JPS6129906B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a porous silicon nitride sintered body at low cost. Reactive sintered silicon nitride has a small coefficient of thermal expansion and excellent thermal shock resistance. Because it has excellent properties such as good corrosion resistance against molten metal and no loss of strength at high temperatures, it is being used in many fields including high-temperature mechanical parts and metal industrial refractories. The present inventors have researched a method for manufacturing reaction-sintered silicon nitride molded products with complex shapes at low cost and in a short time. A molded product obtained by adding 25 to 50% by weight of both in a non-oxidizing nitrogen-containing atmosphere.
It has been discovered that by firing at 1400°C to 1500°C, the above objectives can be achieved by eliminating the decrease in the deposition rate and the occurrence of drying cracks during cast molding without impairing the firing properties or plasticity. This led to the present invention. That is, in the present invention, Si is 30 to 75% by weight, clay is 20 to 45% by weight, and kaolin is 5 to 30% by weight, and the total amount of both is 25 to 50%.
% by weight, the balance being Si ₃ N ₄ was molded, and the molded body was placed in a non-oxidizing nitrogen-containing atmosphere.
It is characterized by being fired at 1400°C to 1500°C. The present invention will be explained in detail below. In the present invention
It is necessary to suppress the amount of Si to 30 to 75% by weight. Since Si is nitrided to form a Si ₃ N ₄ bond, there is a minimum amount necessary to obtain satisfactory strength, and the value is 30% by weight. As the amount of Si increases
As the amount of Si ₃ N ₄ bond increases, the strength of the sintered body increases, but as the amount of Si increases, it becomes difficult to proceed with a uniform nitriding reaction within the compact. The reason for this is that nitriding silicon is a significantly exothermic reaction, and the heat of reaction causes the reaction to proceed rapidly, raising the temperature inside the compact, causing unreacted silicon to melt and close the pores. , inhibiting further nitridation or producing large pores and making the structure non-uniform. For this reason, for products with a large amount of Si, it is necessary to pay close attention to the heating rate of the furnace and the nitrogen supply rate.
Nitriding firing takes a long time. From the standpoint of producing an inexpensive silicon nitride sintered body, it is desirable to be able to fire it in a short time, and the amount of Si must be kept at 75% by weight or less. The addition of clay and kaolin in the present invention has the following effects. The first effect is that clay and kaolin have the effect of absorbing the heat generated by the nitrogenization of Si during the firing process and suppressing the unevenness of the structure of the fired body due to the melting of Si, resulting in a reduction in the firing process. This means that it has become possible to Dry compact _N2 gas or _N2 + _H2 gas,
1150 ~ in non-oxidizing nitrogen-containing atmosphere such as _NH3 gas
The temperature is gradually raised in the 1400°C range to allow the nitriding reaction to proceed, and then the temperature is raised to a maximum of 1500°C to complete the reaction. In this process, clay and kaolin are
It reacts with Si to form Si ₃ N ₄ , O' phase, X phase, etc. Clay and kaolin not only absorb reaction heat as fillers when Si is nitrided, but also
It also absorbs heat when Si ₃ N ₄ , O′ phase, and X phase are generated.
This suppresses the temperature rise caused by Si nitriding. Conventionally, clay has generally had the function of imparting plasticity to the base material, thereby facilitating the demolding, finishing, and drying processes in casting molding, and at the same time, making it possible to mold products with complex shapes. However, in this manufacturing method, we discovered that clay and kaolin have the ability to absorb the heat generated during nitriding of Si and suppress the non-uniformity of the structure of the fired product due to Si melting, thereby reducing the exothermic reaction of Si. This prevents the generation of pores due to the melting of unreacted Si and the generation of large pores that make the structure non-uniform, making it possible to perform nitriding sintering in a short time. Incidentally, the firing temperature is suitably 1400 to 1500°C.
At temperatures above 1500°C, the fired body foams and a homogeneous sintered body cannot be obtained. On the other hand, below 1400°C, the above-mentioned nitriding reaction is difficult to complete. The second effect of adding clay and kaolin is that as a result of imparting plasticity to the base material, demolding, finishing, and drying steps in casting molding are facilitated, thereby making it possible to mold products with complex shapes. When casting and molding a base material made only of Si, Si ₃ N ₄ and containing no clay or kaolin, it is important to dry the demolded molded product uniformly; even the slightest uneven drying will cause cracks. do. The reason for this is that the base material has almost no plasticity; for example, if the amount of moisture loss on the surface of the molded product is even slightly greater than that on the inside, the surface will shrink more than the inside, creating stress and causing cracks. Therefore, for such base materials, the demolded molded product must be immediately placed in a saturated humid temperature atmosphere to prevent surface drying.
