CA1152536A - Dense sintered silicon carbide ceramic - Google Patents
Dense sintered silicon carbide ceramicInfo
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- CA1152536A CA1152536A CA000381702A CA381702A CA1152536A CA 1152536 A CA1152536 A CA 1152536A CA 000381702 A CA000381702 A CA 000381702A CA 381702 A CA381702 A CA 381702A CA 1152536 A CA1152536 A CA 1152536A
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- silicon carbide
- aluminum
- aluminum oxide
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Abstract
ABSTRACT OF THE DISCLOSURE
A dense sintered silicon carbide ceramic which has high strength and a flexural strength of at least 25 kg/mm2 at room temperature and 1400°C is obtained by molding a mixture of an aluminum oxide source in an amount of 0.5 to 35 wt.% as A?2O3 and silicon carbide in a substantial remainder portion followed by pressureless sintering.
A dense sintered silicon carbide ceramic which has high strength and a flexural strength of at least 25 kg/mm2 at room temperature and 1400°C is obtained by molding a mixture of an aluminum oxide source in an amount of 0.5 to 35 wt.% as A?2O3 and silicon carbide in a substantial remainder portion followed by pressureless sintering.
Description
The present invention relates to a sintered silicon carbide. More particularly, the present invention relates to a dense sintered silicon carbide having high strength obtained By pressureless sintering such as normal sinter-ing.
Silicon carb~de is well known as a useful ceramic source having high hardness, excellent wear resistance, a low thermal expansion coefficient, a high decomposition temperature, h~gh oxidation resistance and chemical resis-tance, and a relatively high electrical conductivity. A
dense sintered silicon carbide has these characteristics and also has high strength even at high temperature, high heat shock resistance, and is considered to be an effec-tive source for high temperature structural products andto be useful for various uses, such as in gas turbines.
Silicon carbide has relatively high covalent bond whereby it is difficult to sinter silicon carbide by itself. In order to obtain a dense sintered product, it is necessary to incorporate a certain sintering additive. In the hot press process, boron, B4C, aluminum, AQN or AQ2O3 has been used as a sintering additive. In, for example, U.S. Patent No. 3,836,673, a sintered silicon carbide having a strength of 104 psi (70 kg/mm2) is obtained by the hot press process with the addition of 0.5 to 5 wt.% of aluminum. However, the use of metallic aluminum and the hot press process have the disadvantages given hereafter.
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In the pressureless sinte~ing, it is known to incor-porate aluminum and carbon. In, or example, U.S. Patent No. 4,23Q,479, a yroduct having a high flexural strength is obtained by pressureless sintering of a mixture of silicon carbide, 0.2 to 2 wt.% of an aluminum component and 0.1 to 2.0 wt.% of a carbon component. In this pro-cess, aluminum is mainly used and carbon is added for accel-eration of the sintering. As the carbon source, a resin is used and hardening of the resin causes problems in the process. Moreover, the conventional pressureless sintered product has not been satisfactory with respect to its character~stics and process.
The present invention provides a sintered silicon carbide having superior characteristics with respect to conventional products, produced by a pressureless sintering process avoiding a hot press process.
According to the present invention there is provided a sintered silicon carbide ceramic having high strength and a flexural strength of at least 25 kg/mm2 at room temp-erature and 1400C which is obtained by pressureless sin-tering of a mixture of 0.5 to 35 wt.% of aluminum oxide and a remainder portion essentially of silicon carbide.
It is known to use aluminum oxide as a sintering addi-tive for silicon carbide. However~ the characteristics and the production of the convent~onal products are quite dif-ferent from those of the present invention. One type of conventional product is refractory ~ire brick as disclosed in U.S. Patent No. 3,759,725 and British Patent No. 1,460,635 and is obtained by molding a mixture of coarse aggregate of silicon carbide and aluminum oxide and then sintering the molded product at about lZ00 to 1500C. The sintered product comprises a thick layer of aluminum oxide or sili-con oxide disposed around silicon carbide grains.
It has also been proposed in J. Am. Ceram. Soc. 39(11) 386-389, 1956 by Alliegro et al to use aluminum oxide as a sintering additive in the hot press process to produce a dense sintered silicon carbide ceramic.
It was initially considered that the effect of aluminum oxide on silicon carbide in the hot press process is similar to that in the pressureless sintering process. It was also considered that the sintered product obtained by the hot press process has higher density and strength than the sintered product obtained by conventional pressureless sintering when the same molded product made of a mixture having the same formulation is sintered at the same temp-erature by the hot press process and the pressureless process because of the pressure effect. However, it has now been found that a sintered product having higher strength is obtained by the pressureless sintering process compared with the hot press process.
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3~52536 As shown in the examples, the strength at high temperature is different to a great extent. It is considered that the sintering mechanism ~n the pressureless sintering process is different from that in the hot press process. The microstructure of the resulting sintered product is also different.
In the hot press process, pressure is applied in the liquid phase comprising aluminum oxide as a main component to easily form a dense structure. However, the sintered product has a microstructure having equiaxed grains (block-like) with aluminum oxide disposed around the silicon car-bide grain. At high temperature, aluminum oxide disposed around the silicon carbide grains softens causing a serious decrease in strength. However, in the pressureless sin-tering process, the sintering mechanism is unknown and it is considered that the desired grain growth of silicon carbide results from the presence of sufficient liquid phase comprising aluminum oxide as a main component during ' the sintering and the decomposition and evaporation of the components comprising aluminum oxide as a main component result and aluminum oxide contributed to the dense structure is separated from the molded product to form a strong microstructure intertwining grown prismatic or plate-like grains.
According to the observation through an electron micro-scope, aluminum oxide grains are often found among silicon car~ide grains in the sintered product but a second phase of aluminum oxide is not found at most of the grain boun-dary between the silicon carbide grains. In some of the sintered products, aluminum oxide is not found.
~Z536 A desired structureof the sintered product obtained in the present invention is of the illtertwined ~- and ~-SiC
grains compristng fine prismati~c or plate-like B-SiC grains as a main component. In most sintered products obtained in the present invention, ~-SiC is 80-10 vol% and ~-SiC
is 20-90 vol%. It is found that such structure is obtained by pressureless sintering of a fine ~-SiC powder having a specific surface area of at least 10 m2/g as a source of the strong structure. When ~-SiC is used as a source, it usually provides the structure of block-like SiC grains as a main component.
The sintered product having high strength at high temperature obtained by the hot press process using aluminum oxide as a sintering additive is disclosed in Example 34 of U.S. Patent No. 3,520,656. The structure of the sintered product is that of the hot press process. The strength of the sintered product is not satisfactory at temperatures higher than 1200aC and the disadvantages of the hot press process are not overcome.
It is disclosed in U.S. Patent No. 3,998,646 to obtain a sintered silicon carbide having high strength. It is not clear, whether the process is a hot press process or a pressureless sintering process, and the necessity of an addition of aluminum oxide is not disclosed. A sintered silicon carbide ceramic having the characteristic structure is not disclosed.
This shows that the sintered silicon carbide ceramic of the present invention is different from the conventional silicon carbide-aluminum oxide refractory, the silicon carbide ceramics obtained by the hot press process using aluminum oxide additive or the silicon carbide ceramic ob-tained by the conventional pressureless sintering process.
D
The sources and the production of the present invention will be illust~ated in detail, The silicon carbide CSic~ source can be both of ~-form and ~-form though ~-form is preferable as described above.
Th~ pur~ty IS most preferably at least 98% and is more pre-ferably at least 95~ though it can be 90 to 95% for prac-tical use. The particle size of the fine grains is usually shown by a specific surface area rather than an average partic e diameter.
