JP2892244B2 - Manufacturing method of silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride sintered body, and more particularly to a method for producing a sintered body having a high density, a high strength and a small variation in characteristics.

[0002]

2. Description of the Related Art Conventionally, silicon nitride-based sintered bodies have attracted attention as materials having excellent strength, toughness, and the like, and their applications have been promoted as industrial structural materials and heat engine structural materials.

[0003] Since silicon nitride itself is difficult to sinter, a silicon nitride-based sintered body is a sintering aid such as an oxide of a Group 3a element of the periodic table such as Al ₂ O ₃ or Y ₂ O _3. And firing at a temperature of 1600 to 2000 ° C. in a non-oxidizing atmosphere such as nitrogen. In the firing, a method of firing in a pressurized atmosphere having a nitrogen gas pressure of 2 atm or more is adopted because silicon nitride is decomposed when the firing temperature is increased.

[0004] Recently, the firing method has been improved with the aim of increasing the density of the sintered body and further improving the characteristics. As one example, a multi-stage firing method has been proposed in which the open porosity is reduced to some extent under a low nitrogen pressure, and then firing is performed in high-pressure nitrogen. According to this method, when the open pores change to closed pores by firing, there is an advantage that a high-density sintered body can be obtained without trapping high-pressure gas in the closed pores.

[0005]

According to the above method,
Although it is effective for the purpose of achieving the densification of the sintered body, the sintering aid is inevitably contained in the silicon nitride compact in the first place, and the open pores and the multi-stage sintering in the multi-stage sintering are required. In the process of disappearing closed pores, the sintering aid component aggregates in the pores as a liquid phase component and locally induces abnormal growth of silicon nitride crystals, thereby deteriorating the strength of the sintered body. There is a problem that adverse effects are caused by the uneven distribution of the auxiliary.

[0006]

Means for Solving the Problems As a result of repeated studies on the above problems, the present inventors have found that a sintered body densified by firing in high-pressure nitrogen is further subjected to a high-temperature low-pressure atmosphere. It has been found that by performing the treatment, the sintering aid component existing at the grain boundary is caused to move, thereby preventing the uneven distribution of the sintering aid component and obtaining a sintered body with small variation in characteristics.

That is, the method for producing a silicon nitride-based sintered body of the present invention comprises a step of forming a mixture containing silicon nitride as a main component and a sintering aid, and forming the formed body from 1600 to 1800
C., a first baking step of baking in an atmosphere of nitrogen pressure of 2 atm or less until the open porosity is 2% or less, and a sintered body obtained by the first baking step is heated at 1700 to 2000 ° C.
Baking in a nitrogen gas pressure atmosphere of 30 to 200 atm to obtain a sintered body having an open porosity of 1% or less; and baking at a temperature higher than the baking temperature in the second baking step. And a third step of firing under a pressure lower than the nitrogen gas pressure in the firing step of (2).

Hereinafter, the present invention will be described in detail. According to the production method of the present invention, first, a silicon nitride powder and a sintering aid powder are prepared as raw material powders. As the silicon nitride powder, any of α-type and β-type raw materials having an average particle diameter of 0.3 to 1.5 μm, an oxygen amount of 3.0% by weight or less, and an impurity amount of 2.0% by weight or less can be used. .

As sintering aids, Y ₂ O ₃ , La
₂ O ₃ , Er ₂ O ₃ , Yb ₂ O ₃ , Dy ₂ O ₃ , Ho ₂
Oxides of Group 3a elements of the periodic table such as O ₃ , Al ₂ O
₃ , known auxiliaries such as MgO and SiO ₂ can be used. These auxiliaries are used in an amount of 0.5 to 20 in the total amount.
If the amount of the auxiliary agent is less than 0.5% by weight, the sinterability cannot be improved, and if it exceeds 20% by weight, the mechanical properties of the sintered body deteriorate.

Next, after mixing the above-mentioned raw material powder at a predetermined ratio, the raw material powder is mixed to a desired shape by a known molding method, press molding, extrusion molding, injection molding, cold isostatic pressing, cast molding or the like. After forming into a compact, the compact is fired.

[0011] The firing step is roughly composed of three steps.
First, in the first firing step, 1600 to 1800
C., especially at a firing temperature of 1650 to 1780.degree. The first firing step aims at sintering the molded body. In the course of the sintering, the open pores of the molded body change to closed pores. High-pressure gas is trapped in the closed pores. Therefore, it is important to control the pressure of the atmosphere to 2 atm or less until the open porosity becomes 2% or less. Note that the pressure of 2 atm or less includes a normal pressure and a reduced pressure atmosphere.

In the first baking step, the baking temperature is limited to the above range. Sintering does not proceed at a temperature lower than 1600 ° C., and when the temperature exceeds 1800 ° C., silicon nitride is formed under a nitrogen pressure of 2 atm. It is for decomposing.

