JPH0397673A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To form the large-sized sintered body excellent in strength by forming a mixture of the silicon nitride material and a specified amt. of a sintering assistant, keeping the formed body at a specified temp. for a specified time when the body is heated to a specified sintering temp., then heating the body to the sintering temp. and sintering the body. CONSTITUTION:A mixture of the silicon nitride material and <=10wt.% of the sintering assistant is formed. The formed body is kept at 1400-1650 deg.C for a specified time when the body is heated to a specified sintering temp., then heated to the sintering temp. and calcined. The keeping time is preferably set at 10-60min. The sintering temp. of the formed body is generally controlled to about 1700-2000 deg.C.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a silicon nitride sintered body, and in particular reduces the influence of free silica contained in the silicon nitride raw material, and improves the strength. The present invention relates to a method for manufacturing a silicon nitride sintered body that can form a large-sized silicon nitride sintered body with excellent properties.

(Prior art) Ceramic sintered bodies such as silicon nitride (Si3N4), silicon carbide (Sic), alumina (A203), and zirconia (ZrO2) have excellent hardness, wear resistance, heat resistance, corrosion resistance, etc. Because of its excellent properties, it is becoming widely used as a high-temperature wear-resistant material and as gas turbine parts for automobile engines, machinery, power generation, and aircraft.

In particular, silicon nitride sintered bodies have attracted attention for their wear resistance and high-temperature corrosion resistance, and are used not only in the above fields but also in bearing balls, precision jigs, and various sliding parts, and have special properties such as electrical insulation. It is widely used in electronic components as a functional ceramic component with added functions.

Such silicon nitride sintered bodies are usually made by using silicon-containing materials such as silicon, silicon dioxide, and other silicate compounds as starting materials, and adding yttrium oxide (Y 2 0 a
) and other sintering aids are added and mixed, and the resulting mixture is molded by a pressure sintering method or a hot press method, and the molded product obtained is 1,700 to 2
It is manufactured by baking at a temperature of about 1,000°C for 2 to 6 hours and then allowing it to cool naturally.

(Problem to be solved by the invention) However, silicon nitride powder, which is a raw material, has extremely low sinterability compared to other ceramic raw materials, and when silicon nitride powder is used alone, pressure sintering and It is difficult to obtain a dense sintered body by any method other than the hot pressing method without adding a sintering aid or the like.

Therefore, when trying to obtain a sintered body that is dense and has high strength under relatively low pressure, rare earth oxides such as yttrium oxide (Y203) or alkaline earth metal oxides are used as sintering aids. 20 for ceramic raw materials
It is generally practiced to add up to 30% by weight.

These sintering aids react with free silica (F...-SiO2) and oxygen attached to the Si3N4 raw material powder to generate a liquid phase and promote densification of the sintered body.

However, while these sintering aids are useful for densifying the Si3N4 raw material during shaping and sintering, when the sintered body is used as a high-temperature structural material, they have the effect of softening the structure under high-temperature conditions. There is a problem that the original strength of S i 3 N4 is reduced. Therefore, Si3
In order to maintain the high-temperature strength characteristics of the N4 sintered body, it is desired to suppress the addition of sintering aids as much as possible and increase the density of the sintered body.

On the other hand, the surface of the Si3N4 powder, which is the raw material for the sintered body, contains a trace amount of free silica (S t
02) is adhered to the silicon nitride sintered body, and even if the raw material composition is adjusted to a constant value, there is a problem that the sintered density of the silicon nitride sintered body changes depending on the amount of this free silica.

For example, Y 2 0 3 and A 2 as sintering aids
When a ceramic raw material made of Si3N4 doped with 03 is pressure sintered by a hot press method, the sintering reaction proceeds as follows during sintering. That is, Si3N4
and free silica (S i02 ) attached to Si3N4.
and the sintering aid to form a St-A-Y-0-N glass layer, and at the same time, a low-density αSi3N4
The densification of the sintered body is promoted by dislocation into high-density β-Si3N4.

However, when a large amount of free silica exists, the softening temperature of the glass layer decreases, and the sintering reaction proceeds rapidly only in the surface layer of the hollow body heated by the external heating method, resulting in shrinkage only in the surface layer. There are problems. Therefore, the above-mentioned dislocation reaction does not proceed sufficiently inside the sintered body, resulting in a decrease in sintered density and a disadvantage that the strength characteristics of the sintered body as a whole deteriorate. This phenomenon does not pose a problem when a relatively small sintered body is formed, since free silica is smoothly volatilized from the center of the compact to the outside of the compact during the heating process. However, when forming a large sintered body, the temperature difference between the surface layer and the inner part of a certain shape becomes large, and the sintering reaction in the surface layer is completed while free silica remains in the inner part. As a result, the sintered density in the inner deep part decreases.

