A gradient composite coating on a surface of a tc4 alloy and a preparation method thereof

Abstract

The application discloses a TC4 alloy surface gradient composite coating and a preparation method thereof, and the method comprises the following steps: cleaning, laser etching and plasma activation treatment are performed on the surface of the TC4 alloy; SiC composite base layer slurry, cobalt-based alloy intermediate layer slurry and Nb modified high-entropy alloy surface layer slurry are respectively prepared; first transition layers are formed between the SiC composite base layer and the cobalt-based alloy intermediate layer, and second transition layers are formed between the cobalt-based alloy intermediate layer and the Nb modified high-entropy alloy surface layer through hierarchical spraying in sequence; and the hierarchical drying, flame spraying densification and vacuum diffusion cladding treatment are performed to obtain the gradient composite coating. Through the in-situ reaction of the SiC composite base layer and the TC4 base body, the chemical anchoring and the metallurgical diffusion composite connection are realized, the hardness, the thermal expansion coefficient and the elastic modulus are continuously and gradiently transitioned through the first and second transition layers, and the cold and hot impact resistance, the anti-falling, the wear resistance and the chlorine ion corrosion resistance of the TC4 alloy surface are effectively improved.

Description

Technical Field

[0001] This invention relates to the field of gradient composite coating on the surface of TC4 alloy and its preparation method. Background Technology

[0002] TC4 alloy is a typical α+β type titanium alloy, characterized by high specific strength, good heat resistance, and excellent machinability. It has been widely used in aero-engine blades, compressor disks, fuselage structural components, ship connectors, and spacecraft load-bearing components. However, as service environments evolve towards higher temperatures, higher salt spray, higher loads, and multi-factor coupling, the shortcomings of TC4 alloy's surface properties are becoming increasingly apparent. These shortcomings manifest in several ways: In high-temperature oxidizing or thermal shock environments, the oxide film on the TC4 alloy surface is prone to cracking or peeling under repeated thermal stress due to the significant difference in thermal expansion coefficients between the TC4 alloy and the substrate; in chloride-containing media, the oxide film on the TC4 alloy surface exhibits localized weak areas, allowing chloride ions to easily penetrate and cause pitting and crevice corrosion; and in friction and wear conditions, the titanium alloy surface has low hardness and a high tendency to adhere, making it susceptible to adhesive wear and ploughing wear. These failure modes reduce the dimensional stability and safety of critical components, and in severe cases, can even lead to safety accidents.

[0003] High-entropy alloy coatings, due to their multi-principal element synergistic effect, hysteresis diffusion effect, and lattice distortion effect, typically exhibit high hardness, good corrosion resistance, and excellent high-temperature stability, making them a potential solution for titanium alloy surface protection. However, there are differences in thermal expansion coefficients, elastic moduli, and chemical compatibility between high-entropy alloy coatings and the TC4 alloy substrate. If high-entropy alloys are directly applied to the TC4 alloy surface, residual tensile stress is easily generated at the interface, and pores and microcracks are formed within the coating, leading to coating failure and detachment during use.

[0004] Existing technologies have proposed a two-gradient coating scheme, namely, setting a metal transition layer and a surface functional layer. While this scheme can alleviate interfacial stress to some extent, it still lacks a reactive substrate between the metal transition layer and the titanium alloy substrate that can simultaneously provide chemical anchoring, diffusion blocking, and hardness gradient transition, resulting in insufficient interfacial bonding strength. Therefore, how to establish a substrate structure on the TC4 alloy surface that combines chemical anchoring, diffusion blocking, and stress buffering functions, and further optimize the composition of the high-entropy alloy surface layer to achieve good compatibility with the substrate, is key to improving the service reliability of the coating.

[0005] SiC possesses high hardness, high thermal stability, and good wear resistance. Under appropriate vacuum diffusion cladding conditions, SiC can undergo in-situ reactions with the surface of titanium alloys to form TiC and Ti-Si intermetallic compounds, thereby generating chemical anchoring and diffusion barrier effects. Based on this, if SiC is designed as a coating base layer, allowing it to undergo in-situ reactions with the TC4 alloy matrix during high-temperature treatment, and combined with a tough cobalt-based alloy intermediate layer and an Nb-modified high-entropy alloy surface layer, a multi-mechanism continuous gradient structure from the matrix to the functional layers can be formed. Summary of the Invention

[0006] To address the problems of insufficient interfacial bonding strength, high residual stress, and limited improvement in high-temperature corrosion and wear resistance of existing TC4 alloy surface coatings, this invention provides a gradient composite coating for TC4 alloy surfaces and its preparation method. This method employs a SiC composite substrate, which undergoes an in-situ reaction with the TC4 alloy substrate during high-temperature treatment, achieving a composite connection of chemical anchoring and metallurgical diffusion. A cobalt-based alloy interlayer serves as a toughening interlayer to alleviate thermal stress and abrupt changes in elastic modulus. An Nb-modified high-entropy alloy surface layer serves as a functional surface layer to improve wear resistance and high-temperature resistance. A stable bond between the TC4 substrate and the surface coating is achieved through a combination of graded spraying, flame densification, and vacuum diffusion cladding processes.

