JPS6141870B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a ceramic sintered body of non-oxide, ie, nitride or carbide, which has high high-temperature strength and high density. Recently, the development of ceramic materials, especially ceramics as heat-resistant materials, has been actively carried out.
Among them, covalent compounds that are heat-resistant substances that are stable at high temperatures, particularly silicon nitride (Si ₃ N ₄ ) and silicon carbide (SiC), are known to be excellent materials. Ceramics are generally used by molding and sintering raw ceramic powder.
In the case of Si ₃ N ₄ , SiC, etc., unlike general oxide ceramics, they are difficult to sinter, so even if only a single composition, for example Si ₃ N ₄ powder, is sintered, a dense sintered body can be obtained. That is difficult. Therefore, in the case of Si ₃ N ₄ powder, MgO,
Sintering is carried out by adding oxide powders such as Al ₂ O ₃ , Y ₂ O ₃ , CeO ₂ , BeO, etc. as sintering aids. The conventional method of press forming using powder containing a sintering aid and heating and sintering it in vacuum or normal pressure has a low sintering cost and can be used industrially, but Si ₃ In the case of N ₄ , the pores remain even after sintering, so it is not possible to obtain a high-density sintered body using this method. On the other hand, the hot press method, in which sintering is carried out under pressure at high temperatures, makes it possible to obtain a denser sintered body, but this sintered body has the disadvantage that its strength decreases at high temperatures; The cost will also be higher. This decrease in strength at high temperatures is thought to be due to the formation of low melting point substances at the Si ₃ N ₄ powder interface due to the addition of the sintering aid, and is unavoidable when a sintering aid is used. Furthermore, the mixing ratio of the sintering aid may be reduced, or the powder may be sintered in a high-pressure gas atmosphere without the addition of a sintering aid, or the powder may be pulverized by instantaneously applying high pressure using explosive molding, etc., and then sintered. Methods such as sintering have been attempted, but all of these methods have the drawbacks of high sintering costs and a reduction in high-temperature strength, and have not been successful as industrial methods. The above problem also applies to SiC. The present inventors added various nitrides to amorphous and crystalline Si ₃ N ₄ and SiC powders, press-molded them, and then
Although the test was repeated by performing sintering under various atmospheres such as vacuum, normal pressure, or high pressure, and by changing the sintering conditions such as temperature, it was not possible to obtain a sintered body with good high-temperature properties. However, as a result of a detailed inspection of the sintered body, the cause was found to be, for example, "Powder and Powder Metallurgy" Volume 18,
It can be assumed that this is due to the surface oxidation phenomenon of nitride powder as shown in the paper (authored by Akio Hara) published in No. 8, page 338. The present inventors have conducted various studies on Si ₃ N ₄ powder and SiC powder in order to obtain a dense sintered body with high high-temperature strength, and as a result, they have arrived at the present invention. That is, the present invention uses one or more metal elements having an oxide standard free energy of formation at 1000°C of -150 Kcal/g・molO ₂ or less as a solid reducing agent, and further uses one or more metal nitrides other than silicon as a solid reducing agent. 2
Si ₃ N ₄ or SiC with seeds or more added as sintering aids
Prior to sintering, the powder compact is placed in a reduced pressure atmosphere of a reducing gas such as H ₂ or CO to remove the oxidized layer caused by the oxidation phenomenon of Si ₃ N ₄ or SiC. This is a method in which sintering is performed after performing an activation treatment such as holding the material in a reduced pressure atmosphere and a vacuum atmosphere alternately one or more times, or holding it in a plasma atmosphere of the above-mentioned reducing gas. This eliminates the drawback of the sintered body properties, that is, the deterioration in high temperature properties, and provides a sintered body with high density and excellent high temperature strength. It is thought that the surface of the powder from which the oxide layer has been removed becomes highly activated and promotes sintering. The present invention will be explained in detail below using SiC as an example. Hydroxide is generated on the SiC surface due to the oxidation phenomenon described above, and SiO ₂
is thought to be generated. And this _SiO2
When _a high vacuum condition of less than 10 ^-6 to ^{10 -7} atm _is applied to remove _the I can do it. However, in a molded body, these gases generated inside the molded body come out through pores of micron size or less. Under high vacuum, the mean free path of gas is large, so it is difficult for gas to come out. Therefore, there is a drawback that it requires a long time under high vacuum. In addition, when using a gas such as H ₂ or CO under normal pressure (1 atm), the following formula SiO ₂ +H ₂ →SiO↑+H ₂ O↑ SiO ₂ +CO→SiO↑+CO ₂ ↑ becomes SiO ₂ Can be removed. The free energy change of this reaction is △G=△G ⁰ +RT ln K K in each reaction is P _H2O P _SiO /P _H2 , P _CO2 P _SiO
/P _CO , it is thought that the progress of these reactions is controlled by the gas partial pressure of each reaction system. Therefore, P _H2 and P _CO in the reaction system are increased, and P _H2O ,
Reducing P _CO2 and P _SiO will advance the reaction. However, inside the pore, input H ₂ and CO gas
Interdiffusion occurs between the generated SiO, H ₂ O, and CO ₂ gases. Since the gas diffusion coefficient D has the following relationship: D∝1/P (P: pressure), low pressure is preferable this time. That is, since the reduction reaction rate within the pores is affected by both the above-mentioned mean free path of the gas and the gas diffusion coefficient, it becomes fastest in a certain pressure range. Therefore, the reaction at 1 atm has the disadvantage that it is slower than the reaction under reduced pressure where the pressure is higher. As a result of repeated studies to eliminate these drawbacks, we found that metal elements whose oxide standard free energy of formation is less than the standard free energy of formation of SiO ₂ at each temperature, that is, the oxide standard free energy of formation at 1000℃ are -150Kcal. /g・molO ₂ or less using one or more metal elements as a solid reducing agent and one or more metal nitrides other than silicon as a sintering aid to form Si ₃ N ₄ or SiC powder. It has been discovered that the above-mentioned drawbacks can be overcome by adding it. The standard free energy of formation of the oxide used in the present invention at 1000℃ is -150Kcal/g・mo
Metal elements with _1O2 or less include Si, Ti, Ce, Al,