In order to obtain a sintered product having a flexural strength of at least 25 kg/mm2, especially at least 30 kg/mm2, at room temperature and l~OO~C and a density of at least 3.0 as at least 90% of theoretical density, it is necessary for the source to have a specific surface area of at least 5 m2/g. In order to obtain a sintered product having a flexural strength of at least 35 kg/mm at room temperature and 1400C, it is preferable for the source to have a specific surface area of at least 10 m2/g in the case of 6 to 35 wt.% of aluminum oxide and at least 15 m2/g in the case of 0.5 to ~ wt.% of aluminum oxide and the flexural strength of at least 40 kg/mm .
The aluminum oxide (AQ2O3) source is preferably corrun-dum ~-AQ2O3, but can be another crystalline material, such as the y-form. Aluminum sources, such as aluminum hydroxide and aluminum sulfate, which can be converted into aluminum oxide by heating in a non-oxidative atmosphere can also be used. Thus aluminum oxide also includes a precursor which easily forms aluminum oxide. Aluminum oxide preferably has a purity of at least 98%, a low sodium content and an average particle diameter of up to l~m, preferably up to 0.2~m.
~2536 In the process of the present invention a mixture of the aluminum oxide source `~Q.5 to 35 wt.% as AQ2O3) and silicon carbide is used, It is possible to incorporate a small amount of anotfier aluminum source, such as aluminum nitride (AQN), aluminum carbide (AQ4C3), aluminum diboride (A~B2), aluminum phosphide (A~P), aluminum silicon carbide (AQ4SiC4) and aluminum (AQ).
The amount of the aluminum oxide source relative to the total of aluminum oxide and silicon carbide is in the range of 0.5 to 35 wt.% as A~2O3. When it is less than 0.5 wt.~, the dense structure is not formed in the sinter-ing and a dense sintered product having a density of at least 90% of the theoretical density cannot be obtained.
When it is more than 35 wt.%, the dense structure is formed but the strength is too low even though it is sintered at a lower temperature of up to 1900C. When it is more than 35 wt.% and the molded product is sintered at 1900 to 2300C, the decomposition is severe to form a porous structure and undesired aluminum oxide remains in the sintered product. The content of the aluminum oxide source is preferably in the range of 2 to 20 wt.% as AQ2O3.
When the content of the aluminum oxide source is in a range of 6 to 35 wt.%, and silicon carbide having a speci-fic surface area of at least 10 m2/g and a purity of at least 95%, preferably 98~, is used, it is easily possible to obtain a sintered product having a density of at least 90% of the theoretical density and a flexural strength of at least 35 kg~mm2 at room temperature and 1400C.
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~2~36 When the content of aluminum oxide is in the range of 0.5to 5 wt~ and silicon carhide having a specific sur~ace area of at le~St 15 m2~g and a pur~ty of at least ~8~ is used, it is easily possible to obtain a sintered product having a density of at least ~0% of the theoretical density and a flexural strength of at least 40 kg/mm2 at room temp-erature and 1400C.
In the present invention, it is preferable to prepare a mixture of aluminum oxide source and the substantial resi-dual portion of silicon carbide. It is one of the advan-tages of the process of the present invention to be able to use such sources. It is possible to contain impurities of the silicon carbide source and a small amount of other components incorporated in a pulverizing step. As des-cribed below, certain components, such as silicon oxide, can he incorporated at a relatively large amount. This is one of the advantages of the process of the present invention.
All molding processes for molding ceramics can be em-ployed in the molding step. A press molding process, a slip casting process, an injection molding process and an extrusion molding process can be employed. The sintering is carried out in a non-oxidative atmosphere under pres-sureless conditions at 1900 to 2300C. The non-oxldative atmosphere is suitably selected from an atmosphere of nitro-gen, argon, helium, carbon monoxide and hydrogen. It is especially preferable to have an atmosphere of argon and helium. As mentioned below, the treatment in the atmos-phere containing the aluminum component is especially pre-ferable. As the method of the formation of the non-oxida-tive atmosphere, it is preferable to form the atmosphere containing certain carbon or silicon components as well as the aluminum component and to - -_ g _ 3~L5Z536 place the molded product made of the aluminum oxide source and silicon car~ide in s~ch an atmosphere The temperature fo~ sinterin~ i`s pre~erafily in the ran~e of 1950 to 2100C.
When the sintering temperature is lower than 1900C, the density is not satisfactory and a desired dense sintered product cannot be obtained, whereas when it is hi~her than 2300QC, the molded product is decomposed yielding a porous product. The sintering time is in the range of 1 to ~8 hours, preferably 2 to 24 hours. When the sintering time is too short, the dense structure is not formed, or a sat-isfactory strength is not obtained even though the dense structure is formed. When the sintering time is too lon~, the decomposition is too great and a porous product is disadvantageously formed.
The desired process for producing the sintered product in the present invention will no~ be illustrated. The aluminum oxide source as AQ203 is incorporated as a sinter-ing additive. In the conventional process, the aluminum oxide source is rapidly decomposed or evaporated to be re-moved before contributing to the densifying of the molded product. Therefore, a satisfactory density is not attained and a dense sintered product is not obtained.
In order to overcome these problems, various tests have been made. As a result, it is found to preferably sinter the molded silicon carbide containing the aluminum oxide source in an atmosphere including aluminum or aluminum component. That is, a dense sintered product is easily o~tained by sintering the molded product in the atmosphere including one or more of aluminum and aluminum compounds.
In accordance with such process, an amount of aluminum oxide removed B
i2536 be~ore completing the densifying of the molded product is reduced to obtain a dense ~intered product having a stable formulation and struc-ture. However, silicon carbide itself begins to decompose at the sintering temperature for the molded silicon carbide. That is, silicon carbide is not meltea at the atmospheric pressure and begins to sublimate at a temperature higher than 2000C and is decomposed into carbon and silicon rich vapor at higher temperature. The sintering temperature for producing the dense sintered product in the process of the present invention is in the range of l900 to 2300C. In the high temperature zone, sublimation and decomposition of silicon carbide are caused to generate a silicon vapor and disilicon carbide Si2C.
When the molded silicon carbide is sintered in the atmos-phere containing the Si vapor and Si2C, the sublimationand decomposition of silicon carbide in the molded product can be reduced. But, the decomposition of silicon carbide is not simple in a practical operation. Thus, it results in certain mutual reactions of the aluminum oxide source as the sintering additive, and a silica layer of the surface of silicon carbide grains and other impurities and a small amount of oxygen in the atmosphere. In order to prevent a decomposition of the molded product during the sintering and to obtain a dense sintered product, it is preferable to maintain a partial pressure of the gas in the atmosphere over the equilibrium vapor pressure of the gas generated by the dec~mposition of the molded product.
When the molded silicon carbide containing the aluminum oxide source is sintered, it is difficult to confirm the type of reactions and gases generated. In various tests, it is found to be preferable to sinter -- 11 ~
~52536 the molded silicon carbide containing aluminum oxide in the atmosphere including aluminum and silicon and/or carbon components to obtain a dense sintered product having uniform formulation and structure.
In the decomposition of the molded product in the sintering it is considered to mainly result in the following reaction:
SiC + AQ203 ~ AQ20 + SiO + CO
When the partial pressure of the gases of AQ2O, SiO and CO in the atmosphere during the sintering is more than the equilibrium vapor pressure of the gases generated bythe decoIrpositlon of themolded product, the decomposition of the molded product is reduced to obtain a sintered product having higher density.
The process will be further illustrated.
