Note: Descriptions are shown in the official language in which they were submitted.
2174309
1
Chemical vapour infiltration process of a material within a fibrous
substrate with creation of a temperature gradient in the latter.
The present invention relates to a chemical vapor
infiltration (CVI) method of infiltrating a material into
a fibrous substrate.
The field of application of the invention is in
particular that of manufacturing pieces made of composite
material comprising a fibrous substrate or "preform" that
has been densified by a matrix. In this application,
chemical vapor infiltration is used to form a deposit of
the matrix-constituting material on the fibers of the
substrate and throughout the volume thereof, in order to
bond the fibers together and fill in the pores initially
accessible in the substrate. Chemical vapor infiltration
may also be used to finish off densification performed in
part by some other method, e.g. using a liquid process in
which the substrate is impregnated with a liquid
precursor for the matrix-constituting material, and then
the precursor is transformed, generally by heat
treatment.
To perform chemical vapor infiltration, the fibrous
substrate is placed in an enclosure. A reaction gas is
admitted into the enclosure. Under determined conditions
of temperature and pressure, the gas diffuses within the
substrate and forms the deposit of matrix material by
means of the components of the gas decomposing or
reacting on making contact with the fibers.
The composition of the gas is selected as a funct-ionw
of the matrix to be made. CVI methods are well known for
forming matrices out of pyrolytic carbon or "pyrocarbon",
or out of ceramic, e.g. silicon carbide, boron nitride,
or refractory oxides.
Several types of CVI method are in existence: the
constant temperature and pressure method, the pressure
gradient method, and the temperature gradient method.
2174309
2
In the constant temperature and pressure method, the
substrate to be densified is placed in an isothermal
enclosure. Heating is provided, e.g. by means of a
graphite susceptor or core surrounding the enclosure and
itself surrounded by an induction winding. Energy is
applied to the substrate essentially by radiation from
the enclosure. The temperature inside the enclosure is
regulated to the desired value by controlling the current
in the winding, while the pressure is adjusted by
connecting the enclosure to a vacuum source and
controlling the rate at which the gas is admitted into
the enclosure. Matrix material is deposited inside the
substrate and on the surface thereof. The temperature
and pressure are selected to have values that are only
slightly greater than those required for a deposit to
form, so as to avoid massive deposition on the surface of
the substrate occurring immediately on contact with the
gas, since that would quickly lead to the surface pores
being shut off, thereby preventing densification taking
place within the substrate.
Nevertheless, it is inevitable that the surface
pores will be closed progressively, thereby stopping the
densification process before it is complete within the
core of the substrate. It is then necessary to remove
surface crust by machining so as to reopen the array of
pores and continue densification. Several intermediate
crust-removal operations may be necessary on a single
piece prior to achieving the desired degree of
densification.
By accurately controlling infiltration conditions,
that method makes it possible to obtain a matrix of
desired quality, and to do so in reproducible manner. It
also has the major advantage of enabling a plurality of
pieces of various shapes to be densified simultaneously
within the same enclosure.
In spite of these advantages which justify its use
on an industrial scale, the constant temperature and
_ 2174309
3
pressure method suffers from drawbacks of lengthy
duration and large cost, in particular when manufacturing
composite pieces of great thickness. Densification
requires deposition to take place slowly, and thus
requires cycles of long duration. In addition, the
intermediate machining operations for crust removal give
rise to losses of material and contribute to increasing
cost price, with alternation between infiltration and
crust removal lengthening the total duration of
manufacture and increasing its cost. Finally, in
particular for pieces of great thickness, it is
inevitable that a considerable densification gradient
remains within a given piece, with the degree of
densification being significantly less deep within the
piece than at its surface.
The pressure gradient method uses a forced flow of
the gas through the preform. The forced flow gives rise
to a pressure difference across the piece.
In addition to requiring the gas transport system to
be specially adapted, the pressure gradient method
suffers from one of the same limitations as the constant
temperature and pressure method. The permeability of the
pores to the gas decreases rapidly with more deposit
being formed on the side facing the gas inlet. It is
necessary to remove crust therefrom periodically in order
to enable densification to continue.
In addition, that method is applicable only to
substrates of shapes that are particularly simple and
limited, with each piece requiring a special gas feed and
circulation device.
