JPH06172011A - Method for manufacturing silicon nitride sintered body - Google Patents

Abstract

(57) [Summary] [Objective] The object of the present invention is to use stable media regardless of the type of raw material powder of silicon nitride, without using a dispersion medium that produces harmful mist in the raw material mixing and grinding process. It is an object of the present invention to provide a method for producing a silicon nitride sintered body capable of producing a sintered body. [Structure] A method for manufacturing a silicon nitride sintered body according to the present invention comprises:
Water was added as a dispersion medium to a composition consisting of a raw material powder of silicon nitride and a sintering aid to prepare a slurry, and the slurry was maintained at a temperature of 2 to 60 ° C. It is characterized in that it is wet-mixed and pulverized, and the obtained mixture is granulated and molded and then sintered. The pH value of the slurry may be 12 or less and the viscosity may be 1000 cps or less, or an oxide film having a predetermined thickness may be formed in advance on the surface of the raw material powder.

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, and in particular, does not use a toxic solvent or the like in a wet mixing and pulverizing step of raw material powders, etc. The present invention relates to a method for producing a silicon nitride sintered body which suppresses and obtains a high quality silicon nitride sintered body.

[0002]

2. Description of the Related Art Sintered ceramics containing silicon nitride as a main component is lightweight, has high strength, and is excellent in wear resistance, and is therefore widely used as a rolling element and a race material for bearings. It is also used in electronic equipment parts, automobile parts and chemical machinery parts. In particular, the silicon nitride sintered body has excellent heat resistance even in a high temperature range up to about 1900 ° C., has a small coefficient of thermal expansion, and is superior in resistance to thermal shock to conventional metal materials. , Its applications are being developed as various high-strength heat-resistant parts materials, including turbine nozzles and internal combustion engine parts.

Conventionally, a silicon nitride sintered body has a predetermined grain size of silicon nitride powder, a rare earth oxide such as yttrium oxide (Y ₂ O ₃ ) as a sintering aid, and aluminum oxide (Al ₂ O _3). ) And a chlorine-based solvent such as trichloroethane as a dispersion medium are mixed and pulverized to form a uniform mixture,
It is manufactured by granulating the obtained mixture and then degreasing and sintering.

The silicon nitride powder used as a raw material here is generally a raw material powder produced by a so-called silica reduction method in which silicon dioxide (SiO ₂ ) is reduced to silicon nitride, or a metal compound of silicon. A raw material powder produced by a so-called metal direct nitriding method, in which is directly nitrided at high temperature, is used.

Further, since the sinterability is extremely poor only with the above-mentioned silicon nitride powder constituting the sintered body, various sintering aids are added to the raw materials. By the way, in order to improve the characteristics of the sintered body, it is necessary to uniformly mix the silicon nitride powder and the sintering aid, and therefore, various dispersion media are added to the raw material powder to uniformly mix them. Is executed.

As a dispersion medium, water generally exerts an excellent function, but conventional general-purpose silicon nitride powder easily reacts with water and is easily decomposed into NH ₃ and SiO ₂ , and the strength characteristics of the sintered body and Often wear resistance is reduced. That is, silicon nitride reacts with water according to the reaction formula shown in the following formula (1) to generate ammonia gas and increase the pH of the slurry to form silica (SiO ₂ ).

[0007]

[Equation 1]

The generated silica (SiO ₂ ) is Si ₃ N ₄
It lowers the high temperature strength and wear resistance of the sintered body. Therefore, an organic chlorine-based solvent such as trichloroethane or a flammable solvent that is difficult to react with the raw material powder is generally used as a dispersion medium.

[0009]

However, according to the conventional method for producing a silicon nitride sintered body, since the chlorine-based solvent such as trichloroethane is used as the dispersion medium as described above, in the mixing step of the raw material powders. There is a large possibility that harmful solvent vapors are discharged, there is a risk of contaminating the work environment and the surrounding environment, and there is a drawback that enormous cost is required for protection measures for the recovery equipment and the like. Further, when a flammable solvent is used, it is necessary to make all the components of the process pressure-resistant and explosion-proof, which causes a problem that the equipment cost rises.

