JPH03183661A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To prevent the environmental pollution at the time of mixing and to shorten the time for mixing by subjecting a mixture formed by adopting specific raw materials, adding a specific ratio of sintering assistant to the materials and using water as a dispersion medium to degreasing calcination. CONSTITUTION:The compsn. prepd. by adding 1 to 6 pts.wt. oxide of a rare earth element contg. >=30wt.% Y2O3 or cerium oxide, 2 to 7 pts.wt. Al2O3 and <=3 pts.wt. >=1 kinds of titanium oxide, zirconium oxide and molybdenum carbide to 100 pts.wt. silicon nitride powder produced by a silicon dimide pyrolysis method is used. The mixture obtd. by adding water as a dispersion medium to this compsn. and mixing the compsn. by a wet process is pelletized and calcined to yield the silicon nitride sintered body. The silicon nitride powder produced by the silicon dimide pyrolysis method is relatively small in the reactivity with water, therefore, water is usable as a dispersion medium. Since the water is superior in dispersion function to the dispersion media of a solvent system to pollute the environment, the time for mixing is shortened.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Field of Application) The present invention relates to a method for producing a silicon nitride sintered body, and in particular does not use toxic solvents or the like during the raw material powder mixing operation. The present invention relates to a method for producing a silicon nitride sintered body that can produce a silicon nitride sintered body with excellent quality characteristics at low cost.

(Prior art) Ceramic sintered bodies mainly composed of silicon nitride are lightweight, have high strength, and have excellent wear resistance, so they have begun to be widely used as rolling elements and race materials for bearings.
It is also used in automobile parts and chemical and mechanical parts.

In particular, silicon nitride sintered bodies have excellent heat resistance in the high temperature range up to about 1900°C and have a small coefficient of thermal expansion, so they have better resistance to thermal shock than conventional metal materials. Applications are being developed as materials for various high-strength, heat-resistant parts, including turbine nozzles and internal combustion engine parts.

Conventionally, silicon nitride sintered bodies are made by combining nitride (silicon powder) with a predetermined particle size, yttrium oxide (Y2O2) as a sintering aid,
Rare earth oxides such as aluminum oxide (AI 20
A) is mixed with a chlorinated solvent such as trichloroethane as a dispersion medium to form a uniform d-coalescence, the resulting mixture is granulated, and then degreased and sintered into two parts. Ru.

The silicon nitride powder used as the raw material here is generally a raw material powder produced by the so-called silica reduction method, in which silicon dioxide (S + 02) is reduced to disilicone nitride, or a silicon nitride powder made by the so-called silica reduction method, and Raw material powder produced by a so-called metal nitriding method in which a metal compound is directly nitrided at high temperature 1 is used.

Further, since the sintering property of the silicon nitride powder 2 which constitutes the sintered body is extremely poor, various sintering aids are added to raw materials. In order to improve the properties of the sintered body, it is necessary to uniformly mix the silicon nitride powder and the sintering aid, and for this purpose it is necessary to add a dispensing medium to the raw material powder and mix it uniformly. executed.

Water generally works well as a dispersion medium, but
Conventional silicon nitride powder easily reacts with water, resulting in NH and S]
It is easy to decompose into 02, and the strength, durability and wear resistance of the sintered body often deteriorate. Therefore, organic chlorine solvents such as trichloroethane that do not easily react with the raw material powder are generally used as dispersion media.

(Problem to be solved by the invention) However, according to the conventional manufacturing method of silicon nitride sintered bodies, a chlorinated solvent such as trichloroethane was used as a dispersion medium, which caused harmful solvents to be used in the mixing process of raw material powder. There is a high possibility that steam will be emitted, and there is a risk of contaminating the working environment and the surrounding environment, and there is a drawback that protective measures require a huge amount of cost.

