CA2038859C

CA2038859C - Process for the manufacture of a thermostructural composite material having a carbon interphase between its reinforcement fibers and its matrix

Info

Publication number: CA2038859C
Application number: CA002038859A
Authority: CA
Inventors: Louis Heraud; Jean-Philippe Rocher
Original assignee: Societe Europeenne de Propulsion SEP SA
Current assignee: Safran Aircraft Engines SAS
Priority date: 1990-04-02
Filing date: 1991-03-22
Publication date: 1997-09-16
Anticipated expiration: 2011-03-22
Also published as: JPH04224182A; FR2660304A1; FR2660304B1; JP2518975B2; EP0451043A1; CA2038859A1; DE69122984D1; DE69122984T2; EP0451043B1

Abstract

The thermostructural composite material is produced by forming a fibrous preform made from refractory fibers coated with a sizing agent. The preform is densified by a refractory matrix, in particular ceramic, while a carbon interphase is provided between the fibers and the matrix. The fibrous preform is made from refractory fibers coated with a sizing agent of the type that leaves behind a carbon residue upon thermal deccmposition. The carbon interphase is produced by the thermal degradation of the sizing agent, occurring for example after the preform is made. The sizing agent is chosen among phenolic, acrylic and cellulose resins, and in particular carboxymethylcellulose.

Description

Z038~35~

F~cess for the ~vr~ a of a ther o~tructural co-posite ~aterial havmg a carbon i,n~l.ase between its reinforce~ent fibers and its ~atrix.

1. Field of the invention.
The present invention relates to a process for the manufacture of a thermostructural composite material, involving the steps of making a fibrous preform from refractory fibers,and densifying the preform with a refractory matrix while providing a carbon interphase between the fibers and the matrix.
A thermostructural composite material consists of a fibrous preform and a matrix which, together provide high mechanical characteristics that can be maintained at elevated temperatures.
The process according to the invention can be applied to any type of fibrous preform. The latter can be made by packing short fibers, e.g. to provide a felt, or by winding fibers. The texture can also comprise a superposition or winding of one-dimensional (lD) plies or two-dimensional (2D) plies made from strands, cables or threads, each consisting of an assembly of fibers. It is also possible to use three-dimensional (3D) preforms, such as those obtained by needling superimposed or wound plies, or by three-dimensional weaving.
Refractory fibers forming the preform are chosen among those fibers that can withstand a temperature of at least 800~C in an atmosphere that is non-reactive for the fiber, without modification or fundamental deterioration. Examples of such fibers include carbon fibers and ceramic fibers (s;l~csn carbide, alumina, zirconia or the like).
As to the refractory material forming the matrix, any refractory ceramic composition of the oxide, carbide, boride, nitride type or the like, as well as carbon, may be used.
The invention is more particularly aimed at composite materials having a ceramic or part-ceramic matrix, known 2038~ 9 as CMCs. These materials are employed in applications that require high-mechanicsl properties, such as in the manufacture of engine or jet components, or structural parts of space vehicles exposed to substantial heating effects.
05 In a CMC, the matrix in itself has a low breaking strain and tensile strength, a high susceptiblity to kirving, owing to its ceramic nature. In order to obtain a material that is resistant to shocks and crack propagation, the fiber-to-matrix link must be made weak, so that any crack arriving at the fiber-matrix interphase cannot continue across the fiber and cause the latter to break.
On the other hand, it is known that if high mechanical properties are to be obtained, and in particular a high resistance to flexing, then the bond between the fibers and the matrix must be rather high, in order to ensure that forces are transmitted to the fibers of the reinforcement.

2. Prior art.
A good compromise between these seemingly conflicting requirements has been found by interposing an intermediate coating, or interphase, between the fibers and the matrix. Such an interphase has a small thickness and a shear resistance which is lower than that of the matrix. Accordingly, when a crack in the matrix reaches the region of the fiber, the strains at the bottom of the crack will be released by the interphase. This interphase may e.g. be formed from a material having a laminar structure, such as laminar pyrolytic carbon or boron nitride, as described in United States Patent N~ 4 752 503.
According to the method taught in that document, the interphase is deposited on the fibers by chemical vapor deposition within the preform, before depositing in the matrix. This makes it necess~ry to carry out at least one operation between the steps of making the preform and forming the matrix. It will be noted that such operations involving chemical vapor deposition are generally long and require complex installations.

