Note: Descriptions are shown in the official language in which they were submitted.
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A COMPOSITE MATERIAL OF THE CARBON/CARBON TYPE HAVING
INCREASED RESISTANCE TO OXIDATION
Field of the invention
The present invention relates to improving the
oxidation resistance of carbon/carbon (C/C) type
composite materials, i.e. composite materials comprising
fiber reinforcement made of carbon fibers or fibers that
are coated in carbon, embedded in a matrix that is
constituted by carbon, at least in the vicinity of the
fibers.
The field of application of the invention is more
particularly, but not exclusively, that of improving the
resistance to oxidation of C/C type composite materials
whose utilization temperature does not exceed about
1000°C and in which the carbon both of the fibers and of
the matrix is exposed at the surface. This can arise
either because surface protection against oxidation has
disappeared, e.g. after machining or wear in service, or
else because of a deliberate gap in surface anti-
oxidation protection, as occurs, for example, in the
friction surfaces of C/C type composite material brake
disks.
Background of the invention
C/C type composite materials are remarkable for
their thermostructural properties, i.e. the mechanical
properties which make them suitable for constituting
structural elements and their ability at retaining such
mechanical properties up to high temperatures.
Nevertheless, when such materials are placed in an
aggressive atmosphere in use, they are liable to be
degraded by corrosion attacking the reinforcing fibers or
the matrix.
In practice, one kind of corrosion that gives rise
to problems that are particularly severe is the action of
oxygen or the combined action of oxygen and water, which
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occurs when C/C type materials are placed at high
temperature in the presence of air, of moisture, rain,
... . It is therefore the practice to provide C/C type
materials with protection.
There exists very abundant state of the art
concerning anti-oxidation protection for composite
materials containing carbon.
Known methods rely in particular on forming external
and/or internal protection having healing properties so
as to plug, fill, or seal any cracks that appear in the
material. Typically, such protection comprises a
compound of boron and/or a compound of silicon for
forming a borosilicate glass, or a glass based on boron
oxide (B~03) or based on silica (Si02) . The protective
composition is chosen so as to provide a glass which has
viscous behavior at the utilization temperature of the
material so as to perform the healing function.
Reference can be made to document US-A-5 246 736
which mentions quite detailed state of the art and which
describes the formation of a continuous phase constituted
by a ternary Si-B-C system on the surface of a composite
material to be protected, or within the matrix thereof.
Those known methods rely on forming protection
which, in spite of the material cracking, serves to
isolate the carbon of the fibers or of the matrix of the
material in sealed manner from the surrounding medium.
This no longer applies when, whether accidentally or
deliberately, the carbon of the fibers or of the matrix
is exposed at the surface of the material.
In particular, for friction elements such as C/C
composite material brake disks, an anti-oxidation
protective coating can be formed selectively on the non-
friction faces, e.g. as described in document
US 5 686 144, but not on the friction faces. Such a
protective coating on the friction faces would spoil the
friction characteristics of the material and would in any
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case be removed very quickly because of the wear due to
the friction.
In present utilizations, in particular for aircraft
brakes, C/C composite friction elements are exposed to
conditions that favor wear by oxidation: high operating
temperature and the presence of aggravating circumstances
such as moisture and the influence of oxidation catalysts
such as the substances used for deicing runways.
Brief summary of the invention
Thus, an object of the present invention is to
increase the oxidation resistance of C/C type composite
materials, particularly when the carbon of the fibers in
the fiber reinforcement and of the~matrix is exposed to
an oxidizing atmosphere, whether the exposure is
accidental or otherwise.
This object is achieved by the fact that an
interphase is present between the fibers of the fiber
reinforcement and the matrix, said interphase being
constituted by 1% to 20% atomic percent of silicon, at
least 30% atomic percent of boron, and any balance being
of carbon and/or of phosphorous. The silicon could be
replaced in full or in part by germanium.
The inventors have shown that when the carbon of the
fibers and of the matrix of a C/C type composite material
is exposed to oxidizing conditions, oxidation occurs
preferentially at the interface between the fiber
reinforcement and the matrix. This results in a loss of
bonding between the fibers and the matrix which severely
degrades the characteristics of the composite material
even when the loss of mass due to oxidation is small.
This loss of bonding which causes the material in the
vicinity of its surface to lose its composite nature also
gives rise to excessive wear when friction occurs.
Thus, for an aircraft brake disk made of C/C
composite material, oxidation takes place after landing
while the material remains hot, thereby degrading the
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structure of the material on its surface, and this
degraded portion shows much less resistance to friction
forces next time the brake is used. As a result, it has
been observed that C/C composite material disks suffer
more wear while taxiing cold after landing itself, than
they do during landing even though the disk is subjected
to very high levels of stress at that time.
