Preparation method of plasma spraying high-entropy alloy coating and method for improving oxidation resistance of coating

Abstract

The invention discloses a preparation method of a plasma spraying high-entropy alloy coating and a method for improving the oxidation resistance of the coating, and belongs to the technical field of surface engineering. High-entropy alloy powder composed of five elements of Al, Co, Cr, Fe and Ni is adopted as feed, the high-entropy alloy coating is prepared through the vacuum plasma spraying technology, and therefore the compact and non-porous coating is obtained; and the prepared plasma spraying coating is placed in a vacuum furnace to be subjected to heat treatment, and the plasma spraying high-entropy alloy improved coating with the oxidation resistance is obtained. The prepared high-entropy alloy coating is expected to become a novel thermal barrier coating bonding layer material with potential, can prolong the service life of key components under the high-temperature condition, can be widely applied to the fields of aerospace, ocean, automobile engineering and the like, and has remarkable economic benefits and social benefits.

Description

Technical Field

[0001] This invention belongs to the field of surface engineering technology, and particularly relates to a method for preparing a plasma-sprayed high-entropy alloy coating and a method for improving the coating's oxidation resistance. Background Technology

[0002] Applying a thermal barrier coating to the blade surface can effectively reduce the temperature experienced by the blade surface, enabling the aero-engine to operate at higher temperatures and increasing its thrust-to-weight ratio and fuel efficiency. Classical thermal barrier coating systems have a two-layer structure, consisting of a ceramic layer and a bonding layer. To alleviate the thermal mismatch stress caused by the difference in thermal expansion coefficients between the ceramic layer and the high-temperature alloy substrate, a bonding layer is introduced between them, effectively preventing the ceramic layer from detaching. Furthermore, the bonding layer should also possess oxidation resistance properties to prevent the high-temperature alloy substrate from oxidizing under high-temperature conditions.

[0003] High-entropy alloys typically consist of five or more main elements in equiatomic or near-equiatomic ratios. Although they contain multiple main elements, they do not form intermetallic compounds but rather tend to form simple solid solutions, which allows them to maintain good phase stability even at high temperatures. This good phase stability at high temperatures is a prerequisite for high-entropy alloys to be used as high-temperature materials. Furthermore, oxidation is inevitable in high-temperature environments, and the diffusion retardation effect of high-entropy alloys can theoretically slow down the oxidation process controlled by ion diffusion, giving them superior oxidation resistance and enabling their use at high temperatures. The AlCoCrFeNi high-entropy alloy system has a similar elemental composition to traditional binder materials and shows promise as a novel binder material.

[0004] To date, most high-entropy alloys have been prepared using arc casting. However, the formation of high-entropy alloys with simple solid solution structures requires rapid cooling, and the alloys contain many expensive precious metal elements, resulting in high production costs for bulk high-entropy alloys. Therefore, researchers have begun to prepare high-performance high-entropy alloy coatings on the surfaces of inexpensive alloys. The application of rapid solidification surface technologies such as thermal spraying and laser cladding to coat high-performance high-entropy alloy coatings on low-cost metal materials shows promising prospects. Because aero-engine blades have curved surfaces and numerous film cooling holes, plasma spraying technology is a suitable method for preparing bonding layers.

[0005] Traditional methods for preparing the binder layer involve atmospheric plasma spraying, a process conducted in an atmospheric environment. Since the binder layer material is mostly metallic, the binder particles, heated to a molten state by the plasma jet, oxidize during flight, ultimately resulting in oxide inclusions within the coating. Although the flight time of the molten particles is extremely short, the high temperature of the plasma jet inevitably leads to oxidation of the binder particles. This is one of the causes of binder layer failure. Furthermore, the high-temperature oxidation process can lead to the formation of a thick thermally grown oxide layer (TGO), and even spinel oxides, further increasing the risk of binder layer failure and reducing the coating's oxidation resistance. Therefore, there is an urgent need for a simple and low-cost method for improving the oxidation resistance of high-entropy alloy coatings to address these issues. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for preparing a high-entropy alloy coating by plasma spraying and a method for improving the coating's oxidation resistance. The high-entropy alloy coating prepared by this invention has a dense surface, low porosity, and is free of oxide inclusions.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for preparing a plasma-sprayed high-entropy alloy coating, comprising the following steps: using high-entropy alloy powder as feedstock, preparing a high-entropy alloy coating on a substrate by vacuum plasma spraying, thereby obtaining a plasma-sprayed high-entropy alloy coating;

