Preparation method of ceramic composite coating on surface layer of light carbon-based composite material

Abstract

The invention discloses a one-step forming preparation method of a light carbon-based composite material (C / CA) surface gradient ultra-high temperature ceramic (UHTC) composite coating, and belongs to the technical field of carbon-based composite material oxidation resistance. According to the invention, a silicon (Si)-silicon dioxide (SiO2) co-cementation ceramization technology is adopted, and a pre-coating layer on a C / CA substrate and a carbon matrix of a C / CA shallow surface layer can be synchronously ceramized through one-time high-temperature heat treatment process and are respectively converted into a compact UHTC-SiC outer layer and a gradient SiC inner layer (i.e., a SiC / UHTC-SiC double-layer composite coating). According to the preparation process, the double-layer coating containing the ultra-high-temperature ceramic can be obtained only through one-time heat treatment, the influence of high temperature on a carbon matrix pore structure and the crystallization degree in C / CA is remarkably reduced, and meanwhile deformation mismatching between the coating and a substrate is relieved. The outer layer of the obtained coating is a SiO2-filled compact UHTC-SiC layer, the inner layer of the obtained coating is a relatively compact gradient SiC layer, the total thickness reaches 500 microns or above, the residual stress is low, and the weight loss of C / CA under the protection of the coating is only 4% after the C / CA is oxidized at 1500 DEG C for 30 h.

Description

Technical Field

[0001] The invention relates to the field of anti-oxidation coatings on the surfaces of lightweight carbon-based composite materials, and in particular to a gradient ultra-high temperature ceramic (UHTC) composite coating and a preparation method thereof. Background Art

[0002] The new lightweight carbon-based composite material is composed of a carbon fiber toughener and a porous carbon network with low crystallinity. It has the characteristics of low density, high specific strength and good thermal insulation performance. It has great application prospects in thermal protection fields such as aerospace vehicles and their power systems. Similar to traditional carbon-based composite materials, the new lightweight carbon-based composite material is very easy to oxidize above 400°C, resulting in the collapse of the porous structure and the decrease of mechanical strength and thermal insulation performance. It can be seen that solving the problem of material oxidation is the key to promoting the application of new lightweight carbon-based composite materials in the field of thermal protection.

[0003] Setting a dense oxygen-blocking and oxygen-consuming ceramic layer on the surface of the material to isolate the carbon matrix from oxygen is an effective measure to achieve anti-oxidation. For carbon-based composite materials, there have been a lot of studies on anti-oxidation coatings. Traditional coatings are mostly prepared by a layer-by-layer method, that is, the coating is sintered layer by layer through multiple heating processes to obtain a composite or multi-layer dense structure to achieve effective oxidation protection; however, for new lightweight carbon-based composite materials with low crystallinity matrix, if the traditional layer-by-layer method is used, repeated heating will induce graphitization of the carbon network, resulting in increased crystallinity and deterioration of thermal insulation performance; at the same time, high temperature will also cause changes in the porous structure, affecting the strength and thermal conductivity of the material. Therefore, it is necessary to develop a new coating process to reduce the thermal damage of the coating preparation to the matrix material, and to achieve the preparation of a composite dense coating with low heat input.

[0004] Therefore, the present invention proposes a one-time forming preparation method for a gradient ultra-high temperature ceramic (UHTC) composite coating on the surface of a lightweight carbon-based composite material (C / CA). By using silicon (Si)-silicon dioxide (SiO2) co-infiltration ceramicization technology, the pre-coating on the C / CA substrate and the carbon matrix of the shallow surface layer of the C / CA can be simultaneously ceramicized through a high-temperature heat treatment process, and converted into a dense UHTC-SiC outer layer and a gradient SiC inner layer (i.e., a SiC / UHTC-SiC double-layer composite coating) respectively. This preparation process only requires one heat treatment process to obtain a double-layer coating containing ultra-high temperature ceramics, while alleviating the deformation mismatch between the coating and the substrate. The outer layer of the obtained coating is a dense UHTC-SiC layer filled with SiO2, and the inner layer is a relatively dense gradient SiC layer, with a total thickness of more than 500μm, low residual stress, and the C / CA under its protection loses only 4% of its weight after oxidation at 1500℃ for 30h, achieving effective anti-oxidation protection. Summary of the invention

