U.S. patent application number 10/693458 was filed with the patent office on 2004-05-20 for thermal protection system.
Invention is credited to Joseph, Brian E..
Application Number | 20040096691 10/693458 |
Document ID | / |
Family ID | 24948164 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096691 |
Kind Code |
A1 |
Joseph, Brian E. |
May 20, 2004 |
Thermal protection system
Abstract
A thermal protection system (TPS) that combines a structural
thermal insulator (carbon foam) with a high specific strength metal
matrix composite. According to a specifically preferred embodiment,
the structural thermal insulator is overcoated with a protective
antioxidant layer or incorporates an antioxidant compound
therein.
Inventors: |
Joseph, Brian E.; (Wheeling,
WV) |
Correspondence
Address: |
McGuire Woods, LLP
Suite 1800
1750 Tysons Blvd.
McLean
VA
22102
US
|
Family ID: |
24948164 |
Appl. No.: |
10/693458 |
Filed: |
October 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10693458 |
Oct 27, 2003 |
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09944273 |
Aug 31, 2001 |
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6689470 |
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09944273 |
Aug 31, 2001 |
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09733566 |
Dec 8, 2000 |
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6455804 |
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Current U.S.
Class: |
428/634 |
Current CPC
Class: |
C22C 13/00 20130101;
B22F 10/10 20210101; C22C 21/02 20130101; B23K 35/282 20130101;
B32B 15/016 20130101; C22C 18/02 20130101; C22C 47/06 20130101;
B32B 15/017 20130101; Y02P 10/25 20151101; Y10T 428/24942 20150115;
B32B 15/01 20130101; Y10T 428/12625 20150115; C22C 18/04 20130101;
B22F 3/005 20130101; B23K 35/286 20130101; C22C 49/06 20130101;
B22F 10/20 20210101; B23K 1/0056 20130101; B23K 26/0846 20130101;
B22F 12/00 20210101; B22F 2998/00 20130101; C22C 47/14 20130101;
B23K 35/262 20130101; B23K 2103/10 20180801; Y10S 428/92 20130101;
Y10T 428/30 20150115; B22F 2999/00 20130101; B23K 35/002 20130101;
B23K 35/0238 20130101; B22F 2999/00 20130101; B22F 10/20 20210101;
B22F 2203/03 20130101; B22F 2999/00 20130101; B22F 3/26 20130101;
B22F 10/20 20210101; B22F 2999/00 20130101; B22F 3/005 20130101;
B22F 2202/01 20130101; B22F 2998/00 20130101; C22C 47/20 20130101;
B22F 2202/11 20130101; B22F 2202/01 20130101; B22F 2998/00
20130101; C22C 47/068 20130101; B22F 2999/00 20130101; B22F 10/20
20210101; B22F 2203/03 20130101; B22F 2999/00 20130101; B22F 3/26
20130101; B22F 10/20 20210101 |
Class at
Publication: |
428/634 |
International
Class: |
B32B 015/04 |
Claims
What is claimed is:
1) A thermal protection system comprising: A) a carbonaceous core
having a first and a second surface; B) a layer of aluminum or an
alloy of aluminum coated upon said at least said first surface; C)
a structural portion coated over said layer of aluminum or an
aluminum alloy comprising at least one pair of alternating layers
of: i) an aluminum brazing alloy; and ii) an aluminum metal matrix
composite.
2) The thermal protection system of claim 1 wherein said layer of
aluminum or an aluminum alloy is coated upon said first
surface.
3) The thermal protection system of claim 2 wherein said
carbonaceous core comprises a semi-crystalline, largely isotropic,
porous coal-based product produced from particulate coal exhibiting
a free swell index of between about 3.5 and about 5.0 and of a
small diameter, having a density of between about 0.1 and about 0.8
g/cm.sub.3 and a thermal conductivity below about 1 W/m/.degree.
K.
4) The thermal protection system of claim 3 wherein said coal
exhibits a free swell index of between about 3.75 and about
4.5.
4) The thermal protection system of claim 2 wherein said
carbonaceous core has a compressive strength below about 6000
psi.
5) The thermal protection system of claim 2 wherein said
carbonaceous core has been carbonized.
6) The thermal protection system of claim 2 wherein said
carbonaceous core has been graphitized.
7) The thermal protection system of claim 2 further including a
protective anti oxidant layer coated on said second surface.
8) The thermal protection system of claim 7 wherein said protective
antioxidant layer comprises a member selected from the group
consisting of metallic layers, and glass forming metal-halide,
carbide or nitride compounds.
9) The thermal protection system of claim 7 wherein said protective
antioxidant layer comprises a member selected from the group
consisting of ZrB.sub.2, SiC, and B.sub.4C.
10) The thermal protection system of claim 7 wherein said
carbonaceous core comprises a semi-crystalline, largely isotropic,
porous coal-based product produced from particulate coal exhibiting
a free swell index of between about 3.5 and about 5.0 and of a
small diameter, having a density of between about 0.1 and about 0.8
g/cm.sub.3 and a thermal conductivity below about 1 W/m/.degree.
K.
9) The thermal protection system of claim 10 wherein said coal
exhibits a free swell index of between about 3.75 and about
4.5.