The present invention has been made in order to solve the above problems, and is capable of producing a large sintered material with excellent strength characteristics, regardless of the amount of free silica that is inevitably mixed in the silicon nitride raw material. An object of the present invention is to provide a method for manufacturing a silicon nitride sintered body that can be used to form a silicon nitride sintered body.

[Structure of the invention]

(Means and Effects for Solving the Problems) In order to achieve the above object, the method for producing a silicon nitride sintered body according to the present invention includes a method for producing a silicon nitride sintered body such that the content of the sintering aid is 10% by weight or less. Hara f. 1 and a sintering aid are mixed to form a molded body, and the molded body is heated to a predetermined sintering temperature. The present invention is characterized in that a holding step is provided in which the holding step is held for a certain period of time, and after the holding step is completed, the rod-shaped body is heated to a predetermined sintering temperature and fired.

Y203, A20a, etc. are used as the sintering aid, and the content thereof is set to 10% by weight or less. 10
This is because if the content exceeds % by weight, the high temperature strength of the sintered body decreases.

In addition, the silicon nitride raw material used has an average particle size of 2 μm.
The following are suitable, more preferably 0.1 to 1 μm silicon nitride powder. This is because if the average particle size exceeds 2 μm, there will be many pores inside the sintered body, making it impossible to obtain sufficient strength.

Furthermore, the temperature conditions for the holding process were set to 1,400℃ or higher and 1,650℃.
The reason for the setting below is that when the temperature is below 1400°C, the diffusion effect of free silica becomes small, but when the temperature exceeds 1650°C, a liquid phase begins to form on the surface of the compact, and the sintering reaction progresses rapidly. This is because the shrinkage rate of the molded body increases and the amount of free silica remaining inside the molded body increases. In order to make the amount of free silica dissipated more appropriate, the temperature should be adjusted to 1500 ml.
It is desirable that the temperature be higher than 1600°C.

Further, the time for the holding step is set to about 10 to 60 minutes. 1
If the time is less than 0 minutes, the diffusion effect of free silica is small, and 60
This is because if the temperature exceeds 100%, free silica useful for forming a liquid phase during sintering will also be dissipated.

Furthermore, the sintering temperature of the molded body is set at 700 to 2000°C, which is generally adopted as the sintering temperature for silicon nitride.

According to the method for manufacturing a silicon nitride sintered body according to the above structure, in the process of heating the silicon nitride molded body to a predetermined sintering temperature, the molded body is heated at a temperature of 1400°C or more and 1650°C or less for a predetermined time. Because a holding process is provided, the free silica remaining on the surface and deep inside of the molded body is efficiently diffused to the outside of the molded body before the liquid phase of the glass layer is formed inside the molded body. Ru.

As a result, there is less difference in softening temperature between the inside and outside of the imposing body, and the amount of human heat is made uniform between the inside and outside, making it possible to form an overall homogeneous and high-density sintered body.

Therefore, even when using many types of silicon nitride raw materials with different contents of free silica, it is possible to produce a large volume silicon nitride sintered body with a high sintered density, regardless of the amount of free silica. I can do it.

(Example) Next, the present invention will be specifically explained with reference to the following examples.

Examples 1 to 3 Examples 1 to 3 have an average particle size of 0.9 μm and an amount of free silica of 0.9 μm. 5, 1. 7, 2. As shown in Table 1, Y203 and Al2 were added as sintering aids to 100 parts by weight of 6% by weight three types of silicon nitride powder.
A mixture containing 5 parts by weight and 2 parts by weight of 03, respectively, was filled into a general-purpose hot press machine and heated at a pressure of 400 kg/c.
Heating while pressurizing with r&, kept at a temperature of 1600℃ for 30 minutes as shown by the solid line in Figure I, and then the temperature was increased to 1.
After reheating to 750℃ and sintering for 3 hours, let it cool naturally.
A large silicon nitride sintered body with a length of 150 mm, a length of 150 mm, and a thickness of 20 m was manufactured. Then, the sintered density of each of the obtained sintered bodies was measured, and the results shown in Table 1 were obtained.

Comparative Examples 1 to 3 Next, Comparative Examples 1 to 3 had the same composition as Examples 1 to 3, and were sintered for 3 hours by heating to a temperature of 1750°C without a holding step while applying the same pressure. After that, it was allowed to cool naturally to produce a large silicon nitride sintered body having the same dimensions as those of Examples 1 to 3, and the sintered density was measured in the same manner, and the results shown in Table 1 were obtained.

Comparative Examples 4 to 6 In addition, Comparative Examples 4 to 6 had the same composition as Examples 1 to 3, and were heated and fired without a holding step in the same manner as Comparative Examples 1 to 3, and had a length of 30 mm, a width of 30 mm, and a thickness of A small 10 mm silicon nitride sintered body was manufactured, and the sintered density of the sintered body was similarly measured, and the results shown in Table 1 were obtained.

