An impedance and thermal expansion coefficient double-gradient type composite coating and a preparation method thereof

Abstract

The present application belongs to the technical field of functional coating materials, and discloses a kind of impedance and thermal expansion coefficient double gradient type composite coating and its preparation method, coating composition includes aluminum zirconium eutectic, silicon carbide wave absorbing material, boron nitride, the balance is silicon dioxide, plus binder;Coating is continuously gradient change from bottom layer to surface layer, aluminum zirconium eutectic content decreases, silicon carbide wave absorbing material content increases;Different proportion of slurry is coated on the substrate layer by layer, each layer is dried at room temperature, heated and dried, and can be coated next layer after curing;The coating of the application is ultrathin and light, realizes the double gradient continuous change of impedance and thermal expansion coefficient, the silicon carbide wave absorbing material is worm-like and whisker-like composite β-SiC nanomaterial, has wideband high-efficiency wave absorbing performance, and the dense structure of the coating also improves its thermal shock resistance and weather resistance, suitable for surface stealth and protection of high-temperature parts, and has important application value in the field of aerospace and high-end electronic equipment electromagnetic compatibility.

Description

Technical Field

[0001] This invention belongs to the field of functional coating materials technology, specifically relating to a dual-gradient composite coating with impedance and thermal expansion coefficient and its preparation method. Background Technology

[0002] In cutting-edge fields such as aerospace, gas turbines, and high-speed aircraft, substrate materials (such as titanium alloys, high-temperature alloys, and ceramic matrix composites) not only need to withstand extremely high thermal and mechanical loads, but also often face the threat of radar detection. In order to give these components radar stealth capabilities without significantly increasing their weight and size, and to ensure reliability in harsh environments such as high temperature and thermal shock, radar-absorbing coatings are often applied to the surface of the substrate materials.

[0003] Traditional microwave absorbing coatings are mostly single-layer or simple multi-layer structures, with their electromagnetic parameters changing abruptly along the thickness direction. This can easily cause impedance abrupt changes at the interlayer interfaces, leading to electromagnetic wave reflection and limiting the absorption bandwidth and efficiency. At the same time, the mismatch in the coefficients of thermal expansion between the coating material and the substrate material can generate huge thermal stress during high-temperature service or thermal cycling, causing the coating to crack, peel off, and fail.

[0004] In recent years, the concept of functionally graded materials (FJTs) has been introduced into coating design. By continuously changing the composition of the material, the coating properties (such as elastic modulus and coefficient of thermal expansion) can be smoothly transitioned from the substrate to the surface to alleviate thermal stress. However, achieving a synergistic effect of "electromagnetic property gradient" and "thermophysical property gradient" in the same ultrathin coating system while meeting the requirements of high temperature resistance and strong adhesion remains a technical challenge. Existing gradient coatings are often thick (>3 mm) or have complex processes, such as plasma spraying and physical vapor deposition, which are costly and difficult to prepare on a large scale. They also cannot simultaneously achieve excellent microwave absorption performance and high-temperature stability. Therefore, there is an urgent need for a lightweight, high-temperature resistant, weather-resistant, and highly microwave-absorbing multifunctional coating. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a dual-gradient composite coating with varying impedance and thermal expansion coefficient, along with its preparation method. Through precise compositional gradient design, a continuous dual-gradient variation of impedance and thermal expansion coefficient is achieved within a total thickness not exceeding 2.5 mm. This ensures excellent adhesion, thermal shock resistance, weather resistance, and wave absorption performance between the coating and the substrate while achieving broadband and efficient wave absorption performance. This solves the problem in existing technologies where multifunctional coatings struggle to synergistically optimize electromagnetic wave absorption, thermal stress matching, high-temperature stability, and bonding strength at ultra-thin scales.

[0006] To achieve the above objectives, the technical solution adopted in this invention is as follows: a composite coating with dual gradient of impedance and coefficient of thermal expansion, the raw material composition of which includes: aluminum zirconium eutectic, silicon carbide microwave absorbing material, boron nitride, silicon dioxide, plus a binder of 20-40% of the total weight of the raw material; at least three layers of coating are applied and cured sequentially from the bottom layer of the substrate surface to the surface layer away from the substrate, with a single layer thickness of 0.05-0.2 mm and a total thickness ≤2.5 mm.

