Preparation method of composite coating and composite coating

Abstract

The preparation method comprises the following steps: S1, dissolving citric acid monohydrate and ethylenediamine in ultrapure water, and uniformly stirring to obtain a first mixed solution; s2, heating the first mixture from room temperature to 200 DEG C in a gradient manner, keeping the temperature for a first preset time, and cooling to obtain a second mixed solution; s3, separating the second mixed solution to obtain a separation solution, purifying the separation solution, and drying to obtain the nitrogen-doped carbon quantum dots with the molecular weight greater than a preset molecular weight threshold; s4, hexafluorodianhydride is added into an organic solvent to be dissolved, and a third mixed solution is obtained; s5, adding aminopropyl terminated polydimethylsiloxane into the third mixed solution to obtain a polyamide acid solution; s6, adding the nitrogen-doped carbon quantum dots into the polyamide acid solution, and uniformly dispersing to obtain a composite coating; and S7, a substrate is coated with the composite coating, heat treatment is conducted for 12 h at 60 DEG C, heat treatment is conducted for 1 h at 100 DEG C, heating is conducted for 1 h at 150 DEG C, curing is conducted for 2 h at 200 DEG C, and the composite coating is obtained.

Description

Technical Field

[0001] This application belongs to the technical field of polyimide materials, and particularly relates to a method for preparing a composite coating and the composite coating itself. Background Technology

[0002] Polyimide (PI) resin is a class of electrically insulating polymer materials whose main chain contains imide bonds (-CO-NR-CO-). It possesses excellent oxygen resistance, high temperature resistance, low dielectric constant, and outstanding mechanical strength, making it widely used in the electronics industry. There are two main methods for preparing polyimide. One involves mixing dianhydrides and diamines, where the dianhydrides undergo ring-opening to form a polyimide acid solution with the diamine. The polyimide acid is then dehydrated through chemical / physical means to obtain the polyimide resin. The other method involves polymerization of monomers containing imide rings. The former, using a two-step process, allows for better control over the molecular structure and is therefore the most widely used method for preparing polyimide. The properties of polyimide are determined by the structural units in the main chain. Rigid structures (such as aromatic rings) impart excellent thermal stability (thermal decomposition temperature typically above 500℃), chemical inertness, mechanical strength, and barrier properties. Flexible structures impart good toughness and tensile properties to PI. In many fields such as aerospace, energy and chemical engineering, and electronics, metallic materials need to withstand long-term high temperatures above 200°C and corrosion from acid / alkali media, oxidizing atmospheres, or molten salts. Traditional metals or polymers are prone to performance deterioration due to thermal oxidation, chemical degradation, or structural failure. Polyimide's stable chemical structure and excellent film-forming properties can effectively block corrosive media and maintain mechanical stability at high temperatures, thus attracting much attention in the field of high-temperature corrosion protection.

[0003] Currently, polyimide has been successfully applied to thermal protection coatings for aero-engines, sealing materials for nuclear reactors, and high-temperature electronic packaging, but it still cannot meet the equipment's requirements for coating strength, corrosion resistance, high-temperature impedance, and recovery after damage. Summary of the Invention

[0004] In a first aspect, this application provides a method for preparing a composite coating, comprising: S1. Dissolve citric acid monohydrate and ethylenediamine in ultrapure water and stir until homogeneous to obtain the first mixture; S2. The first mixture is heated from room temperature to 200°C and held for a first preset time, then cooled to obtain the second mixture. S3. Separate the second mixture to obtain a separation liquid, purify the separation liquid, and dry it to obtain nitrogen-doped carbon quantum dots with a molecular weight greater than the preset molecular weight threshold. S4. Dissolve hexafluorodianhydride in an organic solvent to obtain the third mixture; S5. Add aminopropyl-terminated polydimethylsiloxane to the third mixture to obtain a polyamic acid solution; S6. Nitrogen-doped carbon quantum dots are added to a polyamic acid solution and dispersed uniformly to obtain a composite coating. S7. Apply the composite coating onto the substrate, heat-treat at 60°C for 12 hours, heat-treat at 100°C for 1 hour, heat at 150°C for 1 hour, and cure at 200°C for 2 hours to obtain the composite coating.

[0005] Furthermore, in steps S4 and S5, the molar ratio of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane is 1:1.

[0006] Furthermore, in step S1, the molar ratio of citric acid monohydrate to ethylenediamine is 1:1.03.

[0007] Furthermore, the molar ratio of nitrogen-doped carbon quantum dots to the total amount of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane is 0.5%-4%.

[0008] Furthermore, in step S2, the rate of gradient heating of the first mixture is 5℃ / min; The initial preset duration is 5 hours.

