Note: Descriptions are shown in the official language in which they were submitted.
20~22~40
5196L 13DV-9231
FIBER REINFORCED CERAMIC MATRIX COMPOSITE
MEMBER AND METHOD FOR MAKING
This invention relates to ceramic composite members
and method for making, and, more particularly in one
form, to ceramic fiber reinforced ceramic matrix
composite members.
CROSS REFERENCE TO RELATED APPLICATION
This application relates to co-pending Canadian
application Serial No. 2,009,595 filed February 8, 1990
entitled °'Consolidated Member and Method and Preform
for Making."
BACKGROUND OF THE INVENTION
Use of ceramics in the form of high temperature
operating articles, such as components for power
generating apparatus including automotive engines,
gas turbines, etc., is attractive based on the light
weight and strength at high temperatures of certain
ceramics. One typical component is a gas turbine
engine strut. However, monolithic ceramic structures,
B
5196L 13DV-9231
- 2 - ~~.~rr~~r
without reinforcement, are brittle. Without
assistance from additional incorporated, reinforcing
structures, such members may not meet reliability
requirements for such strenuous use.
In an attempt to overcome that deficiency, certain
fracture resistant ceramic matrix composites have been
reported. These have incorporated fibers of various
size and types, for example long fibers or filaments,
short or chopped fibers, whiskers, etc. All of these
types are referred to for simplicity herein as
"fibers". Some fibers have been coated with certain
materials which have been applied to prevent strong
reactions from occurring between the reinforcement and
matrix. However, some coatings are of carbon, or
forms of carbon, or other material which will oxidize
if exposed to air at an intended elevated operating
temperature. Inclusion of such fibers within the
ceramic matrix was made to resist brittle fracture
behavior.
One problem with the use of such oxidizing fibers,
such as carbon, as reinforcement in ceramic composites
is that the system can become environmentally unstable
in use: cracks in the ceramic matrix, even
microcracks, can make the oxidizable fiber available
to contact with oxygen in air at elevated operating
temperatures experienced in the hot sections of power
producing engines. Such oxidation of reinforcing
fibers weakens or destroys the fiber structure or its
function, leading to unacceptable weakening of the
structural member.
5196L 13DV-9231
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Another problem relates to the fact that high
sintering temperatures for ceramic particles about
reinforcing fibers limit the kind of fibers which can
be used. For example, many fibers deteriorate above
about 1000°C, well below required ceramic particle
sintering temperatures.
SUMMARY OF THE INVENTION
Briefly, in one form, the present invention
provides a method for making an environmentally
stable, fiber reinforced ceramic matrix composite
member comprising oxidation stable reinforcing fibers,
for example ceramic fibers, and a matrix interspersed
about the fibers. As used herein, "oxidation stable"
in respect to fibers means fibers which substantially
will not experience substantial oxidation and/or
environmental degradation, at intended operating
conditions of temperature and atmosphere. such as
air.- The matrix is a mixture including ceramic
particles bonded together with a ceramic phase.
In the method form, the present invention provides
a ceramic matrix precursor, which transforms upon
heating to a ceramic phase, mixed in a substantially
uniform distribution in a matrix mixture slurry of
discontinuous material comprising ceramic particles in
a liquid compatible with the precursor. This slurry
is interspersed about the oxidation stable fibers, as
a matrix mixture, to provide a prepreg preform which
is heated in an oxidizing atmosphere, such as air, at
a processing temperature, at least at the temperature
required to transform the precursor to a ceramic phase
and less than that which will result in degradation of
ceramic in the preform. Through the present
invention, such
5196L 13DV-9231
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temperature can be in the range of about 600-1000°C.
