CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
61/639,629, filed April 27, 2012, the contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to ceramic matrix composite (CMC)
articles and processes for their production. More particularly, this invention is directed to
a process of producing a silicon-containing CMC article by melt infiltration of a porous
preform that was produced with the use of a matrix slurry composition capable of
promoting the infiltration of the preform.
Higher operating temperatures for gas turbine engines are continuously sought
in order to increase their efficiency. Though significant advances in high temperature
capabilities have been achieved through formulation of iron, nickel and cobalt-base
superalloys, alternative materials have been investigated. CMC materials are a notable
example because their high temperature capabilities can significantly reduce cooling air
requirements. CMC materials generally comprise a ceramic fiber reinforcement material
embedded in a ceramic matrix material. The reinforcement material may be
discontinuous short fibers dispersed in the matrix material or continuous fibers or fiber
bundles oriented within the matrix material, and serves as the load-bearing constituent of
the CMC in the event of a matrix crack. In tum, the ceramic matrix protects the
reinforcement material, maintains the orientation of its fibers, and serves to dissipate
loads to the reinforcement material. Individual fibers (filaments) are often coated with a
release agent, such as boron nitride (BN) or carbon, to form a weak interface or de-bond
- 2 -
layer that allows for limited and controlled slip between the fibers and the ceramic matrix
material. As cracks develop in the CMC, one or more fibers bridging the crack act to
redistribute the load to adjacent fibers and regions of the matrix material, thus inhibiting
or at least slowing further propagation of the crack.
Of particular interest to many high-temperature applications are silicon-based
composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material.
Notable examples of CMC materials and particularly SiC/Si-SiC (fiber/matrix)
continuous fiber-reinforced ceramic composites (CFCC) materials and processes are
disclosed in U.S. Patent Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898,
6,258,737, 6,403,158, and 6,503,441, and U.S. Patent Application Publication No.
2004/0067316, whose contents are incorporated herein by reference. Such processes
generally entail the fabrication of CMCs using multiple prepreg layers, each in the form
of a "tape" comprising the desired ceramic fiber reinforcement material, one or more
precursors of the CMC matrix material, and one or more organic resin binders that
promote the pliability of the tapes. According to conventional practice, prepreg tapes can
be formed by impregnating the reinforcement material with matrix slurry that contains
the ceramic precursor(s) and binder(s). Preferred precursor materials will depend on the
particular composition desired for the ceramic matrix of the CMC component, for
example, SiC powder and a carbon and/or carbon-containing particulate material, for
example, carbon black, if the desired matrix material is SiC. Other typical slurry
ingredients include solvents, also called solvent vehicles, for the binders to promote the
fluidity of the slurry to enable impregnation of the fiber reinforcement material.
After allowing the matrix slurry to partially dry and, if appropriate, partially
curing the binders (B-staging), the resulting tape is laid-up with other tapes, debulked
and, if appropriate, cured while subjected to elevated pressures and temperatures to
produce a cured prepreg preform. The preform is then heated (fired) in a vacuum or inert
atmosphere to remove solvents, decompose the binders, and convert the precursor to the
- 3 -
•
desired carbon-containing ceramic matrix material, yielding a fired porous preform that is
ready for melt infiltration. During melt infiltration, silicon and/or a silicon alloy is
typically applied externally to the porous preform and melted, and the molten silicon
and/or silicon alloy infiltrates into the porosity of the preform. A portion of the molten
silicon is reacted with elemental carbon present in the porous preform, such as the
aforementioned carbon black originally present in the slurry as a precursor, and/or any
carbon char formed by pyrolysis of organic binders. The molten silicon and carbon black
react to form additional silicon carbide that fills the porosity to yield the final CMC
component.
Specific processing techniques and parameters for the above process will
depend on the particular composition of the materials. Conventional melt infiltration
conditions for the production of CMC components require a carefully controlled
atmosphere in terms of both pressure and content of the atmosphere to remove excess
carbon, which can cause excessive and undesirable outgassing during melt infiltration.