[0007] To achieve the above objectives, the specific solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for preparing a gradient composite coating on the surface of a TC4 alloy, comprising the following steps: Step (1): Clean the surface of the TC4 alloy substrate, perform laser etching and plasma activation treatment to obtain a roughened active surface; Step (2): Prepare SiC composite base layer slurry, cobalt-based alloy intermediate layer slurry and Nb-modified high-entropy alloy surface layer slurry respectively; all three slurries contain powder raw materials and organic carriers, the organic carriers including glycerol and hydroxypropyl methylcellulose; Step (3): The SiC composite base slurry, cobalt-based alloy intermediate layer slurry, and Nb-modified high-entropy alloy surface layer slurry are sequentially sprayed onto the roughened active surface of the TC4 alloy substrate in an order from the inside out; and a first transition layer slurry formed by the mixture of the two is sprayed between the SiC composite base slurry and the cobalt-based alloy intermediate layer slurry, and a second transition layer slurry formed by the mixture of the two is sprayed between the cobalt-based alloy intermediate layer slurry and the Nb-modified high-entropy alloy surface layer slurry, thereby forming a composite pre-coated coating; Step (4): The composite precast coating is subjected to graded drying and flame spraying densification treatment; Step (5): The TC4 alloy substrate with composite pre-coated material after step (4) is placed in a vacuum or inert protective atmosphere for high-temperature diffusion cladding treatment, so that the composite pre-coated material and the TC4 alloy substrate, as well as the adjacent coatings, form a metallurgical diffusion bond. Finally, a gradient composite coating is formed on the surface of the TC4 alloy substrate, consisting of SiC composite base layer, first transition layer, cobalt-based alloy intermediate layer, second transition layer and Nb modified high-entropy alloy surface layer from the inside to the outside. The first transition layer and the second transition layer respectively form a compositional gradient transition between adjacent coatings.

[0008] Furthermore, in the first transition layer slurry, the mass ratio of SiC composite base layer slurry to cobalt-based alloy intermediate layer slurry is any one of 70:30, 50:50 or 30:70; The second transition layer slurry, the mass ratio of the cobalt-based alloy intermediate layer slurry to the Nb-modified high-entropy alloy surface layer slurry is any one of 70:30, 50:50 or 30:70.

[0009] Furthermore, the powder raw materials in the SiC composite base slurry include, by mass fraction: 65-85% SiC, 8-22% Ti, 5-12% cobalt-based alloy powder, and 0.5-3% rare earth oxides; the rare earth oxides are selected from at least one of Y2O3, La2O3, or CeO2.

[0010] Furthermore, the powder raw materials in the cobalt-based alloy powder and the cobalt-based alloy intermediate layer slurry, by mass fraction, include: Mo 0.15~0.50%, Mn 0.30~0.80%, C 0.30~0.90%, B 1.00~2.20%, Si 1.80~3.20%, Fe 1.50~4.00%, W 5.50~9.50%, Ni 12.00~18.50%, with the balance being Co.

[0011] Furthermore, the powder raw materials in the Nb-modified high-entropy alloy surface slurry comprise, by mass fraction: B 1.20~2.80%, Si 1.80~3.60%, Mo 2.40~4.50%, C 1.20~2.80%, Ni 4.50~7.50%, Cr 24.00~32.00%, Nb 3.00~8.00%, with the balance being Fe.

[0012] Furthermore, when preparing each layer of slurry, the ratio of the powdered raw material to glycerol in the organic carrier is 10g:(5~18)mL, and the amount of hydroxypropyl methylcellulose added is 0.15~0.50wt.% of the mass of the powdered raw material; the SiC composite base layer slurry also contains 0.05~0.30wt.% of silane coupling agent or polyvinylpyrrolidone, accounting for 0.05~0.30wt.% of the mass of the powdered raw material.

[0013] Further, in step (1), the parameters of laser etching are: laser power 150~600W, scanning speed 100~800mm / s, scanning spacing 0.02~0.12mm, and surface roughness Ra after etching is 35~110μm; the plasma activation treatment is argon plasma treatment, and the treatment time is 1~10min.

[0014] Furthermore, in step (4), the temperature of the flame spraying densification treatment is 450~950℃, the treatment time is 0.05~1.0h, the spraying distance is 120~260mm, the spraying angle is 75~90°, and the combustion air flow rate is 1.0~2.5m. 3 / h, and the flame spraying uses a weak reducing atmosphere; in step (5), the vacuum degree of the vacuum diffusion cladding process is ≤5×10 -2 Pa or a high-purity argon protective atmosphere is used, with a heating rate of 5~15℃ / min, a holding temperature of 1080~1380℃, and a holding time of 0.5~4h.

[0015] Furthermore, after step (5), the gradient composite coating is further subjected to laser remelting, ultrasonic rolling or low-stress shot peening post-treatment.

[0016] Secondly, the present invention provides a gradient composite coating on the surface of a TC4 alloy. The gradient composite coating is prepared by the above-described preparation method. The gradient composite coating comprises, from the surface of the TC4 alloy substrate outwards, a SiC composite base layer, a first transition layer, a cobalt-based alloy intermediate layer, a second transition layer, and an Nb-modified high-entropy alloy functional surface layer. The thickness of the SiC composite base layer is 40-160 μm, the thickness of the cobalt-based alloy intermediate layer is 120-350 μm, and the thickness of the Nb-modified high-entropy alloy functional surface layer is 150-450 μm. TiC and Ti-Si intermetallic compounds are distributed at the interface between the SiC composite base layer and the TC4 alloy substrate to form an in-situ reaction connection region. The Nb-modified high-entropy alloy surface layer contains a BCC / FCC solid solution matrix phase, as well as an NbC reinforcing phase and a boride reinforcing phase dispersed in the matrix phase.

