JPH10167861A - Antioxidation treatment of carbon material reinforced carbon fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a carbon material which shows excellent oxidation resistance in a rapid high temp. oxidative atmosphere under low pressure by forming a SiC coating film on a carbon material reinforced a SiC coated carbon fiber by combining CVD precipitation coating with heat treatment. SOLUTION: A carbon fiber reinforced carbon material is used as a base body. (1) A SiC coating layer is formed by bringing SiO2 gas into contact with the surface of the base body by conversion method. (2) A mixed gas of halogenated org. silicon and hydrogen, or a mixture of silicon halide, hydrocarbon and hydrogen is used to deposit SiC to 50 to 100μm thickness by CVD method by heating at 1100 to 1550 deg.C. Then the carbon material is cooled to room temp., heat treated at the same temp. as the film forming temp. in an inert atmosphere for >=1 hours and then cooled. After this procedure is carried out at least once, SiC is further applied by CVD method and the carbon material is cooled to room temp. and then heat treated at 1600 to 1900 deg.C for >=1 hours in an inert gas atmosphere. (3) The carbon material is impregnated with a glass precursor soln. containing at least one of Si, Al, B and Zr, dried and heated at 500 to 1000 deg.C to form a vitreous film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a carbon fiber reinforced carbon material (hereinafter referred to as "C / C material") which exhibits excellent oxidation resistance in a severe high-temperature oxidizing atmosphere which is rapidly heated under low pressure. The present invention relates to an oxidation resistant treatment method.

[0002]

2. Description of the Related Art C / C materials have excellent specific strength and specific elastic modulus, as well as excellent heat resistance and chemical stability. Therefore, they are used as structural materials in many fields including aerospace. Although useful, it has a material defect inherent in carbon materials such as easy oxidizability, which is the biggest bottleneck in versatility. For this reason, various attempts to improve the oxidation resistance by forming an oxidation-resistant coating layer on the surface of the C / C material have been conventionally studied. For example, ZrO ₂ , Al ₂ O ₃ ,
A method of coating a ceramic material such as SiC or Si ₃ N ₄ has been proposed. However, except for the SiC coating layer, delamination and cracks occur at the coating interface in a thermal cycle during use, and the oxidation resistance cannot be sufficiently imparted.

[0003] As a method of forming a SiC coating layer on the surface of a C / C substrate, a CVD method (chemical vapor deposition method) in which SiC generated by a gas phase reaction is directly deposited and a carbon of the substrate are reacted. There is known a conversion method of converting SiC by reacting with a silicon component by using it as a source.
Among them, the SiC coating layer formed by the CVD method has a clear separation at the interface with the base material. When a thermal shock is applied, delamination easily occurs due to a difference in thermal expansion between the SiC coating layers. It cannot exhibit oxidation resistance. In order to overcome this drawback and reduce the difference in the coefficient of thermal expansion, carbon is deposited on the surface of the carbon / carbon composite material by vapor phase pyrolysis, and then the ceramic is deposited on this surface by vapor phase pyrolysis. A method for producing a carbon fiber reinforced composite material having oxidation resistance has been proposed (JP-A-2-111681).

On the other hand, in the case of the conversion method, since the surface layer of the base material is made of a functionally graded material forming a SiC layer as a continuous structure, no interface separation occurs.
Compared with the VD method, there is a problem that the structure is inferior in density and that a minute crack is generated in the coating layer during the reaction for converting to SiC. In order to solve such a problem, the applicant of the present invention has a first coating step of forming a first SiC coating layer by a conversion method by bringing an SiO gas into contact with a C / C substrate surface, and then a CVD method. And a second coating step for sequentially depositing and depositing amorphous SiC by reducing and pyrolyzing the halogenated organosilicon compound (JP-A-4-12078). (2) An oxidation-resistant treatment method in which SiC is deposited and deposited by a pulse CVI method in which a substrate structure is intermittently filled with a halogenated organosilicon compound in a substrate structure and reduced and thermally decomposed (JP-A-4-42878) has been proposed.

