Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CERAMIC MATRIX COMPOSITE STRUCTURES, COMPONENTS
FORMED THEREWITH, AND METHODS OF PRODUCING
CROSS REFERENCE TO RELATED APPLICATIONS
[00011 This application claims the benefit of U.S. Provisional Application No.
61/345,402, filed May 17, 2010, the contents of which are incorporated herein
by
reference.
BACKGROUND OF THE INVENTION
[00021 The present invention generally relates to ceramic matrix composite
(CMC)
materials. More particularly, this invention relates to ceramic matrix
composite sandwich
structures, gas turbine engine components comprising such structures, and
processes of
making such structures.
[00031 The growth in size of new aircraft engines, coupled with the increasing
cost and
scarcity of exotic raw materials used in high temperature metal alloys, is
driving the
utilization of new materials for making components suitable for use in
intermediate to
high temperature environments. One alternative to metal alloys for making
engine
components for such environments is ceramic matrix composites (CMCs).
100041 Generally, a CMC is a material that includes a second ceramic phase
embedded in
a ceramic matrix. The second ceramic phase imparts different thermal, elastic,
and
structural properties to the ceramic matrix thereby making the resulting CMC
tougher, in
other words, the CMC is not as susceptible to surface and bulk flaws as the
ceramic
matrix alone. This second ceramic phase can include a variety of forms
including
particulates, whiskers, chopped fibers, fiber tows, fiber cloths, and any
combination
thereof. CMCs may also include laminates of different materials as long as
those
laminates contain at least one ceramic layer. Though there are a variety of
CMCs
available, the aviation industry typically turns to CMCs comprising continuous
fiber
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tows, either in tape or cloth forms, due to the higher strains-to-failure that
these forms of
reinforcements are capable of imparting to a ceramic matrix.
[0005] High grades of continuous ceramic fiber utilized by the aviation
industry can be
expensive. Thus, the use of large quantities of these materials is not cost-
effective.
Moreover, backside cooling is often necessary or desirable, in which case CMCs
may
experience thermal stresses due to their relatively low thermal
conductivities. Often,
thousands of small holes are drilled throughout a CMC, or fugitive threads are
incorporated into a CMC, to provide needed cooling to the finished component.
This
can further increase the cost of the material.
[0006] Another consideration when looking to alternate materials is engine
noise. The
ability to produce a quieter engine is becoming a differentiating factor for
airframers
when selecting turbine engines for next generation aircraft. Noise from a
turbine engine
can be attributed to numerous sources, including the fan, turbine, combustor,
aft-turbine
engine component vibration, and high-speed exhaust gases. While a variety of
alternatives have been explored to address such noise issues, these
alternatives can often
result in added weight and/or require cooling air, both of which can decrease
the
efficiency of the engine.
[0007] Accordingly, there remains a need for material systems suitable for use
in
intermediate to high temperature turbine engine applications that can provide
strains-to-
failure comparable to continuous fiber reinforced CMCs, while reducing both
the
quantity of ceramic reinforcement needed, as well as the affect of through-
thickness
thermal stresses. In addition, it would be desirable that such systems also be
capable of
providing noise damping benefits.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The present invention provides ceramic matrix composite sandwich
structures,
gas turbine engine components that comprise such sandwich structures, and
methods of
producing such sandwich structures.
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[0009] According. to a first aspect of the invention, a gas turbine engine
component
comprises a ceramic matrix composite sandwich structure that includes a core
having
oppositely-disposed first and second surfaces, a first facesheet bonded to the
first surface,
and a member bonded to the second surface so that the core is between the
first facesheet
and the member. The first facesheet and the core comprise, respectively, first
and second
ceramic matrix composite materials, and the second ceramic matrix composite
material of
the core comprises a ceramic reinforcement material in a ceramic matrix
material.
[0010] According to a second aspect of the invention, a ceramic matrix
composite
sandwich structure comprises a core having oppositely-disposed first and
second
surfaces, a first facesheet bonded to the first surface, and a member bonded
to the second
surface so that the core is between the first facesheet and the member. The
first facesheet
and the core comprise, respectively, first and second ceramic matrix composite
materials,
and the second ceramic matrix composite material of the core has a ceramic
reinforcement material in the form of a felt or an open weave fabric.
