A wear-resistant and friction-reducing high-temperature-resistant composite coating for stainless steel surface, a preparation method and application thereof

Abstract

This invention provides a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces, its preparation method, and its application. The wear-resistant, friction-reducing, and high-temperature resistant composite coating of this invention comprises: graphite powder, SiO2 powder, polyimide solution (20% solid content), diluent, and dispersant. This invention also relates to a method for constructing a composite coating on a stainless steel surface by spraying. This invention improves interfacial density and adhesion stability by constructing a polyimide underlayer on the stainless steel substrate surface, and combines the solid lubrication effect of graphite with the reinforcing effect of modified SiO2 in a composite functional layer on top, significantly improving the wear-resistant, friction-reducing, and high-temperature resistant performance and structural stability of the coating. It is suitable for the field of wear-resistant, friction-reducing, and high-temperature resistant protection of metal surfaces.

Description

Technical Field

[0001] This invention relates to the field of metal surface lubrication and protection technology; and more particularly to a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces, its preparation method, and its application. Background Technology

[0002] With the rapid development of equipment manufacturing, aerospace, and high-end machinery, stainless steel is widely used in friction pairs and structural components due to its excellent corrosion resistance and mechanical properties. However, in actual service, stainless steel is prone to severe adhesive wear and surface failure under dry friction or high-temperature conditions, manifested as high friction coefficient, high wear rate, and easy galling, which seriously affects its service life and reliability. Therefore, constructing high-performance friction-reducing and wear-resistant coatings on stainless steel surfaces has become an important research direction.

[0003] While existing polyimide-based friction-reducing and high-temperature resistant composite coatings can improve the friction and wear properties of metal surfaces to some extent, they still suffer from problems such as insufficient interfacial bonding, poor filler dispersion, and low coating density. In particular, when directly constructing composite coatings on stainless steel substrates, the limited wettability of the metal surface and insufficient interfacial compatibility can easily lead to poor coating adhesion, local delamination, and decreased service stability.

[0004] In addition, although existing technologies introduce solid lubricant fillers such as graphite to reduce the coefficient of friction, the single lubricant phase system is insufficient in terms of load-bearing capacity and is prone to accelerated wear under high load or long-term friction conditions. At the same time, although the introduction of inorganic particles (such as SiO2 or TiO2) can improve wear resistance to a certain extent, their poor interfacial compatibility with the polyimide matrix makes them prone to agglomeration, which leads to an increase in internal defects in the coating and affects the overall performance.

[0005] To address the aforementioned shortcomings, this invention proposes a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces and its preparation method. By constructing a two-layer structure system of "polyimide underlayer + composite functional layer" and combining it with KH550 silane coupling agent to modify the surface of SiO2, its dispersibility and interfacial compatibility in the polyimide system are improved. Summary of the Invention

[0006] The purpose of this invention is to provide a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces, its preparation method, and its application. This invention addresses the problems of accelerated wear, performance degradation, and unstable coating adhesion on existing stainless steel substrates under complex service conditions such as friction, wear, and high temperatures. It also solves the problems of interfacial voids, localized porosity, filler agglomeration, and insufficient structural density that often occur during the deposition of existing composite coatings on metal substrates.

[0007] This invention is achieved through the following technical solution:

[0008] This invention relates to a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces, comprising: graphite powder, SiO2 powder, a polyimide solution with a solid content of 20%, a diluent, and a dispersant;

[0009] The wear-resistant, friction-reducing, and high-temperature resistant composite coating includes: a polyimide underlayer disposed on the surface of a stainless steel substrate and a composite functional layer disposed on top of it;

[0010] The composite functional layer includes: graphite powder, SiO2 powder, cured polyimide (PI), diluent, and dispersant;

[0011] The diluent is N,N-dimethylformamide (DMF).

[0012] The dispersant is polyvinylpyrrolidone (PVP);

[0013] The SiO2 powder is SiO2 powder that has undergone surface modification treatment with silane coupling agent KH550;

[0014] The cured polyimide in the composite functional layer accounts for 60 wt% to 80 wt% of the total mass of the composite functional layer components;

[0015] Graphite powder constitutes 15 wt% to 30 wt% of the total filler mass;

[0016] The SiO2 powder constitutes 5 wt% to 10 wt% of the total filler mass.

