CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No.
611639,617, filed April 27,2012, the contents of which are incorporated herein by
reference.
STATEMENT REGARDING FEDERALLY SPOSNORED RESEARCH
This invention was made with government support under Contract No.
DE-FC26-05NT42643 awarded by Department of Energy. The Government has
certain rights in this invention.
BACKGROUND OF THE INVENTION
The present invention generally relates to ceramic matrix composite
(CMC) articles and processes for their production.
CMC materials have become of particular interest for use in
turbomachinery as higher operating temperatures are sought to increase their
efficiency. CMC materials, and particularly those proposed for gas turbine
engine applications, typically comprise a ceramic fiber reinforcement material
embedded in a ceramic matrix material. The reinforcement material serves as
the load-bearing constituent of the CMC, and the ceramic matrix protects the
reinforcement material, maintains the orientation of its fibers, and serves to
dissipate loads to the reinforcement material.
Of particular interest to high-temperature applications are silicon-based
composites, such as silicon carbide (Sic) as the matrix andlor reinforcement
material. Notable examples of SiCISi-Sic (fiberlmatrix) CMC materials and
processes are disclosed in commonly-assigned 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 commonly-assigned U.S. Patent Application Publication No.
200410067316. One such process is known as "prepreg" melt-infiltration (MI),
which in general terms entails the fabrication of CMCs using multiple prepreg
layers, each in the form of a tape-like structure comprising the desired
reinforcement material, a precursor of the CMC matrix material, binders, and
other possible ingredients. The prepregs must undergo processing (including
curing, also known as firing) to convert the precursor to the desired ceramic.
Multiple plies of prepregs are stacked and debulked to form a laminate preform, a
process referred to as "lay-up." Following lay-up, the laminate preform will
typically undergo debulking and curing while subjected to applied pressure and
an elevated temperature, such as in an autoclave. The melt-infiltration process
generally entails heating the laminate preform in a vacuum or an inert
atmosphere to decompose (burnout) the binders and produce a porous preform
ready for melt infiltration, after which the preform can be melt infiltrated with, for
example, molten silicon supplied externally to the preform. The molten silicon
infiltrates into the porosity and preferably reacts with constituents (for example, a
carbon source) within the matrix to form a silicon-based ceramic (for example,
silicon carbide) that fills the porosity to yield the desired CMC component.
CMC articles having inner cavities are desirable or necessary for some
applications, including but not limited to cavities that define cooling slotslholes
and complex cooling passages within airfoil components, as well as cavities
intended to generally achieve weight reduction. Inner cavities can be produced
in a CMC article by forming the laminate preform around a mandrel. However,
the mandrels must be removed prior to melt infiltration. Mandrels that remain
solid during burnout must be physically removed, which can be impossible if the
desired cavity has twists or tapers. FIG. 1 schematically shows an example
where a conventional steel mandrel 30 is intended to form a subsequent cavity in
a section 20 of a laminate preform 10. The steel mandrel 30 cannot be removed
from the preform 10 due to its being captured by a shoulder 22 defined by plies
at one end of the preform 10. To address this issue, polymeric mandrels have
been proposed that are formed of fugitive resins. Fugitive polymeric resins, in
the context of this description, are typically hydro-carbon based solids which
upon heating to a sufficiently high temperature, typically 400-800 "C, volatilize
leaving little or no carbon residue. Notable examples of fugitive resins include
poly-methyl methacrylate and ply-vinyl alcohol. However, these resins have
thermal expansion coefficients that may be five to ten times greater than the
material of the CMC preform. The higher expansion coefficient of the fugitive
resins can cause the CMC preform to distort during heating to decompose the
binder resins. During burnout, the fugitive resins melt and the molten resin must
be removed from the resultant cavity within the interior of the CMC article. Some
of the molten resin may form a carbonaceous coating inside the cavity which,
when reacted with silicon during subsequent melt infiltration, can alter the cavity
dimensions. When using fugitive resins with larger-size CMC components, the
amount of gases which must escape from or through the preform as the
polymeric mandrel decomposes also increases. This necessitates using slower
pyrolysis cycles which increases processing cycle time for the CMC components.
