CA2107422A1 - Method of joining silicon carbide bodies - Google Patents
Method of joining silicon carbide bodiesInfo
- Publication number
- CA2107422A1 CA2107422A1 CA 2107422 CA2107422A CA2107422A1 CA 2107422 A1 CA2107422 A1 CA 2107422A1 CA 2107422 CA2107422 CA 2107422 CA 2107422 A CA2107422 A CA 2107422A CA 2107422 A1 CA2107422 A1 CA 2107422A1
- Authority
- CA
- Canada
- Prior art keywords
- silicon
- brazing
- brazing material
- joint
- alloy
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B37/00—Joining burned ceramic articles with other burned ceramic articles or other articles by heating
- C04B37/003—Joining burned ceramic articles with other burned ceramic articles or other articles by heating by means of an interlayer consisting of a combination of materials selected from glass, or ceramic material with metals, metal oxides or metal salts
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
- C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
- C04B2235/652—Reduction treatment
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
- C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
- C04B2235/658—Atmosphere during thermal treatment
- C04B2235/6581—Total pressure below 1 atmosphere, e.g. vacuum
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
- C04B2237/02—Aspects relating to interlayers, e.g. used to join ceramic articles with other articles by heating
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
- C04B2237/02—Aspects relating to interlayers, e.g. used to join ceramic articles with other articles by heating
- C04B2237/04—Ceramic interlayers
- C04B2237/08—Non-oxidic interlayers
- C04B2237/086—Carbon interlayers
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
- C04B2237/02—Aspects relating to interlayers, e.g. used to join ceramic articles with other articles by heating
- C04B2237/16—Silicon interlayers
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
- C04B2237/30—Composition of layers of ceramic laminates or of ceramic or metallic articles to be joined by heating, e.g. Si substrates
- C04B2237/32—Ceramic
- C04B2237/36—Non-oxidic
- C04B2237/365—Silicon carbide
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
- C04B2237/50—Processing aspects relating to ceramic laminates or to the joining of ceramic articles with other articles by heating
- C04B2237/72—Forming laminates or joined articles comprising at least two interlayers directly next to each other
Abstract
Abstract of the Disclosure This invention is related to a method of joining silicon carbide bodies. An assembly is formed of silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon. The assembly is heated to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material, and form the joint.
Description
~'7~
This invention is related ~o a brazing me~hod for joining silicon carbide bodies.
U.S. Patent Nos. 4,889,686; 4,944,904;
4,981,822; 5,015,540; 5,021,367; and 5,043,303, incorporated herein by reference, disclose fiber reinforced silicon carbide composites. The composites are formed by infiltration of molten silicon, or an alloy of silicon and boron, into porous carbonaceous preforms. Carbon or silicon carbide reinforcement fibers are coated with boron nitride, and incorporated into a carbonaceous preform, for example, by tape casting. An assembly is formed of the carbonaceous preform and a means for contacting the preform with the infiltrant, either by placing the infiltrant directly on the preform or by placing the preform and a deposit of the infiltrant on a wicking material such as carbon cloth.
The assembly is heated to an infiltration temperature, about 10 to 20 C above the melting point of the infiltrant, for a period of time so that the entire preform is at ~he infiltration ~emperature to provide complete infiltration of the molten silicon or silicon alloy into the preform. The silicon or silicon alloy infiltrant reacts with the carbon in the preform to form silicon carbide. Residual porosity in the reacted silicon carbide body is filled with the silicon or silicon alloy infiltrant. Some of the applications for such silicon carbide composites will require joining of the composites. Preferably, the joint should be formed below the melting temperature 2:~7~
R~0021976 of the infiltrant to minimize damage to the reinforcement fibers in the composites.
The residual infiltrant in the matrix of the composite can react with and degrade the properties of the coated fiber reinforcement if the composite is heated above the melting temperature of the infiltrant. For example, ceramic fibers such as boron nitride fibers, or boron nitride coated carbon or silicon carbide reinforcement fibers are resistant to reaction with residual silicon in the silicon carbide body. However, boron has a limited solubility in molten silicon. When the bodies are heated above the melting point of silicon, the free silicon can dissolve boron and react with the fiber reinforcement.
As a result, the fiber reinforcement properties can be degraded. Therefore, it is desirable to form the joint below the melting point of silicon, about 1414-C, to minimize degradation of the fiber reinforcement within the body.
An aspect of this invention is to provide a method for brazing silicon carbide bodies below the melting temperature of silicon.
~ie~ De~ ti~n Q~_th~_InYentiQn A method of joining silicon carbide bodies comprises, forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon. The assembly is heated to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material and form the joint.
