Multifunctional protective coating for aerospace composite thin-walled structures and method of making the same

Abstract

The present application belongs to the technical field of aerospace fiber / polymer composite thin-walled structure surface protection and functional coating, and particularly relates to a multifunctional protective coating for aerospace composite thin-walled structure and a preparation method thereof. The coating comprises, from inside to outside, a polysilazane bonding layer for enhancing the interfacial adhesion with the composite material matrix, a polyimide-based damping buffer layer having the functions of heat buffering and vibration energy dissipation, and a ceramic protective layer with aluminum oxide and 8Y yttrium stabilized zirconia as main components, providing thermal protection and wear resistance and erosion resistance. The multifunctional protective coating is prepared by using a wet spraying and medium-temperature staged curing process, and the whole process does not require high-temperature sintering equipment, and is suitable for composite thin-walled structures. The present application solves the problem that traditional single polymer or single ceramic coating cannot simultaneously consider thermal protection and damping performance.

Description

Technical Field

[0001] This invention belongs to the field of surface protection and functional coating technology for thin-walled structures of aerospace fiber / polymer composites, specifically relating to a multifunctional protective coating for thin-walled structures of aerospace composites and its preparation method. Background Technology

[0002] Currently, aerospace thin-walled structures extensively utilize fiber / polymer materials (composed of carbon fiber, aramid fiber, polyimide fiber, silicon carbide fiber, etc., combined with epoxy resin, bismaleimide resin, polyetheretherketone resin, phenolic resin, polyimide resin, etc.). These composite thin-walled structures include aero-engine blades, casings, integral bladed disks, spacecraft solar panels, missile fairings, military transport aircraft panels, fighter jet internal weapons bay panels, and various aircraft skins. During service, these structures endure complex conditions such as aerodynamic heating, particle erosion, vibration, impact, and thermal cycling. On one hand, the structural matrix typically uses fiber-reinforced composite materials to reduce weight, but their inherent high-temperature resistance, wear resistance, and corrosion resistance are limited. On the other hand, the superposition of localized thermal and mechanical loads easily leads to damage and delamination on the structural surface, potentially causing structural failure in severe cases. Therefore, for aerospace composite thin-walled structures, surface coating technology, based on multi-functional protection requirements, has become an important technical approach to improve the service life and safety margin of these structures.

[0003] In recent years, surface coating technology has become an important means to improve the surface protection and mechanical properties of thin-walled fiber / polymer composite structures. By applying multifunctional coatings to the surface of thin-walled composite structures, multiple protective functions such as thermal protection, vibration damping and impact resistance, and corrosion resistance can be integrated without changing the structural dimensions. Compared with traditional coatings, advanced coating materials that integrate multiple functions can significantly improve the overall performance of components in complex working environments, such as simultaneously coping with harsh conditions such as high temperature, vibration, impact, and particle erosion. However, most commonly used coating materials currently suffer from the problem of single-function, making it difficult to meet multiple protection needs. For example, common thermal protection coatings mainly focus on heat insulation, lacking vibration damping and impact resistance capabilities. Although vibration damping coatings can effectively suppress vibration, their performance is greatly reduced in high-temperature environments. Such single-function coating materials cannot effectively cope with the multi-field coupling effects faced by aerospace structures. Therefore, there is an urgent need to develop new coatings with comprehensive protective performance to meet the pressing needs of modern aerospace equipment for high efficiency, multifunctionality, and long service life.

[0004] Patent CN110791200A (Application No.: 2019110715817, Application Date: November 5, 2019, Publication Date: February 14, 2020, Patent Title: A High-Temperature Resistant Coating for Polyimide Composite Material Protection and a Coating Preparation Method) discloses a high-temperature resistant protective coating system for the surface of polyimide composite materials, which can improve the heat protection capability of the composite material in medium and high temperature environments. However, this patent mainly targets the high-temperature resistance and ablation resistance requirements of composite materials, and the overall coating design still leans towards resin or resin-modified systems, resulting in insufficient wear resistance and erosion resistance. Furthermore, this patent focuses on thermal protection evaluation and lacks specific design and verification for the thermal-vibration coupling and thermal-vibration fatigue conditions common in thin-walled aerospace structures.

[0005] Patent CN114411080A (Application No.: 2021116377046, Application Date: November 29, 2021, Publication Date: April 29, 2022, Patent Title: A Thermal Protective Composite Coating and Its Manufacturing Method) discloses a thermal protective coating structure for a metal surface and its preparation method, improving the thermal insulation performance of the structure. However, the patent mainly addresses the issues of high-temperature oxidation and thermal cycling life, with limited improvement in surface wear resistance and resistance to particle erosion. Furthermore, the patent lacks discussion on the applicability of resin-based fiber-reinforced composites and does not conduct experimental tests on wear resistance, vibration reduction, impact resistance, or thermal shock or thermal-vibration fatigue performance.

[0006] Patent CN116180019A (Application No.: 2023100237139, Application Date: November 9, 2023, Publication Date: May 30, 2023, Patent Title: A High-Temperature Resistant Long-Life Thermal Resistance Coating and Its Preparation Method) proposes to construct a long-life thermal resistance coating on the surface of high-temperature alloys using methods such as EB-PVD, mainly emphasizing the stability of the coating at temperatures above 1000 degrees Celsius. However, the preparation process of this patent relies on vacuum high-temperature deposition equipment such as EB-PVD, which has high requirements for the process environment and is not conducive to the low-cost promotion of large-size thin-walled aerospace components. At the same time, this coating does not consider wear resistance, vibration reduction, and impact resistance, and is not suitable for low-temperature preparation on the surface of resin-based composite materials.

[0007] Patent CN106116702A (Application No.: 2016104789256, Application Date: June 27, 2016, Publication Date: November 16, 2026, Patent Title: A Method for Preparing a High-Temperature Anti-Oxidation Thermal Barrier Coating for Cf / SiC Composite Materials) proposes a method for preparing a high-temperature anti-oxidation thermal barrier coating for Cf / SiC composite materials. Its drawback is that the substrate is a ceramic-based or silicon carbide-based composite material, which is incompatible with the resin-based CFRP structure widely used in aerospace. Furthermore, the preparation process of this coating typically involves high-temperature sintering or CVD steps, making the process relatively complex and unsuitable for resin-based composites.

[0008] Patent CN119736572A (Application No.: 2024119741273, Application Date: December 30, 2024, Publication Date: April 1, 2025, Patent Title: A Thermal Protective Coating and Its Preparation Method and Application) improves the crack resistance and oxidation resistance of the coating by introducing a nanoparticle reinforcing phase. However, the preparation process of this coating involves multiple spraying steps and complex material ratios, and its performance improvement is mainly focused on oxidation resistance and crack resistance, without fully considering the heat insulation effect and vibration reduction performance at high temperatures.

