A carbon-based surface high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating and a preparation method thereof

Abstract

The application discloses a carbon-based surface high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating and a preparation method thereof. The coating comprises a gradient SiC layer, a gradient SiCN layer and a gradient SiBCN layer which are sequentially formed on the surface of a carbon-based body and have continuous composition gradient, and no obvious interface exists between the layers. The gradient SiC layer is formed by depositing a Si transition layer and reacting in situ with the carbon-based body, thereby realizing chemical bonding and composition transition from the carbon-based body to SiC. The gradient SiCN layer and the gradient SiBCN layer are obtained by a chemical vapor dynamic co-deposition process, and the ratio of Si, C, N and B reaction gases is accurately controlled, so that the composition realizes continuous gradient from SiC to SiCN and from SiCN to SiBCN. The preparation method effectively alleviates the thermal expansion mismatch between the coating and the carbon-based body, greatly enhances the interface bonding strength and the integrity of the coating. The obtained coating has excellent high-temperature oxidation resistance, corrosion resistance and thermal shock resistance, and can significantly prolong the service life of the carbon-based composite material in an extreme high-temperature environment such as aerospace.

Description

Technical Field

[0001] This invention relates to the field of high-temperature protective coating technology, and in particular to a carbon-based surface high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating for use in high-temperature and high-corrosion environments such as aerospace and military industries, and its preparation method. Background Technology

[0002] Carbon-based composite materials possess high specific strength and specific modulus, and maintain high strength, frictional properties, and thermal shock resistance even at temperatures above 2000℃, making them one of the most promising high-temperature structural materials with broad application prospects in fields such as aero-engines and gas turbine blades. However, carbon-based composite materials are prone to oxidation corrosion in high-temperature oxygen-containing environments, which severely limits their direct application under harsh conditions. To improve the service life and reliability of carbon-based composite materials in high-temperature environments, applying a high-temperature protective coating to their surface is a common and effective technical approach. Through the physical barrier and chemical inertness of the coating, the contact and reaction between the matrix material and the corrosive medium can be significantly delayed or prevented, thereby achieving effective protection.

[0003] Among various coating systems, ceramic coatings have attracted much attention due to their excellent high-temperature resistance, erosion resistance, and corrosion resistance. They can remain stable in high-temperature environments above 1600℃, providing an effective antioxidant barrier for the substrate. However, due to the significant difference in thermal expansion coefficients between the carbon substrate and the ceramic coating, enormous thermal stress is generated during drastic temperature changes; furthermore, the coating and substrate often only have a mechanical bond or a weak physical bond, resulting in insufficient interfacial bonding strength. These two factors combined mean that simply applying a ceramic coating to a carbon substrate surface can easily lead to the coating peeling off from the substrate during application, thereby losing its protective function and reducing the service life of the component.

[0004] Therefore, in view of the shortcomings of the existing technology, it is necessary to design a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface and its preparation method to solve the above problems.

[0005] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solution of the present invention and for the convenience of those skilled in the art to understand it. It should not be assumed that the above content is known to those skilled in the art simply because it has been described in the background of the present invention. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the present invention aims to disclose a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface and its preparation method.

[0007] This invention discloses a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating for carbon-based surfaces, comprising a gradient SiC layer, a gradient SiCN layer, and a gradient SiBCN layer formed sequentially on the surface of a carbon substrate, with continuously varying compositions and no obvious interfaces between the layers.

[0008] The gradient SiC layer contains free C derived from the carbon matrix and SiC generated by the reaction. From the side adjacent to the carbon matrix to the gradient SiCN layer, the content of free C decreases and the content of SiC increases. The gradient SiCN layer contains SiC and SiCN. From the side adjacent to the gradient SiC layer to the gradient SiBCN layer, the content of SiC decreases and the content of SiCN increases. The gradient SiBCN layer contains SiCN and SiBCN. From the side adjacent to the gradient SiCN layer to the outside of the coating, the content of SiCN decreases and the content of SiBCN increases.

[0009] Preferred technical solution: The thickness of the gradient SiC layer is 1-10 μm; the thickness of the gradient SiCN layer is 10-100 μm; the thickness of the gradient SiBCN layer is 100-180 μm; and the total thickness of the gradient SiC layer, gradient SiCN layer and gradient SiBCN layer does not exceed 200 μm.