In addition, chlorinated solvents such as trichloroethane have a low ability to disperse the silicon nitride powder raw material and the sintering aid, so a long mixing operation is required to uniformly mix the raw material powder. Therefore, the problem of low manufacturing efficiency of the sintered body and low mass productivity remained.

The present invention was made to solve the above problems, and it is possible to produce a silicon nitride sintered body with excellent quality characteristics at a low cost and without using a dispersion medium that generates harmful mist vapor etc. in the raw material mixing process. An object of the present invention is to provide a method for manufacturing a silicon nitride sintered body that can be manufactured efficiently.

[Structure of the invention]

(Means and effects for solving the problem) From the above viewpoint, the present inventors do not use harmful dispersion media,
In addition, compared to conventional sintered bodies, without compromising quality characteristics,
In order to efficiently obtain a silicon nitride sintered body, we repeated experiments by varying the type of silicon nitride powder, the type and content of additives as sintering aids, and the type of dispersion medium. By using specific raw materials, adding a sintering aid in a predetermined ratio, and uniformly mixing water as a dispersion medium before degreasing and firing, we can efficiently nitridize the product, comparable to the quality characteristics of conventional sintered bodies. A silicon sintered body could be obtained.

The present invention was completed based on the above findings.

That is, the method for producing a silicon nitride sintered body according to the present invention uses a rare earth element containing 30% by weight or more of either yttrium oxide or cerium oxide based on 100 parts by weight of silicon nitride powder produced by silicon diimide pyrolysis method. For a composition comprising 1 to 6 parts by weight of an oxide, 2 to 7 parts by weight of aluminum oxide, and 3 parts by weight or less of at least one compound selected from the group of titanium oxide, zirconium oxide, and molybdenum carbide. The method is characterized in that water is added as a dispersion medium, wet-mixed, and the resulting mixture is granulated and fired.

Further, the average particle size of silicon nitride is preferably set to 2 μm or less.

The reasons for the limitations of the present invention will be described below.

The silicon nitride powder used as the raw material for the silicon nitride sintered body, which is the object of the present invention, is produced by a silicon diimide pyrolysis method. The silicon diimide thermal decomposition method is a method in which silicon diimide (Si(NH)2) is thermally decomposed under high temperature conditions to decompose it into silicon nitride (SiN) and hydrogen (H2). Since the silicon nitride powder produced by this silicon diimide pyrolysis method has relatively low reactivity with water, water can be used as a dispersion medium in the raw material mixing step described below.

Further, the average particle size of the silicon nitride powder used is preferably set to 2 μm or less. The reason for this is that when the thickness exceeds 2 μm, the density of the sintered body decreases.

Rare earth element oxides such as yttrium oxide and cerium oxide are added as sintering aids to improve sinterability, along with aluminum oxide and titanium oxide, which will be described later. 1 to 6 parts by weight.

If the amount added is less than 1 part by weight, the function as a sintering accelerator will be insufficient and sintering will not be successful. On the other hand, if the amount added exceeds 6 parts by weight, the mechanical strength and thermal shock resistance of the sintered body at high temperatures will decrease, so the amount added is set within the above range.

The rare earth element oxide powder used as this sintering aid does not need to be of high purity; it contains 30% by weight or more of either yttrium oxide or cerium oxide, and the balance is a rare earth element such as lanthanum or scandium. It may also be a crude raw material powder containing oxides and other impurities.

By using this crude raw material, it is possible to set the sintering temperature lower than when using high-purity yttrium oxide, and it has the advantage that sufficient sintering can be achieved with a small amount of addition. In addition, the amount of expensive yttrium and cerium used is reduced, and the manufacturing cost of the sintered body can be significantly reduced.

Aluminum oxide (A1203) is then added as a sintering aid along with yttrium oxide and cerium oxide. The amount added is 2 parts by weight per 100 parts by weight of silicon nitride.
~7 parts by weight.

