JPS63190759A - Manufacture of silicon nitride sintered body - 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 [Industrial Field of Application] The present invention relates to a method for producing a silicon nitride sintered body, and more particularly to a method for producing a silicon nitride sintered body, which allows firing at low pressure and high temperature.

[Prior art and its problems]

Sintered bodies mainly made of silicon nitride have atoms that are mainly covalently bonded, and have excellent properties in terms of strength, hardness, and thermal and chemical stability, so they are used as engineering ceramics, especially for heat engines such as gas turbines, etc. However, as heat engines become more efficient, the operating temperature of heat engines can reach 1400° C. or higher, and materials that can be used under these conditions are desired.

When producing silicon nitride sintered bodies, it is known that sintering with silicon nitride alone is difficult, so sintering aids such as metal oxides and nitrides are added to the sintering process. For this purpose, the main efforts are to reduce the amount of sintering aid or to generate a glass phase with high heat resistance in the grain boundary phase. As one of the methods, firing at a high temperature has been attempted, but it is known that silicon nitride decomposes and loses weight at high temperatures according to the reaction expressed by the following formula (1).

Therefore, with the aim of suppressing the thermal decomposition of silicon nitride in this high temperature range and enabling high temperature sintering, we published Japanese Patent Application Laid-Open No. 52-470.
As described in Publication No. 15, the nitrogen gas pressure in the firing atmosphere is set to about 10% of the decomposition equilibrium pressure of silicon nitride at the firing temperature.
It has been proposed to set q to be more than double.

However, although it is possible to increase the density to some extent by increasing the pressure of the firing atmosphere, conversely, high-pressure gas is trapped in the pores of the sintered body during firing, which suppresses sintering. It has the disadvantage that pores remain inside, which hinders high density. Particularly when the molded product is a large product, it is currently difficult to uniformly increase the density as a whole.

Therefore, as a firing method, a method that allows high temperature sintering under low pressure is desired.

[Means for solving problems]

As a result of intensive research, the present inventors solved the above problem and found that the above-mentioned formula (1) was used as a factor for the weight reduction of silicon nitride during firing.
Simultaneously with the decomposition reaction of the following formula (2) SiJ< +3Si
Oz-6SiOi 1,200 NZ ↑ ・ ・ ・ Focusing on the progress of the reaction (2), N in the atmosphere! It has been discovered that by controlling the partial pressure and SiO, the decomposition of formula (L) (2) can be suppressed and high temperature sintering can be performed under low pressure, leading to the present invention.

That is, in the present invention, when sintering is performed at 1800 to 2200°C, the sintering atmosphere in the temperature range of 1500°C to 2200°C is composed of at least nitrogen and oxygen or SiO,
The nitrogen gas pressure is set above atmospheric pressure and above the decomposition equilibrium pressure of silicon nitride at the firing temperature, and oxygen or S
The gas pressure of iO is the same as that of silicon nitride and SiO at the firing temperature.
The pressure is set to be higher than the equilibrium vapor pressure of SiO in the reaction with SiO.

The present invention will be explained in detail below.

When firing silicon nitride, the decomposition reaction of formula (1) described above occurs at high temperatures, and oxygen compounds mainly composed of SiO□ inevitably exist in the silicon nitride powder, which is the raw material powder. The reaction of formula (2) proceeds between S+J4 and SiO□, and gases of N2 and SiO are generated.

For these reactions, approximately 1 of the decomposition equilibrium pressure of silicon nitride is required.
The progress of the reaction can be suppressed by introducing nitrogen gas at a pressure higher than 0 times into the firing atmosphere, but as described above, the use of high pressure gas has a negative effect on the promotion of densification.

According to the present invention, first, at least nitrogen gas and oxygen or SiO are introduced into the firing atmosphere for the purpose of suppressing the progress of these decomposition reactions. Therefore, the partial pressures of these gases will be explained based on FIG. 1.

FIG. 1 is a diagram showing the relationship between temperature and the decomposition equilibrium pressure of silicon nitride, and FIG. 2 is a diagram showing the relationship between the equilibrium vapor pressure of SiO in the reaction between silicon nitride and 5i02.

According to FIG. 1, the decomposition equilibrium pressure line ^ of silicon nitride and the equilibrium vapor pressure line B of SiO in the reaction between silicon nitride and SiO□ tend to increase as the temperature rises.

According to the present invention, first, the nitrogen gas partial pressure is set to be at least atmospheric pressure and higher than the decomposition equilibrium pressure of silicon nitride at the firing temperature.

Therefore, the decomposition reaction caused by reaction (1) is suppressed. On the other hand, the partial pressure of oxygen or SiO is set to be higher than the equilibrium vapor pressure of SiO in reaction (2) between silicon nitride and SiO□. The progress of reaction (2) is suppressed by this.Reaction (2)
The suppression effect of N2 is more effective when controlled by SiO pressure than by controlling N2 pressure, and this also means thermodynamically that N2
This shows that SiO has a 6-residue effect, whereas SiO has a 6-residue effect. Therefore, when the partial pressure of SiO becomes high, decomposition is extremely suppressed.