[0007] The binder is a mixture of silica sol, ammonium chloride accelerator and silica nitride sol prepared in a mass ratio of (7-9):(0.5-1.5):(0.5-1.5). The silica sol contains more than 30% SiO2 solid solution; the silica nitride sol contains more than 20% Si3N4 solid solution, with an average particle size of less than 100 nm and a pH value of 9-11; and the concentration of ammonium chloride is 0.8 wt%.

[0008] Among them, the silicon carbide microwave absorbing material is made from a mixture of nano-sized amorphous silica and nano-carbon black in a mass ratio of 1:3 as raw materials; a platinum-rhodium metal mixture accounting for 1%-3% of the total weight of the raw materials is added as a catalyst, and worm-like and whisker-like composite β-SiC nanomaterials are obtained by sintering at 2200℃ under the protection of inert gas argon; the mass ratio of platinum to rhodium in the platinum-rhodium metal mixture is 3:7-7:3.

[0009] The aluminum-zirconium eutectic contains 80 wt% Al2O3 and 20 wt% ZrO2.

[0010] The aluminum-zirconium eutectic has a particle size of 0.5-1 μm.

[0011] Among them, the particle size of silicon carbide absorbing material is 5-10μm.

[0012] Among them, boron nitride has a particle size of 1-3 μm.

[0013] The silica particles have a diameter of 1-3 μm.

[0014] Among them, amorphous silica is nanoscale with a particle size ≤100nm and a purity of 99.99%; carbon black is nanoscale with a particle size ≤80nm and a purity of 99.99%.

[0015] The coating composition varies in a gradient direction in the thickness direction. Specifically, the gradient distribution is as follows: from the first layer to the outermost layer on the substrate surface, by mass fraction, the aluminum-zirconium eutectic decreases from 55% to 25%; the silicon carbide microwave absorbing material increases from 30% to 50%; boron nitride is 10-20%, and the balance is silicon dioxide to make up 100%.

[0016] A method for preparing a composite coating with dual gradients of impedance and thermal expansion coefficient includes the following steps:

[0017] Step 1: Raw material preparation: Based on the gradient distribution of raw material components in each layer of the coating, accurately calculate and weigh multiple sets of raw materials with different proportions; each set of raw materials includes: silicon carbide microwave absorbing material, boron nitride, silicon dioxide, and aluminum-zirconium eutectic; the binder is a mixture of silica sol, ammonium chloride coagulant and silica sol.

[0018] Step 2, Slurry preparation: Mix each group of raw materials with the binder separately, and then ball mill or high-speed disperse them to prepare uniform and stable coating slurries with a viscosity of 500-5000 mPa·s.

[0019] Step 3, Substrate Pretreatment: Surface treatment of the metal or ceramic substrate material, including cleaning, degreasing, and sandblasting roughening, to improve adhesion;

[0020] Step 4: Layer-by-layer coating and stepped curing:

[0021] S1. Apply the first layer of slurry to the pretreated substrate surface;

[0022] S2. Place the coated workpiece at room temperature and allow the coating surface to dry for 24 hours.

[0023] S3. Place in an oven and dry at 100-120℃ for 2-6 hours to further remove moisture and some volatile organic compounds;

[0024] S4. Heat to 180-220℃ and cure for 20-40 minutes to initially gel and develop a certain strength.

[0025] S5. Apply a second layer of slurry to the surface of the first cured layer;

[0026] S6. Repeat steps S2-S4 for room temperature drying, heating drying and curing.

[0027] S7. Apply the third layer, the fourth layer, and so on until the surface layer. Before applying each layer, ensure that the previous layer has been cured. The composition ratio of the coating material changes gradually in each layer to form a gradient distribution.

[0028] The preparation of silicon carbide microwave absorbing material includes the following steps:

[0029] Step 1: Preparation of composite powder:

[0030] According to the mass ratio, calculate and weigh the required nano-sized amorphous silica and carbon black respectively. Place the weighed amorphous silica and carbon black in a mixer and mix them thoroughly at 80-120 r / min for 4-6 h. Then add the catalyst and continue mixing until uniform to obtain composite powder.

[0031] Step 2: High-temperature sintering:

[0032] The composite powder is placed in the crucible of the high-temperature reaction device, argon gas is introduced into the reaction chamber and the argon atmosphere is maintained until the reaction is completed. The temperature is increased to 2200℃ at a rate of 10-20℃ / min and held at this temperature for 12-16 hours.