[0009] Furthermore, the separation process in step S3 is as follows: solid impurities in the second mixture are filtered out using a nylon microporous membrane, and the filtrate is collected; Dynamic dialysis was performed for 72 hours in the dialysis solution using a dialysis bag with a preset molecular weight threshold, and the dialysis solution was changed every 8 hours.

[0010] Vacuum drying is used, and the drying temperature is 50-60℃.

[0011] Furthermore, the preset molecular weight threshold is 1000 Da.

[0012] Secondly, this application proposes a composite coating prepared by the method for preparing a composite coating according to any of the above technical solutions.

[0013] Furthermore, the thickness of the composite coating is 130μm-150μm.

[0014] The above-described technical solution of the present invention has at least the following beneficial technical effects: (1) Introducing N-CQDs into PI resin: The surface of N-CQDs contains various oxygen and nitrogen functional groups, which can form covalent / non-covalent crosslinks with PI, greatly alleviating the problem of low coating strength caused by the structure of PI resin. At the same time, after being baked at 200℃ / 150℃ for 7 days, the N-CQDs@PI composite coating still has a resistance of 10%. 11 Ω·cm 2The above demonstrates excellent anti-corrosion properties.

[0015] (2) The flexible siloxane chain gives its molecular chain segments good mobility. The non-covalent bonds formed by the introduced N-CQDs in the resin also respond in the thermal environment. Therefore, the prepared N-CQDs@PI composite coating has high self-healing ability, and the self-healing efficiency of the first damage can reach more than 91.1%. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or in the conventional technology, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram illustrating the synthesis of polyimide in one embodiment of this application.

[0018] Figure 2 This is an infrared spectrum of polyimide (PI coating) in one embodiment of this application.

[0019] Figure 3 The thermogravimetric analysis (TGA) diagrams of the embodiments of this application are as follows: (a) PI coating, (b) N-CQDs@PI composite coating.

[0020] Figure 4 These are electrochemical impedance spectroscopy (EIS) diagrams of different coatings in the embodiments of this application at room temperature: (a) EIS diagram; (b) Bode plot; (c) impedance modulus histogram.

[0021] Figure 5 These are electrochemical impedance spectroscopy (EIS) diagrams of different coatings in the embodiments of this application after 16 hours at 150°C: (a) EIS diagram; (b) Bode plot; (c) Impedance modulus histogram.

[0022] Figure 6 These are electrochemical impedance spectroscopy (EIS) diagrams of different coatings in the embodiments of this application after 16 hours at 200°C: (a) EIS diagram; (b) Bode plot; (c) Impedance modulus histogram.

[0023] Figure 7 These are electrochemical impedance spectroscopy (EIS) diagrams of the 1% N-CQDs@PI composite coating at different aging times and temperatures in the embodiments of this application.

[0024] Figure 8 This is a diagram showing the results of neutral salt spray tests on different coatings in the embodiments of this application.

[0025] Figure 9The following are tensile test results of the 1%N-CQDs@PI composite coating in the embodiments of this application: (a) is the tensile curve after different self-healing cycles; (b) is the self-healing efficiency after different cycles; and (c) is a physical image of the self-healing process.

[0026] Figure 10 These are impact test and adhesion test diagrams of different coatings before and after heat treatment in the embodiments of this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the various embodiments of this application will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details are presented in the various embodiments of this application to facilitate a better understanding of the application. However, the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments. The division of the various embodiments below is for ease of description and should not constitute any limitation on the specific implementation of this application. The various embodiments can be combined with and referenced by each other without contradiction.

[0028] One aspect of this application provides a method for preparing a composite coating, wherein steps S1-S3 are the process of preparing nitrogen-doped carbon quantum dots (N-CQDs); steps S4-S5 are the process of preparing a polyamic acid solution (polyimide precursor); and steps S4-S5 are the process of preparing an N-CQDs@PI composite coating using nitrogen-doped carbon quantum dots and polyamic acid solution; specifically: S1. Citric acid monohydrate (C6H8O7·H2O) and ethylenediamine (C2H8N2) are dissolved in ultrapure water and stirred until homogeneous to obtain the first mixture. Both citric acid monohydrate and ethylenediamine have high solubility in ultrapure water. Citric acid monohydrate is a carbon source precursor for nitrogen-doped carbon quantum dots, while ethylenediamine provides the nitrogen source.

[0029] The molar ratio of citric acid monohydrate to ethylenediamine is 1:1.03; the slight excess of ethylenediamine ensures sufficient nitrogen doping and helps to form abundant functional groups such as amino groups on the carbon core surface. These functional groups provide the possibility for subsequent interfacial interactions (such as hydrogen bonding) with polyamic acid molecules.