10) The thermal protection system of claim 7 wherein said
carbonaceous core has a compressive strength below about 6000
psi.
11) The thermal protection system of claim 7 wherein said
carbonaceous core has been carbonized.
12) The thermal protection system of claim 7 wherein said
carbonaceous core has been graphitized.
13) The thermal protection system of claim 2 further including an
anti oxidant blended into said carbonaceous core. 14) The thermal
protection system of claim 13 wherein said anti oxidant comprises a
member selected from the group consisting of glass forming
metal-halide, carbide or nitride compounds.
15) The thermal protection system of claim 14 wherein said
protective antioxidant layer comprises a member selected from the
group consisting of ZrB.sub.2, SiC, and B.sub.4C.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/733,566 filed Dec. 8, 2000 and entitled
"Continuous Metal Matrix Composite Consolidation".
FIELD OF THE INVENTION
[0002] The present invention relates to enhanced thermal protection
systems for applications that demand high strength and high
temperature protection such as in spacecraft, and more particularly
to multi-layer thermal protection systems that utilize carbonaceous
foam as the insulating material and consolidated metal matrix
materials as the provider of strength.
BACKGROUND OF THE INVENTION
[0003] The design of improved thermal protection systems
particularly for spacecraft has been a long and demanding one.
Historically, there have been two approaches in the design of such
systems. Either the thermal protection system (TPS hereinafter) has
offered little structural benefit and simply served to transfer
loads to an underlying, primary "cold structure" often fabricated
from, for example, a graphite-reinforced polymer material, or the
structural and thermal properties of a single element system have
been compromised to allow the structure to withstand elevated
temperatures.
[0004] Such demands are probably no more apparent than in the
requirements imposed by NASA in the development of materials
systems and design approaches to support the development of
integral cryogenic tank structures and thermal protection systems
such as those required by reusable launch vehicles such as the
X-33. These types of spacecraft experience lengthy reentry profiles
and are thus exposed to high total heat input while also being
exposed to relatively extreme structural demands imposed upon the
"airframe" during launch and reentry.
[0005] In the design of such vehicles, among the long list of
concerns are 1) weight; 2) cost; 3) oxidation resistance of the
TPS; 4) waterproofing; and 5) structural efficiency. There are
number of currently proposed systems to meet the stingent
requirements in each of these areas. One such proposed system
utilizes a ceramic-infiltrated woven fiber preform that offers
excellent oxidation resistance and thermal protection however, at
the expense of strength and weight. To improve the performance of
such materials it has been proposed to use such composites as an
exterior layer backed up by a primary load bearing structure. A
similar, but light weight, structurally more capable and relatively
lower cost solution is provided by the TPS of the present
invention.
SUMMARY OF THE INVENTION
[0006] The present invention provides a multi-layer thermal
protection system (TPS) comprising the combination of an oxidation
resistant, high temperature capable and relatively high strength
carbonaceous foam core with a high specific strength metal matrix
composite layer thereon to provide a TPS that reduces the bulk and
weight of currently proposed material systems. This novel TPS
provides the opportunity to reduce the vehicle size and weight or
increase payload capability while equaling or exceeding the
performance of prior art or proposed alternative fabrication
systems.
DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a graph of showing the general relationship
between gas evolution and time/temperature at various operating
pressures and temperatures for the process of the present
invention.
[0008] FIG. 2 is a cross-sectional view of the composite-thermal
protection system of the present invention.
[0009] FIG. 3 is a schematic perspective view of apparatus suitable
for the manufacture of AMC structures in accordance with the
process of the present invention.
[0010] FIG. 4 is a schematic depiction of the area of contact
between the mandrel surface, and the incoming prepeg tape at the
point of application of infrared laser radiation in accordance with
the process of the present invention.
[0011] FIG. 5 is a graph of thermal conductivity (W/mk) versus
temperature (.degree. C.) for some of the carbon foam materials
that form the carbonaceous core of the TPS of the present
invention.
DETAILED DESCRIPTION
[0012] The thermal protection system (TPS) of thee present
invention comprises a carbonaceous foam core combined with a high
specific strength metal matrix composite and optionally, as
described hereinafter, and optionally, protective layer(s), e.g. an
anti-oxidant layer over the carbon-base foam core.
[0013] Referring now to FIG. 2, the TPS of the present invention 60
comprises a carbonaceous foam core 62, a metallic intermediate
layer 64, and a structural portion 63 comprising alternating
brazing layer(s) 66, and high-strength metal matrix composite
layer(s) 68. Carbonaceous foam core 62 is preferably overcoated
with an anti-oxidant layer 70. While in the description that
follows, structural portion 63 is commonly referred to and
described as comprising as single pair of layers 66 and 68, it will
be apparent to the skilled artisan and is fully intended as within
the scope of the present invention that a plurality of alternating
layers 66 and 68 providing a single integrated structural portion
63 can and will be used in the TPS 60 as described and claimed
herein.