As is clear from the results shown in Table 1, in Examples 1 to 3, since the holding step was provided, free silica in the silicon nitride raw material was effectively dissipated during the heating process, resulting in a high sintered density. It is possible to produce a large silicon nitride sintered body having the following properties.

On the other hand, in the case of Comparative Examples 1 to 3 in which no holding step is provided, the sintered density is low. However, in the case of small-sized sintered bodies as shown in Comparative Examples 4 to 6, high-density sintered bodies almost as shown in Examples 1 to 3 can be obtained without the holding step. Therefore, it can be seen that the method of this embodiment is particularly effective when manufacturing large-sized silicon nitride sintered bodies.

Examples 4 to 6 As shown in Table 1 as Examples 4 to 6, the average particle diameter was 0.
A raw material mixture containing 100 parts by weight of 9 μm silicon nitride powder and 7 parts by weight and 3 parts by weight of Y203 and Al203 as sintering aids, respectively, was filled into a normal molding machine and compressed at a pressure of 400 kg/d. A large number of three types of molded bodies were produced. Next, the obtained molded bodies were divided into three groups, and SiN sintered bodies, Al203 sintered bodies, and Al
It was placed in a firing container (pod) made of 34N sintered body, heated in an electric furnace, held at a temperature of 1550°C for 30 minutes, then reheated to a temperature of 1800°C, fired for 4 hours, and then allowed to cool naturally. 200mm long, 200mm wide, 30+nm thick
A large silicon nitride sintered body was manufactured. Then, the sintered density of each of the obtained sintered bodies was measured, and the results shown in Table 1 were obtained.

Comparative Examples 7 to 9 On the other hand, as shown in Table 1, the samples were fired in a certain shape under the same conditions as Example 4 without the holding process, and Comparative Example 7 was prepared by adding a large amount of sintering aid and carrying out the holding process. Comparative Example 8 was prepared by molding and firing without heating, and Example 6 was prepared by setting the temperature in the holding step at 1700°C.
A large silicon nitride sintered body was produced as Comparative Example 9, which was sintered in the same manner as above. Then, the sintered density was measured in the same manner, and the results shown in Table 1 were obtained.

Margin below C] As is clear from the results shown in Table 1, in Examples 4 to 6 in which a holding process was provided, large silicon nitride sintered bodies with high sintered density and excellent strength were obtained. can get. On the other hand, as shown in Comparative Example 7, the sintered density decreases when the holding step is not provided, and as shown in Comparative Example 8, even if a relatively large amount of sintering aid is added, the sintered density decreases. The improvement effect is small.

Furthermore, as shown in Comparative Example 9, if the temperature of the holding step is set higher than the lower limit temperature at which a liquid phase is generated, the density inside the sintered body does not increase and the density of the sintered body as a whole decreases. I understand.

In Examples 4 to 6, a large number of prepared samples or shapes were placed in a sintering container (pod) made of Si3N4 sintered body.
The surface of the S13N4 sintered body produced by separately housing the sample was extremely smooth, and no uneven coloring occurred. On the other hand, the St3N4 sintered bodies manufactured by being housed in firing containers made of Al203 and AN had large surface roughness, and many had partial color unevenness.

Therefore, when sintering a silicon nitride molded body by pressureless sintering, the sintering surface can be made smooth by using a sintering container made of a sintered body of silicon nitride, which is the same material. It is possible to obtain a sintered body with little color unevenness. As a result, it was found that the amount of finish polishing after sintering was reduced, the finishing process was shortened, and the manufacturing cost of Si3N4 sintered products could be significantly reduced.

〔Effect of the invention〕

As explained above, according to the method for manufacturing a silicon nitride sintered body according to the present invention, in the process of heating a silicon nitride molded body to a predetermined sintering temperature,
Because we have a holding process in which the molded body is held for a predetermined period of time under the following temperature conditions, free particles remaining on the surface and deep inside of the molded body are released before the liquid phase of the glass layer is formed inside the molded body. Silica is efficiently diffused to the outside of the molded body.

As a result, there is less difference in softening temperature between the inside and outside of the molded body, and the amount of human heat is made uniform between the inside and outside, making it possible to form a sintered body that is homogeneous as a whole and has a high density. .

Therefore, even when using many types of silicon nitride raw materials with different contents of free silica, it is possible to produce a large volume silicon nitride sintered body with a high sintered density, regardless of the amount of free silica. I can do it.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between processing time and heating temperature in the case of heating and firing a molded body using the method of the present invention in comparison with the case of a conventional method. Figure 1

Claims (1)

[Claims] A process of mixing a silicon nitride raw material and a sintering aid to form a compact so that the content of the sintering aid is 10% by weight or less, and heating the compact to a predetermined sintering temperature. So, 1
A silicon nitride sintered body characterized in that a holding step is provided in which the compact is held for a predetermined time under a temperature condition of 400° C. or more and 1650° C. or less, and after the holding step is completed, the compact is heated to a predetermined sintering temperature and fired. Production method.