Preparation Method and Application of an Alumina-Doped High-Temperature Radar Absorbing Coating

Abstract

The present invention belongs to the field of microwave absorbing materials, and particularly relates to a preparation method and application of a high-temperature radar absorbing coating doped with alumina. By doping alumina to modify CBS glass-ceramics, the present invention solves the problem of coating cracking caused by volume shrinkage due to crystallization during the heat treatment of CBS-based absorbing coatings prepared by traditional atmospheric plasma spraying, further improves the density of the coating, prevents the oxidation of Fe-based magnetic absorbents in high-temperature environments, enhances the high-temperature stability of the coating, and the alumina raw material is easily obtained; the preparation method of the present invention has simple process and is easy for industrial production, and has the advantages of lower density, wider radar absorption frequency band and strong antioxidant property compared with the same type of coatings, and can meet the stable use in an environment of 700 °C.

Description

Technical Field

[0001] The present invention belongs to the field of microwave absorbing materials, and particularly relates to a preparation method and application of an alumina-doped high-temperature radar absorbing coating. Background Art

[0002] With the rapid development of radar detection technology, the survival of combat equipment and personnel faces severe challenges. Therefore, improving the radar absorbing performance of aircraft, especially the radar absorbing performance of high-temperature components at temperatures of 700 °C and above, has become the key to addressing this problem. For aircraft components with specific shapes, surface modification of alloy substrates through cold / hot coating (spraying) technology can significantly improve their radar absorbing effect without affecting aerodynamic performance. Traditional radar absorbing coatings use organic materials (such as resins, silicones, etc.) as the coating matrix material and magnetic alloys (such as Fe-based magnetic alloys) as the filler material. By adjusting the ratio of the two and the preparation process, a dense coating can be formed at room temperature, showing excellent radar absorbing performance and remaining stable below 350 °C. However, in a high-temperature environment, the organic materials will undergo carbonization and cracking, and at the same time, due to the mismatch of thermal expansion coefficients between the coating and the substrate, thermal stress will accumulate and cause cracking and failure, making it difficult to meet current usage requirements.

[0003] Using high-temperature ceramic materials such as Al2O3, Al2O3-TiO2, and YSZ as the matrix material of the radar absorbing coating and high-temperature resistant Fe-based magnetic alloys as the filler material, and making them completely melt through thermal spraying technology and form a metallurgical bond with the alloy substrate can significantly improve the bonding strength of the coating. However, these high-temperature ceramic materials usually have a large dielectric constant and a high density, which is not conducive to the lightweight and broadband design of the coating. In addition, the layered structure formed by thermal spraying has natural oxygen channels that can allow oxygen to enter the interior of the coating, and Fe-based magnetic alloys are extremely prone to oxidation and corrosion in a high-temperature environment, reducing the high-temperature stability of the coating.

[0004] In contrast, CaO-B2O3-SiO2 (CBS) glass-ceramics are regarded as ideal high-temperature matrix materials due to their low density, thermal conductivity, and dielectric constant. However, due to the rapid cooling and heating effects during the thermal spraying process, the CBS phase in the coating is usually an amorphous phase and a porous structure. The coating can be softened and the pores can be filled during heat treatment at 800 °C (and above) to make the coating denser, but the high temperature enables the internal energy of the coating to reach the crystallization activation energy, resulting in the precipitation of β-CaSiO3 grains. During the crystallization process, the glass phase will undergo volume shrinkage, forming tensile stress between the coating and the alloy substrate, leading to cracking and failure of the coating. Therefore, there is an urgent need to develop a new method to overcome the cracking problem of CBS-based radar absorbing coatings during heat treatment and prepare a high-temperature radar absorbing coating with higher densification that can be stably used in a 700 °C high-temperature environment. Summary of the Invention

[0005] Aiming at the above problems or deficiencies, in order to solve the cracking failure problem caused by volume shrinkage during the heat treatment of the current CaO-B2O3-SiO2 (CBS)-based composite absorbing coating, the present invention provides a preparation method and application of a high-temperature resistant radar absorbing coating doped with alumina, which improves the density and high-temperature stability of the coating, and provides a feasible technical approach for the lightweight and broadband design of high-temperature absorbing coatings at 700 °C.

