A method for preparing BN coating on ceramic fiber surface

Abstract

The present invention discloses a method for preparing a BN coating on the surface of a ceramic fiber, comprising the following steps: 1. modifying chemical groups on the ceramic fiber surface; 2. preparing an in-situ reaction impregnation solution for a boron nitride precursor; and 3. rapid wet chemical deposition of the BN coating. The present invention enables the safe and rapid mass deposition of high-quality BN coatings under conventional conditions. The prepared BN coatings are dense, complete, and firmly bonded to the ceramic fibers, resolving the issues currently faced by BN coatings prepared on ceramic fibers, such as low quality, numerous pores, and prone to cracking and delamination. Furthermore, this technology simplifies the preparation process for the BN coating, simplifies process parameter control, and avoids multi-step hazardous atmosphere heat treatment, laying the foundation for the industrialized production of high-quality, continuous BN coatings.

Description

Technical Field

[0001] The present application relates to the technical field of composite materials, and in particular to a method for preparing a BN coating on the surface of a ceramic fiber. Background Art

[0002] Boron nitride (BN) exhibits excellent oxidation resistance, thermal shock resistance, and chemical stability. Research has shown that BN coatings are the preferred interface material for fiber-reinforced ceramic matrix composites (CMCs), improving the thermal stability and mechanical properties of the composites. Furthermore, BN is a good insulator with a low dielectric constant, making its use as an interface material potentially beneficial for optimizing the overall electromagnetic properties of the material. BN has a hexagonal lamellar structure similar to graphite, with density, structure, and chemical properties similar to those of carbon fibers, and a thermal expansion coefficient matching that of SiC. Under high temperature conditions, BN exhibits a certain self-healing ability. Its oxidation product, B2O3, and SiC's oxidation product, SiO2, rapidly form glassy borosilicates. These fluid borosilicates can heal pores and cracks in the coating or matrix, further improving the material's strain tolerance and lifetime. Therefore, the preparation of BN coatings on various fibers is of great significance.

[0003] Among the many methods for preparing BN interface coatings, the coating obtained by chemical vapor deposition (CVD) is uniform and dense, has good continuity, and is tightly bonded to the substrate, with stable quality. However, in the preparation process of CVD-BN interface coatings, due to the rapid reaction rate and complexity of the boron source and nitrogen source during high-temperature deposition, its deposition mechanism cannot be accurately analyzed, resulting in the deposition process being difficult to control. In addition, the CVD process is unstable, the deposition rate is slow, and it is impossible to prepare in large quantities, which is the main problem that currently exists. On the other hand, after obtaining a stable CVD-BN preparation process, the low crystallinity of the obtained BN coating is another problem that limits the application of this process. The impregnation pyrolysis method is simple in process, low in cost, and suitable for the large-scale preparation of interface coatings. It is the preparation method of BN interface coatings with the greatest potential for promotion and application. However, the continuity, uniformity and bonding of the coating prepared by this method with the substrate need to be further improved.

[0004] At present, the precursors used in the dip coating process to prepare BN materials on the fiber surface mainly include binary precursors (H3BO3 and CO(NH2)2), polyborazine and polyborazine. Binary precursors (H3BO3 and CO(NH2)2) are easy to obtain and non-toxic, but due to the influence of the raw material ratio and solution fluidity, the prepared coating thickness is often uneven, the coating quality is not high, and the coating is easy to peel off. Polyborazine is active, flammable and explosive, which is not conducive to industrial production and application. Because polyborazine precursors contain more impurities, they need special atmosphere heat treatment, the ceramic yield is low, and the prepared BN coating is not dense, incomplete and easy to fall off. In summary, the precursors used in the current dip coating process mainly have the disadvantages of low ceramic yield, complex heat treatment process, and unstable properties.

