JPH0224787B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a silicon carbide sintered body suitable for engineering ceramics. Silicon carbide has high stability at high temperatures, excellent wear resistance, high thermal conductivity, low coefficient of thermal expansion, and is a chemically stable material with excellent oxidation resistance. It is seen as promising for use in structural materials, heat engines, heat exchangers, and hot sliders. Since silicon carbide is a compound with strong covalent bonding properties, it is difficult to obtain a high-density sintered body using silicon carbide alone. For this reason, conventionally, in order to improve the sinterability, boron, aluminum, beryllium, or their nitrides or carbides were added as a sintering aid singly or in combination, and the reaction sintering method was used. , pressureless sintering method, hot press, or
Sintering has been carried out by applying the HIP method. However, in the reactive sintering method, unreacted silicon remains, resulting in a significant decrease in strength at high temperatures.In addition, in the pressureless sintering method, hot press, HIP, etc. using the above additives, unreacted silicon remains in the silicon carbide. There has been a problem in that the high temperature strength and creep resistance of the sintered body are extremely adversely affected by the polarization of additives or the formation of brittle phases at grain boundaries. In order to solve the problems in the conventional sintering of silicon carbide, the present invention aims to clean the grain boundaries without adding a sintering aid and achieves densification sintering of silicon carbide. To obtain a high-density, high-strength silicon carbide sintered body whose density is 93% or more of the theoretical density and whose three-point bending strength at 1400°C is 60 Kg/mm ² or more. The purpose is to Further, the present invention involves coating the silicon carbide surface with an aluminum nitrogen compound such as aluminum nitride or aluminum oxynitride in a nitrogen atmosphere pressureless sintering method, and controlling the nitrogen partial pressure during the sintering process. This method was completed based on the knowledge that shrinkage sintering of silicon carbide can be promoted by suppressing non-shrinkage initial sintering and reducing nitrogen partial pressure in a high temperature range. In the present invention, the nitrogen compound film on aluminum controls the nitrogen pressure and suppresses the surface diffusion and evaporation condensation of silicon carbide up to a high temperature range.
In addition to maintaining the sintering driving force in the sintering temperature range of ℃ to 2350℃ to obtain a dense sintered body, the nitride film is gradually decomposed by lowering the nitrogen partial pressure, and the decomposed Al is carbonized. It also has the effect of reducing harmful residual grain boundary phases by solid solution in silicon particles. but,
Since the generation of a large amount of decomposition vapor inhibits sintering, the film needs to be extremely thin. Usually, AlCl ₃ , AlI ₃ ,
A so-called chemical vapor phase reaction method is applied in which an aluminum compound such as (NH ₄ ) ₃ AlF ₆ or (CH ₃ ) ₆ Al ₂ is reacted in NH ₃ or N ₂ gas to form AlN on the surface of silicon carbide. In this case, if the concentration of the reaction gas is too high, significant agglomeration of grains and generation of AlN powder will occur, so it is necessary to use a reaction gas at a low concentration in order to form a uniform film. When using N ₂ gas, the reaction will not proceed unless H ₂ gas is mixed with the flow. Also, if there is a large amount of water in NH ₃ gas,
It is desirable that the amount of water in the NH ₃ gas is 10 ppm or less since the formation of a AlN film is inhibited. Furthermore, in order to produce a uniform film, it is desirable that the silicon carbide powder be stirred during the reaction, and it is effective to use a sample rolling device in a high temperature furnace or a fluidized bed.
When using AlCl ₃ and NH ₃ , uniform film formation requires
1000℃~1150℃ is desirable. In addition, as a method for coating silicon carbide powder with such nitrides, a film of a metal alone or a compound such as an oxide or halide that is easily converted into nitride in a nitrogen atmosphere is provided, and then a film of a compound such as an oxide or a halide is provided, and then the molded body is coated in a nitrogen atmosphere. A method of converting it into nitride during the sintering process can also be adopted. For example, using an aluminum compound that is soluble in a solvent such as ammonium aluminum sulfate, aluminum isopropoxide, aluminum nitrate, or aluminum sulfate, the aluminum compound dissolved in the solvent is coated on silicon carbide powder, and the aluminum compound is placed in a nitrogen atmosphere. A method is adopted in which the material is thermally decomposed inside to form a nitride film. Specifically, when using ammonium aluminum sulfate, ammonium hydrogen carbonate is added at the same time to synthesize a film of NH ₄ AlO (OH) HCO ₃ , which is then thermally decomposed at 180°C or higher to form an Al ₂ O ₃ film. shall be. When aluminum and propoxide are used, they are dissolved in ethanol and decomposed by heating at 150°C or higher. Through such thermal decomposition, silicon carbide powder is
Covered with an Al compound containing Al as the main component or an Al single film. The silicon carbide powder coated as described above is