According to the manufacturing method of the present invention, the total pressure of the firing atmosphere is 10
This is because it is desirable to set the pressure to below the atmospheric pressure, and if it exceeds 10 atmospheric pressure, high-pressure gas will be trapped in the sintered body and densification will tend to be inhibited.

According to the manufacturing method of the present invention, silicon nitride powder and known sintering aid powder are first used as raw material powders. It is desirable that the silicon nitride powder contains 70% or more of α-silicon nitride, and if the content is less than 70χ, the sinterability will decrease. As the sintering aid, oxides, nitrides, oxynitrides of elements of Group 111a of the periodic table, such as scandium, yttrium, and lanthanide elements, or Be.

In addition to oxides and nitrides of Group 11a elements of the periodic table such as Mg, Ca, Sr, and Ba, 8IzO++AIN, 5i
Oz, Zr (h + ZrN,) If (h etc. 1 or 2
Can be used in combination of more than one species.

These sintering aids are contained in an amount of 0.1 to 20% by weight based on the total amount.
The mixture is uniformly mixed with silicon nitride powder at a ratio of 1,000 ml, and then pulverized and molded into a desired shape by known molding means such as cast molding, injection molding, injection molding, etc., and then transferred to a firing process.

In the firing process, the sintering temperature is set at 1800 to 2200°C. The firing atmosphere is set so that N2 and oxygen or Sin have the predetermined partial pressures mentioned above. When N2 gas and oxygen gas are used, they may be introduced into the firing furnace from a nitrogen cylinder or an air cylinder and maintained at a predetermined pressure. When adjusting the SiO atmosphere, it is difficult to introduce SiO into the furnace from the outside, so it is adjusted by performing a reaction to generate SiO in the furnace. Specifically, silicon nitride and 5iO are used in addition to the molded body placed in the firing furnace.
By placing the mixed powder of It is set to the equilibrium vapor pressure in the reaction between silicon and 5i02. This suppresses the reaction (2) between silicon nitride and SiO□ inside the molded body.

Another method is to place Si0g powder, SiO powder, or a mixed powder of Si powder and SiO□ powder in the firing furnace in advance, and decompose and volatilize the SiO□ during firing, or the following reaction (3) Si+SiO,→2SiO ↑... By proceeding with (3), an SiO atmosphere can be created. The inventors have confirmed that reaction (2) is suppressed by the Si0 partial pressure set at this time.

The reason for this is that the SiO vapor pressure in reaction (3) is
This is thought to be because it is higher than the SiO vapor pressure in 2).

Still another method is to mix Si powder at a ratio of 0.5 to 10% by weight when the raw material powder for producing a sintered body is not prepared, and generate a certain amount of SiO by volatilizing SiO□ during firing. However, it is difficult to suppress decomposition using this method alone, and it is desirable to use it in combination with the above-mentioned method.

When performing the above-mentioned firing, it is desirable that no carbonaceous material be present in the firing atmosphere. Instead, it is desirable to use a material that does not contain carbon at all, such as silicon nitride or boron nitride, or a material that does not contain carbon at all, such as silicon carbide, but exists as a constituent atom of a compound, such as silicon carbide. The pot to be used may be made of the above-mentioned material, or may have an inner wall formed with a membrane or layer made of the above-mentioned material. When manufacturing such pots, for example, when using silicon carbide, a molded body containing silicon is fired in a carbon body, and its inner wall is silicified to produce silicon carbide, or CVD, PVD, etc.
A silicon carbide film can be produced by a known thin film method such as. Other materials can also be manufactured in the same manner.

According to the firing process of the present invention, especially when trying to obtain a large-sized, dense sintered body, the minimum pressure at which silicon nitride does not decompose is applied at the stage where many open pores remain in the compact after the start of firing. It is desirable to hold the material under the same conditions and sinter it until closed pores are generated and at the same time the open pores decrease to a certain amount or less, and then to perform sintering by further holding the material at high temperature and pressure. Thereby, the gas pressure sealed in the pores of the sintered body during firing can be reduced, and a dense and homogeneous sintered body can be obtained. To explain in detail, the firing process is divided into at least two stages, and in the first stage, the open porosity of the molded body is reduced to 1.
The pressure of the entire atmosphere adjusted under the conditions described above is fired at a low pressure of atmospheric pressure to 5 atm until the concentration of nitrogen gas and oxygen or SiO becomes 0 volume % or less, and then the second step is firing at a high pressure of 5 atm or more. do. The temperature in each stage depends on the open porosity reduction efficiency, and is preferably set at 1900°C or lower in the first stage and 1900°C or higher in the second stage. In addition, in this firing process, it is also possible to divide the firing pattern into more finely divided patterns as long as the purpose of each stage is not deviated from.

The present invention will be explained below with reference to Examples.

First, in carrying out the experiment, five types of containers shown in Table 1 below were prepared as saggers to be placed in the firing furnace.

