Manufacturing process for a composite material

Abstract

Method for manufacturing a composite material. The present invention relates to a method for manufacturing a composite material, comprising at least: - the formation of a boron nitride interphase having a coherence length greater than 3 nm in the porosity of a fibrous structure (2) by chemical vapor infiltration in a cold-walled reactor (4), the interphase being formed from a gaseous phase (10) comprising a precursor mixture of triethylamine borane and ammonia and by imposing a temperature greater than or equal to 1225°C and a pressure less than or equal to 100 mbar. Figure for the abstract: Fig. 2.

Description

Title of the invention: Method for manufacturing a composite material. Technical field.

[0001] The present exposition relates to a method of manufacturing a composite material making it possible to modulate the strength of the fiber / matrix interfacial bond of a part made of ceramic matrix composite material (“CMC”) by controlling the pressure imposed during the formation of the interphase by chemical vapor infiltration (“CVI”). Previous technique

[0002] Ceramic matrix composite materials (CMCs) withstand temperatures ranging from 600°C to 1400°C. Due to their superior high-temperature resistance, CMCs require less cooling. Since this cooling is traditionally obtained from the compressor, which impacts the turbomachine's efficiency, CMCs improve engine efficiency, thereby reducing fuel consumption. Furthermore, their use helps optimize turbomachine performance, notably by reducing the overall mass of the turbomachine, which further contributes to lower fuel consumption and thus a significant reduction in pollutant emissions.

[0003] CMC parts can be manufactured by forming a silicon carbide pre-densification matrix within the porosity of a fibrous preform, followed by the introduction of silicon carbide particles and silicification. This method makes it possible to obtain a fully dense, high-modulus Si-SiC matrix and a high linearity limit composite. In the aforementioned range, the fibers are initially coated with an isotropic boron nitride interphase with a low degree of crystallization, which acts as a mechanical fuse, primarily through a fiber / BN debonding that allows the energy of cracking to be absorbed and the cracks to be deflected from the matrix to prevent them from reaching the fibrous reinforcement.

[0004] However, the fracture behavior (elongation at break) of the parts obtained by this range can be variable, thus reducing the damage tolerance zone of the material.

[0005] More generally, it is desirable to allow modulation of the strength of the interfacial fiber / matrix bond of the CMCs so as to adapt it to the intensity of the thermomechanical stresses encountered in the intended application. Description of the invention

[0006] The present description relates to a method for manufacturing a composite material, comprising at least: - the formation of a boron nitride (BN) interphase having a coherence length greater than 3 nm in the porosity of a fibrous structure by chemical vapor infiltration in a cold-walled reactor, the interphase being formed from a gaseous phase comprising a precursor mixture of triethylamine borane and ammonia (NH3) and by imposing a temperature on the surface of the fibrous structure greater than or equal to 1225°C and a pressure less than or equal to 100 mbar.

[0007] Triethylamine borane, designated by "TEAB", corresponds to a complex having the following chemical formula: ((C2H5)3N:BH3).

[0008] The invention is based on the combined use of a specific precursor mixture with a cold-wall CVI technique which, by varying the pressure applied in the reactor, allows modulation of the degree of (structural) anisotropy and, secondarily, of the porosity of the BN, and thus modulation of the fiber / matrix interfacial bond strength to adapt it to the intensity of the thermomechanical stresses encountered in the intended application. In particular, the invention makes it possible to access a wide range of working pressures, notably above 10 mbar, without leading to undesirable by-products such as are generally encountered in hot-wall CVI techniques at these pressures. Generally speaking, in the prior art of hot-wall furnaces, pressure does not influence the anisotropy of the formed material, unlike the present invention, as will be detailed below.

[0009] Furthermore, heating the fibrous structure with cold walls prevents homogeneous phase nucleation phenomena, which lead to powder formation in the gas phase and therefore to poor BN interphase formation efficiency and reactor contamination. This eliminates the need to limit the temperature conditions applied to control this phenomenon, thus contributing to a more flexible process. Moreover, BN has the advantage of oxidizing to form a liquid oxide, B2O3, which acts as a diffusion barrier against oxygen, preventing the surrounding atmosphere from affecting the fibrous reinforcement. The conditions implemented also allow the formation of a boron nitride with an improved degree of crystallization compared to the range mentioned above, this boron nitride having a coherence length greater than 3 nm.