[0009] Patent CN115044875A (Application No.: 2022105947784, Application Date: May 27, 2022; Publication Date: September 13, 2022, Patent Title: A Multilayer Gradient Composite Hydrogen Barrier Coating and Its Preparation Method) proposes a multilayer gradient oxide ceramic / metal dispersion layer composite coating for hydrogen barrier and high-temperature oxidation protection of metal components. This patent achieves the transition between ceramic and metal through a multilayer gradient design and interfacial metallurgical integration. However, the large number of coating layers and the complex material system involved make the deposition process highly demanding in terms of thickness, compositional gradient, and uniform interlayer bonding, resulting in significant challenges for large-scale preparation.

[0010] Patent CN120060771A (Application No.: 2025105271510, Application Date: April 25, 2025, Publication Date: May 30, 2025, Patent Title: A High-Temperature Resistant Lightweight Stealth Coating Material and Its Preparation Method) mign A novel high-temperature resistant lightweight stealth coating material and its preparation method are described. This material modulates infrared properties and improves density by introducing metal-coated carbides or specific fluxes into the coating system. However, the design focus of this system remains on stealth and thermal protection, with weak support for wear resistance, and it does not incorporate a structured energy-dissipating layer for vibration reduction and impact resistance. Furthermore, its preparation largely relies on high-temperature spraying or sintering processes, which presents compatibility issues with the upper temperature limits and molding processes of resin-based thin-walled composite materials, requiring additional process matching for practical applications.

[0011] Patent CN103058654A (Application No.: 201210571293X, Application Date: December 26, 2012, Publication Date: April 24, 2013, Patent Title: Gradient Nanocoating for Corrosion Protection Function of Thermal Barrier Coating and its Preparation Method) uses sol-gel and wet chemical routes to prepare functionally graded thermal barrier coatings with continuously varying YSZ and Al2O3 compositions. However, wet chemical gradient film formation is sensitive to precursor stability and the gel curing process, making it difficult to guarantee composition and thickness consistency in thick coatings and on complex curved surfaces. Furthermore, this coating primarily focuses on thermal barrier or corrosion protection, lacking reinforcement phases and structural designs for particle wear and impact damage during aerospace service, and it does not establish a reliability evaluation system oriented towards thermal-vibration coupling.

[0012] Patent CN103572192A (Application No.: 2013105610199, Application Date: November 13, 2013, Publication Date: February 12, 2014, Patent Title: A Method for Preparing a Ceramic Damping Coating on the Surface of a Thin-Shell Component of Rotating Machinery) prepared a ceramic coating with damping properties on the surface of a thin-walled component of rotating machinery using plasma spraying technology. Although this coating can effectively improve the vibration reduction capability of thin-shell structures, its coating design and process rely heavily on high-temperature spraying equipment and have poor adaptability to complex curved surfaces and large-sized components. Furthermore, the patent only focuses on the single vibration reduction function, lacking comprehensive consideration of thermal protection performance and impact resistance.

[0013] In 2024, Kim H, Praveen K, Lee MW, et al. Performance-tunable thermal barrier coating for carbon fiber-reinforced plastic composites via flamespraying[J]. Composites Part B: Engineering, 2024, 287: 111842.2024.09.17 proposed a double-layer thermal barrier coating scheme for carbon fiber composite panels. This scheme uses flame spraying to deposit a dense YSZ layer and a YSZ / PEEK sacrificial pore-forming layer on the surface of carbon fiber fabric. By controlling the porosity, the thermal conductivity of the coating is reduced, thereby improving the thermal protection capability of the structure. However, this method relies on flame spraying and layered pore-forming processes, requiring high precision in controlling the spraying window, interlayer bonding, and pore uniformity. This makes it sensitive to equipment and process parameters, and the overall preparation process is relatively complex, hindering large-scale fabrication.

[0014] In 2024, Chen J, Parsi PK, Marklund P, et al. Graphene-enhanced, wear-resistant, and thermal-conductive, anti- / de-icing gelcoat composite coating[J]. Advanced Composites and Hybrid Materials, 2024, 7(1): 9. 2024.01.08 By physically mixing functional fillers with a gelcoat system, an anti- / de-icing composite coating with both wear resistance and enhanced thermal conductivity was prepared, and the influence of the filler network structure on thermal conduction and wear behavior was systematically analyzed. However, the design focus of this system is to improve thermal conductivity to accelerate heat distribution and transfer, which is different from the thermal protection performance required for thermal insulation. At the same time, its coating structure is not built around the energy dissipation mechanism required for vibration reduction and impact resistance, and the comprehensive protective coverage of thin-walled composite materials under impact-vibration coupled loads is still insufficient.

[0015] In 2024, Wang Y, Ding L, Lin J, et al. Recent developments in polyurea research for enhanced impact penetration resistance and blast mitigation[J].Polymers, 2024, 16(3): 440.2024.02.05 systematically reviewed the research progress and application mechanism of polyurea in the field of explosion and impact protection, emphasizing the high strain rate energy dissipation capability brought about by its hard and soft segment microphase separation structure, and pointing out that sprayed polyurea has engineering advantages such as fast curing speed and easy formation of thick coatings. However, the design focus of this type of coating is mainly on impact / blast resistance performance, and there is a lack of coordinated design and systematic consideration of multi-functional indicators such as wear resistance and thermal protection. In addition, polyurea coatings still have the problem of relatively high material and construction and maintenance costs in practical engineering applications, which further limits their promotion and application in large-area, large-scale protection scenarios.

[0016] In summary, while existing coating technologies have made progress in thermal protection and vibration damping, most coatings still have limitations, such as complex preparation processes, poor applicability to specific substrates, and a lack of thermal-vibration coupling reliability assessment. Therefore, developing a coating system that is simple to process, suitable for fiber / polymer thin-walled structures, and can simultaneously provide effective thermal protection, vibration damping, and shock resistance has become an urgent need to improve the overall performance and reliability of aerospace equipment. Summary of the Invention

[0017] To address the problems of insufficient thermal protection, unstable interfacial bonding, and difficulty in simultaneously satisfying wear resistance and vibration reduction performance in existing aerospace composite thin-walled structures under thermal-vibration service conditions, this invention proposes a multifunctional protective coating for aerospace composite thin-walled structures and its preparation method. This method is simple in process and uses readily available raw materials. It can provide thermal protection, wear resistance, vibration reduction, and impact resistance in medium-high temperature and strong vibration environments. It is especially suitable for surface protection of various composite thin-walled shells, compartments, and skin structures in the aerospace field.

[0018] The technical solution of this invention is:

[0019] This invention provides a multifunctional protective coating for thin-walled composite structures in aerospace applications, the coating comprising, from the inside out:

[0020] Polysilazane bonding layer is used to enhance interfacial adhesion with the composite matrix;

[0021] Polyimide-based vibration damping and buffer layer, which combines heat buffering and vibration energy dissipation functions;

[0022] The ceramic protective layer, with alumina and 8Y yttrium stabilized zirconium oxide as the main components, provides thermal protection and wear and erosion resistance.

[0023] The multifunctional protective coating is prepared by wet spraying and medium-temperature staged curing process, which does not require high-temperature sintering equipment and is suitable for thin-walled composite material structures.