[0005] Therefore, it is of great significance to explore the process of preparing continuous, uniform, high-quality BN coatings in large quantities on an industrial scale under safe conditions. Summary of the Invention

[0006] The present invention provides a method for preparing a BN coating on the surface of a ceramic fiber, which is used to solve the problems that the BN coating currently prepared on ceramic fibers is of low quality, has many holes and is easy to crack and peel; and the precursors used in the coating preparation process are flammable and explosive, have poor stability and low safety.

[0007] To achieve the above object, the present invention provides a method for preparing a BN coating on the surface of a ceramic fiber, comprising the following steps:

[0008] S1, immersing the ceramic fiber in a nitric acid solution and heating it under reflux, then immersing the ceramic fiber in an organic solvent, and adding thionyl chloride dropwise under an inert atmosphere for further reaction. After the reaction is completed, the ceramic fiber is removed and dried to obtain a ceramic fiber with surface chemical group modification;

[0009] S2, dissolving a polyborazine precursor PABB in a solvent to prepare a PABB precursor impregnation solution;

[0010] S3. The ceramic fiber with surface chemical group modification in step S1 is ultrasonically treated in the PABB precursor impregnation solution in step S2, and then vacuum dried at room temperature, and pre-calcined, solidified, and pyrolyzed to ceramicize to obtain a BN coating.

[0011] The present invention is more advanced than the prior art in that:

[0012] 1. This method first modifies the ceramic fiber, introduces hydroxyl and carboxyl active groups on the fiber surface through the organic solvent of nitric acid, and converts the active groups into acyl chloride groups by adding dichloride. The purpose is to obtain functional groups that can react with the precursor in the impregnation solution, so that the precursor molecules are evenly coated on the fiber surface through chemical reaction and form chemical bonds, making the impregnation effect better than the pure physical coating effect.

[0013] 2. This method requires only the addition of thionyl chloride under an inert atmosphere; the remaining steps are not required. This means that BN coatings can be safely and rapidly deposited in large quantities under conventional conditions. The resulting BN coatings are dense, pore-free, and have a complete morphology. They adhere tightly to the fibers and are not easily detached. Furthermore, this process allows for controllable BN coating thickness.

[0014] 3. The BN coating preparation technology provided by the present invention has the advantages of high safety and stability, high energy utilization efficiency, low equipment requirements, and low environmental pollution. It simplifies the preparation process of the BN coating, does not require strict process parameter control, and does not require multi-step harmful atmosphere heat treatment, and is suitable for industrial mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0016] Figure 1 This is the SEM of the BN coating obtained in Example 1 of the present invention;

[0017] Figure 2 This is a statistical chart of the thickness of the BN coating obtained in Example 1 of the present invention;

[0018] Figure 3 This is the XRD spectrum of the BN coating obtained in Example 1 of the present invention;

[0019] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0021] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in this field can implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0022] A method for preparing a BN coating on a ceramic fiber surface comprises the following steps:

[0023] S1. Immersing the ceramic fiber in a nitric acid solution and heating it under reflux, then immersing the ceramic fiber in an organic solvent, and adding thionyl chloride dropwise under an inert atmosphere for further reaction. After the reaction is completed, taking out the ceramic fiber and drying it to obtain a ceramic fiber with surface chemical group modification.

[0024] Among them, a rotary evaporator can be used for drying.

[0025] S2. Dissolving a polyborazine precursor PABB in a solvent to prepare a PABB precursor impregnation solution.

[0026] PABB refers to polyaminoborazine, which can be prepared using the preparation method disclosed in Patent No. CN115477309B. It is a water-soluble polyborazine precursor with high ceramic yield and stable properties. Its structural formula is:

[0027]

[0028] According to its structural formula, PABB does not contain carbon, so it has a higher ceramic yield, and PABB contains sp 3 The hybrid boron-nitrogen bridge structure provides a six-membered ring and more shared electrons, which stabilizes the electron-deficient boron-nitrogen six-membered ring and can improve the precursor's resistance to water and oxygen.

[0029] After PABB is dissolved in the solvent, ultrasonic treatment can be performed to uniformly disperse and dissolve the PABB, thereby obtaining a colorless and transparent / white and translucent in-situ reaction impregnation solution.