The temperature gradient method consists in
performing non-uniform heating of the substrate so that
its temperature in the vicinity of its exposed surface is
lower than its inside temperature remote from the
surface. Since the deposition reaction is thermally
activated, deposition speed or matrix growth increases
with temperature. As a result, more densification takes
2174309
4
place in the hotter portions within the substrate than in
the cooler portions at the exposed surface of the
substrate. This prevents a greater deposit being formed
at the surface with premature shutting of the pores, and
thus prevents the need for intermediate operations. This
is the type of infiltration method to which the present
invention relates.
A CVI device using a temperature gradient was
presented and described by W.V. Kotlensky to the "16th
National SAMPE Symposium, Anaheim, California, April
21-23, 1971" under the title "A review of CVD carbon
infiltration of porous substrates" and in a work entitled
"Chemistry and physics of carbon", published in the
United States of America by P.L. Walker, Vol. 9, pp. 198-
199. That device is shown very diagrammatically in
Figure 1.
The substrate 10 to be densified is applied via an
inside face l0a against a graphite core 12. The
substrate 10 and the core 12 are received inside an
enclosure 14 defined by a quartz tube 16. A water-cooled
induction winding 18 surrounds the tube 16. Gas is
admitted via the base of the tube 16 and it flows
upwards.
The core 12 is heated by electromagnetic coupling
with the winding 18 and it in turn heats the substrate 10
with which it is in contact. The substrate 10 is a
carbon fiber felt which is not directly heated by
induction because of its low density and poor electro-
magnetic coupling with the winding. In addition, its low
thermal conductivity means that in this configuration
temperature gradients of several hundreds of °C are
observed.
That technique suffers from several drawbacks. In
particular, only substrates of relatively small thickness
can be densified thereby in satisfactory manner. Also,
the efficiency with which the preform is heated is
2114309
strongly dependent on the quality of its contact with the
core.
The above, in association with the existence of
temperature differences of several hundreds of °C, makes
5 it difficult to control deposition conditions
sufficiently accurately to guarantee that a matrix is
formed which has the desired characteristics exactly.
Another CVI technique using a temperature gradient
has been described by J.J. Gebhardt et al. in an article
entitled "Formation of carbon-carbon composite materials
by pyrolytic infiltration", published in Petroleum
derived carbons ACS Series No. 21 6/73.
In that case (Figure 2), the substrate 20 to be
densified is constituted by bundles of intermeshed
graphite fibers oriented in seven different directions.
The substrate is suspended inside an enclosure 24 with
the gas being admitted through the base thereof. The
graphite fibers conduct electricity sufficiently to
enable the substrate to be heated by direct coupling with
an induction winding 28 surrounding the enclosure.
The hottest zone of the substrate is situated inside
the substrate, since its outside surface is cooled by
radiation and by the upward flow of gas through the
enclosure. A temperature gradient of a few °C per
centimeter is obtained going away from the internal
portion which is the hottest.
In order to maintain a sufficient temperature
gradient, the gas circulates at high speed so as to cool
the surface, and the induction winding 28 is restricted
to a few turns in order to heat a limited zone of the
substrate, such that a temperature gradient is also
established between the portion of the substrate situated
in the induction field and the portion of the substrate
lying outside the field. Densification of the entire
substrate is obtained by moving the substrate within the
enclosure, parallel to the axis of the winding. These
CA 02174309 2004-06-09
6
constraints mean that the method can be of limited use
only, since it is difficult to industrialize.
Thus, an object of the present invention is to
provide a CVI method in which the substrate is heated by
electromagnetic coupling, but without the drawbacks of
the above-mentioned methods, and in particular a method
that can be used on an industrial scale for substrates
that may be of great thickness.
According to the invention, this object is achieved
by means of a method of the type in which the substrate
is heated by direct induction, in which method:
the substrate is constituted by a fiber fabric in
which the ratio pr/p~ of transversal electrical
resistivity over longitudinal electrical resistivity is
not less than 1.3, and the ratio 7~r/~,~ of transversal
thermal conductivity over longitudinal thermal
conductivity is not more than 0.9; and
the substrate is situated entirely within the
field produced by the induction winding, the substrate
and the induction winding occupying positions that are
stationary relative to each other.
