Process for treating a silicon carbide fiber

Abstract

Process for treating a silicon carbide fiber. The invention relates to a process for treating at least one silicon carbide fiber (16) comprising a surface layer comprising carbon and / or a silicon oxycarbide, the treatment comprising at least the removal of the surface layer of the fiber by contacting it with an ammonia phase (22) at a temperature greater than or equal to 100°C and at a pressure greater than or equal to 1 bar. Figure for the abstract: Fig. 1.

Description

Title of the invention: Process for treating a silicon carbide fiber 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 the formation of a coating on the surface of the fiber. Previous technique

[0002] Ceramic matrix composite materials (“CMC materials”) exhibit good mechanical properties, making them suitable for use in structural components, and they advantageously retain these properties at high temperatures. They represent an attractive alternative to commonly used metal parts, as they allow for a reduction in structural weight.

[0003] A CMC material can be produced by creating a fibrous preform with a shape similar to that of the final part, which is then densified by a ceramic matrix. The performance of a CMC material requires specific management of the interfacial bonds between the fibers and the matrix in order to achieve the required durability of the final composite. This modulation of the interfaces is conventionally achieved by interposing an interphase between the fiber and the matrix. In thermostructural applications, the use of boron nitride for the interphase may be preferred over pyrocarbon (PyC) due to its more favorable oxidation behavior.

[0004] It is known that prior to the interphase deposition step, surface treatment of the fibers to remove surface heterogeneities significantly improves the properties of the final composite material. US 2018194686A1 is known in particular. This document discloses a method for surface cleaning of "Hi-Nicalon S" type SiC fibers prior to interphase formation. In this method, the fiber surface is first oxidized to form a surface silica layer, followed by treatment with an acidic liquid medium containing at least hydrofluoric acid (HF) to remove the silica layer. After this treatment, the resulting cleaned fibers have a homogeneous silicon carbide surface, providing improved bonding of the treated fiber to the deposited interphase. This solution provides satisfactory results.However, the environmental compatibility of this method, which uses HF acid, could be improved. Furthermore, this method requires various fiber handling steps: rinsing and drying the treated fibers, or moving them. Given the fibers from one enclosure to another, it would be desirable to have a simpler and quicker treatment to carry out. Description of the invention

[0005] 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 removal of the surface layer of the fiber by contacting it 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.

[0006] To the inventors' knowledge, the use of such an ammonia phase to remove the surface layer containing surface heterogeneities is not known or suggested in the literature. This solution provides good results for removing the surface layer of the fiber, which is responsible for reduced adhesion of a coating formed on it, without the use of toxic compounds. The ammonia phase can be in a gaseous, liquid, or supercritical state.

[0007] In one 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 chamber, from a treatment medium comprising at least ammonia.

[0008] This feature advantageously allows the heterogeneity removal and coating steps to be carried out in the same chamber, thus eliminating the need to expose the fiber to ambient air and reducing handling. It also prevents any risk of functionalization of the outermost surface of the fibers and results in even better adhesion of the coating to the fibers. Furthermore, the use of ammonia in both cases eliminates any chemical compatibility issues between the two steps.

[0009] In one embodiment, the temperature of the ammonia phase may be greater than or equal to 600°C, for example between 600°C and 1600°C.

[0010] Implementing such a temperature is advantageous because it allows the reaction between carbon and ammonia to be activated more and therefore increases the kinetics of the treatment.

[0011] 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.

[0012] Such a characteristic makes it possible to make the processing compatible with an industrial scale realization.

[0013] In one embodiment, said at least one fiber is heated by microwaves during the treatment.

[0014] The microwave field allows the surface of the treated fiber to be heated to a sufficient temperature to locally achieve the desired conditions. The microwave field heats the fiber as a whole to ensure homogeneous treatment. Furthermore, the energy required to treat the fiber is reduced, since the fiber is heated directly rather than the entire treatment chamber. However, the use of another heating method, such as radiant heating of a susceptor, does not depart from the scope of the invention.

[0015] In one embodiment, said at least one fiber circulates through a treatment chamber during the treatment. The circulation of the fiber within the treatment chamber may be in the same direction or counter-current to the circulation of the ammonia phase or the treatment medium.

[0016] The use of an ammonia phase as described above, which allows the kinetics to be accelerated, is particularly advantageous for carrying out continuous treatment on a fiber moving in the treatment chamber, and thus greatly improving treatment rates.

[0017] In one embodiment, said at least one fiber is made of silicon carbide having an oxygen content less than or equal to 1% atomic percentage.

[0018] The invention also relates to a 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 of the fibrous reinforcement having been treated by implementing a process as described above.