[0030] S2. The first mixture is gradually heated from room temperature to 200°C at a rate of 5°C / min, and held at 200°C for 5 hours, then allowed to cool naturally to obtain the second mixture. Step S2 involves hydrothermal carbonization and nitrogen doping. Gradient heating facilitates the stable and uniform execution of complex chemical reactions such as carbonization, condensation, and nitrogen doping in the reaction system, avoiding uneven product development or excessive byproducts due to excessively rapid heating. Maintaining the temperature at 200°C for 5 hours ensures sufficient crystallization and growth of the nitrogen-doped carbon quantum dots, and the acquisition of the desired surface states.

[0031] S3. Separate the second mixture to obtain a separated liquid, purify the separated liquid, and dry it to obtain nitrogen-doped carbon quantum dots with a molecular weight greater than a preset molecular weight threshold; preferably, the preset molecular weight threshold is 1000 Da.

[0032] The separation process in step S3 is as follows: Solid impurities in the second mixture are filtered out using a nylon microporous membrane, and the filtrate is collected. Dynamic dialysis is then performed in ultrapure water for 72 hours using a dialysis bag with a preset molecular weight cutoff, with the ultrapure water replaced every 8 hours. Filtration through the nylon microporous membrane removes unreacted large solid particles. Long-term dynamic dialysis using a dialysis bag with a 1000 Da molecular weight cutoff effectively removes residual small molecule salts, unreacted citric acid, ethylenediamine and its oligomers, and other byproducts and residual monomers, obtaining a high-purity aqueous solution of nitrogen-doped carbon quantum dots with a relatively concentrated size and molecular weight distribution. Vacuum drying at 50-60℃ removes moisture, yielding nitrogen-doped carbon quantum dots (N-CQDs) in solid powder form, which facilitates subsequent dispersion in a polyamic acid solution containing organic solvents. The molecular weight threshold is controlled to obtain carbon quantum dots with specific structural sizes and surface functional group densities.

[0033] S4. Add hexafluorodianhydride (6FDA) to N-methyl-2-pyrrolidone (NMP) and stir until completely dissolved to obtain the third mixture. Hexafluorodianhydride has a phenyl ring-containing dianhydride structure, which provides stability and mechanical properties to the material.

[0034] S5. The aminopropyl-terminated polydimethylsiloxane (AP-PDMS) is slowly added to the third mixture, and the mixture is continuously stirred until homogeneous. The mixture is then stirred at room temperature for another 24 hours to obtain a homogeneous polyamic acid solution. The aminopropyl-terminated polydimethylsiloxane is a flexible diamine monomer. Its terminal amino group (-NH2) undergoes a ring-opening addition polymerization reaction with the anhydride group of hexafluorodianhydride to generate organosilicon-modified polyamic acid chains. The diamine is selected with a flexible molecular chain structure that is resistant to high temperatures as the soft segment, and a structure containing a benzene ring dianhydride as the hard segment, providing both stability and mechanical properties to the material.

[0035] The molar ratio of hexafluorodianhydride to aminopropyl-terminated polydimethylsiloxane (AP-PDMS) is 1:1. An equimolar ratio or a slight excess of AP-PDMS is used to control the molecular weight, ensuring that the polymer chain ends with amino groups. This facilitates subsequent reactions with any residual carboxyl groups on the carbon quantum dot surface or the formation of stronger interactions. The introduction of AP-PDMS embeds flexible siloxane segments into the polymer backbone, significantly improving the flexibility, impact resistance, hydrophobicity, and thermal stability of the final composite coating.

[0036] S6. Nitrogen-doped carbon quantum dots are added to a polyamic acid solution and uniformly dispersed to obtain a composite coating; wherein the molar ratio of nitrogen-doped carbon quantum dots to the total amount of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane is 0.5%-4%. The surface of nitrogen-doped carbon quantum dots contains a variety of oxygen and nitrogen functional groups, which can form covalent / non-covalent crosslinks with PI, greatly alleviating the problem of low coating strength caused by the structure of PI resin, while giving it good corrosion resistance and multifunctionality.

[0037] S7. Apply the composite coating onto the substrate, which may be a silicon wafer, glass, metal, or flexible polymer film, etc.

[0038] Then, heat treatment at 60℃ for 12 h gently removes most of the organic solvent NMP. Heat treatment at 100℃ for 1 h further removes residual solvent and initiates preliminary molecular chain rearrangement and a low degree of chemical imidization. Heating at 150℃ for 1 h promotes the chemical imidization reaction of polyamic acid to polyimide, accelerating dehydration and ring closure, and increasing molecular chain rigidity. Curing at 200℃ for 2 h yields a composite coating, allowing the polymer molecular chains to be fully oriented and stacked, forming a dense polyimide network structure. The polyimide resin is a thermoplastic resin and incorporates flexible siloxane chains, giving its molecular chain segments good mobility. The non-covalent bonds formed by the introduced N-CQDs in the PI resin also respond to thermal conditions, thus the prepared N-CQDs@PI composite coating exhibits high self-healing ability.

