JP3193762B2 - Oxidation resistant treatment of carbon fiber reinforced carbon material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxidation-resistant method for a carbon fiber reinforced carbon material (hereinafter referred to as "C / C material") exhibiting excellent oxidation resistance in a severe high-temperature oxidizing atmosphere.

[0002]

2. Description of the Related Art C / C materials exhibit excellent specific strength and specific elasticity even in a high temperature range of 1000 ° C. or higher, and have unique material characteristics such as a small coefficient of thermal expansion. Aviation /
Application as space material is considered. However, this material has an inherent property of carbon material, which is easily oxidizable, and is oxidized in the atmosphere at around 500 ° C, and its material properties rapidly decrease. It was not possible except in applications that required only a short time. For this reason, C /
Attempts have been made actively to modify the surface of the C material by applying an oxidation-resistant coating with various heat-resistant and corrosion-resistant ceramic-based materials to modify the material.
C is the most preferred coating material.

Usually, as a method of coating SiC on the surface of a carbon material, a CVD method (chemical vapor deposition method) in which SiC generated by a gas phase reaction is directly deposited is used as a general coating method. . Although it is possible to form a dense oxidation-resistant film by this CVD method, the coating layer peels off or cracks during cooling because the carbon material used as the base material must be heated to 1000 ° C or more in the coating process. Phenomena are likely to occur. These phenomena are caused by the base carbon and the coated Si
This is due to the fact that the difference in thermal expansion of C is so large that the maximum strain cannot follow. Therefore, the problem can be solved by adjusting the thermal expansion coefficient of the base carbon to be equal to that of SiC. However, in the case of the C / C material, the thermal expansion coefficient of the material is the thermal expansion coefficient of the carbon fiber as the composite reinforcing material. Can not be adjusted freely. For this reason, C / C
When a material is used as a base material, it is extremely difficult to form a normal SiC coating layer by the CVD method.

On the other hand, in a conversion method in which carbon of a base material is used as a reaction source and reacted with a Si component to convert the Si component into SiC, the surface layer of the base material has a continuous structure of S
Since the material becomes a functionally graded material for forming the iC layer, even if the C / C material is used as a coating substrate, the SiC coating layer does not undergo interface peeling as seen in the CVD method. However, in the conversion method, the formed SiC coating layer is C
In addition to the inferior compactness as compared with the VD method, there is a problem that fine cracks occur in the coating layer during the reaction.

The present applicant has developed many techniques for solving such a problem. Among them, the present inventor has found that molybdenum disilicide as an outer layer is formed on an SiC coating layer formed by a conversion method. An oxidation-resistant treatment method for a C / C material that forms an oxidation-resistant film has been developed and has already been proposed as Japanese Patent Application No. 3-25644.

[0006]

The oxidation-resistant treatment method disclosed in Japanese Patent Application No. 3-25644 consists of forming a SiC coating layer on a substrate surface of a carbon fiber reinforced carbon material by contacting SiO gas with a conversion method. And an outer layer coating step of applying a water-based slurry containing particulate molybdenum disilicide and a silicon-containing binder and then heating to form a porous MoSi ₂ coating layer. High oxidation resistance is imparted by the structure. But,
In subsequent evaluation studies, the outer layer of MoSi ₂ showed excellent oxidation resistance under static conditions in an electric furnace, but the coating layer scattered under severe dynamic oxidation conditions such as plasma irradiation. It was found that sufficient oxidation resistance was not exhibited.

The present inventor has continued research on the cause elucidation and coating improvement. As a result, in the prior art, the cause of the scattering of the outer layer of MoSi ₂ under the dynamic oxidation condition is that the molybdenum disilicide and the aqueous solution containing the silicon-containing binder are contained. There is insufficient adhesion between the MoSi ₂ outer layer and the SiC inner layer formed by applying the slurry and then heating to about 300 ° C. In order to increase the adhesion between both layers to a level that can withstand dynamic oxidation conditions, It has been clarified that it is extremely effective to once melt the applied MoSi ₂ layer and diffuse it into the underlying SiC layer structure to interpose the intermediate composite layer of SiC—MoSi ₂ .

The present invention has been developed on the basis of the above findings, and its object is to improve the prior application (Japanese Patent Application No. Hei 3-25644) to improve even under severe dynamic oxidation conditions such as plasma irradiation. C / which can provide stable high oxidation resistance
An object of the present invention is to provide an oxidation-resistant treatment method for C material.

[0009]

According to the present invention, there is provided an oxidation-resistant treatment method for a C / C material according to the present invention. An inner layer coating step of forming a SiC coating layer by applying, an aqueous slurry containing granular molybdenum disilicide and an organic binder is applied, and then heated to a high temperature of 2000 ° C. or more under an inert atmosphere to melt as MoSi ₂ , An intermediate layer coating step of diffusing into the inner layer SiC coating structure to form a composite film of SiC-MoSi ₂ , and then applying the same aqueous slurry as the intermediate layer coating step, followed by a temperature of up to 2000 ° C. in an inert atmosphere. And an outer layer coating step of forming a porous MoSi ₂ film by heating the film.

