Modified rb-sic ceramic and method for producing the same

Abstract

The application discloses a modified RB-SiC ceramic, which comprises a modified RB-SiC ceramic base and a SiC coating layer; wherein the modified RB-SiC ceramic base is composed of an alpha-SiC phase, a beta-SiC phase and free Si, the ratio of the alpha-SiC phase to the beta-SiC phase in the cross-section photo of the modified RB-SiC ceramic base is 3:7-7:3, and the volume ratio of the free Si in the modified RB-SiC ceramic base is less than 1%; and the thickness of the SiC coating layer is 1-100 microns. The application further discloses a preparation method of the modified RB-SiC ceramic, and the steps are as follows: high-purity RB-SiC is subjected to vacuum heat treatment at 1500-1700 DEG C and 1900-2100 DEG C respectively; the RB-SiC ceramic after heat treatment is processed into a required size and shape; the RB-SiC ceramic is subjected to acid pickling by using a mixed acid solution and then is subjected to rinsing by using pure water; SiC is deposited in the internal pores of the RB-SiC ceramic by using a positive and negative forced convection CVI process for densification; and the SiC coating layer is deposited on the surface of the RB-SiC ceramic by using a CVD process. The modified RB-SiC ceramic has a significantly reduced porosity, a significantly improved mechanical property at high temperature, and can be normally used in an environment above 1400 DEG C.

Description

Technical Field

[0001] This invention relates to the field of reaction sintering SiC ceramics technology, and particularly to a modified RB-SiC ceramic and its preparation method. Background Technology

[0002] CVD is a chemical technology that uses one or more gaseous compounds or elements containing thin film elements to chemically react on the surface of a substrate to generate a thin film.

[0003] CVI method is an extension of CVD method, which refers to the method of densifying materials by decomposing and condensing one or more hydrocarbon gaseous compounds at high temperature and then depositing them inside a porous medium.

[0004] SiC ceramics are widely used in aerospace, petrochemical, machinery manufacturing, nuclear industry and semiconductor fields due to their excellent thermal conductivity, high-temperature mechanical strength, low coefficient of thermal expansion, good thermal stability and corrosion resistance.

[0005] Reaction-bonded SiC ceramics, also known as RB-SiC ceramics, not only inherit the excellent properties of SiC ceramics but also possess advantages such as simple processing, short sintering time, complex shape forming, and net-size sintering. However, because the free Si content in RB-SiC ceramic materials prepared by reaction sintering is difficult to reduce, RB-SiC ceramics are only suitable for environments below 1400℃, limiting their application range.

[0006] Chinese Patent Publication No. CN114956852A discloses a method for preparing silicon carbide ceramics with extremely low residual silicon content obtained through multi-step reaction sintering. The reaction-sintered silicon carbide ceramics with extremely low residual silicon content prepared by this method can be widely used in key components of nuclear reactors, high thermal conductivity parts, and integrated circuit core equipment. This method reduces the free Si content in the ceramic by optimizing the preparation process; however, it requires the introduction of a low-temperature phase, which cannot guarantee the density and mechanical properties of the modified RB-SiC ceramic.

[0007] Therefore, in view of the shortcomings of the existing technology, it is necessary to design a modified RB-SiC ceramic and its preparation method to solve the above problems.

[0008] It should be noted that the above introduction to the technical background is only for the purpose of clearly and completely explaining the technical solution of the present invention and facilitating the understanding of those skilled in the art. 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 section of the present invention. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the present invention discloses a modified RB-SiC ceramic and its preparation method to solve the problem that RB-SiC ceramics cannot be used normally in environments above 1400℃. The modified RB-SiC ceramic prepared by this method has improved high temperature resistance while still having high density and mechanical properties.

[0010] This invention discloses a modified RB-SiC ceramic that can be used normally in environments above 1400 degrees Celsius, comprising a modified RB-SiC ceramic matrix and a SiC coating; wherein, the modified RB-SiC ceramic matrix is ​​composed of α-SiC phase, β-SiC phase and free Si, and the ratio of α-SiC phase to β-SiC phase in the cross-sectional photograph of the modified RB-SiC ceramic matrix is ​​3:7-7:3, and the volume percentage of free Si in the modified RB-SiC ceramic matrix is ​​less than 1%; the thickness of the SiC coating is 1-100 μm.