Examples include Mg, Be, Zr, and Ba. These metal elements remove SiO ₂ according to the following formula: SiO ₂ +M=SiO↑+MO (M: added metal element). MO in the above formula may become a gas and be removed, or it may become a solid and remain inside the molded body. The high-temperature properties are better if the material can be removed as a gas, and from this point of view, Si and Al containing gases such as SiO and Al ₂ O are best. The amount of these metal elements added is preferably in the range of 0.1 to 20% by weight. That is, if it is less than 0.1% by weight, there is no effect of addition (effect of removing SiO ₂ ), and if it is used in an amount greater than 20% by weight, the metal element will remain in the molded body and the high-temperature properties will deteriorate. However, the amount of this metal element added varies depending on the SiC used and the oxygen content contained in the metal element powder. If it is within the above range, the effect of addition is significant. However, the amount of addition that can remove the oxygen content (approximately
Even when a metal element is added in an amount of 10% by weight or more, especially in the case of Si, SiO _1-X is generated and does not remain as a metal element, so there is no decrease in high temperature strength. The range in which the amount of Si does not remain as a metallic element is 20
% by weight or less. When more than 20% by weight of Si metal is added, the metal elements remain and the high temperature strength decreases. In the case of metals other than Si, the added metal forms a solid solution in SiC and carbides of the added metal are formed, resulting in the
If it is within 20% by weight, it will not remain as a metallic element and the high temperature strength will not decrease as in the case of Si. If more than 20% by weight of metal is added, the metal elements will remain as in the case of Si, resulting in a decrease in high temperature strength. The present invention further eliminates the deterioration of high-temperature properties by adding metal nitrides other than silicon in addition to the addition of metal elements that exhibit the above-mentioned effects. Metal nitrides include Li, Be, Mg, Al, Ti, Y, Zr,
One or more nitrides such as Ba, Ce, and Ta are particularly good. It has been experimentally determined that when these metal elements and metal nitrides are added, a high vacuum, which is a drawback when not added, is not required, and a vacuum of 1.3×10 ^-4 atm or less is sufficient. Furthermore, it has been found that the effect becomes even greater when the reduction reaction using the CO and H ₂ gases is promoted in conjunction with the addition of these metal elements and metal nitrides. In this case, as mentioned above, the reaction under a reduced pressure atmosphere is good, and it has been experimentally determined that a range of 1.3×10 ⁻⁴ to 0.8 atm is appropriate. It has also been found that it is highly effective to alternately repeat the vacuum and reduced pressure atmospheres one or more times. That is, the reaction of SiO ₂ with H ₂ or CO gas is
As mentioned above, H ₂ O/H ₂ , CO ₂ /CO inside the molded body
However, by creating a vacuum, CO ₂ or H ₂ O inside the molded body is reduced, and by introducing CO and H ₂ gases, the ratio of CO ₂ /CO and H ₂ O / H ₂ is lowered. This is thought to increase the rate of reduction. Further, when the compact is treated in a plasma atmosphere of the reducing gas, the reduction reaction of SiO ₂ is promoted and the powder surface is further activated. The processing temperature of the molded body to which these metal elements and metal nitrides are added is limited to a temperature equal to or lower than the sintering temperature of the molded body. This is because if the activation treatment is performed at a temperature higher than the sintering temperature, sintering will proceed simultaneously with the removal of the oxide layer, making it difficult to uniformly remove the oxide layer from the entire compact. The above-mentioned activation treatment is performed before sintering the molded body to activate the powder surface inside the molded body, thereby moving on to the next sintering step. The above explanation has been made using SiC as an example, but it goes without saying that the same applies to Si ₃ N ₄ as well. Next, the sintering process will be explained. In the case of SiC, the conditions are that the atmosphere is non-oxidizing and that there is little oxygen in the atmosphere. That is, although the molded body is activated by the treatment before sintering, if the atmosphere during sintering is bad, the effect of the activation treatment disappears. In this case, high purity
A gas atmosphere such as Ar, He, H ₂ , N ₂ or the like may be used. In particular, it has been experimentally determined that a vacuum of 1.3×10 ^-4 atm or less is better. A suitable sintering temperature is in the range of 1600 to 2300°C. Limiting to such range is 1600℃
If it is less than 2300°C, sufficient densification cannot be obtained, and if it is higher than 2300°C, the decomposition reaction of SiC itself becomes significant, and pores remain, making it impossible to obtain sufficient densification. Next, in the case of Si ₃ N ₄ , it is preferable to use a high pressure N ₂ gas atmosphere of 1 atm or higher in order to suppress the decomposition reaction of Si ₃ N ₄ . There is a relationship between the sintering temperature and the atmospheric pressure.
It is suitable to sinter at 1600-2300° C. under a N ₂ glass atmosphere pressure of 1 atm to 3×10 ³ atm. Limiting the sintering temperature range in this way is
This is because if the temperature is less than 2300°C, the densification is not sufficient as in the case of SiC, and if the temperature is 2300°C or higher, the decomposition reaction of Si ₃ N ₄ becomes intense and the densification is not sufficient. Further, the particle size of the Si ₃ N ₄ or SiC powder used in the present invention is 0.5 μ or less, preferably 0.2 μ or less, which is good for promoting densification. The present invention will be explained in detail below with reference to Examples. Example 1 For 100g of Si ₃ N ₄ (manufactured by Advanced Materials Engineering, UK) containing 85% α type
5 g of AlN and 2 g of Si were mixed and pulverized using a ball mill. When this powder was analyzed for oxygen, it was found to be 3.2% by weight. This powder is applied at a pressure of 2 tons/cm ² to a length of 40 mm and a width of 30 mm.
After forming it into a plate with a thickness of 10 mm and a thickness of 10 mm, it was loaded into a sintering furnace. After making the inside of the furnace a vacuum (degree of vacuum x 10 ^-5 atm), the temperature began to rise and was maintained at 1400°C for 1 hour. Afterwards, high-purity _N2 gas (purity 99.999%) was introduced into the furnace. After setting the pressure inside the furnace to 40 atm, the temperature was raised to 1850
Sintering was carried out by holding at ℃ for 2 hours. For comparison, a molded body to which no Si metal was added was also sintered in the furnace at the same time. After the furnace was sufficiently cooled, the sintered bodies were taken out from the furnace and tested for oxygen content, bending strength, etc., and the results shown in Table 1 were obtained.