[0039] Another aspect of this application provides a composite coating prepared by the method for preparing a composite coating according to any of the above-mentioned technical solutions. Preferably, the thickness of the composite coating is 130μm-150μm.

[0040] The following specific embodiments are further illustrations of this application. The examples given do not represent all the implementation methods of this application; only some are used as examples for illustration. Specific embodiments are as follows: Example 1 Preparation of 0.5% N-CQDs@PI composite coating: (1) Synthesis of N-CQDs: 15.72 g (0.075 mol) of citric acid monohydrate was weighed as the carbon source precursor, and 4.62 g (0.077 mol) of ethylenediamine was used as the nitrogen source. The solutions were dissolved in 150 mL of ultrapure water at a molar ratio of 1:1.03. After 20 min of thorough mixing with magnetic stirring at 800 rpm, the solution became a homogeneous and transparent state, which is the first mixture.

[0041] The prepared reactant solution was transferred to a 200 mL polytetrafluoroethylene-lined container, and then the high-pressure reactor was assembled. The temperature was then increased in a stepped manner in a temperature-controlled chamber furnace: the temperature was increased to 200°C at a rate of 5°C / min, and maintained at 200°C for 5 hours. After the reaction was completed, the system was allowed to cool naturally, yielding a brownish-red colloidal solution, which is the second mixture.

[0042] The second mixture was subjected to primary filtration through a 0.22 μm nylon microporous membrane to remove unreacted solid impurities. Dynamic dialysis was then performed in ultrapure water using a dialysis bag with a molecular weight cutoff of 1000 Da for 72 h, with the ultrapure water being replaced every 8 h to obtain a purified separation solution.

[0043] The separated liquid was vacuum dried to obtain nitrogen-doped carbon quantum dot powder.

[0044] (2) Synthesis of polyamic acid solution: Weigh 1.33 g (3 mmol) of hexafluorodianhydride and add it to 10 ml of N-methyl-2-pyrrolidone (NMP). Stir for 15 min until completely dissolved to obtain the third mixture.

[0045] Weigh 3 mmol of aminopropyl-terminated polydimethylsiloxane using a precision balance, slowly add it to the third mixture, and continue stirring until homogeneous. Then, continue stirring at room temperature for 24 h to form a homogeneous polyamic acid solution.

[0046] (3) Synthesis of 0.5% N-CQDs@PI composite coating: Weigh 0.5% of N-CQDs powder equivalent to the total of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane, add it to the prepared polyamic acid solution, disperse it rapidly by ultrasound, and then continue to stir at room temperature until uniformly dispersed. Remove bubbles by vacuuming to obtain composite coating.

[0047] The composite coating was applied to the steel plate and cured using a stepped heating method. The coating was heat-treated at 60℃ for 12 hours, at 100℃ for 1 hour, at 150℃ for 1 hour, and finally cured at 200℃ for 2 hours to obtain a 0.5% N-CQDs@PI composite coating.

[0048] Example 2: Preparation of 1% N-CQDs@PI composite coating: (1) Synthesis of N-CQDs: Same as in Example 1.

[0049] (2) Synthesis of polyamic acid solution: Same as in Example 1.

[0050] (3) Synthesis of 1% N-CQDs@PI composite coating: Weigh 1% of N-CQDs powder equivalent to the total of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane. The subsequent operations are the same as in Example 1, and finally a 1% N-CQDs@PI composite coating is obtained.

[0051] Example 3 Preparation of 2% N-CQDs@PI composite coating (1) Synthesis of N-CQDs: Same as in Example 1.

[0052] (2) Synthesis of polyamic acid solution: Same as in Example 1.

[0053] (3) Synthesis of 2% N-CQDs@PI composite coating: Weigh 2% of N-CQDs powder equivalent to the total amount of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane. The subsequent operations are the same as in Example 1, and finally a 2% N-CQDs@PI composite coating is obtained.

[0054] Example 4 Preparation of 4% N-CQDs@PI composite coating: (1) Synthesis of N-CQDs: Same as in Example 1.

[0055] (2) Synthesis of polyamic acid solution: Same as in Example 1.

[0056] (3) Synthesis of 4% N-CQDs@PI composite coating: Weigh 4% of N-CQDs powder equivalent to the total of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane. The subsequent operations are the same as in Example 1, and finally a 4% N-CQDs@PI coating is obtained.

