A METHOD FOR PRODUCING HIGH-DENSITY SINTERED SILICON CARBIDE ARTICLES

Abstract: Described is a method for producing high-density sintered silicon carbide articles useful as wear-resistant materials, corrosion-resistant materials and high-­ temperature structural materials which comprises mixing powdery silicon carbide composed mainly of α-phase silicon carbide, a boron compound of from about 0.05 to about 0.15 wt.%, as converted to boron, based thereon and a carbonizable organic substance so that the total of combinable carbon content of the powdery silicon carbide and combinable carbon content of the carbonizable organic substance is from about 4 to about 8 wt.%, molding the resulting mixture, and subjecting the molded product to heating at a temperature of from about 1,000°C to about 2,000°C in a vacuum and then sintering at a temperature of about 2,300°C or less in an inert atmosphere to obtain a sintered silicon carbide article having a sintered density of about 90% or more of the theoretical density.

Description: The present invention relates to a method for producing novel sintered silicon carbide articles, and more particularly, to a method for producing high density sintered silicon carbide articles which comprises incorporating particular amounts of a carbonaceous substance and a boron compound, which are densification aids in powdery silicon carbide and molding the resulting mixture, after which the molded product is heated in a vacuum and then sintered in an inert atmosphere. Silicon carbide has excellent physical and chemical properties, and particularly, it has high hardness and chemical stability and besides, its mechanical properties, even at high temperatures exceeding 1,000°C, do not change from those at room temperature, so that it has been regarded as promising as wear-resistant materials, corrosion-resistant materials and high-temperature structural materials. However, sintering of silicon carbide is so much difficult that it is difficult to sinter silicon carbide in high density by the usual methods, and therefore, sintering by the hot press method, sintering with densification aids, etc. have been proposed. For example, JP-A-148,712/1976 discloses that high density sintered silicon carbide articles are obtained by mixing 91 to 99.35 parts by weight of silicon carbide having a specific surface area of 1 to 100 m²/g and containing at least 5 wt.% or more of α-phase silicon carbide, 0.67 to 20 parts by weight of a carbonizable, organic solvent-soluble organic material having a carbonization ratio of 25 to 75 wt.%, a boron source containing 0.15 to 3.0 parts by weight of boron and 5 to 15 parts by weight of a temporary binding agent, and sintering the resulting mixture. The carbonization ratio is defined by the following equation: This method surely produces high-density sintered silicon carbide articles, but they are not satisfactory in mechanical strength. For example, no articles having a bending strength exceeding 490 MPa (50 kg/mm²) are obtained. In order to solve this problem, the present inventors previously found that sintered articles of excellent mechanical strength can be obtained without reducing the sintered density by using in combination a particular amount of a particular carbonaceous substance and boron wherein the latter is less in amount than that so far generally regarded as necessary to raise the sintered density in the case of silicon carbide, and applied for a patent (JP-A-200861/1985). In the above method, however, there is a problem that the sintered articles obtained thereby are inferior in hardness. Thus it is the object of the invention to provide novel sintered silicon carbide articles of high hardness, high density and high strength. This object has been achieved by the surprising finding that when calcination and sintering of the molded product are carried out under particular conditions, as described in the claims, said sintered silicon carbide articles can be obtained. In the calcination and sintering under these conditions, a phenol resin can be used as a carbonizable organic substance, which has not served as such conventionally with low content in boron for obtaining a high-density sintered article. The present invention provides a method for producing high-density sintered silicon carbide articles characterized in that powdery silicon carbide composed of α-phase silicon carbide containing up to 30 wt.% of β-phase powdery silicon carbide, a boron compound of from 0.05 to 0.15 wt.%, as converted to boron, based on said powder and a carbonizable organic substance are mixed so that the total of the uncombined carbon content of the powdery silicon carbide and the incombined carbon content of the carbonizable organic substance base on said powder is from 4 to 8 wt.