[0006] A preparation method of a high-temperature resistant absorbing coating doped with alumina includes the following steps:

[0007] Step 1, preparation of alumina-CBS glass-ceramic precursor.

[0008] Mix alumina and CBS glass-ceramic in proportion to obtain an alumina-CBS glass-ceramic precursor powder with a particle size ≤ 2 μm; wherein the doping amount of alumina accounts for 5-10 wt.% of the total mass of alumina and CBS glass-ceramic.

[0009] Step 2, preparation of magnetic absorber-alumina-CBS glass-ceramic slurry.

[0010] Mix the alumina-CBS glass-ceramic precursor obtained in Step 1, magnetic absorber, polyethyleneimine (PEI) and solvent to obtain a slurry for spray granulation, and the solid content of the slurry is 30-50 wt.%.

[0011] The magnetic absorber accounts for 10-25% of the total volume of the solid, and PEI accounts for 1-5 wt.% of the total mass of the solid. The solid refers to the alumina-CBS glass-ceramic precursor and the magnetic absorber; the magnetic absorber is a high-temperature resistant magnetic absorber (such as Fe-based), with a particle size less than 5 μm and a Curie temperature greater than 700 °C.

[0012] Step 3, preparation of magnetic absorber-alumina-CBS glass-ceramic composite powder.

[0013] Perform spray granulation, drying and screening treatment on the slurry prepared in Step 2 to obtain a magnetic absorber-alumina-CBS glass-ceramic composite powder.

[0014] Step 4, preparation of magnetic absorber-alumina-CBS glass-ceramic as-sprayed composite coating.

[0015] Pre-treat and clean the surface of the target substrate. After preparing an adhesive layer on the surface of the target substrate by using atmospheric plasma spraying technology, then spray the magnetic absorber-alumina-CBS glass-ceramic composite powder prepared in Step 3 to obtain the corresponding as-sprayed composite coating.

[0016] Step 5: Preparation of magnetic absorbent-alumina-CBS glass-ceramic composite coating.

[0017] The magnetic absorbent-alumina-CBS glass-ceramic spray composite coating obtained in Step 4 is subjected to two-step heat treatment to obtain the magnetic absorbent-alumina-CBS glass-ceramic composite coating.

[0018] Furthermore, the mixing method in Step 1 is wet ball milling, which makes the mixing effect of the alumina-glass-ceramic precursor better. Among them, the solid content of the slurry is 30-50 wt.%; zirconia grinding balls with a diameter of 2-5 mm are used, and the ball-to-material ratio is 2:1; the rotation speed of the ball mill is 350-450 RPM, and the ball milling mixing time is 24-48 h; the ball milled slurry is dried at 100-120 °C in the atmosphere.

[0019] Furthermore, the crystal structure of alumina in Step 1 is α-Al2O3; the glass-ceramic is CaO-B2O3-SiO2 glass-ceramic with a density less than 2.3 g / cm 3 , a dielectric constant of 5-6, and an initial melting temperature greater than 800 °C.

[0020] Furthermore, the mixing method in Step 2 uses wet ball milling with a horizontal ball mill; the horizontal ball mill is selected because its kinetic energy is low and it will not generate strong external forces on the shape of the magnetic absorbent to make it flaky, while flaky powders will not be very suitable for the subsequent processing technology. For example, plasma spraying curing will cause the flaky magnetic absorbents to be arranged orderly to form relatively high electrical connectivity, and the relatively high electrical connectivity will have a negative impact on the magnetic wave absorption performance due to better conductive effect.

