Composite material protected against oxidation by a self-healing matrix, and a method of manufacturing it

A composite material protected by oxidation at intermediate temperatures exceeding 850.degree. C. comprises fiber reinforcement densified by a matrix which includes at least one self-healing phase including a glass-precursor component such as B.sub.4 C or an Si--B--C system, together with excess free carbon (C) at a mass percentage lying in the range 10% to 35%. The, or each, self-healing phase can be interposed between two ceramic matrix phases, e.g. of SiC. While the material is exposed to an oxidizing medium, oxidation of the free carbon promotes oxidation of the precursor and transformation thereof into a glass capable of plugging the cracks in the matrix by self-healing.

FIELD OF INVENTION 
The present invention relates to composite materials, and more specifically 
it relates to protecting said materials against oxidation. 
BACKGROUND OF THE INVENTION 
The field concerned by the invention is that of thermostructural composite 
materials, and more particularly ceramic matrix composite (CMC) materials. 
These materials are characterized by their mechanical properties which 
make them suitable for building structural elements, and by their ability 
to maintain these mechanical properties at high temperature. 
Thermostructural composite materials are used in particular for making 
parts that are subjected to high thermomechanical stresses in aviation or 
space applications, e.g. parts of engines or fairing elements, or in 
friction applications, e.g. disk brakes for land vehicles or for aircraft. 
CMC type thermostructural composite materials are constituted by fiber 
reinforcement densified with a matrix, the reinforcing fibers being of a 
refractory material such as carbon or a ceramic, and the matrix being a 
ceramic. Densifying the fiber reinforcement consists in filling the 
accessible pores of the matrix. It is performed by chemical vapor 
infiltration or by impregnation using a liquid precursor for the matrix 
and then transforming the precursor, generally by heat treatment. An 
intermediate coating or "interphase", in particular of pyrolytic carbon 
can be deposited on the fibers to optimize bonding between the matrix and 
the fibers, e.g. as described in document EP-A-0 172 082. 
It is necessary to protect thermostructural materials against oxidation, 
particularly when they contain carbon, even when carbon is present only in 
an interphase between ceramic fibers and a ceramic matrix. The 
thermomechanical stresses to which such materials are subjected in use 
inevitably give rise to the matrix cracking. The cracks then provide 
access for oxygen in the ambient medium all the way to the core of the 
material. 
A well-known method of protecting composite materials against oxidation 
consists in forming a coating having self-healing properties, which 
coating may be external or internal, i.e. it may be a coating anchored in 
the residual accessible pores. The term "self-healing" is used herein to 
designate properties whereby the material at its operating temperature 
passes to a viscous state that is sufficiently fluid to fill cracking of 
the matrix and thus block access to ambient oxygen. The self-healing 
coatings used are typically glasses or vitreous compounds, or else 
precursors therefor, i.e. substances capable of forming a glass by 
oxidizing at the operating temperature of the composite material (in situ 
glass formation). 
Proposals have also been made in document FR-A-2 688 477 to form at least 
one continuous phase at the surface of the matrix or within the matrix, 
which phase is constituted by a ternary Si--B--C system. The relative 
proportions of silicon, boron, and carbon are selected so as to make it 
possible, by oxidation, to form a glass having the required viscosity 
characteristics for healing cracks at the intended operating temperatures, 
which temperatures may be as much as 1700.degree. C. 
Undeniably, that protection technique considerably increases the lifetime 
of thermostructural materials in an oxidizing atmosphere. Nevertheless, it 
has been observed that protection is less effective at intermediate 
temperatures, i.e. about 450.degree. C. to 850.degree. C., than it is at 
higher temperatures. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to further increase the 
effectiveness of anti-oxidation protection as provided by incorporating at 
least one self-healing phase in the matrix of a composite material, 
specifically in the intermediate temperature range. 
This object is achieved by the fact that the phase having the self-healing 
property (hereinafter the self-healing phase) comprises, at least within 
the composite material as initially formed, a mixture including a glass 
precursor suitable for forming a glass by oxidation at a temperature not 
exceeding 850.degree. C., together with free carbon, the mass percentage 
of free carbon in the mixture lying initially in the range 10% to 35%, and 
preferably being greater than 15%. It will be observed that the figures 
given throughout this text relating to the mass percentage of free carbon 
are relative to the composite material as prepared prior to any loss of 
free carbon by oxidation. 
