Method for processing silicon carbide fibers

Abstract

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

Description

TECHNICAL FIELD

[0001] The present invention relates to a method 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. BACKGROUND

[0002] Ceramic matrix composites ("CMC materials") have good mechanical properties that make them suitable for forming structural elements and advantageously maintaining these properties at high temperatures. They are an interesting alternative to the metal parts commonly used, as they make it possible to lighten the structures.

[0003] CMC materials can be produced by forming a fibrous preform, the shape of which is similar to that of the final part, then densifying by a ceramic matrix. The functioning of CMC materials requires a specific management of the interfacial bond between the fibers and the matrix, to obtain the properties of the final composite that are difficult to damage. This interfacial modulation is generally achieved by providing an interphase between the fibers and the matrix. In the context of thermal structural applications, the use of boron nitride as an interphase can be advantageous compared to pyrolytic carbon (PyC), for reasons related to its more advantageous oxidation behavior.

[0004] It is known that, prior to the interphase deposition step, a surface treatment of the fibers to eliminate the non-uniformities present on the surface can significantly improve the properties of the final composite. In particular, the document US2018194686A1 is known. This document discloses a method for peeling the surface of a SiC fiber of the "Hi-Nicalon S" type, before forming the interphase. In this method, the surface of the fiber is first oxidized to form a surface layer of silicon dioxide, then treated by an acidic liquid medium containing at least hydrofluoric acid (HF), to remove the silicon dioxide layer formed. After this treatment, a peeled fiber is obtained having a uniform silicon carbide surface, thus improving the bonding between the treated fiber and the deposited interphase. This solution provides satisfactory results. However, the environmental compatibility of this method using HF acid leaves room for improvement. In addition, this method requires the execution of various fiber handling steps: rinsing and drying the treated fiber, or moving the fiber from one chamber to another. It would therefore be desirable to have a treatment that is simpler and faster to carry out. SUMMARY

[0005] The present invention relates to a method for treating at least one silicon carbide fiber comprising a surface layer containing carbon and / or silicon oxycarbide, wherein the treatment comprises removing at least the surface layer from the fiber by contacting the fiber with ammonia at a temperature greater than or equal to 100°C and a pressure greater than or equal to 1 bar.

[0006] To the best of the inventors' knowledge, the use of such an ammonia phase to eliminate surface layers containing surface heterogeneities has not been known or suggested in the literature. This solution provides good results for the removal of the surface layer of the fibres that cause low adhesion of the coating formed on the fibres, while avoiding the use of toxic compounds. The ammonia phase can be gaseous, liquid or supercritical.

[0007] The use of a supercritical ammonia phase has advantages. The kinetics of the removal of carbon species present on the surface of the fibres is significantly greater under supercritical conditions, since the combination of high temperature and high ammonia concentration is used, which favours the reaction. In the technologies using gaseous or liquid phases, only one of these effects is used: high temperature but low ammonia concentration in the case of gaseous treatment, or high concentration but low temperature during the use of the liquid phase, which makes the treatment slower. Furthermore, if surface carbon and other oxide contaminants are present, their removal using supercritical ammonia will be such that they will no longer adhere and will be removed. The use of supercritical ammonia has the advantage of recovery compared to the use of gaseous ammonia, returning to its initial state after treatment, thus enabling the ammonia to be recovered and used again, whereas high-temperature gaseous ammonia would crack into by-products that cannot be reused. Furthermore, these by-products, such as HCN, would need to be neutralised in a waste liquid, which can be avoided using supercritical ammonia. Thus, the treatment in the supercritical phase enables a more efficient and simpler treatment of the silicon carbide fibres.

[0008] In one embodiment, the removal of the surface layer is carried out in a treatment chamber, and the treatment further comprises, in the treatment chamber, forming a coating on said at least one fibre from a treatment medium comprising at least ammonia after said removal.

[0009] This feature advantageously makes it possible to carry out the elimination of the heterogeneities and the coating operation in the same chamber, thus making it possible to avoid the return of the fibres to the air environment and reducing the operations. It also makes it possible to avoid the risk of functionalisation of the end faces of the fibres and to obtain better adhesion between the coating and the fibre. Furthermore, ammonia is used in both cases, eliminating the chemical compatibility problem between the two steps.

[0010] In one exemplary embodiment, the temperature of the ammonia phase can be greater than or equal to 600°C, for example between 600°C and 1600°C.

[0011] The use of such a temperature is advantageous since it further activates the reaction between carbon and ammonia, thus increasing the kinetics of the treatment.