[0010] In one embodiment, the imposed pressure is less than or equal to 60 mbar, preferably less than or equal to 55 mbar, preferably less than or equal to 50 mbar, preferably less than or equal to 45 mbar, preferably less than or equal to 40 mbar, preferably less than or equal to 35 mbar, preferably less than or equal to 30 mbar, preferably less than or equal to 25 mbar, preferably less than or equal to 20 mbar, preferably less than or equal to 15 mbar, preferably less than or equal to 10 mbar, preferably less than or equal to 5 mbar.

[0011] Reducing the pressure applied during interphase formation helps to accentuate the structural anisotropy of the resulting BN, thus improving its crack deflection capacity and making the interphase more suitable for demanding applications in terms of thermomechanical stresses. Indeed, the interphase can essentially consist of sp2 hybridized BN, locally composed of stacks of atomic planes separated by weak interplanar bonds (van der Waals interactions). The more the planes are preferentially oriented parallel to the axis or surface of the fiber, i.e., the higher the structural anisotropy, the more the weak atomic interplanar bonds weaken the bond between the fiber and the matrix (the deflection of matrix cracks is then favored).Conversely, the more isotropic the BN structure, the more locally there are planes with strong intra-plane covalent bonds oriented perpendicular to the fiber axis or surface, which strengthen the bond between the fiber and the matrix.

[0012] According to one example, the imposed pressure can be between 5 mbar and 60 mbar, or between 5 mbar and 55 mbar, or between 5 mbar and 50 mbar, or between 5 mbar and 45 mbar, or between 5 mbar and 40 mbar, or between 5 mbar and 35 mbar, or between 5 mbar and 30 mbar, or between 5 mbar and 25 mbar, or between 5 mbar and 20 mbar, or between 5 mbar and 15 mbar, or between 5 mbar and 10 mbar.

[0013] According to one example, the imposed pressure can be between 10 mbar and 60 mbar or between 10 mbar and 55 mbar, or between 10 mbar and 50 mbar, or between 10 mbar and 45 mbar, or between 10 mbar and 40 mbar, or between 10 mbar and 35 mbar, or between 10 mbar and 30 mbar, or between 10 mbar and 25 mbar, or between 10 mbar and 20 mbar, or between 10 mbar and 15 mbar.

[0014] According to one example, the imposed pressure can be between 15 mbar and 60 mbar or between 15 mbar and 55 mbar, or between 15 mbar and 50 mbar, or between 15 mbar and 45 mbar, or between 15 mbar and 40 mbar, or between 15 mbar and 35 mbar, or between 15 mbar and 30 mbar, or between 15 mbar and 25 mbar, or between 15 mbar and 20 mbar.

[0015] According to one example, the imposed pressure can be between 20 mbar and 60 mbar or between 20 mbar and 55 mbar, or between 20 mbar and 50 mbar, or between 20 mbar and 45 mbar, or between 20 mbar and 40 mbar, or between 20 mbar and 35 mbar, or between 20 mbar and 30 mbar, or between 20 mbar and 25 mbar.

[0016] According to one example, the imposed pressure can be between 25 mbar and 60 mbar or between 25 mbar and 55 mbar, or between 25 mbar and 50 mbar, or between 25 mbar and 45 mbar, or between 25 mbar and 40 mbar, or between 25 mbar and 35 mbar, or between 25 mbar and 30 mbar.

[0017] According to one example, the imposed pressure can be between 30 mbar and 60 mbar or between 30 mbar and 55 mbar, or between 30 mbar and 50 mbar, or between 30 mbar and 45 mbar, or between 30 mbar and 40 mbar, or between 30 mbar and 35 mbar.

