A high-thickness, high-purity max phase coating and a method of making the same

Abstract

This invention discloses a high-thickness, high-purity MAX phase coating and its preparation method. The method includes: 1. Mixing a metal oxide and a carbon source, followed by high-energy ball milling and spray granulation to prepare spherical precursor powder; 2. Depositing a transition layer on a sandblasted substrate surface using plasma spraying, and then depositing the spherical precursor powder on the transition layer to form a precursor coating; 3. Immersing the substrate with the precursor coating in molten salt for electrolysis, causing the precursor coating to be electro-deoxidized and converted into a MAX phase coating. This invention employs a synergistic process of "plasma spraying-molten salt electrochemical conversion," converting the sprayed precursor coating into a MAX phase coating through in-situ electro-deoxidation in molten salt electrochemistry. This process has high deposition efficiency and avoids the defects of carbon loss, phase decomposition, and oxidation in the coating product when directly thermally spraying MAX materials. It achieves the preparation of a high-thickness, high-purity MAX phase coating, suitable for the field of surface protection.

Description

Technical Field

[0001] This invention belongs to the field of protective coating technology, specifically relating to a high-thickness, high-purity MAX phase coating and its preparation method. Background Technology

[0002] M n+1 AX n It is a ternary metal-ceramic phase (where M represents a transition metal element, A represents a group 3 or 4 element, X is generally carbon and nitrogen, and n is usually 1 to 3). It not only has the electrical conductivity, thermal conductivity, thermal shock resistance and machinability similar to metals, but also has the high hardness, oxidation resistance, wear resistance and corrosion resistance of ceramic materials. It is suitable for surface protective coatings in harsh and extreme environments and has shown broad application prospects in aerospace, nuclear industry, defense and other fields.

[0003] Currently, the main technologies for preparing MAX phase coatings include cold spraying, vapor deposition, magnetron sputtering, and thermal spraying. However, each method has certain limitations. For example, vapor deposition and magnetron sputtering can prepare high-purity MAX phase coatings, but these processes have low deposition efficiency, and the thin coatings are insufficient to meet the surface protection requirements under complex working conditions such as high temperature, heavy load, and high wear. While thermal spraying can achieve the preparation of thick coatings, MAX phase powders undergo oxidation, phase transformation, and decarburization during high-temperature spraying, resulting in excessively high oxygen content and low MAX phase composition in the coating product, making it difficult to achieve high-purity MAX phase coatings. Cold spraying can avoid phase transformation and decomposition of MAX phase powders during thermal spraying, but it can lead to cracks and delamination when preparing thick coatings. Furthermore, cold spraying has certain requirements on powder particle size and impurity content, which also poses challenges to the preparation process of MAX phase sprayed powders.

[0004] Therefore, due to the limitations of each preparation technology, efficiently synthesizing MAX phase coatings with both high purity and thickness remains a challenge. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a method for preparing a high-thickness, high-purity MAX phase coating, addressing the shortcomings of the prior art. This method employs a synergistic process of "plasma spraying-molten salt electrochemical conversion." First, spherical precursor powder is sintered and deposited onto the substrate surface via plasma spraying. Then, in-situ electrochemical deoxidation through molten salt electrochemistry converts it into a MAX phase coating. This avoids the defects of low coating purity caused by the decomposition, oxidation, and decarburization of MAX phase powder in the high-temperature flame during thermal spraying, and improves the deposition efficiency of the coating. It achieves the preparation of a high-thickness, high-purity MAX phase coating, offering advantages such as short process, low cost, cleanliness, and high efficiency. It solves the problem of the inability to simultaneously achieve deposition efficiency and product purity in existing MAX phase coating preparation processes.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing a high-thickness, high-purity MAX phase coating, characterized in that the method includes the following steps:

[0007] Step 1: Preparation of precursor powder: The metal oxide and carbon source are mixed and then ball-milled to obtain a uniform composite powder. Spherical precursor powder is then prepared by spray granulation.

