USE OF AN OPTICAL FIBER COMPRISING A BORON NITRIDE-BASED COATING IN A PROCESS FOR THE ADDITIVE MANUFACTURING OF CERAMIC STRUCTURES.

Abstract

The present invention relates to the use of optical fibers comprising a boron nitride (BN)-based coating, in a method for the additive manufacturing of ceramic structures. The present invention also relates to ceramic structures obtained by additive manufacturing comprising an optical component comprising one or more optical fibers as defined above. No figure in the abstract

Description

Title of the invention: USE OF AN OPTICAL FIBER COMPRISING A BORON NITRIDE-BASED COATING IN AN ADDITIVE MANUFACTURING PROCESS CERAMIC STRUCTURES. Technical field

[0001] The present invention relates to the use of optical fibers comprising a boron nitride (BN)-based coating, in a method for the additive manufacturing of ceramic structures. The present invention also relates to ceramic structures obtained by additive manufacturing comprising an optical component comprising one or more optical fibers as defined above. State of the art

[0002] Additive Manufacturing (AM) processes allow the layer-by-layer production of ceramic parts with complex geometries such as, for example, parts comprising recesses, or parts made up of a lattice structure, or parts having geometric singularities that are difficult to obtain using a subtractive manufacturing process.

[0003] There are different types of Additive Manufacturing processes. Examples include material extrusion, plasma spraying, thermal spraying, direct material deposition, selective powder bed fusion, selective powder bed sintering, binder jetting, and photopolymerization.

[0004] The choice of the AM process is generally dictated by the material of interest, the geometric constraints of the model to be manufactured, the final characteristics of the part (mechanical, thermal, aesthetic). Structures made by AM in ceramic material can be exposed to extreme environmental conditions. Instrumenting these parts at the core makes it possible to offer in situ monitoring of the AM process and subsequently to monitor the material health or the different thermomechanical loads to which they may be subjected during their life cycle.

[0005] These ceramic structures produced by FA can be used, for example, in the aeronautical industries in order to carry out measurements in environments operating at high temperatures such as engines, or aerospace (turbine / stator blades, cryogenic environments such as liquid phase gas storage tanks), in the energy field (gas turbines), or even in the nuclear industry (measurements in high temperature environments and / or under radiation). These processes can be used in the manufacture of porous structures, turbine architectures, rotors, foundry molds, connecting elements such as gears [1].

[0006] Fiber optic sensors (FOS) allow physical parameters such as temperature and / or deformation or even pressure to be measured in a distributed manner. They are not very intrusive (diameter of the order of a hundred microns), are insensitive to electromagnetic disturbances and allow these measurements to be carried out over wide temperature ranges, typically up to T > 800°C.

[0007] Thus, unlike sensors such as thermocouples for example, CFOs allow multipoint and multiparametric measurement of the surrounding environment.

[0008] The integration of CFOs within AM processes is typically achieved by a one-time interruption of printing, suitable positioning of the sensor, and then completion of the construction of the part.

[0009] Ceramic parts made by AM, particularly in material extrusion processes, directed energy deposition, laminated object manufacturing, material jetting including material extrusion, directed energy deposition, laminated object manufacturing, material jetting including thermal spraying and plasma spraying, selective melting or selective powder bed sintering, photopolymerization can be subjected to very high temperatures (T > 800°C). The integrated CFO must thus form a mechanically favorable interface with the surrounding matrix in order to accurately measure the physical parameters of interest (good thermal contact to minimize response time, good grip to optimize the transduction of mechanical deformations and avoid problems of fiber slippage relative to the host structure).A coating deposited on the glass sheath of the optical fiber protects it during the sensor integration phase (manipulation by an operator or during implementation of the process), but also forms this interface.

[0010] However, the materials currently available for these coatings do not withstand such high temperatures over long periods of use (typically 350°C for polymeric coatings and at most 700°C for metallic coatings).

[0011] Furthermore, these materials have a roughness that is far too low to reliably form sufficient mechanical adhesion to the printed part, jeopardizing the transfer of mechanical forces or vibrations, or even the temperature response time (interstitial air pockets). This results in CFOs that do not have the qualities required for their use.

[0012] These materials also have a coefficient of thermal expansion which is very different from that of the printed ceramic matrix. This results in constraints linked to the differential expansion of the two materials which can occur during the setting in room temperature. These constraints can cause decohesion of the fiber / matrix interface, degradation of the coating or even breakage of the optical fiber.

[0013] There is therefore a need to propose an optical fiber whose coating makes it sufficiently resistant to withstand the extreme conditions of AM processes, in particular for processes allowing the integration: extrusion of material, directed energy deposition, manufacturing of laminated objects, projection of material including thermal projection and plasma projection, selective melting or selective sintering on a powder bed or even photopolymerization.

[0014] Existing solutions are based on the use of polyimide [2] or metallic [3,4] coatings. These materials nevertheless have the disadvantage of degrading at temperatures that are too low (around 350°C) compared to those to which parts made from ceramic AM can be exposed. They are therefore not suitable for use in an AM process [5].

[0015] Wnuk et al. [2] present the integration of CFOs with Bragg Gratings (BGRs), inscribed in an optical fiber coated with polyimide, in projected alumina (A12O3). Although the temperature resistance of the integrated sensor has not been investigated, the long-term resistance of this coating material is only guaranteed for temperatures < 350°C, which is not sufficient.