[0011] Preferred technical solution: The overall density of the modified RB-SiC ceramic is 3.0-3.2 g / cm³. 3 The porosity is less than 5%, and the pore size distribution of RB-SiC ceramics is concentrated in 1-10μm. The porosity and pore size of RB-SiC ceramics gradually decrease from the inside to the outside until the outer layer is completely dense and the porosity becomes 0.

[0012] Preferred technical solution: The content of metal impurities in the modified RB-SiC ceramic is less than 5 ppm.

[0013] This invention also discloses a method for preparing the above-mentioned modified RB-SiC ceramic, comprising the following steps:

[0014] S1. Select high-purity RB-SiC ceramic and place it in a vacuum furnace. Perform vacuum heat treatment at 1500℃-1700℃ to remove free Si from the RB-SiC ceramic and obtain RB-SiC ceramic with a porous structure.

[0015] S2. Then, the temperature inside the vacuum furnace is raised to 1900℃-2100℃ for vacuum heat treatment, which causes the SiC particles inside the RB-SiC ceramic to recrystallize and generate a continuous three-dimensional network structure.

[0016] S3. Take out the heat-treated RB-SiC ceramic and process it into the required size and shape.

[0017] S4. Prepare a mixed acid solution with a concentration of 3%-10% using hydrofluoric acid and nitric acid. Place the processed RB-SiC ceramic in the mixed acid solution for pickling, and then rinse it with pure water 3-10 times to remove residual free Si and metal impurities on the RB-SiC ceramic.

[0018] S5. Place the cleaned RB-SiC ceramic in a horizontal reactor, making the porous structure of the RB-SiC ceramic perpendicular to the flow direction of the reaction gas in the horizontal reactor and fixing it with a baffle so that the reaction gas can only flow in from the front pores of the RB-SiC ceramic and flow out from the back pores. Then, use the forced convection CVI process to deposit SiC in the internal pores of the RB-SiC ceramic to increase density.

[0019] S6. After step S5 is completed, the flow direction of the reaction gas in the horizontal reactor is reversed, so that the reaction gas flows in from the back pores of the RB-SiC ceramic and flows out from the front pores. SiC is deposited in the internal pores of the RB-SiC ceramic by forced convection CVI process to increase density.

[0020] After the densification of S7 and RB-SiC ceramics is completed, the baffles in the horizontal reactor are removed, and a SiC coating is deposited on the surface of the RB-SiC ceramics using CVD process.

[0021] S8. After the RB-SiC ceramic cools down, it is taken out to obtain the modified RB-SiC ceramic that can be used in environments above 1400℃.

[0022] Preferred technical solution: The heat treatment time in step S1 is 5-10 hours, and the vacuum degree in the vacuum furnace is <10Pa.

[0023] Preferred technical solution: The heat treatment time in step S2 is 10-20 hours, and the vacuum degree in the vacuum furnace is <10 Pa.

[0024] Preferred technical solution: The molar ratio of HF to HNO3 in the mixed acid solution in step S4 is 1:1 to 1:5, and the acid washing time is 12 to 24 hours.

[0025] Preferred technical solution: The reaction gases in steps S5 and S6 are all mixed gases of CH3Cl3Si-Ar-H2, wherein the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the deposition time, the temperature in the horizontal reactor is controlled at 900-1000℃, and the deposition time is 10-50h.

[0026] Preferred technical solution: The deposition conditions for the CVD sealing process in step S7 are as follows: CH3Cl3Si gas is introduced into a horizontal reactor. The deposition temperature in the horizontal reactor is 900-1000℃. After deposition for 3-10 hours, the deposition temperature is increased to 1400-1600℃ at a rate of 3-5℃ / min. After the temperature rise is completed, deposition continues for 5-20 hours. Finally, a dense SiC coating is deposited on the surface of the RB-SiC ceramic.

[0027] Preferred technical solution: The volume percentage of CH3Cl3Si gas decreases linearly from 50% to 10% over time during the heating phase.

[0028] Due to the application of the above technical solution, the beneficial effects of this invention are as follows:

[0029] (1) The present invention effectively removes the low-temperature phase free Si inside RB-SiC ceramic by vacuum heat treatment at 1500℃-1700℃. Further heating to 1900℃-2100℃ can cause the small SiC particles inside RB-SiC ceramic to sublimate and fuse at the large particles, resulting in recrystallization. This can not only further improve the mechanical strength of the material, but also open up the loose and fine pores left after the removal of free Si to form large pores, providing a channel for subsequent CVI process treatment.