[0021] Furthermore, the solvent in Step 2 is a mixed solution of ammonia water and deionized water with a pH value of 8-10 to facilitate later heating and volatilization; and since PEI will undergo chemical changes in a strong alkaline environment (pH value ≥ 11), the pH value of the solvent is controlled.

[0022] Furthermore, the specific process of spray granulation in Step 3 is as follows: the inlet air temperature of the spray granulator is 200-250 °C, the outlet air temperature is 110-130 °C, the feeding speed is 20-40 mL / min, and the atomizer frequency is 23-30 Hz; subsequent drying is carried out at 120 °C in the atmosphere, and after drying, it is sieved to obtain magnetic absorbent-alumina-CBS glass-ceramic composite powder with a particle size of 40-80 μm.

[0023] Furthermore, the fluidity of the magnetic absorbent-alumina-CBS glass-ceramic composite powder obtained in Step 3 is not less than 80 s / 50 g.

[0024] Further, the surface pretreatment of the target substrate in step 4 specifically includes using shot peening to remove rust from the target substrate, sequentially using kerosene, acetone, alcohol, and deionized water to remove oil stains from the target substrate, and finally using corundum to perform sandblasting on the target substrate to make the surface of the target substrate have a uniform roughness.

[0025] Further, the bonding layer is a NiCrAlY layer, and the atmospheric plasma spraying process parameters are: spraying distance is 120 mm, argon gas flow rate is 160 SCFH, nitrogen gas flow rate is 80 SCFH, hydrogen gas flow rate is 60 SCFH, powder feeding rate is 30 g / min; the thickness of the bonding layer is 60 - 100 μm.

[0026] Further, the atmospheric plasma spraying process of the magnetic absorber-alumina-CBS glass-ceramic sprayed composite coating in step 4 is: spraying distance is 100 - 120 mm, argon gas flow rate is 100 - 200 SCFH, nitrogen gas flow rate is 60 - 100 SCFH, hydrogen gas flow rate is 60 - 80 SCFH, powder feeding rate is 10 - 25 g / min, spraying preheating temperature is 200 °C; during the cyclic spraying process, the temperature of the current spraying surface does not exceed 450 °C, and the specific designed thickness of the wave-absorbing coating is achieved through cyclic spraying.

[0027] Further, the two-step heat treatment in step 5 is specifically as follows: the atmosphere is argon, and the gas flow rate is 10 - 30 mL / min; in the first step, it is heated to 800 - 830 °C and held for 2 - 5 h, with a heating rate of 5 - 10 °C / min; in the second step, it is heated to 900 - 930 °C and held for 0.5 - 2 h, with a heating rate of 1 - 4 °C / min.

[0028] The two-step heat treatment is to reduce the coating cracking problem caused by the thermal stress concentration caused by direct rapid heating. Among them, in the first step of heating and holding, a glassy phase will be generated, which penetrates into the pores and cracks on the surface of the bonding layer through capillary force to improve the coating bonding strength, enabling the Al 3+ ions in alumina to diffuse into the glass matrix, inhibiting the crystallization of β-CaSiO3 and promoting the formation of the liquid phase. Supplementary heating in the second step, with a relatively slow heating rate, ensures a more uniform temperature gradient between the coating and the target substrate, reduces the thermal stress concentration caused by local high temperature, further reduces the viscosity of the glassy liquid phase, enables the glassy liquid phase to infiltrate the magnetic absorber and form a dense protective film on its surface, and flows towards the low-pressure area, filling the pores inside the coating and discharging air, thereby forming a denser coating. Finally, it further avoids cracks or peeling caused by the thermal stress change between the coating and the substrate due to high temperature in the application scenario.