[0014] U.S. patent application Ser. No. 09/453,729, filed Dec. 12,
1999 entitled "Cellular Coal Products and Processes" and U.S.
patent application Ser. No. (not yet assigned), filed Jul. 10, 2001
and entitled "Cellular Coal Products and Processes" both owned by a
common assignee describe coal-based cellular or porous products
having a density of preferably between about 0.1 g/cm.sup.3 and
about 0.8 g/cm.sup.3 that are produced by the controlled heating
and decomposition of coal particulate preferably up to 1 mm in
diameter in a "mold" and under a non-oxidizing atmosphere.
According to specifically preferred embodiments, the coal-based
starting materials exhibit a "free swell index" as determined by
ASTM test D720 of between about 3.5 and about 5.0. The porous
products produced by these processes, preferably as a net shape or
near net shape, can be readily machined using conventional
techniques, adhered and otherwise fabricated to produce a wide
variety of low cost, low density products, or used in their
preformed shape. Such cellular products have been shown to exhibit
compressive strengths of up to about 6000 psi. It is these carbon
materials that form core 62 of TPS 60 shown in FIG. 2 and described
herein.
[0015] U.S. patent application Ser. No. 09/733,566 filed Dec. 8,
2000 and entitled "Continuous Metal Matrix Composite Consolidation"
describes a method for the fabrication of unique large aluminum
metal matrix composite (AMC) structures comprising the continuous
brazing of aluminium matrix braze-clad tape using an infrared laser
to melt the braze clad on the tape while applying pressure to the
tape and simultaneously contacting it with previously applied tape
layers on a rotating mandrel. The apparatus utilized to accomplish
this fabrication process may include a variety of pre and
post-contact heaters and preferably includes instruments for the
continuous monitoring and control of the process. It is the
materials produced in accordance with this process that form the
high specific strength metal matrix composite portion, structural
portion 63, comprising layers 66 and 68 of TPS 10 of the present
invention.
[0016] U.S. patent applications Ser. Nos. 09/733,566; 09/453,729
and (Ser. No. not yet assigned) filed Jul. 10, 2001 and entitled
"Cellular Coal Products and Processes" are all incorporated by
reference in their entirety into this specification.
[0017] Core 62 of TPS 60 of the present invention comprises
coal-based cellular or porous product, i.e. a foam, having a
density of preferably between about 0.1 g/cm.sup.3 and about 0.8
g/cm.sup.3. According to a highly preferred embodiment the foam is
produced by the controlled heating of coal particulate preferably
up to 1/4 inch in diameter in a "mold" and under a non-oxidizing
atmosphere. According to a further specifically preferred
embodiment, the starting material is a coal having a free swell
index as determined by the standard ASTM D720 tests of between
about 3.5 and about 5.0. Such carbon based foams, without further
treatment and/or the addition of strengthening additives exhibit
compressive strengths of up to about 4000 psi. Impregnation with
appropriate materials or the incorporation of various strength
improving additives can further increase the compressive, tensile
and other properties of these cellular materials. Although a wide
variety of coals meeting the foregoing requirements can be use to
produce the carbon foam materials described herein, they are
preferably bituminous, agglomerating coals that have been
comminuted to an appropriate particle size, preferably to a fine
powder below about -60 to -80 mesh.
[0018] The utility of these coal-based carbon foams as insulating
materials is clearly shown in FIG. 5 wherein carbon foams in
accordance with the present invention having a density of between
about 0.40 and 0.45 g/cc exhibit thermal conductivities in the
range of 0.3-0.9 at test temperatures of between about 25.degree.
C. and 300.degree. C.
[0019] The preferred coal-based materials described herein are
semi-crystalline or more accurately turbostratically-ordered and
largely isotropic i.e., demonstrating physical properties that are
approximately equal in all directions. The coal-based products of
the present invention typically exhibit pore sizes on the order of
less than 300.mu., although pore sizes of up to 500.mu. are
possible within the operating parameters of the processes
described. The thermal conductivities of the coal-based foams are
generally less than about 1.0 W/m/.degree. K. Typically, these
cellular coal-based products demonstrate compressive strengths on
the order of from about 2000 to about 6000 psi at densities of from
about 0.4 to about 0.5 g/cm.sup.3.
[0020] It is most desirable to the successful production of TPS 60
of the present invention from the coal-based foams described herein
that the coal starting material exhibit the previously specified
free swell index of between about 3.5 and about 5.0 and preferably
between about 3.75 and about 4.5. Selection of starting materials
within these parameters was determined by evaluating a large number
of coals characterized as ranging from high to low volatiles. In
general, it has been found that bituminous coals exhibiting free
swell indexes within the previously specified ranges provided the
best foam products for the production of thermal protection systems
in that they exhibit the lowest calcined foam densities and the
highest calcined foam specific strengths (compressive
strength/density). Coals having free swell indices below these
preferred ranges may not agglomerate properly leaving a powder mass
or sinter, but not swell or foam, while coals exhibiting free swell
indices above these preferred ranges may heave upon foaming and
collapsed upon themselves leaving a dense compact.