However, micro cracks are also generated in the SiC of the second coating layer formed by such a method, and it is necessary to plug these cracks in order to provide higher oxidation resistance. Was done. Therefore, the present applicant has
A first coating layer made of a polycrystalline SiC coating having a gradient function on the surface of the C base material, a second coating layer made of an amorphous or fine polycrystalline SiC coating, and B ₂ O ₃ —Si
Oxidation-resistant carbon fiber reinforced carbon material with a third coating layer of O ₂ glass coating laminated thereon was developed (Japanese Patent Laid-Open No. 4-243989).
No.).

[0006] Further, the present applicant has disclosed the above-mentioned Japanese Patent Application Laid-Open No.
No. 3989, the first coating step of forming a SiC film on a C / C substrate surface by a conversion method, and an amorphous material formed by a pulse CVI method using a mixed gas of a halogenated organosilicon compound and hydrogen. A second step of sequentially performing a first step operation of forming a SiC film of No. 1 and a second step operation of forming a fine polycrystalline SiC film at a heating temperature of 1200 to 1400 ° C., and then B (OC ₁₂ H ₂₇ )
₃ and Si (OC ₂ H ₅ ) ₄ are hydrolyzed and polymerized by the alkoxide method, and a glass precursor solution is vacuum impregnated with B.
_A method for producing an oxidation-resistant C / C composite material comprising a third coating step of forming a surface layer made of a ₂ O ₃ —SiO ₂ glass coating was developed (Japanese Patent Laid-Open No. 6-48872).

[0007]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 4-24 / 1990.
According to the inventions of JP-A-3989 and JP-A-6-48872, sufficiently stable oxidation resistance is exhibited even in a severe high-temperature oxidizing atmosphere. In the oxidation resistance test (low-pressure volatilization test) under high temperature and low pressure conducted on the assumption of the back side of the nose cap etc. of the spacecraft, it was found that the erosion resistance was not sufficient. In particular, it has been found that the crystal state of the SiC coating formed as the second coating layer has a great effect on the erosion resistance.

Therefore, the applicant of the present invention has proposed that S / C
a first coating step of forming a SiC coating layer by a conversion method by contacting an iO gas, and using a mixed gas of a halogenated organosilicon compound and hydrogen or a silicon halide, a hydrocarbon and hydrogen by a CVD method to form a 1400 to 150
A second coating step of performing a first-stage operation of heating to a temperature of 0 ° C. to deposit and coat SiC and a second-stage operation of performing a heat treatment at a temperature of 1600 to 1900 ° C. in an inert atmosphere; A third coating step of impregnating a glass precursor solution obtained by hydrolyzing a metal alkoxide containing at least one of B and Zr, followed by heat treatment at a temperature of 500 to 1000 ° C. to form a vitreous coating. C /
An oxidation-resistant treatment method for C material (Japanese Patent Application No. 8-20438) was proposed.

According to the method of Japanese Patent Application No. 8-20438, crystal defects and crystal irregularities of the SiC film deposited and coated in the second coating step are corrected, and high crystallization is achieved. Oxidation treatment with high erosion resistance can be performed even in the low pressure volatilization test.

However, subsequent studies have shown that CVD
When the SiC coating layer is thickened by the reaction, the residual stress is insufficiently removed and the residual stress accumulates, so that a phenomenon of peeling off from the C / C substrate surface easily occurs.
It was found that it became remarkable above m.

The present invention has been completed on the basis of the above findings, and has as its object to be free from peeling from a substrate even when exposed to a severe high-temperature oxidizing atmosphere rapidly heated under a low pressure, and to have an erosion resistance. An object of the present invention is to provide a method for an oxidation-resistant treatment of a C / C material having excellent properties.

[0012]