The carbon fibers constituting the C / C base material include woven fabrics such as plain weave, satin weave and twill weave made of various materials such as polyacrylonitrile, rayon and pitch materials in one-dimensional or multi-dimensional directions. Oriented fiber bodies, felts and tows are used.
A highly carbonized liquid thermosetting resin such as furan or a thermoplastic substance such as tar pitch is used. The carbon fiber is sufficiently wetted with a matrix resin by means of impregnation, coating, or the like, then semi-cured to form a prepreg, and then laminated and pressed. The molded body is heated to completely cure the resin component, and subsequently calcined or carbonized or graphitized in a conventional manner to obtain a C / C base material. Depending on the application, the process of impregnation, curing, and carbonization of the matrix resin may be repeated,
The structure can be densified by depositing pyrolytic carbon using methane, propane, or the like as a raw material by using the method.

The C / C substrate is first coated with an inner layer for forming a SiC film by a conversion method by bringing a SiO gas into contact with the substrate surface. This inner layer coating step
Put SiO ₂ powder in a closed heating system is mixed with Si or C powder, sets C / C substrate to the system by 1700-2000 ° C.
Is carried out by heating to a temperature of By heating, SiO ₂ is reduced by Si or C components, and the SiO gas generated by the reaction undergoes an interfacial reaction with the carbon structure constituting the C / C material to convert the surface layer into SiC. By the coating treatment, C /
A polycrystalline SiC film having a functionally graded structure in which SiC gradually becomes denser as the surface layer portion of the C base material moves toward the outer surface is formed. Suitable thickness of SiC film to be formed is 100 to 300 μm
If the thickness is less than 100 μm, the oxidation resistance may be insufficient, and if it exceeds 300 μm, the coating may peel off during a rapid thermal cycle load.

In the intermediate layer coating step, a slurry containing granular molybdenum disilicide and an organic binder is applied at the stage of forming a SiC-MoSi ₂ composite film having a continuous structure on the SiC coating layer formed in the inner layer coating step. Thereafter, the mixture is heated to a high temperature of 2000 ° C. or more in an inert atmosphere to melt as MoSi ₂ , which is diffused into the SiC coating structure to form SiC-Mo.
This is performed in the step of forming a composite film of Si ₂ . Molybdenum disilicide uses a fine powder having a particle size of 20 μm or less and is dispersed in water together with a silicon-containing binder such as sodium silicate, ethyl silicate and colloidal silica to form an aqueous slurry. It is desirable to adjust the viscosity of the aqueous slurry to 500 to 2000 cp by controlling the composition. The coating is performed using a doctor blade method, a spatula coating, a brush coating, a spray injection, and the like, and is sufficiently processed until the entire surface is uniformly coated. The C / C substrate after application is dried and then dried under an inert atmosphere in a temperature range of 2000 ° C. or higher, preferably
Once heated to the temperature range of 2050-2150 ° C, Mo
The molten MoSi ₂ is melted as Si ₂ , and heating is continued in this state to diffuse the melted MoSi ₂ into the inner SiC coating layer structure to form S 2.
It is converted into a composite film of iC-MoSi ₂ .

Next, a porous MoSi ₂ film is formed on the upper surface of the SiC—MoSi ₂ composite layer as an outer coating layer. In this outer layer coating step, an aqueous slurry containing the same composition of granular molybdenum disilicide and an organic binder having the same composition as that used in the above-mentioned intermediate layer coating step is applied, and a temperature range up to 2000 ° C. under an inert atmosphere, preferably It is formed as a porous MoSi ₂ film partially heated by heating to a temperature range of 1500 to 1600 ° C. In this step, M is used to form molybdenum disilicide powder.
The oSi ₂ film is blended and adjusted to have a particle size of a porous structure, and is formed so that the film thickness is in the range of 50 to 200 μm. Film thickness
When the thickness is less than 50 μm, a sufficient function as an oxidation-resistant coating is not exhibited. When the thickness exceeds 200 μm, the function of relaxing thermal stress is reduced and delamination is liable to occur. Therefore,
When the film thickness in the above range cannot be obtained by one coating operation, it is preferable to repeat the coating operation to adjust the film thickness.

