O3 particles added to the nano-adhesive layer not only improve the hardness and wear resistance of the coating, but also fill the microscopic voids and pores in the coating, increase the density of the coating, and reduce losses caused by mechanical friction and wear. At the same time, the use of an electroplated nickel layer as a transition layer can effectively alleviate the difference in thermal expansion coefficient between the titanium alloy substrate and the high-entropy alloy coating, reduce the stress and cracks caused by thermal expansion mismatch, provide a good base, and enhance the adhesion and high-temperature stability of the coating.

[0038] Secondly, the present invention adopts oxyacetylene flame semi-melting-solidification treatment, which can achieve metallurgical bonding between the coating material and the substrate, forming a dense bonding layer, and significantly improving the adhesion and high temperature resistance of the coating. At the same time, the cold spraying process is adopted to avoid the common thermal stress and heat-affected zone problems in the thermal spraying process, reduce thermal damage and deformation of the substrate, improve the chemical purity and physical properties of the coating, and have higher deposition efficiency and thickness control accuracy. The step-by-step heating vacuum sintering process gradually adapts the substrate and coating to temperature changes through slow heating and staged temperature control, avoiding thermal stress concentration and material cracks caused by rapid heating, and finally forming a uniform and dense structure, thereby improving the comprehensive performance of the coating.

[0039] In particular, the present invention, through the combination of multiple processes and the design of the composition of the gradient coating, can prepare a gradient coating with excellent high-temperature oxidation resistance and adhesion under the cold spray process, significantly improving the oxidation resistance of titanium alloy in high-temperature environments and effectively extending its service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the structure of the high-temperature oxidation-resistant gradient coating on the surface of TC11 titanium alloy;

[0041] Figure 2 is the XRD pattern of FeCoCrNiMoBSi powder;

[0042] Figure 3 It is a structural schematic diagram of a nickel electroplating device on a titanium alloy substrate;

[0043] Figure 4 It is the step temperature rising curve of vacuum sintering process;

[0044] Figure 5 is a histogram of the microhardness of each coating in the embodiments and comparative examples;

[0045] Figure 6The figure is a bar graph showing the oxidation weight gain rate of each coating in the examples and comparative examples. DETAILED DESCRIPTION

[0046] Exemplary embodiments of the present invention are described in detail below. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be implemented and that various changes may be made to the invention without departing from the spirit and scope of the invention. The following more detailed description of the embodiments of the invention is not intended to limit the scope of the claimed invention, but is merely for illustrative and non-limiting purposes, to describe the features and characteristics of the invention, to set forth the best mode for carrying out the invention, and to enable those skilled in the art to practice the invention. Therefore, the scope of the invention is limited solely by the appended claims.

[0047] A high-temperature oxidation resistant gradient coating is suitable for high-temperature working environments in the aerospace field. It can effectively improve the high-temperature oxidation resistance and wear resistance of the coating, extend the service life of the equipment, and is particularly suitable for titanium alloy materials. The gradient coating consists of a nickel plating layer, a nano-adhesive layer, and a working layer. The structural diagram of the gradient coating is shown in the figure below. Figure 1 shown.

[0048] Among them, the chemical composition and mass percentage of the nickel plating layer are: nickel: 98-99%, sulfur: 0.01-0.05%, and the remainder is other impurities (such as hydrogen, oxygen, etc.); the nickel plating layer is a coating covering the surface of the titanium alloy substrate and bonded to the substrate, with a thickness of generally 30-50 μm.

[0049] The nano-adhesive layer's chemical composition and weight percentages are: polyvinyl alcohol (50-70%), plasticizer (5-10%), typically glycerin, ceramic nano-Al2O3 particles (10-20%) as a filler, and deionized water (10-20%). The nano-adhesive is evenly applied to the nickel-plated surface using a cold spray process, typically to a thickness of 10-20 μm.

[0050] The working layer includes multiple high-entropy alloy layers, with a nano-adhesive between adjacent high-entropy alloy layers. The chemical composition and mass percentage of the high-entropy alloy layers are as follows: carbon: 0.1-0.5%, boron: 1-1.3%, chromium: 19-21%, nickel: 21-24%, silicon: 1-1.5%, molybdenum: 8-10%, cobalt: 22-24%, sulfur: 0.005-0.015%, phosphorus: 0.015-0.045%, and the balance is iron and unavoidable impurities. The phase structure of the FeCoCrNiMoBSi powder is FCC, such as Figure 2The modified high entropy alloy powder is evenly sprayed on the nano adhesive layer using a cold spraying process. The thickness of a single layer is generally 100 to 200 μm. The nano adhesive and high entropy alloy powder are cold sprayed layer by layer to a thickness of 1 to 1.2 mm.

[0051] The above-mentioned gradient coating uses an electroplated nickel layer as the base coating, and a nano-adhesive and high-entropy alloy layer is cold-sprayed layer by layer on the base coating as the working layer. Its main raw material is CoCrFeNiMBSi high-entropy alloy, which has excellent oxidation resistance, corrosion resistance and wear resistance in high-temperature environments, and the performance of the coating itself is guaranteed to a certain extent. The ceramic nano-Al2O3 particles in the nano-adhesive are not easy to decompose or react in high-temperature environments, and can form a stable oxide film at high temperatures to prevent oxygen from penetrating into the interior of the coating, thereby further improving the high-temperature stability and oxidation resistance of the coating, making it suitable for high-temperature working environments in the aerospace field. At the same time, the nano-Al2O3 particles have a high hardness, which improves the hardness and wear resistance of the coating, can effectively reduce the loss of the coating in mechanical friction and wear, and extend the service life of the coating.