[0008] Step 2, Precursor Coating Deposition: The substrate surface is sandblasted and plasma spraying is used to deposit a transition layer on the sandblasted substrate surface. Then, the spherical precursor powder from Step 1 is deposited on the transition layer to form a precursor coating.

[0009] Step 3, molten salt electrochemical conversion: The substrate with the precursor coating from Step 1 is immersed in molten salt, and an electrolysis is performed under an inert atmosphere, so that the precursor coating is electrodeoxidized and converted into a MAX phase coating with a thickness greater than 200 μm and a single phase.

[0010] This invention first mixes metal oxide and carbon source by high-energy ball milling, and then prepares spherical precursor powder with good flowability by spray granulation. Then, the substrate surface is sandblasted and a transition layer is sprayed to improve the bonding strength between the subsequent coating and the substrate. Then, the spherical precursor powder is deposited on the surface by plasma spraying to form a precursor coating. Finally, under the protection of an inert atmosphere, the precursor coating is deoxidized in situ by molten salt electrochemical method to obtain the MAX phase coating.

[0011] The above-mentioned method for preparing a high-thickness, high-purity MAX phase coating is characterized in that the metal oxides in step one are TiO2 and Al2O3, and the carbon source is graphite powder.

[0012] The method for preparing a high-thickness, high-purity MAX phase coating is characterized in that the molar ratio of TiO2, Al2O3, and graphite powder is 3:0.75:1.5. By controlling the composition of the raw material powder, the molar ratio of Ti, Al, and C in the spherical precursor powder is ensured to be 2:1:1, consistent with the Ti2AlC composition of the MAX phase coating, thus avoiding the formation of impurity phases due to excessively high or low raw material powder ratios.

[0013] The method for preparing a high-thickness, high-purity MAX phase coating described above is characterized in that the particle size of the spherical precursor powder in step one is 20 μm to 60 μm. This particle size effectively improves the flowability of the spherical precursor powder and reduces carbon combustion loss during the subsequent ion spraying process.

[0014] The method for preparing a high-thickness, high-purity MAX phase coating described above is characterized in that the current of the plasma spraying method in step two is 200A. By controlling the current of the plasma spraying method, while ensuring the sintering and melting of the TiO2 / Al2O3 / C precursor powder, excessive current is avoided to prevent carbon combustion loss in the high-temperature flame, thereby preventing a decrease in the carbon content in the TiO2 / Al2O3 / C precursor coating and the formation of a large amount of TiAl impurity phase during electrolysis.

[0015] The method for preparing a high-thickness, high-purity MAX phase coating described above is characterized in that the thickness of the precursor coating in step two does not exceed 250 μm. By controlling the thickness of the precursor coating, the bonding strength between the coating and the substrate is ensured, avoiding the risk of peeling and cracking during subsequent electrolysis.

[0016] The method for preparing a high-thickness, high-purity MAX phase coating, as described above, is characterized in that the molten salt in step three is CaCl2, which is first dried under vacuum at 300°C for 12 hours and then pre-electrolyzed at 3V for 12 hours. By subjecting the molten salt to prolonged drying and pre-electrolysis, impurities in the molten salt are effectively reduced, which is beneficial for the preparation of a high-purity MAX phase coating.

[0017] The method for preparing a high-thickness, high-purity MAX phase coating described above is characterized in that the electrolysis temperature in step three is 900℃. By controlling the electrolysis temperature, the conversion effect of the MAX phase coating is ensured, avoiding the inability to form the MAX phase, such as Ti2AlC phase, at excessively low temperatures, and avoiding adverse effects on the substrate material at excessively high temperatures.