Such heating transforms the ceramic precursor, such as
by decomposition, to a ceramic phase, for example of
amorphous or crystalline form, which bonds together
the ceramic particles from the slurry into a ceramic
matrix about the fibers. Because components of this
reinforced, ceramic matrix composite member are
stabilized in an oxidizing atmosphere, preferably
being substantially all ceramic oxides bonded
together, the member is environmentally stable, and of
high strength and high resistance to fracture.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a graphical comparison of fracture
resistance data for an unreinforced matrix, another
reinforced matrix and a composite. reinforced member
of the present invention.
Figure 2 is a fragmentary, sectional perspective
view of a portion of a gas turbine engine strut.
Figure 3 is a fragmentary, diagrammatic sectional
view of plies of ceramic matrix composite disposed
about forming blocks.
Figure 9 is a fragmentary, sectional perspective
view of the member of Figure 3 disposed in forming die
portions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fracture resistant, fiber reinforced ceramic
matrix composites offer the designers of high
temperature components for power generating engines,
5196L 13DV-9231
_5_ ~~...r~~~~
such as components for automotive engines, turbine
engines, etc., an opportunity to specify strong,
lightweight members. However, certain of such known
composites are environmentally unstable upon the
occurrence of cracks which expose oxidizable portions
to air. In addition, certain known processing results
in an undesirable level of porosity in the product.
Also, the kind of fibers which can be included in
sintered ceramic reinforced composites has been
limited based on the relatively high sintering
temperatures required and a fiber deterioration
temperature lower than the required sintering
temperature.
The present invention provides an improved method
for avoiding such known problems and for making an
environmentally stable reinforced ceramic matriz
composite member of high strength and high fracture
resistance at lower processing temperatures. A
principal basis for the invention is providing
ingredients which can be stabilized at a lower
temperature; and, after a stabilizing heating, one
product is a member preferably having substantially
all ceramic oxides bonded together. Use of such
ingredients eliminates the potential for member
deterioration in use due to oxidation.
Typical of the ceramic particles used for ceramic
matricies are the oxides of such elements as A1, Si,
Ca, Hf, B, Ti, Y and Zr, and their mixtures and
combinations. Such commercially available materials
include A1203, Si02, CaO, Zr02, Hf02, BN, Ti02,
3A1203~2Si02, Y203 Ca0~A1203 and various clays and
glass frits. Ceramic particle sizes in the range
between about 75 microns to 0.2 micron in diameter
519 6L 1,~3D~V~-9~2~31~
-6-
have been tested as a matrix ingredient in the
evaluation of the present invention. One form of the
present invention addresses the fact that each of such
ceramics, when used as a structure, will shrink when
fired to an elevated consolidating temperature. For
example, a form of alumina will experience a linear
shrinkage in the range of about 3-4% at 1400°C.
Evaluated in connection with the present invention
were a variety of ceramic precursors, which can be
used as a matrix precursor as well as an infiltrant
precursor, as described later herein. Use of such a
precursor as a bonding agent in combination with the
ceramic particles enables generation of a stable
composite at a significantly lower processing
temperature. Such precursors which transform, for
example by decomposition upon heating, to a ceramic
phase, can be in solid or liquid form, or their
mixtures, for practice of portions of the present
invention. Generally they are classified as
organometallics, sol gels or metal salts. Included in
the evaluation were the following ceramic precursors:
polycarbosilanes, silicones, metal salts (including
vinylic polysilane, dimethyl siloxane, and hafnium
oxychloride), silica and alumina sols, aluminum
isopropoxide, mono aluminum phosphate and other
phosphates. The following Table I identifies specific
forms of such precursors.
-7- 13DV-9231
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5196L 13DV-9231
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According to the method of the present invention,
discontinuous material, comprising the ceramic
particles, ceramic precursor, and optionally a binder,
are dispersed in a liquid to provide a matrix mixture
slurry. As used herein, the term "discontinuous
material" is intended to mean powder, particles, small
fragments, flakes of material, whiskers, etc. A
characteristic of the liquid of the slurry is that it
be compatible with, and preferably a solvent for, the
ceramic precursor, and for the binder if one is used.