7 ~ ~ ~
Preferably, the contiguous surfaces are coated with a carbon particulate to promote wicking of the alloy throughout the joint zone, and formation of silicon carbide in the joint.
The brazing material is a material that is soluble in, and lowers the melting temperature of silicon, such as germanium or boron.
Figure 1 is a perspective view of two silicon carbide blocks.
Figure 2 is a perspective view of the silicon carbide blocks positioned to have contiguous surfaces defining a joint zone.
Figure 3 is a perspective view of the silicon carbide blocks positioned to have contiguous surfaces defining a joint zone, and.a wicking means in contact with a deposit of a brazing material and the joint zone.
We have discovered a method of joining silicon carbide bodies at a temperature belsw the melting point of silicon or silicon carbide. As a result, melting of silicon or silicon carbide in the silicon carbide body is minimized, and damage, for example, to a reinforcement phase in the body is minimized.
Two or more silicon carbide bodies are positioned to have contiguous surfaces defining a joint zone. An assembly comprised of the positioned silicon carbide bodies, and a brazing means in contact with the joint zone is heated to the brazing temperature where a molten alloy of the brazing X~7 ~22 material and silicon can form, and the molten alloy can infiltrate the joint zone. Since the brazing material is soluble in silicon, the brazing material can diffuse from the joint zone into the silicon carbide bodies comprised of free silicon until the melting temperature of the alloy in the joint zone is increased above the brazing temperature and forms the joint.
The joint is formed with an alloy of the brazing material and silicon. Suitable brazing materials are germanium and boron, which form liquid solutions with silicon at temperatures from about 937-and 1385-C, respectively, up to the melting point of silicon, about 1414 C. Conventional germanium-silicon, or boron-silicon phase diagrams show that as the amount of br~zing material increases in the alloy, the melting temperature of the alloy decreases toward the lower end of the range. Most preferably, the brazing material is germanium, because germanium forms only solid solutions with silicon. As a result, germanium does not form additional silicon compounds that can produce residual stresses in the joint from a thermal expansion mismatch with the silicon carbide or silicon in the joint.
A brazing means supplies the brazing material to ~he joint zone to form a braze that joins the bodies together. The brazed joint is comprised of at least one of silicon, an alloy of qilicon and the brazing material, or silicon carbide.
Silicon carbide bodies or composites comprised of excess free silicon, such as the molten ~ilicon infiltration formed silicon carbide, can be coated with the brazing material on the surfaces that are to be jo~ned to form the brazing means.
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Preferably, the amount of brazing material is minimized to minimize the amount of the brazing material that will be diffused into the bodies to form the joint. The brazing material is in contact with the free silicon at the surfaces. An assembly is formed of the bodies positioned so the coated surfaces are contiguous. The assembly is heated to a brazing temperature to melt the brazing material, and form the joint. Preferably, the contiguous surfaces are urged together during the heating.
The molten brazing material diffuses into free silicon at the contiguous surfaces to form a molten alloy that fills the joint zone to form the joint. The brazing material continuously diffuses into the free silicon in the body, depleting the joint zone of the brazing material. The melting point of the alloy of brazing material and silicon in the joint zone increases as the brazing material diffuses therefrom. When the melting point increases above the brazing temperature the alloy solidifies and forms the joint.
In another example, at least two silicon carbide bodies are positioned to have contiguous surfaces defining a joint zone. The brazing means is comprised of a wick such as a carbon fiber in contact with the joint zone, and a deposit of the brazing material and silicon. Preferably, the deposit provides an amount of brazing material and silicon to completely fill the joint zone. An assembly of the silicon carbide bodies, and brazing means in contact with the joint zone is heated to the brazing ~emperature. An alloy of the brazing material and silicon is wicked into the joint zone by the carbon fiber wick. The alloy fills the joint zone and as the 2~ ~i7 ~2~
brazing material diffuses therefrom into the free silicon in the silicon carhide bodies, the melting temperature of the alloy increaqes above the brazing temperature and the joint forms.
Preferably, the suraces to be ~oined are coated with a particulated caxbon to improve wicking and filling of the alloy of silicon and brazing material throughout the joint zone. The coating of particulated carbon also reacts with silicon to form silicon carbide in the joint.
The carbon particulate wicks the brazing material a~d silicon into the joint zone, and serves as a source of carbon to react with the i~filtrant and form silicon carbide. The carbon particulate can have a density of about 1.2 to 2.2 grams per milliliter.