The term "transversal" applied to electrical
resistivity or thermal conductivity is used to mean the
resistivity or conductivity as measured perpendicularly
to the outer side faces of the substrate, and in
particular radial resistivity or conductivity for a
substrate that is cylindrical. The term "longitudinal"
is used with respect to electrical resistivity or thermal
conductivity to mean resistivity or conductivity measured
parallel to the outer side faces of the substrate, in
particular circumferential resistivity or conductivity
for a cylindrical substrate.
It has been established by the Applicant that a
substrate having the above characteristics as to the
ratios pr/p~ and 7~r/7~~ is particularly suitable for being
densified with heating by direct electromagnetic
coupling. Its electrical resistivity and thermal
2174309
7
conductivity are such that sufficient heating is produced
by the direct coupling while simultaneously creating a
temperature gradient of amplitude that is sufficiently
great to make it possible to do without the artifice of
incomplete coupling as described by Gebhardt et al.
One type of substrate that is particularly suitable
for implementing the method of the invention, and that
satisfies the above conditions, is constituted by a
needled fibrous structure having a fiber volume ratio of
at least 20$.
The fiber volume ratio in the substrate, i.e. the
percentage of the apparent volume of the substrate that
is indeed occupied by fibers, is preferably not less than
25%.
The use of needled fiber fabrics to constitute
reinforcing fabrics for pieces made of composite material
is well known. A method of making needled fabrics out of
refractory fibers, and in particular carbon fibers, is
described in patent applications FR-A-2 584 106 and
FR-A-2 584 107. Such fabrics are built up of superposed
two-dimensional plies, e.g. layers of cloth or sheets of
threads that are stacked up flat, or turns of cloth wound
on a mandrel. The plies are bonded together by needling.
Needling is advantageously performed at constant density
while the fabric is being built up.
Such needled fabrics are particularly suitable for
making pieces of composite material. The bonding
together of plies by needling makes it possible to
withstand delamination, i.e. destruction or damage by the
plies separating or by the plies sliding relative to one
another. In addition, needling imparts pore sizes to the
fiber fabric of dimension and distribution throughout the
volume that are relatively uniform, which is favorable to
densification.
The association of a substrate made of needled fiber
fabric with heating by direct coupling can have numerous
applications on an industrial scale whereas the prior art
CA 02174309 2004-06-09
8
mentioned above relating to heating by electomagnetic
coupling is more of an experimental technique.
In accordance with an aspect of the present
invention there is provided, a CVI method of infiltrating
a material into a fibrous substrate including
electrically conductive fibers, the method comprising the
following steps:
~ placing the substrate in an enclosure;
~ heating the substrate by direct electromagnetic
coupling between the substrate and an induction winding
generating an electromagnetic field to enable a
temperature gradient to be established within the
substrate such that the substrate has a temperature that
is greater in its portions furthest from its exposed
surfaces than in the vicinity of its exposed surfaces;
and
admitting a reaction gas into the enclosure which
gas is a precursor for the material to be infiltrated,
with formation of said material being enhanced in those
portions of the substrate that are at higher temperature;
the method being characterized in that:
the substrate is constituted by a fiber fabric in
which the ratio pr/p~ of transversal electrical
resistivity over longitudinal electrical resistivity is
not less than 1.3, and the ratio ~r/~~ of transversal
thermal conductivity over longitudinal thermal
conductivity is not more than 0.9; and
the substrate is situated entirely within the
field produced by the induction winding, the substrate
and the induction winding occupying positions that are
stationary relative to each other.
Other features of the method of the invention appear
on reading the following description given by way of non-
limiting indication.
In the accompanying drawings:
Figures 1 and 2 are described above, and show prior
art chemical vapor infiltration installations in highly
CA 02174309 2004-06-09
8a
diagrammatic form, in which the substrate to be densified
is heated by electromagnetic coupling;
Figure 3 shows in highly diagrammatic manner, an
installation enabling the method of the invention to be
implemented; and
Figure 4 is a graph showing the temperature gradient
and the density gradient in a cylindrical piece densified
by a method of the invention.
In the installation of Figure 3, a substrate to be
densified 30 is disposed inside an enclosure 34. In the
example shown, the substrate is in the form of an annular
cylinder having a rectangular meridional section. The
substrate is heated by electromagnetic coupling by means
of induction from winding 38 surrounding the enclosure.
The substrate 30 and the winding 38 are coaxial. The
winding 38 extends axially over a length that is equal to
or greater than the axial length of the substrate 30 so
that the substrate lies entirely within the electro-
magetic field generated by the winding.