[0011] Other aspects of the invention include methods of producing the
component and
ceramic matrix composite sandwich structure comprising their respective
elements
described above.
[0012] A technical effect of the invention is the ability of the ceramic
matrix composite
sandwich structure to exhibit desirable properties at intermediate to high
temperatures,
such as those existing in gas turbine engines. The ceramic matrix composite
sandwich
structure achieves such properties in part due to the ceramic reinforcement
material
within its core. Preferred constructions for the ceramic reinforcement
material serve to
minimize the quantity of reinforcement material required by the core while
enabling the
sandwich structure to exhibit desired properties, for example, an elastic
modulus of
greater than 70 GPa and a strain-to-failure of greater than about 0.2% at a
temperature of
at least 425 C. Other potential benefits of the sandwich structure include
reduced
through-thickness thermal stresses and noise damping effects.
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[0013] Other aspects and advantages of this invention will be better
appreciated from the
following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. I schematically represents a cross-sectional view of a high-bypass
turbofan
engine.
[0015] FIG. 2 schematically represents a cross-sectional view of a CMC
sandwich
structure in accordance with an embodiment of the invention.
[0016] FIGS. 3, 4 and 5 are scanned images showing ceramic fabrics suitable
for use-in
facesheets of the sandwich structure represented in FIG. 2.
[0017] FIG. 6 schematically represents a cross-sectional view of a CMC
sandwich
structure in accordance with another embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Embodiments described herein generally relate to ceramic matrix
composite
sandwich structures, gas turbine engine components comprising the same, and
methods
for making the same. Such structures are suitable for use in intermediate to
high service
operating temperatures and are capable of exhibiting strain-to-failure
properties that are
comparable to conventional continuous fiber reinforced CMCs, while also having
the
potential for reducing the quantity of ceramic reinforcement needed and/or the
adverse
effects of through-thickness thermal stresses. In addition, sandwich
structures described
herein are capable of exhibiting desirable acoustic damping characteristics.
While various
applications are foreseeable and possible, applications of particular interest
include high
temperature applications, for example, components of gas turbines, including
land-based
and aircraft gas turbine engines. While specific references will be made to
certain
components for use within the turbine sections of gas turbine engines, those
skilled in the
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art will appreciate that the teachings of this invention are applicable to a
variety of
components.
[00191 For illustrative purposes, FIG. I schematically represents a high-
bypass turbofan
engine 10 of a type known in the art. The engine 10 is schematically
represented as
including a fan assembly 12 and a core engine 14. The fan assembly 12 is shown
as
including a composite fan casing 16 and a spinner nose 20 projecting forwardly
from an
array of fan blades 18. Both the spinner nose 20 and fan blades 18 are
supported by a fan
disc (not shown). The core engine 14 is represented as including a high-
pressure
compressor 22, a combustor 24, a high-pressure turbine 26 and a low-pressure
turbine 28.
A large portion of the air that enters the fan assembly 12 is bypassed to the
rear of the
engine 10 to generate additional engine thrust. The bypassed air passes
through an
annular-shaped bypass duct 30 and exits the duct through a fan nozzle 32. The
fan blades
18 are surrounded by a fan cowling or nacelle that defines an inlet duct 34 to
the engine
10, as well as the fan nozzle 32. An exhaust nozzle 36 and a centerbody 38
extend
aftward from the core engine 14. The centerbody 38 is concentrically aligned
with the
exhaust nozzle 36 along the centerline of the core engine 14. The centerbody
38 defines
a convergent-divergent path through the nozzle 36, such that the inner surface
of the
exhaust nozzle 36 defines an outer boundary of the engine exhaust flowpath and
the outer
surface of the centerbody 38 defines an inner boundary of the exhaust
flowpath.
100201 As previously described, embodiments herein generally relate to CMC
sandwich
structures suitable for use in producing components that are capable of
operating in
intermediate and high temperature engine environments, including the turbine
section of
the engine 10 of FIG. 1. As used herein, "intermediate temperature" refers to
temperatures of about 800 F (about 425 C) to about 1500 F (about 815 C), while
"high
temperature" refers to temperatures greater than about 1500 F (815 C). CMC
sandwich
structures of this invention generally comprise a core that is "sandwiched"
between two
facesheets. With reference to FIG. 2, a sandwich structure 40 is represented
as a multi-
layered structure that generally comprises first and second facesheets 42 and
44 separated
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by a core 46 and attached to first and second surfaces 48 and 50,
respectively, of the core
46. The facesheets 42 and 44, as well as the core 46, may comprise different
combinations of CMC materials, as will be explained below. Mechanically, such
sandwich structure 40 is similar to an I-beam in which the bending stiffness
of the
structure 40 is highly dependent on the modulus of each facesheet 42 and 44
and the
distances of the facesheets 42 and 44 from the center of the core 46 in a
balanced I-beam
system.