This allows a substantially uniform distribution of
the precursor in the slurry, along with the ceramic
particles and optional binders to provide the matrix
mixture. For example, the liquid can be aqueous or it
can be organic, depending upon the precursor or
mixture of precursors, and optional binder. As was
stated, preferably the precursor will dissolve in the
liquid, which in that case acts as a solvent. Typical
organic liquids used as solvents include ethyl
alcohol, trichlorethane, methyl alcohol, toluene and
methyl ethyl ketone which allow the binders, polymers
and/or infiltrants to dissolve into a solution. The
quantity of solvent required depends upon the
solubility and saturation limit of the
binders/polymers and the desired viscosity of the
slurry. The preferred limits range from 20-30 wt
solvent. Additional solvent quantities will only
induce prolonged drying times to evaporate the excess
solvents.
In respect to the ceramic particles in the slurry,
it has been recognized that such particles should be
included in the range of greater than 40 wt% up to
about 90 wt% of-the sum of ceramic particles and
precursor. At 40 wt% or less, there is insufficient
5196L 13DV-9231
ceramic to provide a matrix about reinforcing fibers
in the composite member and results in too much
porosity; at greater than about 90 wt$. there is
insufficient bonding, by the transformed precursor's
ceramic phase, of the ceramic particles about the
reinforcing fibers. The preferred range for the
ceramic particles in that sum of particles and
precursor is 50-80 wt%, and more specifically about
70-80 wt%.
5196L 13DV-9231
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4~~~
In the matrix mixture slurry, it has been
recognized that the ceramic precursor should be
included in the range of about 10-40 wt% of the sum of
precursor and ceramic particles, preferably 10-30 wt%,
to provide adequate flow and bonding. Less than about
wt% provides insufficient ceramic phase for flow
and bonding together the ceramic particles after
precursor decomposition heating; at greater than about
40 wt%, decomposition of the precursor results in
excessive porosity in the matrix phase.
The balance of the slurry generally is the
liquid. However, such other materials as binders and
plasticizers, herein generally called "binders", used
temporarily to hold an uncured matrix together, can be
included in the slurry. Binders to hold the preform
together prior to heating at the processing
temperature can be included up to about 20 wt% of the
sum of ceramic particles, precursor and binder.
Greater than that will result in too much porosity.
Examples of such binders and of plasticizers evaluated
(and one commercial source) are Prestoline Master Mix
(P.B.S. Chemical), cellulose ether (Dow Chemical),
polyvinyl butyral (Monsanto) butyl benzyl phthalate
(Monsanto), polyalkylene glycol (Union Carbide) and
polyethylene glycol (Union Carbide). Binding systems
also used were epoxy resins, for example general
purpose epoxy resin manufactured by Ciba-Geigy,
silicones, for example polysiloxane (GE), RTV (GE) and
polycarbosilane (Union Carbide). Included as required
were dispersants such as glycerol trioleate, marine
oil, adipate polyester, sodium polyacrylate and
phosphate ester. When epoxy resin was used as a
binding system with the above preferred precursor and
5196L 0 ~ G G. ~ ~ 13DV-9231
- 11 r,
ceramic ranges, the epoxy was about 1-10 wt~, in
respect to the mixture of precursor and ceramic
particles.