[0029] In the present invention, a uniformly distributed alumina-glass-ceramic precursor is obtained through step 1 to ensure that the contact surface between alumina and CBS glass-ceramic is increased in the subsequent heat treatment so that the ions at the interface diffuse more fully. Step 2 is to obtain a radar absorbing coating with excellent performance, which requires the magnetic absorbent to be uniformly distributed inside the coating. However, the density of the magnetic absorbent is relatively large, and sedimentation will occur during the slurry preparation process. In an alkaline environment, the metal oxide layer formed by the magnetic absorbent and the surface of the alumina-CBS glass-ceramic precursor can absorb [OH] - , making the particle surface electronegative, improving the dispersibility of the magnetic absorber and the alumina-CBS glass-ceramic precursor under the action of electrostatic repulsion; at the same time, PEI is a cationic dispersant, which can be adsorbed on the particle surface to form a stable adsorption layer, forming a steric hindrance, further improving the dispersibility of the magnetic absorber and the alumina-CBS glass-ceramic precursor, and improving the stability of the slurry; and, during the spray granulation process, the viscosity of PEI increases after losing water, forming a sticky film on the particle surface, which can play a role in agglomerating the powder. Finally, the final CBS glass-ceramic-based high-temperature resistant wave-absorbing coating was obtained by the synergistic effect of PEI in an alkaline environment supplemented with ammonia water and alumina and CBS glass-ceramic through a two-step heat treatment.

[0030] In the two-step heat treatment process of the present invention, the formation of the CBS phase liquid phase at high temperature is regulated by controlling the alumina doping content, thereby avoiding the cracking and failure of the coating caused by the volume shrinkage of the CBS phase during the crystallization process. 3+ Ions can diffuse into the glass matrix through the alumina / CBS interface and combine with non-bridging oxygen in the glass to form stable [AlO4] - In order to maintain the neutrality of the overall charge, the free cations in the glass matrix (such as Ca 2+ ) will participate in charge compensation, thereby inhibiting the crystallization of β-CaSiO3 and promoting the formation of liquid phase. However, due to the high melting point of alumina and Al 3+ Diffusion in the glass matrix is ​​limited, so when the content of alumina is too high, the alumina in the coating will hinder the flow of the glass liquid phase. However, at higher temperatures, the CaAl2Si2O8 phase will precipitate in the glass. Compared with β-CaSiO3, this phase has a smaller volume shrinkage and a lower dielectric constant, further optimizing the structural stability and functionality of the coating.

[0031] In summary, the present invention solves the problem of coating cracking caused by volume shrinkage during heat treatment due to crystallization in the CBS-based microwave absorbing coating prepared by traditional atmospheric plasma spraying by doping and modifying CBS glass-ceramics with alumina, further improving the coating density, preventing the oxidation of Fe-based magnetic absorbers in high-temperature environments, enhancing the high-temperature stability of the coating, and the alumina raw material is easily obtained; the preparation method of the present invention has simple process and is easy to industrialize, and has the advantages of lower density, wider radar absorption frequency band and strong oxidation resistance compared with the same type of coating, and can meet the stable use in the environment of 700 °C. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Optical photographs after heat treatment of Examples 1-4 and Comparative Examples 1-4;

[0033] Figure 2 SEM image of the magnetic absorber-alumina-CBS glass-ceramic composite coating prepared in Example 1;

[0034] Figure 3 Flat reflectivity test diagrams of the radar absorbing coatings prepared in Examples 1-4 and Comparative Example 4 at room temperature;

[0035] Figure 4 Flat reflectivity test diagrams of the radar absorbing coating prepared in Example 1 at room temperature and 700 °C;

[0036] Figure 5 Flat reflectivity test diagrams of the radar absorbing coating prepared in Example 1 after isothermal oxidation at 700 °C for 0 h, 100 h, 200 h and 300 h. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be further described in detail below through specific examples, comparative examples and drawings.

[0038] Example 1

[0039] The preparation method of the high-temperature resistant microwave absorbing coating doped with alumina in this example includes the following steps:

[0040] Step 1. Preparation of alumina-CBS glass-ceramic precursor.