[0030] (2) The forced convection CVI process is controllable. The reactant gas can easily enter the internal pores of the RB-SiC ceramic to react and generate SiC to fill the pores. The porosity and pore diameter gradually decrease with the increase of reaction time. The supply of reactant gas gradually decreases with the increase of reaction time to ensure the overall densification of the RB-SiC ceramic matrix. Through the forced convection CVI process, the pores of the modified RB-SiC ceramic can be filled to the maximum extent, and the porosity of the modified material is significantly reduced, and the mechanical properties at high temperature are significantly improved.

[0031] (3) CVD process is used to seal the surface of RB-SiC ceramics. During the process, the deposition temperature is gradually increased to ensure that the deposited SiC coating can be embedded into the pores on the surface of the RB-SiC ceramic substrate in the initial stage. Then, the SiC coating is rapidly deposited to achieve dense filling of the surface pores, which further improves the density and mechanical strength of the modified RB-SiC ceramics. Attached Figure Description

[0032] Figure 1 This is an internal cross-sectional photograph of SiC ceramic before modification by the forced convection CVI process in this invention;

[0033] Figure 2 This is an internal cross-sectional photograph of SiC ceramic modified by the forced convection CVI process in this invention;

[0034] Figure 3 This is a schematic diagram of the transverse cross-section inside the horizontal reactor in step S5 of the present invention;

[0035] Figure 4 This is a schematic diagram of the longitudinal cross-section inside the horizontal reactor in step S5 of the present invention;

[0036] Figure 5This is a schematic diagram of the longitudinal cross-section inside the horizontal reactor in step S6 of the present invention.

[0037] In the attached diagrams above, 1 represents RB-SiC ceramic; 2 represents a horizontal reactor; 3 represents the reaction gas; and 4 represents a baffle. Detailed Implementation

[0038] 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.

[0039] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance. The terms "horizontal," "vertical," and "suspended," etc., do not indicate that the component must be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0040] Example 1:

[0041] A modified RB-SiC ceramic with an overall density of 3.0 g / cm³. 3 The metal impurity content is less than 5 ppm, the porosity is less than 5%, and the pore size distribution is concentrated in the range of 1-10 μm. Furthermore, the porosity and pore size of the RB-SiC ceramic gradually decrease from the inside to the outside, with the outermost layer having a porosity of 0. Modified RB-SiC ceramics consist of a modified RB-SiC ceramic matrix and a SiC coating, wherein:

[0042] The modified RB-SiC ceramic matrix is ​​composed of α-SiC phase, β-SiC phase and free Si. In the cross-sectional photograph of the modified RB-SiC ceramic matrix, the ratio of α-SiC phase to β-SiC phase is 3:7, and the volume percentage of free Si in the modified RB-SiC ceramic matrix is ​​less than 1%.

[0043] The thickness of the SiC coating is 1-100 μm.

[0044] A method for preparing the above-mentioned modified RB-SiC ceramic:

[0045] S1. Select high-purity RB-SiC ceramic and place it in a vacuum furnace. Perform vacuum heat treatment at 1500℃ for 10 hours. The vacuum degree in the vacuum furnace is <10Pa. This is used to remove free Si from the RB-SiC ceramic and obtain RB-SiC ceramic with a porous structure.

[0046] S2. Subsequently, the temperature inside the vacuum furnace is raised to 1900℃ for vacuum heat treatment for 20 hours, with the vacuum level inside the furnace being <10Pa. This causes the SiC particles inside the RB-SiC ceramic to recrystallize and form a continuous three-dimensional network structure.

[0047] S3. Take out the heat-treated RB-SiC ceramic and process it into the required size and shape.

[0048] S4. Prepare a 3% mixed acid solution using hydrofluoric acid and nitric acid, with a molar ratio of HF to HNO3 of 1:1. Place the processed RB-SiC ceramic in the mixed acid solution for pickling for 24 hours, and then rinse it three times with pure water to remove residual free Si and metal impurities on the RB-SiC ceramic.