[0021] The preferred coal-based foam production method of the
present invention comprises: 1) heating a coal particulate of
preferably small i.e., less than about 1/4 inch particle size in a
"mold" and under a non-oxidizing atmosphere at a heat up rate of
from about 1 to about 20.degree. C. to a temperature of between
about 300 and about 700.degree. C.; 2) soaking at a temperature of
between about 300 and 700.degree. C. for from about 10 minutes up
to about 12 hours to form a preform or flushed product; and 3)
controllably cooling the preform or finished product to a
temperature below about 100.degree. C. The non-oxidizing atmosphere
may be provided by the introduction of inert or non-oxidizing gas
into the "mold" at a pressure of from about 0 psi, i.e., free
flowing gas, up to about 500 psi. The inert gas used may be any of
the commonly used inert or non-oxidizing gases such as nitrogen,
helium, argon, CO.sub.2, etc.
[0022] It is generally not desirable that the reaction chamber be
vented or leak during the heating and soaking operation. The
pressure of the chamber and the increasing volatile content therein
tends to retard further volatilization while the cellular product
sinters at the indicated elevated temperatures. If the furnace is
vented or leaks during soaking, an insufficient amount of volatile
matter may be present to permit inter-particle sintering of the
coal particles thus resulting in the formation of a sintered powder
as opposed to the desired cellular product. Thus, according to a
preferred embodiment of the present process, venting or leakage of
non-oxidizing gas and generated volatiles is inhibited consistent
wit the production of an acceptable cellular product.
[0023] Additional more conventional blowing agents may be added to
the particulate prior to expansion to enhance or otherwise modify
the pore-forming operation.
[0024] The term "mold, as used herein is meant to define a
mechanism for providing controlled dimensional forming of the
expanding coal. Thus, any chamber into which the coal particulate
is deposited prior to or during heating and which, upon the coal
powder attaining the appropriate expansion temperature, contains
and shapes the expanding porous coal to some predetermined
configuration such as: a flat sheet; a curved sheet; a shaped
object; a building block; a rod; tube or any other desired solid
shape can be considered a "mold" for purposes of the instant
invention.
[0025] As will be apparent to the skilled artisan familiar with
pressurized gas release reactions, as the pressure in the reaction
vessel, in this case the mold increases, from 0 psi to 500 psi, as
imposed by the non-oxidizing gas, the reaction time will increase
and the density of the produced porous coal will increase as the
size of the "bubbles" or pores produced in the expanded coal
decreases. Similarly, a low soak temperature at, for example about
400.degree. C. will result in a larger pore or bubble size and
consequently a less dense expanded coal than would be achieved with
a soak temperature of about 600.degree. C. Further, the heat-up
rate will also affect pore size, a faster heat-up rate resulting in
a smaller pore size and consequently a denser expanded coal product
than a slow heat-up rate. These phenomenon are, of course, due to
the kinetics of the volatile release reactions which are affected,
as just described, by the ambient pressure and temperature and the
rate at which that temperature is achieved. These process variables
can be used to custom produce the expanded coals of the present
invention in a wide variety of controlled densities, strengths etc.
These results are graphically represented in FIG. 1 wherein the X
axis is gas release, the Y axis is time and the individual curves
represent different pressures of inert gas P.sub.1, P.sub.2, and
P.sub.3, different heat-up rates HR.sub.1, HR.sub.2, and HR.sub.3,
and P.sub.1<P.sub.2<P.sub.3 and
HR.sub.1<HR.sub.2<HR.sub.3.
[0026] Cooling of the coal-based foam after soaking is not
particularly critical except as it may result in cracking of
thereof as the result of the development of undesirable thermal
stresses. Cooling rates less than 10.degree. C./min to a
temperature of about 100.degree. C. are typically used to prevent
cracking due to thermal shock. Somewhat higher, but carefully
controlled, cooling rates may however, be used to obtain a "sealed
skin" on the open cell structure of the product foam as described
below. The rate of cooling below 100.degree. C. is in no way
critical.
[0027] After expanding the coal particulate as just described, the
porous coal product is an open celled material. Several techniques
have been developed for "sealing" the surface of the open celled
structure to improve its adhesive or joining capabilities, for
example, for the application of facesheets or protective
antioxidant layers of dissimilar materials, for further fabrication
and assembly of a number of parts of for formation of TPS 60
described herein through the attachment of additional layers 64 and
70. For example, a layer of a commercially available
graphitic-epoxy or graphitic-phenolic adhesive can be coated onto
the surface and cured at elevated temperature or allowed to cure at
room temperature to provide an adherent skin for attachment of
layers 64 and 70. Alternatively, the expansion operation can be
modified by cooling the expanded coal product or preform rapidly,
e.g., at a rate of 10.degree. C./min or faster after expansion. It
has been discovered that this process modification results in the
formation of a more dense skin on the preform or product which
presents a closed pore surface to the outside of the preform or
product. At these cooling rates, care must be exercised to avoid
cracking of the preform or product.
[0028] After expanding, the porous coal-based preform or product is
readily machineable, sawable and otherwise readily fabricated using
conventional fabrication techniques to fabricate the composite
tooling described herein.
[0029] Subsequent to production of the preform or product as just
described, the preform or product may be, and in the current
application as core 62 of TPS 60 preferably is subjected to
carbonization and/or graphitization according to conventional
processes to obtain particular properties desirable for specific
applications, for example, to render the carbon foam more heat or
ablation resistant as will be desirable in many TPS
applications.