Consequently, melt infiltration is typically performed in a controlled furnace atmosphere,
necessitating the use of batch operations that increase production costs. Attempts to
infiltrate preforms by immersion in molten silicon without atmosphere control have
required extended treatments (for example, eight hours or more) and resulted in
unacceptable preforms having an infiltrated shell surrounding an un-infiltrated core. This
is a direct result of a phenomenon called "choking." Choking is a term used to denote a
condition in which infiltration coupled with reaction of the molten silicon with carbon
black in the preform leads to SiC formation in such a way that pore radii are reduced
making further infiltration difficult or completely halting all further infiltration.
This phenomenon of inhibited infiltration due to excessive formation of SiC
utilizing the carbon black present in the preform is illustrated in FIG. I, which
schematically represents a portion of a fired porous preform 10 that has been fabricated in
accordance with the prior art. The depiction shown in Fig 1 represents a snap-shot at a
- 4-
•
moment after the melt infiltration has begun. The portion of the preform 10 comprises a
matrix 12 that contains SiC particles 14 and as yet unreacted carbon 15. For the purpose
of producing a CMC article, the matrix 12 would also surround a fiber reinforcement
material, which is not shown in FIG. 1 the preform are also intended to encompass the
presence of a fiber reinforcement material). The matrix 12 is further represented as
containing porosity in the form of pores 16 that formed as a result of decomposition of
the resinous binders during firing. FIG. 1 also schematically represents the effect of melt
infiltration, during which additional SiC 18 forms on the surfaces of the pores 16 as a
result of molten silicon (not shown) reacting with carbon black within the matrix 12. As
represented in FIG. 1, the additional SiC 18 can significantly reduce the cross-sectional
areas of the pores 16, which slow and may even prevent further infiltration by the molten
silicon. Such choking is a result of two main characteristics in processes of the type
described above: use of carbon black or high carbon content in the matrix slurry which
reduces the porosity needed for infiltration, and the rapid reaction of the carbon with the
molten silicon, forming the additional SiC 18 which inherently has a higher molar
volume than silicon. As mentioned above, another limiting aspect of the prior art is that
preforms produced with conventional matrix slurry materials require controlled pressures
and atmospheres during melt infiltration to achieve good surface wetting and full melt
infiltration. It is generally known that the presence of unreacted carbon and the existence
of unfilled porosity remaining after the melt infiltration process as a result of choking
have an adverse effect on the mechanical properties of the CMC article resulting from the
phenomenon illustrated in Fig 1.
Accordingly, there is a desire for improved methods capable of producing
melt-infiltrated CMC components. These methods would be preferably capable of
achieving substantial improvements in infiltration speed and completeness, as well as
reduce or eliminate the need for carefully controlled pressures and atmospheres to control
outgassing during melt infiltration. Addressing both of these issues would provide the
possibility for substantial improvement in the robustness of the CMC article and reduce
- 5 -
•
processing costs.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a method of producing a porous composite
preform that can be melt infiltrated by direct immersion in or exposure to molten silicon,
preferably at relatively high infiltration speeds and preferably without requiring the use of
a controlled pressure or atmosphere during infiltration.
According to a first aspect of the invention, a method is provided that entails
producing a matrix slurry composition that contains at least one resin binder and a SiC
powder. The SiC powder is a precursor for a SiC matrix of the CMC article and the at
least one resin binder is a precursor for a carbon char of the SiC matrix. A fiber
reinforcement material is impregnated with the matrix slurry composition to yield a
preform, which is then heated to form a porous preform that contains the SiC matrix and
porosity and to convert the at least one resin binder to the carbon char that is present
within the porosity. Melt infiltration of the porosity within the porous preform is then
performed with molten silicon or a molten silicon-containing alloy to react with the
carbon char and form silicon carbide that at least partially fills the porosity within the
porous preform. The carbon char constitutes essentially all of the elemental carbon in the
porous preform.
