JPS60171268A - 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] [Technical field] The present invention relates to a method for manufacturing a silicon nitride sintered body.

Silicon nitride sintered bodies have excellent properties such as strength and heat resistance, so their uses have been rapidly expanding in recent years, including their use in automobile engine parts.

(Prior art) Since sintering of silicon nitride generally involves rotation, magnesia (MgO) is used as a sintering aid when sintering silicon nitride.
, alumina (Al2O2>, or magnesia alumina spinel (fVIQAlz).

A small amount of spinel and rare earth oxides such as 4) are added. That is, after molding a molded body from a raw material powder that is a mixture of silicon nitride powder and the above-mentioned sintering aid powder, the molded body is sintered in a nitrogen gas atmosphere, thereby producing a silicon nitride sintered body. was. The sintering aid acts with silicon nitride to improve sinterability.

Silicon nitride sintered bodies using conventional sintering aids have a problem in that although their strength properties are high at room temperature, they deteriorate at high temperatures. For example, at a high temperature of about 1300° C., there is a problem that the strength decreases to about 1/3 to 2/3 of that at room temperature. The main reason is that silicon nitride containing sintering aids softens at lower temperatures than pure silicon nitride.

[Object of the Invention] The present invention was devised in view of the above circumstances, and provides a method for producing a silicon nitride sintered body whose strength does not decrease much even at high temperatures.

[Summary of the structure of the invention] As a result of intensive research on the manufacturing method of a silicon nitride sintered body, the present invention has found that before final firing of silicon nitride by heating the molded body in a nitrogen gas atmosphere, It has been discovered that the reduction in high-temperature strength of silicon nitride sintered bodies can be suppressed by temporarily performing firing in a hydrogen gas atmosphere at °C.

The reason for this is presumed to be as follows. That is, the sintering aid reacts with silicon nitride to locally create a melt-like substance, thereby promoting sintering of the silicon nitride and densifying the silicon nitride sintered body. Therefore, a silicon nitride sintered body containing a sintering aid has the advantage of improved sinterability, lower porosity, and larger room-temperature bullet holes. However, there are also negative aspects due to the inclusion of sintering aids. For example, silicon nitride containing a sintering aid softens at a lower temperature than pure silicon nitride, resulting in lower high temperature strength.

Therefore, before the silicon nitride sintered body is sufficiently densified, that is, while there are many pores in the sintered body, it is heated in a gas atmosphere containing hydrogen gas, thereby causing the sintering aid to evaporate through the pores. The aim is to prevent the strength from decreasing at high temperatures.

The present invention was completed based on the above discovery. That is, the method for manufacturing a silicon nitride sintered body of the present invention includes a molding step of forming a compact by molding a raw material powder that is a mixture of silicon nitride powder and sintering aid powder, and a molding step in which the compact is placed in a nitrogen gas atmosphere. a first firing step of preliminarily firing the molded body in an atmosphere mainly containing nitrogen gas; ~165
A second step of firing the molded body in an atmosphere containing hydrogen gas at 0°C, and a third step of finally firing the molded body in an atmosphere mainly composed of nitrogen gas. That is.

Detailed explanation of the 4μ formation of the invention] The forming step, which is a component of the present invention, involves mixing silicon nitride powder, sintering aid powder, etc. to form a raw material powder, and molding the raw material powder to form a compact. This is the process of forming. The molding process is
Conventionally employed methods can be used. That is,
Injection molding or extrusion molding, in which a mixture of silicon nitride powder, sintering aid powder, and resin is molded into a predetermined shape, or slurry casting, in which a chiller of silicon nitride powder and sintering aid powder is poured and molded. Further, methods such as isostatic pressing method and mechanical press molding method can be used.

When injection molding or extrusion molding is used as the molding process, it is desirable to perform a 1:2 fat process in which organic substances such as resin contained in the molded body are heated and removed.

When silicon nitride powder has coarse grains, its sinterability decreases. Therefore, it is desirable that the silicon nitride powder has a maximum particle size of 10 μm or less and an average particle size of 2 or less. The most preferred maximum particle size range of silicon nitride powder is 1 to 3 microns. In addition, the most desirable average particle size of silicon nitride powder is 0.5μ to 0.8μ.
It is.