[0016] Duo et al. [4] integrated aluminum-coated optical fibers by alumina flame thermal spraying (thermal spraying) onto an aluminum substrate. Although no temperature tests were conducted on the instrumented samples, the optical transmission of the fiber is monitored during the deposition process. The potential exposure of the sample to high temperatures is limited, on the one hand, by the choice of the material on which the optical fiber is fixed (aluminum: melting at ~660°C), and on the other hand by the choice of the fiber coating (also made of aluminum: use at T < 400°C), which is not sufficient.

[0017] Lei et al. [6] integrated an intrinsic FPI (Fabry-Pérot) inscribed with a femtosecond laser within a silica glass fiber. The uncoated CFO (i.e. 0125 pm of silica) is placed in a groove machined in an alumina substrate, then embedded in an alumina-filled paste. The whole is then locally heated using a CO2 laser allowing the consolidation of the filler material, i.e. the alumina-filled paste. Measurements are carried out up to 800°C on the instrumented sample. The optical fiber has not undergone any heat treatment prior to its integration, and the output of the uncoated silica part is supposedly very fragile after exposure to 800°C, making the handling of the sample very delicate and is therefore not satisfactory. Furthermore, the process described, which allows the insertion of the optical fiber into the ceramic matrix, is very complex to implement.

[0018] The work of Petrie et al. [7,8] focuses on the integration of silica optical fibers within silicon carbide (SiC). A part is first constructed by binder jetting, a process in which layers of SiC particles are bound by a specific material, resulting in a mechanically very fragile part. It is then dried (at approximately 190°C) and then densified using the Chemical Vapor Infiltration (CVI) process. A chemical reaction under neutral gas and at approximately 1000°C leads to the formation of crystalline SiC as well as the evacuation of binders, resulting from AM by binder jetting. These steps are very complex to implement.In addition, the method described (CVI step) does not allow a priori to produce large parts, requires a very long manufacturing time (crystallization reaction of at least 5 hours), has a very high associated cost, and has the additional disadvantage of implementing reagents with high toxicity. First, the authors present material compatibility tests between optical fiber coatings and SiC obtained by CVI, on sections of silica optical fiber: uncoated, coated with gold, and silver. They show that after exposure to approximately 1000°C during the CVI process, a partial melting of the metallic coatings is visible ( [Fig.6] of Ref. [7]). This observation corroborates the limit of use of metallic coatings at temperatures T < 700°C as indicated by the different suppliers.The "bare" optical fiber option is described as the most suitable for their needs (better interface with SiC), however the authors clearly mention the difficulties of handling a bare silica optical fiber after exposure to such temperatures (extreme fragility in the absence of a protective coating). In a second step, during the integration by CVI of silica optical fibers coated with different materials - acrylate (polymer) and ormocer (organically modified ce ramie) - placed in Mo capillaries, the coatings, as expected, completely burned. Unexpected distortions of the Mo capillaries at high temperature led to their breakage. This breakage is described as accidental by the authors, and solely due to a defect in the fixation of the capillaries, not sufficiently accommodating the thermal expansions to which the structures were exposed during CVI.

[0019] In the case of this work, the use of metal coatings proved to be irrelevant with regard to the temperatures imposed by the CVI, and the option of using a “bare” optical fiber is not a long-term solution given its extreme fragility after exposure to high temperatures.

[0020] Thus, to the applicants' knowledge, there is no optical fiber which has the properties required to be implemented in an FA process.

[0021] Certain steps linked to the integration process are limited in temperature due to the thermal resistance of the coatings used (approximately 350°C for polyimide and around 400°C for aluminum): - Stabilization of residual stresses within the optical fiber resulting from the fiber drawing process, - Annealing of the part or stabilization treatment of the ceramic matrix.

[0022] Then, these limits affect the range of use of the instrumented parts. It is also shown that the use of a "bare" optical fiber allows, in certain cases, to reach these high operating temperatures (T > 800°C), but that this option is not viable from the perspective of part instrumentation in difficult environments, where the systems are intended to be handled, which can induce bends and / or scratches in the optical fibers. These stresses, applied to an uncoated silica optical fiber, inevitably lead to their breakage.

[0023] There is therefore a need for the implementation of the FA process, for an optical fiber whose resistance allows its integrity to be maintained up to its point of entry into the instrumented part, and also to guarantee a healthy ceramic matrix / CFO interface up to at least 1000°C.

[0024] However, none of the documents mentioned teaches a fiber having the properties required for such use.

[0025] The present invention thus proposes the use of an optical fiber comprising an outer coating comprising a mixture of hexagonal boron nitride and bentonite, in a proportion of at least 10% by weight of bentonite relative to the total weight of said outer coating, in a process for the additive manufacturing of ceramic structures. The use according to the invention makes it possible to develop and manufacture optical fiber sensors coated with a ceramic material of controllable thickness with a view to their integration within ceramic structures produced by additive manufacturing, forming a mechanically reliable interface with the matrix up to very high temperature levels, for example 800°C or higher. The invention also relates to ceramic structures comprising an optical component comprising one or more optical fibers as defined above.

[0026] Description of the invention

[0027] In order to solve the problems mentioned above, the applicant has developed the use of a fiber in a process for the additive manufacturing of ceramic structures, said fiber comprising a core made of fiber-forming material and having an external surface, said fiber being characterized in that it further comprises an external coating comprising a mixture of hexagonal boron nitride and bentonite, in a proportion of at least 10% by weight of bentonite relative to the total weight of said external coating.

[0028] Below 10% by weight of bentonite relative to the total weight of said outer coating, the coating does not adhere to the fiber, while above 35% by weight of bentonite relative to the total weight of said outer coating, the fiber thus coated is no longer flexible enough.