According to another aspect of the invention, a method is provided that entails
producing a matrix slurry composition that contains at least two resin binders and a SiC
powder and does not contain any carbon particulate. The SiC powder is a precursor for a
SiC matrix of the CMC article, and the at least two resin binders are precursors for a
carbon char of the SiC matrix and have an effective char yield of 9.5 to 25%. A fiber
reinforcement material is then impregnated with the matrix slurry composition to yield a
preform, which is then heated to form a porous preform that contains the SiC matrix and
- 6-
•
porosity and to convert at least one of the at least two resin binders to the carbon char that
is present within the porosity. The porosity is then melt infiltrated with molten silicon or
a molten silicon-containing alloy to react the carbon char and form silicon carbide that
partially fills the porosity within the porous preform.
A technical effect of the invention is that the porous preform has a resultant
porosity that promotes a more rapid infiltration. Speedy infiltration is achieved by a
porous structure that contains a controlled amount of carbon char content that is less
prone to choking during melt infiltration and does not lead to significant blockage during
melt infiltration. Instead, a preferred aspect of the invention is the ability to yield a pore
structure in which SiC formed by reaction with particulate carbon is absent in the
preform, and in which spaces between SiC matrix formed otherwise contain threads of
resin-derived carbon char and porosity. A surprising and unexpected technical effect of
the invention is the ability to eliminate the need for a controlled atmosphere of a type
usually required due to the high amount of carbon (as resin and carbon black) in the
matrix slurry composition. Another technical effect resulting from the invention is that
there appears to be a more controlled formation of SiC resulting from the reaction
between molten silicon and the resin-derived carbon char, as compared to that which
occurs between molten silicon and carbon black or other particulate carbon that is
intentionally present in the prior art. The more controlled formation of SiC made
possible with the invention is believed to also reduce the likelihood of choking. Yet
another technical effect is that the resulting porosity appears to promote infiltration as a
result of exhibiting unique connectivity, believed to be due to the way that the SiC is
exclusively formed from molten silicon and the carbon char.
Other aspects and advantages of the invention will be better appreciated from
the following detailed description.
- 7 -
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. I schematically represents a porous geometry of a portion of a preform
(at a moment during the melt infiltration process) that is typical of prior art processes
where carbon black is a component of a matrix slurry used to produce the preform. As
illustrated, the porosity within the preform is inadequate for speedy infiltration due to
channels being substantially blocked, leading to the phenomenon referred to herein as
choking during melt infiltration.
FIG. 2 schematically represents a porous geometry of a portion of a preform
(immediately preceding the infiltration process) produced from a matrix slurry that does
not contain carbon black in accordance with an embodiment of the invention, and depicts
a desirable porosity and minimal blockage in that it would not tend to promote choking.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in terms of processes for producing
CMC articles through melt infiltration techniques performed on a porous preform to yield
a matrix containing SiC. According to a preferred aspect of the invention, the preform is
produced by firing a prepreg preform formed by impregnating a fiber reinforcement
material with a matrix slurry composition that contains one or more ceramic precursors.
The slurry composition is formulated to reduce the tendency for the reaction of the
ceramic precursors to inhibit ("choke-off') the subsequent infiltration of molten silicon
into the fired porous preform during a melt infiltration process, so that the molten silicon
is able to more quickly infiltrate the porous preform. In addition, melt infiltration of the
preform preferably does not require a carefully controlled atmosphere or pressure to
achieve full infiltration. Instead, the preform can be melt infiltrated in a protective
atmosphere, for example, flowing argon, instead of a controlled furnace atmosphere.
Furthermore, with some embodiments of the invention, it is possible for melt infiltration
- 8 -
to be completed in as little as two to ten minutes.
