JPH07126070A - Production of silicon carbide sintered material - Google Patents

Abstract

PURPOSE:To produce a high-density sintered material by forming a preliminary sintered material so that the surface layer may have a higher density than that of the inside and that the inside and the surface layer may have >=a specified relative density and applying an isostatic hot pressing treatment thereto. CONSTITUTION:A specific amount of a sintering assistant such as powdery B4C is admixed with a powdery alpha- or beta-type SiC having about 0.1 to 3.0mum average particle diameter and a forming binder is applied to the resultant mixture to form a specific shape and this process is repeated two or more times. The obtained shaped material is then calcined in a non-oxidative atmosphere at about 1900 to 2200 deg.C for about 0.5 to 4hr to prepare a preliminary sintered material having >=90% relative density of the inside and >=96% relative density of the surface layer. The resultant preliminary sintered material is then sintered in a non-oxidative atmosphere under conditions of about 500 to 2000atm and about 1600 to 2000 deg.C according to the isostatic hot pressing method, thus producing the objective dense SiC sintered material having >=about 99% relative density.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon carbide based sintered body, and more particularly to a method for producing a large-sized sintered body.

[0002]

2. Description of the Prior Art Sintered silicon carbide has been attracting attention as a ceramic material excellent in mechanical strength, high temperature strength, hardness, etc., and is being applied to various mechanical parts in place of metals.

Such a silicon carbide sintered body is generally solid phase sintered by adding carbon or boron as a sintering aid, or liquid phase sintering by adding an oxide such as Al ₂ O ₃ or the like. Higher densities have been achieved by methods such as Among them, a hot isostatic pressing method (HIP method) has recently been known as a method for obtaining a high-density sintered body. This method is a method in which a pre-sintered body obtained by hot press firing or normal pressure firing is fired while applying a high pressure gas of 2000 atm.

[0004]

However, in the above HIP method, unless the relative density of the pre-sintered body is 95% or more, the pressure by the high pressure gas does not act on the sintered body, and It cannot be densified. Such a high-density pre-sintered body can be relatively easily manufactured by a conventional method in the case of a small shape product.

However, in the case of a large-sized product, it is very difficult to obtain a high-density sintered body from the inside to the surface. Therefore, even if the HIP method is applied, a high-density product cannot be obtained. .

Therefore, an object of the present invention is to provide a method for producing a silicon carbide based sintered body, which is capable of obtaining a high density sintered body by hot isostatic firing even in a large-sized product.

[0007]

The inventors of the present invention have studied a method for increasing the density of a pre-sintered body of a large-sized product, and found that the pre-sintered body must be applied to apply hydrostatic pressure. The present invention has been completed by discovering that it is not necessary for the whole to have a high density, and if only the vicinity of the surface of the sintered body has a high density, HIP treatment can achieve high density.

That is, according to the method for producing a silicon carbide-based sintered body of the present invention, a mixed powder obtained by adding a sintering aid to silicon carbide powder is molded into a predetermined shape, and then the molded body is placed in a non-oxidizing atmosphere. In order to obtain a pre-sintered body by calcining the pre-sintered body, and further subjecting the pre-sintered body to hot isostatic pressing, the density of the surface layer portion is higher than that of the interior and the relative density of the interior is 90% or more. ,
The relative density of the surface layer portion is 96% or more, and further, after the step of applying the hydrostatic pressure to the mixed powder and molding the pre-sintered body is repeated at least twice or more. It is characterized by being obtained by firing the molded body in a non-oxidizing atmosphere.

The present invention will be described in detail below. According to the production method of the present invention, first, the silicon carbide powder used as the raw material powder may be either α-type or β-type, and an average particle size of about 0.1 to 3.0 μm is preferable. . Also,
As the sintering aid added to the silicon carbide powder, known auxiliaries can be used. For example, carbon powder, boron compounds such as B and B ₄ C, Al, Be, Al ₂ O ₃ and Y ₂ O _{3 can be used.} And the like can be used. The amount of these sintering aids is appropriately determined depending on the kind of the aid, and specifically, it is described in JP-A-54-1.
44411, JP-A-57-42577, JP-A-55.
It may be added in a ratio as described in No. 116667.

After the silicon carbide powder and the sintering aid are mixed as described above, a binder for molding and the like are added, and the mixture is molded into a predetermined shape. As the ceramics molding means, die press molding, cold isostatic pressing, extrusion molding, injection molding and the like are known. According to the present invention, among them, cold isostatic pressing is used. In this cold isostatic press, the mixed powder is placed in a deformable container and water or oil is used as a pressure medium to obtain 300 to 5000 kg / cm ^2.
The pressure is applied to mold. Further, according to the present invention, it is important to repeat such isostatic press molding twice or more. Specifically, after the pressure is applied and molded, the pressure is released, then the pressure is applied again and the pressure is released, and this is repeated in some cases. In addition, before performing the isostatic pressing, it may be formed into a desired shape by another forming method such as a die press.

