JPS63270358A - Production of sintered silicon carbide - Google Patents

Abstract

PURPOSE:To obtain the titled sintered material having high density and large size at a low cost, by adding boron(-containing compound) and carbon(- containing compound) to beta-phase SiC powder produced e.g. by silica reduction process, forming the obtained raw material powder and calcining in a mixed gas atmosphere of N2 and an inert gas at a specific ratio under normal pressure. CONSTITUTION:SiC powder produced by silica reduction process or direct carburization of silicon, containing >=95wt.% of beta-phase and <=0.2wt.% of N2 as an impurity, having a specific surface area of >=10m<2>/g and composed mainly of beta-phase is compounded with 0.2-3wt.% of B or a boron-containing compound (e.g. B4C or BN) and 0.5-4wt.% (in terms of C) of C or a carbon-containing compound (e.g. phenolic resin) to obtain raw material powder. The powder is formed in a prescribed form and calcined at 2,000-2,200 deg.C under normal pressure in a mixed gas atmosphere obtained by mixing N2 gas with an inert gas (e.g. He, Ne or Ar) at a ratio of 1/10-2/3 to obtain the titled dense sintered product having uniform composition and a density of >=93% of theoretical density.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a silicon carbide sintered body, and more particularly, to a manufacturing method for obtaining a large-sized, high-density sintered body. .

[Prior art]

Silicon carbide sintered bodies have oxidation resistance, corrosion resistance, thermal shock resistance,
It has attracted attention as a heat-resistant material with excellent strength, and is applied to various high-temperature materials, wear-resistant materials, etc.

Generally, this silicon carbide sintered body is obtained by adding a sintering aid to α-phase or β-phase silicon carbide powder, forming the powder, and then firing it at a high temperature. Specifically, as a sintering aid, Japanese Patent Publication No. 57-3203
As disclosed in Japanese Patent Publication No. 57-170877, etc., it is proposed to use boron or a boron-containing compound such as boron carbide or boron nitride in combination with carbon or an organic compound capable of producing carbon by firing. has been done. Firing methods include vacuum firing, normal pressure firing, reduced pressure firing, and true press methods, all of which are heated at 2000°C in an inert gas atmosphere such as argon, helium, or neon.
However, among the above firing methods, it is difficult to produce sintered bodies with complex shapes using the hot press method, so vacuum firing, reduced pressure firing, and normal pressure firing methods are used. Most commonly adopted.

However, according to the above-mentioned normal pressure firing method, densification tends to vary depending on the type of silicon carbide used. For example α
When performing atmospheric pressure firing in an argon atmosphere using silicon carbide powder as a phase and both boron-based and carbon-based sintering aids, a highly dense sintered body is produced regardless of the size of the compact. However, if similar firing is performed using β-phase silicon carbide powder, there is a phenomenon in which it is possible to densify small products to some extent, but it is difficult to densify large products. arise. This is considered to be due to the fact that the production temperature of the β-phase silicon carbide powder itself is lower than that of the α-phase silicon carbide powder, that is, the grain growth initiation temperature is low. That is, in a system using β-phase silicon carbide powder, 20
In the process of raising the temperature to a firing temperature of around 00℃, the grain growth of β-phase silicon carbide is intense, so the silicon carbide grains become larger when the firing temperature is reached, which reduces the driving force for densification at the firing temperature. Therefore, it becomes difficult to obtain a highly dense sintered body. Particularly when the compact is large, the grain growth becomes particularly intense due to the slow rate of temperature rise, and densification cannot be achieved.

To solve this problem, Japanese Patent Publication No. 57-32035 discloses that grain growth of β-phase silicon carbide can be suppressed by introducing nitrogen into the atmosphere, which is particularly effective when manufacturing large products. It is disclosed.

In particular, according to the embodiment, high-quality silicon carbide powder manufactured by a vapor phase method, doped with boron, and with free carbon uniformly dispersed is used in a vacuum, a reduced pressure nitrogen atmosphere, a reduced pressure argon atmosphere, or an atmospheric pressure nitrogen atmosphere. It is disclosed that the material is fired in the same manner as above.

