FLEXIBLE WIRE FOR THERMAL SPRAY GUN AND THERMAL SPRAY DEVICE

Description

technical field

[0001] The invention relates to a flexible cord for supplying a thermal spray torch, for the purpose of applying a coating. It also relates to a method for manufacturing said cord and a thermal spraying device using said cord. Previous art

[0002] Thermal spraying involves using a source of thermal and kinetic energy to melt a collection of inorganic particles, initially in the form of wire or powder, and then projecting the at least partially molten particles onto a substrate. Upon impact, the partially molten particles spread out and cool on the substrate. As they cool, they can then effectively adhere to each other and to the substrate, forming a coating.

[0003] There are different technologies, which can be classified into the classes of "enthalpy of combustion", "electrical discharge" and "momentum density".

[0004] The "enthalpy of combustion" class includes "detonation gun" technologies: "flame-powder" (generating particle velocities < 80 m / s and only able to be fueled by powders), "flame-wire" (generating particle velocities > 150 m / s and able to be fueled by a medium such as wire, cord, or rod), and finally "high-velocity flame" (generating particle velocities > 500 m / s). Among the "high-velocity flame" technologies, we distinguish "D-Gun detonation," "HVOF" (High Velocity Oxy-Fuel), "HVSFS" (High Velocity Suspension Flame Spray), and "HVAF" (High Velocity Air Fuel), which can be fueled by powders, suspensions (as in the case of HVSFS), or even wires.

[0005] The "electric discharge" class includes "electric arc" and "plasma" technologies. The latter category encompasses so-called "atmospheric" or APS (Atmospheric Plasma Spray) processes, as well as processes in controlled atmospheres and temperatures, such as "VPS / LPPS" (Vacuum or Low Pressure Plasma Spray), "VLPPS / PS-PVD" (Very low pressure plasma spray and Plasma Spray enhanced Physical Vapor Deposition), "SPS" (Suspension Plasma Spray), "SPPS" (Solution Precursor Plasma Spray), "Induction Plasma," and "WSP" (Water Stabilized Plasma). These systems propel particles at speeds ranging from 150 m / s to 500 m / s, depending on the variant and process parameters used.These "plasma" systems can be used with dry powders whose particle size is such that the median size D 50 is greater than 10 micrometers typically, or with liquids (chemical precursors such as salts, or suspensions comprising inorganic particles whose median size D 50 is less than 10 micrometers).

[0006] The "momentum density" class includes "Cold Spray" technologies, which include "low pressure", "high pressure" and "recycled helium" technologies.

[0007] All these technologies are well known, and in particular described in the book ASM Handbook vol 5a - Thermal Spray Technology.

[0008] Each technology is associated with its own specific constraints, so the problems encountered, and the solutions provided to address them, are generally different depending on the technology considered.

[0009] The invention is particularly concerned with the set comprising: in the "enthalpy of combustion" class, the "flame-wire" and "high-speed flame" technologies; and the technologies of the "electrical discharge" class, and in particular the "plasma" technologies.

[0010] For clarity, in this description and in a generic manner, a "torch" refers to any torch or gun used in these technologies. Specifically, "torch" includes a wire flame spray (WFS), a plasma torch system, or a high-velocity flame projection device as defined above. A recent development in these technologies involves feeding these devices with liquid precursor suspensions or solutions (the suspensions being composed of a solvent and fine inorganic particles, typically with a median size D50 of less than 5 micrometers). This evolution towards very fine particles (or even the formation of the material) is a significant advancement. in situ(In the case of liquid precursors), this has enabled the generation of new types of microstructures, for example, very finely structured, very dense, or even columnar, or "feathery" (having the characteristics of juxtaposed feathers when observed in micrographic cross-section). These developments open new perspectives, but encounter constraints and limitations in process robustness, stability, and reproducibility due to the fact that particle suspensions are not always perfectly stable. Solvent vaporization, however, reduces energy efficiency during the spraying phase. Furthermore, when the suspensions contain particles of different compositions, particularly those with different densities, formulating such stable suspensions without sedimentation or particle agglomeration is difficult, if not impossible, to achieve industrially.

[0011] JP2016156058A describes a feed medium consisting of a composite wire. The coating is a dense electrolytic film for the fabrication of fuel cells. Such a wire can be difficult to manufacture and does not allow for the production of ceramic coatings (oxides, for example) with precise control of the size of the sprayed particles. In particular, this medium contains fine particles embedded in a matrix. Tests have shown that the large amount of matrix leads to detrimental enthalpy variations during spraying. US 4 593 856 also describes a feed medium in the form of a wire. However, the wire fusion may be incomplete, which impairs the quality of the coating.

[0012] Brazing rods with a sheathed core are known to feed brazing torches to deposit thick, metallurgically bonded layers (known as "overlays" or "hardfacings") onto a metallic substrate. They are not suitable for thermal spray coatings, which can be applied without a brazing step and can be made of ceramics and deposited onto any type of substrate. These brazing rods consist of inorganic particles larger than 10 micrometers.