8 5 ~

S mYr~lURY OF I~IE INrVE~rllO N WTIlH OEJEC~rS
Itis an ~ m ofan aspectof~he present mven~on tD
provide a ~rocess ~h~ the ~ n ~ ~he fibers and the nnabix can be produced more simply and rapidly, so reducing the time 05 during which the chemical vapor deposition installations are used, and thus reducing the manufacturing cost of the composite materials without significantly affecting their mechanical and thermal properties.
Accolding bo an a~ ofthein~ on,these ~ ms ~ achieved by making the fibrous preform from refractory fibers coated with a sizing agent of a type that leaves behind a carbon residue upon thermal decomposition, w:,~leby the carbon interphase is provided by thermal decomposition of the sizing agent.
The carbon interphase results from a thermal treatment carried out after making the preform. The treatment is conducted under an inert atmosph~l-e and at a sufficiently high temperature (in general greater than 300~C) to cause pyrolysis of the sizing agent. This temperature should not, of course, exceed the limit beyond which the fibers can be damaged.
Z0 Advantageously, when the matrix is at least partly formed by a chemical vapor deposition ope,~Lion conducted in an enclosure containing the preform, the thermal treatment of the sizing agent is achieved during the temperature rise within the enclosure necess~ry for the chemical vapor deposition of the matrix material.
The matrix can also be at least partially formed by liquid implegnaLion of the preform, by means of a matrix precursor, and can subsequently ulld~l-go a thermal treatment to yield a ceramic material constituting the matrix, by a h ansfolmation of the precursor. In this case, the preform is advantageously made from fibers coated with the sizing agent and impleyllaLed with the matrix precursor prior to the heat treatment, the latter thereby producing both the carbon intel-phase, by thermal decomposition of the sizing agent, and the ceramic material constituting the matrix, by transformation of the 4 D 2 ~ ~ 8 8 5 ~

precursor.
Thus, irrespective of whether the matrix is formed by a gsseous or liquid process, the carbon inte~-phase can be created advantageously during the matrix-forming process, without calling 05 upon additional operations.
The l?r~c~ss acco~ ng to an aspect of ~e invention l~Uil~,S the use of fibers coated with a sizing agent susceptible of leaving behind a carbon residue by thermal decomposition, i.e. having a non-negl;9i~le coke content. This is not the case with the sizing agents normally used for refractory fibers such as polyvinyl acetates, polyvinyl alcohols, or epoxies heving a zero or neglis;hle coke content. Moreover, it has in some cases been necessa y to remove the sizing agent prior to forming the interphase : such an operation is obviated in the process accoLding to the invention.
Different types of sizing agents may be used for carrying out the invention. Generally, the sizing agent is selected amongst polymers having long carbon chains, such as those used as precursors in the manufacture of carbon fibers. The coke content should be sufficient to leave behind a substantially continuous coating on the elementary fibers, after the thermal process. Preferably, the coke content should be not less than 20%
(pe-cer,~age weight of the carbon residue with respect to the weight of sizing agent). As a comparison, the coke contents of polyvinyl acetate or epoxy sizing agents normally used are o% and 5,7% ~-espe~ively after thermal processing at 900~C.
Candidate materials for the sizing agent include acrylic polymers and cellulose polymers having a coke content generally in the range of 20 to 60% by weight. Among the latter, carboxymethylcelluloses can be chosen, these already being used as a sizing agent.
The p.~cess accor~ing to an aspect of the invention may also be used in the manufacture of sequenced-matrix composite materials following a p~-ocedure comprising the steps of making a fibrous preform by means of strands or threads made of refractory fibers . ~ . ~~

- 2038~3S9 coated with a sizing agent, and densifying the preform with a part-ceramic sequenced matrix, with a carbon interphase formed between the fibers and the matrix, and at least a second interphase formed between two ceramic phases of the matrix. In 05 this case, according to the invention, the preform is made from refractory fibers coated with a sizing agent of the type that leaves behind a carbon residue upon thermal decomposition, and the threads or xLl-ands are coated, prior to forming the matrix, by a substance capable of yielding the material constituting the second 1~ interphase through a thermal treatment, thereby producing both the first interphase by decomposition of sizing agent, and the second interphase by a transformation of the substance coating the strands or threads.
The strands or threads may be coated with a product susceptible of leaving behind a carbon residue upon thermal decomposition, so as to form a second interphase that is also made of carbon.
As in the foregoing, when the matrix is at least partially formed by chemical vapor deposition, the thermal treatment that simultaneously yields the first and second interphases is advantageously conducted during temperature rise of the enclosure into which the preform is inserted for the chemical vapor deposition operation. The infiltration by the ceramic material forming the matrix then occurs, in particular within the residual pores of each strand or thread, so as to constitute a ceramic phase between the first and the second interphases.
When the matrix is at least partially formed by a liquid process, the impregnaLion of the preform by a matrix precursor is obtained after the thermal treatment that gives arise to the first and second interphases. In particular, this impregnation takes place within the residual pores of each strand or thread.
Accordingly, there is formed a ceramic phase, between the first and second interphases, during a second thermal treatment through which the ceramic matrix material is obtained by transfoI Lion of the precursor.