[0052] Secondly, the present invention performs a gradient treatment on the coating, and the nickel plating layer serves as a transition layer, which can effectively alleviate this difference in thermal expansion coefficients and reduce the stress and cracks caused by thermal expansion mismatch in high temperature environments. The thermal expansion coefficient of nickel is between that of the titanium alloy and the coating material, playing a buffering and transitional role. In addition, the nickel plating layer provides a good substrate, which helps to enhance the adhesion of the subsequent high entropy alloy coating. A layer of oxide film is easily formed on the surface of the titanium alloy, and coating deposition directly thereon may result in poor adhesion. The electroplated Ni layer can effectively cover the surface of the titanium alloy, making it easier for the high entropy alloy coating to adhere thereto evenly and firmly. The nickel plating layer not only enhances the adhesion of the coating, but also nickel has good corrosion resistance, which can provide additional protection in high temperature and corrosive environments to prevent the titanium alloy substrate from being corroded.

[0053] The working layer is cold-sprayed with nano-adhesive and high-entropy alloy coatings layer by layer. This layered structural design ensures the stability and durability of the coating in high-temperature environments. Layered deposition controls the thickness and uniformity of each layer, effectively dispersing thermal stress and avoiding stress concentration and structural defects. In addition, the ceramic nano-Al2O3 particles in the nano-adhesive can fill the microscopic voids and pores in the coating, increase the density of the coating, and reduce the porosity of the coating. The uniform distribution of Al2O3 particles can make the coating structure more uniform and avoid performance degradation caused by local defects. At the same time, nano-Al2O3 particles can play a toughening role in the coating, improving the toughness of the coating by refining the grains and preventing crack propagation.

[0054] The method for preparing the high-temperature oxidation resistant gradient coating for titanium alloy comprises the following steps:

[0055] 1. Prepare a titanium alloy substrate for cold spraying, immerse the titanium alloy substrate in anhydrous ethanol solution for ultrasonic cleaning to remove oil and pollutants.

[0056] Second, the cleaned titanium alloy substrate is immersed in an acid pickling solution for surface activation treatment as required to remove the surface oxide layer and expose the pure metal substrate. Then, the surface of the activated titanium alloy substrate is electroplated with nickel to form a bottom coating. The preparation process of the electroplated nickel layer in this step is as follows: Figure 3 As shown, the following steps are included:

[0057] (1) Add 240-300 g / L of nickel sulfate (NiSO4·6H2O), 30-90 g / L of nickel chloride (NiCl2·6H2O), and 30-45 g / L of boric acid (H3BO3) to the plating bath. Stir thoroughly with a glass rod or magnetic stirrer until all chemicals are completely dissolved. Adjust the pH of the plating solution to 4-4.5 using dilute sulfuric acid or sodium hydroxide. Heat the plating solution to 50-60°C and maintain a constant temperature.

[0058] (2) Immerse the titanium alloy substrate in a pickling solution (HF 30-40 mL, HNO3 70-80 mL) for 1-3 minutes. After activation, rinse the substrate thoroughly with deionized water and dry it in preparation for plating.

[0059] (3) Use high-purity nickel as the anode material to ensure that the anode can stably provide nickel ions. Clamp the surface-activated titanium alloy substrate on the electroplating rack, and place the titanium alloy substrate and nickel anode in the electroplating tank respectively to ensure that the electroplating solution completely covers the substrate and the anode.

[0060] (4) Set the current density to 2-4 A / dm 2 The electroplating time is generally controlled within 1 to 2 hours to ensure that the Ni layer thickness reaches 30 to 50 μm. During the electroplating process, the plating solution temperature and pH value are regularly checked to ensure that they are within the set range. The plating solution is gently stirred with a glass rod to maintain uniform plating conditions.

[0061] (5) After the electroplating is completed, the titanium alloy substrate is taken out from the electroplating tank, and the substrate is thoroughly rinsed with deionized water to remove the electroplating solution residue. Finally, the substrate is placed in a constant temperature drying oven for drying to obtain a titanium alloy substrate with a nickel-plated layer on the surface.

[0062] Third, prepare the nanoadhesive according to the set composition. Using cold spray equipment, spray the nanoadhesive evenly onto the surface of the electroplated nickel layer at a spray pressure of 0.2-0.4 MPa, a nozzle diameter of 0.2-0.4 mm, a spray distance of 10-20 cm, and a spray speed of 100-200 mm / s, ensuring that the single layer thickness is controlled within 10-20 μm. Next, use cold spray equipment at a spray pressure of 1.5-3 MPa, a nozzle diameter of 0.8-1.2 mm, a spray distance of 25-35 cm, and a spray speed of 150-300 mm / s to directly cold spray the high-entropy alloy powder onto the nanoadhesive layer. The high-speed impact uniformly mixes the high-entropy alloy powder and the adhesive to form a base layer. The first working layer is called the base layer.

[0063] Compared to thermal spraying, cold spraying not only avoids the thermal stress and heat-affected zone issues common in thermal spraying, but also reduces thermal damage and deformation of the substrate. It also avoids the oxidation and phase change issues common in high-temperature coating processes, resulting in coatings with improved chemical purity and physical properties. Furthermore, the high-speed impact of cold spraying allows powder particles to firmly adhere to the substrate surface, forming a high-density, high-bonding-strength coating with high deposition efficiency. Coating thickness can also be precisely controlled by adjusting spraying parameters, making it suitable for preparing coatings ranging from micrometers to millimeters to meet diverse application requirements.