Surprisingly, it has been observed that the presence of excess free carbon 
in the self-healing phase provides a very great improvement in the 
effectiveness of protection against oxidation. This improvement is 
remarkable specifically at intermediate temperatures, i.e. while the free 
carbon is associated with a precursor suitable for forming a glass having 
self-healing properties in a temperature range beginning at around 
450.degree. C., which is a value that is unusually low for a refractory 
ceramic. 
A possible explanation for this paradoxical result might be as follows. 
When the composite material is used in an oxidizing medium with the matrix 
cracked to a greater or lesser extent, it is possible to distinguish two 
oxidation phenomena that can take place. The first is harmful: this is 
oxidation of the carbon contained in the fibers and/or the interphase of 
the composite material, which oxidation progressively destroys the 
mechanical potential of the material. The second is beneficial: this is 
oxidation of the glass precursor contained in the matrix, thereby causing 
cracks to be plugged by self-healing and greatly reducing the access of 
oxygen from the surrounding to the core of the material, thus increasing 
its lifetime. 
These two phenomena are in competition. If oxidation of the glass precursor 
is established too slowly, then the mechanical potential of the material 
begins to be degraded. If the glass precursor oxidizes quick enough, then 
it dominates over the phenomenon of harmful oxidation. The presence of 
excess free carbon in the self-healing phase encourages oxidation of the 
precursor. When the material is exposed to an oxidizing medium, then free 
carbon is lost as soon as a temperature is reached at which the carbon 
begins to oxidize. This increases the specific surface area for oxidation 
of the precursor, thereby increasing conversion thereof into glass at any 
given temperature. Also, the oxidation of free carbon takes place by 
trapping a significant portion of the oxygen that would otherwise have 
reached and oxidized the fibers and/or the interphase, and by creating a 
partial pressure of CO or of CO.sub.2 it can also have the effect of 
reducing the partial pressure of oxygen in the crack, thereby giving the 
self-healing glass time to form and perform its function. 
The beneficial effect of the presence of excess free carbon is particularly 
remarkable when it is associated with a precursor suitable for producing a 
glass that is self-healing at intermediate temperatures, since conversion 
into glass takes place more slowly at lower temperatures. By way of 
example, such a precursor can be constituted by boron carbide B.sub.4 C. 
The mass percentage of the free carbon in the self-healing phase formed by 
B.sub.4 C and free carbon is then preferably greater than 15%, or even 
greater than 20%. 
The beneficial effect of the presence of excess free carbon also exists 
when it is associated with a precursor suitable for producing a 
self-healing glass at higher temperatures. By way of example, such a 
precursor is the ternary Si--B--C system for temperatures greater than 
about 650.degree. C., as described in document FR-A-2 668 477 when the 
mass percentage of free carbon is around 20%, or for temperatures 
exceeding 850.degree. C. when the mass percentage of free carbon is no 
more than 10%. Such a precursor can also be silicon carbide SiC for 
temperatures exceeding 1000.degree. C. when the mass percentage of free 
carbon is about 20%. In one embodiment at least one phase having 
self-healing properties is formed in which the glass-precursor component 
is suitable for oxidizing to form a glass having self-healing properties 
starting at approximately 450.degree. C. 
In general, for a given self-healing phase, the mass percentage of free 
carbon must decrease with increasing desire to achieve maximum 
effectiveness at high temperature. Firstly the conversion of the precursor 
into glass takes place more quickly at high temperatures, and secondly too 
much free carbon could give rise to the formation of a large quantity of 
glass resulting in excessive consumption of the matrix. 
The effectiveness of the protection against oxidation can be optimized over 
a broad range of temperatures by forming within the matrix at least one 
first self-healing phase that is effective at intermediate temperatures, 
e.g. based on B.sub.4 C, and at least one second self-healing phase that 
is effective at high temperatures, e.g. based on Si--B--C, at least the 
first of these self-healing phases containing a sufficient quantity of 
free carbon. To ensure protection for carbon interphase and/or fibers in 
the composite material, these phases are formed in the order in which they 
are mentioned by interposing ceramic matrix phases that withstand 
oxidation better than the self-healing phases, so as to maintain 
reinforcement secured to the matrix and so as to limit crack propagation. 