[0029] By fiber-forming material is meant a material that allows fiberization, i.e. that can undergo a transformation from a solid material into a fiber. It may be a glassy material having a glass transition allowing it to be drawn. Preferably, the core may be made of a material chosen from glass transition materials and sapphire glass.

[0030] Advantageously, the outer coating can be in direct contact with the core.

[0031] Advantageously, whatever the embodiment envisaged, the core 11 of the fiber of the use according to the present invention can have a diameter comprised in an interval going from 20 μm to 10 mm, preferably from 80 μm to 500 μm and more preferably from 125 μm.

[0032] Advantageously, the outer coating may have a thickness of between 5 μm and 240 μm. If the core is cylindrical in shape, the thickness of the outer coating will then be a radial thickness of between 5 μm and 240 μm.

[0033] Advantageously, the optical fiber comprising an outer coating comprising a mixture of hexagonal boron nitride and bentonite, at a rate of at least 10% by weight of bentonite relative to the total weight of said outer coating, may be chosen from a standard optical fiber, a multi-core fiber, a micro-structured fiber, a tapered fiber, an optical coupler comprising one or more input fibers and one or more output fibers, a laser fiber, without this list being limiting.

[0034] Advantageously, the additive manufacturing method implemented according to the invention can comprise the steps: a) manufacturing a ceramic matrix from a ceramic material, b) bringing at least one fiber 1 into contact with the ceramic matrix obtained in step a); c) fixing at least one fiber 1 to the surface of the ceramic matrix, possibly using elements on the periphery of the manufacturing zone, so as to limit any relative movement of said at least one fiber 1 with respect to the ceramic matrix. d) manufacturing a complementary matrix totally or partially covering the at least one fiber 1. The assembly formed by the ceramic matrices and the fiber forms a ceramic structure according to the invention.

[0035] Advantageously, the ceramic matrix is ​​composed of an inorganic material, generally composed of metallic, metalloid or non-metallic atoms. These may be oxides (for example: aluminum oxide, zirconium oxide, doped or non-oxides (carbides, borides, nitrides), ceramics composed of silicon and atoms such as tungsten, magnesium, platinum or titanium, and composite ceramics (combination of oxides and non-oxides). The choice of the material used to form the matrix is ​​generally dictated by the geometric constraints of the model to be manufactured, the final characteristics of the ceramic structure which is manufactured (mechanical, thermal and aesthetic constraints).

[0036] Elements on the periphery of the manufacturing zone are understood to mean mechanical or measuring systems located outside the volume inside which the matrix is ​​manufactured by the method, and assisting in carrying out said method; it being understood that the manufacturing zone is the volume inside which the matrix is ​​manufactured using the manufacturing method.

[0037] By limitation of any relative movement of the fiber is meant a technique for maintaining the fiber in a fixed position, for example using mechanical fixing systems or adhesive materials. The amplitude of variation of the local position of the fiber acceptable around this so-called fixed position in space as well as its frequency depend on the experimental conditions of the process investigated.

[0038] Advantageously, the additive manufacturing process can be chosen from material extrusion, directed energy deposition, manufacturing of laminated objects, material projection including thermal projection and plasma projection, selective powder bed fusion or selective powder bed sintering, binder projection, photopolymerization.

[0039] Advantageously, the method for manufacturing a ceramic structure is a method for manufacturing a ceramic structure instrumented with a CFO by atmospheric plasma projection, and comprises the steps: (a) manufacturing a ceramic matrix from a ceramic material by atmospheric plasma projection, b') bringing at least one fiber 1 into contact with the ceramic matrix obtained in step a'), and obtaining an instrumented matrix; c') positioning the instrumented matrix obtained in step a) in a layer-by-layer deposition chamber of a ceramic material, by atmospheric plasma projection on the instrumented matrix and integration of the at least one fiber 1, by totally or partially covering the at least one fiber 1 of said ceramic material; and obtaining an instrumented ceramic structure of a CFO.

[0040] A ceramic matrix is ​​understood to mean a three-dimensional object, or a volume of material, manufactured using an additive manufacturing process. An instrumented matrix is ​​understood to mean the material manufactured using the additive manufacturing process and capable of undergoing various post-treatments, and a volume of said material of a defined geometry comprising a CFO or a fiber on its surface or within it.

[0041] Ceramic material means an inorganic material composed of metallic, metalloid or non-metallic atoms. It may be oxides (for example: aluminum oxide, zirconium oxide, doped or not), non-oxides (carbides, borides, nitrides), ceramics composed of silicon and atoms such as tungsten, magnesium, platinum or even titanium, and composite ceramics (combination of oxides and non-oxides). It may be, for example, alumina (A12O3), zirconia (ZrO2), silicon carbide (SiC), tungsten carbide (WC), boron carbide (B4C), silicon nitride (S3N4), aluminum nitride (AIN), zirconium diboride (ZrB2).

[0042] A deposition enclosure is understood to mean a volume within which the deposition is carried out using the additive manufacturing process. This volume is physically delimited by a wall which may or may not be sealed against the ambient atmosphere. Said deposition enclosure has a volume which depends on the additive manufacturing process, generally between 0.001 m3 and 200 m3.

[0043] Advantageously, in the case where the deposition enclosure is sealed against the external atmosphere, the gaseous composition and the pressure of the atmosphere contained inside the deposition enclosure can be controlled.

[0044] Layer-by-layer deposition means the production of a volume of material of predefined geometry by incremental or successive deposition of intermediate volumes of material circumscribed in said volume of predefined geometry.

