Preform for CVI densification with directed flow

Abstract

Preform for directed flow CVI densification The invention relates to an annular fibrous preform (200) extending between an inner edge (220) and an outer edge (230) characterized in that it comprises at least one through groove (212) on at least one face (210, 240). Figure for abstract: Fig. 2

Description

Title of the invention: Preform for CVI densification with directed flow Technical field

[0001] The invention relates to the field of preforms for the manufacture of composite materials, and more particularly those densified by directed flow chemical vapor infiltration processes. Prior art

[0002] Thermostructural composite materials are remarkable for their properties high mechanical properties and their ability to retain these properties at high temperatures. Typical examples of such thermostructural composite materials are carbon / carbon composites comprising a porous substrate densified by a carbon matrix and ceramic matrix composites comprising a porous substrate densified by a ceramic matrix.

[0003] Chemical vapor infiltration densification processes are known. One or more porous substrates are placed inside an enclosure. A gas phase comprising one or more precursors of the matrix constituent material is introduced into the enclosure. The temperature and pressure conditions are adjusted to allow the gas phase to diffuse within the porosity of the substrates in order to deposit the matrix constituent material there by decomposition of a constituent of the gas phase or by reaction between several constituents.

[0004] Processes for densifying porous substrates require high temperatures to be reached, and in order to increase the productivity of the process, it is often considered to densify many porous substrates in a single densification chamber. In the case of annular porous substrates, it is generally considered to introduce them into the densification chambers in the form of stacks, the substrates having all their central passages aligned vertically and each of the substrates being separated from adjacent substrates by spacers.

[0005] In a known manner, the shims can be made of inconel or composite material and they make it possible to ensure the conditions for circulation of the reactive gas phase in the substrates under the desired pressure conditions and to ensure that the substrates are not in contact with each other.

[0006] However, the spacers arranged between each of the preforms represent a significant loss of space in the densification furnace.

[0007] The very advantageous thermomechanical characteristics of parts made of composite materials are sufficient to make them competitive despite a costly and complex manufacturing process, but the improvement of the manufacturing processes of such parts is still sought after. Statement of the invention

[0008] The invention aims specifically at improving existing directed flow chemical vapor infiltration processes, and proposes new preforms making it possible to avoid the use of spacers, which allows space to be saved in the densification furnace.

[0009] According to a first of its aspects, the invention relates to an annular fibrous preform extending between an internal edge and an external edge, characterized in that it comprises at least one through groove on at least one face.

[0010] For the purposes of the invention, a through groove is understood to be a groove made on the face of the preform and extending from its internal edge to its external edge.

[0011] Due to the presence of at least one through groove, when an annular preform according to the invention is stacked with another annular preform, according to the invention or not, the internal space of the stack and the external space remain in fluid communication.

[0012] The presence of the groove therefore makes it possible to use the preforms in stacks for conventional furnaces for chemical vapor infiltration processes with directed flow, without requiring the presence of spacers.

[0013] Indeed, in directed flow chemical vapor infiltration processes, openings are usually provided in the spacers between the internal space of the stack of preforms and the external space.

[0014] Such openings make it possible to ensure a controlled pressure difference between the external space and the internal space of the fiber preforms.

[0015] It is to the credit of the inventors to have envisaged that the presence of the grooves directly on the fiber preforms nevertheless makes it possible to obtain parts with thermomechanical characteristics and a shape identical to those of the methods of the prior art, while doing away with the need for spacers.

[0016] Indeed, conventionally, the external faces of the preforms are machined after the chemical vapor infiltration process, because these faces have been in contact with the gas phase and do not have exactly the same properties as the core of the preforms.

[0017] Furthermore, the dimensions of the densified parts must be precisely adapted to their final applications, for example their arrangement in a heat sink in the case where they are used as a friction member, for example for a brake disc.

[0018] Thus, and in particular in an embodiment where the grooves have dimensions smaller than the thickness of material removed by the final machining, the preforms of the invention allows the production of composite material parts in all respects identical to those of the methods of the prior art, but with a simplified process.

[0019] In one embodiment, the through groove(s) are straight grooves, i.e. the width of the groove is constant along the entire length of the groove, from the inner edge to the outer edge. For example, the recess created by a through groove on the face of the fiber preform may be rectangular in shape.

[0020] In one embodiment, the preform comprises a plurality of through grooves on at least one of its faces.