[0017] In the gradient composite coating prepared by this invention, SiC in the SiC composite substrate undergoes an in-situ reaction with the TC4 alloy matrix during high-temperature treatment, generating TiC and Ti-Si reaction products, forming an interfacial bonding layer that provides chemical anchoring and diffusion barrier. Simultaneously, Ti powder promotes the above reaction and improves interfacial bonding, while cobalt-based alloy powder enhances the metallic continuity of the substrate. Rare earth oxides refine the oxide film and reduce porosity defects. The thermal expansion coefficient and toughness of the cobalt-based alloy intermediate layer are between those of the SiC composite substrate and the Nb-modified high-entropy alloy surface layer, mitigating thermal stress and abrupt changes in elastic modulus. Furthermore, the interdiffusion of Co-Ni-Fe-Cr elements improves the metallurgical bonding with adjacent layers. After high-temperature treatment, the Nb-modified high-entropy alloy surface layer forms a composite structure where a solid solution coexists with fine reinforcing phases such as NbC and borides. The Nb element also promotes the formation of a stable passivation film containing Nb, Cr, and Si, thereby improving resistance to chloride ion corrosion and high-temperature oxidation. By sequentially spraying SiC composite base slurry, cobalt-based alloy intermediate layer slurry, and Nb-modified high-entropy alloy top layer slurry, and spraying a transition layer slurry between adjacent layers, the hardness, coefficient of thermal expansion, elastic modulus, and chemical composition exhibit a continuous gradient change, avoiding crack initiation caused by abrupt performance changes at the interface. A combination of flame spraying densification and vacuum diffusion cladding is employed. Flame spraying is used to quickly remove organic carriers and achieve particle pre-bonding, while vacuum diffusion cladding is used to promote interfacial reaction, diffusion connection, and pore closure, significantly improving the overall density and anti-peeling ability of the coating.

[0018] Beneficial effects: (1) In this invention, SiC composite substrate is used as the substrate for TC4 alloy surface coating, so that SiC participates in in-situ reaction during vacuum diffusion cladding to form a reaction connection zone containing TiC and Ti-Si intermetallic compounds, thereby upgrading the traditional mechanical bonding to a composite connection with the combined effect of chemical anchoring and metallurgical diffusion. A good interfacial bond can be formed between TiC and TC4 substrate, and Ti-Si intermetallic compounds fill the interfacial micro-regions. The combined effect of the two significantly improves the bonding strength between the coating and the substrate, effectively resisting the initiation and propagation of interfacial cracks.

[0019] (2) This invention consists of a three-layer main structure comprising a SiC composite base layer, a cobalt-based alloy intermediate layer, and an Nb-modified high-entropy alloy surface layer, with a transition layer introduced between adjacent layers. The transition layer achieves a gradual change in chemical composition, thereby realizing a continuous gradient transition of hardness, coefficient of thermal expansion, and elastic modulus along the coating thickness direction, avoiding crack initiation caused by abrupt performance changes at the interface. Compared to a single high-entropy alloy coating or a two-gradient coating, it can more effectively alleviate abrupt changes in the coefficient of thermal expansion and hardness, reduce residual stress concentration, and improve the coating integrity after thermal shock cycling.

[0020] (3) This invention adds Nb to the powder raw material of the Nb-modified high-entropy alloy surface slurry, taking into account solid solution strengthening, carbide / boride dispersion strengthening, and passivation film stabilization. After the addition of Nb, it combines with C and B during the high-temperature diffusion cladding process to form NbC and boride reinforcing phases, which together with the solid solution matrix form a composite structure, playing a dispersion strengthening role and improving the wear resistance of the surface layer. Under preferred conditions, the surface hardness of the coating can reach 1020 HV0.5, and it shows a gradual decreasing trend along the thickness direction.

[0021] (4) In this invention, Nb element can also promote the formation of stable passivation film containing Nb, Cr and Si, thereby improving the resistance to chloride ion corrosion and high temperature oxidation, so that the coating has more stable surface passivation behavior and lower corrosion tendency in chloride ion environment.

[0022] (5) The process of this invention is highly adaptable and can be achieved by combining equipment such as flame spraying, vacuum cladding, vacuum diffusion sintering, laser remelting, ultrasonic rolling or low-stress shot peening. It is easy to adjust the total thickness, interlayer ratio and post-treatment intensity according to the size of the part and service requirements, and has high engineering application value. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the three-gradient coating structure of the present invention.

[0024] Figure 2 This is a schematic diagram of the preparation process of the present invention.

[0025] Figure 3 This is a schematic curve showing the microhardness distribution of the gradient composite coating cross section obtained in Example 3 of the present invention. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the following embodiments are used to illustrate the present invention, and not to limit the scope of protection of the present invention. Without departing from the concept of the present invention, those skilled in the art can make equivalent substitutions or optimizations to the powder particle size, layer thickness, number of spraying times and heat treatment regime.

[0027] In this embodiment of the invention, an intelligent high-speed flame spraying system, a vacuum diffusion cladding system, a laser remelting equipment, and an ultrasonic rolling equipment may be used. The purity of the argon gas used is preferably not less than 99.99%, and the purity of the hydrogen gas is preferably not less than 99.9%. The glycerol, hydroxypropyl methylcellulose, and coupling agent used can all be of analytical grade or industrial high-purity grade.

[0028] This invention provides a method for preparing a gradient composite coating on the surface of TC4 alloy. Please refer to [link / reference]. Figure 2The preparation method is described in detail below.

[0029] Step (1) Surface pretreatment of TC4 alloy The TC4 alloy substrate surface was cleaned sequentially with acetone and anhydrous ethanol using ultrasonic cleaning for 5–20 min to remove surface oil and impurities, followed by hot air drying. Subsequently, laser etching was performed with a laser power of 150–600 W, a scanning speed of 100–800 mm / s, and a scanning interval of 0.02–0.12 mm. Through the synergistic control of these parameters, a roughened surface with micro-pits and micro-grooves was formed on the TC4 alloy surface, resulting in a surface roughness Ra of 35–110 μm after etching. The effects of laser etching are twofold: firstly, it increases the contact area between the coating and the substrate, improving the mechanical interlocking effect of subsequent coatings; secondly, the micro-pits and micro-grooves can capture powder particles in the slurry during spraying, creating a microscopic interlocking structure between the coating and the substrate. After etching, the surface was activated using argon plasma for 1–10 min. Plasma activation removes adsorbed contaminants from the surface, and the surface activity is enhanced under the action of high-energy particles, improving the wettability and spreadability of the subsequent slurry spraying. Through the above cleaning, etching and activation processes, an active surface with micron-level roughness is obtained.