[0030] S3. The ceramic fiber with surface chemical group modification in step S1 is ultrasonically treated in the PABB precursor impregnation solution in step S2, and then vacuum dried at room temperature, and pre-calcined, solidified, and pyrolyzed to ceramicize to obtain a BN coating.

[0031] The purpose of ultrasonic treatment is to fully impregnate the ceramic fiber. The fiber can be repeatedly impregnated to obtain a high-quality BN coating with adjustable thickness.

[0032] Preferably, in step S1, the concentration of the nitric acid solution is 5-50 wt%. By controlling the concentration of nitric acid, the number of active groups grafted onto the fiber surface is regulated. Different types of fibers have different reactivity with nitric acid solutions, so the concentration of nitric acid needs to be adjusted to graft an appropriate number of active groups and control the number of precursor molecules chemically bonded to the fiber surface.

[0033] Preferably, in step S1, the reflux temperature is 30-150°C for 1-24 hours. By controlling the temperature and time range, efficient grafting of an appropriate amount of active groups can be achieved in the shortest possible time. Excessive reaction temperatures and long times will result in excessive introduction of active groups, causing the fiber surface to be coated with too many precursor molecules. Conversely, insufficient introduction of active groups will result in less precursor coating on the fiber surface.

[0034] Preferably, in step S1, the organic solvent is one or more of benzene, toluene, xylene, chloroform, and carbon tetrachloride.

[0035] Preferably, in step S1, the amount of thionyl chloride added does not exceed 5% of the mass of the ceramic fiber. By controlling the addition ratio of thionyl chloride, the type of grafted active groups can be adjusted. By reacting different active groups with different positions of the precursor molecule, the type of chemical bonding between the fiber and the precursor can be controlled.

[0036] Preferably, in step S1, the ceramic fiber includes one or more of SiC fiber, SiBN fiber, carbon fiber and Al2O3 fiber.

[0037] Preferably, in step S2, the solvent is a mixed solvent of ethanol and deionized water; the volume ratio of ethanol to deionized water is (10:1)-(1:10).

[0038] Preferably, in step S2, the mass fraction of the PABB precursor in the PABB precursor impregnation solution is 1-15 wt %. By controlling the mass fraction of the PABB precursor impregnation solution, the coating thickness can be regulated, and a high-thickness coating can be prepared with as few repeated impregnations as possible.

[0039] Preferably, in step S3, the pre-calcination temperature is 150-450°C for 1-60 seconds, and the pyrolysis-ceramicization temperature is 1000-1600°C for 1-60 seconds. Controlling the pre-calcination temperature and time primarily achieves the dehydrogenation and inorganicization of the precursor molecules, achieving densification and integrity of the coating through the coupling of temperature and time. Controlling the pyrolysis-ceramicization temperature and time primarily achieves close integration of the crystallization of the BN coating and the fibers. Excessively high temperatures and times can easily cause cracking and incompleteness of the coating, while excessively low temperatures and times can lead to poor crystallinity of the coating.

[0040] Preferably, in step S3, the number of dipping times is 1-5 times.

[0041] Example 1

[0042] 1. Chemical group modification of ceramic fiber surface:

[0043] First, SiC fibers were immersed in a nitric acid solution with a certain concentration of 10wt%. After heating and refluxing at 100°C for 5 hours, the fibers were immersed in solvent benzene. Then, dichlorothionyl was added dropwise in an inert atmosphere according to one thousandth of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0044] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0045] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 1:1 to prepare a precursor impregnation solution with a concentration of 5 wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a colorless and transparent in-situ reaction impregnation solution.

[0046] 3. Rapid wet chemical deposition of BN coating:

[0047] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to ensure full impregnation of the fiber. The fiber was then vacuum-dried at room temperature, pre-calcined and cured at 250°C, and then pyrolyzed and ceramicized at 1400°C. After one impregnation, the fiber obtained a high-quality, uniform BN coating with a thickness of 0.4 μm and a standard deviation of 0.2 μm, without any cracking or shedding.