[0017] The dispersant accounts for 5 wt% to 8 wt% of the total mass of the filler.

[0018] A polyimide solution with a solid content of 20% is mixed with N,N-dimethylformamide at a volume ratio of 1:1 to 1:1.5 to obtain a slurry for the polyimide underlayer.

[0019] Preferably, the graphite powder is flake graphite with a particle size ≤10 μm.

[0020] Preferably, the SiO2 powder is nano-sized SiO2.

[0021] This invention also relates to a method for preparing the aforementioned wear-resistant, friction-reducing, and high-temperature-resistant composite coating for stainless steel surfaces, comprising the following steps:

[0022] Step 1, Modification of SiO2

[0023] SiO2 powder was added to a mixed solvent of ethanol and deionized water in a volume ratio of 90:10 and ultrasonically dispersed for 10–20 min; glacial acetic acid was added to adjust the pH to 4–5; silane coupling agent was added at an amount of 3%–8% of the mass of SiO2; the reaction was stirred at room temperature or 40–50 °C for 2–4 h; after the reaction was completed, the modified SiO2 powder was obtained by separation, washing and drying.

[0024] Step 2, Preparation of the bottom slurry

[0025] The polyimide precursor was mixed with N,N-dimethylformamide to obtain a polyimide underlayer slurry;

[0026] Step 3, Preparation of composite functional layer slurry

[0027] First, graphite powder and modified SiO2 powder are added to N,N-dimethylformamide, followed by polyvinylpyrrolidone. The mixture is then pre-dispersed using mechanical stirring / ultrasonic dispersion. Subsequently, a polyimide solution with a solid content of 20% is added, and the mixture is stirred and mixed evenly to obtain a graphite / SiO2 / polyimide composite functional layer slurry.

[0028] This invention also relates to the application of the aforementioned wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces, applied to stainless steel surfaces by spraying.

[0029] The specific steps for applying the coating to a stainless steel surface via spraying are as follows:

[0030] Step A, Matrix Pretreatment

[0031] The stainless steel substrate is polished, cleaned, and dried to remove surface contaminants;

[0032] Step B, base coat spraying

[0033] Place the substrate 2 on the spraying workbench 3 and use a pneumatic spray gun 1 to spray the polyimide base layer slurry onto the surface of the substrate 2.

[0034] Step C, Pre-baking

[0035] After spraying the polyimide base coat, the fasteners are pre-baked at 70°C for 5 to 20 minutes to pre-cure the base coat.

[0036] Step D: Multiple thin-spraying of composite functional layer slurry

[0037] The graphite / SiO2 / polyimide composite functional layer slurry was sprayed onto the surface of substrate 2 using a spraying method. During the spraying process, the spraying path was in a "Z" shape. After spraying, a pre-baking treatment was performed for 5 to 20 minutes. The spraying and pre-baking operation was repeated 2 to 3 times.

[0038] Step E, heat treatment curing

[0039] The fasteners are heat-treated and cured in a step-by-step manner to remove some of the solvent, causing the polyimide solution to undergo an imidization reaction, ultimately forming a wear-resistant, friction-reducing, and high-temperature resistant composite coating on the surface of substrate 2.

[0040] Preferably, during the spraying process, the spraying distance between the pneumatic spray gun 1 and the substrate 2 is 13 cm to 17 cm.

[0041] Preferably, during the spraying process, the pneumatic spray gun 1 sprays a "Z" shaped path on the substrate surface 2.

[0042] Preferably, the thickness of the wear-resistant, friction-reducing, and high-temperature resistant composite coating is 20–30 μm; wherein the thickness of the polyimide underlayer is 3–6 μm, and the thickness of the composite functional layer is 14–27 μm.

[0043] Preferably, the stepped curing is specifically performed under the following conditions: curing at 80°C for 30 min, curing at 120°C for 1 h, curing at 220°C for 1 h, and curing at 300°C for 1 h.

[0044] The functions of each component involved in this invention are as follows:

[0045] The polyimide described in this invention serves as a film-forming matrix, imparting excellent heat resistance, film-forming properties, and adhesion stability to the coating.

