JPH0229628B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a heat-resistant ceramic material, particularly a composite ceramic material in which a silicon nitride or silicon carbide sintered body is reinforced with silicon carbide whiskers. Such composite ceramic materials are useful in applications such as turbine and diesel engine components. [Prior Art] When constructing structural parts such as turbines and diesel engines that are used under harsh conditions in high-temperature gases from ceramic materials, silicon nitride is used because of its excellent high-temperature strength and chemical inertness at high temperatures. Alternatively, silicon carbide appears to be the most likely material. Furthermore, the inherent brittleness of ceramics, which is much greater than that of metal materials, is expected to be significantly improved by incorporating heat-resistant fiber materials, particularly silicon carbide whiskers. According to this, for example, by combining silicon nitride with silicon carbide whiskers, the reliability of its mechanical properties is significantly improved, and it is also possible to perform electric discharge machining, which was impossible with a sintered product made of silicon nitride alone. become. [Problems to be solved by the invention] However, silicon nitride fine particles with a diameter of around 1 μm have a diameter of 0.1 to 0.5 μm and a length of 50 to 50 μm.
It is not easy to uniformly mix silicon carbide whiskers, which are fine needle-like substances with a length of 200 μm, that is, extremely long compared to the diameter, so in the case of mixtures of whiskers and molten materials such as light metals and plastics, In some cases, there is no significant increase in strength as seen previously, and on the contrary, there is a decrease in strength. The reason for this is that the voids that tend to occur when composited with whiskers cannot be reduced by mixing with microscopic solid particles such as silicon nitride, and this point is the biggest problem in the production of composite morami materials. It's summery. An object of the present invention is to basically solve the problem of the generation of voids due to the mixing of fine solid particles and whiskers, and to provide a method that can produce a composite ceramic material with extremely high strength. [Means for Solving the Problems] In the present invention, instead of mixing separately prepared silicon carbide whiskers with silicon nitride or silicon carbide particles that are the matrix of the composite ceramic material, the surface of the matrix particles is first coated with dioxide. Silicon carbide whiskers are mixed by depositing fine particles of silicon and then reacting the deposited silicon dioxide with excess carbon to produce silicon carbide whiskers. [Summary of the Invention] As described above, according to the present invention, silicon dioxide whiskers are generated by first attaching fine particles of silicon dioxide to the surface of matrix particles and reacting the silicon dioxide with carbon. That is, the silicon carbide whiskers grow radially from the surface of the matrix particles and exhibit a state similar to the burs of a chestnut. Therefore, a step of solid mixing the matrix particles and whiskers is not necessary, and a significantly higher level of uniformity can be achieved than in solid mixing. It is also possible to generate silicon carbide whiskers by mixing silicon dioxide and carbon, which are the raw materials for silicon carbide whiskers, with matrix fine particles in the form of their respective powders, but since the specific gravity of each component is large, it is difficult to Uniformity is already lost at this stage. Adhesion of silicon dioxide particles to the surface of matrix particles can be carried out industrially very easily and inexpensively by treatment with an aqueous dispersion of silica sol. As long as this deposition is uniform, the mixing of other components necessary to form silicon carbide whiskers does not necessarily have to be uniform. This is because the growth source of silicon carbide whiskers is silicon dioxide, which greatly contributes to maintaining the uniformity of whisker dispersion after sintering. The amount of silicon dioxide attached to the matrix particles is determined depending on the desired whisker mixing ratio in the final composite ceramic material.
Based on the limit value of the effect of improving physical properties such as strength due to whisker combination, for 100 parts by weight of matrix powder,