According to the present invention, there is provided an oxidation-resistant treatment method for a carbon fiber reinforced carbon material, which comprises the steps of:
a first coating step of forming an SiC coating layer by a conversion method by bringing an iO gas into contact therewith; 1100 to 155 by a CVD method using a mixed gas of a halogenated organosilicon compound and hydrogen or a mixed gas of a silicon halide, a hydrocarbon and hydrogen.
A second coating step of heating to a temperature of 0 ° C. to deposit and coat SiC, impregnating and drying a glass precursor solution obtained by hydrolyzing a metal alkoxide containing at least one of Si, Al, B and Zr. A third coating step of forming a glassy film by heat treatment at a temperature of 500 to 1000 ° C .;
A carbon fiber reinforced carbon material having an oxidation resistance of 1100 to 1550 by the CVD method in the second coating step.
The thickness of the SiC film formed by heating to a temperature of
After forming a film to a thickness of 100 μm, the film is cooled to room temperature, and then heated in an inert atmosphere at the same temperature as the film forming temperature by the CVD reaction.
Heat treatment for more than an hour and cool to room temperature again, CVD Si
After performing at least one C precipitation-heat treatment operation,
SiC is deposited and coated by the VD reaction and then cooled to room temperature, and finally in an inert atmosphere at 1600 to 1900 ° C.
The structure is characterized in that a heat treatment is performed at a temperature of 1 hour or more.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The C / C material serving as the base material of the present invention is:
A prepreg is formed by impregnating or applying a matrix resin solution with a high carbonization residual rate to carbon fiber woven fabric, felt, tow, and other reinforcing fibers, and the prepreg is formed by lamination, then cured and fired and carbonized. Is done. Various kinds of carbon fibers such as polyacrylonitrile, rayon and pitch are used as the reinforcing carbon fiber, and phenol, furan and other liquid thermosetting resins with good carbonization are used as the matrix resin. You. The produced C / C material can be densified by repeating the process of impregnating, hardening, and carbonizing the matrix resin liquid as necessary.

In a first coating step of forming a SiC coating layer by a conversion method, SiO ₂ powder is mixed with Si or C powder, stored in a closed heating system, and a C / C material is set or buried in the system. The reaction is performed by a heating reaction. The conditions include Si or C for SiO ₂ .
In a weight ratio of 2: 1, and a heating temperature of 1800 to 20
It is preferable that the temperature is set to 00 ° C. and the inside of the system is kept in a reducing or neutral atmosphere. During heating, SiO ₂ is reduced by heating with Si or C components to generate SiO gas,
O gas reacts with the carbon structure of the C / C material to form a surface layer of SiC.
Converts into a functionally graded coating layer whose concentration changes continuously at the interface. The SiC coating layer formed in the first coating step functions as a thermal stress relieving layer between the SiC coating layer formed in the second coating step by the CVD method and the C / C base material. The thickness of the SiC coating layer is preferably set in the range of 50 to 100 μm in order to minimize a decrease in the strength of the / C base material.

In the second coating step, SiC is deposited and deposited on the SiC coating layer formed in the first coating step by CVD, and the halogenated organic silicon compound and hydrogen or the silicon halide and hydrocarbon and hydrogen are deposited. By continuously contacting the mixed gas with the C / C material heated to a temperature of 1100 to 1550 ° C. in a gas state in the quartz reaction chamber, SiC is deposited and coated. Trichloromethylsilane (CH ₃ SiCl ₃ ) is preferably used as the halogenated organosilicon compound, and tetrachlorosilane, trichlormethylsilane and the like are used as the silicon halide, and methane and ethane are used as the hydrocarbon. . These raw materials are mixed at an appropriate ratio, and the mixed gas is supplied to a reaction chamber in which the C / C material is heated, and SiC is deposited and coated by a CVD reaction. If the heating temperature is lower than 1100 ° C., the film forming rate is low, which is not practical. If the heating temperature is higher than 1550 ° C., grain growth and whisker formation are involved, so that it is difficult to form a uniform film.

According to the present invention, the CVD reaction is interrupted and cooled to room temperature before the film thickness of SiC deposited and coated in the second coating step is increased, that is, when the film is formed to a thickness of 50 to 100 μm. Heating in an inert atmosphere
This is a heat treatment operation in which the film is kept at the same temperature as the film formation temperature by the VD reaction for 1 hour or more and cooled to room temperature again. In this CVD SiC deposition-heat treatment operation, the SiC coating layer deposited by the CVD reaction cracks in the course of being cooled to room temperature, and the residual stress accumulated during the CVD reaction is released. The cracks in the SiC coating layer are closed at the time of the heat treatment in the subsequent heat treatment operation, and become fine cracks in the process of cooling to room temperature again, and the residual stress is further released. This CVD SiC deposition-heat treatment operation results in CV
The residual stress accumulated during the D reaction is efficiently released, and the stress can be effectively relaxed. Si by CVD reaction
It is necessary to carry out the operation of C deposition-heat treatment at least once until a coating layer of a predetermined thickness is formed. Thereafter, SiC is deposited and coated by the above-mentioned CVD reaction, cooled to room temperature, and finally inertized. 1600-1900 ° C in atmosphere
Is heated for 1 hour or more to form a SiC coating layer.

