JPS6110070A - Manufacture of silicon nitride base ceramics - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Field of Application of the Invention) The present invention relates to a method for producing silicon nitride ceramics, and more specifically to a method for producing dense silicon nitride ceramics that are dense and have excellent properties such as strength. Regarding the method.

(Background of the Invention) Silicon nitride (Si3N4) ceramics have excellent heat resistance, wear resistance, etc., and are therefore widely used in gas turbines, mechanical seals, burner tips, and the like.

Hitherto, known methods for sintering silicon nitride ceramics include hot pressing, pressureless sintering, and reactive sintering.

In the hot press method, a molded body made of silicon nitride powder is
Because it is fired at a high temperature and pressure of 00 to 1800℃ and 200 to 300 kg/-, it can be densified with a relatively small amount of sintering aid, and a sintered body with excellent strength and corrosion resistance can be obtained. There are restrictions on the size of the body, and there are also problems in that only simple shapes can be sintered.

The pressureless sintering method is a method in which a molded body made of silicon nitride powder is fired at 1700 to 1800°C under normal pressure, and a relatively high-strength sintered body can be obtained, but Y2O3, MgO, Aβ
Since it contains a large amount of sintering aid such as 203, there are problems in that it shrinks greatly during sintering and also suffers from a decrease in strength at high temperatures.

In addition, the reaction sintering method is a method in which a compact made of silicon powder is fired in a nitrogen atmosphere and made into silicon nitride ceramics through a nitriding reaction.There is almost no shrinkage during sintering, and even complex shapes can be sintered. Also, since it does not require a sintering aid, there is almost no strength loss at high temperatures, and the firing temperature is 1300-1400°C, which is lower than other manufacturing methods, making it possible to reduce manufacturing costs. It has the advantage of

However, since the reactive sintering method uses silicon powder as a starting material, it is virtually impossible to obtain a completely dense sintered body. In other words, if the silicon molded body is completely nitrided, the density of the silicon nitride ceramic will be 1.67 times that of the silicon molded body, but the density of the silicon nitride ceramic (
In order to obtain a density of 3.18 g/aJ), the density of the starting silicon molded body needs to be 82% of the true density of silicon.

However, as long as powder is used as a raw material, it is almost impossible to create a compact with such high density. For this reason, the porosity of reactive sintered silicon nitride ceramics is usually 2.
The content is relatively high at 0 to 30%, and as a result, only porous ceramics can be obtained. Therefore, the strength is significantly lower than that of sintered bodies produced by the conventional hot pressing method and pressureless sintering method, and there is a big problem in terms of practicality.

(Objective of the Invention) The object of the present invention is to eliminate the drawbacks of the prior art described above and to manufacture silicon nitride-based ceramics that are dense and have excellent properties such as strength while taking advantage of the characteristics of the reaction sintering method. The purpose is to provide a method.

(Summary of the Invention) In order to achieve the above object, the present inventors have conducted various studies and found that pores in a silicon nitride (Si3N4) reaction sintered body can be filled with an organic silicon compound having a silicon-carbon (Si-C) bond. Or, by filling the solution with a solution of the amorphous pyrolysis reaction product of the compound and heating it to a temperature range in which the organosilicon compound or the amorphous pyrolysis product crystallizes into silicon carbide, silicon nitride ceramics can be formed. The present invention was achieved by discovering that it is possible to fill the pores and make the material dense.

The method for producing dense silicon nitride ceramics of the present invention includes:
A step of heating a molded body made of silicon powder in a nitrogen atmosphere to cause a nitriding reaction to produce a porous silicon nitride sintered body, impregnating the silicon nitride sintered body with an organosilicon compound having a silicon-carbon bond, or It is characterized by comprising a step of filling, and then a step of reheating it under vacuum or an inert atmosphere to a temperature range at which at least a portion of the organosilicon compound is converted to silicon carbide.

When carrying out the method of the present invention, first, a molded body made of silicon powder is heated and fired in a nitrogen atmosphere to obtain a porous silicon nitride sintered body through a nitriding reaction.