[0010] Preferred technical solution: In the gradient SiC layer, from the carbon matrix side to the gradient SiCN layer side, the free C content decreases from 100% to 0%, and the SiC content increases from 0% to 100%; in the gradient SiCN layer, from the adjacent gradient SiC layer side to the gradient SiBCN layer side, the SiC content decreases from 100% to 0-50%, and the SiCN content increases from 0% to 50-100%; in the gradient SiBCN layer, from the adjacent gradient SiCN layer side to the outside of the coating, the SiCN content decreases from 50-100% to 0-50%, and the SiBCN content increases from 0% to 50-100%.

[0011] Preferred technical solution: The gradient SiBCN layer also contains SiC, and the SiC content is 0-50%.

[0012] A method for preparing a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface includes the following steps:

[0013] (1) Clean the surface of the carbon matrix with a surface roughness Ra greater than 2 μm and dry it thoroughly;

[0014] (2) A Si transition layer is deposited on the surface of a carbon substrate using chemical vapor deposition; then the gas supply is stopped, allowing the silicon layer to react in situ with the carbon substrate to form a gradient SiC layer, and the excess Si is volatilized in the form of Si vapor.

[0015] (3) Using a chemical vapor phase dynamic co-deposition process, a gradient SiCN layer is deposited on the surface of the gradient SiC layer formed in step (2) by controlling the ratio of silicon, carbon and nitrogen reactive gases;

[0016] (4) Using a chemical vapor phase dynamic co-deposition process, a gradient SiBCN layer is deposited on the surface of the gradient SiCN layer formed in step (3) by controlling the proportion of silicon, boron, carbon and nitrogen reactive gases;

[0017] (5) Cool the chemical vapor deposition reaction chamber to room temperature to complete the coating preparation.

[0018] Preferred technical solution: The carbon matrix is ​​graphite or C / C substrate.

[0019] Preferred technical solution: In step (2), the deposition process conditions of the Si transition layer are: silicon tetrachloride as Si source, hydrogen as carrier gas, argon as dilution gas, deposition temperature 900-1400℃, deposition pressure 500-5000Pa, and deposition time 10-60 minutes.

[0020] Preferred technical solution: In step (2), the in-situ reaction conditions are: heat preservation for 1-5 hours at 1600-1800℃ and under vacuum conditions; the vacuum condition is a vacuum degree of less than 10Pa.

[0021] Preferred technical solution: In step (3), the proportion of silicon, carbon and nitrogen reaction gases is controlled as follows: silicon tetrachloride is used as the Si source, methane as the C source, ammonia as the N source, hydrogen as the carrier gas, and argon as the dilution gas. The deposition temperature is 900-1400℃ and the deposition pressure is 500-5000Pa. In the initial stage of deposition, methane, silicon tetrachloride, hydrogen and argon are introduced, and the molar ratio of methane to silicon tetrachloride is 1:1. The deposition time is 1-2h. Then, the amount of methane introduced is gradually reduced and the amount of ammonia introduced is increased within 1-10h. The total amount of methane and ammonia introduced is kept constant until the molar ratio of ammonia to methane is 1:1 and the molar ratio of ammonia, methane and silicon tetrachloride is 1:1:2. Finally, after the introduced gas is stable, the deposition continues for 1-2h.

[0022] Preferred technical solution: In step (4), the deposition temperature is 900-1400℃, the deposition pressure is 500-5000Pa, and the proportion of reaction gases containing silicon, boron, carbon and nitrogen is controlled by: based on the final gas proportion in step (3), boron trichloride is introduced as the B source; and the amount of boron trichloride introduced is gradually increased within 1-10 hours, while keeping the total amount of boron trichloride and silicon tetrachloride introduced unchanged, until the molar ratio of boron trichloride to silicon tetrachloride is 1:1, and the molar ratio of ammonia, methane, silicon tetrachloride and boron trichloride is 1:1:1:1. Finally, after the introduced gas is stable, deposition continues for 5-20 hours.

[0023] Due to the application of the above technical solutions, the beneficial effects of this invention compared with the prior art are as follows:

[0024] This invention generates a gradient SiC layer by depositing a Si transition layer and reacting it in situ with a carbon matrix, forming a strong chemically bonded interface that significantly improves the adhesion between the coating and the substrate, effectively preventing peeling under thermal shock or stress. By removing residual free Si at high temperatures, the gradient SiCN layer grows directly on the Si surface, reducing thermal stress during subsequent coating preparation. Simultaneously, the three-layer gradient structure and the continuous gradual change in composition within each layer ensure a smooth transition in the overall thermal expansion coefficient from the substrate to the outer layer, greatly alleviating internal stress caused by thermal mismatch. The gradient SiCN layer and the gradient SiC layer have similar compositions at their interface (both contain SiC), and the gradient SiBCN layer and the gradient SiCN layer also have similar compositions at their interface (both contain SiCN). This homogeneous transition ensures a strong bond and seamless overall structure, further enhancing the coating's integrity and reliability. The outermost gradient SiBCN layer, especially the surface with high SiBCN content, exhibits excellent high-temperature stability, oxidation resistance, and corrosion resistance, effectively blocking the intrusion of external corrosive media such as oxygen and moisture, providing long-term protection for the internal substrate. By employing chemical vapor deposition (CVD) and dynamic co-deposition techniques, and through precise control of the type, flow rate, and ratio of reactive gases, the composition and thickness of each gradient layer can be precisely controlled, resulting in good process repeatability. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the structure of a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface according to the present invention.

[0027] Figure 2 This is a flowchart illustrating the preparation method of a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface according to the present invention.

[0028] In the above figures, 1 is a carbon matrix; 2 is a gradient SiC layer; 3 is a gradient SiCN layer; and 4 is a gradient SiBCN layer. Detailed Implementation

[0029] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0030] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be used interchangeably where appropriate for the description of embodiments of this application herein. Furthermore, the terms "comprising" and "having," and their synonyms, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0031] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "middle," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing the invention and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0032] Furthermore, in addition to indicating direction or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in certain situations to indicate a dependency or connection. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0033] Furthermore, the terms "installation," "setting," "equipped with," "connection," "linking," "fitting," and "fitting" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, components, or parts. Similarly, "fitting" can mean completely or partially fitted. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0035] Example:

[0036] like Figure 1 As shown, this embodiment prepares a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating for application on the surface of C / C composite materials. From the inside out, the coating consists of a gradient SiC layer 2, a gradient SiCN layer 3, and a gradient SiBCN layer 4. The three layers, as well as the gradient SiC layer and the carbon matrix 1, are seamlessly bonded through a continuous gradient of composition, with no obvious interface.

[0037] Gradient SiC layer: approximately 5 μm thick. Its side adjacent to the carbon matrix is ​​rich in free C derived from the matrix, while the free C content decreases to 0% towards the gradient SiCN layer side, and the SiC content generated by the reaction increases from 0% to 100%, achieving a smooth transition from the carbon matrix to the ceramic coating and forming an interface connection achieved by chemical bonding, thereby improving the bonding strength.

[0038] Gradient SiCN layer: approximately 50 μm thick. The composition adjacent to the gradient SiC layer is dominated by SiC (approximately 100%), while the SiC content decreases to approximately 30% towards the gradient SiBCN layer, and the SiCN content increases from 0% to approximately 70%.

[0039] Gradient SiBCN layer: approximately 130 μm thick. The side adjacent to the gradient SiCN layer is predominantly composed of SiCN (approximately 70%), decreasing to approximately 20% towards the outer edges, while the SiBCN content increases from 0% to approximately 80% (surface layer). In some embodiments, the gradient SiBCN layer also contains SiC, with a SiC content of 0-50%.

[0040] Total coating thickness: approximately 185 μm

[0041] like Figure 2 As shown, this embodiment also provides a method for preparing a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface, including the following steps:

[0042] (1) Select a C / C substrate with a surface roughness Ra of 3 μm as the carbon matrix or obtain a carbon matrix with a surface roughness Ra of 3 μm by sanding the C / C composite material. Then clean and dry the carbon matrix for later use.

[0043] (2) Place the pretreated C / C substrate in a chemical vapor deposition (CVD) reaction chamber.

[0044] First, a Si transition layer is deposited: silicon tetrachloride (SiCl4) is used as the Si source, hydrogen (H2) as the carrier gas, and argon (Ar) as the dilution gas. The deposition temperature is controlled at 1200℃, the deposition pressure at 2000Pa, and the deposition time at 30 minutes. During this process, SiCl4 is reduced by H2, forming a pure Si transition layer on the C / C substrate surface.

[0045] Subsequently, all gas supply was stopped, the system was evacuated to a vacuum level of 5 Pa, and the temperature was raised to 1700℃ and held for 3 hours. Under these high-temperature vacuum conditions, the Si transition layer reacts in situ with the carbon on the surface of the C / C substrate to generate SiC. Simultaneously, unreacted free Si evaporates and is removed as vapor. Ultimately, a gradient SiC layer with a thickness of approximately 5 μm is formed on the substrate surface. At the interface between this gradient SiC layer and the C / C substrate, free carbon from the C / C substrate is present, and the free carbon content gradually decreases to 0% away from the C / C substrate, while the SiC content gradually increases from 0% to 100%, achieving a continuous chemical transition from the carbon matrix to SiC.