[0057] Example 5 (1) Synthesis of polyamic acid solution: 1.33 g (3 mmol) of hexafluorodianhydride was weighed and added to 10 ml of N-methyl-2-pyrrolidone (NMP) and stirred for 15 min until completely dissolved to obtain the third mixture. 3 mmol of aminopropyl-terminated polydimethylsiloxane was weighed using a precision balance and slowly added to the third mixture. The mixture was stirred continuously until homogeneous. Then, the mixture was stirred at room temperature for 24 h to form a homogeneous polyamic acid solution.

[0058] (2) Synthesis of PI coating: Polyamic acid solution was coated onto a steel plate, and the cyclic polyamic acid was fused together by step-by-step heating. First, it was heat-treated at 60°C for 12 h, then heated to 100°C and held for 1 h, then heated at 150°C for 1 h, and finally cured at 200°C for 2 h to obtain a PI coating with a thickness of 150 μm.

[0059] This application conducted performance tests on the coatings prepared in Embodiments 1-5, and the test results and performance comparisons are as follows: Figure 3 The following are thermogravimetric analysis (TGA) diagrams from the embodiments of this application: (a) is the PI coating of Example 5, and (b) is the 1% N-CQDs@PI composite coating of Example 2.

[0060] from Figure 3 As can be seen, the 1% N-CQDs@PI composite coating of Example 2 exhibits excellent thermal stability, with an initial decomposition temperature as high as 511.42℃, which is higher than the 507.72℃ of the PI coating of Example 5.

[0061] Figure 4 The following are electrochemical impedance spectroscopy (EIS) diagrams of different coatings at room temperature in the embodiments of this application: (a) EIS diagram; where the horizontal axis represents the real part of the impedance value and the vertical axis represents the imaginary part of the impedance value. The larger the curve radius, the higher the charge transfer resistance and the better the corrosion resistance. (b) Bode plot is a graphical representation of the frequency response of the coating; the horizontal axis is the frequency, tested from the high frequency region to the low frequency region, the left vertical axis is the impedance value, the higher the impedance value, the better the performance and the better the corrosion resistance, and the right vertical axis is the phase angle. The closer to 90°, the denser the protective layer and the stronger the capacitance. (c) Impedance modulus histogram.

[0062] from Figure 4 It can be seen that the pure polyimide (PI) resin coating has a high impedance modulus, with an impedance value as high as 1.045 × 10⁻⁶. 10 The impedance value is ohm・cm², indicating that the prepared resin has excellent protective properties. However, compared with the pure PI resin coating, the resin coating with added N-CQDs showed a higher impedance value, with the coating containing 1% N-CQDs reaching an impedance value of 1.232 × 10⁻⁶ ohms・cm². 12 The improvement in corrosion resistance is approximately one order of magnitude compared to a pure PI coating, indicating that the introduction of N-CQDs enhances the coating's corrosion resistance. Analysis of the performance improvement of the N-CQDs@PI composite coating reveals that the abundant functional groups on the N-CQDs surface can form excellent interactions with some polar groups or chemical bonds in the PI resin, allowing the quantum dots to integrate well into the resin and form a dense protective layer. The N-CQDs themselves also possess corrosion inhibitor properties, further enhancing the coating's anti-corrosion performance.

[0063] Figure 5 These are electrochemical impedance spectroscopy (EIS) diagrams of different coatings in the embodiments of this application after 16 hours at 150°C: (a) EIS diagram; (b) Bode diagram; (c) impedance modulus histogram.

[0064] from Figure 5 (a) It can be seen that the 1% N-CQDs@PI composite coating (red) in Example 2 has the largest radius and the best corrosion resistance. The 0.5% N-CQDs@PI composite coating (black) is next. The 2% N-CQDs@PI composite coating (blue) and the 4% N-CQDs@PI composite coating (green) have extremely small radii and significantly deteriorated corrosion resistance.

[0065] from Figure 5 (b) It can be seen that the proportion of N-CQDs in the N-CQDs@PI composite coating is 1% (red) > 0.5% (black) > 2% (blue) > 4% (green); the phase angle peak is the highest at 1% and closest to 90°, while 4% has almost no capacitance.

[0066] from Figure 5 (c) It can be seen that the 1% N-CQDs@PI composite coating has the highest initial impedance value (approximately 1.2 × 10⁻⁶). 12 The second best is 0.5% N-CQDs@PI composite coating (approximately 8×10 ohm・cm²). 11 ohm・cm 2 ); 2% plummeted to 3.351×10 9 ohm・cm 2 The corrosion resistance further decreased by 4%.

[0067] When the heating time was extended to 16 hours, the impedance of the composite coating with a proportion of 0.5%-2% did not show significant changes and remained at 10. 11 Above ohm·cm². However, the 4% decrease is significant, reducing the impedance value to 10. 8 ohm・cm²; at the same time, two time constants also appeared in the Bode plot ( Figure 5 (b) indicates that the coating has entered the middle stage of corrosion, but still maintains a certain protective effect.