[0009] The vacuum degree during vacuum plasma spraying is 25-35 mbar, the working voltage is 68-70V, the working current is 700-720A, the spraying distance is 280-300mm, the argon flow rate is 60-65 slpm, and the hydrogen flow rate is 5-10 slpm.

[0010] Furthermore, the elemental composition of the high-entropy alloy powder is Al, Co, Cr, Fe and Ni.

[0011] Furthermore, the molar ratio of Al, Co, Cr, Fe and Ni is 1:1:1:1:1.

[0012] Furthermore, the particle size of the high-entropy alloy powder is 15-53 μm.

[0013] Furthermore, the substrate is selected from 316L stainless steel.

[0014] The present invention also provides a method for improving the oxidation resistance of a coating, comprising the following steps: performing vacuum heat treatment on the plasma-sprayed high-entropy alloy coating described in the above technical solution.

[0015] Furthermore, the vacuum degree of the vacuum heat treatment is ≤5mbar, the temperature of the vacuum heat treatment is 1000-1100℃, and the holding time of the vacuum heat treatment is 5-6h.

[0016] Furthermore, the process of vacuum heat treatment of the plasma-sprayed high-entropy alloy coating is as follows: the plasma-sprayed high-entropy alloy coating described in the above technical solution is placed in the heating chamber of a vacuum heat treatment furnace, and the furnace chamber is sealed; the vacuum system is started, the vacuum degree in the furnace chamber is evacuated to ≤5mbar and maintained, and then heated to 1000-1100℃ at a heating rate of 6℃ / min, and then the heat treatment stage is entered. After the heat treatment is completed, the coating is naturally cooled to a safe furnace exit temperature.

[0017] Furthermore, the heat preservation time during the heat preservation stage is 5-6 hours.

[0018] Compared with the prior art, the present invention has the following advantages and technical effects:

[0019] The plasma-sprayed high-entropy alloy coating preparation method provided by this invention is carried out under vacuum conditions, effectively isolating the atmospheric environment and reducing the oxidation and nitriding of molten particles during the spraying process, thus ensuring the accuracy and purity of the coating composition. Simultaneously, the plasma jet in a vacuum environment has higher energy density and stability, which is beneficial for the full melting and high-speed deposition of the high-entropy alloy powder, thereby obtaining a coating with a dense structure, low porosity, strong interlayer bonding, and high bonding strength with the substrate.

[0020] The method for improving the antioxidant properties of coatings provided by this invention employs vacuum heat treatment, which completely avoids oxidation damage to the coating during the treatment process under a high vacuum environment. High-temperature, long-term heat preservation effectively promotes the full diffusion and homogenization of elements within the coating, eliminating microscopic defects formed during spraying. Simultaneously, this process optimizes the phase composition and elemental distribution of the coating, enriching key antioxidant elements near the coating surface. This significantly enhances the coating's ability to form a stable, continuous, and highly protective oxide film during high-temperature service, thereby greatly strengthening its long-term antioxidant properties. Attached Figure Description

[0021] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 The X-ray diffraction pattern of the AlCoCrFeNi coating prepared in Example 1;

[0023] Figure 2 The X-ray diffraction pattern of the AlCoCrFeNi coating prepared in Example 2;

[0024] Figure 3 This is a cross-sectional morphology image of the AlCoCrFeNi coating prepared in Example 1;

[0025] Figure 4 The average hardness of the AlCoCrFeNi coatings in Examples 1 and 2 is given, wherein the plasma-sprayed AlCoCrFeNi coating is Example 1, and the vacuum-sprayed AlCoCrFeNi coating is Example 2.

