JPS6213309B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a heat-resistant ceramic sintered body, particularly a silicon nitride sintered body. Among heat-resistant ceramics, silicon nitride and silicon carbide have particularly excellent heat resistance, so they are promising materials for the structures and parts of high-temperature gas turbines, diesel engines, etc., and are of great interest. be. When heat-resistant ceramic sintered bodies are used for these structural materials, chemical and physical stability at high temperatures is required. Among these, high mechanical properties at high temperatures are desired. However, silicon nitride and silicon carbide are both covalent compounds and are considered to be difficult to sinter. Therefore, it is difficult to sinter silicon nitride and silicon carbide alone, and by adding a sintering aid of several percent to several tens of percent, a low melting point compound is formed and sintering is promoted. I'm letting you do it. For example, in the case of silicon nitride, it is used as a sintering aid.
By adding 5 to 20% of MgO, Al ₂ O ₃ , Y ₂ O ₃ and the like and performing hot pressing, a sintered body with a density close to the theoretical density can be obtained. However, the sintered body thus obtained has insufficient strength at high temperatures. That is, while MgO, Al ₂ O ₃ or Y ₂ O ₃ added as a sintering aid has the advantage of forming a low melting point compound and accelerating sintering as described above, on the other hand, this low melting point compound As a result, the strength at high temperatures decreases. For this reason, studies have been made to minimize the type and amount of sintering aids used during the production of silicon nitride and silicon carbide sintered bodies, but the drawback of reduced strength at high temperatures has not yet been resolved. The current situation is that this has not been done. Furthermore, as an industrially advantageous method for producing silicon nitride powder, a method is known in which a mixed powder of silicon oxide and carbon is heat-treated in nitrogen gas. However, with this method, the purity of the silicon nitride powder produced is low and only a mixed powder of α-type and β-type silicon nitride can be obtained. Furthermore, silicon carbide as an impurity,
Silicon oxynitride may also be present. There are also many problems with the particle size, such as the inability to obtain fine powder due to the high processing temperature. In view of the above-mentioned points, the present inventors have conducted intensive studies to obtain a silicon nitride sintered body having excellent heat resistance and high-temperature strength without using a sintering aid, and have arrived at the present invention. That is, the present invention provides a method for producing a silicon nitride sintered body that can eliminate all of the above-mentioned drawbacks and problems in addition to the characteristics of silicon nitride sintered bodies produced by conventional methods. . The present invention will be described in detail below. According to the present invention, for example, silicon tetrachloride and ammonia are reacted to obtain silicon nitride powder using a gas phase synthesis method. This powder has high purity and a fine particle size of 0.01 to 0.1μ. This fine powder has an amorphous structure and has a
It transforms into the α form at 1650°C and into the β form above 1650°C.
This fine powder compact having an amorphous structure is sintered in two stages: at a temperature at which it transforms into the α-type, and then at a temperature at which the α-type transforms into the β-type. The present invention is characterized in that it provides a silicon nitride sintered body that does not contain carbon dioxide, has high density, and does not lose strength at high temperatures. Silicon nitride decomposes into silicon and nitrogen at high temperatures. In order to suppress this decomposition, it goes without saying that the sintering atmosphere should be kept in a non-oxidizing atmosphere with a partial pressure of nitrogen gas. In the method of the present invention, when hot pressing is performed, the sintering time for each stage is approximately 10 to 20 minutes when the press shaft stops moving, making it possible to complete the two stages of sintering within one hour. . The present invention will be explained below using examples. Example 1 Ammonia gas (purity 99.99%) and silicon tetrachloride (purity 99.99%) using nitrogen gas (purity 99.99%) as a carrier gas were introduced into a reaction tube heated to 1050°C. As a result, white powder was deposited at the bottom of the reaction tube. When this powder was subjected to X-ray diffraction, it was found to be amorphous with no significant peaks, and it was confirmed that it was amorphous silicon nitride based on the results of infrared absorption analysis and silicon and nitrogen content analysis. did. Next, this powder was hot pressed using a graphite mold. First, the inside of the furnace was set to a vacuum of 10 ^-3 Torr, and nitrogen gas was introduced to adjust the vacuum to 50 Torr.The furnace temperature was then raised to 1450℃, and the pressure was 30℃ at a pressure of 250Kg/ ^cm2 .
