JPS6065766A - Method of sintering silicon nitride ceramic - 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 The present invention relates to a method for sintering silicon nitride ceramics, and particularly to a method for preventing a carbonization reaction on the surface of a sintered body during the sintering process in pressureless sintering. Silicon nitride-based ceramics are mainly composed of silicon nitride (S i 3 N 4 ), with addition of sintering aids such as magnesium oxide (
MgO), alumina (AA'+03), yttrium oxide (Y2O2), zirconium oxide (Zr02), etc., or a mixture of two or more thereof.

Mix and knead silicon nitride powder and sintering aid powder (add appropriate forming aid) and form into the desired shape. After that, the molded body was heated to 1650 to 1800°C with nitrogen gas (N
2) It is carried out by holding the sintering furnace for a predetermined period of time in a sintering furnace adjusted to an atmosphere (usually 1 atm to several tens of atm).

To explain the sintering process, first, the molded body is placed in a suitable container (such as a graphite crucible) as it is molded, or after degreasing or calcining treatment, and then placed in a sintering furnace (sintering container).
Insert it inside. After charging the molded bodies, the inside of the furnace is evacuated and evacuated, and the temperature is started to rise.The atmosphere is kept in a vacuum until the temperature reaches 400 to 800°C, and at this temperature, nitrogen gas is sealed up to a predetermined pressure. The purpose of keeping the furnace in vacuum up to this temperature is to remove moisture and organic components released from the compact from the atmosphere, and the purpose of filling in nitrogen gas at this temperature is to prevent the decomposition of 5i3N4. .

After filling with nitrogen gas, the temperature is raised to a predetermined sintering temperature and held for a predetermined time to complete sintering, and then cooled to obtain a sintered body. This is the normal process.

However, the sintered body obtained in this way has carbide (S) on the surface.
iC) layer is often involved. This carbide layer is fragile and peels off, causing rough skin. Even if it does not peel off, the formation of such a carbide layer is not desirable.

The reason why such a phenomenon occurs is considered to be as follows.

That is, in the sintering process, the inside of the furnace is evacuated and vacuumed before filling with nitrogen gas, but the vacuum is not a complete vacuum and is generally 10-4 to 1O-5 torr, so there is a small amount of air inside the furnace. Alternatively, oxygen released from the molded product remains.

Therefore, this trace amount of residual oxygen remains mixed in the sealed nitrogen gas atmosphere. These oxygen components include H2O, Co', 02, etc. On the other hand, inside a sintering furnace, graphite is usually used for heating elements, supporting members, etc., and graphite containers such as graphite crucibles are often used as containers when charging compacts into the furnace. .

Therefore, as the temperature inside the furnace becomes high, carbon reacts with the oxygen mixed in the atmosphere on the surface of the container, etc., as shown in the reaction formula below, and CO is generated, and this CO is mixed with the molded body. Carbonizes silicon nitride by causing a surface reaction.

2C (graphite) + 02 → 2CO...Mountain 2Si 3
N4 + 6CO→6S iC+4N2 + 802
...The oxygen liberated by the carbonization reaction of the extracted Si3N4 reacts with carbon (graphite) to become CO, and further S
Participates in the carbonization reaction of i3N4. By repeating this reaction, carbonization progresses on the surface of the sintered body, resulting in growth of a carbide layer, peeling, and roughening of the surface.

There are generally two methods for preventing carbonization of the surface of the sintered body. One method is to wrap the compact in silicon nitride powder when placing it in a graphite container. In this way, even if CO is generated on the surface of the container, the generated CO will react with the silicon nitride powder before reaching the surface of the compact, making it difficult for the CO to reach the compact and preventing surface carbonization. Another method is to increase the atmospheric nitrogen gas pressure, e.g.
This is a method of adjusting Q/cA or higher. If the nitrogen gas pressure is increased, the 00 partial pressure will decrease and S i 3N4
Zero decomposition carbonization is suppressed. However, in the first method,
Silicon nitride powder for filling is required, and the number of steps for charging increases. Furthermore, the second method requires the sintering furnace to have a high pressure-resistant structure, which inevitably increases equipment costs.

The present invention solves the above problems and provides a sintering method that can inexpensively and reliably prevent surface carbonization of a sintered body.

