EP4359365A1

EP4359365A1 - Method for treating a silicon carbide fibre

Info

Publication number: EP4359365A1
Application number: EP22740948.9A
Authority: EP
Inventors: Adrien Delcamp; Marie LEFEBVRE; Cyril Aymonier; Nicolas BISCAY
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite de Bordeaux; Institut Polytechnique de Bordeaux; Safran Ceramics SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite de Bordeaux; Institut Polytechnique de Bordeaux; Safran Ceramics SA
Priority date: 2021-06-23
Filing date: 2022-06-20
Publication date: 2024-05-01
Also published as: FR3124508A1; WO2022269178A1; CN117881644A

Abstract

The invention relates to a method for treating at least one silicon carbide fibre (16) comprising a surface layer containing carbon and/or a silicon oxycarbide, wherein the treatment comprises at least removing the surface layer from the fibre by contacting with an ammonia phase (22) in the supercritical state.

Description

Title of the invention: Process for the treatment of a silicon carbide fiber

Technical area

The present invention relates to a process for treating at least one silicon carbide fiber having a surface layer to be removed before forming a coating on the surface of the fiber.

Prior technique

Ceramic matrix composite materials (“CMC materials”) have good mechanical properties making them suitable for forming structural elements and advantageously retain these properties at high temperatures. They constitute an interesting alternative compared to the metal parts commonly used, because they allow a lightening of the structure.

A CMC material can be produced by producing a fibrous preform whose shape is similar to that of the final part which is then densified by a ceramic matrix. The functioning of a CMC material requires specific management of the interfacial bonds between fibers and matrix, in order to access the damageable nature of the final composite. This modulation of the interfaces is obtained, conventionally by interposition of an interphase between the fiber and the matrix. In the context of thermostructural applications, the use of boron nitride for the interphase can be favored over pyrocarbon (PyC) for its more interesting oxidation behavior.

It is known that before the interphase deposition step, a treatment of the surface of the fibers aimed at removing the heterogeneities present on the surface makes it possible to significantly improve the properties of the final composite material. In particular, US 2018194686A1 is known. This document discloses a method for pickling 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 in order to form a layer of surface 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, pickled fibers are obtained having a homogeneous surface of silicon carbide conferring improved bonding of the treated fiber to the deposited interphase. This solution provides satisfactory results. However, the environmental compatibility of this method, using HF acid, could be improved. In addition, this method requiring the performance of different fiber handling steps: rinsing and drying of the treated fibers or movement of the fibers from one enclosure to another, it would be desirable to have a treatment that is simpler and quicker to perform.

Disclosure of Invention

The present invention relates to a process for treating at least one silicon carbide fiber comprising a surface layer comprising carbon and/or a silicon oxycarbide, the treatment comprising at least the elimination of the surface layer of the fiber by in contact with an ammonia phase at a temperature greater than or equal to 100°C and at a pressure greater than or equal to 1 bar.

To the knowledge of the inventors, the use of such an ammonia phase to eliminate the surface layer comprising the surface heterogeneities is not known or suggested in the literature. This solution provides good results for eliminating the surface layer of the fiber, which is responsible for a drop in adhesion of a coating formed thereon, by dispensing with the use of toxic compounds. The ammonia phase can be in the gaseous, liquid or supercritical state.

The use of a supercritical ammonia phase has advantages. The kinetics of elimination of the carbonaceous species present on the surface of the fibers is distinctly higher in supercritical conditions due to the combination of the application of a high temperature and a high concentration of ammonia, this combination favoring the reaction. In the techniques using a gaseous phase or a liquid phase, only one of these two effects is used: high temperature but with a low concentration of ammonia in the case of gaseous treatment, or high concentration but with a low temperature during the treatment. use of a liquid phase, which results in lower processing speeds. Additionally, the use of supercritical ammonia will remove surface carbon and other oxide contaminants if present which will no longer adhere and will be evacuated. Compared to the use of gaseous ammonia, the use of supercritical ammonia has the advantage of being recycled, returning to its starting state after treatment and thus allowing the recovery of ammonia which can then be reused, while high temperature ammonia gas cracks into by-products that are not reusable. These by-products (like HCN) also need to be neutralized in the effluents, a treatment that can be dispensed with when supercritical ammonia is used. Thus, the supercritical phase treatment allows a more efficient and simpler treatment of the silicon carbide fiber.