[0021] For example, the preform may comprise a plurality of through grooves distributed angularly over the entire face of the fiber preform.

[0022] Through grooves will be said to be “angularly distributed” if the angle formed by the directions of two neighboring grooves is identical for all the grooves present on the face of the fiber preform.

[0023] In one embodiment, at least one through groove is present on each of the faces of the fiber preform.

[0024] In the case where grooves are present on several faces, they may be identical or different.

[0025] In one embodiment, the through groove(s) have a variable width between the inner edge and the outer edge. This embodiment allows the geometry of the opening to be more precisely adjusted to the desired flow of the densification gas phase passing through the through groove.

[0026] For example, the edges of the groove(s) may be aligned with a radial direction of the preform.

[0027] In one embodiment, the through groove(s) have a width less than or equal to 20 mm.

[0028] In the case where the through groove(s) have a variable width, the groove may have a width greater than or equal to 10 mm and less than or equal to 20 mm.

[0029] In one embodiment, the width of the groove may increase from the inner edge of the preform to the outer edge of the preform.

[0030] In this embodiment, the amount of material to be removed after densification is reduced, which facilitates machining.

[0031] In one embodiment, the width of the groove may decrease from the inner edge of the preform to the outer edge of the fiber preform.

[0032] In one embodiment, the through groove(s) have a depth less than or equal to 2.5 mm.

[0033] This depth ensures that the groove has a depth less than the preform thickness removed during final machining.

[0034] In one embodiment, the thickness of removed fiber preform is less or equal to 5 mm, for example between 2.8 mm and 3.2 mm, or even equal to 3 mm.

[0035] In one embodiment, a face of the fiber preform comprising a through groove may further comprise a circular groove on this face of the fiber preform, the circular groove being in contact with the internal edge.

[0036] When such a groove is present, the through grooves are therefore located between the circular groove and the outer edge of the fiber preforms. In other words, the through grooves open into the groove.

[0037] This embodiment makes it possible to avoid the deposition of pyrocarbon on the internal diameter of the preform which can form a blister at the end of the process as has been observed for preforms not having such a groove.

[0038] In one embodiment, a face of the preform comprising a through groove may further comprise a circular groove on a face of the preform comprising a through groove, the circular groove being in contact with the outer edge.

[0039] When such a groove is present, the through grooves are therefore located between the circular groove and the internal edge of the preforms.

[0040] This embodiment makes it easier to disassemble the stacks of preforms after carrying out a densification process.

[0041] If it is or if they are present, the circular grooves in contact with the internal and / or external edge may have a depth less than or equal to 2.5 mm, and a width less than or equal to 5 mm.

[0042] As with through grooves, the furrows may be present on one face or on both faces of the preform.

[0043] The inventors further found that these dimensions allowed good circulation of the gas phase in a directed flow chemical vapor infiltration process.

[0044] Preferably, the annular fibrous preform may be a preform comprising silicon carbide SiC fibers or carbon fibers.

[0045] In one embodiment, the fibrous preform is a friction part preform, for example an aircraft brake disc preform.

[0046] In one embodiment, the annular fibrous preform has an internal diameter of between 15 and 25 cm, an external diameter of between 40 and 50 cm and a thickness of between 20 and 30 mm.

[0047] According to another of its aspects, the invention relates to a method of manufacturing an annular fibrous preform extending between an internal edge and an external edge, said preform comprising at least one through groove on at least one face, said method comprising at least the following steps: (a) a step of curing the annular fiber preform; then (b) a step of sculpting at least one through groove on at least one face of the hardened preform.

[0048] Unexpectedly, the inventors found that it was possible to simply sculpt the grooves on the preform provided that a first step (a) of hardening the preform is carried out.

[0049] Indeed, the sculpting of the preform takes place before densification thereof and the preform then has low mechanical strength. It is therefore not possible to sculpt the grooves on the face of the preform without special precautions due to its low mechanical strength.

[0050] However, once hardened, the preform can be sculpted precisely, and the fibrous nature of the preform is not sufficient for it to return to its initial shape.

[0051] In one embodiment, step (a) of curing the preform may be carried out by impregnation with a curing compound, for example polyvinyl alcohol (often referred to as PVA), polyethylene glycol (often referred to as PEG) or a mixture of these compounds.

