EP3717440A1

EP3717440A1 - Method for treating silicon carbide fibres

Info

Publication number: EP3717440A1
Application number: EP18819536.6A
Authority: EP
Inventors: Sylvie Loison; Lucile HENRY; Cyril Aymonier; Chrystel HUGUET
Original assignee: Centre National de la Recherche Scientifique CNRS; Safran Ceramics SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Safran Ceramics SA
Priority date: 2017-11-29
Filing date: 2018-11-16
Publication date: 2020-10-07
Also published as: US11498877B2; FR3074169B1; WO2019106257A1; US20200377416A1; CN111630022A; FR3074169A1

Abstract

The invention relates to a method for treating at least one silicon carbide fibre (10), said method comprising at least the following steps: a) formation of a silica layer (22) on the surface of a silicon carbide fibre, having an oxygen content lower than or equal to 1 atomic percent, the silica layer being formed by bringing said fibre into contact with an oxidising medium having a temperature higher than or equal to 50°C and a pressure higher than or equal to 1MPa, and step a) being such that the treated fibre has a surface layer predominantly consisting of carbon and during step a) the surface carbon is removed by oxidation and the silicon carbide is oxidised in order to form the silica layer, or the treated fibre has a surface layer comprising a silicon oxycarbide and during step a) the oxycarbide is oxidised in order to form the silica layer, and b) removal of the silica layer formed, by hydrothermal treatment of the fibre obtained after carrying out step a), in which the fibre is treated with water at a pressure between the saturated vapour pressure and 30 MPa and at a temperature lower than or equal to 400°C.

Description

Process for treating silicon carbide fibers

The present invention relates to a method for treating at least one silicon carbide (SiC) fiber for improving the quality of the bonding of this fiber to a coating covering it.

Background of the invention

The manufacture of composite material parts reinforced with silicon carbide fibers is well known. It comprises the production of a fibrous preform based on silicon carbide fibers whose shape is close to that of the part to be manufactured and the densification of the preform by a matrix.

It is well known that, in composite materials with fibrous reinforcement, the characteristics of the fiber-matrix interface have a great influence on the mechanical properties of the material. It has been shown that satisfactory behavior can be obtained, in particular with respect to shocks and propagation of cracks, by forming on the fibers, before formation of the matrix, a thin layer of interphase by Example pyrolytic carbon deposited vapor phase or boron nitride. However, it remains interesting to further improve the mechanical properties of the composite material parts.

A solution has previously been proposed to this problem in WO 2016/207534. This document discloses a method for stripping the surface of SiC fibers of the "Hi-Nicalon S" type prior to the formation of an interphase. In this method, there is first oxidation of the surface of the fibers to form a surface layer of silica, then treatment with an acidic liquid medium comprising at least hydrofluoric acid (HF) in order to eliminate the Silica layer formed. After this treatment, etched fibers having a homogeneous surface of silicon carbide imparting improved bonding of the treated fiber to the deposited interphase are obtained.

This solution provides satisfactory results. However, the environmental compatibility of this method, using HF acid, could be improved. In addition, this method requires the realization of different fiber handling steps: rinsing and drying treated fibers or fiber displacements of an enclosure to another, it would be desirable to have a simpler and faster treatment to achieve.

Object and summary of the invention

According to a first aspect, the invention aims at a process for treating at least one silicon carbide fiber, the method comprising at least the following steps:

a) forming a silica layer on the surface of a silicon carbide fiber having an oxygen content of less than or equal to 1 atomic%, the silica layer being formed by contacting this fiber with a an oxidizing medium having a temperature greater than or equal to 50 ° C. and a pressure greater than or equal to 1 MPa ("megapascal", 1 MPa = 10 ⁶ Pa), the oxidizing medium being in particular under supercritical conditions, and

b) removal of the silica layer formed, by hydrothermal treatment of the fiber obtained after implementation of step a) in which the fiber is treated with water at a pressure between the saturation vapor pressure and MPa and at a temperature of less than or equal to 400 ° C.

Silicon carbide fibers having an oxygen content of less than or equal to 1 atomic% have a surface layer which is responsible for reducing the quality of fiber adhesion to a coating formed on their surface. This reduction in the quality of adhesion results in a reduction of the mechanical properties of the composite material part formed from these fibers.