[0030] Step (2): Preparation of three types of slurry SiC composite base slurry, cobalt-based alloy intermediate layer slurry, and Nb-modified high-entropy alloy top layer slurry were prepared respectively. All three slurries included powder raw materials and organic carriers, with glycerol and hydroxypropyl methylcellulose (HMCMC) serving as the organic carriers. Glycerol was used to adjust the viscosity and flowability of the slurry, ensuring suitable flow for spraying; HMCMC acted as a thickener and binder, improving the cohesiveness and adhesion to the substrate, and preventing sedimentation and agglomeration of powder particles during drying after spraying. The ratio of powder raw materials to glycerol was 10 g:(5~18) mL, and the amount of HMCMC added was 0.15~0.50 wt.% of the corresponding powder raw material mass. The preparation methods of the three slurries are described below.

[0031] Preparation of SiC composite base layer slurry: By mass fraction, the SiC composite base layer powder raw materials include 65-85% SiC, 8-22% Ti, 5-12% cobalt-based alloy powder, and 0.5-3% rare earth oxides, which are selected from at least one of Y2O3, La2O3, or CeO2. The SiC powder particle size is 5-45 μm, and the Ti powder particle size is 15-75 μm. SiC is a hard ceramic phase, providing the wear-resistant skeleton and high-temperature stability of the coating; Ti powder can react with the SiC and TC4 surfaces during subsequent high-temperature treatment to generate TiC and Ti-Si reaction products, which are key components for achieving chemical anchoring; the addition of cobalt-based alloy powder can improve the metallic continuity of the base layer, and the composition of the cobalt-based alloy powder is consistent with the composition of the powder raw materials in the cobalt-based alloy intermediate layer slurry; rare earth oxides can refine the oxide film and reduce porosity defects during high-temperature treatment. Preferably, the organic carrier in the SiC composite base slurry includes glycerol and hydroxypropyl methylcellulose, as well as a silane coupling agent or polyvinylpyrrolidone. The above-mentioned powdered raw materials are mixed with glycerol at a ratio of 10 g:(5~18) mL, and 0.15~0.50 wt.% of hydroxypropyl methylcellulose (based on the mass of the powdered raw materials) is added. Further, 0.05~0.30 wt.% of silane coupling agent or polyvinylpyrrolidone (based on the mass of the powdered raw materials) is added to improve the dispersion stability of SiC particles in the slurry. After ball milling for 2 hours, the mixture is placed in a constant-temperature shaker and shaken for 12 hours to obtain a uniformly dispersed SiC composite base slurry.

[0032] Preparation of cobalt-based alloy interlayer slurry: By mass fraction, the cobalt-based alloy interlayer powder raw materials include Mo 0.15~0.50%, Mn 0.30~0.80%, C 0.30~0.90%, B 1.00~2.20%, Si 1.80~3.20%, Fe 1.50~4.00%, W 5.50~9.50%, Ni 12.00~18.50%, with the balance being Co. This cobalt-based alloy composition exhibits excellent toughness and resistance to thermal fatigue. Its coefficient of thermal expansion and elastic modulus are between those of the SiC composite base layer and the high-entropy alloy surface layer, making it suitable as a stress buffer layer. W and Mo provide solid solution strengthening, while B and Si form borides and silicides as reinforcing phases during high-temperature treatment. Ni and Cr improve corrosion resistance. The above powder raw materials were mixed with glycerin at a ratio of 10g:(5~18)mL, and 0.15~0.50wt.% of hydroxypropyl methylcellulose was added. The mixture was shaken for 12h to obtain a cobalt-based alloy intermediate layer slurry.

[0033] Preparation of Nb-modified high-entropy alloy surface coating slurry: The Nb-modified high-entropy alloy surface coating powder raw material, by mass fraction, is a B-Si-Mo-C-Ni-Cr-Nb-Fe system, comprising B 1.20~2.80%, Si 1.80~3.60%, Mo 2.40~4.50%, C 1.20~2.80%, Ni 4.50~7.50%, Cr 24.00~32.00%, Nb 3.00~8.00%, with the balance being Fe. The particle size of the Nb-modified high-entropy alloy surface coating powder raw material is 150~320 mesh. In this composition system, the addition of Nb can simultaneously achieve solid solution strengthening, carbide / boride dispersion strengthening, and passivation film stabilization; B and Si form boride and silicide reinforcing phases during high-temperature treatment, further improving the surface coating hardness and wear resistance; Mo improves corrosion resistance and high-temperature strength; Ni improves toughness and resistance to thermal shock. The above powder raw materials were mixed with glycerol at a ratio of 10g:(5~18)mL, and hydroxypropyl methylcellulose accounting for 0.15~0.50wt.% of the powder raw materials was added. The mixture was shaken for 16h to obtain Nb modified high entropy alloy surface slurry.