The atmosphere including the ~luminum component or the aluminum and silicon and/or carbon components is provided by feeding the gas of these components in to the sintering furnace. The aluminum component gas can be fed as AQ, AQCQ3, AQ2C or AQO etc.; the silicon component gas can be fed as Si, SiCQ4, SiH4 or SiO etc. and the carbon component gas can be fed as a hydrocarbon or CO etc.
Usually, the atmosphere is prepared by mixing the gas of these compo-nents with a non-oxidative gas, such as nitrogen, ar~n and helium.
In another method, it is effective to place a powder or a molded product or a sintered product for generating the gas of these compo-nents at the sintering temperature around the molded silicon carbide.
,. c,,~, Thus for example;
~ 1) The atmosphere is ~ormed by aluminum powder, one or more aluminum compound powders or a molded product of the powder which is placed around the molded silicon carbide;
Silicon carb~de is well known as a useful ceramic source having high hardness, excellent wear resistance, a low thermal expansion coefficient, a high decomposition temperature, h~gh oxidation resistance and chemical resis-tance, and a relatively high electrical conductivity. A
dense sintered silicon carbide has these characteristics and also has high strength even at high temperature, high heat shock resistance, and is considered to be an effec-tive source for high temperature structural products andto be useful for various uses, such as in gas turbines.
Silicon carbide has relatively high covalent bond whereby it is difficult to sinter silicon carbide by itself. In order to obtain a dense sintered product, it is necessary to incorporate a certain sintering additive. In the hot press process, boron, B4C, aluminum, AQN or AQ2O3 has been used as a sintering additive. In, for example, U.S. Patent No. 3,836,673, a sintered silicon carbide having a strength of 104 psi (70 kg/mm2) is obtained by the hot press process with the addition of 0.5 to 5 wt.% of aluminum. However, the use of metallic aluminum and the hot press process have the disadvantages given hereafter.
~' ..
,. - -:- - .
.
.
In the pressureless sinte~ing, it is known to incor-porate aluminum and carbon. In, or example, U.S. Patent No. 4,23Q,479, a yroduct having a high flexural strength is obtained by pressureless sintering of a mixture of silicon carbide, 0.2 to 2 wt.% of an aluminum component and 0.1 to 2.0 wt.% of a carbon component. In this pro-cess, aluminum is mainly used and carbon is added for accel-eration of the sintering. As the carbon source, a resin is used and hardening of the resin causes problems in the process. Moreover, the conventional pressureless sintered product has not been satisfactory with respect to its character~stics and process.
The present invention provides a sintered silicon carbide having superior characteristics with respect to conventional products, produced by a pressureless sintering process avoiding a hot press process.
According to the present invention there is provided a sintered silicon carbide ceramic having high strength and a flexural strength of at least 25 kg/mm2 at room temp-erature and 1400C which is obtained by pressureless sin-tering of a mixture of 0.5 to 35 wt.% of aluminum oxide and a remainder portion essentially of silicon carbide.
It is known to use aluminum oxide as a sintering addi-tive for silicon carbide. However~ the characteristics and the production of the convent~onal products are quite dif-ferent from those of the present invention. One type of conventional product is refractory ~ire brick as disclosed in U.S. Patent No. 3,759,725 and British Patent No. 1,460,635 and is obtained by molding a mixture of coarse aggregate of silicon carbide and aluminum oxide and then sintering the molded product at about lZ00 to 1500C. The sintered product comprises a thick layer of aluminum oxide or sili-con oxide disposed around silicon carbide grains.
It has also been proposed in J. Am. Ceram. Soc. 39(11) 386-389, 1956 by Alliegro et al to use aluminum oxide as a sintering additive in the hot press process to produce a dense sintered silicon carbide ceramic.
It was initially considered that the effect of aluminum oxide on silicon carbide in the hot press process is similar to that in the pressureless sintering process. It was also considered that the sintered product obtained by the hot press process has higher density and strength than the sintered product obtained by conventional pressureless sintering when the same molded product made of a mixture having the same formulation is sintered at the same temp-erature by the hot press process and the pressureless process because of the pressure effect. However, it has now been found that a sintered product having higher strength is obtained by the pressureless sintering process compared with the hot press process.
L~
3~52536 As shown in the examples, the strength at high temperature is different to a great extent. It is considered that the sintering mechanism ~n the pressureless sintering process is different from that in the hot press process. The microstructure of the resulting sintered product is also different.
In the hot press process, pressure is applied in the liquid phase comprising aluminum oxide as a main component to easily form a dense structure. However, the sintered product has a microstructure having equiaxed grains (block-like) with aluminum oxide disposed around the silicon car-bide grain. At high temperature, aluminum oxide disposed around the silicon carbide grains softens causing a serious decrease in strength. However, in the pressureless sin-tering process, the sintering mechanism is unknown and it is considered that the desired grain growth of silicon carbide results from the presence of sufficient liquid phase comprising aluminum oxide as a main component during ' the sintering and the decomposition and evaporation of the components comprising aluminum oxide as a main component result and aluminum oxide contributed to the dense structure is separated from the molded product to form a strong microstructure intertwining grown prismatic or plate-like grains.
According to the observation through an electron micro-scope, aluminum oxide grains are often found among silicon car~ide grains in the sintered product but a second phase of aluminum oxide is not found at most of the grain boun-dary between the silicon carbide grains. In some of the sintered products, aluminum oxide is not found.
~Z536 A desired structureof the sintered product obtained in the present invention is of the illtertwined ~- and ~-SiC
grains compristng fine prismati~c or plate-like B-SiC grains as a main component. In most sintered products obtained in the present invention, ~-SiC is 80-10 vol% and ~-SiC
is 20-90 vol%. It is found that such structure is obtained by pressureless sintering of a fine ~-SiC powder having a specific surface area of at least 10 m2/g as a source of the strong structure. When ~-SiC is used as a source, it usually provides the structure of block-like SiC grains as a main component.
The sintered product having high strength at high temperature obtained by the hot press process using aluminum oxide as a sintering additive is disclosed in Example 34 of U.S. Patent No. 3,520,656. The structure of the sintered product is that of the hot press process. The strength of the sintered product is not satisfactory at temperatures higher than 1200aC and the disadvantages of the hot press process are not overcome.
It is disclosed in U.S. Patent No. 3,998,646 to obtain a sintered silicon carbide having high strength. It is not clear, whether the process is a hot press process or a pressureless sintering process, and the necessity of an addition of aluminum oxide is not disclosed. A sintered silicon carbide ceramic having the characteristic structure is not disclosed.
This shows that the sintered silicon carbide ceramic of the present invention is different from the conventional silicon carbide-aluminum oxide refractory, the silicon carbide ceramics obtained by the hot press process using aluminum oxide additive or the silicon carbide ceramic ob-tained by the conventional pressureless sintering process.
D
The sources and the production of the present invention will be illust~ated in detail, The silicon carbide CSic~ source can be both of ~-form and ~-form though ~-form is preferable as described above.
Th~ pur~ty IS most preferably at least 98% and is more pre-ferably at least 95~ though it can be 90 to 95% for prac-tical use. The particle size of the fine grains is usually shown by a specific surface area rather than an average partic e diameter.
In order to obtain a sintered product having a flexural strength of at least 25 kg/mm2, especially at least 30 kg/mm2, at room temperature and l~OO~C and a density of at least 3.0 as at least 90% of theoretical density, it is necessary for the source to have a specific surface area of at least 5 m2/g. In order to obtain a sintered product having a flexural strength of at least 35 kg/mm at room temperature and 1400C, it is preferable for the source to have a specific surface area of at least 10 m2/g in the case of 6 to 35 wt.% of aluminum oxide and at least 15 m2/g in the case of 0.5 to ~ wt.% of aluminum oxide and the flexural strength of at least 40 kg/mm .