[0030] Additionally, a variety of additives and structural
reinforcers may be added to the coal-based preforms or products
either before or after expansion to enhance specific mechanical
properties such as fracture strain, fracture toughness and impact
resistance should these be required for a particular TPS
application. For example, particles, whiskers, fibers, plates, etc.
of appropriate carbonaceous or ceramic composition can be
incorporated into the porous coal-based preform or product to
enhance its mechanical properties.
[0031] The open celled, coal-based foams of the present invention
can additionally be impregnated with, for example, petroleum pitch,
epoxy resins or other polymers using a vacuum assisted resin
transfer type of process. The incorporation of such additives
provides load transfer advantages similar to those demonstrated in
carbon composite materials. In effect a 3-D composite is produced
that demonstrates enhanced impact resistance and load transfer
properties should these be required by a particular TPS
application.
[0032] The cooling step in the expansion process results in some
relatively minimal shrinkage on the order of less than about 5% and
generally in the range of from about 2% to about 3%. This shrinkage
must be accounted for in the production of near net shape preforms
or final products of specific dimensions and is readily
determinable through trial and error with the particular coal
starting material being used. The shrinkage may be further
minimized by the addition of some inert solid material such as coke
particles, ceramic particles, ground waste from the coal expansion
process etc. as in common practice in ceramic fabrication so long
as such additions do not adversely affect the thermal conductivity
or elevated temperature performance of the foam as intended for its
end use.
[0033] Carbonization, sometimes referred to as calcining, is
conventionally preformed by heating the preform or product foam
under an appropriate inert gas at a heat-up rate of less than about
5.degree. C. per minute to a temperature of between about
800.degree. C. and about 1200.degree. C. and soaking for from about
1 hour to about three or more hours. Appropriate inert gases are
those described above that are tolerant of these high temperatures.
The inert atmosphere is supplied at a pressure of from about 0 psi
up to a few atmospheres. The carbonization/calcination process
serves to remove all of the non-carbon elements present in the
preform or product such as sulfur, oxygen, hydrogen, etc that might
adversely affect TPS 60 in its application.
[0034] Graphitization, commonly involves heating the preform or
product either before or after carbonization at a heat-up rate of
least than about 10.degree. C. per minute, preferably from about
1.degree. C. to about 5.degree. C. per minute, to a temperature of
between about 1700.degree. C. and about 3000.degree. C. in an
atmosphere of helium or argon and soaking for a period of less than
about one hours. Again, the inert gas may be supplied at a pressure
ranging from about 0 psi up to a few atmospheres. Graphitization as
just described, results in the formation of a carbonaceous core 62
that is extremely heat resistant and free of volatiles that could
reduce its utility in high temperature insulating applications.
[0035] The preferred, coal-based porous TPS core 12 preforms and
products of the present invention can be produced in any solid
geometric shape. Such production is possible using any number of
modified conventional processing techniques such as extrusion,
injection molding, etc. In each of such instances, the process
must, of course, be modified to accommodate the processing
characteristics of the starting material coal. For example, in
extruding such products, as described below, the coal powder
starting material is fed by an auger into an expansion chamber
where it is expanded and from which it is extruded while still
viscous. Upon exiting the extrusion die, the material is cooled to
provide a solid shape of the desired and precalculated dimension.
To improve the efficiency, i.e., cycle time of the process, the
input material can be preheated to a temperature below the
expansion point e.g., below about 300.degree. C., fed into the
auger chamber where additional heat is imparted to the powder with
final heating being achieved just before extrusion through the
die.
[0036] Similar relatively minor process modifications can be
envisioned to fabricate carbon foam cores 62 in injection molding,
casing and other similar conventional material fabrication
processes.
[0037] Additional details and examples of the fabrication process
of carbonaceous core 62 contained in aforementioned U.S. patent
application Ser. No. 09/453,729, filed Dec. 12, 1999 and entitled
"Cellular Coal Products and Processes" which is incorporated herein
in its entirety.
[0038] As mentioned above, the carbonaceous foam cores 62 of the
present invention may be coated with a wide variety of protective
layers 70 such as antioxidant layers. Such protective layers 70
which act as oxidation preventive barriers include, for example,
but not exclusively, thermal spray, plasma spray or laser
deposition processes of applied coatings of a metal, for example,
aluminium or inconel etc. to achieve specific heat resistant,
ablation resistant, heat transfer or thermal expansion properties
compatible with the specific end use requirements for TPS 60. As
will be described below, it is the application of metals in this
fashion that permits the joining of layers 66 and 68 to foam core
62 to provide TPS 60 described herein. Additionally, other similar
systems that achieve the same antioxidant results include those
based upon glass forming metal-halide, carbide or nitride compounds
that can be applied onto the foam by vapor or mechanical
deposition. Examples of such materials include, but are not limited
to ZrB.sub.2, SiC, and B.sub.4C. When exposed to oxidizing
conditions at elevated temperatures, these compounds form
protective glass-like films such as ZrO.sub.2, SiO.sub.2 and
B.sub.2O.sub.3. Such layers can also be adhered to the carbonaceous
foam core using any of a wide variety of, for example,
graphite-epoxy, or graphite-phenolic adhesives available
commercially, if the final end-use application of TPS 60 will such
adhesives without resulting in their decomoposition or
volatilization in high temperature applications. Alternatively,
various spray deposition techniques are commercially available for
the application of such layers to carbonaceous foam core 62.