As shown below, forming an interphase that contains
silicon, boron, and optionally carbon and/or phosphorous,
makes it possible to oppose effectively any attack on the
fiber-matrix interface, providing the interphase is
relatively rich in boron. The composition of the
interphase is preferably selected so that the increase in
volume that results from the formation of the oxides B203,
Si02 (and/or Ge02) , and optionally P205, compensates
substantially for the volume of carbon consumed by
oxidation at the utilization temperatures. The presence
of phosphorous facilitates providing protection against
oxidation at lower temperatures and thus extends the
effectiveness of the interphase to the range where carbon
oxidation commences. Typically the interphase is
constituted by 5o to loo atomic percent of silicon and/or
germanium, 50o to 70o atomic percent of boron, and loo to
30o atomic percent of carbon and/or phosphorous.
The interphase constitutes a thin layer and not a
phase of the matrix. Its thickness is thus less than
1 um, and is preferably not more than 300 nm. Also
preferably, this thickness is not less than 10 nm. A
refractory layer that does not contain boron, e.g. a
refractory carbide such as silicon carbide, zirconium
carbide, or hafnium carbide, can be interposed between
the fibers of the fiber reinforcement and the interphase.
The refractory layer serves to prevent boron diffusing
into the carbon of the fibers or the carbon coating the
fibers of the fiber reinforcement, since that would have
the effect of affecting the mechanical properties of the
reinforcement.
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The invention is remarkable in that in the expected
range (utilization temperature up to 1000°C), a C/C
composite material is provided with protection against
oxidation by a thin interphase layer between the fibers
5 and the matrix, whereas the usual methods of protecting
C/C composite materials require an external protective
coating to be formed, or at least require protective
barriers to be incorporated within the matrix.
The field of application of the invention is not
limited to the case where the carbon of the fibers in the
fiber reinforcement and of the matrix is exposed to the
surrounding medium, either deliberately or following
normal wear, but also covers the case where such exposure
can be accidental. Under such circumstances, it is
possible to provide the material with further protection
against oxidation by external protection and/or internal
protection incorporated in the matrix. Such protection
can be provided by layers having healing properties and
formed by compounds of boron and/or of silicon, such as
those in the above-mentioned prior art, or layers based
on phosphates, such as those described in documents
EP-A-0 619 801 and EP A-0 789 677.
Another object of the invention is to provide a
method of improving resistance to oxidation of C/C type
composite materials.
This object is achieved by a method in which an
interphase constituted by 1~ to 20% atomic percent, of
silicon, at least 30% atomic percent of boron, and any
remainder being carbon and/or phosphorous, is formed on
the carbon of the fibers of the fiber reinforcement. The
silicon can be replaced in full or in part by germanium.
The composition of the interphase is preferably selected
so that the increase in volume that results from
oxidation of its silicon, boron, and optionally
phosphorous elements compensates for the volume of carbon
that is consumed by oxidation. The interphase can be
formed by chemical vapor infiltration using a reactive
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gas containing precursors for the elements constituting
the interphase.
In another implementation of the method, the
interphase can be formed solely by adding silicon and
boron from gaseous precursors. The silicon and the boron
can then diffuse and form an interphase constituted by a
ternary silicon-boron-carbon (Si-B-C) system, the carbon
coming from the reinforcing fibers. By way of example,
the precursors can be chlorides of silicon (SiCl9) and of
boron (BC13) giving silicon and boron in the presence of
hydrogen gas (H2). When germanium replaces all or part of
the silicon, the precursor used can be germanium chloride
(GeClq), for example. When phosphorous replaces all or
part of the carbon, the gaseous precursor for the
phosphorous can be phosphorous bromide PBr3, for example.
In yet another implementation of the invention, the
interphase can be formed by pack-cementation, in
particular by thermochemical treatment serving to
generate a ternary interphase of Si-B-C and/or P by
activated cementation.
The interphase can be formed after the fiber
reinforcement has been prepared and prior to the matrix
being formed. In a variant, the C/C type composite
material is made and is then subjected to heat treatment
to graphitize the carbon at a temperature lying in the
range 1200°C to 3000°C, thereby causing decohesion
between the carbon matrix and the fibers of the fiber
reinforcement, with the interphase being formed
subsequently at the interface between the matrix and the
fibers of the fiber reinforcement by chemical vapor
infiltration or by one of the other methods mentioned
above.
Implementations of the method and embodiments of the
resulting material are described below by way of non-
limiting indication.