[0045] In a first variant, the method for manufacturing a ceramic structure according to the invention comprises the steps: (i) manufacturing a ceramic matrix from a ceramic material in an enclosure via layer-by-layer deposition of the ceramic material, ii) bringing at least one fiber 1 into contact with the ceramic matrix produced in step i) and obtaining an instrumented matrix; iii) positioning the instrumented matrix obtained in step ii) in a deposition chamber and deposition, layer by layer, of the ceramic material on the instrumented matrix in order to integrate the at least one fiber 1, by totally or partially covering the at least one fiber 1 of said ceramic material; and obtaining an instrumented structure of a CFO, iv) bringing at least one other fiber 1 into contact with the matrix manufactured during step iii) as described in step ii) and depositing a new thickness of matrix to integrate these fibers as described in step iii), the number of iterations of step iv) being greater than or equal to 1, preferably from 1 to 5 iterations, v) optionally post-physical-chemical treatment of the part obtained following the previous steps, by immersion in an organic solvent and / or exposure to a temperature above 200°C, vi) optionally heat treatment of the part obtained in step v), said treatment consisting of exposing the part to a temperature above 600°C.

[0046] In a second variant, the method of manufacturing a ceramic structure according to the invention comprises the steps: i') manufacturing a ceramic matrix from a ceramic material, in a deposition chamber, via layer-by-layer deposition of the ceramic material, ii') bringing at least one fiber 1 into contact with the ceramic matrix produced in step i'), inside the deposition chamber; iii') depositing, layer by layer, a ceramic material on the instrumented matrix by totally or partially covering the at least one fiber 1 of said ceramic material in order to integrate the fibers 1, and obtaining an instrumented structure of a CFO, the number of iterations of step iii') being greater than or equal to 1, preferably from 1 to 5 iterations, iv') optionally post-physical-chemical treatment of the part obtained following the previous steps, by immersion in an organic solvent, and / or exposure to a temperature above 200°C. v') optionally heat treatment of the pretreated part obtained in step iv'), said treatment consisting of exposing the part to a temperature above 600°C.

[0047] The present invention also relates to a ceramic structure comprising an optical component comprising one or more optical fibers according to the invention. The ceramic structure according to the invention is preferably chosen from a turbine / stator blade, a rotor, a foundry mold, a connecting element such as gears, a porous structure such as a filter.

[0048] Advantageously, the ceramic structure according to the invention is obtained by an additive manufacturing process, in particular a process as described above.

[0049] Advantageously, the ceramic structure according to the invention is composed of at least one optical component comprising one or more optical fibers according to the invention and a ceramic matrix, said matrix being composed of an inorganic material such as oxides, non-oxides, or a combination of oxides and non-oxides.

[0050] In the context of the invention, the term optical component is understood to mean a fiber optic sensor of the Bragg grating type, strings of spectrally or temporally multiplexed Bragg gratings, quasi-continuous Bragg gratings that can be interrogated in frequency reflectometry, regenerated Bragg gratings, type II or strings of microbubbles, Rayleigh probes with or without amplification by nanoparticles integrated into the vitreous matrix or by nano-gratings obtained by femtosecond insolation of the vitreous matrix, intrinsic or extrinsic Fabry-Pérot, etched or not using a femtosecond laser. These fiber optic sensors can be manufactured from supports such as: standard optical fiber, multi-core fiber, microstructured fiber, tapered fiber (or "taper" in English), optical coupler with one or more fibers at the input and one or more fibers at the output, laser fiber, without this list being exhaustive. Said optical components are integrated within ceramic structures produced by additive manufacturing, forming a mechanically reliable interface with the matrix up to very high temperature levels and ensuring the protection of the optical fiber before its insertion within or sub-surface of the part.

[0051] The fibers and CFOs implemented according to the use or the method according to the invention also make it possible to carry out in situ monitoring of the additive manufacturing process used for the integration. This in situ monitoring can be carried out by interrogating the CFO using an acquisition system adapted to the type of CFO integrated using the additive manufacturing process. The measured quantities can be, for example, temperature and / or deformation.

[0052] The fibers and CFOs implemented according to the use or the method according to the invention can withstand the heat treatments potentially applied to the parts resulting from additive manufacturing intended to stabilize their thermomechanical properties (debinding, densification, annealing for example, within the limit of ~1000°C for silica optical fibers). This treatment is generally accompanied by a shrinking / compaction of the material, favorable to the mechanical strength of the integrated CFO because it is more resistant in compression than in tension.

[0053] The instrumented ceramic material part of the integrated CFO allows measurements to be taken, for example of temperature and / or deformation, in a difficult environment, and particularly at high temperature (T > 800°C), with the aim of Material Health Monitoring (SHM).

[0054] The fiber or CFO coated with the ceramic material retains its reliability (metrological and mechanical) at high temperatures, as well as its compatibility with the material of the instrumented part. It also has low intrusiveness (typically 100-500 pm in diameter) within the structures and allows multipoint (multiplexing) and multiparametric measurements to be carried out. The coated CFO can also be integrated along a complex path within the instrumented part. It is also possible to integrate several CFOs within the same part, at different sites of interest.

[0055] Advantageously, the shape of the ceramic structures produced using the AM process can be more or less complex depending on the intended application. It can be, for example, discs, parallelepipeds, shapes of revolution such as hollow or non-hollow cylinders, shapes of revolution with added elements such as fins, without this list being limiting.

[0056] The dimensions of the manufactured ceramic structures are - along one of the axes of space - between 0.1 mm and 1 m and preferably between 1 mm and 500 mm.

[0057] According to reference [9], the ceramic materials applicable by thermal spraying are materials with a high melting point such as ceramics (oxides and Technical ceramics are divided into three different categories: oxides (for example: aluminum oxide, zirconium oxide, doped or not), non-oxides (carbides, borides, nitrides, ceramics composed of silicon and atoms such as tungsten, magnesium, platinum or titanium), and composite ceramics (combination of oxides and non-oxides).