As a means for solving the above-mentioned problems, by selecting a silicon nitride powder having a low reactivity with water,
A manufacturing method for preventing deterioration of quality is disclosed in Japanese Patent Application No. 1-317835. The disclosed method for producing a ceramic sintered body is characterized by using a silicon nitride raw material powder produced by a silicon diimide pyrolysis method. Here, the silicon diimide thermal decomposition method thermally decomposes silicon diimide (Si (NH) ₂ ) under high temperature conditions to obtain silicon nitride (Si ₃ N ₄ ) and hydrogen (H
₂ ) It is a method of decomposing into and. Since the silicon nitride powder produced by this imide pyrolysis method has a relatively low reactivity with water, it becomes possible to use water as a dispersion medium in the wet mixing and pulverizing step of the raw materials.

However, the silicon nitride raw material powder produced by the imide pyrolysis method is relatively expensive as compared with the raw material powder produced by other production methods such as the silica reduction method and the metal direct nitriding method, and the production of the sintered body. There is a problem that raises the cost. Further, even the silicon nitride raw material powder produced by the imide pyrolysis method has a problem that the strength characteristics and the like of the sintered body are deteriorated depending on the conditions of the wet mixing and pulverizing operation.

The present invention has been made to solve the above problems, and it is possible to use water in a mixing and pulverizing step of raw materials without using a dispersion medium that generates harmful mist, steam and the like. Further, there is provided a method for producing a silicon nitride sintered body capable of inexpensively producing a silicon nitride sintered body having excellent quality characteristics, the characteristics of which are not affected by the difference in the method of producing the raw material powder of silicon nitride and the type. The purpose is to

[0013]

From the above viewpoints, the present inventor does not use a harmful dispersion medium, does not impair the quality characteristics as compared with the conventional sintered body, and has a simple process. For the purpose of obtaining a silicon nitride sintered body, the type of silicon nitride powder, the temperature of the slurry during wet mixing and pulverization, p
When the experiment was repeated while changing various process conditions such as H and viscosity, the above process conditions were set within a predetermined range, and an oxide film was formed on the surface of the raw material powder of silicon nitride. It was possible to obtain a high-quality silicon nitride sintered body which was comparable to the quality characteristics of the conventional sintered body without being affected by the difference in the manufacturing method.

The present invention has been completed based on the above findings. That is, in the method for producing a silicon nitride sintered body according to the present invention, a slurry is prepared by adding water as a dispersion medium to a composition consisting of a silicon nitride raw material powder and a sintering aid, and the resulting slurry is obtained. It is characterized in that the slurry is wet-mixed and pulverized while the temperature is maintained at 2 to 60 ° C., and the obtained mixture is granulated and then sintered.

Further, the pH value of the slurry in the wet mixing and pulverizing step is set to 12 or less, and the viscosity of the slurry is set to 10 or less.
It should be set to 00 cps (centipoise) or less.

Further, it is preferable to previously form an oxide film having a thickness of 10 to 500 angstroms on the surface of the silicon nitride raw material powder.

It is preferable to use a silicon nitride raw material powder having a particle size of 0.1 μm or more.

However, the grain size of the silicon nitride raw material powder is 2 μm.
Set the following.

The reasons for limiting the present invention will be described below.

The silicon nitride raw material powder used as the raw material of the silicon nitride sintered body which is the subject of the present invention is not limited to the one produced by the silicon diimide pyrolysis method, but it is also produced by the metal direct nitriding method. Any type of silicon nitride powder, such as those manufactured by the silica reduction method, can be used.

