JPH0456792B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a silicon carbide sintered body, and more particularly, the present invention relates to a method for producing a silicon carbide sintered body, and more specifically, after a specific carbon-containing substance and a boron compound are present in a specific amount range as a sintering aid in silicon carbide powder. The present invention relates to a method for manufacturing a high-density silicon carbide sintered body containing specific amounts of carbon and boron in the sintered body by molding, heating in vacuum, and sintering in an inert atmosphere. Silicon carbide has excellent physical and chemical properties, particularly high hardness, corrosion resistance, and mechanical properties that are the same even at high temperatures as at room temperature, making it a promising candidate for wear-resistant materials and high-temperature structural materials. was being watched. However, since it is difficult to sinter to a high density by normal methods due to the difficulty of sintering, sintering by hot pressing, sintering by adding a sintering aid, etc. have been proposed. For example, in Japanese Patent Application Laid-Open No. 51-148712, 1 to 100
α-type silicon carbide 91~ having a specific surface area of m ² /g
99.35 parts by weight of a carbonizable organic solvent-soluble organic material with a carbonization rate of 25-75% by weight, 0.67-20 parts by weight, a boron source containing 0.15-3.0 parts by weight of boron and 5-15 parts by weight of a temporary binder. It is taught that a high-density sintered silicon carbide body can be obtained by mixing and sintering the two parts. However, according to this method, although a high-density silicon carbide sintered body is obtained, the mechanical strength is not satisfactory, for example, the transverse rupture strength is 50 kg/
Nothing larger than mm ² is seen. Therefore, the inventors of the present invention have proposed that when adding a specific carbon-containing substance in a specific amount range to silicon carbide in an amount smaller than the amount of boron that is generally required to increase the sintered density of silicon carbide. discovered that a sintered body with excellent mechanical strength could be obtained without causing a decrease in sintered density, and filed a patent application earlier. However, as we continued to study this method, we found that while it was easy to obtain a high-density product for thin sintered bodies, it was not necessarily possible to obtain a high-density sintered body for thick compacts. I have faced some inconveniences. In view of these circumstances, the inventors of the present invention have intensively investigated the reason why a high-density sintered body cannot be obtained when the thickness of the molded body is thick. We believe that this is because the carbon-containing material that has been sintered tends to remain inside the sintered body, and as a result, densification is hindered. As a result of considering the method of removing excess residual carbon present in the molded body, as well as the appropriate amount of carbon, the method of sintering the molded body was specified to achieve high density even when the thickness of the molded body is thick, for example, exceeding 10 mm. It was discovered that a sintered body having a large size and high strength could be obtained, and the method of the present invention was completed. In other words, the present invention provides silicon carbide powder with a boron compound as a sintering aid in an amount exceeding 0.03% by weight and less than 0.15% by weight in terms of boron, and a total amount of carbon contained in the silicon carbide powder exceeding 4% by weight. Tarpitz is added and mixed so that the carbon content is less than 8% by weight, and the formed compact is heated in a vacuum to a temperature of 1500°C or higher, and then sintered in an inert atmosphere at a temperature of 2300°C or lower to reduce the boron content. More than 0.03% by weight and 0.15% by weight
Production of a high-density silicon carbide sintered body, characterized in that the carbon content is less than 2.0% by weight, and the sintered body has a density of 90% or more of the theoretical density. We are here to provide you with a method. The method of the present invention will be explained in more detail below. As the silicon carbide powder used in the method of the present invention, it is preferable to use a silicon carbide powder consisting of silicon carbide having an average particle size of 1 μm or less and mainly having an alpha or anisometric crystal form, but a beta type silicon carbide powder is also preferable. They can be used alone or in combination. These silicon carbide powders generally contain 0.2 to 2% by weight of additional carbon, and the silicon carbide powder applied to the method of the present invention is no exception. In the method of the present invention, the total amount of boron compounds, including more than 0.03% by weight and less than 0.15% by weight in terms of boron, and additional carbon contained in the silicon carbide powder, is more than 4% by weight and 8% by weight, based on the silicon carbide powder. However, if the amount of the boron compound added to the silicon carbide powder is less than 0.03% by weight in terms of boron, the sintered density will decrease, which is undesirable; In the above case, although a high-density sintered body can be obtained, the mechanical strength of the sintered body decreases, which is not preferable. Furthermore, if the amount of tar pitch added to the silicon carbide powder is less than 4% by weight, including the additional carbon contained in the silicon carbide powder, the silicon carbide will cause grain growth before sintering. A high-density sintered body cannot be obtained, and if the content exceeds 8% by weight, the mechanical strength of the sintered body decreases, which is not preferable. There are no particular restrictions on boron compounds that can be used in the method of the present invention, but in general, compounds that stably exist up to the sintering temperature of the target sintered body and have a high boron content are desirable. Examples include boron and boron carbide. This is not preferable because it lowers the degree of oxidation. Also, there are no particular restrictions on tarpitz.
