JPH0225867B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a pressureless silicon carbide sintered body by sintering silicon carbide powder in a pressureless state. (Prior art) The conventional method for producing a pressureless silicon carbide sintered body includes the steps of weighing raw materials, suspending and dispersing, drying, molding, and sintering, as shown in JP-A No. 56-92167. . In the raw material weighing process, a predetermined amount of SiC powder, boron-containing additive, carbonaceous additive, etc. is weighed using a balance, etc. As the boron-containing additive, either boron powder or boron carbide powder is selected. Common. As carbon additives, various organic substances existing in a carbon state at the start of sintering such as resol type or novolak type phenolic resin, coal tar pitch, and polyphenylene, or pyrolytic carbon such as carbon black and acetylene black are used. . There is no established theory regarding the effects of boron and carbon, but according to Prochazka et al., who succeeded in high-density atmospheric pressure sintering of β-SiC for the first time using this auxiliary agent system, silicon carbide powder is difficult to sinter. The reason is that the ratio of grain boundary energy to surface energy (γ _gb /γ _sv ) is the thermodynamic limit value √
It is argued that this is because the grain boundary energy decreases and the surface energy increases due to the action of boron and carbon, so that it becomes less than this limit value and densification occurs thermodynamically. . In this case, if it is less than 0.2%, it will not be effective, and if it is added more than 0.3%, it will only become expensive and will not be very effective, and if it is too much, the strength of the obtained sintered body will decrease. Furthermore, regarding carbon, silicon carbide particles are always covered with a silica film at room temperature, and this silica film inhibits the self-sintering of silicon carbide powder. It is also considered to have the effect of increasing the sinterability between silicon particles. The amount added is preferably 0.1% to 1.0%; if it is too small, the above effect will not be achieved, and if it is too large, the strength of the sintered body will be reduced. In the suspension and dispersion step, the weighed raw materials are suspended and dispersed in a dispersion solvent using a dispersion device such as a ball mill. Acetone or ethanol is commonly used as a dispersion solvent. SiC is extremely hard with a Vickers hardness of 2800, so during dispersion and mixing, the containers and balls that form the dispersion device are subject to severe wear and tear. mixed into the suspension,
This causes problems in the sintering process. Therefore,
The portion of the dispersion device that comes into contact with the suspension is generally made of superhard material (tungsten carbide). The drying process is a process in which the dispersion solvent in the suspension is removed by drying and finally a mixed powder in which the raw ingredients are uniformly mixed is obtained. It is common practice to obtain a dry powder by spraying a suspension into a stream.During this process, the material (from suspension to dry powder) is ,100℃
heated to temperatures between 200°C and 200°C. The molding process can be industrially divided into the following two types. (1) Hydrostatic press molding (CIP molding) (2) Injection molding CIP molding is a molding method used when molding parts with relatively simple shapes, and the dry powder obtained is filled into a rubber mold. This is a method of applying hydrostatic pressure of 1 to 2 t/cm ² to this and molding it into the desired shape. Injection molding is a molding method used to mold parts with complex shapes, and involves kneading and pulverizing the resulting mixed powder and a binder component consisting of thermoplastic plastic, wax, etc. This is a method of molding this using a method similar to injection molding, which is commonly used for plastics.
The molded body thus obtained is heat treated to decompose and remove the binder component before being sintered. In either molding method, the theoretical density ratio of the powder compact is 59 wt% or less. In the sintering process, the molded body obtained in is heated at a rate of 5°C per minute up to 1650°C in a non-oxidizing atmosphere such as an Ar gas stream or a vacuum.