%, the resulting mixture is molded and the molded product is heated in a vacuum at a temperature of from 1,000°C to 2,000°C and then sintered in an inert atmosphere at a temperature of 2,300°C or less to obtain a sintered silicon carbide article having a sintered density of 90% or more of the theoretical density. The method of the present invention will be explained in more detail. Preferred powdery silicon carbide used in the method of the present invention is one consisting of silicon carbide of 1 µm or less in average particle diameter having an α-phase, namely, a crystal structure of non-cubic polytypes. Generally, silicon carbide contains from 0.2 to 2.0 wt.% of uncombined carbon; and this is also the same with the powdery silicon carbide used in the present invention. In the present invention, the amount of the boron compound incorporated is from 0.05 to 0.15 wt.%, preferably from 0.08 to 0.13 wt.%, as converted to boron, based on the powdery silicon carbide. The amount of the carbonizable organic substance incorporated is from 4 to 8 wt.%, preferably from 4.5 to 6 wt.%, as expressed by total of uncombined carbon content of the powdery silicon carbide and uncombined carbon content of the carbonizable organic substance. The uncombined carbon content of the carbonizable organic substance is an amount expressed by a carbonization ratio which the organic substance possesses. When the amount of the boron compound added is less than 0.05 wt.%, as converted to born, based on the powdery silicon carbide, high-density sintered articles are difficult to obtain. While when the amount exceeds 0.15 wt.%, coarse grains are easy to grow in the sintered articles depending upon the sintering temperature and time, thereby lowering the mechanical strength of the articles. When the amount of the carbonizable organic substance is less than 4 wt.% based on the powdery silicon carbide, as expressed by the total of the uncombined carbon content of the powdery silicon carbide and the uncombined carbon content of the carbonizable organic substance, high-density sintered articles cannot be obtained. While when the amount exceeds 8 wt.%, the mechanical strength of the sintered articles lowers, which is not preferred. There is no particular limitation to the boron compound usable in practicing the present invention, but generally, those which remain stable up to the sintering temperature for the sintered articles aimed at, and besides have a high boron content are preferred. Specifically, there may be mentioned boron, boron carbide, etc. The carbonizable organic substance may be any of organic solvent-soluble organic materials having a carbonization ratio of 30 wt.% or more, preferably 40 to 60 wt.%. For example, there may be mentioned synthetic resins such as phenol resins, furan resins, resorcinol resins, aniline resins, cresol resins, polyimide, polyacrylonitrile, polyphenylene, polymethylphenylene, etc., and tar pitches such as coal tar pitch, oil tar pitch, etc. In the present invention, for producing molded products of a mixture of the powdery silicon carbide, boron compound and carbonizable organic substance blended in the proportion as shown above, the following method will suffice: The mixture is uniformly mixed with an organic solvent (e.g. benzene, quinoline, acetone) and/or water and (i) molded by slip casting, or (ii) granulated by spray drying and molded under pressure on a press, or (iii) mixed with an organic binder and molded by extrusion molding, injection molding, etc. The molded product thus obtained, after being subjected to machining or to treating the removal of binder if necessary, is heated to a temperature of 1,000°C or higher, preferably 1,500°C or higher in a vacuum, and then sintered at a temperature of 2,300°C or less in an inert atmosphere such as argon, helium, nitrogen, etc. The degree of vacuum is not particularly limited so far as the pyrolysis of the carbonizable organic substance is carried out uniformly and sufficiently, and it is generally 1333 Pa (10 Torr) or higher, preferably 13 Pa (10 _1 Torr) or higher, more preferably from 13 (10 _1 ) to 13.10 _5 (10 _6 Torr). In the method of the present invention, when the heating temperature in a vacuum is lower than 1,000°C, pyrolysis of the carbonizable organic substance is not sufficient. While when it exceeds 2,000°C, with sintering furnaces made of graphite heaters and refractory which are commonly used in industry, the durability of the heaters and refractory becomes a problem, which is not preferred. The sintering temperature is 2,300°C or less. When it exceeds 2,300°C, no advantages are found in raising the density of sintered articles, and besides pyrolysis of silicon carbide takes place, which is not preferred. While when the sintering temperature is 2,000°C or less, high-density sintered articles are not sometimes