[0024] Furthermore, in the aforementioned multifunctional protective coating, the raw materials for preparing the polysilazane bonding layer include, by mass percentage: 15%-30% polysilazane precursor, 5%-15% nano-silica sol, 0.5%-3% fumed silica, 40%-65% xylene solvent, and 0.1%-0.5% defoamer. Optionally, a silane coupling agent may be pre-coated onto the surface of the composite matrix before spraying to activate the interface and enhance interfacial bonding.

[0025] The polysilazane precursor can be a SiN or SiON system organic-inorganic hybrid polymer. The amount of nano-silica sol added is 10% of the mass of the polysilazane precursor, and the amount of hydrophilic fumed silica added is 1.5%. This formulation ensures that the polysilazane bonding layer exhibits good leveling and film-forming continuity during spraying, and forms a dense and stable Si-based network structure during curing, thereby improving the cohesive strength of the bonding layer and the interfacial adhesion strength between it and the composite matrix.

[0026] Furthermore, the raw materials for preparing the aforementioned multifunctional protective coating and the polyimide-based vibration damping buffer layer include, by mass percentage: 45%-65% polyimide precursor solution, 5%-15% alumina powder, 0.6%-5% 8Y yttrium stabilized zirconium oxide powder, 3%-10% nano silica sol, 0.5%-2% hydrophilic fumed silica, 15%-35% N-methylpyrrolidone solvent, 0.2%-1.0% dispersant, and 0.1%-0.5% defoamer.

[0027] The mass ratio of alumina powder to 8Y yttrium stabilized zirconia powder can be adjusted between 3:1 and 8:1 to balance thermal conductivity, intralayer stiffness, and damping performance. The optional carbon nanotubes or graphene nanosheets have a mass fraction of 0.5%-0.8% to improve intralayer energy dissipation and crack resistance.

[0028] Furthermore, in the aforementioned multifunctional protective coating, the raw materials for preparing the ceramic protective layer include, by mass percentage, 20%-40% alumina powder, 10%-15% 8Y yttrium stabilized zirconia powder, 5%-15% nano-silica sol, 5%-20% binder resin, 30%-40% anhydrous ethanol and N-methylpyrrolidone mixed solvent, 0.5%-2% hydrophilic fumed silica, 0.2%-1.0% dispersant, and 0.1%-0.5% defoamer; wherein the mass ratio of alumina powder to 8Y yttrium stabilized zirconia powder is 3:1 to 8:1, and the binder resin is polyvinyl butyral or polyimide precursor.

[0029] The alumina powder is preferably α-phase fine particles. The binder resin is partially or completely pyrolyzed during the curing process to form a wear-resistant skeleton mainly composed of ceramic phase. Under the premise of satisfying the integrity and adhesion of the coating, the mass percentage of the binder resin is preferably 8%-12% to improve the upper limit of temperature resistance.

[0030] Furthermore, the aforementioned multifunctional protective coating also includes a polyimide-based sealing layer on the outer surface of the ceramic protective layer. This layer has a thickness of 20μm-40μm and is prepared from a low-solids-content polyimide precursor solution, fine-grained alumina powder, 8Y yttrium-stabilized zirconium oxide powder, and nano-silica. This layer is used to improve surface smoothness, color uniformity, high-temperature stability, and additional wear resistance.

[0031] The present invention also provides a method for preparing the above-mentioned multifunctional protective coating, specifically including the following steps:

[0032] Step 1: Pre-treat the surface of the thin-walled composite material structure, including sanding to adjust the surface roughness, removing the resin-rich layer, wiping or cleaning with acetone and anhydrous ethanol in sequence to remove oil and impurities, and drying at medium temperature for later use.

[0033] Step 2: Prepare a polysilazane bonding layer coating and spray it onto the pretreated substrate surface. The coating is then cured in stages to form a polysilazane bonding layer.

[0034] Step 3: Prepare a polyimide-based vibration damping and buffer layer coating, spray it onto the bonding layer, and cure it to form a polyimide-based vibration damping and buffer layer;

[0035] Step 4: Prepare the ceramic protective coating and spray it onto the vibration damping buffer layer, then cure it to form the ceramic protective layer;

[0036] All spraying processes employ a multi-coat thin-spray technique, with each coat left to stand to allow the solvent to evaporate after each coat.

[0037] Furthermore, in the above preparation method, step 2, the preparation of the polysilazane bonding layer coating includes: mixing the polysilazane precursor and xylene solvent at a mass ratio of 1:3-1:4, adding nano-silica sol and fumed silica, stirring evenly, and then adding defoamer; using a spray gun to uniformly spray the polysilazane bonding layer coating onto the surface of the pretreated composite material, using a multi-pass thin spraying method to form a bonding layer with a thickness of 20μm-50μm; placing the sprayed structural part in an oven, heating and curing in stages, first keeping at 80℃ for 30 minutes to remove the solvent, then heating to 150℃ and keeping for 1 hour, and then heating to 200-220℃ and keeping for 1 hour, to promote the cross-linking of polysilazane to form a continuous Si-based network, thereby obtaining a polysilazane bonding layer that is firmly bonded to the composite material matrix.

[0038] Furthermore, in step 3 of the above preparation method, the preparation of the polyimide-based vibration damping and buffer coating includes: adding N-methylpyrrolidone solvent, dispersant, and nano-silica sol to a polyimide precursor solution; optionally, adding carbon nanotubes or graphene nanosheets as reinforcing fillers; slowly adding alumina powder and 8Y yttrium-stabilized zirconia powder while stirring; then adding a small amount of hydrophilic fumed silica to adjust and form a ceramic-reinforced polyimide slurry with certain thixotropic properties; removing large agglomerates by allowing it to stand to remove bubbles and filtering it through a sieve; and uniformly spraying the polyimide-based ceramic slurry onto the surface of the polysilazane bonding layer using a spray gun. A multi-pass thin-spraying process is used to control the coating thickness, which is approximately 30μm-50μm. Between each spraying, the coating is left to stand at room temperature or medium temperature to allow the solvent to evaporate. Then, it is cured in an oven according to a predetermined temperature regime to promote the imidization reaction of polyimide and form a polyimide-based vibration damping buffer layer with good toughness, thermal stability and vibration damping properties. The curing process includes: first, holding at 80°C for 30 minutes, then raising the temperature to 150°C and holding for 1 hour, and then raising the temperature to 200°C and holding for 1 hour.

[0039] Furthermore, in the above preparation method, step 4, the preparation of the ceramic protective coating includes: mixing alumina powder and 8Y yttrium stabilized zirconia powder according to a predetermined mass ratio, adding a dispersant and nano-silica sol, then adding anhydrous ethanol and N-methylpyrrolidone mixed solvent and binder resin, stirring evenly, and then adding hydrophilic fumed silica to adjust it into a ceramic spraying slurry with moderate viscosity and thixotropy. After filtration to remove excessively large particles, the ceramic protective coating is sprayed onto the surface of the polyimide-based vibration damping buffer layer using a spray gun. Multiple thin sprays are used to build up layers one by one, controlling the total thickness within the range of 20μm-50μm. During the spraying process, the spraying pressure, spray distance, and moving speed are adjusted to ensure a uniform, finely sanded surface, avoiding localized excessive thickness that could lead to cracking. The multi-layered composite material structural component, after spraying, is placed in an oven for staged temperature curing. During the medium-temperature stage, solvents and some binders are removed. During the higher-temperature stage, densification between ceramic particles and cross-linking of the binder phase are promoted. After curing, a dense, continuous ceramic protective layer that is firmly bonded to the underlying layer is obtained. The curing process includes: first, holding at 60℃-80℃ for approximately 30-60 minutes, then raising the temperature to 150℃ and holding for approximately 1 hour, and finally raising the temperature to 200℃-220℃ and holding for approximately 1 hour.

