KR101129314B1 - Forming method of silicon carbide coating layer on carbon reinforcement material - Google Patents

Abstract

The present invention relates to a method for easily forming a SiC coating layer as a protective layer on the surface of short carbon fibers used as a reinforcing material of a metal-based composite material, and to a short carbon fiber produced by the method.
In the SiC coating layer forming method according to the present invention, the carbon reinforcing material is dispersed in a silica sol so that the silica particles are attached to the surface of the carbon reinforcing material and dried, and then the silicon (Si) of the attached silica particles and carbon (C) of the carbon reinforcing material It is characterized by forming a SiC coating layer through a heating carbon reduction reaction.

Description

Forming SiC coating layer on carbon reinforcing material {FORMING METHOD OF SILICON CARBIDE COATING LAYER ON CARBON REINFORCEMENT MATERIAL}

The present invention deteriorates the carbon reinforcing material by reaction with a molten metal on the surface of the carbon reinforcing material that can be used as the reinforcing material of the metal base composite such as carbon short fibers, carbon particles, carbon whiskers. The present invention relates to a method of forming a stable SiC coating layer that protects against the formation of a SiC coated carbon reinforcing material, and more particularly, to a silica sol in which fine silica particles are colloidal, and to which a carbon reinforcing material is added and dispersed. After the particles are attached to the surface of the carbon reinforcing material and the carbon of the carbon reinforcing material and the silicon of the silica react with each other through a thermal carbon reduction reaction to form a dense SiC coating layer on the surface of the carbon reinforcing material and the carbon produced by the method It is about reinforcement.

Composites using carbon fiber as the reinforcing material for aluminum bases have been developed to increase the strength and stiffness of the material without compromising the aluminum specific properties such as light weight and high thermal conductivity.

However, in the case of a metal matrix composite (MMC) such as an aluminum matrix composite, using short fibers rather than using long fibers as a reinforcing material can increase the degree of freedom in shape. It is desirable because it can be diversified in application fields.

On the other hand, when the carbon fiber is complexed to the aluminum base by using the casting method, a phenomenon in which the carbon fiber gradually deteriorates due to the mutual reaction of aluminum and carbon in a high temperature molten aluminum, and thus deteriorates the carbon fiber to function as a reinforcing material. If you do not do so, the physical properties of the composite material will be greatly reduced.

As a method for preventing carbon fiber deterioration in the casting process as described above, a method of forming a coating layer on the surface of the carbon fiber with a ceramic stable at high temperature such as SiC has been proposed.

As a method of coating SiC on the surface of the carbon fiber, a method of coating SiC on the surface of the carbon long fiber is known by dipping the carbon long fiber into a raw material solution capable of synthesizing SiC, followed by drying and heat treatment. However, a suitable method for forming a SiC coating layer at a low cost on the surface of high carbon short fibers as a reinforcing material is not known until now.

An object of the present invention is to provide a method for easily forming a dense SiC coating layer capable of protecting the carbon reinforcing material from the reaction with the molten metal on the surface of the carbon reinforcing material, especially short carbon fibers.

Another object of the present invention is to provide a carbon reinforcing material having a SiC coating layer formed on the surface.

As a means for solving the problems of the present invention, the present invention, by dispersing the carbon reinforcing material in the silica sol so that the silica particles adhere to the surface of the carbon reinforcing material and dried, the silicon (Si) and carbon of the silica particles through heat treatment It provides a method for reacting the carbon (C) of the reinforcing material to form a SiC layer.

In the present invention, 'silica sol' refers to a material in which fine silica (SiO ₂ ) particles are dispersed including a colloidal state in a solvent.

In addition, in the method according to the present invention, the heat treatment is preferably carried out for 30 minutes to 12 hours at a temperature of 1480 ~ 1900 ℃, when the heat treatment temperature is less than 1480 ℃ thermal carbon reduction reaction is not enough to complete SiC This is because a coating layer is difficult to obtain and a loss of energy and carbon is large when it exceeds 1900 ° C. Also, if the heat treatment time is less than 30 minutes, the reaction may not be completed sufficiently, and if it exceeds 12 hours, the energy cost may be excessively increased and the carbon loss of the carbon reinforcing material may increase.

In the method according to the invention, the carbon reinforcing material is characterized in that the short carbon fibers, carbon particles or carbon whiskers.