The aluminum oxide (AQ2O3) source is preferably corrun-dum ~-AQ2O3, but can be another crystalline material, such as the y-form. Aluminum sources, such as aluminum hydroxide and aluminum sulfate, which can be converted into aluminum oxide by heating in a non-oxidative atmosphere can also be used. Thus aluminum oxide also includes a precursor which easily forms aluminum oxide. Aluminum oxide preferably has a purity of at least 98%, a low sodium content and an average particle diameter of up to l~m, preferably up to 0.2~m.
~2536 In the process of the present invention a mixture of the aluminum oxide source `~Q.5 to 35 wt.% as AQ2O3) and silicon carbide is used, It is possible to incorporate a small amount of anotfier aluminum source, such as aluminum nitride (AQN), aluminum carbide (AQ4C3), aluminum diboride (A~B2), aluminum phosphide (A~P), aluminum silicon carbide (AQ4SiC4) and aluminum (AQ).
The amount of the aluminum oxide source relative to the total of aluminum oxide and silicon carbide is in the range of 0.5 to 35 wt.% as A~2O3. When it is less than 0.5 wt.~, the dense structure is not formed in the sinter-ing and a dense sintered product having a density of at least 90% of the theoretical density cannot be obtained.
When it is more than 35 wt.%, the dense structure is formed but the strength is too low even though it is sintered at a lower temperature of up to 1900C. When it is more than 35 wt.% and the molded product is sintered at 1900 to 2300C, the decomposition is severe to form a porous structure and undesired aluminum oxide remains in the sintered product. The content of the aluminum oxide source is preferably in the range of 2 to 20 wt.% as AQ2O3.
When the content of the aluminum oxide source is in a range of 6 to 35 wt.%, and silicon carbide having a speci-fic surface area of at least 10 m2/g and a purity of at least 95%, preferably 98~, is used, it is easily possible to obtain a sintered product having a density of at least 90% of the theoretical density and a flexural strength of at least 35 kg~mm2 at room temperature and 1400C.
.~
~ .
:
. .
~2~36 When the content of aluminum oxide is in the range of 0.5to 5 wt~ and silicon carhide having a specific sur~ace area of at le~St 15 m2~g and a pur~ty of at least ~8~ is used, it is easily possible to obtain a sintered product having a density of at least ~0% of the theoretical density and a flexural strength of at least 40 kg/mm2 at room temp-erature and 1400C.
In the present invention, it is preferable to prepare a mixture of aluminum oxide source and the substantial resi-dual portion of silicon carbide. It is one of the advan-tages of the process of the present invention to be able to use such sources. It is possible to contain impurities of the silicon carbide source and a small amount of other components incorporated in a pulverizing step. As des-cribed below, certain components, such as silicon oxide, can he incorporated at a relatively large amount. This is one of the advantages of the process of the present invention.
All molding processes for molding ceramics can be em-ployed in the molding step. A press molding process, a slip casting process, an injection molding process and an extrusion molding process can be employed. The sintering is carried out in a non-oxidative atmosphere under pres-sureless conditions at 1900 to 2300C. The non-oxldative atmosphere is suitably selected from an atmosphere of nitro-gen, argon, helium, carbon monoxide and hydrogen. It is especially preferable to have an atmosphere of argon and helium. As mentioned below, the treatment in the atmos-phere containing the aluminum component is especially pre-ferable. As the method of the formation of the non-oxida-tive atmosphere, it is preferable to form the atmosphere containing certain carbon or silicon components as well as the aluminum component and to - -_ g _ 3~L5Z536 place the molded product made of the aluminum oxide source and silicon car~ide in s~ch an atmosphere The temperature fo~ sinterin~ i`s pre~erafily in the ran~e of 1950 to 2100C.
When the sintering temperature is lower than 1900C, the density is not satisfactory and a desired dense sintered product cannot be obtained, whereas when it is hi~her than 2300QC, the molded product is decomposed yielding a porous product. The sintering time is in the range of 1 to ~8 hours, preferably 2 to 24 hours. When the sintering time is too short, the dense structure is not formed, or a sat-isfactory strength is not obtained even though the dense structure is formed. When the sintering time is too lon~, the decomposition is too great and a porous product is disadvantageously formed.
The desired process for producing the sintered product in the present invention will no~ be illustrated. The aluminum oxide source as AQ203 is incorporated as a sinter-ing additive. In the conventional process, the aluminum oxide source is rapidly decomposed or evaporated to be re-moved before contributing to the densifying of the molded product. Therefore, a satisfactory density is not attained and a dense sintered product is not obtained.
In order to overcome these problems, various tests have been made. As a result, it is found to preferably sinter the molded silicon carbide containing the aluminum oxide source in an atmosphere including aluminum or aluminum component. That is, a dense sintered product is easily o~tained by sintering the molded product in the atmosphere including one or more of aluminum and aluminum compounds.
In accordance with such process, an amount of aluminum oxide removed B
i2536 be~ore completing the densifying of the molded product is reduced to obtain a dense ~intered product having a stable formulation and struc-ture. However, silicon carbide itself begins to decompose at the sintering temperature for the molded silicon carbide. That is, silicon carbide is not meltea at the atmospheric pressure and begins to sublimate at a temperature higher than 2000C and is decomposed into carbon and silicon rich vapor at higher temperature. The sintering temperature for producing the dense sintered product in the process of the present invention is in the range of l900 to 2300C. In the high temperature zone, sublimation and decomposition of silicon carbide are caused to generate a silicon vapor and disilicon carbide Si2C.
When the molded silicon carbide is sintered in the atmos-phere containing the Si vapor and Si2C, the sublimationand decomposition of silicon carbide in the molded product can be reduced. But, the decomposition of silicon carbide is not simple in a practical operation. Thus, it results in certain mutual reactions of the aluminum oxide source as the sintering additive, and a silica layer of the surface of silicon carbide grains and other impurities and a small amount of oxygen in the atmosphere. In order to prevent a decomposition of the molded product during the sintering and to obtain a dense sintered product, it is preferable to maintain a partial pressure of the gas in the atmosphere over the equilibrium vapor pressure of the gas generated by the dec~mposition of the molded product.
When the molded silicon carbide containing the aluminum oxide source is sintered, it is difficult to confirm the type of reactions and gases generated. In various tests, it is found to be preferable to sinter -- 11 ~
~52536 the molded silicon carbide containing aluminum oxide in the atmosphere including aluminum and silicon and/or carbon components to obtain a dense sintered product having uniform formulation and structure.
In the decomposition of the molded product in the sintering it is considered to mainly result in the following reaction:
SiC + AQ203 ~ AQ20 + SiO + CO
When the partial pressure of the gases of AQ2O, SiO and CO in the atmosphere during the sintering is more than the equilibrium vapor pressure of the gases generated bythe decoIrpositlon of themolded product, the decomposition of the molded product is reduced to obtain a sintered product having higher density.
The process will be further illustrated.
The atmosphere including the ~luminum component or the aluminum and silicon and/or carbon components is provided by feeding the gas of these components in to the sintering furnace. The aluminum component gas can be fed as AQ, AQCQ3, AQ2C or AQO etc.; the silicon component gas can be fed as Si, SiCQ4, SiH4 or SiO etc. and the carbon component gas can be fed as a hydrocarbon or CO etc.