[0040] Furthermore, the above preparation method also includes step 5: preparing a sealing top coating, spraying a layer of low-solids polyimide-based sealing coating onto the surface of the ceramic protective layer. The coating contains a small amount of fine ceramic powder and nano-silica. After thin spraying and medium-temperature curing, a polyimide-based sealing top layer with a smooth surface, uniform color, and additional wear resistance is formed.

[0041] Advantages and beneficial effects of the present invention:

[0042] This invention utilizes a multi-layer structure design consisting of a polysilazane bonding layer, a polyimide-based vibration damping buffer layer, and a ceramic protective layer with added alumina and 8Y yttrium-stabilized zirconium oxide. This design creates a multi-system protective coating for thin-walled aerospace composite materials, which combines high interfacial bonding strength, thermal protection, and wear resistance and vibration damping. This solves the problem that traditional single polymer or single ceramic coatings cannot simultaneously achieve both thermal protection and vibration damping performance.

[0043] The polysilazane bonding layer of the present invention can form a Si-based ceramic network under medium temperature conditions, which significantly improves the interfacial bonding strength between the coating and the composite matrix and reduces the risk of interfacial debonding during thermal shock service.

[0044] The polyimide-based vibration damping buffer layer of the present invention has a high glass transition temperature and stable damping performance. Combined with alumina and yttrium-stabilized zirconia particles and optional carbon nanofillers, it can effectively dissipate energy under high-temperature vibration environment and improve the vibration damping and impact resistance of the composite thin-walled shell.

[0045] The ceramic protective layer of the present invention uses alumina and yttrium-stabilized zirconium oxide as the main components. It has low thermal conductivity and excellent wear and erosion resistance. It can provide stable thermal protection and surface wear protection for thin-walled composite material structures under high-temperature airflow, particle erosion and thermal shock conditions.

[0046] This invention employs a wet spraying and medium-temperature curing process, eliminating the need for high-temperature sintering and high-energy thermal spraying equipment. The process is simple, the temperature level is suitable for aerospace composite material matrices, and it does not introduce significant thermal damage. The coating formulation and thickness are easily adjusted according to different components and working conditions, making it suitable for widespread application in thin-walled aerospace composite material structures. Attached Figure Description

[0047] Figure 1 The diagram shows a model of the multifunctional protective coating prepared according to the present invention. In the diagram, 1-T700 carbon fiber sheet, 2-polysiloxane bonding layer, 3-polyimide vibration damping buffer layer, 4-ceramic protective layer, and 5-polyimide-based sealing layer.

[0048] Figure 2 The images show the samples; among them, 2(a) is a T700 carbon fiber sample without coating. Figure 2 (b) is the T700 carbon fiber sample with reduced damping buffer layer thickness in Comparative Example 1; Figure 2 (c) is the T700 carbon fiber sample of Example 2 without the introduction of nano-reinforced filler and without the setting of the sealing layer; Figure 2 (d) is the T700 carbon fiber sample with reduced ceramic protective layer thickness in Example 3;

[0049] Figure 3 The images show the surface morphology of Examples 1, 2 and Comparative Example 1 of the present invention; wherein, 3(a) is a T700 carbon fiber sample coated with the multifunctional protective coating of Example 1; 3(b) is a T700 carbon fiber sample coated with Example 2; and 3(c) is a T700 carbon fiber sample coated with Comparative Example 1.

[0050] Figure 4 The images shown are infrared images of thermal protection tests under constant temperature thermal radiation for Embodiments 1 and 2, Comparative Example 1, and the uncoated test specimen of the present invention; wherein, 4(a) is an infrared thermal imaging comparison image 20 minutes after the start of the test; 4(b) is an infrared thermal imaging comparison image 21 minutes and 30 seconds after the start of the test; 4(c) is an infrared thermal imaging comparison image 22 minutes after the start of the test; 4(d) is an infrared thermal imaging comparison image 32 minutes and 30 seconds after the start of the test; 4(e) is an infrared thermal imaging comparison image 23 minutes and 30 seconds after the start of the test; 4(f) is an infrared thermal imaging comparison image 34 minutes after the start of the test; 4(g) is an infrared thermal imaging comparison image 24 minutes and 30 seconds after the start of the test; and 4(h) is an infrared thermal imaging comparison image 25 minutes after the start of the test.

[0051] Figure 5 A comparison of the time-domain response curves of the T700 carbon fiber sheet sample coated with the multifunctional protective coating of Example 1 and the T700 carbon fiber sheet sample without coating under resonance conditions.

[0052] Figure 6 The image shows a comparison of the impact contact force of a T700 carbon fiber sheet sample coated with the multifunctional protective coating of Example 1 and an uncoated T700 carbon fiber sheet sample under low-speed impact. In the image, 6(a) is the time-impact contact force curve of the uncoated specimen and 6(b) is the contact force curve of the T700 carbon fiber sheet sample coated with the coating of Example 1 under low-speed impact. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0054] The purity of the raw materials used in this invention is not particularly limited, but commercially available raw materials of industrial purity or commonly used purity in the field are preferred. The spraying device, drying device, and stirring device used in this invention are all commonly used equipment in the field and are not particularly limited, such as conventional gravity or pressure spray guns, experimental vacuum drying ovens, and magnetic stirrers.

[0055] In the following embodiments, the composite material matrix used is a T700 carbon fiber reinforced resin matrix sheet, which can be provided by Weihai Guangwei Composite Materials Co., Ltd. or other composite material manufacturers.

[0056] The polysilazane precursor is a commercially available polysilazane solution, provided by Aladdin Biochemical Technology Co., Ltd. or other chemical companies.

[0057] The polyimide precursor solution uses commercially available aromatic polyimide coating precursors with N-methylpyrrolidone as the main solvent, and is provided by Aladdin Biochemical Technology Co., Ltd. or other chemical companies.

[0058] The alumina powder and 8Y yttrium stabilized zirconium oxide powder are preferably high-purity fine-particle powders provided by Shanghai McLean Co., Ltd., while the nano silica sol and hydrophilic fumed silica can be products supplied by Aladdin Biochemical Technology Co., Ltd.

[0059] The preferred dispersant is an aqueous solution of sodium polyacrylate suitable for dispersing inorganic powders, and the defoamer is a commonly used industrial silicone defoamer, such as defoamer type 204.

[0060] Example 1

[0061] This embodiment describes a method for preparing a multifunctional protective coating for thin-walled composite structures in aerospace applications, comprising the following steps:

[0062] Step 1: Matrix Pretreatment. Cut T700 carbon fiber sheets into 50mm × 50mm samples. Wipe the surface with anhydrous ethanol and acetone sequentially to remove oil and impurities. Lightly sand the sample surface with fine sandpaper to remove the resin-rich layer and create a slightly rough surface that facilitates mechanical bonding. Wipe clean again with anhydrous ethanol and dry in a 60℃ oven for about 20 minutes. Remove and allow to cool naturally for later use.