[0052] For example, the hardening step (a) can be carried out by dipping the preform in a bath comprising a hardening compound, then optionally passing it through an oven.

[0053] Alternatively, the hardening step (a) can be carried out by impregnating the face(s) of the preform to be sculpted with a hardening compound, for example by means of a spray.

[0054] Such an embodiment allows the use of a reduced amount of hardening compound, which lowers the overall cost of the process.

[0055] In both previous cases, the use of a hardening agent allows relatively simple preparation of the preform.

[0056] In particular, it is not necessary to treat the sculpted hardened preform to remove the hardening agent, since the temperature required for the subsequent step of densifying the preform will be sufficient to achieve the removal by pyrolysis of the hardening compound, whether it impregnates all or only part of the preform.

[0057] In one embodiment, step (a) of hardening the preform can be broken down into two sub-steps (a1) and (a2).

[0058] In one embodiment, step (a) may comprise at least the following steps: (al) a step of saturating the annular fibrous preform with moisture; then (a2) a step of hardening the preform by lowering the temperature below 0°C, preferably less than or equal to -5°C, for example equal to -5°C.

[0059] Lowering the temperature during step (a2) of the preform saturated with moisture during step (a1) allows the introduced moisture to freeze and resulting in the hardening of the preform.

[0060] Such an embodiment is particularly preferred because it avoids the use of chemical compounds, which makes the process more environmentally friendly.

[0061] Furthermore, after step (b) of sculpting the fiber preform, the humidity saturating the preform will require a simple baking in order to have dry preforms before introducing them into the densification oven.

[0062] In one embodiment and regardless of the embodiment of step (a), step (b) may be carried out by means of a cutting object or by abrasion, for example by grinding.

[0063] All of steps (a) and (b) make it possible to obtain a preform as described above, and having at least one through groove.

[0064] According to another of its aspects, the invention relates to a method for manufacturing a part made of composite material comprising at least the following steps: (c) densification by a directed flow chemical vapor infiltration process of a stack of annular fibrous preforms, at least one of which is as described above; then (d) separation of the densified preforms obtained at the end of the directed flow chemical vapor infiltration process; then (e) a final machining step of the faces of the densified fiber preforms.

[0065] The method of the invention makes it possible to avoid the use of spacers, unlike the directed flow chemical vapor infiltration methods of the prior art.

[0066] Thus, in one embodiment, the stack of annular fiber preforms comprises annular fiber preforms directly in contact with each other.

[0067] Given the usual dimensions of a stack of preforms, a densification furnace for a chemical vapor infiltration process, and a densification shim, the absence of the shims can represent an increase in the number of preforms impregnated during a complete densification cycle of at least 7.5%, or even between 7.5 and 15%. The absence of the spacer shims allows, for a process of the invention, a substantial saving of space in the furnace.

[0068] Indeed, the groove(s) present on the face of the fiber preforms make it possible to ensure the role usually assigned to spacer wedges.

[0069] In one embodiment, all the preforms of the stack conform to those described above. Such a characteristic is however not necessary and it may be advantageous to have the usual annular fiber preforms, i.e. without through grooves, in order to optimize the pressure difference between the inside and the outside of the preforms, this being a function of the di- dimensions and the number of grooves present in the final stack.

[0070] Due to the absence of spacers, densification by the chemical vapor infiltration process, however, causes the preforms of the same stack to stick together.

[0071] It is to the credit of the inventors to have noted that a separation of the preforms obtained nevertheless made it possible to obtain a part made of composite material by a process which was generally more efficient than the processes of the prior art.

[0072] In one embodiment, spacers, for example made of ceramic material, may be present at the top and bottom of the stack of preforms.

[0073] Such spacers in fact make it possible to avoid any sticking of the preforms at the ends of the stack with elements of the furnace, for example trays on which the preforms are arranged.

[0074] Step (d) of separating the preforms can be carried out in different ways.

[0075] In one embodiment, separation step (d) may be carried out by means of of a mechanical separation.

[0076] For example, the separation step (d) can be carried out by a mechanical cutting operation, for example by means of a saw, for example a circular saw of sufficient diameter or a saw whose blade is driven in a back-and-forth movement.

[0077] Indeed, the mechanical cutting step allows easy separation of the preforms, and does not damage the preform over a thickness greater than that which will be taken up during the final machining step (e).