[0049] S5, such as Figure 3 and Figure 4 As shown, the cleaned RB-SiC ceramic 1 is placed in a horizontal reactor 2 for CVI process. The porous structure of the RB-SiC ceramic 1 is perpendicular to the flow direction of the reaction gas 3 in the horizontal reactor and fixed by a baffle 4, so that the reaction gas can only flow in from the front pores of the RB-SiC ceramic 1 and flow out from the back pores. The reaction gas 3 is a mixture of CH3Cl3Si-Ar-H2 gas, in which the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the deposition time. The temperature in the horizontal reactor 2 is controlled at 900-1000℃ and the deposition time is 10h. SiC is deposited in the internal pores of the RB-SiC ceramic 1 to increase density through the front forced convection CVI process.

[0050] S6, such as Figure 3 and Figure 5 As shown, after step S5 is completed, the flow direction of the reaction gas 3 in the horizontal reactor 2 is reversed, so that the reaction gas 3 flows in from the back pores of the RB-SiC ceramic 1 and flows out from the front pores. The reaction gas 3 is a mixture of CH3Cl3Si-Ar-H2 gas, in which the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the deposition time. The temperature in the horizontal reactor is controlled at 900-1000℃, and the deposition time is 10h. SiC is deposited in the internal pores of the RB-SiC ceramic through the back forced convection CVI process to increase density, thus ensuring the consistency of the performance of the RB-SiC ceramic in both directions.

[0051] After the densification of S7 and RB-SiC ceramics is completed, the baffles in the horizontal reactor are removed, and CVD process is used. The CVD sealing process is as follows: CH3Cl3Si gas is introduced into the horizontal reactor. After deposition at 900℃ for 10 hours, the deposition temperature is increased to 1400℃ at a rate of 3℃ / min. The volume percentage of CH3Cl3Si gas decreases linearly from 50% to 10% as the heating stage progresses. After the heating is completed, deposition continues for 20 hours, and finally a dense SiC coating is deposited on the surface of the RB-SiC ceramic.

[0052] S8. After the RB-SiC ceramic cools down, it is taken out to obtain the modified RB-SiC ceramic that can be used in environments above 1400 ℃.

[0053] like Figure 1 and Figure 2 As shown, in this embodiment, the porosity of RB-SiC ceramic modified by the forced convection CVI process is significantly reduced.

[0054] Example 2:

[0055] A modified RB-SiC ceramic with an overall density of 3.2 g / cm³. 3 The metal impurity content is less than 5 ppm, the porosity is less than 5%, and the pore size distribution is concentrated in the range of 1-10 μm. Furthermore, the porosity and pore size of the RB-SiC ceramic gradually decrease from the inside to the outside, with the outermost layer having a porosity of 0. Modified RB-SiC ceramics consist of a modified RB-SiC ceramic matrix and a SiC coating, wherein:

[0056] The modified RB-SiC ceramic matrix is ​​composed of α-SiC phase, β-SiC phase and free Si. In the cross-sectional photograph of the modified RB-SiC ceramic matrix, the ratio of α-SiC phase to β-SiC phase is 7:3, and the volume percentage of free Si in the modified RB-SiC ceramic matrix is ​​less than 1%.

[0057] The thickness of the SiC coating is 1-100 μm.

[0058] A method for preparing the above-mentioned modified RB-SiC ceramic:

[0059] S1. Select high-purity RB-SiC ceramic and place it in a vacuum furnace. Perform vacuum heat treatment at 1700℃ for 5 hours. The vacuum degree in the vacuum furnace is <10Pa. This is used to remove free Si from the RB-SiC ceramic and obtain RB-SiC ceramic with a porous structure.

[0060] S2. Subsequently, the temperature inside the vacuum furnace is raised to 2100℃ for vacuum heat treatment for 20 hours, with the vacuum level inside the furnace being <10Pa. This causes the SiC particles inside the RB-SiC ceramic to recrystallize and form a continuous three-dimensional network structure.

[0061] S3. Take out the heat-treated RB-SiC ceramic and process it into the required size and shape.

[0062] S4. Prepare a 3% mixed acid solution using hydrofluoric acid and nitric acid, with a molar ratio of HF to HNO3 of 1:1. Place the processed RB-SiC ceramic in the mixed acid solution for pickling for 24 hours, and then rinse it three times with pure water to remove residual free Si and metal impurities on the RB-SiC ceramic.