[0058] The at least one fiber 1 implemented in the present use according to the invention can be manufactured from a pasty composition for fiber. The method for manufacturing a pasty composition for fiber coating can comprise the following steps: A) dispersing in water a dry mixture of hexagonal boron nitride BN and bentonite to ensure good mixing of the bentonite and the boron nitride, the dry mixture comprising at least 10% by weight of bentonite relative to the total weight of said dry mixture, to form an aqueous suspension; B) evaporation of the water contained in said aqueous suspension, until a dry powdery extract is obtained; C) dispersion of said powdered dry extract in water to form a pasty composition, at a rate of at least 40% by mass of dry extract in water.

[0059] Advantageously, step B) of the process for manufacturing a pasty composition for fiber coating according to the invention can be carried out under primary vacuum or under atmospheric pressure, and at a temperature which can be between 50°C and 90°C, preferably between 60°C and 80°C, and better still of the order of 60°C.

[0060] The pasty composition for optical fiber coating used in the present use according to the invention can be obtained by the manufacturing method mentioned above.

[0061] Advantageously, the pasty composition may further comprise a dopant, which may advantageously be based on carbon, zirconium oxides, titanium oxides and nanoparticles of metals or semiconductors, organic fillers (organic and organometallic molecular compounds), inorganic fillers and mixtures thereof.

[0062] The fiber used in the use according to the invention can be manufactured according to a manufacturing method using such a pasty composition to obtain the deposition of an external coating on the external surface of a fiber, the method comprising the following steps: A) supply or production of a fiber core made of fibrable material (without protective coating); B) providing a pasty composition for fiber coating according to the invention; C) coating at least a portion of said fiber with said pasty composition so as to form a wet layer on said fiber; D) heat treatment of said optical fiber coated with said wet layer at a temperature between 100°C and 250°C for a time sufficient to form an outer coating layer 2 capable of being handled (in this case rolled up and handled).

[0063] Advantageously, steps C and D can be repeated one or more times until the desired thickness of exterior coating is obtained.

[0064] Advantageously, the method for manufacturing the fiber may further comprise a step A' of stripping the fiber according to the invention, to remove, over at least part of the length of the fiber, the protective sheath present in the case of a commercial fiber supply. Preferably, this step A' may be carried out by bringing the protective sheath into contact with a dichloromethane solution, in the case of a polyacrylate protective sheath. Other methods of stripping the fiber are possible, for example by mechanical stripping with pliers or a razor blade. However, with regard to optical fibers intended to be handled at least once, it is preferable to use chemical stripping.

[0065] Other advantages and particularities of the present invention will result from the description which follows, given by way of non-limiting example and made with reference to the appended figures and to the examples. Brief description of the figures

[0066] The following examples illustrate the invention, in conjunction with the figures commented on above, without however limiting its scope:

[0067] [Fig-1]: [Fig.l] represents a cross-sectional view (A) and a view in perspective (B) of a first example of fiber according to the invention (fiber without protective sheath); the fiber 1 comprises a core 11 made of fiber-forming material and has an external surface 111, covered by an external coating 2 based on hexagonal boron nitride and bentonite.

[0068] [Fig.2]: [Fig.2] represents two optical microscope photographs of the fiber 1, covered by the outer coating 2 obtained by the manufacturing process described in paragraph

[60] after heat treatment at 1000°C.

[0069] [Fig.3]: [Fig.3] represents the relative variation over time of the response of a Bragg grating (AXBragg) under 800°C for 800 hours, for a bare fiber (in solid line) and a fiber according to the invention, provided with a coating comprising three layers of boron nitride-based coating (in dotted lines).

[0070] [Fig.4]: [Fig.4] represents the relative variation over time of the response of a Bragg grating inscribed in an optical fiber coated with boron nitride-based material during its integration using the atmospheric plasma projection process.

[0071] [Fig.5]: [Fig.5] represents, on the one hand, the relative variation over time of the response of five Bragg gratings integrated within a parallelepiped sample subjected to repeated bending loads at four points. [Fig.5] also presents the evolution over time of the temperature in the test chamber as well as that of the mechanical load applied to the sample.

[0072] [Fig.6]: [Fig.6] represents a synopsis of the different stages allowing the manufacture of a structure in ceramic material produced by additive manufacturing, comprising at least one CFO protected by the boron nitride-based coating, according to the invention.

[0073] EXAMPLES

[0074] The nature of the products used for the manufacture of the fibers and the process implemented, as well as the characterization processes are detailed below.

[0075] Products, raw materials: - solvent for chemical stripping: dichloromethane, isopropanol; - hexagonal BN powder; - bentonite of general formula Al2H20i2Si4; - samples of optical fibers (in particular silica, sapphire, or chalcogenide) including or not a protective sheath in organic polymer (for example polyacrylate).

[0076] Devices and tests for structural and microstructural characterization A complete physicochemical characterization was carried out with complementary techniques at different scales to characterize the applied coating layer using: - optical microscopy, - X-ray diffraction (XRD) analysis, - high temperature resistance test comprising heating the fiber samples according to the invention to 1000°C, with a heating ramp at 10°C / min, followed by inertia or instantaneous cooling; - determination of the behavior of the Bragg response of the fiber samples according to the invention by analyzing the reflectivity at the Bragg wavelength via a broadband laser source and an optical spectrum analyzer.

[0077] EXAMPLE 1: Production of an example of pasty composition C for fiber re-coating.