[0048] Figure 1 This is the SEM of the BN coating obtained in this example. As can be seen from the figure, the coating is relatively dense and complete without cracks, and is tightly bonded to the SiC fiber.

[0049] Figure 2 Statistical graph of the thickness of the BN coating obtained in this example. As can be seen from the graph, the coating thickness is relatively uniform, with an average thickness of 0.4 μm and a standard deviation of 0.2 μm.

[0050] Figure 3 This is the XRD spectrum of the BN coating obtained in this example. It can be seen from the figure that the coating structure is hexagonal BN.

[0051] Example 2

[0052] 1. Chemical group modification of ceramic fiber surface:

[0053] First, SiBN fibers were immersed in a nitric acid solution with a certain concentration of 5wt%. After heating and refluxing at 30°C for 24 hours, the fibers were immersed in a solvent chloroform. Then, under an inert atmosphere, dichlorothionyl was added dropwise at a ratio of 0.5% of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0054] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0055] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 10:1 to prepare a precursor impregnation solution with a concentration of 2 wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a colorless and transparent in-situ reaction impregnation solution.

[0056] 3. Rapid wet chemical deposition of BN coating:

[0057] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to ensure full impregnation of the fiber. The fiber was then vacuum-dried at room temperature and pre-calcined at 150°C for curing, followed by pyrolysis and ceramicization at 1600°C. After five impregnations, the fiber obtained a high-quality, uniform BN coating with a thickness of 0.8 μm and a standard deviation of 0.2 μm, without any cracking or shedding.

[0058] Example 3

[0059] 1. Chemical group modification of ceramic fiber surface:

[0060] First, SiBN fibers were immersed in a nitric acid solution with a certain concentration of 5wt%. After heating and refluxing at 30°C for 24 hours, the fibers were immersed in a solvent chloroform. Then, under an inert atmosphere, dichlorothionyl was added dropwise at a ratio of 0.5% of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0061] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0062] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 10:1 to prepare a precursor impregnation solution with a concentration of 2 wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a colorless and transparent in-situ reaction impregnation solution.

[0063] 3. Rapid wet chemical deposition of BN coating:

[0064] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to ensure full impregnation of the fiber. The fiber was then vacuum-dried at room temperature and pre-calcined at 150°C for curing, followed by pyrolysis and ceramicization at 1600°C. After five impregnations, the fiber obtained a high-quality, uniform BN coating with a thickness of 0.8 μm and a standard deviation of 0.2 μm, without any cracking or shedding.

[0065] Example 4

[0066] 1. Chemical group modification of ceramic fiber surface:

[0067] First, Al2O3 fibers were immersed in a nitric acid solution with a certain concentration of 20wt%. After heating and refluxing at 120℃ for 15h, the fibers were immersed in solvent toluene. Then, dichlorothionyl was added dropwise in an inert atmosphere according to 5% of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0068] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0069] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 1:1 to prepare a precursor impregnation solution with a concentration of 15 wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a white translucent in-situ reaction impregnation solution.

[0070] 3. Rapid wet chemical deposition of BN coating:

[0071] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to ensure full impregnation of the fiber. The fiber was then vacuum-dried at room temperature and pre-calcined at 450°C for curing and then pyrolyzed and ceramicized at 1000°C. After one impregnation, the fiber obtained a high-quality, uniform BN coating with a thickness of 1.2 μm and a standard deviation of 0.2 μm, without any cracking or shedding.

[0072] Example 5

[0073] 1. Chemical group modification of ceramic fiber surface:

[0074] First, SiC fibers were immersed in a nitric acid solution with a certain concentration of 40wt%. After heating and refluxing at 50°C for 15 hours, the fibers were immersed in solvent benzene. Then, dichlorothionyl was added dropwise in an inert atmosphere according to 2% of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0075] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0076] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 1:2 to prepare a precursor impregnation solution with a concentration of 2.5wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a colorless and transparent in-situ reaction impregnation solution.