[0041] (1) Weigh the raw materials according to 5 wt.% of alumina in the total mass of the alumina-CBS glass-ceramic powder;

[0042] (2) Alumina (crystal structure α-Al2O3) and CBS glass-ceramic powder (CaO-B2O3-SiO2, molar ratio of Ca:Si:B is 4:4:1.5) are added into a nylon ball mill tank. Deionized water and grinding balls (zirconia grinding balls, including 2 mm and 5 mm zirconia grinding balls) are respectively added for ball milling and mixing evenly to obtain a mixed material; among them, the mass ratio of deionized water, zirconia grinding balls to alumina-CBS glass-ceramic powder is 2:8:1, and the mass ratio of 2 mm zirconia grinding balls to 5 mm zirconia grinding balls is 2:1; the ball milling is carried out in a planetary ball mill at a rotation speed of 400 r / min for 48 h; (3) The zirconia grinding balls in the mixed material obtained by ball milling are filtered, and the remaining components of the mixed material are dried in an atmospheric environment at 100 °C to obtain an alumina-glass-ceramic precursor.

[0043] Step 2. Preparation of magnetic absorbent-alumina-CBS glass-ceramic slurry.

[0044] The alumina-CBS glass-ceramic precursor, magnetic absorbent, polyethyleneimine PEI and solvent obtained in Step 1 are mixed evenly to prepare a slurry for spray granulation, and the solid content of the slurry is 50 wt.%.

[0045] The magnetic absorbent accounts for 25 vol.% of the total solid volume, and PEI accounts for 2 wt.% of the total solid mass. The solid refers to the alumina-CBS glass-ceramic precursor and the magnetic absorbent; the magnetic absorbent is a high-temperature resistant Fe 90 Si7Cr3 magnetic absorbent, with a particle size less than 5 μm and a Curie temperature greater than 700 °C to avoid deterioration of the wave absorption performance caused by oxidation in subsequent processes.

[0046] Among them, the mass ratio of zirconia grinding balls to magnetic absorbent-alumina-CBS glass-ceramic powder is 4:1; among them, the solvent is a mixed solution of ammonia water and deionized water with a pH of 10; the ball milling is carried out in a horizontal ball mill at a rotation speed of 100 r / min for 12 h.

[0047] After filtering the zirconia grinding balls in the mixed material obtained by ball milling, magnetic absorbent-alumina-CBS glass-ceramic slurry is obtained.

[0048] Step 3. Preparation of magnetic absorbent-alumina-CBS glass-ceramic composite powder.

[0049] (1) The slurry obtained by mixing in Step 2 is subjected to spray granulation; among them, the specific spray granulation process parameters are: the inlet air temperature is 235 °C, the outlet air temperature is 115 °C, the atomizer frequency is 25 Hz, and the feeding speed is 20 mL / min.

[0050] (2) Dry the residual water vapor of the powder obtained by spray granulation in an atmospheric environment at 120 °C, and use 40 μm and 80 μm sieves to obtain a magnetic absorbent-aluminum oxide-CBS glass-ceramic composite powder with a particle size of 40-80 μm.

[0051] Step 4: Preparation of the as-sprayed composite coating of magnetic absorbent-aluminum oxide-CBS glass-ceramic.

[0052] (1) Surface pretreatment of the spraying substrate (made of 310 stainless steel): First, use shot peening to remove the surface rust layer, then successively use kerosene, acetone, alcohol, and deionized water to remove oil stains on the surface, and then use corundum to perform sandblasting on the substrate to obtain a uniform surface roughness.

[0053] (2) Prepare the bonding layer on the substrate. The bonding layer is a NiCrAlY layer. The process parameters of atmospheric plasma spraying are: spraying distance is 120 mm, argon gas flow rate is 160 SCFH, nitrogen gas flow rate is 80 SCFH, hydrogen gas flow rate is 60 SCFH, powder feeding rate is 30 g / min, and the thickness of the bonding layer is 80 μm.