After holding for 30 minutes, the temperature was raised again at a rate of 5°C per minute, and when it reached 2100°C, it was held for 30 minutes, followed by firing and densification using a cooling temperature program. For structural ceramics, in order to obtain sufficient mechanical strength, it is necessary to have a densification of 95% or more of the theoretical density. Among the above firing temperature conditions, the intention of holding at a temperature of 1650℃ for 30 minutes is to start densification (approximately 1750℃ or higher)
Previously, (1) The added boron was uniformly diffused onto the surface of the silicon carbide powder. (2) Promote reduction of silica on the silicon carbide surface by carbon. It is. (Problems to be Solved by the Invention) However, in such a conventional method for producing a pressureless silicon carbide sintered body, tungsten mixed in due to wear and tear of the cemented carbide member forming the suspension device, β-WB or
In addition to producing a compound called W ₂ B ₅ and wasting the boron added as a sintering aid, the density of the powder compact was not sufficient. There was a problem in that it was not possible to obtain a sintered body with a theoretical density ratio of 90 wt% or more. (Means for Solving the Problems) This invention was made by focusing on such conventional problems, and is made by adding a sintering aid powder containing a boron component and a carbon component to silicon carbide powder. A pressureless silicon carbide sintered body that is produced by suspending and mixing raw material powder in a dispersion medium using a dispersion device, removing the dispersion medium, molding the mixture, and then sintering it in a non-oxidizing atmosphere. In the manufacturing method, the above problems are solved by reducing the amount of tungsten mixed in from the dispersion device to 100 ppm or less to prevent wastage of boron, and by increasing the theoretical density ratio of the powder compact before sintering to 60 wt% or more. The purpose is to Furthermore, in the sintering step, it is preferable that the molded body is heated at a temperature equal to or lower than the sintering shrinkage starting temperature for a time longer than the time calculated by the following formula before the sintering shrinkage starts. t ₀ =83.6×(ρ _B γ _B /ρ _G W _B ) ^2/3 /exp(−31900/T
)...(1) t: Heating holding time (hours) ρ _B : Density of added boron (g/cm ³ ) γ _B : Average particle size of added boron (cm) ρ _G : Density of powder compact (g/cm 3 ) cm ³ ) W _B : Addition amount of boron (weight %) T : Heating temperature (〓) (Effect) 1 Effect by reducing the amount of tungsten mixed The added boron and the mixed tungsten combine to form β-WB or W ₂ A compound called _B5 is generated and the boron added as a sintering aid is wasted (confirmed by X-ray diffraction). Therefore, reducing the amount of tungsten mixed in will reduce the amount of boron wasted, and as a result, this implementation Even with a low boron addition amount of less than 0.3wt% in the example,
It is believed that a sufficiently dense sintered body can be obtained. 2. Effects of setting the theoretical density ratio of the compact to 60% or more Distance between boron powders in the compact x ₀ (1
(assuming it exists as secondary particles) is expressed by the following formula, and as a matter of course, the smaller the amount of boron added, the longer the distance becomes. However, the symbol content is the same as in equation (1).By increasing the density of the compact, the value of ρ in equation (2) increases, and as a result, X ₀
The value of becomes smaller. Therefore, the time t ₀ required for boron diffusion is shortened. Furthermore, since the molding pressure (hydrostatic pressure) was increased when producing the powder compact, secondary aggregates of the raw material components that were not destroyed by conventional pressure are thought to be destroyed, making them easier to diffuse. 3 Effect of heat treatment before sintering shrinkage In pressureless silicon carbide sintering, the added boron must be uniformly diffused on the silicon carbide powder surface in the powder compact before sintering shrinkage starts. However, weight is considered to be an important factor in uniformly densifying the material and obtaining a sintered body with a high density. The distance between the boron powders in the compact is expressed by equation (2) as described above. On the other hand, if the surface diffusion coefficient of boron in the molded body is D, the diffusion distance x when the temperature is maintained at T (〓) for t (seconds) is approximated by the following equation. x≒√ ...(3) ∴t≒x ² /D ...(3)' Therefore, the smaller the amount of boron added, the longer the heat treatment time for uniform diffusion. From the above, before the start of sintering shrinkage, the temperature is below the sintering shrinkage starting temperature for boron diffusion, but higher than 200°C below the sintering shrinkage starting temperature, preferably.
When heat treated at 100℃ below the sintering shrinkage start temperature, boron diffusion is promoted, and as a result, even with a low boron content of less than 0.3wt% (0.2wt% in the example), sufficiently dense sintering is possible. I think it will give you a solid body. Equation (1) expresses the surface diffusion coefficient D of boron in the compact as follows: D≒5.46×10 ^-4 exp{−63.34×10 ³ /1.986×T}...(4
), substituting equations (2) and (4) into equation (3)′, and further multiplying by the correction coefficient obtained from the results of this experiment. 4. Effects of freeze-drying method Because of its drying principle, freeze-drying method can reduce the dispersion solvent without impairing the dispersion uniformity of the raw material components in the suspension dispersion, compared to the conventional heating drying method using a spray dryer etc. can be removed by drying.