[0078] In one embodiment, it is possible to carry out steps (d) and (e) by means of a single cutting step, but with a saw whose blade has a thickness corresponding to the thickness to be removed between two preforms.

[0079] Alternatively, the inventors have found that step (d) of separating the fiber preforms can be carried out in a heat treatment step carried out at a temperature greater than or equal to 1600°C, preferably greater than or equal to 2000°C.

[0080] Such a step (d) of separating the preforms is particularly preferred, insofar as a high temperature heat treatment is recommended to finalize the obtaining of parts made of composite material.

[0081] Indeed, a high-temperature heat treatment makes it possible to cause a shrinkage of the matrix formed on the faces of the fiber preform, which causes the separation of two successive preforms of a stack which have attached to each other during the chemical vapor infiltration process due to the deposition of reactive gas phase on their respective faces. In the case where the preforms are not completely separated, their adhesion is sufficiently reduced to that a moderate mechanical effort allows the separation to be completed.

[0082] In one embodiment, the separation step (d) may be a mechanical separation step followed by a heat treatment carried out at a temperature greater than or equal to 1600°C, preferably greater than or equal to 2000°C.

[0083] The method of manufacturing a part made of composite material finally comprises a step (e) of machining the face of the preform.

[0084] Indeed, the presence of the through grooves is not desired in the final part and it is therefore necessary to remove the grooves from the final part.

[0085] It should however be noted that this step does not introduce any additional complexity compared to prior art methods of manufacturing a composite material part.

[0086] Indeed, in a conventional manner, after they have been obtained by a chemical vapor infiltration process, the faces of composite material parts are machined, because the faces of the fiber preforms have been more exposed to the reactive gas phase during the chemical vapor infiltration process and therefore do not have the same properties as the rest of the part.

[0087] For example, step (e) makes it possible to machine the faces of the fiber preform to a thickness of more than 3.0 mm, for example between 3.0 mm and 5.0 mm.

[0088] This machining step (e) makes it possible to remove the face of the preforms which has been in contact with the reactive gas phase of the chemical vapor infiltration process and thus ensures good homogeneity of the properties of the composite material parts obtained at the end of the process by removing the part most exposed to the reactive gas phase. In addition, this step (e) makes it possible to bring the parts into conformity with the desired dimensional tolerances to allow their assembly within the heat sinks.

[0089] In one embodiment of the method of the invention, the composite material part comprises carbon fibers and a carbon matrix.

[0090] In one embodiment of the method of the invention, the composite material part is a friction part, for example a brake disc for an aircraft.

[0091] The above-mentioned characteristics and advantages, as well as others, will appear on reading the detailed description which follows, of exemplary embodiments of the proposed device and method. This detailed description refers to the attached drawings. Brief description of the drawings

[0092] [Fig. 1] [Fig. 1] schematically represents an installation for the realization of a directed flow chemical vapor infiltration process.

[0093] [Fig.2] [Fig.2] schematically represents a fiber preform of the invention in one embodiment of the invention.

[0094] [Fig.3] [Fig.3] schematically represents a fiber preform of the invention in one embodiment of the invention.

[0095] [Fig.4] [Fig.4] schematically represents a stack of fiber preforms for carrying out a method of the invention.

[0096] [Fig.5] [Fig.5] schematically represents a fiber preform of the invention in one embodiment of the invention.

[0097] [Fig.6] [Fig.6] is a flowchart comprising the steps of a method for manufacturing a part made of composite material according to the invention. Description of the embodiments

[0098] The invention is now described by means of figures, present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0099] An embodiment of an installation 100 for the chemical vapor infiltration of porous preforms is described in relation to [Fig.l].

[0100] [Fig.l] schematically shows a gas phase chemical infiltration densification installation 100, the loading zone 140 of which is delimited by a cylindrical side wall 101, a bottom wall 102 and an upper wall 103.

[0101] Substrates to be densified 130, here annular fibrous preforms, can be arranged in the loading zone 140 in a plurality of annular vertical stacks 131 which rest on a loading tray 120. The latter comprises a plurality of passages 121 aligned with the internal volumes 130a of the stacks and each stack is closed at its upper part by a cover 132.