[0064] Fourth, use an oxyacetylene flame spray gun to remove the adhesive from the base coat in step 3. Use a flame temperature of 200-300°C, a neutral flame (equal ratios of oxygen and acetylene), a nozzle diameter of 1.0-1.5 mm, and a distance of 10-15 cm from the coating surface. Slowly and evenly scan the base coat surface, heating for 5-10 seconds each time, gradually heating until the adhesive is completely volatilized. Then, use an oxyacetylene flame spray gun to semi-melt and solidify the base coat. Use a flame temperature of 600-800°C, a neutral flame (equal ratios of oxygen and acetylene), a nozzle diameter of 1.0-1.5 mm, and a distance of 10-20 cm from the coating surface. Slowly and evenly scan the base coat surface, heating for 5-10 seconds each time, gradually heating until a semi-melted state is achieved. After stopping heating, cool the base coat under a protective atmosphere (such as nitrogen or argon).

[0065] The coating is subjected to a debindering and semi-melting-solidification treatment using an oxyacetylene flame. The oxyacetylene flame can efficiently remove organic components from the nanobinder, ensuring that no residual adhesive remains in the coating, thereby improving the purity and stability of the coating. Furthermore, the flame temperature is increased, and the oxyacetylene flame is used to locally heat the coating, causing it to reach a semi-molten state. This means that some areas of the coating material melt, while other areas remain solid. In the semi-molten state, metal atoms at the interface between the Ni-plated layer, the coating material, and the titanium substrate begin to diffuse into each other, forming a metallurgical bond. After heating stops, the material cools rapidly, and the semi-molten areas resolidify to form a dense, strong bond layer, enhancing the density and adhesion of the coating. The semi-melting-solidification treatment can achieve metallurgical bonding between the coating material and the substrate, forming a dense bond layer, significantly improving the adhesion and high-temperature resistance of the coating.

[0066] 5. On the basis of the base layer, the nano-adhesive and high-entropy alloy powder are cold-sprayed layer by layer, ensuring that the single layer thickness is controlled within 100-200μm. Except for the base layer, after each spraying layer, the coating is subjected to de-adhesive treatment and semi-melting-solidification treatment using an oxyacetylene flame spray gun until the cyclic deposition reaches a working layer of 1-1.2mm.

[0067] The working layer is cold-sprayed with nano-adhesive and high-entropy alloy coatings layer by layer. This layer-by-layer deposition controls the thickness and uniformity of each layer, ensuring the stability and consistency of the coating in a high-temperature environment. Layer-by-layer deposition helps to disperse thermal stress, avoid stress concentration and structural defects. In addition, the ceramic nano-Al2O3 particles in the nano-adhesive can fill the microscopic voids and pores in the coating, increase the density of the coating, and reduce the porosity of the coating. The uniform distribution of Al2O3 particles can make the coating structure more uniform and avoid performance degradation caused by local defects. At the same time, nano-Al2O3 particles can play a toughening role in the coating, improving the toughness of the coating by refining the grains and preventing crack propagation.

[0068] 6. Vacuum sinter the cold-sprayed working layer coating. Use a step-by-step heating method to slowly raise the temperature to the sintering temperature and keep it at the sintering temperature (1200-1300°C) for 1-2 hours to allow the coating material to fully diffuse and combine to form a uniform and dense structure. Finally, slowly cool it to room temperature in a vacuum furnace to complete the coating preparation. The step-by-step heating curve of the vacuum sintering process in this step is as follows: Figure 4 As shown, the following steps are included:

[0069] (1) Place the titanium alloy substrate sprayed with the working layer in a vacuum sintering furnace and evacuate to 10 -3 Pa or lower to ensure that there is no oxygen or other gases affecting the coating quality during the sintering process.

[0070] (2) Initial low-temperature stage: From room temperature to 500°C, the temperature is increased at a rate of 10°C / min and maintained at 500°C for 30 minutes. In the lower temperature range, the slow heating rate helps to gradually heat the substrate and coating, avoiding thermal shock and stress concentration caused by rapid heating. This stage is mainly to eliminate moisture and volatile impurities on the material surface, while preparing for the intermediate temperature stage.

[0071] (3) Intermediate temperature stage: From 500°C to 800°C, the temperature is increased at a rate of 10°C / min and maintained at 800°C for 30 minutes. In the intermediate temperature range, the thermal stress inside the material is further released to ensure uniform temperature distribution. At the same time, it is also to promote the uniform diffusion of Ti and Ni elements in the titanium matrix and nickel plating layer to form TiNi phase. This stage is also to fully prepare for the final high-temperature sintering stage, and the temperature is gradually increased.

[0072] (4) High-temperature sintering stage: From 800°C to 1200-1300°C, the temperature is increased at a rate of 5°C / min. In the high-temperature range, the heating rate is further reduced to avoid thermal stress concentration caused by differences in thermal expansion coefficients. After reaching the sintering temperature of 1200-1300°C, a 1-2 hour heat preservation treatment is performed to allow the coating material to fully diffuse and combine to form a uniform and dense structure.