[0018] The method for preparing a high-thickness, high-purity MAX phase coating described above is characterized in that the electrolysis time in step three is 3 to 5 hours. By controlling the electrolysis time, the conversion effect of the MAX phase coating is ensured, avoiding situations where the electrolysis time is too short to form a high-purity MAX phase such as Ti2AlC phase, and avoiding situations where the electrolysis time is too long, causing the molten salt to have an adverse effect on the coating and the substrate.

[0019] Meanwhile, this invention also discloses a high-thickness, high-purity MAX phase coating prepared by the above-described method, characterized in that the coating is a Ti2AlC coating converted from a TiO2 / Al2O3 / C precursor coating. Typically, the TiO2 / Al2O3 / C precursor coating is used as the cathode, and in-situ electro-deoxidation is performed at a voltage lower than that of the molten salt decomposition to convert it into a Ti2AlC coating, and the Ti2AlC coating has a thickness similar to that of the TiO2 / Al2O3 / C precursor coating.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. This invention employs a synergistic process of "plasma spraying-molten salt electrochemical conversion". Through in-situ electrodeoxidation of molten salt electrochemistry, the precursor coating sprayed on the substrate surface is converted into a MAX phase coating. This effectively avoids the defects such as carbon loss, phase decomposition and oxidation of the coating product caused by instantaneous high temperature when directly thermally spraying MAX phase powder, and realizes the preparation of high-purity MAX phase coating.

[0022] 2. Compared with traditional coating preparation processes that often use expensive MAX phase powders as raw materials and have strict requirements on powder particle size and purity, this invention uses spherical precursor powders prepared by high-energy ball milling of metal oxides and carbon sources followed by spray granulation as raw materials, which reduces raw material costs. Furthermore, the preparation of the target component MAX phase can be achieved by controlling the ratio of precursor powders. The equipment required for subsequent electrolysis is simple in structure, requires less investment, and has low maintenance costs, thus realizing the low-cost preparation of MAX phase coatings.

[0023] 3. To address the shortcomings of the direct molten salt electrochemical method in that most MAX phases prepared are micron-sized powder particles, this invention proposes a combined technology route of plasma spraying-molten salt electrolysis. By using plasma flame to achieve full sintering of spherical precursor powder, the pulverization phenomenon during the electrolysis process is effectively suppressed, and high-quality MAX phase coatings are prepared.

[0024] 4. To address the shortcomings of low deposition efficiency in conventional vapor deposition and magnetron sputtering deposition processes, this invention employs a synergistic process of "plasma spraying-molten salt electrochemical conversion," which improves the deposition efficiency of the coating and enables the preparation of high-thickness MAX phase coatings, meeting the surface protection requirements under load conditions.

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0026] Figure 1 The XRD pattern of the MAX phase coating prepared in Example 1 of this invention.

[0027] Figure 2 This is a SEM image of the MAX phase coating prepared in Example 1 of the present invention.

[0028] Figure 3 The image shows the XRD pattern of the MAX phase coating prepared in Comparative Example 1 of this invention.

[0029] Figure 4 The image shows the XRD pattern of the coating prepared in Comparative Example 2 of this invention.

[0030] Figure 5 The image shows the XRD pattern of the coating prepared in Comparative Example 3 of this invention.

[0031] Figure 6The image shows the XRD pattern of the coating prepared in Comparative Example 4 of this invention. Detailed Implementation

[0032] Example 1

[0033] This embodiment includes the following steps:

[0034] Step 1: Preparation of precursor powder: TiO2 powder, Al2O3 powder and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then ball-milled to obtain a uniform composite powder. Spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is then prepared by spray granulation.

[0035] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water and then roughened by sandblasting. Then, plasma spraying is used. The main process parameters include a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer. The main process parameters include a current of 200A, a powder feed rate of 6rpm, and a main gas flow rate of 140SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 250μm.