[0077] 3. Rapid wet chemical deposition of BN coating:

[0078] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to ensure full impregnation of the fiber. The fiber was then vacuum-dried at room temperature and pre-calcined at 200°C for curing, followed by pyrolysis and ceramicization at 1500°C. After four impregnations, a high-quality, uniform BN coating with a thickness of 1.0 μm and a standard deviation of 0.2 μm was obtained, without any cracking or shedding.

[0079] Example 6

[0080] 1. Chemical group modification of ceramic fiber surface:

[0081] First, SiBN fibers were immersed in a nitric acid solution with a certain concentration of 15wt%. After heating and refluxing at 75°C for 6 hours, the fibers were immersed in solvent benzene. Then, under an inert atmosphere, thionyl chloride was added dropwise according to 0.3% of the fiber mass for further reaction. The fibers were then dried using a rotary evaporator to obtain ceramic fibers with surface chemical group modification.

[0082] 2. Preparation of boron nitride precursor in-situ reaction impregnation solution:

[0083] The water-soluble polyborazine precursor PABB was dissolved in a mixed solvent of ethanol and deionized water in a ratio of 3:1 to prepare a precursor impregnation solution with a concentration of 1.5wt%. Ultrasonic treatment was performed to uniformly disperse and dissolve the precursor to obtain a colorless and transparent in-situ reaction impregnation solution.

[0084] 3. Rapid wet chemical deposition of BN coating:

[0085] The modified ceramic fiber obtained in step S1 was ultrasonically treated in the in-situ reaction impregnation solution obtained in step S2 to fully impregnate the fiber. The fiber was then vacuum-dried at room temperature, pre-calcined and cured at 350°C, and then pyrolyzed and ceramicized at 1300°C. After one impregnation, a high-quality, uniform BN coating with a thickness of 0.2 μm and a standard deviation of 0.1 μm was obtained. The coating was free of cracking or shedding.

[0086] While various embodiments of the present invention have been described above, the foregoing description is intended to be illustrative, non-exhaustive, and not limiting of the disclosed embodiments. Numerous modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. Therefore, the scope of the present invention shall be determined by the scope of the claims.

Claims

1. A method for preparing a BN coating on a ceramic fiber surface, characterized in that: The following steps are involved: S1, immersing the ceramic fiber in a nitric acid solution and heating it under reflux, wherein the concentration of the nitric acid solution is 5-50 wt%, the heating reflux temperature is 30-150 ° C, and the time is 1-24 h, then immersing the ceramic fiber in an organic solvent, and adding thionyl chloride dropwise under an inert atmosphere for further reaction, wherein the amount of thionyl chloride added does not exceed 5% of the mass of the ceramic fiber. After the reaction is completed, the ceramic fiber is removed and dried to obtain a ceramic fiber with surface chemical group modification; The organic solvent is one or more of benzene, toluene, xylene, chloroform and carbon tetrachloride; The ceramic fibers include one or more of SiC fibers, SiBN fibers, carbon fibers, and Al2O3 fibers; S2. dissolving a polyborazine precursor (PABB) in a solvent, wherein the solvent is a mixed solvent of ethanol and deionized water, and the volume ratio of ethanol to deionized water is (10:1) to (1:10), to prepare a PABB precursor impregnation solution, wherein the mass fraction of the PABB precursor in the PABB precursor impregnation solution is 1-15 wt%; The structural formula of the PABB is as follows: ； S3, ultrasonically treating the ceramic fiber with surface chemical group modification in step S1 in the PABB precursor impregnation solution in step S2, and then vacuum drying at room temperature, and pre-calcining, curing, and pyrolysis-ceramicizing to obtain a BN coating; The temperature of pre-calcination and curing is 150-450°C, and the time is 1-60s; the temperature of pyrolysis and ceramicization is 1000-1600°C, and the time is 1-60s.

2. The method for preparing a BN coating on a ceramic fiber surface according to claim 1, wherein: In step S3, the number of immersions is 1-5 times.

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