Further, the temperature of the slurry in the wet mixing and pulverizing step has a great influence on the reactivity between water and a silicon nitride raw material powder, which is a kind of chemical reaction, so that it should be set in a low temperature range of 2 to 60 ° C. Required, and more preferably 4
By setting the temperature to 0 ° C. or lower, and further to 30 ° C. or lower, the reaction between the raw material powder and water as the dispersion medium can be effectively suppressed. The slurry temperature is preferably kept at 2 ° C. or higher in order to prevent the solidification of the dispersion medium.

Further, when the pH value of the slurry in the wet mixing and pulverizing step exceeds 12, the reaction between water and the silicon nitride raw material powder is promoted, so that it is set to 12 or less in order to suppress the reaction. However, the slurry p
Even when the H value is as low as less than 6, ammonia generation by the reaction proceeds, so the pH value of the slurry is preferably set in the range of 7 to 10, and more preferably in the range of 8 to 9.5. However, if a strong alkali such as NaOH or KOH is added to adjust the pH, the reaction is suppressed, but residual metal ions such as Na and K adversely affect the sinterability. As the above, NH ₃ salt or the like that volatilizes during degreasing sintering is preferable. However, it should be noted that the density of the sintered body may be similarly reduced when an excessive amount of NH ₃ salt is added.

It has been confirmed that the viscosity of the slurry in the wet mixing and pulverizing step has a great influence on the reactivity between water and the silicon nitride raw material powder, and in the method of the present invention, it is set to 1000 centipoise (cps) or less. To be done. When the viscosity of the slurry exceeds 1000 centipoise, the mechanical impact force exerted on water and the silicon nitride raw material powder during mixing and pulverization increases, and the contact efficiency between both increases and the reactivity increases. In particular, when an oxide film is formed on the surface of the silicon nitride raw material powder as described later, there is a high possibility that the oxide film will be destroyed by the impact force during the mixing and pulverization. Therefore, the viscosity of the slurry is set to 1000 cps or less, more preferably 500 cps or less.
The viscosity of the slurry can be arbitrarily adjusted by changing the mixing ratio of water, raw material powder, and various binders.

By suppressing the viscosity of the slurry to be low as described above, there is also a secondary effect that the granulated powder can be easily formed.

In addition, a silicon nitride raw material powder having a thickness of 1
By forming an oxide film of 0 to 500 Å (A), it is possible to prevent direct contact between water and silicon nitride components during wet mixing and pulverization, and to effectively suppress the reaction with water. Is.

The silicon nitride raw material powder contains a certain amount of oxygen and oxide depending on the storage state.
In the method of the present invention, an oxide film is positively formed to suppress the reactivity with water. The method of forming the oxide film is performed, for example, by using silicon nitride raw material powder in air at a temperature of 500 to 600.
By heating at ° C for 2-3 hours. Thickness 10-500
A SiO ₂ film of A can be formed. The thickness of the oxide film can be measured by observation with a transmission electron microscope (TEM). When the thickness of the oxide film is less than 10 A, the effect of suppressing the reaction is not sufficient, while when the thickness exceeds 500 A, the inside of each silicon nitride raw material powder is oxidized, so that the nitriding is performed. The high temperature strength originally possessed by the silicon sintered body is impaired. Therefore, the thickness of the oxide film is set within the above range.

According to the knowledge of the present inventors, the size of the silicon nitride powder used leads to a large contact area with water, which greatly affects the reactivity with water and the compactness of the sintered body. It is known to do. In particular, when using the raw material powder produced by the metal direct nitriding method, the grain size is 0.1
It is important to use one that is substantially free of silicon nitride raw material powder of less than μm. Therefore, in order to suppress the reaction with water, the particle size of the raw material powder needs to be 0.1 μm or more. On the other hand, from the viewpoint of compactness and strength of the sintered body, the particle size of the silicon nitride powder used is 2
It is recommended to set it to less than μm. The reason is that if the thickness exceeds 2 μm, the sintering becomes insufficient and the compactness of the sintered body decreases.