In an exemplary embodiment, the removal of the surface layer is carried out in a treatment chamber, and the treatment further comprises, after this removal, the formation of a coating on said at least one fiber, in the treatment, from a treatment medium comprising at least ammonia.

This characteristic advantageously makes it possible to carry out the steps of removal of heterogeneities and of coating in the same enclosure, which makes it possible to dispense with putting the fiber back into the ambient air and to reduce handling. This also makes it possible to avoid any risk of functionalization of the extreme surface of the fibers and to obtain even better adhesion of the coating to the fibers. In addition, ammonia is used in both cases, which makes it possible to eliminate any problem of chemical compatibility between the two stages.

In an exemplary embodiment, the temperature of the ammonia phase may be greater than or equal to 600°C, for example between 600°C and 1600°C.

The implementation of such a temperature is advantageous because it makes it possible to further activate the reaction between the carbon and the ammonia and therefore to increase the kinetics of the treatment.

In particular, the temperature of the ammonia phase can be between 800° C. and 1200° C., and the pressure of the ammonia phase can be between 100 bar and 150 bar. In general, the pressure of the ammonia phase can be higher than the critical pressure.

Such a characteristic makes it possible to make the processing compatible with production on an industrial scale.

In an exemplary embodiment, said at least one fiber is ^heated by microwaves during the treatment.

The microwave field makes it possible to bring the surface of the treated fiber to a sufficient temperature to locally achieve the desired conditions. The microwave field makes it possible to heat the fiber as a whole in order to ensure a homogeneous treatment. In addition, the energy required to treat the fiber is reduced, because the fiber is heated directly and no longer the entire treatment enclosure. However, it does not depart from the scope of the invention if another means of heating is used, such as heating by radiation of a susceptor.

In an exemplary embodiment, said at least one fiber circulates through a treatment enclosure during the treatment. The circulation of the fiber in the treatment enclosure can take place in the same direction or in counter-current to a circulation of the ammonia phase or of the treatment medium.

The use of an ammonia phase as described above, which makes it possible to accelerate the kinetics, is particularly advantageous for carrying out the treatment continuously on a fiber traveling through the treatment enclosure, and thus greatly improving the treatment rates. .

In an exemplary embodiment, said at least one fiber is made of silicon carbide having an oxygen content less than or equal to 1% in atomic percentage.

The invention also relates to a process for manufacturing a part made of composite material, comprising at least the formation of a matrix in the porosity of a fibrous reinforcement, the fibers of the fibrous reinforcement having been treated by implementing a process as described above.

It will be noted that the fibrous reinforcement can be obtained after treating a plurality of fibers in the manner described above. Alternatively, the fibrous reinforcement can first be formed from a plurality of fibers, then this reinforcement is treated in the manner described above.

The composite material part can for example be a turbomachine part, for example a turbomachine blade or a turbine ring sector.

Brief description of the drawings

[Fig. 1] FIG. 1 schematically represents the treatment of a fiber in a treatment enclosure within the framework of an example of a method according to the invention.

[Fig. 2] Figure 2 is a sectional view showing, schematically, the structure of a silicon carbide fiber initially having a surface layer.

[Fig. 3] Figure 3 is a sectional view showing, schematically, the structure of the silicon carbide fiber of Figure 2 after removal of the surface layer and before deposition of the coating.

[Fig. 4] Figure 4 is a cross-sectional view showing, schematically, the structure of the coated treated fiber.

[Fig. 5] FIG. 5 schematically represents an example of a processing installation that can be used for implementing the method according to the invention.

Description of embodiments

There is shown in FIG. 1 the treatment of a fiber 16 making it possible to treat its surface in order to eliminate heterogeneities with a view to then coating it in the same enclosure with a coating, for example with an interphase of boron nitride.