[Problem that the invention seeks to solve]

However, since the raw material powder used here is very expensive, it is desirable to use inexpensive β-phase silicon carbide powder obtained by other manufacturing methods, such as silica reduction method or silicon direct carbonization method. .

Therefore, when boron-based and carbon-based auxiliary agents are added to β-phase silicon carbide powder obtained by silica reduction method or silicon direct carbonization method, and fired by the method disclosed in the above-mentioned example, both cases result in a highly dense sintered body. The problem arises that it is not possible to obtain

Specifically, these raw material powders are particularly susceptible to decomposition in reduced pressure or vacuum atmospheres, so a carbon layer is likely to be formed on the surface of the sintered body, and nitrogen atoms are doped into the crystals in an atmospheric nitrogen atmosphere, causing sintering to deteriorate. Since diffusion, which is one of the mechanisms, is suppressed, it has the disadvantage of not being densified.

[Purpose of the invention]

Therefore, the present invention eliminates the above-mentioned drawbacks when using silicon carbide powder mainly composed of β phase produced by silica reduction method or silicon direct carbonization method, and provides stable molding regardless of its size. It is an object of the present invention to provide a method for manufacturing a silicon carbide sintered body that can be fired in a manner that allows the firing of the silicon carbide sintered body.

[Means for solving problems]

As a result of repeated research on the above-mentioned problems, the present inventors found that
Using a specific β-phase silicon carbide powder produced by the silica reduction method and the silicon straight line method and having a low nitrogen content, boron and carbon-based auxiliary agents are added to it, and a large amount of gas consisting of a mixture of nitrogen gas and inert gas is used. By firing in an atmospheric pressure atmosphere, a large-sized, high-density sintered body can be obtained.

〔Example〕

The present invention will be explained in detail below.

The silicon carbide powder used in the present invention is produced by a silica reduction method or a silicon direct conversion method. In the silica reduction method, silica (Si02) powder and fine carbon powder are synthesized by heating to 1500 to 1700°C in an inert atmosphere, and if desired, after synthesis, decarburization treatment by oxidation and desilica treatment with hydrofluoric acid are performed. be done. On the other hand, in the silicon direct carbonization method, metallic silicon powder and carbon powder are
This is a method of heating and reacting at a temperature of 0 to 1400°C. In the present invention, among the raw materials obtained by such a method, the content of β phase is 95% by weight or more, and the content of nitrogen mixed into the raw material as an impurity during synthesis is 0.2% by weight or less, especially 0. .15% by weight or less is used. The reason why the nitrogen content is limited is that, as will be clear from the examples described later, if the nitrogen content exceeds 0.2% by weight, densification, especially diffusion, is suppressed, making it impossible to achieve high densification. Moreover, this silicon carbide powder has a stopping surface area of 10n.
+”/g or more fine powder with a purity of 98% or more is desirable, and impurities include Free-Sin□, Fr
ee-C, Δ1. It may also contain Fe and the like.

Examples of the auxiliary agent added to this silicon carbide powder include a bow-based auxiliary agent and a carbon-based auxiliary agent.

As boron-based auxiliaries, amorphous boron, boron, or boron-containing compounds such as boron carbide, boron nitride, and phosphorous boride are used.As carbon-based auxiliaries, in addition to carbon and carbon black, phenolic resins and coal tar pitch are used. Carbon-containing compounds that can generate carbon during firing, such as petroleum pitch, are used.

Among these sintering aids, sintering will be insufficient even if either the boron-based aid or the carbon-based aid is missing, and the boron-based aid may be used at a ratio of 0.2 to 3% by weight, or The carbon-based auxiliary agent is controlled depending on the amount of Sin contained in the β-phase silicon carbide powder, but it is preferably blended in an amount of approximately 0.5 to 4% by weight in terms of carbon.

The mixed powder prepared with the above composition is shaped by a known forming means and then transferred to a firing process.