[0005] The purpose of the present invention is to provide a gradient ultra-high temperature ceramic composite coating on the surface of a lightweight carbon-based composite material and a preparation method thereof to meet the thermal protection requirements in an ultra-high temperature aerobic environment.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A one-step molding preparation method for a gradient ultra-high temperature ceramic composite coating on the surface of a lightweight carbon-based composite material comprises the following steps:

[0008] (1) The density is 0.2 to 0.8 g / cm 3 The lightweight carbon-based composite material (C / CA) is processed into a desired shape, ultrasonically cleaned with alcohol, and then placed in an oven at 80–120°C for 24–48 hours for use;

[0009] (2) weighing carbon powder (0.1-2 μm) and ceramic powder (0.1-2 μm) in proportion and mixing them in a ball mill at a rotation speed of 100-120 r / min for a ball milling time of 10-12 h to obtain a mixed powder for an outer pre-coating layer;

[0010] (3) Prepare a 5-20wt.% polyvinyl butyral (PVB) ethanol solution. Add the outer pre-coating mixed powder obtained in step (2) to the PVB ethanol solution, and stir mechanically to obtain a slurry. The mass fraction of the mixed powder in the slurry is 30-60wt.%;

[0011] (4) Apply the slurry to the C / CA surface described in step (1) and dry it at 100-120°C for 3-5 hours. Repeat the application and drying process until a 100-150 μm pre-coating layer is obtained on the surface;

[0012] (5) Weigh a certain proportion of Si powder (1-10 μm) and SiO2 powder (1-10 μm), mechanically mix to obtain an embedded powder and fill it into a graphite crucible. Embed the C / CA with pre-coating described in step (4) in the embedded powder and place it in a high-temperature furnace for high-temperature treatment in step (6);

[0013] (6) High temperature treatment: In a vacuum environment, heat the sample to 1300–1600°C at a rate of 5–10°C / min, keep the sample at the target temperature for 1–3 hours, and then take it out after cooling it to room temperature. During the heat treatment, Si in the embedded powder reacts with the carbon powder in the pre-coating and the carbon matrix in the C / CA to generate SiC. At the same time, SiO2 will fill and densify the outer layer, and eventually the carbon matrix of the pre-coating and the C / CA superficial layer will be simultaneously converted into a dense UHTC-SiC outer layer and a gradient SiC inner layer, thus obtaining a SiC / UHTC-SiC double-layer composite coating.

[0014] In step (2), the types and proportions of carbon powder and ceramic powder are as follows, which can be adjusted according to the service environment of the materials:

[0015] Pre-coating powder 1: C (40-60 wt.%), ZrB2 (30-60 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%);

[0016] Pre-coating powder 2: C (40-55 wt.%), HfB2 (25-50 wt.%), Al2O3 (5-15 wt.%), SiO2 (10-20 wt.%);

[0017] Pre-coating powder 3: C (20-60 wt.%), ZrC (30-50 wt.%), Al2O3 (5-10 wt.%), SiO2 (20-40 wt.%);

[0018] Pre-coating powder 4: C (20-35 wt.%), HfC (40-60 wt.%), Al2O3 (5-10 wt.%), SiO2 (10-20 wt.%);

[0019] Pre-coating powder 5: C (20-35 wt.%), TaC (20-35 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%);

[0020] Pre-coating powder 6: C (5-10 wt.%), B4C (35-60 wt.%), ZrO2 (15-30 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%);

[0021] Pre-coating powder 7: C (10-20 wt.%), B4C (30-60 wt.%), HfO2 (20-40 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%);

[0022] Pre-coating powder 8: C (20-35 wt.%), ZrO2 (20-40 wt.%), B (10-20 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%);

[0023] Pre-coating powder 9: C (10-20 wt.%), Zr (10-30 wt.%), B4C (20-35 wt.%), Al2O3 (5-10 wt.%), SiO2 (10-20 wt.%);

[0024] Pre-coating powder 10: C (10-20wt.%), zirconium disilicide (ZrSi2) (30-40wt.%), B4C (20-30wt.%), Al2O3 (5-10wt.%), SiO2 (5-10wt.%).

