JPH0462001A - Manufacture of silicon nitride powder molded product - Google Patents

Abstract

PURPOSE:To obtain a silicon nitride sinter having high strength and uniform structure without causing voids or defects by sintering in simple molding by using an organic solvent with a specific melting point as a solvent for wet mixing, and using a metallic compound soluble in the organic solvent as a sintering assistant. CONSTITUTION:Silicon nitride powder, an organic solvent whose melting point is 0-40 deg.C, and a sintering assistant composed of a metallic compound soluble in the organic solvent are mixed with one another at a temperature not lower than the temperature at which the organic solvent is substantially melted, and the mixture thus obtained is introduced into a mold at a temperature not lower than the temperature at which the organic solvent is substantially melted. Then. the mixture is cooled at a temperature not higher than the temperature at which the organic solvent is substantially solidified to be cured in the mold, and the organic solvent is dried and removed to obtain a molded product. The temperature at which the organic solvent is substantially melted and the temperature at which the organic solvent is substantially solidified mean the respective temperatures varied from the melting temperature and solidifying temperature proper to the organic solvent by the melting of the sintering assistant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a silicon nitride powder molded body, which is an intermediate in the production of a silicon nitride sintered body.

[Conventional technology]

Silicon nitride (SiN) has high strength and toughness at room temperature, and since it maintains these excellent properties even at high temperatures of 1000C or higher, its application to turbine parts and automobile engine parts is being considered.

Conventionally, silicon nitride is a substance with strong covalent bonds and is difficult to sinter alone, so A10, MgO, Y is used as a sintering aid.
Dense, high-strength sintered bodies were produced by liquid-phase sintering with the addition of oxides of elements from groups IIA, IA, and IB of the periodic table, such as O.

However, at present, silicon nitride sintered bodies do not have enough strength to be applied to automobile engine parts that require high reliability. It is essential to establish a manufacturing process for sintered bodies with In the manufacturing process, the mixing process of the sintering aid powder and the Si N powder and the molding process occupy a large proportion of research and development.

In the conventional general method, sintering aid powder and Si N powder are wet-mixed with a solvent such as ethyl alcohol using a ball mill, etc., and the mixture is dried using a spray dryer etc. and simultaneously cut into an appropriate size. It was granulated, filled into a mold, and then pressure-molded.

However, in this method, it is impossible to completely disperse the powder in the solvent even if a dispersant is used, so local grain growth occurs due to segregation of the sintering aid during sintering, which causes a decrease in strength. It became. Furthermore, when dry granulation is performed after wet mixing, the granulated particles become hard, which tends to cause defects during molding, and these defects remain even after sintering, resulting in a decrease in strength.

Therefore, in recent years, sintering aids that are soluble in solvents such as alphoxide, which is an element in groups IA, IA, and IB of the periodic table, have been used.
A method has been attempted in which the sintering aid is dissolved in a solvent, mixed wet with powder, and then dried and granulated to form the powder. According to this method, it is possible to uniformly disperse and mix the sintering aid in the solvent, but during drying and granulation after mixing, compounds such as metal alkoxides, which are sintering aids, may polymerize due to hydrolysis. There is a phenomenon in which the particles are precipitated as ultrafine particles or as ultra-fine particles, and this has the disadvantage that the filling properties of the granulated particles are significantly reduced and molding becomes extremely difficult.

[Problem to be solved by the invention]

In view of such conventional circumstances, the present invention enables uniform dispersion and mixing of sintering aids, is easy to mold with a mold, and has a uniform structure free of holes or defects by subsequent sintering. It is an object of the present invention to provide a method for manufacturing a silicon nitride powder molded body, which allows a high-strength silicon nitride sintered body to be obtained.

[Means to solve the problem]

In order to achieve the above object, in the method for manufacturing a silicon nitride powder compact according to the present invention, silicon nitride powder has a melting point of oC.
An organic solvent of ~40 C and a sintering aid consisting of a metal compound soluble in the organic solvent are mixed at a temperature higher than the temperature at which the organic solvent substantially melts, and the resulting mixture is heated until substantially the organic solvent melts. The mixture is introduced into a mold at a temperature higher than that at which the organic solvent substantially solidifies, and then the mixture is solidified in the mold by cooling to a temperature lower than that at which the organic solvent substantially solidifies, and the organic solvent is removed by drying to obtain a molded body. do.

In addition, the temperature at which the organic solvent substantially melts and the temperature at which the organic solvent substantially solidifies mean the respective temperatures that change from the melting temperature and solidification temperature inherent to the organic solvent due to dissolution of the sintering aid. do.