Next, the obtained silicon nitride sintered body is impregnated and filled with a solution of an organosilicon compound having a silicon-carbon bond.

The organosilicon compound having a silicon-carbon bond used in this case may be either a hexamer type or a polymer type, and examples thereof include hexamethyldisiloxane, benqmethyldisiloxane, organopolysiloxanes, and the like.

The sintered body impregnated as described above is placed in a vacuum or in an inert atmosphere (e.g. nitrogen, argon, etc.) at a temperature at which part or all of the organosilicon compound is thermally decomposed and crystallized and converted into silicon carbide. The dense silicon nitride-based ceramic of the present invention can be obtained by heating to a certain temperature and re-firing.

The above resintering temperature is 8
00 to 1500°C is preferable. The number of times of impregnation treatment may vary depending on the amount of volatile components of the organosilicon compound used, etc., but it is usually about 1 to 10 times.

In the method of the present invention, the silicon nitride sintered body may be impregnated and filled with a solution of an amorphous thermal decomposition reaction product obtained by thermally decomposing the organic silicon compound.

The amorphous pyrolysis reaction product comprises the organosilicon compound,
It is obtained by heating in advance in a vacuum or inert atmosphere to a temperature range in which an amorphous thermal decomposition reaction product is produced. The sintered body impregnated with a solution of the amorphous pyrolysis reaction product is heated to a temperature range in which the amorphous pyrolysis reaction product is crystallized and converted into silicon carbide, and then re-sintered. By doing so,
This is the silicon nitride ceramic of the present invention.

Further, in the method of the present invention, the nitriding step, the thermal decomposition/filling step, and the crystallization step of the molded body made of silicon powder may be performed simultaneously. For example, converting the organosilicon compound into an amorphous pyrolysis reaction product and impregnating and filling the amorphous pyrolysis reaction product into a porous silicon nitride sintered body in the presence of the porous silicon nitride sintered body. It is also possible to carry out these steps simultaneously under the pyrolysis reaction temperature. The holding temperature in the firing furnace when filling the pores with the thermal decomposition and amorphous thermal decomposition reaction products is selected depending on the thermal decomposition temperature of the organosilicon compound, but is generally preferably 300 to 800°C.

(Effects of the Invention) According to the method of the present invention, the pores of silicon nitride (Si3N4) produced by the reaction sintering method, which has the problem of high porosity, are filled with a silicon carbide component and bonded, thereby achieving a low porosity. Dense silicon nitride ceramics can be obtained by a simple method. In addition, by increasing the densification, it is possible to obtain ceramics with strength characteristics similar to those of the Honto Press method and the auxiliary sintering method, which require advanced technology. Therefore, ceramics with superior wear resistance etc. can be obtained compared to conventional silicon nitride reaction sintered bodies.

(Embodiments of the Invention) Example 1 50 g of 200 mesoyu silicon powder was added with 50 g as a binder.
% pyrivinyl alcohol aqueous solution (5 m A+) was added, and after mixing in an automatic mortar for about 1 hour, the resulting mixture was sized using a 32-mesh sieve. Next, about 2 g of the sized powder was put into a 16φ mold and press molded under a load of 1,000 kg/- to obtain a molded body made of silicon powder with a density of 1.69 g/d. This molded body was placed in a nitrogen atmosphere for 1350'
C. for 8 hours to obtain a silicon nitride sintered body through a nitriding reaction. The nitriding rate at this time was approximately 75%, and there was no dimensional change.
The density of the sintered body is 2.35 g/L: 11. The porosity was 26%.

Next, the obtained silicon nitride sintered body was immersed in a hexamethyldisiloxane solution, impregnated in an ultrasonic cleaner for about 3 hours, and then heated at 1000°C for 2 hours in a nitrogen atmosphere to re-fire. did.

Table 1 shows the density, porosity, and hardness of the sintered bodies obtained by carrying out this impregnation-re-firing step up to three times.

FIG. 3 of Table 1 is a graph showing the relationship between the number of impregnations and the rate of decrease in porosity.