10021] Each facesheet 42 and 44 comprises a CMC composition that contains a
ceramic
reinforcement material, preferably in the form of one or more ceramic fabrics
(not
shown). The fabrics may be formed of a variety of known fabric materials used
in CMC
structures. For example, in one embodiment each facesheet 42 and 44 may have
an
oxide-oxide CMC composition, in which case each facesheet 42 and 44 has a
reinforcing
fabric, cloth, or paper that is formed of oxide-based material and is
contained within a
matrix formed of an oxide-based material. The oxide-based materials may be,
for
example, aluminum oxide (A1203), silicon dioxide (Si02), aluminosilicates, and
mixtures
thereof. Nonlimiting examples of aluminosilicates include crystalline
materials such as
mullite (3A1203.2SiO2), as well as glassy aluminosilicates. Other nonlimiting
examples
of suitable ceramic materials for the facesheets 42 and 44 include silicon
carbide
reinforcing fabrics, cloths or papers contained in an oxide-based material,
for example, a
SiC-oxide CMC composition.
10022] Ceramic fabrics suitable for use in the facesheets 42 and 44 include
fabrics having
a tight weave or an open weave. As used herein, a "tight weave" refers to
fabrics in
which there is contact between adjacent tows in the final CMC sandwich
structure, as
illustrated in FIG. 3, whereas the term "open weave" refers to fabrics having
a visible
space between the tows in the final CMC sandwich structure, as illustrated in
FIGS. 4 and
5. Nonlimiting examples of tight weave fabrics suitable for use in the
facesheets 42 and
44 include oxide-fiber cloths such as AF-10, BF-20, XN-513 and DF-1 I
(commercially
available from the 3M Company), as well as SiC-fiber cloths such as PN-
SI5HI6PX and
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TM-517E08PX (commercially available from UBE America, Inc.). Nonlimiting
examples of open weave ceramic fabrics suitable for use in the facesheets 42
and 44
include AF-8 (commercially available from the 3M Company), as well as
specialty
woven cloths such as a 14xl4 plain weave using 800 filament NexteITM720 tows
that are
individually twisted 0.5 times/inch (about 0.2 times/cm), a 10x10 plain weave
using two
800 NextelTM720. filament tows that are twisted together 1.5 times/inch (about
0.6
times/cm), and a 8x8 plain weave using four 750 NextelTM440 filament tows that
are
twisted together 1.5 times/inch (about 0.6 times/cm). The facesheets 42 and 44
may
comprise the same ceramic materials, or the facesheets 42 and 44 may be formed
of
different ceramic materials. Moreover, in some instances it may be desirable
for one of
the facesheets 42 or 44 to comprise a tight weave fabric and the other to
comprise an
open weave fabric, for example, to tailor the acoustic damping characteristics
of the
sandwich structure 40, as will be discussed below. The ceramic reinforcement
material
typically constitutes about 5 to about 45 volume percent of each facesheet 42
and 44,
depending on the type of reinforcement. Tight weave reinforcement materials
preferably
constitute about 35 to about 45 volume percent of a facesheet 42 or 44, while
open weave
and paper reinforcement materials preferably constitute about 5 to about 20
volume
percent of a facesheet 42 or 44.
100231 The ceramic matrix of the facesheets 42 and 44 can be formed from a
ceramic
slurry, in which case the facesheets 42 and 44 are initially in the form of a
prepreg. As
used herein, a "ceramic slurry" refers to any fluid material containing a
mixture of one or
more types of polymer materials and one or more types of ceramic particles
that are
mixed in a solvent to form a substantially uniform mixture that is capable of
being
dispersed around the ceramic fabrics of the facesheets 42 and 44, and then
subsequently
transformed into a ceramic material through the application of heat. As an
example, the
polymer materials can be converted to form a ceramic material during a
sintering
operation performed on the facesheets 42 and 44 prior to assembly with the
core 46, or
during a sintering operation performed on the sandwich structure 40 during
which the
facesheets 42 and 44 can be bonded to the core 46 for high temperature
applications. US
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Patent 5,601,674 provides a nonlimiting description of a ceramic slurry of a
type that can
be used with the present invention.