The sintering method of the present invention prevents surface carbonization by reducing the residual amount of oxygen in the atmospheric nitrogen gas in the sintering furnace. The purpose is to repeat the operation of removing the filled nitrogen gas under reduced pressure and filling it with new nitrogen gas once or multiple times.

According to the present invention, after charging the compact, the atmosphere is removed under reduced pressure and the enclosed nitrogen gas is removed under reduced pressure at least once and replaced with new nitrogen gas. For example, assuming that the nitrogen gas (e.g., 1 atm) that was filled in the first time is mixed with oxygen at Ap pm, this is +11-6 mI engineering rr? d
j DG +41: M l +-M7, 甫yN 1.
s "IL-Id7r- If you simply estimate the amount of enzyme mixed in when sealed and the pressure is 1 atm, A X i O'ppm
becomes. If nitrogen gas is repeatedly charged, removed under reduced pressure, and refilled, the oxygen content will further decrease. It is clear from the above description that as the oxygen content decreases, surface carbonization of the sintered body becomes less likely to occur.

Of course, the amount of residual oxygen in a nitrogen gas atmosphere cannot be calculated simply as described above.
Although it is necessary to take into consideration the moisture released from the compact during the repeated removal and refilling operations, other oxygen content, and the oxygen content that accompanies the filled nitrogen gas as an impurity, it is necessary to carry out the nitrogen gas refilling operation an appropriate number of times. By carrying out the process (for example, twice to several times), as shown in the later examples, a sufficient reduction in oxygen content and a reliable carbonization prevention effect can be achieved.

The operations of enclosing nitrogen scum, removing it under reduced pressure, and re-enclosing it are performed in a temperature range of room temperature to 1400°C.

In order to promote the removal of oxygen such as moisture released from the compact from the furnace, it is best to carry out the process while the temperature is rising, and high mixing is advantageous, but if the pressure inside the furnace is reduced in a temperature range exceeding 1400°C , S i 3N4 decomposition occurs. Therefore, 1400°
C is the upper limit. Promotion of release of enzyme components such as water, and S i
Considering both aspects of preventing decomposition of 3N4, 500 to 11
A range of 00°C is optimal.

Note that these operations may be performed continuously or intermittently. Further, during the operation, the temperature increase may be stopped or may be carried out concurrently with the temperature increase.

Next, examples of the present invention will be described.

Example 1 Silicon nitride powder (average particle size 1.0 μm0 alpha conversion rate 91%)
) and 9% by weight of magnesium oxide powder (average particle size 1.3 μm) were mixed and kneaded in a ball mill, and formed into a disk-shaped compact (diameter 60 mm x thickness 7-8 mm) using a -shaft press. was dried in air at 300°C.

[I] The above molded body was placed in a graphite crucible with a lid and charged into a sintering furnace, and the atmospheric atmosphere in the furnace was adjusted to 1O-5 torr at room temperature.
Eliminate the pressure by reducing the pressure. Raise the temperature while maintaining reduced pressure to 600°
Fill with nitrogen gas to a pressure of 1.1 atm.

Then, after reducing the pressure to 10 torr at 650°C, nitrogen gas is immediately filled in to 1.1 atm, the pressure is reduced again to 10 -5 Lorr, and nitrogen gas is immediately filled in. 170 while maintaining the atmospheric pressure at 1.1 atm.
The sample was heated to 0°C, maintained at the same temperature for 1 hour to complete sintering, and then naturally cooled while maintaining the pressure at 1.1 atm.

In the same manner as in [I] above, the compact was charged into a sintering furnace, the atmospheric atmosphere inside the furnace was reduced to 10-5 torr, and the temperature was raised to 600°C while maintaining the reduced pressure, and nitrogen gas was added to 1.1 torr. Fill to atmospheric pressure. Then, at 800°C, 10-5t
Immediately after reducing the pressure to IQ torr, nitrogen gas was filled to 1.1 atm, and after removing the pressure to IQ torr, 1.
Immediately fill in nitrogen gas to 1.1 atm. Sintering was completed under the same conditions as in [I] above while maintaining the nitrogen gas atmosphere at 1.1 atm to obtain a sintered body.

Comparative Example 1 A compact formed and dried in the same manner as in Example 1 was placed in a graphite crucible with a lid and charged into a sintering furnace, and the atmospheric atmosphere inside the furnace was reduced to 10 -5 torr at room temperature.