[0046] The graphite powder, as a solid lubricating phase, forms a low-shear lubrication interface due to its layered structure, thereby reducing frictional resistance and improving the friction-reducing and wear-resistant properties of the coating.

[0047] The modified SiO2 is used as a reinforcing phase to improve the hardness and load-bearing capacity of the coating.

[0048] The KH550 silane coupling agent is used to modify the surface of SiO2, thereby improving its dispersion uniformity and interfacial compatibility in the polyimide system, reducing filler agglomeration and interfacial defects, and improving coating density and service stability.

[0049] To achieve stable adhesion and uniform deposition of the wear-resistant, friction-reducing, and high-temperature resistant composite coating involved in this invention on the stainless steel surface, a combination of a polyimide underlayer and a multi-layer thin-spray process is adopted to form a continuous and dense structure on the substrate surface. This effectively avoids the interface voids and local porosity problems that are easy to occur during direct deposition, thereby giving full play to the friction-reducing, wear-resistant, and heat-resistant protective properties of the composite coating.

[0050] The present invention has the following advantages:

[0051] (1) This invention constructs a wear-resistant, friction-reducing, and high-temperature resistant composite coating system for stainless steel substrates. Polyimide serves as the film-forming matrix, providing excellent heat resistance, film-forming properties, and adhesion stability. Graphite, as a solid lubricating phase, provides a low-shear lubrication interface. SiO2, as a reinforcing phase, improves the coating's load-bearing capacity and wear resistance. The synergistic effect of these three components results in a coating with good friction-reducing properties, wear resistance, and thermal stability. According to relevant research, graphite filling can reduce the coefficient of friction from approximately 0.4 to the range of 0.1–0.2.

[0052] (2) By introducing a polyimide underlayer onto the surface of a stainless steel substrate, this invention effectively improves the wettability of the substrate surface and fills its micro-defects, reducing the interfacial voids between the composite functional layer and the substrate, thereby significantly improving the structural density and adhesion stability of the coating. Typically, this type of structure can enable the coating adhesion to reach the 0-1 level of the cross-cut adhesion test and significantly reduce the risk of interfacial delamination.

[0053] (3) This invention modifies the surface of SiO2 with the silane coupling agent KH550, introducing an organosilicon structure onto its surface to improve its dispersion uniformity and interfacial compatibility in the polyimide matrix. Test results show that after surface modification and interfacial reinforcement treatment, the wear rate of the polyimide composite material can be reduced by about 40%, and the friction performance is significantly improved. Therefore, this invention further reduces filler agglomeration and porosity, and improves the coating density and service stability.

[0054] (4) This invention employs a synergistic design of graphite powder and modified SiO2 powder to improve wear resistance while ensuring a low coefficient of friction. Test results show that the coefficient of friction of the composite material is stable in the range of approximately 0.10 to 0.20, while the wear rate can reach 10. -5 ~10 -6 The effect is on the order of mm³ / (N·m), achieving synergistic optimization of friction reduction and wear resistance.

[0055] (5) The present invention adopts a preparation process that combines multiple thin spraying and pre-baking, so that the composite coating is deposited layer by layer to form a continuous and dense structure, effectively avoiding problems such as sagging, pores and local accumulation that are easy to occur in the single thick coating process, thereby improving the uniformity and structural integrity of the coating, and the coating thickness can be stably controlled within the range of 20 to 30 μm.

[0056] (6) The present invention employs a gradient temperature curing process (curing is performed sequentially as follows: curing at 80℃ for 30 min, curing at 120℃ for 1 h, curing at 220℃ for 1 h, and curing at 300℃ for 1 h), which facilitates the gradual evaporation of the solvent and the staged curing of the polyimide, avoiding defects such as blistering, pinholes, and cracking caused by rapid temperature rise. Polyimide material itself has excellent heat resistance and can maintain structural stability at around 350℃. The coating of the present invention can maintain stable friction performance under high temperature conditions, remaining within the range of 0.15 to 0.20.

