U.S. patent application number 11/039123 was filed with the patent office on 2006-06-22 for composite components for use in high temperature applications.
This patent application is currently assigned to Advanced Ceramics Research, Inc.. Invention is credited to Greg E. Hilmas, Anthony C. Mulligan, Mark M. Opeka, Marlene Platero-AllRunner, Mark J. Rigali, Manish P. Sutaria.
Application Number | 20060135344 11/039123 |
Document ID | / |
Family ID | 27359904 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060135344 |
Kind Code |
A1 |
Rigali; Mark J. ; et
al. |
June 22, 2006 |
Composite components for use in high temperature applications
Abstract
Fibrous monolith composites suitable for use in high temperature
environments and/or harsh chemical environments are provided, along
with methods of preparation thereof. The fibrous monolith
composites exhibit such beneficial properties as enhanced strength,
corrosion resistance, thermal shock resistance and thermal cycling
tolerance.
Inventors: |
Rigali; Mark J.; (Carlsbad,
NM) ; Sutaria; Manish P.; (Malden, MA) ;
Hilmas; Greg E.; (Rolla, MO) ; Mulligan; Anthony
C.; (Tucson, AZ) ; Platero-AllRunner; Marlene;
(Tucson, AZ) ; Opeka; Mark M.; (Bethesda,
MD) |
Correspondence
Address: |
BANNER & WITCOFF, LTD.
TEN SOUTH WACKER DRIVE
SUITE 3000
CHICAGO
IL
60606
US
|
Assignee: |
Advanced Ceramics Research,
Inc.
Tucson
AZ
|
Family ID: |
27359904 |
Appl. No.: |
11/039123 |
Filed: |
January 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10013601 |
Dec 4, 2001 |
6847699 |
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11039123 |
Jan 18, 2005 |
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60251133 |
Dec 4, 2000 |
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60251170 |
Dec 4, 2000 |
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Current U.S.
Class: |
501/95.1 ;
264/639; 264/640; 264/681; 264/682; 264/683; 428/366; 428/367;
428/373; 501/96.1; 501/96.3 |
Current CPC
Class: |
C04B 2235/40 20130101;
C04B 2235/5436 20130101; C04B 35/5622 20130101; C04B 2235/5264
20130101; C04B 35/62855 20130101; C04B 2235/3843 20130101; C04B
41/4584 20130101; C04B 35/645 20130101; C04B 35/5607 20130101; C04B
2235/3244 20130101; C04B 2235/5244 20130101; C04B 35/6261 20130101;
C04B 35/62873 20130101; C04B 2235/3839 20130101; C04B 2235/524
20130101; C04B 2235/526 20130101; Y10T 428/2918 20150115; C04B
2235/425 20130101; C04B 41/009 20130101; Y10T 428/2929 20150115;
C04B 2235/3826 20130101; C04B 2235/405 20130101; C04B 30/02
20130101; C04B 2235/5252 20130101; H01J 2235/088 20130101; C04B
2235/6021 20130101; C04B 35/62865 20130101; C04B 2235/3813
20130101; C04B 35/6286 20130101; B33Y 70/00 20141201; C04B 35/6263
20130101; C04B 2235/404 20130101; C04B 2235/3847 20130101; C04B
35/632 20130101; C04B 2235/3817 20130101; H01J 35/08 20130101; C04B
2235/3852 20130101; Y10T 428/2916 20150115; B33Y 80/00 20141201;
C04B 2235/77 20130101; C04B 35/62868 20130101; C04B 35/62876
20130101; H01J 2235/081 20130101; C04B 30/02 20130101; C04B 14/46
20130101; C04B 20/1062 20130101; C04B 30/02 20130101; C04B 14/46
20130101; C04B 20/1055 20130101; C04B 41/009 20130101; C04B 14/46
20130101; C04B 41/009 20130101; C04B 30/02 20130101 |
Class at
Publication: |
501/095.1 ;
264/639; 264/640; 264/681; 264/682; 264/683; 501/096.1; 501/096.3;
428/366; 428/373; 428/367 |
International
Class: |
C04B 35/00 20060101
C04B035/00; C04B 33/32 20060101 C04B033/32; B28B 1/00 20060101
B28B001/00 |
Goverment Interests
[0002] The present invention was made with U.S. Government support
under grant Number DASG60-00-C-0069 awarded by the Ballistic
Missile Defense Organization, grant Number NAS8-40553 awarded by
the National Aeronautics and Space Administration, grant Number
NAS8-97002 awarded by the National Aeronautics and Space
Administration, and grant Number DAS-60-00-C-0069 awarded by the
Ballistic Missile Defense Organization. Accordingly, the Government
may have certain rights in the invention described herein.
Claims
1. A method of manufacturing a high temperature fibrous monolith
composite comprising: blending a first composition of one or more
powders with a processing compound to provide a core material;
blending a second composition of one or more powders with a
processing compound to provide an intermediate shell material;
blending a third composition of one or more powders with a
processing compound to provide an outer shell material; forming a
feed rod from the core material, intermediate shell material and
outer shell material; extruding the formed feed rod to create a
reduced diameter filament; and sintering the filament to provide
the high temperature fibrous monolith composite, the core material
and intermediate and outer shell materials being generally discrete
in the composite and each composition being selected from the group
consisting of materials that remain in the solid state to allow use
of the composite at an operating temperature of at least about
1000.degree. C. or more.
2. The method of claim 1, wherein at least one of the first
composition, second composition and third composition includes a
powder selected from the group consisting of metal, metal alloy,
carbide, nitride, boride, oxide, phosphate and silicide and
combinations thereof.
3. The method of claim 1, wherein the processing compound includes
a polymer binder and wherein the polymer binder is removed from the
filament after extrusion.
4. The method of claim 1 further including: consolidating two or
more reduced diameter filaments to create a multi-filament rod; and
extruding the multi-filament rod to create a reduced diameter
second filament.
5. An article of manufacture suitable for use in a high temperature
environment comprising fibrous monolith composite materials,
fibrous monolith composite materials comprising: a first
composition selected from the group consisting of metal, metal
alloy, carbide, nitride, boride, oxide, phosphate and silicide and
combinations thereof; and a second composition generally
surrounding the first composition and selected from the group
consisting of metal, metal alloy, carbide, nitride, boride, oxide,
phosphate and silicide and combinations thereof, a third
composition generally surrounding the first and second composition
and selected from the group consisting of metal, metal alloy,
carbide, nitride, boride, oxide, phosphate and silicide and
combinations thereof, the compositions being generally discrete in
the composite materials and each composition being selected from
the group consisting of materials that remain in the solid state to
allow use of the article at an operating temperature of at least
about 1000.degree. C. or more.
6. The article of claim 5, wherein each composition is selected
from the group consisting of materials that remain in the solid
state to allow use of the article at an operating temperature of at
least about 1600.degree. C. or more.
7. The article of claim 5, wherein each composition is selected
from the group consisting of materials that remain in the solid
state to allow use of the article at an operating temperature of at
least about 2500.degree. C. or more.
8. The article of claim 5, wherein each composition is selected
from the group consisting of materials that remain in the solid
state to allow use of the article at an operating temperature of at
least about 3000.degree. C. or more.
