JPS62235261A - Manufacture of high density sintered body of silicon nitride - 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 use! l! This invention relates to a method for producing a high-density sintered body of silicon nitride (Si3N4), and particularly to improving the strength of the sintered body.

[Prior Art] Sintered bodies of silicon nitride, along with sintered bodies of silicon carbide (SiC) and sintered bodies of zirconia (ZrO Sintering non-oxides such as silicon nitride and silicon carbide requires a higher temperature than the sintering temperature of oxides; for example, silicon carbide requires a high temperature of 1950 to 2050°C.

However, silicon nitride has an equilibrium value of about 0 at 1700°C.
．． 1 atm or less, and at 1900°C, it decomposes thermally in nitrogen at about 1 atm or less, so the 1950-205
It is difficult to sinter at a high temperature of 0°C.

To sinter silicon nitride, powder of oxides such as Y2O2, alumina (Y2O2), magnesia (Mg0), etc. is added as a sintering aid to the silicon nitride powder, and the sintering It is necessary to lower the temperature to 1,700 to 1,900°C and apply a pressure higher than the decomposition pressure of silicon nitride to the sintered body in a nitrogen gas atmosphere.

In this case, the pressure of the nitrogen gas in the τ atmosphere must be considerably higher than the equilibrium pressure of thermal decomposition of silicon nitride in order to prevent weight loss. For example, in order to sinter silicon nitride at 1800° C., the pressure of nitrogen gas must be higher than atmospheric pressure.

Taking these constraints into account, the sintered lead was sintered in an atmosphere of nitrogen gas from 03 atm to 21 atm, and the density ratio (actual density of the sintered body xlOO/sintered body theoretical density)
A method of obtaining a sintered body of 88 to 94% is known.

However, the sintered body obtained by such a method contains 6 to 1
Since 2% of pores remain, variations in strength are likely to occur, and it is difficult to produce a sintered body with high strength and little variation in strength using this method.

Therefore, as a means to compensate for this imperfection, the sintered body is heated under pressure in the above-mentioned nitrogen gas atmosphere, and the pores remaining in the sintered body are removed from the pores that are not connected to the atmosphere. , that is, after making the pores closed, further pressurize with nitrogen gas at a higher pressure than the first stage (10 to 100 atm) and pressurize at 1700~
A method of heating to 2000°C is known.

In this case, since the sintered body does not contain open pores, the sintered body can be compressed in the same direction by gas pressure, and in this process, nitrogen gas in the pores inside the sintered body is released into the solid surrounding the pores. , the pores shrink, and densification progresses. With this method, a high-density sintered body with a density ratio of 95 to 98% can be obtained.

Furthermore, this method was further advanced and the second stage of densification process was carried out using a hot isostatic pressing (HIP) device, applying ultra-high pressure of about 1000 atm with nitrogen gas, and achieving a density rate of 98.
Methods for producing high density sintered bodies of ~100% are also known. This HIP treatment using nitrogen gas is considered to be the most excellent method for obtaining a sintered body with a density close to the theoretical density.

[Problems to be solved by the invention] However, at high temperatures of 1700°C or higher, the pressure of nitrogen gas = surrounding air exceeds 10 atm, and the higher the nitrogen gas pressure, the more a part of the surrounding nitrogen becomes a sintered body. A part of this solid solution nitrogen becomes a gas in the process of cooling the sintered body, and this gas forms micropores containing high-pressure nitrogen inside the sintered body. , this high-pressure nitrogen generates internal stress within the sintered body and reduces the strength of the sintered body, for example,
The strength of the high-density sintered body manufactured in a nitrogen gas atmosphere of 1240 atm and 1750°C is lower than that of the high-density sintered body manufactured at 100 atm and 1750°C, and the strength still varies. There was a problem that there was.

The present invention was made to solve these problems, and an object of the present invention is to provide a method for producing a high-density sintered body of silicon nitride that has high strength and minimizes variations in strength.

[Means for Solving the Problems] The method for producing a high-density sintered body of silicon nitride according to the present invention is a method of pressurizing and heating a sintered body of silicon nitride without open pores with a mixed gas. The mixed gas consists of an inert raw gas and nitrogen gas, the total pressure of the mixed gas is 800 to 2150 atm, the partial pressure of the nitrogen gas is 0.3 to 10 atm, and the temperature of the mixed gas is 1650 to 2050 atm. ℃.

[Function] In this invention, = pressure of surrounding gas is 800 to 21
Although the pressure is as high as 50 atm, the partial pressure of nitrogen gas is as low as 0.3 to 10 atm, so the Ichinomiya ward of spiritual air nitrogen has a large amount of energy. Hardly one mineral is lost per day, and micropores containing high-pressure nitrogen are not formed during the cooling process of the sintered body, so no internal stress is generated within the sintered body.

[Example] The present invention is a method for manufacturing a high-density sintered body of silicon nitride by pressurizing and heating a sintered body of silicon nitride without open pores with a mixed gas.

This silicon nitride sintered body without open pores is produced by sintering a porous silicon nitride molded body in a nitrogen gas atmosphere at a comparatively low pressure of, for example, 10 atmospheres or less.

The mixed gas that pressurizes and heats the silicon nitride sintered body consists of an inert gas and nitrogen gas, and this inert gas does not react with the sintering raw material and is not solidly dissolved in the sintered body, or contains nitrogen gas. Use gases such as argon and helium whose solid solution amount can be ignored.