[0068] In summary, at 150℃, composite coatings with different N-CQDs contents all exhibit certain high-temperature corrosion resistance. However, as time progresses, the filler ratio begins to show its influence on coating performance. For coatings with high filler content, the change in impedance modulus is not significant initially, but the performance deteriorates significantly after 16 hours of heating. In contrast, composite coatings with low N-CQDs content show more stable performance, with the 1% N-CQDs@PI composite coating exhibiting the best performance.

[0069] Figure 6 These are electrochemical impedance spectroscopy (EIS) diagrams of different coatings in the embodiments of this application after 16 hours at 200°C: (a) EIS diagram; (b) Bode plot; (c) Impedance modulus histogram.

[0070] from Figure 6 (a) It can be seen that the 1% N-CQDs@PI composite coating (red) in Example 2 still has the largest radius and the best corrosion resistance. The 0.5% N-CQDs@PI composite coating (black) is second. The 2% N-CQDs@PI composite coating (blue) and the 4% N-CQDs@PI composite coating (green) have the largest radii compared to the group at 150℃-16h. Figure 5 (It has shrunk.)

[0071] from Figure 6 (b) It can be seen that the proportion of N-CQDs in the N-CQDs@PI composite coating is 1% (red) > 0.5% (black) > 2% (blue) > 4% (green). The Z value of 1% is slightly higher than that of the 150℃-16h group; the peak phase angle of 1% is still the highest, but slightly lower than that of the 150℃-16h group, and the capacitance decreases slightly.

[0072] from Figure 6 (c) It can be seen that the impedance value of the 1% N-CQDs@PI composite coating is slightly increased (≈1.4×10). 12 ohm・cm 2 The 0.5% N-CQDs@PI composite coating is similar to the 150℃ group (≈8×10). 11 ohm・cm 2 The 2% N-CQDs@PI composite coating drastically decreased to 1.193×10⁻⁶. 9 ohm・cm 2 (Below 150℃ group), the 4% N-CQDs@PI composite coating has a strength of 3.54 × 10⁻⁶. 9 ohm・cm 2 Higher addition levels further reduce corrosion resistance. The impedance characteristics of 2% N-CQDs@PI and 4% N-CQDs@PI show a significant decrease, with a clear dual-time-constant characteristic appearing in the Bode plot. Figure 6 (b) indicates that the coating has entered the mid-stage of corrosion; however, its impedance modulus remains at 10. 9 ohm・cm 2The coating still exhibits a certain degree of protection. In contrast, the 1% doped sample maintained a single time constant throughout the aging process, and its impedance modulus was consistently higher than that of other samples, indicating that it possesses the best long-term thermal stability. This performance difference due to content variations may be attributed to the aggregation of N-CQDs at high doping levels, leading to defects and performance degradation. Under prolonged high temperatures, the micro-defects caused by filler aggregation in the high-doped coating accelerated its degradation, while the moderately doped (1%) coating achieved optimized impedance performance due to the reconstruction of the dynamic non-covalent network.

[0073] Will Figure 4 , Figure 5 and Figure 6 The comparison shows that the corrosion resistance of the pure PI coating without N-CQDs modification in Example 5 is far lower than that of the 1% N-CQDs@PI composite coating. The introduction of N-CQDs significantly improves the corrosion resistance of the PI matrix, and the corrosion resistance of the 1% N-CQDs@PI composite coating far exceeds that of the pure PI matrix. The modification effect of 0.5% N-CQDs addition is insufficient; the impedance modulus and charge transfer resistance are only about 2 / 3 of those of the 1% N-CQDs group, and the density of the protective layer is also slightly lower. High additions of 2% or 4% may cause excessive agglomeration of N-CQDs, destroying the density of the PI matrix and resulting in a sharp drop of 3 orders of magnitude in the impedance modulus. The 1% N-CQDs@PI composite material has a stable structure and the best N-CQDs dispersion. After heat treatment at 150℃ and 200℃, 1% N-CQDs@PI consistently maintains the highest impedance and optimal protection, with only slight fluctuations at 200℃.

[0074] Figure 7 This is the EIS diagram of the 1% N-CQDs@PI composite coating in Example 2 of this application under different aging times and temperatures. The 1% N-CQDs@PI composite coating with the best early-stage performance was selected for the experiment, which lasted 168 hours at temperatures of 150°C and 200°C. Figure 7 It can be seen that after being subjected to high temperatures, the impedance modulus of the sample at 150℃-168h can reach 7.57×10⁻⁶. 11 ohm・cm 2 The impedance modulus of the sample obtained at 200℃ for 168 hours was also 1.39 × 10⁻⁶. 11 ohm・cm 2 All of them have a certain degree of high-temperature protection effect for a certain period of time.