[0025] The present invention adopts silicon (Si)-silicon dioxide (SiO2) co-infiltration ceramicization technology, and the pre-coating layer on the C / CA substrate and the carbon matrix of the shallow surface layer of C / CA can be ceramicized synchronously through a high-temperature heat treatment process, and converted into a dense UHTC-SiC outer layer and a gradient SiC inner layer (i.e., a SiC / UHTC-SiC double-layer composite coating) respectively. This preparation process only requires one heat treatment process to obtain a double-layer coating containing ultra-high temperature ceramics, which significantly reduces the influence of high temperature on the pore structure and crystallinity of the carbon matrix in C / CA, and alleviates the deformation mismatch between the coating and the substrate. The outer layer of the obtained coating is a dense UHTC-SiC layer filled with SiO2, and the inner layer is a relatively dense gradient SiC layer, with a total thickness of more than 500μm, low residual stress, and the C / CA under its protection loses only 4% of its weight after oxidation at 1500℃ for 30h.

[0026] The design mechanism of the present invention is as follows:

[0027] The present invention utilizes the diffusion and reaction of silicon to realize the ceramicization of the pre-coating layer and the carbon matrix, and utilizes the diffusion with silicon dioxide to realize the densification of the coating layer. The specific mechanism is as follows: At high temperature, gaseous silicon gradually enters the pre-coating layer and the carbon matrix through diffusion, and reacts with the carbon source in the pre-coating layer to generate SiC. It is worth mentioning that at this time, there are still residual pores in the outer layer. Then Si continues to diffuse into the matrix and ceramicizes the porous carbon matrix into a SiC network to form an inner coating, and due to the concentration gradient of the Si diffusion process, the inner coating has a dual gradient of composition and density. As the reaction proceeds, the silicon dioxide in the embedded powder diffuses into the pre-coating layer, and gradually cools during the cooling process, wrapping the SiC and ultra-high temperature ceramic components, filling the pores between the components to form a dense outer coating. Finally, a dense outer layer containing ultra-high temperature ceramic components and an inner layer with a gradient of composition and density are obtained, providing antioxidant protection for the lightweight carbon-based composite material C / CA.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention uses silicon (Si)-silicon dioxide (SiO2) co-infiltration ceramicization technology to prepare a double-layer coating containing ultra-high temperature ceramics on the surface of a new lightweight carbon-based composite material C / CA with only one heating, and the lower heat input can inhibit the graphitization of the carbon network.

[0030] 2. As an effective coating preparation method, the inner and outer coatings are simultaneously transformed from carbon to ceramic, from loose to dense, and the thermal deformation is coordinated, which effectively reduces the hard interface stress of the inner and outer coatings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The cross-sectional microstructure morphology of the double-layer coating formed in one step;

[0032] Figure 2 Comparison diagram of residual stress of double-layer coating formed in one step and stress of coating formed by traditional layer-by-layer method (comparative example 1 method);

[0033] Figure 3 This is a curve of mass loss rate versus time when the sample with coating is oxidized in air atmosphere at 1500℃;

[0034] Figure 4 This is the cross-sectional microstructure morphology of the one-step formed HfB2-SiC / SiC double-layer coating;

[0035] Figure 5 Cross-sectional microstructure morphology of the comparative double-layer coating;

[0036] Figure 6 (a) is a comparison chart of the residual stress of a double-layer coating formed in one step and (b) the crystallinity of the coating substrate formed by the traditional layer-by-layer method (Comparative Example 1 method); DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments.

