JPH03177358A - Carbon fiber-reinforced carbon composite material and its production - Google Patents

Abstract

PURPOSE:To provide the subject composite material having high strength and high oxidation resistance by forming a porous silicon carbide film on the surface of the subject composite material treated with silicon carbide for sealing and having a bulk density of >=a specified value and forming a chemical vapor deposition film of silicon carbide thereon. CONSTITUTION:Pore parts 2' of a carbon fiber-reinforced carbon composite material (C/C composite) 1 are impregnated with silicon carbide 2 preferably by the chemical vapor deposition method to make its bulk density >=1.85g/cm<3>. The above impregnated composite material is used as a substrate and the surface part of the substrate is subjected to the diffusion reaction method for conversion into silicon carbide so as to form a porous silicon carbide layer 3 followed by further formation of a silicon carbide layer 4 thereon using the chemical vapor deposition method. If the bulk density of the above-mentioned substrate is <1.85g/cm<3>, diffusion of the Si source for formation of silicon carbide into the central part of the C/C composite 1 during the diffusion reaction process can not be prevented effectively. In addition, the above-mentioned layer (modified layer) 3 can relieve thermal stress produced by the difference between the thermal expansion coefficient of the layer 4 and that of the C/C composite material 1.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a carbon fiber-reinforced carbon composite material that can withstand repeated use in the aerospace industry or in high-temperature atmospheres such as parts for nuclear reactors.

<Conventional technology> Carbon materials are good conductors of electricity and heat, and at the same time have many unique physical and chemical properties such as heat resistance, corrosion resistance, and lubricity. It is used in a wide range of fields, including carbon graphite products, electrodes for electrical discharge machining, brushes for electrical equipment, and carbon products for machinery.

Carbon fiber-reinforced carbon composite materials (hereinafter referred to as C/C composites) have a particularly high specific strength, are lightweight, and have the excellent properties of carbon materials, so they can be used in areas where conventional materials cannot be used. For example, it can be applied to rocket nose cones and nozzles.

However, the C/C composite itself is heated to 500°C.
Various oxidation-resistant treatments have been considered, as the excellent physical and chemical properties it possesses deteriorate due to oxidation.
actually applied.

Among them, chemical vapor deposition method (hereinafter referred to as CVD method)
Ceramic coating is one of the most commonly used methods, including carbides such as silicon carbide, titanium carbide, hafnium carbide, tantalum carbide, silicon nitride, titanium nitride, zirconium nitride, and boron nitride. Various coatings such as nitrides such as, alumina, zirconia, oxides such as silica, borides such as hafnium hydride, etc. can be applied.

However, in this method, it is often necessary to heat the carbon material serving as the base to a salinity of 1000°C or higher, and the ceramic coating on the surface often cracks or peels off when the base is cooled. this is,
There is a large difference in thermal expansion coefficient between the substrate and the ceramic to be deposited.
This is due to the fact that the maximum strain cannot be followed.

This phenomenon can be solved by making the thermal expansion coefficient of the base material almost the same as that of the ceramic that is deposited, but when a C/C composite is used as the base material, the thermal expansion coefficient of the carbon fiber This method has not been effective because it is restricted by its own coefficient of thermal expansion and cannot be freely adjusted, and there is no ceramic material with heat and oxidation resistance that matches the coefficient of thermal expansion.

JP-A No. 61-26563 discloses that after a C/C composite is forcibly impregnated with an organosilicon polymer compound in a molten state, the impregnated material is carbonized by firing at a temperature of 1200 to 2000°C in an inert gas atmosphere. A method of converting to silicon is disclosed. However, with this method, when the density of the C/C composite is high, it is not possible to uniformly impregnate the organosilicon polymer compound in the molten state, and as a result, a film with high adhesion due to the anchor effect cannot be obtained. , no improvement in oxidation resistance can be expected.

JP-A-61-27248 discloses a method in which a C/C composite is not coated with silicon carbide by a diffusion method, and a silicon nitride film is formed on the outer surface by a CVD method. According to this method, it is possible to form a somewhat effective oxidation-resistant film, but since silicon nitride is used, the usable temperature is as low as approximately 1400°C, and furthermore, the strength decreases significantly at high temperatures, so heat load It is difficult to apply to large areas. In addition, in this method, C/
When converting the surface layer of a C/C composite into silicon carbide, silicon carbide sources such as silicon vapor diffuse from the pores, so the silicon carbide is converted not only to the surface layer but also to the center of the C/C composite. As a result, the strength of the C/C composite itself decreases due to the occurrence of defects due to volumetric expansion.

This tendency is particularly noticeable for -dimensional and two-dimensional C/C composites.