The treated fiber is made of silicon carbide optionally having an oxygen content less than or equal to 1% in atomic percentage. By way of example of such fibers, mention may be made of the fibers marketed under the reference “Hi-Nicalon S” or “Hi-Nicalon”. As an alternative, Si-C-O fibers which have a higher oxygen content can be treated. By way of example of such fibers, mention may be made of the fibers marketed under the reference “Nicalon”. FIG. 2 very schematically illustrates the section of a fiber 16 of silicon carbide, before the pre-treatment. The fiber 16 has a surface layer 12 comprising a silicon oxycarbide (compound based on silicon, carbon and oxygen) and/or carbon which it is preferable to remove before depositing the coating. The surface layer 12 can be enriched in carbon with respect to the stoichiometry of silicon carbide. The surface layer 12 may be mainly formed of carbon, with an atomic proportion of carbon in the surface layer 12 greater than 50%, for example greater than or equal to 60%. The thickness el2 of the surface layer 12 may generally be between 1 nm and 1 mm, for example between 1 nm and 1 μm. Silicon carbide fiber 16 consists of a silicon carbide core 11 and a surface layer 12 located close to the surface of fiber 16. Surface layer 12 has a heterogeneous surface state. The surface layer 12 may be responsible for a reduction in the quality of the adhesion of the fiber to a coating covering it.

In the example considered, the fiber 16 circulates through a treatment enclosure 10 in the direction shown by the arrow D in FIG. 1 during the treatment. The processing of the fiber can be carried out continuously without interrupting the movement of the fiber. As a variant, the fiber can be treated segment by segment, the segment to be treated being immobilized in the treatment chamber 10 and the surface layer of the fiber being eliminated, then an adjacent fiber segment 16 being introduced into the chamber 10 so as to to eliminate the surface layer of this adjacent segment.

The treated fiber 16 is unwound from a reel (not shown), passes through the treatment enclosure 10 and is then wound up in the form of a reel after treatment. The running speed of the fiber 16 in the treatment enclosure 10 can be between 0.1 cm/s and 50 cm/s.

The treatment enclosure 10 is filled with a fluid medium 20 comprising ammonia, for example consisting essentially of ammonia. The fluid medium 20 can be in liquid form. The fluid medium 20 can be pressurized, for example at a pressure greater than or equal to the critical pressure of ammonia. The fiber 16 is immersed in the fluid medium 20 during its treatment in the enclosure treatment 10. In the example shown, the fiber 16 is heated directly, for example by applying a microwave field. The heating makes it possible to increase the temperature at least in the vicinity of the fiber 16, in order to obtain the ammonia phase 22 useful for the treatment.

The ammonia phase 22 carrying out the treatment has a temperature greater than or equal to 100° C. and a pressure greater than or equal to 1 bar. The surface layer 12 can be removed by bringing the fiber 16 into contact with the ammonia phase at a temperature between 600° C. and 1600° C. and a pressure between 1 bar and 300 bar. The temperature of the ammonia phase can be greater than or equal to 800°C. The pressure of the ammonia phase can be greater than or equal to 100 bar, for example between 100 bar and 150 bar. The fiber 16 can first be brought into contact with ammonia under pressure, at a pressure between 1 bar and 300 bar, then the heating of the fiber can be carried out so as to bring the ammonia to the desired temperature. and thus proceed to the elimination of the surface layer 12. The ammonia may or may not be injected continuously into the enclosure 10 during the treatment. In the case where there is continuous circulation of the fluid medium, it is possible to impose during all or part of the treatment a ratio [fluid medium flow introduced into the enclosure] / [volume of the enclosure] greater than or equal to 0, 00016 s ¹ , for example between 0.0016 s ¹ and 0.016 s ¹ or between 0.0016 s ¹ and 0.16 s ¹ . The surface carbon of the fiber 16 can react with the ammonia so as to form hydrogen cyanide (HCN) which is evacuated. Oxycarbon compounds are also eliminated by the ammonia medium. The fiber 11 is thus obtained with an improved surface condition, for example free of heterogeneities, which is illustrated in FIG. 3, which is intended to be subsequently coated. The duration of the treatment for eliminating the surface layer 12 can be greater than or equal to 10 seconds, for example between 5 minutes and 30 minutes.

