JPH04272237A - Composite carbon fiber - Google Patents

Abstract

PURPOSE:To provide the title fiber modified in its surface layer, capable of improving the mechanical properties of the composite materials reinforced therewith. CONSTITUTION:The objective carbon fiber with part or the whole of carbon fiber converted into silicon carbide, characterized by the crystal structure of said silicon carbide consisting mainly of 2H or 3C polytype or a mixture of said polytypes.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to composite carbon fibers,
Specifically, the invention relates to composite carbon fibers used in fiber-reinforced composite materials for space, aviation, and defense, and fiber-reinforced composite materials for automobile parts.

0002

[Prior art] Strength, which is a disadvantage of metals and ceramics,
Materials with improved properties such as rigidity, wear resistance, and thermal expansion using carbon fiber are carbon fiber reinforced metals, carbon fiber reinforced ceramics, and carbon bonded carbon fiber composite materials (C/C composites). It is well known that it is commonly used as a high-strength, lightweight structure, especially at high temperatures.

However, when producing carbon fiber-reinforced metals, carbon fibers and metal matrix, especially aluminum, have high reactivity and easily react at high temperatures to form Al4.
A major problem is that C3 and the like are generated and the strength is reduced. This is particularly noticeable in carbon fibers with a low graphitization rate. Furthermore, when producing carbon fiber reinforced ceramics, the wettability between carbon fibers and ceramics is poor, and it is not possible to sufficiently increase the adhesive force at the interface between the two. Carbon-bonded carbon fiber composite materials (C/C composites) also have limitations in terms of wear resistance, oxidation resistance, and the like.

[0004] Conventionally, therefore, carbon fiber surfaces were subjected to CVD treatment, PVD treatment, plating, or thermal spraying to form SiC, WC,
Attempts have been made to deposit and coat TiC, W, Mo, Cu, etc. to suppress reactions with matrix substances and oxidizing gases, and to improve sliding properties. In addition, recently, as disclosed in Japanese Patent Application Laid-Open No. 62-107038, metal oxides such as MgO and BeO are coated on carbon fibers to prevent reactions with metals and improve wettability with ceramics. A method has been thought of to do so.

[0005]

[Problems to be Solved by the Invention] However, conventional CVD treatments, PVD treatments, plating treatments with high melting point oxides, non-oxides, metals, etc. on the surface of carbon fibers,
Deposited coating treatment such as thermal spraying, or MgO or BeO
As shown in FIG. 4, the method of applying a metal oxide such as
Because they are bonded by physical adhesion such as Waals force,
There was a problem in that the interfacial adhesion between the surface of the carbon fiber 19 and the coating substance 21 was insufficient, and when the carbon fiber 19 was made into a composite with a matrix metal and used repeatedly under load at high temperatures, the strength deteriorated quickly. In particular, there is a problem in that when the material covering the carbon fibers itself reaches a high temperature of approximately 1200° C. or higher, its strength begins to deteriorate rapidly.

[0006]

[Means for Solving the Problems] The present invention has been made to address the above-mentioned problems, and provides a composite material that improves the wettability of carbon fibers with metals, ceramics, etc., and that can be produced even at high temperatures. The aim is to find carbon fibers that can be used as raw materials for carbon fiber-reinforced materials that do not cause strength deterioration. That is, the present invention provides a composite carbon fiber obtained by converting a part or all of carbon fiber into silicon carbide, the silicon carbide crystal structure being a polytype of 2H or 3C, or a mixture of polytypes of 2H and 3C. The gist of the invention is a composite carbon fiber characterized by having as a main component.

Now, methods for converting the carbon fiber surface layer into silicon carbide include reacting with silicon vapor or various silicon compounds, or applying pack cementation, but the most preferable method is to react with silicon monoxide gas and silicon carbide. A method of reacting carbon fibers as shown in the following equation is mentioned. SiO (g) + 2C = SiC + CO (g) By using this method, 2H according to Ramsdale notation can be obtained while maintaining the shape and dimensions of the carbon fiber 19 as shown in Fig. 3.
A silicide layer 20 having a crystal structure (polytype) such as 3C, 4H, 15R, or 6H can be formed.

