JP2737591B2 - Whisker surface modification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a whisker used as a reinforcing material for a composite material, and more particularly to a whisker surface modification method.

[0002]

2. Description of the Related Art As one method of modifying the surface of a whisker used as a reinforcing material for a composite material, for example, Japanese Unexamined Patent Publication No.
As described in Japanese Patent No. 2099, polysilazane is dissolved in an organic solvent such as toluene, SiC whiskers are dispersed in the solution, filtered and dried, and then the SiC whiskers are heated to 1100 to 1600 ° C. in a nitriding atmosphere. A method of forming a film of silicon nitride on the surface of a SiC whisker by baking at a temperature is already known.

[0003] As one of the surface modification methods for whiskers generally known in the art, whiskers and fumed silica are mixed at a predetermined ratio, and the mixture is mixed with a hydrocarbon. There is a method of forming a silicon nitride film on the surface of a whisker by heating to a high temperature of about 1400 ° C. in a mixed gas of a gas and an ammonia gas.

According to these surface modification methods, a chemically stable silicon nitride film is formed on the surface of a whisker,
Since the chemical reaction between the whisker and the matrix metal during the production of the composite material can be suppressed, the fracture toughness of the composite material is improved by relaxing the interfacial bonding force between the whisker and the matrix metal. Or deterioration due to whisker reaction can be prevented.

[0005]

However, since the polysilazane used in the method described in the above publication is very expensive, it is difficult to carry out surface modification of whiskers at low cost. In addition, since polysilazane is a polymer and has high viscosity, whiskers cannot be efficiently dispersed in an organic solvent in which polysilazane is dissolved,
Therefore, it is difficult to efficiently perform the surface modification of the whisker.

The silicon nitride of the film formed by the above-mentioned conventional general surface modification method is formed polycrystalline and the surface of the film becomes uneven, so that deterioration due to whisker reaction is effectively prevented. For this purpose, the thickness of the coating must be increased. As a result, the aspect ratio of the whisker decreases and the effect of improving the strength of the whisker decreases, and the volume ratio of the whisker must be increased in order to produce a high-strength composite material. In the production of the whiskers, the permeability of the matrix metal into the whisker compact is likely to deteriorate, and the machinability of the composite material is likely to deteriorate.

In view of the above-mentioned problems in the conventional whisker surface reforming method, the present invention can efficiently and inexpensively perform the whisker surface modification as compared with the conventional whisker. An object of the present invention is to provide an improved whisker surface modification method capable of forming a good silicon nitride film on the surface.

[0008]

According to the present invention, as shown in FIG. 1, tetraethoxysilane is dissolved in an organic solvent (step 1), and whiskers are added to the solution. In the dispersed state, the tetraethoxysilane is hydrolyzed (Step 2), the whiskers are filtered and dried (Step 3), and the whiskers are heated to a high temperature in a nitriding atmosphere and fired ( Step 4), which is achieved by a whisker surface modification method, which comprises forming a silicon nitride film on the surface of the whisker.

[0009]

According to the surface modification method of the present invention, the silicon-containing substance is not polysilazane, which is a polymer, but tetraethoxysilane, which is a very inexpensive monomer (about 1/500 of the price). Also, tetraethoxysilane has a lower viscosity than polysilazane and can efficiently disperse whiskers in the solution, so that the surface modification of whiskers can be performed efficiently and inexpensively as compared with the above-mentioned conventional method. Can be implemented.

Further, since the surface of the silicon nitride film formed by the surface modification method of the present invention is very smooth, according to the method of the present invention, the surface of the whisker can be satisfactorily modified with a small amount of the film. Can be surface-modified without substantially changing the aspect ratio of the whisker,
Therefore, if a composite material having the same strength is to be obtained, the volume ratio of the whisker can be reduced as compared with the case where a whisker whose surface has been modified by a conventional general method is used, and a whisker molded body is obtained. The molten metal of the matrix metal can be satisfactorily permeated therein, thereby improving the production efficiency of the composite material.

According to the method of the present invention, as described above, the aspect ratio of the whisker is substantially the same as the aspect ratio before the film is formed. It is possible to improve the properties such as the strength of the composite material as compared with the case where it is performed, and it is possible to reduce the volume ratio of the whisker required to obtain the same strength. In the case of a high-strength composite material, machinability can be improved.

[0012]

According to the results of an experimental study conducted by the present inventors, when the heating and sintering temperature for whiskers is less than 1000 ° C., tetraethoxysilane adhering to the surface of whiskers cannot be removed. In some cases, the hydrolysis compound is not completely converted to silicon nitride, and the formed silicon nitride film is easily peeled off from the whiskers.
Conversely, if the heating and firing temperature for the whisker is too high, the whisker is liable to deteriorate. In particular, when the whisker is an aluminum borate whisker (alumina-boria whisker), the whisker tends to deteriorate when the firing temperature exceeds 1400 ° C.