In addition, the method according to the invention is characterized in that a heat treatment for removing the binder attached to the surface of the carbon reinforcement is further performed before the drying process.

The present invention provides a carbon reinforcing material in which the silicon of the silica particles attached to the surface of the carbon reinforcing material and the carbon of the carbon reinforcing material to form a SiC coating layer by the thermal carbon reduction (carbothermal reduction) do.

The method according to the present invention can reduce the cost of SiC coating compared to other methods because the SiC coating layer is formed through a simple method of dipping, drying and heat treating short carbon fibers in a silica sol.

The method according to the invention is also suitable for coating short carbon fibers which increases the shape freedom of the metal matrix composite.

In addition, the present invention provides a carbon reinforcing material formed with a dense SiC coating layer to effectively block the reaction between the hot aluminum molten metal and the carbon reinforcing material.

1 is a SEM photograph of the short carbon fiber bundle used in the embodiment of the present invention.
FIG. 2 is an optical photograph after dispersing the short carbon fiber bundle of FIG. 1.
3A is a photograph after coating silica particles using a silica sol on the surface of short carbon fibers.
3B is an enlarged photograph of FIG. 3A.
Figure 4a shows the XRD analysis of the short carbon fibers coated with silica particles.
Figure 4b shows the XRD analysis result after the thermal carbon reduction reaction of the short carbon fibers coated with silica particles.
5A is a SEM photograph of SiC coated short carbon fibers according to an embodiment of the present invention.
5B is a TEM photograph of SiC coated short carbon fiber according to an embodiment of the present invention.
5C is an enlarged photograph of portion 'A' of the TEM photograph of FIG. 5B.
FIG. 5D illustrates an electron diffraction state of '(1)' and '(2)' parts of FIG. 5C.
6 is a graph showing the results of TGA analysis of SiC-coated short fibers and SiC-coated short fibers according to an embodiment of the present invention.
FIG. 7A shows SEM images and mapping results of the short carbon fibers heated and cooled until the molten aluminum temperature including the short carbon fibers not coated with SiC was 700 ° C. FIG.
FIG. 7B shows SEM images and mapping results of short carbon fibers not coated with SiC and heated to a melt temperature of 700 ° C. and maintained for 180 minutes.
FIG. 7C shows SEM images and mapping results of the short carbon fibers heated and cooled until the melt temperature of aluminum including short carbon fibers not coated with SiC was 800 ° C. FIG.
FIG. 7d illustrates SEM images and mapping results of the short carbon fibers heated until the molten metal temperature of the aluminum including short carbon fibers not coated with SiC was 800 ° C. and maintained for 180 minutes.
FIG. 8A illustrates SEM images and mapping results of short carbon fibers heated and cooled to SiC-coated short carbon fibers according to an embodiment of the present invention until the molten aluminum temperature is 800 ° C. FIG.
Figure 8b shows the SEM image and mapping results of the short carbon fibers after heating and holding for 30 minutes until the melt temperature of aluminum containing SiC-coated short carbon fibers to 800 ℃ according to an embodiment of the present invention .
Figure 8c shows the SEM image and the mapping results of the short carbon fibers after heating and holding for 60 minutes the molten aluminum of aluminum containing SiC-coated short carbon fibers according to an embodiment of the present invention to 800 ℃ .
FIG. 8D shows SEM images and mapping results of short carbon fibers after heating and maintaining for 180 minutes until the molten metal temperature of aluminum including SiC-coated short carbon fibers is 800 ° C. according to an embodiment of the present invention. .

Hereinafter, embodiments of the present invention for forming a SiC coating layer on the surface of short carbon fibers will be described in detail with reference to the accompanying drawings.

carbon In short fibers SiC  coating

In an embodiment of the present invention, a short carbon bundle of Toho Tenax America Inc. (TENAX 201-pelletized chopped product) as a starting material was used as a starting material, and the average diameter of the short carbon fibers was used. And length were 8 micrometers and 3 mm, respectively.

The short carbon fiber bundle was soaked in acetone for dispersion and sonicated for about 5 hours, and then dried at 70 ° C. for 24 hours using a vacuum oven. On the other hand, if there is a lot of binder material on the surface of the short carbon fiber bundle, it is preferable to remove the binder material by pyrolysis by heating at a temperature of about 400 ° C. for several hours before the drying process.