9. A coating material comprising fibrous monolith materials
including: a first composition of one or more powders selected from
the group consisting of metal, metal alloy, carbide, nitride,
boride, oxide, phosphate and silicide and combinations thereof and
a processing compound; and a second composition of one or more
powders selected from the group consisting of metal, metal alloy,
carbide, nitride, boride, oxide, phosphate and silicide and
combinations thereof and a processing compound, said second
composition generally surrounding the first composition, the
compositions being generally discrete in the fibrous monolith
materials and each composition being selected from the group
consisting of materials that remain in the solid state to allow use
of the coating material at an operating temperature of at least
about 1000.degree. C. or more.
10. The coating material of claim 9, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 1600.degree. C. or more.
11. The coating material of claim 9, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 2500.degree. C. or more.
12. The coating material of claim 9, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 3000.degree. C. or more.
13. The coating material of claim 9, including a third composition
of one or more powders selected from the group consisting of metal,
metal alloy, carbide, nitride, boride, oxide, phosphate and
silicide and combinations thereof and a processing compound, said
third composition generally surrounding the second composition.
14. The coating material of claim 13, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 1600.degree. C. or more.
15. The coating material of claim 13, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 2500.degree. C. or more.
16. The coating material of claim 13, wherein each composition is
selected from the group consisting of materials that remain in the
solid state to allow use of the coating material at an operating
temperature of at least about 3000.degree. C. or more.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 10/013,601, filed on Dec. 4, 2001, and
entitled "Composite Components for Use in High Temperature
Applications," which is based on and claims the benefit of U.S.
Provisional Application Ser. No. 60/251,170, filed on Dec. 4, 2000,
and entitled "High Performance Fibrous Monolith X-Ray Target," and
U.S. Provisional Application Ser. No. 60/251,133, filed on Dec. 4,
2000, and entitled "High Temperature Carbide, Oxide, Nitride,
Silicide, And Boride Based Fibrous Monoliths For High Temperature
Application." These applications are incorporated by reference in
their entirety.
FIELD OF THE INVENTION
[0003] The present invention relates to multi-component composites,
such as fibrous monolith ceramic composites, suitable for use in
materials and structures that are subject to harsh environmental
conditions, including extreme temperatures, chemical atmospheres
and thermal shock, and methods of preparing the same. The high
temperature FM composites have increased thermal shock resistance
and increased thermal cycling tolerance.
BACKGROUND OF THE INVENTION
[0004] Certain carbides, nitrides, borides, oxides, phosphates and
silicides exhibit enhanced mechanical properties, including
enhanced strength, oxidation resistance, damage tolerance and wear
resistance. As a result, these materials have found use in high
temperature applications where the materials are subject to extreme
temperatures (greater than 3000.degree. C.), as well as corrosive
environments. For example, many of the carbides, nitrides, borides,
oxides, phosphates and silicides of the elements from Groups IVb,
Vb, and VIb of the periodic table, as well as carbides, nitrides,
borides, oxides, and silicides of boron, aluminum, and silicon have
been used in industrial and other applications where such
conditions are likely to be encountered. Generally, structures
formed of these materials exhibit improved strength and hardness at
ambient and elevated temperatures, improved toughness and wear
resistance, high melting points, thermal shock resistance, and
oxidation resistance.
[0005] Historically, ZrB.sub.2 and HfB.sub.2 based materials have
been the choice for high-temperature ablation resistance in
oxidizing environments. They have high melting points (about
3000.degree. C.), excellent oxidation resistance, elevated
temperature creep resistance, and moderate resistance to thermal
shock. The addition of SiC boosts their resistance to oxidation at
intermediate temperatures to produce the best performing diboride
material. Above 2200.degree. C. it is the high melting point
carbides of Zr, Ta, and Hf (3540.degree. C., 3880.degree. C., and
3890.degree. C., respectively) and not the diborides that exhibit
the best oxidation resistance. TaC-HfC solid solutions (e.g. 80%
TaC-20% HfC) have high melting temperatures and even better
oxidation resistance than the individual Hf and Ta carbides.
However, the use of these monolithic materials has been limited due
to their poor resistance to thermal shock.
[0006] As a more specific application, materials capable of
withstanding high temperatures are desired for use in X-ray system
design, particularly for the X-ray target. The maximum X-ray power
output from an X-ray tube is an important parameter in the
operation and maintenance of a radiological system. The time period
required to inspect an object is inversely proportional to the
X-ray power output. For a given X-ray power output of the X-ray
tube, the tube lifetime is directly proportional to its maximum
power rating. Accordingly, higher values for the maximum X-ray
power output are desirable to reduce the inspection times and the
throughput of patients or objects examined with the radiological
system, as well as to reduce the maintenance and operating costs as
a result, in part, of the longer tube lifetimes. Because of the
inefficiencies related to X-ray sources, storage and movement of
waste heat from the radiation source is an important consideration
in the design of X-ray systems. The thermal expansion match between
the substrate and the target material and the ability of the target
material to contribute to the high voltage stability are important
material characteristics to be considered when designing an X-ray
target.
[0007] Target materials for X-rays have been made of Cu or similar
materials and cooled with circulating oil or water. Other targets
utilize standard carbon backed metal targets, which provide
improved performance compared to Cu-based targets by eliminating
the required cooling but have the disadvantage of an inability of
the braze composition to withstand the temperature profiles that
are experienced during operation. Where Cu or similar targets with
low melting temperatures are used, active target cooling is
required to withstand the high temperature during operation,
thereby increasing the complexity.
[0008] There remains a need for materials exhibiting improved
strength, hardness, thermal shock resistance, oxidation resistance
and fracture toughness, as compared to presently known materials,
for use in high temperature applications and/or harsh chemical
environments.
SUMMARY OF THE INVENTION
[0009] The present invention relates to structures that utilize
fibrous monolith ("FM") composites to provide the structures with
excellent thermal shock resistance, excellent erosion and
oxidation/corrosion resistance, enhanced thermal cycling tolerance,
enhanced strength at elevated temperatures, and graceful,
non-catastrophic failure at room and elevated temperatures. The
present invention also relates to methods of preparing such
composites and structures.
[0010] The composites of the present invention may be used as
coatings or external surface component materials in combination
with existing structures or with particular substrate structures to
impart the benefits of the composites to the structures.
Additionally, a more substantial portion of, or even a complete,
structure may be formed from the FM composites.