[0075] Figure 8 These are graphs showing the results of neutral salt spray tests on different coatings from Examples 1-5 of this application. The rectangular coating samples had intersecting scratches drawn along their two diagonals using a cross-cutting tool, and were tested in neutral salt spray for 0 days, 5 days, and 23 days.

[0076] from Figure 8 As can be seen, in the initial state, all sample surfaces had pre-treated scratches and uniform surface color: the PI coating was light yellowish-gray, the 0.5% N-CQDs@PI composite coating was light gold, the 1% N-CQDs@PI composite coating was dark gold, the 2% N-CQDs@PI composite coating was reddish-brown, and the 4% N-CQDs@PI composite coating was brownish-yellow.

[0077] Five days after corrosion, corrosion products began to adhere to the surfaces of all samples, resulting in a darker color and decreased uniformity: the PI coating and the 0.5% N-CQDs@PI composite coating turned grayish, lost their luster, and scratches were still visible but blurred. The 1% N-CQDs@PI composite coating was dark brown, and the 2% N-CQDs@PI composite coating was dark brown, with minimal change in scratches. The 4% N-CQDs@PI composite coating showed light-colored corrosion streaks, uneven overall color, and the scratches began to change. Overall, the 1% N-CQDs@PI, 2% N-CQDs@PI, and 4% N-CQDs@PI composite coatings showed color changes but still maintained a certain level of luster.

[0078] After 23 days of corrosion, the corrosion intensified significantly, with a large amount of reddish-brown corrosion products appearing on the surface of all samples. The PI coating and the 0.5% N-CQDs@PI composite coating showed the most severe corrosion, with the surface covered with reddish-brown rust and crack-like corrosion areas. The 1% N-CQDs@PI composite coating showed the least corrosion, with only slight rust, a relatively uniform surface, and even shallower scratches, indicating signs of recovery. The 2% N-CQDs@PI composite coating and the 4% N-CQDs@PI composite coating showed corrosion products accumulating in strips / patches. This set of experiments shows that N-CQDs has a significant impact on the corrosion resistance of the coating. When the addition amount is 1%, the surface condition of the coating is the best after 23 days of corrosion, with the fewest corrosion products and the most uniform surface; when the addition amount is too low (0, 0.5%) or too high (2%, 4%), the corrosion resistance decreases significantly, and severe rust and coating peeling are more likely to occur. When the addition amount was 2% and 4%, corrosion started earlier at the scratches, and the corrosion spread was significantly greater than that of the 1% N-CQDs@PI composite coating on day 23.

[0079] Figure 9 The following are tensile test results of the 1% N-CQDs@PI composite coating in this application: (a) is the tensile curve after different self-healing cycles; (b) is the self-healing efficiency after different cycles; (c) is a photograph of the self-healing process. The sample is a 130 μm dumbbell-shaped film prepared by a mold.

[0080] Figure 9 In (a), the horizontal axis represents strain, and the vertical axis represents stress. From Figure 9(a) It can be seen that the original stress and strain are the highest; after the 1st / 4th / 8th thealing, the mechanical properties gradually decrease, but after the 8th repair, significant stress and strain are still retained, indicating that the coating can be repaired multiple times and still has load-bearing capacity.

[0081] Figure 9 In (b), the horizontal axis represents the number of repairs, and the vertical axis represents the self-healing efficiency (the ratio of the performance after repair to the original performance). The self-healing efficiency is approximately 100% after the first repair, slightly decreases after the fourth repair, and remains at approximately 70% after the eighth repair, indicating that the coating still has high repair efficiency after multiple repairs.

[0082] Figure 9 In (c), the sample was initially completely broken. The fractured parts were brought into contact, and after heating at 150°C for 10 minutes, the fractured parts healed into a complete sample. The healed sample was then stretched, and it remained intact, visually verifying the self-healing effect.

[0083] Figure 10 These are impact and adhesion test images of different coatings before and after heat treatment in this application embodiment. The impact test uses a drop hammer impact method with a uniform impact height of 60cm. Immediately after impact, the impact area is observed with the naked eye and an optical microscope, focusing on checking for defects such as cracks, peeling, breakage, and dents. The adhesion test includes simultaneous scratch and quantitative pull-out tests. Scratching is performed using a standard cross-cutting tool, creating a 1mm × 1mm grid pattern on the coating surface (scratching depth penetrates the coating to the substrate). The pressure and spacing of the scratches are uniform. For quantitative pull-out testing, a scratched sample is used. A standard pull-out test column is adhered to the coating surface using high-strength adhesive, ensuring a bubble-free and gap-free bond between the test column and the coating. The test is conducted after the adhesive has fully cured. A uniform stretching rate of 1mm / min is set, and the tester clamps the pull-out test column, stretching it uniformly along a direction perpendicular to the coating surface until the coating peels off from the steel plate. Record the maximum pull-out force at each test point, calculate the bond strength using the formula: Bond Strength = Maximum Pull-out Force / Bond Area of ​​Test Column, and take the average value as the final result.