After molding with the addition of carbonaceous additives in the same way as AlN-coated silicon carbide powder, the temperature was raised from room temperature to 1000°C using the same flow method as above, and kept at a temperature of 1200 to 1600°C for several hours, followed by N ₂ By nitriding in an air stream, the coated alumina is converted to nitride, which functions in the same manner as the aluminum nitride described above in the subsequent temperature raising process and sintering process. Next, in the present invention, the carbonaceous additive is used to reduce and remove oxide-based impurities that are present in a slight surplus and form a glass phase at high temperatures and deteriorate high-temperature strength, and also to functions as a trap. Furthermore, it has the effect of suppressing the decomposition of silicon carbide and promoting the decomposition of the nitride coating layer during sintering. For this purpose, in order to obtain fixed carbon with good dispersion, fine particles of carbon black, phenol resin, coal tar pitch, etc.
Acetylene black can be used. However, if used in a large amount, it will adversely affect the strength and bulk density of the sintered body, so it is desirable to add 2% by weight or less based on the total amount of silicon carbide. The silicon carbide surface-coated as described above is mixed with a carbonaceous additive using a solvent such as acetone, dibutyl phthalate, or polyvinyl alcohol by the method described below, and after drying, isostatic pressure is applied using a rubber press. Shape by applying pressure, etc. The first step of heating the compact is mainly to remove decomposed gas by passing nitrogen, hydrogen, argon, helium, ammonia gas, etc.
The temperature is raised to about 500℃ at a slow rate of ℃/h. From 500℃, the temperature increases at a rate of 100 to 200℃/h.
Start pressurizing the nitrogen atmosphere at 1000-1200°C. The nitrogen atmosphere is nitrogen alone or a mixed gas containing nitrogen and a small amount of at least one of He, H ₂ and hydrocarbons. When the nitrogen partial pressure is 3 atm or less, non-shrinkage initial sintering occurs, so the effect of suppressing the initial sintering of silicon carbide by the nitride film becomes weaker, and
When the pressure exceeds 10 atmospheres, no further effect is observed in maintaining the sintering driving force of silicon carbide. After nitrogen pressurization, the molded body is heated at a rate of 200 to 400° C./h, suppressing surface diffusion and evaporation condensation to maintain sintering driving force, and suppressing crystal growth of silicon carbide. When the temperature exceeds 2000°C and reaches the sintering temperature, vacuum is applied to lower the nitrogen partial pressure and replacement with an inert gas such as argon gas is performed to reduce decomposition of the nitride film. however,
When the temperature exceeds 2350℃, grain growth of silicon carbide occurs, and growth of plate-shaped crystals of 100 microns or more is observed.
This inhibits the densification of the sintered product and reduces its strength. Therefore, the nitride-coated silicon carbide powder compact in the present invention is sintered at a temperature in the range of 2000°C to 2350°C, particularly at 2100°C.
It is desirable that the temperature range is between 2200℃ and 2200℃. Specific embodiments and effects of the present invention will be described below based on Examples. [Example 1] As the silicon carbide powder, α-type silicon carbide fine powder having a silicon carbide content of 97%, a specific surface area of 10 m ² /g, and an average particle size of 0.5 μm was used. Approximately 30 g of this powder was coated with an AlN surface film in a vapor phase. The temperature of the sample part in the furnace is 1000℃ ~
A double structure was used to prevent AlCl ₃ gas and NH ₃ gas from coming into contact near the sample portion at _{1200°C, and solid AlCl 3 was} heated to 170°C and led to the reactor using H ₂ gas as a carrier. The AlCl ₃ transport pipe was kept warm to prevent precipitation during the process. NH ₃ gas was mixed with Ar gas and flowed at a flow rate of 9 c.c./min for NH ₃ gas and 3 c.c./min for AlCl ₃ for 1 hour while continuously rolling the sample in the furnace. The reaction treatment was carried out for 4 hours. _NH3
The gas used was purified to a moisture content of 10 ppm or less. Table 1 shows the weight increase rate due to coating treatment and the Al content determined by chemical analysis. Coating powder thus obtained100
A polyvinyl alcohol solution was added to 1 part by weight of carbon black having an average particle size of 200 Å or 4 parts by weight of novolak phenol resin, mixed for 2 hours in a pot mill, and dried under vacuum heat to remove the alcohol content. This raw material mixture was rubber pressed at a pressure of 1400 Kg/cm ² to obtain a molded product of 50 x 25 x 10 mm. The bulk density of the molded product was 54 to 56% TD. Next, the molded body was placed in a carbon-heating furnace where the atmosphere could be adjusted, using purified nitrogen gas with a dew point of -70°C and _O2 of 1 ppm or less, and heated to 550 °C in a _N2 gas stream containing 0.1% hydrogen gas.
60℃/h to ℃, 150℃/h from 550℃ to 1050℃
The temperature was increased to 1050℃ and _N2 atmosphere pressure was 10 atm.
The temperature was raised at 400°C/h. Thereafter, the atmosphere was replaced with Ar gas at a temperature of 1900 to 2100°C. Replacement was performed in the order of nitrogen depressurization, evacuation (10 ³ torr), and Ar gas replacement over a period of about 30 minutes, followed by firing at 2100° C. for 4 hours. Table 2 shows the processing conditions and the physical and chemical properties of the sintered body. For comparison, sample No. 1 is the result of sintering in the same manner using silicon carbide powder without coating treatment and adding 2% by weight of AlN fine powder with a purity of 99.6% and 1% by weight of carbon black as additives. It is. Samples No. 2 to No. 6 are the results when coating powder sample C was used, and sample No. 7 is the results when coating powder sample F was used. As is clear from Table 2, replacing nitrogen with Ar gas is effective at temperatures above 2000°C. In addition, sample No. 5,
Those with a residual Al content of 1% by weight or more, such as No. 7, show a decrease in strength.