Usually, the atmosphere is prepared by mixing the gas of these compo-nents with a non-oxidative gas, such as nitrogen, ar~n and helium.
In another method, it is effective to place a powder or a molded product or a sintered product for generating the gas of these compo-nents at the sintering temperature around the molded silicon carbide.
,. c,,~, Thus for example;
~ 1) The atmosphere is ~ormed by aluminum powder, one or more aluminum compound powders or a molded product of the powder which is placed around the molded silicon carbide;
(2) The atmosphere is formed by one or more of alu-minum powders, aluminum compound powders and one or more of silicon powders, one or more silicon compound powders, car-bon powder, and carbon compound powders or an unsintered molded product of the powder which is placed around the molded silicon carbide; or
(3) The atmosphere is formed by a sintered product of silicon carbide containing aluminum and or an aluminum compound which is placed around the molded silicon carbide.
As the method of placing the powder around the molded silicon carbide, the molded product is buried in the powder or is placed in a casing having a carbon or silicon car-bide powder coating on the inner wall. The method of burying the molded product in the powder is preferable be-cause the decomposition of the molded product is greatly reduced. However, this method is not suitable for a large molded product or a molded product having a complicated shape. The method of coating the powder on the inner wall of the casing is suitable for molded products having various shapes. The surface condition of the sintered product is superior and the dense sintered product having the charac-teristics similar to the burying method can be obtained.
The coating method is attained by coating a slurry of the powder and an organic medium, such as alcohol and acetone, or water on the casing. It is possible to incor-porate a binder, such as polyvinyl alcohol, in the slurry.
B
.
In the method of burying into the powder or coating with the powder, the above-mentioned powders can be used.
It is preferable to use a mixture o~ aluminum or an alum-inum co~pound and silicon carbide powder and/or carbon powder. As the aluminum powder or the aluminum compound powder, it is preferable to use aluminum oxide powder though it is possible for example t~ use aluminum hydroxide, aluminum nitride or aluminum car~ide. In the powder coat-ing method, it is possible to use an aromatic polymer which leaves high carbon residue, such as phenol resins, and polymethyl phenylene instead of carbon powder.
When the silicon carbide powder and/or carbon powder is mixed with aluminum powder or the aluminum compound powder, an amount of the aluminum component is in the range of 2 to 40 wt.% as AQ. When it is less than 2 wt.%, the prevention of the decomposition of the molded product is not sufficiently high and a desired dense sintered product is not obtained. When it is more than 40 wt.%, the decom-position velocity of the powder is too high and the weightloss is, disadvantageously, too great even though it has high density.
When the molded product is buried in aluminum powder or aluminum oxide powder, the immersion of the liquid phase of the aluminum component is disadvantageously caused.
It is preferable to use a sintered product instead of the powder or the unsintered molded product. It is also, preferable to use the sintered silicon carbide containing aluminum or the ' - .
~2536 aluminu~ compound. In this case~ it is preferable to place the sintered product having a similar formulation axound the molded product for sintering though it is possible to place a sintered product having different formulation.
When a sintered product is used, the surface area of the sintered product is small and the decomposition velocity is low which has the effect of maintaining the atmosphere in a desired condition for a long time in comparison with the use of the powder or the unsintered molded product.
This is suitable for sintering over a long period of time.
The eharacteristic feature and advantage of the present invention are further illustrated.
1) A dense sintered silicon carbide ceramic having high strength can be easily obtained by the pressureless sintering process.
For example, when a sintering additive of AQ, A~N, B
or B4C is used, the hot press process is needed and a pro-duct having a complicated shape or a large size cannot be produced. Aluminum is easily oxidized so as to be diffi-cult for use and reacts to cause foaming or contact with water, and a fine aluminum powder is dangerously explosive.
However, B, B4C and AQN are expensive and fine powder there-of is not easily available, and these materials are not easily ground.
2) A sintered product having higher strength than the conventional product can ~e obtained.
For example, a sintering additive, such as B+C, B4C+C, AR+C and AQN+C, can be used for the pressureless sintering.
~,~
~2536 However, an aromatic pol~mer, such as polyphenylmethylene and phenol resins, is usually used as the C source. The handling is not easy and the C source should be uniformly mixed re~uiring a long operating time. The strength of the product in the case of s-C additive, is relatively low, such as 40 to 50 kg~mm2, at room temperature. In the case of such sintering additive, the silicon carbide source is quite fine, such as a specific surface area of at least 15 m /g. It is necessary to use the sources containing less SiO2 though the sources having a high SiO2 content can be used in the present invention.
3) Aluminum oxide is stable and is not reactive with water. A step of contacting with water can be employed.
Wet mixing, grinding and a slip casting using water can be employed and the atmosphere is not limited.
As the method of placing the powder around the molded silicon carbide, the molded product is buried in the powder or is placed in a casing having a carbon or silicon car-bide powder coating on the inner wall. The method of burying the molded product in the powder is preferable be-cause the decomposition of the molded product is greatly reduced. However, this method is not suitable for a large molded product or a molded product having a complicated shape. The method of coating the powder on the inner wall of the casing is suitable for molded products having various shapes. The surface condition of the sintered product is superior and the dense sintered product having the charac-teristics similar to the burying method can be obtained.
The coating method is attained by coating a slurry of the powder and an organic medium, such as alcohol and acetone, or water on the casing. It is possible to incor-porate a binder, such as polyvinyl alcohol, in the slurry.
B
.
In the method of burying into the powder or coating with the powder, the above-mentioned powders can be used.
It is preferable to use a mixture o~ aluminum or an alum-inum co~pound and silicon carbide powder and/or carbon powder. As the aluminum powder or the aluminum compound powder, it is preferable to use aluminum oxide powder though it is possible for example t~ use aluminum hydroxide, aluminum nitride or aluminum car~ide. In the powder coat-ing method, it is possible to use an aromatic polymer which leaves high carbon residue, such as phenol resins, and polymethyl phenylene instead of carbon powder.
When the silicon carbide powder and/or carbon powder is mixed with aluminum powder or the aluminum compound powder, an amount of the aluminum component is in the range of 2 to 40 wt.% as AQ. When it is less than 2 wt.%, the prevention of the decomposition of the molded product is not sufficiently high and a desired dense sintered product is not obtained. When it is more than 40 wt.%, the decom-position velocity of the powder is too high and the weightloss is, disadvantageously, too great even though it has high density.
When the molded product is buried in aluminum powder or aluminum oxide powder, the immersion of the liquid phase of the aluminum component is disadvantageously caused.
It is preferable to use a sintered product instead of the powder or the unsintered molded product. It is also, preferable to use the sintered silicon carbide containing aluminum or the ' - .
~2536 aluminu~ compound. In this case~ it is preferable to place the sintered product having a similar formulation axound the molded product for sintering though it is possible to place a sintered product having different formulation.
When a sintered product is used, the surface area of the sintered product is small and the decomposition velocity is low which has the effect of maintaining the atmosphere in a desired condition for a long time in comparison with the use of the powder or the unsintered molded product.
This is suitable for sintering over a long period of time.
The eharacteristic feature and advantage of the present invention are further illustrated.
1) A dense sintered silicon carbide ceramic having high strength can be easily obtained by the pressureless sintering process.
For example, when a sintering additive of AQ, A~N, B
or B4C is used, the hot press process is needed and a pro-duct having a complicated shape or a large size cannot be produced. Aluminum is easily oxidized so as to be diffi-cult for use and reacts to cause foaming or contact with water, and a fine aluminum powder is dangerously explosive.
However, B, B4C and AQN are expensive and fine powder there-of is not easily available, and these materials are not easily ground.