The substrate is densified by chemical vapor
infiltration (CVI) by admitting a reaction gas into the
enclosure, with the composition of the gas being selected
as a function of the kind of matrix that is to be
deposited within the volume of the substrate. The gas is
admitted to the bottom portion of the enclosure. The
fraction of the gas that has not reacted together with
any gaseous reaction products are extracted from the top
portion of the enclosure which is connected to a vacuum
source (not shown).
35
CA 02174309 2004-06-09
9
A CVI installation of the above type is described in
European patent application EP-A-0 256 073. The
installation of Figure 3 for implementing the method of
the invention differs from that known installation in
that the substrate is heated by direct electromagnetic
coupling between the winding and the substrate, and not
indirectly by coupling between the winding and a graphite
core situated at the periphery of the enclosure and
serving to heat it.
The substrate 30 is made up of electrically
conductive fibers, such as graphite or carbon fibers.
The substrate 30 has characteristics of electrical
resistivity and of thermal conductivity that make it
suitable for being heated to the desired temperature by
direct coupling with the winding 38 while simultaneously
presenting a significant temperature gradient between its
hottest inner portion and its exposed outer surfaces.
These characteristics are such that the ratio between its
radial electrical resistivity pr and its circumferential
electrical resistivity p~ is not less than 1.3, while the
ratio between its radial thermal conductivity ~r and its
circumferential thermal conductivity ~° is not more than
0.9.
Its radial electrical resistivity preferably lies in
the range 1 mSZ/cm to 20 mSZ/cm, while its radial thermal
conductivity lies in the range 0.1 W/m.°K to 20 W/m.°K.
One type of structure that is particularly suitable
for the substrate 30 is a needled structure of graphite
or carbon fibers. A method of manufacturing such a
three-dimensional cylindrical structure by needling is
described in above-mentioned document FR-A-2 584 107.
That method consists in winding a two-dimensional fabric
on a mandrel and needling while winding is taking place.
By way of example, the two-dimensional fabric is
constituted by a woven cloth. Winding and needling are
performed using fabric fibers in a state that constitutes
a precursor for carbon, e.g. in the pre-oxidized
2174309
to
polyacrylonitrile state. Needling performed directly on
carbon fibers would have an excessively destructive
effect on the fabric (it would break its threads). Heat
treatment is applied after the needled structure has been
built up in order to transform the precursor into carbon.
The treatment may be continued-at higher temperatures,
where appropriate, for the purpose of converting the
fibers into graphite, at least to some extent.
The volume ratio of the fibers in the needled
structure, i.e. the percentage of the apparent volume of
the structure actually occupied by the fibers, is a
function both of the fiber ratio in the wound basic
two-dimensional fabric and of needling characteristics
since needling has a compacting effect on superposed
wound layers.
The volume ratio of the fibers in the needled
structure should not be less than 20$, and is preferably
not less than 25$, in order to satisfy the conditions
relating to electrical resistivity and to thermal
conductivity.
It is well known that a body is heated by induction
because of the Joule effect due to induced currents, and
that such currents are concentrated at the surface (skin
effect). The greater the frequency of the current
powering the induction winding, the more marked the
phenomenon whereby heat is concentrated at the surface.
To optimize densification of the substrate by CVI,
the objective is to establish a temperature gradient
between a zone deep within the substrate and the surface
thereof.
In spite of the skin effect, by selecting an
appropriate frequency, and by taking account of the
surface of the substrate being cooled by radiation and by
convection (into the flow of gas), it is possible with a
substrate having the above-specified characteristics to
obtain a temperature gradient within the substrate.
2114309
11
Clearly the best frequency depends on several
parameters: the nature of the fibers constituting the
substrate, the thickness of the substrate, its electrical
resistivity and thermal conductivity values, ... .
As an indication, for a needled substrate made of
carbon fibers obtained in the manner described above, the
optimum frequency lies in the range about 150 Hz to about
3000 Hz, depending on the fiber volume ratio and on the
thickness of the substrate.
In the example shown in Figure 3, the substrate 30
is in the form of a right circular cylinder. The method
of the invention can be implemented with substrates of
other shapes, in particular cylindrical substrates that
are not circular in section or non-cylindrical axially-
symmetrical substrates, with the shape of the winding
being adapted accordingly, where appropriate.
Two example implementations of the method of the
invention are described below.