Evaluated in connection with the present invention
were a variety of ceramic reinforcement fibers
including those shown in the following Table II, along
with each of their coefficients of thermal expansion
( CTE )
TABLE II
REINFORCEMENT FIBERS
TYPE CTE x 10-6 per °C
A. MONOFILAMENTS
Sapphire 7-9
Avco SCS-6TM 4.8
Sigma 4.8
B. ROVINGS/YARN
Nextel~ 440 4.4
Nextel 480 4.4
Sumitomo~ 8.8
DuPont FP 7.0
DuPont PRD-166 9.0
UBE 3.1
Nicolon 3.1
Carbon 0
C. CHOPPED FIBERS/WHISKERS
Nextel 440 4.4
SaffilTM 8.0
In an Example 1 evaluated in connection with the
present invention, the matrix mixture slurry included
A1203 particles in the size range of about 0.2 - 50
B
5196L 13DV-9231
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microns as the ceramic particles, a silicone
commercially available as RTV as the ceramic
precursor, and an epoxy resin marketed as bisphenol as
the binder. In this typical mixture, by weight, A1203
was 70-80%. silicone was 10-30% and epo$y was 1-10% of
the sum of A12~3, silicone and binder. With this
mixture was the combination solvent trichloroethane
and ethanol as the liquid in the amount of about 20-30
wt%, the balance, 70-80 wt%. being the above mixture
of ceramic, precursor, and binder to provide the
matrix mixture slurry.
In an Example 2, a combination of ceramic
precursors were included. Such mixture included, by
weight, 70-80% A1203 as the ceramic particles, 5-15%
silicone and 5-15% aluminum isopropoxide as the
ceramic precursors. and, as the binder, epoxy in the
amount of 1-10% of the sum of ceramic particles,
precursor and binder. With this mixture w.as the
combination solvent trichloroethane and ethanol as the
liquid in the amount of about 20-30 wt%, the balance,
70-80 wt%. being the mixture of ceramic, precursors,
and binder to provide the matrix mixture slurry.
In one form of the method of the present
invention, each of the matrix mixture slurries of
Examples 1 and 2 above was interspersed about
reinforcing ceramic fibers in the form of a fabric.
In these examples, the reinforcing fibers were made of
the Sumitomo yarn or rovings identified above,
included in the range of 20 - 40 volume % of the
member. In other forms and examples, the reinforcing
ceramic fibers were filament wound. In the present
invention, it has been recognized that the reinforcing
fibers be included in the range of about 10-50 vol% of
5196L 13DV-9231
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the member, and preferably 30-90 vol$. Less than 10
vol% provides insufficient reinforcement strength, and
at greater than about 50 vol% the fibers are spaced
too closely for the disposition about them of adequate
matriz.
After allowing this prepeg to dry, to enable the
majority of the solvent to be evaporated, the prepreg
plies thus created were shaped and molded into a
member, such as through use of compression molds, or
an autoclave, to apply temperature and pressure, as is
well known and practiced in the art. Thereafter, the
member was cooled into a solid preform shape.
The preform was then heated at a processing
temperature in the range of 600° - 1000°C rather than
at the generally much higher sintering temperature
used in known methods, for ezample in the range of
about 1300 - 1650°C. This heating is conducted to
remove organics such as the temporary binder and to
transform through decomposition, the ceramic precursor
into a ceramic bonding phase or phases. Through
practice of the present invention of including a
bonding precursor with the ceramic particles, the
processing temperature can be maintained in a range
much lower than that required to sinter together
ceramic particles about reinforcement fibers. Also,
it enables use of fibers which otherwise would be
degraded or thermochemically damaged at the known,
higher sintering temperatures.
In the above Examples 1 and 2, heating at the
processing temperature was conducted in the range of
about 600 - 800°C. Such heating results in a ceramic
matrix of ceramic particles bonded together through a
519 6L ~1
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ceramic phase or phases. Generally the matrix has an
open gorosity in the range of about 5-30 vol%.
The present invention, in another form, includes
additional steps for reducing or eliminating such
porosity. In such form, additional ceramic precursor
in liquid form, or dispersed in a liquid generally in
high concentration, is applied to the above described
ceramic matrix and infiltrated into the porosity. For
example, the matrix can be immersed in the liquid
ceramic precursor infiltrant and a vacuum applied to
facilitate precursor penetration into the pores.
After drying, the infiltrated matrix is heated, as
described above, to transform the infiltrant ceramic
precursor into a ceramic phase or phases thereby
eliminating certain porosity. Such pore infiltration
and transformation heating can be repeated, as
desired, to reduce or eliminate porosity from the
matrix to a desired level.