Rare earth element oxides such as yttrium oxide and cerium oxide are added together with aluminum oxide and titanium oxide as a sintering aid for improving the sinterability, and the addition amount thereof is silicon nitride powder 100. About 1 to 6 parts by weight is preferable with respect to parts by weight.

After adding the above-mentioned respective components in a composition ratio within a predetermined range, water as a dispersion medium is added, and the respective components are uniformly mixed and pulverized. The amount of water added is set to about 40 to 65% with respect to the total weight of the slurry mixture. If it is less than 40%, the dispersion effect is small, while if it exceeds 65%, the drying time is prolonged.

Here, an attritor having a higher mixing efficiency than that of a conventional ball mill may be used as the mixing and crushing machine.
As the attritor, for example, as shown in FIG. 1, a large number of grinding and stirring balls 1 are loaded, a grinding tank 2 containing silicon nitride raw material powder, a sintering aid, a binder and a dispersion medium, and a raw material powder layer contained therein. An agitator arm 3 that is rotatably disposed on the agitator shaft, an agitator shaft 4 that rotates the agitator arm 3, and a jacket 5 that is integrally attached to the outer surface of the crushing tank 2 and that controls the temperature of the raw material slurry and the like.
It is composed of a circulation pump 6 and a circulation pipe 7 for circulating the raw material powder and the like in the crushing tank 2.

The raw material powder, the sintering aid and the water contained in the crushing tank 2 are mixed by the rotation of the agitator arm 3. The respective raw material powders are mixed with each other and further finely divided by the impact force of the ball 1.

Here, the mixing time is set to 1 to 24 hours. When the mixing time is less than 1 hour, the dispersive mixing of the silicon nitride powder and the sintering aid is insufficient, while when the mixing time is longer than 24 hours, a part of the silicon nitride powder reacts with water. This is because SiO ₂ is generated and the strength of the sintered body is reduced.

Next, the uniform slurry obtained by the mixing operation is sprayed in an atmosphere at a temperature of 200.degree. In this granulation step, only water in the slurry evaporates to form raw material particles. Next, the obtained raw material particles are press-molded into a predetermined shape,
Further, after performing raw processing and degreasing operation as necessary, the temperature is set to 1650 to 1850 in a non-oxidizing atmosphere.
Sinter at ° C.

This sintering is also carried out by the atmospheric pressure sintering method.
Alternatively, even by other sintering methods such as a hot pressing method and a hot isostatic pressing method (HIP) performed by pressurizing an atmosphere, a sintered silicon nitride body that is dense and has excellent high temperature strength and thermal shock resistance can be obtained. can get.

According to the method for producing a silicon nitride sintered body of the present invention, since water is used as a dispersion medium for uniformly mixing and pulverizing the raw material powder, harmful vapor may be generated during the mixing and pulverizing process. It does not pollute the work environment and the surrounding environment.

Further, the temperature and pH of the slurry which have a great influence on the reactivity between the silicon nitride raw material powder and water as a dispersion medium.
By strictly controlling the value, viscosity, and particle size of the raw material powder, and forming an oxide film on the surface of the raw material powder, various silicon nitride raw material powders are used regardless of the difference in the manufacturing method of the raw material powder. In this case, it is possible to effectively prevent the reaction between water and the silicon nitride raw material powder, and it becomes possible to use water as the dispersion medium. A sintered body excellent in high temperature strength and heat resistance can be obtained without causing deterioration.

Since water has a very excellent dispersion function of the raw material powder as compared with a conventional solvent-based dispersion medium such as trichloroethane, the raw material powder can be uniformly mixed by a short-time mixing operation. . Therefore, the manufacturing efficiency of the sintered body is improved, and the mass productivity of the sintered body can be improved.

[0038]

EXAMPLES Next, the present invention will be specifically described with reference to the following examples.