A range of 10 to 150 parts is appropriate. Carbon black as a carbon source added to the matrix on which silicon dioxide is deposited (hereinafter referred to as the "deposit") has a stoichiometric amount of carbon necessary to convert the silicon dioxide of the deposit into silicon carbide. The amount may be in excess of the theoretical proportion, and is generally in the range of 10 to 600 parts by weight per 100 parts by weight of the matrix powder. The alkali metal or alkaline earth metal halide added to this deposit improves the physical strength, especially the bending strength, of the final sintered product, as is clear from the examples described later. is essential. According to the experimental results, good effects were obtained when 10 to 300 parts by weight of halide were added to 100 parts by weight of matrix powder. The effect of addition is not significant if it is less than 10 parts by weight, and the effect remains unchanged even if it is added in excess of 300 parts by weight. As a sintering aid, one commonly used for sintering silicon nitride or silicon carbide, such as yttoria (Y ₂ O ₃ ), can be used in the amount commonly used. Similarly, commonly known cobalt chloride or the like is used as a silicon carbide whisker formation promoter. The mixture of these components is heated to a temperature of 1300-1700° C. in an inert atmosphere to form silicon carbide whiskers on the surface of the matrix particles. When the temperature is below 1,300°C, almost no whiskers are produced, and on the other hand, when the temperature is above 1,700°C, the whiskers that are produced become shorter and chemical decomposition begins. The mixture thus produced, in which silicon carbide whiskers are uniformly distributed, is formed into a desired shape in the same manner as in the production of ordinary ceramic materials, and is generally applied to the sintering of silicon nitride or silicon carbide. Sintered under conditions. Since the conditions for molding and sintering are obvious to those skilled in the art, detailed explanation thereof will be omitted. [Example] 20 pieces of matrix fine powder (average particle size 1.0 μm)
By repeating the operation of immersing in ~40% silica sol aqueous dispersion (change the concentration as appropriate depending on the amount of adhesion), drying it, and immersing it again as many times as necessary, the matrix particles are coated with various proportions on the surface of the matrix particles. Silicon dioxide was deposited. Cobalt chloride containing cobalt equivalent to 2wt% of silica is added to the silica sol aqueous dispersion as a whisker generation promoter, and a small amount of cobalt is added as a sintering aid.
_Y2O3 was added _. Carbon black and sodium chloride were added and mixed in various proportions to the matrix powder to which silica was attached in this way. The various mixtures obtained were packed into artificial graphite crucibles,
The material was maintained at 1600±10° C. for 1.5 hours in a nitrogen stream using an electric furnace to generate silicon carbide whiskers on the matrix surface. Next, this whisker-generating treated product is placed in the air.
After removing unreacted carbon black by holding at 600℃±20℃ for 3 hours, it was filled into a graphite mold whose inner wall was coated with boron nitride.
Hot pressing was performed at 1800° C., 400 Kg/cm ² and 60 minutes. Square rod-shaped samples with dimensions of 3 x 3 x 40 mm were cut out from the obtained sintered products. For comparison, a similar sample was cut out from a sintered product of a comparative example that was not based on the method of the present invention. The three-point bending strength of each sample was determined under the conditions of a bottom span of 20 mm and a crosshead speed of 0.5 mm/min, and the results are shown in Table 1 along with the components of each sample. Note that all numerical values indicating compounding ratios in the table are parts by weight. Comparative Example 1 is a case in which 6.5 parts of SiC whiskers are mixed with 10 parts of Si ₃ N ₄ fine powder, Comparative Example 2 is a case in which no silicon carbide whisker formation promoter is added, and Comparative Example 3 is a case in which 10 parts of SiC fine powder is mixed with 6.5 parts of SiC whiskers. Comparative Example 4 is a case in which 6.5 parts of SiC whiskers were mixed and no silicon carbide whisker formation promoter was added.
For reference, the room temperature bending strengths of the Si ₃ N ₄ sintered body containing no SiC whiskers and the SiC sintered body were 67 Kg/mm ² and 65 Kg/mm ² , respectively.

〔Effect of the invention〕

As described above, according to the present invention, fine particles of silicon dioxide are attached to the surface of matrix particles, and silicon carbide whiskers are generated in the process of converting the attached silicon dioxide into silicon carbide in the presence of carbon. , compared to the case of mixing matrix particles and silicon carbide whiskers,
This not only simplifies the process, but also provides the effect of obtaining a sintered product with extremely high strength due to the uniform dispersion of the whiskers.

Claims (1)

[Claims] 1. Fine powder of silicon dioxide is applied to the surface of matrix particles of silicon nitride and/or silicon carbide,
Adhere at a ratio of 10 to 150 parts by weight of silicon dioxide to 100 parts by weight of silicon nitride or silicon carbide,
10 to 600 parts by weight of carbon, 10 to 300 parts by weight of an alkali metal or alkaline earth metal halide, and an appropriate amount of A sintering aid for silicon nitride or silicon carbide and an appropriate amount of a silicon carbide whisker formation promoter are added and mixed, and the mixture is heated to
A method for producing a composite ceramic material, which comprises heating to 1700°C to generate silicon carbide whiskers on the surface of the matrix particles, removing excess carbon by oxidation, shaping into a desired shape, and sintering. .