[Table] From the above table, the sintered body of the present invention is superior to the comparative sintered body.
A sintered body was obtained which was densified with a much reduced oxygen content and whose high-temperature properties hardly deteriorated. On the other hand, in the comparative example, the oxygen content hardly decreased, densification did not occur, and the high-temperature properties were not strong. Example 2 The additive metal Si in Example 1 was replaced with Al, Be, Mg,
A sintered body was obtained in the same manner as in Example 1 except that Ba was used. For comparison, in the present invention, metal elements are outside the range.
Sintered bodies to which Mn, Co, Cr, Sn, etc. were added were also made into sintered bodies and tested in the same manner as in Example 1, and the results shown in Table 2 were obtained.

[Table] From the above table, in the sintered body of the present invention, those using Al as an additive metal element have the same effect as Si, and those using Be, Mg, and Ba have a denser sintered body. It was observed that the oxygen content was also reduced by half as the temperature increased. On the other hand, all of the sintered bodies of Comparative Examples had almost no decrease in oxygen content, were not densified, and had poor properties. Example 3 Weight ratio of SiO ₂ powder (average particle size 12 mμ) 1, C
Powders (average particle size 29 mμ) mixed at a blending ratio of 0.6 were uniformly mixed in a ball mill, then placed in a reactor and heat treated in _H2 gas at 1800°C for 5 hours to synthesize SiO powder. did. Next, 5% by weight of AlN and 2% by weight of YN were added to this SiO powder.
and 2% by weight of Si were added and mixed and pulverized using a ball mill. Oxygen analysis of this powder revealed that the amount of oxygen was
It was 2.5% by weight. This powder is applied at a pressure of 3 tons/cm ² to a length of 40 mm and a width of 40 mm.
After forming it into a plate with a thickness of 10 mm and a thickness of 10 mm, it was loaded into a sintering furnace. After creating a vacuum of 4×10 ⁻⁴ atm in the furnace, the temperature was raised and maintained at 1700° C. for 1 hour. Thereafter, the temperature was raised while maintaining the degree of vacuum, and the temperature was held at 2000°C for 2 hours to perform sintering. For comparison, a molded body to which no Si metal was added was also sintered in the same manner. After sufficiently cooling the furnace, the sintered body was taken out and tested. Similar to Example 1, the sintered body of the present invention obtained good results as shown in Table 3.