[0063] Step 2: Preparation of the polysilazane bonding layer coating. Place 30% of the polysilazane precursor in a clean beaker, add 40% xylene solvent, and then add 15% N-methylpyrrolidone solvent. Stir magnetically for approximately 20 minutes to ensure thorough mixing. Add nano-silica sol to the above solution, approximately 12% of the total mass, and a small amount of hydrophilic fumed silica, controlling the total addition to approximately 2.5% of the total coating mass. Continue stirring for 20 minutes. Add 0.5% defoamer, stir gently, and let stand for 10 minutes to remove air bubbles, obtaining the polysilazane bonding layer coating for spraying.

[0064] Step 3: Spraying and Curing of the Polysilazane Bonding Layer. The polysilazane coating prepared in Step 2 was uniformly sprayed onto the pretreated T700 carbon fiber sheet surface using a spray gun. During spraying, the distance between the nozzle and the sample was controlled to be approximately 15 cm. Multiple thin sprays were applied to form a uniform wet film with a thickness of approximately 20 μm. After spraying, the sample was placed in a room temperature environment and allowed to stand for approximately 20 minutes to allow most of the solvent to evaporate. Then, it was placed in an oven and cured according to the following process: first, it was kept at 80℃ for approximately 30 minutes, then the temperature was increased to 150℃ and kept for approximately 1 hour, and then the temperature was increased to 200℃ and kept for approximately 1 hour. After removal and natural cooling, a polysilazane bonding layer firmly bonded to the composite matrix was obtained.

[0065] Step 4: Preparation of the polyimide-based vibration damping and buffer coating. Add 45% (by mass) of a polyimide precursor solution to a clean beaker, followed by 35% N-methylpyrrolidone solvent. Adjust the coating viscosity to a slow-flowing state and stir magnetically for 20 minutes. Add sodium polyacrylate aqueous solution as a dispersant, approximately 0.9% (by mass) of the inorganic powder, and stir until homogeneous. Add approximately 3% (by mass) of nano-silica sol to the system, followed by approximately 10% (by mass) of pre-weighed alumina powder and 5% (by mass) of 8Y yttrium-stabilized zirconia powder. Slowly add the above inorganic powders while stirring to ensure uniform dispersion. Optionally, add a small amount of carbon nanotubes or graphene nanosheets, approximately 0.5% (by mass) of the total coating mass, to enhance vibration damping and crack resistance. Then add a small amount of hydrophilic fumed silica, approximately 0.5% (by mass) of the total coating mass, to adjust the thixotropic properties of the slurry. Continue stirring for 20 minutes and perform ultrasonic dispersion as needed. Finally, add 0.1% defoamer, let stand for 20 minutes to remove air bubbles, and then filter through a 200-mesh sieve to remove large particle agglomerates, thus obtaining a polyimide-based vibration damping and buffer layer spray coating.

[0066] Step 5: Spraying and curing the polyimide-based vibration damping layer. Lightly dust the surface of the polysilazane bonded layer sample obtained in Step 3, and if necessary, lightly roughen it with fine sandpaper, then wipe it clean with anhydrous ethanol. Using a spray gun, uniformly spray the polyimide-based ceramic slurry prepared in Step 4 onto the polysilazane bonded layer using a multi-pass thin-spray process, each spray forming a uniform wet film. After spraying, allow it to stand at room temperature for approximately 15 minutes to allow the solvent to evaporate. Repeat the above thin-spray and standing process until the predetermined thickness is reached; the total thickness can be controlled within 50 μm. After spraying, place the sample in an oven, first holding it at 80℃ for approximately 30 minutes, then raising the temperature to 150℃ and holding it for approximately 1 hour, and finally raising the temperature to 200℃ and holding it for approximately 1 hour to complete the imidization reaction of the polyimide, forming a polyimide-based vibration damping layer with certain toughness and damping properties.

[0067] Step 6: Preparation of the ceramic protective coating. In a clean beaker, mix alumina powder and 8Y yttrium-stabilized zirconia powder at a predetermined mass ratio, where the alumina powder mass fraction is approximately 40% and the 8Y yttrium-stabilized zirconia powder mass fraction is approximately 10%. Add approximately 0.2 wt% of sodium polyacrylate aqueous solution as a dispersant, stir until homogeneous, then add nano-silica sol, accounting for approximately 5% of the total coating mass. Add anhydrous ethanol and N-methylpyrrolidone at a mass ratio of 3:1 as a solvent to the above system, adjusting the total solid content to 30%. Then add a small amount of polyvinyl butyral solution (approximately 13.8%), followed by hydrophilic fumed silica, accounting for approximately 0.5% of the total coating mass, to adjust thixotropy. Continue stirring for 20 minutes, and perform ultrasonic dispersion if necessary. Add 0.5% defoamer, let stand for 20 minutes, and filter through a 200-mesh sieve to remove large particle agglomerates, obtaining the ceramic protective coating.

[0068] Step 7: Ceramic Protective Layer Spraying and Drying / Curing. The surface of the polyimide-based vibration damping layer sample obtained in Step 5 is lightly roughened and wiped clean. The ceramic coating prepared in Step 6 is then evenly sprayed onto the surface of the vibration damping layer using a spray gun. During spraying, the spray gun movement speed and spraying distance are controlled, employing a multi-pass thin-film spraying method to ensure each pass forms a thin wet film, approximately 50 μm in thickness. After spraying, the sample is left to stand at room temperature for about 15 minutes to allow solvents such as ethanol to fully evaporate. This process of spraying and standing is repeated several times until the target thickness is achieved. After spraying, the sample is placed in an oven and first kept at 80°C for about 60 minutes to remove most of the solvent. Then, the temperature is increased to 150°C and held for about 1 hour, followed by a further increase to 200°C and held for about 1 hour to promote the curing of the binder resin and the formation of a ceramic protective framework primarily composed of alumina and yttrium-stabilized zirconia. The sample is then removed and allowed to cool naturally to obtain the ceramic protective layer.

[0069] Step 8: Preparation and spraying of the waterborne polyurethane sealing layer. When further improvement in surface smoothness and color consistency is required, waterborne polyurethane is used as the film substrate for the sealing layer, with a thickness of approximately 20 μm. A certain amount of waterborne polyurethane is placed in a clean beaker, and appropriate amounts of deionized water or a matching diluent are added to adjust the viscosity, making it suitable for spraying or leveling.

[0070] like Figure 1 As shown, through the above steps, a multifunctional protective coating system consisting of a polysilazane bonding layer 2, a polyimide-based vibration damping buffer layer 3, a ceramic protective layer 4, and a polyimide-based sealing layer 5 can be prepared on the surface of T700 carbon fiber sheet 1. This coating has high interfacial bonding strength, good thermal protection performance, wear resistance and erosion resistance, and vibration damping and impact resistance under thermal-vibration service conditions, and has good applicability to aerospace composite thin-walled structures.