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Brief description of the drawings
In the accompanying drawings:
Figures lA and 1B show the successive steps in
making a C/C type composite material that is protected
against oxidation by two implementations of the
invention;
Figure 2 shows, highly diagrammatically, an
installation for chemical vapor infiltration that is
suitable for use in making a composite material in
accordance with the method of Figure lA;
Figures 3A and 3B show, highly diagrammatically,
how the interphase between the fibers and the matrix of
the composite material is exposed after machining or
wear;
~ Figures 4 and 5 are views obtained by an electron
microscope showing the appearance of a C/C composite
material after oxidation in dry air at 600°C for 6 hours,
respectively with a 200 nm thick Si-B-C interphase and
with an Si-C type interphase;
~ Figures 6 to 9 are views obtained with an optical
microscope showing the appearance of a C/C composite
material after oxidation in dry air at 600°C for 16 h,
respectively with a 100 nm thick Si-B-C interphase, with
a 30 nm thick Si-B-C interphase, with no interphase, and
with a B-C type interphase;
Figures 10 and 11 are views obtained with an
optical microscope showing the appearance of a tensile
fracture in a C/C composite material after oxidation in
dry air at 600°C for 16 hours, with a 100 nm thick Si-B-C
interphase, and with a 30 nm thick Si-B-C interphase;
- Figure 12 is a graph showing how the relative loss
of mass of a C/C composite material that is exposed to
wet air at 600°C varies with time, respectively with a
100 nm thick Si-B-C interphase and without an interphase;
~ Figure 13 is a diagram showing preferential
oxidation at the fiber-matrix interface of a C/C
composite material; and
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Figure 14 is a diagram showing the oxidation
behavior of a C/C type composite material that has an
interphase in accordance with the invention.
Description of preferred embodiments
The field of application of the invention is that of
C/C type composite materials. The term "C/C composite
material" is used herein to mean a composite material
having fiber reinforcement made of carbon fibers or
fibers that are coated in carbon, and densified by a
matrix that is made of carbon, or at least in part of
carbon in the vicinity of the reinforcing fibers.
Carbon-coated fiber reinforcement can, for example, be
constituted by ceramic reinforcing fibers, e.g. silicon
carbide fibers (SiC), coated in a layer of carbon such as
a layer of pyrolytic carbon or "pyrocarbon" as formed by
deposition or by chemical vapor infiltration. A matrix
formed in part out of carbon can, for example, be a
matrix having a plurality of phases comprising at least
one carbon phase in the vicinity of the fibers of the
fiber reinforcement and at least one phase of a material
other than carbon, such as a ceramic phase, e.g. of SiC,
of boron carbide (B4C), or of a ternary Si-B-C system such
as described in above-mentioned document US A-5 246 736,
where an SiC phase can be obtained by chemical vapor
infiltration or by siliciding using silicon that is
infiltrated in the molten state.
A first step 1 (Figure lA) in implementing a method
of the invention for making a C/C type composite material
piece having increased oxidation resistance consists in
making fiber reinforcement.
The fiber reinforcement can be of the one-
dimensional (1D) type, of the two-dimensional (2D) type,
or of the three-dimensional (3D) type. With 2D
reinforcement, the fibers form a two-dimensional system
constituted by a woven cloth or by sheets of threads or
cords optionally in a plurality of superposed layers.
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With 3D reinforcement, the fibers form a three-
dimensional system, e.g. formed by three-dimensional
weaving or by superposing and bonding together two-
dimensional plies of cloth, felt, sheets of threads or
cords, or indeed by rolling helical cloth or deformable
braid flat into superposed turns and bonding the
superposed turns together. Superposed turns or plies can
be bonded together by needling or by implanting threads.
The fiber reinforcement can be implemented in the form of
a fiber preform whose shape corresponds to the shape of
the composite material part that is to be made.
Preferably, but optionally, a second step 2 consists
in providing the fibers of the fiber reinforcement with a
refractory coating that forms a protective barrier
against the boron contained in the interphase that is
subsequently formed in accordance with the invention.
The protective coating is, for example, a carbide of
silicon, of zirconium, or of hafnium. It is in the form
of a thin layer, e.g. of thickness lying in the range
10 nm to 300 nm, and it is made by chemical vapor
infiltration, by cementation, or by a reactive method by
infiltrating silicon monoxide (Si0) in the gaseous state.
Thereafter, an Si-B-C type interphase is formed
(step 3) on the fibers of the fiber reinforcement
optionally provided with a refractory protective coating.