[0033] Step 3, Post-processing:

[0034] After the reaction was completed, the product was naturally cooled to room temperature under an argon atmosphere. The product was then removed, crushed, sieved, and impurity removed to obtain worm-like / whisker-like composite β-SiC nanomaterials, i.e. silicon carbide microwave absorbing materials.

[0035] The resistivity of silicon carbide absorbing materials is 0.5-159 Ω·cm. In the frequency range of 2-18 GHz, the real part ε′ of its composite dielectric constant is 6-15 and the imaginary part ε″ is 2-8.

[0036] The preferred mass ratio of whisker-like structures to worm-like structures in silicon carbide microwave absorbing materials is 60%:40%.

[0037] The coating of this invention employs a dual-gradient synergistic design: 1. Electromagnetic wave impedance gradient: From the bottom layer with high metal content (relatively high conductivity and high dielectric constant) to the surface layer with high SiC fiber / high ceramic content (relatively low conductivity and moderate loss), an impedance gradient with continuously changing real and imaginary parts of the dielectric constant is formed. This perfectly simulates the "gradual impedance change-loss absorption" structure of an ideal absorber, allowing electromagnetic waves to enter the coating to the maximum extent and be absorbed layer by layer, resulting in extremely low reflectivity and effectively broadening the absorption bandwidth. 2. Thermal expansion coefficient gradient: The content of the metallic phase (Al-Zr eutectic, high CTE) decreases from the inside to the outside, while the content of the ceramic phase (SiO2, BN, SiC, low CTE) increases, causing the overall thermal expansion coefficient of the coating to gradually decrease from the side closer to the substrate to the surface side. This design can greatly alleviate the thermal stress between the coating and the substrate (usually metal with moderate CTE) at high temperatures, significantly improve the coating's thermal shock resistance and bonding strength, and prevent high-temperature peeling.

[0038] The total thickness of the coating of this invention is ≤2.5mm, consisting of at least three ultra-thin coating layers, achieving precise gradient control at the microscale. This ultra-thin characteristic minimizes its impact on the weight and size of the substrate, making it ideal for weight- and size-sensitive aerospace applications. The binder addition ratio of this invention is adjusted according to the required thickness of each coating layer. When the required coating thickness is small, the binder addition ratio is relatively high, and the slurry concentration is reduced to achieve thin coating.

[0039] This invention employs silicon carbide absorbing material, which is a worm-like and whisker-like composite β-SiC nanomaterial. Using amorphous silica and carbon black as raw materials, it undergoes a high-temperature sintering reaction under a specific catalyst and argon protection. This allows for precise control of the ratio of worm-like to whisker-like β-SiC in the product, thereby customizing the complex dielectric parameters of the material and optimizing its impedance matching and electromagnetic wave loss capability. The worm-like / whisker-like composite β-SiC nanomaterial achieves complementary and synergistic loss mechanisms, enabling the material to have high absorption efficiency over a wide frequency band.

[0040] The coating of this invention contains a high proportion of high-temperature resistant components (SiC, BN, SiO2, and Al-Zr eutectic), enabling it to withstand temperatures above 1000℃. The addition of BN and SiO2 enhances the coating's high-temperature thermal conductivity, facilitating heat dissipation. The unique stepped curing process and the application of silicon nitride sol binder ensure strong adhesion within the coating and between the coating and the substrate. The dense structure of the coating also endows it with excellent weather resistance.

[0041] The beneficial effects of this invention are as follows: Through precise compositional gradient and process design, this invention achieves a dual gradient change in electromagnetic wave impedance and thermal expansion coefficient of the coating within an ultra-thin dimension with a total thickness not exceeding 2.5 mm. The silicon carbide absorbing material used is a worm-like and whisker-like composite β-SiC nanomaterial, thereby ensuring excellent adhesion between the coating and the substrate while obtaining broadband and efficient wave absorption performance. The dense structure of the coating also improves its thermal shock resistance and weather resistance. The slurry coating and stepped curing process used in this invention is simple, low-cost, and easy to apply to large-area and complex curved surface components. It has good process adaptability and repeatability, and has important application value in the field of electromagnetic compatibility of aerospace and high-end electronic equipment. Attached Figure Description

[0042] Figure 1 The reflection loss curve of the gradient coating prepared in Example 3 of the present invention in the 2-18GHz frequency band is shown.