[0057] (7) The wear-resistant, friction-reducing, and high-temperature resistant composite coating prepared by this invention has good environmental adaptability and stability. It can still form a stable transfer film under dry friction and high-temperature conditions, thereby continuously reducing frictional resistance and inhibiting wear. Test results show that the coating is more likely to form a stable lubricating film under conditions of 200-250℃, thereby further reducing friction and wear. Attached Figure Description

[0058] Figure 1 This is a schematic diagram of the spraying of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel substrates, which is involved in this invention;

[0059] The attached diagram is labeled as follows: 1 is the pneumatic spray gun, 2 is the substrate, and 3 is the spraying worktable. Detailed Implementation

[0060] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are merely further illustrations of the present invention, but the scope of protection of the present invention is not limited to the following embodiments.

[0061] Example 1

[0062] This embodiment relates to a wear-resistant, friction-reducing, and high-temperature resistant composite coating for a stainless steel substrate, including the following steps:

[0063] The first step is to weigh 5 g of SiO2 powder and add it to a mixed solvent consisting of 95 mL of anhydrous ethanol and 5 mL of deionized water, and then ultrasonically disperse it for 20 min to form a uniform suspension.

[0064] Then, a small amount of glacial acetic acid was added to adjust the pH of the system to 4-5, and then 0.25 g of KH550 silane coupling agent was added. After hydrolysis at room temperature for 20 min, the reaction was stirred for 2 h to allow KH550 to undergo a condensation reaction with the hydroxyl groups on the surface of SiO2 after hydrolysis.

[0065] After the reaction was completed, the mixture was separated by centrifugation and washed 2-3 times with anhydrous ethanol. Finally, it was dried in an oven at 80℃ for 6 h to obtain modified SiO2 powder.

[0066] The second step involves mixing the polyimide solution (20% solid content) with N,N-dimethylformamide (DMF) solvent at a volume ratio of 1:1, and then stirring the mixture in a stirring device for 30 minutes to obtain the polyimide bottom layer slurry.

[0067] The third step involves weighing 2.7 g of flake graphite (particle size ≤ 10 μm) and placing it in a dispersion container. 0.15 g of PVP is added as a dispersant, and 20 ml of DMF solvent is added as a diluent. Simultaneously, 0.3 g of the modified SiO2 powder obtained in step 1 is added to 20 ml of DMF solvent. After ultrasonic dispersion for 20 min, the mixture is combined and mechanically stirred for 20 min. Then, 36.0 g of polyimide solution (20% solid content) is added, and the mixture is mechanically stirred at 600 r / min for 4 h. Ultrasonic dispersion is then performed again for 20 min to obtain the graphite / SiO2 / polyimide composite coating slurry.

[0068] The fourth step is to sand the stainless steel substrate 2 with sandpaper, then ultrasonically clean it in anhydrous ethanol for 20 minutes each to remove surface oil and impurities, and then dry it in an oven at 70°C for later use.

[0069] Step 5: Fix the pretreated stainless steel substrate 2 onto the spraying workbench 3. Using a small pneumatic spray gun 1, evenly spray the polyimide undercoat slurry prepared in step 2 onto the substrate surface, following a "Z" shaped spraying path; control the spraying distance to 13 cm. Figure 1 As shown;

[0070] Step 6: Place the stainless steel substrate with the base coat applied in a 70°C oven for 10 minutes to pre-bake, in order to remove some of the solvent and allow the base coat to stabilize initially.

[0071] Step 7: Using a small pneumatic spray gun 1, spray the graphite / SiO2 / polyimide composite slurry prepared in step 2 onto the substrate surface that has been coated with the underlayer; the spraying distance is about 13 cm. After each spraying, place it in a 70℃ oven for pre-baking for 10 min; repeat the above "spraying-pre-baking" operation twice in total, and deposit the composite coating layer by layer in a multi-thin spraying manner;

[0072] Step 8: Place the pre-baked sample in a high-temperature oven and perform step-by-step curing as follows: hold at 80℃ for 30 min, raise the temperature to 120℃ and hold for 1 h, raise the temperature to 220℃ and hold for 1 h, and then raise the temperature to 300℃ and hold for 1 h; after curing, cool to room temperature with the oven to obtain a stainless steel sample with a wear-resistant, friction-reducing, and high-temperature resistant composite coating. The thickness of the obtained polyimide wear-resistant, friction-reducing, and high-temperature resistant composite coating is 28 μm, of which the thickness of the polyimide underlayer is 5 μm and the thickness of the composite functional layer is 23 μm.