These fibers generally have a surface layer predominantly formed of carbon. Such a surface layer typically has a thickness of between 1 nm and 1 μm. Within a given set of fibers, however, some fibers may have a surface layer of different composition. In particular, it is possible to find differences in terms of composition of the surface layer within the same yarn formed by a plurality of fibers and / or between several yarns of the same batch. The fibers may thus have a surface layer comprising a silicon oxycarbide (compound base of silicon, carbon and oxygen). In the latter case, the surface layer typically has a thickness of less than or equal to 100 nm.

The present invention provides a solution for removing this surface layer regardless of the composition thereof and, therefore, for improving the quality of the fiber / coating bond in order to improve the mechanical properties of the obtained composite material part.

When a fiber having a surface layer predominantly formed of carbon is treated, there is during step a) elimination of the surface carbon by oxidation, and oxidation of the silicon carbide to form the silica layer. When a fiber having a surface layer comprising a silicon oxycarbide is treated, there is during step a) oxidation of the oxycarbide to form the silica layer. In both cases, the silica layer formed is then removed during step b). It is also possible to treat a set of fibers, a first portion of the fibers initially having a silica surface layer, and a second portion of the fibers initially having a surface layer of carbon and / or oxycarbide of silicon. In the latter case, there is no evolution of the surface layer during step a) for the first part of the fibers, and after step b) a homogeneous surface is obtained where the silica has removed for each of the first and second parts of the fibers.

In the invention, a surface silica layer is, during step a), formed by voluntary oxidation and then a hydrothermal treatment is used to obtain a homogenous etched surface of silicon carbide. In addition, the pressure and temperature conditions described above for the oxidizing medium during step a) make it possible to obtain a layer of surface silica that the fiber initially has a surface layer predominantly made of carbon, or a surface layer comprising a silicon oxycarbide. This surface silica layer is then removed by the hydrothermal treatment of step b). Step b) is a step of hydrothermal treatment of the fiber obtained after implementation of step a) so as to remove the silica layer formed in step a).

Thus, the treatment according to the invention makes it possible to obtain the same composition on the surface of the treated fiber, whatever the composition of its superficial layer before treatment. In other words, the treatment according to the invention produces, after pickling, the same surface composition despite the existing diversity in terms of surface composition for the fibers before treatment. This improves the quality of the fiber / coating bond, and therefore the mechanical properties of the part.

Furthermore, the fact of combining the use of an oxidizing medium under pressure and temperature during step a) and the use of a hydrothermal treatment during step b) advantageously makes it possible to carry out steps a) and b) in the same enclosure, and therefore to remove manipulation steps of the treated fiber, thus simplifying the process and thus decreasing its duration.

In addition, this treatment makes it possible to dispense with the use of HF acid and to improve the environmental compatibility of the pickling process. The fact of no longer using HF acid eliminates the need for rinsing and drying done after the acid treatment, thus further shortening the treatment time.

The thickness of the silica layer formed during step a) may be greater than or equal to 1 nm, for example at 5 nm, for example at 10 nm. The thickness of the silica layer formed during step a) may be between 1 nm and 1 μm or between 1 nm and 100 nm.

In an exemplary embodiment, the oxidizing medium has a dioxygen content by volume of between 5% and 100%, the temperature of the oxidizing medium being between 50 ° C and 700 ° C and the pressure of the oxidizing medium being between 1 MPa and 30 MPa.

These conditions of step a) make it possible to efficiently form the silica layer while oxidizing the underlying SiC as little as possible, while implementing a relatively low temperature and working pressure. These conditions are compatible with industrialization of the process.

In an exemplary embodiment, the oxidizing medium has a dioxygen content by volume of between 5% and 50%, the temperature of the oxidizing medium being between 200 ° C and 700 ° C and the pressure of the oxidizing medium being between 13 MPa and 25 MPa, for example between 15 MPa and 25 MPa. These conditions of step a) make it possible to further optimize the control of the formation of the silica layer.

In an exemplary embodiment, the oxidizing medium is a mixture between an inert compound and dioxygen.