First, it is necessary to equalize the moisture content in the molded body, and then gradually lower the humidity to uniformly reduce the moisture content in the molded body. For this reason, there is a drawback that careful attention is required for drying and the drying time is long. Furthermore, since clay-free base materials have low plasticity, they tend to crack when demolded, and there is also the problem that only simple shapes that can be easily demolded can be molded. Furthermore, since the molded product is fragile, great care must be taken when processing it. These problems can be solved by adding clay and kaolin to the Si, Si ₃ N ₄ matrix, and the lower limit of the amount added is 10% by weight. The plasticity of the matrix increases as the amount of clay and kaolin increases. In such a base material, even if some moisture difference occurs in the molded body during the drying process and stress is generated due to the difference in shrinkage, the stress is alleviated by plastic deformation of the base material, so cracks are less likely to occur during demolding. Therefore, no cracks occur even when the molded product is dried in heated air, which not only simplifies the drying process but also shortens the time. In addition, since the molded product can withstand deformation to a certain extent, it is possible to demold even more complex shapes without causing cracks, and the range of shapes that can be cast can be significantly expanded. Moreover, if only the amount of clay increases, the amount of fine particles in the base material will become too large, resulting in a decrease in the deposition rate and the occurrence of dry cracks. Because it is added, it is possible to reduce the deposition speed and prevent the occurrence of dry cracks without sacrificing plasticity. However, since kaolin has less plasticity than clay, the amount used should be kept as low as possible within the range that can solve the problems of slow inking speed and easy cracking. The amount of clay is preferably at least 20% by weight of the total weight, and the upper limit of the amount of kaolin is 30% by weight when the total amount of clay and kaolin is within the range of 25 to 50% by weight. If the total amount of clay and kaolin exceeds 50% by weight, the coefficient of thermal expansion of the fired base material will increase and the thermal shock resistance will decrease, which is not preferable. The third effect is to reduce raw material costs. This is because the price of clay and kaolin is
It is cheaper, less than 1/10 compared to Si and Si ₃ N ₄ . The fourth effect is that the grinding time can be shortened. In casting molding, there are 20 particles of 1μ or less in the base material.
% or more is necessary to obtain an excellent molded product, but since clay and kaolin are aggregates of fine particles of 1μ or less and easily disperse when stirred in water, it is necessary to finely pulverize Si and Si ₃ N ₄ . This eliminates the need for pulverization, making it possible to simplify and shorten the pulverization process. The next effect of clay addition is the effect of clay and kaolin addition on the properties of the sintered body. This can be reduced by the presence of 30% by weight or more of Si together with clay and kaolin. This seems to be because Si reduces the clay Al ₂ Si ₂ O ₅ (OH) ₄ during the firing process and promotes the reaction, producing Si ₃ N ₄ , O' phase, and X phase. When the compact is fired at 1500℃ in _N2 , the clay and kaolin-added base material undergoes reaction sintering.
It showed almost the same porosity and bending strength as the Si ₃ N ₄ base material, had no significant difference in thermal shock resistance, and even showed excellent oxidation resistance. The reason why such properties are obtained is that the sintered body is mainly composed of Si ₃ N ₄ crystals, which are the same as the reaction sintered products, and in addition, Si-Al
This is because only a small amount of O' phase and X phase consisting of -O-N atoms are added. Next, specific examples will be described in order to better understand the present invention. Example 1 Commercially available metallic silicon (average diameter 6μ, purity 98.5%)
42.3wt%, _Si3N4 ( _60mesh purity 98.5%) 17.7
Put each weight% into a ball mill, add ethanol and grind for 4 hours and 40 hours to obtain average diameters of 3 μ and 1.5 μ.
of powder was obtained. In the air after removing ethanol,
Si, Si ₃ N ₄ dried at 150℃, clay (Iga Frogme clay), and kaolin (North Korean kaolin) were mixed as shown in Table 1, water and sodium acrylate were added, and the mixture was stirred in a pot for 16 hours to reduce the moisture content to 55 % slurry was prepared. 10×
A 5 x 40 mm test piece was cast and its properties investigated, and the molding properties shown in Table 1 were obtained. It is observed that as the amount of kaolin is gradually increased compared to the amount of clay, the inking rate increases without decreasing the plasticity coefficient.