Through this process, as shown in FIG. 2, the short carbon fibers dispersed in the bundle state of FIG. 1 and the binder material was removed were obtained.

The short carbon fibers thus dispersed were immersed in a silica sol and dried through a solid / liquid separation process so that the fine silica particles of the silica sol adhere to the short carbon fibers. The silica sol used was water dispersed colloidal silica containing 40% by weight of silica (SiO ₂ ), particle size at 10 nm and 20 ° C., specific gravity at 1200, pH at 9.5 to 10.5, and viscosity at 20 ° C. It was 40 cps or less and used the thing which shows an apparent transparent state.

3A and 3B are scanning electron microscope images of short carbon fibers after silica (SiO ₂ ) particles are attached to the surface of short carbon fibers. As can be seen in Figures 3a and 3b, it can be seen through the above method that the white fine particles are smoothly attached to the surface of the short carbon fiber.

In addition, in order to confirm the coating of the silica particles on the whole short carbon fibers, the short carbon fibers subjected to the silica pre-coating treatment were analyzed using XRD. As a result, it was confirmed that the silica particles were evenly attached to the entire short carbon fiber from the silica peak indicated by 'o' on the drawing of FIG. 4A.

The short carbon fibers having silica particles attached thereon are heated at 1460 ° C. and 1500 ° C. for 1 hour using an argon gas atmosphere kiln, and through carbon thermal reduction, carbon constituting the short carbon fibers (C) And Si (Si) components constituting the silica react to generate SiC.

The thermal carbon reduction reaction is a continuous reaction of silica and carbon of short carbon fiber reacting to form gaseous SiO (1) and gaseous SiO and carbon of short carbon fiber reacting to form SiC (2). Caused by the reaction.

SiO ₂ + C → SiO (gas) + CO (gas) --------------- (1)

SiO (gas) + 2C → SiC + CO (gas) ---------------- (2)

Since the partial carbon constituting the short carbon fibers is removed through the reaction, the diameter of the short carbon fibers decreased from about 8 μm to about 6 μm after the thermal carbon reduction reaction.

Table 1 shows the results of analyzing the chemical composition of the SiC coating layer formed on the surface of the short carbon fiber by EDS by heating at 1460 ° C. and 1500 ° C. for 1 hour through thermal carbon reduction (carbothermal reduction).

element 1460 ℃ 1500 ℃ weight% atom% weight% atom% C 52.41 70.42 47.41 67.83 O 5.14 5.19 - - Si 42.45 24.39 52.59 32.17

As a result of the EDS analysis, when the thermal carbon reduction reaction was performed at 1460 ° C., since some oxygen remained in the formed coating layer, it was found that the thermal carbon reduction reaction was not completed at this temperature. In comparison, when the thermal carbon reduction reaction was performed at 1500 ° C., the complete thermal carbon reduction reaction was performed.

Accordingly, the following SiC coating layer analysis and protection characteristics analysis was performed using a SiC-coated short carbon fiber that was subjected to a thermal carbon reduction reaction at 1500 ℃ completely made a SiC formation reaction.

SiC  Analysis of the coating layer

First, XRD analysis was performed to determine whether the SiC coating layer was formed on the entire short carbon fiber through the method according to the embodiment of the present invention. As a result, as can be seen from Figures 4a and 4b, all of the silica particles pre-coated to the short carbon fibers became SiC through the reaction.

5A and 5B are photographs of the surface and the cross-section of the short carbon fibers on which the SiC coating layer is formed according to the embodiment of the present invention, respectively, by scanning electron microscopy and transmission electron microscopy. As shown in Figure 5a, according to the embodiment of the present invention through the thermal carbon reduction method, a coating layer having irregularities on the surface of the short carbon fibers is formed, as shown in Figure 5b, although the coating layer has a difference in thickness It is formed continuously. FIG. 5C is an enlarged photograph of the portion indicated by 'A' in FIG. 5B, and FIG. 5D shows that the coating layer (the portion indicated by (1) on the drawing) is made of SiC.

On the other hand, TGA analysis was performed to determine whether the SiC coating layer prepared according to the embodiment of the present invention is dense and continuously formed, the results are shown in FIG.