[0011] Applications for the fibrous monolith composite materials of
the present invention include use in structures such as flat
plates, solid hot gas containment tubes, radiant burner tubes,
radiant burner panels, rocket nozzles, body armor panels, X-ray
targets for CT scanner X-ray tubes, high temperature furnace
equipment, antimatter containment vessels, furnace furniture,
solar-thermal-propulsion components, internal combustion engine
components, turbine engine, turbomachinery components and steering
vanes for vectored thrust control, which can all be readily formed
from the green material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a perspective cross-sectional view of a uniaxial
fibrous monolith composite in accordance with the present
invention;
[0013] FIG. 2 is a graphical illustration of flexural stress as a
function of displacement for a fibrous monolith composite in
accordance with the present invention;
[0014] FIG. 3 is a schematic flow diagram showing a process of
preparing filaments in accordance with the present invention;
[0015] FIG. 4 is a photomicrograph of an axial cross-section of an
FM composite in accordance with the present invention;
[0016] FIG. 5 is a photograph showing preparation of a structure
using green fibrous monolith filaments in accordance with the
present invention;
[0017] FIG. 6 is a schematic illustration of an X-ray target
including a FM composite in accordance with the present invention;
and
[0018] FIG. 7 is a photomicrograph of an axial cross-section of a
second FM composite in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention is directed to the application of FM
composites in high temperature and/or harsh chemical environments
and to methods of preparing FM composites and structures for use in
such environments. The FM composites exhibit mechanical properties
including excellent thermal shock resistance, excellent erosion and
oxidation/corrosion resistance, enhanced thermal cycling tolerance,
enhanced strength at elevated temperatures, and graceful,
non-catastrophic failure at room and elevated temperatures. More
particularly, the structures of the present invention include
fibrous monolithic ceramic and/or metallic composites that include
a plurality of filaments having a core surrounded by a shell. The
composites may be formed into structures and/or provided as a
coating for or layered onto a surface of structures subject to high
temperature and/or harsh environments to impart the desired
characteristics to the structure.
[0020] As used herein, "fibrous monolithic composite" and "fibrous
monolith" are intended to mean a ceramic and/or metallic composite
material that includes a plurality of monolithic fibers, or
filaments, each having at least a cell phase surrounded by a
boundary phase but may include more than one core and/or shell
phase. Fibrous monoliths exhibit the characteristic of non-brittle
fracture, such that they provide for non-catastrophic failure.
[0021] As used herein, "cell phase" is intended to mean a centrally
located primary material of the monolithic fiber, that is dense,
relatively hard and/or strong. The cell phase extends axially
through the length of the fiber, and, when the fiber is viewed in
transverse cross-section, the cell phase forms the core of the
fiber. The "cell phase" also may be referred to as a "cell" or
"core".
[0022] As used herein, "boundary phase" is intended to mean a more
ductile and/or weaker material that surrounds the cell phase of a
monolithic fiber in a relatively thin layer and that is disposed
between the various individual cell phases, forming a separating
layer between the cell phase and surrounding cell phases when a
plurality of fibers are formed in a fibrous monolithic composite.
The "boundary phase" also may be referred to as a "shell," "cell
boundary" or "boundary".
[0023] Fibrous monoliths ("FMs") are a unique class of structural
ceramics that have mechanical properties similar to continuous
fiber reinforced ceramic composites (CFCCs). Such properties
include relatively high fracture energies, damage tolerance, and
graceful failures. In contrast to CFCCs, FMs can be produced at a
significantly lower cost. FMs, which are monolithic ceramics,
generally are manufactured by powder processing techniques using
inexpensive raw materials. As a result of the high performance
characteristics of FMs and the low costs associated with
manufacture of FMs, FMs are used in a wider range of applications
than heretofore typical for ceramic composites.
[0024] As shown in FIG. 1, the macroarchitecture of an FM composite
10 generally includes multiple filaments 12 each comprising at
least two distinct materials--a primary phase in the form of
elongated polycrystalline cells 14 separated by a thin secondary
phase in the form of cell boundaries 16. Typical volume fractions
of the two phases are between about 50 to about 99% of the fiber
for the primary phase (polycrystalline cell) and between about 1 to
about 50% of the fiber for the interpenetrating phase (cell
boundary). Preferably, the volume fractions are between about 80 to
about 95% for the primary phase (polycrystalline cell) and between
about 5 to about 20% for the interpenetrating phase (cell
boundary). The primary or cell phase typically consists of a
structural material of a metal, metal alloy, carbide, nitride,
boride, oxide, phosphate or silicide and combination thereof. The
cells are individually surrounded and separated by cell boundaries
of a tailored secondary phase. Powders that may be used in the
secondary phase include compounds to create weak interfaces such as
fluoromica, and lanthanum phosphate; compounds to create porosity
in a layer which function to create a weak interface; graphite
powders and graphite-containing powder mixtures; and hexagonal
boron nitride powder and boron nitride-containing powder mixtures.
If a metallic debond phase is desired, reducible oxides of metals
may be used, e.g., nickel and iron oxides, or powders of metals,
e.g., nickel, iron, cobalt, tungsten, aluminum, niobium, silver,
rhenium, chromium, or their alloys.
[0025] Advantageously, powders which may be used in the cell and/or
boundary phase composition to provide the green matrix filament
include diamond, graphite, ceramic oxides, ceramic carbides,
ceramic nitrides, ceramic borides, ceramic silicides, metals, and
intermetallics. Preferred powders for use in that composition
include aluminum oxides, barium oxides, beryllium oxides, calcium
oxides, cobalt oxides, chromium oxides, dysprosium oxides and other
rare earth oxides, hafnium oxides, lanthanum oxides, magnesium
oxides, manganese oxides, niobium oxides, nickel oxides, tin
oxides, aluminum phosphate, yttrium phosphate, lead oxides, lead
titanate, lead zirconate, silicon oxides and silicates, thorium
oxides, titanium oxides and titanates, uranium oxides, yttrium
oxides, yttrium aluminate, zirconium oxides and their alloys; boron
carbides, iron carbides, hafnium carbides, molybdenum carbides,
silicon carbides, tantalum carbides, titanium carbides, uranium
carbides, tungsten carbides, zirconium carbides; aluminum nitrides,
cubic boron nitrides, hexagonal boron nitrides, hafnium nitride,
silicon nitrides, titanium nitrides, uranium nitrides, yttrium
nitrides, zirconium nitrides; aluminum boride, hafnium boride,
molybdenum boride, titanium boride, zirconium boride; molybdenum
disilicide; lithium and other alkali metals and their alloys;
magnesium and other alkali earth metals and their alloys; titanium,
iron, nickel, chromium, cobalt, molybdenum, tungsten, hafnium,
rhenium, rhodium, niobium, tantalum, iridium, platinum, zirconium,
palladium and other transition metals and their alloys; cerium,
ytterbium and other rare earth metals and their alloys; aluminum;
carbon; lead; tin; and silicon
[0026] Compositions comprising the cell phase differ from those
comprising the boundary phase in order to provide the benefits
generally associated with FMs. For example, the compositions may
include formulations of different compounds (e.g., HfC for the cell
phase and WRe for the boundary phase or WC--Co and W--Ni--Fe) or
formulations of the same compounds but in different amounts (e.g.,
WC-3% Co for the cell phase and WC-6% Co for the boundary phase) as
long as the overall properties of the compositions are not the
same. For example, the compositions can be selected so that no
excessively strong bonding occurs between the two phases in order
to limit crack deflection. Examples of FM composites include but
are not limited to: HfC/graphite, HfC/Wre, ZrC/BN, TaC(20% HfC)/BN,
HfC/Re--Ir, ZrC/Re, WC/W, WC--Co/Nb, NbC/Nb, VC/Mo, TiC/W,
HfN/graphite, HfB.sub.2/BN, ZrB.sub.2/BN, HfC/HfO.sub.2,
HfC/HfN.sub.0.33, HfC/HfC.sub.0.67.