The temperature is raised while maintaining reduced pressure, and nitrogen gas is filled at 600°C, and the temperature is raised to 1700°C while maintaining the pressure at 1.1 atm. After completing sintering by holding at the same temperature for 1 hour, 1.1
Cooled naturally while maintaining atmospheric pressure.

The presence or absence of surface carbide and its layer thickness were measured for each of the sintered bodies obtained in Example 1 [■], Interval, and Comparative Example 1 (number of test repetitions N-2).

Carbide is extremely brittle and easily peels off during sandblasting. The carbide layer thickness was calculated by measuring the thickness before and after sandblasting in each test. The measurement results are shown in Table 1.

Example 2 Silicon nitride powder (average particle size 1.0 μm1 alpha conversion rate 92%)
)) 8% by weight of this zirconium oxide (Zr0z) powder (average particle size 1.3 μm) was added as a sintering aid, mixed and kneaded in a ball mill, and then formed into a disc-shaped compact (diameter 60 mm) in a -shaft press. × thickness 7~B ytrm ) was molded and dried at 300°C in the air.

Mouth] To1--1Zli-n Geisha Ko”Murtsuze Lr
Charge the reactor into a λHinkushun Tatekyo Shirigo and remove the atmospheric atmosphere inside the reactor by reducing the pressure to 10” torr at room temperature.The temperature is increased while maintaining the reduced pressure, and when it reaches 600°C, nitrogen gas is filled in. 1,
After setting the pressure to 1 atm, immediately reduce the pressure to 10-5 torr,
Fill in nitrogen gas again to 1.1 atm. This is 10
After reducing the pressure to -5 torr, nitrogen gas was further filled in.
．． The temperature is raised to 1730°C while maintaining the pressure at 1 atm. After completing sintering by maintaining the same temperature for 1 hour, it was naturally cooled while maintaining the temperature at 1 atm.

[111 In the same manner as in [I] above, the compact was charged into a sintering furnace, the atmospheric atmosphere inside the furnace was reduced to 10-5 torr, and
While maintaining reduced pressure, raise the temperature to 600°C and add nitrogen gas to 1.
Enclose to 1 atm. Then at 700°C 10-
After reducing the pressure to 5 torr, immediately fill it with nitrogen gas to a pressure of 1° and 1 atm. Furthermore, 10”t at 1100°C
The pressure is reduced to 1.1 atm and nitrogen gas is immediately filled in to 1.1 atm. While maintaining the nitrogen gas atmosphere at 1.1 atm, the temperature was raised to 1730°C, maintained at the same temperature for 1 hour, and then naturally cooled to obtain a sintered body.

Comparative Example 2 A molded body formed and dried in the same manner as in Example 2 was placed in a graphite crucible with a lid and charged into a sintering furnace, and sintered in the same process as in Comparative Example 1. However, the sintering temperature is 1780'C.

The thickness of the carbide layer on the surface of each of the sintered bodies (N=2) obtained in Example 2 [Moon, Kawa, and Comparative Example 2] was measured in the same manner as above, and the results shown in Table 1 were obtained. .

Table 1 As shown in Table 1, in the comparative example in which nitrogen gas was not removed under reduced pressure and re-enclosed, a carbide layer was significantly formed, whereas in the inventive example, the carbonization reaction on the surface was virtually complete. It can be seen that this is prevented.

As described above, according to the present invention, it is possible to prevent surface carbide formation and obtain a healthy silicon nitride sintered body by a simple operation of removing the nitrogen gas atmosphere under reduced pressure and re-enclosing it. The nitrogen gas atmosphere pressure during the sintering process in the present invention is arbitrary,
Although the pressure is adjusted appropriately within the range of 1 atm to several tens of atm, it is not necessarily necessary to increase the pressure, and the fact that the carbonization reaction can be prevented in a low pressure range of about 1 to 3 atm eliminates the need for a high pressure-resistant structure. It reduces equipment and manufacturing costs and facilitates operation, making it very useful industrially.

Claims (1)

[Claims] (1) During pressureless sintering of silicon nitride-based ceramics, the operation of removing atmospheric nitrogen gas under reduced pressure at a temperature up to 1400'C and then filling in new nitrogen gas is performed at least once during temperature rise. A method for sintering silicon nitride ceramics.