[0063] S5, such as Figure 3 and Figure 4 As shown, the cleaned RB-SiC ceramic 1 is placed in a horizontal reactor 2 for CVI process. The porous structure of the RB-SiC ceramic 1 is perpendicular to the flow direction of the reaction gas 3 in the horizontal reactor and fixed by a baffle 4, so that the reaction gas can only flow in from the front pores of the RB-SiC ceramic 1 and flow out from the back pores. The reaction gas 3 is a mixture of CH3Cl3Si-Ar-H2 gas, in which the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the deposition time. The temperature in the CVI horizontal reactor is controlled at 900-1000℃ and the deposition time is 10h. SiC is deposited in the internal pores of the RB-SiC ceramic to increase density through the front forced convection CVI process.

[0064] S6, such as Figure 3 and Figure 5 As shown, after step S5, the flow direction of the reaction gas 3 in the horizontal reactor 2 is reversed, so that the reaction gas 3 flows in from the back pores of the RB-SiC ceramic 1 and flows out from the front pores. The reaction gas 3 is a mixture of CH3Cl3Si-Ar-H2 gas, in which the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the deposition time. The temperature inside the horizontal reactor 2 is controlled at 900-1000℃, and the deposition time is 10h. SiC is deposited in the internal pores of the RB-SiC ceramic 1 through the back forced convection CVI process to increase density, thus ensuring the consistency of the performance of the RB-SiC ceramic 1 in both directions.

[0065] After the densification of S7 and RB-SiC ceramics is completed, the baffles in the horizontal reactor are removed, and CVD process is used. The CVD sealing process is as follows: CH3Cl3Si gas is introduced into the horizontal reactor. After deposition at 1000℃ for 3 hours, the deposition temperature is increased to 1600℃ at a rate of 5℃ / min. The volume percentage of CH3Cl3Si gas decreases linearly from 50% to 10% as the heating stage progresses. After the heating is completed, deposition continues for 5 hours, and finally a dense SiC coating is deposited on the surface of the RB-SiC ceramic.

[0066] S8. After the RB-SiC ceramic cools down, it is taken out to obtain the modified RB-SiC ceramic that can be used in environments above 1400 ℃.

[0067] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A modified RB-SiC ceramic that can be used in environments above 1400 degrees Celsius, characterized in that: Including modified RB-SiC ceramic matrix and SiC coating; The modified RB-SiC ceramic matrix is ​​composed of α-SiC phase, β-SiC phase and free Si. In the cross-sectional photograph of the modified RB-SiC ceramic matrix, the ratio of the α-SiC phase to the β-SiC phase is 3:7-7:

3. The volume percentage of free Si in the modified RB-SiC ceramic matrix is ​​less than 1%. The thickness of the SiC coating is 1-100 μm. The preparation method of this modified RB-SiC ceramic includes the following steps: S1. Select high-purity RB-SiC ceramic and place it in a vacuum furnace. Perform vacuum heat treatment at 1500℃-1700℃ to remove free Si from the high-purity RB-SiC ceramic and obtain RB-SiC ceramic with a porous structure. S2. Subsequently, the temperature inside the vacuum furnace is raised to 1900℃-2100℃ for vacuum heat treatment, which causes the SiC particles inside the RB-SiC ceramic to recrystallize and generate a continuous three-dimensional network structure. S3. Take out the RB-SiC ceramic after heat treatment and process the RB-SiC ceramic into the required size and shape; S4. Prepare a mixed acid solution with a concentration of 3%-10% using hydrofluoric acid and nitric acid. Place the processed RB-SiC ceramic in the mixed acid solution for acid washing, and then rinse it with pure water 3-10 times to remove residual free Si and metal impurities on the RB-SiC ceramic. S5. The cleaned RB-SiC ceramic is placed in a horizontal reactor, with the porous structure of the RB-SiC ceramic perpendicular to the flow direction of the reaction gas in the horizontal reactor and fixed by a baffle, so that the reaction gas can only flow in from the front pores of the RB-SiC ceramic and flow out from the back pores. SiC is deposited in the internal pores of the RB-SiC ceramic by forced convection CVI process to increase density. S6. After step S5 is completed, the flow direction of the reaction gas in the horizontal reactor is reversed, so that the reaction gas flows in from the back pores of the RB-SiC ceramic and flows out from the front pores; SiC is deposited in the internal pores of the RB-SiC ceramic by forced convection CVI process to increase density; S7. After the densification of the RB-SiC ceramic is completed, the baffle in the horizontal reactor is removed, and a SiC coating is deposited on the surface of the RB-SiC ceramic using CVD process. S8. After the RB-SiC ceramic has cooled down, it is taken out to obtain a modified RB-SiC ceramic that can be used in environments above 1400℃.