Through the above impregnation-re-firing process, dense silicon nitride ceramics, which could not be obtained by conventional reaction sintering methods, could be obtained.

Example 2 A molded body 1 made of silicon powder of 16φ x 3t obtained in the same manner as in Example 1 was placed in the apparatus shown in FIG. 2 and fired at 1350°C for 8 hours in a nitrogen atmosphere to undergo a nitriding reaction. After that, the temperature inside the firing furnace 2 was lowered to 600°C. On the other hand, a vaporizer 4 filled with hexamethyldisiloxane 3 in advance was maintained at a constant temperature of 90° C., which is below the boiling point of hexamethyldisiloxane (99.7° C.), by a constant temperature bath 5.

When the temperature inside the firing furnace 2 has fallen to 600°C, the three-way door is opened, nitrogen gas is introduced from the nitrogen cylinder 7 as a carrier gas into the vaporizer 4, and hexamethyldisiloxane vapor is transferred to the firing furnace 2. introduced inside. Thermal decomposition of hexamethyldisiloxane vapor starts at around 350°C, so by maintaining the inside of the firing furnace 2 at 600°C for 4 hours,
Pyrolysis and filling of the pyrolysis reaction product into the silicon nitride sintered body was carried out.

Next, the three-way switch was switched to create a nitrogen atmosphere inside the firing furnace 2, and then the temperature inside the firing furnace 2 was raised to 1000°C and held for 2 hours, so that the nitrogen atmosphere present in the pores of the silicon nitride sintered body was Amorphous silicon carbide was crystallized. The pattern of the above steps is shown in FIG.

The broken line portion in the figure shows the case of a repeated process.

As a result of the above operation, the porosity of the silicon nitride sintered body was significantly reduced from 26% to 7%. Compared to the method of impregnating an organosilicon compound in Example 1, this method does not contain volatile components in the thermal decomposition product, and the particle size is 0.01 to 0.
This is thought to be because the silicon carbide component can be efficiently filled because it is as fine as 5 μm (using the same principle as the CVD method).

Furthermore, according to this process, the nitriding process of the molded body made of silicon powder, the thermal decomposition/filling process, and the crystallization process are carried out as a series of continuous processes, which has the advantage that the processing time can be significantly shortened. There is also.

[Brief explanation of drawings]

FIG. 1 is a pattern diagram of the manufacturing process of the present invention, FIG. 2 is an explanatory diagram of an apparatus for carrying out the manufacturing process of Example 2 of the present invention,
FIG. 3 is a graph showing the relationship between the number of impregnations and the reduced porosity in Example 1 of the present invention. l... Molded body made of silicon powder, 2... Firing furnace, 3
...hexamethyldisiloxane, 4...vaporizer, 5
・・・Thermostat, 6... Three-way cock, 7... Nitrogen cylinder. Calligraphy School (Iiii7)

Claims (3)

[Claims] (1) A step of heating a molded body made of silicon powder in a nitrogen atmosphere to cause a nitriding reaction to form a porous silicon nitride sintered body; A silicon nitride comprising the steps of impregnating or filling with a silicon compound, and then reheating it under vacuum or an inert atmosphere to a temperature range at which at least a portion of the organosilicon compound is converted to silicon carbide. A method for manufacturing ceramics. (2) In claim 1, the organosilicon compound is heated in a vacuum or in an inert atmosphere to a temperature range in which an amorphous pyrolysis product is produced. The method is characterized in that the amorphous thermal decomposition product is suspended in a solution consisting of an organic solvent and allowed to penetrate into the porous silicon nitride sintered body produced by the above nitriding reaction, and then the above reheating is performed. A method for producing silicon nitride ceramics. (3) In claim 2, the transformation of the organosilicon compound into an amorphous pyrolysis product and the infiltration of the amorphous pyrolysis product into a porous silicon nitride sintered body are described as 1. A method for producing silicon nitride ceramics, characterized in that the process is carried out in the presence of a porous silicon nitride sintered body maintained in a decomposition reaction temperature range.