Thus, according to one embodiment of the present invention, when the whiskers are aluminum borate whiskers,
The heating and firing temperature for whiskers is 1000 to 1400 ° C
Is set to

When the thickness of the silicon nitride film formed on the surface of the whisker is less than 0.001 μm, the effect of forming the silicon nitride film on the surface of the whisker becomes insufficient. If the thickness is set to a value exceeding 0.2 μm, it takes a long time to form a coating film, and it becomes difficult to efficiently perform the surface modification. Therefore, the thickness of the silicon nitride film formed on the surface of the whisker is 0.00
It is preferable to set the thickness to 1 to 0.2 μm.

The organic solvent used in the method of the present invention may be any organic solvent as long as it can dissolve tetraethoxysilane well and can hydrolyze tetraethoxysilane. Organic solvents generally used as organic solvents such as methyl alcohol and toluene may be used.

Furthermore, the whiskers to be surface-modified by the method of the present invention are not limited to the above-mentioned TiC whiskers and aluminum borate whiskers, and their performance is improved by forming a smooth silicon nitride film on the surface. Other whiskers such as potassium titanate whiskers and SiC whiskers may be used as long as they can be improved.

[0017]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an embodiment of the present invention.

EXAMPLE 1 Although not shown in the figure, 70 g of tetraethoxysilane was dissolved in a solvent of 95 g of ethyl alcohol to form a solution of tetraethoxysilane in ethyl alcohol. 6g of 0.1N HC
1 and 33.6 g of water were added. Then, as shown in FIG. 2, 460 g of aluminum borate whisker 12 (“Albolex G” manufactured by Shikoku Chemicals Co., Ltd., whisker diameter 0.5 to 1 μm) was added to the ethanol solution 10 of tetraethoxysilane thus formed. Whisker length 10
30 .mu.m) and uniformly dispersed, and the dispersion was stirred for about 2 hours to hydrolyze the tetraethoxysilane.

Next, the dispersion treated as described above is subjected to suction filtration, whereby aluminum borate whiskers having a tetraethoxysilane hydrolyzing compound adhered to the surface are substantially formed at a volume ratio of about 10%. A molded article having dimensions of 16 × 38 × 100 mm oriented at random was formed, and the molded article was dried at room temperature and then dried.
Dry at 0 ° C. for 1 day. Further, the molded body was vacuum-dried at 170 ° C. for 2 hours, thereby completely curing the substance adhered to the surface of the aluminum borate whisker.

Next, the aluminum borate whisker was heated at 1400 ° C. for 3 hours in a mixed gas of a hydrocarbon gas and an ammonia gas (volume ratio of 1:20) and calcined. FIG.
FIG. 4 is a schematic view showing a cross section of an aluminum borate whisker 12 having a 0.01 μm-thick silicon nitride film 18 on the surface formed by baking in this manner;
12A indicates a portion substantially containing only aluminum borate. Although the surface of the silicon nitride film 18 is smooth, it is presumed that the interface between the film 18 and the portion 12A containing only aluminum borate is actually complicated.

COMPARATIVE EXAMPLE 1 For the purpose of comparison, the same aluminum borate whisker used in Example 1 and the same aluminum borate whisker and fumed silica were mixed at a predetermined ratio, and the mixture was mixed with a hydrocarbon. The mixture was heated to 1400 ° C. in a mixed gas of gas and ammonia gas (1:20 by volume), thereby forming a 0.4 μm-thick silicon nitride film on the surface of aluminum borate whiskers. Then, the aluminum borate whisker on which the coating was formed was suction-molded to form a molded body having a whisker volume ratio of 10%, but the molded body could not maintain its own shape.
It was not possible to form a molded article usable for producing a composite material.

Example 2 In order to evaluate the performance of an aluminum borate whisker surface-modified by the surface modification method of the present invention, a composite material was prepared using the aluminum borate whisker surface-modified by the method of the present invention. It was manufactured and its strength was measured.

First, a surface modification treatment of aluminum borate whiskers was performed in the same manner as in Example 1 except that the concentration of the ethyl alcohol solution in which tetraethoxysilane was dissolved was changed. As a result, two types of aluminum borate whiskers having a thickness of 0.001 μm (Example 2A) and 0.2 μm (Example 2B) of the silicon nitride film were formed, and the surface was modified in Example 1 described above. A quality aluminum borate whisker was prepared.

Next, as shown in FIG. 4, the aluminum borate whisker 12 having the silicon nitride film formed on its surface is charged into distilled water 20 and distilled by a propeller 22 until the whisker is sufficiently defibrated. After stirring the water and the whiskers, the whiskers are transferred into a molding jig to perform compression molding to form a molded body having a size of 100 × 38 × 16 mm, and the molded body is kept at −50 ° C. together with the jig. And kept in that state until the water in the molded body was sufficiently frozen.

Next, the compact was taken out of the jig, and thereafter, as shown in FIG. 5, the inner dimensions were 38 × 16 mm, the length was 140 mm, the openings were opened at both ends, and the weight 24 was provided at one end. The molded body 28 formed as described above was filled in a case 26 made of stainless steel (JIS standard SUS304) provided integrally. Next, the formed body is heated by a heater together with the case to sufficiently dry the formed body, thereby forming an aluminum borate whisker having a silicon nitride film on its surface and having a whisker volume ratio of 30.
% Of the molded body was formed.