SiC Precipitation Coating-In the heat treatment operation, 1
The reason why the film thickness formed by one CVD reaction is set in the range of 50 to 100 μm is that if the film thickness is less than 50 μm, the processing efficiency is poor, while if it exceeds 100 μm, the release of residual stress is not sufficient. The formation of the SiC coating layer by the CVD reaction is performed at least once, but the total thickness of the SiC coated in the second coating step is preferably 100 μm or more. By performing the CVD SiC deposition-heat treatment operation, the residual stress during film formation can be effectively removed by giving a thermal history without changing the crystallinity of SiC. In addition, the final heat treatment temperature is 160
The reason for setting the temperature in the range of 0 to 1900 ° C. is that if the temperature is lower than 1600 ° C., the release of residual stress is insufficient and the correction of crystal defects and crystal irregularities of SiC is not sufficient. It is. Preferably, the temperature is set in the range of 1700 to 1800 ° C. Thus, the residual stress accumulated by the CVD reaction can be effectively released, and the SiC film can be highly crystallized. Further, the diffusion and penetration of Si into the C / C substrate are promoted, and the adhesion to the substrate is improved.

In the second coating step, the internal stress accumulated in the C / C base during the CVD reaction is efficiently released, so that the residual stress is reduced, and the diffusion of Si into the C / C base is promoted. This improves the adhesion to the substrate, thereby preventing the SiC coating layer from peeling off, and improving the erosion resistance at high temperatures and low pressures by improving the crystal structure of SiC by heat treatment. Become.

The third coating step is a step of forming and coating a vitreous coating on the SiC coating layer on the surface of the C / C substrate which has been subjected to the treatment of the second coating step in this way. The composition of the vitreous coating is SiO ₂ , Al ₂ O ₃ , B ₂ O ₃ , ZrO ₂
Or a simple compound such as Si, Al, B,
A glass precursor solution is prepared by hydrolyzing a metal alkoxide containing at least one of Zr, and C / C
A vitreous film is formed by a method of impregnating a substrate by dipping or coating, drying, and then heat-treating the substrate at a temperature of 500 to 1000 ° C. The glass precursor solution is
Si (OC ₂ H ₅ ) ₄ , B (OC ₄ H ₉ ) ₃ , Zr (OC ₄
It is prepared by an alkoxide method in which water is added dropwise to a solution obtained by adding an alcohol to a metal alkoxide such as H ₉ ) ₄ and stirring and mixing the mixture, followed by hydrolysis. The vitreous coating thus formed is filled and filled with fine cracks generated in the SiC coating layer formed in the second coating step, and the entire surface is sealed, and the outside air is shut off under an oxidizing atmosphere. Functions as a barrier for oxygen that diffuses and enters.

[0020]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples.

Example 1 (1) Preparation of C / C substrate A polyacrylonitrile-based high-strength and high-elasticity type plain woven carbon fiber woven fabric was matrixed with a phenol resin precondensate (manufactured by Dainippon Ink and Chemicals, Inc.). It was sufficiently applied as a resin and air-dried for 48 hours to prepare a prepreg sheet. Fourteen of these prepreg sheets were stacked and placed in a mold to form a composite under the conditions of a heating temperature of 110 ° C. and a pressure of 20 kg / cm ² . Next, after the molded body was heated to a temperature of 250 ° C. to completely cure the matrix resin, it was transferred into a firing furnace maintained in a nitrogen gas atmosphere, and was heated at a temperature rising rate of 5 ° C./hr to 2000 ° C.
The temperature was raised to ℃, and the temperature was maintained for 5 hours to carry out calcining and carbonization. Thus, the carbon fiber volume content (Vf) 65
%, And a C / C substrate having an apparent specific gravity of 1.65 g / cc was prepared.