[00241 Nonlimiting examples of suitable polymers for use in the ceramic slurry
include
polymers that harden and convert to silica, and more preferably polymers that
have a
conversion efficiency to silica of at least 30 weight percent. Nonlimiting
examples of
suitable polymers include silicone resins, for example, methyl sesquisiloxane
mixtures of
the polysiloxane family commercially available from sources such as General
Electric
Silicone Products Div. (for example, SR350 and SR355) and Dow Corning (for
example, 249 silicone resin). Nonlimiting examples of ceramic particle
constituents that
are suitable for use in the ceramic slurry include oxides of such elements as
Al, Si, B, and
combinations thereof, including such commercially available materials as
A1203, Si02,
B203 and 3AI203.2Si02. Typically, preferred ceramic particles sizes are
believed to have
diameters of less than one micrometer, though the use of larger particles is
also
foreseeable. Nonlimiting examples of suitable solvents for use in the ceramic
slurry
include liquids that are capable of dissolving the polymer(s) and uniformly
distributing
the ceramic particles and dissolved polymer(s) around the ceramic fabrics of
the
facesheets 42 and 44. Nonlimiting examples of suitable solvents organic
solvents such as
ethyl alcohol, isopropyl alcohol and acetone.
100251 While the relative quantities of the polymer(s), ceramic particles, and
solvent can
vary depending on the solubility and saturation limit of the polymer and the
desired
viscosity of the slurry, an example of a suitable composition for the ceramic
slurry
comprises, but weight, about 30% to about 60% ceramic particles, about 10% to
about
60% polymer, and about 25% to about 50% solvent. More preferred ranges for the
constituents of the ceramic slurry are believed to be, by weight, about 35% to
about 40%
ceramic particles, about 15% to about 20% polymer, and about 30% to about 45%
solvent.
100261 The core 46 of the sandwich structure 40 represented in FIG. 2 is a CMC
composition that contains a ceramic reinforcement material, preferably in the
form of a
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ceramic felt material. As will be discussed below in reference to FIG. 6,
another type of
sandwich structure 140 contains a core 146 having a different type of CMC
compositions,
in which the ceramic reinforcement material is in the form of a fabric. In
either case, the
ceramic reinforcement material is dependent upon the particular application
but
preferably does not constitute more than 20 volume percent of the core 46/146.
More
particularly, the ceramic reinforcement material preferably constitute at
least 5 to about
20 volume percent of the core 46/146, and more preferably about 7 to about 15
volume
percent of the core 46/146. Suitable thicknesses for the cores 46 and 146 will
vary
depending on the particular application, though core thicknesses in a range of
about 3
mm to about 37 mm, more preferably about 12 mm to about 25 mm, are believed to
be
particularly well suited for use in gas turbine applications.
[00271 In the embodiment of FIG. 2, in which the ceramic reinforcement
material of the
core 46 is a felt material, the core 46 can be made by first suspending and
blending
ceramic fibers, optional glass fibers, and one or more fugitive bulking
materials in a
soapy water solution to make a core suspension. The core suspension may
include
varying combinations of materials, a nonlimiting example of which is, by
weight, about
50% to about 83% ceramic fibers, about 0% to about 30% glass fibers, and about
10% to
about 35% fugitive material(s).
100281 Preferred ceramic fibers for the felt core 46 of FIG. 2 are capable of
promoting
the strain properties of the core 46. Notable examples include silicon-
containing CMC
fibers such as silicon carbide, silicon nitride, silicon oxycarbides, silicon
oxynitrides, and
mixtures thereof. Other notable examples include oxide materials previously
noted as
suitable for use in the facesheets 42 and 44. The ceramic fibers of the felt
core 46 can
have various lengths, typically ranging from about 4 mm (about 1/8 inch) to
about 38 mm
(about 1.5 inch), for example, about 20 mm (about 0.75 inch).