[0027] The cell boundary phase may be selected to create pressure
zones, microcrack zones, ductile-phase zones, or weak debond-type
interfaces in order to increase the toughness of the composite. For
example, low-shear-strength materials such as graphite and
hexagonal boron nitride make excellent weak debond-type cell
boundaries and are present in Si.sub.3N.sub.4/BN and SiC/Graphite
FM composites. The weak BN and graphite interfaces deflect cracks
and delaminate thereby preventing brittle failure of these
composites and increasing their fracture toughness. As a result, FM
structures exhibit fracture behavior similar to CFCCs, such as C/C
and SiC/SiC composites, including the ability to fail in a
non-catastrophic manner. By way of example, typical flexural stress
as a function of displacement for a Si.sub.3Ni.sub.4/BN FM material
is shown in FIG. 2, which illustrates that fibrous monolith
composites are non-brittle and retain significant load bearing
capability after fracturing is initiated.
[0028] Composites of the present invention include multifilament FM
layers and FM substrates with unique fiber orientations. These
composites may be used to fabricate various structures, may be
applied as coatings on the desired structures, or may be provided
as a structural layer to provide thermal shock resistance. Use of
the composites also may increase the erosion/oxidation resistance
lifetime of structures used in harsh chemical environments.
[0029] The composite materials of the present invention are capable
of tolerating operating temperatures approaching 5400.degree. F.
(3000.degree. C.), while maintaining excellent thermal, physical,
and mechanical properties. In addition, these materials satisfy
material requirements such as low density, high elastic modulus,
low coefficient of thermal expansion, high thermal conductivity,
excellent erosion and oxidation/corrosion resistance, and
flaw-insensitivity. They also possess the ability to be joined, to
survive thermal cycling and multi-axial stress states, and for
reusable applications, the materials maintain the above attributes
after prolonged exposures to harsh chemical environments.
[0030] Low shear strength cell boundaries, such as BN and graphite,
accommodate the expansions and contractions that occur during
thermal cycling of the FM composite components, thereby resulting
in improved thermal shock resistance. From the mechanical behavior
viewpoint, such cell boundaries enable non-catastrophic failure due
to stress delocalization and crack deflection mechanisms at both
room and elevated temperatures. In addition, the presence of a
ductile or relatively ductile cell boundary phase increases the
damage tolerance and wear resistance of the FM composite. Without
intending to be limited by any theories, it is believed that the
improvement in damage tolerance of the present composites is
attributed to the FM architecture in which the cell boundary
absorbs and deflects crack energy during impact so that damage,
when it occurs, is isolated to the individual micron-sized
cells.
[0031] Various methods of preparing fibrous monolithic filaments
are known in the art, including the methods disclosed in U.S. Pat.
No. 5,645,781, which is incorporated by reference herein in its
entirety. Generally, as illustrated in FIG. 3, the process of
preparing fibrous monolithic filaments in accordance with the
present invention includes separately blending the starting
materials for a core 20 and shell 22, forming the core 24 having a
first composition and forming the shell 26 having a second
composition, forming the feed rod 28 from the core and shell, and
extruding the feed rod 30 one or more times to provide a ceramic
filament 32. The filaments may then be formed and/or arranged to
provide the desired structure in accordance with the present
invention.
[0032] Fibrous monolith composites are fabricated using
commercially available ceramic and metal powders using a process
for converting ordinary ceramic powder into a "green" fiber that
include the powder, a thermoplastic polymer binder and other
processing aids. The fiber is compacted into the "green" state to
create the fabric of elongated polycrystalline cells that resemble
a fiber after sintering or hot pressing. The process is widely
applicable, and allows a cell/cell boundary bi-component fiber to
be made from a thermodynamically compatible set of materials
available as sinterable powders. The scale of the microstructure is
determined by the green fiber diameter (cell size) and coating
thickness (cell boundary). Once the green composite fiber is
fabricated it can be formed using any method known to those skilled
in the art into the shape of the desired component having, for
example, conventional composite architecture (e.g., uniaxial
lay-up, biaxial lay-up, woven fabric, etc.). The thermoplastic
binder is removed in a binder burnout step, and the component is
hot pressed or sintered to obtain a fully consolidated and
densified component.
[0033] The core and shell of the feed rod are formed of
mechanically activated and agglomerate-free powders. The powders,
such as the metals, alloys, carbides, nitrides, borides, oxides,
phosphates and silicides listed above, are selected to provide the
desired mechanical properties in the final composite. A wide
variety of combinations of powders may be used for the core and
shell materials. Powders having particle size distributions in the
range of about 0.01 to about 100 microns (.mu.m) in size may be
used. Preferably, the particle size of the powder is between about
1 to about 10 microns.
[0034] Milling stations such as commercially available from Boston
Gear, Boston, Mass. may be used as needed to ball mill the ceramic
powder to obtain the desired size distribution. The desired ceramic
powder preferably is ball milled with ethanol. The ceramic/ethanol
blend is ball milled with milling media such as silicon nitride
(Si.sub.3N.sub.4) or zirconium oxide (ZrO.sub.2) thus creating a
ball-mill slurry. Sintering aids such as, for example, aluminum
oxide (Al.sub.2O.sub.3) and yttrium oxide (Y.sub.2O.sub.3)
additions to Si.sub.3N.sub.4, when necessary, are added and milled
together with the ball mill slurry. The powders are milled for a
time effective for providing desired particle sizes and
distribution. Typical milling times are between about 24 to about
120 hours, depending on the starting ceramic material. For example,
boron nitride (BN) powder is milled for about 12 to 24 hours,
silicon nitride powder is milled for about 24 hours, and zirconium
carbide (ZrC), purchased as a fairly coarse refractory ceramic, is
typically milled for a longer period, about 72 to 120 hours.
[0035] Upon completion of the milling operation, the ball mill
slurry is collected from the milling station and the
ceramic/ethanol mixture is separated from the milling media using a
perforated mill jar lid as a "strainer". The ethanol is separated
from the ceramic powder using a Buchi Rotavapor separator
commercially available from Brinkman Instruments Inc. of Westbury,
N.Y. Solvent is evaporated from the ball-milled slurry in the Buchi
Rotavapor separator and the ceramic powder dried. Ethanol solvent
may be reclaimed as desired for reuse or proper disposal according
to local, state, and federal waste disposal requirements. The
ceramic powders are removed from the separator jar and placed in
labeled plastic jars.
[0036] The individual ceramic powders are blended with
thermoplastic melt-spinnable polymer binders, as well as one or
more processing aids such as plasticizers as necessary, using a
high shear mixer commercially available from C. W. Brabender of
South Hackensack, N.J. or from Thermo Haake of Paramus, N.J., to
form a smooth, uniformly suspended composite blend also referred to
as a "dope". Examples of thermoplastic binders include ethylene
ethylacetate (EEA) commercially available as DPDA-618NT from Union
Carbide, ethylene vinylacetate (EVA) commercially available as
ELVAX 470 from E.I. DuPont Co., and Acryloid Copolymer Resin (B-67)
commercially available from Rohm and Haas, Philadelphia, Pa.