In an exemplary embodiment, during the hydrothermal treatment, the fiber is treated with water at a temperature of between 100 ° C. and 370 ° C. and, for example, at a pressure of between 5 MPa and 30 MPa.

These conditions allow an effective removal of the silica layer and are compatible industrialization of the process.

In an exemplary embodiment, before step a), a step of desizing or disintegrating the fiber is carried out by carrying out a preliminary hydrothermal or solvothermal treatment.

In cases where the conditions implemented during step a) do not make it possible to carry out the disintegration or desizing, such a preliminary treatment can be carried out. When this preliminary treatment is performed, it can advantageously be implemented in the same chamber as steps a) and b), which limits the overall duration of treatment.

In an exemplary embodiment, the method further comprises the following step:

c) deposition of an interphase layer on the surface of the fiber obtained after implementation of step b), the interphase layer being for example boron nitride, optionally doped with silicon, silicon nitride or pyrolytic carbon.

Preferably, the interphase layer is a boron nitride layer.

In an exemplary embodiment, a plurality of silicon carbide fibers each having an oxygen content of less than or equal to 1% atomic percentage can be processed.

The present invention also relates to a process for manufacturing a fibrous preform comprising at least one step of treating a plurality of silicon carbide fibers by implementing a method as described above and a step of forming a fiber preform. a fiber preform by implementing one or more textile operations from said plurality of fibers thus treated. The present invention also relates to a process for manufacturing a fibrous preform comprising at least one step of forming a fibrous preform by implementing one or more textile operations from a plurality of silicon carbide fibers exhibiting each an oxygen content less than or equal to 1% atomic percentage and a processing step of said plurality of fibers, once the preform formed, by carrying out a method as described above.

The present invention also relates to a method of manufacturing a composite material part comprising at least one step of manufacturing a fiber preform by implementing a method as described above followed by a forming step of at least one carbon matrix phase or a ceramic material densifying said fiber preform.

The composite material part may for example be a turbomachine part, for example a turbomachine blade.

Brief description of the drawings

Other features and advantages of the invention will emerge from the following description, given in a non-limiting manner, with reference to the appended drawings, in which:

FIGS. 1A-1C are sectional views showing, in a schematic and partial manner, the evolution of the structure of a silicon carbide fiber initially having a surface layer comprising a silicon oxycarbide during the implementation of FIGS. steps a) and b) according to the invention,

FIGS. 1D to 1F are sectional views showing, schematically and in part, the evolution of the structure of a silicon carbide fiber initially having a superficial layer formed mainly of carbon during the implementation of the steps a) and b) according to the invention,

FIG. 2 represents the evolution, as a function of depth, of the atomic percentages of the silicon elements (atomic percentage noted SIA), carbon (atomic percentage noted as CA) and oxygen (atomic percentage noted as OA) of a fiber before treatment according to the invention, and FIG. 3 represents the evolution, as a function of depth, of the atomic percentages of the elements silicon (atomic percentage noted Si _B ), carbon (atomic percentage noted C _B ) and oxygen (atomic percentage noted 0 _B ) of a fiber after completion of an example of step a).

Detailed description of embodiments

The invention relates to the treatment of silicon carbide fibers having an oxygen content of less than or equal to 1% atomic percentage. The invention therefore relates to the treatment of silicon carbide fibers relatively low in oxygen, these fibers being distinguished from Si-C-0 fibers which have an oxygen content outside the range mentioned above.

The fibers treated by the process according to the invention may, for example, have a C / Si atomic ratio of between 1 and 1.1, for example between 1 and 1.05. So-called third generation silicon carbide fibers, such as "Hi-Nicalon S" type fibers, have such an atomic ratio as well as an oxygen content of less than or equal to 1% atomic percentage. Other types of silicon carbide fibers can be treated by the process according to the invention as "Hi-Nicalon" type fibers which have a C / Si atomic ratio outside the ranges mentioned above but which have a negative oxygen content less than or equal to 1% atomic percentage.

FIG. 1A very schematically illustrates the section of a silicon carbide fiber 10 having an oxygen content of less than or equal to 1% in atomic percentage before implementing the method according to the invention. FIGS. 1A-1C illustrate the treatment of a fiber initially having a surface layer 11 comprising a silicon oxycarbide.