Table 1 Example l 5ixNa powder (specific surface area 15 m”/g, cx content 90χ, oxygen content 1.2 wt%) 92 wt% + Y!0
After a mixed powder consisting of 35% by weight and 3% by weight of 5iOz was press-molded into a 5 N4 x 45 molded body, the same molded body was placed in each of the saggers ■■■ and fired in the same pattern. The firing pattern is 1770℃, N2 partial pressure 1
Atm for 3 hours, 1850°C and N2 pressure of 2.5 atm for 2 hours, and 1970°C and N2 pressure of 9.8 atm for 2 hours.

The amount of decomposition and specific gravity of the obtained sintered body were measured. The results are shown in Table 2.

Example 2 Using the same raw material powder as in Example 1, a molded body was produced in the same manner as in Example 1 with a composition of 93% by weight of 5iJn, 2.5% by weight of YtCh, and 4.5% by weight of Ah (h).

The obtained molded bodies were individually placed in saggers ■■■ and fired in the same firing pattern. The firing pattern is 1
Calcined for 1 hour at 720°C and 1atm of N2 pressure, then heated to 19°C.
10℃, N! It was fired for 3 hours at a pressure of 9.8 atm.

The decomposition amount specific gravity of the obtained sintered body was measured in the same manner as in Example 1.

The results are shown in Table 2.

Example 3 Using the same raw material powder as in Example 1, Si+Na 87
wt%, 'It(h 4 wt%, Al2O35 wt%,
A molded body was prepared in the same manner as in Example 1 with a composition of 4% by weight of 5iOz.

The obtained molded bodies were each placed in a sagger pot and fired in the same firing pattern as in Example 2. The decomposition amount specific gravity of the obtained sintered body was measured in the same manner as in Example 1.

The results are shown in Table 2.

Example 4 Using the same raw material powder as in Example 1, a molded body was produced in the same manner as in Example 1 with a composition of 5iJ493.5% by weight and YrO36.5% by weight.

The obtained molded bodies were individually placed in saggers ■■■ and fired in the same firing pattern. The firing pattern is 1
Calcined for 3 hours at 750°C and 1atm of N2 pressure, then heated to 18
50°C, Nt pressure 2.5 atm for 1 hour, and then 1 hour.
It was fired for 2 hours at 970°C and N2 pressure of 10at++.

The decomposition amount specific gravity of the obtained sintered body was measured in the same manner as in Example 1.

The results are shown in Table 2.

[Margin below]

Table 2 As is clear from Table 2, the sintered body obtained by firing in a carbon pot (■) with a simple N2 atmosphere was
Decomposition was observed even when the pressure was above atmospheric pressure, and the surface of the obtained sintered body was discolored, making it impossible to obtain a dense body.

Furthermore, even when SiC pot (2) was used, the decomposition inhibiting effect was insufficient despite the addition or inclusion of 0 g Si powder.

In contrast to these comparative examples, in the saggers ■■ in which the mixed powder of 5i3Na powder and SiO□ powder was placed, the mixed powder reacted, and the amount of decomposition could be suppressed to 7% by weight or less. This proves that the atmosphere formed by the reaction of Si, N, powder and SiOg powder, that is, the atmosphere consisting of N2 and SiO, is effective in suppressing decomposition. In addition, by using a SiC pot as the material of the sagger, the decomposition suppressing effect was further improved and the amount of decomposition could be suppressed to 4% by weight or less.

Furthermore, SiO powder was also placed in the sagger pot
Most of the powder was melted continuously, and it was confirmed that an SiO atmosphere was formed in the sagger. The decomposition amount of the sintered body fired in this atmosphere was also suppressed to 4% by weight or less, and it was confirmed that disposing the SiO powder is effective in suppressing decomposition.

C Effects of the Invention As detailed above, the method for producing a silicon nitride sintered body of the present invention can be performed even in a low-pressure atmosphere by controlling the firing atmosphere to a predetermined pressure in N2, oxygen, or SiO atmosphere. High-temperature sintering is possible while suppressing the decomposition of silicon nitride, thereby making it possible to obtain a highly dense sintered body.

Further, since the manufacturing apparatus itself does not require a complicated and large-sized high-pressure container, a sintered body with low manufacturing cost can be safely obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between temperature and the decomposition equilibrium pressure of silicon nitride, and FIG. 2 is a diagram showing the relationship between temperature and the equilibrium vapor pressure of SiO in the reaction between silicon nitride and 5in2.

Claims (1)

[Claims] After mixing silicon nitride powder and sintering aid and molding, 1800~
In a method for producing a silicon nitride sintered body which is sintered at a temperature of 2200°C, the firing atmosphere in the temperature range of 1500 to 2200°C contains nitrogen and oxygen or SiO, and the nitrogen gas pressure is at least atmospheric pressure, and the firing temperature is The partial pressure of oxygen or SiO is higher than the decomposition equilibrium pressure of silicon nitride at
A method for producing a silicon nitride sintered body, characterized in that the equilibrium vapor pressure is set at or above the equilibrium vapor pressure of .