[0026] Figure 5 This is a surface morphology image of the AlCoCrFeNi coating in Example 1 after oxidation at 1000℃ for 10 hours;

[0027] Figure 6 This is a surface morphology image of the AlCoCrFeNi coating in Example 2 after oxidation at 1000℃ for 10 hours;

[0028] Figure 7 The image shows the surface morphology of the AlCoCrFeNi coating in Example 1 after oxidation at 1000℃ for 50 hours.

[0029] Figure 8 The image shows the surface morphology of the AlCoCrFeNi coating in Example 2 after oxidation at 1000℃ for 50 hours. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] This invention provides a method for preparing a plasma-sprayed high-entropy alloy coating, comprising the following steps: using high-entropy alloy powder as feedstock, a high-entropy alloy coating is prepared on a substrate by vacuum plasma spraying, thereby obtaining a plasma-sprayed high-entropy alloy coating.

[0033] This invention employs vacuum plasma spraying technology to prepare high-entropy alloy coatings, effectively isolating the atmospheric environment and reducing oxidation and nitriding of molten particles during the spraying process, thus ensuring the accuracy and purity of the coating composition. Simultaneously, the plasma jet in a vacuum environment possesses higher energy density and stability, which facilitates the full melting and high-speed deposition of the high-entropy alloy powder, resulting in a coating with a dense structure, low porosity, strong interlayer bonding, and high adhesion strength to the substrate.

[0034] In a preferred embodiment, the vacuum degree during vacuum plasma spraying is 25-35 mbar, the working voltage is 68-70 V, the working current is 700-720 A, the spraying distance is 280-300 mm, the argon flow rate is 60-65 slpm, and the hydrogen flow rate is 5-10 slpm. More preferably, the vacuum degree during vacuum plasma spraying is 30 mbar, the working voltage is 69 V, the working current is 720 A, the spraying distance is 290 mm, the argon flow rate is 60 slpm, and the hydrogen flow rate is 8 slpm. This invention, by controlling the process parameters of vacuum plasma spraying, facilitates the acquisition of high-entropy alloy coatings with dense structure, low porosity, strong interlayer bonding, and high bonding strength with the substrate.

[0035] In a preferred embodiment, the high-entropy alloy powder has an elemental composition of Al, Co, Cr, Fe and Ni; the molar ratio of Al, Co, Cr, Fe and Ni is 1:1:1:1:1.

[0036] In a preferred embodiment, the particle size of the high-entropy alloy powder is 15-53 μm.

[0037] In a preferred embodiment, the substrate is selected from 316L stainless steel.

[0038] In a preferred embodiment, a pretreatment step for the substrate is further included before vacuum plasma spraying; the pretreatment includes: sandblasting the substrate with 46# corundum sand, and after sandblasting, blowing the substrate surface to be sprayed with compressed air, followed by wiping with anhydrous ethanol.

[0039] The present invention also provides a method for improving the oxidation resistance of a coating, comprising the following steps: performing vacuum heat treatment on the plasma-sprayed high-entropy alloy coating described in the above technical solution.

[0040] The method for improving the antioxidant properties of coatings provided by this invention employs vacuum heat treatment, which completely avoids oxidative damage to the coating during the treatment process under a high vacuum environment. Simultaneously, this process optimizes the phase composition and elemental distribution of the coating, enriching key antioxidant elements near the coating surface. This significantly enhances the coating's ability to form a stable, continuous, and highly protective oxide film during high-temperature service, thereby greatly strengthening its long-term antioxidant properties.

[0041] In a preferred embodiment, the vacuum degree of the vacuum heat treatment is ≤5 mbar, the temperature of the vacuum heat treatment is 1000-1100℃, and the holding time of the vacuum heat treatment is 5-6 hours. High temperature and long-term heat treatment effectively promotes the full diffusion and homogenization of elements within the coating, eliminating microscopic defects formed during spraying.

