JPH046671B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-density silicon nitride reaction sintered body. Among silicon nitride Si ₃ N ₄ products, those called reaction sintered bodies are usually made by applying nitrogen gas to a molded body of Si powder or a molded body of (Si + Si ₃ N ₄ ) powder mixture while nitriding it. Manufactured by sintering. This kind of product has thermal shock resistance, hardness,
It has excellent electrical insulation and chemical stability at high temperatures, and has almost no shrinkage during reaction sintering.
Because it has the advantage of achieving high dimensional accuracy,
Widely used in applications such as fireproof materials, wear-resistant materials, corrosion-resistant materials, and insulation materials. The drawback of conventional silicon nitride reaction sintered bodies is that they are mechanically weak, with a bending strength of around 20Kgf/ ^mm2 , or at most 30Kgf/ ^mm2 , which is unsatisfactory as a material for heat-resistant structures. be. This is because even products obtained by completely nitriding silicon are only relatively low-density sintered bodies with a porosity of 230 to 30%. If a reaction sintered body with higher density can be produced, it will be possible to improve various properties such as strength from room temperature to high temperature, making it extremely useful as a material for heat-resistant structures by taking advantage of its characteristic of not losing strength even at high temperatures. Become something. The measures taken so far to increase the density of reaction sintered bodies are to increase the density of Si or (Si+Si ₃ N ₄ ) compacts. Specifically, the first step is to increase the powder compaction pressure, but even if the pressure is as high as practical, the density of the product after nitriding is only 2.39 g/cm ³ (75% of the theoretical density). .
Attempts have been made to adjust the particle size of the powder and mix powders with various particle sizes, but the density of the reaction sintered body was still limited to 2.54 g/cm ³ . In order to obtain a reaction sintered body with even higher density, pre-sintering of the Si molded body, that is, sintering in an inert atmosphere prior to nitriding, is introduced to create a high-density sintered body of the Si molded body. It was proposed that
121613). However, the method disclosed above does not allow Si by pre-sintering.
In order to effectively achieve high density of the compact, it is essential to use extremely fine Si powder with an average particle size of 0.2μ or less. Apart from the problem that such fine powder is not easy to manufacture, the density of the resulting reaction sintered body is still 2.39 g/cm ³
(92% of the theoretical density) was the limit. The present inventor has conducted extensive research with collaborators with the intention of further increasing the density of Si molded bodies using pre-sintering, and has achieved a maximum density of 3.05 g/cm ³ (96% of the theoretical density). We succeeded in obtaining an extremely high-density reaction sintered body, which we have already disclosed (Japanese Unexamined Patent Application Publication No. 1987-1999).
188465 and 57-188466). The method is Si
Adding a certain amount of boron to the powder to improve sinterability or adding certain elements i.e. Fe, Co,
Ni, Cr, Mo, Mn, W, Ti, Zr, Ta, Nb, V,
The gist is to promote nitriding by adding a certain amount of one or more elements selected from Mg, Ca, Cu, Zn, and Sn, or a compound thereof. Subsequent research revealed that nitridation promoters such as Fe have the effect of improving sinterability and are useful over a wide range of addition amounts. Additives such as B and Fe mentioned above are
Of course, they may be used in combination, which is preferred. Furthermore, the present inventor conducted various studies with the aim of increasing the strength of the silicon nitride reaction sintered body, and found that, along with the density, impurities are generated in the reaction sintered body.