Preferably, the carbon particulate is a low density amorphous carbon having a density of about 1.2 to 1.95 grams per milliliter. A suitable carbon partlculate is a Dylon aqueous graphite powder suspension, Dylon Industries, Inc., Ohio. Other suitable carbon particulates can be formed from carbonized binders such as epoxy, phenolic resin, or furfuryl alcohol, or by crushing or chopping carbon fibers or felt.
For example, the particulated carbon can be mixed in an organic polymer binder such as epoxy resin. The organic binder may include plasticizers and dispersants. The mixture of organic binder and particulated carbon can be deposited on the surfaces that are to be joined, and the surfaces are urged together. Any lubricants, binders, plasticizers, dispersants or similar materials used in forming the mixture are of the type which decompose on heating at temperatures below the joint formation temperature, preferably below 500 C, without leaving a residue that ~ RD0021976 degrades the infiltration of the joint. I~ should be understood a suitable binder may leave a porous carbon deposit that does not degrade the infiltration of the joint. The bodies are heated to decompose the organic S binder, and a particulated carbon deposit remains on the contiguous surfaces.
The carbon particulate coating can also be formed from a water based slurry mixture. A suitable water based slurry mixture is formed by mixing crushed carbon felt, or carbon powder, in an aqueous solution comprised of about 2 to 6 weight percent of a nonionic poly(ethylene oxide) homopolymer ranging in weight average molecular weight from about one-hundred thousand to five million. A suitable ethylene oxide lS polymer is Polyox WSR-205 or WSR Coagulant, Union Carbide.
The water based slurry mixture can be spread with a straight edge to form a sheet or tape.
The liquid is allowed to evaporate in air, and the polymer is decomposed by heating to 300 C in air.
Additional strength is provided to the tape by infiltrating a furfuryl alcohol, or tetrahydro~urfuryl alcohol, for example, 931 graphite adhesive binder, Cotronics, N.Y. Alternatively, the furfuryl alcohol, 2S or tetrahydrofurfuryl alcohol can be mixed into the slurry prior to tape casting in amounts up to about 50 weight percent of the solution. The tape is dried in air, and can be heated to lOO C to strengthen the tape. A section of the tape is positioned between the surfaces that are to be joined, pressed therebetween, and heated to 300 C in air to decompose the polymer.to ~orm the particulated carbon coating on the contiguous surfaces.
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The assembly is heated in an inert atmosphere or partial vacuum. Suitable inert atmospheres include argon, or reducing atmospheres such as hydrogen or carbon monoxide. Atmospheres that react with molten silicon, such as oxygen or nitrogen, are avoided. The remaining atmosphere of the partial vacuum should be inert, such as argon, or reducing such as carbon monoxide. Preferably, the nonoxidizing partial ~acuum is provided before heating is initiated. The partial vacuum is at least sufficient to avoid the entrapment of pockets of gas, and minimizes porosi_y in the ~oin~. Generally, such a partial vacuum ranges from about 0.01 torr to about 2 torr, and usually from about 0.01 torr to about 1 torr to remove gas evolving in the joint during infiltration with the molten alloy of silicon and the brazing material.
Preferably, the furnace used to heat the assembly is a carbon furnace, i.e. a furnace fabricated from elemental carbon. Such a furnace acts as an oxygen getter for the atmosphere within the furnace reacting with oxygen to produce C0 or C02 and thereby provides a nonoxidizing atmosphere, i.e.
reaction between the atmosphere, and infiltrant is minimized. In such instance where a carbon furnace is not used, it is preferable to have an oxygen getter present in the furnace chamber, such as elemental carbon, in order to provide a nonoxidizing atmosphere.
Alternatively, o~her nonoxidizing atmospheres inert to the molten silicon and brazing material can be used at partial vacuums of about 10-2 torr to 2 torr.
-~ The assembly is heated to the brazing temperature where the brazing material is molten, but below the melting temperature of silicon. Preferably s~7 ~2 RDoo21976 the brazing temperature is at least about 5 C above the melting temperature of the alloy, and at least about S C below the melting temperature of silicon.
The brazing temperature can range from about 950 C to about 1410 C when the brazing material is germanium, and from about 1390-C to about 1410 C when the brazing material is boron. Preferably, the alloy of brazing material and silicon that is infiltrated into the joint zone has a melting temperature at least about 10 C below the melting temperature of silicon.
One method of joining silicon carbide bodies is shown by making reference to Figure 1. A
first silicon carbide block 2, and a second silicon carbide block 4 are coated on surfaces 6 and 8, respec~ively, with a brazing material such as germanium. The coating can be formed by any conventional means, such as chemical vapor deposition or physical deposition processes such as sputtering, that can deposit the amount required to form the desired alloy of brazing material and silicon.
Referring to Figure 2, an assembly is formed comprised of the silicon carbide blocks 2 and 4 positioned so that surfaces 6 and 8 are contiguous.
The assembly is heated to a brazing temperature where the molten alloy of silicon and brazing material forms. For example, when the brazing material is germanium, the assembly can be heated to about 950 to 1410 C, and when the brazing material is boron the assembly can be heated to about 1390- to 1~10 C. Free silicon, within the silicon carbide bodies and adjacent to the brazing material on the contiguous -~urfaces 6 and 8, melts to form an alloy of the silicon and brazing material. The molten alloy wicks throughout the joint zone 5 filling space.
S~7 ~22 The brazing material is soluble in solid silicon within blocks 6 and 8, and diffuses from the joint zone into the blocks. The assembly is heated for a period of time to provide for diffusion of the brazing material from the joint zone 5. As the brazing material diffuses from the joint zone 5, the melting temperature of the silicon alloy increases above the brazing temperature, and the alloy in the joint zone 5 solidifies to form the joint. The assembly can be heated to a temperature where the brazing alloy forms, but below the melting temperature of silicon.
Referring to Fig. 1, another method of joining silicon carbide blocks 2 and 4 comprises coating a particulated carbon on surfaces 6 and 8.
Referring to Figure 3, an assembly is formed comprised of blocks 2 and 4 positioned to have contiguous surfaces 6 and 8 defining a joint zone 5, and a brazing means comprised of a carbon fiber wick 14 in contact with a deposit 12 on a carbon cloth 10 and the joint zone 5. The deposit 12 is comprised of the brazing material and silicon, such as silicon comprised of about 50 weight percent germanium. The assembly is heated to a temperature where the deposit 12 melts, about 1350 C for the silicon alloy comprised of about 50 weight percent germanium, and wicks into joint zone 5. The coating of particulated carbon on the contiguous surfaces improves the wicking of the molten alloy throughout the joint zone 5. The silicon alloy reacts with the particulated carbon to form silicon carbide. As the germanium diffuses into free silicon in the body, the melting point of the alloy remaining in the joint zone 5 increases above the brazing temperature and isothermally freezes forming ~ RD0021976 the joint. The joint is formed comprised of silicon carbide, and porosity filling alloy.
Additional features and advantages of the method of this invention are shown in the following examples where, unless otherwise stated, the following materials and equipment were used. The carbon particulate was chopped WDF carbon felt about 1.45 g/ml in density obtained from Union Carbide. The epoxy resin was Epon 828, Shell Chemical Co., Tx., and the hardener for the epoxy was diethylenetriamine also known as DTA.
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A deposit of an alloy comprised of 50 weight percent germanium, and the balance silicon was formed by heating a mixture of 5 grams of silicon and 5 grams of germanium in a vacuum to 1400-C for 10 minutes.
A surface on each of two carbon blocks about 3 millimeters by 3 millimeters were coated with a mixture comprised of 12 grams of carbon particulate, about 4 grams of epoxy, about 4 grams of xylene, and 0.4 grams of hardener. The coa~ed surfaces were squeezed together, and excess mixture was wiped from the blocks. The mixture was cured under an infrared lamp overnight, forming a joint zone be~ween contiguous surfaces of the blocks having a carbon particulate thereon. An assembly of the blocks and 0.05 grams of the alloy deposit placed on top of the joint zone was heated in a vacuum at 50 C per hour to 550 C, and at 4 C per minute to 1365 C. The assembly was heated at the brazing temperature of 1365-C for 2 hours, and furnace cooled forming a joint between the blocks.
B~2 .,.
2~ 42Z
A joint was formed between silicon carbide blocks according to the method in Example 1 except as noted herein. An assembly was formed as shown in Fig.
This invention is related ~o a brazing me~hod for joining silicon carbide bodies.
U.S. Patent Nos. 4,889,686; 4,944,904;
4,981,822; 5,015,540; 5,021,367; and 5,043,303, incorporated herein by reference, disclose fiber reinforced silicon carbide composites. The composites are formed by infiltration of molten silicon, or an alloy of silicon and boron, into porous carbonaceous preforms. Carbon or silicon carbide reinforcement fibers are coated with boron nitride, and incorporated into a carbonaceous preform, for example, by tape casting. An assembly is formed of the carbonaceous preform and a means for contacting the preform with the infiltrant, either by placing the infiltrant directly on the preform or by placing the preform and a deposit of the infiltrant on a wicking material such as carbon cloth.
The assembly is heated to an infiltration temperature, about 10 to 20 C above the melting point of the infiltrant, for a period of time so that the entire preform is at ~he infiltration ~emperature to provide complete infiltration of the molten silicon or silicon alloy into the preform. The silicon or silicon alloy infiltrant reacts with the carbon in the preform to form silicon carbide. Residual porosity in the reacted silicon carbide body is filled with the silicon or silicon alloy infiltrant. Some of the applications for such silicon carbide composites will require joining of the composites. Preferably, the joint should be formed below the melting temperature 2:~7~
R~0021976 of the infiltrant to minimize damage to the reinforcement fibers in the composites.
The residual infiltrant in the matrix of the composite can react with and degrade the properties of the coated fiber reinforcement if the composite is heated above the melting temperature of the infiltrant. For example, ceramic fibers such as boron nitride fibers, or boron nitride coated carbon or silicon carbide reinforcement fibers are resistant to reaction with residual silicon in the silicon carbide body. However, boron has a limited solubility in molten silicon. When the bodies are heated above the melting point of silicon, the free silicon can dissolve boron and react with the fiber reinforcement.
As a result, the fiber reinforcement properties can be degraded. Therefore, it is desirable to form the joint below the melting point of silicon, about 1414-C, to minimize degradation of the fiber reinforcement within the body.
An aspect of this invention is to provide a method for brazing silicon carbide bodies below the melting temperature of silicon.
~ie~ De~ ti~n Q~_th~_InYentiQn A method of joining silicon carbide bodies comprises, forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon. The assembly is heated to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material and form the joint.
7 ~ ~ ~
Preferably, the contiguous surfaces are coated with a carbon particulate to promote wicking of the alloy throughout the joint zone, and formation of silicon carbide in the joint.
The brazing material is a material that is soluble in, and lowers the melting temperature of silicon, such as germanium or boron.
Figure 1 is a perspective view of two silicon carbide blocks.
Figure 2 is a perspective view of the silicon carbide blocks positioned to have contiguous surfaces defining a joint zone.
Figure 3 is a perspective view of the silicon carbide blocks positioned to have contiguous surfaces defining a joint zone, and.a wicking means in contact with a deposit of a brazing material and the joint zone.
We have discovered a method of joining silicon carbide bodies at a temperature belsw the melting point of silicon or silicon carbide. As a result, melting of silicon or silicon carbide in the silicon carbide body is minimized, and damage, for example, to a reinforcement phase in the body is minimized.
Two or more silicon carbide bodies are positioned to have contiguous surfaces defining a joint zone. An assembly comprised of the positioned silicon carbide bodies, and a brazing means in contact with the joint zone is heated to the brazing temperature where a molten alloy of the brazing X~7 ~22 material and silicon can form, and the molten alloy can infiltrate the joint zone. Since the brazing material is soluble in silicon, the brazing material can diffuse from the joint zone into the silicon carbide bodies comprised of free silicon until the melting temperature of the alloy in the joint zone is increased above the brazing temperature and forms the joint.
The joint is formed with an alloy of the brazing material and silicon. Suitable brazing materials are germanium and boron, which form liquid solutions with silicon at temperatures from about 937-and 1385-C, respectively, up to the melting point of silicon, about 1414 C. Conventional germanium-silicon, or boron-silicon phase diagrams show that as the amount of br~zing material increases in the alloy, the melting temperature of the alloy decreases toward the lower end of the range. Most preferably, the brazing material is germanium, because germanium forms only solid solutions with silicon. As a result, germanium does not form additional silicon compounds that can produce residual stresses in the joint from a thermal expansion mismatch with the silicon carbide or silicon in the joint.
A brazing means supplies the brazing material to ~he joint zone to form a braze that joins the bodies together. The brazed joint is comprised of at least one of silicon, an alloy of qilicon and the brazing material, or silicon carbide.
Silicon carbide bodies or composites comprised of excess free silicon, such as the molten ~ilicon infiltration formed silicon carbide, can be coated with the brazing material on the surfaces that are to be jo~ned to form the brazing means.
~7~æ~
Preferably, the amount of brazing material is minimized to minimize the amount of the brazing material that will be diffused into the bodies to form the joint. The brazing material is in contact with the free silicon at the surfaces. An assembly is formed of the bodies positioned so the coated surfaces are contiguous. The assembly is heated to a brazing temperature to melt the brazing material, and form the joint. Preferably, the contiguous surfaces are urged together during the heating.
The molten brazing material diffuses into free silicon at the contiguous surfaces to form a molten alloy that fills the joint zone to form the joint. The brazing material continuously diffuses into the free silicon in the body, depleting the joint zone of the brazing material. The melting point of the alloy of brazing material and silicon in the joint zone increases as the brazing material diffuses therefrom. When the melting point increases above the brazing temperature the alloy solidifies and forms the joint.
In another example, at least two silicon carbide bodies are positioned to have contiguous surfaces defining a joint zone. The brazing means is comprised of a wick such as a carbon fiber in contact with the joint zone, and a deposit of the brazing material and silicon. Preferably, the deposit provides an amount of brazing material and silicon to completely fill the joint zone. An assembly of the silicon carbide bodies, and brazing means in contact with the joint zone is heated to the brazing ~emperature. An alloy of the brazing material and silicon is wicked into the joint zone by the carbon fiber wick. The alloy fills the joint zone and as the 2~ ~i7 ~2~
brazing material diffuses therefrom into the free silicon in the silicon carhide bodies, the melting temperature of the alloy increaqes above the brazing temperature and the joint forms.
Preferably, the suraces to be ~oined are coated with a particulated caxbon to improve wicking and filling of the alloy of silicon and brazing material throughout the joint zone. The coating of particulated carbon also reacts with silicon to form silicon carbide in the joint.
The carbon particulate wicks the brazing material a~d silicon into the joint zone, and serves as a source of carbon to react with the i~filtrant and form silicon carbide. The carbon particulate can have a density of about 1.2 to 2.2 grams per milliliter.
Preferably, the carbon particulate is a low density amorphous carbon having a density of about 1.2 to 1.95 grams per milliliter. A suitable carbon partlculate is a Dylon aqueous graphite powder suspension, Dylon Industries, Inc., Ohio. Other suitable carbon particulates can be formed from carbonized binders such as epoxy, phenolic resin, or furfuryl alcohol, or by crushing or chopping carbon fibers or felt.
For example, the particulated carbon can be mixed in an organic polymer binder such as epoxy resin. The organic binder may include plasticizers and dispersants. The mixture of organic binder and particulated carbon can be deposited on the surfaces that are to be joined, and the surfaces are urged together. Any lubricants, binders, plasticizers, dispersants or similar materials used in forming the mixture are of the type which decompose on heating at temperatures below the joint formation temperature, preferably below 500 C, without leaving a residue that ~ RD0021976 degrades the infiltration of the joint. I~ should be understood a suitable binder may leave a porous carbon deposit that does not degrade the infiltration of the joint. The bodies are heated to decompose the organic S binder, and a particulated carbon deposit remains on the contiguous surfaces.
The carbon particulate coating can also be formed from a water based slurry mixture. A suitable water based slurry mixture is formed by mixing crushed carbon felt, or carbon powder, in an aqueous solution comprised of about 2 to 6 weight percent of a nonionic poly(ethylene oxide) homopolymer ranging in weight average molecular weight from about one-hundred thousand to five million. A suitable ethylene oxide lS polymer is Polyox WSR-205 or WSR Coagulant, Union Carbide.
The water based slurry mixture can be spread with a straight edge to form a sheet or tape.
The liquid is allowed to evaporate in air, and the polymer is decomposed by heating to 300 C in air.
Additional strength is provided to the tape by infiltrating a furfuryl alcohol, or tetrahydro~urfuryl alcohol, for example, 931 graphite adhesive binder, Cotronics, N.Y. Alternatively, the furfuryl alcohol, 2S or tetrahydrofurfuryl alcohol can be mixed into the slurry prior to tape casting in amounts up to about 50 weight percent of the solution. The tape is dried in air, and can be heated to lOO C to strengthen the tape. A section of the tape is positioned between the surfaces that are to be joined, pressed therebetween, and heated to 300 C in air to decompose the polymer.to ~orm the particulated carbon coating on the contiguous surfaces.
t`~
The assembly is heated in an inert atmosphere or partial vacuum. Suitable inert atmospheres include argon, or reducing atmospheres such as hydrogen or carbon monoxide. Atmospheres that react with molten silicon, such as oxygen or nitrogen, are avoided. The remaining atmosphere of the partial vacuum should be inert, such as argon, or reducing such as carbon monoxide. Preferably, the nonoxidizing partial ~acuum is provided before heating is initiated. The partial vacuum is at least sufficient to avoid the entrapment of pockets of gas, and minimizes porosi_y in the ~oin~. Generally, such a partial vacuum ranges from about 0.01 torr to about 2 torr, and usually from about 0.01 torr to about 1 torr to remove gas evolving in the joint during infiltration with the molten alloy of silicon and the brazing material.
Preferably, the furnace used to heat the assembly is a carbon furnace, i.e. a furnace fabricated from elemental carbon. Such a furnace acts as an oxygen getter for the atmosphere within the furnace reacting with oxygen to produce C0 or C02 and thereby provides a nonoxidizing atmosphere, i.e.
reaction between the atmosphere, and infiltrant is minimized. In such instance where a carbon furnace is not used, it is preferable to have an oxygen getter present in the furnace chamber, such as elemental carbon, in order to provide a nonoxidizing atmosphere.
Alternatively, o~her nonoxidizing atmospheres inert to the molten silicon and brazing material can be used at partial vacuums of about 10-2 torr to 2 torr.
-~ The assembly is heated to the brazing temperature where the brazing material is molten, but below the melting temperature of silicon. Preferably s~7 ~2 RDoo21976 the brazing temperature is at least about 5 C above the melting temperature of the alloy, and at least about S C below the melting temperature of silicon.
The brazing temperature can range from about 950 C to about 1410 C when the brazing material is germanium, and from about 1390-C to about 1410 C when the brazing material is boron. Preferably, the alloy of brazing material and silicon that is infiltrated into the joint zone has a melting temperature at least about 10 C below the melting temperature of silicon.
One method of joining silicon carbide bodies is shown by making reference to Figure 1. A
first silicon carbide block 2, and a second silicon carbide block 4 are coated on surfaces 6 and 8, respec~ively, with a brazing material such as germanium. The coating can be formed by any conventional means, such as chemical vapor deposition or physical deposition processes such as sputtering, that can deposit the amount required to form the desired alloy of brazing material and silicon.
Referring to Figure 2, an assembly is formed comprised of the silicon carbide blocks 2 and 4 positioned so that surfaces 6 and 8 are contiguous.
The assembly is heated to a brazing temperature where the molten alloy of silicon and brazing material forms. For example, when the brazing material is germanium, the assembly can be heated to about 950 to 1410 C, and when the brazing material is boron the assembly can be heated to about 1390- to 1~10 C. Free silicon, within the silicon carbide bodies and adjacent to the brazing material on the contiguous -~urfaces 6 and 8, melts to form an alloy of the silicon and brazing material. The molten alloy wicks throughout the joint zone 5 filling space.
S~7 ~22 The brazing material is soluble in solid silicon within blocks 6 and 8, and diffuses from the joint zone into the blocks. The assembly is heated for a period of time to provide for diffusion of the brazing material from the joint zone 5. As the brazing material diffuses from the joint zone 5, the melting temperature of the silicon alloy increases above the brazing temperature, and the alloy in the joint zone 5 solidifies to form the joint. The assembly can be heated to a temperature where the brazing alloy forms, but below the melting temperature of silicon.
Referring to Fig. 1, another method of joining silicon carbide blocks 2 and 4 comprises coating a particulated carbon on surfaces 6 and 8.
Referring to Figure 3, an assembly is formed comprised of blocks 2 and 4 positioned to have contiguous surfaces 6 and 8 defining a joint zone 5, and a brazing means comprised of a carbon fiber wick 14 in contact with a deposit 12 on a carbon cloth 10 and the joint zone 5. The deposit 12 is comprised of the brazing material and silicon, such as silicon comprised of about 50 weight percent germanium. The assembly is heated to a temperature where the deposit 12 melts, about 1350 C for the silicon alloy comprised of about 50 weight percent germanium, and wicks into joint zone 5. The coating of particulated carbon on the contiguous surfaces improves the wicking of the molten alloy throughout the joint zone 5. The silicon alloy reacts with the particulated carbon to form silicon carbide. As the germanium diffuses into free silicon in the body, the melting point of the alloy remaining in the joint zone 5 increases above the brazing temperature and isothermally freezes forming ~ RD0021976 the joint. The joint is formed comprised of silicon carbide, and porosity filling alloy.
Additional features and advantages of the method of this invention are shown in the following examples where, unless otherwise stated, the following materials and equipment were used. The carbon particulate was chopped WDF carbon felt about 1.45 g/ml in density obtained from Union Carbide. The epoxy resin was Epon 828, Shell Chemical Co., Tx., and the hardener for the epoxy was diethylenetriamine also known as DTA.
~am~_L
A deposit of an alloy comprised of 50 weight percent germanium, and the balance silicon was formed by heating a mixture of 5 grams of silicon and 5 grams of germanium in a vacuum to 1400-C for 10 minutes.