Hot press was performed for a minute. Next, increase the amount of nitrogen gas introduced and adjust it to 250 Torr, then raise the furnace temperature to 1700℃.
Then, a second hot press was performed for 30 minutes at a pressure of 250 kg/cm ² . Next, after cooling the furnace temperature, wait until the mold is sufficiently cooled, and then remove the mold from the furnace.
The hot-pressed sintered body was taken out from the mold using a hand press. When the density of the silicon nitride sintered body thus obtained was measured, the density was 2.86 g/cm ³ , which was 90% of the theoretical density. Furthermore, when we conducted a three-point bending test at room temperature and 1200℃, the bending strength was 95Kg/
cm ² and 91 Kg/cm ² , demonstrating that a sintered body with no decrease in strength at high temperatures can be obtained. Example 2 Ammonia gas (purity 99.99%) and silicon tetrachloride (purity 99.99%) using nitrogen gas (purity 99.99%) as carrier gas were introduced into a reaction tube heated to 1200°C. As a result, white powder was deposited at the bottom of the reaction tube. This powder was amorphous silicon nitride as in Example 1. This powder was hot pressed using a graphite mold as follows. First, the inside of the furnace was set to a vacuum of 10 ^-3 Torr, and then the nitrogen gas pressure was set to 1 atm. The furnace temperature was raised to 1600℃, and the temperature was increased to 250℃.
Hot pressing was carried out at a pressure of Kg/cm ² for 15 minutes, then the furnace temperature was raised to 1800° C., and the pressure was maintained at 250 Kg/cm ² for 10 minutes. Next, the furnace was cooled and the sintered body was taken out. The density of the obtained sintered body is 3.10g/
The bending strengths at room temperature and 1200° C. were almost the same at 103 Kg/cm ² and 98 Kg/cm ² at 96% of the theoretical density in cm ³ . Example 3 Ammonia gas (purity 99.99%) and silicon tetrachloride (purity 99.99%) using nitrogen gas (purity 99.99%) as carrier gas were introduced into a reaction tube heated to 1100°C. As a result, white powder was deposited at the bottom of the reaction tube. When this powder was subjected to X-ray diffraction, it was confirmed that it was amorphous. Infrared absorption analysis and analysis of silicon and nitrogen confirmed that it was amorphous silicon nitride. Next, this powder was sintered using a hot press method and an atmospheric pressure sintering method, and the obtained sintered bodies were evaluated, and the results shown in Table 1 were obtained. The hot press method uses a graphite mold to first create a vacuum of 10 ^-3 Torr in the furnace, then introduce nitrogen gas to create a pressure of 1 atm, then raise the temperature to 1500°C, and
Hot pressing was carried out for 30 minutes at a pressure of Kg/cm ² .
Next, the pressure of nitrogen gas was set to 2 atmospheres, the temperature was raised to 1750°C, and a second hot press was performed at a pressure of 150 kg/cm ² . In addition, in the pressureless sintering method, the inside of the furnace is first vacuumed to 10 ^-3 Torr, nitrogen gas is introduced to bring the temperature to 1 atm, and then the temperature is increased to 155°C.
After raising the temperature to
The temperature was raised to 1750°C. 【table】

Claims (1)

[Claims] 1. A molded silicon nitride fine powder having an amorphous structure obtained by a gas phase synthesis method is held in a non-oxidizing atmosphere having a partial pressure of nitrogen gas at a temperature range of 1450 to 1650°C for 0.1 to 1 hour. Then, sintering is performed while transforming into an α-type structure, followed by
A method for producing a silicon nitride sintered body, characterized in that sintering is carried out while holding the body for ~1 hour to transform it into a β-type structure.