The basic structure of the present invention is a three-layer coating structure by the above-described steps.
By performing the outermost layer coating step of coating the _two glass films, the oxidation resistance can be further improved. In this outermost layer coating step, tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ ) is converted into a sol by a sol-gel method in which the pH is adjusted to pH 1 to 3 in advance with a mixed solution of hydrochloric acid and water to hydrolyze, and the resulting SiO 2 is formed. ₍₂ ) This is carried out by immersing the C / C substrate subjected to the outer layer coating treatment in the precursor, subjecting the precursor to a reduced pressure treatment if necessary, and then heating the precursor to a temperature of 400 ° C. or higher.

[0015]

According to the oxidation-resistant treatment method of the present invention, a polycrystalline SiC coating layer is formed on the surface of the C / C substrate by the conversion method in the inner layer coating step. This SiC coating layer has a relatively dense and functionally gradient functional structure, and does not easily peel off at the interface. The SiC-MoSi ₂ composite film formed in the intermediate layer coating step is extremely strongly bonded as a functionally graded structure continuous to the inner layer SiC structure through the melting / diffusion action at the time of formation, and is combined with the outer layer MoSi ₂ layer. Functions to enhance adhesion. Next, the MoSi ₂ layer coated as the outer layer is formed as a strong coating layer that withstands dynamic oxidation conditions because it sinters and bonds well with the MoSi ₂ structure in the intermediate layer. In addition, since the outer layer MoSi ₂ film has a porous structure in which the powders are partially sintered, the thermal stress at the interface caused by the difference in thermal expansion with the inner layer SiC is skillfully alleviated to prevent the occurrence of cracks and the like. At the same time, when exposed to a high-temperature oxidizing atmosphere, MoSi ₂ is converted into SiO ₂ glass and covers the surface layer as a uniform protective film.
Further, when a SiO ₂ glass film is formed as the outermost layer coating, the oxidation resistance is further improved.

Through such an action, extremely stable high oxidation resistance is provided even under severe and dynamic high-temperature oxidation conditions such as plasma irradiation.

[0017]

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Example 1 (1) Preparation of C / C base material A polyacrylonitrile-based high-strength, high-elasticity type plain woven carbon fiber cloth was sufficiently coated with a phenol resin precondensate (manufactured by Dainippon Ink Industries, Ltd.) as a matrix. It was applied and air-dried for 48 hours to prepare a prepreg sheet. The prepreg sheets were laminated and placed in a mold, and composite-molded under the conditions of a heating temperature of 110 ° C. and an applied pressure of 20 kg / cm ² . Next, after heating the composite molded body to a temperature of 250 ° C. to completely cure it, the composite molded body is transferred to a firing furnace maintained in a nitrogen atmosphere, raised to 2000 ° C. at a rate of 5 ° C./hr, and held for 5 hours. Fired and carbonized. Thus, a C / C base material having a carbon fiber volume content (Vf) of 65% and an apparent specific gravity of 1.65 g / cc was produced.

(2) Inner Layer Coating Step The SiO ₂ powder and the Si powder are mixed at a molar ratio of 2: 1 and the mixed powder is put into a graphite crucible and C
/ C substrate was set. The graphite crucible was transferred into an electric furnace, and the inside was sufficiently replaced with Ar gas. Then, the temperature was raised to 1900 ° C. at a rate of 50 ° C./hr and held for 2 hours to provide a tilting function to the surface layer of the C / C base material. Was formed. Although the thickness of the formed SiC coating layer was about 200 μm, it was recognized that cracks having a width of about 10 μm were generated on the surface thereof.

(3) Intermediate layer coating step An organic silicon-based binder [Daiichi Kogyo Seiyaku Co., Ltd.] Co., Ltd., Semora TB-72] 1.5% by weight and 16.5% by weight of water were added and mixed well with stirring to obtain a
A viscous slurry of 00 cp was prepared. This aqueous slurry was uniformly applied to the surface of the C / C substrate having been subjected to the inner layer coating treatment, and air-dried at room temperature for 2 hours. Then, it was placed in a heating furnace maintained in an argon gas atmosphere and subjected to a heat treatment at 2050 ° C. for 30 minutes. By this treatment, the coating material was once melted as MoSi ₂ , diffused into the SiC coating structure of the inner layer as it was, and converted into a SiC—MoSi ₂ composite film. The diffusion depth of MoSi ₂ into the SiC coating structure was about 80 to 90 μm, and this portion exhibited a functionally graded structure of SiC—MoSi ₂ composition.

(4) Outer Layer Coating Step The aqueous slurry used in the intermediate layer coating step was uniformly applied to the upper surface of the SiC-MoSi ₂ composite film subjected to the intermediate layer coating treatment, and air-dried at room temperature for 2 hours. Then, it was placed in a heating furnace maintained in an argon gas atmosphere and subjected to a heat treatment at 1500 ° C. for 30 minutes. The formed MoSi ₂ film has a porous structure in which the powders are partially sintered, and the average film thickness is 13
It was 0 μm.