[00291 Chopped glass fibers are desirable in the core 46 for having-the
ability to bond the
ceramic fibers together within the felt core 46. The glass fibers do so by
softening, yet
not melting completely, when heated, for example, during the above-noted
sintering
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operation performed to yield the sandwich structure 40 represented in FIG. 2.
While the
component use temperature can help determine specific chopped glass fibers
suitable for
use in a particular application, the chopped glass fibers can be generally
selected from a
variety of commercially-available fiber glass materials, including but not
limited to E-
glass, M-glass, S-glass and Pink insulation (Owens-Corning ). Suitable
lengths for the
chopped glass fibers will typically range from about 4 mm (about 1/8 inch) to
about 25
mm (about 1 inch), for example, about 20 mm (about 0.75 inch). In a particular
example,
fiberglass insulation can be cut into pieces of about 1 xI inch (about 25x25
mm), so that
its glass fibers are dispersed during blending with the ceramic fibers and
fugitive
material.
[00301 One or more fugitive materials are preferably chosen on the basis of
providing a
bulking effect to the felt core 46 and later creating porosity during
sintering of the core
46. While various different types of fugitive materials can be used, preferred
fugitive
materials decompose when heated, for example, during the above-noted sintering
operation performed on the sandwich structure 40. As an example, cellulose
fillers can
be used as a fugitive material to promote ceramic slurry migration during
prepregging or
infiltrating the felt core 46, as will be described below. Alternatively or in
addition,
chopped rayon fibers or bulk aramid can be used as a fugitive material, with
the benefit
of minimizing the final density of the core 46. Still other notable fugitive
materials
include aramid particles, for example, having particles diameters of about 4
mm (1/8
inch).
[00311 Core suspensions comprising blends of ceramic fibers, glass fibers and
fugitive
material can be poured into a conventional felt-making machine to consolidate
the fibers
and extract any excess water solution. The resulting wet felt core can then be
placed in a
felt dryer to remove any residual water and yield a dried felt core. A ceramic
slurry, as
previously defined, may then be applied to the dried felt core using a
prepregging process
or vacuum-assisted infiltration to yield a preliminary core in the form of a
slurry-
impregnated core. Suitable prepreg and infiltration processes for use with
this invention
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are known in the art and therefore will not be discussed in any detail here.
The ceramic
slurry may have the identical composition as that described for use in the
facesheets 42
and 44, or it may comprise a different combination of polymer and ceramic
particles than
that used in the facesheets 42 and 44.
[0032] In general, lay-up of the sandwich structure 40 can entail individually
applying
the facesheets 42 and 44 to the surfaces of the slurry-impregnated preliminary
core,
which upon sintering will yield the structure 40 represented in FIG. 2. As
previously
noted, the facesheets 42 and 44 can be sintered prior to application to the
preliminary
core, or the facesheets 42 and 44 and the preliminary core can undergo
sintering together
by applying the facesheets 42 and 44 to the preliminary core while still in
the form of
prepregs. Another alternative is to apply the facesheets 42 and 44 to the
preliminary core
after being heated to cure the polymers of their respective ceramic slurries
without
converting the polymers to a ceramic. If the facesheets 42 and 44 are applied
to the
preliminary core in the form of prepregs, sintering of the resulting structure
can be
carried out so that the ceramic slurries of the facesheets 42 and 44 and the
slurry-
impregnated preliminary core bond the facesheets 42 and 44 to the core 46.
Optionally,
prepregs of the facesheets 42 and 44 may be bonded to the preliminary core
with a
suitable adhesive. On the other hand, if the facesheets 42 and 44 are applied
to the
preliminary core in a cured or presintered condition, the use of an adhesive
will typically
be necessary to bond the facesheets 42 and 44 to the core 46, in which case
further curing
and/or sintering steps may be carried out as necessary.