The interphase is constituted by 1~ to 20~ atomic percent
of silicon, at least 30~ atomic percent of boron, with
any remainder being constituted by carbon. The
interphase is in the form of a thin layer, e.g. of
thickness not less than 10 nm and less than 1 um. It can
be made by chemical vapor infiltration (CVI), by a
reactional process, or by cementation. A portion at
least of the carbon can be replaced by phosphorous. On
oxidizing, phosphorous generates the oxide P205 which is
known for its role as an agent for providing carbon with
protection against catalytic oxidation. In addition, at
least a portion of the silicon can be replaced by
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germanium. On oxidation, germanium generates the oxide,
Ge02, which forms a vitreous structure in the same manner
as silica, Si02.
Thereafter, the fiber reinforcement is densified
5 (step 4) by the matrix of the composite material by a
liquid process or by a gas process, the matrix being made
of carbon, at least in a first stage adjacent to the
fiber reinforcement, and may optionally be finished off
using a ceramic phase. Liquid process densification
10 consists in impregnating the fiber reinforcement with a
matrix precursor in the liquid state, e.g. a resin, and
subsequently in performing heat treatment whereby the
matrix-constituting material is obtained by transforming
the precursor. Several successive impregnation and heat
treatment cycles can be performed. Densification by a
gas process is implemented by chemical vapor
infiltration. Methods of forming matrix phases out of
pyrolytic carbon or out of ceramic, by a liquid process
or by a gaseous process are well known, as are
combinations thereof.
Figure 2 is a diagram showing a chemical vapor
infiltration installation suitable for implementing steps
2, 3, and 4 in order to form a refractory protective
coating of silicon carbide SiC on the fibers of the fiber
reinforcement, followed by an Si-B-C type interface, and
then by a matrix having at least one carbon phase and
optionally having one or more ceramic phases containing
C, Si, and/or B.
The installation is analogous to that described in
document US A-5 246 736, to which reference has already
been made.
The installation has a graphite susceptor 10
situated inside an enclosure 12 and defining a reaction
chamber 14 in which the pieces of fiber reinforcement are
placed on a turntable 16. The susceptor is heated by an
induction coil 18 placed around it.
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The reaction chamber 14 is fed with gas for giving
the desired deposit via a pipe 20 which passes through
the wall of the enclosure 12 and terminates in the
chamber 14 via a cover 14a that closes the top portion
thereof .
Residual gas is extracted from the reaction chamber
by means of one or more ducts 22 opening out into the
bottom 14b of the chamber and connected, outside the
enclosure, to a pipe 24 leading to a pump 26.
- 10 The volume situated around the susceptor 10, inside
the enclosure 12, is swept by means of an inert gas, such
as nitrogen N2, which forms a buffer around the reaction
chamber.
Gas sources 32, 34, 36, 38 supply the components of
the gas which is introduced into the reaction chamber.
Each source is connected to the pipe 20 via a duct which
includes an automatically controlled stop valve given
respective reference 42, 44, 46, 48, and a mass flow rate
meter given respective reference 52, 54, 56, 58, with the
flow meters serving to regulate the relative proportions
of the components of the gas.
For the purpose of depositing the elements Si, B,
and C, the gas is made up of a mixture of precursors for
the elements Si, B, and C, to which gas there is added a
reducing element such as hydrogen Hz.
The elements C and Si can be generated by precursors
belonging respectively to the family of hydrocarbons and
to the family of silanes or chlorosilanes. They can also
be generated together by decomposing an organo-silicate
precursor such as methyltrichlorosilane (MTS).
The element B is generated by a borane or a halide,
such as boron trichloride (BC13).
The gas sources 32, 34, and 36 are consequently
respectively sources of H2, of MTS, and of BC13.
The gas source 38 is a source of hydrocarbon, e.g.
of methane, enabling the element carbon to be generated.
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When the carbon of the interphase is replaced at
least in part by phosphorous, an additional source of gas
is used which is a precursor of phosphorous, e.g. the
bromide PBr3; under such circumstances, the precursor gas
selected for boron is preferably the bromide BBr3.
When the silicon of the interphase is replaced at
least in part by germanium, an additional source of gas
is used as a precursor of germanium, for example
germanium chloride GeCl9.
The SiC refractory protective coating is initially
formed by admitting a gas into the chamber 14 constituted
by a mixture of MTS and of H2, with deposition being
continued until the desired thickness is reached.
Thereafter, the Si-B-C interphase is made by
admitting a mixture of MTS, BC13, and H2 into the chamber,
with the relative proportions of MTS and BC13 in the gas
being selected as a function of the relative proportions
of Si and B to be obtained in the interphase.