The silicon carbide fiber 10 consists of a silicon carbide core 12 and a surface layer 11 situated in the vicinity of the surface of the fiber 10. The surface layer 11 has a heterogeneous surface state and here comprises less a silicon oxycarbide. The surface layer 11 is responsible for a decrease in the quality of the adhesion of the fiber to a coating covering it. The thickness e 1 of the surface layer 11 may typically be between 1 nm and 100 nm, for example between 5 nm and 100 nm, for example between 10 nm and 100 nm. The surface layer 11 is intended to be eliminated by implementing the method according to the invention.

Silicon carbide fibers may be processed in any form, for example, yarns, tows, strands, cables, fabrics, felts, mats and even two- or three-dimensional preforms. The silicon carbide fibers treated according to the process of the invention may advantageously be used for producing fiber preforms of composite material part.

In order to form the fibrous preform, a fibrous texture may first be obtained by carrying out one or more textile operations and then this fibrous texture may be shaped in order to obtain a fibrous preform having the desired shape. The fibrous texture can be obtained by three-dimensional weaving, for example "interlock" weave, that is to say a weave weave in which each layer of weft son binds several layers of warp son with all the son of the same column of weft having the same movement in the plane of the armor. Other types of three-dimensional weaving may of course be used to make the fibrous texture. When the fibrous texture is woven, weaving can be performed with warp yarns extending in the longitudinal direction of the texture, being noted that weaving with weft yarns in this direction is also possible. Various modes of weaving that can be used to produce the fiber texture are described in particular in document WO 2006/136755.

The fibrous texture may be further formed by assembling at least two fibrous structures. In this case, the fibrous structures can be bonded together, for example by sewing or needling. The fibrous structures may in particular be each obtained from a layer or a stack of several layers of:

- one-dimensional fabric (UD),

- two-dimensional fabric (2D),

- braid,

- knit,

- felt, - Unidirectional web (UD) of son or cables or multidirectional webs (nD) obtained by superposition of several UD webs in different directions and UD web connection between them for example by sewing, by chemical bonding agent or by needling.

In the case of a stack of several layers, they are interconnected for example by sewing, by implantation of son or rigid elements or by needling.

The silicon carbide fibers can be treated by the process according to the invention before or after the production of the preform.

In a preliminary manner, it is possible to carry out, prior to the implementation of step a), a preliminary treatment aimed at eliminating the size or wrapping present on the fiber or fibers. Such preliminary treatment is optional insofar as the conditions implemented during step a) may, in certain cases, make it possible to carry out desizing and / or disintegrating in addition to forming the silica layer. These fibers can be initially sized or wrapped with polyvinyl alcohol (PVA), for example.

When the preliminary treatment is carried out, it may consist of a hydrothermal or solvothermal treatment. The solvothermal route may use one or more alcohols such as methanol or ethanol, or a mixture of water and alcohol. The medium used to carry out this step may be in the liquid state. Alternatively, the medium is in supercritical conditions.

The pressure imposed during the preliminary treatment may be greater than 1 bar, or even greater than or equal to 1 MPa, or even greater than or equal to 5 MPa. This pressure can be between 5 MPa and 30 MPa.

The temperature imposed during the preliminary treatment may be greater than or equal to 100 ° C, or even be between 100 ° C and 370 ° C or be between 100 ° C and 250 ° C.

When water is used to carry out the preliminary treatment, it is possible to impose a temperature of between 100 ° C. and 370 ° C. and a pressure of between 5 MPa and 30 MPa.

When an alcohol is used to carry out the preliminary treatment, it is possible to impose a temperature of between 100 ° C. and 250 ° C. and a pressure of between 5 MPa and 30 MPa. It is also possible to use a mixture of water and alcohol to carry out the preliminary treatment and to impose a temperature of between 100 ° C. and 370 ° C. and a pressure of between 5 MPa and 30 MPa. The volume percentage of water in such a mixture of water and alcohol may, for example, be between 25% and 75%.

The duration of the preliminary treatment of desizing or disintegrating may be greater than or equal to 5 minutes, or even be between 5 minutes and 30 minutes.