8~5 ~
5a Other aspects of this invention are as follows:
A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a refractory matrix with a carbon interphase being provided between the fibers of the preform and the matrix, said ~oce~- comprising the steps of:
providing elementary refractory fibers substantially coated with a sizing agent, said sizing agent being a material capable of leaving a carbon residue to form an interphase layer upon thermal decomposition;
forming a fibrous preform from said coated elementary refractory fibers formed into threads;
providing the carbon interphase layer as a substantially continllollc coating on the elementary refractory fibers in the preform by thermal decomposition of the sizing agent upon heating of the fibrous preform; and subsequently densifying said fibrous preform with a refractory matrix material.
A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a part-ceramic se~l~nc~ matrix, with a first carbon interphase being provided between the fibers of the preform and the matrix, and with a second interphase being provided between two ceramic ph~C~C of the matrix, the process comprising the steps of:
providing elementary refractory fibers substantially coated with a sizing agent capable of leaving a carbon residue to form the first carbon interphase upon thermal decomposition;
fabricating a fibrous preform from strands or threads made of said coated elementary refractory fibers;
impregnating the fibrous preform with a precursor of the material of the second interphase capable of ~ ~ ~ 3 8 8 5 ~

5b being transformed into the second interphase material by thermal treatment, whereby each strand or thread of the preform is covered with said precursor;
heating the fibrous preform to obtain the first carbon interphase as a substantially continuous coating on said elementary fibers by thermal decomposition of the sizing agent and the second interphase by transformation of its ~e~L~or, each strand or thread showing a residual porosity; and subsequently densifying the fibrous preform with a ceramic matrix material to form a first ceramic phase between the first and second interphases by introduction of the ceramic matrix material into the residual porosity of the strands or threads, and a second ceramic phase separated from the first ceramic phase by the second interphase.
A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a refractory matrix with a carbon interphase being provided between the fibers of the preform and the matrix, said process comprising the steps of:
providing elementary refractory fibers;
coating said elementary fibers with a sizing agent, said sizing agent being a material capable of leaving a carbon residue to form a carbon interphase layer upon thermal decomposition;
forming a fibrous preform from said individually coated elementary refractory fibers formed into threads;
providing the carbon interphase layer as a substantially continuous coating on the elementary refractory fibers by thermal decomposition of the sizing agent upon heating of the fibrous preform; and subsequently densifying said fibrous preform with a refractory matrix material.

Z038~3S9 There will now be given different examples explaining how the process according to the invention can be implemented.
These examples are given as an indication only and do not limit the invention. Comparative examples are also given.