Thus, for example, to make an interphase in which
the atomic percentage of Si is no more than 20% and the
atomic percentage of B is not less than 300, the ratio
between the volume flow rate of MTS and the volume flow
rate of BC13 must be less than 1.5 when the temperature in
the chamber is equal to 1000°C.
It will be observed that it is extremely easy to
switch from depositing SiC to depositing Si-B-C since it
suffices to introduce the precursor BC13 into the gas
mixture previously constituted by MTS and H~, and to
adjust the mass flow rates. This change in the gas
mixture can take place progressively, thereby enabling a
continuous transition to be obtained between the SiC and
Si-B-C deposits, or else it can be instantaneous.
The SiC coating and the Si-B-C interphase are
deposited at a temperature lying in the range 800°C to
1150°C and under a pressure lying in the range 0.1 kPa to
100 kPa.
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Processes other than chemical vapor infiltration as
described above can be used for making the Si-B-C
interphase.
Thus, a method combining a reactive technique and
infiltration, or a method of the CVR (I) type can be used
which consists in generating the elements silicon and
boron from gaseous precursors, which elements diffuse
over a limited depth from the surface of the carbon
fibers and form an Si-B-C type interphase in which the
carbon is taken from the fibers. The precursors used
are, for example the chlorides SiCl4 and BC13.
Infiltration is performed in the presence of hydrogen gas
(Hz) at a temperature lying in the range about 900°C to
1150°C and under a total pressure lying in the range
about 1 kPa to 15 kPa. It is possible to use an
installation of the kind shown in Figure 2, replacing the
source of MTS by a source of SiCl4.
Another possible method consists in thermochemical
treatment producing the Si-B-C interphase by pack
cementation, e.g. a method of the type described by the
Applicant in international patent application
PCT/FR97/01890. In that method, the treatment is
performed at a temperature lying in the range 700°C to
1300°C under a pressure lying in the range 0.1 kPa to
30 kPa of hydrogen, of rare gas, or of a mixture of those
gases in the presence of a donor cement and a solid
activating compound. The donor cement is a mixture of
powders constituted by silicon and boron alloyed with an
element M selected from aluminum, calcium, chromium,
yttrium, and magnesium, and optionally a moderator
element M' selected from iron, nickel, chromium, cobalt,
molybdenum, and tungsten. The solid activating compound
is a halide (chloride or fluoride, preferably fluoride)
of the selected element M. As an indication, it is
possible to select a donor cement constituted by 55% by
weight silicon, 25% by weight boron, and 20% by weight
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magnesium, with the activating compound being magnesium
fluoride MgFz.
Once an Si-B-C interphase having the desired
thickness has been obtained, the carbon matrix, or at
least a first carbon-containing phase of the matrix, is
made by admitting the carbon precursor from the source 38
into the chamber 14. With a methane precursor, chemical
vapor infiltration is performed at a temperature lying in
the range 700°C to 2100°C under a pressure lying in the
range 0.5 kPa to 100 kPa. It is possible to incorporate
a phase in the matrix other than pyrolytic carbon, in
particular an SiC or a B4C or an Si-B-C phase, by acting
on the composition of the gas mixture admitted into the
chamber 14.
In the above, it is assumed that an Si-B-C type
interphase is formed prior to forming the matrix of the
composite material. Nevertheless, it is possible to
proceed otherwise, as shown diagrammatically in Figure
1B.
After step 1 in which the fiber reinforcement is
formed and optional step 2 in which a refractory
protective coating is formed, the reinforcement is
densified by a carbon matrix or a matrix having at least
one carbon phase adjacent to the reinforcing fibers (step
5). The composite material obtained in this way is
subjected to high temperature heat treatment (step 6) for
the purpose of graphitizing the carbon of the matrix and
possibly also that of the fibers. This treatment is
performed at a temperature lying in the range about
1200°C to 3000°C, typically at about 2200°C. This gives
rise to decohesion between the fibers of the fiber
reinforcement and the carbon matrix.
It is then possible, e.g. by chemical vapor
infiltration, to form an Si-B-C type interphase deposit
at the interface between the fibers of the fiber
reinforcement and the matrix (step 7). This chemical
vapor infiltration can be performed by means of an
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installation as shown in Figure 2. This results in the
same sequence: carbon fibers - (optional refractory
protective coating) - Si-B-C type interphase - and carbon
matrix. Forming the Si-B-C interphase by processes using
5 the reactive technique or pack cementation as described
above is also possible.