[0054] (3) Use the magnetic absorbent-aluminum oxide-CBS glass-ceramic composite powder obtained in Step 3 to prepare the as-sprayed coating by atmospheric plasma spraying; among them, the specific spraying process parameters are: argon gas flow rate is 180 SCFH, nitrogen gas flow rate is 70 SCFH, argon gas flow rate is 50 SCFH, spraying power is 50 kW, spraying distance is 100 mm; among them, the spraying preheating temperature is 200 °C, the temperature on the back of the substrate during spraying does not exceed 450 °C, and the coating thickness reaches 1 mm by cyclic spraying, that is, the as-sprayed composite coating of magnetic absorbent-aluminum oxide-CBS glass-ceramic is obtained.

[0055] Step 5: Heat treatment of the magnetic absorbent-aluminum oxide-CBS glass-ceramic composite coating.

[0056] Put the as-sprayed composite coating of magnetic absorbent-aluminum oxide-CBS glass-ceramic obtained in Step 4 into a tube furnace for heat treatment: the heat treatment temperature is 800 °C, hold for 5 h, and the heating rate is 10 °C / min; then raise the temperature from 800 °C to 900 °C and hold for 1 h, and the heating rate is 2 °C / min; among them, the heat treatment atmosphere is argon, and the gas flow rate is 20 mL / min. Finally, cool it to room temperature in the furnace and take it out to obtain the magnetic absorbent-aluminum oxide-CBS glass-ceramic composite coating.

[0057] Example 2

[0058] The difference between this Example 2 and Example 1 is that the added alumina content accounts for 7 wt.% of the total mass of alumina-CBS glass-ceramic.

[0059] Example 3

[0060] The difference between Example 3 and Example 1 is only that: alumina is added, and the content of alumina accounts for 9 wt.% of the total mass of the alumina-CBS glass-ceramics.

[0061] Example 4

[0062] The difference between Example 4 and Example 1 is only that: alumina is added, and the content of alumina accounts for 10 wt.% of the total mass of the alumina-CBS glass-ceramics.

[0063] Comparative Example 1

[0064] The difference between Comparative Example 1 and Example 1 is only that: alumina is added, and the content of alumina accounts for 4 wt.% of the total mass of the alumina-CBS glass-ceramics.

[0065] Comparative Example 2

[0066] The difference between Comparative Example 2 and Example 1 is only that: alumina is added, and the content of alumina accounts for 11 wt.% of the total mass of the alumina-CBS glass-ceramics.

[0067] Comparative Example 3

[0068] The difference between Comparative Example 3 and Example 1 is only that: the heat treatment is carried out by a one-step method, the heat treatment temperature is 900 °C, the heating rate is 10 °C / min, the holding time is 1 h, the heat treatment atmosphere is argon, the gas flow rate is 20 mL / min, and finally it is cooled to room temperature in the furnace and taken out.

[0069] Comparative Example 4

[0070] The difference between Comparative Example 4 and Example 1 is only that: in Step 2, the solvent is only deionized water.

[0071] The samples of Examples 1-4 and Comparative Example 4 were respectively subjected to performance tests, including heat resistance, density, and perpendicular reflectivity. Among them, there are no test results for Comparative Examples 1-4 because the coating cracked and failed after heat treatment (see Figure 1 ), so they are not included in the calculation. The specific test characterization analysis is as follows:

[0072] The heat resistance performance was tested at 700 °C / 300 h, cooled to room temperature, and the qualified standard was that no cracking, crazing, and peeling occurred on the coating surface. The test results are shown in Table 1; the density of the coating was tested by the Archimedes method, and the test results are shown in Table 1.

[0073] Table 1:

[0074] Sample Number Temperature Resistance <![CDATA[Density (g / cm 3 )]]> Example 1 Qualified 3.79 Example 2 Qualified 3.82 Example 3 Qualified 3.84 Example 4 Qualified 3.85 Comparative Example 4 Qualified 3.73

[0075] As can be seen from Table 1, the coatings of Examples 1-4 can be used for a long time at 700 °C.