[0036] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0037] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode, and the TiO2 / Al2O3 / C precursor coating was used as the cathode. Electrolysis was performed for 3 hours under an inert atmosphere at a temperature of 900℃, with a voltage of 3.2V applied. This allowed the TiO2 / Al2O3 / C precursor coating to be electro-deoxidized and transformed into a MAX phase coating. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly washed with 0.5mol / L dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Finally, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a MAX phase coating on the surface.

[0038] Figure 1 The XRD pattern of the MAX phase coating prepared in this embodiment is shown below. Figure 1It can be seen that the MAX phase coating has a single Ti2AlC phase, indicating that a high-purity Ti2AlC coating has been successfully synthesized.

[0039] Figure 2 SEM images of the MAX phase coating prepared in this embodiment, from Figure 2 It can be seen that the thickness of the MAX phase coating is approximately 200 μm.

[0040] Example 2

[0041] This embodiment includes the following steps:

[0042] Step 1: Preparation of precursor powder: TiO2 powder, Al2O3 powder and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then ball-milled to obtain a uniform composite powder. Spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is then prepared by spray granulation.

[0043] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water and then roughened by sandblasting. Then, plasma spraying is used. The main process parameters include a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer. The main process parameters include a current of 200A, a powder feed rate of 6rpm, and a main gas flow rate of 140SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 250μm.

[0044] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0045] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode, and the TiO2 / Al2O3 / C precursor coating was used as the cathode. Electrolysis was performed for 5 hours under an inert atmosphere at a temperature of 900℃, with a voltage of 3.2V applied. This allowed the TiO2 / Al2O3 / C precursor coating to be electro-deoxidized and transformed into a MAX phase coating. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly washed with 0.5mol / L dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Finally, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a MAX phase coating on the surface.

[0046] Testing revealed that the MAX phase coating prepared in this embodiment has a single Ti2AlC phase, and the thickness of the MAX phase coating is approximately 200 μm.

[0047] Comparative Example 1

[0048] This comparative example includes the following steps:

[0049] Step 1: Preparation of precursor powder: TiO2 powder, Al2O3 powder and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then ball-milled to obtain a uniform composite powder. Spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is then prepared by spray granulation.

[0050] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water and then roughened by sandblasting. Then, plasma spraying is used. The main process parameters include a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer. The main process parameters include a current of 200A, a powder feed rate of 6rpm, and a main gas flow rate of 140SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 250μm.

[0051] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0052] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode, and the TiO2 / Al2O3 / C precursor coating was used as the cathode. Electrolysis was performed for 6 hours under an inert atmosphere at a temperature of 900℃, with a voltage of 3.2V applied. This allowed the TiO2 / Al2O3 / C precursor coating to be electro-deoxidized and transformed into a MAX phase coating. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly washed with 0.5mol / L dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Finally, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a MAX phase coating on the surface.

[0053] Figure 3 The XRD pattern of the MAX phase coating prepared in this comparative example is shown below. Figure 3 It can be seen that the MAX phase coating has a high content of Ti2AlC phase and a small amount of TiC and C impurities, indicating that excessive electrolysis time will lead to carbon accumulation and increased impurities, making it impossible to obtain a high-purity MAX phase coating. This shows that the appropriate electrolysis time is crucial to the purity of the MAX phase coating product.

[0054] The thickness of the MAX phase coating prepared in this comparative example was measured to be approximately 200 μm.

[0055] Comparative Example 2

[0056] This comparative example includes the following steps:

[0057] Step 1: Preparation of precursor powder: TiO2 powder, Al2O3 powder and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then ball-milled to obtain a uniform composite powder. Spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is then prepared by spray granulation.

[0058] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water, and then roughened by sandblasting. Next, plasma spraying is used, with the main process parameters including a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer, with the main process parameters including a current of 800A, a powder feed rate of 3rpm, and a main gas flow rate of 70SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 150μm.

[0059] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0060] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode and the TiO2 / Al2O3 / C precursor coating as the cathode. Electrolysis was carried out for 3 hours under an inert atmosphere at a voltage of 3.2V and a temperature of 900℃. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly washed with 0.5mol / L dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Finally, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a surface coating.