[0012] In particular, the temperature of the ammonia phase is 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 greater than the critical pressure.

[0013] Such a feature makes it possible to make the treatment compatible with industrial-scale operations.

[0014] In one exemplary embodiment, the at least one fiber is heated by microwaves during the treatment.

[0015] The microwave field can bring the surface of the treated fiber to a sufficiently high temperature to locally reach the desired conditions. The microwave field can heat the entire fiber to ensure a homogeneous treatment. Moreover, since the fiber is directly heated and not the entire treatment chamber, the energy required for treating the fiber is reduced. However, it is not beyond the scope of the invention if another heating means is used, such as radiation from a susceptor.

[0016] In one exemplary embodiment, the at least one fiber is moved through the treatment chamber during the treatment. The movement of the fiber in the treatment chamber can be performed in the same direction or counter-current with respect to the circulation of the ammonia phase or the treatment medium.

[0017] The use of the ammonia phase described above, which can accelerate the kinetics of the process, is particularly advantageous for performing a continuous deposition on the fiber moving in the treatment chamber and thus greatly increases the treatment rate.

[0018] In one exemplary embodiment, the at least one fiber is made of silicon carbide having an oxygen content less than or equal to 1% (atomic percent).

[0019] The present invention also relates to a method of manufacturing a composite part, comprising forming a matrix at least in the pores of a fibrous reinforcement, the fibers of which have been treated by implementing the method described above.

[0020] It is to 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 be formed from a plurality of fibers first, then treated in the manner described above.

[0021] The composite part can be, for example, a turbomachine part, such as a turbomachine blade or vane or turbomachine ring. BRIEF DESCRIPTION OF DRAWINGS

[0022] [ Figure 1 ] Figure 1 is a cross-sectional view schematically representing a structure of a silicon carbide fiber initially having a surface layer.

[0023] [ Figure 2 ] Figure 2 is a cross-sectional view schematically representing a structure of a silicon carbide fiber initially having a surface layer.

[0024] [ Figure 3 ] Figure 3 is a cross-sectional view schematically representing a structure of a silicon carbide fiber initially having a surface layer. Figure 2a cross-sectional view of the structure of a silicon carbide fiber.

[0025] [ Figure 4 ] Figure 4 is a cross-sectional view schematically representing the structure of a treated coated fiber.

[0026] [ Figure 5 ] Figure 5 schematically represents an embodiment of a treatment installation that can be used to implement the method of the application. DETAILED DESCRIPTION

[0027] Figure 1 The treatment of the fiber 16 is shown, the surface of which is treated to eliminate non-uniformities, so as to be subsequently coated in the same chamber, for example with a boron nitride interphase.

[0028] The treated fiber is made of silicon carbide, optionally with an oxygen content less than or equal to 1% (atomic percentage). Such fibers include, for example, the fibers sold under the name "Hi-Nicalon S" or "Hi-Nicalon". Alternatively, a Si-C-O fiber with a higher oxygen content can be treated. Such fibers include, for example, the fibers sold under the name "Nicalon".

[0029] Figure 2 A cross-section of a silicon carbide fiber 16 before pre-treatment is shown very schematically. The fiber 16 has a surface layer 12 comprising silicon oxycarbide (composed of silicon, carbon and oxygen) and / or carbon, the surface layer 12 being preferably removed before depositing a coating. The surface layer 12 can be carbon-rich compared to the stoichiometry of silicon carbide. The surface layer 12 can be formed mainly of carbon, the atomic proportion of carbon in the surface layer 12 being greater than 50%, for example greater than or equal to 60%. The thickness e12 of the surface layer 12 can generally be between 1 nm and 1 mm, for example between 1 nm and 1 μιη. The silicon carbide fiber 16 is composed of a silicon carbide core 11 and a surface layer 12 located near the surface of the fiber 16. The surface layer 12 is in a heterogeneous surface state. The surface layer 12 can cause a reduction in the quality of adhesion of a coating deposited thereon.

[0030] In the considered embodiment, the fiber 16 is moved along the direction indicated by the arrow D in the figure through the treatment chamber 10 during the treatment. The treatment of the fiber can be carried out continuously, without interrupting the movement of the fiber. Alternatively, the fiber can also be treated in sections, a section to be treated being fixed in the treatment chamber 10 and the surface layer removed from this section, then an adjacent fiber section 16 is introduced into the chamber 10 to remove the surface layer from this adjacent section. Figure 1 The fiber is moved along the direction indicated by the arrow D in the figure through the treatment chamber 10 during the treatment. The treatment of the fiber can be carried out continuously, without interrupting the movement of the fiber. Alternatively, the fiber can also be treated in sections, a section to be treated being fixed in the treatment chamber 10 and the surface layer removed from this section, then an adjacent fiber section 16 is introduced into the chamber 10 to remove the surface layer from this adjacent section.