[0019] It should be noted that the fibrous reinforcement can be obtained after processing a plurality of fibers in the manner described above. Alternatively, the fibrous reinforcement can first be formed from a plurality of fibers, and then this reinforcement is processed in the manner described above.

[0020] The composite material part may, for example, be a turbomachine part, such as a turbomachine blade or a turbine ring sector. Brief description of the drawings

[0021] [Fig.1] Fig.1 represents schematically the treatment of a fiber in a treatment chamber as part of an example of a process according to the invention.

[0022] [Fig.2] The [Fig.2] is a cross-sectional view schematically representing the structure of a silicon carbide fiber initially having a surface layer.

[0023] [Fig.3] The [Fig.3] is a cross-sectional view schematically representing the structure of the silicon carbide fiber of the [Fig.2] after removal of the surface layer and before deposition of the coating.

[0024] [Fig.4] The [Fig.4] is a cross-sectional view schematically representing the structure of the coated treated fiber.

[0025] [Fig. 5] Figure 5 schematically represents an example of a treatment plant that can be used to implement the process according to the invention. Description of embodiments

[0026] Figure 1 shows the treatment of a fiber 16 allowing its surface to be treated in order to eliminate heterogeneities in order to then coat it in the same enclosure with a coating, for example with a boron nitride interphase.

[0027] The treated fiber is made of silicon carbide, possibly with an oxygen content of 1% or less. Examples of such fibers include those marketed under the reference "Hi-Nicalon S" or "Hi-Nicalon". Alternatively, Si-C-O fibers with a higher oxygen content can be treated. Examples of such fibers include those marketed under the reference "Nicalon".

[0028] Figure 2 schematically illustrates the cross-section of a silicon carbide fiber 16 before pretreatment. The fiber 16 has a surface layer 12 comprising silicon oxycarbide (a compound based on silicon, carbon, and oxygen) and / or carbon, which is preferably removed before coating deposition. The surface layer 12 may be enriched in carbon relative to the stoichiometry of silicon carbide. The surface layer 12 may be predominantly composed 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 pm. The silicon carbide fiber 16 consists of a silicon carbide core 11 and a surface layer 12 located in the vicinity of the surface of the fiber 16.The surface layer 12 has a heterogeneous surface condition. This surface layer 12 may be responsible for a decrease in the quality of the fiber's adhesion to the coating covering it.

[0029] In the example considered, the fiber 16 travels through a processing chamber 10 in the direction indicated by arrow D in [Fig. 1] during processing. The fiber processing can be carried out continuously without interrupting the fiber's movement. Alternatively, the fiber can be processed segment by segment, with the segment to be processed being immobilized in the processing chamber 10 and its surface layer being removed, then an adjacent fiber segment 16 being introduced into the chamber 10 so as to remove the surface layer of this adjacent segment.

[0030] The treated fiber 16 is unwound from a reel (not shown), passes through the processing chamber 10, and is then wound into a reel after processing. The speed at which the fiber 16 travels through the processing chamber 10 can be between 0.1 cm / s and 50 cm / s.

[0031] The treatment chamber 10 is filled with a fluid medium 20 comprising ammonia, for example, consisting essentially of ammonia. The fluid medium 20 may be in liquid form. The fluid medium 20 may be pressurized, for example, to 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 treatment chamber 10. In the illustrated example, the fiber 16 is heated directly, for example, by applying a microwave field. The heating increases the temperature at least in the vicinity of the fiber 16, in order to obtain the ammonia phase 22 useful for the treatment.

[0032] The ammonia phase 22 performing 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 contacting the fiber 16 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 pressurized ammonia, at a pressure between 1 bar and 300 bar, then the fiber can be heated so as to bring the ammonia to the desired temperature and thus proceed to the removal of the surface layer 12. The ammonia may or may not be continuously injected into the enclosure 10 during the treatment.In the case of continuous circulation of the fluid medium, a ratio [flow rate of fluid medium introduced into the chamber] / [volume of the chamber] greater than or equal to 0.00016 s⁻¹ can be imposed 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. The surface carbon of fiber 16 can react with ammonia to form hydrogen cyanide (HCN), which is removed. Oxycarbon compounds are also eliminated by the ammonia medium. This yields fiber 11 with an improved surface finish, for example, free of heterogeneities, as illustrated in [Fig. 3], which is intended to be subsequently coated. The duration of the surface layer removal treatment 12 can be greater than or equal to 10 seconds, for example between 5 minutes and 30 minutes.