It was found that SiO ₂ and SiON ₂ have a significant effect. That is, since these impurities are present in the Si ₃ N ₄ structure and form weak points in terms of strength, their amount must be reduced as much as possible. As a countermeasure to this problem, the method for manufacturing a silicon nitride reaction sintered body proposed here uses boron or its compound as B of 0.15 to 5.0% by weight, and/or Fe, Co,
Ni, Cr, Mo, Mn, W, Ti, Zr, Ta, Nb, V,
One or more elements of Mg, Ca, Cu, Zn, and Sn or their compounds are used as the above elements (2
Silicon powder containing 0.05 to 2.0% by weight (if the total amount is more than The obtained preliminary sintered body is
In the method for producing a high-density silicon nitride reaction sintered body, which consists of heating to a temperature of 1100 to 1500°C and nitriding by applying _N2 , the pre-sintering atmosphere is hydrogen gas or 0.5% by volume or more of hydrogen. It is characterized by the use of a mixed gas with argon or helium. In the conventional manufacturing method, the oxygen contained in the raw material Si powder is thought to exist mainly in the form of SiO ₂ , which is volatilized by the reaction of SiO ₂ +Si→2SiO↑ during preliminary sintering, and does not remain until the subsequent nitriding process. It is understood that it was lost. However, when fine Si powder is used to produce a high-density reaction sintered body, the amount of oxygen contained therein inevitably increases, for example, due to an increase in the oxide film due to an increase in surface area. Then, during the pre-sintering process, oxygen is
It is likely that the above mechanism will not be able to volatilize it completely. This has prevented improvements in the strength of reaction sintered bodies despite efforts to use fine powders. By making hydrogen present in the pre-sintering step according to the present invention, oxygen in the raw Si powder is removed, and SiO ₂ remains in the subsequent nitriding step.
Generation of SiON ₂ is avoided. This effect is due to the volume in the pre-sintering atmosphere gas.
Obtained when more than 0.5% _H2 is present. Preferably, H ₂ is present in an amount of 1% or more, several % to about 10%. The limits and reasons for the contents of the various substances having the effect of promoting sintering or nitriding are the same as in the invention disclosed above. That is, in order to expect the effect of boron, it is necessary to contain at least 0.15% by weight. However, boron generates boron nitride BN in the nitriding process, and in large amounts this inhibits reaction sintering. Therefore, it must be kept within 5.0% by weight. The content of Fe and other substances is 0.05 to Si powder.
If the amount is less than % by weight, no effect will be obtained. Below this lower limit, the density of the pre-sintered body may become high and the time required to nitridize Si to a high degree becomes impractically long. On the other hand, a content exceeding 2.0% causes significant grain growth and prevents high density during preliminary sintering, so it must be avoided. The preferred range varies depending on the element used, but is usually 0.1 to 0.6% by weight. In the case of boron, the existing form may be metallic boron, an amorphous substance, or a metallic boride, and Fe and others may be in an elemental state or in a compound such as an oxide. Well, compounds between them are of course preferred. When both are used together, it is preferable to select a compound of boron and these elements because both can be present at once. The raw material Si powder should have an average particle size of 15 μm or less, and preferably a fine powder of 1 μm or less in order to facilitate high density during preliminary sintering. The sintering accelerator and the nitriding accelerator are also desirably fine particles with a particle size equal to or smaller than that of the Si powder. The techniques for powder compaction and pre-sintering are the same as previously disclosed. That is, it goes without saying that the raw material powder or powder mixture can be formed by any means such as conventional die forming, isostatic pressing, slip casting, injection molding, and the like. The density of the compact to be pre-sintered is set at 0.82 g/cm ³ (the theoretical density