It is suitable to use organic solvent-soluble coal tar pitch or oil tar pitch having a carbonization rate of 40 to 60% by weight. In the present invention, the silicon carbide powder, boron compound, and tar pitch having the above-mentioned composition are uniformly mixed using an organic solvent such as benzene, quinoline, anthracene, or water, and then subjected to slip casting molding or spray drying. A molded product may be obtained by granulation by a method and pressure molding by a press molding method, or by mixing an organic binder and by extrusion molding, injection molding, or the like. The molded body thus obtained is subjected to mechanical processing and debinding treatment as necessary, and then placed in a vacuum.
The temperature is raised to 1,500°C or higher, and sintering is performed at a temperature of 2,300°C or lower in an inert atmosphere such as argon, helium, nitrogen, etc. The temperature and time for switching from vacuum to inert atmosphere are not unique depending on the amount of carbon contained in the compact and the degree of vacuum used, but the carbon content in the sintered compact after sintering is 2% by weight or less. The conditions are selected. Such conditions include heating at a vacuum level of 10 ^-1 torr or more, preferably 10 ^-3 to ^{10 -6} torr, and a temperature of 1500 to 2000°C for 1 to 10 hours, followed by further sintering in an inert atmosphere. Can be mentioned. Generally, when the carbon content is the same, the degree of vacuum is higher, and the higher the heating temperature, the shorter the heating time under vacuum, but these conditions can be easily set by simple preliminary experiments. In the method of the present invention, the heating temperature in vacuum is 1500℃.
If the temperature is less than ℃, the remaining amount of carbon in the sintered body is 2% by weight.
If the temperature exceeds 2000° C., it is not preferable because a highly densified product cannot be obtained. On the other hand, if the temperature exceeds 2000° C., evaporation of silicon carbide occurs, which is not preferable. In addition, the processing temperature under an inert atmosphere is 2300℃ or less, preferably 2050 to 2300℃.
Therefore, if the sintering temperature exceeds 2300°C, silicon carbide evaporates and crystal grains become coarser, which is undesirable because the mechanical strength of the sintered body decreases. Although the method of the present invention is particularly suitable for sintering thick molded bodies, it can also be applied to thin molded bodies. It is not clear why the method of the present invention can increase the density of a thick molded body without deteriorating its strength, but when the thickness of the molded body is thick, the carbon evaporated in the molded body Since it is difficult to diffuse out of the molded body and reaches an equilibrium vapor pressure within the molded body, further evaporation of carbon is suppressed. Therefore
It suppresses sintering and particle growth at temperatures above 1900℃, and does not result in high-density products, but when heated to temperatures above 1500℃ under vacuum, the carbon evaporated in the compact will diffuse out of the compact. However, during sintering in an inert atmosphere in the next step, it is possible to reduce the amount of carbon to an amount that does not inhibit the sintering of silicon carbide, that is, to less than 2% by weight. A high-density silicon carbide sintered body with no reduction in mechanical strength can be obtained because the sintering reaction of silicon carbide is not suppressed and the sintering reaction is carried out while the inert atmosphere suppresses the decomposition of silicon carbide at high temperatures. It is assumed that. According to the method of the present invention detailed above, a molded body is formed by adding a specific carbon-containing substance and a boron compound as sintering aids in a specific amount range to silicon carbide powder, and then in a vacuum at a temperature above a specific temperature. Heat to
By adopting a two-step heating method of successively heating and sintering in an inert atmosphere, the boron content is more than 0.03% by weight and less than 0.15% by weight, the carbon content is less than 2.0% by weight, and the theoretical density is of
