JPH0429626B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method that can produce a high-strength silicon nitride sintered body even if the amount of sintering aid used is reduced. As is well known, silicon nitride (Sl ₃ N ₄ ) or a sintered body of silicon nitride has (1) extremely high mechanical strength and hardness, and high high-temperature strength, compared to conventional ceramic products.
(2) It is resistant to thermal shock and has high fire resistance. (3) Chemically stable and highly corrosion resistant. (4) Great electrical insulation. It has properties such as. Therefore, its applications are wide, and it is used in high-grade refractories, fireproof materials, wear-resistant materials, electrical insulation materials, etc. for metal smelting, ceramics, and machinery industries. In recent years, it has also attracted attention as a raw material for high-temperature materials such as gas turbines, which require high strength, heat resistance, and mechanical impact resistance over an extremely wide temperature range. Since silicon nitride is inherently difficult to sinter, various attempts have been made to improve its sinterability, but nothing satisfactory has yet been developed. And in conventional technology,
If the sinterability is improved, the high-temperature strength is reduced, which is a contradiction in terms, and it is difficult to obtain a sintered body that satisfies both of these properties at the same time. For example, when silicon nitride powder obtained by various manufacturing methods that have been developed so far is sintered under normal conditions such as pressureless sintering, almost no shrinkage occurs due to sintering, and the resulting sintered The density of the aggregate is small and the strength is low, so it cannot withstand the intended use.
In general, in order to directly sinter difficult-to-sinter materials such as silicon nitride and silicon carbide to a high density without using sintering aids, it is necessary to hot press under ultra-high pressure, as in diamond synthesis. Afterwards,
Although it is possible to obtain the desired product, this method must be carried out under ultra-high pressure, and therefore the sintering cost is high and it cannot be adopted from an economical and industrial perspective. For this reason, MgO, Al ₂ O ₃ ,
It is common practice to add 10 to 20% by weight of oxides such as Y ₂ O ₃ and ZrO ² . However, the silicon nitride sintered body obtained by adding these sintering aids forms a low melting point substance at the grain boundaries of silicon nitride.
This accelerates sintering and makes it possible to obtain a high-density sintered body, but on the other hand, the formation of this low-melting point substance causes a decrease in high-temperature strength properties, making it possible to obtain a sintered body that satisfies both properties at the same time. It is difficult to obtain. The present inventors previously filed a patent application for a method for producing silicon nitride powder with excellent sinterability, which is characterized by heating a mixture of a nitrogen-containing silane compound and a sintering aid in a non-oxidizing atmosphere ( (Special application 1987-213995). When the powder obtained by this method was sintered, a sintered body with high density and excellent high-temperature strength properties was obtained even if the amount of sintering aid added was reduced. However, the room temperature strength properties are
There was no major difference compared to conventional silicon nitride powder. Therefore, as a result of intensive research to further improve the above-mentioned powder, it was found that a sintered body of silicon nitride that has high density and satisfies both room temperature and high temperature strength properties can be obtained by an economical and simple sintering method commonly used. The present invention was completed by developing a method that made this possible. That is, the present invention involves mixing a nitrogen-containing silane compound and a sintering aid, heating the mixture in a non-oxidizing atmosphere to crystallize the nitrogen-containing silane compound, and then molding and sintering to produce a sintered body. In doing so, the present invention provides a method for producing a silicon nitride sintered body, which is characterized in that a nitrogen-containing silane compound and a sintering aid are mixed and heated in a method that has the functions of crushing and dispersing. The nitrogen-containing silane compound used in the present invention is isolated by washing a mixture of silicon diimide (Si(NH) ₂ ), which is a reaction product of silicon halide and ammonia, and ammonium halide with liquid ammonium. Decomposition products Sl ₂ N ₃ H, amorphous Si 3 N obtained by heating a mixture of silicon diimide or Si(NH) ₂ and ammonium halide in a non-oxidizing atmosphere, for example at 400°C to 1300° _C . In the _present invention, these raw materials used for subsequent thermal decomposition do not substantially contain halogen, and even if they do contain halogen, it is in a very small amount of 1% by weight or less. Sintering aids include Y ₂ O ₃ , Al ₂ O ₃ , MgO
oxides such as carbonates and oxalates that can be converted into oxides by heating, and non-oxides such as AlN and TiC.
The mixing ratio of these is 17% by weight or less, preferably 2 to 8% by weight, and more preferably 3 to 5% by weight. The present invention is characterized by the method of mixing the nitrogen-containing silane compound and the sintering aid. That is, in the present invention, when mixing using a V-type mixer or a horizontal cylindrical mixer, which are mixers that do not destroy the shape of the powder, the nitrogen-containing silane compound and the sintering aid are dispersed unevenly, so it is difficult to crush and disperse the powder. It is necessary to use a mixing method such as an agitator and crusher, an attrition mill, and a wet ball mill, which are mixers that have both functions at the same time. In order to confirm the difference in the degree of mixing between these mixing methods, the mixed powder was heated in a non-oxidizing atmosphere and subjected to a powder X-ray diffraction test and chemical analysis. In the former case, that is, simple mixing, powder X-ray diffraction tests revealed that some of the sintering aids remained unreacted, and chemical analysis revealed that some of the sintering aids remained unreacted depending on the sample location. While considerable variation in the amount was confirmed, such results were not obtained with the latter method, that is, a mixing method that combines crushing and dispersion. The reason for this is not clear, but because the specific surface area of the nitrogen-containing silane compound is extremely large at 100 to 1000 m ² /g, the cohesive force between particles is extremely strong. It is assumed that mixing is impossible. As for the form of the mixture, the mixed powder itself or the mixed powder granulated and molded can be used as appropriate. In the present invention, the mixture of the nitrogen-containing silane compound and the sintering aid is heated at a temperature of 1200°C to 1700°C in a non-oxidizing atmosphere. The heating temperature is determined by the atmosphere during heating. That is,
The heating temperature in an atmosphere mainly composed of nitrogen is 1400℃.