[0061] Figure 4 The XRD pattern of the coating prepared in this comparative example is shown below. Figure 4 It can be seen that the coating consists of a TiC phase and a small amount of TiAl phase. Compared with Example 1, the increased spraying current in preparing the TiO2 / Al2O3 / C precursor coating in this comparative example led to an increase in flame temperature. The reduced powder feed rate and main gas flow rate resulted in a longer residence time of the spherical TiO2 / Al2O3 / C precursor powder in the flame. These changes exacerbated the carbon combustion loss in the spherical TiO2 / Al2O3 / C precursor powder. The lack of C led to the subsequent electrolysis products first forming TiAl, and the C formed by the side reaction during the electrolysis process carbonized to form TiC. This indicates that the selection of appropriate plasma spraying parameters, especially the spraying current for preparing the precursor coating, is crucial for the synthesis of the Ti2AlC coating during electrolysis. Under unsuitable spraying parameters, the carbon in TiO2 / Al2O3 / C is severely lost, resulting in the inability to obtain the MAX phase through electrolysis.

[0062] The thickness of the coating prepared in this comparative example was measured to be approximately 100 μm.

[0063] Comparative Example 3

[0064] This comparative example includes the following steps:

[0065] Step 1: Preparation of precursor powder: TiO2 powder, Al2O3 powder and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then ball-milled to obtain a uniform composite powder. Spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is then prepared by spray granulation.

[0066] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water and then roughened by sandblasting. Then, plasma spraying is used. The main process parameters include a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer. The main process parameters include a current of 200A, a powder feed rate of 3rpm, and a main gas flow rate of 70SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 150μm.

[0067] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0068] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode and the TiO2 / Al2O3 / C precursor coating as the cathode. Electrolysis was carried out for 3 hours under an inert atmosphere at a voltage of 3.2V and a temperature of 900℃. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly washed with 0.5mol / L dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Finally, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a surface coating.

[0069] Figure 5 The XRD pattern of the coating prepared in this comparative example is shown below. Figure 5 It can be seen that the coating product is mainly TiAl phase, with a small amount of Ti2AlC and TiC, and a high-purity Ti2AlC coating cannot be obtained. Compared with Example 1, the reduced powder feed rate and main gas flow rate in the preparation of the TiO2 / Al2O3 / C precursor coating in this comparative example led to increased carbon combustion loss in the spherical TiO2 / Al2O3 / C precursor powder, resulting in the formation of TiC and the inability to form a high-purity Ti2AlC phase through electrolysis. This indicates that the present invention needs to suppress carbon powder loss through the synergistic effect of plasma spraying parameters to ensure that a high-purity Ti2AlC coating is obtained through subsequent electrolysis.

[0070] The thickness of the coating prepared in this comparative example was measured to be approximately 100 μm.

[0071] Comparative Example 4

[0072] This comparative example includes the following steps:

[0073] Step 1: Preparation of precursor powder: TiO2, Al2O3 and graphite powder are mixed in a molar ratio of 3:0.75:1.5 and then subjected to high-energy ball milling to obtain a uniform composite powder. Then, spherical TiO2 / Al2O3 / C precursor powder with a particle size of 20μm~60μm is prepared by spray granulation.

[0074] Step 2, Precursor Coating Deposition: The surface of the 304 stainless steel substrate is first cleaned with acetone and deionized water, and then roughened by sandblasting. Next, plasma spraying is used, with the main process parameters including a current of 700A, a powder feed rate of 2rpm, and a main gas flow rate of 80SCFH. A Ni-18Cr-6Al transition layer is deposited on the sandblasted 304 stainless steel substrate surface. Then, the spherical TiO2 / Al2O3 / C precursor powder from Step 1 is deposited on the Ni-18Cr-6Al transition layer, with the main process parameters including a current of 800A, a powder feed rate of 3rpm, and a main gas flow rate of 70SCFH, forming a TiO2 / Al2O3 / C precursor coating with a thickness of about 150μm.