[0034] Step (3): Graded spraying to form a composite pre-coated layer The spray gun is used to perform graded spraying in the following order: "SiC composite base layer - first transition layer - cobalt-based alloy intermediate layer - second transition layer - Nb-modified high-entropy alloy surface layer", as detailed below: The SiC composite base layer slurry is sprayed onto the surface of the TC4 alloy after step (1) to form a SiC composite base layer with a thickness of 40~160μm; A first transition layer slurry, formed by mixing SiC composite base slurry and cobalt-based alloy intermediate layer slurry, is sprayed between SiC composite base slurry and cobalt-based alloy intermediate layer slurry. The mass ratio of the two slurries in the first transition layer slurry is any one of 70:30, 50:50 or 30:70, to form the first transition layer. A cobalt-based alloy intermediate layer slurry is sprayed onto the surface of the first transition layer to form a cobalt-based alloy intermediate layer with a thickness of 120~350μm. A second transition layer slurry, formed by mixing a cobalt-based alloy intermediate layer slurry and an Nb-modified high-entropy alloy surface layer slurry, is sprayed onto the surface of the cobalt-based alloy intermediate layer. The mass ratio of the two slurries in the second transition layer slurry is any one of 70:30, 50:50 or 30:70, to form the second transition layer. Nb-modified high-entropy alloy slurry is sprayed onto the surface of the second transition layer to form an Nb-modified high-entropy alloy surface layer with a thickness of 150~450μm.

[0035] By setting a first transition layer and a second transition layer, the chemical composition between adjacent layers changes gradually, leading to a continuous gradient change in hardness, coefficient of thermal expansion, and elastic modulus along the coating thickness direction. This avoids stress concentration and crack initiation caused by abrupt changes in properties at the interface. This forms a composite pre-coated layer. The total coating thickness is 310~961μm.

[0036] The spraying sequence is designed with a clear logic: the innermost SiC composite base layer is in direct contact with the TC4 alloy substrate, undergoing an in-situ reaction with the substrate during high-temperature treatment to achieve chemical anchoring; the middle cobalt-based alloy layer has excellent toughness, absorbing and dispersing thermal stress and mechanical loads; the outermost Nb-modified high-entropy alloy layer has high hardness and excellent corrosion resistance, directly resisting abrasion and corrosive media erosion. The progressive function of the three layers, together with the setting of the transition layer, constitutes a complete protective system.

[0037] Step (4), graded drying and flame spraying densification treatment The composite pre-coated coating obtained in step (3) is subjected to graded drying. Graded drying adopts a stepped heating method, first drying at a lower temperature to slowly remove the organic carrier, and then drying at a higher temperature to completely remove the residual solvent, preventing cracks or blistering on the coating surface due to excessively rapid drying. The specific temperature and time of graded drying can be selected according to the actual slurry formulation and coating thickness used.

[0038] After drying, densification is performed using flame spraying equipment at a temperature of 450~950℃ for 0.05~1.0h, a spraying distance of 120~260mm, a spraying angle of 75~90°, and a combustion air flow rate of 1.0~2.5m. 3 / h. Flame spraying uses a weak reducing atmosphere to reduce the oxidation of TC4 alloy and coating powder. The purpose of flame spraying is to decompose and release the organic carrier, while simultaneously allowing the powder particles to pre-bond under the heat of the flame, preventing the coating from peeling off or cracking before subsequent vacuum diffusion cladding.

[0039] Step (5) Vacuum diffusion cladding treatment The TC4 alloy substrate with a composite pre-coated layer, after flame densification, is subjected to high-temperature diffusion cladding in a vacuum or inert protective atmosphere. Vacuum diffusion cladding is performed at a vacuum level ≤ 5 × 10⁻⁶. -2 The process is carried out under a protective atmosphere of Pa or high-purity argon, with a heating rate of 5~15℃ / min, a holding temperature of 1080~1380℃, a holding time of 0.5~4h, and then cooled with the furnace to below 600℃ before being cooled with inert gas.

[0040] A vacuum or inert protective atmosphere is used to prevent oxidation of the coating powder and the TC4 alloy substrate at high temperatures, ensuring the smooth progress of the interfacial reaction. During this high-temperature treatment, an in-situ reaction occurs between the SiC composite substrate and the TC4 alloy substrate, generating TiC and Ti-Si intermetallic compounds. These compounds together form an in-situ reaction connection zone between the SiC composite substrate and the TC4 alloy substrate, providing chemical anchoring and diffusion barrier functions. Simultaneously, diffusion and sintering occur between adjacent powder particles within each layer, forming a continuous and dense coating structure. The Nb-modified high-entropy alloy surface layer forms a composite structure containing a BCC / FCC solid solution matrix phase, as well as NbC reinforcing phases and boride reinforcing phases dispersed in the matrix phase during the high-temperature treatment.

[0041] Finally, a gradient composite coating is formed on the surface of the TC4 alloy substrate, consisting of, from the inside out, a SiC composite base layer, a first transition layer, a cobalt-based alloy intermediate layer, a second transition layer, and an Nb-modified high-entropy alloy functional surface layer. Please refer to [reference needed]. Figure 2 In this structure, an in-situ reaction bonding zone containing TiC and Ti-Si intermetallic compounds is formed between the SiC composite base layer and the TC4 alloy matrix, providing chemical anchoring and diffusion barrier functions; the cobalt-based alloy intermediate layer forms a continuous toughness buffer zone, mitigating thermal stress and abrupt changes in elastic modulus; and the Nb-modified high-entropy alloy surface layer forms a composite functional zone containing BCC / FCC solid solution, NbC, and boride reinforcing phases. The combined effect of these three elements significantly improves the coating's resistance to thermal shock, spalling, wear, and chloride ion corrosion.

[0042] Following vacuum diffusion cladding, at least one of the following post-treatments can be performed, depending on service requirements: laser remelting, ultrasonic rolling, or low-stress shot peening. Laser remelting can further refine the microstructure of the Nb-modified high-entropy alloy surface layer and create a more continuous Cr-Nb-Si composite passivation film. Ultrasonic rolling can reduce surface roughness, seal surface pores, and introduce compressive stress. Low-stress shot peening can also introduce compressive stress on the coating surface and improve surface integrity. These post-treatment methods can be flexibly selected or combined according to part size and service requirements.

[0043] This gradient composite coating can be applied to key TC4 alloy components such as aero-engine blades, compressor disks, fuselage structural parts, ship connectors, and spacecraft load-bearing components. It provides effective surface protection under complex working conditions such as high-temperature oxidation, chloride ion corrosion, and friction and wear, thereby improving the service stability and service life of the components.