[0033] Adhesives used to bond the facesheets 42 and 44 to the core 46 can
comprise a
mixture of a polymer and ceramic particles processed with a solvent to form a
substantially uniform mixture that does not readily infiltrate any open
porosity of the
slurry-impregnated preliminary core or core 46. Particularly suitable polymers
include
those capable of being converted to a ceramic material that is compatible with
the
ceramic materials of the facesheets 42 and 44 and core 46. Nonlimiting
examples of
suitable polymers include those mentioned above for use in the ceramic
slurries of the
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facecoats 42 and 44, including the SR350 and SR35 silicone resins commercially
available from General Electric Silicone Products Div. and the 249 silicone
resin
commercially available from Dow Corning , in which case the polymer has a
conversion
efficiency to silica of at least 30 weight percent. Particularly suitable
compositions for
the solvent and ceramic particles of the adhesive include those mentioned
above for use
in the ceramic slurries of the facecoats 42 and 44. Suitable particle sizes
for the ceramic
particles are on the order of -325mesh (less than 44 micrometers). Suitable
amounts of
solvent in the adhesive are generally on the order of about 8 to about 20
weight percent.
Additionally, up to about 10 weight percent chopped oxide or silicon carbide
ceramic
fibers can be added to the adhesive to reduce the incidence of shrinkage
cracks that might
occur within the ceramic formed by the adhesive if the sandwich structure 40
is subjected
to temperatures of greater than about 815 C (1500 F).
10034] The ratio of ceramic particles to polymer within the adhesive may
depend on the
ceramic (e.g., silica) yield of the polymer. For example, the SR350 silicone
resin has a
conversion efficiency of about 83wt% SiO2 while the SR355 and 249 silicone
resins have
conversion efficiencies of about 60wt% Si02. However, suitable results are
believed to
be achieved with an adhesive that comprises about 3 to 4 parts by weight of
ceramic
particles for every I part by weight of the silica yield from the polymer.
10035] In embodiments in which prepregs of the facecoats 42 and 44 are applied
to a
prepreg of the core 46, the prepregs can be placed in a mold and cured and
sintered
together to yield the structure 40 of FIG. 2. This approach can be
advantageous for
fabricating large, complex ceramic sandwich structures, in that machining of
the core 46
can be avoided or minimized. In one embodiment, the prepreg stack (with or
without an
adhesive) is bagged under vacuum, placed in an oven, and heated at a
temperature of
about 250 F (about 120 C) for approximately two hours to cure the polymers of
the
prepregs and, if present, the adhesive. The cured structure can then be
sintered by
heating the cured structure to a temperature at least about 74 C and up to a
temperature of
between about 600 C and 1000 C for about two hours to convert the matrix
materials of
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the facesheets 42 and 44 and core 46 to their respective ceramic materials and
produce
the final sandwich structure 40 represented in FIG. 2. The temperature
employed during
sintering will determine in part whether the structure 40 is suitable for use
in an
intermediate or high temperature engine environment.
[0036] In the embodiment of FIG. 6 utilizing a fabric core material, a
sandwich structure
140 is formed that can comprise the above-described facesheets 42 and 44
bonded to a
fabric core 146. In this embodiment, a ply of an open weave fabric (as
described above
in reference to the facesheets 42 and 44) can be prepregged with a ceramic
slurry of a
type previously described for prepregging or infiltrating the felt core
material used to
form the felt core 46 of FIG. 2. The continuous tows of an open weave fabric
contribute
strength to the core 146 and sandwich structure 140, while the open
interstices between
tows of the weave permit air migration and cooling. Plies of the prepregged
open weave
fabric can be laid-up to a desired thickness, depending on the particular
requirements of
the application, to yield a preliminary core. The final fabric core 146 can
comprise a
plurality of layers of the same open weave fabric, or a plurality of layers of
different open
weave fabrics. The facesheets 42 and 44, which may be produced with tight
weave or
open weave fabrics that have been prepregged with a ceramic slurry (for
example, as
described above) can then be applied to the preliminary core, preferably
bonded with the
use of an adhesive (for example, as described above), after which the prepreg
stack can
be cured and sintered as previously described to yield the sandwich structure
140.
[0037] Sandwich structures 40 and 140 of the types described above can be used
to
manufacture an entire engine component, or can be produced as discreet strips
that can be
selectively placed within or on a component during the fabrication thereof to
provide
tailored density, cooling, and/or acoustic properties. In addition, sandwich
structures 40
and 140 of the types described above can be fabricated directly on a metal or
ceramic
component such that the component can serve as one of the two facesheets 42 or
44 of
the structure 40 or 140. This approach can reduce the use of costly tooling,
even for
components having complex geometries, and yield a sandwich structure 40 or 140
that
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form-fits to the component, thereby reducing variations in the thickness of
the adhesive
and improving attachment between the component and the sandwich structure 40
or 140.