As shown in FIG. 6, in the case of short carbon fibers not coated with SiC, the weight reduction starts from 400 ° C. by the reaction of carbon and oxygen, and the weight reduction ends at about 760 ° C. FIG. In contrast, in the case of SiC-coated short carbon fibers according to an embodiment of the present invention, no weight loss occurs up to 720 ° C. From this it can be seen that the SiC coating layer formed according to the embodiment of the present invention is dense and continuous enough to effectively protect the short carbon inside.

SiC  Protective characterization of coating layer

In order to confirm the protective effect in the molten aluminum of the SiC coating layer formed in accordance with an embodiment of the present invention, the reaction test between the short carbon fiber without SiC and the short carbon fiber and SiC coated carbon in accordance with an embodiment of the present invention Was performed.

Figure 7a shows the SEM image and the mapping results of the carbon short fibers cooled as soon as the aluminum melt temperature reaches 700 ℃, Figure 7b is a SEM image of the carbon short fibers cooled after maintaining for 180 minutes at an aluminum melt temperature of 700 ℃ It shows the mapping result.

As shown in FIGS. 7A and 7B, in the 700 ° C. test, there was no difference in the immediately cooled carbon short fibers cooled after being maintained for 180 minutes. That is, it can be seen that deterioration of short carbon fibers hardly occurs in an environment of 700 ° C.

Figure 7c shows the SEM image and the mapping results of the short carbon fibers cooled as soon as the melt temperature reaches 800 ℃, Figure 7d shows the SEM image and mapping results of the short carbon fibers maintained for 180 minutes at 800 ℃ melt temperature. .

As shown in FIG. 7C, partial degradation occurs even in the case of the short carbon fiber cooled as soon as 800 ° C. is reached. As a result of EDS analysis, not only carbon is detected in the aluminum base near the short carbon fiber, but also the short carbon fiber The aluminum content in it was found to be about 21 wt%, which means that as soon as 800 ° C. was reached, the short carbon fibers cooled had a significant reaction with the aluminum matrix.

On the other hand, as can be seen in Figure 7d, in the case of the short carbon fibers maintained at 800 ℃ for 180 minutes, it can be seen that also severely degraded on the SEM image, most of the short carbon fibers are reacted with aluminum in the mapping results. Indicates.

From the above results, it can be seen that when the molten metal temperature is close to 800 ° C, the role of the reinforcing material cannot be expected by using the short carbon fiber as it is.

8A shows SEM images and mapping results of the SiC-coated short carbon fibers of the present invention cooled as soon as the aluminum melt temperature reaches 800 ° C., and FIGS. 8B to 8D show 30 minutes and 60 minutes at 800 ° C. of aluminum melt temperature, respectively. SEM images and mapping results of the cooled SiC-coated short carbon fibers were maintained after 180 minutes.

As can be seen in Figures 8a and 8b, the SiC coating layer retains its original shape until after 30 minutes at 800 ° C melt. As can be seen from the result of Si mapping, as the holding time increases, the SiC coating layer gradually degrades, and when only 180 minutes have elapsed, only a slight SiC coating layer remains on the surface of the short carbon fiber. However, no change in the composition and shape of the short carbon fibers occurs.

From the above results, it can be seen that the SiC coating layer formed according to the embodiment of the present invention can effectively suppress the reaction of the short carbon fibers with aluminum in the molten aluminum of high temperature.

Claims (6)

Dispersing a carbon reinforcing material consisting of carbon particles, carbon whiskers or short carbon fibers in a silica sol so that the silica particles adhere to the surface of the carbon reinforcing material and dried, and then heat-treated the silicon (Si) of the silica particles and carbon of the carbon reinforcing material A method of reacting (C) to form a SiC layer. The method of claim 1, wherein the heat treatment is performed for 30 minutes to 12 hours at a temperature of 1480 ~ 1900 ℃. delete The method according to claim 1 or 2, further comprising a heat treatment to remove the binder adhering to the surface of the carbon reinforcing material before the drying process. A carbon reinforcing material in which silicon of silica particles adhered to a surface of a carbon reinforcing material made of carbon particles, carbon whiskers, or short carbon fibers and a carbon of the carbon reinforcing material form a SiC coating layer by thermal carbon reduction. 6. The carbon reinforcing material according to claim 5, wherein the carbon reinforcing material is short carbon fiber.

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