[0008] This reaction proceeds by heating in a temperature range of 1200°C to 2000°C. Here, in order to generate silicon monoxide gas, a mixture of silicon powder and silicon dioxide powder, a mixture of silicon carbide powder and silicon dioxide powder, a mixture of carbon powder and silicon dioxide powder, and other various silicon This can be carried out by heating the compound to 1200°C to 2000°C.

[0009] When carbon fibers and silicon monoxide are reacted to convert the carbon fiber surface into silicon carbide, the treatment temperature is set to 12
By selecting the temperature within the range of 00°C to 1650°C, it is possible to leave unreacted carbon in the silicified layer on the surface of the carbon fiber, and to generate silicon carbide whose main component is a polytype with a crystal structure of 2H. It is possible to make products with various silicification ratios, which are the weight proportions of silicon carbide. Further, the thickness of the silicified layer on the carbon fiber surface can be controlled by adjusting the treatment time as well as the treatment temperature. In addition, the silicification rate and the thickness of the silicide layer can be controlled by adjusting the concentration of silicon monoxide.

Similarly, by selecting the reaction temperature between carbon fiber and silicon monoxide in the range of 1650°C to 2000°C, unreacted carbon remains in the silicified layer on the surface of the carbon fiber, and the crystal structure changes to 2H. It is possible to produce a mixture of the 3C and 3C polytypes, or a silicon carbide mainly composed of the 3C polytype.

[0011] At least 10% by weight of unreacted carbon in the entire composite carbon fiber whose carbon fiber surface layer is converted to silicon carbide.
It is desirable to leave the above remaining. This makes it possible to ensure the flexibility of the carbon fiber due to the sea-island structure of carbon and silicon carbide.

Next, a method for manufacturing carbon fiber by continuously firing it will be explained with reference to the drawings. FIG. 1 is a schematic diagram of an apparatus for manufacturing the composite carbon fiber of the present invention. In FIG. 1, 1 is a pre-carbonized fiber or carbon fiber, which is treated at 150°C to 300°C using a preheating heater 2. Atmospheric gas inside the furnace is introduced through the gas supply port 3, and exhaust gas is taken out through the exhaust gas ports 7 and 13 inside the furnace.

[0013] Also, sealing gas is supplied from a water seal section in the furnace in which a sealing water bath 17 is arranged through a supply port 15 and taken out through exhaust gas ports 7 and 13 in the furnace.

The preheated fibers are heated and carbonized at 1000° C. to 3000° C. by a firing/carbonizing heater 5. The carbon fibers subjected to the above treatment are transferred to a silicification zone separated by slits 12 and 14, and the surface layer is converted into silicon carbide. Here, the temperature of the silicification zone is set at 1200 DEG C. to 2000 DEG C. using the silicification heater 6. Further, silicon monoxide gas can be generated by heating the silicon monoxide gas generation source 10 in the graphite crucible 9 to 1200°C to 2000°C, and it is introduced from the silicon monoxide gas supply port 11 to generate carbon. React with fibers. 1
Induction heating coil 8 to heat between 200℃ and 2000℃
The graphite crucible 9 may be heated using. The residual silicon monoxide gas is exhausted from the exhaust gas port 13 inside the furnace.

The carbon fibers whose surface layer has been converted to silicon carbide are cooled through a cooling zone defined by slits 14 and 18, and then emerge from a water seal section provided with slits 16.

[0016] Although there are no particular restrictions on the carbon fibers used here, pitch-based carbon fibers are suitable from the viewpoint of ease of conversion reaction into silicon carbide.