The graphical comparison of Figure 1 is a stress
vs. strain curve which shows the fracture resistance
and toughness of the member made according to the
present invention. The data in this Figure 1 were
obtained by testing at room temperature. The
specimens used were 0.5" x 6" x 0.1" rectangular test
bars.
The data represented by curve 1 was from testing
of a specimen made from the mixture of the above
Example l, as described, without dispersing the slurry
about reinforcing fibers. The material in curve 1 is
a monolithic matrix of ceramic particles, ceramic
precursor and epoxy binder which is low in strength
and fails catastrophically in a brittle manner.
5196L 13DV-9231
_15_ ~~~.2r~~~
Monolithic ceramics of this type are not viable
candidates for critical shapes in structural
applications due to their intolerance to defects and
subsequent low toughness.
The data represented by curve 2 of Figure 1 was
from testing of a specimen of the same size and shape
as that used for curve 1 data, made from that same
mixture. However, the mixture was interspersed about
a reinforcing fiber fabric of Sumitomo yarn included
at about 30 vol% of the member. The material in curve
2 is a ceramic composite in which the same monolithic
matrix material in curve 1 has been incorporated
throughout and around the fiber reinforcements. The
material has high strength because the load is now
transferred to the high strength fibers and the
material exhibits graceful fracture and toughness.
This type of composite behavior allows a part to have
extended life after the initial onset of fracture.
As can be seen from Figure 1, the reinforced
ceramic matrix composite member of curve 2 is
significantly stronger and tougher than that of curve
1.
Included for comparison in Figure 1 is a curve 3
representing use of saphhire reinforcing fibers in a
matrix of A1203 and sintered at about 1450-1500°C,
well above the temperature capability of the fibers
identified in Table II. No precursor was included in
such a composite, which was 55 vol.% aluminosilicate
and 45 vol % sapphire fibers. Accordingly, this
mixture necessitated use of the sintering,
consolidation temperature significantly higher than
the processing temperature used in the method of the
5196L 13DV-9231
_16_
invention, generally about 600° - 1000°C. The
material in curve 3 exhibits higher strength than
curve 2 with tough behavior. These improved
properties represent the benefits of using a higher
strength reinforcing fiber with a thermally computable
matrix to enable load transfer from the matrix to the
fiber in an efficient manner.
As can be seen from the comparison of curve 2,
representing the present invention, and curve 3,
representing a member made by a different method, the
present invention provides a high strength, tough,
reinforced ceramic composite made without ultra high
temperature consolidation processing. This occurs
through use, in the present invention, of a ceramic
precursor which decomposes at a lower temperature to a
ceramic phase which bonds together the ceramic
particles and reinforcing fibers into a composite
member.
Typical of members which can be made according to
the present invention is an airfoil shaped strut,
useful in a gas turbine engine hot section, and shown
in the fragmentary, sectional perspective view of
Figure 2. The strut, shown generally at 10, includes
a strut body 12 having leading edge 14 and trailing
edge 16. Strut 10 is sometimes referred to as a
hollow strut because of the presence of a plurability
of cavities 18 therein separated by ribs 20.
Strut 10 can be made by groviding a plurality of
plies such as laminations, sheets, tape, etc., made as
described above. The fragmentary sectional view of
Figure 3 is diagrammatic and representative of
disposition of such plies, identified at 22, about
forming blocks 24, such as of aluminum, as an initial
5196L 13DV-9231
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formation of the preform configuration of a portion of
the strut of Figure 2 in relation to the shape of that
finished strut. In reality, each ply for this member
will have a thickness dependent on fiber and form, as
is well known in the art. For example, typical
thicknesses are in the range of about 0.008-0.020
inches. However, as is well known in the art, the
number of plies actually required to provide such a
laminated structure would be many more than those
presented for simplicity in Figure 3. Additional
individual fibers 25 are disposed between plies within
potential spaces between plies at the edge curvature
regions of blocks 24 to reduce voids.