When a plurality of self-healing phases containing free carbon are formed, 
the mass percentage of free carbon in the self-healing phases decreases 
starting from the phase closest to the fibers.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
With reference to FIGS. 1 and 2, there follows a description of a method of 
making CMC type thermostructural composite material parts having SiC fiber 
reinforcement together with an SiC matrix (SiC/SiC material), with 
self-healing phases constituted by boron carbide (B.sub.4 C) and excess 
free carbon incorporated in the matrix. 
A two-dimensional fabric in the form of a plain weave cloth is made from 
fibers essentially constituted of silicon carbide (SiC fibers) sold by the 
Japanese company Nippon Carbon under the name "Nicalon NLM 202" (step 1). 
The cloth is treated chemically, e.g. as described in document FR-A-2 640 
258 (step 2). 
Rectangular plies are cut from the cloth, are stacked on one another, and 
are pressed together in tooling (a "shaper") made of graphite to obtain a 
preform in the form of a rectangular parallelepiped having a thickness of 
3 mm and in which the volume fraction occupied by the fibers, i.e. the 
percentage by volume of the preform that is indeed occupied by fibers, is 
40% (step 3). 
The preform held in this way in the tooling is placed in a chemical vapor 
infiltration oven to form an interphase coating of pyrolytic carbon on the 
fibers of the preform. By way of example, reference may be made to 
document EP-0 172 082 (step 4). 
The preform provided in this way with its interphase coating and still held 
in the tooling within the infiltration oven is then subjected to a first 
densification sequence comprising, in succession, forming a silicon 
carbide (SiC) matrix phase, forming a self-healing phase constituted by 
boron carbide (B.sub.4 C) and by excess free carbon (C), and forming an 
SiC matrix phase that does not contain free carbon (step 5). 
The first densification sequence confers sufficient consolidation to the 
preform to enable it to be removed from the tooling outside the 
infiltration oven (step 6), i.e. this sequence establishes adequate 
bonding between the fibers. 
The consolidated preform is strong enough to be capable of being handled 
while retaining its shape. It is put back into the infiltration oven to be 
subjected to a second densification sequence similar to the first, i.e.: 
SiC/B.sub.4 C+C/SiC (step 7), and then to a third similar sequence 
SiC/B.sub.4 C+C/SiC (step 8). 
The densified preform is then removed from the oven and cut up into a 
plurality of parts in the form of rectangular parallelepipeds having 
dimensions of 20 mm.times.10 mm.times.3 mm for making mechanical test 
pieces (step 9). 
The cut-apart pieces are put back into the infiltration oven to be 
subjected to a fourth and last densification sequence of SiC/B.sub.4 
C+C/SiC similar to the preceding sequences (step 10). 
This provides composite material parts in which the matrix comprises 
alternating phases of SiC and of B.sub.4 C+C. The SiC phases are of a 
thickness that can increase with increasing distance from the fibers, e.g. 
lying in the range 0.5 .mu.m close to the fibers and several tens of .mu.m 
(e.g. 20 .mu.m to 50 .mu.m) at the surface of the material. The thickness 
of the B.sub.4 C+C phases may also increase with increasing distance from 
the fibers, e.g. in the range 0.5 .mu.m close to the fibers and several 
tens of .mu.m at the surface of the material (e.g. 20 .mu.m to 50 .mu.m). 
A chemical vapor infiltration installation can be used for implementing the 
above-described densification sequences, as shown diagrammatically in FIG. 
2. 
This installation comprises a graphite susceptor 10 situated inside an 
enclosure 12 and defining a reaction chamber 14 in which composite 
material parts to be treated are placed on a turntable 16. The susceptor 
is heated by an inductor 18 disposed around it. 
The reaction chamber 14 is fed with gas for generating the desired deposit 
by means of a pipe 20 that passes through the wall of the enclosure 12 and 
that terminates inside the chamber 14 via a cover 14a that closes the top 
end thereof. 
Residual gases are extracted from the reaction chamber by means of one or 
more pipes 22 that open out into the bottom 14b of the chamber and that 
are connected outside the enclosure to a pipe 24 connected to a pump 26. 
The volume situated around the susceptor 10 inside the enclosure 12 is 
swept by an inert gas, such as nitrogen N.sub.2 that forms a buffer around 
the reaction chamber. 