As Examples 1 to 3, 100 parts by weight of each silicon nitride powder having an average particle diameter of 0.8 μm produced by the metal direct nitriding method, the silicon diimide pyrolysis method and the silica reduction method was added to a sintering aid. As a raw material powder mixture, 5 parts by weight of yttrium oxide, 5 parts by weight of aluminum oxide, and 1.5 parts by weight of titanium oxide are added to form a raw material powder mixture. Ion exchange corresponding to 80% by weight of the obtained mixture weight. Water was added as a dispersion medium to prepare a slurry, and wet mixing and pulverization were carried out for 0 to 4 hours with an attritor shown in FIG. And at the start of mixing and crushing, 30 minutes, 60 minutes, 1
The slurry mixture was sampled at 20 minutes and 240 minutes, respectively.

Next, 7% by weight of a wax emulsion as a binder and a water-soluble acrylic resin were added to each of the obtained slurry mixtures, and the drying temperature was 20 with a spray dryer.
Granulated at 0 ° C. Next, each of the obtained granulated powder samples was placed in an oxygen analyzer and calcined at a temperature of 1500 ° C. to measure the oxygen concentration generated. This oxygen concentration is generated by the decomposition of silica (SiO ₂ ) produced by the reaction between the silicon nitride raw material powder and water according to the above formula (1), and the reaction between the raw material powder and water during the mixing and pulverizing process. It is proportional to the quantity. Therefore, the reactivity and stability of the raw material powder and water can be evaluated by the magnitude of the change with time of the oxygen concentration.

Table 1 shows the measurement results of the oxygen concentration of the granulated powder prepared from each slurry mixture.

On the other hand, using the silicon nitride raw material powders used in Examples 1 to 3 as Comparative Examples 1 to 3, the temperature, pH and Each granulated powder was prepared by treating in the same manner as in Examples 1 to 3 except that the mixture was pulverized within the range of viscosity, and the oxygen concentration was measured in the same manner. The measurement results are shown in Table 1 below.

[0043]

[Table 1]

As is clear from the results shown in Table 1, according to the granulated powders of Examples 1 to 3 in which the temperature, pH and viscosity of the slurry during wet mixing and pulverization were strictly controlled and controlled, Comparative Examples 1 to 1 were not controlled. Compared to the case of 3, the oxygen concentration is less affected by the difference in the manufacturing method of the raw material, the decomposition due to the reaction between the silicon nitride raw material powder and water during the wet mixing and pulverization is effectively suppressed. It turned out that Therefore, a high-quality silicon nitride sintered body can be obtained, and the degree of freedom in selecting the silicon nitride raw material powder can be greatly increased.

Next, the influence of the particle size of the silicon nitride powder on the reactivity will be specifically described with reference to Example 4 and Comparative Example 4.

As Example 4, except that the silicon nitride raw material powder formed by the direct metal nitriding method and having an average grain diameter of 1.0 μm and having a grain diameter of less than 0.1 μm was removed were used. Each granulated powder was prepared by treating in the same manner as in Example 1, and the oxygen concentration generated from the granulated powder was measured in the same manner.

On the other hand, as Comparative Example 4, each granulated powder was prepared in the same manner as in Example 4, except that the powder having a particle size of less than 0.1 μm was used as it is without being removed. The oxygen concentration generated from the granular powder was measured and the results shown in Table 2 were obtained.

As is clear from the results shown in Table 2, in the case of Example 4 in which the fine raw material powder having a large contact area with water was removed, the oxygen concentration It is found that the rate of increase with time is small and the reaction of the raw material powder with water is suppressed.

Next, the effect of the oxide film formed on the surface of the raw material powder will be specifically described based on Example 5 and Comparative Example 5. That is, as Example 5, the silicon nitride raw material powder prepared in Example 4 (powder having a particle size of less than 0.1 μm was removed in advance) was preheated in air at a temperature of 600 ° C. for 2 hours.
20 hours thick silica (Si
Each granulated powder was prepared by performing wet mixing and pulverization and granulation treatment in the same manner as in Example 4 except that the O ₂ ) film was formed.
Then, the oxygen concentration contained in each granulated powder was measured in the same manner as in Example 4.