As the sintering aid powder, Yztria (Yz0
3), Alumina (Al2O2) Magnesia (MQO)
, Magnesium Narminium Subinel (tvlGA +
20′4> or the like can be used. The blending ratio of the sintering aid is preferably about 4 to 10 parts by weight per 100 parts by weight of silicon nitride powder, as in the conventional manufacturing method of silicon nitride sintered bodies, but it is not limited to this range. Note that it is desirable that the silicon nitride powder and the sintering aid powder are uniformly mixed. For this purpose, it is preferable to thoroughly mix the silicon nitride powder and the sintering aid powder using a ball mill or other known appropriate method.

The firing steps that characterize the manufacturing method of the present invention include a first firing step in which the molded body is preliminarily fired in an atmosphere mainly containing nitrogen gas, and a first firing step in which the molded body is preliminarily fired in an atmosphere containing hydrogen gas at 1400 to 1650°C. The method is characterized by comprising a second firing step in which the molded body is fired, and a third step in which the molded body is finally fired in an atmosphere mainly containing nitrogen gas. In this case, the first firing step, the second firing step, and the third firing step are performed in this order. Note that hydrogen gas best evaporates the sintering aid.

In the first firing step, the molded body is densified to some extent.
This is a step of preliminarily firing the molded body.

Therefore, the first firing step is preferably carried out by heating the compact to about 1500° C. in a nitrogen gas atmosphere of about 0.1 to 2 atm, preferably 1 atm.

The sintered body subjected to the first firing step has a relatively large number of pores. In the case of the first firing step, the molded body is preferably placed in a sintering furnace, and the sintering furnace is heated in this state. In this case, for example, after placing the compact in a sintering furnace, the inside of the furnace is vacuum degassed, heated to 1500°C at a temperature increase rate of 15°C/min or less, and then nitrogen gas is injected into the furnace. However, it is preferable to stop the injection of nitrogen gas at the time when the nitrogen gas pressure in the furnace reaches 1 atmosphere.

The second firing step is a step in which hydrogen permeates into the interior of the sintered body before final firing through the pores in the sintered body, thereby causing the sintering aid to evaporate outward through the pores. Therefore, in the second firing step, the molded body is heated for 15 minutes in an atmosphere containing hydrogen gas.
This is preferably carried out by heating and holding at a temperature of 0.000.

The holding period varies depending on the pressure of the atmosphere containing hydrogen gas, the hydrogen gas concentration in the atmosphere, the heating temperature, etc., but is usually about 1 to 3 hours. An atmosphere containing hydrogen gas is
It may be entirely hydrogen gas, or it may be a mixture of nitrogen gas and hydrogen gas, for example.

When all hydrogen gas is used, the pressure of the hydrogen gas atmosphere is preferably 0.1 to 1 atm. The second firing step is preferably carried out by switching the atmosphere inside the sintering furnace containing the compact to nitrogen gas or hydrogen gas.

Here, the second firing step needs to be performed at 1400 to 1650°C. The reason for this is that the molded silicon nitride becomes denser than 1650°C (a)! This is because as hydrogen gas progresses, it becomes difficult for hydrogen gas to enter the inside of the molded body, and therefore the effect of suppressing the decline in high temperature strength becomes smaller. Also, 100
This is because if the temperature is lower than 0° C., the sinterability of silicon nitride decreases, resulting in a decrease in the strength of the sintered body.

The third firing step is the final firing step. Therefore, in the third firing step, after the second firing step is finished and the molded body is slowly cooled, the molded body is heated again at 1750° C. in a nitrogen gas atmosphere to densify the molded body, and then the final firing is performed. In this case, it is preferable to introduce nitrogen gas into the sintering furnace, heat it to 1750°C at a temperature rate of 5°C/min, and maintain the temperature at 1750°C for two hours.

[Effects of Bimei] The method for making a silicon nitride sintered body in a lit manner according to the present invention can suppress a decrease in the high temperature strength of the silicon nitride sintered body compared to the conventional technology.

Further, in the method for manufacturing a silicon nitride sintered body according to the present invention, the room temperature strength of the silicon nitride sintered body can also be improved.

[Example] Examples will be explained below by dividing into a molding process and a firing process.

(1) Molding process First, silicon nitride powder with an average particle size of 1.2μ and a maximum particle size of 8μ, ittria powder with an average particle size of 0.9μ and a maximum particle size of 2μ, and an average particle size of 0.4μ and a maximum particle size of A 2μ spinel powder was prepared. Then, 92 wt % of silicon nitride powder, 5 wt % of ittria, and 3 wt % of spinel were mixed in a ball mill to produce a raw material powder.