[0078] Boron nitride and bentonite (at least 10% by weight of bentonite) are ground using a planetary mill, with the direction of rotation reversed every 5 minutes (for a satisfactory particle size).

[0079] The grinding product thus obtained is dispersed in a large quantity of water (approximately 250 mL) to form a suspension.

[0080] The suspension thus obtained is evaporated to dryness in a 500 mL Schlenk tube. Evaporation is carried out under primary vacuum (103 Pa) using a vacuum / argon ramp. Throughout the operation, the Schlenk tube is maintained at 60°C in a water bath, via an oil bath. After 4 to 6 hours of evaporation: the dry extract obtained is ground manually (mortar and pestle). The powder obtained can be stored in an oven at 50°C or in a desiccator for several months.

[0081] When making the deposit on fiber, the powder obtained is dispersed in at least 20 mL of distilled water.

[0082] The pasty composition according to the invention C is obtained.

[0083] EXAMPLE 2: Manufacture of a fiber coated with boron nitride-based material

[0084] Step A

[0085] Optical fiber samples without protective sheath are used. In the case of a supply of commercial optical fiber samples (in particular made of silica, sapphire, or chalcogenide) comprising a polyacrylate protective sheath, an additional stripping step is necessary during a step A'.

[0086] Step A'

[0087] As a reminder, optical fibers, during their manufacture, are conventionally protected by organic polymers: without this protective coating, optical fibers are extremely vulnerable to mechanical contact, making them difficult to handle. However, this organic coating is by nature incompatible with the deployment of optical fiber in a harsh environment.

[0088] It is therefore preferable to remove at least partially this coating. This stripping operation A' is preferably carried out by chemical attack. The advantage of this step A' is to strip a specific portion of the optical fiber, either at one end or on a previously defined area. Generally, at each end of the fiber, the initial coating is retained over a sufficient length so as to be able to at least maintain the fiber in position during the coating deposition step without weakening it. The lengths are adjusted according to the type of application targeted.

[0089] The solvent used is dichloromethane, when it is an original protective sheath of polyacrylate type (standard case).

[0090] If the commercial optical fiber samples include a protective sheath made of a polymer other than a polyacrylate and which is not sensitive to dichloromethane, another solvent capable of dissolving this polymer will be used. If the protective sheath is, for example, made of polyimide, hot hydrochloric acid or sulfuric acid will be used to dissolve it.

[0091] Step A' of chemical stripping makes it possible to avoid weakening of the fiber, unlike mechanical stripping (with pliers or razor blade).

[0092] Step B

[0093] The pasty composition C of example 1 is used.

[0094] Step C

[0095] At least part of a stripped fiber sample is then coated with the pasty composition C so as to form a wet layer on the fiber, for example by immersion or directly on a fiberizing tower.

[0096] Step D

[0097] The sample is then dried. It can be placed in an oven at 100°C. The coating is dry to the touch after 15 seconds. After this treatment, the fiber can be wound onto a standard reel (typically 158 mm radius). It can also be dried in a vertical tubular furnace directly on the fiberizing tower, below the spinneret holder. The hot zone is approximately 250 mm. The oven temperature is 250°C.

[0098] EXAMPLE 3: characterization of coatings

[0099] Various tests were then carried out to characterize the BN and bentonite coatings in accordance with the invention.

[0100] In order to detect possible physicochemical modifications of the coating (prohibitive for the intended applications), the samples are observed under an optical microscope, characterized by DRX, and under different temperature conditions. The optomechanical behavior is also studied.

[0101] A first temperature resistance test of the coatings formed in Example 2 was carried out at 1000°C, increased by 10°C / minute up to 1000°C, for a period of 500 hours, then inertia cooling. [Fig.2] is an observation of the sample under an optical microscope after this heat treatment. These observations show that the coating does not show any alteration of its integrity (crack or fracture).

[0102] Other samples of fibers having BN-coated Bragg gratings are also studied under different isotherms (at high and low temperatures), in order to validate the criterion of non-modification of the opto-mechanical properties of the fiber. Indeed, it is essential that the coating does not alter the sensitivity of the sensor that it protects. Successive heating and cooling cycles are also repeated on samples with and without coating in order to validate the good dynamic behavior (thermal expansion of the different materials).

[0103] Similarly, the behavior of the Bragg response is compared with and without coating, as illustrated in [Fig.3] during a cycle over 800 hours at 800°C.

[0104] EXAMPLE 4: Fiber Optic Sensor integrated into a mechanical test specimen by an additive manufacturing process implemented with the fiber obtained in Example 2

[0105] Manufacturing of CFOs

[0106] In this example, the CFOs are made up of multiplexed Bragg Gratings (RBs) in wavelength, with a physical length of 1 mm.

[0107] These RdBs are inscribed in the heart of a silica optical fiber using laser pulses of a unit duration, here, between 100 and 200 fs.

[0108] This registration method makes it possible to obtain RdBs resistant to high environmental temperatures (T> 800°C).

[0109] The RdBs are written through the initial coating of the optical fiber (acrylate polymer), here transparent to wavelengths belonging to the visible light range. This makes it possible to preserve the mechanical integrity of the fibers during their transport to the coating application stage.

[0110] The RdBs-inscribed optical fibers are stripped of their initial coating and then coated with the boron nitride-based protective material as described in Example 2.

[0111] A stabilizing heat treatment is applied to the CFOs coated with the protective material.

[0112] This heat treatment comprises the step presented in Example 2, i.e. a first step at 100°C.

[0113] This heat treatment is completed by a step at 500°C for 1 hour then at 750°C for 2 hours. These steps serve to stabilize the coating material but also the RdB inscribed in the heart of the optical fiber.