Further, for sintering the silicon nitride powder compact, a conventionally known sintering method may be used.

[Effect]

In the present invention, the solvent for wet mixing has a melting point of 0°C to
A 40C organic solvent is used, and a metal compound soluble in the organic solvent is used as a sintering aid.

Such organic solvents include tertiary butyl alcohol (melting point: about 25C), n-dodecyl alcohol (melting point: about 24C),
0), acetophenone (melting point: about 201?), cyclohexane (melting point: about 6.5C), etc. Although water has a melting point of OC, it is not preferable as a solvent because it may oxidize the Si N powder and there are restrictions on the selection of sintering aids, dispersants, and the like.

In addition, the above-mentioned sintering aids include Mg, (!A, group IA elements such as Be, etc., group IA elements such as Sc, Y, La, Oe, etc., alphoxides of group IB elements such as A!, etc.), and chelating agents. Metal compounds such as complexes, organic acid salts such as carboxylates, and inorganic acid salts such as nitrates can be used singly or in combination of two or more.

In this method, the organic solvent and the sintering aid are wet mixed together with the SiN powder, and the sintering aid is dissolved in the molten organic solvent at the same time as 31. N. Disperse and mix the powder. The mixing means is a known one, such as a ball mill or a pressure kneader.
A roll mill or the like may be used, and the selection is made depending on the viscosity of the mixture and the amount to be processed. During this mixing, a conventional dispersant to promote the dispersion and reduce the viscosity of the SiN powder, a conventional binder to improve the moldability, and a compound with a melting point below OC used to adjust the solidification temperature of the mixture are added. An organic solvent or the like may be added as necessary.

Although the mixture obtained in the above mixing step takes the form of a slurry, it is introduced into a mold as it is without undergoing a drying or granulation step using an organic solvent. Even if the mold is a solvent-permeable one that absorbs or permeates only the solvent, such as a plaster mold for cast molding, a transparent mold has a low powder concentration and a relatively low viscosity similar to normal cast molding. It is possible to use mixtures ranging from mixtures with a high powder concentration and high viscosity similar to those used in ordinary injection molding, and can be expected to improve sinterability because the powder is densely packed. However, since it takes time for the solvent to permeate, the molding time becomes longer, and there are other problems such as the need for sophisticated quality control to prevent compositional fluctuations due to the permeation of the sintering aid dissolved in the solvent. On the other hand, in a solvent-impermeable mold, since the powder density is the same as that of the mixture, it is necessary to increase the powder concentration of the mixture to the same level as in ordinary injection molding or low-pressure molding.

After introduction into the mold, the mold is directly cooled or the pressure inside the mold is reduced to evaporate the organic solvent to a temperature below which the organic solvent substantially solidifies, thereby solidifying the organic solvent. The mixture is allowed to solidify in the mold.

Finally, the organic solvent is removed by drying from the solidified mixture to obtain a molded body. Since organic solvents generally have a higher evaporation rate than water, they absorb a large amount of heat of evaporation even at a low degree of vacuum, so they can be easily removed by drying. When the fluidity is poor and the strength of the compact is high, such as when the powder concentration is high or the sintering aid solidifies, the organic solvent should be kept at a temperature above the substantial melting point of the organic solvent to dry and remove it. Although freeze-drying under reduced pressure is preferred in many cases. According to freeze-drying, the shape of the compact is easily maintained regardless of the powder concentration because the compact is frozen and solidified, and the evaporation site of the organic solvent moves from the surface of the compact to the inside of the compact, so the compact There are no restrictions on the wall thickness or shape.

The obtained compact is transferred to the sintering process, but if a dispersant or binder is added in advance, it is decomposed and removed by heating in an inert gas atmosphere or an oxygen-containing atmosphere as usual. Even when these are not added, it is preferable to decompose the metal compound of the sintering aid by the same heat treatment.

〔Example〕

Example 1 Average primary particle diameter 0.2 μm produced by imide decomposition method
Si N powder, 7% by weight of yttrium isopropoxide in terms of YO as a sintering aid, 3% by weight of aluminum isopropoxide in terms of AlO, and 2% by weight of polyvinyl butyral as a binder were mixed in a solvent. 12 in a ball mill using tertiary butyl alcohol as
It took an hour and a minute. The initial concentration of S1N powder was 38% by volume, the room temperature was about 25C, and the solvent was molten. The resulting slurry-like mixture was introduced into a 10100 x 100 x 8 plaster mold with backup, maintained at 30C, at a pressure of 10 atm, and maintained for 15 minutes to allow the solvent to permeate. Next, the plaster mold was cooled to 5C and the inside of the mold was evacuated to solidify the mixture, followed by freeze-drying at a pressure of 1 torr for 3 hours.