As a result of intensive research into the causes of this strength reduction, it was found that the following causes were mainly involved. That is, C/
When the C composite is one-dimensional or two-dimensional, the voids between the laminations are particularly large, so that Si vapor and S
iO diffuses and mainly converts the interlayers into SiC, reducing the adhesion between the laminated layers. Therefore, during the bending test, the sample breaks not at the point where stress is most concentrated, but from between the layers.
It is thought that this results in a decrease in strength.

<Problem to be solved by the invention> In the oxidation-resistant coating technology for C/C composites, in order to obtain an effective coating, a diffusion reaction method is used to modify the surface layer to create a so-called thermal stress relaxation layer. However, this method had the disadvantage of significantly reducing the strength of the C/C composite itself during the diffusion reaction.Also, if the CVD method is used to obtain a dense film, heat exchange between the C/C composite and the Due to the difference in expansion coefficient, peeling occurs and the film does not become an effective oxidation-resistant film.

In order to solve these problems, the present invention aims to provide a carbon fiber-reinforced carbon composite material that hardly causes a decrease in the strength of the C/C composite itself and has excellent oxidation resistance, and a method for manufacturing the same. shall be.

Means for Solving the Problems> According to the first aspect of the present invention, a carbon fiber-reinforced carbon composite material that has been sealed with silicon carbide and has a bulk density of 1.85 g/cm' or more is used as a base material. A carbon fiber-reinforced carbon composite material is provided, which has a porous silicon carbide film on its surface layer and a chemical vapor deposition film of silicon carbide thereon.

According to the second aspect of the present invention, a carbon fiber-reinforced carbon composite material that has been sealed with silicon carbide and has a bulk density of 1.85 g/cr or more is used as a base material, and the surface layer thereof is silicon carbide-treated. A carbon fiber-reinforced carbon composite material is provided, comprising a layer and a chemical vapor deposited silicon carbide coating thereon.

According to a third aspect of the present invention, the pores of the carbon fiber reinforced carbon composite material are impregnated with silicon carbide by chemical vapor deposition to substantially increase the bulk density to 1.8537 cm or more;
Thereafter, the surface layer of the carbon fiber-reinforced carbon composite material is converted to silicon carbide by a diffusion reaction method, and a silicon carbide film is formed on the silicon carbide layer by a chemical vapor deposition method. A method of manufacturing a fiber-reinforced carbon composite material is provided.

In the above, the thickness of the porous silicon carbide film or silicon carbide layer is preferably 10 to 700 μm, and the thickness of the chemical vapor deposition film of silicon carbide is 50 μm.
It is preferable that it is 500 micrometers.

The present invention will be explained in more detail below.

Figure 1 shows the carbon fiber reinforced carbon composite material (C/C
FIG. 2 is an example of a schematic cross-sectional view of a composite.

The toothpick shown in the same figure, the C/C composite of the present invention, is obtained by laminating and press-molding carbon fibers using a binder, hardening and firing the molded body, and making it rough. A porous layer is provided on the surface layer of the base material consisting of C/C composite 1 and a sealing material such as silicon carbide 2 which is impregnated into the pores 2' of C/C composite 1. A silicon carbide film or a silicon carbide layer (modified layer) 3 obtained by surface modification is formed, and a silicon carbide layer 3 obtained by chemical vapor deposition (CVD) is formed on this silicon carbide layer 3. It has a silicon layer 4, that is, a chemical vapor deposition film.

Here, if the base material consisting of the C/C composite 1 and the pore-sealing silicon carbide 2 has been pore-sealed so that its density is substantially 1.85 g/m3 or more, the pores 2' It is not necessary that all of the silicon carbide 2 be impregnated, and the pores 2' may remain.

As the carbon fibers constituting the C/C composite used in the present invention, two-dimensional woven fabrics such as plain weave, satin weave, and twill weave, one-dimensional oriented materials, three-dimensional oriented materials, felt, tow, etc. are used, and as the binder, Thermosetting substances such as phenolic resins and furan resins, and thermoplastic substances such as tar pitch can be used. The carbon fiber is made into a prepreg by impregnation with a binder, coating, etc., and then laminated and pressure-molded to form a molded body.

This molded body is made by hardening the binder through heat treatment.
Thereafter, it is fired in accordance with a conventional method, and further graphitized as required to obtain a C/C composite. after that,
Depending on the application, a C/C composite with even higher density and higher strength can be made by repeating densification using an impregnation method, a CVD method, or the like. Through the above steps, C/C
The bulk density of the composite is practically 1,6 g7cm
Increases to around 3.