obtained. A preferred sintering temperature is therefore from 2,050°C to 2,250°C. The temperature and time required for heating in a vacuum prior to sintering in an inert atmosphere depend upon the carbon content of the molded products and the degree of vacuum applied, so that they cannot be determined unconditionally. Generally, however, there may be employed a method in which heating is carried out while raising the temperature from room temperature to from 1,500°C to 2,000°C over from 1 to 10 hours under a vacuum of from 13 (10 _1 ) to 0,13 Pa (10 _3 Torr), and then sintering is carried out in an inert atmosphere. A reason why high-density sintered silicon carbide articles are obtained in the present invention is not clear, but it may be considered that, by evacuating the atmosphere at a temperature of 1,000°C or less at which pyrolysis of the carbonizable organic substance takes place, carbon produced by the pyrolysis has a favorable structure to promote sintering. Further, the evacuated atmosphere may be considered to act favorably at a temperature of 1,000°C or higher in removing the oxide layer from the surface of silicon carbide powders. This effect acts more effectively with an increasing temperature in a range of up to 1,800°C at which sintering begins to proceed. Also, the hardness of sintered articles obtained by sintering according to the present invention is higher than that of sintered articles subjected to no treatment under vacuum. The reason for this is not also clear, but the hardness of sintered articles may be considered to become high for the following reasons: The treatment under vacuum promotes the decomposition of the carbonizable organic substance to reduce the amount of residual carbon in the sintered articles and besides creates a state in which residual fine carbon particles have uniformly been dispersed within the sintered articles. The content of residual boron in the sintered article of the present invention is from 0.03 to 0.15 wt.%, preferably from 0.07 to 0.11 wt.%. When the content is less than 0.03 wt.%, high-density sintered articles cannot be obtained. While when it exceeds 0.15 wt.%, the mechanical strength of the sintered articles lowers. The physical properties of the sintered articles are substantially determined by the powdery silicon carbide and the boron and carbonaceous compounds to be added thereto, but minute control by operation conditions is also possible. According to the method of the present invention described above in detail, by employing a two-stage heating schedule in which particular amounts of the carbonaceous compound and boron compound are incorporated as densification aids in the powdery silicon carbide, the resulting mixture is molded and the molded product is heated in a vacuum to a particular temperature or higher and subsequently sintered in an inert atmosphere, it became possible to obtain high-density sintered silicon carbide articles of excellent mechanical strength and hardness having a boron content of from more than 0.03 wt.% to less than 0.15 wt.% and a density of 90% or more, generally 95% or more of the theoretical density. The method of the present invention for producing high-density sintered silicon carbide articles is of a very high industrial value to produce industrial materials such as turbine blades, pump parts, paper-making machine parts, etc. The present invention will be illustrated in more detail with reference to the following examples. Example 1 After dissolving 17 g of coal tar pitch (carbonization ratio, 53%) in 17 g of quinoline, 400 g of benzene was added, followed by thorough mixing. To the resulting solution were added 200 g of α-phase silicon carbide containing 0.34 wt.% of uncombined carbon and having a silicon carbide content of 98% and a BET specific surface area of 10 m²/g (UF-10®; a product of Lonza, Ltd.) and 0.3 g of boron carbide of 1200-mesh through (Denka Boron®; a product of Denki Kagaku Kogyo Co.), and the mixture was milled and mixed for 3 hours by means of a plastic ball mill. The mixture was then dried at 60°C in a nitrogen gas stream, crushed and passed through a 180-mesh sieve. The mixed powder obtained was cold-pressed, charged in a rubber mold and molded on a hydrostatic pressure press under a molding pressure of 147 MPa (1.5 tons/cm²) to obtain a molded product of 50 mm × 35 mm × 5 mm. This molded product was heated, under a vacuum of from 0.13 (10 _3 ) to 13.10 _3 Pa (10 _4 Torr), from room temperature to 600°C at a rate of 100°C/hour and then from 600°C to 1,800°C at a rate of 400°C/hour, after which an argon gas was introduced while maintening the temperature at 1,800°C for 30 minutes. After the pressure reached 1013 hPa (760 Torr), sintering was carried out at a temperature of 2,125°C for 2 hours in an argon gas atmosphere. The resulting sintered article had the following physical properties: Sintered density, 3.17 g/cm³; bending strengh by the three-point bending test (sample size, 4 mm × 3 mm × 50 mm; span, 30 mm) 628 MPa (64 kg/mm²); and micro vickers hardness (load 9.8 N (1 kg) × 15 sec), 23438 MPa (2390 kg/mm²). The residual boron content of this sintered article was 0.09 wt.