[0114] Integration of CFOs

[0115] The manufacturing process discussed in the present example is atmospheric plasma spraying of ceramic material.

[0116] A ceramic material powder, here cordierite (Al3Mg2AlSi50i8) is introduced into a plasma torch. This plasma is generated by circulating gases between electrodes between which an electric voltage is applied, generating an electric arc.

[0117] The particles of ceramic material melt on contact with the plasma. They are transported by the latter at a speed depending on the process parameters known to those skilled in the art.

[0118] The scanning of the plasma torch relative to a manufacturing surface makes it possible to deposit layers, with a thickness of a few microns for example, on said surface.

[0119] In this example, a first step a) consists of depositing a millimetric thickness of material in order to form the CFO integration support, i.e. a ceramic matrix.

[0120] This support has a surface area of ​​15 x 45 mm2.

[0121] A second step b) consists of positioning the CFO coated with the boron nitride-based protective material on the ceramic matrix. The CFO is held in position using point additions of adhesive during a third step c). It is essential to ensure tension of the fiber so that it is pressed against the ceramic matrix and thus limit any relative movement of the fiber in relation to said ceramic matrix.

[0122] A fourth step d) consists of depositing an additional thickness of cordierite which is projected onto the surface of the instrumented matrix during step c) to embed the CFO in the material.

[0123] In situ monitoring of the process

[0124] Continuous interrogation of the RdBs using a suitable instrument makes it possible to monitor the progress of the manufacturing process, i.e. the successive deposition of each layer of material.

[0125] For example, [Fig.4] shows the Bragg wavelength shift measured by an RdB during the plasma spraying process. This response is sensitive to variations in deformation and temperature within the material.

[0126] The advantage of process monitoring by CFO compared to the techniques usually used, such as pyrometry for example, lies in the volume probed by the CFOs (a few pm3) which is much lower than the so-called usual techniques.

[0127] High temperature tests

[0128] The quality of the interface formed between the coated CFO and the material deposited by the plasma spraying process is studied by subjecting the instrumented sample to mechanical bending loads, while varying the test temperature.

[0129] The test temperatures are between room temperature and 800°C, more precisely 27°C; 148°C; 344°C; 572°C; 782°C.

[0130] The mechanical loads are applied using four-point bending supports positioned in the temperature-controlled enclosure.

[0131] Five mechanical loads of 60 N each are applied at each test temperature.

[0132] The response of the RdBs under the effect of mechanical loads is presented in [Fig.5]. This response differs according to the respective position of the RdBs along the length of the plot, because the four-point bending test induces a deformation field dependent on the longitudinal position.

[0133] The absence of significant drop in the response of the RdBs under the effect of mechanical loading shows that the interface retains its mechanical integrity up to the maximum test temperature.

[0134] Indeed, a drop in the response of the RdBs under the effect of mechanical loadings would indicate a loss of transfer of deformations between the material deposited by plasma spraying and the CFOs coated with the boron nitride-based material.

[0135] The different steps described in the context of the present example are summarized in [Fig.6]. In this figure, the boxed steps are considered necessary to obtain a ceramic part produced by FA and instrumented with a CFO, and the steps between brackets are optional or can vary, for example, depending on the chosen process.

[0136] LIST OF REFERENCES 1. Z. Chen, Z. Li, J. Li, C. Liu, C. Lao, Y. Fu, C. Liu, Y. Li, P. Wang, and Y. He, “3D printing of ceramics: A review,” Journal of the European Ceramic Society 39, 661-687 (2019). 2. VP Wnuk, A. Mendez, S. Ferguson, and T. Graver, “Process for mounting and packaging of fiber Bragg grating strain sensors for use in harsh environment applications,” in E. Udd and D. Inaudi, eds. (2005), p. 46. 3. D. Havermann, J. Mathew, WN MacPherson, RRJ Maier, and DP Hand, “Temperature and Strain Measurements With Fiber Bragg Gratings Embedded in Stainless Steel 316,” Journal of Lightwave Technology 33, 2474-2479 (2015). 4. Y. Duo, S. Costil, P. Pfeiffer, and B. Serio, "Embedding properties of optical fibers integrated into ceramic coatings obtained by wire flame thermal spray," Smart Mater. Struct. 24, 035027 (2015). 5. L. Huang, R. S. Dyer, R. J. Lago, A. A. Stolov, and J. Li, "Mechanical properties of polyimide coated optical fibers at elevated températures," in Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XVI (International Society for Optics and Photonics, 2016), Vol. 9702, p. 97020Y. 6. J. Lei, Q. Zhang, Y. Song, J. Tang, J. Tong, F. Peng, and H. Xiao, "Laser-assisted embedding of all-glass optical fiber sensors into bulk ceramics for high-temperature applications," Optics & Laser Technology 128, 106223 (2020). 7. C. M. Petrie, A. M. Schrell, D. N. Leonard, Y. Yang, B. C. Jolly, and K. A. Terrani, "Embedded sensors in additively manufactured Silicon Carbide," Journal of Nuclear Materials 153012 (2021). 8. CM Petrie, A. Schrell, D. Leonard, BC Jolly, and KA Terrani, Demonstration of Embedded Sensors in Ceramic Structures (Oak Ridge National Lab.(ORNL), Oak Ridge, TN (United States), 2020). 9. P. Fauchais, "Ceramic deposits by PVD or CVD assisted or by plasma projection," Engineering techniques Friction, wear and lubrication documentary base: TIP574WEB., (2013).