[0031] The treated fibres 16 are unwound from a reel (not shown), pass through the treatment chamber 10 and are then wound onto a reel after treatment. The travel speed of the fibres 16 in the treatment chamber 10 can be between 0.1 cm / s and 50 cm / s.

[0032] The treatment chamber 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 fibres 16 are immersed in the fluid medium 20 during their treatment in the treatment chamber 10. In the illustrated embodiment, the fibres 16 are directly heated, for example by the application of a microwave field. The heating causes the temperature to rise at least in the vicinity of the fibres 16 to obtain an ammonia phase 22 for the treatment.

[0033] The temperature of the ammonia phase 22 in which the treatment is performed is greater than or equal to 100°C and the pressure is greater than or equal to 1 bar. The surface layer 12 can be removed by contacting the fibres 16 with the ammonia phase at a temperature of between 600°C and 1600°C and a pressure of between 1 bar and 300 bars. 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 bars, for example between 100 bars and 150 bars. The fibres 16 can first be contacted with pressurized ammonia at a pressure of between 1 bar and 300 bars and then heated to bring the ammonia to the desired temperature in order to remove the surface layer 12. The ammonia can be injected continuously into the chamber 10 during the treatment or not. When the fluid medium is continuously circulated during all or part of the treatment, a [flow rate of fluid medium introduced into the chamber] / [volume of the chamber] ratio greater than or equal to 0.00016 s -1 may be applied, 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 fibres 16 can react with the ammonia to form hydrogen cyanide (HCN) which is removed. Carbon oxide compounds are also removed by the ammonia medium. Thus, fibres 11 having an improved surface state, for example free of heterogeneities, as Figure 3 illustrated, and which will be ready for coating, are obtained. The duration of the treatment to remove the surface layer 12 can be greater than or equal to 10 seconds, for example between 5 minutes and 30 minutes.

[0034] As mentioned above, the process can continue so as to coat the obtained fibres 11 in the same chamber 10 after removal of the surface layer 12. Generally, a coating can be formed on the fibres in the treatment chamber 10 starting from a treatment medium comprising a precursor of the coating to be formed dissolved in ammonia. The treatment medium can be in gaseous, liquid or supercritical state. Ammonia can be the solvent of the precursor of the coating to be formed. Thus, for example, a boron nitride intermediate phase can be deposited from a fluid medium comprising borane ammonia (chemical formula BH3NH3) dissolved in ammonia. The decomposition of borane ammonia in contact with the fibres thus heated can be initiated by direct heating of the fibres by microwaves and the boron nitride intermediate phase is formed. This intermediate phase is formed by decomposition of borane ammonia under the effect of temperature. This decomposition can be used to obtain hexagonal boron nitride and release molecular hydrogen. In particular, in the case where borane ammonia dissolved in ammonia is used, the temperature of the treatment medium can be greater than or equal to 600°C, for example between 600°C and 1600°C, and the pressure of the treatment medium can be greater than or equal to 10 bars, for example between 10 bars and 300 bars. The temperature of the treatment medium can 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 can be greater than or equal to 100 bars, for example between 100 bars and 150 bars. These conditions constitute a compromise that makes it possible to benefit from the high solubility of borane ammonia while benefiting from improved mass transfer and therefore higher deposition kinetics. Furthermore, these pressures are feasible for the industrial development of the process.

[0035] The fluid medium comprising the precursor of the coating can be continuously moved through the treatment chamber 10, or the treatment chamber 10 can initially be filled with the fluid medium, then the treatment is carried out without introducing fluid medium into the treatment chamber or without emptying. In the case of a continuous circulation of the fluid medium comprising the precursor, a ratio greater than or equal to 0.00016 s -1 between the flow rate of the fluid medium introduced into the chamber and the volume of the chamber can be applied during all or part of the treatment, 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 between 0.1 mL and 100 mL, the fluid medium can be introduced into the treatment chamber 10 at a flow rate between 0.1 mL / minute and 10 mL / minute, for example between 0.1 mL / minute and 3 mL / minute, during all or part of the treatment. Generally, the molar concentration of the precursor of the coating in the fluid medium 20 can be greater than or equal to 0.001 mol / L, for example between 0.001 mol / L and 10 mol / L.