Examples of plasticizers include heavy mineral oil (HMO)
commercially available as Mineral Oil White, Heavy, Labguard.RTM.
and methoxy polyethyleneglycol having a molecular weight of about
550 (MPEG-550) commercially available from Union Carbide. The
composite blend is compounded at about 150.degree. C. while
metering a viscosity-modifying additive until a viscosity is
obtained that will ensure desired rheology for a molten fiber
extrusion process.
[0037] Because the mixers have fixed volume reservoirs, the recipes
for the thermoplastic/ceramic blends produced in batches are
formulated on a volumetric, as opposed to a gravemetric basis. As
an example, one blend consists of between about 50 to about 62 vol.
% of the ceramic powder, between about 37 to about 50 vol. % of the
thermoplastics, and between about 0 to about 12 vol. % of the
plasticizers. Thus, the mass of a batch of ceramic/thermoplastic
dope varies with the density of the ceramic powder. By way of
example, a batch of Si.sub.3N.sub.4 with a density of 3.44 g/cc
produces approximately 1 kg of "green" compound material.
[0038] After mixing, the composite blends are warm-pressed into a
green composite feed rod. A composite feed rod consists of a "core"
of a primary ceramic material enclosed by a cladding or "shell" of
a second ceramic material. A preferred feed rod pressing station
includes a hydraulic vertical press with one or more heated
cylindrical dies, which allows the cores to be pressed. A heated
uniaxial platen press, such as commercially available from Carver
Inc., of Wabash, Ind., is used to press the shells for the
composite feed rods. The volume ratio of the core and shell of a
composite feed rod can be systematically varied to any desired
ratio by using different sets of machine tooled core and shell
dies. By way of example, "green" composite feed rods (22 mm in
diameter) with the following core/shell volume ratios are commonly
produced: 90/10, 82.5/17.5, 69/31, and 50/50.
[0039] A pressed feed rod is extruded. One extrusion process
includes a computer numerically controlled (CNC) ball-screw
extruder, including a ball screw from Thomson Saginaw of Saginaw,
Mich., connected to a CNC directed current (DC) servomotor from
Compumotor, Rohnert Park, Calif. The ball screw is connected to a
brass metal rod that is used to pressurize and extrude the contents
of the heated cylindrical die. The entire assembly is mounted and
held vertically in a metal framework. Composite feed rods are
extruded through a spinneret to produce a green fiber filament or
"spaghetti". This process also is referred to as "single filament
co-extrusion" (SFCX).
[0040] Typical filament sizes are 100 .mu.m, 250 .mu.m, 320 .mu.m,
500 .mu.m, 750 .mu.m, 1 mm, 2 mm or 4 mm in diameter. Filaments
having diameters between about 0.01 and about 10 mm may be extruded
using a spinneret fabricated with the appropriate orifice diameter.
The extruded filaments maintain the volume ratio of the original
feed rod despite significant differences in diameters, such as a
starting feed rod diameter of 22 mm and an extruded filament
diameter of 250 .mu.m (which is approximately 100 times smaller
than the starting feed rod diameter). Use of thermoplastic/ceramic
blends having appropriate rheological properties for the cores and
shells maintain the volume ratio of the original feed rod.
Preferably, the viscosity of the core material is approximately
equivalent to the viscosity of the shell material. Use of core and
shell materials with approximately equivalent viscosities provides
improved flow stability and control to assist with maintaining the
original geometry of the feed rod.
[0041] In general, filaments having diameters of no more than about
250 .mu.m can be obtained by single filament co-extrusion. Smaller
diameter green filaments may be readily broken during the winding
and extrusion process, thereby limiting the ability to produce
filaments having smaller diameters. To obtain cell sizes smaller
than 250 .mu.m, filaments having diameters of between about 1 to
about 2 mm may be extruded and bundled together to form a
multifilament feed rod having a diameter of about 22 mm. This feed
rod is then extruded through a spinneret to produce multifilament
spaghetti. Using this multifilament co-extrusion (MFCX) procedure,
cell sizes approaching 10 microns or less can be produced.
[0042] Filaments having more than one cell composition and/or more
than one shell composition can also be prepared to provide the
benefits of the properties of the additional composition and/or to
insulate the shell material. As an example, a layer of a second
cell composition may be disposed around the shell, such that the
filament includes a central cell, an intermediate shell and an
outer shell. Other combinations of cells and shells also may be
prepared as desired. For example, a core material in combination
with a plurality of different shells may be used.
[0043] A plurality of filaments may be bundled together and
disposed within another shell. This arrangement of filaments
results in essentially a "honeycomb" configuration when arranged to
form the FM composite, as shown in FIG. 7. In this architecture, a
plurality of filaments each having a core and shell are bundled
together and coated in a second shell. This architecture can be
obtained using a modified co-extrusion process where an individual
filament is formed in a first pass through the extruder. A
plurality of fibers of a predetermined length are bundled, encased
in a second, common shell and again passed through the extruder. As
an example, the volume ratios of the individual filament in the
first pass is 82.5% for the core and 17.5% for the shell, and the
bundled fibers are coated with a 17.5% volume common shell. The
bundled arrangement maintains the mechanical behavior of the
filaments but insulates a "weaker" shell material from the external
environment and any harsh conditions.
[0044] Numerous modifications and adjustments to the process for
preparing filaments may be made to allow for variations in the
particular compositions used to prepare the filaments. For example,
viscosities may be adjusted, the diameter of the extrusion die may
be changed, or the relative volumes of the core and shell may be
changed. Other methods for extruding and/or otherwise forming the
filaments known to those of skill in the art also may be utilized.
For example, any modified process for continuous co-extrusion may
be used.
[0045] Generally, filaments may be bundled, woven, wound, braided,
chopped, pressed, or laid up to produce essentially a near net
shape pre-form. In a typical two-dimensional (2D) lay-up, the
composite filament is wound on a computer numerically controlled
(CNC) drum winder. The winder includes a rotating plastic drum
driven by a servomotor from Compumotor, Rohnert Park, Calif. A
single axis motion controller from Compumotor, Rohnert Park, Calif.
is used to adjust the filament position as the filament is wound
around the drum. Two-dimensional parts having a desired fiber
alignment can be fabricated using the CNC drum winder.
[0046] After winding, composite filament sheets are cut to the
desired shape and dimensions. The cut sheets can then be laid up in
any standard 2D architecture (i.e. uniaxial, 0.degree./90.degree.,
quasi-isotropic, etc.). After the 2D pre-form is laid-up, a
uniaxial platen press is used to warm laminate the component part.
3D components such as rocket nozzles, rocket throats, combustor
liners and the like can be built using helical, axial and
circumferential lay ups and windings and using any combination
thereof.
[0047] In another embodiment of the present invention, the green
filaments are grouped or bundled into a cylinder or other
predetermined shape. The bundled filaments are cut to a
predetermined length and machined, molded or otherwise formed to
provide a structure having a desired shape and having the FM
filaments extending generally parallel to the axis of the
structure. One or more FM layers may be disposed across a surface
of the structure as desired to provide increased enhancement of the
mechanical properties of the structure. The FM composite structure
is warm laminated to consolidate the FM filaments.