[0073] (5) Cooling stage: After vacuum sintering is completed, slowly cool to room temperature at a rate of 5°C / min to avoid thermal stress and cracks caused by rapid cooling. During the cooling process, nitrogen is introduced to atmospheric pressure to prevent rapid oxidation of the coating in the air. Maintain a stable nitrogen flow to ensure that the coating surface is not exposed to oxygen.

[0074] The vacuum sintering process employs a stepped heating rate and phased temperature control, allowing the substrate and coating to gradually adapt to temperature changes, avoiding thermal stress concentration and cracking caused by rapid heating. Particularly during the low and medium temperature stages, the slow heating helps eliminate surface moisture and residual binder, releasing thermal stress within the coating. Simultaneously, it promotes uniform diffusion of Ti and Ni elements within the titanium substrate and nickel coating, forming a TiNi phase. During the high-temperature sintering stage (1200-1300°C), the coating material diffuses and bonds fully, forming a uniform and dense structure. At high temperatures, the nickel coating diffuses between the Ti, Ni, Co, Cr, Fe, and Mo atoms in the coating and titanium substrate, promoting metallurgical bonding. This bonding not only enhances the coating's adhesion but also improves its mechanical properties and high-temperature oxidation resistance. In high-entropy alloy coatings, in particular, the diffusion and mixing of different elements contributes to the formation of a stable multi-phase structure, enhancing the overall performance of the material. Furthermore, slow cooling avoids the thermal stress and microcracking caused by rapid cooling, maintaining the integrity and stability of the coating.

[0075] Example 1

[0076] Step 1: Substrate preparation and surface activation:

[0077] Prepare a TC11 titanium alloy substrate. Immerse the titanium alloy substrate in anhydrous ethanol solution for ultrasonic cleaning. Then, immerse the cleaned titanium alloy substrate in a mixed pickling solution of 30 mL HF and 70 mL HNO3 for 3 minutes for surface activation. Rinse thoroughly with deionized water and dry.

[0078] Step 2: Electroplating nickel layer:

[0079] Prepare a plating solution containing 280g / L nickel sulfate (NiSO4·6H2O), 60g / L nickel chloride (NiCl2·6H2O), and 40g / L boric acid (H3BO3) in a plating tank. Adjust the pH of the plating solution to 4.3 and heat it to 55°C. Then place the titanium alloy substrate and nickel anode in the plating tank respectively, and set the current density to 3A / dm 2 The electroplating time is 1 hour, and a nickel plating layer with a thickness of 30 μm is formed.

[0080] Step 3: Cold spraying nano adhesive layer:

[0081] Prepare a nano-adhesive with the following chemical composition by weight: 70% polyvinyl alcohol, 7% plasticizer (glycerol), 15% filler (ceramic nano-Al2O3 particles), and 8% deionized water. Apply the nano-adhesive to the electroplated nickel layer using a cold spray process at a pressure of 0.3 MPa, a nozzle diameter of 0.3 mm, a spray distance of 15 cm, and a spray speed of 100 mm / s. The adhesive layer thickness is 10 μm.

[0082] Step 4: Cold spraying high entropy alloy base layer:

[0083] Prepare a high-entropy alloy powder with the following chemical composition by mass: 0.1% carbon, 1% boron, 19% chromium, 21% nickel, 1% silicon, 8% molybdenum, 22% cobalt, 0.005% sulfur, 0.015% phosphorus, and the balance iron. Use a cold spray process to apply the high-entropy alloy powder to the nanobinder layer at a spray pressure of 2 MPa, a nozzle diameter of 1 mm, a spray distance of 30 cm, and a spray speed of 200 mm / s, to form a 200-μm-thick base layer.

[0084] Step 5: Debonding and semi-melting-curing treatment:

[0085] Use an oxyacetylene flame spray gun to remove the adhesive from the base layer in step 4, flame temperature: 200°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.0mm, distance between nozzle and coating surface: 10cm, scan the surface of the base layer for 5 seconds, and then use an oxyacetylene flame spray gun to semi-melt and solidify the base layer, flame temperature: 800°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.5mm, distance between nozzle and coating surface: 20cm, scan the surface of the base layer for 10 seconds.

[0086] Step 6: Cold spraying high entropy alloy working layer layer by layer:

[0087] Nanobinder and high-entropy alloy powders are cold-sprayed layer by layer on top of the base coat, ensuring a single layer thickness of 200 μm. Except for the base coat, each layer is then debindered and semi-melted, then solidified using an oxyacetylene flame spray gun until a 1 mm working layer is deposited.

[0088] Step 7: Vacuum sintering:

[0089] Place the cold-sprayed working layer coating in a vacuum sintering furnace and evacuate to 10-3Pa. Use a step-by-step heating method, heating from room temperature to 500℃ at 10℃ / min and keeping at 500℃ for 30 minutes; heating from 500℃ to 800℃ at 10℃ / min and keeping at 800℃ for 30 minutes; heating from 800℃ to 1200℃ at 5℃ / min and keeping at 1200℃ for 1.5 hours. Slowly cool to room temperature at a rate of 5℃ / min. The coating is prepared and numbered. Figure 5 and Figure 6 Coating #1 in.

[0090] Example 2

[0091] Step 1: Substrate preparation and surface activation:

[0092] Prepare a TC11 titanium alloy substrate. Immerse the titanium alloy substrate in anhydrous ethanol solution for ultrasonic cleaning. Then, immerse the cleaned titanium alloy substrate in a mixed pickling solution of 30 mL HF and 70 mL HNO3 for 3 minutes for surface activation. Rinse thoroughly with deionized water and dry.