Matrix slurry compositions of the present invention preferably contain a SiC
powder as at least one precursor for the SiC matrix of the CMC component. The slurry
composition may contain the SiC powder at higher contents than is conventional as a
replacement for carbon and/or carbon-containing particulate (for example, carbon black),
which the slurry composition preferably does not contain or contains at much lower
contents than in prior art slurry compositions. As a particular example, the slurry
composition may contain, by weight, more than 70% SiC powder and no carbon
particulate as the solids in the slurry composition. A suitable but nonlimiting particle size
range for the SiC powder is about 10 micrometers or less.
According to another aspect of the invention, the matrix slurry composition
contains one or more primary resin binders that not only serve as a binder for the prepreg
preform prior to firing to form the porous preform, but also as a precursor to char that
preferable constitutes the sole source of elemental carbon in the porous preform after the
prepreg preform is fired. The slurry composition preferably contains a solvent for the
primary resin binder to promote the fluidity of the matrix slurry composition and promote
impregnation of the fiber reinforcement material to yield the prepreg preform. The slurry
composition may also contain one or more pore formers and/or catalysts. If present, the
pore former may be a resin that also serves as a binder for the prepreg preform, but does
not contribute to char formation in the porous preform or contributes to char formation to
a much lesser extent than the primary resin binder(s). For reasons discussed below, the
primary resin binder(s) and any pore former(s) preferably have an effective char yield
range of at least 9.5 up to about 25%.
As a nonlimiting example, a suitable matrix slurry composition produced
during an investigation leading to the present invention contained, in weight percent,
about 78% SiC powder, about 15% resin binder(s), and 7% pore former(s). For purposes
- 9 -
of clarity, only the proportion of the SiC powder, primary resin binder and pore former
are indicated, and the amount of any desired solvent is omitted and determination of the
amount of solvent suitable for inclusion in the slurry composition is within the capability
of those skilled in the art. Upon firing, this particular composition resulted in a matrix
material containing, by volume, about 53% SiC, about 16% char (elemental carbon), and
the remainder essentially porosity. Importantly, the slurry composition contained char in
an amount that is higher than is conventional for prior art SiC-forming matrix slurry
compositions, but at a level sufficiently high to serve as a complete replacement for
carbon and/or carbon-containing particulate materials included in the slurry compositions
of the prior art. The slurry composition contains an amount of primary resin binder(s)
that is sufficient to yield a char content in the matrix of the porous preform of, by
volume, about 7% to about 30%. Char yields less than 9.5% may not produce the desired
minimum amount of char content in the preform 20 while char yields exceeding 25%
may lead to unreacted carbon in the matrix which is undesirable form the standpoint of
mechanical strength of the CMC. Thus, the primary resin binder(s) and any pore
former(s) preferably have an effective char yield range of at least 9.5 up to about 25%.
These percentages are for the matrix of the preform excluding the fiber reinforcement
material. Further, the matrix slurry composition is preferably tailored to result in at least
45% by volume of SiC in the porous preform, with a preferred upper limit being about
80% by volume. Aside from the reinforcement material, SiC powder, and char content,
the balance of the porous preform following burnout is preferably essentially porosity, for
example, at least 20% and more preferably on the order of about 25 volume percent or
more. A preferred aspect of the invention is the elimination of carbon particulate from
the matrix slurry composition.
Primary resin binders useful in the invention are carbon-yielding resins such
as thermosetting furan (C4H40) -based resins, though it is foreseeable that other resins
could be used, for example, phenolics, novolacs, polyester or epoxies, if their volumetric
percentages for char yield and porosity can be used to attain the char content and porosity
- 10 -
noted above. In preferred embodiments, two or more resins are used to attain char
contents and porosities within the ranges prescribed for the invention, in which case at
least one of the resins may have a substantially higher char yield than one or more other
resins used in the slurry composition. For example, one of the resins may have a high
char yield and be the predominant source of carbon char in the porous preform, while
another resin may predominantly serve as a binder in the prepreg preform and produce
porosity in the fired porous preform without being a significant source of carbon char. In
this case, the first resin may be considered the primary resin binder in the slurry
composition, while the second is considered to be a pore former. The resins and their
amounts in the slurry composition can be chosen on the basis of having an effective
(combined) char yield capable of attaining the char content and porosity within the ranges
prescribed for the invention. Furthermore, it may be practical to select the resins on the
basis of having different burnout temperatures, such that carbon char forms at varying
temperatures within the preform to provide another parameter that can be exploited in
order to achieve desired porosity conditions at a given stage ofthe process.