[0073] Example 2

[0074] This embodiment relates to a wear-resistant, friction-reducing, and high-temperature resistant composite coating for a stainless steel substrate, including the following steps:

[0075] The first step is to weigh 4 g of SiO2 powder and add it to a mixed solvent consisting of 95 mL of anhydrous ethanol and 5 mL of deionized water, and then ultrasonically disperse it for 20 min to form a uniform suspension.

[0076] Then, a small amount of glacial acetic acid was added to adjust the pH of the system to 4-5, and then 0.25 g of KH550 silane coupling agent was added. After hydrolysis at room temperature for 20 min, the reaction was stirred for 2 h to allow KH550 to undergo a condensation reaction with the hydroxyl groups on the surface of SiO2 after hydrolysis.

[0077] After the reaction was completed, the product was separated by centrifugation and washed 2-3 times with anhydrous ethanol. Finally, it was dried in an oven at 80℃ for 7 h to obtain modified SiO2 powder.

[0078] The second step is to mix the polyimide solution (20% solid content) and DMF solvent at a volume ratio of 1:1.2, and then stir in a stirring device for 30 minutes to obtain the polyimide bottom layer slurry.

[0079] The third step involves weighing 3 g of flake graphite (particle size ≤ 10 μm) and placing it in a dispersion container. 0.2 g of PVP is added as a dispersant, and 20 ml of DMF solvent is added as a diluent. Simultaneously, 0.2 g of the modified SiO2 powder obtained in step 1 is added to 20 ml of DMF solvent. After ultrasonic dispersion for 20 min, the mixture is stirred together mechanically for 20 min. Then, 36.0 g of polyimide solution (20% solid content) is added, and the mixture is mechanically stirred at 600 r / min for 4 h. Ultrasonic dispersion is then performed again for 20 min to obtain the graphite / SiO2 / polyimide composite coating slurry.

[0080] The fourth step is to sand the stainless steel substrate 2 with sandpaper, then ultrasonically clean it in anhydrous ethanol for 20 minutes each to remove surface oil and impurities, and then dry it in an oven at 70°C for later use.

[0081] Step 5: Fix the pretreated stainless steel substrate 2 onto the spraying workbench 3. Using a small pneumatic spray gun 1, evenly spray the polyimide undercoat slurry prepared in step 2 onto the substrate surface, following a "Z" shaped spraying path; control the spraying distance to 14 cm. Figure 1 As shown;

[0082] Step 6: Place the stainless steel substrate with the base coat applied in a 70°C oven for 10 minutes to pre-bake, in order to remove some of the solvent and allow the base coat to stabilize initially.

[0083] Step 7: Using a small pneumatic spray gun 1, spray the graphite / SiO2 / polyimide composite slurry prepared in step 2 onto the substrate surface that has been coated with the underlayer; the spraying distance is about 14 cm. After each spraying, place it in a 70℃ oven for pre-baking for 10 min; repeat the above "spraying-pre-baking" operation three times in total, and deposit the composite coating layer by layer in a multi-thin spraying manner.

[0084] Step 8: Place the pre-baked sample in a high-temperature oven and perform step-by-step curing as follows: hold at 80℃ for 30 min, raise the temperature to 120℃ and hold for 1 h, raise the temperature to 220℃ and hold for 1 h, and then raise the temperature to 300℃ and hold for 1 h; after curing, cool to room temperature with the oven to obtain a stainless steel sample with a wear-resistant, friction-reducing, and high-temperature resistant composite coating. The thickness of the obtained polyimide wear-resistant, friction-reducing, and high-temperature resistant composite coating is 30 μm, of which the thickness of the polyimide underlayer is 5 μm and the thickness of the composite functional layer is 25 μm.