This is because if the amount added is less than 2 parts by weight, the sinterability decreases, while if the amount added exceeds 7 parts by weight, the high temperature strength of the sintered body decreases.

Further, as an additive component, at least one compound selected from the group of titanium oxide, zirconium oxide, and molybdenum carbide is added in an amount of 3 parts by weight or less (excluding zero). These compounds are added to increase the toughness of the sintered body and to improve the color tone of the sintered body. If the amount added exceeds 3 parts by weight, the high temperature strength of the sintered body will decrease, so it is set within the above range.

After adding each of the above components in a composition ratio within a predetermined range, water as a dispersion medium is added to uniformly mix each component. The amount of water added is set to about 40 to 65% based on the total weight of the slurry mixture. If it is less than 40%, the dispersion effect is small;
This is because if it exceeds 65%, the drying time becomes long.

Here, as the mixer, it is preferable to use an attritor, which has higher mixing efficiency than a conventional ball mill. For example, as shown in FIG. 2, the attritor includes a crushing tank 2 loaded with a large number of crushing and stirring balls 1 and containing raw material powder and a dispersion medium, and an agitator arm 3 rotatably disposed within the contained raw material powder layer. , an agitator shaft 4 that rotates the agitator arm 3, a jacket 5 that is integrally attached to the outer surface of the grinding tank 2 and adjusts the temperature of the raw material slurry, etc., and a circulation system that circulates the raw material powder, etc. in the grinding tank 2. It is composed of a pump 6 and circulation piping 7.

The raw material powder, sintering aid, and water contained in the crushing tank 2 are mixed by rotation of the agitator arm 3.

Each of the raw material powders is mixed with each other and further refined by the impact force of the ball 1.

Here, the mixing time is set to 1 to 24 hours.

If the mixing time is less than 1 hour, the dispersion and mixing of the silicon nitride powder and the sintering aid will be insufficient, while if the mixing time exceeds 24 hours, a part of the silicon nitride powder will react with water and form SiO.
This is because □ is produced, leading to a decrease in the strength of the sintered body.

Next, the uniform slurry obtained by the mixing operation was mixed at a temperature of 2
The mixture is subjected to a spray granulation process in which the mixture is sprayed into an atmosphere at 00°C and granulated. In this granulation step, only the water in the slurry evaporates to form raw material particles. Next, the obtained raw material particles are press-molded into a predetermined shape, and then subjected to raw processing and degreasing operations as necessary, and then sintered at a temperature of 1650 to 1850 °C in non-oxidizing recovery 2. However, this sintering can be performed by pressureless sintering, or by other sintering methods such as hot pressing, hot isostatic pressing (HIP, etc.)
Moreover, a silicon nitride sintered body having excellent high-temperature strength and thermal shock resistance can be obtained.

According to the method for producing a silicon nitride sintered body according to the present invention, water is used as a dispersion medium to uniformly mix the raw material powder, so there is no risk of generating harmful vapor during the mixing operation, and the work environment is and will not pollute the surrounding environment.

In addition, the silicon nitride raw material powder produced by the silicon diimide pyrolysis method has low reactivity with water and is stable, making it possible to use water as a dispersion medium, and sintering by reaction with water. A sintered body with excellent high-temperature strength and heat resistance can be obtained without deteriorating the quality characteristics of the body.

Since water has an extremely superior ability to disperse the raw material powder compared to conventional solvent-based dispersion media such as trichloroethane, the raw material powder can be uniformly mixed in a short mixing operation. Therefore, the manufacturing efficiency of the sintered body can be greatly improved, and the mass productivity of the sintered body can be improved.

(Example) Next, the present invention will be specifically explained using examples.

As Example 1, to 100 parts by weight of silicon nitride powder with an average particle size of 0.8 μm produced by silicon diimide pyrolysis method, 5 parts by weight of yttrium oxide as a sintering aid, 5 parts by weight of aluminum oxide, 1 titanium oxide
, 5 parts by weight were added to form a raw material powder mixture, ion-exchanged water equivalent to 80% of the weight of the resulting mixture was added as a dispersion medium, and mixed for 4 hours in an attritor shown in Figure 2. .