[0043] Figure 2 The reflection loss curve of the equal-thickness double-layer coating prepared for comparison is shown in the 2-18GHz frequency band.

[0044] Figure 3 This is a schematic diagram showing the matching relationship between the coefficient of thermal expansion (CTE) of the gradient coating on the metal substrate material of the present invention and temperature. Detailed Implementation

[0045] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0046] To avoid repetition, the raw materials involved in this specific embodiment will be described uniformly first, and will not be repeated in the embodiments:

[0047] The aluminum-zirconium eutectic contains 80 wt% Al2O3 and 20 wt% ZrO2, and has a particle size of 0.5-1 μm.

[0048] The silica sol contains >30% SiO2 solid solution; the silica nitride sol contains >20% Si3N4 solid solution, with an average particle size <100nm and a pH value of 9-11; the concentration of ammonium chloride is 0.8wt%.

[0049] Boron nitride has a particle size of 1-3 μm; silicon dioxide has a particle size of 1-3 μm.

[0050] Amorphous silica is nanoscale with a particle size ≤100nm and a purity of 99.99%; carbon black is nanoscale with a particle size ≤80nm and a purity of 99.99%.

[0051] Example 1:

[0052] A method for preparing a composite coating with dual gradients of impedance and thermal expansion coefficient includes the following steps:

[0053] Step 1: Raw material preparation:

[0054] Preparation of silicon carbide microwave absorbing material: Amorphous silica and carbon black were calculated and weighed according to a mass ratio of 1:3. The weighed amorphous silica and carbon black were placed in a mixer and thoroughly mixed at 100 r / min for 5 h. Then, a platinum-rhodium metal mixture accounting for 2% of the total weight of the raw materials was added as a catalyst. The mass ratio of platinum to rhodium in the platinum-rhodium metal mixture was 6:4. The mixture was continued to be mixed until uniform to obtain composite powder. The composite powder was placed in the crucible of a high-temperature reaction device, and argon gas was introduced into the reaction chamber and maintained at the argon atmosphere until the reaction was completed. The temperature was increased to 2200℃ at a heating rate of 15℃ / min and held at this temperature for 15 h. After the reaction was completed, the mixture was naturally cooled to room temperature under the protection of argon atmosphere. The product was taken out and, after crushing, sieving, and impurity removal, worm-like / whisker-like composite β-SiC microwave absorbing material with a particle size of 5-10 μm was obtained.

[0055] Based on the gradient distribution of raw material components in each layer of the coating, five sets of raw materials with different proportions were accurately calculated and weighed:

[0056] Layer 1 (near substrate): 55% aluminum-zirconium eutectic, 30% SiC microwave absorbing material, 10% boron nitride, and 5% silicon dioxide;

[0057] Layer 2: 50% aluminum-zirconium eutectic, 33% SiC microwave absorbing material, 10% boron nitride, and 7% silicon dioxide;

[0058] Layer 3: 46% aluminum-zirconium eutectic, 35% SiC microwave absorbing material, 15% boron nitride, and 4% silicon dioxide;

[0059] 4th layer: 42% aluminum-zirconium eutectic, 38% SiC microwave absorbing material, 15% boron nitride, and 5% silicon dioxide;

[0060] 5th layer: 38% aluminum-zirconium eutectic, 42% SiC microwave absorbing material, 15% boron nitride, and 5% silicon dioxide;

[0061] 6th layer: 34% aluminum-zirconium eutectic, 45% SiC microwave absorbing material, 18% boron nitride, and 3% silicon dioxide;

[0062] 7th layer: 30% aluminum-zirconium eutectic, 48% SiC microwave absorbing material, 20% boron nitride, and 2% silicon dioxide;

[0063] 8th layer (surface layer): 25% aluminum-zirconium eutectic, 50% SiC microwave absorbing material, 20% boron nitride, and 5% silicon dioxide;

[0064] The binder is a mixture of silica sol, ammonium chloride coagulant, and silica nitride in a mass ratio of 9:1.5:1.5, and the amount of binder added to each layer is 40% of the total weight of the raw materials in each layer;

[0065] Step 2, Slurry preparation:

[0066] Each group of raw materials is mixed with a binder and then ball-milled or high-speed dispersed to prepare multiple uniform and stable coating slurries with a viscosity of 500 mPa·s.

[0067] Step 3: Substrate Pretreatment

[0068] Using 316 stainless steel sheet as the base, surface treatment is carried out, including cleaning, degreasing, and sandblasting roughening.