As indicated above, this preliminary treatment of desizing or disintegrating is optional insofar as the conditions employed during stage a) can, in certain cases, allow both the elimination of the size and the covering and the formation of the silica layer.

We will now describe in more detail the steps a) and b) of formation of the silica layer and elimination thereof.

The fiber 10 is first brought into contact with an oxidizing medium under pressure and at a temperature, in particular under supercritical conditions, in order to form the surface silica layer. Details of the oxidizing medium used during step a) will be described below. Following this contacting, an oxidized surface fiber is obtained. In the case of the fiber of FIG. 1A, the surface layer 11 comprising the oxycarbide is oxidized and is chemically converted into a silica layer 22 having a thickness e ₂ which, in the example illustrated, is substantially equal to thickness ei of the surface layer 11 (see FIG. 1B). The thickness of the silica layer formed may, alternatively, be greater than the thickness of the surface layer 11. The diameter of the fiber remains substantially constant after implementation of step a), in the case of FIGS. IA and IB.

In the case of the fiber 101 of FIG. 1D which has a surface layer 111 predominantly formed of carbon, there is a decrease in the diameter of the fiber following step a). The carbon may be the major element in atomic proportion in the surface layer 111. The atomic carbon content in the surface layer 111 may be greater than 50%, for example 60%. Initially, the surface layer 111 has a thickness of typically between 1 nm and 1 miti. Following step a), this surface layer 111 is eliminated and the SiC is oxidized, so as to form the silica layer 22. The thickness e _2i of the silica layer 22 obtained can typically be a few nm. or tens of nm. The fiber obtained after step a) is referenced 201. Following step a), there is here reduction of the diameter of the fiber, due to the elimination of the layer 111.

The pressure of the oxidizing medium during step a) may be greater than or equal to 1 MPa, or even greater than or equal to 5 MPa. This pressure can be between 5 MPa and 30 MPa.

The temperature of the oxidizing medium during step a) may be greater than or equal to 50 ° C., for example greater than or equal to 200 ° C., for example greater than or equal to 400 ° C. This temperature can be between 50 ° C and 700 ° C, for example be between 200 ° C and 700 ° C, or between 400 ° C and 700 ° C.

The oxidizing medium may comprise at least one compound chosen from: oxygen, hydrogen peroxide, ozone, an alkali metal permanganate, or an alkali metal dichromate. When the oxidizing medium is different from the oxygen, the values described above for the temperature and the pressure of this oxidizing medium during step a) remain valid. By way of example, the oxidizing medium may be an aqueous solution comprising hydrogen peroxide at a rate of 3% to 90% by weight, or an aqueous solution of a permanganate or an alkali metal dichromate at a concentration less than the limit of solubility in water.

Advantageously, the oxidizing medium comprises at least oxygen.

Advantageously, the oxidizing medium may have a dioxygen content by volume of between 5% and 100%, the temperature of the oxidizing medium being between 50 ° C and 700 ° C and the pressure of the oxidizing medium being between 1 MPa and 30 MPa.

The oxidizing medium may be a mixture of an inert compound, such as nitrogen, argon or carbon dioxide, and dioxygen. The oxidizing medium may, in particular, be air.

The silicon carbide fiber may be brought into contact with the oxidizing medium during step a) for a duration greater than or equal to 1 minute, for example greater than or equal to 5 minutes, for example greater than or equal to 10 minutes, for example greater than or equal to 15 minutes. This duration is for example between 15 minutes and 5 hours.

Once the silica layer obtained, it is then removed in step b) by contacting with water under hydrothermal conditions. During this treatment, the silicon atoms contained in the silica layer are hydrolysed. After step b), a homogeneous fiber surface of SiC is obtained. Operating conditions that can be used for the hydrothermal treatment of step b) have been described in application WO 2014/114874 for the formation of a microporous carbon layer on Nicalon® SiC fibers having an oxygen content greater than 1 % atomic percentage.

The water used in step b) is at a pressure between the saturation vapor pressure and 30 MPa and at a temperature of less than or equal to 400 ° C. The pressure of the water used in step b) can be between 5 MPa and 30 MPa. The temperature of the water used in step b) may be between 100 ° C. and 400 ° C., or even be between 100 ° C. and 370 ° C., or even between 200 ° C. and 370 ° C.