Next, 7% by weight of a wax emulsion and a water-soluble acrylic resin were added as binders to the obtained slurry mixture, and the mixture was granulated using a spray dryer at a drying temperature of 200°C. Next, the obtained granulated powder was put into a press at a rate of 700 kg/cn.
A cylindrical shaped body was formed at a molding pressure of f, and was further processed into a spherical shape. The resulting formed body is 70
After degreasing by heat treatment in a non-oxidizing atmosphere at 0°C, normal pressure sintering was performed at 1800°C for 2 hours, and then further 100°C in a nitrogen gas atmosphere at 1750°C.
A hydrostatic pressure of 0 atmospheres was applied and HIP treatment was performed for 1 hour to obtain a sintered body.

Next, the obtained sintered body is subjected to surface processing, rough processing and finishing processing to achieve a final sphericity of 0.25 μm.
The spherical sintered body was processed to an accuracy of 0.25 pm for diameter variation, 0.5 μm for mutual difference, and 0.025 μm for surface roughness, and formed a spherical sintered body as a ball for a bearing.

Then, the obtained spherical sintered body 8 was loaded into a thrust load rolling fatigue testing apparatus as shown in FIG. 3, and the rolling fatigue characteristics of the spherical sintered body 8 were measured. Here, in the thrust load rolling fatigue test apparatus, a plurality of spherical sintered bodies 8 are arranged at a distance from each other via a cage 10 on a substrate 9 made of, for example, 5UJ2, and an inner ring 11 is placed on the upper surface of the spherical sintered bodies 8. Rotating shaft 12 through
It is configured by providing. The test conditions of this example are as follows:
The load added to 2 is 400 kg, and the rotation speed is 1500 per minute.
It was rotated.

On the other hand, as Comparative Example 1, trichloroethane was added as a dispersion medium to the raw material powder having the same composition as in Example 1, and it was processed under the same conditions to form a spherical sintered body, which was subjected to a thrust load rolling fatigue test. provided.

As a result, the test time was 2 for both Example 1 and Comparative Example 1.
Even after 00 hours, no defects such as peeling occurred on the surface of the spherical sintered body, and it was confirmed that the sintered body had sufficient rolling fatigue properties.

Further, test pieces for a three-point bending test were prepared under the same conditions as in Example 1 and Comparative Example 1, and the three-point bending strength was measured at different temperatures, and the results shown in Table 1 below were obtained. Note that measurements at each temperature were performed on five samples, and the average value is displayed.

[Margin below]

Table 1 As is clear from the results shown in Table 1, even when using ion-exchanged water as a dispersion medium, strength characteristics comparable to those of the conventional example using trichloroethane were obtained, and water mixing It can be seen that there is no influence due to

Next, as Comparative Examples 2 and 3, silicon nitride powder rice raw materials produced by the silica reduction method and the metal nitriding method were used, and treated under the same treatment conditions as in Example 1, that is, using ion-exchanged water as the dispersion medium. The raw material powders were mixed and prepared to obtain a spherical sintered body.

In addition, as Comparative Example 3, the amount of rare earth oxide added was reduced to 0.
．． 5 parts by weight, Comparative Example 4 where the amount of aluminum oxide added was too small, 1.5 parts by weight, Comparative Example 5
As for the samples in which the amount of titanium oxide added was an excessive 5 parts by weight, the raw materials were mixed and sintered under the same processing conditions as in Example 1 to obtain spherical sintered bodies of the same size.