Accordingly, there is a need for improved methods capable of forming
internal cavities within CMC articles.
BRIEF DESCRIPTION OF THE INVENTION
The present invention provides a method capable of forming an
internal cavity within a CMC article through the use of a mandrel that can be
advantageously reactive to constituents of the CMC material so as to be
incorporated into the CMC article.
A first aspect of the invention is a method of creating an internal cavity
in a CMC article by the use of a reactive mandrel to achieve a cavity in a CMC
article. The mandrel material wets the CMC preform and reacts with the preform
and/or is absorbed into the preform during a thermal treatment. In a preferred
embodiment of the invention, a reactive mandrel is made of elemental silicon or a
silicon alloy, which melts during melt infiltration to provide a source of silicon for
the melt infiltration process and/or is mostly eliminated from the resultant hollow
inner cavity.
A second aspect of the invention is a mandrel suitable for the purpose
of creating an internal cavity in a CMC article. In a preferred embodiment of the
invention, the mandrel comprises silicon or an alloy of silicon.
A third aspect of the invention is to create a CMC article with a desired
internal cavity through a method of using a mandrel made of a material that is
chemically reactive with a constituent of the CMC preform and/or absorbable by
the CMC preform, and melt-infiltrating the preform wherein the mandrel material
wets the CMC preform, reacts with a constituent of the perform and is
substantially absorbed.
A technical effect of the invention is that, because the mandrel is
melted to react with or be absorbed into a CMC preform, an internal cavity in a
CMC article can be produced without undesired distortion or deformation of the
CMC preform used to produce the article. Further, long processing times typically
needed when fugitive resins are used for producing a cavity can be eliminated.
Another technical effect of the invention is that cavities of desired and
complex shapes can be achieved in a CMC article for purposes of weight
reduction and/or cooling purposes without difficulties typically associated with the
removal of non-reactive metallic mandrels or problems associated with
elimination of fugitive resins used in non-reactive mandrels.
Other aspects and advantages of this invention will be further
appreciated from the following detailed description
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically represents a cross-section of a CMC preform with
a non-reactive mandrel.
FIG. 2 schematically a cross-section of a CMC preform with a reactive
or otherwise absorbable mandrel.
FIG. 3 schematically represents a cross-section CMC article with a
cavity achieved through the use of a reactive or otherwise absorbable mandrel.
DETAILED DESCRIPTION OF THE INVENTION
This invention is directed to the creation of internal cavities within CMC
articles, for example, to create cooling channels, achieve weight reduction and/or
any other desired purpose. Conventional processes of creating such cavities
have utilized mandrels formed of materials such as fugitive resins or non-reactive
metals. Both of these methods have several limitations and disadvantages as
described previously. The current invention addresses difficulties and
disadvantages of the prior art by methods that incorporate use of mandrels made
of materials that can be absorbed by and preferably reacted with a CMC preform
used in the manufacture of a CMC article. In particular, preferred materials for
mandrels employed with the invention are molten at a thermal treatment
temperature of the preform, for example, during melt infiltration performed after a
curing (firing) step carried out on a laminate preform to form a porous preform.
Preferred characteristics for materials for mandrels that can be
advantageously eliminated by absorption and reaction with a CMC preform
include the ability to be formed into a suitable shape for a mandrel, wet the CMC
preform at melt infiltration temperatures, react with constituents of the CMC
preform to form reaction products that are advantageous or at least not
detrimental to the final article, and be absorbed nearly completely by the CMC
preform either by reaction, by infiltration, or both.
Preferred materials for such mandrels are silicon and alloys of silicon.
Sintered silicon-containing mandrels can be manufactured by, for example, damp
pressing and sintering a powder material in large lots to minimize costs.