[0038] Example 1

[0039] This embodiment is a one-step forming preparation method for a gradient ultra-high temperature ceramic composite coating on the surface of a lightweight carbon-based composite material. The specific process is as follows:

[0040] (1) The density is 0.6 g / cm 3 Lightweight carbon-based composite materials (C / CA) are processed into 10×10×10mm 3 The blocks were ultrasonically cleaned with alcohol and then dried in an oven at 100°C for 24 h before use;

[0041] (2) Weigh carbon powder (C: 1 μm, 50 wt.%), zirconium diboride (ZrB2: 1 μm, 30 wt.%), aluminum oxide (Al2O3: 50 nm, 10 wt.%), and silicon dioxide (SiO2: 50 nm, 10 wt.%) powders according to the following average particle size and mass fraction; mix them in a ball mill at a speed of 100 r / min for 2 h to obtain a mixed powder for the outer pre-coating layer;

[0042] (3) Prepare a 10 wt.% polyvinyl butyral (PVB) ethanol solution. Add the uniformly mixed outer pre-coating mixed powder to the PVB ethanol solution, and stir mechanically to obtain a slurry. The mass fraction of the mixed powder in the slurry is 50 wt.%;

[0043] (4) Apply the slurry on the dried C / CA surface and dry it at 100°C for 5 hours. Repeat the application and drying process until a pre-coating layer with a thickness of 100 to 150 μm is obtained on the surface to obtain C / CA with a pre-coating layer;

[0044] (5) Si powder (average particle size 1 μm, Sinopharm Group) and SiO2 powder (average particle size 1 μm, Sinopharm Group) were weighed in a mass ratio of 1:1, mechanically (stirred) mixed to obtain an embedded powder and filled into a graphite crucible. The C / CA with the pre-coating layer was completely immersed and embedded in the embedded powder;

[0045] (6) High temperature treatment: Place the graphite crucible containing the sample in a vacuum furnace (i.e., high temperature furnace), evacuate to a vacuum state (less than 0.5 Pa), and heat from room temperature to 1550°C at a rate of 5-10°C / min (here 8°C / min). Keep the temperature at the target temperature of 1550°C for 2 h, then stop the furnace, cool to room temperature, and take out the sample.

[0046] The cross-sectional microstructure of the gradient ultra-high temperature ceramic composite coating on the surface of the lightweight carbon-based composite material obtained in the above steps is as follows: Figure 1 As shown, the grayish white is the ZrB2-SiC outer coating with a thickness of about 100 μm formed by the sintering of the pre-coating layer and the SiC inner coating with a thickness of about 400 μm formed by the in-situ reaction of the carbon network and Si, with a total thickness of about 500 μm, and the black part is the carbon matrix that has not been ceramicized. It can be seen that the inner and outer coatings are tightly bonded, without transverse and longitudinal cracks, and there is no cracking at the junction of the coating and the carbon substrate. Figure 2 As shown in the figure, the Raman spectroscopy microscopic residual stress test results show that the residual stress at the interface of the double-layer coating is significantly lower than the traditional method of preparing the inner layer first and then the outer layer. Therefore, cracks are not easy to initiate and expand when the temperature changes. After oxidation at 1500℃ for 30h, the weight loss is only 4%. Figure 3 shown.

[0047] Example 2

[0048] This embodiment is a one-step forming preparation method for a gradient ultra-high temperature ceramic composite coating on the surface of a lightweight carbon-based composite material. The specific process is as follows:

[0049] (1) The density is 0.6 g / cm 3 Lightweight carbon-based composite materials (C / CA) are processed into 10×10×10mm 3 , cleaned with alcohol ultrasonically, and then placed in an oven at 100°C for 24 hours for use;

[0050] (2) Weigh carbon powder (C: 1 μm, 50 wt.%), zirconium diboride HfB2: 1 μm, 30 wt.%), aluminum oxide (Al2O3: 50 nm, 10 wt.%), and silicon dioxide (SiO2: 50 nm, 10 wt.%) powders according to the following average particle size and mass fraction; mix them in a ball mill at a speed of 100 r / min for 2 h to obtain a mixed powder for the outer pre-coating layer;