[0069] Step 4: Layer-by-layer coating and stepped curing:

[0070] S1. Apply the first layer of slurry to the pretreated substrate surface;

[0071] S2. Let the coated workpiece stand at room temperature for 24 hours until the coating surface is dry.

[0072] S3. Place in an oven and dry at 120℃ for 4 hours to further remove moisture and some volatile organic compounds;

[0073] S4. Heat to 220℃ and keep warm for 20 minutes to solidify, gelling and generating a certain strength;

[0074] S5. Apply a second layer of slurry to the surface of the first cured layer;

[0075] S6. Repeat steps S2-S4 for room temperature drying, heating drying, and curing.

[0076] S7. Apply the 3rd layer, 4th layer, and so on until the surface layer. Before applying each layer, ensure that the previous layer has been cured. The composition ratio of the coating material changes gradually in each layer to form a gradient distribution.

[0077] The coating prepared in this embodiment consists of 8 layers, and the total coating thickness was measured to be 1.6 mm.

[0078] Example 2:

[0079] A method for preparing a composite coating with dual gradients of impedance and thermal expansion coefficient includes the following steps:

[0080] Step 1: Raw material preparation:

[0081] Preparation of silicon carbide microwave absorbing material: Amorphous silica and carbon black were calculated and weighed according to a mass ratio of 1:3. The weighed amorphous silica and carbon black were placed in a mixer and thoroughly mixed at 80 r / min for 4 h. Then, a platinum-rhodium metal mixture accounting for 1% of the total weight of the raw materials was added as a catalyst. The mass ratio of platinum to rhodium in the platinum-rhodium metal mixture was 7:3. The mixture was continued to be mixed until uniform to obtain composite powder. The composite powder was placed in the crucible of a high-temperature reaction device, and argon gas was introduced into the reaction chamber and maintained at the argon atmosphere until the reaction was completed. The temperature was increased to 2200℃ at a heating rate of 10℃ / min and held at this temperature for 12 h. After the reaction was completed, the mixture was naturally cooled to room temperature under the protection of argon atmosphere. The product was taken out and, after crushing, sieving, and impurity removal, worm-like / whisker-like composite β-SiC microwave absorbing material with a particle size of 5-10 μm was obtained.

[0082] Based on the gradient distribution of raw material components in each layer of the coating, accurately calculate and weigh the raw materials in different proportions for each group:

[0083] Layer 1 (near substrate): 55% aluminum-zirconium eutectic, 30% SiC microwave absorbing material, 10% boron nitride, and 5% silicon dioxide;

[0084] Layer 2: 48% aluminum-zirconium eutectic, 35% SiC microwave absorbing material, 10% boron nitride, and 7% silicon dioxide;

[0085] Layer 3: 40% aluminum-zirconium eutectic, 40% SiC microwave absorbing material, 15% boron nitride, and 5% silicon dioxide;

[0086] 4th layer: 32% aluminum-zirconium eutectic, 45% SiC microwave absorbing material, 15% boron nitride, and 8% silicon dioxide;

[0087] 5th layer (surface layer): 25% aluminum-zirconium eutectic, 50% SiC microwave absorbing material, 20% boron nitride, and 5% silicon dioxide;

[0088] The binder is a mixture of silica sol, ammonium chloride coagulant and silica nitride in a mass ratio of 7:0.5:0.5; the amount of binder added to each layer is 20% of the total weight of the raw materials of each layer.

[0089] Step 2, Slurry preparation:

[0090] Each group of raw materials is mixed with a binder and then ball-milled or high-speed dispersed to prepare multiple uniform and stable coating slurries with a viscosity of 5000 mPa·s.

[0091] Step 3: Substrate Pretreatment

[0092] Using 316 stainless steel sheet as the base, surface treatment is carried out, including cleaning, degreasing, and sandblasting roughening.

[0093] Step 4: Layer-by-layer coating and stepped curing:

[0094] S1. Apply the first layer of slurry to the pretreated substrate surface;

[0095] S2. Let the coated workpiece stand at room temperature for 24 hours until the coating surface is dry.

[0096] S3. Place in an oven and dry at 100℃ for 6 hours to further remove moisture and some volatile organic compounds;

[0097] S4. Heat to 180℃ and keep warm for 40 minutes to cure, gelling and generating a certain strength;

[0098] S5. Apply a second layer of slurry to the surface of the first cured layer;

[0099] S6. Repeat steps S2-S4 for room temperature drying, heating drying, and curing.