%. Example 2 After dissolving 16 g of oil tar pitch (carbonization ratio, 55%) in 16 g of quinoline, 400 g of benzene was added, followed by thorough mixing. To the resulting solution were added 200 g of α-phase silicon carbide containing 0.34 wt.% of uncombined carbon and having a silicon carbide content of 98% and a BET specific surface area of 10 m²/g and 0.3 g of boron carbide of 1200-mesh through, and the mixture was milled and mixed for 3 hours by means of a plastic ball mill. The mixture was then dried at 60°C in a nitrogen gas stream, crushed and passed through a 180-mesh sieve. The mixed powder obtained was cold-pressed, charged in a rubber mold and moded on a hydrostatic pressure press under a molding pressure of 147 MPa (1.5 tons/cm²) to obtain a molded product of 50 mm × 35 mm × 5 mm. This molded product was heated, unter a vacuum of from 0.13 (10 _3 ) to 13.10 _3 Pa (10 _4 Torr), from room temperature to 600°C at a rate of 100°C/hour and then from 600°C to 1,800°C at a rate of 400°C/hour, after which an argon gas was introduced while maintaining the temperature at 1,800°C for 30 minutes. After the pressure reached 1013 hPa (760 Torr), sintering was carried out at a temperature of 2,125°C for 2 hours in an argon gas atmosphere. The resulting sintered article had the following physical properties: Sintered density, 3.08 g/cm³; three-point bending strength, 569 MPa (58 kg/mm²); and micro vickers hardness, 22555 MPa (2,300 kg/mm²). The residual boron content of this sintered article was 0.09 wt.%. Example 3 Eleven grams of a liquid novolak type phenol resin (Sumilite Resin® PR-940C; a product of Sumitomo Bakelite Co.; carbonization ratio, 41%), 10 g of 10 wt.% aqueous polyvinyl alcohol solution (Kurare Poval® 205; a product of Kurare Co.), 190 cm³ of water, 100 g of α-phase silicon carbide containing 0.34 wt.% of uncombined carbon and having a silicon carbide content of 98% and a BET specific surface area of 10 m²/g (UF-10®; a product of Lonza, Ltd.) and 0.15 g of boron carbide of 1200-mesh through (Denka Boron®; a product of Denki Kagaku Kogyo Co.) were added to a plastic ball mill and milled and mixed for 3 hours. The resulting mixture was dried at 60°C in a nitrogen gas stream, crushed and passed through a 180-mesh sieve. The mixed powder obtained was cold-pressed, charged in a rubber mold and molded on a hydrostatic pressure press under a molding pressure of 147 MPa (1.5 tons/cm²) to obtain a molded product of 50 mm × 35 mm × 5 mm. This molded product was heated, under a vacuum of from 0.13 (10 _2 ) to 13.10 _2 (10 _3 ) Torr, from room temperature to 600°C at a rate of 100°C/hour and then from 600°C to 1,800°C at a rate of 400°C/hour, after which an argon gas was introduced while maintaining the temperature at 1,800°C for 30 minutes. After the pressure reached 1013 hPa (760 Torr), the temperature was further raised to 2,125°C at a rate of 50°C/hour in an argon gas atmosphere, after which sintering was carried out at a temperature of 2,125°C for 2 hours. The resulting sintered article had the following physical properties: Sintered density, 3.17 g/cm³; bending strength by the three-point bending test (sample size, 4mm × 3 mm × 40 mm; span, 30 mm), 588 (60 kg/mm²); and micro vickers hardness (load 9.8 N (1 kg) × 15 sec), 24124 MPa (2460 kg/mm²). This sintered article had a residual boron content of 0.08 wt.%, and its structure was found to be uniform microscopically. Example 4 Preparation, calcination and sintering of the molded product were carried out in the same condition as in Example 1 except that the amount of the β-phase powdery silicon carbide was decreased to 75 parts by weight, and that 25 parts by weight of α-phase powdery silicon carbide containing 0.59 wt.% of uncombined carbon and having a silicon carbide content of 98% and a BET specific surface area of 20 m²/g (Beta-rundum Ultrafine®; a product of Ibiden Co.) was used. The resulting sintered article had the following physical properties: Sintered density, 3.14 g/cm³; bending strength, 539 MPa (55 kg/mm²); and micro vickers hardness, 23340 MPa (2380 kg/mm²). The residual boron content of this sintered article was 0.08 wt.%. Comparative example 1 A molded product prepared in the same condition as in Example 1 was calcined at a temperature of 600°C for 3 hours in an argon gas stream, and then sintered at a temperature of 2,125°C for 2 hours in an argon gas atmosphere. The resulting sintered article had the following physical properties: Sintered density, 3.10 g/cm³; bending strength, 569 MPa (58 kg/mm²); and micro vickers hardness, 20300 MPa (2070 kg/mm²). The residual boron content of this sintered article was 0.09 wt.