(2) First coating step The SiO ₂ powder and the Si powder are mixed in a mixing ratio of 2: 1 (weight ratio), and the mixed powder is put into a graphite crucible and a C / C material (vertical and horizontal) 50 mm and a thickness of 5 mm). The graphite crucible was transferred into an electric furnace, and the inside was sufficiently replaced with argon gas. Then, the temperature was raised to 1850 ° C. at a rate of 50 ° C./hr, and the temperature was maintained for 1 hour to incline the surface layer of the C / C base material. A polycrystalline SiC coating layer having a function was formed. Although the thickness of the formed SiC coating layer was about 50 μm, it was recognized that fine cracks having a width of about several μm were generated on this surface.

(3) Second coating step A C / C material having a SiC coating layer formed by the conversion method in the first coating step is set in a reaction tube of a CVD apparatus.
After sufficiently replacing the inside of the tube with argon gas, the temperature of the C / C material was raised to 1300 ° C. by high-frequency induction heating. Then, a mixed gas of trichloromethylsilane (CH ₃ SiCl ₃ ) and H ₂ (molar ratio CH ₃ SiCl ₃ / H ₂ = 1: 20) is introduced, and CVD is performed.
After depositing and coating polycrystalline SiC to a thickness of 70 μm on the SiC coating layer formed in the first coating step by the reaction,
The introduction of the raw material gas was stopped to interrupt the CVD reaction, and the system was cooled to room temperature. Next, the inside of the reaction tube was replaced with an argon gas atmosphere, heat-treated at a temperature of 1300 ° C. for 1 hour, and then cooled to room temperature. After performing the CVD SiC deposition-heat treatment operation twice in this manner, the SiC is deposited by the above-described CVD reaction.
After C was deposited and then cooled to room temperature, it was finally heated in an argon gas atmosphere at a temperature of 1800 ° C. for 1 hour to deposit and coat 140 μm-thick SiC.

(4) Third coating step Si (OC ₂ H ₅ ) ₄ and ethanol were mixed in a molar ratio of 2: 1 and the mixture was refluxed and stirred at a temperature of 70 ° C. A mixed aqueous solution of 25 moles of water and 0.2 moles of NH ₄ OH was added dropwise to 1 mole of Si (OC ₂ H ₅ ) ₄ (pH: 12.0), and the mixture was stirred to form a spherical SiO 2 of about 0.2 μm. A suspension in which _two fine particles were uniformly dispersed was prepared. The C / C material treated in the second coating step was immersed in the suspension and impregnated under reduced pressure for 15 minutes. Next, after air-drying, the operation of applying the suspension and air-drying was repeated three times, and then dried at a temperature of 100 ° C. to form an intermediate layer composed of SiO ₂ fine particles. This C / C material is B (OC
₄ H ₉ ) ₃ solution, impregnated under reduced pressure for 15 minutes, air-dried all day and night, and hydrolyzed with water in the air.
After drying at ° C. of temperature, to form a further 500 ° C. deeds for 15 minutes heat treatment at a temperature B ₂ O ₃ glassy coating.

[0025] Then, Si (OC ₂ H _{5) 4} and ethanol in a molar ratio of 1: included in an amount ratio of 4.5, the mixed solution which had been stirred under reflux at room temperature Si (OC ₂ H _{5) 4} A mixed aqueous solution of 2.5 mol of water and 0.03 mol of HCl per 1 mol was added dropwise and stirred (pH: 3.0) to prepare a SiO ₂ glass precursor solution. A C / C material on which a B ₂ O ₃ vitreous film was formed was put into the glass precursor solution, impregnated under reduced pressure for 15 minutes, and then dried at 100 ° C. This C / C material was again charged into the B (OC ₄ H ₉ ) ₃ solution and
Impregnation under reduced pressure was performed for 5 minutes, air-dried all day and night, and hydrolyzed with moisture in the air. Next, it was dried at a temperature of 100 ° C. and further heat-treated at a temperature of 800 ° C. for 60 minutes in an argon gas atmosphere to form a B ₂ O ₃ —SiO ₂ glassy film.

(5) Oxidation resistance test An oxidation resistance test was carried out on the C / C material subjected to the above-mentioned oxidation treatment by using a low-pressure volatilization test apparatus by the following method. The pressure in the low-pressure volatilization test apparatus was reduced to 1000 Pascal (Pa), the light emitted by the xenon lamp was condensed, and irradiated to the surface of the C / C material, and the temperature was raised to 1700 ° C., and the state was maintained for 1100 seconds. After the irradiation, the peeling state of the SiC coating layer, the weight reduction of the C / C material, and the thickness reduction of the SiC coating layer were measured.