[0044] Regarding the thickness of each layer: In this invention, the thickness of the SiC composite base layer is 40~160μm, the thickness of the cobalt-based alloy intermediate layer is 120~350μm, and the thickness of the Nb-modified high-entropy alloy surface layer is 150~450μm. These refer to the thickness of each main layer. The transition layer is a compositional overlap region formed by spraying between adjacent main layers; its thickness is measured separately (generally below 500nm) and is not included in the thickness of the aforementioned main layers. In the microstructure of the coating cross-section, the transition layer appears as an interface region where the composition gradually changes between adjacent main layers. The existence of this region allows for a continuous gradient transition in chemical composition and properties between adjacent main layers.

[0045] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and comparative examples.

[0046] Example 1

[0047] This embodiment provides a method for preparing a gradient composite coating on the surface of TC4 alloy, including the following steps: (1) The TC4 alloy sample was ultrasonically cleaned in acetone and anhydrous ethanol for 10 min in sequence and dried with hot air; laser etching was performed with a laser power of 300W, a scanning speed of 500mm / s and a scanning interval of 0.1mm to make the surface roughness Ra 70μm; then it was activated in argon plasma for 3 min. (2) Weigh the powder raw materials according to the composition of SiC composite base layer powder (SiC 76%, Ti 15%, cobalt-based alloy powder 8%, Y2O3 1%), and add glycerol, 0.25 wt.% of hydroxypropyl methylcellulose and 0.10 wt.% of silane coupling agent. The ratio of powder raw materials to glycerol is 10 g: 8 mL. After ball milling for 2 h, place it in a constant temperature shaker and shake for 12 h to obtain SiC composite base layer slurry. (3) Weigh the powder raw materials according to the composition of the cobalt-based alloy intermediate layer powder raw materials (Mo 0.25%, Mn 0.47%, C 0.62%, B 1.48%, Si 2.33%, Fe 2.68%, W 7.52%, Ni 15.79%, Co balance), mix with glycerol at a ratio of 10g:7mL, add 0.20wt.% hydroxypropyl methylcellulose according to the mass of the powder raw materials, and shake for 12h to obtain the cobalt-based alloy intermediate layer slurry; (4) Weigh the powder raw materials according to the composition of the Nb modified high entropy alloy surface layer powder raw materials (B 1.65%, Si 2.20%, Mo 3.35%, C 1.85%, Ni 6.20%, Cr 27.50%, Nb 5.80%, Fe balance), mix with glycerol at 10g:9mL, add 0.25wt.% hydroxypropyl methylcellulose according to the mass of the powder raw materials, and shake for 16h to obtain Nb modified high entropy alloy surface layer slurry; (5) Use a spray gun to perform graded spraying in sequence: the thickness of the SiC composite base layer is about 80 μm, the thickness of the cobalt-based alloy intermediate layer is about 220 μm, and the thickness of the Nb-modified high-entropy alloy surface layer is about 300 μm; a first transition layer with a mass ratio of 50:50 is set between the SiC composite base layer and the cobalt-based alloy intermediate layer, and a second transition layer with a mass ratio of 50:50 is set between the cobalt-based alloy intermediate layer and the Nb-modified high-entropy alloy surface layer to obtain a composite pre-coated coating; (6) The composite pre-coated layer is dried at 80℃ for 40 min and at 100℃ for 30 min, and then densified using a flame spraying device with an air flow rate of 1.5 m³ / min. 3 / h, spraying distance is 200mm, spraying angle is 90°, treatment temperature is 850℃, and treatment time is 0.25h; (7) The TC4 alloy substrate with composite pre-coated coating after flame densification is placed in a vacuum diffusion cladding system, with a vacuum degree ≤5×10 -2 The temperature was increased to 1260℃ at 10℃ / min under Pa, held for 2 hours, and then cooled to below 600℃ in the furnace and cooled by argon gas.

[0048] The surface microhardness of the coating is 980 HV0.5. The cross-sectional hardness of the coating decreases gently from 980 HV0.5 on the surface layer towards the TC4 substrate, with the hardness at the interface between the substrate and the base layer being approximately 420 HV0.5. In a 3.5% NaCl solution, the self-corrosion potential of the coated sample is -0.31 V, and the self-corrosion current density is 2.8 × 10⁻⁶. -8 A / cm 2 In the dry reciprocating sliding friction and wear test (load 5N, sliding speed 0.1m / s, sliding distance 500m), the wear amount of the coating was 0.52mg.

[0049] Example 2

[0050] This embodiment provides a method for preparing a gradient composite coating on the surface of TC4 alloy, including the following steps: (1) The surface of TC4 alloy was treated according to step (1) of Example 1, but the surface roughness after laser etching was controlled to 100 μm to improve the mechanical interlocking ability of the thick coating. (2) Preparation of SiC composite base layer slurry, cobalt-based alloy intermediate layer slurry and Nb-modified high-entropy alloy surface layer slurry, wherein the raw material composition of Nb-modified high-entropy alloy surface layer powder is B 1.80%, Si 2.45%, Mo 3.60%, C 2.10%, Ni 6.80%, Cr 26.50%, Nb 6.20%, Fe balance; (3) The composite pre-coated coating is obtained by spraying the SiC composite base layer, the first transition layer (the mass ratio of SiC composite base layer to cobalt-based alloy intermediate layer is 70:30), the cobalt-based alloy intermediate layer, the second transition layer (the mass ratio of cobalt-based alloy intermediate layer to Nb-modified high-entropy alloy surface layer is 30:70), and the Nb-modified high-entropy alloy surface layer in that order. (4) The composite pre-coated coating is dried at 70℃ for 30 min and at 110℃ for 20 min, and then subjected to flame densification treatment at a temperature of 650~900℃ for 0.35 h. (5) Vacuum diffusion cladding treatment was carried out under the protection of high-purity argon gas, with a holding temperature of 1300℃ and a holding time of 1.5h; after cooling, ultrasonic rolling treatment was carried out with a rolling force of 300N and a rolling speed of 80mm / s.