After formation of the member of Figure 3
assembled about forming blocks 24, the assembly is
placed within appropriately shaped, mating forming
dies 26A and 26B in Figure 4 for the purpose of
laminating the member into an article preform.
Typically, a pressure, represented by arrows 28, in
the range of about 150-1000 pounds per square inch, is
applied to the member while it is heated, for ezample
in the range of 150-400°F, for a time adequate to
allow proper lamination to occur. Such a temperature
is not adequate to enable consolidation of the
materials of construction to occur.
After lamination, the preform thus provided is
removed from the forming dies and the forming blocks
are removed. The preform then is placed in a furnace,
and heated to a temperature below 1000°C in a
controlled manner to remove binders and plasticizers,
and then to a processing temperature at which no
degradation of fibers occurs, such as 1000°C or above
to sinter the preform into a substantially dense
ceramic matrix composite article of Figure 2.
20 ~I 2 2~4 0
5196L 13DV-9231
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The cross referenced related Canadian application
Serial No. 2,009,595 addresses the problem of shrinkage
of consolidated ceramic particles and resultant
porosity. According to the invention of such cross
referenced application, such shrinkage is counteracted
by mixing with the ceramic particles, prior to
consolidation, particles of an inorganic filler which
will exhibit net expansion relative to the ceramic
particles during heating to the consolidation
temperature. Tested in the evaluation of that
invention are the inorganic filler materials, of lathy-
type crystal shape, and identified in the following
Table III.
TABLE III
FILLER MATERIALS
IDENTIFICATION LATHY-TYPE
MINERALOGICAL CRYSTAL
NAME COMPOSITION SHAPE
Pyrophyllite A1203 ~ 4Si02 ~ H20 laminar
Wollastonite Ca0~Si02 bladed/elongated with
circular crystals
Mica KZO ~ 3A1203 ~ 6Si02 ~ 2H20 plate-like
Talc 3Mg0~4Si02~H20 flat flake
Montmorillonite (Al,Fe,Mg)OZ~4SiO2~H20 elongated
Kyanite 3A1203 ~ 3Si02 bladed/elongated
,, ,
p12~~p
5196L 13DV-9231
- 19
Such filler materials can be used in one form of the
present invention to counteract porosity created during
heating at the processing temperature. The proportion of the
filler in that above mixture is selected so that expansion of
' the filler counteracts such porosity however generated.
When the inorganic filler of the related application is
included in the matrix mixture of the present invention of
ceramic particles and precursor, and optional. binder, such filler
can be included in an amount, for exartg~le up to about 50 wt ~
of the sum of ceramic, precursor, optional binder and filler.
The proportion a.s selected so that expansion of the filler
counteracts porosity. Such porosity could result frown shrinkage
of the ceramic particles but primarily occurs at the lower
processing temperature frcan transformation or volume change of
materials during heating of the preform of the present invention
in an oxidizing atmosphere, as has been described herein.
Typically, the porosity control mixture of ceramic particles
and filler will be, by weight, 50-93~ ceramic particles and
7-50~ inorganic filler, with the porosity control mixture
representing, by weight, greater than 40~ up to about 90~ of
the matrix mixture of particles, precursor and optiar~al. binder.
Preferred as inorganic filler materials are those shown in the
above Table III, and having a lathy-type crystal shape. In
particular, pyrophyllite and wollastonite have been found to be
especially useful as fillers. Also, as described in the
disclosure of Canadian application No. 2,009,595, reinforcing
fibers which will expand relative to the matrix mixture enhance
the capability of processing the preform at ambient pressure.
B
fizz
t
5196L 13DV-9231
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The present invention has been described in
connection with typical, though not limiting, examples
and embodiments, and their related data. However,
those skilled in the art will readily recognize that
the present invention is capable of a variety of
modifications and variations within the scope of the
appended claims.