[0093] Step 2: Electroplating nickel layer:

[0094] Prepare a plating solution containing 280g / L nickel sulfate (NiSO4·6H2O), 60g / L nickel chloride (NiCl2·6H2O), and 40g / L boric acid (H3BO3) in a plating tank. Adjust the pH of the plating solution to 4.3 and heat it to 55°C. Then place the titanium alloy substrate and nickel anode in the plating tank respectively, and set the current density to 3A / dm 2 The electroplating time is 1.5 hours, and a nickel plating layer with a thickness of 40 μm is formed.

[0095] Step 3: Cold spraying nano adhesive layer:

[0096] Prepare a nano-adhesive with the following chemical composition by weight: 70% polyvinyl alcohol, 7% plasticizer (glycerol), 15% filler (ceramic nano-Al2O3 particles), and 8% deionized water. Apply the nano-adhesive to the electroplated nickel layer using a cold spray process at a pressure of 0.3 MPa, a nozzle diameter of 0.3 mm, a spray distance of 15 cm, and a spray speed of 100 mm / s. The adhesive layer thickness is 10 μm.

[0097] Step 4: Cold spraying high entropy alloy base layer:

[0098] Prepare a high-entropy alloy powder with the following chemical composition by mass: 0.1% carbon, 1% boron, 19% chromium, 21% nickel, 1% silicon, 8% molybdenum, 22% cobalt, 0.005% sulfur, 0.015% phosphorus, and the balance iron. Use a cold spray process to apply the high-entropy alloy powder to the nanobinder layer at a spray pressure of 2 MPa, a nozzle diameter of 1 mm, a spray distance of 30 cm, and a spray speed of 200 mm / s, to form a 200-μm-thick base layer.

[0099] Step 5: Debonding and semi-melting-curing treatment:

[0100] Use an oxyacetylene flame spray gun to remove the adhesive from the base layer in step 4, flame temperature: 200°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.0mm, distance between nozzle and coating surface: 10cm, scan the surface of the base layer for 5 seconds, and then use an oxyacetylene flame spray gun to semi-melt and solidify the base layer, flame temperature: 800°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.5mm, distance between nozzle and coating surface: 20cm, scan the surface of the base layer for 10 seconds.

[0101] Step 6: Cold spraying high entropy alloy working layer layer by layer:

[0102] Nanobinder and high-entropy alloy powders are cold-sprayed layer by layer on top of the base coat, ensuring a single layer thickness of 200 μm. Except for the base coat, each layer is then debindered and semi-melted, then solidified using an oxyacetylene flame spray gun until a 1 mm working layer is deposited.

[0103] Step 7: Vacuum sintering:

[0104] Place the cold-sprayed working layer coating in a vacuum sintering furnace and evacuate to 10-3Pa. Use a step-by-step heating method, heating from room temperature to 500℃ at 10℃ / min and keeping at 500℃ for 30 minutes; heating from 500℃ to 800℃ at 10℃ / min and keeping at 800℃ for 30 minutes; heating from 800℃ to 1200℃ at 5℃ / min and keeping at 1200℃ for 1.5 hours. Slowly cool to room temperature at a rate of 5℃ / min. The coating is prepared and numbered. Figure 5 and Figure 6 Coating #2 in.

[0105] Example 3

[0106] Step 1: Substrate preparation and surface activation:

[0107] Prepare a TC11 titanium alloy substrate. Immerse the titanium alloy substrate in anhydrous ethanol solution for ultrasonic cleaning. Then, immerse the cleaned titanium alloy substrate in a mixed pickling solution of 30 mL HF and 70 mL HNO3 for 3 minutes for surface activation. Rinse thoroughly with deionized water and dry.

[0108] Step 2: Electroplating nickel layer:

[0109] Prepare a plating solution containing 280g / L nickel sulfate (NiSO4·6H2O), 60g / L nickel chloride (NiCl2·6H2O), and 40g / L boric acid (H3BO3) in a plating tank. Adjust the pH of the plating solution to 4.3 and heat it to 55°C. Then place the titanium alloy substrate and nickel anode in the plating tank respectively, and set the current density to 3A / dm 2 The electroplating time is 1 hour, and a nickel plating layer with a thickness of 40 μm is formed.

[0110] Step 3: Cold spraying nano adhesive layer:

[0111] Prepare a nano-adhesive with the following chemical composition by weight: 65% polyvinyl alcohol, 7% plasticizer (glycerol), 20% filler (ceramic nano-Al2O3 particles), and 8% deionized water. Apply the nano-adhesive to the electroplated nickel layer using a cold spray process at a pressure of 0.3 MPa, a nozzle diameter of 0.3 mm, a spray distance of 15 cm, and a spray speed of 100 mm / s. The adhesive layer thickness is 10 μm.

[0112] Step 4: Cold spraying high entropy alloy base layer:

[0113] Prepare a high-entropy alloy powder with the following chemical composition by mass: 0.1% carbon, 1% boron, 19% chromium, 21% nickel, 1% silicon, 8% molybdenum, 22% cobalt, 0.005% sulfur, 0.015% phosphorus, and the balance iron. Use a cold spray process to apply the high-entropy alloy powder to the nanobinder layer at a spray pressure of 2 MPa, a nozzle diameter of 1 mm, a spray distance of 30 cm, and a spray speed of 200 mm / s, to form a 200-μm-thick base layer.