[Table] Example 2 The sample of Example 1 was placed in an alumina crucible and fired in a Tammann furnace. After creating a N ₂ atmosphere in the furnace by vacuum displacement, the temperature was raised at a rate of 400°C/hr up to 1150°C, and at a rate of 100°C/hr from 1150 to 1350°C to advance the nitriding reaction. The temperature was further increased to 1500°C at a rate of 400°C/hr, and the current was then turned off and allowed to cool. The properties of the sintered body are as follows.

[Table] As the amount of kaolin added increases, the porosity and strength remain constant within the range, but the coefficient of thermal expansion slightly increases. Also, by adding clay and kaolin, α, β
In addition to Si ₃ N ₄ , the formation of O' phase and X phase was observed. Example 3 The samples shown in Table 2 were sintered to produce sintered bodies in the same manner as in Example 2. The properties of the sintered body are as follows.

[Table] In the case of this example, when clay and kaolin are added, in addition to α and βSi ₃ N ₄ , O' phase and X phase are observed to be formed. Compared to the reaction sintered base with 0% clay and kaolin added, when kaolin and clay are added, the porosity, strength, and thermal expansion coefficient vary within the range up to 50%, and there is no major difference, but when the added amount is 60%. When straining the weight%, the coefficient of thermal expansion becomes significantly large,
Undesirable. As described above, the present invention is based on 30 to 75% by weight of Si and clay.
A mixed powder consisting of 20 to 45% by weight of kaolin, 5 to 30% by weight of kaolin for a total of 25 to 50% by weight, and the balance of Si ₃ N ₄ was molded, and the molded body was heated in a non-oxidizing nitrogen-containing atmosphere for 1400 min. Since it is fired at temperatures between ℃ and 1500℃, it has various advantages such as easier molding due to improved plasticity, shorter nitriding process, lower cost, and simpler crushing process without impairing the properties of the sintered body. Alternatively, if only the amount of clay increases, the amount of fine particles in the base material will become too large, resulting in a decrease in the deposition rate and the occurrence of dry cracks. Because of this addition, it is possible to reduce the deposition rate and prevent the occurrence of drying cracks without sacrificing the plasticity, making it possible to further improve the yield, making it possible to produce silicon nitride sintered bodies at low cost and in a short time. The purpose of manufacturing can be achieved.

Claims (1)

[Claims] 1 A mixed powder consisting of 30 to 75% by weight of Si, 20 to 45% by weight of clay, and 5 to 30% by weight of kaolin in a total amount of 25 to 50% by weight, with the balance being Si ₃ N ₄ is molded, and the molded body is 1400℃~1500℃ in non-oxidizing nitrogen atmosphere
1. A method for producing a silicon nitride sintered body, the method comprising: firing a silicon nitride sintered body.