2. The modified RB-SiC ceramic according to claim 1, characterized in that: The overall density of the modified RB-SiC ceramic is 3.0-3.2 g / cm³. 3 The porosity is less than 5%, and the pore size distribution of the RB-SiC ceramic is concentrated in 1-10 μm. The porosity and pore size of the RB-SiC ceramic gradually decrease from the inside to the outside until the outer layer is completely dense and the porosity is 0.

3. The modified RB-SiC ceramic according to claim 1, characterized in that: The modified RB-SiC ceramic contains less than 5 ppm of metallic impurities.

4. A method for preparing modified RB-SiC ceramics as described in any one of claims 1-3, characterized in that... Includes the following steps: S1. Select high-purity RB-SiC ceramic and place it in a vacuum furnace. Perform vacuum heat treatment at 1500℃-1700℃ to remove free Si from the high-purity RB-SiC ceramic and obtain RB-SiC ceramic with a porous structure. S2. Subsequently, the temperature inside the vacuum furnace is raised to 1900℃-2100℃ for vacuum heat treatment, which causes the SiC particles inside the RB-SiC ceramic to recrystallize and generate a continuous three-dimensional network structure. S3. Take out the RB-SiC ceramic after heat treatment and process the RB-SiC ceramic into the required size and shape; S4. Prepare a mixed acid solution with a concentration of 3%-10% using hydrofluoric acid and nitric acid. Place the processed RB-SiC ceramic in the mixed acid solution for acid washing, and then rinse it with pure water 3-10 times to remove residual free Si and metal impurities on the RB-SiC ceramic. S5. The cleaned RB-SiC ceramic is placed in a horizontal reactor, with the porous structure of the RB-SiC ceramic perpendicular to the flow direction of the reaction gas in the horizontal reactor and fixed by a baffle, so that the reaction gas can only flow in from the front pores of the RB-SiC ceramic and flow out from the back pores. SiC is deposited in the internal pores of the RB-SiC ceramic by forced convection CVI process to increase density. S6. After step S5 is completed, the flow direction of the reaction gas in the horizontal reactor is reversed, so that the reaction gas flows in from the back pores of the RB-SiC ceramic and flows out from the front pores; SiC is deposited in the internal pores of the RB-SiC ceramic by forced convection CVI process to increase density; S7. After the densification of the RB-SiC ceramic is completed, the baffle in the horizontal reactor is removed, and a SiC coating is deposited on the surface of the RB-SiC ceramic using CVD process. S8. After the RB-SiC ceramic has cooled down, it is taken out to obtain a modified RB-SiC ceramic that can be used in environments above 1400℃.

5. The method for preparing modified RB-SiC ceramics according to claim 4, characterized in that: The heat treatment time in step S1 is 5-10 hours, and the vacuum degree in the vacuum furnace is <10 Pa.

6. The method for preparing modified RB-SiC ceramics according to claim 4, characterized in that: The heat treatment time in step S2 is 10-20 hours, and the vacuum degree in the vacuum furnace is <10 Pa.

7. The method for preparing modified RB-SiC ceramics according to claim 4, characterized in that: In step S4, the molar ratio of HF to HNO3 in the mixed acid solution is 1:1 to 1:5, and the acid washing time is 12-24 h.

8. The method for preparing modified RB-SiC ceramics according to claim 4, characterized in that: The reaction gases in steps S5 and S6 are all mixed gases of CH3Cl3Si-Ar-H2, wherein the volume percentage of CH3Cl3Si gas decreases linearly from 10% to 2% with the reaction time. The temperature inside the horizontal reactor is controlled at 900-1000℃, and the deposition time is 10-50h.

9. The method for preparing modified RB-SiC ceramics according to claim 4, characterized in that: The CVD sealing process deposition conditions in step S7 are as follows: CH3Cl3Si gas is introduced into the horizontal reactor. After deposition at 900-1000℃ for 3-10 hours, the deposition temperature is increased to 1400-1600℃ at a rate of 3-5℃ / min. After the temperature increase is completed, deposition continues for 5-20 hours, and finally a dense SiC coating is deposited on the surface of the RB-SiC ceramic.

10. The method for preparing modified RB-SiC ceramics according to claim 9, characterized in that: The volume percentage of CH3Cl3Si gas decreased linearly from 50% to 10% over time during the heating phase.

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