However, since the density of graphite, which has the greatest anisotropy and the highest degree of crystallinity, is 2.25 g/cm3, it can be said that the above-mentioned C/C composite is quite porous, and it can be used as is in the subsequent process. When diffusion reaction treatment is applied, diffusion of silicon vapor etc. from the pores reaches the center of the C/C composite, causing deterioration of the carbon fibers and matrix,
The strength of the C/C composite is significantly reduced due to the introduction of defects due to volume expansion when carbon is converted to silicon carbide.

The present inventors have discovered that by sealing the gaps between the laminated layers existing in the C/C composite in advance, it is possible to minimize the decrease in strength caused by the diffusion reaction method.

A chemical vapor deposition method is preferred as a method for sealing the voids between the laminated layers. In the liquid impregnation method, which is generally used as a method for treating voids (including voids between laminated layers), a solvent is often added to reduce the viscosity and surface tension of the impregnant, and the volatilization of low-boiling components causes C/ This is not preferable because cracks occur in the C composite, and the end stop due to impregnation and firing is low and takes a long time. On the other hand, with chemical vapor deposition, since the reactive species is a gas, there is almost no limit to the size of the pores that can diffuse, and it can achieve the same or better effect in a much shorter time than the liquid impregnation method. preferred for obtaining.

The material to be impregnated into the pores (including the voids between the laminates) of the carbon fiber-reinforced carbon composite material by chemical vapor deposition (CVD) is preferably silicon carbide. Silicon carbide takes in oxygen that has entered through diffusion etc., forms silicon oxide, and prevents further diffusion. This is because it has an affinity with the elements. The bulk density of the C/C composite obtained by impregnating silicon carbide must be substantially 1.85 g/cm" or more. In other words, if the bulk density is less than 1.85 g/cm", as described below In the diffusion reaction process, it is not possible to effectively prevent the silicon source for forming silicon carbide from diffusing to the center of the C/C composite.

If the bulk density range of the C/C composite obtained by impregnating silicon carbide is further limited, the upper limit is a bulk density of 2 to 15.
g/cm" or less. In other words, if the densification by silicon carbide progresses further, the C/C will decrease due to thermal expansion of silicon carbide under severe heat load conditions.
This is because the composite causes peeling between layers.

The C/C composite obtained by the above-described method, in which the pores are impregnated with silicon carbide, needs to have its surface layer modified to silicon carbide by a diffusion reaction method. In other words, this layer (modified layer) is necessary as a so-called thermal stress relaxation layer that relieves the thermal stress caused by the difference in thermal expansion coefficient between the C/C composite and the dense silicon carbide film deposited by the CVD method described below. be. The diffusion reaction method for obtaining this thermal stress relaxation layer, that is, the modified layer, involves embedding a C/C composite in ceramic powder containing metallic silicon, and causing a reaction at a temperature of 1500 to 1900°C for 30 to 30 minutes. A method for producing SiO, for example, adding C/C/
Examples include a method in which a C composite is placed and a reaction is carried out at a temperature of 1600 to 2000° C. for 5 to 30 minutes, but the method is not necessarily limited to these methods. According to these methods, since the C/C composite itself is converted into silicon carbide, the adhesion of the film itself becomes strong and has the advantage that it does not easily peel off even when subjected to thermal shock.

The thickness of the C/C composite surface layer modified by the diffusion reaction method is preferably in the range of 10 to 700 μm. That is, when the modified layer is 10 μm or less, the C/C composite and the outer layer CVD1. ”, the adhesion with the silicon carbide film obtained from 700 μm was not strong.
If it exceeds m, cracks are likely to occur due to deterioration of the modified layer, and the silicon carbide film is likely to be destroyed due to rapid thermal cycle loads, which is not preferable.

The degree of porosity of the silicon carbide film obtained by the diffusion method,
In other words, is the porosity measured using a cross-sectional scanning electron microscope? This can be carried out by image analysis using an RI mirror photograph, pore distribution measurement using a mercury porosimeter, or the like. Image analysis of cross-sectional scanning electron micrographs shows that the porosity of the film is 10 to 80 vo.
It is preferable that it be within the range of JZ%. If it is less than 10 voIL%, the thermal stress relaxation ability of the film will decrease. Furthermore, if the porosity exceeds 80 voIL%, the strength of the film itself will drop significantly, which is not preferable.