[0051] The surface microhardness of the coating is 935 HV0.5. The cross-sectional hardness of the coating decreases gently from 935 HV0.5 on the surface layer towards the TC4 substrate, with the hardness at the interface between the substrate and the base layer being approximately 400 HV0.5. In a 3.5% NaCl solution, the self-corrosion potential of the coated sample is -0.35 V, and the self-corrosion current density is 3.5 × 10⁻⁶. -8 A / cm 2 In the dry reciprocating sliding friction and wear test (load 5N, sliding speed 0.1m / s, sliding distance 500m), the wear amount of the coating was 0.61mg.

[0052] Example 3

[0053] This embodiment provides a method for preparing a gradient composite coating on the surface of TC4 alloy, including the following steps: (1) The composite pre-coating was prepared using the sequential spraying method of Example 1, but the thickness of the Nb-modified high-entropy alloy surface layer was controlled to be 450 μm; (2) The composite pre-coated coating was subjected to graded drying and flame spraying densification treatment using the method of Example 1; (3) After flame densification treatment, it is kept at 1240℃ for 3 hours in a vacuum diffusion cladding system; (4) After vacuum diffusion cladding, the Nb-modified high-entropy alloy surface layer was subjected to laser remelting treatment. The laser power was 900W, the scanning speed was 800mm / s, the spot diameter was 2.0mm, and the overlap rate was 50%. (5) After laser remelting, the product is subjected to low-temperature stress relief treatment at 650℃ for 2 hours under argon protection.

[0054] The surface microhardness of the coating is 1020 HV0.5; please refer to... Figure 3The hardness of the coating cross-section decreases gently from 1020 HV0.5 (highest) on the surface layer towards the TC4 substrate, with the hardness at the interface between the base layer and the substrate being approximately 430 HV0.5. In a 3.5% NaCl solution, the self-corrosion potential of the coated sample is -0.28V, and the self-corrosion current density is 2.1 × 10⁻⁶. -8 A / cm 2 In the dry reciprocating sliding friction and wear test (load 5N, sliding speed 0.1m / s, sliding distance 500m), the wear amount of the coating was 0.43mg.

[0055] Comparative Example 1 Take a TC4 alloy sample of the same specifications as in Example 1, and ultrasonically clean it in acetone and anhydrous ethanol for 10 minutes in sequence. After hot air drying, it is ready for use without any coating treatment on the surface.

[0056] The TC4 matrix has a microhardness of 340 HV0.2. In a 3.5% NaCl solution, the self-corrosion potential of the TC4 matrix is ​​-0.72 V, and the self-corrosion current density is 6.8 × 10⁻⁶. -7 A / cm 2 In the dry reciprocating sliding friction and wear test (load 5N, sliding speed 0.1m / s, sliding distance 500m), the wear amount of the TC4 substrate was 6.72mg.

[0057] The test results of Examples 1-3 and Comparative Example 1 show that the gradient composite coating prepared by the present invention significantly improves the overall performance of the TC4 alloy surface. Compared with the untreated TC4 substrate, the microhardness of the coating surface layer in Examples 1-3 is significantly improved, and the cross-sectional hardness decreases gradually from the surface layer to the substrate, indicating that the transition layer effectively alleviates the abrupt hardness change between the coating and the substrate. In 3.5% NaCl solution, the self-corrosion potential of Example 13 shifted positively by 0.41V, 0.37V, and 0.44V, respectively, and the self-corrosion current density decreased by about one order of magnitude, indicating that the coating's resistance to chloride ion corrosion is significantly better than that of the TC4 substrate. In the tribological test, the wear amount of Examples 1-3 is significantly lower than that of the TC4 substrate, indicating that the wear resistance of the coating is significantly improved compared to the TC4 substrate. Due to the use of laser remelting post-treatment in Example 3, the surface microstructure is further refined, and all performance indicators are superior to those of Examples 1 and 2.