[0018] According to one example, the imposed pressure can be between 35 mbar and 60 mbar or between 35 mbar and 55 mbar, or between 35 mbar and 50 mbar, or between 35 mbar and 45 mbar, or between 35 mbar and 40 mbar.

[0019] According to one example, the imposed pressure can be between 40 mbar and 60 mbar or between 40 mbar and 55 mbar, or between 40 mbar and 50 mbar, or between 40 mbar and 45 mbar.

[0020] According to one example, the imposed pressure can be between 45 mbar and 60 mbar, or between 45 mbar and 55 mbar, or between 45 mbar and 50 mbar.

[0021] According to one example, the imposed pressure can be between 50 mbar and 60 mbar, or between 50 mbar and 55 mbar, or between 55 mbar and 60 mbar.

[0022] According to a particular case of the invention, the imposed pressure may be less than or equal to 2.5 mbar.

[0023] Such operating conditions allow the obtaining of turbostratic BN with greater anisotropy and with single-crystal zones of r-BN.

[0024] In one embodiment, the fibrous structure is heated by microwaves during the formation of the interphase. In this case, the material of the fibrous structure is chosen to couple with the microwaves and can, for example, be silicon carbide or carbon. Alumina fibers can also be used in this embodiment provided that the heating is initiated indirectly at a low temperature with a susceptor made of SiC, for example.

[0025] The invention is not limited to such an embodiment, however, heating in a cold-walled reactor can be achieved by induction, by laser heating or by resistive heating, for example in contact with the fibrous structure.

[0026] In one embodiment, the temperature imposed on the surface of the fibrous structure is greater than or equal to 1300°C. This temperature may be between 1225°C and 1375°C or between 1300°C and 1375°C or between 1225°C and 1300°C.

[0027] In one embodiment, the fibrous structure corresponds to one or more strands, each strand being formed of a plurality of filaments, and the interphase is formed in the interfilament porosity of each strand.

[0028] This case corresponds to the formation of the interphase at the strand stage, before the formation of the fibrous preform from such strands.

[0029] In particular, the strand(s) can each have a core temperature Te which satisfies the following relationship: Te > l,l*Ts, where Ts denotes the temperature at the surface of the strand considered. In particular, we can have Te > l,2*Ts, or l,l*Ts < Te < l,3*Ts or l,2*Ts < Te < l,3*Ts.

[0030] A higher core temperature can compensate for the difficulty of filling the strand(s) and thus contribute to obtaining an even more uniform interphase in the interfilament porosity. The temperature gradient between the core and the surface results from cold-wall operation and the tension imposed on the strand(s).

[0031] In particular, the interphase can be formed while the strand(s) are transported through the reactor.

[0032] Alternatively, the fibrous structure corresponds to a fibrous preform of a part made of composite material to be obtained.

[0033] This case corresponds to the formation of the interphase in the porosity of the already formed fibrous preform.

[0034] In particular, the fibrous preform can be obtained by three-dimensional weaving.

[0035] Such a feature makes it possible to further improve the mechanical properties of the part by using a non-delaminating reinforcement.

[0036] The present description also relates to a method for manufacturing a part made of ceramic matrix composite material, comprising: - the formation of the boron nitride interphase having a coherence length greater than 3 nm in the interfilament porosity of a plurality of strands by implementing a process as described above, - the production of a fibrous preform forming the fibrous reinforcement of the part to be obtained from said strands, after formation of the interphase, comprising the formation of a fibrous blank by implementing one or more textile operations, and the shaping of this blank, and - the densification of the fibrous preform thus obtained by a ceramic matrix.

[0037] The present description also relates to a method for manufacturing a part made of ceramic matrix composite material, comprising: - the production of a fibrous preform forming the fibrous reinforcement of the part to be obtained, including the formation of a fibrous blank by implementing one or more textile operations, and the shaping of this blank, - the formation of the boron nitride interphase having a coherence length greater than 3 nm in the porosity of the fibrous preform thus obtained by implementing a process as described above, and - the densification of the fibrous preform thus obtained by a ceramic matrix.