[Table] Example 4 Sintering was carried out under the same conditions as in Example 1, except that the temperature for removing oxides from the compact under vacuum in Example 1 was changed in several steps between 1000 and 2000°C. When we investigated the relationship between this oxide layer removal temperature and oxygen content, we obtained the results shown in Figure 1.
It was found that the range (shaded area) between 1050°C and the sintering temperature (1800°C) was effective. Example 5 The amount of added metal element Si in Example 3 was reduced to 0.
Sintering was carried out under the same conditions as in Example 3 except that the amount was varied between ~40% by weight, and the relationship between the amount of Si added and the bending strength of the obtained sintered body was investigated. The effect shown in the figure is obtained, and the amount of Si added is
It was found that a range of 0.1 to 20% by weight (shaded area) is preferable. Example 6 The conditions were the same as in Example 1, except that the activation treatment atmosphere for removing oxides in Example 1 was changed to CO gas or H ₂ gas, and the atmosphere pressure was changed as shown in Table 4. A sintered body was obtained. When this was tested in the same manner as in Example 1, the results shown in Table 4 were obtained.

[Table] Example 7 Activation treatment atmosphere for oxide removal was set to 1.5×10 ^-4 atm
From vacuum to CO gas reduced pressure atmosphere (0.15 atm) 5
A sintered body was obtained in the same manner as in Example 1 except that the process was repeated several times. The results are shown in Table 5, and when compared with Tables 1 and 4, it was found that the present example had the lowest oxygen content and little deterioration in high-temperature strength.

[Table] Example 8 Sintering was carried out using the SiC powder of Example 3 under all the same conditions as Example 3 except for changing the sintering temperature, and the results shown in Figure 3 were obtained. It was confirmed that the range of 1600 to 2300°C (shaded area) is the optimum range for sintering. Although not shown, the same results were obtained for Si ₃ N ₄ as well. In addition, we investigated the relationship between this sintering temperature and the pressure in the _N2 gas atmosphere, and as shown in Figure 4, it was found that
The results show that the shaded area in the range of 2300°C is preferable.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the oxide removal treatment temperature and the oxygen content in the sintered body in the present invention, and Figure 2 is a graph showing the relationship between the amount of Si metal added and the sintered body at 1400°C in one embodiment of the present invention. FIG. 3 is a chart showing the relationship between sintering temperature and density in an embodiment of the present invention, and FIG _{. 4} is a chart showing the relationship between sintering temperature and density in an embodiment of the present invention. ₄ sintering temperature and
It is a chart showing the relationship with _N2 gas atmosphere pressure.

Claims (1)

[Claims] 1. Si, Ti, Ce, which has an oxide standard free energy of formation at 1000°C of −150 kcal/g・molO ₂ or less,
One or more metal elements such as Al, Mg, Be, Ca, Zr, Ba, and Li are added as a solid reducing agent, and one or more metal nitrides other than Si are added as a sintering aid. A compact of Si ₃ N ₄ or SiC powder is activated at a temperature below the sintering temperature of the compact, and then sintered in a non-oxidizing atmosphere.
Method for producing Si ₃ N ₄ or SiC sintered bodies. 2. The method for producing a Si ₃ N ₄ or SiC sintered body according to claim 1, wherein the activation treatment is performed in a reduced pressure atmosphere of a reducing gas at a temperature below the sintering temperature. 3. Si ₃ N ₄ according to claim 1, characterized in that the activation treatment is performed at a temperature below the sintering temperature under conditions in which a reduced pressure atmosphere of a reducing gas and a vacuum atmosphere are alternately repeated one or more times. Or a method for manufacturing SiC sintered bodies. 4. The method for producing a Si ₃ N ₄ or SiC sintered body according to claim 1, wherein the activation treatment is performed in a reducing gas plasma atmosphere at a temperature below the sintering temperature. 5. The method for producing a Si ₃ N ₄ or SiC sintered body according to any one of claims 2 to 4, wherein the reducing gas is CO and/or H ₂ .