A silicon carbide film is further formed on the silicon carbide modified layer obtained by the diffusion reaction method by the CVD method.
That is, the silicon carbide modified layer obtained by the diffusion reaction method is more porous than that obtained by the CVD method,
This is because it cannot form a sufficient oxidation-resistant film. CVD
Coating with silicon carbide by this method is the most commonly performed method, and is characterized by the fact that a dense film can be obtained. As a specific method, for example, a mixed gas of silicon tetrachloride, methane, hydrogen, and argon (volume ratio 1:1:5:1) is
It can be obtained by reacting at a temperature of 0 to 40 mmHg and a temperature of 900 to 1500°C for about 20 to 900 minutes. However, the reaction conditions of CVD are not necessarily limited to the above-mentioned reaction conditions because they vary greatly depending on the crystal structure of silicon carbide to be deposited, the type of raw material gas used, etc.

The thickness of the silicon carbide film coated by the CVD method is
It is necessary that the thickness be in the range of 50 to 500 μm. That is, if the film thickness is thinner than 50 μm, it will not function sufficiently as an oxygen diffusion barrier to the C/C composite, and if it exceeds 500 μm, the time required for precipitation will be extremely long, making it impractical and causing thermal cycle load. This is because cracking and chipping of the film are likely to occur.

By the above method, the oxidation resistance of the C/C composite can be dramatically improved, and the decrease in strength caused by the oxidation-resistant coating can be minimized.

<Example> The present invention will be specifically described below based on Examples.

(Example 1) A C/C composite used as a base material was produced by the method described below. A thermosetting phenol formaldehyde resin (Kanbo ■, trade name Belbar) was
After dissolving and diluting the resin with methanol to a concentration of 5% by weight, a Toshi carbon fiber cloth (using high-modulus carbon fibers) was impregnated with the resin to a basis weight of 85 g/m'. Thereafter, it was dried in an oven at 80° C. for 30 minutes to volatilize methanol and obtain a resin-impregnated carbon fiber sheet (prepreg).

This sheet was cut out to a size of 310 x 310 mm, 18 sheets were stacked, and heated to a weight of 5 kg/cm2.
The material was heated and pressed at 150° C. for 60 minutes to form a hollow-bottom, carbon fiber-reinforced plastic. Next, the carbon edge 1.1 was made of reinforced plastic under argon gas flow for 10'C/hr.
Baked to 1000"℃ at a heating rate of 310X310
A C/C composite of x1.5 mm was obtained. The C/C composite thus obtained was further subjected to a densification process of pitch impregnation and firing four times, resulting in a bending strength of 36.4 kg/mm' and a bulk density of 1.61 g/mm'.
A high-strength C/C composite of Cm3 was used.

The C/C composite obtained by the above method was
It was cut out to a size of 50 mm and impregnated with silicon carbide using the CVD method. Hydrogen gas containing about 5% by weight of methyltrichlorosilane (CH35iCj2s) was used as a reaction gas at 12 Il/min and Ar gas was heated at 2°C/m1n.
It was sent to a CV D reaction chamber, and under the reaction conditions of a reaction temperature of 980°C and a reaction pressure of 15 Torr, the holding time was set to three levels of 3.5.7 hours, and the bulk density was 1.86 g/cm' for each.
, 1.94 g/cm 32.08 g/cm 3 C/C composite was obtained. The C/C composite thus obtained was mixed with an equimolar mixture of high purity coke (trade name: MC coke) and silicon dioxide (manufactured by Wako Pure Chemical Industries, Ltd., special grade) (
Place it in a graphite container containing a weight ratio of 27:73) and
The C/C composite surface layer was converted into silicon carbide by reacting at 1850° C. for 60 minutes while gas was flowing at 15° C./min. The thickness of the modified layer converted to silicon carbide was determined by electron micrographs and EPMA analysis.
Furthermore, hydrogen gas containing 30% by weight of methyltrichlorosilane was added to the sample at 12°C/20°C using the aforementioned CVD apparatus.
ll1in, and Ar gas at 2 It/m1nCV
D is fed into reaction chamber, reaction temperature is 1240°C, reaction pressure is 5.
A dense silicon carbide film was coated under conditions of 0 Torr and a holding time of 3 hours.

Oxidation-resistant coated C obtained by the method described above
/C composite was cut into a size of 50 x 10 mm, IL/d = 27 (j2: distance between fulcrums, d: thickness)
A three-point bending test was conducted under the condition that the crosshead descending speed was 2 mm/min. In addition, the sample size of 50 x 50 mm was irradiated with Matsuha 2 argon plasma in the air for 30 minutes, and the weight loss at that time was also investigated.

The results are shown in Table 1.

In addition, Fig. 2 shows that E in the C/C composite of the present invention.
A cut plane 5 for EPMA analysis and an EPMA scan line 6 of the C/C composite on which PMA analysis is performed are shown. Here, the cut surface 5 for EPMA analysis was 15 mm from the surface. FIG. 3 shows E of invention example 1 shown in Table 1.
A chart showing Si intensity by PMA analysis is shown. This shows that Si exists only in the surface layer and does not diffuse into the interlayers.