[0042] In a preferred embodiment, the process of vacuum heat treatment of the plasma-sprayed high-entropy alloy coating is as follows: the plasma-sprayed high-entropy alloy coating is placed in the heating chamber of a vacuum heat treatment furnace, and the furnace chamber is sealed; the vacuum system is started, the vacuum degree in the furnace chamber is evacuated to ≤5mbar and maintained, and then heated to 1000-1100℃ at a heating rate of 6℃ / min, and then the heat treatment stage is entered. After the heat treatment is completed, the coating is naturally cooled to a safe furnace exit temperature.

[0043] In a preferred embodiment, the vacuum heat treatment furnace is selected from horizontal vacuum heat treatment furnaces.

[0044] In a preferred embodiment, the heat preservation time during the heat preservation stage is 5-6 hours.

[0045] Unless otherwise specified, all raw materials used in the embodiments of this invention were purchased through commercial channels.

[0046] Example 1

[0047] A method for preparing a high-entropy alloy coating by plasma spraying, comprising the following steps:

[0048] (1) Using a 316L stainless steel disc with a diameter of 25.4 mm and a thickness of 6 mm as the substrate, the 316L stainless steel disc was sandblasted with 46# corundum sand. The working pressure of the sandblasting was 0.4 MPa. After the sandblasting was completed, the substrate surface to be coated was first blown with compressed air, and then wiped with anhydrous ethanol.

[0049] (2) Using high-entropy alloy powder as feedstock, a high-entropy alloy coating is prepared on a 316L stainless steel disc by vacuum plasma spraying to obtain a plasma-sprayed high-entropy alloy coating; wherein, the high-entropy alloy powder is composed of Al, Co, Cr, Fe and Ni in a molar ratio of 1:1:1:1:1, and the powder particle size is 15-53μm; the vacuum degree of the vacuum plasma spraying chamber is 30mbar, the working voltage is 69V, the working current is 720A, the spraying distance is 290mm, the argon flow rate is 60slpm, and the hydrogen flow rate is 8slpm.

[0050] Example 2

[0051] A method for improving the antioxidant properties of a coating, comprising the following steps:

[0052] The plasma-sprayed high-entropy alloy coating obtained in Example 1 was placed in the heating chamber of a horizontal vacuum heat treatment furnace, and the furnace chamber was sealed. The vacuum system was started to pump the vacuum in the furnace chamber to ≤5mbar and maintain it. Then, it was heated to the target heat treatment temperature of 1050℃ at a constant heating rate of 6℃ / min. After the target temperature was reached, it entered the heat preservation stage. After the heat preservation was completed, it was naturally cooled to the safe furnace exit temperature. The heat preservation stage lasted for 5 hours.

[0053] Figure 1 The image shows the X-ray diffraction pattern of the AlCoCrFeNi coating prepared in Example 1. Figure 1 As can be seen, the AlCoCrFeNi coating prepared by vacuum plasma spraying is composed of single-phase BCC, which is consistent with the phase composition of AlCoCrFeNi powder. This indicates that no phase change process occurred in the raw materials during the preparation process, and the phase composition of the raw material powder was preserved.

[0054] Figure 2 The image shows the X-ray diffraction pattern of the AlCoCrFeNi coating prepared in Example 2. Figure 2 As can be seen, after vacuum heat treatment, the AlCoCrFeNi coating underwent a phase transformation. In addition to the BCC phase, the FCC phase and σ-FeCr phase also appeared. This indicates that under high temperature conditions, the enhanced atomic diffusion rate in the multi-principal high-entropy alloy coating can induce lattice recombination during heat treatment, thereby promoting the coating to transform into the FCC phase with higher atomic packing density.