[0046] (3) After the gradient SiC layer is formed, without changing the deposition temperature (1200℃) and deposition pressure (2000Pa), a chemical vapor phase dynamic co-deposition process is adopted to deposit a gradient SiCN layer on the surface of the gradient SiC layer by controlling the proportion of silicon, carbon and nitrogen-containing reactive gases. The reactive gases are SiCl4 (Si source), CH4 (C source) and NH3 (N source), H2 is the carrier gas and Ar is the dilution gas.

[0047] In the initial stage, CH4, SiCl4, H2, and Ar were introduced, with the molar ratio of CH4 to SiCl4 controlled at 1:1, and deposition lasted for 1 hour. During this stage, SiC was primarily deposited on the surface of the gradient SiC layer. Over the next 5 hours, the flow rate of CH4 was precisely controlled using a flow meter, gradually decreasing while the flow rate of NH3 was gradually increased, maintaining a constant total molar flow rate of CH4 and NH3. The molar ratio of the reactant gases linearly transitioned from the initial (CH4:SiCl4 = 1:1, NH3 = 0) to the final (NH3:CH4:SiCl4 = 1:1:2). Maintaining the final gas ratio (NH3:CH4:SiCl4 = 1:1:2), deposition continued for 1.5 hours. After the entire deposition process was completed, a gradient SiCN layer with a thickness of approximately 50 μm was formed on the surface of the gradient SiC layer. The SiCN gradient layer gradually decreases from 100% to about 30% from the side adjacent to the SiC gradient layer to the side adjacent to the SiBCN gradient layer, with the SiC content gradually increasing from 0% to about 70%, and the composition changing continuously.

[0048] (4) Keep the deposition temperature (1200℃) and deposition pressure (2000Pa) unchanged, and continue to use the chemical vapor phase dynamic co-deposition process to deposit a gradient SiBCN layer on the surface of the gradient SiCN layer by controlling the proportion of silicon, boron, carbon and nitrogen reactive gases.

[0049] Based on the final gas ratio (NH3:CH4:SiCl4=1:1:2, H2 and Ar) in step (3), boron trichloride (BCl3) is introduced as the B source.

[0050] Initially, the BCl3 flow rate was 0. Over the next 6 hours, the BCl3 flow rate was gradually increased while the SiCl4 flow rate was gradually decreased, maintaining a constant total molar flow rate of BCl3 and SiCl4. The gas ratio linearly transitioned from (NH3:CH4:SiCl4=1:1:2, BCl3=0) to a final ratio of (NH3:CH4:SiCl4:BCl3=1:1:1:1). Maintaining the final gas ratio (NH3:CH4:SiCl4:BCl3=1:1:1:1), deposition continued for 12 hours. A gradient SiBCN layer with a thickness of approximately 130 μm was formed on the surface of the gradient SiCN layer. From the adjacent gradient SiCN layer outwards, the SiCN content gradually decreased from 70% to 20%, while the SiBCN content gradually increased from 0% to approximately 80%, with a continuous gradual change in composition.

[0051] (5) After the gradient SiBCN layer deposition is completed, all reactive gases are stopped. Under Ar atmosphere protection, the chemical vapor deposition (CVD) reaction chamber is cooled to room temperature to obtain a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface. The total coating thickness is approximately 185 μm.

[0052] The coating cross-section was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed no obvious interfaces between the three coating layers or within each layer, and the composition exhibited a continuous gradient change. The adhesion between the coating and the substrate was tested using the scratch test, and the coating and substrate were found to be firmly bonded. After thermal shock cycling testing, the coating did not detach from the carbon substrate.

[0053] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface, characterized in that: This includes gradient SiC layers, gradient SiCN layers, and gradient SiBCN layers, which are sequentially formed on the surface of a carbon matrix, have continuously and gradually changing compositions, and have no obvious interfaces between layers. The gradient SiC layer comprises free C derived from the carbon matrix and SiC generated by the reaction, with the free C content decreasing and the SiC content increasing from the side adjacent to the carbon matrix towards the gradient SiCN layer; the gradient SiCN layer comprises SiC and SiCN, with the SiC content decreasing and the SiCN content increasing from the side adjacent to the gradient SiC layer towards the gradient SiBCN layer; the gradient SiBCN layer comprises SiCN and SiBCN, with the SiCN content decreasing and the SiBCN content increasing from the side adjacent to the gradient SiCN layer towards the outside of the coating.