The spherical sintered bodies obtained in Example 1 and Comparative Examples 1 to 5 were loaded into a crushing test apparatus as shown in FIG. 4, and the mechanical strength of each sintered body was measured. Here, the crushing test apparatus has an upper plate 14 and a lower plate 15 arranged so as to be slidable in the axial direction so as to face each other inside the cylinder 13, and a recess formed in the center of the opposing surfaces of the upper plate 14 and the lower plate 15. With the spherical sintered bodies 8 placed in the respective parts, a load is applied to the upper plate 14,
This device tests the strength of the sintered body by determining the load W when the spherical sintered body 8 is crushed.

Here, the strength characteristics of each spherical sintered body were expressed by the ST value calculated by the following equation (1) in order to eliminate the influence of the size of the sintered body.

ST value=W/D” (1) Here, W is the load when the spherical sintered body is crushed, and D is the diameter of the spherical sintered body.

The crushing test results of each of the spherical sintered bodies of Example 1 and Comparative Examples 1 to 5 are shown in FIG.

As is clear from the results shown in Figure 1, even in Example 1 where ion-exchanged water was used as the dispersion medium, the ST value was comparable to that of the conventional example (Comparative Example 1) where trichloroethane was used as the dispersion medium. can get. On the other hand, in Comparative Example 2.3, in which raw material powder produced by the silica reduction method or metal nitriding method was used as silicon nitride powder, the reaction between the raw material powder and ion-exchanged water partially occurred, resulting in the formation of a sintered body. ST
The value will decrease. Also, the ST value decreases slightly when there is too little aluminum oxide as a sintering aid (Comparative Example 4) or when there is too much titanium oxide (Comparative Example 5).
It can be seen that sufficient strength characteristics cannot be obtained.

〔Effect of the invention〕

As explained above, according to the method for producing a silicon nitride sintered body according to the present invention, water is used as a dispersion medium to uniformly mix the raw material powder, so there is a risk that harmful vapor may be generated during the mixing operation. There is no risk of contamination of the work environment or surrounding environment.

In addition, the silicon nitride raw material powder produced by the silicon diimide pyrolysis method has low reactivity with water and is stable, making it possible to use water as a dispersion medium, and sintering by reaction with water. A sintered body with excellent high-temperature strength and heat resistance can be obtained without deteriorating the quality characteristics of the body.

Since water has an extremely superior ability to disperse the raw material powder compared to conventional solvent-based dispersion media such as trichloroethane, the raw material powder can be uniformly mixed in a short mixing operation. Therefore, the manufacturing efficiency of the sintered body can be greatly improved, and the mass productivity of the sintered body can be improved.

[Brief explanation of drawings]

Fig. 1 is a graph showing the ST values of sintered bodies manufactured by the method of the present invention together with comparative examples, Fig. 2 is a cross-sectional view showing the configuration of the attritor mixer, and Fig. 3 is the configuration of the thrust load plate fatigue test device. FIG. 4 is a cross-sectional view showing the configuration of the crushing test apparatus. DESCRIPTION OF SYMBOLS 1... Ball, 2... Grinding tank, 3... Agitator arm, 4... Agitator shaft, 5... Jacket, 6... Circulation pump, 7... Circulation piping, 8
... Spherical sintered body, 9 ... Substrate, 10 ... Cage,
11... Inner ring, 12... Rotating shaft, 13... Cylindrical body,
14... Upper plate, 15... Lower plate.

Claims (2)

[Claims] 1. 1 to 6 parts by weight of a rare earth element oxide containing 30% by weight or more of either yttrium oxide or cerium oxide, and 2 to 2 parts by weight of aluminum oxide for 100 parts by weight of silicon nitride powder produced by silicon diimide pyrolysis method.
7 parts by weight and 3 parts by weight or less of at least one compound selected from the group of titanium oxide, zirconium oxide, and molybdenum carbide. A method for producing a silicon nitride sintered body, which comprises granulating and firing the resulting mixture. 2. 2. The method of manufacturing a silicon nitride sintered body according to claim 1, wherein the average particle size of the silicon nitride is set to 2 μm or less.