JPH044995B2 - - 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, almost no shrinkage during reaction sintering, and high dimensional accuracy. Widely used as an insulating material.

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 20 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 blend 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.

The inventors have noted that even with the aid of these additives, the density of the pre-sintered body must be appropriate in order to obtain a high-density reactive sintered body.

That is, while the density of the preliminary sintered body is relatively low, the density of the reaction sintered body increases as the density increases, and the density of the reaction sintered body obtained by conventional technology is 2.2 to 2.5. When trying to obtain a product with a density higher than g/ ^cm3 , the density of the pre-sintered body should be 1.70
g/cm ³ or more.

On the other hand, as the density of the pre-sintered body increases,
Since it becomes difficult to nitrid the inside of the reaction sintered body, a reaction sintered body cannot be obtained, or only a product with insufficient nitridation can be obtained, and an increase in the density of the reaction sintered body cannot be achieved as a whole. Regardless of the shape or dimensions of the pre-sintered body, the density of the pre-sintered body must be 2.05 in order to be able to be nitrided under practical nitriding conditions.
It must be kept at around g/ ^cm3 .

Therefore, in order to produce a silicon nitride reaction sintered body with desirable properties, the density of the Si pre-sintered body must be within the above range of 1.70 to 2.05 g/cm ³ .
Furthermore, it is required to precisely control the density to a value that provides a reaction sintered body with a density appropriate for the intended use.

The method for producing a high-density silicon nitride reaction sintered body of the present invention addresses this need, and includes boron or a compound thereof as B of 0.15 to 5.0% by weight, and/or Fe, Co, Ni, Cr, Mo, Mn, W, Ti,
Zr, Ta, Nb, V, Mg, Ca, Cu, Zn and Sn
One or more elements or compounds thereof as the above elements (in the case of two or more, the total amount)
Silicon powder containing 0.05 to 2.0% by weight is molded, and the molded body is heated to 1100℃ or higher in an inert gas atmosphere.
Pre-sintering is performed by heating to a temperature lower than the melting point of Si.
In a method for manufacturing a high-density silicon nitride reaction sintered body, which consists of heating the obtained pre-sintered body to a temperature of 1100 to 1500°C and nitriding it by applying _N2 , easily sinterable Si is used as silicon powder. The method is characterized in that a mixture of powder and hard-to-sinter Si powder is used, and the density of the pre-sintered body is controlled by selecting the mixing ratio.

A conventional technique for controlling the density of a sintered body is to select the sintering temperature and time. However, since the implementation conditions for such a method vary depending on the shape and dimensions of the compact to be sintered, the method must be determined after many experiments, and is not universally applicable. In this respect, the control method according to the present invention has the advantage that it can be applied to the sintering of any molded body by examining the sintering characteristics of the two types of Si powder to be mixed.

Here, "easily sinterable" and "difficult to sinter" silicon powders are defined by the following test method. That is, Si powder (without additives) was molded using a 2 ton/cm ² pressure blur press and heated at 1360°C x 2 in an Ar flow.
When sintering for hours, the shrinkage rate (linear shrinkage rate)
When it is 8% or less...difficult to sinter; when it is 15% or more...easily sintered. For reference, when a molded body with a molded density of 55% of the true density is sintered, assuming that there is no weight change, the achieved density will be approximately at a shrinkage rate of 8%.
71%, 15% is about 90%.

The present inventor has not yet clarified what powder properties determine the difference in the sintering properties of Si powder. Generally, the smaller the particle size, the higher the sinterability, but it is also true that there is a considerable difference in sinterability even between particles of the same size. In the experience of the present inventors, fine Si powder obtained from the gas phase has higher sinterability than one of comparable particle size obtained by a pulverization method. In any case, the sinterability of each powder must be determined by the test method described above, and it is easy to do so.