[0039] Whatever the antioxidant material applied as layer 70 over
foam core 62, since many of these materials are relatively brittle,
it is important to the successful practice of the present invention
in its most demanding applications, that the thermal expansion of
any protective layer 62 be matched to the thermal expansion of
underlying foam core 62, or perhaps more properly, the thermal
expansion of foam core 62 be matched to that of antioxidant
protective layer 70, as this may indeed be the simpler approach
given the relative capability of foam core 62 to be tailored in its
thermal expansion properties.
[0040] According to an alternative preferred embodiment, the
antioxidant property can be imparted directly to the carbon foam
through the incorporation of appropriate oxidation inhibitors
directly into the carbon foam, either with or without the
additional protection afforded by the presence of a separate
protective antioxidant layer 70. Appropriate oxidation inhibitors
usable in this context include those described above in connection
with the application of protective antioxidant layers.
[0041] As a further enhancement of the properties of foam core 62
described herein, functionally graded foams of varying density at
their surfaces or throughout their structure may be prepared as
described in copending U.S. patent application Ser. No. 09/733,602,
filed Dec. 8, 2000. According to this invention, coal-based
cellular products having integral stiffeners or load paths,
directed heat transfer paths and directed mass transfer paths are
provided through the placement of coal-based cells of a different
size and/or density than those making up the matrix of the product
during manufacture. There is also provided a method for the
production of coal-based cellular products possessing these
characteristics. The method described in this application utilizes
the ability to select and design such properties through the proper
selection and control of cell size and density. Such control of
cell size and density is in turn achieved through appropriate
selection of starting materials, starting material particle size,
mold packing and processing parameters. This application is
incorporated herein in its entirety.
[0042] In TPS 60 of the present invention, the foregoing carbon
foam thermally insulating/structural member 62 is married to or
combined with a continuously brazed aluminum metal-matrix
composite. In fabrication, the above-described carbon foam material
that forms layer 62 is plasma sprayed or otherwise (thermally,
laser, vapor deposition) coated with a layer of layer of aluminum
64, and then brazed onto a continuously-wound or otherwise applied
and brazed aluminum matrix structure (AMC), e.g. tubes or tank
sections, as described in aforementioned U.S. patent application
Ser. No. (not yet assigned) filed ______, and entitled "Continuous
Metal Matrix Composite Consolidation".
[0043] Layer 68 of TPS 10 is a metal matrix composite (MMC),
specifically preferred is an aluminum matrix composite (AMC), in
prepeg tape form comprised of alumina (Al.sub.2O.sub.3), or other
suitable ceramic fibers, in an aluminum/aluminum alloy matrix. The
prepeg tape is coated with a "brazing" alloy, layer 66 of an
aluminum alloy having a lower melting point than the aluminum
matrix of the prepeg tape/composite, prior to application in the
process of the present invention. Fabrication is accomplished by
applying the braze material coated prepeg tape comprising layers 66
and 68 to a fabrication structure with the simultaneous application
of pressure while melting the braze coating at the junction between
the prepeg tape and the material interface (layer 14) surface using
a laser, preferably an infrared or diode laser that provides very
limited and very localized heating and melting of the braze coat.
The laser beam of infrared radiation preferably has a rectangular
cross section to enhance heating efficiency in the area of the
junction. As will be seen from the detailed description that
follows, a variety of pre and post-contact heaters and process
control devices are preferably used to control and monitor the
process. The braze-coated feedstock just described and comprising
layers 66 and 68 can be prefabricated at a remote location and
provided in coil form, or, as described hereinafter, can be
prepared just prior to fabrication by coating the AMC prepeg, layer
68, with the braze coat, layer 66 in line just prior to exposure to
the laser radiation and application to the fabrication
structure.
[0044] While any number of techniques such as spraying (thermal,
arc, plasma, etc.), surface alloying, etc. can be used to apply the
lower melting braze coating to the prepeg tape, in the case where
the braze coating is applied in line with the consolidation
operation, the prepeg tape is guided through a pot of molten
brazing, i.e. lower melting, metal, extracted from the pot of metal
through a coating thickness control device such as a die or air
knife, and then through a cooling chamber to solidify the coating.
Preferably, the pot of molten metal is equipped with an ultrasonic
pulse inducing element comprising a power supply, a transducer and
a probe to facilitate coating of the matrix of the prepeg tape with
the braze coating. When used, the ultrasonic probe is inserted into
the pot of lower melting molten metal it produces a cavitation
field that results in pressure waves that reduce the contact angle
and improve the wetting of the lower melting material to the
prepeg. The cooling chamber can be highly sophisticated, but can be
as simple a metal tube through which is flowed a chilled gas such
as nitrogen and through which the braze coated prepeg travels on
exit from the coating pot and the thickness control device.
[0045] Referring now to FIG. 3 that depicts one preferred method
for the application of the process described in aforementioned U.S.