[0055] Figure 3 This is a cross-sectional morphology image of the AlCoCrFeNi coating prepared in Example 1. From... Figure 3 As can be seen, the AlCoCrFeNi coating prepared by vacuum plasma spraying is dense, with almost no pores inside, and no oxides are generated during the preparation process. The dense coating structure promotes the improvement of the material's mechanical properties and oxidation resistance, as it eliminates the rapid diffusion of metal ions along defects within the coating. Furthermore, the porosity of the AlCoCrFeNi coating prepared in Example 1 was measured using the Archimedes method, and it was found to be <0.5%.

[0056] Hardness tests were performed on the coatings in Examples 1 and 2. The specific testing method was as follows: the sample was sanded and polished using 400#-2000# SiC sandpaper, followed by a hardness test. The applied force during the measurement was 0.5 kg, and the holding time was 10 s. Measurements were taken at three different vertical heights within the coating, and five horizontal measurements were performed at each vertical height, resulting in a total of 15 hardness values. The average hardness was then calculated, and the results are shown below. Figure 4 .

[0057] Figure 4 The average hardness is represented by the AlCoCrFeNi coating in Examples 1 and 2, where the plasma-sprayed AlCoCrFeNi coating is Example 1, and the vacuum-sprayed AlCoCrFeNi coating is Example 2. Figure 4 It can be seen that the average microhardness of the AlCoCrFeNi coatings in Examples 1 and 2 is 500 HV and 390 HV, respectively. The AlCoCrFeNi coating after heat treatment contains an FCC phase. The FCC phase typically exhibits lower hardness but stronger deformability, which is beneficial for stress relaxation of the thermally grown oxide layer (TGO), thereby preventing coating cracking during oxidation.

[0058] The antioxidant properties of the coatings in Examples 1 and 2 were tested. The specific testing procedure was as follows: An isothermal oxidation experiment was conducted on the test samples in an air atmosphere using a muffle furnace. The heating process was carried out at a heating rate of 6℃ / min. After the furnace temperature reached 1000℃, the test sample (i.e., the AlCoCrFeNi coating surface) was placed face up in a corundum crucible. The crucible was then placed in the center of the furnace. After the corresponding oxidation time was reached, the samples were cooled using furnace cooling to complete the antioxidant property test. The test results are shown below. Figure 5-8 .

[0059] Figure 5 The image shows the surface morphology of the AlCoCrFeNi coating in Example 1 after oxidation at 1000℃ for 10 hours. Figure 6 This is a surface morphology image of the AlCoCrFeNi coating in Example 2 after oxidation at 1000℃ for 10 hours. From... Figure 5 and Figure 6 It can be seen that the AlCoCrFeNi coating prepared by vacuum plasma spraying in Example 1 forms needle-like and lamellar oxide θ-Al2O3 after oxidation. In Example 2, the AlCoCrFeNi coating after vacuum heat treatment directly forms blocky α-Al2O3 under the same oxidation conditions. α-Al2O3 is more stable than θ-Al2O3, indicating that vacuum heat treatment can improve the oxidation resistance of the coating.

[0060] Figure 7 The image shows the surface morphology of the AlCoCrFeNi coating in Example 1 after oxidation at 1000℃ for 50 hours. Figure 8 This is a surface morphology image of the AlCoCrFeNi coating in Example 2 after oxidation at 1000℃ for 50 hours. From... Figure 7 and Figure 8It can be seen that the TGO layer of the AlCoCrFeNi coating after vacuum heat treatment in Example 2 is thinner, indicating that heat treatment can suppress the growth rate of the TGO layer, maintain the AlCoCrFeNi coating as thin and dense with good adhesion, and demonstrate improved anti-oxidation performance of the coating. Measurements showed that the thicknesses of the TGO layers in Examples 1 and 2 were 2.1 μm and 1.8 μm, respectively.

[0061] Comparative Example 1

[0062] A method for preparing a plasma-sprayed high-entropy alloy coating differs from Example 1 in that, in step (2), the vacuum degree of the vacuum plasma spraying chamber is 50 mbar, while the rest is the same as in Example 1.

