JPH06279124A - Production of silicon nitride sintered compact - Google Patents

Abstract

PURPOSE:To provide a method for producing a silicon nitride sintered compact by which a dense silicon nitride sintered compact can be obtd. with high reproducibility at a low cost by enhancing the reproducibility of densification of a silicon nitride sintered compact in a two-stage sintering method. CONSTITUTION:Silicon powder contg. at least oxide of a rare earth element is used as starting material and at least one of iron and calcium is added to the starting material in the form of a simple metal, an alloy, a compd., etc., by 0.03-2wt.% (expressed in terms of iron or calcium). A compact of the starting material contg. iron or calcium is subjected to nitriding treatment and the resulting silicon nitride reacted sintered compact is subjected to densification sintering at a high temp. to obtain the objective high density silicon nitride sintered compact.

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 nitride sintered body by applying a two-step sintering method which combines a reaction sintering method and a densification sintering method.

[0002]

2. Description of the Related Art A silicon nitride (Si ₃ N ₄ ) sintered body has properties such as high strength and fracture toughness and excellent thermal shock resistance (low thermal expansion coefficient, high thermal conductivity) and the like. It is being applied to various structural materials such as parts materials, bearing materials, and cutting tool materials, and is being put to practical use.

As the industrially used sintering method of the above-mentioned silicon nitride, there are (a) a reaction sintering method in which Si powder is used as a starting material, and sintering is performed with a nitriding reaction, (b) Si ₃ N
₄ densification sintering method in which additives are added to the powder and sintering is advanced via the low melting point compound generated by the reaction of both
Is common.

In the reaction sintering method (a), inexpensive Si powder can be used as a raw material, the shrinkage rate can be suppressed to a low level, and a sintered body with high dimensional accuracy can be obtained. Although it has an advantage, it has a drawback that it is difficult to densify the sintered body and the obtained sintered body becomes porous, and sufficient strength and oxidation resistance cannot be obtained. On the other hand, (b)
Although the densified sintering method of 1 can easily obtain a high-density silicon nitride sintered body, it has a drawback that the manufacturing cost is high because the dimensional shrinkage rate is large and the number of post-processing steps is increased. is doing.

As described above, the reaction sintering method and the densification sintering method have advantages and disadvantages, respectively.
A two-stage sintering method that combines these two types of sintering methods is also used in some areas. In the two-stage sintering of silicon nitride, a system in which an additive is added to Si powder is used as a starting material, and first, nitriding reaction sintering is performed as the first step to increase the weight by the nitrogen content to produce Si ₃ N.
_4- Convert to additive system to increase density. Next, in a second step, the Si ₃ N ₄ -additive-based reaction sintered body is fired at a high temperature to densify it. Such a two-stage sintering method can keep the shrinkage rate low, that is, the dimensional accuracy is high,
It has advantages that post-processing can be reduced (reduction of manufacturing cost) and that a dense sintered body can be easily obtained.

[0006]

However, the current two-stage sintering method of silicon nitride has the following problems. That is, the result of the nitriding reaction in the first step is
It is said that it has a great influence on the densification and sintering in the second step, and it is necessary to obtain fine Si ₃ N ₄ particles in the first step, but the conditions therefor have not been sufficiently examined. Therefore, with the current two-step sintering method, it has not been possible to obtain a dense silicon nitride sintered body with good reproducibility. Specifically, there are many phenomena such as failure of densification in the second step, further melting of Si and leaching on the surface.

The present invention has been made in order to solve such a problem, and by improving the reproducibility of the densification of the silicon nitride sintered body in the two-step sintering method, the cost is low and the density is high. It is an object of the present invention to provide a method for producing a silicon nitride sintered body, which enables the silicon nitride sintered body to be obtained with good reproducibility.

[0008]

The method for producing a silicon nitride sintered body according to the present invention comprises a first step of subjecting a compact made of silicon powder containing at least a rare earth oxide as a starting material to a nitriding treatment, and In the method for producing a silicon nitride sintered body, which comprises the second step of densifying and sintering the silicon nitride-based reaction sintered body obtained in the first step, at the starting material,
It is characterized in that at least one selected from iron and calcium is added in the form of a simple metal, an alloy, a compound or the like in the range of 0.03 to 2% by weight as the converted amount of iron and calcium.