Figure 3A shows in highly diagrammatic manner how,
after densification, a fiber F coated in the interphase I
is completely covered by the matrix. As already
10 mentioned, the invention is particularly applicable when,
after machining or wear, the carbon of the fibers is
exposed to the surrounding medium. Under such
circumstances, as shown in Figure 3B, a fiber F and the
interphase I formed thereon is flush with the surface S
15 that has been produced by machining or wear.
Examples of making a C-C type composite material
having increased resistance to oxidation are described
below.
In all of the Examples, the initial fiber
reinforcement is 3D type reinforcement constituted by
plies of carbon cloth based on the pre-oxidized poly-
acrylonitrile precursor, the plies being superposed and
bonded together by needling. The plies are superposed
and needled together while in the precursor stage, as
described for example in document US-A-4 790 052. The
precursor is transformed by heat treatment after the
fiber reinforcement has been formed.
In addition, in all of the examples, the matrix of
the composite material is made entirely out of pyrolytic
carbon, by chemical vapor infiltration.
Example lA
An interphase constituted by about 10% atomic
percent of Si, about 40% atomic percent of B, and about
50% atomic percent of C was formed on the fiber
substrate.
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The interphase was made by chemical vapor
infiltration by using a gas mixture constituted by MTS,
BC13, and Hz with the volume flow rates of MTS and BC13
being at a ratio of 1. The volume flow rate of HZ was
five times that of the other gases. The temperature and
the total pressure in the infiltration chamber were
respectively 920°C and 5 kPa. Chemical vapor
infiltration was continued for 5 hours (h) until the
interphase had reached a thickness of 300 nm.
The fiber reinforcement provided with the interphase
was then densified with the matrix of pyrolytic carbon.
Example 1B
The procedure was the same as in Example lA except
that the thickness of the interphase was limited to
30 nm. Duration of infiltration was equal to 30 minutes
(min) .
Example 2
An interphase constituted by about 1.5% atomic
percent of Si, about 70% atomic percent of boron, about
18.5% atomic percent of carbon, and about 10% atomic
percent of phosphorous was formed on the fiber substrate.
The interphase was made by chemical vapor
infiltration using a gas constituted by a mixture of MTS,
BBr3, PBr3, and H2, with volume flow rates respectively
equal to 40 cm3/min, 110 cm3/min, 15 cm3/min, and
200 cm3/min. The temperature in the infiltration chamber
was 920°C and the total pressure was 5 kPa. Infiltration
was continued for 1 h, until the interphase had reached a
thickness of about 700 nm.
Example 3A
The procedure was as in Example lA, however the
temperature was 950°C and an interphase was formed that
was constituted by about 9% (atom) of Si, 50% (atom) of
B, and 41% (atom) of C, using a gas in which the ratio
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between the volume flow rates of MTS and BC13 was equal to
0.5. The thickness of the interphase was 250 nm after
2.5 h of infiltration.
Example 3B
The procedure was as in Example 3A, however the
thickness of the interphase was limited to 100 nm, by
reducing the duration of infiltration to 1 h.
Example 3C
The procedure was as in Example 3A, but the
thickness of the interphase was limited to 30 nm, by
reducing the duration of infiltration to 20 min.
Example 4
The procedure was as in Example lA, however the
interphase formed was constituted by 18% (atom) of Si,
37% (atom) of B, and 45% (atom) of C, using a gas in
which the ratio between the volume rates of MTS and BC13
was equal to 1. The temperature and the pressure in the
infiltration chamber were respectively about 950°C and
5 kPa.
Example 5 (comparative)
The procedure was as in Example lA, however the
interphase formed was constituted by 30% (atom) of Si,
30% (atom) of B, and 40% (atom) of C, using a gas in
which the ratio between the volume flow rates of MTS and
BC13 was equal to 2. The temperature and the pressure in
the Infiltration chamber were respectively about 950°C
and 5 kPa.
Example 6 (comparative)
Fiber reinforcement such as that of Example lA was
densified by a matrix of pyrolytic carbon without forming
an interphase.
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Example 7A (comparative)
Fiber reinforcement such as that of Example lA was
provided with an interphase constituted by 55% (atom) of
Si, and 45% (atom) of C.
The interphase was made by chemical vapor
infiltration using a gas constituted by a mixture of MTS
and H2. The temperature and the pressure in the
infiltration chamber were respectively 950°C and 3 kPa.
Infiltration was continued until an SiC interphase was
obtained having a thickness of 250 nm.
The fiber reinforcement with the SiC interphase was
then densified with a matrix of pyrolytic carbon.
Example 7B (comparative)
The procedure was as in Example 7A, but the
thickness of the interphase was limited to 100 nm.