The obtained compact was further heated to 600 'C' in the atmosphere to decompose or remove the sintering aid and binder, and then
Sintered at 1750 tel' in a nitrogen atmosphere at atmospheric pressure for 5 hours, and then sintered at 1750 C in a nitrogen atmosphere at 1000 atmospheric pressure.
A Si N sintered body of an example of the present invention was obtained by performing H-P treatment for 1 hour.

For comparison, 5% ffi% of YO powder and At
A sintered body (comparative example A) prepared in the same manner as in the above example except that 2% by weight of O powder was added, and a powder obtained by freeze-drying the slurry mixture was molded into C-P at a pressure of 1000 atmospheres. A sintered body (Comparative Example B) produced in the same manner as in the above example was prepared.

A bending test piece was cut out from each of the obtained Si N sintered bodies, and the four-point bending strength was measured based on JIS.

Table 1 shows the bending strength (average value of 25 samples) and Weibull coefficient of each sintered body, together with the relative density of the compact and sintered body.

Table 1 Also, after lapping each sintered body, the structure was observed using a scanning electron microscope, and the sintered bodies of the examples of the present invention did not have any coarse grains with a major axis length of 10 Am or more. Only a few pores with a diameter of 3 μm or less were observed. On the other hand, in Comparative Example A, many coarse grains with a major axis length of 10 μm or more were observed, and the number of pores was also greater than that of the inventive example. Further, in Comparative Example B, large pores with a diameter of about 10 μm were observed, and there were also more pores with a diameter of about 3 μm than in the present invention example.

Example 2 Average primary particle diameter 002 μm produced by imide decomposition method
of SiN powder, 5% by weight of yttrium isopropoxide in terms of YO as a sintering aid, 2% by weight of aluminum isopropoxide in terms of A10, and 2% by weight of borivinyl phthalate as a binder. Tertiary butyl alcohol was used as a solvent and mixed for 20 minutes using a three-roll mill.

The initial concentration of Si N powder was 45% by volume, the roll temperature was adjusted to about 3 Or, and the solvent was melted.

The resulting slurry mixture was freeze-dried for 8 hours at 5C. The obtained molded body was subjected to decomposition or removal of the sintering aid and binder in the same manner as in Example 1, and then sintered and HIF-treated to obtain a Si N sintered body of an example of the present invention.

The four-point bending strength was measured in the same manner as in Example 1 for the obtained sintered bodies of the present invention example and the sintered bodies produced in the same manner as the comparative example described in Example 1, and the results are shown in Table 2. Bending strength of the body (
The average value of 25 samples) and the same Wipul coefficient are shown together with the relative density of the compact and sintered compact.

Table 2 Also, after lapping each sintered body, the structure was observed using a scanning electron microscope, and the sintered bodies of the examples of the present invention had no coarse grains with a major axis length of 10 μm or more. Only a few pores with a diameter of 3/jm or less were observed, but in Comparative Example A, many coarse particles with a major axis length of 10 μm or more were observed, and the number of pores was also larger than in the inventive example. .

〔Effect of the invention〕

According to the present invention, a silicon nitride powder molded body in which a sintering aid is uniformly dispersed and mixed can be molded easily using a mold without causing any defects.

By using the silicon nitride powder compact of the present invention, it is possible to obtain a high-strength sintered body that is free from pores and defects and has a uniform structure. It is particularly effective for manufacturing silicon nitride sintered bodies used in

Applicant: Sumitomo Electric Industries, Ltd.

Claims (2)

[Claims] (1) silicon nitride powder, an organic solvent with a melting point of 0°C to 40°C,
and a sintering aid consisting of a metal compound soluble in the organic solvent at a temperature substantially equal to or higher than the temperature at which the organic solvent melts, and the resulting mixture is molded into a mold at a temperature equal to or higher than the temperature at which the organic solvent substantially melts. A silicon nitride powder molded body is obtained by solidifying the mixture in a mold by cooling the mixture to a temperature substantially below the solidification temperature of the organic solvent, and drying and removing the organic solvent. Production method. (2) The method for producing a silicon nitride powder compact according to claim (1), wherein the dry removal of the organic solvent is freeze-drying under reduced pressure.