【table】

【table】

[Example 2]

Using the powder sample C of Example 1, under the same conditions as Example 1, 1 part by weight of carbon black was added as an additive, and the N ₂ gas was replaced with Ar gas at a temperature of 2150°C.
Table 3 shows the results of sintering at 2400°C and the same temperature.

[Example 3]

As the silicon carbide powder, α-type silicon carbide fine powder having a silicon carbide content of 97%, a specific surface area of 10 m ² /g, and an average particle size of 0.5 μm was used. To 100 parts by weight of this silicon carbide powder, 15 parts by weight of aluminum isopropoxide was added as an ethanol solution, mixed in a pot mill for 4 hours,
It was dried by heating at 150°C. As a result, the silicon carbide powder showed a weight increase of 2.8% by weight.The Al content according to chemical analysis was 1.0% by weight. and 4 parts by weight of novolac phenol resin were mixed with dissolved acetone, mixed in a pot mill for 2 hours, and then dried in vacuum. This blend was rubber pressed at a pressure of 1400 kg/cm ² to obtain a molded article of 50 x 25 x 10 mm. The bulk density of the molded product was 50% TD. Next, the carbon heating element was heated in a nitrogen gas stream containing 0.1% _NH3 gas with a _H2O content of 10ppm or less for 50 minutes.
Increase temperature to 500℃ at ℃/h and from 500℃ to 1550℃
The temperature was increased at a rate of 100°C/h and held at 1550°C for 2 hours to perform reduction nitridation. Then _N2 atmosphere pressure is 10 atm and 300
The temperature was raised to 2100℃ at a rate of ℃/h, and at this temperature Example 1
Replace with Ar gas in the same way. Replacement took about 30 minutes, followed by sintering at 2100°C for 4 hours. The resulting sintered body has a bulk density of 96.8% TD.
Room temperature bending strength 62.0Kg/ with residual Al content 0.4% by weight
It was warm in ^mm2 . [Example 4] 35 parts by weight of an aluminum ammonium sulfate solution (0.2 mol) per 100 parts by weight of the same silicon carbide powder as in Example 3 and an ammonium hydrogen carbonate solution
20mol/ and mixed in a pot mill for 2 hours,
After drying by heating at 150°C, heat treatment was performed at 900°C in an Ar atmosphere. As a result of this treatment, the silicon carbide powder showed a weight increase of 3.1% by weight, and the Al content was 1.2% by weight.
The main component of the compound was alumina. The thus obtained coating powder was sintered in the same manner as in Example 3. The resulting sintered body had a bulk density of 96.2%, a TD residual Al content of 0.5% by weight, and a room temperature bending strength of 63.3 Kg/mm ² . [Example 5] Sample No. 4 of Example 1 and a sample coated using the same coating method as Examples 3 and 4 were coated with an atmospheric nitrogen pressure of 1
Sintering experiments were conducted between ~30 atm. We also conducted sintering experiments by changing the atmosphere. The results are shown in Table 4. As is clear from the above examples, by the method of the present invention, a silicon carbide sintered body having a target density of 93% or more of the theoretical density and a three-point bending strength of 60 kg/mm ² or more at 1400°C can be obtained. As engineering ceramics, it is possible to obtain materials that are sufficiently durable for various harsh purposes.

【table】

Claims (1)

[Claims] 1 A powder compact made by adding a small amount of carbonaceous additive to a silicon carbide powder having a film of simple aluminum or an aluminum compound that can be converted into an aluminum nitrogen compound or an aluminum nitrogen compound is heated to a nitrogen gas-containing non-oxidizing atmosphere of 3 atmospheres or more. 1. A method for sintering a silicon carbide powder compact, which comprises heating to at least 2000°C in a neutral atmosphere, and then sintering in an atmosphere with a reduced partial pressure of nitrogen gas.