[0084] (a) Impact test of PI coating; (b) Impact test of 1% N-CQDs@PI composite coating; (c) Impact test of 1% N-CQDs@PI composite coating after heating at 150°C for 16 hours; (d) Adhesion test of 1% N-CQDs@PI composite coating; (f) Adhesion test of 1% N-CQDs@PI composite coating after heating at 150°C for 16 hours.

[0085] from Figure 10It can be seen that both the pure PI coating and the composite coating with added N-CQDs remained intact and without cracks after being impacted from a height of 60cm, indicating that both the pure PI coating and the N-CQDs@PI composite coating have excellent impact resistance. Subsequently, after heating the composite coating at 150℃ for 16h, it was subjected to impact under the same conditions, and no cracks appeared in the composite coating, indicating that the coating performance was not significantly reduced after being subjected to high temperature. Figure 10 (d) and (f) Adhesion tests of the 1% N-CQDs@PI composite coating showed no significant coating peeling near the scratches, indicating excellent adhesion. The coating also exhibited excellent adhesion after being heated at 150℃ for 16 hours. A pull-out test quantified the adhesion, revealing a bond strength of 2.95 MPa between the 1% N-CQDs@PI composite coating and the substrate. After high-temperature aging, the bond strength reached 3.21 MPa, slightly higher than the high-temperature strength, likely due to a tighter bond between the coating and substrate at higher temperatures. In conclusion, the mechanical properties of the N-CQDs@PI composite coating before and after high-temperature aging showed no significant decline; some properties even showed slight improvement, indicating that the 1% N-CQDs@PI composite coating is well-suited for high-temperature environments.

[0086] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of this application and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of this application should be included within the protection scope of this application. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A method for preparing a composite coating, characterized in that, include: S1. Dissolve citric acid monohydrate and ethylenediamine in ultrapure water and stir until homogeneous to obtain the first mixture; S2. The first mixture is heated from room temperature to 200°C and held for a first preset time, then cooled to obtain the second mixture. S3. Separate the second mixture to obtain a separation liquid, purify the separation liquid, and dry it to obtain nitrogen-doped carbon quantum dots with a molecular weight greater than the preset molecular weight threshold. S4. Dissolve hexafluorodianhydride in an organic solvent to obtain the third mixture; S5. Add aminopropyl-terminated polydimethylsiloxane to the third mixture to obtain a polyamic acid solution; S6. Nitrogen-doped carbon quantum dots are added to a polyamic acid solution and dispersed uniformly to obtain a composite coating. S7. Apply the composite coating onto the substrate, heat-treat at 60°C for 12 hours, heat-treat at 100°C for 1 hour, heat at 150°C for 1 hour, and cure at 200°C for 2 hours to obtain the composite coating.

2. The method for preparing the composite coating according to claim 1, characterized in that, In steps S4 and S5, the molar ratio of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane is 1:

1.

3. The method for preparing the composite coating according to claim 2, characterized in that, In step S1, the molar ratio of citric acid monohydrate to ethylenediamine is 1:1.

03.

4. The method for preparing the composite coating according to claim 3, characterized in that, The molar ratio of nitrogen-doped carbon quantum dots to the total amount of hexafluorodianhydride and aminopropyl-terminated polydimethylsiloxane is 0.5%-4%.

5. The method for preparing the composite coating according to claim 1, characterized in that, In step S2, the rate of gradient heating of the first mixture is 5℃ / min; The initial preset duration is 5 hours.

6. The method for preparing the composite coating according to claim 1, characterized in that, The separation process in step S3 is as follows: solid impurities in the second mixture are filtered out using a nylon microporous membrane, and the filtrate is collected. Dynamic dialysis was performed for 72 hours in dialysis fluid using a dialysis bag with a preset molecular weight threshold, and the dialysis fluid was changed every 8 hours. Vacuum drying is used, and the drying temperature is 50-60℃.

7. The method for preparing the composite coating according to claim 1, characterized in that, The preset molecular weight threshold is 1000 Da.

8. A composite coating, characterized in that, The composite coating is prepared by the method described in any one of claims 1-7.

9. The composite coating according to claim 8, characterized in that, The thickness of the composite coating is 130μm-150μm.

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