[0085] Example 3

[0086] This embodiment relates to a wear-resistant, friction-reducing, and high-temperature resistant composite coating for a stainless steel substrate, comprising the following steps: First, 4 g of SiO2 powder is weighed and added to a mixed solvent consisting of 95 mL of anhydrous ethanol and 5 mL of deionized water, and ultrasonically dispersed for 20 min to form a uniform suspension; then, a small amount of glacial acetic acid is added to adjust the pH of the system to 4-5, and then 0.25 g of KH550 silane coupling agent is added. After hydrolysis at room temperature for 20 min, the reaction is continued with stirring for 2 h, so that the hydrolyzed KH550 undergoes a condensation reaction with the hydroxyl groups on the surface of SiO2; after the reaction is completed, the mixture is centrifuged and washed 2-3 times with anhydrous ethanol, and finally dried in an oven at 80℃ for 7 h to obtain modified SiO2 powder.

[0087] The second step involves mixing the polyimide solution (20% solid content) with DMF solvent at a volume ratio of 1:1.3, stirring in a stirring device for 30 minutes to obtain the polyimide bottom slurry.

[0088] The third step involves weighing 2 g of flake graphite (particle size ≤ 10 μm) and placing it in a dispersion container. 0.2 g of PVP is added as a dispersant, and 20 ml of DMF solvent is added as a diluent. Simultaneously, 0.1 g of the modified SiO2 powder obtained in step 1 is added to 20 ml of DMF solvent. After ultrasonic dispersion for 20 min, the mixture is combined and mechanically stirred for 20 min. Then, 35.0 g of polyimide solution (20% solid content) is added, and the mixture is mechanically stirred at 600 r / min for 4 h. Ultrasonic dispersion is then performed again for 20 min to obtain the graphite / SiO2 / polyimide composite coating slurry.

[0089] The fourth step is to sand the stainless steel substrate 2 with sandpaper, then ultrasonically clean it in anhydrous ethanol for 20 minutes each to remove surface oil and impurities, and then dry it in an oven at 70°C for later use.

[0090] Step 5: Fix the pretreated stainless steel substrate 2 onto the spraying workbench 3. Using a small pneumatic spray gun 1, evenly spray the polyimide undercoat slurry prepared in step 2 onto the substrate surface, following a "Z" shaped spraying path; control the spraying distance to 15 cm. Figure 1 As shown;

[0091] Step 6: Place the stainless steel substrate with the base coat applied in a 70°C oven for 10 minutes to pre-bake, in order to remove some of the solvent and allow the base coat to stabilize initially.

[0092] Step 7: Using a small pneumatic spray gun 1, spray the graphite / SiO2 / polyimide composite slurry prepared in step 2 onto the substrate surface that has been coated with the underlayer; the spraying distance is about 15 cm. After each spraying, place it in a 70℃ oven for pre-baking for 10 min; repeat the above "spraying-pre-baking" operation three times in total, and deposit the composite coating layer by layer in a multi-thin spraying manner.

[0093] Step 8: Place the pre-baked sample in a high-temperature oven and perform step-by-step curing as follows: hold at 80℃ for 30 min, raise the temperature to 120℃ and hold for 1 h, raise the temperature to 220℃ and hold for 1 h, and then raise the temperature to 300℃ and hold for 1 h; after curing, cool to room temperature with the oven to obtain a stainless steel sample with a wear-resistant, friction-reducing, and high-temperature resistant composite coating. The thickness of the obtained polyimide wear-resistant, friction-reducing, and high-temperature resistant composite coating is 25 μm, of which the thickness of the polyimide underlayer is 3 μm and the thickness of the composite functional layer is 22 μm.

[0094] Experimental results show that the coefficient of friction of this composite coating can be as low as 0.13 at room temperature and can still be kept below 0.12 at a high temperature of 300℃. At the same time, the wear rate can be reduced by 1 to 2 orders of magnitude, and the coating adhesion reaches the level of 0 to 1 in the cross-cut test, which is significantly better than pure polyimide coating.

[0095] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A wear and friction reducing high temperature resistant composite coating for stainless steel surfaces, characterized by, include: Graphite powder, SiO2 powder, polyimide solution with 20% solid content, diluent and dispersant; The wear-resistant, friction-reducing, and high-temperature resistant composite coating includes: a polyimide underlayer disposed on the surface of a stainless steel substrate and a composite functional layer disposed on top of it; The composite functional layer includes: graphite powder, SiO2 powder, cured polyimide, diluent, and dispersant; The diluent is N,N-dimethylformamide; The dispersant is polyvinylpyrrolidone; The SiO2 powder is SiO2 powder that has undergone surface modification treatment with a silane coupling agent; The cured polyimide in the composite functional layer accounts for 60 wt% to 80 wt% of the total mass of the composite functional layer components; Graphite powder constitutes 15 wt% to 30 wt% of the total filler mass; The SiO2 powder constitutes 5 wt% to 10 wt% of the total filler mass. The dispersant accounts for 5 wt% to 8 wt% of the total mass of the filler. A polyimide solution with a solid content of 20% is mixed with N,N-dimethylformamide at a volume ratio of 1:1 to 1:1.5 to obtain a slurry for the polyimide underlayer.