On the other hand, as Comparative Example 5, a granulated powder was prepared in the same manner as in Example 5 except that the film was not positively formed, and the oxygen concentration was measured in the same manner as shown in Table 2 below. I got the result. When the surface of the raw material powder of Comparative Example 5 was observed in advance with a transmission electron microscope, the thickness of the oxide film was 5 μm on average.

[0051]

[Table 2]

As is clear from the results shown in Table 2, in the case of Example 5 in which the fine powder having a particle size of less than 0.1 μm was removed to form the oxide film on the surface of the raw material powder, the silicon nitride was formed by the oxide film. This has the effect of preventing direct contact between the element and water, and effectively suppressed the decomposition of the silicon nitride raw material powder.

[0053]

As described above, according to the method for producing a silicon nitride sintered body of the present invention, since water is used as the dispersion medium for uniformly mixing and pulverizing the raw material powder, the mixing and pulverizing operation is performed. Occasionally there is no danger of producing harmful vapors and there is no pollution of the working environment and the surrounding environment.

Further, the temperature and pH of the slurry, which greatly affects the reactivity between the raw material powder of silicon nitride and water as a dispersion medium.
By strictly controlling the value, viscosity and particle size of the raw material powder, and by forming an oxide film of a predetermined thickness on the surface of the raw material powder, regardless of the difference in the manufacturing method of the raw material powder, various silicon nitrides can be produced. Even when using the raw material powder, it is possible to effectively prevent the reaction between water and the silicon nitride raw material powder, it is possible to use water as a dispersion medium,
A sintered body excellent in high-temperature strength and heat resistance can be obtained without deteriorating the quality characteristics of the sintered body due to the reaction with water.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration example of an attritor mixing pulverizer used in the method of the present invention.

[Explanation of symbols]

 1 Ball 2 Grinding Tank 3 Agitator Arm 4 Agitator Shaft 5 Jacket 6 Circulation Pump 7 Circulation Piping

Claims (5)

[Claims] 1. A slurry is prepared by adding water as a dispersion medium to a composition composed of a raw material powder of silicon nitride and a sintering aid, and the temperature of the obtained slurry is kept at 2 to 60 ° C. A method for producing a silicon nitride sintered body, comprising wet-mixing and pulverizing the slurry in the above state, granulating the obtained mixture, and then sintering. 2. A slurry is prepared by adding water as a dispersion medium to a composition composed of a raw material powder of silicon nitride and a sintering aid, and the pH value of the obtained slurry is maintained at 12 or less. A method for producing a silicon nitride sintered body, which comprises wet-mixing and pulverizing a slurry in a state, granulating the obtained mixture, and then sintering. 3. A state in which water as a dispersion medium is added to a composition consisting of a raw material powder of silicon nitride and a sintering aid to prepare a slurry, and the viscosity of the obtained slurry is maintained at 1000 cps or less. 2. A method for producing a silicon nitride sintered body, which comprises: wet-mixing and pulverizing the slurry in step 1, granulating the obtained mixture, and then sintering the mixture. 4. A silicon nitride raw material powder surface having a thickness of 10 in advance.
~ 500 angstrom oxide film is formed, and then water is added as a dispersion medium to the composition consisting of the silicon nitride raw material powder and the sintering aid to prepare a slurry,
A method for producing a silicon nitride sintered body, which comprises wet-mixing and pulverizing the obtained slurry, granulating the obtained mixture, and then sintering the mixture. 5. The method for producing a silicon nitride sintered body according to claim 4, wherein a raw material powder of silicon nitride having a particle diameter of 0.1 μm or more is used.