When the pressure application-release is repeatedly performed as described above, the density of the molded product gradually increases as a whole, but the density of the surface layer part of the molded product increases, and finally the surface layer part and the inside thereof. With and, a molded product having a density difference can be obtained. The relative density of the obtained molded body is preferably 60% or more in the inside, and higher than 70% in the surface layer portion.

According to the present invention, the internal relative density is 9 by pre-baking the molded body obtained as described above.
It is important to obtain a sintered body in which the relative density of the surface layer portion is 0% or more and 96% or more. To obtain a sintered body having such a density, it is obtained by firing the above-mentioned molded body in vacuum or in a non-oxidizing atmosphere such as Ar or N ₂ at 1900 to 2200 ° C. for about 0.5 to 4 hours. be able to.

Next, the presintered body was subjected to a pressure of 1600 at a pressure of 500 to 2000 atm using argon gas as a pressure medium.
By firing at a temperature of up to 2000 ° C, a sintered body having a relative density of 99% or more can be obtained.

In the above-mentioned manufacturing method, the pre-firing temperature and the hot isostatic firing temperature are limited to the above ranges, because the desired density cannot be obtained when the temperature is lower than the specified conditions, and conversely, when the temperature is too high. This is because the decomposition of silicon carbide occurs and the surface is roughened and the characteristics are deteriorated.

[0015]

According to the manufacturing method of the present invention, when the cold isostatic pressing method is used as the molding method to repeatedly apply pressure and release pressure, the overall density of the molded body gradually increases with the number of times. However, once the density is increased to a certain level, the density does not increase any further. However, at this time, a phenomenon occurs in which the surface layer portion of the molded body is higher than the inner portion of the surface layer portion. When a compact having such a high surface layer density is fired, a sintered body having a denser surface layer can be obtained.

Pre-firing-When densifying by hot isostatic pressing, a relative density of at least 95% or more is required as a pre-sintered body. Is difficult to make.

Therefore, according to the present invention, a sintered body having a high density only in the surface layer portion is obtained by the above-mentioned forming-prefiring, so that even a large-sized product can be sintered by hot isostatic pressing to have a high density. Can be obtained.

Therefore, it is not necessary to prepare a pre-sintered body having a relative density as a whole of 95% or more for a large-sized product, and if only the surface layer portion has a high density, a high pressure is applied to the whole sintered body. It is possible to obtain a high density and homogeneous sintered body.

[0019]

EXAMPLE 0.4% by weight of B ₄ C powder was added to β-type SiC powder.
And 2% by weight of carbon powder and mixed by a ball mill for 24 hours, phenol resin as a binder was added to this mixed powder, and the mixture was uniaxially pressed at a pressure of 0.1 kg / cm ² , and then hydrostatically pressed to obtain a diameter. 60mm, thickness 30m
A molded body of m was obtained. At this time, the hydrostatic press is 2000k
It was carried out 1 to 4 times with a pressure of g / cm ² . Then, the molded body was pre-baked in argon at 2000 ° C. for 3 hours and then hot isostatically baked in argon at 196 MPa at 1950 ° C. for 1 hour.

During the above steps, the densities of the molded body, the pre-sintered body and the final sintered body were measured. As for the measurement of the density, since the hardness and the density have a linear relationship, a calibration curve is created, and the hardness is measured at 1 mm intervals from the surface layer portion of the molded body to the inside, and the measured hardness is relative by the created calibration curve. Converted to density. The characteristics of the sintered body are JIS R16.
A test piece based on No. 01 was cut out from the final sintered body and the three-point bending strength was measured. The results are shown in Table 1.

[0021]

[Table 1]

According to the results shown in Table 1, in samples No. 1 and 2 in which the relative density of the surface layer portion of the pre-sintered body has not reached 96%, high densification cannot be achieved in the final sintered body. On the other hand, the sintered body having a relative density of the surface layer portion of the pre-sintered body produced according to the present invention of 96% or more has sufficiently densified to the inside in the final sintered body, and has a higher strength than the comparative example. Was also expensive.

[0023]

As described in detail above, according to the manufacturing method of the present invention, a sintered body having an internal density lower than the density of the pre-sintered body required to obtain a high density sintered body by HIP firing is used. Even if it exists, a high-density sintered body can be obtained. Therefore,
In producing a large-sized product, a high-density and homogeneous sintered body can be easily obtained.

Claims (2)

[Claims] 1. A mixed powder obtained by adding a sintering aid to silicon carbide powder is molded into a predetermined shape, and the molded body is fired in a non-oxidizing atmosphere to obtain a pre-sintered body. In the method for manufacturing a silicon carbide-based sintered body in which a sintered body is hot isostatically fired, the density of the surface layer portion of the pre-sintered body is higher than that of the inside,
Internal relative density is 90% or more, surface layer relative density is 96
% Or more, a method for producing a silicon carbide-based sintered body. 2. The pre-sintered body is obtained by repeating the step of applying a hydrostatic pressure to the mixed powder and molding it at least twice, and then firing the molded body in a non-oxidizing atmosphere. A method for manufacturing a silicon carbide-based sintered body, characterized by comprising: