Note: Descriptions are shown in the official language in which they were submitted.
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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
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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.
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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
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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
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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
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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
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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.
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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
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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.
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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.