Therefore, the state of dispersion of the sintering aid (particularly boron) in the dry powder is considered to be more uniform than in a spray dryer or the like. Therefore, if sintering is performed using the same sintering temperature program, the distribution of boron due to diffusion will be more uniform. In addition, according to this method, the atmosphere during drying of the suspension is low temperature and low oxygen partial pressure, so that the above-mentioned
We believe that the waste of boron oxidation can also be reduced.
On the other hand, when drying with a spray dryer, the atmosphere is at normal pressure and the temperature is low.
Since the temperature is as high as 180°C, the added boron is oxidized and wasted, so it is thought that it will be difficult to increase the density by adding less than 0.3 wt% boron. (Example) As shown in Table 1, amount of boron added, amount of tungsten mixed, theoretical density ratio of powder compact, slurry drying conditions (temperature and oxygen partial pressure of drying atmosphere), oxygen partial pressure of sintering atmosphere, boron Pressure-pressure sintered silicon carbide sintered bodies were obtained by changing the diffusion treatment conditions (temperature and holding time), and the theoretical density ratio of each sintered body was determined from the measurement of the weight and volume. The results are also shown in Table 1. Firing was performed in the following steps. Weighing raw materials Suspending the raw powder in a dispersion solvent using a ball mill to create a slurry Drying the slurry Filling the dry powder into a rubber mold Press molding using hydrostatic pressure Pre-sintering heat treatment Sintering In the process, silicon carbide powder is used as silicon carbide powder. β-Random (Ultra Fine), boron powder from Cerac Co., Ltd. was used as the boron component, and resol type phenol resin PR50404 from Sumitomo Duretsu Co., Ltd. was used as the carbon component. In the process, a total of 100 g of silicon carbide powder, boron powder, and the above-mentioned phenol resin (the above are referred to as raw materials) and 200 c.c. of 1,4-dioxane as a dispersion medium are placed in a pot of a ball mill with a capacity of 0.7, and the pot is heated at 70 rpm. and mixed for 25 hours. Ball is 10mm
250 pieces of φ were used. In order to change the amount of tungsten mixed in, two types of balls, cemented carbide balls (tungsten carbide) and silicon carbide balls, were mixed and used, and the amount of tungsten mixed was adjusted by the mixing ratio. Under the above conditions, we determined the relationship between the mixing ratio of cemented carbide balls and the amount of tungsten mixed in through preliminary experiments, and found that it was 22 ppm at 10%, 40 ppm at 15%, and 40 ppm at 20%.
We obtained results of 40ppm, 69ppm for 25%, and 109ppm for 30%, so based on these results, we set the predetermined tungsten content. In the drying process, Examples 13, 14, 15, 25,
Freeze-drying was used for 26 and 27, and vacuum heating drying was used for the other examples. 4-dioxane) was removed by drying. The degree of vacuum at this time is approximately 1 mmHg (oxygen partial pressure: 0.2 mmHg). In the freeze-drying method, the slurry is once frozen, the frozen product is placed under reduced pressure, and a dispersion solvent (1,4-dioxane) is added.
was dried using the freeze-drying method of sublimation drying. The drying atmosphere in this case has a temperature of 10°C and a degree of vacuum of 13 mmHg (oxygen partial pressure: 2.7 mmHg). In the process, inside a spherical rubber mold with a diameter of 30 mm,
After filling 10g of dry powder after completing the process,
Sealed. In the pressing process with a hydrostatic pressure of , the theoretical density ratio of the molded body was adjusted by the magnitude of the hydrostatic pressure.