The firing process in the present invention is based on the normal pressure firing method, and the firing atmosphere is composed of a mixed gas of nitrogen gas and an inert gas such as He, Ne, ^r, etc., and the gas partial pressure ratio is (nitrogen gas / It is extremely important to set the inert gas) ratio between 1710 and 2/3, especially between 115 and 1/3.

That is, if the gas ratio is less than 1/10, the effect of suppressing grain growth will be insufficient, and if it exceeds 2/3, diffusion will be suppressed as described above, making it impossible to obtain a dense sintered body in either case.

In addition, in the present invention, the firing temperature is 2000 to 2200.
It is desirable to set the temperature to 2050 to 2150°C.

According to the manufacturing method of the present invention, not only small molded objects but also
Even large molded objects can be sufficiently densified.
As is clear from the examples described later, the theoretical density ratio is 93.
% or more of uniform composition can be obtained.

The invention will now be explained with the following examples.

Example raw material: 97.5% by weight of silicon carbide powder shown in Table 1 produced by silica reduction method or silicon direct carbonization method;
1.0% by weight of amorphous boron powder and 1.5% by weight of novolak phenol resin as a carbon source in terms of carbon were mixed with acetone as a solvent.

The acetone is evaporated and the dry powder is made into a large, thick molded body with a diameter of 100 mm and a thickness of 80 mm (
A molded product with a weight of 1320 g was obtained. This molded body was fired under normal pressure (atmosphere) under the firing conditions shown in Table 1.

Table 1 The specific gravity (ratio to theoretical density) of the obtained sintered body was measured by the Archimedes method.

As a comparative example, the firing atmosphere was set to argon only (kl, 2) and nitrogen gas only (kl,2).
The specific gravity of No. 7) was measured in the same manner.

The results are shown in Table 2.

According to the results in Table 2, 1.2 samples were fired using the conventional method in an argon atmosphere, and 2 samples were fired in an atmosphere containing only nitrogen.
All of the N117 samples fired at a high temperature of 200"C were insufficiently densified, while the floors 3 to 6.8.10, which were mixed with nitrogen and argon at a specific ratio,
It was possible to obtain a highly dense body with a density of more than %. Moreover, the sample of No. 9, which used raw material C with a nitrogen content exceeding 0.2 wtχ as a raw material, was also insufficiently densified.

Incidentally, an enlarged view of the crystalline state of the sintered body of the sample 11h4 in Table 2 is shown in FIG. As is clear from Figure 1, it has a structure in which columnar crystals unique to β-phase silicon carbide are intertwined, and there is no feather-like abnormal grain growth that is noticeable when firing in an argon-only atmosphere. , it can be seen that it is a highly dense body.

〔Effect of the invention〕

As detailed above, according to the method for manufacturing a silicon carbide sintered body of the present invention, a large-sized sintered body can be manufactured using inexpensive β-phase silicon carbide synthesized by a silica reduction method or a silicon direct conversion method. During manufacturing, by setting the atmosphere during firing to normal pressure with a mixture of inert gas and nitrogen gas in a specific ratio, it is possible to perform firing while preventing the harmful effects of nitrogen gas and suppressing grain growth. As a result, a large, high-density sintered body can be obtained at low cost without densification being inhibited by grain growth.

[Brief explanation of drawings]

FIG. 1 is an enlarged view showing the crystal structure of a silicon carbide sintered body obtained according to the present invention.

Claims (1)

[Claims] (1) Boron or a boron-containing compound and carbon or carbon are added to silicon carbide powder produced by a silica reduction method or a silicon direct carbonization method and having a nitrogen content of 0.2% by weight or less and a β phase of 95% by weight or more. After molding the raw material powder made by adding the containing compound, the ratio of nitrogen and inert gas is 1.
:200 under normal pressure in a mixed atmosphere of 10 to 2:3.
A method for producing a silicon carbide sintered body, the method comprising firing at a temperature of 0 to 2200°C.