In the production method of the present invention, first, at least one selected from iron and calcium is added to silicon powder containing a rare earth oxide as a starting material. Iron or calcium accelerates the nitriding reaction of silicon, and also makes the silicon nitride particles generated in the first step fine and granular, facilitating the densification of the reaction sintered body in the second step, and It suppresses contraction.

As described above, the particle size of silicon nitride produced by adding iron or calcium to the starting material is
The silicon powder used can be refined to 50% or less,
And more than 60% of them become granular. By refining the generated silicon nitride particles to 50% or less of the silicon powder, the reaction sintered body can be easily densified in the second step, and it is not necessary to use ultrafine silicon powder. Therefore, it contributes to the reduction of manufacturing cost.
Further, if the degree of granulation of the produced silicon nitride particles is less than 60%, the densification of the reaction sintered body is hindered, and the densification becomes difficult. Granularity of silicon nitride particles is 90%
It is more preferable to set it as above.

The above-mentioned iron and calcium can be added in various forms such as elemental metal powder, alloy powder such as Fe-Si alloy, compound powder such as oxide and nitric acid compound. In addition, iron and calcium are 0.03 to 2% by weight in the starting material in terms of iron and calcium equivalent.
To be added within the range. If the addition amount of these is less than 0.03% by weight, the above effects cannot be sufficiently obtained, and if it exceeds 2% by weight, the sintering of silicon nitride is adversely affected or the obtained silicon nitride sintered body is adversely affected. May deteriorate the characteristics of. The added amount of iron and calcium (converted amount) is 0.05
More preferably, it is in the range of 0.5% by weight.

The silicon powder used as the main component of the starting material preferably has an average particle size of 0.5 to 10 μm, more preferably 1 to 7 μm.
This makes it easier to densify the reaction sintered body in the second step.

As the starting material mainly containing silicon used in the present invention, in addition to the rare earth oxide, at least 1 selected from aluminum oxide, aluminum nitride, titanium oxide, hafnium oxide, zirconium oxide, chromium oxide and magnesium oxide. Seeds can be added. As the rare earth oxide, yttrium oxide or ytterbium oxide is preferably used, and it is particularly desirable to use yttrium oxide and aluminum oxide in combination. These additive compounds function as a sintering aid for silicon nitride, and contain 1 to 10% by weight of rare earth oxide, and others.
It is preferably added to the starting material in the range of 0.5 to 10% by weight.

In the method for producing a silicon nitride sintered body of the present invention, first, a rare earth oxide, if necessary, another compound, and iron and calcium are mixed in a predetermined amount with silicon powder, and they are sufficiently mixed. Then, an organic binder or the like is further added, and after mixing and granulating, a molded body having a required shape is produced by various known molding methods such as a press molding method. Here, in the manufacturing method of the present invention, the shrinkage ratio due to sintering is, for example,
Since it can be kept as low as 5 to 12%, it is possible to approximate the shape of the molded body to the required shape of the sintered body.

In the present invention, Si ₃ N ₄ can be added up to about 50% by weight as a Si source in the starting material. In this case, the shrinkage rate is slightly increased, but it contributes to improvement of uniformity and reduction of manufacturing cost.

Next, the molded body is subjected to a nitriding treatment as a first step. This nitriding treatment is performed by performing a degreasing treatment in the air or a nitrogen atmosphere, and then heat-treating in nitrogen at a temperature of about 1200 to 1450 ° C. for about 2 to 40 hours. Here, the temperature raising process up to the heat treatment temperature is important, and it is preferable to select a condition such that the surface of the molded body is covered with silicon nitride in this temperature rising process. The condition is the composition, the shape of the molded body, It depends on the size, but for example, 11
After raising the temperature to about 00 ℃ at a heating rate of about 1 to 3 ℃ / hr, temporarily hold that temperature for about 1 to 5 hours, or raise the temperature from that temperature to about 50 to 100 ℃ / hr. It is preferable to raise the temperature at a rate.