[0048] In other embodiments, articles having various filament
characteristics and orientations may be provided. For example, in
preparing an FM layer, different combinations of cell and/or
boundary materials, filament size, filament shape, and filament
orientation are contemplated as being within the scope of the
invention. Such variations in these variables can be applied on a
layer-to-layer basis, that is, a first layer is formed of filaments
of uniform composition, size, shape and orientation and a second
layer is formed with filaments having a composition, size, shape
and/or orientation different from that of the first layer. Such
variations also can be applied within a particular layer, that is,
the layer is formed of filaments of differing compositions, sizes,
shapes, and/or orientations. Any number and combination of layers
may be used in forming the FM composite article in order to achieve
the benefits of desired properties of the FM materials.
[0049] In another embodiment, the FM structure can be produced
using rapid prototyping techniques as known to those skilled in the
art or any modified rapid prototyping technique. For example, the
feed rod can be loaded into a freeform fabrication apparatus having
a heated barrel. A molten material is formed and directly feeds
into a fine deposition nozzle having a volumetric flow rate that
can be adjusted for high raw material throughput dispensing. The
molten material is extruded through a high pressure nozzle onto a
foam pad. The foam pad is mounted on a 4-axis, motorized, computer
numerically controlled (CNC) platen. The solid freeform fabrication
technique provides a complex part from one single, continuous
fiber.
[0050] Although the invention is described with reference to
generally cylindrical-shaped FM filaments that are bundled together
to form FM composites wherein the shape of the filaments become
essentially hexagonal in cross-section as a result of processing,
other configurations are contemplated, as will be appreciated by
those skilled in the art. For example, filaments having square,
rectangular or triangular cross-sections may be obtained by varying
the shape of the extrusion die accordingly. Additionally, the shape
of the die used in the laminating step also may be modified
accordingly as desired. Thus, different shapes and configurations
of filaments in the FM composite may be obtained, which can impact
the resultant mechanical properties of the FM composite.
[0051] A binder burnout furnace, such as commercially available
from Lindberg, Watertown, Wis. is used to remove polymer binder
from the formed composite coatings and FM composite structures.
Sintering processes, including hot pressing, hot isostatic pressing
or pressureless sintering, provide final consolidation and
densification of the composite coatings and FM composite
structures. A typical induction hot-press such as commercially
available from Vacuum Industries, Somerville, Mass. is capable of a
maximum temperature of 2400.degree. C. and a maximum load of 100
tons and can be operated in several different environments
including vacuum, argon, and nitrogen atmospheres.
[0052] In another embodiment of the present invention, FM
composites are used in high performance X-ray targets for X-ray
vacuum tubes X-rays used in the medical, defense, industrial, and
security industries. FM composites in accordance with the present
invention provide target designs that are capable of withstanding
high power and high temperature and that have a relatively high
heat capacity. The high conductivity FM composite materials spread
the heat through the target, which lowers the maximum temperature
at the "spot" and increases the useful time for the target. The
target spins at speeds up to 10,000 rpm while undergoing local
surface heating to temperatures in excess of 1600.degree. C. FM
composites are able to withstand the thermal shock and cyclic
thermal fatigue for typical operation inside the X-ray tube.
[0053] As shown in FIG. 6, an X-ray target 50 generally is
disc-shaped. An annular layer of FM composite material 52 is
disposed adjacent the outer perimeter at or near an upper surface
of the target substrate 54. A monolithic layer 56 may be disposed
over the FM composite material 52. Preferably the layer of FM
composite material 52 is relatively thin (less than about 1
mm).
[0054] Direct bonding of a FM composite material onto a substrate
material such as graphite is possible with the composites of the
present invention, thereby eliminating intermediate layers (such as
titanium-zirconium-molybdenum alloy, or "TZM") which are typically
required in conventional systems as an interface between the target
surface material and the substrate to compensate for thermal
expansion differences between the two. The need for high
temperature brazing to attach the intermediate layer to the
substrate is eliminated. Consequently, the temperature capability
of the target material is improved considerably. Furthermore,
elimination of the intermediate layer results in a target of
reduced weight, thereby leading to less wear on X-ray target
components and longer life of the X-ray tube.
[0055] Coefficient of thermal expansion (CTE) analysis may be used
to assist with determining the feasibility of a system as a coating
material. CTE mismatch leads to the buildup of inherent stress as
materials react to temperature change and also may lead to
catastrophic failure. CTE of a composite can be manipulated based
on the volume fraction of the core and shell compositions. Thermal
stresses developed in coating result at least in part to
differences in CTE between the substrate and coating. Most coatings
will not inherently fail if the CTE mismatch does not exceed about
.+-.10%.
[0056] Rapid prototyping techniques such as extrusion freeform
fabrication (EFF) deposits the HfC/W-Re fibrous monolith materials
on graphite substrates. A retrofitted high pressure extruder head
used with a fused deposition modeler provides for rapid prototyping
of the FM material. The EFF process entails pressing a feedrod of
the compounded engineering thermoplastic raw material. The feedrod
is loaded into a heated barrel on the EFF apparatus. The molten FM
material directly feeds into a fine deposition nozzle whose
volumetric flow rate can be adjusted for high raw material
throughput dispensing. The molten FM material is subsequently
extruded through the nozzle onto a CAD computer interfaced, movable
X-Y-Z platen located directly below the deposition head. The
computer controlled EFF apparatus lays the FM material as spirals
or in the radial directions.
[0057] In other embodiments, alternative methods of preparing FM
filaments and composite materials may be utilized. Alternative
compositions and methods, including those described in the
co-pending U.S. patent applications listed in Table 1, which are
incorporated by reference herein in their entireties, are
contemplated for use with the present invention. TABLE-US-00001
TABLE 1 ATTY FILING DOCKET TITLE INVENTORS DATE NO. ALIGNED
COMPOSITE Anthony C. Mulligan Dec. 04, 2001 03248.00038 STRUCTURES
FOR MITIGATION Mark J. Rigali OF IMPACT DAMAGE AND Manish P.
Sutaria RESISTANCE TO WEAR IN Dragan Popovich DYNAMIC ENVIRONMENTS
CONSOLIDATION AND Manish P. Sutaria Dec. 04, 2001 03248.00039
DENSIFICATION METHODS FOR Mark J. Rigali FIBROUS MONOLITH Ronald A.
Cipriani PROCESSING Gregory J. Artz Anthony C. Mulligan METHODS AND
APPARATUS FOR Anthony C. Mulligan Dec. 04, 2001 03248.00040
PREPARATION OF THREE- Mark J. Rigali DIMENSIONAL BODIES Manish P.
Sutaria Gregory J. Artz Felix H. Gafner K. Ranji Vaidayanathan
COMPOSITIONS AND METHODS Mark J. Rigali Dec. 04, 2001 03248.00044
FOR PREPARING MULTIPLE- Manish P. Sutaria COMPONENT COMPOSITE Felix
Gafner MATERIALS Ron Cipriani Randy Egner Randy C. Cook
MULTI-FUNCTIONAL COMPOSITE Anthony C. Mulligan Dec. 04, 2001
03248.00045 STRUCTURES John Halloran Dragan Popovich Mark J. Rigali
Manish P. Sutaria K. Ranji Vaidyanathan Michael L. Fulcher Kenneth
L. Knittel
EXAMPLES
[0058] The following examples are intended to illustrate the
present invention and should not be construed as in any way
limiting or restricting the scope of the present invention.