Examples 2 to 9, Comparative Examples 1 to 7 Conditions such as the deposition temperature of SiC by CVD in the second coating step, the thickness of the coating, the presence or absence of a heat treatment in an argon gas atmosphere, and the processing temperature. Change to C
The first coating was performed in the same manner and under the same conditions as in Example 1 except that the VD method SiC deposition-heat treatment was performed, the coating was performed by changing the number of repetitions of this operation, and the final heat treatment temperature was changed. The process of the process and the 3rd coating process was performed. An oxidation resistance test was performed on these C / C materials in the same manner as in Example 1.

With respect to the obtained results, the coating treatment conditions in the second coating step are shown in Table 1, and the results of the oxidation resistance test are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

From the results of Tables 1 and 2, the C / C materials of the examples subjected to the oxidation-resistant treatment by applying the conditions of the present invention were the C / C of Comparative Examples 1 to 7 which did not satisfy the conditions of the present invention. High temperature compared to the material,
It can be seen that it has excellent oxidation resistance under low pressure, and has high adhesion to the substrate because the residual stress is effectively released, and it is difficult to peel off even when exposed to a severe oxidizing atmosphere. Further, in Comparative Examples 2 to 4 in which the final heat treatment was performed without performing a heat treatment operation after the SiC was deposited and coated by the CVD method, it was found that the oxidation resistance and the adhesion to the substrate were further inferior.

[0032]

As described above, according to the oxidation-resistant treatment method of the present invention, the SiC coating layer of the functionally graded structure formed by the conversion method in the first coating step is formed on the SiC coating layer by the CVD method as the second coating step. After forming the film to a thickness of 50 to 100 μm, the film is cooled to room temperature, and then the CVD is performed in an inert atmosphere.
After performing at least one CVD SiC deposition-heat treatment operation of heating at the same temperature as the film formation temperature by the reaction and cooling to room temperature again, SiC is deposited and coated by the CVD reaction, and after cooling to room temperature, it is finally unreacted. 1600 in an active atmosphere
By performing a heat treatment at a temperature of about 1900 ° C. and forming a vitreous film as a third coating step, the residual stress accumulated during the CVD reaction is smoothly released, thereby improving the adhesion of the SiC film and improving the crystal structure. , And exfoliation does not occur even in a severe oxidizing atmosphere under high temperature and low pressure, and excellent oxidation resistance can be imparted. Therefore, it is extremely useful as an oxidation-resistant treatment method for structural members exposed to severe conditions of high temperature and low pressure in outer space.

Claims (2)

[Claims] 1. A first coating step in which a carbon fiber reinforced carbon material is used as a base material and an SiO gas is brought into contact with the surface of the base material to form a SiC coating layer by a conversion method. 110 by a CVD method using a mixed gas of silicon carbide, hydrocarbon and hydrogen.
A second coating step of heating to a temperature of 0 to 1550 ° C. to deposit and coat SiC, and impregnating a glass precursor solution obtained by hydrolyzing a metal alkoxide containing at least one of Si, Al, B and Zr. 500-100 after drying
3rd heat treatment at a temperature of 0 ° C. to form a vitreous film
A coating step, wherein the carbon fiber reinforced carbon material is subjected to the oxidation-resistant treatment by the CVD method in the second coating step.
The SiC film formed by heating to a temperature of １５1550 ° C. is formed to a thickness of 50 to 100 μm, then cooled to room temperature, and then at least one hour at the same temperature as the film forming temperature by the CVD reaction in an inert atmosphere. Heat treated and cooled to room temperature again, C
After performing the VD method SiC deposition-heat treatment operation at least once, the SiC is deposited and coated by the above-described CVD reaction, and then cooled to room temperature, and finally is placed in an inert atmosphere at 1600 to 1600.
A heat treatment method for a carbon fiber reinforced carbon material, wherein the heat treatment is performed at a temperature of 1900 ° C. for 1 hour or more. 2. The method according to claim 1, wherein the thickness of the SiC deposited and coated in the second coating step is 100 μm or more.