In advance, we determined the relationship between hydrostatic pressure and the theoretical density ratio of the compact through preliminary experiments, and found that the hydrostatic pressure for forming
At 1 t/cm ² it is 56.5%, at 2 t/cm ² it is 59.1%, at 3 t/cm ² it is 60.3%, and at 4 t/cm ² it is 61%, and the theoretical density ratio of the compact was adjusted using this hydrostatic pressure. . In the pre-sintering process, the compact is placed in a high-frequency induction heating sintering furnace at a vacuum level of 5×10 ^-3
In an atmosphere of mmHg, the temperature was raised at a rate of 5°C/min to 1650°C, and after being held at 1650°C for a predetermined time,
The temperature has dropped. In addition, in preliminary experiments, the sintering shrinkage starting temperature was
It was confirmed that the temperature was 1800℃. In this example, t ₀ obtained by equation (1) is WB
= 0.25%, t ₀ will be 3.1 hours. Therefore, in Example 1, the retention time (t) is 0.5 hours and t/t ₀ is 16.1%. In the sintering process, the degree of vacuum during sintering is controlled by controlling the oil temperature of the oil diffusion pump in the vacuum exhaust system (oil diffusion pump - mechanical booster oil rotary vacuum pump) attached to the high frequency induction heating sintering furnace. adjusted. Sintering temperature is 2100℃ for 30 minutes
The temperature was raised at a rate of 5°C/min. Comparative Examples 1 to 6 Sintered bodies were prepared in the same manner as in the example except that the amount of tungsten mixed was increased, and the bending strength was measured. The results are shown in Table 2. Comparative Example 7 A sintered body was prepared in the same manner as in Example except that the theoretical density ratio of the molded body was 59%, and the bending strength was measured. The results are shown in Table 2.

【table】

【table】

[Table] As can be seen from the data in Tables 1 and 2, the amount of tungsten mixed in the mixed powder after drying is
When it exceeded 100 ppm, the theoretical density ratio of the obtained sintered body became less than 90%. Furthermore, it is recognized that the smaller the amount of tungsten mixed, the higher the theoretical density ratio achieved and the higher the bending strength. In addition, the theoretical density ratio of the powder compact is 60%.
Even if the amount of tungsten mixed was less than 100 ppm, the theoretical density ratio of the obtained sintered body was less than 90%. Furthermore, it is recognized that the higher the theoretical density ratio of the powder compact, the higher the theoretical density ratio of the sintered body, and the higher the bending strength. Furthermore, if the compact is treated for a long time at a temperature below the sintering shrinkage start temperature, the bending strength increases, and if a freeze-drying method is used to remove the dispersion medium from the slurry, the bending strength can be further improved. Recognize. (Effects) As described above, in the present invention, in the method for producing a pressureless silicon carbide sintered body, the amount of tungsten mixed into the raw material powder is controlled to 100 ppm or less, and the molded body is When producing the sintered body, the theoretical density ratio was set to 60% by weight or more, making it possible to obtain a sintered body with high strength. Furthermore, before sintering the compact, a sintered compact with even higher strength can be obtained by heat-treating the compact at a temperature below the sintering shrinkage start temperature for a time longer than that calculated by the above formula (1). Furthermore, if a freeze-drying method is used to remove the dispersion medium from a slurry in which raw material powder is dispersed in the dispersion medium, a sintered body with even higher strength can be obtained.

Claims (1)

[Scope of Claims] 1. Silicon carbide powder and at least 0.2wt% or more 0.3wt
After suspending and mixing raw material powder consisting of a boron component of less than % and a sintering aid containing a carbon component of 0.1 wt% or more and 1.0 wt% or less in a dispersion medium using a dispersion device, the dispersion medium is In a method for producing a pressureless silicon carbide sintered body in which the removed mixture is molded and then sintered in a non-oxidizing atmosphere, when the raw material component is suspended in a dispersion medium, the raw material component is suspended in the raw material component. A method for producing a pressureless silicon carbide sintered body, which comprises controlling the amount of tungsten mixed in to 100 ppm or less, and forming the obtained mixture so that the theoretical density ratio is 60% by weight or more. 2. The sintering step is characterized in that, before the sintering shrinkage starts, the molded body is heated at a temperature below the sintering shrinkage starting temperature for a time longer than the time calculated by the following formula. The method for producing the pressureless silicon carbide sintered body described above. t ₀ =83.6×(ρ _B γ _B /ρ _G W _B ) ^2/3 /exp(−31900/T
) However, t: Heating time (hours) ρ _B : Density of added boron (g/cm ³ ) γ _B : Average particle size (cm) ρ _G : Density of powder compact (g/cm ³ ) W _B : Added amount of boron (wt%) T: Heating temperature (〓) 3. Claim 1 or 2, characterized in that a freeze-drying method is used when removing the dispersion medium from the suspension mixture. The method for producing the pressureless silicon carbide sintered body described above.