[0017]

[Operation of the invention] The present invention is characterized in that silicon monoxide gas permeates and diffuses through the carbon fiber surface layer, reacts with the carbon fiber itself, and is converted into silicon carbide. Alternatively, it is fundamentally different from methods such as plating, thermal spraying, and coating, in which the same substance or a different substance is deposited on the surface of the carbon fiber.

[0018] In other words, the carbon fiber surface obtained by the CVD method, PVD method, plating, thermal spraying, coating, etc. is bonded to the deposited coating material and the carbon fiber surface only by physical adhesion due to van der Waals force or the like. When used as a fiber filler in composite materials, repeated use at high temperatures causes the deposited coating material to peel off due to differences in thermal expansion, resulting in a loss of strength.

However, since the silicon carbide layer on the surface of the carbon fiber of the present invention has been changed by reacting with silicon monoxide, the boundary has a completely continuous structure, and it cannot be used repeatedly at high temperatures. The silicified layer will not peel off.

Furthermore, since the silicon carbide layer on the surface of the carbon fiber of the present invention has the same porosity as the carbon fiber, it has higher thermal shock resistance than a film deposited by CVD or PVD, which has almost no pores. When the matrix penetrates into the minute pores of the silicon carbide layer, a so-called anchoring effect works, so that it is more firmly bonded to the matrix.

On the other hand, as for the effect of the crystal structure, it has been found that by making the crystal structure of silicon carbide on the surface of the carbon fiber of the present invention mainly composed of 2H polytype, the flexibility of this composite carbon fiber is increased. did. Although the reason is not clear, it can be assumed that this is due to the large contribution of the ionic cohesive energy of the 2H silicon carbide crystal component.

In addition, by making the crystal structure of silicon carbide on the surface of carbon fibers mainly composed of 3C polytype, the characteristic of increasing strength at temperatures above 1200°C, which is unique to the 3C silicon carbide crystal component, is exhibited, and the composite material is It becomes possible to increase high temperature strength.

Similarly, by making the crystalline structure of silicon carbide on the carbon fiber surface mainly composed of a mixture of 2H and 3C polytypes, composite carbon fibers having both flexibility and high-temperature high strength properties can be obtained. Can be done.

In addition, in terms of the wear resistance of the composite material, compared to the case of ordinary carbon fiber fillers, the composite carbon fiber filler of the present invention has a much greater wear resistance due to the effect of the silicon carbide component having a 3C polytype crystal structure. was found to improve.

[0025] The present invention includes not only carbon fibers alone, but also those assembled in the form of various mats and cloths with one-dimensional or more dimensional structures, and various non-woven fabrics and yarns. It is also extremely effective. Next, the present invention will be specifically explained using examples.

[0026]

[Example] Example 1 Pitch fiber (2 denier, number of filaments 10,000
) was subjected to firing carbonization and silicification using the apparatus shown in FIG. Nitrogen gas containing a predetermined amount of oxygen was sent from the gas supply port 3, and nitrogen gas was sent from the gas supply port 15.

The silicon monoxide gas generation source 10 is made by putting 300 g of a mixture of silicon powder and silicon dioxide powder (molar ratio 1:1) into a graphite crucible 9 and heating it to 1600° C. by induction heating.
Generated silicon monoxide. The temperature inside the furnace is determined by preheating heater 2.
, a calcination carbonization heater 5, and a silicification heater 6 as shown in FIG. The surface layer obtained in this way is 2
The composite carbon fiber made of silicon carbide mainly composed of polytype H was highly flexible. This composite carbon fiber was laminated with aluminum foil and molded into a predetermined shape, and then a carbon fiber reinforced aluminum composite was produced using a hot press method. The carbon fiber content volume fraction of the prepared sample was 15
vol%, 20vol%, and 30vol%.

The tensile strength of the obtained composite at room temperature was 90% to 95% of the value calculated from the composite rule. Furthermore, as a result of measuring high temperature tensile strength, it showed a nearly constant value up to 580°C.