[0038] In one embodiment, the part made of composite material is a turbomachine part. Brief description of the drawings

[0039] [Fig.1] Fig.1 represents a succession of steps of a first example of a process for manufacturing a part in CMC according to the invention.

[0040] [Fig.2] Fig.2 represents, schematically and partially, an example of device for forming the interphase which can be used in the process of [Fig.1]. [Fig.3] Fig.3 represents, schematically and partially, a cross-sectional view of a strand in which the interphase has been formed in the interfilament porosity as part of the process corresponding to Figures 1 and 2. [Fig.4] Fig.4 represents two electron-selected area diffraction (ESAD) images of two boron nitride interphases having an improved degree of crystallization and having different microstructures, in particular different degrees of structural anisotropy, obtained in two processes according to the invention. [Fig.5] Fig.5 represents two high-resolution transmission electron microscopy images of the interphases corresponding to Fig.4.

[0041] [Fig.6] The [Fig.6] is a selected area electron diffraction pattern of another boron nitride interphase obtained in a process according to the invention. [Fig.7] Fig.7 represents, schematically and partially, an example of a part made of composite material according to the invention. [Fig.8] Fig.8 represents a succession of steps of a second example of a process for manufacturing a part in CMC according to the invention. [Fig.9] Fig.9 represents, schematically and partially, an example of a turbomachine part that can be obtained within the framework of the invention. Description of the implementation methods

[0042] The invention is now described by means of figures, which are provided for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0043] Fig. 1 represents a succession of steps of a first example of a manufacturing process for a CMC part according to the invention.

[0044] An interphase is first made in the porosity of a plurality of strands each formed of a plurality of filaments by chemical infiltration in vapor phase in a cold wall reactor (step E10).

[0045] The implementation of this step is illustrated in [Fig. 2], in which the device 1 comprises a reactor 4 in which a plurality of strands 2 are transported by being driven by a conveying system 6, comprising here first 6a and second 6b sets of pulleys. Each set 6a or 6b comprises one or more pulleys. During the formation of the interphase, the strands 2 are transported by the conveying system 6 from the inlet end 5a to the outlet end 5b. The Conveyor system 6 is configured to transport the strands 2 through reactor 4 along a conveyor axis Y. In the example shown, the conveyor axis Y is parallel to the longitudinal axis X of device 1. The strands 2 are tensioned between pulleys 6a and 6b and between the inlet and outlet ends 5a and 5b. Due to the applied tension, the filaments 20 of the strands 2 can separate, leading to the inter-filament porosity of each strand 2 being filled by the interphase (filling the internal porosity of the strands 2). In one variant, a minimum mechanical tension allows the filaments 20 of the strands 2 to be tightened, making heating more efficient and enabling a temperature gradient between the core and the strand surface that compensates for the difficulty of filling. The strands 2 can be continuously transported through reactor 4 during interphase formation.In this case, the strands 2 do not stop while they are being transported through reactor 4.

[0046] The strands 2 may be made of silicon carbide. Examples of usable silicon carbide strands are commercially available under the references "Nicalon", "Hi-Nicalon", "Hi-Nicalon-S", or Tyranno SA3 from UBE Industries. The ceramic strands 2 may have an oxygen content of 1% or less. The "Hi-Nicalon-S" strands, for example, have this characteristic. Alternatively, the strands 2 may be made of carbon or alumina.

[0047] Figure 3 is a cross-sectional view taken perpendicular to the X and Y axes and representing the strand 2 after the formation of the interphase 30, which is formed, in particular, within the strand 2 and between the filaments 20 constituting it. The interphase 30 can be formed at the contact of the filaments 20 of the strands. As illustrated, the interphase 30 can also cover the external surface of the strand 2.

[0048] The interphase 30 is obtained by injecting a gaseous phase 10 into the treatment reactor 4 through an inlet orifice 7. The unreacted gaseous phase is pumped out through an outlet orifice 8 (arrow 11). The device 1 also includes a heating system configured to heat the strands 2 to form the interphase 30. The reactor 4 has cold walls, and the heating system is configured to directly heat the strands 2, for example, by microwaves, laser, induction, or resistive heating.