The water used in step b) may have a temperature below the critical temperature, which is 374 ° C, and a pressure between the saturated vapor pressure and 30 MPa. Such a case corresponds to water in subcritical condition.

The water used in step b) may have a temperature of between 350 ° C. and 400 ° C. and a pressure of between 15 MPa and 30 MPa. Such a case corresponds to water under conditions in the vicinity of the critical point.

Advantageously, the water used in step b) is at a temperature between 100 ° C and 370 ° C and for example at a pressure between 5 MPa and 30 MPa.

The duration of the hydrothermal treatment may be greater than or equal to 15 minutes, for example between 15 minutes and 5 hours.

The water used in step b) may or may not be supplemented with alcohol. The use of alcohol in the water makes it possible to slow the kinetics of elimination of the silica, which can be advantageous if one seeks to control in a fine manner the kinetics of step b). FIGS. 1C and 1F show the result obtained after implementing steps a) and b) for the two types of fibers. In both cases, it is obtained a silicon carbide fiber having a surface state and a homogeneous composition. In the illustrated examples, after step b), the entire surface layer 11 or 111 is removed, regardless of its chemical nature.

The steps which have just been described can be carried out in closed, semi-continuous or continuous mode.

In closed mode, the treated fiber (s) and the treatment medium are kept in a closed chamber. The system is maintained under the desired temperature and pressure conditions for the desired time in order to perform the treatment. Then, the medium is removed from the reactor and the fiber or fibers are recovered.

In semi-continuous mode, the fibers are held in an enclosure and are subjected to a continuous flow of the treatment medium. The treatment medium circulates continuously through the enclosure and evacuates from it loaded with the material to be extracted.

Continuous mode is similar to semi-continuous mode with the difference that the fibers also circulate through the chamber during processing. The fiber or fibers are unwound from a reel of untreated fibers, pass into the treatment zone and then are wound in coil form after treatment.

For example, when the treatment medium circulates through the chamber during the aforementioned treatments, the flow rate of this treatment medium through the chamber may be between 1 mL / minute and 6 mL / minute.

An interphase layer may then be deposited in contact with the surface of the fiber obtained after implementing steps a) and b).

The deposition of the interphase layer directly on the surface of the etched fiber is carried out in a manner known per se by running in a reactor or in closed mode.

The fiber treated by the process according to the invention has an improved bond with the interphase layer. The interphase layer may be a boron nitride (BN) layer or a pyrolytic carbon (PyC) layer. The thickness of the interphase layer may for example be greater than or equal to 20 nm, for example be between 20 nm and 1500 nm. One or more additional layers may be deposited on the interphase layer, for example made of ceramic material such as SiBC, BNSi or silicon carbide.

Once the interphase layer has been deposited, a piece of composite material with improved mechanical properties can then be formed by densifying, by at least one matrix phase, a fiber preform comprising the treated fibers coated with the interphase layer. The fibrous preform forms the fibrous reinforcement of the composite material part and the matrix phase is formed in the porosity of the fibrous preform. The matrix phase may for example be silicon carbide or carbon.

This densification is carried out in a manner known per se. The densification of the fiber preform can thus be carried out by a liquid route (impregnation with a precursor resin of the matrix and transformation by crosslinking and pyrolysis, the process being repeatable) or by a gaseous route (chemical vapor infiltration of the matrix). The invention is particularly applicable to the production of ceramic matrix composite material (CMC) parts formed by a fibrous reinforcement of silicon carbide fibers densified by a ceramic matrix, in particular carbide, nitride, refractory oxide, etc. Typical examples of such CMC materials are SiC-SiC materials (reinforcement of silicon carbide fibers and silicon carbide matrix). The matrix phase can also be carried out by infiltration of silicon in the molten state ("Melt-Infiltration" process).

Alternatively, the matrix could be formed directly in contact with the surface of the treated fibers (no interphase layer between the fibers and the matrix).