[0100] S7. Apply the 3rd, 4th, and 5th layers in sequence, ensuring that the previous layer has been cured before applying each layer; the composition ratio of the coating material gradually changes in each layer to form a gradient distribution.

[0101] In this embodiment, the coating consists of 5 layers, and the total coating thickness is measured to be 1.0 mm.

[0102] Example 3:

[0103] A method for preparing a composite coating with dual gradients of impedance and thermal expansion coefficient includes the following steps:

[0104] Step 1: Raw material preparation:

[0105] Preparation of silicon carbide microwave absorbing material: Amorphous silica and carbon black were calculated and weighed according to a mass ratio of 1:3. The weighed amorphous silica and carbon black were placed in a mixer and thoroughly mixed at 120 r / min for 6 h. Then, a platinum-rhodium metal mixture accounting for 3% of the total weight of the raw materials was added as a catalyst. The mass ratio of platinum to rhodium in the platinum-rhodium metal mixture was 3:7. The mixture was continued to be mixed until uniform to obtain composite powder. The composite powder was placed in the crucible of a high-temperature reaction device, and argon gas was introduced into the reaction chamber and maintained at the argon atmosphere until the reaction was completed. The temperature was increased to 2200℃ at a heating rate of 20℃ / min and held at this temperature for 16 h. After the reaction was completed, the mixture was naturally cooled to room temperature under the protection of argon atmosphere. The product was taken out and, after crushing, sieving, and impurity removal, worm-like / whisker-like composite β-SiC microwave absorbing material with a particle size of 5-10 μm was obtained.

[0106] Based on the gradient distribution of raw material components in each layer of the coating, accurately calculate and weigh the raw materials in different proportions for each group:

[0107] Layer 1 (near substrate): 55% aluminum-zirconium eutectic, 30% SiC microwave absorbing material, 10% boron nitride, and 5% silicon dioxide;

[0108] Layer 2: 53% aluminum-zirconium eutectic, 32% SiC microwave absorbing material, 10% boron nitride, and 5% silicon dioxide;

[0109] Layer 3: 50% aluminum-zirconium eutectic, 34% SiC microwave absorbing material, 10% boron nitride, and 6% silicon dioxide;

[0110] 4th layer: 48% aluminum-zirconium eutectic, 36% SiC microwave absorbing material, 10% boron nitride, and 6% silicon dioxide;

[0111] 5th layer: 45% aluminum-zirconium eutectic, 38% SiC microwave absorbing material, 15% boron nitride, and 2% silicon dioxide;

[0112] 6th layer: 43% aluminum-zirconium eutectic, 40% SiC microwave absorbing material, 15% boron nitride, and 2% silicon dioxide;

[0113] 7th layer: 40% aluminum-zirconium eutectic, 42% SiC microwave absorbing material, 15% boron nitride, and 3% silicon dioxide;

[0114] 8th layer: 38% aluminum-zirconium eutectic, 43% SiC microwave absorbing material, 15% boron nitride, and 4% silicon dioxide;

[0115] 9th layer: 35% aluminum-zirconium eutectic, 45% SiC microwave absorbing material, 15% boron nitride, and 5% silicon dioxide;

[0116] Layer 10: 32% aluminum-zirconium eutectic, 46% SiC microwave absorbing material, 20% boron nitride, and 2% silicon dioxide;

[0117] 11th layer: 28% aluminum-zirconium eutectic, 48% SiC microwave absorbing material, 20% boron nitride, and 4% silicon dioxide;

[0118] 12th layer (surface layer): 25% aluminum-zirconium eutectic, 50% SiC microwave absorbing material, 20% boron nitride, and 5% silicon dioxide;

[0119] The binder is a mixture of silica sol, ammonium chloride coagulant and silica nitride in a mass ratio of 8:1:1; the amount of binder added to each layer is 30% of the total weight of the raw materials of each layer.

[0120] Step 2, Slurry preparation:

[0121] Each group of raw materials is mixed with a binder and then ball-milled or high-speed dispersed to prepare multiple uniform and stable coating slurries with a viscosity of 2000 mPa·s.

[0122] Step 3: Substrate Pretreatment

[0123] Using 316 stainless steel sheet as the base, surface treatment is carried out, including cleaning, degreasing, and sandblasting roughening.