The single appended figure illustrates very schematically how two interphases are formed when the process according to the invention is applied to the manufacture of a composite material having a sequenced matrix.
DETAILED DESCRIPTION OF THE PRt~tK~Eu EMBODIMENTS
In all the examples that follow, the fibers used for the manufacture of the fibrous preform are silicon carbide fibers coated with a carboxymethylcellulose sizing agent whose coke content is 33% at 900~C.
Example 1 A one-dimensional (lD) fibrous preform, made from silicon carbide fibers coated with a carboxymethylcellulose sizing agent, is msnufactured by winding filamentary fibers around a tool serving to keep the fibers aligned along planar sections.
The preform is inserted inside a chemical vapor deposition furnace to be densified by a silicon carbide matrix.
The chemical vapor deposition of silicon carbide within the fibrous preform is a well-known operation, described e.g. in document FR-A-2 401 888.
Prior to the gaseous phase injection in the furnace, the preform is raised to the temperature required for the deposition process, i.e. around 1,000~C. During this temperature raising phase, the preform is maintained in the furnace, where it is exposed to an inert atmosphere, e.g. a partial vacuum containing nitrogen. As a result, the sizing agent on the fibers undergo a pyrolysis prior to the start of the matrix formation, leaving a carbon residue on the fibers that is to serve as an interphase between the fibers and the matrix.
The densification of the preform is then started and continues until the residual porosity reaches a ratio of 10% by volume.
In a bending test conducted on the thus-obtained composite material (lD-SiC/SiC with a C interphase arising from the thermal decomposition of the sizing agent) the measured 05 bending resistance yields a value Rf = 950 MPa.
Comparative example la The procedure is the same as for example 1, except that fibers coated with a polyvinyl acetate having a 0% coke content used in place of the carboxymethylcellulose sizing agent. The sizing agent is totally eliminated during the temperature rise prior to the silicon carbide infiltration.
The resistance Rf measured during a bending test is 750 MPa only, i.e. 20% less than in the previous example.
Comparative example lb The process is identical to that of example la, except that an interphase is formed by vapor phase infiltration of laminar pyrolytic carbon, prior to forming the matrix, as in the process described in document FR-A-2 567 874 already mentioned.
The sizing agent is totally eliminated during the temperature rise prior to the formation of the pyrocarbon interphase.
The bending resistance Rf measured during a bending test yields a value of 1,000 MPa.
The process according to the invention thel-efol-e grants a bending resistance which is very close to that obtained by chemical vapor deposition of a pyrolytic carbon interphase, without making such an operation necessary.
Example 2 A two-dimensional (2D) fibrous preform made from silicon carbide fibers coated with a carboxymethylcellulose sizing agent is produced by the following steps:
- piling flat plies of SiC fiber cloth of the type sold under the trade-name "Nicalon" by Nippon Carbon of Japan, and - holding the pile of plies in an adapted tool to maintain a fiber volume ratio of around 40%.
The preform is placed inside a chemical vapor deposition Z0~8859 furnace to be densified by the silicon carbide matrix. The densification is achieved as in example 1, the sizing agent undergoing pyrolysis during the temperature rise preceding the silicon carbide infiltration.
û5 A tractive test is conducted on the final composite, during which are measured: the tensile breaking strain ~R~ the Young's modulus E, and the tensile breaking strain ~ R. The results obtained are as follows:
~ R = 170 MPa lû E = 210 GPa ~ R = 0-35%
Comparative example 2a The process is identical to that of example 2 except that the fibers are coated with a polyvinyl acetate sizing agent ha~ing a 0% coke content, in place of the carboxymethylcellulose sizing agent. The sizing agent is totally eliminated during the temperature rise preceding the silicon carbide infiltration.
A tractive test is conducted under the same conditions as in example 2, giving the following results:
~ R = 140 MPa E = 230 GPa '~ R = 0.12%
Comparative example 2b The process is identical to that of example 2a, except that the interphase is produced by vapor phase infiltration of laminar pyrolytic carbon, prior to forming the matrix, as in the process described in document FR-A-2 567 874 mentioned above. The sizing agent is totally eliminated during the temperature rise prior to the formation of the pyrolytic carbon interphase.
A tractive test conducted under the same conditions as in example 2 gives the following results:
R = 170 MPa E = 210 GPa R = 0 3%