[0058] In summary, all embodiments of the present invention can successfully prepare a gradient composite coating on the surface of a TC4 alloy substrate, consisting of a SiC composite base layer, a first transition layer, a cobalt-based alloy intermediate layer, a second transition layer, and an Nb-modified high-entropy alloy surface layer. In each embodiment, the SiC composite base layer forms an in-situ reaction connection zone containing TiC and Ti-Si intermetallic compounds with the TC4 alloy substrate, achieving a composite connection of chemical anchoring and metallurgical diffusion; the cobalt-based alloy intermediate layer forms a continuous toughness buffer zone, effectively alleviating thermal stress and abrupt changes in elastic modulus; the Nb-modified high-entropy alloy surface layer forms a composite structure containing BCC / FCC solid solution, NbC, and boride reinforcing phases. The resulting gradient composite coating exhibits a gentle gradient transition in hardness along the thickness direction, avoiding stress concentration and crack initiation caused by abrupt changes in performance at the interface, and significantly improving the resistance to thermal shock, spalling, wear, and chloride ion corrosion of the TC4 alloy surface. Those skilled in the art can adjust the thickness of each layer, the proportion of the transition layer, and the post-treatment method according to actual working conditions, referring to the above embodiments, all of which fall within the protection scope of the present invention.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. All equivalent transformations or modifications made in accordance with the essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a gradient composite coating on the surface of a TC4 alloy, characterized in that, Includes the following steps: Step (1): Clean the surface of the TC4 alloy substrate, perform laser etching and plasma activation treatment to obtain a roughened active surface; Step (2): Prepare SiC composite base layer slurry, cobalt-based alloy intermediate layer slurry and Nb-modified high-entropy alloy surface layer slurry respectively; all three slurries contain powder raw materials and organic carriers, the organic carriers including glycerol and hydroxypropyl methylcellulose; Step (3): The SiC composite base slurry, cobalt-based alloy intermediate layer slurry, and Nb-modified high-entropy alloy surface layer slurry are sequentially sprayed onto the roughened active surface of the TC4 alloy substrate in an order from the inside out; and a first transition layer slurry formed by the mixture of the two is sprayed between the SiC composite base slurry and the cobalt-based alloy intermediate layer slurry, and a second transition layer slurry formed by the mixture of the two is sprayed between the cobalt-based alloy intermediate layer slurry and the Nb-modified high-entropy alloy surface layer slurry, thereby forming a composite pre-coated coating; Step (4): The composite precast coating is subjected to graded drying and flame spraying densification treatment; Step (5): The TC4 alloy substrate with composite pre-coated material after step (4) is placed in a vacuum or inert protective atmosphere for high-temperature diffusion cladding treatment, so that the composite pre-coated material and the TC4 alloy substrate, as well as the adjacent coatings, form a metallurgical diffusion bond. Finally, a gradient composite coating is formed on the surface of the TC4 alloy substrate, consisting of SiC composite base layer, first transition layer, cobalt-based alloy intermediate layer, second transition layer and Nb modified high-entropy alloy surface layer from the inside to the outside. The first transition layer and the second transition layer respectively form a compositional gradient transition between adjacent coatings.

2. The preparation method according to claim 1, characterized in that, In the first transition layer slurry, the mass ratio of SiC composite base layer slurry to cobalt-based alloy intermediate layer slurry is any one of 70:30, 50:50 or 30:70; In the second transition layer slurry, the mass ratio of the cobalt-based alloy intermediate layer slurry to the Nb-modified high-entropy alloy surface layer slurry is any one of 70:30, 50:50, or 30:

70.

3. The preparation method according to claim 1, characterized in that, The powder raw materials in the SiC composite base slurry include, by mass fraction: 65-85% SiC, 8-22% Ti, 5-12% cobalt-based alloy powder, and 0.5-3% rare earth oxides; the rare earth oxides are selected from at least one of Y2O3, La2O3, or CeO2.

4. The preparation method according to claim 3, characterized in that, The powder raw materials in the cobalt-based alloy powder and the cobalt-based alloy intermediate layer slurry, by mass fraction, include: Mo 0.15~0.50%, Mn 0.30~0.80%, C 0.30~0.90%, B 1.00~2.20%, Si 1.80~3.20%, Fe 1.50~4.00%, W 5.50~9.50%, Ni 12.00~18.50%, with the balance being Co.

5. The preparation method according to claim 1, characterized in that, The powder raw materials in the Nb-modified high-entropy alloy surface slurry comprise, by mass fraction: B 1.20~2.80%, Si 1.80~3.60%, Mo 2.40~4.50%, C 1.20~2.80%, Ni 4.50~7.50%, Cr 24.00~32.00%, Nb 3.00~8.00%, with the balance being Fe.

6. The preparation method according to claim 1, characterized in that, When preparing each layer of slurry, the ratio of the powdered raw material to glycerol in the organic carrier is 10g:(5~18)mL, and the amount of hydroxypropyl methylcellulose added is 0.15~0.50wt.% of the mass of the powdered raw material; the SiC composite base layer slurry also contains 0.05~0.30wt.% of silane coupling agent or polyvinylpyrrolidone, which accounts for 0.05~0.30wt.% of the mass of the powdered raw material.

7. The preparation method according to claim 1, characterized in that, In step (1), the parameters of laser etching are: laser power 150~600W, scanning speed 100~800mm / s, scanning spacing 0.02~0.12mm, and surface roughness Ra after etching is 35~110μm; the plasma activation treatment is argon plasma treatment, and the treatment time is 1~10min.

8. The preparation method according to claim 1, characterized in that, In step (4), the temperature for flame spraying densification is 450~950℃, the treatment time is 0.05~1.0h, the spraying distance is 120~260mm, the spraying angle is 75~90°, and the combustion air flow rate is 1.0~2.5m. 3 / h, and the flame spraying uses a weak reducing atmosphere; in step (5), the vacuum degree of the vacuum diffusion cladding process is ≤5×10 -2 Pa or a high-purity argon protective atmosphere is used, with a heating rate of 5~15℃ / min, a holding temperature of 1080~1380℃, and a holding time of 0.5~4h.

9. The preparation method according to claim 1, characterized in that, After step (5), the gradient composite coating is further subjected to laser remelting, ultrasonic rolling or low-stress shot peening post-treatment.

10. A gradient composite coating on the surface of TC4 alloy, characterized in that, The gradient composite coating is prepared by the preparation method according to any one of claims 1 to 9. The gradient composite coating comprises, from the surface of the TC4 alloy substrate outward, a SiC composite base layer, a first transition layer, a cobalt-based alloy intermediate layer, a second transition layer, and an Nb-modified high-entropy alloy functional surface layer; wherein, the thickness of the SiC composite base layer is 40~160μm, the thickness of the cobalt-based alloy intermediate layer is 120~350μm, and the thickness of the Nb-modified high-entropy alloy functional surface layer is 150~450μm. TiC and Ti-Si intermetallic compounds are distributed at the interface between the SiC composite base layer and the TC4 alloy matrix to form an in-situ reaction bonding region. The Nb-modified high-entropy alloy surface layer contains a BCC / FCC solid solution matrix phase, as well as NbC reinforcing phases and boride reinforcing phases dispersed within the matrix phase.

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