It has made it possible to obtain a high-density silicon carbide sintered body with a density of 90% or more and excellent mechanical strength, and it is used as a manufacturing method for industrial materials such as turbine blades, pump parts, paper machine parts, etc. Its industrial value is enormous. The present invention will be explained in more detail with reference to Examples below. Example 1 After dissolving 12 g of coal tar pitch (50% carbon yield) in 18 g of quinoline, 400 g of benzene was added and thoroughly mixed. Silicon carbide content 97% with carbon 1.5% by weight in this solution, BET specific surface area 15
200 g of α-type silicon carbide of m ² /g and 0.3 g of boron carbide of 1200 mesh passes were added and dispersed and mixed for 3 hours using a plastic ball mill. Dry at 60℃ while flowing nitrogen gas, crush, pass through a 180-mesh sieve, cold press the resulting mixed powder, charge it into a rubber mold, and hydrostatically press at a molding pressure of 1.5 tons/ ^cm2 . Press molding was performed to produce a molded body of 80φ x 15mm. This molded body was heated from room temperature to 1700°C at a heating rate of 200°C/hr under a vacuum of 10 ^-3 to ^{10 -4} torr.
Argon gas was introduced while holding for a minute.
2100℃ under argon gas atmosphere after reaching 760torr
It was sintered for 1 hour under the temperature conditions of . The sintered density of the obtained sintered body was 3.11 g/cm ³ , and the bending strength in a 3-point bending test (sample dimensions 4 x 3 x 50 mm, span 30 mm) was 60.
Kg/cm ² , and the amount of residual carbon in the sintered body was 0.8% by weight. Example 2 After 24 g of oil tar pitch (carbon yield 50%) was dissolved in 36 g of quinoline, 400 g of benzene was added and thoroughly mixed. Silicon carbide content 97.5%, BET specific surface area containing 0.6% carbon by weight in this solution
200 g of α-type silicon carbide of 10 m ² /g and 0.3 g of boron carbide of 1200 mesh passes were added and dispersed and mixed for 3 hours using a plastic ball mill. Dry at 60℃ while flowing nitrogen gas, crush, pass through a 180-mesh sieve, cold press the resulting mixed powder, charge it into a rubber mold, and hydrostatically press at a molding pressure of 1.5 tons/ ^cm2 . Press molding was performed to produce a molded body of 80φ x 15mm. This molded body was heated from room temperature to 1900°C under a vacuum of 10 ^-3 to ^{10 -4} torr at a heating rate of 200°C/hr.
Argon gas was introduced while holding for a minute.
2100℃ under argon gas atmosphere after reaching 760torr
It was sintered for 1 hour under the temperature conditions of . The sintered density of the obtained sintered body was 3.08 g/cm ³ , and the 3-point bending strength was 58 Kg/mm ²
The amount of residual carbon in the sintered body was 1.2% by weight. Comparative Example 1 A molded body prepared under the same conditions as Example 1 was sintered at 600° C. for 3 hours while flowing argon gas, and then sintered at 2100° C. for 1 hour in an argon gas atmosphere. The obtained sintered body had a sintered density of 2.80 g/cm ³ , a bending strength of 27 Kg/mm ² , and a residual carbon content of 2.5% by weight. Examples 3 and 4 and Comparative Examples 2 to 5 Molded bodies were formed and sintered under the conditions listed in Table 1, and the physical properties of the obtained sintered bodies were measured. The results are shown in Table 1. 【table】

Claims (1)

[Claims] 1 Silicon carbide powder as a sintering aid in terms of boron
The total amount of boron compounds exceeding 0.03% by weight and below 0.15% by weight and carbon contained in the silicon carbide powder is 4
Tar pitch is added and mixed so that the carbon content exceeds 8% by weight, and the molded product is placed in a vacuum.
Heating to a temperature above 1500℃ and then sintering at a temperature below 2300℃ in an inert atmosphere reduces the boron content.
Carbon content is more than 0.03% by weight and less than 0.15% by weight
2.0% by weight or less, and the sintered body has a density of 90% or more of the theoretical density.