35%) or more. If the density is lower than this, it is difficult to obtain a sintered body having a uniform structure even if the density can be increased by preliminary sintering. As a preliminary sintering method, in addition to free sintering, ordinary methods such as uniaxial pressure sintering (so-called hot press) and hot isostatic pressure sintering can be used. Pre-sintering is performed at a temperature of 1100°C or higher. At temperatures lower than this, high density cannot be expected even if fine powder is used. The upper limit temperature is, of course, the melting point of Si. The atmosphere is preferably an inert gas such as argon, but may also be a vacuum. The degree of densification at the preliminary sintering stage is expressed by the amount of shrinkage caused by sintering. This does not depend on the density of the compact to be sintered, but in order to provide the high-density reaction sintered product that the present invention aims to achieve, the volumetric shrinkage rate must be at least 10%, and 20%. % or more is preferable. The pre-sintered body is somewhat more porous and less dense than that produced by conventional pre-sintering methods as a result of the presence of H ₂ aiding the volatilization of SiO ₂ . If this is exposed to an oxidizing atmosphere, the oxygen that has been so painstakingly removed will be added again, and the purpose of pre-sintering by controlling the atmosphere will be diminished, so it must be placed in a non-oxidizing atmosphere. After the preliminary sintering is completed, it is preferable to immediately place it in a nitriding atmosphere and proceed to the nitriding step. In practice, nitriding may be carried out using the same furnace and changing the atmospheric gas and temperature following preliminary sintering. Taking the above points into consideration, the nitriding of the Si pre-sintered body can be carried out in the present invention in the same manner as in the production of conventional silicon nitride reaction sintered bodies. That is, it is generally heated to a temperature of 1100 to 1500°C under a nitrogen gas atmosphere at atmospheric pressure. The temperature can also be increased stepwise from the low temperature side of 1100 to 1350°C. In order to adjust the reaction rate, the nitrogen pressure may be selected from a range of reduced pressure (up to about 1/100 atm) to increased pressure (up to 2000 atm). In addition to pure nitrogen gas, hydrogen-mixed nitrogen gas and ammonia can also be used. The time required for nitriding varies greatly depending on the density of the preliminary sintered body, average grain size, nitriding temperature, and atmospheric conditions, as well as on the allowable amount of residual Si, but ranges from several hours to about 200 hours. Examples and Comparative Examples The following additives were added to silicon fine powder with an average particle size of 0.15μ. Example 1 and Comparative Example 1 B 0.4% Example 2 and Comparative Example 2 Fe ₂ O ₃ 1.2% Example 3 and Comparative Example Example 3 B 0.4% + Nb ₂ O ₃ 1.2% Example 4 B 0.4% Molded using a rubber press, diameter 30 mm x height 10
I got a mm tablet. The density of the molded body is
It was 52%. These molded bodies were heated in the following atmosphere and at a temperature of
Preliminary sintering was performed at 1375°C for 1 hour. Examples 1, 2, 3 Ar + 10% H ₂ Example 1 Ar + 1% H ₂ Comparative Examples 1, 2, 3 Ar The densities of the preliminary sintered bodies were as follows. Example 1 80% Comparative Example 1 82% 2 82 2 82 3 85 3 86 Example 4 80% These preliminary sintered bodies were heated to 1350% in an N ₂ atmosphere.
Nitriding was carried out by heating at 1380°C for 20 hours → 1410°C for 20 hours. For each reaction sintered body obtained, the density, impurity content (ground and subjected to X-ray diffraction) and strength were measured. The results are shown below. The silicon nitride reaction sintered body of the present invention contains SiO ₂ and
It is clear from this data that impurities such as Si ₂ ON ₂ are not generated and the strength is excellent. 【table】

Claims (1)

[Claims] 1 Boron or its compound as B is 0.15 to
5.0% by weight, and/or Fe, Co, Ni,
Cr, Mo, Mn, W, Ti, Zr, Ta, Nb, V,
One or more elements of Mg, Ca, Cu, Zn, and Sn or their compounds are used as the above elements (2
Silicon powder containing 0.05 to 2.0% by weight (if the total amount is more than The obtained preliminary sintered body is
In the method for producing a high-density silicon nitride reaction sintered body, which consists of heating to a temperature of 1100 to 1500°C and nitriding by applying _N2 , the pre-sintering atmosphere is hydrogen gas or 0.5% by volume or more of hydrogen. A manufacturing method characterized by the use of a gas mixture with argon or helium. 2. The manufacturing method according to claim 1, wherein after preliminary sintering, the obtained preliminary sintered body is transferred to a nitriding atmosphere and nitrided without being placed in an oxidizing atmosphere. 3. The manufacturing method according to claim 1 or 2, wherein the pre-sintered body has a volumetric shrinkage rate of 10% or more.