Example 1
[0059] A HfC/W--Re composite for a solar thermal propulsion engine
application was fabricated using FM technology. As mentioned above,
monolithic HfC has a good oxidation resistance at elevated
temperature and exhibits a high melting temperature. However, the
use of HfC is limited by its poor thermal shock resistance. With an
addition of ductile metal with high melting temperature, W--Re, the
thermal shock resistance of this composite was improved
dramatically. Table 2 provides coextrudable formulations of
HfC/thermoplastic and W--Re/thermoplastic for the fabrication of
HfC/WRe FM composite components. FIG. 5 shows a `green` nozzle
assembly in which HfC/W--Re fibers are in the process of being
wound onto a graphite mandrel prior to hot pressing. TABLE-US-00002
TABLE 2 Material Density (g/cc) Volume % Mass (g) HfC 12.67 54.00
287.36 EEA.sup.1 0.93 18.00 7.11 EVA.sup.2 0.94 18.00 7.03 Heavy
Mineral 0.88 10.00 4.44 Oil .sup.1Ethylene Ethyl Acrylate, Union
Carbide .sup.2Ethylene Vinyl Acetate, DuPont
[0060] TABLE-US-00003 TABLE 3 Material Density (g/cc) Volume % Mass
(g) W-3.6% Re-0.26% HfC 19.33 49.00 404.31 EEA 0.93 40.00 18.75
MPEG 550 1.10 10.2 4.73
Example 2
[0061] Tantulum-based FM composites for high temperature propulsion
applications were prepared. The FM composites included propulsion
nozzles and thrusters, and hot gas valves for use in rocket
engines. Tables 4 and 5 present co-extrudeable formulations for the
production of TaC-HfC/BN fibrous monolith composite filament.
TABLE-US-00004 TABLE 4 Material Density (g/cc) Volume % Mass (g)
TaC20% HfC 13.63 54.00 309.13 EEA 0.93 18.00 7.11 EVA 0.94 18.00
7.03 Heavy Mineral 0.88 10.00 4.44 Oil
[0062] TABLE-US-00005 TABLE 5 Material Density (g/cc) Volume % Mass
(g) BN 2.29 54.00 51.94 EEA 0.93 46.00 17.97 MPEG 550 1.10 minimal
Minimal
Example 3
[0063] Various HfC-based FM composite systems were prepared.
Mechanical properties of the HfC-based composites are presented in
Table 6. TABLE-US-00006 TABLE 6 Theoretical Measured Fracture EMOD
EMOD Stress FM System Architecture (GPa) (GPa) (MPa) HfC/BN/HFC
Honeycomb 313.02 336.0 .+-. 10.6 183.3 .+-. 16.3 (82.5/17.5)/ 17.5
HfC/G/HfC Honeycomb 302.48 324.8 .+-. 10.9 220.5 .+-. 18.9
(82.5/17.5)/ 17.5 HfC/W3.6Re Bi-layer 459.2 453.1 .+-. 7.8 325.2
.+-. 80.8 82.5/17.5 HfC/W3.6Re Bi-layer 463.4 461.2 .+-. 17.1 362.5
.+-. 74.9 90/10 HfC/W3.6Re Bi-layer 382.5 293 .+-. 11.2 197.1 .+-.
10.8 50/50
[0064] The HfC FM composite systems were heated to temperatures of
2000.degree. C. (3632.degree. F.) with 34.5 MPa (5 ksi) of pressure
for soak times of 1 hour in order to densify the FM composites.
Under these conditions, HfC combined with the cell boundary
materials (hBN, graphite and W3.6Re) to roughly 90% of full
theoretical density. Table 7 summarizes these results.
TABLE-US-00007 TABLE 7 Theoretical Measured % Full Density Density
Theoretical HfC System Architecture (g/cc) (g/cc) Density HfC/BN/
Honeycomb 11.17 10.69 95.70 HFC (82.5/17.5)/17.5 HfC/G/HfC
Honeycomb 11.1 10.4 93.69 (82.5/17.5)/17.5 HfC/W3.6Re Bi-layer
13.84 13.12 93.58 82.5/17.5 HfC/W3.6Re Bi-layer 13.336 12.62 92.70
90/10 HfC/W3.6Re Bi-layer 16.00 13.76 86 50/50 G = graphite.
[0065] To increase the composite density, the consolidation
temperature was raised to 2200.degree. C., while maintaining the
previous pressure and soak time. The results for the HfC/BN/HfC
test coupons are presented in Table 8. TABLE-US-00008 TABLE 8
Theoretical Measured % Full Density Density Theoretical HfC System
Architecture (g/cc) (g/cc) Density HfC/BN/ Honeycomb 11.261 11.259
99.98 HFC (82.5/17.5)/17.5 HfC/BN/ Honeycomb 11.261 11.264 100 HFC
(82.5/17.5)/17.5 HfC/BN/ Honeycomb 11.382 11.056 97.14 HFC
(82.5/17.5)/25
Example 4
[0066] Various TaC-based FM composites were prepared and sintered
using a uni-axial hot press. A 1''.times.3''.times.0.25" thick
billet of TaC/W3.6Re was consolidated to 99% theoretical density at
1900.degree. C. (3452.degree. F.) under 27.6 MPa (4 ksi) of
pressure. The maximum temperature used to consolidate any TaC-based
FM composite was 1950.degree. C. (3542.degree. F.) and 34.4 MPa (5
ksi) pressure.
[0067] TaC contains 20 wt % HfC. Because TaC-HfC solid solutions
(e.g. 80% TaC-20% HfC) have high melting temperatures and even
better oxidation resistance than Ta carbides or Hf carbides alone,
HfC was added to the TaC. The success in densifying the TaC-based
systems may be a result of the HfC additions that may be acting as
a sintering aid.
[0068] Mechanical properties of the TaC-based composites are
presented in Table 9. The results of these densification
experiments are summarized in Table 10. TABLE-US-00009 TABLE 9
Theoretical Measured Fracture EMOD EMOD Stress FM System
Architecture (GPa) (GPa) (MPa) TaC(HfC)/ Honeycomb 318.15 370.6
.+-. 16.9 291.3 .+-. 16.6 BN/TaC(HfC) (82.5/17.5)/ 17.5 TaC(HfC)/
Honeycomb 307.61 340.6 .+-. 14.5 257.5 .+-. 61.7 G/TaC(HfC)
(82.5/17.5)/ 17.5 TaC(SiC)/ Honeycomb 318 378.1 .+-. 4.0 400.7 .+-.