The total pressure of the mixed waste that pressurizes and heats the silicon nitride sintered body is 8
00 to 2150 atmospheres. The reason why the total pressure of the mixed gas is set to 800 atm or more is because if the total pressure of the mixed gas is less than 800 atm, it is difficult to increase the density of the sintered body. In addition, the reason why the total pressure of the mixed gas was set to 2150 atm or less is that when the total pressure of the mixed gas is increased to 800 atm or more, densification is promoted as the pressure increases, but if the total pressure of the mixed gas exceeds 2150 atm, This is because even if the pressure is increased, there is no further effect of increasing the density. Figure 1 shows the relationship between gas pressure and theoretical density ratio, which supports this fact.

The nitrogen gas partial pressure of the mixed gas for pressurizing and heating the silicon nitride sintered body is 0.3 to 10 atm. The partial pressure of nitrogen gas is 03
The reason why the partial pressure of the nitrogen gas is set to be at least 0.3 atm is because the amount of thermal decomposition of silicon nitride cannot be ignored and furthermore, the densification of the sintered body is inhibited. The reason why the partial pressure of nitrogen gas was set to be less than 10 atm is because if the partial pressure of nitrogen gas exceeds 10 atm, the amount of nitrogen gas that diffuses into the inside of the sintered body cannot be ignored, and the strength of the sintered body decreases. It is from.

The temperature of the mixed gas that pressurizes and heats the silicon nitride sintered body is 1
The temperature shall be 650 to 2050°C. The temperature of this mixed gas is 16
The range was set at 50 to 2050°C because pressurizing and heating the sintered body at temperatures below 1650°C and above 2050°C is not suitable for forming the oxide glass phase, which is the grain boundary phase. be.

Experimental example 1 Silicon nitride 92%, alumina 6%, ittria 2%
Powder consisting of (impurities 1300 ppm, oxygen 1.8
%, carbon 0.40%, gelatinization rate 94%, specific surface area 8.5
Add 1% paraffin to m27g), pulverize and mix in acetone for 4 hours using a stirring ball mill, spray dry, degrease, and use the resulting powder as a raw material for monoglaze press molding and CIP treatment to obtain a density ratio of 57. 0% molded body (size 60 mm x 60 mm
Sintering was carried out for 2.5 hours in a nitrogen gas atmosphere of 1 atm to obtain a sintered body with a density ratio of 925%.

Next, this sintered body is buried in aluminum nitride powder, and this is charged into the furnace of the HIP device. Nitrogen gas is introduced into the furnace, and the pressure of the nitrogen gas τ atmosphere is set to lθ atmospheric pressure. At the same time, the temperature inside the furnace was raised to 1,750°C, and while maintaining this temperature, argon gas was injected into the furnace using a compressor to bring the total pressure of the gas atmosphere to 2,001 atm, and this state was maintained for 1.75 hours. He was then ordered released.

The density ratio of the sintered body obtained by this treatment was 987%. In addition, a bending test specimen (3 m
When we cut out 20 specimens (mx 4mm x 40mm) and conducted a three-point bending test at room temperature, we found that
Bending strength is 98.1kg/mm', Weibull coefficient is 27
It was 0.

Experimental Example 2 Density rate 57.0% obtained by the same procedure as in Experimental Example 1 above,
A molded body of 60 mm x 60 mm x 15 mm was buried in aluminum nitride powder, placed in a HIP device, heated to 1000°C while vacuum suction, and heated to 0.2 Torr.
Hold for 2 hours to degas, then add nitrogen gas and maintain the pressure at 1 atm while raising the temperature to 1750°C and hold for 2.5 hours. While maintaining the furnace temperature at 1750°C, argon gas is introduced using a compressor. The pressure was set to 1001 atm, and in this state 1.
It was held for 75 hours and then allowed to cool.

It was 3%. In addition, 20 test pieces for bending tests (Nmm x 4mm x 40mm) were cut out from this sintered body.
When these test pieces were subjected to a three-point bending test at room temperature, the bending strength was 102.0 kg/mm2. The Weibull coefficient was 25.6.

Comparative Example Density rate 57.0% obtained by the same procedure as in Experimental Example 1 above.
A molded body of 60 mm x 60 mm x 15+ nm was buried in aluminum nitride powder and sintered at 1750°C for 2.5 hours in an atmosphere of nitrogen gas τ of 1 atm to obtain a sintered body with a density rate of 92.5%. This sintered body was buried in aluminum nitride powder, charged into a HIP device, and heated at 1750°C for 11 hours.
Sintering was carried out for 1.75 hours in a nitrogen gas atmosphere of 0.00 atm.

The density rate of the sintered body obtained by this treatment is 99.2%
Met. In addition, 20 test pieces for bending tests (size 3 mm x 4 mm x 40 mm) were cut out from this sintered body, and a 3-point bending test at room temperature was performed on these test pieces, and the bending strength was 68.2 kg/mm2. The Weibull coefficient was 18.7.

As explained above, this invention: increases the pressure of surrounding gas to 8
Although the pressure was high (00 to 2150 atm), the partial pressure of nitrogen gas was kept low at 03 to 100 atm, so that part of the nitrogen gas was absorbed into the crystal grains and grain boundary phase of the sintered body during sintering. There is almost no diffusion and solid solution, and micropores containing high-pressure nitrogen are not formed in the sintered body during the cooling process, causing internal stress in the produced high-density sintered body, resulting in high-strength sintered bodies. It has the effect of being able to produce solids.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between gas pressure and theoretical density ratio. Agent Patent Attorney Tadashi Sato Year 0 500 1α) O15002α Force Repulsion (Ki/E)

Claims (1)

[Claims] This is a method of pressurizing and heating a sintered body of silicon nitride without open pores with a mixed gas, the mixed gas consists of an inert gas and nitrogen gas, and the total pressure of the mixed gas is 800 to 2150 atmospheres,
A method for producing a high-density sintered body of silicon nitride, wherein the partial pressure of the nitrogen gas is 0.3 to 10 atm, and the temperature of the mixed gas is 1650 to 2050°C.