JP2696735B2 - 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 useful as a material for a high-temperature structure.

[0002]

2. Description of the Related Art Silicon nitride sintered bodies are considered to be promising as high-temperature structural materials because of their excellent mechanical properties at high temperatures. In order to manufacture this silicon nitride sintered body at a high density, since the main component silicon nitride is a difficult-to-sinter material and may be thermally decomposed at a high temperature, the hot pressing method and the hot static It is indispensable to sinter the compact under a high temperature and a high pressure such as a hydraulic pressing method and an atmospheric pressure sintering method. Among them, the atmospheric pressure sintering method is an optimal means for mass-producing a dense sintered body with a small amount of a sintering aid without being restricted by the product shape.

[0003] However, when the product becomes considerably large, pores remain in the sintered body during the sintering process even by the atmospheric pressure sintering method, and it becomes difficult to obtain a dense sintered body. As a method of improving this, Japanese Patent Application Laid-Open Nos. 59-18165 and 62-113969 disclose a method of performing a primary sintering of a compact mainly containing silicon nitride in a pressurized nitrogen atmosphere and then performing a primary sintering. Also, a two-stage sintering method in which secondary sintering is performed under high pressure has been proposed.

However, even with the two-stage sintering method, it is difficult to completely remove the pores remaining during the primary sintering by the secondary sintering, and therefore the mechanical strength of the obtained sintered body is not sufficient. Not something.

[0005]

An object of the present invention is to solve the above problems,
An object of the present invention is to provide a method for producing a dense silicon nitride sintered body having excellent mechanical strength.

[0006]

According to the present invention, a mixture of silicon nitride powder and a sintering aid is molded, and when the molded body is sintered, 1.0 to 20 kg / cm ² of nitrogen or nitrogen is used. Primary sintering in an inert gas atmosphere at a temperature in the range of 1650 to 1950 ° C. and at least 250 ° C. higher than the melting point of the grain boundary phase generated by sintering, and having a porosity of 0.5 to 5% After making the sintered body a temperature that is 100 ° C. or more lower than the temperature of the primary sintering and 0 to 250 ° C. higher than the melting point of the grain boundary phase, the gas pressure is 5 to 50 times the pressure of the primary sintering. A second sintering at a temperature of 1650 to 1950 ° C. and a sintering temperature of 100 ° C. or more higher than the temperature of the second sintering. It is about the law.

The silicon nitride powder used in the present invention is not particularly limited, but the amorphous silicon nitride powder and / or the nitrogen-containing silane compound are fired in a nitrogen-containing inert gas atmosphere or a nitrogen-containing reducing gas atmosphere. The crystalline silicon nitride powder obtained in this manner is preferably used.

[0008] Examples of the sintering aid in the present invention include magnesia, alumina, yttria, beryllia, ceria, zirconia, silica, erbium oxide, ytterbium oxide, aluminum nitride and mixtures thereof. The amount of the sintering aid is 1 to 1 with respect to the silicon nitride powder.
5% by weight, preferably 4 to 9% by weight is desirable.

The mixed powder raw material is formed into a desired shape by a die press molding method, a slurry casting method, a rubber press method, an injection molding method, or the like.

[0010] Next, the molded body is weighed 1.0 to 20 kg /
165 cm ^{2 in} an atmosphere of nitrogen or a nitrogen-containing inert gas.
Primary sintering at a temperature in the range of 0 to 1950 ° C. and at least 250 ° C. higher than the melting point of the grain boundary phase generated by sintering,
The sintered body has a porosity of 0.5 to 5%.

The pressure of the atmosphere gas is 1.0 to 20 kg / c.
m ² , preferably 1.5 to 10 kg / cm ² . When the pressure is lower than 1.0 kg / cm ^{2, the} effect of suppressing the thermal decomposition of silicon nitride is not exhibited, and when the pressure is higher than 20 kg / cm ² , the pressure is confined in closed pores in the sintered body. The pressure of the obtained gas is increased, and the densification is not promoted.
The firing temperature is in the range of 1650 to 1950 ° C. If the firing temperature is lower than 1650 ° C., densification of the sintered body is insufficient, and if it is higher than 1950 ° C., thermal decomposition of silicon nitride occurs, which is not preferable.

Further, when sintering a compact formed of silicon nitride and an auxiliary, a grain boundary phase is formed by a reaction between silica on the surface of silicon nitride particles and the auxiliary. It is necessary to perform sintering at a temperature higher than ℃. If the sintering temperature is not higher than the melting point of the grain boundary phase by 250 ° C. or more, a sufficient liquid phase is not generated, or even if a liquid phase is generated, the viscosity is too high and sintering does not sufficiently proceed. Not preferred.

Further, in the present invention, it is preferable to perform the primary sintering while flowing an atmospheric gas. This allows CO gas generated by the reaction between the oxygen-containing component volatilized from the auxiliary agent and the carbon of the furnace material to flow out of the furnace,
The CO gas concentration in the atmosphere gas can be suppressed to 0.2% or less, the generation of silicon carbide due to the reaction between the CO gas and silicon nitride can be suppressed, and harmful pores remain in the obtained sintered body. Disappears.

Next, after the temperature is lowered by 100 ° C. or more from the temperature of the primary sintering and higher by 0 to 250 ° C. than the melting point of the grain boundary phase, the gas pressure is reduced by 5 to 50 times the pressure of the primary sintering. Next, sinter. The gas pressure for the secondary sintering is, for example, 7.5 to 75 kg / cm ² when the gas pressure for the primary sintering is 1.5 kg / cm ^2.
2, and when the gas pressure in the primary sintering is 10 kg / cm ² becomes 50~500kg / cm ^2.