Gas sources 32, 35, 36, and 38 deliver the components of the gas that is 
injected into the reaction chamber. Each source is connected to the pipe 
via a duct that includes a respective automatically controlled stop valve 
42, 44, 46, and 48 together with a respective mass flow meter 52, 54, 56, 
and 58, the flow meters enabling the relative proportions of the 
components of the gas to be controlled. 
To deposit SiC, the gas is made up of methyltricholorosilane (MTS) having a 
reducing element such as hydrogen H.sub.2 added thereto. 
For depositing B.sub.4 C+C, the element B is taken from a borane or a 
halide, such as boron trichloride (BCl.sub.3), while the element C comes 
from a hydrocarbon such as methane (CH.sub.4) or from a mixture of 
hydrocarbons, such as methane and propane, for example. 
Consequently, the gas sources 32, 34, 36, and 38 are respectively sources 
of H.sub.2, MTS, BCl.sub.3, and CH.sub.4 (or a mixture of CH.sub.4 
+C.sub.3 H.sub.8). 
The source 38 is also used for forming the pyrocarbon interphase on the SiC 
fibers. 
The composition of the B.sub.4 C+C mixture, i.e. the percentage of excess 
free carbon, is controlled by selecting the proportions of the BCl.sub.3, 
CH.sub.4 (or mixture of CH.sub.4 +C.sub.3 H.sub.8), and H.sub.2 precursors 
in the gas. 
Chemical vapor infiltration of the SiC ceramic phases of the matrix takes 
place at a temperature lying in the range about 800.degree. C. to about 
1150.degree. C., under a pressure lying in the range about 
0.1.times.10.sup.3 N/m.sup.2 to 50.times.10.sup.3 N/m.sup.2, while 
chemical vapor infiltration of the B.sub.4 C+C self-healing phases is 
implemented at a temperature lying in the range about 800.degree. C. to 
about 1150.degree. C. and at a pressure lying in the range about 
0.1.times.10.sup.3 N/m.sup.3 to 50.times.10.sup.3 N/m.sup.2. 
Various test pieces A to D of composite material have been made in the 
manner described above using the following respective mass percentages of 
free carbon in the various self-healing phases: 0%, 8%, 12%, 18% and 26% 
(the percentage being the same for all of the self-healing phases in a 
given material). 
The test pieces were subjected to traction fatigue tests at 600.degree. C. 
in air with the traction stress exerted being caused to vary from 0 to 120 
MPa at a frequency of 2 Hz. The lifetime was measured as the time that 
elapsed between the beginning of the test and the test piece breaking. 
FIG. 3 shows the relationship between the mass percentage of free carbon 
and lifetime. Lifetime increases with increasing mass percentage of free 
carbon, in the range examined. When the mass percentage of free carbon is 
26%, the test was stopped after 100 hours have elapsed, test piece D still 
being unbroken. Naturally the mass percentage of excess carbon cannot 
exceed a limit which is about 35%, beyond which the disappearance of 
carbon cannot be compensated sufficiently by the increase in volume that 
results from oxidation of the precursor, which could lead to defective 
plugging. 
FIG. 3 shows the remarkable effectiveness of adding excess free carbon in 
the self-healing phase for an intermediate temperature (600.degree. C.). 
For the material in which the self-healing phase was constituted by boron 
carbide containing 26% by mass of free carbon, the lifetime under the same 
test conditions was likewise greater than 100 hours at 500.degree. C., 
whereas it was only about 50 hours for a material in which the 
self-healing phase was constituted by boron carbide without excess free 
carbon. 
In order to test the effectiveness of adding excess free carbon in a higher 
temperature self-healing phase, test pieces were made using the same 
method as that described with reference to FIG. 1, but in which the 
B.sub.4 C+C self-healing phases were replaced by self-healing phases made 
up of a ternary Si--C--B system with a free carbon mass percentage equal 
to 8% and a B/Si ratio of about 6.5. 