[0124] Step 4: Layer-by-layer coating and stepped curing:

[0125] S1. Apply the first layer of slurry to the pretreated substrate surface;

[0126] S2. Let the coated workpiece stand at room temperature for 24 hours until the coating surface is dry.

[0127] S3. Place in an oven and dry at 110℃ for 2 hours to further remove moisture and some volatile organic compounds;

[0128] S4. Heat to 200℃ and keep warm for 30 minutes to solidify, gelling and generating a certain strength;

[0129] S5. Apply a second layer of slurry to the surface of the first cured layer;

[0130] S6. Repeat steps S2-S4 for room temperature drying, heating drying, and curing.

[0131] S7. Apply the 3rd layer, 4th layer, and so on until the surface layer. Before applying each layer, ensure that the previous layer has been cured. The composition ratio of the coating material changes gradually in each layer to form a gradient distribution.

[0132] In this embodiment, the coating consists of 12 layers. Performance tests were conducted on the coating, and the total coating thickness was measured to be 2.0 mm. The cross-cut adhesion test showed a grade 1 coating bond. Thermal shock resistance: The coating sample was held in a 1000℃ muffle furnace for 10 minutes, then immediately quenched in 20℃ water. After 50 cycles, the coating showed no cracking or peeling. Microwave absorption performance: The bow-shaped method was used for testing. Figure 1 As shown, within the 2-18GHz range, the bandwidth with reflection loss less than -10dB exceeds 3GHz, and the minimum reflection loss reaches -32dB. High-temperature resistance: after exposure to static air at 1000℃ for 100 hours, a dense oxide layer forms on the coating surface without blistering or peeling, and the absorption performance decreases by less than 15%. The coating prepared in this embodiment exhibits high-temperature resistance, thermal shock resistance, strong adhesion, weather resistance, and excellent absorption performance.

[0133] Comparative example:

[0134] A double-layer coating of equal thickness was prepared: the bottom layer had a high metal content (Al-Zr 60%, SiC fiber 20%, BN 20%), and the top layer had a high SiC fiber content (Al-Zr 20%, SiC fiber 60%, BN 20%). Other processes were the same as in Example 3, and the total coating thickness was 2.3 mm. Figure 2 As shown, its minimum reflection loss is -20dB, its absorption bandwidth is significantly narrower than that of the gradient coating in Example 3, and cracks appear between the layers after thermal shock testing.

[0135] This invention, through precise component gradient design and a unique layer-by-layer curing process, successfully prepared an ultrathin, lightweight impedance gradient composite coating with excellent microwave absorption properties, high temperature resistance, thermal shock resistance, and strong adhesion. Figure 3 As shown, the coefficient of thermal expansion of the coating changes continuously in a gradient from the bottom layer to the surface layer of the near-metal substrate, solving the key technical problem of multifunctional protection and stealth integration of high-temperature component surfaces.

[0136] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the technical solution of the present invention.

Claims

1. A composite coating with dual gradients of impedance and coefficient of thermal expansion, characterized in that, By mass fraction, the raw material composition includes: 25-55% aluminum-zirconium eutectic, 30-50% silicon carbide microwave absorbing material, 10-20% boron nitride, with the balance being silicon dioxide to make up 100%, plus 20-40% binder by the total weight of the raw material; The coating composition exhibits a gradient change along the thickness direction, specifically as follows: from the first layer to the outermost layer on the substrate surface, the aluminum-zirconium eutectic decreases from 55% to 25% by mass fraction; while the silicon carbide microwave absorbing material increases from 30% to 50%. Apply and cure at least three coating layers sequentially from the bottom layer of the substrate to the top layer away from the substrate, with each layer having a thickness of 0.05-0.2 mm and a total thickness of ≤2.5 mm. The aluminum-zirconium eutectic is composed of Al2O3 and ZrO2, and the silicon carbide microwave absorbing material is a worm-like and whisker-like composite β-SiC nanomaterial.

2. The composite coating with dual gradient of impedance and thermal expansion coefficient according to claim 1, characterized in that: The binder is a mixture of silica sol, ammonium chloride coagulant and silica nitride in a mass ratio of (7-9):(0.5-1.5):(0.5-1.5).

