JPS6041634B2 - Method for manufacturing high-density silicon nitride reaction sintered body - 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 the products of silicon nitride Sj, N, there are products called reaction sintered bodies.
What is exposed is usually a molded body of Si powder or (Si
+Si3N4) It is manufactured by applying nitrogen gas to a molded body of a powder mixture to nitridize it and sinter it.

This type of product has excellent thermal shock resistance, hardness, electrical insulation properties at high temperatures, and chemical stability, as well as having the advantage of almost no shrinkage during reaction sintering and high dimensional accuracy. It is widely used in applications such as fireproof materials, wear-resistant materials, corrosion-resistant materials, and insulation materials. The disadvantage of conventional silicon nitride reaction sintered bodies is that they are mechanically weak.
The bending strength is about 20.9fIrfI1ft, and at most it is only 30.9fl-, which is unsatisfactory as a heat-resistant structural material.

Even for products obtained by completely nitriding silicon, this is 20 to 3
This is because it is a relatively low-density sintered body with a porosity of 0%. 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, so it can be used as a material for heat-resistant structures by taking advantage of its characteristic of not losing strength even at high temperatures. It becomes very useful. Measures taken so far to improve the density of reaction sintered bodies include Si (or Si+Si3N)
4) The density of the molded body may be increased.

Specifically, first of all, the powder compaction pressure is increased. Even if a high pressure is applied to the practical limit, the density of the product after nitriding is only 2.39y1cr1 (75% of the theoretical density) at most. 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 2.54y1a1. 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 S1 molded body. It was proposed (Japanese Unexamined Patent Publication No. 52-121613).

Previously, in the production of silicon nitride reaction sintered bodies, a process called "k sintering" was added, in which the silicon nitride was heated to approximately 1200°C in an argon atmosphere prior to nitriding; The purpose was to increase the strength of the steel and improve its workability before nitriding, and in fact, almost no shrinkage occurred during Ar sintering.

In this respect, the adoption of pre-sintering can be said to be a noteworthy idea. However, in the method disclosed above, in order to effectively increase the density of the Si molded body by preliminary sintering, the average grain size is 0.
．． It is essential to use extremely fine Si powder of 2μ or less.

Even leaving aside the problem that it is not easy to produce such a fine powder, the density of the resulting reaction sintered body is still 2.39.
The limit was gIa1 (92% of the theoretical density).

As a result of repeated research with the intention of further increasing the density of the S1 molded body using pre-sintering, the present inventors have now reached a maximum of 3.05yId (96% of the theoretical density). We have succeeded in obtaining an extremely high-density reaction sintered body, which we would like to disclose here. be. The above-mentioned limit to the density that can be achieved in the production of reaction sintered bodies in which a molded body is made using fine Sl powder, which is then pre-sintered and nitrided, is due to the nitriding rate of the pre-sintered body. It's slow and progressing! The inventors of the present invention have found that this is because Si is saturated at a certain temperature, and even if heating is continued for a long time, a considerable amount of Si remains unreacted.

The measures adopted in the present invention to overcome this problem are as follows:
It involves adding a certain amount of a specific element or its compound as an additive that promotes nitriding.

That is, the method for producing a high-density silicon nitride reaction sintered body of the present invention is based on Fe. C.O. Ni. ．． Cr. M.O. ．． MnlW.
．． Ti. ．． Zr. ．． Ta. ．． Nb,. V. ．． Mg. ．． C
a. ．． Cu,. One or more elements selected from Zn and Sn or compounds thereof are used as the above elements (2
0.05 to 0.85% by weight (total amount if more than 1 species),
and boron or its compound as B from 0.15 to 5
．． Silicon powder containing 0% by weight is molded and sintered in an inert gas atmosphere or vacuum at a temperature of 1100°C or higher but lower than the melting point of silicon, and the resulting silicon pre-sintered body is It consists of nitriding by applying nitrogen at a temperature of 1100 to 1500°C. Si, which is easily available on the market, contains a small amount of impurities such as Fe.

Conventionally, its presence has been associated with the generation of liquid during sintering of Si powder compacts, which facilitates sintering but leads to rapid crystal growth.
As a result, it was considered undesirable in terms of increasing the porosity of the sintered body, and it was considered that the impurity content should be kept as small as possible. However, the present inventors conducted detailed experiments focusing on the nitridation promoting effect of characteristic impurities such as Fe, and found that if the amount was within the above range, the above-mentioned adverse effects did not become much of a problem. I discovered that it can be of great help.

In other words, the present invention is characterized in that, contrary to existing ideas, the effect is achieved by using an appropriate amount of impurities as a positive substance. Boron has the effect of preventing the S1 grains from becoming coarse during pre-sintering, contributing to higher density of the pre-sintered body, and also makes the Sl particles in the pre-sintered body finer. It will become easier to proceed. This effect is the first finding obtained by the present inventors. As mentioned above, since the Si powder contains a small amount of the aforementioned nitridation promoting elements such as Fe as impurities, it is sufficient to first utilize this and supplement as necessary if there is a deficiency.

Therefore, in the present invention, the term "contains" includes what is originally contained, what has come to be contained by addition, and both. These substances may be in an elemental state or may be a compound such as an oxide, and compounds of the same are, of course, preferred.

Naturally, materials that easily volatilize at high temperatures must be used in consideration of their yield. The content of the substance that has a nitriding promoting effect is 0.05% by weight based on the Si powder.
Otherwise, the effect will not 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 0.85% 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
~1.5% by weight. In order to expect the above-mentioned effects by adding boron, the content must be 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, 5. It must be stopped within % of the call volume. The existing form of boron may be metal boron, amorphous material, metal boride, or the like. It is preferable to select a compound with the element that acts as the nitriding promoter, since both can be present at once. The raw material Si powder has an average particle size of 15 to facilitate densification during preliminary sintering.
A value of less than μ should be used, preferably 1 μ
It is the following fine powder.

Substances of nitriding accelerators and boron or its compounds are also
It is desirable that the particles have a particle size equal to or smaller than that of one powder.

S1 powder usually contains 0.5 to several percent by weight of oxygen, and this is said to have little effect on pre-sintering and nitriding because it volatilizes as SiO etc. when heated. , is not a desirable presence, so it is best to remove it by acid treatment or the like.

The raw material powder or powder mixture can be formed by any means such as conventional die forming, isostatic pressing, slip casting, and injection molding. The density of the compact to be pre-sintered is set to 0.8 in order to facilitate its handling and processing and to ensure sinterability during pre-sintering.
It should be greater than 2y1cT1 (35% of the theoretical density).

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. In addition to free sintering, uniaxial pressure sintering (
Conventional methods such as so-called hot press sintering and hot isostatic pressure sintering can be used. Pre-sintering is carried out 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 determined by the amount of shrinkage caused by sintering.

Although this does not depend on the density of the compact to be sintered, a volumetric shrinkage rate of at least 5% is required in order to provide the high-density reaction sintered product that is intended to be achieved in the present invention. The density of the pre-sintered body varies greatly depending on the particle size of the raw material powder, molding conditions, and sintering conditions, and by adjusting the density, it is possible to obtain a value close to the theoretical density.

However, the density of the Si sintered body corresponding to the silicon nitride reaction sintered body with the theoretical density (3.18yIc!l) is 1.91g1d.
(82% of the theoretical density), and it is obvious that sintering to a higher density than this is meaningless, regardless of whether or not subsequent nitriding is possible. According to the experience of the present inventors, the difficulty of nitriding the pre-sintered body is 1.84yIc!
It becomes noticeable when it exceeds 1 (79% of the theoretical density). On the other hand, the density is 1.37yIc! Starting from a pre-sintered body with a density of less than 1, the density of the reaction sintered body is at most 2.29f1cT1
(72% of the theoretical density) was also experienced. Therefore, the density that the pre-sintered body should have is between 1.37 and 1.
．． It is in the range of 84yIc11.

The nitriding of the S1 preliminary sintered body can be carried out in the same manner as has been carried out in the production of conventional silicon nitride reaction sintered bodies.

That is, in general, under a nitrogen gas atmosphere at atmospheric pressure, 1
Heat to a temperature of 100-1500°C. The temperature is 110
The temperature can also be raised stepwise from a low temperature side of 0 to 1350°C. In order to adjust the reaction rate, the nitrogen pressure may be selected within the range of reduced pressure (up to about 1/1007 atm) to increased pressure (up to 200 msec). In addition to pure nitrogen gas, hydrogen-mixed nitrogen gas and ammonia can also be used. The time required for nitriding depends on the density of the pre-sintered body,
Although it varies greatly depending on the average grain size, nitriding temperature, atmospheric conditions, and the allowable amount of residual Si, it is from several hours to about 2 (4) hours.

As shown in the following examples, according to the method of the present invention,
The amount of unnitrided residual Si is set to 0.0, which is a practically preferable allowable limit.
Easy to reduce to 5% by weight or less, maximum 3.05y
A reactive sintered body having a high density of 'Clt (96% of the theoretical density) can be produced.

This is a height that could not be achieved with conventional technology, and the mechanical strength of the sintered body also reaches a level that is two to three times higher than that of conventional products. In this way, the present invention greatly expands the applications of silicon nitride.

Example 1 1% by weight of Fe2O3 and 0.4% by weight of amorphous boron powder were further added to Si powder having an average particle size of 0.15μ and containing 0.0% by weight of Fe, Thoroughly mixed by wet method in a polyethylene coated ball mill.

This powder mixture was diced using a stainless steel die with a diameter of 1 mm.
5? × It was molded into a cylindrical body with a height of 1 h.

The density of the compact was 1.05yld (45% of the theoretical density). The molded body was placed in an argon atmosphere of 1 atm at 1370°C.
Pre-sintered at ℃×1 hour, density 1.84yIC77
A pre-sintered body of f (theory, 79% of density) was obtained.

The above pre-sintered body was heated for 1 hour in a nitrogen gas atmosphere of 1 atm.
350℃ x 4 (4) → 1380℃ x 80I Terama → 14
When nitrided by 50°C x 201 Terama treatment, a reaction sintered body with a density of 3.05y1cd (96% of the theoretical density) was obtained.

The amount of residual Si therein was 0.5% by weight or less. Comparative Example 1 A molded body prepared in the same manner as in Example 1 except that no boron was added was pre-sintered at 1405°C for 1 hour, resulting in a density of 1.83y1cI1 (79% of the theoretical density). A preliminary sintered body was obtained.

In order to obtain a reaction sintered body with a density of 3.05 fIa1 (96% of the theoretical density), it is necessary to
It was necessary to perform nitriding under the conditions of 38 (generations) x 120 hours → 1450° C. x 30 hours, that is, a longer sintering operation.

This shows the effect of boron in Example 1. Example 2 1 Using the following additives in place of the Fe2α powder and amorphous boron powder of Example 1, forming → pre-sintering and mononitriding were performed under the same conditions, and the amount of residual Si was 0.5 in both cases. It was possible to produce a completely nitrided reaction sintered body of less than 1% by weight.

In addition, the density of the preliminary sintered body is 1.75 to 1.8491c.
T1 (75~79% of the theoretical density), the density of the reaction sintered body is 2.85~3.06yIc11 (~96% of the theoretical density)
). ---I no I Roro'8υ
υ． 1Iυ. 0Tυ. 0 Example 3 Using the raw material composition and processing conditions of Example 1, a diameter of approximately 5 h was obtained.
×Thickness 6? A reaction sintered body was produced, and its three-point bending strength at room temperature was measured.

Claims (1)

[Claims] 1 Fe, Co, Ni, Cr, Mo, Mn, W, Ti,
One or more elements or compounds selected from Zr, Ta, Nb, V, Mg, Ca, Cu, Zn, and Sn are used as the above elements (in the case of two or more, the total amount).0.
Silicon powder containing 0.05 to 0.85% by weight and 0.15 to 5.0% by weight of boron or its compound as B is molded and heated to an inert state at a temperature of 1100°C or higher but lower than the melting point of silicon. Sintered in a gas atmosphere or vacuum to obtain a silicon pre-sintered body at 1100-1500°C.
A method for producing a high-density silicon nitride reaction sintered body, which comprises nitriding by applying nitrogen at a temperature of . 2. The manufacturing method according to claim 1, wherein the density of the molded body is selected to be 0.82 g/cm^3 or more. 3. The manufacturing method according to claim 1, in which sintering is performed such that the volumetric shrinkage rate during preliminary sintering is 5% or more. 4 The density of the preliminary sintered body is 1.37 to 1.84 g/cm^
3. The manufacturing method according to claim 1, wherein the manufacturing method is sintered so as to be in the range of 3.