As described in above-mentioned document FR-A-2 668 477, the ternary 
Si--B--C system is obtained by chemical vapor infiltration using a gas 
that comprises a mixture of MTS, BCl.sub.3, and H.sub.2 precursors. The 
excess free carbon is obtained by selecting the relative proportions of 
MTS, BCl.sub.3, and H.sub.2, and optionally while adding the CH.sub.4 (or 
CH.sub.4 +C.sub.3 H.sub.8) precursor. The relative proportions of Si, B, 
and C in the ternary system Si--B--C determine the temperature at which 
the borosilicate glass formed by oxidation possess the required 
self-healing properties. For the above-envisaged materials of the SiC--SiC 
type, appropriately selecting these proportions makes it possible to cover 
a broad range of high temperatures starting at about 650.degree. C. and 
extending to about 1200.degree. C., at which temperature the "Nicalon NLM 
202" fiber becomes unstable. A test piece E obtained in this way was 
subjected to fatigue testing under the conditions described above for test 
pieces A to D, with the exception that the temperature was raised to 
1200.degree. C. After 50 hours of testing, test piece E still had not 
broken. 
By way of comparison, a test piece identical to test piece D which had the 
highest performance at intermediate temperatures, was subjected to fatigue 
testing at 1200.degree. C., and it broke after 8 hours, while a test piece 
identical to test piece E was subject to fatigue testing at 600.degree. 
C., and it broke after 7 hours. 
Thus, in order to provide effective protection against oxidation over a 
wide range of temperatures, it is advantageous to combine the performance 
provided by a self-healing phase that is effective at intermediate 
temperature with the performance provided by a self-healing phase that is 
effective at high temperature. 
To this end, test pieces F were made using the method described with 
reference to FIG. 1, with the exception that in the last two densification 
sequences, the B.sub.4 C+C self-healing phase was replaced by an Si--B--C 
self-healing phase, thus giving a sequenced matrix as shown in FIG. 4. In 
addition, the mass percentage of free carbon in the self-healing phases 
decreased from a value of 26% in the first-formed phase (the phase closest 
to the fibers) to a value of 8% in the last-formed phase, passing through 
values of 20% and 15% (B/Si equal to about 4.6) in the second and third 
phases, as shown in FIG. 4. 
Test pieces F were subjected to fatigue testing as described above, 
respectively at 600.degree. C. and at 1200.degree. C. No breakage was 
observed after 100 hours at 600.degree. C., or after 50 hours at 
1200.degree. C. 
Finally, test pieces G having an SiC+B.sub.4 C matrix without any excess 
free carbon were likewise subjected to fatigue testing at 600.degree. C. 
and at 1200.degree. C. and 85 hours. Breakage was observed after 7 hours 
at 600.degree. C. and after 35 hours at 1200.degree. C. 
The results of tests performed on test pieces D, E, F, and G are summarized 
in the following table. 
______________________________________ 
% free C in 
Fatigue Fatigue 
testingng 
testing 
Material phases 
at 600.degree. C. 
at 1200.degree. C. 
______________________________________ 
D 26% 100 h, break at 
SiC/B.sub.4 C + C 
no break 
8 h 
matrix 
E 50 h, 
SiC/Si - B - C + C 
7 h no break 
matrix 
F 100 h, 
50 h, 
SiC/B.sub.4 + C/ 
starting no break no break 
Si - B - C + C 
matrix from the 
fibers 
SiC/B.sub.4 C 
0% break at 
matrix 35 h 
______________________________________ 
Naturally, the invention can be implemented with glass precursors other 
than those mentioned in the above implementations, and using a ceramic 
other than SiC for completing the matrix. Examples of glass precursors and 
of ceramic matrix precursors for CMC composite materials that are 
protected against oxidation are abundant in the state of the art. 
In addition, the number of self-healing phases interposed in the matrix 
together with ceramic phases may be other than four. This number must be 
at least 1 when only one type of self-healing phase is provided, and it 
must be not less than the number of different types of self-healing phase 
that are provided. In this respect, more precise coverage of a very wide 
range of temperatures can be sought by forming successive self-healing 
phases of different compositions, beginning by those that are most 
effective at low temperatures and terminating by those that are most 
effective at high temperatures. 
Finally, it will be observed that when a plurality of self-healing phases 
that are effective at different temperatures are formed with precursors 
for glasses of different compositions, there is no need for all of them to 
include excess free carbon. Thus, for example, in a material having one or 
more self-healing phases that are effective at intermediate temperatures 
and comprising boron carbide and excess free carbon, it is possible to 
incorporate one or more self-healing phases that are effective at higher 
temperatures comprising an Si--B--C system but without any excess free 
carbon.