[0033] As indicated above, the process can be continued so as to coat the fiber 11 obtained after removal of the surface layer 12 in the same chamber 10. Generally, a coating can be formed on the fiber, in the treatment chamber 10, from a treatment medium comprising at least one precursor of the coating to be formed dissolved in ammonia. The treatment medium can be in a gaseous, liquid, or supercritical state. Ammonia can be the solvent for the precursor of the coating to be formed. For example, a nitride interphase can be deposited. Boron is produced from a fluid medium containing borazane (or "ammonia borane" in English-language literature, with the chemical formula BH3NH3) dissolved in ammonia. Direct microwave heating of the fiber induces the decomposition of the borazane upon contact with the heated fiber, forming the boron nitride interphase. This interphase is formed by the decomposition of borazane under the influence of temperature. This decomposition can yield hexagonal boron nitride and releases dihydrogen. In particular, when using borazane dissolved in ammonia, 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 bar, for example, between 10 bar and 300 bar.The temperature of the treatment medium can 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 bar, for example, between 100 bar and 150 bar. These conditions represent a compromise that allows us to take advantage of the high solubility of borazane while benefiting from improved mass transfer and therefore higher deposition kinetics. Furthermore, these pressures are feasible for the industrial development of the process.

[0034] The fluid medium comprising the coating precursor can circulate continuously through the treatment chamber 10, or alternatively, the treatment chamber 10 can be initially filled with the fluid medium and the treatment can then be carried out without any fluid medium being introduced into or removed from the chamber. In the case where there is continuous circulation of the fluid medium comprising the precursor, a ratio [flow rate of fluid medium introduced into the chamber] / [volume of the chamber] greater than or equal to 0.00016 s1 can be imposed during all or part of the treatment, for example, between 0.0016 s1 and 0.016 s1 or between 0.0016 s1 and 0.16 s1. 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 chamber 10 during all or part of the treatment with a flow rate between 0.1 mL / minute and 10 mL / minute, for example between 0.1 mL / minute and 3 mL / minute.In general, the molar concentration of coating precursor 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.

[0035] The interphase 24 can 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 pm. The resulting interphase 24 has a controlled and homogeneous thickness over the entire circumference of the treated fiber with a stoichiometric boron:nitrogen ratio close to unity. The boron nitride obtained can be crystalline. The use of a crystalline material is advantageous. To increase crack deflection properties, a hexagonal boron nitride interphase can be obtained. The fiber coating treatment time can be greater than or equal to 10 seconds, for example, between 5 and 30 minutes or between 1 and 10 minutes.

[0036] The formation of a boron nitride interphase in the treatment chamber 10 from a medium containing ammonia has just been described. Other types of coatings are conceivable, such as the deposition of a tantalum nitride (TaN) coating from a treatment medium comprising, for example, Tris(diethylamino)(tert-butylimino)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 bar, for example, between 10 bar and 300 bar.

[0037] The advantage of this treatment is that the heterogeneity removal and coating steps are carried out in the same chamber 10, thus eliminating the need to expose the fiber to ambient air and reducing handling. Furthermore, ammonia is also used to form the coating, which eliminates any chemical compatibility issues between the two steps.

[0038] 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 excess unreacted precursor.

[0039] Fig. 5, which will be described below, illustrates more fully an example of an installation that can be implemented for the realization of a process according to the invention.

[0040] The installation 1 illustrated in [Fig. 5] includes a syringe pump 4, which allows operation at a constant flow rate or pressure. It further includes a microwave generation device for heating the fiber 11 by means of this radiation, as well as a tube 9 made of microwave-transparent material containing the fiber 11 to be heated. The installation 1 includes a pressure regulator 15 which allows the pressure throughout the device to be fixed when the syringe pump is operating at a constant flow rate.

[0041] In the case where the fiber 11 is treated to coat it with the boron nitride interphase, the reservoir 6 is initially filled with 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, while valves 7, 8, 13, and 14 are closed. The syringe pump operates at constant pressure and thus fills the reservoir 6 with ammonia at working pressure. The valve of the reservoir 5 is then closed and valve 7 is opened, which puts the entire device under pressure. Next, 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, valve 7 is closed and the reservoir valve 5 and valve 8 are opened to inject the solution composed of borazane and solvent into the treatment chamber 10. When the constant flow operation has stabilized, the microwave heating device is used to bring the fiber 11 contained in the chamber 10 up to working temperature for a specified time. When all of the precursor has been injected, pressurized solvent is injected (using the syringe pump) to remove any remaining traces of precursor. A similar operating mode is used for the ammonia phase treatment to remove the surface layer without the use of borazane.