2. The high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface according to claim 1, characterized in that: The thickness of the gradient SiC layer is 1-10 μm; the thickness of the gradient SiCN layer is 10-100 μm; the thickness of the gradient SiBCN layer is 100-180 μm; and the total thickness of the gradient SiC layer, gradient SiCN layer and gradient SiBCN layer does not exceed 200 μm.

3. The high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface according to claim 2, characterized in that: In the gradient SiC layer, from the carbon matrix side to the gradient SiCN layer side, the free C content decreases from 100% to 0%, and the SiC content increases from 0% to 100%; in the gradient SiCN layer, from the side adjacent to the gradient SiC layer to the gradient SiBCN layer side, the SiC content decreases from 100% to 0-50%, and the SiCN content increases from 0% to 50-100%; in the gradient SiBCN layer, from the side adjacent to the gradient SiCN layer to the outside of the coating, the SiCN content decreases from 50-100% to 0-50%, and the SiBCN content increases from 0% to 50-100%.

4. The high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface according to claim 1, characterized in that: The gradient SiBCN layer also contains SiC, and the SiC content is 0-50%.

5. A method for preparing a high-temperature corrosion-resistant SiC-SiCN-SiBCN gradient composite coating on a carbon-based surface, characterized in that, Includes the following steps: (1) Clean the surface of the carbon matrix with a surface roughness Ra greater than 2 μm and dry it thoroughly; (2) A Si transition layer is deposited on the surface of a carbon substrate using chemical vapor deposition; then the gas supply is stopped, allowing the silicon layer to react in situ with the carbon substrate to form a gradient SiC layer, and the excess Si is volatilized in the form of Si vapor. (3) Using a chemical vapor phase dynamic co-deposition process, a gradient SiCN layer is deposited on the surface of the gradient SiC layer formed in step (2) by controlling the ratio of silicon, carbon and nitrogen reactive gases; (4) Using a chemical vapor phase dynamic co-deposition process, a gradient SiBCN layer is deposited on the surface of the gradient SiCN layer formed in step (3) by controlling the proportion of silicon, boron, carbon and nitrogen reactive gases; (5) Cool the chemical vapor deposition reaction chamber to room temperature to complete the coating preparation.

6. The preparation method according to claim 5, characterized in that: The carbon matrix is ​​graphite or a C / C substrate.

7. The preparation method according to claim 5, characterized in that: In step (2), the deposition process conditions of the Si transition layer are as follows: silicon tetrachloride is used as the Si source, hydrogen is used as the carrier gas, argon is used as the dilution gas, the deposition temperature is 900-1400℃, the deposition pressure is 500-5000Pa, and the deposition time is 10-60 minutes.

8. The preparation method according to claim 7, characterized in that: In step (2), the conditions for the in-situ reaction are: heat preservation for 1-5 hours at 1600-1800℃ and under vacuum; the vacuum condition is a vacuum degree of less than 10 Pa.

9. The method according to claim 5, characterized in that: In step (3), the control of the proportion of silicon, carbon, and nitrogen-containing reaction gases includes: using silicon tetrachloride as the Si source, methane as the C source, ammonia as the N source, hydrogen as the carrier gas, and argon as the dilution gas, with a deposition temperature of 900-1400℃ and a deposition pressure of 500-5000Pa; wherein, at the initial stage of deposition, methane, silicon tetrachloride, hydrogen, and argon are introduced, and the molar ratio of methane to silicon tetrachloride is 1:1, with a deposition time of 1-2h; then, within 1-10h, the amount of methane introduced is gradually reduced and the amount of ammonia introduced is increased, while keeping the total amount of methane and ammonia introduced unchanged, until the molar ratio of ammonia to methane is 1:1, and the molar ratio of ammonia, methane, and silicon tetrachloride is 1:1:2, and finally, after the introduced gases stabilize, deposition continues for 1-2h.

10. The preparation method according to claim 9, characterized in that: In step (4), the deposition temperature is 900-1400℃ and the deposition pressure is 500-5000Pa. The control of the proportion of silicon, boron, carbon and nitrogen reaction gases includes: based on the final gas proportion in step (3), boron trichloride is introduced as the B source; and the amount of boron trichloride introduced is gradually increased within 1-10 hours, while keeping the total amount of boron trichloride and silicon tetrachloride introduced unchanged, until the molar ratio of boron trichloride to silicon tetrachloride is 1:1, and the molar ratio of ammonia, methane, silicon tetrachloride and boron trichloride is 1:1:1:

1. Finally, after the introduced gas is stable, deposition continues for 5-20 hours.

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