[0063] Comparative Example 2

[0064] A method for preparing a plasma-sprayed high-entropy alloy coating differs from Example 1 in that, in step (2), the working current of the vacuum plasma spraying chamber is 650A, while the rest is the same as in Example 1.

[0065] Comparative Example 3

[0066] A method for preparing a plasma-sprayed high-entropy alloy coating differs from Example 1 in that, in step (2), the working current of the vacuum plasma spraying chamber is 780A, while the rest is the same as in Example 1.

[0067] The hardness, porosity, and thickness of the TGO layer after 50 h of oxidation of the coatings prepared in Comparative Examples 1-3 were tested using the same test method as in Example 1. The results are shown in Table 1.

[0068] Table 1

[0069] Hardness / HV TGO layer thickness / μm Porosity / % Comparative Example 1 420 3.2 1.2 Comparative Example 2 465 2.9 1.0 Comparative Example 3 540 2.5 0.7

[0070] As shown in Table 1, the density and oxidation resistance of the coating are closely related to the vacuum level and working current of the vacuum plasma spraying chamber. Under low vacuum conditions, the plasma jet can expand fully, allowing the sprayed particles to achieve higher speeds and more complete melting. When impacting the substrate, this results in a denser coating structure, significantly reducing porosity. Similarly, when the current is too low, the plasma energy is insufficient, leading to incomplete melting of powder particles. Unmelted particles and pores remain in the coating, reducing its density and bonding strength. This also creates more diffusion channels during oxidation, accelerating TGO layer growth. Conversely, when the current is too high, although particle melting is more complete, the excessive energy can cause excessive evaporation or decomposition of powder particles. This can also lead to overheating of the substrate, generating thermal stress that may cause the coating to crack or peel off, thus reducing its oxidation resistance.

[0071] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a high-entropy alloy coating by plasma spraying, characterized in that, Includes the following steps: A high-entropy alloy coating is prepared on a substrate by vacuum plasma spraying using high-entropy alloy powder as feedstock. The vacuum degree during vacuum plasma spraying is 25-35 mbar, the working voltage is 68-70V, the working current is 700-720A, the spraying distance is 280-300mm, the argon flow rate is 60-65 slpm, and the hydrogen flow rate is 5-10 slpm.

2. The preparation method according to claim 1, characterized in that, The high-entropy alloy powder has an elemental composition of Al, Co, Cr, Fe, and Ni.

3. The preparation method according to claim 2, characterized in that, The molar ratio of Al, Co, Cr, Fe and Ni is 1:1:1:1:

1.

4. The preparation method according to claim 2, characterized in that, The high-entropy alloy powder has a particle size of 15-53 μm.

5. The preparation method according to claim 1, characterized in that, The substrate is selected from 316L stainless steel.

6. A method for improving the antioxidant properties of a coating, characterized in that, Includes the following steps: The plasma-sprayed high-entropy alloy coating described in claim 1 is subjected to vacuum heat treatment.

7. The method for improving the antioxidant properties of a coating according to claim 6, characterized in that, The vacuum degree of the vacuum heat treatment is ≤5mbar, the temperature of the vacuum heat treatment is 1000-1100℃, and the holding time of the vacuum heat treatment is 5-6h.

8. The method for improving the antioxidant properties of a coating according to claim 6, characterized in that, The specific process of vacuum heat treatment of the plasma-sprayed high-entropy alloy coating is as follows: the plasma-sprayed high-entropy alloy coating of claim 1 is placed in the heating chamber of a vacuum heat treatment furnace and the furnace chamber is sealed; the vacuum system is started to pump the vacuum degree in the furnace chamber to ≤5mbar and maintain it, and then the temperature is heated to 1000-1100℃ at a heating rate of 6℃ / min, and then the heat treatment stage is entered. After the heat treatment is completed, the coating is naturally cooled to a safe temperature for removal from the furnace.

9. The method for improving the antioxidant properties of a coating according to claim 8, characterized in that, The heat preservation stage lasts for 5-6 hours.

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