2. The wear and friction reducing high temperature resistant composite coating for stainless steel surfaces as claimed in claim 1, wherein, The graphite powder is flake graphite with a particle size ≤10 μm.

3. The wear and friction reducing high temperature resistant composite coating for stainless steel surfaces as claimed in claim 1 wherein, The SiO2 powder is nano-sized SiO2.

4. A method for preparing a wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 1, characterized in that, Includes the following steps: Step 1, Modification of SiO2 SiO2 powder was added to a mixed solvent of ethanol and deionized water in a volume ratio of 90:10 and ultrasonically dispersed for 10–20 min; glacial acetic acid was added to adjust the pH to 4–5; silane coupling agent was added, with the amount added being 3%–8% of the mass of SiO2; the reaction was stirred at room temperature or 40–50 °C for 2–4 h; after the reaction was completed, the modified SiO2 powder was obtained by separation, washing and drying. Step 2, Preparation of the bottom slurry The polyimide precursor was mixed with N,N-dimethylformamide to obtain a polyimide underlayer slurry; Step 3, Preparation of composite functional layer slurry First, graphite powder and modified SiO2 powder are added to N,N-dimethylformamide, followed by polyvinylpyrrolidone. The mixture is then pre-dispersed using mechanical stirring / ultrasonic dispersion. Subsequently, a polyimide solution with a solid content of 20% is added, and the mixture is stirred and mixed evenly to obtain a graphite / SiO2 / polyimide composite functional layer slurry.

5. The application of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 1, characterized in that, Application on stainless steel surfaces via spraying.

6. The application of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 5, characterized in that, Includes the following steps: Step A, Matrix Pretreatment The stainless steel substrate is polished, cleaned, and dried to remove surface contaminants; Step B, base coat spraying Place the substrate (2) on the spraying workbench (3) and use a pneumatic spray gun (1) to spray the polyimide base layer slurry onto the surface of the substrate (2); Step C, Pre-baking After spraying the polyimide base coat, the fasteners are pre-baked at 70°C for 5 to 20 minutes to pre-cure the base coat. Step D: Multiple thin-spraying of composite functional layer slurry The graphite / SiO2 / polyimide composite functional layer slurry was sprayed onto the surface of the substrate (2) by spraying. During the spraying process, the spraying path was a "Z" shaped path. After spraying, the pre-baking treatment was carried out for 5 to 20 minutes. The spraying and pre-baking operation was repeated 2 to 3 times. Step E, heat treatment curing The fasteners are heat-treated and cured in a step-by-step manner to remove some of the solvent, so that the polyimide solution undergoes an imidization reaction, and finally a wear-resistant, friction-reducing, and high-temperature resistant composite coating is formed on the surface of the substrate (2).

7. The application of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 6, characterized in that, During the spraying process, the spraying distance between the pneumatic spray gun (1) and the substrate (2) is 13 cm to 17 cm.

8. The application of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 6, characterized in that, During the spraying process, the pneumatic spray gun (1) sprays a "Z" shaped path on the substrate surface (2).

9. The wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 1, characterized in that, The thickness of the wear-resistant, friction-reducing, and high-temperature resistant composite coating is 20–30 μm; wherein the thickness of the polyimide underlayer is 3–6 μm, and the thickness of the composite functional layer is 14–27 μm.

10. The application of the wear-resistant, friction-reducing, and high-temperature resistant composite coating for stainless steel surfaces as described in claim 6, characterized in that, The stepped curing process specifically involves curing under the following conditions in sequence: curing at 80℃ for 30 min, curing at 120℃ for 1 h, curing at 220℃ for 1 h, and curing at 300℃ for 1 h.

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