(5) Evaluation of Oxidation Resistance A cycle in which the C / C substrate subjected to the above coating treatment is irradiated with a plasma flame (substrate surface temperature: 1900 ° C.) for 6 minutes in the air is repeated five times. Oxidation resistance was evaluated under dynamic oxidation conditions by measuring the weight change of the material. The results are shown in Table 1 as the weight change rate for each cycle.

Example 2 The outermost layer of a SiO ₂ glass film was coated on the upper surface of the C / C material having the three-layer coating structure obtained in the process of Example 1 by the following process. A mixed solution of tetraethoxysilane (Si (OC ₂ H ₅ ) ₄ ) [manufactured by Toshiba Silicone Co., Ltd.] and ethanol (molar ratio 1:
To 7), an aqueous hydrochloric acid solution was added so as to have a pH of 3.0, and the mixture was stirred at room temperature for 1 hour to carry out hydrolysis to prepare a SiO ₂ glass precursor. The C / C substrate treated in Example 1 was immersed in the glass precursor, impregnated for 1 hour under reduced pressure, and dried at room temperature for 24 hours. After drying, it was heated at a temperature of 500 ° C. for 10 minutes to be converted into a SiO ₂ glass film. With respect to the C / C substrate having been subjected to the coating treatment of four layers in this manner, an oxidation resistance test was performed using a plasma flame in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 1 Using the same inner layer coating process as in Example 1, a 200 μm-thick SiC layer was formed by a conversion method. Then, 60% by weight of molybdenum disilicide powder having a particle size of 37 μm or less,
A slurry having a viscosity of 1500 cp prepared by sufficiently stirring and mixing 11% by weight of sodium silicate and 29% by weight of water is uniformly applied, air-dried at room temperature for 2 hours, and then heated at a rate of 100 ° C./hr at a heating rate of 300 ° C./hr.
C. and kept for 30 minutes. MoSi ₂ formed
The thickness of the coating layer was 80 μm. An oxidation resistance test using a plasma flame was performed on the C / C material subjected to the two-step coating process in the same manner as in Example 1 and the results are shown in Table 1.

Comparative Example 2 Using the same inner layer coating process as in Example 1, a 200 μm-thick SiC layer was formed by a conversion method. Then, a SiO ₂ glass film was formed under the same conditions as in Example 2. C / C subjected to two-step coating process in this way
The material was subjected to an oxidation resistance test using a plasma frame in the same manner as in Example 1, and the results are shown in Table 1.

[0026]

[Table 1]

From the results shown in Table 1, it is found that the C / C substrate according to the embodiment of the present invention has a slight weight increase in the process of converting the outer layer of MoSi ₂ into SiO ₂ glass, but the surface temperature is 1900 ° C.
The weight loss due to oxidation is extremely small even if the plasma flame reaching the temperature of 5 is repeated for 5 cycles. On the other hand, in the comparative example, a phenomenon was observed in which oxidation was rapidly accelerated by irradiation for 1 to 2 cycles, and holes penetrated the material.

[0028]

As described above, according to the present invention, a polycrystalline SiC coating layer having a functionally graded structure as an inner layer on the surface of a C / C substrate, and a functionally graded structure by melting and diffusion of MoSi ₂ as an intermediate layer. By sequentially forming a SiC-MoSi ₂ composite layer, an MoSi ₂ layer obtained by partially sintering MoSi ₂ as an outer layer, and an outermost layer of a SiO ₂ glass film as necessary, severe conditions such as plasma irradiation can be obtained. A high oxidation resistance treatment that can withstand various dynamic oxidation conditions can be performed. Therefore, the present invention is applied to structural members that are exposed to severe conditions in a high-temperature oxidizing atmosphere, and exhibits effects such as securing stable performance and extending the durable life.

Claims (2)

(57) [Claims] 1. The method according to claim 1, wherein the substrate surface of the carbon fiber reinforced carbon material is made of SiO
An inner layer coating step of forming a SiC coating layer by a conversion method by contacting a gas, and applying an aqueous slurry containing particulate molybdenum disilicide and an organic binder, followed by heating to a high temperature of 2000 ° C or higher in an inert atmosphere. MoS by
an intermediate layer coating step of melting as i ₂ and diffusing into the inner layer SiC coating structure to form a composite film of SiC—MoSi ₂ , and then applying the same aqueous slurry as in the intermediate layer coating step and then under an inert atmosphere An outer layer coating step of forming a porous MoSi ₂ film by heating to a temperature of up to 2000 ° C. in an oxidation-resistant treatment method of carbon fiber reinforced carbon material. 2. An outermost layer coating step of impregnating an outer layer coating surface with an SiO ₂ glass precursor obtained by hydrolytic polymerization of tetraethoxysilane and then heating to form a SiO ₂ glass film. An oxidation-resistant treatment method for the carbon fiber-reinforced carbon material described in the above.