In the second step, the silicon nitride-based reaction sintered body obtained in the first step is fired at a high temperature to densify and sinter the desired high-density silicon nitride sintered body. obtain. Specific conditions for this second step include, for example, 1
~ 1750 in a pressurized atmosphere of nitrogen at about 10 atm ~
It is preferable to bake at a temperature of about 1900 ° C. for about 2 to 10 hours.

[0018]

In the method for producing a silicon nitride sintered body of the present invention, at least one selected from iron and calcium is added to the starting raw material containing silicon and a compound that serves as a sintering aid. By including iron and calcium in the starting material, the nitriding reaction of silicon in the first step is promoted, and the silicon nitride particles produced are finely and granulated with good reproducibility. Iron and calcium generate a silicide (for example, FeSi ₂ ) together with Si and SiO _{2 in} addition to the catalytic effect of removing the oxide film (SiO ₂ ) on the surface of silicon particles as SiO. This Fe or Ca silicide is
At the nitriding reaction temperature, it becomes a liquid phase and the N ₂
Si reacts with Si to form and precipitate Si ₃ N ₄ . It is considered that the silicon nitride particles produced are refined and granulated through the liquid phase of the silicide of Fe and Ca. Such a change in particle morphology requires an appropriate size of Si powder, and if it exceeds 10 μm, miniaturization becomes difficult. As a result, it becomes possible to densify the reaction sintered body easily and with good reproducibility in the second step, and it is possible to reproducibly produce a low-cost silicon nitride sintered body having a low shrinkage ratio and containing a sintering aid. It is possible to get well.

Although iron and calcium have been conventionally used as nitriding reaction accelerators for silicon, only their effect on the nitriding reaction is known, and iron and calcium are produced by using them in the two-step sintering method. It was discovered for the first time by the present invention that the silicon nitride particles obtained were finer than the silicon powder, and the reproducibility of densification and sintering was significantly improved. Only under the limited conditions of the present invention, the newly proposed effect is possible.

[0020]

EXAMPLES Examples of the present invention will be described below.

Example 1 First, Si powder containing 0.3% by weight of Fe and 0.02% by weight of Ca and having an average particle size of 5 μm was added to Y powder having an average particle size of 0.5 μm.
5% by weight of ₂ O ₃ powder and Al ₂ O with an average particle size of 0.2 μm
2% by weight of ₃ powders were added and thoroughly mixed by a wet mixing method using a ball mill to prepare a starting material powder. Next, an organic binder was added to the above starting material powder, and the mixture was further thoroughly mixed and then granulated to a particle size of # 48 mesh. This granulated powder was press-molded with a molding pressure of 1 ton / cm ² and a diameter of 15 m.
A pellet-shaped compact having a size of m × 5 mm was produced.

Next, after degreasing the pellet-shaped compact in a nitrogen atmosphere at 600 ° C., it is heat-treated in nitrogen as a first step to produce a Si ₃ N ₄ -additive type reaction sintered body. Got In this first step, the temperature was raised from normal temperature to 1100 ° C in 2 hours in the flow of N ₂ gas, and 1100 ° C to 1400 ° C.
The temperature was raised to 2 ° C in 2 hours, held at 1400 ° C for 6 hours, and then cooled in the furnace.

Then, the reaction sintered body obtained in the first step was densified and sintered in the second step under the firing condition of 1800 ° C. × 6 hours in a pressurized nitrogen atmosphere of 9 atm. Was applied. Through the second step, the desired densified sintered body of silicon nitride was obtained.

Further, as a comparison with the present invention, the above-mentioned Examples were used except that Si powder having an Fe content of 0.04% by weight and a Ca content of 0.001% by weight (average particle size is the same as that of the Examples). Two-stage sintering was performed under the same composition and under the same conditions as in No. 1 to produce a silicon nitride sintered body (Comparative Example 1).