Example 8A (comparative)
Fiber reinforcement such as that of Example lA was
provided with an interphase constituted by 35% (atom) of
B and 65% (atom) of C.
The interphase was made by chemical vapor
infiltration using a gas constituted by a mixture of BC13
and of propane C3H8. The temperature and pressure in the
infiltration chamber were respectively 950°C and 1 kPa.
Infiltration was continued until an interphase was
obtained having a thickness of 250 nm.
The fiber reinforcement with the interphase was then
densified by a matrix of pyrolytic carbon.
Example 8B (comparative)
The procedure was as in Example 8A, but the
thickness of the interphase was limited to 100 nm.
Example 9
Fiber reinforcement such as that of Example lA was
CA 02271177 1999-OS-OS
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densified by a matrix of pyrolytic carbon, without prier
formation of an interphase.
The resulting C/C material was subjected to heat
treatment at a temperature of 2800°C for a duration of
2 h, thereby graphitizing the carbon.
After the high temperature heat treatment, the C/C
material was subjected to chemical vapor infiltration to
obtain an Si-B-C system under the same conditions as in
Example 3A, with the duration of infiltration being
limited to 2 h. An interphase deposit was formed having
the same composition as that of Example 3A in the gaps
left between the carbon fibers and the carbon matrix by
the decohesion resulting from the heat treatment. An S1-B-C
deposit was also formed in the pores of the carbon matrix
and at the surface thereof.
~~nnl a 10
The procedure,was the same as in Example 9, but the
duration of the chemical vapor infiltration of the Si-H-C
system was raised to 4 h.
Example 11 (comparative)
The procedure was as in Example 9, but without
performing chemical vapor infiltration of an Si-B-C
system after the high temperature heat treatment.
Tests
Oxidation tests were performed on samples of the
materials obtained from the preceding examples, after
surface machining to avoid being in a situation of
external protection.
The results of the tests are given below, the
samples being identified by the number of the
corresponding example.
Test 1 (oxidation in dry air at 600°C)
Samples 3A, 5, 7, 8, 9, 10, and 11 were exposed to
CA 02271177 1999-OS-OS
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temperature of 600°C in dry air for 6 h or 8 h, and
relative mass variation was measured (i.e. the ratio of
absolute mass variation over initial mass). The
following results were obtained:
Sample 3A 5 6 7A 8A 9 10 11
Time 6 6 6 6 6 8 8 8
(h)
Inter- Si-B-C Si-B-Cnone Si-C B-C Si-B-C Si-B-C none
phase
Mass -1.2 -1.5 -5.5 -6.7 -2.4 -3.4 -1.1 -5.7
change
(%)
Figures 4 and 5 are electron microscope views
showing the appearance of samples 3A and 7A after
oxidation. Fiber-matrix decohesion is clearly visible in
10 Figure 5, whereas Figure 4 shows no decohesion.
Test 2 (oxidation in dry air at 600°C for 16 h)
Samples lA, 1B, 6, and 8A were exposed to 600°C in
dry air for 16 h.
15 The photographs of Figures 6 to 9 show the
appearance of the samples after oxidation, observation
being performed with an optical microscope at
magnification 20 (except for the detail of Figure 8 where
the magnification is 40).
20 The fiber-matrix decohesion is clearly visible in
Figures 8 and 9 (black outlines around the fibers). It
is also visible, but to a lesser extent, in Figure 7.
However no decohesion is visible in Figure 6.
Test 3 (tensile testing after oxidation)
Samples 3A, 3B, and 3C were exposed at 600°C to dry
air for 16 h, and then subjected to tensile testing to
failure. The following were measured: Young~s modulus,
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breaking stress, and breaking deformation. The following
results were obtained:
Sample 3A 3B 3C
Interphase Si-B-C Si-B-C Si-B-C
Thickness 250 nm 100 nm 30 nm
Young's 11 24 14
modulus (GPa)
Breaking 30 59 40
stress (MPa)
Breaking 0.91 0.41 0.97
deformation
Figures 10 and 11 are photographs showing the
appearance of the breaks for samples 3B and 3C. In
Figure 11, it can be seen that the fibers have been laid
bare, indicating relatively weak bonding between the
fibers and the matrix after oxidation. In contrast, this
cannot be seen in Figure 10, thus indicating a strong
fiber-matrix bond, as confirmed by the relatively low
breaking deformation.