[0071] Comparative Example 1

[0072] This comparative example describes a method for preparing a protective coating for thin-walled composite structures in aerospace applications. The basic process flow is the same as that in Example 1, with the main differences being the adjustment of the thickness of the vibration damping buffer layer and the formulation and spraying process parameters of the ceramic protective layer. Specifically, it includes the following steps.

[0073] Step 1, matrix pretreatment, is the same as in Example 1.

[0074] Step 2: Preparation of polysilazane bonding layer coating. Take about 10% polysilazane precursor, about 70% xylene solvent, 19.8% nano-silica sol, and control the amount of hydrophilic fumed silica to 0.2 wt% of the total coating mass. After stirring slightly, let stand for 10 minutes to obtain the polysilazane bonding layer coating for spraying.

[0075] Step 3: Spraying and curing of the polysilazane bonding layer. Similar to Example 1, the spraying distance was controlled at 15cm and the spraying thickness was approximately 20μm. This comparative example used room temperature natural curing, and a polysilazane bonding layer was obtained after complete curing.

[0076] Step 4: Preparation of polyimide-based vibration damping and buffer coating. Take approximately 40% of the polyimide precursor solution and thoroughly mix it with 50% of N-methylpyrrolidone solvent. The mass fraction of nano-silica sol is 4 wt% of the total coating mass. Add approximately 3% alumina powder, 2% 8Y yttrium-stabilized zirconium oxide powder, and 1% carbon nanotubes or graphene nanosheets of the total coating mass. Stir the above solution thoroughly and allow it to stand for 10 minutes after stirring.

[0077] Step 5: Apply the polyimide-based vibration damping coating prepared in Step 4 to the surface of the specimen using a spray gun. Each spray forms a uniform wet film with a thickness of approximately 20 μm. After spraying, allow the sample to stand at room temperature for about 10 minutes to allow the solvent to evaporate. After spraying, place the sample indoors for room temperature curing.

[0078] Step 6: Preparation of the ceramic protective coating. The ceramic protective coating was prepared according to the method in Example 1, but the inorganic powder ratio was adjusted: the mass fraction of alumina powder was 15%, and the mass fraction of 8Y yttrium stabilized zirconia powder was 8%. The amount of sodium polyacrylate aqueous solution added was 1.5 wt% of the total coating mass, the amount of nano-silica sol added was 25%, the amount of anhydrous ethanol and N-methylpyrrolidone mixed solvent was 50%, the amount of binder resin was 0.1% of the total coating mass, and the amount of hydrophilic fumed silica added was 0.4%. The above solution was stirred thoroughly, and after stirring, it was allowed to stand for 10 minutes. The mixed solution was then filtered through 200-mesh filter paper to obtain the final ceramic protective coating.

[0079] Step 7: Ceramic Protective Layer Spraying and Drying / Curing. After surface treatment of the vibration damping buffer layer sample obtained in Step 5, the ceramic protective layer coating prepared in Step 6 is sprayed onto its surface using a spray gun. Unlike Example 1, the single-coat wet film thickness in this comparative example is slightly thinner than in Example 1, and the number of spray coats is controlled to three, with a total thickness of approximately 30 μm. After each coat, the coating is allowed to stand at room temperature for 10 minutes. After completing multiple thin coats, natural curing is carried out at room temperature.

[0080] Example 2

[0081] This embodiment describes a method for preparing a multifunctional protective coating for thin-walled composite structures in aerospace applications. The basic process flow is the same as in Embodiment 1, except that no carbon nanotubes or graphene are added to the polyimide-based vibration damping layer, and no polyimide-based sealing layer is provided. The specific preparation steps are as follows.

[0082] Step 1: Matrix Pretreatment. T700 carbon fiber reinforced resin matrix composite sheet was selected as the matrix. The matrix surface was cleaned with acetone and anhydrous ethanol in sequence to remove oil and impurities. Then, the surface was lightly sanded with fine sandpaper to improve the surface roughness. Finally, it was allowed to air dry in a clean environment for later use.

[0083] Step 2: By mass percentage, add 15% of the polysilazane precursor to a mixing container, along with 35% N-methylpyrrolidone solvent, 5% nano-silica sol, and then sequentially add 0.5 wt% hydrophilic fumed silica, 0.1 wt% defoamer, and approximately 44.4% xylene solvent. Mix thoroughly under mechanical stirring to obtain the polysilazane bonding layer coating.

[0084] Step 3: Spraying and Curing of the Polysilazane Bonding Layer. The polysilazane bonding layer coating prepared in Step 2 is uniformly sprayed onto the substrate surface using a spray gun. The spraying distance is controlled at approximately 15 cm, and multiple thin sprays are used to form a uniform coating with a thickness of approximately 50 μm. After spraying, the coating is cured in stages at 80℃ for 30 minutes, 150℃ for 1 hour, and 200℃ for 1 hour, respectively, to form a dense polysilazane bonding layer.

[0085] Step 4: By mass percentage, add 45% of the polyimide precursor solution to a mixing container, followed by 15% alumina powder and 0.6% 8Y yttrium stabilized zirconia powder, then 10% nano-silica sol and 2 wt% hydrophilic fumed silica. Next, add approximately 25.9% N-methylpyrrolidone solvent, 1 wt% dispersant, and 0.5 wt% defoamer. Mix thoroughly under mechanical stirring and ultrasonic dispersion to obtain a polyimide-based vibration damping and buffering layer coating. In this embodiment, no reinforcing fillers such as carbon nanotubes or graphene nanosheets are added.

[0086] Step 5: Spraying and curing of the polyimide-based vibration damping layer. After surface dust removal treatment of the bonding layer sample obtained in Step 3, the vibration damping layer coating prepared in Step 4 is sprayed onto its surface using a spray gun. A multi-coat thin spraying process of three to five coats is adopted, with a spray thickness of approximately 20 μm. After each coat, the coating is allowed to stand at room temperature for 15 minutes. After the spraying is completed, the coating is cured sequentially at 80℃ for 30 minutes, 150℃ for 1 hour, and 200℃ for 1 hour to obtain the polyimide-based vibration damping layer, with a total thickness controlled at approximately 40 μm.

[0087] Step 6: By mass percentage, add 20% alumina powder and 11.9% 8Y yttrium stabilized zirconia powder to a mixing container. Then add 15% nano-silica sol, 20% binder resin, approximately 30% anhydrous ethanol and N-methylpyrrolidone mixed solvent, and 2 wt% hydrophilic fumed silica, 1 wt% dispersant, and 0.1 wt% defoamer. Mix thoroughly under stirring and dispersion conditions to obtain a ceramic protective coating.

[0088] Step 7: Ceramic protective layer spraying and drying / curing. After surface treatment of the sample obtained in Step 5, the ceramic protective layer coating prepared in Step 6 is sprayed onto its surface using a spray gun in three thin coats, with a coating thickness of approximately 40 μm. After each coat, the sample is allowed to stand at room temperature for 15 minutes. After spraying, the sample is dried and cured sequentially at 80℃ for 80 minutes, 150℃ for 1 hour, and 200℃ for 1 hour to form the ceramic protective layer. In this embodiment, a polyimide sealing layer is not used.