3. The composite coating with dual gradient of impedance and coefficient of thermal expansion according to claim 2, characterized in that: The silica sol contains >30% SiO2 solid solution; the silica nitride sol contains >20% Si3N4 solid solution, with an average particle size <100nm and a pH value of 9-11; the concentration of ammonium chloride is 0.8wt%.

4. The composite coating with dual gradient of impedance and coefficient of thermal expansion according to claim 1, characterized in that: The aluminum-zirconium eutectic contains 80 wt% Al2O3 and 20 wt% ZrO2; the particle size of the aluminum-zirconium eutectic is 0.5-1 μm.

5. The composite coating with dual gradient of impedance and coefficient of thermal expansion according to claim 1, characterized in that: The silicon carbide microwave absorbing material is a worm-like and whisker-like composite β-SiC nanomaterial obtained by sintering a mixture of nano-sized amorphous silica and nano-carbon black at a mass ratio of 1:3, with an additional 1%-3% platinum-rhodium metal mixture as a catalyst, under the protection of inert argon gas at 2200℃; the mass ratio of platinum to rhodium in the platinum-rhodium metal mixture is 3:7-7:

3.

6. The composite coating with dual gradient of impedance and coefficient of thermal expansion according to claim 5, characterized in that: Amorphous silica is nanoscale with a particle size ≤100nm and a purity of 99.99%; carbon black is nanoscale with a particle size ≤80nm and a purity of 99.99%; silicon carbide microwave absorbing material has a particle size of 5-10μm.

7. The composite coating with dual gradient of impedance and coefficient of thermal expansion according to claim 1, characterized in that: Boron nitride has a particle size of 1-3 μm; silicon dioxide has a particle size of 1-3 μm.

8. A method for preparing a dual-gradient composite coating of impedance and thermal expansion coefficient as described in claim 1, characterized in that, Includes the following steps: Step 1: Raw material preparation: Based on the gradient distribution of raw material components in each layer of the coating, accurately calculate and weigh multiple sets of raw materials with different proportions; each set of raw materials includes: silicon carbide microwave absorbing material, boron nitride, silicon dioxide, and aluminum-zirconium eutectic; the binder is a mixture of silica sol, ammonium chloride coagulant and silica sol. Step 2, Slurry preparation: Mix each group of raw materials with the binder separately, and then ball mill or high-speed disperse them to prepare uniform and stable coating slurries with a viscosity of 500-5000 mPa·s. Step 3, Substrate Pretreatment: Surface treatment of the metal or ceramic substrate material, including cleaning, degreasing, and sandblasting roughening, to improve adhesion; Step 4: Layer-by-layer coating and stepped curing: S1. Apply the first layer of slurry to the pretreated substrate surface; S2. Place the coated workpiece at room temperature and allow the coating surface to dry for 24 hours. S3. Place in an oven and dry at 100-120℃ for 2-6 hours to further remove moisture and some volatile organic compounds; S4. Heat to 180-220℃ and cure for 20-40 minutes to allow the silica sol and silica nitride sol to initially gel and develop a certain strength. S5. Apply a second layer of slurry to the surface of the first cured layer; S6. Repeat steps S2-S4 for room temperature drying, heating drying and curing. S7. Apply the third layer, the fourth layer, and so on until the surface layer. Before applying each layer, ensure that the previous layer has been cured. The composition ratio of the coating material changes gradually in each layer to form a gradient distribution.

9. The method for preparing a dual-gradient composite coating of impedance and thermal expansion coefficient according to claim 8, characterized in that: The preparation of silicon carbide microwave absorbing materials includes the following steps: Step 1: Preparation of composite powder: According to the mass ratio, calculate and weigh the required amorphous silica and carbon black respectively. Place the weighed amorphous silica and carbon black in a mixer and mix them thoroughly at 80-120 r / min for 4-6 hours. Then add 1%-3% of the total weight of the raw materials as a catalyst and continue mixing until uniform to obtain composite powder. Step 2: High-temperature sintering: The composite powder is placed in the crucible of the high-temperature reaction device, argon gas is introduced into the reaction chamber and the argon atmosphere is maintained until the reaction is completed. The temperature is increased to 2200℃ at a rate of 10-20℃ / min and held at this temperature for 12-16 hours. Step 3, Post-processing: After the reaction was completed, the product was naturally cooled to room temperature under an argon atmosphere. The product was then removed, crushed, sieved, and impurity removed to obtain worm-like / whisker-like composite β-SiC nanomaterials, i.e. silicon carbide microwave absorbing materials.

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