~vavrnr ~ i
A fiber substrate in the form of an annular cylinder
with an inside diameter of 90 mm and an outside diameter
of 350 mm, and intended to constitute a reinforcing
fabric or preform for a piece of composite material was
made as follows.
A two-dimensional fabric made up of pre-oxidized
polyacrylonitrile (PAN) fiber cloth was wound to build up
superposed layers on a mandrel.
Each new layer was needled to the underlying fabric.
For this purpose, a needle board was used extending
axially over a distance of not less than the width of the
plies, which width was equal to the height (100 mm) of
the preform to be made. Needling was performed while
winding was taking place, with each new layer being
needled to a constant depth equal to the thickness of a
plurality of needled layers. Once the preform had
reached the desired thickness, 130 mm in this example,
one or more finishing needling passes were performed in
2174309
12
conventional manner to obtain a constant density of
needling throughout the preform.
The weight per unit area of the cloth used lay in
the range 100 g/mz to 600 g/mz. Needling was performed at
a density suitable for achieving a fiber volume ratio in
the preform equal to 28$, after heat treatment had been
applied to transform the pre-oxidized PAN into carbon.
Such a fabric has radial electrical resistivity pr
estimated at 13 mS2/cm, and circumferential electrical
resistivity p~ equal to 7 mf~/cm, giving a ratio pr/p~ equal
to 1.85, and having radial thermal conductivity ~,
estimated at 0.25 W/m.°K and circumferential thermal
conductivity ~,~ = 0. 5 W/m. °K, giving a ratio ~,r/~,~ = 0.50.
The preform was placed in the enclosure 34 and was
heated by powering the winding 38 at a frequency of
1800 Hz. A temperature field was established within the
preform as a result both of heat sources generated by the
induced currents due to electromagnetic coupling and heat
losses from the outer surface of the preform. Heat
losses took place by convection as the gas swept through,
and also by radiation. The gas admitted into the
enclosure was constituted by methane which gave rise to a
pyrolytic carbon matrix on decomposing.
The temperature distribution within the thickness of
the preform (in the radial direction) was measured at the
beginning of densification (time tl = 0), after
densification had been performed partially (time t2 =
310 h) and at the end of densification (time t3 = 820 h).
Curves T1, T2, and T3 in Figure 4 show the respective
temperature gradients at times tl, t2, and t3.
The distribution of density across the thickness of
the preform (in the radial direction) was also measured
at times tz and t3. Curves DZ and D3 in Figure 4 show the
respective density gradients at times t2 and t3. Density
measurements were performed by taking samples from the
middle portion of the densified preform.
2174309
13
A maximum temperature was achieved in a zone
situated at about 35 mm from the outer surface of the
preform, and this continued throughout the densification
process. Carbon matrix densification gave rise to
progressive variation in the radial and circumferential
values of electrical resistivity and of thermal
conductivity. At the end of the densification process,
the ratio pr/p~ had reached the value 1.85 while the ratio
/~,~ had reached the value 0.80. Clearly, carbon matrix
densification gives rise to a progressive increase in
radial thermal conductivity and correspondingly to a
decrease in thermal gradient.
At the end of densification, at time t3, the
resulting piece had a mean density of 1.72 g/cm3, with a
core density of 1.73 g/cm3. The method thus made it
possible to achieve densification that was almost
uniform, and without requiring any intermediate machining
operation.
~xann~r.F 7
An annular cylindrical preform having an outside
diameter of 1100 mm, a thickness of 125 mm, and a height
of 400 mm was made by winding and needling cloth, as in
Example 1. The preform was intended for manufacturing
the nozzle throat of a thruster.
After the preform fibers had been carbonized, the
preform was placed in an enclosure of a densification
installation of the kind shown in Figure 3. The preform
was heated by direct electromagnetic coupling using the
winding, which was powered at a frequency of 150 Hz. The
frequency was lower than that used in Example 1 because
the volume of the preform was greater.
The preform was densified by admitting into the
enclosure a gas essentially constituted by methane,
giving rise to a matrix of pyrolytic carbon.
After 920 hours of densification, the density
reached was equal to 1.62 g/cm3 and there was no need for
intermediate machining. At this stage, it was possible
2174309
14
to machine the outer profiles of the throat and perform
surface treatment to make the piece gastight, after which
the piece was ready for use.