[0076] The main difference between Examples 1-4 and Comparative Examples 1-2 is the different doping contents of aluminum oxide. Figure 1 It can be seen that when the alumina content is lower than 5wt.% of the total volume of alumina-glass-ceramics, due to Al 3+ The ion content is low and the precipitation of β-CaSiO3 phase cannot be effectively suppressed, which leads to cracking of the coating. When the alumina content is lower than 10wt.% of the total volume of alumina-microcrystalline glass, due to the high melting point of alumina and effective diffusion, the alumina phase in the coating forms an obstacle to the glass liquid phase, making it impossible for the glass liquid phase to infiltrate the bonding layer and magnetic alloy, so the coating cracks.

[0077] The main difference between Example 1 and Comparative Example 3 is that different heat treatment methods are used. Figure 1 It can be seen that when the coating is rapidly heated to 900°C for heat treatment, the rapid change in temperature will cause the local stress of the coating to increase, resulting in cracking of the coating.

[0078] The main difference between Example 1 and Comparative Example 4 is the different solvent selections during the preparation of the magnetic alloy / alumina@microcrystalline glass slurry. Figure 3 As can be seen from the third column of Table 1, when the solvent is neutral, the radar absorption peak of the coating shifts to the right and the absorptivity decreases, and the density of the coating decreases, which indicates that the content of magnetic alloy in the coating is significantly reduced.

[0079] Figure 4 This is a comparison chart of the vertical reflectivity of Example 1 at room temperature and 700°C. The test results show that during the temperature change process, the vertical reflectivity changes slightly and can meet the requirements of use under high temperature.

[0080] Figure 5 This is a comparison chart of the vertical reflectivity of Example 1 after 100h, 200h and 300h. The test results show that the radar absorption effect of the coating is stable, and the vertical reflectivity changes little after serving in a long-term high-temperature environment, which meets the application requirements.

[0081] It can be seen from the above embodiments that: compared with the prior art, the alumina-doped high-temperature resistant absorbing coating used in the present invention solves the cracking problem of the CaO-B2O3-SiO2-based absorbing coating due to volume shrinkage during heat treatment. The density of the coating is improved by the heat treatment method, and the liquid phase CBS can infiltrate the adhesive layer and the magnetic alloy, thereby improving the high-temperature stability of the coating. Moreover, by adjusting the amount of alumina doping, the electromagnetic properties of the coating can be regulated within a certain range. At the same time, the method has the advantages of convenient raw material acquisition, green and environmentally friendly manufacturing process, and suitability for mass production. The above proves the feasibility of the alumina-doped high-temperature resistant absorbing coating prepared by the present invention.

Claims

1. A method for preparing an aluminum oxide-doped high temperature resistant radar absorbing coating, characterized in that: The following steps are involved: Step 1, preparing alumina-CBS glass-ceramics precursor; Alumina and CBS glass-ceramics are mixed in proportion to obtain an alumina-CBS glass-ceramics precursor powder with a particle size of ≤2 μm; wherein the amount of alumina doped accounts for 5 to 10 wt.% of the total mass of alumina and CBS glass-ceramics; Step 2, preparation of magnetic absorbent-alumina-CBS glass-ceramic slurry; The alumina-CBS glass-ceramics precursor obtained in step 1, the magnetic absorbent, polyethyleneimine PEI and the solvent are mixed to prepare a slurry for spray granulation, wherein the solid content of the slurry is 30-50wt.%; The magnetic absorbent accounts for 10-25% of the total volume of the solid, and PEI accounts for 1-5wt.% of the total mass of the solid. The solid refers to the alumina-CBS glass-ceramic precursor and the magnetic absorbent; the magnetic absorbent is a high temperature resistant magnetic absorbent, with a particle size of less than 5 μm and a Curie temperature greater than 700°C; Step 3, preparation of magnetic absorbent-alumina-CBS glass-ceramics composite powder; The slurry obtained in step 2 is spray granulated, dried, and screened to obtain a magnetic absorber-alumina-CBS glass-ceramic composite powder; Step 4, preparation of magnetic absorbent-alumina-CBS glass-ceramics sprayed composite coating; The target substrate surface is pre-treated and cleaned, and an adhesive layer is prepared on the target substrate surface by using an atmospheric plasma spraying technique, and then the magnetic absorber-alumina-CBS glass-ceramic composite powder prepared in step 3 is sprayed to prepare a corresponding sprayed composite coating; Step 5, preparation of magnetic absorber-alumina-CBS glass-ceramics composite coating; The magnetic absorber-alumina-CBS glass-ceramics sprayed composite coating obtained in step 4 is subjected to a two-step heat treatment to obtain a magnetic absorber-alumina-CBS glass-ceramics composite coating.