Patent Application entitled "Continuous Metal Matrix
Consolidation", consolidation apparatus 10 comprises a rotating
mandrel 12 supported on legs 14 (or any other suitable support
system), a laser 16 that directs a beam of infrared radiation 18 to
the junction 20 between braze coated prepeg tape 22 and surface 24,
a carriage unit 26 that supports and imparts lateral traversing
motion to compaction wheel 28, pre-heaters 30 and post heater(s)
32. According to a preferred embodiment of the invention, an
optical pyrometer 33 can be used to monitor the temperature at
junction 20 and the signal therefrom used to control either the
mandrel rotation an/or carriage unit traverse speeds or the
intensity of laser 16, to thereby control the temperature of the
molten braze coating 36 (see FIG. 4) that occurs at junction
20.
[0046] Referring now to FIG. 4 that schematically depicts a side
view of consolidation apparatus 10 and shows the relative positions
of laser 16, infrared radiation beam 18, compaction wheel 28,
mandrel 12 that in the case of the instant invention may comprise a
structure of carbonaceous core 62, and incoming braze-coated prepeg
tape 22 at junction 20, it is readily observed that at junction 20,
there exists a "front" of molten metal 34 that comprises the molten
or liquid form of braze coating 36 on prepeg tape 22. Front 34 is
produced by the localized heating induced by the impact of infrared
radiation beam 18 upon the surface of braze coating 36. It must be
noted, that although not specifically depicted in FIG. 4, surface
24 of mandrel 12 (carbonaceous core 62) includes at least one wrap
of previously applied prepeg tape 22, or, as in the case of the
instant application, a previously applied layer of metal/aluminum
64 to which incoming feedstock prepeg tape 22 is adhered as braze
coating 36 melts due to the localized and controlled heating action
of infrared radiation beam 18, and subsequently cools as it is
removed from the area of front 34 due to rotation of mandrel 12 in
the direction shown by arrow 38 thereby building serial overlying
layers of AMC joined to each other by alternating layers of braze
material 36. Simultaneously with the creation of front 34 and the
movement of prepeg tape 22 in the direction indicated by arrow 38,
compaction wheel 28 pushes prepeg tape 22, and consequently
associated melted braze coating 36, into intimate contact with
surface 24 on mandrel 12 causing prepeg tape 22 to adhere firmly
thereto. The specific conditions under which such fabrication can
occur are described in greater detail hereinafter. Conversion of
this fabrication process and apparatus to one that involves the
continuous fabrication of TPS 60 of the present invention in
"sheet" or other structural shape by the simultaneous application
of pressure and melting force as applied by a suitable laser is
clearly within the capabilities of those skilled in the art given
the description contained herein.
[0047] Consolidation apparatus 10 fundamentally comprises a 2-axis
filament winder of the type used in the fabrication of polymer
matrix composites. According to a preferred embodiment, mandrel 12
can be up to 48 inches long and up to about 36 inches in diameter.
Of course, larger dimensioned devices can be used in those cases
where larger structural members are being fabricated. The
rotational movement of mandrel 12 and the linear traverse of
compaction wheel 28 on carriage unit 26 are controlled and
coordinated by means of "Pattern Master" software or the like that
are supplied with the filament winder unit, or custom deigned and
implemented if a specific non-standard wrap pattern is required or
desired.
[0048] Laser 16 preferably comprises a stacked multi-bar infrared
laser. An array of optical lenses 38 are used to shape infrared
radiation beam 18 into a rectangular pattern that matches the
cross-sectional dimension of prepeg tape 22. According to a
preferred embodiment of the invention, laser 16 is powered by a DC
power supply capable of delivering 75 amps to the preferred stacked
multi-bar diode laser 16. Laser 16 in this configuration is
designed to operate in a continuous wave mode at a power of up to
500 watts. Water cooling of the laser head is required to maintain
the life of the diodes and is conventionally accomplished by means
of a water-to-air chiller unit (not shown). Multi-bar diode lasers
of this type are commercially available from Opto Power
Corporation, 3321 E. Global Loop, Tucson, Az. 85706.
[0049] Mandrel 12 may, of course be collapsible or otherwise
removable once the finished structure is completed by completion of
the wrapping operation in the case of the production of a tubular
structure as described herein, but may simply comprise a suitable
shape of carbonaceous core 62 in the case where a "flat" or
otherwise suitably configured structural shape of carbonaceous core
62 is being "laminated" with prepeg tape 22 as described
herein.
[0050] As shown in FIG. 3, immediately after junction 20 prepeg
tape 22 is contacted on its reverse side 40 by compacting wheel 28
to accomplish consolidation. Compacting wheel is preferably
fabricated from a ceramic material to minimize conductive heat loss
from junction 20 during consolidation. A highly preferred material
for compaction wheel 28 is zirconium phosphate which exhibits these
and other suitable properties. Of course, suitable alternative
process controls can make the selection of materials for compaction
wheel 28 less critical. Compaction wheel 28 is arranged to ride at
top dead center of mandrel 12 or in whatever fashion may be
appropriate when an alternative shape is being "laminated" and is
guided in its movement by carriage assembly 26. Compaction wheel 28
in addition to providing compressive energy for consolidation also
has a second important function, in that it provides a V-shaped
cavity at junction 20 thereby reducing reflective losses by
trapping some of the infrared radiation of beam 18 and creating a
"multiple bounce" situation where most of the incoming radiation is
used for heating and less of such radiation is lost due to
reflection from the various surfaces at junction 20.