[0042] We have just described an example of a process according to the invention in which a fiber is treated, but we do not, of course, depart from the scope of the invention if several fibers are treated simultaneously so as to remove their surface layer and possibly then form a coating on each of them. It should be noted that each fiber can be in the form of a strand comprising a plurality of filaments. We also do not depart from the scope of the invention if we treat not one or more unbonded fibers, but a pre-formed texture comprising a plurality of fibers, mobile or immobile within the treatment chamber. Thus, fibers can be treated in any form whatsoever, for example, yarns, strands, twists, cables, fabrics, felts, mats, and even two- or three-dimensional textures.The fibers treated according to the process of the invention can advantageously be used for the production of fibrous preforms for composite parts. The conditions described above for the treatment remain applicable regardless of the form in which the fiber(s) are treated.

[0043] We have just described the treatment of silicon carbide fibers to remove their surface layer using an ammonia phase and then to form a coating on the surface of the cleaned fiber. The following section describes the continuation of the process for obtaining a composite part from the fibers thus treated.

[0044] 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 involves textile operations that are known per se, for example, weaving, possibly three-dimensional weaving. Thus, the preform may, for example, have an "interlock" weave, that is, a weave in which each layer of weft yarns connects several layers of warp yarns, with all the yarns in the same weft column having the same movement in the plane of the weave. Other types of three-dimensional weaving can, of course, be used to manufacture the preform. As indicated above, this does not depart from the scope of the invention. if the preform is first formed from fibers, and then the fibers of the preform thus obtained are processed as described above.

[0045] The process can continue by the formation of at least one matrix phase in the porosity of the fibrous preform whose fibers have been treated as described above.

[0046] The resulting matrix may be at least partially ceramic, for example predominantly ceramic by mass, for example entirely ceramic. The formation of the matrix employs techniques that are known per se, being carried out for example 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 gaseous densification (chemical vapor phase infiltration of the matrix), or even by melt-infiltration technique (“Melt-Infiltration”; “MI”).

[0047] The invention relates in particular to the production of ceramic matrix composite parts formed by a fibrous reinforcement of silicon carbide fibers densified by a ceramic matrix, in particular a refractory carbide, nitride, or oxide. Typical examples of such CMC materials are SiC-SiC materials (silicon carbide fiber reinforcement and silicon carbide matrix).

[0048] The resulting part may be a turbomachine, aeronautical or industrial component. This part may be a turbomachine blade or a turbine ring sector, for example. Examples

[0049] A fiber was fed through a processing chamber 10, schematically represented in [Fig. 1], which had a volume of 1 cm³. The processed fiber was a silicon carbide fiber with an oxygen content of 1% or less by atomic percentage, corresponding to a commercially available fiber under the reference "Hi-Nicalon S". Before processing, the fiber had a surface layer 12 with a thickness of approximately 100 nm.

[0050] A pretreatment step of the fiber was first carried out by subjecting it in chamber 10 to a supercritical ammonia phase. During the pretreatment, the fiber was heated by a microwave field to a temperature of 1000 °C, and the supercritical ammonia phase was pressurized to 120 bar. Ammonia was continuously injected into the treatment chamber during the pretreatment at a flow rate of 6 mL / min, and the fiber moved at a speed of 30 cm / minute. This resulted in a de-coated silicon carbide fiber surface as shown in [Fig. 3].

[0051] The boron nitride interphase was then deposited onto the fiber as follows The coating was applied in the same treatment chamber. During this deposition, the fiber moved through the treatment chamber at a speed of 30 cm / minute. A mixture of ammonia and borazane, present at a molar concentration of 1 mol / L, was continuously introduced into the treatment chamber at a flow rate of 1 mL / min. During the treatment, the fiber surface was heated to 1100 °C by the microwave field, and the fluid medium was at a pressure of 120 bar. The treatment was carried out for 15 minutes, resulting in a 1000 nm BN interphase on the surface of the silicon carbide fiber.

[0052] The expression "between ... and ..." should be understood as including the bounds.

Claims

Demands

1. A process for treating at least one silicon carbide fiber (16) comprising a surface layer (12) comprising carbon and / or silicon oxycarbide, the treatment comprising at least the removal of the surface layer of the fiber by contacting it with a supercritical ammonia phase (22).

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

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

4. A method according to claim 3, wherein 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.

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

6. Method according to any one of claims 1 to 5, said at least one fiber (16; 11) flows through a treatment chamber (10) during the treatment.

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 less than or equal to 1% atomic percentage.

8. A method 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 (11) of the fibrous reinforcement having been treated by implementing a method according to any one of claims 1 to 7.

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