[0102] Preferably, the stacks 131 of substrates 130 rest on the loading tray 120 and can be divided into several superimposed sections separated by one or more intermediate trays 122 having central passages 122a aligned with those of the substrates 130. [Fig.l] shows stacks 131 in which the fiber preforms 130 are in direct contact with each other. Indeed, each stack of substrates 131 comprises at least one preform as described and comprising through grooves, which ensures fluid communication between the internal volumes 130a and the external volume 141, and makes it possible to dispense with the spacers usually used.

[0103] The dimensions of the through grooves of the preforms 130 of the invention can be chosen so as to substantially balance the pressure between the volumes 130a and 141.

[0104] A gaseous flow, represented by arrows, containing one or more gaseous precursors of the material constituting the matrix, is admitted into the furnace through an inlet orifice 104 delimited by a conduit 106.

[0105] The gas phase is then conveyed through the passages 121 of the loading tray 120 into the internal volumes 130a of the cells 131. The gas phase then passes into the volume 141 external to the cells inside the loading zone 140. The effluent gas is extracted through a passage 105 formed in the upper wall 103, the passage 105 being connected by a pipe 107 to suction means, such as a vacuum pump (not shown).

[0106] In one embodiment, the gas phase may pass through a preheating zone, for example arranged between the inlet orifice 104 and the loading plate 120.

[0107] In the embodiment shown, the gas phase simply enters a distribution zone 170, which allows good distribution of the phase between the different internal volumes 130a of the stacks 131 of preforms 130.

[0108] In the example described here, the heating means 110 of the installation is an induction heating element. More precisely, the cylindrical side wall 101 delimiting the loading zone 140 constitutes an armature, or susceptor, for example made of graphite, which is coupled with an inductor 108 located outside the furnace and formed of at least one induction coil. An insulator 109 is interposed between the inductor 108 and the wall 101. In a well-known manner, the heating of the furnace is ensured by heating the armature 101 when the inductor 108 is supplied with an alternating voltage. For this purpose, the coil(s) of the inductor are connected to an alternating voltage generator (not shown).

[0109] The magnetic field created by the inductor 108 induces in the wall 101 (susceptor) an electric current which causes the latter to heat up by the Joule effect, the elements present inside the wall 101 being heated by radiation.

[0110] The heating means 110 of the installation 100 can be provided by other means such as electrical heating means consisting for example of heating resistors embedded in the wall 101.

[0111] [Fig.2] shows an annular fiber preform in a first embodiment lization of the invention.

[0112] The annular fibrous preform 200 comprises an inner edge 220 and an outer edge 230.

[0113] Preferably, and as in the mode shown here, the preform 200 has a circular shape.

[0114] The preform further comprises two flat faces 210 and 240 which will be called respectively upper face 210 and lower face 240. However, it should be understood that this name is in no way limiting.

[0115] In the embodiment of [Fig.2], the preform 200 comprises through grooves 212 on its upper face 210.

[0116] Here the through grooves 212 are straight grooves, but this is not limiting of the invention.

[0117] [Fig.3] represents an alternative embodiment of a preform 200 according to the invention.

[0118] In the alternative embodiment shown in [Fig. 3], the preform comprises grooves 212 on both its upper face 210 and its lower face 240.

[0119] In this embodiment, the through grooves 212 are identical on both faces but this is not necessary.

[0120] As shown in Figures 2 and 3, it should not be understood from the terms used that the through groove 212 passes through the preform from the upper face 210 to the lower face 240. Indeed, and as shown, the through groove(s) pass through the preform 200 from the inner edge 220 to the outer edge 230.

[0121] In an alternative embodiment to that shown in [Fig.4], the preforms can also be arranged so that the through grooves are not opposite each other.

[0122] The two fiber preforms correspond to preforms 200 according to [Fig.2], the through grooves of which have been arranged opposite each other. In other words, the two upper faces 210 of the preforms are arranged in contact with each other.

[0123] In the embodiment shown, a single channel is formed by the joining of the through grooves 212 of the two preforms, allowing fluid communication between the interior of the preforms and the exterior of the preforms 200.

[0124] In an alternative embodiment to that shown in [Fig.4], two preforms 200 can also be arranged in the manner shown but in such a way that the through grooves of one face are not opposite the through grooves of the other face.

[0125] [Fig.5] represents a preform in one embodiment of the invention.

[0126] The preform 200 comprises a circular groove 250 at the inner edge 220 on the upper face 210 of the preform. The through grooves 212 then extend between the circular groove 250 and the outer edge 230.