It goes without saying that the ratio of the easily sinterable Si powder and the difficult to sinterable Si powder to be blended should be selected depending on the sinterability of each powder and the intended density of the pre-sintered body. However, the density range mentioned above is 1.70~2.05g/
The blending ratio giving cm ³ will usually be found in the range of easily sinterable powder: difficult to sinter powder = 20:80 to 60:40 by weight. However, other blending ratios may be appropriate depending on the powder.

The limits and reasons for the contents of the 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 when it becomes large, it 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. , compounds among 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 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 0.82 g/cm ³ (the theoretical density) to facilitate handling and processing and to ensure sinterability during pre-sintering.
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.

Preliminary sintering can be carried out by conventional methods such as free sintering, uniaxial pressure sintering (so-called hot press), and hot isostatic pressure sintering.

Pre-sintering is performed at a temperature of 1100°C or higher. 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. When following the invention,
The achieved density is determined mostly by the sinterability and blending ratio of the easily sinterable and difficult-to-sinter Si powders, and a difference of about ±10°C in sintering temperature has no effect, and the sintering time is also 30°C. It approaches saturation in about a minute, and there is no substantial change even if heated for a long time.

In the present invention, the nitriding of the Si preliminary sintered body can be carried out in the same manner as in the production of conventional silicon nitride reaction sintered bodies. In other words, generally under a nitrogen gas atmosphere at atmospheric pressure,
Heat to a temperature of 1100-1500 ° C. The temperature is 1100~
It is also possible to gradually increase the temperature from the low temperature side of 1350℃. In order to adjust the reaction rate, the nitrogen pressure may be selected within the range from reduced pressure (up to about 1/100th of an atmosphere) 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 pre-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 Si lumps with a purity of 99.999% are crushed to have an average particle size of 0.23μ
powder was prepared. This Si powder showed a linear shrinkage rate of 3% in the sintering test described above, and is classified as a difficult-to-sinter powder.

Separately, a commercially available Si powder obtained from the gas phase with an average particle size of 0.15 μm was subjected to a sintering test, and it was confirmed that it had a linear shrinkage rate of 26% and belonged to easily sinterable powder.

Both were blended in various proportions, placed in a polyethylene ball mill, wet mixed with n-hexane medium, dried by evaporating the n-hexane, and rubber press molded into a cylindrical body with a diameter of 10 mm and a height of 10 mm. (pressure 2 ton/cm ² ).

Each material was sintered by heating at 1360°C for 2 hours in a high-purity Ar air flow.

The density of the obtained pre-sintered body was measured. The results are shown in the figure so that control by blending ratio can be confirmed.

Next, each pre-sintered body was heated at 1370℃ in a nitrogen stream.
Nitriding treatment was performed by heating for ×48 hours → 1385°C × 96 hours → 1420°C × 24 hours.

The density of the reaction sintered body was measured, and the amount of residual Si therein was determined by X-ray diffraction. The results are shown below together with the density of the preliminary sintered body.

Preliminary sintered body density Reacted sintered body density Residual Si (g/cm ³ ) (g/cm ³ ) (%) 1.62 2.43 0.5 or less 1.74 2.58 〃 1.85 2.73 〃 1.98 2.98 〃 2.01 3.00 0.7 2.10 2.75 15.4

[Brief explanation of drawings]

The drawings show that easily sinterable Si is used in an embodiment of the present invention.
It shows how the density of the pre-sintered body changes depending on the blending ratio of powder and hard-to-sinter Si powder, and what kind of density the pre-sintered body gives a reaction sintered body by nitriding. It is a graph.

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 manufacturing method of 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 , easily sinterable Si powder and hard-to-sinter Si powder are used as silicon powder. A manufacturing method characterized by using a mixture of and controlling the density of the pre-sintered body by selecting the mixing ratio. 2. Select the mixing ratio of easily sinterable Si powder to difficult to sinter Si powder in the range of 20:80 to 60:40 by weight, and set the density of the pre-sintered body in the range of 1.7 to 2.05 g/ ^cm3 . The manufacturing method according to claim 1.