[0114] Step 5: Debonding and semi-melting-curing treatment:

[0115] Use an oxyacetylene flame spray gun to remove the adhesive from the base layer in step 4, flame temperature: 200°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.0mm, distance between nozzle and coating surface: 10cm, scan the surface of the base layer for 5 seconds, and then use an oxyacetylene flame spray gun to semi-melt and solidify the base layer, flame temperature: 800°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.5mm, distance between nozzle and coating surface: 20cm, scan the surface of the base layer for 10 seconds.

[0116] Step 6: Cold spraying high entropy alloy working layer layer by layer:

[0117] Nanobinder and high-entropy alloy powders are cold-sprayed layer by layer on top of the base coat, ensuring a single layer thickness of 200 μm. Except for the base coat, each layer is then debindered and semi-melted, then solidified using an oxyacetylene flame spray gun until a 1 mm working layer is deposited.

[0118] Step 7: Vacuum sintering:

[0119] Place the cold-sprayed working layer coating in a vacuum sintering furnace and evacuate to 10-3Pa. Use a step-by-step heating method, heating from room temperature to 500℃ at 10℃ / min and keeping at 500℃ for 30 minutes; heating from 500℃ to 800℃ at 10℃ / min and keeping at 800℃ for 30 minutes; heating from 800℃ to 1200℃ at 5℃ / min and keeping at 1200℃ for 1.5 hours. Slowly cool to room temperature at a rate of 5℃ / min. The coating is prepared and numbered. Figure 5 and Figure 6 Coating #3 in.

[0120] Comparative Example 1

[0121] Step 1: Substrate preparation and surface roughening:

[0122] Prepare a TC11 titanium alloy substrate, polish the surface of the titanium alloy substrate with 800# metallographic sandpaper to remove the surface oxide film, immerse the titanium alloy substrate in anhydrous ethanol solution for ultrasonic cleaning and blow dry.

[0123] Step 2: Cold spraying nano adhesive layer:

[0124] Prepare a nano-adhesive with the following chemical composition and percentage by mass: 80% polyvinyl alcohol, 10% plasticizer (glycerol), and 10% deionized water. Apply the nano-adhesive to the electroplated nickel layer using a cold spray process at a spray pressure of 0.3 MPa, a nozzle diameter of 0.3 mm, a spray distance of 15 cm, and a spray speed of 100 mm / s. The adhesive layer thickness is 10 μm.

[0125] Step 3: Cold spraying high entropy alloy base layer:

[0126] Prepare a high-entropy alloy powder with the following chemical composition by mass: 0.1% carbon, 1% boron, 19% chromium, 21% nickel, 1% silicon, 8% molybdenum, 22% cobalt, 0.005% sulfur, 0.015% phosphorus, and the balance iron. Use a cold spray process to apply the high-entropy alloy powder to the nanobinder layer at a spray pressure of 2 MPa, a nozzle diameter of 1 mm, a spray distance of 30 cm, and a spray speed of 200 mm / s, to form a 200-μm-thick base layer.

[0127] Step 4: Debonding and semi-melting-curing treatment:

[0128] Use an oxyacetylene flame spray gun to remove the adhesive from the base layer in step 4, flame temperature: 200°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.0mm, distance between nozzle and coating surface: 10cm, scan the surface of the base layer for 5 seconds, and then use an oxyacetylene flame spray gun to semi-melt and solidify the base layer, flame temperature: 800°C, flame type: neutral flame (equal ratio of oxygen and acetylene), nozzle diameter: 1.5mm, distance between nozzle and coating surface: 20cm, scan the surface of the base layer for 10 seconds.

[0129] Step 5: Cold spraying high entropy alloy working layer layer by layer:

[0130] Nanobinder and high-entropy alloy powders are cold-sprayed layer by layer on top of the base coat, ensuring a single layer thickness of 200 μm. Except for the base coat, each layer is then debindered and semi-melted, then solidified using an oxyacetylene flame spray gun until a 1 mm working layer is deposited.

[0131] Step 6: Vacuum sintering:

[0132] The cold sprayed working layer coating is placed in a vacuum sintering furnace and vacuumed to 10-3 Pa. The temperature is quickly raised to 1200℃ and maintained at 1200℃ for 1.5 hours. Nitrogen is directly injected to quickly cool the coating. The coating is prepared and numbered. Figure 5 and Figure 6 Coating #4 in.

[0133] The thickness and bonding strength of each coating in the above-mentioned embodiments and comparative examples are shown in Table 1. The thickness of coating 1# in the embodiment is 1.037mm, and the bonding strength between the substrate and the coating is 54MPa. The thickness of coating 2# in the embodiment is 1.071mm, and the bonding strength between the substrate and the coating is 59MPa. The thickness of coating 3# in the embodiment is 1.055mm, and the bonding strength between the substrate and the coating is 55MPa. The thickness of coating 4# in comparative example 1 is 0.973mm, and the bonding strength between the substrate and the coating is 32MPa. In contrast, in comparative example 1, the nickel layer was not electroplated, resulting in a serious decrease in the bonding strength between the substrate and the coating. At the same time, the thickness of the electroplated nickel layer increases, which further alleviates the difference in thermal expansion coefficient and improves the adhesion of the coating. In addition, as Figure 5 As shown in the figure, the microhardness of coating 3# is greater than that of coatings 1# and 2#, because the Al2O3 particle content in coating 3# (20%) is higher than that in coatings 1# and 2# (15%). This shows that Al2O3 particles in the coating can significantly improve the microhardness and wear resistance of the coating.