Appropriate selection of the primary resin binder(s) and any pore former can
serve to control the reaction so that large amounts of SiC are not formed quickly during
melt infiltration with molten silicon, which if allowed to occur would promote blockage
of the internal porosity within a preform due to the increased molar volume of SiC over
silicon. The reaction control referred above can be accomplished by effecting a gradual
formation of char from the primary resin binder(s), rather than having a significant
amount of particulate carbon readily available in the slurry compositions that is believed
to promote rapid and excessive formation of SiC within the porosity of the preform.
Another preferred and desirable aspect of providing all available elemental carbon in the
preform as carbon char produced by decomposing the primary resin binder(s) is that the
resultant char has been found to be discontinuous or otherwise present in a form such that
the resultant porosity is maintained more open than is believed to have been previously
possible with the use ofprior art slurries containing carbon particulate.
- 11 -
Consequently, a preferred aspect of the invention is to select one or more
primary resin binders and any pore formers for use in a matrix slurry composition such
that it/they generate(s) a controlled amount of carbon char and open porosity within a
porous preform that is capable of promoting infiltration by molten silicon as a result of
inhibiting choking during infiltration. Such a result is schematically represented in FIG.
2, which shows a portion of a fired porous preform 20 (at a stage immediately preceding
the infiltration process) comprising a matrix 22 that contains SiC particles 24 (the fiber
reinforcement material is not shown). The matrix 12 is further represented as containing
interconnected porosity 26. FIG. 2 also schematically represents thread-like and
discontinuous carbon char 30 resulting from the firing process, during which the primary
resin binder has been converted to char that will react during a subsequent melt
infiltration process to produce additional SiC (not shown) within the porosity 26. As
represented in FIG. 2, the carbon char 30 does not significantly reduce the cross-sectional
areas of the porosity 26, which would inhibit infiltration by molten silicon. Such a result
is contrary to the scenario schematically represented in FIG. 1, which shows restricted
porosity as a result of the prepreg preform having contained both carbon particulate and
carbon char prior to infiltration in accordance with prior practices. Instead, FIG. 2
represents porosity 26 within the matrix 22 containing only carbon char 30 predominantly
formed during burnout of the primary resin binder(s) in accordance with a preferred
aspect of the invention. It is believed that the resulting carbon char 30 advantageously
has a thread-like appearance that further inhibits choking.
The volumetric ratios of the SiC powder, char-forming primary reSIn
binder(s), and pore former(s) (if present), are preferably controlled so that the porous
preform 20 does not suffer from choking during silicon melt infiltration and does not
require a controlled atmosphere or pressure to achieve infiltration, as has been required
for preforms infiltrated with previous slurry compositions containing carbon and/or
carbon-containing particulate, for example, carbon black. The need for performing the
- 12 -
melt infiltration in a controlled atmosphere is eliminated with the compositions described
here because of the near or complete absence of the particulate carbon in the slurry
composition used to produce the prepreg preform that undergoes firing, and consequently
the resulting porous preform 20 that undergoes melt infiltration. Further, it is believed
that the carbon char 30 produced with the primary resin binder(s) is much more reactive
than particulate carbon employed in prior art matrix slurry compositions. As an example,
it may be possibly to simply immerse the porous preform 20 in molten silicon under an
inert (for example, argon or nitrogen) blanket at low pressures, for example, 30 torr
(about 40 millibar) or less above atmospheric pressure. The preform 20 is capable of
being quickly infiltrated, potentially in as little as about two to ten minutes, in contrast to
the more typical forty minutes or more required for preforms produced with conventional
slurry compositions.