Heating is carried out at a temperature of 1200°C to 1350°C under vacuum or in an atmosphere containing at least one of hydrogen and argon as a main component. The reason is that if the heating temperature is less than 1400℃ in an atmosphere containing nitrogen as the main component,
This is because thermal decomposition of the nitrogen-containing silane compound is incomplete and amorphous silicon nitride is produced, and if the temperature exceeds 1700°C, grain growth of silicon nitride occurs, which is undesirable. Also, heating temperature under vacuum or hydrogen or argon atmosphere is
If the temperature is lower than 1200°C, amorphous silicon nitride will similarly be produced, and if the temperature exceeds 1350°C, it will decompose into metallic silicon, which is not preferable. When silicon nitride powder obtained by heating a mixture of a nitrogen-containing silane compound and a sintering aid is sintered, a high-density sintered body is obtained even when the amount of the sintering aid added is small, as described below. The reason for this is not clear, but (1) the nitrogen-containing silane compound has an extremely large specific surface area, quickly reacts with the added sintering aid, and becomes homogeneous. (2)
This is presumed to be because the silicon nitride powder reacts with the sintering aid, so decomposition into metallic silicon during sintering is suppressed. The sintered body obtained by sintering the silicon nitride powder produced by heating the mixture of the nitrogen-containing silane compound and the sintering aid under specific conditions has the sinterability shown in Table 1. This sintered body has excellent strength properties both at room temperature and at high temperatures. The characteristic feature of the present invention is that, as a surprising fact, according to the present invention, it is not only possible to obtain an excellent high-density sintered body even with a small amount of sintering aid added; A sintered body with several orders of magnitude better bending strength at high and room temperatures than sintered bodies made using conventional methods was obtained. This is presumed to be because the uniformity of the structure of the sintered body was improved by reacting the silicon nitride and the sintering aid in the powder state. The silicon nitride obtained by the present invention is particularly suitable as a raw material for engine parts that require high strength and reliability. Next, the present invention will be explained in further detail with reference to Examples. Examples 1 to 4 and Comparative Examples 1 to 4 33 g/hr of silicon tetrachloride saturated steam (25°C) with general nitrogen gas supplied to the outer pipe of the double charging pipe, and 20 g/hr of ammonia gas to the inner pipe A reaction tube (60 m/mφ x 280 m/
m), the two were continuously reacted, and the produced fine powder was transported by nitrogen gas and collected in a container at the bottom of the reaction tube. Next, the above powder was heated at 200℃/hr in an ammonia atmosphere filled in a 140m/mφ tubular furnace made of quartz, and kept at a temperature of 1000℃ for 2 hours to form an amorphous powder that turned white. A powder was obtained.
Chemical analysis showed that the composition of the resulting powder was very close to Sl ₂ N ₃ H. The above powder and the sintering aid shown in Table 1 were mixed for 1 hour using a porcelain stirrer and crusher, and then the powder was pressure-molded using a 25 m/mφ mold press at a molding pressure of 200 Kg/cm ² under a nitrogen atmosphere. Heat to 4°C and hold for 0.5 hours.
Seed powder was obtained. This was crushed using a polyethylene ball mill, and then 5 x 50 x 4 pieces were crushed using a mold press.
(mm) after press forming to 1650℃~1750℃
A sintered body was obtained by firing in a nitrogen atmosphere for 4 hours.
After the surface of this sintered body was ground with a #400 diamond grindstone, the density and bending strength were measured. The results are shown in Table 1. As a comparative example, a nitrogen-containing silane compound and a sintering aid were mixed in a V-type mixer to obtain a sintered body in the same manner as in the example, and the measured values of the sintered body are shown in Table 1. The bending strength is between the fulcrums.
Each is the average of 10 values obtained by a 30 mm three-point bending test. Further, chemical analysis values of Y ₂ O ₃ in Example 3 and Comparative Example 3 are shown in Table 2. Approximately 0.2 from 5 arbitrary locations
Each g is taken out. Examples 5-8 Si(NH) ₂ obtained in the same manner as Examples 1-4,
The NH ₄ Cl mixed powder was washed with -70°C liquid ammonia to remove by-product NH ₄ Cl, and Si(NH) ₂ was isolated. This Si(NH) ₂ powder and the sintering aid shown in Table 2 were mixed in a wet ball mill for 2 hours, vacuum dried and pressure molded, and then held at 1300°C in an H ₂ atmosphere for 1 hour to form the four types of powder. Obtained. This was sintered in the same manner as in Example 1, and the density and bending strength were measured. The results are shown in Table 3. Examples 9 to 12 Silicon tetrachloride saturated vapor (25
℃) at a rate of 33 g/hr and ammonia gas at a rate of 20 g/hr.
m/m), the two were continuously reacted, and the resulting fine powder was collected in a container kept at 500°C using nitrogen gas. The above powder and the sintering aid shown in Table 3 were mixed for 20 minutes using a silicon nitride attrition mill, and the powder was held at 1300° C. for 1 hour under vacuum to obtain four types of powder. This was sintered in the same manner as in Example 1, and the density and bending strength were measured. The results are shown in Table 4.

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Claims (1)

[Claims] 1. When producing a sintered body by mixing a nitrogen-containing silane compound and a sintering aid and heating the mixture in a non-oxidizing atmosphere to crystallize the nitrogen-containing silane compound, it is then molded and sintered. A method for producing a silicon nitride sintered body, which comprises mixing and heating a nitrogen silane compound and a sintering aid in a method that has the functions of crushing and dispersing.