2) A sintered product having higher strength than the conventional product can ~e obtained.
For example, a sintering additive, such as B+C, B4C+C, AR+C and AQN+C, can be used for the pressureless sintering.
~,~
~2536 However, an aromatic pol~mer, such as polyphenylmethylene and phenol resins, is usually used as the C source. The handling is not easy and the C source should be uniformly mixed re~uiring a long operating time. The strength of the product in the case of s-C additive, is relatively low, such as 40 to 50 kg~mm2, at room temperature. In the case of such sintering additive, the silicon carbide source is quite fine, such as a specific surface area of at least 15 m /g. It is necessary to use the sources containing less SiO2 though the sources having a high SiO2 content can be used in the present invention.
3) Aluminum oxide is stable and is not reactive with water. A step of contacting with water can be employed.
Wet mixing, grinding and a slip casting using water can be employed and the atmosphere is not limited.
4~ Even though the purity of the silicon carbide source is low (even lower than 95%) or the grain size is relatively large teven a specific surface area of less than lOm /g), such factors do not greatly affect the sintering process and characteristics of the product. It is unneces-sary to remove the silicon oxide layer from the surface of the silicon carbide powder. It is possible to add silicon oxide.
The present invention is remarkably advantageous in an industrial scale.
The present invention will be further illustrated, by the following Examples and References.
.
- ;
EXAMP~ES 1 to 6 and RE:FERENCES 1 to 3:
In Examples 1 to 6, ~- o:r c~- silicon carbide powder and aluminum oxide powder (corundum) having a purity higher than 95% and an average particle diameter less than l,um shown in Table 1 were thoroughly mixed with ethanol and each mixture was molded by hydraulic isostatic pressure molding under a pressure of 2000 kg/cm2 to form a molded product having a size of 20 x 40 x 15 mm. Each molded product was held in a carbon casing having a cover slightly :
larger than the volume of the molded product. The carbon casing was placed in an argon gas atmosphere and the molded product was sintered under the conditions shown in Table 1.
In References 1 to 3, each mixture was treated by a hot press in a carbon mold having an inner diameter of 30 mm under a pressure of 200 kg/cm2. The densities and flexural strengths of the sintered products are shown in Table 1.
B
.. ~ ..
. . . . .;. . . .
. ~ . .. ~. . ..
~. ` .
. ... ~. ~ ~ , Table i SiC Exp.l Exp~ 2 _~E~ ~ Exp. S Exp. S :~
Ma~in crystalline ~ ~ ~ ~ ~ a Purity (%) 99< 99< 99< 99< 99< 95<
Specific surface 13 . 4 13 . 4 13 . 413 . 4 18.1 7 . 0 A Q 2 3 ~ ~
Content ( %) 25 15 3 2 3 13 Sinterin g Temperature~C) 2000 2000 2000 2000 2000 1950 Time (hr. ) 5 5 5 5 5 5 Density (g/cm3) 3. 06 3.11 3.13 3.14 3.18 3.13 Flexural strength (kg/mm2) Room temperature 76 . 7 87 . 9 61. 6 45 . 6 65 . 1 56 . 2 1400C 44.7 53. 0 34.3 32.0 46. 1 38. 7 :
., :
- , ~' ` ' ~
~5;~536 Table 1 (cont'à) ¦ Ref. 1 Ref. 2 Ref. 3 sic Main crystalline ~ ~ ~
Purity (%) 99< 99< 99<
Speci~lc surface 7 4 13. 4 7. 4 area (m2/g) .
A~?~
Content (96) 13 Sintering Temperature (C) 1950 2100 2200 Time (hr. ) 1 Density (g/cm3) 3.19 3, 20 3.19 Flexural strength (kg/mm2) Room temperature 43 . 0 54. 9 77. 0 1400C 15. 8 19. 3 43. 2 ~ . ~
:~:
S3~i The samples were respectiYely cut and the structures of the cut sur~aces of the samples were obseryed under a microscope~ The results are a~ ~ollows.
Samples l, 2~ ~ and 5:
A microstructure uni~ormly intertwining prismatic or plate-like SiC grains having a long axis of about 2-5~ which comprises fine SiC grains.
Sample 4:
The same as Samples 1, except containing, large grown grains and having slightly non-uniform grain sizes.
Sample 6:
r A microstructure closely bonding SiC grains in a block-like manner having a diametex of l to 5~.
EXAMPLES 7 to 16:
~ ilicon carbide powder having a purity of 99% and a specific surface area of at least 10 m2/g (commercially available) was used. Each sintering additive shown in Table 2 was mixed in the amount shown in Table 2. Each mixture was charged in a plastic pot and thoroughly mixed with plastic balls in the presence of acetone and the mixture was dried and pressed under a pressure of 300 kg/cm to 30 prepare tne molded product having a size of 20 x 20 x 40 mm.
Each molded product was sintered in an electric resistance furnace under the conditions shown in Table 2 at 2000C
for 3 hours.
. ~, ~, .
~52S36 Table 2 Exp. 7 Exp. 8 I Exp. 9Exp. 10Exp . 11 Sintering addi~ive Kind AQ2O3 AQ2O3 AQ2O3 AQ2O3 AQ2O3 Content (wt . %)4 4 4 4 4 Atmosphere Method Bury Coat*3 Coat Coat Coat Kind AQ2O3 20 AQ2 3 AQ2O3 30AQ2O3 40 AQ2O3 30 C 80 100 SiC 70SiC 60 resin30 Content (wt. %) SiC 40 Density (g/cm3) 3.16 3. 06 3 14 3. 09 3 .13 Flexural stren gth .
(kg/mm2) Room temperature 70 . 2 58. 3 69. 4 52 . 6 64 . 7 1400C 41. 5 38. 1 40 . 3 41. 5 39. 7 B
31~
Table 2 (c~nt'd) Exp. 12 Exp. 13 Exp. 14 Exp. 15 Exp. 16 _ Sintering additive Kind 2 3 AQ2O3 AQ2O3 AQ2O3 ~Q23 Content (wt. %) 4 6 9 12 1 Atmosphere *~
l 0 Method Coat pMrodd . B ury B ury B ury Kind AQ2O3 50 AQ2O3 20 AQ2O3 10 AQ2O3 10 AQ2O3 lQ
Content (wt . ~ ) SiO2 50 SiC 80 SiC 90 iC 90 SiC 90 _ Density (g/cm3) 3.04 3.14 3.12 3.11 2.92 Flexural strength (kg/mm2) Room temperature 50.5 75.4 72.8 fi9.3 27.1 1400C 35.8 45.1 42.9 ~7.3 25.3 Note: *1 ~-SiC was used as the silicon carbide source (the other sources are ,B-SiC) *2 Burying method: A molded product was buried in a - mixed powder.
(the kind and content are shown in the Table) *3 Coating method: A slurry of the mixed powder and ~ ethanol was coated on an inner wall of a carbon casing and was dried.
The molded product was held therein.
me coated thickness was akout 0.5 rr,m.
, .. .. . ~ .
, -~152536 *4 Molded product: A molded product was placed in anunsintered molded product casing made of the mixed powder.
The structures of the cut surfaces of the sintered products are as follows:
Samples 7 to 9 and 11 to 15:
The same as Sample 1.
Sample 10:
The same as Sample 6.
Sample 16:
The same as Sample 4.
~5 ~:
The present invention is remarkably advantageous in an industrial scale.
The present invention will be further illustrated, by the following Examples and References.
.