(Comparative Example 1) A C/C composite obtained in the same manner as in Example 1 was held for 3 hours for impregnation with silicon carbide by the CVD method.
Silicon carbide was impregnated under the same reaction conditions as in Example 1 except that the reaction time was 0 minutes. C obtained in this way
The bulk density of the /C composite was 1.66 g/cm3.

Further, under the same conditions as in Example 1, an oxidation-resistant coated C/C composite having a modified layer formed by a diffusion reaction method and a silicon carbide film formed by a CVD method was obtained. The three-point bending test and the oxidation test were conducted in the same manner as in Example 1. The results obtained are shown in Table 1.

(Comparative Example 2) Si strength according to EPMA analysis of a C/C composite obtained in the same manner as in Example 1 and subjected to the same treatment as in Example 1 without pre-impregnating SiC by CVD method The chart is shown in Figure 4. This shows that the St acid component has penetrated too much into the pores (including the voids in the laminate).

<Effects of the Invention> C/C composites are widely used in the aerospace field, nuclear power industry, etc. as an excellent material not found in other materials, such as light weight, heat resistance, and high specific strength at high temperatures. Furthermore, in recent years, advances in the types of carbon-Fa fibers and matrices used, and in manufacturing methods have significantly increased the strength of C/C composites.

However, in the conventional oxidation-resistant coating method, although the oxidation-resistant properties of the C/C composite are improved, the mechanical strength decreases even more sharply, and an improvement has been expected.

According to the present invention, it is possible to obtain a high-strength and oxidation-resistant C/C composite that could not be obtained using conventional oxidation-resistant coating methods, and it can be applied to areas subject to severe thermal and mechanical loads. It becomes possible. In other words, it is expected that the degree of freedom in material design will increase and that extremely flexible application fields will open up.

[Brief explanation of drawings]

Figure 1 shows the oxidation-resistant C/
FIG. 2 is a schematic diagram of a C composite. Figure 2 shows the EP of the surface-modified C/C composite of the present invention.
It is a schematic perspective view which shows the state of the cut surface for MA analysis. Figure 3 shows that the C/C composite is pre-sold by the CVD method.
It is a figure showing St intensity by EPMA analysis when a diffusion reaction is performed after impregnating with iC. Figure 4 shows that the C/C composite is pre-sold by the CVD method.
It is a figure showing St intensity by EPMA analysis when a diffusion reaction is performed without impregnating with iC. Explanation of symbols 1...C/C composite, 2...Silicon carbide impregnated into pores, 2'...Vacancies, 3...Silicon carbide layer obtained by surface modification ( modified layer), 4... silicon carbide layer obtained by CVD method, 5...
・Cut section for EPMA analysis of the surface-modified C/C composite of the present invention, 6... EPMA scanning line

Claims (5)

[Claims] (1) Sealed with silicon carbide and has a bulk density of 1.85
A carbon fiber-reinforced carbon composite material having a carbon fiber reinforced carbon composite material of g/cm^3 or more as a base material, and having a porous silicon carbide film on the surface layer and a chemical vapor deposition film of silicon carbide thereon. Fiber reinforced carbon composite material. (2) Bulk density is 1.85 due to pore sealing treatment with silicon carbide
A carbon fiber-reinforced carbon fiber-reinforced material characterized by having a silicon carbide layer on the surface layer and a chemical vapor deposition film of silicon carbide on the surface layer, using a carbon fiber-reinforced carbon composite material having a carbon fiber-reinforced carbon composite material with a carbon fiber strength of 3 g/cm^3 or more as a base material. Carbon composite material. (3) The carbon fiber reinforced carbon composite material according to claim 1 or 2, wherein the porous silicon carbide film or silicon carbide layer has a thickness of 10 to 700 μm. (4) The thickness of the chemical vapor deposition film of silicon carbide is 50 to 5
The carbon fiber-reinforced carbon composite material according to any one of claims 1 to 3, wherein the carbon fiber reinforced carbon composite material has a diameter of 00 μm. (5) The pores of the carbon fiber-reinforced carbon composite material are impregnated with silicon carbide by chemical vapor deposition to substantially reduce the bulk density to 1.
．． 85 g/cm^3 or more, and then converting the surface layer of the carbon fiber reinforced carbon composite material to silicon carbide using a diffusion reaction method, and further forming a silicon carbide film on the top of the silicon carbide layer using a chemical vapor deposition method. A method for producing a carbon fiber-reinforced carbon composite material.