[0036] The thickness e24 of the mesophase 24 can be greater than or equal to 1 nm, for example greater than or equal to 10 nm (see Figure 4 ). The thickness e24 can be between 10 nm and 1 mm, for example between 10 nm and 10 pm. The resulting mesophase 24 has a controlled and uniform thickness over the entire circumference of the treated fiber, with a stoichiometric ratio of boron: nitrogen close to 1. The boron nitride obtained can be crystalline. The use of a crystalline material is beneficial in order to improve the crack deflection properties. A hexagonal boron nitride mesophase can be obtained. The duration of the coating treatment of the fiber can be greater than or equal to 10 seconds, for example between 5 minutes and 30 minutes, or between 1 minute and 10 minutes.

[0037] The formation of a boron nitride mesophase from a medium containing ammonia in the treatment chamber 10 has just been described. Other types of coating are also possible, such as the deposition of a tantalum nitride (TaN) coating from a treatment medium containing, for example, (diethylamino)(tert-buty limino) tantalum TBTDET dissolved in 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 bars, for example between 10 bars and 300 bars.

[0038] The advantage of this treatment is that the steps of eliminating non-uniformities and performing the coating are performed in the same chamber 10, which makes it possible to avoid the return of the fiber to ambient air and to reduce the operations. In addition, ammonia is also used to form the coating, thus eliminating any chemical compatibility problems between the two steps.

[0039] Once the coating has been deposited, the treatment chamber 10 can be cleaned by injecting pressurized liquid ammonia at a flow rate of 0.5 mL / min to 10 mL / min in order to remove the unreacted excess precursor.

[0040] The device 1 shown in the annexed drawings illustrates an embodiment of a device that can be used to implement the method of the present application. Figure 5 More complete embodiments of a device that can be used to implement the method of the present application will be described hereinafter.

[0041] Figure 5 The device 1 shown in the annexed drawings comprises an injection pump 4, which can work at constant flow or pressure. It also comprises means for generating microwaves capable of heating the fiber 11 by this type of radiation, and a tube 9 made of a material transparent to microwaves and containing the fiber 11 to be heated inside it. The device 1 comprises a pressure regulator 15, which can fix the pressure of the entire apparatus when the injection pump is operated at constant flow rate.

[0042] In the case of treatment of the fibres 11 to remove the boron nitride interphase, the reservoir 6 is first filled with borane ammonia and then connected to the rest of the device. The syringe pump 4 is filled with solvent and cooled by means of a cryostat. The syringe pump is then opened and the valves 7, 8, 13 and 14 are closed. The syringe pump works at constant pressure, thus filling the reservoir 6 with ammonia at the working pressure. The valve of the reservoir 5 is then closed and the valve 7 is opened, leaving the entire device under pressure. Next, the valve 13 is opened and the syringe pump is used in constant flow mode. The pressure is then fixed by means of the pressure regulator 15. Once the pressure in the system has stabilised, the valve 7 is closed and the valve of the reservoir 5 and the valve 8 are opened, in order to inject the solution composed of borane ammonia and solvent into the treatment chamber 10. When the constant flow rate operation has stabilised, the fibres 11 contained in the chamber 10 are heated to the working temperature using the microwave heating device over a given time. When all the precursors have been injected, the pressurised solvent is injected (by means of the syringe pump) in order to remove any traces of precursor that can remain. For the treatment by means of the ammonia phase, a similar mode of operation is used in order to remove the surface layer without using borane ammonia.

[0043] An embodiment of the method according to the application has just been described in which one fibre is treated. However, it goes without saying that it is also within the scope of the application to treat a plurality of fibres simultaneously in order to remove their surface layer and optionally to form a coating on each of the fibres. It is noted that each of the fibres can be in the form of a roving comprising a plurality of filaments. It is also not beyond the scope of the application to treat a formed texture of a plurality of fibres, which can be mobile or immobile in the treatment chamber, instead of treating one fibre or a plurality of fibres which are not joined together. Thus, the fibres can be treated in any form, such as threads, rovings, strands, cords, fibre fabrics, felts, mats and even two- or three-dimensional textures. The fibres treated according to the method of the application can advantageously be used to produce fibre preforms of composite parts. The treatment conditions described above remain applicable regardless of the form in which the fibres are treated.

[0044] The treatment of silicon carbide fibres has just been described in order to remove their surface layer using the ammonia phase and then to form a coating on the surface of the fibres cleaned in this way. The effort will now be made to describe other methods of obtaining a composite part from the fibres thus treated.