Next, as a second firing step, the sintered body obtained in the first firing step is fired in a high-pressure nitrogen atmosphere at a nitrogen gas pressure of 30 to 200 atm, preferably 50 to 150 atm. . According to the sintering under such a high pressure, the decomposition of silicon nitride can be suppressed. Therefore, by increasing the sintering temperature to 1700 to 1900 ° C., the open porosity is further reduced by the external pressure and the high-temperature sintering. And finally sintering sufficiently to an open porosity of 1% or less.

Further, according to the present invention, as the third firing step, the sintered body obtained in the second sintering step is heated at a temperature higher than the firing temperature in the second firing step, and the nitrogen pressure is reduced. The firing is performed under a lower pressure than in the second firing step. However, it goes without saying that the firing temperature and the nitrogen gas pressure in the third firing step should be set to be equal to or higher than the decomposition equilibrium pressure of the nitrogen gas of silicon nitride. Specifically, 1700
The treatment is carried out at a temperature of 2,000 to 2000 ° C. and a nitrogen pressure of 10 to 100 atm.

According to the present invention, it is economically advantageous that the first to third sintering steps are successively performed in the same sintering furnace. There is no problem if it is performed in another firing furnace.

In the present invention, the sintering step can increase the fluidity of the grain boundary component of the sintered body and obtain a sintered body without coagulation of the auxiliary component at the grain boundary. 3 followed by hot isostatic firing (HI
(P method), etc., in a high-pressure nitrogen gas atmosphere of 1000 atm or more at a temperature of 1500 to 1900 ° C. to achieve higher densification.

[0017]

According to the present invention, a high-density sintered body having an open porosity of 1% or less without trapping high-pressure gas can be obtained by the first and second firing steps. According to the process, by increasing the temperature, the viscosity of the grain boundary component is reduced, and by reducing the pressure, the internal stress is relaxed, whereby the grain boundary component can be fluidized. The agglomeration portion of the auxiliary component present in the sintered body after the firing step of (1) is diffused, whereby the auxiliary component can be made uniform.

Thus, according to the present invention, if the firing temperature in the third firing step is lower than the temperature in the second firing step,
When the viscosity of the vitreous component at the grain boundary increases, and when the atmospheric pressure is higher than that in the second firing step, the internal stress of the sintered body is not relaxed and the fluidization of the grain boundary component is hindered in any case.

In the sintered body obtained by the present invention, since the auxiliary components are always uniformly dispersed, generation of a fracture source due to unevenness of the structure due to coagulation of the auxiliary is suppressed, and high strength is obtained. In addition, it is possible to reduce variation in characteristics even during mass production.

[0020]

The present invention will be described below with reference to the following examples. Example As silicon nitride powder, the average particle size was 0.7 μm, the amount of oxygen was 1.1% by weight, the amount of metal impurities was 0.3% by weight, and the rate of α conversion was 93
% Powder is prepared and the average particle size is 0.2 to
Prepare 2.5 μm of various metal oxide powders shown in Table 1,
After weighing and mixing them at the ratios shown in Table 1, 1 ton / cm
Press molding was performed at a pressure of ² . The obtained molded body was fired under the firing conditions shown in Table 1 to obtain a sintered body.

In the firing process, the open porosity of the sintered body after the first and second firing steps was measured by Archimedes' method and is shown in Table 1. In addition, for each of the finally obtained sintered bodies, JISR1601
, A four-point bending test was performed to measure the strength, and the average and minimum values are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

As is clear from Tables 1 and 2, Samples No. 4 to 9 in which the conditions in the first and third firing steps in the firing step are out of the range of the present invention have low average strengths. Further, it is understood that the minimum strength is extremely lower than the average value and the variation in characteristics is large.

On the other hand, all the samples produced by the method of the present invention show high strength, and the minimum value is almost the same as the average value, indicating that the characteristics are stable.

[0026]

As described above in detail, according to the present invention, it is possible to obtain a sintered body having a uniform structure, high density and high strength by diffusing agglomeration of auxiliary components in the sintering process. In addition to this, variations in characteristics during mass production can be reduced.

Claims (1)

(57) [Claims] 1. A step of forming a mixture containing silicon nitride as a main component and containing a sintering aid, and subjecting the formed body to an open porosity of 2% in an atmosphere at 1600 to 1800 ° C. and a nitrogen pressure of 2 atm or less. A first sintering step of sintering until the temperature becomes below, and sintering the sintered body obtained in the first sintering step in a nitrogen gas pressure atmosphere of 1700 to 2000 ° C. and 30 to 200 atm. A second firing step of obtaining a sintered body of 1% or less, and firing at a temperature higher than the firing temperature in the second firing step and at a pressure lower than the nitrogen gas pressure in the second firing step. And a third step for producing a silicon nitride-based sintered body.