In the secondary sintering, the melting point of the grain boundary phase is 0 to 2 degrees.
Since it is performed at a temperature higher by 50 ° C., the viscosity of the grain boundary phase becomes higher than at the time of the primary sintering. it can. In the present invention, the pressure-temperature setting of the secondary sintering is very important. When the gas pressure is increased while maintaining the primary sintering temperature, only the grain boundary phase component is pushed into the closed pores, and the sintering is performed. The tissue of the body becomes heterogeneous.

Further, in the range of 1650 to 1950 ° C.,
The third sintering is performed at a temperature 100 ° C. or more higher than the temperature of the second sintering. Thereby, densification can be further advanced and a homogeneous sintered body can be obtained. Further, the temperature of the tertiary sintering may be higher or lower than the temperature of the primary sintering as long as it is within the range of 1650 to 1950 ° C.

[0017]

The present invention will be described more specifically with reference to the following Examples and Comparative Examples. Examples 1 to 12 α-type silicon nitride powder (manufactured by Ube Industries, Ltd .: specific surface area: 11 m ²⁾
/ G, oxygen content 1.3 wt%) 92.5 wt%, yttria (Shin-Etsu Chemical Co., Ltd.) 5 wt% and alumina (Sumitomo Chemical Co., Ltd .: AKP-30) 2.5 wt% The added compounded powder was wet-mixed for 48 hours using ethanol as a medium, and then dried under reduced pressure.

The resulting mixture was preliminarily formed into a rectangular shape using a mold having a cross section of 50 × 80 mm square.
Rubber pressed in cm ² . The obtained compact was placed in an atmosphere pressure sintering furnace and subjected to three-stage sintering under the conditions of temperature and nitrogen gas pressure shown in Table 1.

By flowing gas at a flow rate of 5 l / min during the primary sintering, the CO gas concentration in the furnace was 0.1% or less. FIG. 1 shows a temperature-nitrogen gas pressure pattern in three-stage sintering. The holding time of the primary sintering and the tertiary sintering are both 1.5 hours, and the holding time of the secondary sintering is 0.1 hour.
5 hours. In the secondary sintering, the nitrogen gas pressure was increased 10 minutes after reaching a predetermined temperature.

Table 1 shows the measurement results of the bulk density and bending strength of the obtained sintered body. The bulk density was measured by the Archimedes method. The bending strength was 3 × 4
Cut out a test piece of × 40mm and insert it into outer span 3
It was set on a 4-point bending test jig having a length of 0 mm and an inner span of 10 mm, and the bending strength at room temperature and 1300 ° C. was measured.
The bending strength at room temperature is the average value of 40 test pieces,
The bending strength at 1300 ° C. was determined by an average value of ten test pieces.

[0021]

[Table 1]

Examples 13 to 24 The starting material was a compounded powder obtained by adding 3.5% by weight of yttria, 2.8% by weight of alumina and 0.5% by weight of zirconia to 93.2% by weight of α-type silicon nitride powder, The same operation as in Examples 1 to 12 was repeated except that the temperature was changed to the nitrogen gas pressure condition shown in Table 2 and the nitrogen gas was not passed during the primary sintering. CO gas concentration in the furnace during primary sintering is 5%
It rose above. Table 2 shows the measurement results of the bulk density and the bending strength of the obtained sintered body. The surface layer of the obtained sintered body was discolored to green-grey. According to the X-ray diffraction measurement, a peak of β-SiC was detected in the surface layer of the sintered body.

[0023]

[Table 2]

Comparative Examples 1 to 6 The same operation as in Example 13 was repeated, except that the temperature-nitrogen gas pressure conditions shown in Table 2 were changed. The holding time for both the primary sintering and the secondary sintering was 1.5 hours. During the primary sintering, the CO gas concentration in the furnace increased to 5% or more. Table 2 shows the measurement results of the bulk density and the bending strength of the obtained sintered body. The surface layer of the obtained sintered body was discolored to green-grey. According to the X-ray diffraction measurement, a peak of β-SiC was detected in the surface layer of the sintered body.

[0025]

According to the present invention, a dense silicon nitride sintered body having excellent mechanical strength can be manufactured by employing the three-stage sintering method.

[Brief description of the drawings]

FIG. 1 is a diagram showing temperature-nitrogen gas pressure patterns of Examples 1 to 24 and Comparative Examples 1 to 6.

Claims (2)

(57) [Claims] 1. A method of molding a mixture of a silicon nitride powder and a sintering aid, and sintering the molded product, wherein the compact is 1.0 to 20 kg / cm.
^{2650 in} a nitrogen or nitrogen-containing inert gas atmosphere.
Primary sintering at a temperature in the range of １９1950 ° C. and at least 250 ° C. higher than the melting point of the grain boundary phase formed by sintering, to obtain a sintered body having a porosity of 0.5 to 5%, 100 ° C. or more lower than the melting temperature and 0 ° C.
After raising the temperature to ~ 250 ° C, the primary sintering pressure is 5-5
Secondary sintering at 0 times gas pressure, followed by 1650-
In the range of 1950 ° C. and 100 degrees below the secondary sintering temperature.
A method for producing a silicon nitride-based sintered body, characterized by performing tertiary sintering at a temperature higher than or equal to ° C. 2. The method according to claim 1, wherein the primary sintering is performed while flowing an atmosphere gas.