Example

A "Hi-Nicalon S" fiber fabric screened and wrapped with PVA was first subjected to preliminary hydrothermal treatment under a continuous flow of water at 300 ° C at 25 MPa for about 20 minutes. This treatment made it possible to carry out the desizing and disintegrating of these fibers. FIG. 2 is an AUGER analysis result showing the evolutions, as a function of the depth, of the proportions in silicon (Si _A ), carbon (C _A ) and oxygen (0 _A ) within SiC fibers "Hi-Nicalon S" before implementation of a step a) according to the invention. The fibers had at the surface before treatment a predominantly carbon layer having a thickness of about 200 nm.

A step a) of oxidation of the surface of the fibers thus obtained was then carried out in closed mode.

The oxidation of the surface of the SiC fibers was carried out using an oxidizing medium under pressure and at temperature. The oxidizing medium used was a CO 2 / O 2 mixture comprising 20% by volume of oxygen. The oxidizing medium used had a temperature of 600 ° C. and a pressure of between 13 MPa and 15 MPa. The fibers were brought into contact with the oxidizing medium for two hours.

FIG. 3 is an AUGER analysis result relating to the fibers obtained after treatment with the oxidizing medium. The removal of the surface carbon and the formation of a silica layer having a thickness of about 100 nm are noted.

The silica layer obtained after the oxidation step was then removed by hydrothermal treatment at a temperature of 300 ° C and a pressure of 25 MPa. This hydrothermal treatment was carried out for 30 minutes.

This process was conducted in a single chamber for the use of fluids under pressure and temperature. The fact of being able to implement this process in the same enclosure made it possible to simplify the process and significantly reduce its duration. All of the SiC fiber base fabric fibers obtained after the treatment had a homogeneous SiC surface.

The expression "understood between ... and ..." must be understood as including boundaries.

Claims

A method of treating at least one silicon carbide fiber (10), the method comprising at least the following steps:

a) forming a layer (22) of silica on the surface of a silicon carbide fiber having an oxygen content of less than or equal to 1% atomic percentage, the silica layer being formed by contacting this fiber with an oxidizing medium having a temperature greater than or equal to 50 ° C and a pressure greater than or equal to 1 MPa, and step a) being such that:

the treated fiber has a surface layer predominantly formed of carbon and there is during step a) removal of the surface carbon by oxidation and oxidation of the silicon carbide in order to form the silica layer, or

the treated fiber has a surface layer comprising a silicon oxycarbide and during step a) oxidation of the oxycarbide to form the silica layer, and

2. Method according to claim 1, wherein the oxidizing medium has a dioxygen content by volume of between 5% and

100%, the temperature of the oxidizing medium being between 50 ° C and 700 ° C and the pressure of the oxidizing medium is between 1 MPa and 30 MPa.

3. Method according to claim 2, wherein the oxidizing medium has a dioxygen content by volume of between 5% and 50%, the temperature of the oxidizing medium being between 200 ° C and 700 ° C and the pressure of the oxidizing medium being between between 13 MPa and 25 MPa.

4. The method of claim 1 to 3, wherein the oxidizing medium is a mixture between an inert compound and dioxygen.

5. Process according to any one of claims 1 to 4, wherein during the hydrothermal treatment, the fiber is treated with water at a temperature of between 100 ° C and 370 ° C.

6. Method according to any one of claims 1 to 5, wherein is carried out, before step a), a step of desizing or disbudding of the fiber by carrying out a preliminary hydrothermal or solvothermal treatment.

The method of any one of claims 1 to 6, wherein the method further comprises the step of:

8. Process according to any one of claims 1 to 7, wherein a plurality of silicon carbide fibers each having an oxygen content of less than or equal to 1% atomic percentage is treated.

9. A method of manufacturing a fiber preform comprising at least one step of treating a plurality of silicon carbide fibers by carrying out a method according to claim 8 and a step of forming a fibrous preform by implementing one or more textile operations from said plurality of fibers thus treated.

A method of manufacturing a fibrous preform comprising at least one step of forming a fibrous preform by carrying out one or more textile operations from a plurality of silicon carbide fibers each having a content of oxygen less than or equal to 1% atomic percentage and a treatment step of said plurality of fibers, once the preform formed, by implementing a method according to claim 8.

11. A method of manufacturing a composite material part comprising at least one step of manufacturing a fiber preform by implementing a method according to claim 9 or 10 followed by a step of forming at least one carbon matrix phase or a ceramic material densifying said fiber preform.