As before, it is observed that the process according to the invention makes it possible to obtain composite materials with the sought-sfter mechanical properties (high resistance and high breaking strain) to a degree comparable to that obtained by 05 chemical vapor deposition of a pyrolytic carbon interphase, without requiring such an expensive operation.
Example 3 This example relates to the manufacture of a material having a sequenced matrix, and more particul~rly to a material in which the matrix comprises at least two ceramic phases separated by an interphase made from a material having a higher breaking strain than the material forming the ceramic phases. Such a material is described in the French patent application filed as FR-89 02718.
A 2D fibrous preform is produced from a cloth made of SiC fibers coated with a carboxymethylcellulose sizing agent and impregnated with a substance capable of leaving behind, after thermal decomposition, the desired material for forming an interphase between the two ceramic phases of the matrix. This material being e.g. carbon, the impregnating product is chosen among polymers having a non-negligible coke content. Typically, the impleyllcLing product can be chosen among those polymers usable as a sizing agent according to the plesen~ invention. In the present example, the preform is impleyn~ed with carboxymethylcellulose, i.e. the same substance as the one constituting the sizing agent for the fibers.
The impreg"a~ion conditions are chosen in such a way that the impregna~ing product covers each strand or thread of the preform, without infiltrating inside these strands or threads, which are each formed from a multitude of elementary fibers. This is achieved by carrying out the impley"aLion under atmospheric pressure, e.g. by a simple dipping operation, using an impregnating product having a sufficiently high viscosity.
The impregnated plies of the cloth are piled flat and held in place by an adapted tool to maintain a volume percentage of 203~38S9 fibers of around 40%.
The thus-obtained preform is placed inside a chemical vapor deposition furnace to be densified by a s;licon carbide matrix using the same process as described with reference to 05 example 1.
During the temperature rise prior to the start of infiltration, the fiber's sizing agent, as well as the impregnaLing product of the preform, both undergo a pyrolysis, leaving behind a carbon residue.
As shown in the annexed drawing, there is then formed a first carbon interphase 11 around the elementary fibers 10, and a second carbon interphase 16 exhibiting a few discontinuities, around each strand of the fibers 15 that constitute a thread of the preform (in the figure, only a few el~ ~nLary fibers are shown, whereas a thread is normally formed from a large number of such fibers). The above results from the fact that the ilr~egnating product covers each thread, but does not penetrate within them.
Next, during the formation of the matrix, the silicon carbide is infiltrated within the accessible pores of the preform, including within the threads or ~LLands 15. There is then formed a ceramic phase 12 between the carbon interphases 11 and 16, and a ceramic phase 18 separated from phase 12 by interphase 16.
A tractive test is conducted on the thus-obtained sequenced matrix composite material. The measurement of the tensile sL~en~Lh ~ R~ Young's modulus E and breaking strain ~ R
yield the following results:
R = 210 MPa E = 230 GPa ~ R = 0 7%
These results show a significant improvement in the tractive behavior compared with the results given in example 2 and comparative example 2b.
Example 3 shows how it is possible to simply obtain a sequence comprising a first carbon interphase on the fibers, a 11 203~3859 first ceramic phase, a second carbon interphase and a second ceramic phase. The sequencing of the matrix can of course be continued by further alternating the formation of an interphase and a ceramic phase, the interphases being obtainable by OS impregnation-pyrolysis or by chemical vapor deposition.
It is obviously possible to produce a sequenced matrix composite material by forming an interphase between the fibers and the matrix using the process according to the invention, and then by forming the different successive phases of the matrix as described in patent application FR 89 02718 already mentioned.
It is economically advantageous to perform the thermal treatment yielding the interphase between the fibers and the matrix, and possibly a second interphase, during the temperature rise prior to the infiltration of the ceramic matrix. However, this thermal treatment may also be obtained independently, and not in association with the densification process. The treatment is in this case conducted in an inert atmosphere (e.g. vacuum or nitrogen).
In the foregoing examples, the ceramic matrix is formed by a gaseous process (chemical vapor deposition). The process according to the invention is equally applicable in the case where the ceramic matrix is formed by a liquid process, i.e. by impregnation of the preform by means of a liquid precursor of the ceramic matrix material, followed by a thermal treatment yielding the ceramic material by a cer ic inducing transformation of the precursor. The methods for obtaining a ceramic matrix by a liquid process are well known.
In the above case, the impregnation by the precursor can be achieved on the preform with the fibers already coated with the sizing agent. Accordingly, a single thermal treatment will yield both the interphase between the fibers and the matrix, by decomposition of the sizing agent, and the ceramic material of the matrix, by transformation of the precursor. The densification of the preform can then be continued by a liquid or a gaseous process, or even by alternating the deposition processes to obtain 12 20~8~3S9 a sequenced matrix.
The impregnation by the precursor can also be achieved on the preform after forming the fiber-matrix interphase by a first initial thermal treatment that is applied independently.
05 This will be the case especially when the carbon-fiber matrix interphase and a second carbon interphase are first produced, as explained in example 3. The impregnation by the matrix precursor is then carried out so that the precursor penetrates within each thread or strand of the fibers forming the preform. Accordingly, after a second thermal process transforming the precursor into a ceramic, there are obtained two ceramic phases separated by a second carbon interphase. The densification can then be continued by a liquid and/or gaseous process, possibly with a sequencing of the matrix.

Claims

1. A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a refractory matrix with a carbon interphase being provided between the fibers of the preform and the matrix, said process comprising the steps of:
providing elementary refractory fibers substantially coated with a sizing agent, said sizing agent being a material capable of leaving a carbon residue to form an interphase layer upon thermal decomposition;
forming a fibrous preform from said coated elementary refractory fibers formed into threads;
providing the carbon interphase layer as a substantially continuous coating on the elementary refractory fibers in the preform by thermal decomposition of the sizing agent upon heating of the fibrous preform; and subsequently densifying said fibrous preform with a refractory matrix material.