A surface on each of two carbon blocks about 3 millimeters by 3 millimeters were coated with a mixture comprised of 12 grams of carbon particulate, about 4 grams of epoxy, about 4 grams of xylene, and 0.4 grams of hardener. The coa~ed surfaces were squeezed together, and excess mixture was wiped from the blocks. The mixture was cured under an infrared lamp overnight, forming a joint zone be~ween contiguous surfaces of the blocks having a carbon particulate thereon. An assembly of the blocks and 0.05 grams of the alloy deposit placed on top of the joint zone was heated in a vacuum at 50 C per hour to 550 C, and at 4 C per minute to 1365 C. The assembly was heated at the brazing temperature of 1365-C for 2 hours, and furnace cooled forming a joint between the blocks.
B~2 .,.
2~ 42Z
A joint was formed between silicon carbide blocks according to the method in Example 1 except as noted herein. An assembly was formed as shown in Fig.
3. The assembly was comprised of a carbon fiber wick S in contact with the joint zone, and a second deposit of the alloy deposit weighing about 3 times the weight of the carbon fiber plus 0.1 gram on a carbon cloth.
A strong joint was formed between the blocks in each Example. The blocks were sectioned through the joint, and minor porosity was found. It is believed air bubbles in the mixture of epoxy and carbon particulate resulted in the porosity in the joint.
A strong joint was formed between the blocks in each Example. The blocks were sectioned through the joint, and minor porosity was found. It is believed air bubbles in the mixture of epoxy and carbon particulate resulted in the porosity in the joint.
Claims (18)
1. A method of joining molten silicon infiltration formed silicon carbide bodies comprising:
forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon, and heating the assembly to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material, and form the joint.
forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon, and heating the assembly to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material, and form the joint.
2. A method according to claim 1 wherein the brazing temperature is at least about 5°C above the melting point of the alloy, and at least about 5°C
below the melting point of silicon.
below the melting point of silicon.
3. A method according to claim 2 wherein the brazing material is at least one of germanium or boron, and the alloy has a melting point at least about 10°C below the melting point of silicon.
4. A method according to claim 3 wherein the brazing means is a deposit of the brazing material on the contiguous surfaces.
5. A method according to claim 4 wherein the bodies are urged together during the heating.
6. A method according to claim 5 wherein the brazing material is germanium.
7. A method according to claim 3 before the step of forming an assembly, comprising, coating the contiguous surfaces with a carbon particulate.
8. A method according to claim 7 wherein the brazing means is a deposit of the brazing material and silicon on a wicking means in contact with the joint zone.
9. A method according to claim 8 wherein the brazing material is germanium.
10. A method of joining molten silicon infiltration formed silicon carbide bodies comprising:
forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon, the contiguous surfaces having a layer of a carbon particulate formed thereon, and heating the assembly to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material, and form the joint.
forming an assembly of the silicon carbide bodies having contiguous surfaces defining a joint zone, and a brazing means for providing to the joint zone a brazing material that lowers the melting point of silicon, the contiguous surfaces having a layer of a carbon particulate formed thereon, and heating the assembly to a brazing temperature in an inert atmosphere or partial vacuum to infiltrate the zone with an alloy of silicon and the brazing material, and form the joint.
11. A method according to claim 10 wherein the brazing temperature is at least about 5°C above the melting point of the alloy, and at least about 5°C
below the melting point of silicon.
below the melting point of silicon.
12. A method according to claim 11 wherein the brazing material is at least one of germanium or boron, and the alloy has a melting point at least about 10°C below the melting point of silicon.
13. A method according to claim 12 wherein the brazing means is a deposit of the brazing material on the contiguous surfaces.
14. A method according to claim 13 wherein the bodies are urged together during the heating.
15. A method according to claim 14 wherein the brazing material is germanium.
16. A method according to claim 12 wherein the brazing means is a deposit of the brazing material and silicon on a wicking means in contact with the joint zone.
17. A method according to claim 16 wherein the brazing material is germanium.
18. The invention as defined in any of the preceding claims including any further features of novelty disclosed.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US95621692A | 1992-10-05 | 1992-10-05 | |
| US956,216 | 1992-10-05 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2107422A1 true CA2107422A1 (en) | 1994-04-06 |
Family
ID=25497930
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2107422 Abandoned CA2107422A1 (en) | 1992-10-05 | 1993-09-30 | Method of joining silicon carbide bodies |
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| Country | Link |
|---|---|
| CA (1) | CA2107422A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10150188B1 (en) * | 2017-08-04 | 2018-12-11 | General Electric Company | Method of ceramic matrix composite repair |
-
1993
- 1993-09-30 CA CA 2107422 patent/CA2107422A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10150188B1 (en) * | 2017-08-04 | 2018-12-11 | General Electric Company | Method of ceramic matrix composite repair |
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