28.9 BN/TaC(SiC) (82.5/17.5)/ 17.5 TaC/G/TaC 3-layer 270.75 311.3
180.8 .+-. 24.2 (50/25/25) TaC/W3.6Re Bi-layer 367.6 475 .+-. 6.9
302.7 .+-. 45.9 82.5/17.5 TaC/W3.6Re Bi-layer 363.5 497.7 .+-. 27.1
440.6 .+-. 38.4 90/10
[0069] TABLE-US-00010 TABLE 10 Theoretical Measured % Full Density
Density Theoretical FM System Architecture (g/cc) (g/cc) Density
TaC(HfC)/ Honeycomb 12.02 11.749 .+-. 0.128 97.75 BN/ (82.5/17.5)/
TaC(HfC) 17.5 TaC(HfC)/G/ Honeycomb 11.96 11.67 .+-. 0.146 97.57
TaC(HfC) (82.5/17.5)/ 17.5 TaC(SiC)/ Honeycomb 11.927 11.035 .+-.
0.049 92.52 BN/ (82.5/17.5)/ TaC(SiC) 17.5 TaC/G/TaC 3-layer 10.7
10.267 .+-. 0.107 95.95 (50/25/25) TaC/W3.6Re Bi-layer 14.57 14.426
.+-. 0.011 99 82.5/17.5 TaC/W3.6Re Bi-layer 14.14 13.776 .+-. 0.737
97.43 90/10 G = graphite
[0070] Three test coupons were prepared using TaC without any
sintering aids to evaluate if a TaC/BN/TaC FM composite system can
be densified without sintering aids. The consolidation temperature
also was increased to 2200.degree. C., with pressures and soak
times remaining constant. The results for these coupons are
presented in Table 11. TABLE-US-00011 TABLE 11 Theoretical Measured
% Full Density Density Theoretical FM System Architecture (g/cc)
(g/cc) Density TaC/BN/TaC Honeycomb 12.218 11.910 97.48
(82.5/17.5)/17.5 TaC/BN/TaC Honeycomb 12.371 12.192 98.55
(82.5/17.5)/25 TaC/BN/TaC Honeycomb 12.493 12.250 98.05
(82.5/17.5)/31
Example 5
[0071] Various ZrC-based FM composites were prepared and sintered
at 1900.degree. C. to 1950.degree. C. at 27.6 MPa to 30.3 MPa (4 to
4.4 ksi). Mechanical properties of the TaC-based composites are
presented in Table 12. The results of these densification
experiments are summarized in Table 13. TABLE-US-00012 TABLE 12
Theoretical Measured % Full Density Density Theoretical FM System
Architecture (g/cc) (g/cc) Density ZrC/BN/ZrC Honeycomb 5.886 5.123
.+-. 0.062 87.03 (82.5/17.5)/ 17.5 ZrC/G/ZrC Honeycomb 5.69 5.333
.+-. 0.01 93.73 (82.5/17.5)/ 17.5 ZrC/BN/ZrC 3-layer 5.94 5.348
.+-. 0.104 90.03 (80/10/10) ZrC/G/ZrC 3-layer 5.895 5.671 .+-.
0.031 96.2 (80/10/10) ZrC/G/ZrC 3-layer 5.213 5.18 .+-. 0.03 99.37
(50/25/25) ZrC/W3.6Re Bi-layer 8.622 8.183 .+-. 0.086 94.91
82.5/17.5 ZrC/W3.6Re Bi-layer 7.648 7.343 .+-. 0.082 96.01 90/10 G
= graphite
[0072] TABLE-US-00013 TABLE 13 Theoreti- cal Measured Fracture EMOD
EMOD Stress FM System Architecture (GPa) (GPa) (MPa) ZrC/BN/ZrC
Honeycomb 356.66 259.9 .+-. 9 213.00 .+-. 32.98 (82.5/17.5)/ 17.5
ZrC/G/ZrC Honeycomb 346.12 329 .+-. 4.9 249.83 .+-. 28.69
(82.5/17.5)/ 17.5 ZrC/BN/ZrC 3-layer 370.9 236.9 .+-. 19.6 297.5
.+-. 29.3 (80/10/10) ZrC/G/ZrC 3-layer 363.6 294.5 .+-. 8.6 288.5
.+-. 54.3 (80/10/10) ZrC/G/ZrC 3-layer 304.5 267.4 .+-. 8.3 248.9
.+-. 13.7 (50/25/25) ZrC/W3.6Re Bi-layer 404.75 420.10.6 118.2 .+-.
28.4 82.5/17.5 ZrC/W3.6Re Bi-layer 404 442.7 .+-. 5.1 243.38.3
90/10
[0073] Although the consolidation parameters were fairly
consistent, the resulting degree of consolidation varied between FM
composite systems depending on the cell boundary material.
Surprisingly, the ZrC FMs containing BN as an interface material
did not consolidate as well as those containing graphite. FM
composites with a graphite interface have in the past proven more
difficult to densify as a result of graphite being very difficult
to sinter. To improve densification, SiC at varying ratios may be
used as a sintering aid for ZrC.
[0074] To increase the composite density, the consolidation
temperature was raised to 2200.degree. C. The results for the
ZrC/BN/ZrC test coupons are presented in Table 14. TABLE-US-00014
TABLE 14 Theoretical Measured % Full Density Density Theoretical FM
System Architecture (g/cc) (g/cc) Density ZrC/BN/ZrC Honeycomb
6.176 6.154 99.64 (82.5/17.5)/25 ZrC/BN/ZrC Honeycomb 6.124 6.183
100 (82.5/17.5)/17.5 ZrC/BN/ZrC Honeycomb 6.124 6.015 98.22
(82.5/17.5)/17.5 ZrC/BN/ZrC Honeycomb 6.124 5.785 94.47
(82.5/17.5)/17.5
Example 6
[0075] This example illustrates how fibrous monolith architecture
protects materials from thermal shock at high temperatures. Various
ZrB.sub.2-based fibrous monolithic ceramics were prepared.
ZrB.sub.2 has a high melting point of about 3000.degree. C. and
exhibits excellent oxidation resistance, creep resistance at
elevated temperatures, and moderate thermal stress resistance for a
monolithic ceramic. To improve the thermal stress resistance of
ZrB.sub.2, the material was prepared as a fibrous monolith with
ZrB.sub.2 as the cell (primary) material of the fibrous monolith
composite structure. The cell boundary (secondary) phase in the
fibrous monolith structure consisted of boron nitride (BN), a low
shear strength material with a melting point of 3000.degree. C. The
low shear strength BN accommodates for expansions and contractions
during thermal cycling of the component, resulting in improved
thermal shock resistance. Furthermore, a zirconia or hafnia scale
that grew from the carbide cell phase at high temperatures
protected the BN from oxidation. The BN or graphite cell boundaries
enables non-catastrophic failure due to stress delocalization and
crack deflection mechanisms.
[0076] Tubes of ZrB.sub.2/BN and HfB.sub.2/BN for use in solar
thermal propulsion applications were prepared and exposed to
temperatures up to 3000.degree. C. Several of these tubes were
tested in an Arc-Lamp test facility under flowing nitrogen gas at a
temperature of 2500.degree. C. The tubes underwent severe thermal
shock for several cycles at a maximum rate of 1000.degree.
C./second with no visible sign of degradation to their structure
after testing. The BN interlayer surrounding the ZrB2 cells
rendered the material insensitive to thermal shock.
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