[0134] Table 1 Thickness of each coating and bonding strength between coating and substrate in Examples and Comparative Examples

[0135] Coating thickness (mm) Bonding strength between coating and substrate (MPa) Example 1# 1.037 54 Example 2# 1.071 59 Example 3# 1.055 55 Example 4# 0.973 32

[0136] Comparison of high temperature oxidation resistance of the coatings of Example 1, Example 2 and Example 3 with that of Comparative Example 1:

[0137] The coating samples prepared in Example 1, Example 2, Example 3 and Comparative Example 1 were subjected to a high temperature oxidation test at 900°C for 5 hours in a muffle furnace. Figure 6As shown, the CoCrFeNiMoBSi coating on the surface of TC11 titanium alloy exhibits good high-temperature oxidation resistance at a temperature of 900°C. The coating 3# prepared in Example 3 is oxidized at 900°C for 5 hours, and the oxidation weight gain rate is the lowest. However, the oxidation weight gain rate of coating 4# prepared in Comparative Example 1 is the highest. First, because the titanium alloy substrate is not electroplated with nickel in Comparative Example 1, due to the large difference in thermal expansion coefficient between the titanium alloy substrate and the high-entropy alloy coating, large thermal stress is generated at the interface, forming defects such as pores and microcracks, which reduces the high-temperature oxidation resistance of the coating. Second, because ceramic nano-Al2O3 particles are not added to the adhesive in the comparative example, the formation of pore defects in the coating is promoted, which reduces the high-temperature oxidation resistance of the coating. Third, because the vacuum sintering process in Comparative Example 1 does not perform step-by-step heating and slow cooling, the diffusion of elements in the coating is uneven, and a large amount of brittle intermetallic compounds are formed at the interface. Rapid cooling causes the thermal stress inside the coating to concentrate, forming thermal cracks, which reduces the high-temperature oxidation resistance of the coating.

[0138] In summary, the high-temperature oxidation resistance of Examples 1, 2, and 3 is significantly better than that of Comparative Example 1.

[0139] It should be noted that the present invention achieves a gradient coating with excellent high-temperature oxidation resistance and adhesion through a cold spray process by combining multiple processes and designing the gradient coating's composition. Each step is coordinated and continuous to achieve the high performance of the final coating. To facilitate the creative description, the principles of the entire process are described below in a continuous manner.

[0140] like Figure 1 As shown, interface I is titanium alloy substrate / nickel plating layer, and interface II is nickel plating layer / high entropy alloy coating.

[0141] First, the electroplated nickel layer on the titanium alloy surface is mainly to provide a new base on the titanium alloy surface to solve the problem of mismatched thermal expansion coefficients between the titanium alloy and the high-entropy alloy coating. However, the electroplated nickel layer on the titanium alloy surface only makes the plated metal and the titanium alloy substrate surface closely contact through van der Waals forces and electrostatic attraction. The bonding strength between them is low and must be improved through metallurgical bonding.

[0142] Therefore, a semi-melting-solidification treatment at 800°C using an oxyacetylene flame spray gun was performed. This, on the one hand, promoted the diffusion of Ti atoms from the titanium substrate into the nickel coating, and Ni atoms from the nickel coating into the titanium substrate. At 800°C, the diffusion coefficients of Ti and Ni atoms are low, resulting in a slow elemental diffusion rate, which promotes the formation of a preliminary Ti-Ni diffusion layer at interface I. Furthermore, Co, Cr, Fe, Ni, and Mo atoms from the high-entropy alloy coating diffuse into the nickel coating, and Ni atoms from the nickel coating diffuse into the high-entropy alloy coating. At 800°C, the diffusion coefficient of Ni atoms is greater than that of Co, Cr, Fe, Ti, and Mo, promoting the formation of a Ni-rich diffusion layer at interface II. Simultaneously, Cr atoms in the high-entropy alloy coating are oxidized in air to form trace amounts of Cr2O3 oxide, which provides heterogeneous nucleation sites for grain growth during subsequent vacuum sintering, similar to the role of ceramic nano-Al2O3 particles doped in the binder. However, short-term elemental diffusion cannot form a reliably connected diffusion layer.

[0143] Therefore, a vacuum sintering process with a step-by-step temperature increase method is adopted.

[0144] The temperature was raised from room temperature to 500°C at a rate of 10°C / min and held at 500°C for 30 minutes. At 500°C, the atomic diffusion coefficients of Ti, Ni, Co, Cr, Fe, and Mo are relatively low. Slow diffusion promotes uniform diffusion of all elements at the interface, preventing defects such as Kirkendall voids that form early due to uneven diffusion. At this stage, all elements at Interfaces I and II are uniformly diffused.

[0145] The temperature was raised from 500°C to 800°C at a rate of 10°C / min and kept at this temperature for 30 minutes. At 800°C, the diffusion coefficients of Ti, Ni, Co, Cr, Fe, and Mo atoms increased, but were still relatively low. This further promoted the uniform diffusion of each element and enhanced the metallurgical bonding performance of the interface. At this stage, according to the TiNi phase diagram, at 800°C, the Ni and Ti elements at interface I interacted to form a NiTi phase. At the same time, due to the increased diffusion rate of each element, the Ni element at interface II segregated to form a γ-Ni phase. Due to the slow diffusion effect of high-entropy alloys, the Ni, Co, Cr, Fe, and Mo elements in the high-entropy alloy coating interacted to form a solid solution with a single FCC phase structure.