Elemental silicon and silicon alloy powder materials are capable of exhibiting
nearly zero shrinkage during sintering, yet exhibit sufficient strength to survive
handling and autoclave curing pressures. Furthermore, a sintered siliconcontaining
mandrel can remain within the preform during the entire process
sequence leading up to melt infiltration and exhibit thermal expansion
characteristics similar to those of the CMC preform. While silicon or siliconbased
materials can be well suited as the material of a mandrel used to create a
cavity within a SiC-based CMC articles, it is foreseeable that different mandrel
materials with different reaction andlor infiltration characteristics may exist or be
developed and would be compatible with the chemistry of a CMC article.
A mandrel of this invention may be formed entirely of elemental
silicon or a silicon alloy. Alternatively, sintered silicon-containing mandrels may
also contain fugitive binders, such as acrylic resins or polyvinyl alcohol. A small
amount of water or alcohol may be added to render the initial powder mixture
damp and suitable for pressing in a mold. The powder mixture can be pressed
under sufficient pressure to yield a desired freestanding shape, herein after
called a core. In a preferred, non-limiting method, this core can be dried and
then loaded into a vacuum furnace to undergo sintering, for example, at a
temperature of about 1385 +/-I0 "C for about ninety minutes, to render it freestanding
and able to survive being covered with ceramic composite precursor
prepreg plies to yield a laminate CMC preform. Since the core shrinks during the
sintering operation, allowance must be made by oversizing the original pressed
core shape. The sintered shape is then removed from the vacuum furnace. One
could also envision using a 3-dimensional (3-D) printer with silicon powder in a
printing ink suitable for use with 3-D printers as a way to make a mandrel with
fine features. A core made with silicon ink can be sintered as described above.
The mandrel is preferably coated to yield a substantially impervious
surface capable of preventing any resins of the CMC preform from penetrating
the sintered silicon mandrel during lay-up and curing of the laminate CMC
preform. An impervious surface is particularly desirable if preform resins are of
the type that would form a silicon compound, such as silicon carbide, and could
therefore react with the silicon in the mandrel and possibly cause the inner cavity
dimensions formed by the mandrel to be altered. Suitable coating materials for
this purpose include, but are not limited to, acrylic spray resins, such as poly
methyl methacrylate.
Ceramic composite precursor prepreg plies may then be wrapped
over the sintered mandrel shape and cured in an autoclave or matched-plate die
set. After curing, the preform and sintered mandrel may be heated to above 500
OC to remove the resinous components to yield a porous preform. The porous
preform plus the sintered silicon core and any additional silicon can then be
further heated in another vacuum or atmosphere furnace to cause the silicon to
melt and infiltrate the porous preform. FIG. 2 schematically represents a sintered
silicon mandrel 40 incorporated into a section 20 of a laminate preform 10.
During a melt infiltration process or other thermal treatment of the preform 10, the
silicon mandrel 40 melts and the resulting molten material wets the CMC preform
10, infiltrates into the preform 10 and chemically reacts with constituents of the
CMC preform to form, for example, Sic or another silicon compound. Any
unreacted silicon may simply remain infiltrated. FIG. 3 schematically represents
a cavity 50 formed in the section 20 of the fully infiltrated CMC preform 10
indicated in FIG. 3 as the final CMC article 100.
It will be noted by those skilled in the art that use of a reactive mandrel
material can result in unconsumed mandrel material. This condition can be
avoided by proper tuning of the processing conditions. The presence of
unconsumed mandrel material within the CMC article may not have any
deleterious effects except in cases wherein the weight of the CMC article has to
be closely controlled. In such cases complete consumption of the mandrel
material can be ensured through additional processing steps as required.
Multiple cavities can be formed in a CMC preform utilizing multiple mandrels and
following the methods described herein. A single cavity or multiple cavities
formed in a CMC article can be utilized for purposes of weight reduction, andlor
as cooling slotslholes.
It is foreseeable that other materials could be used that wet the CMC
preform 10 during a melt-infiltration process and are completely absorbed
through penetration into the CMC preform 10, but would not necessarily react as
silicon does with the CMC preform 10. However, in such cases one needs to
ensure that such materials do not contribute to any ill effects either due to
thermal expansion characteristics or other physical properties. Preferred
embodiments of the invention are thus directed to the utilization of siliconcontaining
materials such that no extraneous materials are used or formed other
than those of conventional CMC melt-infiltration processes, resulting in a SiCbased
CMC article. It is to be further noted that the methods disclosed may be
used to create internal cavities in CMC articles based on silicon compounds
other than Sic, such as SIN as anon-limiting example.