[0051] (3) Prepare a 10 wt.% polyvinyl butyral (PVB) ethanol solution. Add the uniformly mixed outer pre-coating mixed powder to the PVB ethanol solution, and stir mechanically to obtain a slurry. The mass fraction of the mixed powder in the slurry is 50 wt.%;

[0052] (4) Apply the slurry on the dried C / CA surface and dry it at 100°C for 5 hours. Repeat the application and drying process until a pre-coating layer with a thickness of 100 to 150 μm is obtained on the surface, thus obtaining a C / CA with a pre-coating layer;

[0053] (5) Si powder (average particle size 1 μm, Sinopharm Group) and SiO2 powder (average particle size 1 μm, Sinopharm Group) were weighed in a mass ratio of 1:1, mechanically (stirred) mixed to obtain an embedded powder and filled into a graphite crucible. The C / CA with the pre-coating layer was completely immersed and embedded in the embedded powder;

[0054] (6) High temperature treatment: Place the graphite crucible containing the sample in a vacuum furnace (i.e., high temperature furnace), evacuate to a vacuum state (less than 0.5 Pa), and heat from room temperature to 1550°C at a rate of 5-10°C / min (here 8°C / min). Keep the temperature at the target temperature of 1550°C for 2 h, then stop the furnace, cool to room temperature, and take out the sample.

[0055] The cross-sectional microstructure of the gradient ultra-high temperature ceramic composite coating on the surface of the lightweight carbon-based composite material obtained in the above steps is as follows: Figure 4As shown, the grayish white is the HfB2-SiC outer coating with a thickness of about 100μm formed by the sintering of the pre-coating layer and the SiC inner coating with a thickness of about 400μm formed by the in-situ reaction of the carbon network and Si, with a total thickness of about 500μm, and the black part is the carbon matrix that has not been ceramicized. It can be seen that the inner and outer coatings are tightly bonded, without transverse and longitudinal cracks, and there is no carbon-ceramic mismatch cracking at the junction of the coating and the carbon substrate.

[0056] Comparative Example 1

[0057] This embodiment is a comparative example of a one-time forming preparation method of a gradient ultra-high temperature ceramic composite coating on the surface of a lightweight carbon-based composite material. The difference is that two high-temperature treatments are required and only Si powder is embedded. The specific process is as follows:

[0058] (1) The density is 0.6 g / cm 3 Lightweight carbon-based composite materials (C / CA) are processed into 10×10×10mm 3 , after ultrasonic cleaning, place in an oven at 100°C for 24 hours for use;

[0059] (2) embedding the C / CA treated in step (1) in a graphite crucible filled with Si powder (average particle size 1 μm, Sinopharm Group);

[0060] (3) High temperature treatment: Place the graphite crucible with the sample embedded in it in a vacuum furnace (i.e., a high temperature furnace), evacuate it to a vacuum state (less than 0.5 Pa), and heat it from room temperature to 1500°C at a rate of 5-10°C / min (here 8°C / min). Keep it at the target temperature of 1500°C for 2 hours, then stop the furnace, cool it to room temperature, take out the sample, and clean the surface.

[0061] (4) Weigh carbon powder (C: 1 μm, 50 wt.%), zirconium diboride (ZrB2: 1 μm, 30 wt.%), aluminum oxide (Al2O3: 50 nm, 10 wt.%), and silicon dioxide (SiO2: 50 nm, 10 wt.%) powders according to the following average particle size and mass fraction; mix them in a ball mill at a speed of 100 r / min for 2 h to obtain a mixed powder;

[0062] (5) Prepare a 10 wt.% polyvinyl butyral (PVB) ethanol solution. Add the uniformly mixed outer pre-coating mixed powder to the PVB ethanol solution, and stir mechanically to obtain a slurry. The mass fraction of the mixed powder in the slurry is 50 wt.%;