2. The method for preparing the aluminum oxide-doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The mixing method in step 1 is wet ball milling, so that the mixing effect of the alumina-microcrystalline glass precursor is better; wherein the solid content of the slurry is 30-50wt.%; 2-5mm zirconia grinding balls are used, and the ball-to-material ratio is 2:1; the ball mill speed is 350-450RPM, and the ball milling mixing time is 24-48h; the ball milled slurry is dried at 100-120°C in an atmospheric environment.

3. The method for preparing the aluminum oxide-doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The crystal structure of the aluminum oxide in step 1 is α-Al2O3; the glass-ceramics is CaO-B2O3-SiO2 glass-ceramics, and the density is less than 2.3 g / cm 3 , dielectric constant is 5-6, and initial melting temperature is greater than 800℃.

4. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The mixing method in step 2 is wet ball milling using a horizontal ball mill.

5. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The solvent in step 2 is a mixture of ammonia water and deionized water, and its pH value is 8-10.

6. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The specific process of spray granulation in step 3 is as follows: the air inlet temperature of the spray granulator is 200-250°C, the air outlet temperature is 110-130°C, the feed rate is 20-40mL / min, and the atomizer frequency is 23-30Hz; subsequent drying is carried out at 120°C in the atmosphere, and after drying, the magnetic absorbent-alumina-CBS microcrystalline glass composite powder with a particle size of 40-80μm is sieved.

7. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The fluidity of the magnetic absorbent-alumina-CBS glass-ceramic composite powder obtained in step 3 is not less than 80s / 50g.

8. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The bonding layer is a NiCrAlY layer, and the atmospheric plasma spraying process parameters are: spraying distance is 120mm, argon flow rate is 160SCFH, nitrogen flow rate is 80SCFH, hydrogen flow rate is 60SCFH, powder feeding rate is 30g / min; the bonding layer thickness is 60-100μm.

9. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The atmospheric plasma spraying process of the magnetic absorber-alumina-CBS microcrystalline glass sprayed composite coating in step 4 is: spraying distance 100-120mm, argon flow rate 100-200SCFH, nitrogen flow rate 60-100SCFH, hydrogen flow rate 60-80SCFH, powder feeding rate 10-25g / min, spraying preheating temperature 200°C; during the cyclic spraying process, the current spraying surface temperature does not exceed 450°C, and the specifically designed absorbing coating thickness is achieved through cyclic spraying.

10. The method for preparing the aluminum oxide doped high temperature resistant radar absorbing coating according to claim 1, characterized in that: The two-step heat treatment in step 5 is specifically as follows: the atmosphere is argon, and the gas flow rate is 10-30 mL / min; the first step is to heat up to 800-830°C, keep warm for 2-5 hours, and the heating rate is 5-10°C / min; the second step is to heat up to 900-930°C, keep warm for 0.5-2 hours, and the heating rate is 1-4°C / min.

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