Claims

Claims

1. Use of an optical fiber in a method of additive manufacturing of ceramic structures, said fiber (1) comprising a core (11) made of a material allowing fibering and having an external surface (111), said fiber being characterized in that it further comprises an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, at a rate of at least 10% by weight of bentonite relative to the total weight of said outer coating (2).

2. Use according to claim 1, according to which the core (11) of the fiber is made of a material chosen from glass transition materials and sapphire glass.

3. Use according to any one of claims 1 and 2, wherein said outer coating (2) of the fiber (1) is directly in contact with the core (11).

4. Use according to any one of claims 1 to 3, wherein the core (11) of said fiber (1) has a diameter in a range from 20 pm to 10 mm, preferably from 80 pm to 500 pm and more preferably 125 pm.

5. Use according to any one of claims 1 to 4, according to which the outer coating (2) of the fiber (1) has a thickness of between 5 pm and 240 pm.

6. Ceramic structure, preferably chosen from - turbine / stator blade, - rotor, - foundry mold, - connecting element, preferably gears, and - porous structure, preferably a filter, the ceramic structure comprising an optical component comprising one or more optical fibers (1), each fiber (1) comprising a core (11) made of a material allowing fiberization and having an external surface (111) and further comprising an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, at a rate of at least 10% by weight of bentonite relative to the total weight of said outer coating (2).

7. Use according to any one of claims 1 to 5, wherein the additive manufacturing method comprises the steps: a) manufacturing a ceramic matrix from a material ceramic, b) bringing into contact at least one fiber (1) comprising a core (11) made of a material allowing fiberizing and having an external surface (111) and further comprising an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, in a proportion of at least 10% by weight of bentonite relative to the total weight of said outer coating (2), with the ceramic matrix obtained in step a); c) fixing the at least one fiber (1) to the surface of the ceramic matrix, possibly using elements at the periphery of the manufacturing zone, so as to limit any relative movement of said fiber (1) relative to the ceramic matrix. d) manufacturing a volume of additional material totally or partially covering the fiber (1).

8. Use according to any one of claims 1 to 5 and 7, wherein the additive manufacturing process is chosen from a plasma spraying or thermal spraying process, material extrusion, directed energy deposition, manufacturing of laminated objects, selective powder bed fusion, selective powder bed sintering, binder spraying, photopolymerization.

9. A method of manufacturing by atmospheric plasma spraying an instrumented ceramic structure of a CFO according to claim 6, comprising the steps: a') manufacturing a ceramic matrix from a ceramic material by atmospheric plasma spraying, b') bringing into contact at least one fiber (1) comprising a core (11) made of a material allowing fiberizing and having an external surface (111) and further comprising an outer coating (2) comprising a mixture of hexagonal boron nitride and bentonite, in a proportion of at least 10% by weight of bentonite relative to the total weight of said outer coating (2), with the ceramic matrix obtained in step a') and obtaining an instrumented material; c') positioning the instrumented matrix obtained in step b') in a deposition chamber and depositing, layer by layer, a ceramic material, by atmospheric plasma spraying on the instrumented matrix and integrating the at least one fiber (1);and obtaining a ceramic structure instrumented with a CFO.;

10. A method of manufacturing a ceramic structure according to claim 6, comprising the steps: (i) manufacturing a ceramic matrix from a ceramic material in an enclosure via layer-by-layer deposition of the ceramic material, ii) bringing into contact at least one fiber (1) comprising a core (11) made of a material allowing fiberizing and having an external surface (111) and further comprising an external coating (2) comprising a mixture of hexagonal boron nitride and bentonite, in a proportion of at least 10% by weight of bentonite relative to the total weight of said external coating (2), with the ceramic matrix produced in step i) and obtaining an instrumented matrix; iii) positioning the instrumented matrix obtained in step ii) in a deposition chamber and deposition, layer by layer, of the ceramic material on the instrumented matrix in order to integrate at least one fiber (1), by totally or partially covering at least one fiber (1) of said ceramic material; and obtaining an instrumented structure of a CFO, iv) bringing at least one other fiber (1) into contact with the matrix manufactured during step iii) as described in step ii) and depositing a new matrix thickness to integrate these fibers as described in step iii), the number of iterations of step iv) being greater than or equal to 1, preferably from 1 to 5 iterations, v) optionally post-physical-chemical treatment of the part obtained following the previous steps, by immersing it or not in an organic solvent, by exposing it or not to temperatures above 200°C, vi) optionally heat treatment of the part obtained in step v), said treatment consisting of exposing the part to a temperature above 600°C.

11. A method of manufacturing a ceramic structure according to claim 6, comprising the steps: (i) manufacturing a ceramic matrix from a ceramic material in a deposition chamber, via layer-by-layer deposition of the ceramic material, ii') bringing into contact at least one fiber (1) comprising a core (11) made of a material allowing fiberizing and having an external surface (111) and further comprising an external coating (2) comprising a mixture of hexagonal boron nitride and bentonite, at a rate of at least 10% by weight of bentonite relative to the total weight of said core (11) outer garment (2), and the ceramic matrix produced in step i'), inside the deposition enclosure; iii') depositing, layer by layer, a ceramic material on the instrumented matrix by totally or partially covering the at least one fiber (1) of said ceramic material, in order to integrate the fibers (1), and obtaining an instrumented structure of a CFO, the number of iterations of step iii') being greater than or equal to 1, preferably from 1 to 5 iterations, iv') optionally post-physical-chemical treatment of the part obtained following the previous steps, by immersion in an organic solvent, and / or exposure to a temperature above 200°C. v') optionally heat treatment of the pretreated part obtained in step iv'), said treatment consisting of exposing the part to a temperature above 600°C.

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