As indicated above, the method can be continued so as to coat the fiber 11 obtained after removal of the surface layer 12 in the same enclosure 10. In general, a coating can be formed on the fiber, in the enclosure treatment 10, from a treatment medium comprising at least a precursor of the coating to be formed dissolved in ammonia. The treatment medium can be in the gaseous, liquid or supercritical state. Ammonia can be the solvent for the precursor of the coating to be formed. It is thus possible, for example, to deposit a boron nitride interphase, from a fluid medium comprising borazane (or “ammonia borane” in the Anglo-Saxon literature, of chemical formula BH ₃ NH ₃ ) dissolved in ammonia. The direct heating of the fiber by microwaves makes it possible to induce the decomposition of the borazane in contact with the fiber thus heated and to form the boron nitride interphase. The interphase is formed by decomposition of borazane under the effect of temperature. This decomposition can make it possible to obtain hexagonal boron nitride and releases dihydrogen. In particular, in the case where borazane dissolved in ammonia is used, a temperature of the treatment medium may be greater than or equal to 600° C., for example between 600° C. and 1600° C., and a pressure of the treatment medium may be greater than or equal to 10 bar, for example between 10 bar and 300 bar. The temperature of the treatment medium may in particular be greater than or equal to 800° C. or greater than or equal to 900° C., for example between 800° C. and 1600° C. or between 900° C. and 1600° C., and the pressure of the treatment medium may be greater than or equal to 100 bar, for example between 100 bar and 150 bar. These conditions constitute a compromise making it possible to take advantage of the high solubility of borazane while benefiting from improved mass transfer and therefore higher deposition kinetics. Moreover, these pressures can be envisaged for an industrial development of the process.

The fluid medium comprising the precursor of the coating can flow continuously through the treatment enclosure 10, or alternatively the treatment enclosure 10 can be initially filled with the fluid medium and the treatment can then be carried out without the fluid medium is introduced into the enclosure or evacuated. In the case where there is continuous circulation of the fluid medium comprising the precursor, it is possible to impose during all or part of the treatment a ratio [flow rate of fluid medium introduced into the enclosure] / [volume of the enclosure] greater than or equal at 0.00016 s ¹ , for example between 0.0016 s ¹ and 0.016 s ¹ or between 0.0016 s ¹ and 0.16 s ¹ . For example for a treatment chamber 10 having a volume of between 0.1 mL and 100 mL, the fluid medium can be introduced into the enclosure 10 during all or part of the treatment with a flow rate comprised between 0.1 mL/minute and 10 mL/minute, for example comprised between 0.1 mL/minute and 3 mL/minute. In general, the molar concentration of coating precursor in the fluid medium 20 may be greater than or equal to 0.001 mol/L, for example between 0.001 mol/L and 10 mol/L.

The interphase 24 may have a thickness e24 greater than or equal to 1 nm, for example greater than or equal to 10 nm (see FIG. 4). This thickness e24 can be between 10 nm and 1 mm, for example between 10 nm and 10 μm. The interphase 24 obtained has a controlled and homogeneous thickness over the entire circumference of the treated fiber with a boron:nitrogen stoichiometric ratio close to unity.

The boron nitride obtained may be crystalline. The use of a crystalline material is advantageous in order to increase the deflection properties of the cracks. A hexagonal boron nitride interphase can be obtained. The duration of the fiber coating treatment can be greater than or equal to 10 seconds, for example between 5 minutes and 30 minutes or between 1 minute and 10 minutes.

The formation of a boron nitride interphase in the treatment enclosure 10 from a medium comprising ammonia has just been described. Other types of coatings can be envisaged, such as the deposition of a coating of tantalum nitride (TaN) from a treatment medium comprising, for example, Tris(diethylamino)(tert-butylimino)tantalum TBTDET dissolved in the ammonia. According to this variant, the temperature of the treatment medium can be greater than or equal to 200° C., for example between 200° C. and 1600° C., and the pressure of the treatment medium can be greater than or equal to 10 bar, for example between 10 bar and 300 bar.