- ;
EXAMP~ES 1 to 6 and RE:FERENCES 1 to 3:
In Examples 1 to 6, ~- o:r c~- silicon carbide powder and aluminum oxide powder (corundum) having a purity higher than 95% and an average particle diameter less than l,um shown in Table 1 were thoroughly mixed with ethanol and each mixture was molded by hydraulic isostatic pressure molding under a pressure of 2000 kg/cm2 to form a molded product having a size of 20 x 40 x 15 mm. Each molded product was held in a carbon casing having a cover slightly :
larger than the volume of the molded product. The carbon casing was placed in an argon gas atmosphere and the molded product was sintered under the conditions shown in Table 1.
In References 1 to 3, each mixture was treated by a hot press in a carbon mold having an inner diameter of 30 mm under a pressure of 200 kg/cm2. The densities and flexural strengths of the sintered products are shown in Table 1.
B
.. ~ ..
. . . . .;. . . .
. ~ . .. ~. . ..
~. ` .
. ... ~. ~ ~ , Table i SiC Exp.l Exp~ 2 _~E~ ~ Exp. S Exp. S :~
Ma~in crystalline ~ ~ ~ ~ ~ a Purity (%) 99< 99< 99< 99< 99< 95<
Specific surface 13 . 4 13 . 4 13 . 413 . 4 18.1 7 . 0 A Q 2 3 ~ ~
Content ( %) 25 15 3 2 3 13 Sinterin g Temperature~C) 2000 2000 2000 2000 2000 1950 Time (hr. ) 5 5 5 5 5 5 Density (g/cm3) 3. 06 3.11 3.13 3.14 3.18 3.13 Flexural strength (kg/mm2) Room temperature 76 . 7 87 . 9 61. 6 45 . 6 65 . 1 56 . 2 1400C 44.7 53. 0 34.3 32.0 46. 1 38. 7 :
., :
- , ~' ` ' ~
~5;~536 Table 1 (cont'à) ¦ Ref. 1 Ref. 2 Ref. 3 sic Main crystalline ~ ~ ~
Purity (%) 99< 99< 99<
Speci~lc surface 7 4 13. 4 7. 4 area (m2/g) .
A~?~
Content (96) 13 Sintering Temperature (C) 1950 2100 2200 Time (hr. ) 1 Density (g/cm3) 3.19 3, 20 3.19 Flexural strength (kg/mm2) Room temperature 43 . 0 54. 9 77. 0 1400C 15. 8 19. 3 43. 2 ~ . ~
:~:
S3~i The samples were respectiYely cut and the structures of the cut sur~aces of the samples were obseryed under a microscope~ The results are a~ ~ollows.
Samples l, 2~ ~ and 5:
A microstructure uni~ormly intertwining prismatic or plate-like SiC grains having a long axis of about 2-5~ which comprises fine SiC grains.
Sample 4:
The same as Samples 1, except containing, large grown grains and having slightly non-uniform grain sizes.
Sample 6:
r A microstructure closely bonding SiC grains in a block-like manner having a diametex of l to 5~.
EXAMPLES 7 to 16:
~ ilicon carbide powder having a purity of 99% and a specific surface area of at least 10 m2/g (commercially available) was used. Each sintering additive shown in Table 2 was mixed in the amount shown in Table 2. Each mixture was charged in a plastic pot and thoroughly mixed with plastic balls in the presence of acetone and the mixture was dried and pressed under a pressure of 300 kg/cm to 30 prepare tne molded product having a size of 20 x 20 x 40 mm.
Each molded product was sintered in an electric resistance furnace under the conditions shown in Table 2 at 2000C
for 3 hours.
. ~, ~, .
~52S36 Table 2 Exp. 7 Exp. 8 I Exp. 9Exp. 10Exp . 11 Sintering addi~ive Kind AQ2O3 AQ2O3 AQ2O3 AQ2O3 AQ2O3 Content (wt . %)4 4 4 4 4 Atmosphere Method Bury Coat*3 Coat Coat Coat Kind AQ2O3 20 AQ2 3 AQ2O3 30AQ2O3 40 AQ2O3 30 C 80 100 SiC 70SiC 60 resin30 Content (wt. %) SiC 40 Density (g/cm3) 3.16 3. 06 3 14 3. 09 3 .13 Flexural stren gth .
(kg/mm2) Room temperature 70 . 2 58. 3 69. 4 52 . 6 64 . 7 1400C 41. 5 38. 1 40 . 3 41. 5 39. 7 B
31~
Table 2 (c~nt'd) Exp. 12 Exp. 13 Exp. 14 Exp. 15 Exp. 16 _ Sintering additive Kind 2 3 AQ2O3 AQ2O3 AQ2O3 ~Q23 Content (wt. %) 4 6 9 12 1 Atmosphere *~
l 0 Method Coat pMrodd . B ury B ury B ury Kind AQ2O3 50 AQ2O3 20 AQ2O3 10 AQ2O3 10 AQ2O3 lQ
Content (wt . ~ ) SiO2 50 SiC 80 SiC 90 iC 90 SiC 90 _ Density (g/cm3) 3.04 3.14 3.12 3.11 2.92 Flexural strength (kg/mm2) Room temperature 50.5 75.4 72.8 fi9.3 27.1 1400C 35.8 45.1 42.9 ~7.3 25.3 Note: *1 ~-SiC was used as the silicon carbide source (the other sources are ,B-SiC) *2 Burying method: A molded product was buried in a - mixed powder.
(the kind and content are shown in the Table) *3 Coating method: A slurry of the mixed powder and ~ ethanol was coated on an inner wall of a carbon casing and was dried.
The molded product was held therein.
me coated thickness was akout 0.5 rr,m.
, .. .. . ~ .
, -~152536 *4 Molded product: A molded product was placed in anunsintered molded product casing made of the mixed powder.
The structures of the cut surfaces of the sintered products are as follows:
Samples 7 to 9 and 11 to 15:
The same as Sample 1.
Sample 10:
The same as Sample 6.
Sample 16:
The same as Sample 4.
~5 ~:
Claims (26)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A dense sintered silicon carbide ceramic having high strength and a flexural strength of at least 25 kg/mm2 at room temperature and 1400°C which is obtained by molding a mixture of an aluminum oxide source in an amount of 0.5 to 35 wt.% as A?2O3 and silicon carbide in a substantial remainder portion followed by pressureless sintering.
2. The dense sintered silicon carbide ceramic accord-ing to Claim 1 which has a flexural strength of at least 35 kg/mm2 at room temperature and 1400°C which is obtained by molding a mixture of aluminum oxide source in an amount of 6 to 35 wt% as A?2O3 and silicon carbide in a substan-tial remainder portion followed by pressureless sintering.
3. The dense sintered silicon carbide ceramic accord-ing to Claim 1 which has a flexural strength of at least 40 kg/mm2 at room temperature and 1400°C which is obtained by molding a mixture of aluminum oxide source in an amount of 0.5 to 6 wt.% as A?2O3 and silicon carbide in a sub-stantial remainder portion followed by pressureless sin-tering.
4. The dense sintered silicon carbide ceramic accord-ing to Claim 1 wherein said aluminum oxide source is incor-porated in an amount of 2 to 20 wt.% as A?2O3.
5. The dense sintered silicon carbide ceramic accord-ing to Claim 1 wherein a mixture of aluminum oxide source and .beta.-silicon carbide is used to form a microstructure having intertwined fine prismatic or plate-like silicon carbide.
6. The dense sintered silicon carbide ceramic accord-ing to Claim 5 which has a flexural strength of at least 40 kg/mm2 at room temperature and 1400°C.