[0045] The fibers obtained after the above treatment can then be used to form fiber preforms of the desired component. The fiber preforms are formed using textile operations known per se, such as weaving, optionally three-dimensional weaving. Therefore, the preforms can, for example, have an "interlocking" weaving pattern; in other words, the weaving pattern is such that each layer of weft yarn connects multiple layers of warp yarn, and all yarns in the same row of weft yarns have the same movement in the same weaving plane. Of course, other types of three-dimensional weaving can be used to produce the preforms. As mentioned above, if the preforms are first formed from fibers, and then the fibers of the preforms thus obtained are subjected to the above treatment, this does not exceed the scope of the invention.

[0046] This method can be carried out by forming at least one matrix phase in the pores of a fiber preform, the fibers of which have been treated as described above.

[0047] The resulting matrix can be made at least partially of ceramic, for example, primarily of ceramic, or entirely of ceramic. The matrix is ​​formed using techniques known per se, such as liquid densification (by impregnation of the matrix precursor resin and by crosslinking and pyrolysis transformation, a process that is repeatable) or gaseous techniques (chemical vapor infiltration of the matrix), or even by melt infiltration (MI).

[0048] This invention is particularly applicable to the production of components made of ceramic matrix composites, which are formed of fiber reinforcements having silicon carbide fibers densified by a ceramic matrix (particularly refractory carbides, nitrides, or oxides). A typical example of such CMC materials is SiC-SiC materials (reinforced by silicon carbide fibers and a matrix made of silicon carbide).

[0049] The resulting component can be an aerospace or industrial turbine component. For example, the component can be a turbine blade or blade, or a turbine annular blade.

[0050] Example

[0051] like Figure 1 As schematically shown, a fiber passes through processing chamber 10, which has a diameter of 1 cm. 3 The volume of the fiber being treated is that of silicon carbide fiber with an oxygen content of less than or equal to 1% atomic percentage, corresponding to the fiber sold under the name "Hi-Nicalon S". Prior to treatment, the fiber has a surface layer 12 with a thickness of approximately 100 nm on its surface.

[0052] The fibres are first subjected to a pre-treatment step by exposing them to a supercritical ammonia phase in a chamber 10. During the pre-treatment, the fibres are heated by a microwave field so that the temperature reaches 1000°C and the pressure of the supercritical ammonia phase is increased to 120 bars. During the pre-treatment, ammonia is continuously injected into the treatment chamber at a flow rate of 6 mL / min and the fibres are moved at a speed of 30 cm / minute. A surface-stripped silicon carbide is thus obtained, as shown in Figure 3

[0053] A boron nitride interphase is then deposited on the said stripped fibres in the same treatment chamber. During this deposition, the fibres are moved at a speed of 30 cm / minute in the treatment chamber. A mixture of ammonia and borane ammine (molar concentration of 1 moL / L in the mixture) is continuously introduced into the treatment chamber at a flow rate of 1 mL / min. During the treatment, the temperature of the surface of the fibres is raised to 1100°C by a microwave field and the fluid medium is at a pressure of 120 bars. The treatment is carried out over a period of 15 minutes and in this way a BN interphase of 1000 nm is obtained on the surface of the silicon carbide fibres.

[0054] The expression "between" should be understood as including the extreme values.​

Claims

1. A method for treating at least one silicon carbide fiber, said silicon carbide fiber comprising a surface layer comprising carbon and / or silicon oxycarbide, said treatment comprising removing at least said surface layer from said fiber by contacting the fiber with ammonia in a supercritical state.

2. The method according to claim 1, wherein the removal of said surface layer is carried out in a treatment chamber, and wherein the treatment further comprises forming a coating on said at least one fiber from a treatment medium comprising at least ammonia in said treatment chamber after said removal.

3. The method according to claim 1, wherein the temperature of said ammonia phase is greater than or equal to 600°C.

4. The method according to claim 3, wherein the temperature of said ammonia phase is comprised between 800°C and 1200°C, and the pressure of said ammonia phase is comprised between 100 bar and 150 bar.

5. The method according to claim 1, wherein said at least one fiber is heated by microwaves during said treatment.

6. The method according to claim 1, wherein said at least one fiber is moved through a treatment chamber during said treatment.

7. The method according to claim 1, wherein said at least one fiber is made of silicon carbide having an oxygen content less than or equal to 1% atomic percent.

8. A method for manufacturing a composite part, comprising at least: - fibers of a fibrous reinforcement are treated by implementing the method according to claim 1; and - a matrix is formed in the pores of said fibrous reinforcement.

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