[0075] Step 3, Electrochemical Conversion of Molten Salt: The molten salt CaCl2 is dried under vacuum at 300℃ for 12h to remove residual bound water in the molten salt. Then the temperature is raised to 900℃, and a graphite rod is used as the anode and a stainless steel rod is used as the cathode. The molten salt is pre-electrolyzed at 3V for 12h to remove impurities.

[0076] The 304 stainless steel substrate with the TiO2 / Al2O3 / C precursor coating from step one was immersed in dried and pre-electrolyzed molten salt CaCl2. Graphite was used as the anode and the TiO2 / Al2O3 / C precursor coating was used as the cathode. Electrolysis was carried out for 12 hours under an inert atmosphere with a voltage of 3.2V and an electrolysis temperature of 900℃. After electrolysis, the cathode was slowly removed from the molten salt and cooled. It was then repeatedly cleaned with dilute hydrochloric acid and deionized water to remove residual molten salt CaCl2 from the surface. Afterward, it was dried under vacuum at 60℃ for 6 hours to obtain 304 stainless steel with a surface coating.

[0077] Figure 6 The XRD pattern of the coating prepared in this comparative example is shown below. Figure 6 It can be seen that the coating product is a TiC phase and a Ti2AlC coating cannot be obtained. Compared with Comparative Example 2, this comparative example further extended the electrolysis time to 12h, indicating that Ti2AlC still cannot be formed after long-term electrolysis when the spraying parameters are not suitable.

[0078] The thickness of the coating prepared in this comparative example was measured to be approximately 100 μm.

[0079] In summary, this invention proposes a method for preparing high-thickness, high-purity MAX phase coatings using a synergistic process of "plasma spraying-molten salt electrochemical conversion." This method provides a new synthetic route for high-quality MAX phase coatings, which is beneficial for promoting the development and application of MAX phase coatings.

[0080] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a high-thickness, high-purity MAX phase coating, characterized in that, The method includes the following steps: Step 1: Preparation of precursor powder: The metal oxide and carbon source are mixed and then ball-milled at high energy to obtain a uniform composite powder. The powder is then spray-granulated to obtain spherical precursor powder. The metal oxide is TiO2 and Al2O3, and the carbon source is graphite powder. Step 2, Precursor Coating Deposition: The substrate surface is sandblasted and plasma spraying is used to deposit a transition layer on the sandblasted substrate surface. Then, the spherical precursor powder from Step 1 is deposited on the transition layer to form a precursor coating. Step 3, molten salt electrochemical conversion: The substrate with the precursor coating from Step 1 is immersed in molten salt, and an electrolysis is performed under an inert atmosphere to convert the precursor coating into a MAX phase coating with a thickness greater than 200 μm and a single phase; the MAX phase coating has a single Ti2AlC phase.

2. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The molar ratio of TiO2, Al2O3 and graphite powder is 3:0.75:1.

5.

3. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The particle size of the spherical precursor powder mentioned in step one is 20μm~60μm.

4. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The current for the plasma spraying method described in step two is 200A.

5. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The thickness of the precursor coating in step two shall not exceed 250 μm.

6. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The molten salt mentioned in step three is CaCl2, which is first dried under vacuum at 300°C for 12 hours and then pre-electrolyzed at 3V for 12 hours.

7. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The electrolysis temperature in step three is 900℃.

8. The method for preparing a high-thickness, high-purity MAX phase coating according to claim 1, characterized in that, The electrolysis time in step three is 3 to 5 hours.

9. A high-thickness, high-purity MAX phase coating prepared by the method described in any one of claims 1 to 8, characterized in that, The coating is a Ti2AlC coating converted from a TiO2 / Al2O3 / C precursor coating.

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