2. The process of Claim 1, wherein the densifying step comprises:
placing the fibrous preform in an enclosure;
heating the fibrous preform; and forming at least part of the matrix by a chemical vapor deposition carried out on the heated fibrous preform in the enclosure, the thermal decomposition of the sizing agent to form the carbon interphase on the fibers being achieved upon heating of the fibrous preform in the enclosure prior to the forming of the matrix.

3. The process of Claim 1, wherein the densifying step comprises:
impregnating the fibrous preform with a liquid precursor of the matrix material capable of being transformed into the matrix material by thermal treatment; and heating the impregnated fibrous preform to obtain both the carbon interphase by thermal decomposition of the sizing agent and the matrix material by transformation of its precursor.

4. The process of Claim 1, wherein the sizing agent comprises phenolic resins, acrylic resins, or cellulose resins.

5. The process of Claim 4, wherein the sizing agent contains carboxymethylcellulose resin.

6. A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a part-ceramic sequenced matrix, with a first carbon interphase being provided between the fibers of the preform and the matrix, and with a second interphase being provided between two ceramic phases of the matrix, the process comprising the steps of:
providing elementary refractory fibers substantially coated with a sizing agent capable of leaving a carbon residue to form the first carbon interphase upon thermal decomposition;
fabricating a fibrous preform from strands or threads made of said coated elementary refractory fibers;
impregnating the fibrous preform with a precursor of the material of the second interphase capable of being transformed into the second interphase material by thermal treatment, whereby each strand or thread of the preform is covered with said precursor;
heating the fibrous preform to obtain the first carbon interphase as a substantially continuous coating on said elementary fibers by thermal decomposition of the sizing agent and the second interphase by transformation of its precursor, each strand or thread showing a residual porosity; and subsequently densifying the fibrous preform with a ceramic matrix material to form a first ceramic phase between the first and second interphases by introduction of the ceramic matrix material into the residual porosity of the strands or threads, and a second ceramic phase separated from the first ceramic phase by the second interphase.

7. The process of Claim 6, wherein the densifying step comprises:
placing the fibrous preform with the strands or threads coated with the precursor in an enclosure;
heating the fibrous preform; and forming at least part of the ceramic matrix by a chemical vapor deposition operation carried out on the heated fibrous preform in the enclosure, the thermal decomposition of the sizing agent to form the first carbon interphase on the fibers and the transformation of the precursor into the second interphase material being achieved upon heating of the fibrous preform in the enclosure prior to the forming of the matrix.

8. The process of Claim 6, wherein the densifying step is carried out after the heating step and comprises:
impregnating the fibrous preform having the first and second interphases with a liquid precursor of the matrix material capable of being transformed into the matrix material by thermal treatment, the impregnation being carried out within the fibrous preform, including within the residual porosity of the strands or threads, and transforming the precursor or into the matrix material by a thermal treatment, whereby the first ceramic phase of the matrix is provided between the first and second interphases and the second ceramic phase of the matrix is provided, separated from the first ceramic phase by the second interphase.

9. The process of Claim 6, wherein the sizing agent comprises phenolic resins, acrylic resins, or cellulose resins.

10. The process of Claim 9, wherein the sizing agent contains a carboxymethylcellulose resin.

11. The process of Claim 6, wherein the strands or threads are coated with a precursor capable of leaving a carbon residue upon thermal treatment.

12. The process of Claim 11, wherein the strands or threads are coated with a resin comprising phenolic resins, acrylic resins, or cellulose resins.

13. The process of Claim 12, wherein the strands or threads are coated with a carboxymethylcellulose resin.

14. A process for the manufacture of a thermostructural composite material having a fibrous preform densified by a refractory matrix with a carbon interphase being provided between the fibers of the preform and the matrix, said process comprising the steps of:
providing elementary refractory fibers;
coating said elementary fibers with a sizing agent, said sizing agent being a material capable of leaving a carbon residue to form a carbon interphase layer upon thermal decomposition;
forming a fibrous preform from said individually coated elementary refractory fibers formed into threads;
providing the carbon interphase layer as a substantially continuous coating on the elementary refractory fibers by thermal decomposition of the sizing agent upon heating of the fibrous preform; and subsequently densifying said fibrous preform with a refractory matrix material.