A technical effect of the invention is that, by significantly reducing and more
preferably eliminating the need for carbon-containing particulate in the slurry
composition and compensating for the absence of carbon-containing particulate by
deriving sufficient carbon char from the one or more primary resin binders, it is believed
that an optimized pore structure can be obtained that facilitates a more rapid infiltration
without the undesirable choking phenomenon observed with prior prepreg materials.
Another preferred aspect of this invention is the ability to eliminate the need for using a
controlled atmosphere during melt infiltration, which allows a preform to be infiltrated by
direct union with molten silicon (or a silicon-containing alloy) using a high volume
process, for example, immersion in molten silicon or a continuous tunnel furnace.
Additionally, labor and cost associated with preform lay-up tooling for use in a
controlled-atmosphere process may also be eliminated.
In preferred embodiments of the invention in which the slurry composition
contains two or more different primary resin binders, the slurry composition may contain
about 5 to about 25 weight percent of a first primary resin binder that has a relatively
- 13 -
high carbon char yield, for example, about 30 to about 70%, and about 2 to about 20
weight percent of a second primary resin binder that has a relatively low carbon char
yield, for example, of about 0 to about 10%. If the char yield of the second primary resin
binder is at or near 0%, it may be considered a pore former if it predominantly serves as a
binder in the prepreg preform and produces porosity in the fired porous preform 20
without being a significant source of the carbon char 30.
The char yield of a resin can be determined by heating a specimen of cured
resin in an inert or vacuum atmosphere to a sufficiently elevated temperature, for
example, above 500°C, and then determining the percentage of residue left from the
original specimen. Those skilled in the art will appreciate that typical furan, novolac and
phenolic resins have char yields near 50%, whereas resins such as epoxies and polyesters
have lower char yields, for example, in a range of about 20 to about 40%. By calculation
and knowledge of the char yield, one can determine preferred ratios for amounts of two
or more primary resin binders, optionally any pore former, and the SiC powder to be
combined in the slurry composition to produce a matrix material which can be melt
infiltrated under a wide variety of pressures and atmospheres in much less time.
It is to be recognized that the underlying principle of this invention can be
practiced by those skilled in the art in several different forms. For example, the slurry
composition may contain a single resin having the desired char yield range of 9.5 to 25%,
or two or more resins may achieve an effective carbon char yield of 9.5 to 25%, which
can be varied by judicious choices of the types and amounts of resins based on their
individual carbon char yields and/or their proportions in the slurry composition.
Table 1 below shows, by way of example, some selected variations of this
principle, identifying certain resins, their carbon char yields, and resin amounts (in
weight percents, ignoring solvents and any catalysts) believed to be capable of promoting
infiltration rates in porous preforms. In these examples, the "resin" is effectively a first
- 14-
primary resin binder and the "pore former" is effectively a second primary resin binder in
the slurry that, due to its char yield, meets the definition used herein for a "pore former."
The examples in Table 1 also report suitable amounts of SiC powder that can be used in
combination with the resins. It should be recognized that an additional benefit of this
invention is that adequate amounts of pore formers can be used as binders in the prepreg
preform without perhaps an undesired concomitant increase in carbon content. Thus
benefits of the current invention can be seen to include smoother unperturbed melt
infiltration by avoiding or at least minimizing one or more of the root causes of the
choking phenomenon. Another notable benefit of the current invention is that
eliminating or at least minimizing the presence of carbon particulate in the slurry can
reduce or completely eliminate the need for a controlled or special atmosphere during
preform melt infiltration of the preform.
TABLE I
Resin Pore Pore Former
Composition Char Former Char Yield,
Example Resin Resin % Yield, % Pore Former % % SiC Type SiC Powder. %
1 Furfuryl Alcohol 17 50 Polyvinyl Butyral 4 0