Example 2 A composite carbon fiber whose surface layer was made of silicon carbide whose main component was a mixture of 2H and 3C polytypes was obtained in the same manner as in Example 1 except that the conversion reaction temperature was 1750°C. It was molded into a predetermined shape together with the SiC powder prepared in . This was sintered at 1900° C. to produce carbon fiber-reinforced SiC composites with carbon fiber content volume fractions of 15 vol%, 20 vol%, and 30 vol%. As a result of measuring the high temperature tensile strength of the obtained composite, it was possible to maintain the strength up to 1800°C.

Example 3 A composite carbon fiber whose surface layer was made of silicon carbide mainly composed of 3C polytype was prepared by the same method as in Example 1 except that the conversion reaction temperature was 1900° C. and was prepared with a binder. It was molded into a predetermined shape together with SiC powder. This was sintered at 1900°C and the carbon fiber content volume fraction was 15vol.
%, 20vol%, 30vol% carbon fiber reinforced SiC
A composite was created. As a result of measuring the high temperature tensile strength of the obtained composite, it was possible to maintain the strength up to 1850°C.

Comparative Example 1 A sample was prepared by carrying out all the same treatments as in Example 1 except that the silicification treatment of the carbon fiber surface layer was not performed. The tensile strength at room temperature of the obtained composite sample was 55% to 65% of the value calculated from the composite rule. Further, as a result of measuring high temperature tensile strength, it showed a nearly constant value up to 280°C.

Comparative Example 2 A sample was prepared by carrying out all the same treatments as in Example 2 except that the silicification treatment of the carbon fiber surface layer was not performed. As a result of measuring the high temperature tensile strength of the obtained composite sample, 125
It was able to maintain its strength down to 0°C.

[0033]

Effects of the Invention As explained above, the composite carbon fiber of the present invention has a part or all crystal structure of 2H or 3C.
Since silicon carbide is mainly composed of polytypes of 2H and 3C, or a mixture of 2H and 3C polytypes, when a carbon fiber reinforced composite is produced, there is no reaction between the matrix metal or ceramics and the carbon fibers. It can be used with confidence because it does not deteriorate in strength due to being raised or carburized.

In addition, by utilizing the flexibility characteristic of the 2H polytype and the increasing mechanical strength at temperatures above 1200°C, which is the characteristic of the 3C polytype, carbon fiber reinforcement is achieved that does not cause strength deterioration even in high temperature ranges. A complex can be obtained.

In the production of the composite carbon fiber of the present invention, carbon fibers with various silicification rates can be obtained by freely adjusting the silicification treatment temperature, treatment time, silicon monoxide gas concentration, etc. The sliding characteristics of mechanical seals made of carbon fiber composites can also be easily controlled.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an apparatus for producing composite carbon fibers of the present invention.

FIG. 2 is a graph showing an example of temperature distribution within the composite carbon fiber manufacturing apparatus shown in FIG. 1;

FIG. 3 is a cross-sectional view of a carbon fiber and a composite carbon fiber of the present invention.

FIG. 4 is a cross-sectional view of a carbon fiber and a composite carbon fiber whose surface has been coated by CVD, PVD, coating, or the like.

[Explanation of symbols]

1 Fiber 2 Preheating heater 3, 15 Gas supply port 4, 12, 14, 16, 18 Slit 5 Burning carbonization heater 6 Silicization heater 7, 13 Exhaust gas port 8 Induction heating coil 9 Graphite crucible 10 Silicon monoxide gas source 11 Silicon monoxide gas supply port 17 Sealing water bath 19 Carbon fiber 20 Silicized layer 21 Coating material

Claims (1)

[Claims] 1. A composite carbon fiber in which a part or all of carbon fiber is converted into silicon carbide, wherein the crystal structure of the silicon carbide is a polytype of 2H or 3C, or 2H and 3C.
Composite carbon fiber characterized by being mainly composed of a mixture of polytypes.