The structure 40 or 140 can be adhesively bonded or mechanically attached to
the metal
or ceramic component by heating to a temperature that will cure the polymer of
the
ceramic slurry (or polymers of the ceramic slurries), after which the
structure 40 or 140
can be sintered to convert the polymer to a ceramic material.
100381 Nonlimiting examples of components that can be manufactured using the
CMC
sandwich structure technology described herein include the core engine exhaust
nozzle
36 and the exhaust centerbody 38 of FIG. 1. For example, the sandwich
structures 40 and
140 of FIGS. 2 and 6 can be used to form an interior skin of the nozzle 36
and/or an
exterior skin of the centerbody 38 to provide these components with desirable
properties,
for example, structural, thermal, and acoustic properties desired for the
nozzle 36 and/or
centerbody 38 at the high temperatures of the exhaust gas flowpath. Either
sandwich
structure 40 or 140 can be laid up on a tool to obtain the desired geometries
for the nozzle
36 and/or centerbody 38. Alternately, the sandwich structures 40 and 140 can
be laid up
directly on the nozzle 36 or centerbody 38 and subsequently adhered thereto.
Acoustic
damping can be realized with the nozzle 36 and centerbody 38 though the use of
the
sandwich structure 140 comprising an open weave fabric, as described
previously.
100391 Sandwich structures of the types described above can be constructed to
have a
lower density as compared to corresponding metal components. For example, a
sandwich
structure having a thickness of about one-half inch (about 13 mm) and
fabricated as
described above is capable of having densities of about 0.3 g/cc (about 19
Ibs/ft3) and
less. In addition, sandwich structures of the types described above are
capable of an
elastic modulus of about 10 Msi (about 70 GPa) or higher and a strain-to-
failure of about
0.2% or higher at intermediate or high temperatures. High strain-to-failure
properties are
achieved in part through the use of appropriate adhesives, for example, of the
types
described above, which are capable of maintaining the bond between the
facesheets 42
and 44 and core 46 and 146 in intermediate and high temperature engine
environments of
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operation. Moreover, the use of fabric cores 146 of the type described in
reference to
FIG. 6 can yield a sandwich structure 140 that exhibits an air flow capability
of about
7x10-4 lbs/sec per square inch (about 5x10-5 kg/s per square centimeter) at a
P/PA ratio of
1.2, where P is the applied cooling pressure and PA is the atmospheric
pressure. This can
allow components made from such sandwich structures to attain desired internal
cooling
needed to reduce thermal stress to acceptable levels without the need to drill
holes or use
fugitive threads, as is currently done. Those skilled in the art will
understand that the
effectiveness of the cooling can relate to both the tow count and the P/PA
ratio, as such
factors can influence how much time air interacts with the internal ceramic
structure and
therefore, the extent of internal cooling.
[0040] The use of fabric cores 146 of the type described in reference to FIG.
6 can also
yield a sandwich structure 140 that provides acoustic benefits. More
particularly, the low
density and modulus of the fabric core 146 can be utilized to damp engine
noise. For
example, the fabric core 146 can be fabricated to contain multiple open-weave
fabrics,
and the thickness of each fabric layer differ from other fabric layers to
modify the density
and modulus of the core 146, which in turn modifies the acoustic properties of
the core
146 and the structure 140 as a whole.
[0041] The use of felt cores 46 of the type described in reference to FIG. 2
can also
provide acoustic benefits. For example, the orientation of the fibers within
the core 40
can be modified to alter the impedance of the core 40 in a specific direction
to match
acoustic design needs. In particular, each layer of felt can be planar in the
draw-down
thickness with some interlocking between the fibers in the perpendicular
direction. The
felt can be cut into the desired dimension, and layers of the felt can be
stacked to re-orient
the planes to provide the desired properties. Additionally, lowering the tow
count of one
or both facesheets 42 and 44 can alter its impedance.
[0042] While the invention has been described in terms of particular
embodiments, it is
apparent that other forms could be adopted by one skilled in the art. For
example, the
physical configurations of the sandwich structures 40 and 140 schematically
represented
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WO 2012/118516 PCT/US2011/036630
in FIGS. 2 and 3 could differ from those shown, and materials and processes
other than
those noted could be used. Therefore, the scope of the invention is to be
limited only by
the following claims.
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