Test 4 (oxidation in dry air at 800°C)
Samples lA, 3A, 3B, 5, 6, 7B, and 8B were exposed to
800°C under dry air, and the rates of radial oxidation of
the fibers and of the matrix were measured by
periodically observing samples with an electronic
scanning microscope. Rate of radial oxidation is equal
to the variation as a function of time in the width of
the zone removed by oxidation in a direction
perpendicular to the fibers. The following results were
obtained:
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Sample lA 3A-3B 5 6 7B 8B
Interphase Si-B-C Si-B-C Si-B-C none Si-C B-C
Radial oxidation0 0 10 29 12 6.8
rate of fiber
(l~mh 1)
Radial oxidation0 0 11 28.2 22 15.3
rate of matrix
(~mh_1)
Test 5 (oxidation under wet air at 600°C)
Samples 3A and 6 were exposed at 600°C to a flow of
wet air containing 3~ by volume of H20, at a flow rate of
500 liters per hour (1/h).
The curves in Figure 12 show variation as a function
of time in relative mass loss. After 8 h, it was 3.9~
for sample 3A and 25.4 for sample 6.
Discussion of test results
Tests 1 (Figures 6, 7, 9), 2, 4, and 5 (Figure 12)
show the undeniable improvement in oxidation resistance
provided by the presence of the Si-B-C interphase between
C fibers and C matrix, compared with the absence of an
interphase, and this applies under oxidation conditions
at 600°C in dry air, with or after prior high temperature
treatment, or at 800°C in dry air, or at 600°C in wet
air.
Test 4 shows that the atomic percentage of Si in the
Si-B-C interphase must be limited. The presence of Si is
nevertheless necessary in order to avoid decohesion as
can be seen in Figure 9. Tests 1 (Figures 4, 5) and 4
also show the better effectiveness provided by the
presence of B, compared with an Si-C interphase. The
interphase must have 1~ to 20~ atomic percent of silicon,
at least 30% atomic percent of boron, with any remainder
being carbon.
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Test 2 (Figure 7) and Test 3 show loss of
performance when the interphase is relatively thin
(30 nm), even though performance remains nevertheless
better than in the absence of any interphase. Test 3
leads to a similar conclusion when the thickness of the
interphase is relatively thick (250 nm), so the thickness
must in any event remain limited in order to conserve the
carbon nature of the matrix material close to the fibers
and avoid having an Si-B-C matrix phase interposed
between the fibers and the phase of a carbon nature (an
interphase constitutes a sheath on the reinforcing fibers
without creating bonds between the fibers capable of
transmitting significant forces, whereas the purpose of
matrix phase is to transmit to the fiber reinforcement
the forces to which the composite material is subjected).
Thus, the thickness of the interphase is preferably at
least 10 nm and less than 1 um.
Although they cannot provide any scientific proof,
the inventors believe that the effectiveness of the Si-B-
C interphase of the invention, as compared with no
interphase or with other interphases, can be explained as
follows .
When there is no interphase at the interface between
carbon fiber and carbon matrix phase, oxidation by the
oxygen (OZ) from the surrounding medium begins in
privileged manner at the interface, as shown very
diagrammatically by Figure 13, and it propagates along
the fiber. This results in loss of bonding between the
fiber and the matrix, and consequently in a significant
reduction in the strength of the material. This loss of
bonding is clearly visible in Figure 8.
However, when an interphase of the invention is
interposed between the fiber and the carbon matrix
(Figure 14), this loss of bonding between the fiber and
the matrix is not observed. It appears that the presence
of a significant quantity of boron which reacts with the
oxygen of the surrounding medium from relatively low
CA 02271177 1999-OS-OS
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temperatures to give the oxide BZO3, which liquefies at
relatively low temperature (about 450°C), favors early
oxidation of the silicon, thereby forming silica Si02.
This enables the BZO3 to be retained by forming a silica
"foam" which encloses the oxide Bz03, greatly limiting
departure thereof, and thus forming a "plug" which
opposes penetration of oxygen from the surrounding
medium.
If the interphase formed between the carbon fiber
and the carbon matrix does not contain any silicon but
only contains boron and carbon, then the oxide B203 and
carbon dioxide COz are formed leaving a void at the
interface. In addition, in the presence of moisture, and
at relatively low temperatures, B203 reacts with water
vapor, and the resulting species (HBOZ) is highly
volatile. The formation of a void at the fiber-matrix
interface is visible in Figure 10.
Still by way of comparison, if the interphase formed
between the carbon fiber and the carbon matrix does not
contain boron, but only silicon and carbon, there is no
significant oxidation of SiC at temperatures below
1000°C. Oxidation then takes place preferentially at the
interfaces between the fiber and the interphase and
between the matrix and the interphase, thereby leading to
a loss of bonding. This loss of bonding is visible in
Figure 5 which gives rise to a significant loss of mass
from the composite material from 600°C.