The advantage of this treatment is to carry out the steps of removal of the heterogeneities and of coating in the same enclosure 10 which makes it possible to dispense with putting the fiber back into the ambient air and to reduce handling. In addition, ammonia is also used to form the coating, which makes it possible to eliminate any problem of chemical compatibility between the two stages. Once the coating has been deposited, the treatment chamber 10 can be cleaned by injecting pressurized liquid ammonia at a flow rate of between 0.5 mL/minute and 10 mL/minute in order to remove the excess precursor having no not reacted.

FIG. 5 which will be described below illustrates, more completely, an example of an installation which can be implemented for carrying out a method according to the invention.

The installation 1 illustrated in FIG. 5 comprises a syringe pump 4, making it possible to work at a constant flow rate or pressure. It further comprises a device for generating microwaves making it possible to heat the fiber 11 by means of this radiation as well as a tube 9 made of a material transparent to the microwaves containing the fiber 11 to be heated. The installation 1 comprises a pressure regulator 15 which makes it possible to fix the pressure in the whole of the device when the syringe pump operates at a constant flow rate.

In the case where the treatment of the fiber 11 is carried out so as to cover it with the boron nitride interphase, the reservoir 6 is initially filled with the borazane and then connected to the rest of the device. The syringe pump 4 is filled with the solvent and cooled by means of a cryostat. The syringe pump 4 is then opened, the valves 7, 8, 13 and 14 being closed. The syringe pump operates at constant pressure and thus fills reservoir 6 with ammonia at working pressure. The tank valve 5 is then closed and the valve 7 opened, which puts the entire device under pressure. Then, valve 13 is opened and the syringe pump is used in constant flow mode. The pressure is thus fixed by means of the pressure regulator 15. Once the pressure in the system has stabilized, the valve 7 is closed and the tank valve 5 and the valve 8 are opened to inject the solution composed of borazane and solvent into the treatment enclosure 10. When operation at constant flow is stabilized, the microwave heating device is used to bring the fiber 11 contained in the enclosure 10 to the working temperature for a given time. When all of the precursor has been injected, pressurized solvent is injected (by means of the syringe pump) to evacuate any traces of precursor that may remain. A mode of similar operation is used for ammonia phase treatment, to remove the surface layer without the use of borazane.

We have just described an example of a method according to the invention in which a fiber is treated but it is of course not departing from the scope of the invention if several fibers are treated simultaneously so as to eliminate their surface layer and possibly then form a coating on each of them. It will be noted that each fiber may be in the form of a roving comprising a plurality of filaments. It does not depart from the scope of the invention either if one treats either one fiber or several fibers not bonded together but an already formed texture comprising a plurality of fibers, mobile or immobile in the treatment enclosure. Thus, the fibers can be treated in any form whatsoever, for example yarns, rovings, strands, cables, fabrics, felts, mats and even two- or three-dimensional textures. The fibers treated according to the method of the invention can advantageously be used for the production of fiber preforms of composite material parts. The conditions which have been described above for the treatment remain applicable whatever the form in which the fiber or fibers are treated.

We have just described the treatment of silicon carbide fibers so as to eliminate their surface layer with an ammonia phase and then form a coating on the surface of the fiber thus cleaned. The following describes the continuation of the process making it possible to obtain a part made of composite material from the fibers thus treated.

Fibers obtained after the treatment described above can then be used to form a fibrous preform of the part to be obtained. The formation of the fibrous preform makes use of textile operations which are known per se, for example weaving, possibly three-dimensional weaving. Thus, the preform can for example have an "interlock" weave, that is to say a weave weave in which each layer of weft yarns binds several layers of warp with all the threads of the same weft column having the same movement in the plane of the weave. Other types of three-dimensional weaving could of course be used to manufacture the preform. As indicated above, it is not beyond the scope of the invention if the preform is first formed from fibers, then the fibers of the preform thus obtained are treated as described above.

The method can continue with the formation of at least one matrix phase in the pores of the fibrous preform, the fibers of which have been treated as described above.