[0089] Example 3

[0090] This embodiment describes a method for preparing a multifunctional protective coating for thin-walled composite structures in aerospace applications, comprising the following steps:

[0091] Step 1: The matrix pretreatment method is the same as in Example 1.

[0092] Step 2: Preparation of the polysilazane bonding layer coating. Place 16.5% of the polysilazane precursor in a clean beaker, add 65% xylene solvent, and stir magnetically for approximately 20 minutes to ensure thorough mixing. Add nano-silica sol (approximately 15% of the total mass), 3% fumed silica, and finally 0.5% defoamer. Continue stirring for another 20 minutes. The resulting polysilazane bonding layer coating is ready for spraying.

[0093] Step 3: The method for spraying and curing the polysilazane bonding layer is the same as in Example 1.

[0094] Step 4: Preparation of polyimide-based vibration damping and buffering layer coating. A 65% polyimide precursor solution was added to a mixing container, followed by 5% alumina powder and 1.5% 8Y yttrium-stabilized zirconia powder, then 10% nano-silica sol and 2 wt% hydrophilic fumed silica. Approximately 15% N-methylpyrrolidone solvent, 0.8% carbon nanotubes, 0.2 wt% dispersant, and 0.5 wt% defoamer were then added. The mixture was thoroughly mixed under mechanical stirring and ultrasonic dispersion to obtain the polyimide-based vibration damping and buffering layer coating. In this embodiment, no reinforcing fillers such as carbon nanotubes or graphene nanosheets were added.

[0095] Step 5: The spraying and curing method for the polyimide-based vibration damping buffer layer is the same as in Example 1.

[0096] Step 6: Preparation of the ceramic protective coating. In a clean beaker, add 33.8% alumina powder and 15% 8Y yttrium stabilized zirconia powder. Add approximately 0.2 wt% sodium polyacrylate aqueous solution as a dispersant to the mixed powder as a dispersant. After stirring evenly, add nano-silica sol, with a mass fraction of approximately 5% of the total coating mass. Then, add anhydrous ethanol and N-methylpyrrolidone at a mass ratio of 3:1 as a solvent to adjust the total solids content of the coating to approximately 40%. Next, add a small amount of polyvinyl butyral solution, with a mass fraction of approximately 5%, and add hydrophilic fumed silica, with a mass fraction of approximately 0.5% of the total coating mass, to adjust the thixotropic properties of the slurry. Mix under continuous stirring for approximately 20 minutes. Finally, add 0.5% defoamer, let stand for approximately 20 minutes to remove air bubbles, and filter through a 200-mesh sieve to remove large agglomerates, obtaining the ceramic protective coating.

[0097] Step 7: Slightly roughen and clean the surface of the polyimide-based vibration damping layer sample obtained in Step 5. Use a spray gun to evenly spray the ceramic protective coating prepared in Step 6 onto its surface. Control the spraying distance and the speed of the spray gun during the spraying process, and use a multi-pass thin spraying process. After each spraying, let it stand at room temperature for about 15 minutes to allow the solvent to fully evaporate, and repeat the spraying.

[0098] Step 8: A waterborne polyurethane sealing layer with a thickness of 40 μm is also set in step 8, and its spraying process parameters and curing conditions are the same as in Example 1.

[0099] Sample images of Examples 2 and 3 and Comparative Example 1 are shown below. Figure 2 As shown, where, Figure 2 (a) is a T700 carbon fiber sample without coating; Figure 2 (b) is the T700 carbon fiber sample with reduced damping buffer layer thickness in Comparative Example 1. A continuous coating layer is formed on its surface, but the overall coating thickness is significantly reduced, resulting in limited interlayer buffering effect. Figure 2 (c) This is the T700 carbon fiber sample from Example 2 without the introduction of nano-reinforced fillers and without a sealing layer. It can be seen that the outermost layer lacks a tight sealing layer, the surface is relatively rough, and the structural integrity and resistance to external erosion need to be improved. Figure 2 (d) is the T700 carbon fiber sample with reduced ceramic protective layer thickness in Example 3. Its outer ceramic layer has good continuity, but the overall ceramic coating content and thickness are lower than those of the standard example, and the density and barrier ability of the surface protective layer are relatively weakened.

[0100] Figure 3 These are surface morphology diagrams of Embodiments 1 and 2 and Comparative Example 1 of the present invention. Figure 3 (a) A T700 carbon fiber sample coated with the multifunctional protective coating of Example 1. Figure 3 (b) is a sample of T700 carbon fiber coated in Example 2. Figure 3 (c) The T700 carbon fiber sample coated with Comparative Example 1. From the three-dimensional surface morphology, it can be seen that there are significant differences in coating roughness and particle size among the three embodiments. (a) The surface in Figure 1 consists of relatively uniform small protrusions and shallow pits, with height fluctuations ranging from approximately 40-69 μm. The overall undulation is small, indicating that the ceramic particles are relatively uniformly dispersed, and the sealing layer has a good leveling effect on the surface. (b) The number of surface protrusions in Figure 1 increases, and particle agglomeration is slightly aggravated. The height variation increases to approximately 45-78 μm, with locally higher peaks, indicating that under the conditions of simplified damping layer and no ceramic filler in the sealing layer, surface controllability is slightly reduced. (c) Figure 1 shows large-sized protrusions and obvious particle accumulation, with the maximum height reaching approximately 104 μm. The surface undulation is the most severe, reflecting that after thinning the damping layer and ceramic layer thickness and adjusting the ceramic ratio, the coating is more prone to local accumulation and roughening. In summary, Example 1 performs best in terms of surface smoothness and roughness controllability, which is more conducive to obtaining stable wear resistance and mechanical properties.

[0101] Figure 4The figures show infrared images of thermal protection tests under constant temperature thermal radiation for Examples 1 and 2, Comparative Example 1, and the uncoated test specimens of the present invention. In the figures, P1 is the infrared thermal image of the uncoated T700 carbon fiber sheet sample; P2 is the infrared thermal image of the T700 carbon fiber sheet sample of Comparative Example 1 with a reduced thickness of the damping buffer layer; P3 is the infrared thermal image of the T700 carbon fiber sheet sample of Example 3 with a reduced thickness of the ceramic protective layer; and P4 is the infrared thermal image of the T700 carbon fiber sheet sample of Example 1 with a multifunctional protective coating. (a) is a comparison of infrared thermal images 20 minutes after the start of the test; (b) is a comparison of infrared thermal images 21 minutes and 30 seconds after the start of the test; (c) is a comparison of infrared thermal images 22 minutes after the start of the test; (d) is a comparison of infrared thermal images 32 minutes and 30 seconds after the start of the test; (e) is a comparison of infrared thermal images 23 minutes and 30 seconds after the start of the test; (f) is a comparison of infrared thermal images 34 minutes after the start of the test; (g) is a comparison of infrared thermal images 24 minutes and 30 seconds after the start of the test; (h) is a comparison of infrared thermal images 25 minutes after the start of the test. The infrared thermal imaging results show significant differences in surface temperature distribution and heating characteristics among the four samples under the same heating conditions. The uncoated test specimen and the central cross-shaped high-temperature area and border area of ​​Example 2 exhibit the brightest color, with a larger high-temperature area and a narrower surrounding low-temperature area, indicating rapid thermal conductivity and a steep temperature gradient in the substrate. In Examples 1 and 3, the overall brightness of the high-temperature zone decreased, the boundary of the cross-shaped hot zone became relatively blurred, and the area of ​​the high-temperature zone shrank, indicating that the coating played a certain role in blocking heat flow. Among them, Example 1 showed the most significant suppression of the highest surface temperature and temperature gradient in the central region. The above analysis shows that the multilayer coating invented in this patent can reduce the surface temperature of the composite material sheet and slow down the heating rate.