[0127] The circular groove 250 is obtained on the face of the preform 200 at the same time and in the same manner as the through grooves 212.

[0128] [Fig.6] represents in the form of a flowchart the different stages of a process for manufacturing a part from composite material.

[0129] The method comprises a plurality of steps numbered (a) to (e) already described above.

[0130] Step (a) is a step of hardening the fiber preform.

[0131] The latter ensures that the preform has sufficient mechanical strength for to be able to be sculpted.

[0132] Step (b) is a step of sculpting the hardened preform.

[0133] It can advantageously be carried out by cutting using a cutting object or by abrasion, for example by grinding. Indeed, the hardening of the fiber preform allows precise cutting of the preform, which would not be possible on the unhardened fiber preform, the latter not having sufficient mechanical strength.

[0134] Step (c) is a step of densifying a stack of preforms by a directed flow chemical vapor infiltration process.

[0135] Such a method has been described in connection with [Fig. 1].

[0136] For example, the method may be carried out with a reactive gas phase comprising, for example, methane, ethane, propane, butane or a mixture of these gases, the reactive gas phase optionally comprising a neutral carrier gas, for example, argon.

[0137] In particular, such a reactive gas phase makes it possible to obtain a pyrocarbon matrix in the annular fibrous preforms, but it is understood that those skilled in the art know how to choose other reactive gas phases to obtain other matrix phases.

[0138] For example, the chemical vapor infiltration process can be carried out in an enclosure at a temperature between 950 and 1050°C for a duration between 100 and 800 hours.

[0139] The method may further comprise a step (d) of separating the densified preforms obtained at the end of the chemical vapor infiltration process.

[0140] As described above, this step can be carried out by mechanical cutting or by means of high-temperature heat treatment.

[0141] Such step (d) can be carried out by heat treatment at a temperature greater than or equal to 2000°C for more than 60 minutes.

[0142] Preferably, this heat treatment can take place, even after a step (d) carried out by mechanical cutting.

[0143] Finally, step (e) is a step of machining the faces of the densified parts. The machining is typically carried out in order to grind the lower and upper faces of the densified parts.

[0144] For example, such step (e) may be performed by grinding the face of the preform 200.

[0145] Machining (e) may aim to remove a thickness of the face of the preform of between 2.0 and 5.0 mm.

[0146] The composite material parts obtained at the end of the process which has just been described may be carbon / carbon composite material parts, for example friction parts, or even brake discs.

Claims

Claims

1. Annular fibrous preform (200) extending between an inner edge (220) and an outer edge (230) characterized in that it comprises at least one through groove (212) on at least one face (210, 240).

2. The preform (200) of claim 1, wherein the at least one through groove (212) has a constant width from the inner edge (220) to the outer edge (230).

3. Preform (200) according to claim 1 or 2, wherein the annular fibrous preform comprises a plurality of through grooves (212) distributed angularly over the entire face of the preform.

4. Preform (200) according to any one of claims 1 to 3, in which at least one through groove (212) is present on each of the faces (210, 240) of the fibrous preform.

5. A method of manufacturing an annular fibrous preform (200) extending between an inner edge (220) and an outer edge (230), said preform comprising at least one through groove (212) on at least one face (210, 240), said method comprising at least the following steps: (a) a step of hardening the annular fibrous preform; then (b) a step of sculpting at least one through groove on at least one face of the hardened preform.

6. A manufacturing method according to claim 5, wherein the curing step (a) comprises at least the following steps: (a1) a step of saturating the annular fibrous preform (200) with moisture; then (a2) a step of curing the preform by lowering the temperature below 0°C.

7. A method of manufacturing a part made of composite material comprising at least the following steps: (c) densification by a directed flow chemical vapor infiltration method of a stack (131) of annular fiber preforms (200, 130), at least one of which is according to claims 1 to 4; then (d) separation of the densified preforms obtained at the end of the directed flow chemical vapor infiltration method; then (e) a step of final machining of the faces of the densified fiber preforms.

8. Method of manufacturing a part made of composite material according to claim 7, in which the stack (131) of annular fibrous preforms comprises annular fibrous preforms directly in contact with each other.

9. A method according to claim 7 or 8, wherein step (d) is a heat treatment step carried out at a temperature greater than or equal to 1600°C.

10. A method according to any one of claims 7 to 9, wherein the composite material part comprises carbon fibers and a carbon matrix.

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