The matrix obtained may be at least partially made of ceramic, for example mainly in ceramic mass, for example entirely made of ceramic. The formation of the matrix implements techniques which are known per se, being for example carried out by liquid densification technique (impregnation with a precursor resin of the matrix and transformation by crosslinking and pyrolysis, the process being able to be repeated) or by a gaseous route (chemical infiltration in the vapor phase of the matrix), or even by a technique of infiltration in the molten state (“Melt-Infiltration”; “MI”).

The invention applies in particular to the production of parts in composite material with a ceramic matrix formed by a fibrous reinforcement in silicon carbide fibers densified by a ceramic matrix, in particular carbide, nitride or refractory oxide. Typical examples of such CMC materials are SiC-SiC materials (silicon carbide fiber reinforcement and silicon carbide matrix).

The part obtained can be a turbomachine, aeronautical or industrial part. This part can be a turbomachine blade or a turbine ring sector, for example.

Example

A fiber was passed through a processing chamber 10 shown schematically in FIG. 1 which had a volume of 1 cm ³ . The treated fiber was a silicon carbide fiber with an oxygen content less than or equal to 1% in atomic percentages, corresponding to a fiber marketed under the reference “Hi-Nicalon S”. The fiber had on the surface before treatment a superficial layer 12 having a thickness of approximately 100 nm.

A fiber pre-treatment step was first carried out by subjecting it in the enclosure 10 to a supercritical ammonia phase. During the pre-treatment, the fiber was heated by a microwave field bringing it to a temperature of 1000°C and the supercritical ammonia phase was brought to a pressure of 120 bar. Ammonia was injected into the treatment enclosure continuously during the pre-treatment with a flow rate of 6 mL/min and the fiber moved at a speed of 30 cm/minute. There was thus obtained a pickled surface of silicon carbide fiber as shown in Figure 3.

The boron nitride interphase was then deposited on the fiber thus pickled in the same treatment enclosure. During this deposition, the fiber moved through the treatment chamber at a speed of 30 cm/minute. A mixture of ammonia and borazane present in a molar concentration of 1 moL/L in the mixture was introduced continuously into the treatment enclosure with a flow rate of 1 mL/min. During the treatment, the surface of the fiber was brought to a temperature of 1100°C by the microwave field and the fluid medium was at a pressure of 120 bar. The treatment was carried out for a period of 15 minutes and a BN interphase of 1000 nm was thus obtained on the surface of the silicon carbide fiber.

The expression "between ... and ..." must be understood as including the limits.

Claims

[Claim 1] A method of treating at least one silicon carbide fiber (16) comprising a surface layer (12) comprising carbon and/or silicon oxycarbide, the treatment comprising at least removing the surface layer of the fiber by bringing it into contact with an ammonia phase (22) in the supercritical state.

[Claim 2] Process according to claim 1, in which the removal of the surface layer (12) is carried out in a treatment chamber (10), and in which the treatment further comprises, after this removal, the formation of a coating (24) on said at least one fiber (11), in the treatment enclosure, from a treatment medium comprising at least ammonia.

[Claim 3] A method according to claim 1 or 2, wherein the temperature of the ammonia phase (22) is greater than or equal to 600°C.

[Claim 4] Process according to claim 3, in which the temperature of the ammonia phase (22) is between 800°C and 1200°C, and the pressure of the ammonia phase is between 100 bar and 150 bar.

[Claim 5] A method according to any of claims 1 to 4, wherein said at least one fiber (16; 11) is heated by microwaves during the treatment.

[Claim 6] A method according to any one of claims 1 to 5, said at least one fiber (16; 11) flows through a treatment enclosure (10) during treatment.

[Claim 7] A method according to any one of claims 1 to 6, wherein said at least one fiber (16; 11) is made of silicon carbide having an oxygen content of less than or equal to 1 atomic percent. [Claim 8] Method of manufacturing a part made of composite material, comprising at least the formation of a matrix in the porosity of a fibrous reinforcement, the fibers (11) of the fibrous reinforcement having been treated by implementing a method according to any one of claims 1 to 7.