7. A process for producing a dense sintered silicon carbide ceramic having high strength and a flexural strength of at least 25 kg/mm2 at room temperature and 1400°C which comprises molding a mixture of an aluminum oxide source in an amount of 0.5 to 35 wt.% as A?2O3 and silicon carbide in a substantial remainder portion followed by pressure-less sintering in a non-oxidative atmosphere at 1900 to 2300°C.
8. The process according to Claim 7 wherein said aluminum oxide source is incorporated in an amount of 2 to 20 wt.% as A?2O3.
9. The process according to Claim 8 wherein aluminum oxide is used as said aluminum oxide source.
10. The process according to Claim 7 wherein a mixture of said aluminum oxide source in an amount of 6 to 35 wt.%
and silicon carbide having a specific surface area of at least 10 m2/g and a purity of at least 95% in a substantial remainder portion is molded and treated by said pressure-less sintering to obtain the product having a flexural strength of at least 35 kg/mm2 at room temperature and 1400°C and a density of at least 3Ø
and silicon carbide having a specific surface area of at least 10 m2/g and a purity of at least 95% in a substantial remainder portion is molded and treated by said pressure-less sintering to obtain the product having a flexural strength of at least 35 kg/mm2 at room temperature and 1400°C and a density of at least 3Ø
11. The process according to Claim 7 wherein a mix-ture of said aluminum oxide source in an amount of 0.5 to 6 wt.% and silicon carbide having a specific surface area of at least 15 m2/g and a purity of at least 98% in a substantial remainder portion is molded and treated by said pressureless sintering to obtain the product having a flexural strength of at least 40 kg/mm2 at room temperature and 1400°C and a density of at least 3Ø
12. The process according to Claim 7 wherein .beta.-silicon carbide having a specific surface area of at least 10 m2/g and a purity of at least 98% is used to obtain a sintered silicon carbide having a flexural strength of at least 40 kg/mm2 at room temperature and 1400°C.
13. The process according to Claim 7 wherein said aluminum oxide source is incorporated in an amount of 2 to 20 wt.% as A?203.
14) A process for producing a sintered silicon carbide ceramic having high strength and high flexural strength of at least 25 kg/mm2 at room temperature and 1400°C which comprises molding a mixture of an aluminum oxide source and silicon carbide in a substantial remainder portion and sintering said molded product in a non-oxidative atmosphere having an aluminum component.
15) The process according to Claim 14 wherein said sintering is carried out at 1900 to 2300°C.
16) The process according to Claim 14 wherein said atmosphere around said silicon carbide molded product is formed by aluminum powder and/or aluminum compound powder or a molded product of said powder or a sintered product obtained by sintering said molded product.
17) The process according to Claim 16 wherein said aluminum compound powder is one or more of alumina, aluminum nitride, aluminum carbide, aluminum silicon carbide (A?4SiC4), aluminum boride and aluminum phosphide.
18) The process according to Claim 16 wherein said aluminum powder and/or aluminum compound powder or a molded product of said powder or a sintered product obtained by sintering said molded product contains 2 to 40 wt.% of aluminum component.
19. The process according to Claim 14 wherein said aluminum source comprises at least a portion of alumina (A?203).
20. The process according to claim 14 wherein said atmosphere comprises aluminum, silicon and/or carbon as components.
21. The process according to Claim 14 wherein said atmosphere comprises an inert gas.
22. A process for producing a sintered silicon car-bide having a high strength and a flexural strength of at least 25 kg/mm2 at a room temperature and 1400°C which comprises molding a mixture of an aluminum oxide source in an amount of 0.5 to 35 wt.% as A?2O3 and silicon carbide having a specific surface area of at least 10 m2/g and a purity of at least 95% and sintering said molded product in an atmosphere comprising an aluminum component by pressureless sintering at 1900 to 2300°C.
23. The process according to Claim 22 wherein a mix-ture of an aluminum oxide source in an amount of 2 to 20 wt.% as A?203 and silicon carbide in the remainder por-tion is sintered to obtain a sintered product having a flexural strength of at least 35 kg/mm2 at a room tempera-ture and 1400°C.
24) The process according to Claim 23 wherein aluminum oxide is used as said aluminum oxide source.
25) The process according to Claim 22 wherein .beta.-silicon carbide is used as said silicon carbide.
26) The process according to Claim 22 wherein said atmosphere comprising an aluminum component is formed by aluminum powder and/or an aluminum compound or a molded product of said pow-der or a sintered product obtained by sintering said molded product.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP9682880A JPS5722171A (en) | 1980-07-17 | 1980-07-17 | Manufacture of high density silicon carbide sintered body |
| JP96828/1980 | 1980-07-17 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1152536A true CA1152536A (en) | 1983-08-23 |
Family
ID=14175406
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000381702A Expired CA1152536A (en) | 1980-07-17 | 1981-07-14 | Dense sintered silicon carbide ceramic |
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|---|---|
| JP (1) | JPS5722171A (en) |
| CA (1) | CA1152536A (en) |
Cited By (1)
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| CN114591085A (en) * | 2022-01-19 | 2022-06-07 | 宁波伏尔肯科技股份有限公司 | Silicon carbide ceramic and preparation method thereof |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0651591B2 (en) * | 1984-07-17 | 1994-07-06 | 株式会社ノリタケカンパニーリミテド | Method for manufacturing SiC-based sintered body |
| JPS6212664A (en) * | 1985-07-09 | 1987-01-21 | 株式会社ノリタケカンパニーリミテド | Method of sintering b4c base composite body |
| JP2632046B2 (en) * | 1989-08-08 | 1997-07-16 | トヨタ自動車株式会社 | Method for producing silicon nitride sintered body |
| CA2071522C (en) * | 1989-10-26 | 1996-12-17 | John Drennan | Dense sic ceramic products |
| US5258793A (en) * | 1990-04-06 | 1993-11-02 | Minolta Camera Kabushiki Kaisha | Structure of cartridge chamber and cover of a camera |
| JP2570384Y2 (en) * | 1991-01-19 | 1998-05-06 | 河村電器産業株式会社 | Terminal block cover structure of circuit breaker |
| JPH0812434A (en) * | 1993-11-01 | 1996-01-16 | Noritake Co Ltd | Production of sintered b4c material and sintered b4c compact |
| JP2004292197A (en) * | 2003-03-26 | 2004-10-21 | Ngk Insulators Ltd | Method of manufacturing honeycomb structure |
| CN115141951A (en) * | 2022-08-08 | 2022-10-04 | 河南瀚银光电科技股份有限公司 | Method for preparing high-performance aluminum-based silicon carbide |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3520656A (en) * | 1966-03-30 | 1970-07-14 | Du Pont | Silicon carbide compositions |
| JPS5165111A (en) * | 1974-11-11 | 1976-06-05 | Norton Co | |
| US4080415A (en) * | 1976-11-22 | 1978-03-21 | The Carborundum Company | Method of producing high density silicon carbide product |
| US4179486A (en) * | 1978-07-21 | 1979-12-18 | Rockwell International Corporation | Method of protecting Si3 N4 ceramic alloy during heating |
| JPS5547269A (en) * | 1978-09-27 | 1980-04-03 | Kagaku Gijutsucho Mukizai | Manufacture of thialon sintered body |
-
1980
- 1980-07-17 JP JP9682880A patent/JPS5722171A/en active Granted
-
1981
- 1981-07-14 CA CA000381702A patent/CA1152536A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114591085A (en) * | 2022-01-19 | 2022-06-07 | 宁波伏尔肯科技股份有限公司 | Silicon carbide ceramic and preparation method thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5722171A (en) | 1982-02-05 |
| JPS621346B2 (en) | 1987-01-13 |
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