[0051] Preheat lamps 30, and where used post heat lamp(s) 32
preferably comprise reflector lamps as line sources of infrared
energy to preheat or post heat prepeg tape 22 prior to or after
exit from junction 20. Preheat lamps 30 preferably heat prepeg tape
22 to a temperature of about 500.degree. F. in order to reduce the
heating load on laser 16. As will be obvious to the skilled
artisan, such preheating may not be required if a higher powered
laser is used. Post heating lamp(s) 32 are similarly configured,
and if and where applied can be used to control the cool down of
prepeg tape 22 as it exits junction 20 to reduce the thermal
stresses that may be induced by the brazing process.
[0052] According to an alternative preferred embodiment of the
present invention, a rotary ball vibrator 42 that induces vibration
in the range of from about 1000 to about 25000 vibrations per
minute is added to consolidation apparatus 10 to provide a more
thorough mixing of molten braze alloy front 34 at junction 20.
Rotary ball vibrator 42 is attached to a metal rod 44 that contacts
prepeg tape 22 just before it enters junction 20. The presence of
rotary ball vibrator 42 causes prepeg tape 22 to vibrate at the
same frequency as vibrator 42 which in turn induces oscillations in
front 34 at junction 20. Thus, these oscillations occur in junction
20 as prepeg tape 22 is addressed by compaction wheel 28.
[0053] According to yet another alternative preferred embodiment of
the present invention, a flow of inert gas is applied over the
heated area at junction 20 to minimize the formation of oxides in
front 34 during brazing. Free flowing argon, nitrogen or the like
inert gas directed to the area of junction 20 appears to provide
such benefit.
[0054] Optical pyrometer 33 may be included to provide temperature
feedback information to the control circuits of laser 16 thereby
assuring that the appropriate amount of heat is being applied at
junction 20 to achieve satisfactory melting of braze coating 36 and
consolidation as described above.
[0055] Finally, at least in process development and refinement
situations, it can be desirable to include a video camera (not
shown) to closely monitor the area of junction 20 to obtain the
appropriate operating parameters for a specific given prepeg tape
22 and braze coating 34 composition.
[0056] In practice, the method of the present invention is carried
out using the above-described apparatus 10 by first applying a
suitable aluminum alloy coating 14 on the surface of carbonaceous
core 62,for example, pure aluminum, 1100 alloy aluminum or any
other suitable aluminum, titanium, magnesium etc. metallic matrix
containing a ceramic reinforcing material, for example, Nextel
610.TM. aluminum oxide 1500 denier fibers commercially available
from the 3M Corporation, Minneapolis, Minn. Prepeg tape 22 is
preferably provided as a coil on a payoff for continuous feeding.
Consolidation apparatus 10 is then activated. Mandrel 12 begins to
turn, or in the case of application to a flat or otherwise shaped
surface oscillated in a direction orthogonal to the width of
compacting wheel 28, the laser 16 is focused on junction 20 and
prepeg 22 is fed into junction 20 for consolidation by compacting
wheel 28. The specific process conditions are largely a matter of
choice as dictated by the materials being consolidated (the AMC
matrix 68 alloy and the braze coating 66 compositions), the power
of laser 16, the rotational or movement speed of mandrel/surface 12
etc. However, in the case of fabrication of the above-described
prepeg tape bearing braze coatings of the types referred to in the
examples below, melting temperatures in the range of from about 375
to about 1200.degree. F. produced by a suitable laser operating at
between about 100 and about 450 watts and prepeg tape feed rates on
the order of between about 0.65 and 1.50 inches/sec. have been
found useful and appropriate.
EXAMPLES
[0057] The following examples when considered in conjunction with
the foregoing detailed description will serve to better illustrate
the successful practice of the present invention.
Examples 1-4
[0058] Prepeg tapes comprising Nextel 610.TM. fibers in pure
aluminum are consolidated as described hereinabove on a mandrel
shape or carbonaceous core 62 coated with a layer 64 of 1100
aluminum alloy applied by thermal spray using the following braze
coatings and under the following tabularly presented operating
conditions:
1 Braze Braze Coating Temperature Laser Power Tape Feed Rate 1)
96.5 Sn/3.5 Ag 430-500.degree. F. 426 Watts 0.70 inches/sec. 2) 70
Sn/30 Zn 389-707.degree. F. 110 Watts 1.06 inches/sec. 3) 84 Zn/11
AL/5 Cu 715-845.degree. F. 268 Watts 0.87 inches/sec. 4) 88 Al/12
Si 1070-1220.degree. F. 373 Watts 1.27 inches/sec.
[0059] Under each of the foregoing conditions, satisfactory
consolidated round structural shapes of the prepeg material
indicated were fabricated.
[0060] As the invention has been described, it will be apparent to
the skilled artisan that the same may be varied in many ways
without departing from the spirit and scope of the invention. Any
and all such modifications are intended to be included within the
scope of the appended claims.
* * * * *