[0146] The temperature was raised from 800°C to 1200°C at a rate of 5°C / min and held at 1200°C for 1.5 hours. From 800°C to 1200°C, the diffusion coefficients of atoms such as Ti, Ni, Co, Cr, Fe, and Mo increased significantly, significantly boosting the diffusion rate of these elements. This promoted the full diffusion of these elements at the interface, forming a uniform diffusion layer. During this stage, as diffusion progressed, a solid-phase reaction occurred, and the atoms at the interface began to rearrange, forming a stable new phase. This new phase gradually formed and grew at the interface, causing the grains to grow and forming a uniform microstructure.

[0147] When the sintering temperature reaches 1200°C, the diffusion of Ni, Co, Cr, Fe, and Mo atoms within the high-entropy alloy coating intensifies. The accumulation of dislocations and strain energy initiates recrystallization. This process begins, with new grains nucleating and growing in high-energy regions (such as dislocation-dense areas and grain boundaries). These new grains grow rapidly, gradually replacing the original deformed grains. The microstructure within the coating becomes more uniform, and the grains become finer. After recrystallization is complete, the coating develops a uniform, fine-grained structure.

[0148] The examples described in the present invention are merely descriptions of the preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high-temperature oxidation resistant gradient coating, characterized in that: The following steps are involved:

1. Prepare the titanium alloy substrate and perform ultrasonic cleaning and pickling activation treatment; 2. electroplating a nickel layer on the titanium alloy substrate to form a nickel-plated layer; 3. Spraying a nano adhesive layer on the nickel plating layer using a cold spraying process; 4. Using a cold spraying process to spray high entropy alloy powder on the nano adhesive layer to form a base layer; 5. Use oxyacetylene flame spray gun to remove adhesive and semi-melt-solidify the base layer; 6. On the basis of the base layer, cold spray the nano adhesive and high entropy alloy powder layer by layer until a working layer is formed; 7. Vacuum sintering of the cold sprayed working layer. This step adopts a step-by-step heating method, specifically: (1) Heating from room temperature to 500°C at a rate of 10°C / min and keeping at 500°C for 30 minutes; (2) Heating from 500°C to 800°C at a rate of 10°C / min and holding at 800°C for 30 minutes; (3) Raise the temperature from 800°C to 1200-1300°C at a rate of 5°C / min and keep it at 1200-1300°C for 1-2 hours; 8. After vacuum sintering is completed, slowly cool to room temperature to complete the coating preparation; The chemical composition and mass percentage of the nickel plating layer are: nickel: 98-99%, sulfur: 0.01-0.05%, and the balance is other impurity elements; The chemical composition and mass percentage of the nano adhesive layer are: polyvinyl alcohol: 50-70%, plasticizer: 5-10%, ceramic nano-Al2O3 particles: 10-20%, deionized water: 10-20%; The working layer includes multiple high entropy alloy layers, with nano adhesives between two adjacent high entropy alloy layers. The chemical composition and mass percentage of the high entropy alloy layers are: Carbon: 0.1-0.5%, boron: 1-1.3%, chromium: 19-21%, nickel: 21-24%, silicon: 1-1.5%, molybdenum: 8-10%, cobalt: 22-24%, sulfur: 0.005-0.015%, phosphorus: 0.015-0.045%, the balance is iron and inevitable impurities.

2. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: The thickness of the nickel plating layer is 30-50 μm.

3. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: The thickness of the nano adhesive layer is 10 to 20 μm.

4. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: The thickness of the high entropy alloy layer is 100-200 μm, and the thickness of the working layer is 1-1.2 mm.

5. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, characterized in that: In the step 2, the specific preparation method of the nickel plating layer is: (1) Add nickel sulfate, nickel chloride and boric acid to the electroplating tank, adjust the pH value of the electroplating solution to 4-4.5, and heat the electroplating solution to 50-60°C; (2) Use high-purity nickel as the anode material for electroplating, the current density is 2 to 4 A / dm², and the electroplating time is 1 to 2 hours.

6. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: In the step three, the spraying pressure is 0.2-0.4 MPa, the nozzle diameter is 0.2-0.4 mm, the spraying distance is 10-20 cm, and the spraying speed is 100-200 mm / s.

7. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: In the step 4, the spraying pressure is 1.5-3 MPa, the nozzle diameter is 0.8-1.2 mm, the spraying distance is 25-35 cm, and the spraying speed is 150-300 mm / s.

8. The method for preparing a high-temperature oxidation resistant gradient coating according to claim 1, wherein: In the step five, when performing the debonding treatment, the flame temperature is 200-300°C, the nozzle diameter is 1.0-1.5 mm, the distance between the nozzle and the surface of the base layer is 10-15 cm, and each heating is 5-10 seconds; when performing the semi-melting-solidification treatment, the flame temperature is 600-800°C, the nozzle diameter is 1.0-1.5 mm, the distance between the nozzle and the surface of the base layer is 10-20 cm, and each heating is 5-10 seconds.

9. A high temperature oxidation resistant gradient coating, characterized by: It is a high-temperature oxidation-resistant gradient coating as described in any one of claims 1 to 8.

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