Next, the width is 100111m, the thickness is lQmm, and the length is e o m.
m molded bodies were formed from the above raw material powder. In this case, it was set to 50.0 kQ/c+n'.

(2> Firing process First, the compact was charged into a sintering furnace and heated to 1500°C in a nitrogen gas atmosphere, thereby performing the first firing process. In this case, the nitrogen gas atmosphere was 1 atm. did.

The temperature increase rate was set at 5°C/min.

Next, hydrogen gas is introduced around the compact, thereby changing the atmosphere from nitrogen gas to hydrogen gas. The humidity of the sintering furnace when hydrogen gas is introduced is 1380°C and 1420°C.
℃, 1550°C, 1650°C, and 1700°C. Then, the temperature was maintained at 1500° C. for 1 hour in a hydrogen gas atmosphere, thereby carrying out a second firing step. in this case,
The hydrogen gas atmosphere was set at 1 atm.

Next, after slow cooling, nitrogen gas was reintroduced into the sintering furnace. And in that state, 1750 at 5℃ 7m1n
℃ and held at 1750 ℃ for 2 hours to perform a third firing step, thereby producing a sample.

[Test Example] A 3-point bending strength test at room temperature and 1250° C. was conducted on a sample of the silicon nitride sintered body produced through the above-described molding process and firing process.

In addition, as a comparative example, a sample that was not fired in a hydrogen gas atmosphere was also prepared and similarly heated to room temperature and 1250°C.
A point bending strength test was conducted. In the case of the comparative example, the conditions of the molding process are the same as those of the above embodiment, and the conditions of the firing process except for the second firing process are also the same as those of the above embodiment.

The test results are shown in the table. As shown in the table, when the hydrogen gas introduction temperature is 1420°C, the strength at 1250°C is 76 kg/ml12, and the strength at room temperature is 8
8k (1/mm2).

When the hydrogen gas introduction temperature is 155 o'C, 1250
The strength at °C was 87 kO/mm2T, and the strength at room temperature was 92 ko/m1. When the hydrogen gas introduction temperature is 1650℃, the strength at 1250℃ is 82kg/m1, and the strength at room temperature is 90kg/m1.
It was ko/IIt. As mentioned above, when the hydrogen gas introduction temperature is within the range of 1400°C to 1650°C, the high temperature strength at 1250°C is 76 k.
L'1l11 or more, therefore, the high temperature strength did not decrease much.

Furthermore, when the hydrogen gas introduction temperature is 1700°C and 1380°C, the high temperature strength at 1250°C is 58 k!, as shown in the table. 1/n+m', 63 kg/m+n'
and was lower than that of the product of the present invention.

On the other hand, in the case of a comparative example in which firing was not performed in a hydrogen gas atmosphere, the high temperature strength at 1250°C was 60 k (
As low as 1/mi, the strength at room temperature is 83 klJ/
1111112te (IFl ivy.

As described above, the high temperature strength at 1250° C. of the product of the present invention increased by 27%, 45%, and 37% compared to the comparative example. For example, if the hydrogen gas introduction temperature is 1420°C, (7
6-60) Total 60 x 100 = 27%.

Also, the strength at room temperature was 83 kg/mm2 for the comparative example.
On the other hand, the product of the present invention has 88 k<1/lnm'
, 92 ko/n++n2.90 ko/mm2.

Claims (3)

[Claims] (1) A molding process in which a raw material powder consisting of a mixture of silicon nitride powder and sintering aid powder is molded to form a molded body, and the molded body is fired in a nitrogen gas atmosphere to produce a silicon nitride sintered body. In the method, the firing step includes a first firing step of preliminarily firing the compact in an atmosphere mainly containing nitrogen gas, and a firing step of 1400 to 165 ml.
A second firing step in which the molded body is fired in an atmosphere containing hydrogen gas at 0'C, and a third step in which the molded body is finally fired in an atmosphere mainly containing nitrogen gas. A method for manufacturing a silicon nitride sintered body. (2) In the first firing step, the compact is heated in a nitrogen gas atmosphere.
This is carried out by heating to about 500°C, the second firing process is carried out by holding the compact at about 1500°C in a hydrogen gas atmosphere, and the third firing process is carried out after the second firing process is finished. The method for producing a silicon nitride sintered body according to claim 1, which is carried out by subsequently heating the compact at 1750° C. in a nitrogen gas atmosphere. (3) Sintering aids include ittria, magnesia, and