JPH0360855B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing a SiC whisker preform used as a fibrous skeleton when forming a fiber-reinforced composite material by infiltrating a matrix material such as metal or plastic. [Prior Art] An effective means for producing fiber-reinforced composite materials using short fibrous substances such as whiskers as reinforcing materials.
There is a method in which a whisker preform formed into a predetermined shape is placed in a mold, and a liquid matrix material is forced into the preform and solidified. The characteristics of the composite material obtained by this method are the structure of the whisker preform, which is the fibrous skeleton;
In particular, it is greatly influenced by its homogeneity. In order to form a whisker preform with a homogeneous structure, the whisker product is dispersed in water or an organic solvent and then subjected to high-speed filtration under pressure, and the resulting wet whisker cake is dried as is or compressed to a predetermined density. Method of
is effective, but when this method is adopted, it is difficult to avoid sedimentation and development of whiskers during the operation process, and there is a drawback that density fluctuations in the vertical direction occur due to differences in whisker Stokes diameters. In order to solve the above problem, it is sufficient to create a state in which whiskers can be uniformly and stably dispersed in solution. The applicant has proposed SiC as a method to meet this objective.
We have developed a technology for stabilizing dispersion by treating whiskers with a strong acid or strong base at a concentration of 1N or higher to modify their surface properties (Patent Application No. 1986-96730,
60-96731). [Problems to be Solved by the Invention] However, the above-mentioned prior art methods of treatment with strong acids and strong bases sometimes did not provide satisfactory dispersion stability, and there were problems in which the effects could not always be reproduced satisfactorily. . As a result of investigating the cause of this, the inventors found that the main cause of the reproducibility variation was the amount of SiO ₂ attached to the surface of the SiC whiskers to be treated, and especially for SiC whiskers with an attached SiO ₂ content of 0.5% by weight or less, the dispersion was affected. It was confirmed that the stability showed a decreasing tendency. Normally, SiO ₂ may be generated and adhered to the surface of SiC whiskers during the manufacturing or post-processing process.
Although the presence of this SiO ₂ component may be tolerated in some composite applications, it becomes a factor in deteriorating properties for purposes such as obtaining a high-performance FRM. Therefore,
Preforming a homogeneous structure using SiC whiskers that do not contain SiO ₂ is also an extremely important issue. [Means for Solving the Problems] The present invention was developed based on the above findings and requests, and is particularly effective in manufacturing a preform when using SiC whiskers with a SiO ₂ adhesion content of 0.5% by weight or less. The present invention provides a method. That is, the present invention is characterized in that SiC whiskers are dispersed in a weak base aqueous solution and then overtreated, and the formed wet whisker cake is dried as it is or compressed to a predetermined density. SiC whiskers have a diameter of 0.1~5.0μm and a length of 10~
The present invention particularly targets β-type single crystals in the form of fine short fibers of 500 μm, with an SiO ₂ adhesion content of 0.5% by weight or less. Therefore, the SiO ₂ adhesion content is
If it exceeds 0.5% by weight, it is used after being purified by means such as HF treatment. After the SiC whiskers are disentangled by an appropriate defibration treatment, they are dispersed in a weak base aqueous solution. The weak base aqueous solution is, for example, NH ₄ OH,
This is an aqueous solution in which a basic substance with a small ionization constant such as CH ₃ NH ₂ or NaHCO ₃ is dissolved, and in this point it differs from the prior art, which requires treatment with a strong base. As the weak base aqueous solution, it is preferable to use an NH ₄ OH aqueous solution whose contained components can be easily volatilized and removed from the preform structure during heat drying. Furthermore, it is desirable to maintain the pH of the aqueous solution at 10 or higher. Dispersion of SiC whiskers into a weak base aqueous solution is performed by stirring and mixing. The dispersion liquid prepared in this manner exhibits a suspension state in which SiC whiskers are homogeneously and stably dispersed. This dispersion liquid is overtreated through an overformer of a desired shape, and the wet whisker cake formed by removing the dispersion medium is compressed to a predetermined density in consideration of the Vf value when forming it as it is or when compounding. By drying, a SiC whisker preform with a homogeneous structure is obtained. [Operation] SiC whiskers in the dispersion medium exhibit semicolloidal behavior, and their dispersibility is greatly influenced by the interfacial potential of the electric double layer at the dispersion medium interface. In the present invention, by maintaining the PH of the dispersion medium at a weak basic level, conditions are provided for the ξ potential of the intervening SiC whiskers to be a negative value, thereby maintaining the ability to maintain a homogeneous and stable dispersion state that is difficult to reagglomerate. This is what it brought. [Example] β with SiO ₂ adhesion content of 0.48 wt% and 0.15 wt%
type SiC whisker (diameter 0.5~1.5μm, length 10~
300μm) in 0.3% concentration NH ₄ OH aqueous solution (PH11.1)
was added at a ratio of 5% by weight, and thoroughly stirred and mixed using a stirrer. After the dispersion was allowed to stand for 1 hour, it flowed into a cylindrical pressure overformer and passed through at high speed. The formed wet whisker cake was dried at 100°C for 16 hours to form a cylindrical SiC with a diameter of 80 mm and a height of approximately 100 mm. A whisker preform was manufactured. After measuring and observing the expansion change rate and surface crack occurrence before and after drying of the manufactured preform, each specimen was horizontally sliced into 6 equal parts from the top.
Vf value fluctuations were investigated. For comparison, an example using pure water (ion exchange treated water) as the dispersion medium (Comparative Example 1) and a dispersion liquid in which SiC whiskers were boiled in a 2N-KOH aqueous solution and then washed with water to adjust the pH to 8.0. Regarding the example using (Comparative Example 2), preforming was also carried out under the same conditions as in the example. These measured and observed data are shown in a table for comparison.

【table】

〔Effect of the invention〕

According to the present invention, it is possible to manufacture a strong SiC whisker preform with a homogeneous structure and without cracking due to dry expansion, especially when using SiC whiskers with a low SiO ₂ adhesion content. . Therefore, a preform for high performance FRM can be manufactured effectively and with good reproducibility.

Claims (1)

[Claims] 1. A method for producing a SiC whisker preform, which comprises dispersing SiC whiskers in a weak base aqueous solution and then over-treating the resulting wet whisker cake, and drying the formed wet whisker cake as it is or by compressing it to a predetermined density. . 2 SiO ₂ adhesion content of SiC whiskers is 0.5% by weight
A method for manufacturing a SiC whisker preform as set forth in claim 1 below. 3. The method for producing a SiC whisker preform according to claim 1, wherein the weak base aqueous solution is an NH ₄ OH aqueous solution.