[0049] According to the invention, the gas phase 10 comprises a precursor mixture of triethylamine borane and ammonia. The gas phase 10 may further comprise a diluent gas, for example, nitrogen. The diluent gas may be inert. The gas phase 10 may essentially comprise triethylamine borane, ammonia, and any diluent gas present.

[0050] The fibrous structure, here formed by the strands 2, is heated, on its surface, to a temperature greater than or equal to 1225°C, for example greater than or equal to 1300°C, For example, the temperature in reactor 4 can be between 1225°C and 1600°C or between 1300°C and 1600°C, and a pressure of 100 mbar or less is applied in reactor 4. The surface temperature of the fibrous structure can be between 1225°C and 1375°C, for example, between 1300°C and 1375°C or between 1225°C and 1300°C. As mentioned above, the core temperature of the fibrous structure can be 20% to 30% higher than its surface temperature. As indicated above, the pressure applied in reactor 4 during the formation of interphase 30 is adjusted to modulate the degree of structural anisotropy of the BN. The temperature applied during the formation of interphase 30 is adjusted according to the material used for the fibrous structure to prevent its degradation.

[0051] Generally, the isotropic or anisotropic nature of boron nitride can be demonstrated in the composite by transmission electron microscopy (TEM) using techniques known to those skilled in the art. X-ray diffraction (XRD) can also be used to assess whether an isolated layer of BN (not part of a composite material) has a higher or lower degree of crystallization. The degree of crystallization of boron nitride is, for example, determined based on the coherence length. A coherence length of 3 nm or less indicates boron nitride with a low degree of crystallization, while a coherence length greater than 3 nm indicates crystalline boron nitride with a higher degree of crystallization.In the invention, the boron nitride interphase has a coherence length greater than 3 nm, for example greater than or equal to 4 nm, or even between 4 nm and 5 nm, or greater than or equal to 6 nm. The coherence length can be measured by XRD for a BN layer deposited in isolation not forming part of a composite material. In the case of a deposit forming an interphase of a composite material, the coherence length can be measured from selected-area electron diffraction patterns (from the thicknesses of the diffraction arcs).

[0052] In general, the crystallographic structure of boron nitride can be highlighted by grazing incidence X-ray diffraction (GIXRD).

[0053] In general, the gas introduction flow rates in reactor 4 can satisfy the following conditions during interphase formation: - ratio [NH3 introduction rate] / [TEAB introduction rate] between 11 and 20, and - ratio [diluent gas introduction rate] / [(NH3 introduction rate) + (TEAB introduction rate)] between 1 and 11, for example between 3 and 7, for example approximately equal to 5.

[0054] Figure 4 corresponds to selected area electron diffraction (SAED) images of the BN deposits obtained on a strand moving through a device of the type that of [Fig. 2]. In both cases, the interphase was formed from a gaseous phase comprising a precursor mixture of triethylamine borane and ammonia, by imposing a surface temperature of 1300°C under microwave radiation. The pressure imposed in reactor 4 was 55 mbar for the left image and 10 mbar for the right image. [Fig. 4] highlights a clear difference in the aperture angle (AO) of the arcs obtained by selected-area electron diffraction between the 55 mbar deposition and the 10 mbar deposition, reflecting a variation in the structural anisotropy of the BN obtained.

[0055] Fig. 5 corresponds to the high-resolution transmission electron microscopy images of Fig. 4. We observe the formation of a porous isotropic turbostratic BN layer in the image on the left (55 mbar), and a dense anisotropic turbostratic BN layer for the image on the right (10 mbar).

[0056] Figure 6 shows a selected-area electron diffraction image of another BN deposit obtained at 1300°C with an applied pressure of 2.5 mbar in reactor 4. In this case, a coherence length of approximately 5.1 nm and an aperture angle of 18° are obtained.