In view of the above, it can be seen that a significant advantage of this
invention is that it solves problems associated with forming hollow internal
cavities within CMC articles without having to physically remove a mandrel from
the resulting cavity after curing, and without introducing potentially deleterious
materials into the final CMC article.
While the invention has been described in terms of specific
embodiments, it is apparent that other forms could be adopted by one skilled in
the art. Accordingly, it should be understood that the invention is not limited to
the specific disclosed embodiments. It should also be understood that the
phraseology and terminology employed above are for the purpose of disclosing
the invention and the embodiments, and do not necessarily serve as limitations
to the scope of the invention. Therefore, the scope of the invention is to be
limited only by the following claims.
TITLE: METHOD OF PRODUCING AN INTERNAL CAVITY IN A CERAMIC
MATRIX COMPOSITE AND MANDREL THEREFOR
10 Preform 110
20 Section 120
22 Shoulder 122
24 124
30 Mandrel 130
40 Mandrel 140
50 Cavity 150
100 Article 1100

We Claims
1. A method of forming a CMC article with at least one internal cavity,
the method comprising:
incorporating at least one mandrel into a CMC preform; and
subjecting the CMC preform to a thermal treatment wherein the at least
one mandrel melts to yield a molten material that wets the CMC preform, and is
reacted with andlor absorbed into the CMC preform leaving behind at least one
internal cavity within the CMC preform.
2. The method according to claim 1, wherein the at least one mandrel
consists of silicon or a silicon alloy.
3. The method according to claim 1, wherein the thermal treatment
comprises a melt-infiltration step.
4. The method according to claim 1, wherein the at least one internal
cavity comprises multiple internal cavities and the at least one mandrel
comprises multiple mandrels.
5. The method according to claim 1, wherein the at least one mandrel
contains a material that is chemically reactive with a constituent of the CMC
preform.
6. A mandrel incorporated into a CMC preform, wherein the mandrel
yields a molten material when melted that reacts with andlor is absorbed into the
CMC preform during a thermal treatment to leave behind an internal cavity in the
CMC preform.
7. The mandrel in claim 6, wherein the mandrel is made of silicon or
silicon alloy.
8. The mandrel in claim 7, where in the mandrel comprises a coating
that is impervious to constituents of the CMC preform.
9. The mandrel in claim 8, wherein the coating comprises an acrylic
resin.
10. The mandrel in claim 9, wherein the acrylic resin is methyl
methacrylate.
11. The mandrel in claim 6, wherein the mandrel comprises a
sintered powder material.
12. The mandrel in claim 6, wherein the mandrel is made by a
process comprising 3-D printing.
13. A CMC article with at least one internal cavity, where in the at
least one internal cavity is made by a method comprising:
incorporating at least one mandrel into a CMC preform, the at least
one mandrel comprising a material that is chemically reactive with a constituent
of the CMC preform; and
melt-infiltrating the CMS preform wherein the at least one mandrel
melts and the material thereof wets the CMC preform, reacts with the constituent
of the CMC preform, and is substantially consumed leaving behind at least one
internal cavity in the CMC preform.
14. The CMC article of claim 13 wherein the material of the at least
one mandrel is elemental silicon.
15. The CMC article of claim 13, wherein the material of the at least
one mandrel is silicon or a silicon alloy.
16. The CMC article of claim 13, wherein the at least one cavity
comprises multiple internal cavities and the at least one mandrel comprises
multiple mandrels.
17. The CMC article of claim 13, wherein the at least one cavity is a
cooling slot/hole.
18. The CMC article of claim 13, wherein the CMC article is an airfoil
component.
19. The CMC article of claim 13, where in the CMC article is based on
silicon compounds.
20. The CMC article of claim 13, wherein the silicon compound is Sic.