Next, after pre-heating the molded body together with the case to 600 ° C. for 1 hour, the molded body 28 together with the case is placed in the mold 32 of the high-pressure casting apparatus 30 as shown in FIG. Plunger 3 for pouring a molten metal 34 of an Al alloy (JIS 2024) and fitting the molten metal into a mold
6 and pressurized at a pressure of about 1000 kgf / cm ² , and the pressurized state was maintained until the molten metal was completely solidified. After the molten metal is completely solidified, the mold 3 is
The solidified body is removed from the solidified body, and the solidified body is machined to cut out a composite material composed of an Al alloy composite-reinforced with aluminum borate whiskers having a silicon nitride film on the surface, and heat-treated the composite material. T6 was applied.

For the purpose of comparison, a composite material was formed in the same manner and under the same conditions as in the embodiment except that aluminum borate whiskers having no silicon nitride film on the surface were used. On the other hand, T6 heat treatment was performed (Comparative Example 2).

Next, a bending test piece was formed from each composite material, and a three-point bending test was performed on each test piece at room temperature with a distance between supporting points of 40 mm. The results of these tests are shown in FIG.
FIG. 7 shows that the strength of the composite material can be significantly improved by forming a silicon nitride film on the surface of the aluminum borate whisker according to the method of the present invention.

Although not shown as an example, even when the whiskers are TiC whiskers or SiC whiskers, the whiskers can be efficiently and inexpensively formed on the surface of the whiskers as compared with the conventional method using polysilazane. A silicon nitride film could be formed, and a smoother and better silicon nitride film could be formed as compared with the method of Comparative Example 1.

Although the present invention has been described in detail with reference to specific embodiments, the present invention is not limited to these embodiments, and various other embodiments may be included within the scope of the present invention. It will be clear to those skilled in the art that is possible.

[0031]

As is apparent from the above description, according to the surface modification method of the present invention, tetraethoxysilane, which is much cheaper than polysilazane, is used as the silicon-containing substance. Since whiskers can be efficiently dispersed in a solution having low viscosity by using polysilazane, whisker surface modification can be performed efficiently and inexpensively compared to the conventional method described in the above-mentioned publication. can do.

According to the method of the present invention, a very smooth silicon nitride film can be formed, and the surface of the whisker can be satisfactorily modified with a small amount of the film. The surface can be modified without substantially changing the ratio, so that if a composite material having the same strength is to be obtained, the whisker which has been surface-modified by a conventional general method is used. As a result, the volume ratio of the whiskers can be reduced, and the molten metal of the matrix metal can be satisfactorily penetrated into the whisker compact, thereby improving the production efficiency of the composite material.

According to the method of the present invention, since the aspect ratio of the whisker is substantially the same as the aspect ratio before the film is formed as described above, a whisker whose surface has been modified by a conventional general method is used. The properties such as the strength of the composite material can be improved as compared with the case where it is performed, and the volume ratio of the whisker required when trying to obtain the same strength can be reduced. For the composite material, machinability can be improved.

[Brief description of the drawings]

FIG. 1 is a process chart showing a series of steps of a whisker surface modification method of the present invention.

FIG. 2 is a schematic diagram showing a state in which aluminum borate whiskers are dispersed after hydrochloric acid and water are added to an ethanol solution of tetraethoxysilane.

FIG. 3 is a schematic view showing a cross section of an aluminum borate whisker having a silicon nitride film on its surface.

FIG. 4 is an explanatory view showing a step of defibrating aluminum borate whiskers.

FIG. 5 is a longitudinal sectional view showing a formed body of aluminum borate whiskers accommodated in a case.

FIG. 6 is a longitudinal sectional view showing a high-pressure casting process performed using the aluminum borate whisker compact shown in FIG. 6;

FIG. 7 is a graph showing the bending strength of the composite materials of Example 2 and Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Ethanol alcohol solution of tetraethoxysilane 12 ... Aluminum borate whisker 12A ... Part of only aluminum borate 18 ... Coating of silicon nitride 20 ... Distilled water 22 ... Propeller 26 ... Case 28 ... Molded body 30 ... High pressure casting device 32 ... Mold 34 ... Metal alloy melt 36 ... Plunger 38 ... Knockout pin

Claims (2)

(57) [Claims] 1. A method for dissolving tetraethoxysilane in an organic solvent, hydrolyzing the tetraethoxysilane in a state where whiskers are dispersed in the solution, filtering and drying the whiskers, and placing the whiskers in a nitriding atmosphere. Whiskers are heated to a high temperature and fired to form a silicon nitride film on the surfaces of the whiskers. 2. The whisker surface modification method according to claim 1, wherein the whisker is an aluminum borate whisker, and the sintering temperature for the whisker is 1000 to 14;
A whisker surface modification method, wherein the temperature is 00 ° C.