[0057] Once the interphase 30 is formed, the process continues by producing a fibrous preform (step E20 in [Fig. 1]) using the strands 2 thus treated. A fibrous blank is thus first formed from these strands 2, for example by three-dimensional weaving, that is, a weaving method in which at least some of the warp strands interlock with weft strands over several weft layers. The three-dimensional weave can, for example, be an "interlock" weave, that is, a weave in which each layer of warp strands interlocks with a plurality of weft strand layers, with all the strands in the same warp column having the same movement in the plane of the weave. The roles of the warp and weft can be reversed, and this reversal is also to be considered as being covered by the claims. The use of other types of 3D weaving is, of course, not outside the scope of the invention.Various suitable weaving techniques are described in document WO 2006 / 136755. It is also possible to start with fibrous textures such as two-dimensional fabrics or unidirectional sheets, and to obtain reinforcement by draping such fibrous textures over a form. These textures can optionally be joined together, for example by sewing or the implantation of threads, to form the reinforcement.

[0058] The blank is then shaped in a shaping tool in a manner known per se, so as to obtain the fibrous preform which substantially has the shape and dimensions of the part to be obtained.

[0059] The process is then continued by densifying the fibrous preform thus obtained with a ceramic matrix, that is to say, filling the porosity of the preforms using such a matrix (step E30 in [Fig. 1]). The formation of the matrix uses techniques known per se.

[0060] The CMC part 100 obtained after the formation of the matrix 40 is illustrated in [Fig. 7]. In the example considered, the interphase 30 is in contact with the strands 2 (filling their internal porosity) and the matrix 40 is in contact with the interphase 30. The interphase 30 is sandwiched between the strands 2 and the matrix 40. The interphase 30 has a debriding function and acts as a mechanical fuse by deflecting cracks propagating towards the strands 2, thus improving the material's lifespan. The local thickness el of the interphase 30 can be greater than or equal to 40 nm, for example, 400 nm; this local thickness el can be between 40 nm and 2 pm, preferably between 400 nm and 600 nm. In general, the local thickness of the interphase can be measured by scanning electron microscopy (SEM) or transmission electron microscopy (TEM) using techniques known to the person skilled in the art.Chemical vapor infiltration allows for the creation of a uniformly thick layer across a cross-section of a strand. Unless otherwise specified, the cross-section of a strand is taken perpendicular to its length. The local thickness values ​​described above can be verified on at least one cross-section of all or part of the strands.

[0061] According to one possible example, the matrix 40 may comprise a first part in silicon carbide, or in silicon nitride, in contact with the interphase 30 and a second part covering this first part in Si-SiC resulting from a silicification.

[0062] The preform can first be consolidated in the forming tool by depositing a consolidation matrix phase onto the interphase 30. This deposition can be carried out by chemical vapor deposition. The consolidation phase can be silicon carbide or silicon nitride. The consolidation phase here corresponds to the portion of the matrix 40 illustrated in [Fig. 7]. The thickness of the consolidation phase is sufficient to consolidate the fibrous preform, that is, to bind the strands together sufficiently to allow the preform to be handled while retaining its shape without the assistance of a holding tool.

[0063] The consolidated preform can then be removed from its forming tooling and a silicon carbide pre-densification matrix can be deposited on the consolidation phase, for example by chemical vapor phase infiltration.

[0064] After formation of the pre-densification matrix, a slip can be introduced into the residual porosity of the preform comprising silicon carbide particles, optionally with added carbon particles, and then infiltration can be carried out with silicon or a silicon alloy in the molten state in order to to obtain a Si-SiC matrix phase. The Si-SiC matrix phase forms a densification matrix which, here in contact with, covers the pre-densification matrix.

[0065] We have just described an example of a possible range for manufacturing the matrix, but those skilled in the art will recognize that variations are possible without departing from the scope of the invention. For example, the matrix can be formed in whole or in part by a polymer infiltration and pyrolysis technique (“Polymer Infiltration and Pyrolysis”; “PIP”), or entirely by chemical vapor infiltration.