Various characteristics were measured and evaluated using the respective silicon nitride sintered bodies according to these Examples and Comparative Examples and the respective reaction sintered bodies after the first step. First, the nitriding rate, density, shrinkage rate, and generated Si ₃ of each reaction sintered body after the first step
The shape of N ₄ particles, the degree of granularity, the average particle diameter of the granular particles, and the constituent phase of the produced Si ₃ N ₄ were measured and evaluated. The nitriding rate was obtained by the X-ray diffraction method. Further, the diameter, thickness and weight of the reaction sintered body were measured, the bulk density was calculated from these, and the shrinkage rate was calculated from the dimensional change in the diameter direction. Si ₃ N ₄
The shape of the particles, the degree of granularity, and the average particle size of the granular particles were evaluated by observing the fracture surface of the reaction sintered body by SEM, and the constituent phase of the produced Si ₃ N ₄ was evaluated from the X-ray diffraction peak. .

Further, the density, shrinkage ratio and constituent phase of each silicon nitride sintered body after the second step were evaluated. The results are summarized in Table 1.

[0027]

[Table 1] As is clear from Table 1, the reaction sintered body according to Example 1 has fine and highly granular Si ₃ N ₄ particles,
It can be seen that the densified sintered body using such a reaction sintered body has a high density, a small shrinkage rate, and a high strength.
On the other hand, from the reaction-sintered body of Comparative Example 1, needle-like crystal growth was observed, and Si ₃ N ₄ particles were not made fine or granular. And, the silicon nitride sintered body after the densification sintering using such a reaction sintered body has a low density,
That is, the porosity was high and the strength was unsatisfactory.

From these results, it is possible to finely and granulate the silicon nitride particles produced in the first step with good reproducibility by allowing Fe and Ca to be present in appropriate amounts in the starting material. It can be seen that it becomes possible to easily and reproducibly densify the reaction sintered body in the second step. Therefore, since it is possible to obtain a high-density silicon nitride sintered body with a small shrinkage rate and high reproducibility by using an inexpensive silicon powder, it is possible to reduce the cost of raw materials and shorten the number of post-processing steps. The manufacturing cost of the high-density silicon nitride sintered body can be significantly reduced.

Examples 2 to 15 Each starting material having the composition shown in Table 2 was subjected to two-stage sintering under the manufacturing conditions shown in Table 2 (other conditions are the same as those in Example 1), and silicon nitride was prepared. A sintered body was produced. The characteristics of each of these silicon nitride sintered bodies were measured and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0030]

[Table 2]

[Table 3]

[0031]

As described above, according to the present invention, the silicon nitride produced in the first step can be finely and granulated with good reproducibility. The binding process is advantageous. That is, it is possible to obtain a silicon nitride sintered body that is dense and has high dimensional accuracy with good reproducibility. As described above, since it is possible to obtain a dense silicon nitride sintered body with high dimensional accuracy while using low-cost silicon powder as a raw material, it is possible to reduce the manufacturing cost of the high-density silicon nitride sintered body. Greatly contribute to.

[0032]

Claims (5)

[Claims] 1. A first step of performing nitriding treatment on a molded body starting from a silicon powder containing at least a rare earth oxide, and a silicon nitride-based reaction sintered body obtained in the first step, In the method for producing a silicon nitride sintered body, which comprises a second step of densifying and sintering below, at least one selected from iron and calcium is used as the starting material in the form of a simple metal, alloy, compound or the like. In the method for producing a silicon nitride sintered body, the iron and calcium are added in a range of 0.03 to 2% by weight in terms of conversion. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the silicon powder has an average particle size in the range of 0.5 to 10 μm. 3. The method for producing a silicon nitride sintered body according to claim 1, wherein the grain size of silicon nitride produced in the first step is reduced to 50% or less of the silicon powder in the starting material, And that
A method for producing a silicon nitride sintered body, characterized in that 60% or more is formed into particles. 4. The method for producing a silicon nitride sintered body according to claim 1, wherein the starting material is further selected from aluminum oxide, aluminum nitride, titanium oxide, hafnium oxide, zirconium oxide, chromium oxide and magnesium oxide. A method for producing a silicon nitride sintered body, which comprises at least one kind. 5. The method for manufacturing a silicon nitride sintered body according to claim 1, wherein the added amount of iron and calcium is 0.05 as a converted amount.
The method for producing a silicon nitride sintered body is characterized in that the content is in the range of 0.5 wt%.