[0102] Figure 5 The time-domain response curves of the T700 carbon fiber sheet sample coated with the multifunctional protective coating of Example 1 and the T700 carbon fiber sheet sample without coating are compared under resonance conditions. As can be seen from the figure, the time-domain vibration response of the T700 carbon fiber sheet sample coated with the coating of Example 1 under resonance excitation is lower than that of the uncoated sample, which proves the vibration reduction performance of the multi-system powder-reinforced damping thermal protective coating proposed in this invention.

[0103] Figure 6The figures show a comparison of the impact contact forces of T700 carbon fiber sheet samples coated with the multifunctional protective coating of Example 1 and those without the coating under low-velocity impact. The impact resistance of the coating was evaluated by comparing the time-contact force curves of the coated and uncoated specimens under an impact energy of 20J. Figure (a) shows the time-impact contact force curve of the uncoated specimen, and Figure (b) shows the contact force curve of the T700 carbon fiber sheet sample coated with the Example 1 coating under low-velocity impact. The results show that the peak impact force of the uncoated specimen reaches 2600N, while the peak impact force of the specimen coated with Example 1 is reduced to 2200N, a reduction of approximately 15.3%. This confirms that the currently prepared multifunctional protective coating can effectively protect the composite matrix by reducing the impact load on the substrate through energy absorption and load buffering.

Claims

1. A multifunctional protective coating for thin-walled composite structures in aerospace applications, characterized in that, The coating, from the inside out, comprises: Polysilazane bonding layer is used to enhance interfacial adhesion with the composite matrix; Polyimide-based vibration damping and buffer layer, which combines heat buffering and vibration energy dissipation functions; The ceramic protective layer, with alumina and 8Y yttrium stabilized zirconium oxide as the main components, provides thermal protection and wear and erosion resistance. The multifunctional protective coating is deposited layer by layer using a wet spraying process, and is heated and cured in stages within the range of 60℃-200℃, without the need for high-temperature sintering equipment; A polyimide-based sealing layer is also provided on the outer surface of the ceramic protective layer to improve surface smoothness, color uniformity and additional wear resistance.

2. The multifunctional protective coating according to claim 1, characterized in that, The raw materials for preparing the polysilazane bonding layer include, by mass percentage, 15%-30% polysilazane precursor, 15%-35% N-methylpyrrolidone solvent, 5%-15% nano-silica sol, 0.5%-3% fumed silica, 40%-65% xylene solvent, and 0.1%-0.5% defoamer.

3. The multifunctional protective coating according to claim 1, characterized in that, The raw materials for preparing the polyimide-based vibration damping buffer layer include, by mass percentage: 45%-65% polyimide precursor solution, 5%-15% alumina powder, 0.6%-5% 8Y yttrium stabilized zirconium oxide powder, 3%-10% nano-silica sol, 0.5%-2% hydrophilic fumed silica, 15%-35% N-methylpyrrolidone solvent, 0.2%-1.0% dispersant, and 0.1%-0.5% defoamer; optionally, carbon nanotubes or graphene nanosheets are added as reinforcing fillers, with a mass percentage of 0.5%-0.8%.

4. The multifunctional protective coating according to claim 1, characterized in that, The raw materials for preparing the ceramic protective layer include, by mass percentage: 20%-40% alumina powder, 10%-15% 8Y yttrium stabilized zirconium oxide powder, 5%-15% nano silica sol, 5%-20% binder resin, 30%-40% anhydrous ethanol and N-methylpyrrolidone mixed solvent, 0.5%-2% hydrophilic fumed silica, 0.2%-1.0% dispersant and 0.1%-0.5% defoamer. The binder resin is polyvinyl butyral or polyimide precursor.

5. The multifunctional protective coating according to claim 1, characterized in that, The polyimide-based capping layer has a thickness of 20μm-40μm and is prepared from a low-solids-content polyimide precursor solution, fine-grained alumina powder, 8Y yttrium-stabilized zirconium oxide powder, and nano-silica.

6. A method for preparing the multifunctional protective coating according to any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Pre-treat the surface of the thin-walled composite material structure, including grinding, cleaning and drying; Step 2: Prepare a polysilazane bonding layer coating and spray it onto the pretreated substrate surface. The coating is then cured in stages to form a polysilazane bonding layer. Step 3: Prepare a polyimide-based vibration damping and buffer layer coating, spray it onto the bonding layer, and cure it to form a polyimide-based vibration damping and buffer layer; Step 4: Prepare the ceramic protective coating and spray it onto the vibration damping buffer layer, then cure it to form the ceramic protective layer; All spraying processes employ a multi-coat thin-spray technique, with a 20-minute settling time after each coat to allow the solvent to evaporate.

7. The method according to claim 6, characterized in that, In step 2, the preparation of the polysilazane bonding layer coating includes: mixing the polysilazane precursor with xylene solvent at a certain mass ratio, then adding nano-silica sol and fumed silica, stirring evenly, and then adding defoamer; the curing process includes: first keeping at 80°C for 30 minutes, then raising the temperature to 150°C and keeping at 1 hour, and then raising the temperature to 200°C and keeping at 1 hour.

8. The method according to claim 6, characterized in that, In step 3, the preparation of the polyimide-based vibration damping and buffer coating includes: adding N-methylpyrrolidone solvent, dispersant, nano-silica sol, alumina powder and 8Y yttrium stabilized zirconia powder to the polyimide precursor solution and stirring evenly; the curing process includes: first holding at 80°C for 30 minutes, then raising the temperature to 150°C and holding for 1 hour, and then raising the temperature to 200°C and holding for 1 hour.

9. The method according to claim 6, characterized in that, The preparation of the ceramic protective coating in step 4 includes: mixing alumina powder and 8Y yttrium stabilized zirconia powder according to a predetermined mass ratio, adding dispersant and nano silica sol, then adding anhydrous ethanol and N-methylpyrrolidone mixed solvent and binder resin, stirring evenly, and then adding hydrophilic fumed silica; the curing process includes: first holding at 80°C for about 60 minutes, then raising the temperature to 150°C and holding for about 1 hour, and then raising the temperature to 200°C and holding for about 1 hour.

10. The method according to claim 6, characterized in that, It also includes step 5: preparing a sealing top coating and spraying it onto the surface of the ceramic protective layer, which is then cured to form a sealing top layer.

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