[0066] Figure 8 illustrates a manufacturing variant of the composite material part of Figure 7 in which the fibrous preform is first formed in a manner similar to that described above (step E200), and then the interphase 30 is formed within the porosity of the already produced fibrous preform using the same conditions as described above, except that the preform is not in motion during deposition (step E100). A ceramic matrix is ​​then formed on the interphase as described above (step E300).

[0067] Generally speaking, part 100 can be a turbine part of an aircraft engine. By way of example, part 100 can be a turbine ring or a turbine ring sector, a moving blade, a fixed blade, a combustion chamber wall, a distributor or part of a distributor.

[0068] Thus, [Fig. 9] schematically illustrates a turbine ring sector 100 obtained within the framework of the invention. The sector 100 comprises a portion 102 forming an annular base defining the inner face of the turbine ring and an outer face from which extend at least two portions 104 forming tabs intended for attachment to the ring support structure. A plurality of sectors 100 are arranged circumferentially to define the gas flow path in a turbine, for example, in a high-pressure turbine.

[0069] The expression "between ... and ..." should be understood as including the bounds.

Claims

Demands

1. A method for manufacturing a composite material, comprising at least: - the formation (E10; El00) of a boron nitride interphase (30) having a coherence length greater than 3 nm in the porosity of a fibrous structure (2) by chemical vapor phase infiltration in a cold-walled reactor (4), the interphase being formed from a gaseous phase (10) comprising a precursor mixture of triethylamine borane and ammonia and by imposing a temperature on the surface of the fibrous structure greater than or equal to 1225°C and a pressure less than or equal to 100 mbar.

2. A method according to claim 1, wherein the imposed pressure is less than or equal to 60 mbar.

3. A method according to claim 2, wherein the imposed pressure is less than or equal to 45 mbar, preferably less than or equal to 35 mbar, preferably less than or equal to 25 mbar.

4. A method according to claim 3, wherein the imposed pressure is less than or equal to 15 mbar, preferably less than or equal to 2.5 mbar.

5. A method according to any one of claims 1 to 4, wherein the fibrous structure (2) is heated by microwaves during the formation (E10; E100) of the interphase (30).

6. A method according to any one of claims 1 to 5, wherein the temperature imposed on the surface of the fibrous structure is greater than or equal to 1300°C.

7. A method according to any one of claims 1 to 6, wherein the fibrous structure (2) corresponds to one or more strands, each strand being formed of a plurality of filaments (20), and wherein the interphase (30) is formed in the interfilament porosity of each strand.

8. Method according to claim 7, wherein the strand or strands each have a core temperature Te which satisfies the following relationship: Te > l,l*Ts, where Ts denotes the temperature at the surface of the strand considered.

9. A method according to any one of claims 1 to 6, wherein the fibrous structure corresponds to a fibrous preform of a part in composite material to be obtained, the fibrous structure being obtained for example by three-dimensional weaving.

10. A method for manufacturing a part (100) of ceramic matrix composite material, comprising: - the formation (E10) of the boron nitride interphase (30) having a coherence length greater than 3 nm in the interfilament porosity of a plurality of strands (2) by implementing a method according to claim 7 or 8, - the production (E20) of a fibrous preform forming the fibrous reinforcement of the part to be obtained from said strands, after formation of the interphase, comprising the formation of a fibrous blank by implementing one or more textile operations, and the shaping of this blank, and - the densification (E30) of the fibrous preform thus obtained by a ceramic matrix.

11. A method for manufacturing a part (100) made of ceramic matrix composite material, comprising: - the production (E200) of a fibrous preform forming the fibrous reinforcement of the part to be obtained, comprising the formation of a fibrous blank by implementing one or more textile operations, and the shaping of this blank, - the formation (E100) of the boron nitride interphase (30) having a coherence length greater than 3 nm in the porosity of the fibrous preform thus obtained by implementing a process according to claim 9, and - the densification (E300) of the fibrous preform thus obtained by a ceramic matrix.

12. Method according to claim 10 or 11, wherein the composite material part (100) is a turbomachine part.

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