[0063] (6) Apply the slurry to the C / CA surface cleaned in step (3) and dry it at 100°C for 5 hours. Repeat the application and drying process until a pre-coating layer with a thickness of 100 to 150 μm is obtained on the surface, thereby obtaining a sample with a pre-coating layer;

[0064] (7) embedding the sample treated in step (6) in a graphite crucible filled with Si powder (average particle size 1 μm, Sinopharm Group);

[0065] (8) High temperature treatment: Place the graphite crucible containing the sample in a vacuum furnace (i.e., a high temperature furnace), evacuate to a vacuum state (less than 0.5 Pa), and heat from room temperature to 1550°C at a rate of 5-10°C / min (here 8°C / min). Keep the temperature at the target temperature of 1550°C for 2 h, then stop the furnace, cool to room temperature, and take out the sample.

[0066] The cross-sectional microstructure of the surface coating of the lightweight carbon-based composite material obtained in the above steps is as follows: Figure 5 As shown in the figure, the grayish white is the outer ZrB2-SiC coating and the inner SiC coating. The outer coating is dense, with a total thickness of about 400μm, but there are obvious transverse cracks at the interface of the inner and outer coatings. This is mainly because the inner and outer coatings are not sintered synchronously. The specific explanation is that when the outer coating changes from carbon to ceramic during the second high-temperature treatment, the inner coating has already changed into ceramic during the first high-temperature treatment. Due to the uncoordinated thermal deformation of carbon and ceramic, large residual stress is formed, such as Figure 2 It is worth mentioning that due to the high heat output brought by the two high-temperature treatments, the microstructure of the carbon matrix undergoes a graphitization transformation, and compared with the matrix in Example 1, a significantly more ordered lamellar structure is formed, such as Figure 6 As shown, the two high-temperature preparations of the coating destroyed the low crystalline structure of the substrate. It can be seen that in Example 1, the double-layer coating was formed by sintering once, which effectively reduced the hard interface stress between the inner and outer coatings, and also maintained the low crystallinity of the substrate due to the lower heat input.

Claims

1. A method for preparing a gradient composite coating on the surface of a lightweight carbon-based composite material, characterized in that: The silicon (Si)-silicon dioxide (SiO2) co-infiltration ceramicization technology is used to obtain a gradient SiC / UHTC-SiC double-layer composite coating through a high-temperature heat treatment process, which specifically includes the following steps: (1) Density 0.2~0.8g / cm 3 A lightweight carbon-based composite material (C / CA) is used as the substrate; (2) weighing carbon powder (0.1-2 μm) and ceramic powder (0.1-2 μm) in proportion and mixing them to obtain an outer pre-coating mixed powder; (3) preparing a 5-20 wt.% polyvinyl butyral (PVB) ethanol solution; adding the outer pre-coating mixed powder obtained in step (2) to the PVB ethanol solution, and mechanically stirring to obtain a slurry; wherein the mass fraction of the mixed powder in the slurry is 30-60 wt.%; (4) applying the slurry to the C / CA surface described in step (1), and drying at 100-120°C for 3-5 hours; repeating the application and drying process until a pre-coating layer with a thickness of 100-150 μm is obtained on the surface; (5) Weigh a certain proportion of Si powder (particle size 1-10 μm) and SiO2 powder (particle size 1-10 μm), mechanically mix to obtain an embedded powder and fill it into a graphite crucible; embed the C / CA with the pre-coating layer described in step (4) in the embedded powder, and place it in a high-temperature furnace for high-temperature treatment in step (6); (6) High temperature treatment: In a vacuum environment, heat the sample to a target temperature of 1300–1600°C at a rate of 5–10°C / min, keep the temperature at the target temperature for 1–3 h, and remove the sample after cooling to room temperature.

2. The preparation method according to claim 1, characterized in that: In step (1), the density is 0.2 to 0.8 g / cm 3 The lightweight carbon-based composite material (C / CA) is used as the substrate, processed into the required shape, ultrasonically cleaned with alcohol, and placed in an oven at 80-120℃ for 24-48h for use.