%. Comparative example 2 A molded product prepared in the same condition as in Example 3 was directly calcined at a temperature of 600°C for 3 hours in an argon gas stream without subjecting to calcination in a vacuum, and then sintered at a temperature of 2,125°C for 2 hours in an argon gas atmosphere. The resulting sintered article had the following physical propertie: Sintered density, 2.85 g/cm³; bending strength, 265 MPa (27 kg/mm²); and micro vickers hardness, 18633 (1900 kg/mm²). The residual boron content of this sintered article was 0.08 wt.%. Examples 5 to 10 and Comparative examples 3 to 8 Preparation, calcination and sintering of the molded product were carried out in the same manner as in Example 1 except that conditions shown in Tables 1 and 2 were used, and the resulting sintered articles were measured for the physical properties. The results are shown in Tables 1 and 2. The size of the molded product was 50 mm × 35 mm × 4 mm except Example 9 and Comparative example 8 wherein it was 50 mm × 35 mm × 10 mm. The temperature was raised at the following rates: From room temperature to 600°C, 100°C/hour; from 600°C to 1,800°C, 400°C/hour; and from 1,800°C to sintering temperature, 50°C/hour. In Example 8, 0.5 g of silicon having an average particle diameter of 3 µm was additionally added to the powdery silicon carbide. In Example 10, a mixture of 75 wt.% of α-phase powdery silicon carbide and 25 wt.% of α-phase powdery silicon carbide, tie both being the same as those used in Example 4, was used as material. Examples 11 to 16 Preparation, calcination and sintering of the molded product were carried out in the same manner as in Example 3 except that conditions shown in Table 3 were used, and the resulting sintered articles were measured for the physical properties. The results are shown in Table 3. Comparative examples 9 to 14 Preparation, calcination and sintering of the molded product were carried out in the same manner as in Example 3 except that conditions shown in Table 4 were used, and the resulting sintered articles were measured for the physical properties. The results are shown in Table 4.

Claims: 1. A method for producing high-density sintered silicon carbide articles which comprises mixing powdery silicon carbide composed of α-phase silicon carbide containing up to 30 wt.% of β-phase powdery silicon carbide a boron compound of from 0.05 to 0.15 wt.%, as converted to boron, based on said powder and a carbonizable organic substance so that total of uncombined carbon content of the powdery silicon carbide and uncombined carbon content of the carbonizable organic substance based on said powder 4 to 8 wt.%, molding the resulting mixture, and subjecting the molded product to heating at a temperature of from 1,000°C to 2,000°C in a vacuum and then sintering at a temperature of 2,300°C or less in an inert atmosphere to obtain a sintered silicon carbide article having a sintered density of 90% or more of the theoretical density. 2. A method according to Claim 1, wherein the powdery silicon carbide composed of α-phase silicon carbide has an average particle diameter of 1 µm or less. 3. A method according to Claim 1, wherein the amount of the boron compound to be added to the powdery silicon carbide composed of α-phase silicon carbide based on said powder is from 0.08 to 0.13 wt.%, as converted to boron. 4. A method according to Claim 1, wherein the amount of the carbonizable organic substance to be added to the powdery silicon carbide composed of α-phase silicon carbide based on said powder 4.5 to 6 wt.%, as expressed by total of uncombined carbon content of the powdery silicon carbide and uncombined carbon content of the carbonizable organic substance. 5. A method according to Claim 1, wherein the carbonizable organic substance is oil tar pitch and/or coal pitch. 6. A method according to Claim 1, wherein the carbonizable organic substance is a phenol resin. 7. A method according to Claim 1, wherein the inert atmosphere is composed of argon, helium or nitrogen. 8. A method according to Claim 1, wherein the sintering of the molded product is carried out in a vacuum of from 13 (10 _1 ) to 0.13 Pa (10 _3 Torr) while raising the temperature from room temperature to from 1,500°C to 2,000°C over from 1 to 10 hours, and then a temperature of 2,300°C or less in an inert atmosphere. 9. A method according to Claim 1, wherein the boron content of the sintered article is from 0.03 to about 0.15 wt.%.

Inventors: MOURI, MASAHIDE, YAMADA, KOICHI
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1986
Main classification: C04B35
IPC codes: C04B35