3. The preparation method according to claim 1, characterized in that: In the step (2), the composition of the pre-coating mixed powder includes, but is not limited to, any one of the following 10 materials, depending on the service environment of the material: pre-coating powder 1: C, zirconium diboride (ZrB2), aluminum oxide (Al2O3), SiO2; pre-coating powder 2: C, hafnium diboride (HfB2), Al2O3, SiO2; pre-coating powder 3: C, zirconium carbide (ZrC), Al2O3, SiO2; pre-coating powder 4: C, hafnium carbide (HfC), Al2O3, SiO2; pre-coating powder 5: C, carbon Tantalum carbide (TaC), Al2O3, SiO2; pre-coating powder 6: C, boron carbide (B4C), zirconium dioxide (ZrO2), Al2O3, SiO2; pre-coating powder 7: C, B4C, hafnium dioxide (HfO2), Al2O3, SiO2; pre-coating powder 8: C, ZrO2, elemental boron (B), Al2O3, SiO2; pre-coating powder 9: C, Zr, B4C, Al2O3, SiO2; pre-coating powder 10: C, zirconium disilicide (ZrSi2), B4C, Al2O3, SiO2; The mixing process uses a ball mill with a rotation speed of 100-120 r / min and a ball milling time of 10-12 h to obtain an outer pre-coating mixed powder.

4. The preparation method according to claim 1 or 2, characterized in that: In step (2), the mass fraction of each component in each pre-coating powder can be adjusted according to the service environment of the material. The composition of the pre-coating mixed powder includes but is not limited to any one of the following 10 types: Pre-coating powder 1: C (40-60 wt.%), ZrB2 (30-60 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%); Pre-coating powder 2: C (30-55 wt.%), HfB2 (25-50 wt.%), Al2O3 (5-15 wt.%), SiO2 (10-20 wt.%); Pre-coating powder 3: C (20-40 wt.%), ZrC (30-50 wt.%), Al2O3 (5-10 wt.%), SiO2 (20-40 wt.%); Pre-coating powder 4: C (20-35 wt.%), HfC (40-60 wt.%), Al2O3 (5-10 wt.%), SiO2 (10-20 wt.%); Pre-coating powder 5: C (20-35 wt.%), TaC (40-55 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-20 wt.%); Pre-coating powder 6: C (5-15 wt.%), B4C (35-60 wt.%), ZrO2 (15-30 wt.%), Al2O3 (5-15 wt.%), SiO2 (5-20 wt.%); Pre-coating powder 7: C (10-20 wt.%), B4C (30-55 wt.%), HfO2 (20-40 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%); Pre-coating powder 8: C (20-35 wt.%), ZrO2 (30-45 wt.%), B (10-20 wt.%), Al2O3 (5-10 wt.%), SiO2 (5-10 wt.%); Pre-coating powder 9: C (10-25 wt.%), Zr (10-30 wt.%), B4C (20-35 wt.%), Al2O3 (5-15 wt.%), SiO2 (10-20 wt.%); Pre-coating powder 10: C (10-20wt.%), zirconium disilicide (ZrSi2) (30-40wt.%), B4C (20-30wt.%), Al2O3 (5-10wt.%), SiO2 (5-10wt.%).

5. The preparation method according to claim 1, characterized in that: In step (5), the mass fraction of Si powder and SiO2 powder is 1:1 respectively.

6. The preparation method according to claim 1, characterized in that: In step (6), during the heat treatment process, Si in the embedded powder reacts with the carbon powder in the pre-coating and the carbon matrix in the C / CA to generate SiC, while SiO2 also fills and densifies the outer layer, and finally the pre-coating and the carbon matrix of the shallow layer of C / CA are simultaneously converted into a dense UHTC-SiC outer layer and a SiC inner layer with a composition and density gradient, thereby obtaining a SiC / UHTC-SiC double-layer composite coating.

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