[0013] US 3,701,444 also contains an arc weld bead. Such a bead is not typically used to feed a thermal spray torch for coating, as these two applications (welding / spraying) are quite different. In particular, the inorganic particles generally have a median diameter greater than or equal to 100 micrometers.

[0014] FR 1 443 142 finally describes a cord that has a core coated with a sheath. The set of inorganic particles has a median size D 50 incompatible with obtaining a very homogeneous or finely structured coating.

[0015] Document FR2673871 describes a cord intended for powering a thermal projection torch, the core of which is protected by a sheath made of polymeric material.

[0016] The US6924007 B2 document describes a thermal spraying device with a torch and a cord injectable by the device into the plasma stream.

[0017] Therefore, there is a constant need for a feed medium suitable for projection by a plasma or flame torch: allowing a substantially continuous supply of the torch, in a reliable manner, making it possible to overcome the constraints and limitations of Suspension or Liquid Solution type media, in particular by being in a solid and stable form, easily handled, preferably windable onto a reel; leading to a very homogeneous or finely structured coating; without excessive torch abrasion; and at the cost of limited energy consumption.

[0018] The present invention aims to satisfy at least partially this need. Summary of the invention

[0019] According to the invention, this goal is achieved by means of a cord having an equivalent outer diameter of between 1 mm and 3.5 mm and consisting of a core, or "soul", in the form of a wire, and a sheath covering the core along its entire length, the core being made up of a set of inorganic particles whose median size D 50 is less than 10 micrometers, the inorganic particles representing more than 40% and less than 80% of the volume of the core; and of a matrix binding said inorganic particles, the matrix comprising a polymer binder and optionally a matrix lubricant, for example glycerin, together representing more than 90% of the volume of the matrix, the remainder to 100% being made up of impurities; the sheath having a thickness between 50 micrometers and 500 micrometers and comprising a sheath polymer and preferably a sheath lubricant, identical or different from the optional matrix lubricant, together representing more than 90% of the volume of the sheath, the remainder to 100% being preferably made up of impurities and an optional coloring pigment, the volume percentages being determined without taking into account the possible presence of residues of a solvent.

[0020] Surprisingly, the inventors discovered that a cord according to the invention allows for a continuous and reliable supply of power to the torch, while also leading to a high-quality coating, with several advantages compared to the use of fine particle suspensions: improved process stability and elimination of problems associated with the stability of particle suspensions (risks of sedimentation, agglomeration); lower energy consumption, due to the absence of evaporation of aqueous solvents used in the suspensions; absence of particle agglomeration at the injection point or in flight, unlike what is observed during thermal projection of a suspension; better control of the flow rate of inorganic material, advantageously proportional to the advance speed of the bead in the projection device, whereas the injection of suspension leads to significant variations in this flow rate.

[0021] Furthermore, a feed medium in the form of a cord advantageously contains much less solvent than a suspension. The low solvent content, particularly water content, of a cord, typically less than 5% by mass, considerably limits energy losses during the thermal projection of the medium, which aims to melt the inorganic particles completely or partially.

[0022] Without being bound by this theory, the inventors ultimately observed that the characteristics of the cord lead to a small enthalpy change during spraying. This enthalpy change therefore disturbs the thermal spraying process much less than with dry powder spraying and the use of prior art cords. In particular, the inventors found that the combination of a core with a low matrix content and a cladding results in a lower enthalpy change than a composite cord without a cladding and with a higher matrix content, as in JP2016156058A.In general, the presence of a minimum amount of polymer is necessary, especially to confer the required flexibility, and the inventors discovered that increasing the concentration of inorganic particles at the core of the cord and adding a sheath to maintain this flexibility were preferable to a lower concentration of inorganic particles within a composite cord.

[0023] In a particularly advantageous embodiment, the ratio R of the equivalent outside diameter of the bead, in micrometers, to the median size of all the inorganic particles, in micrometers, is between 200 and 20,000, preferably between 200 and 1600. Remarkably, this configuration, combined with an inorganic particle volume content relative to the core volume of said bead greater than 40%, preferably greater than 50%, or even greater than 60%, and less than 80%, preferably less than 78%, preferably less than 75%, allows for better particle dispersion during their expulsion in the plasma jet or flame, after decomposition of the cladding. This improved dispersion is particularly advantageous for high-velocity plasma or flame torches, and especially for axial injection torches.

[0024] This results in a better compromise between the following properties and characteristics: optimal dispersion of inorganic particles in the spray jet when using this cord in a thermal spraying process, without agglomeration of inorganic particles together, which allows the obtaining of coatings with fine microstructures; flexibility of the cord allowing its easy handling without risk of breakage; low enthalpy required for the decomposition of organic compounds and their conversion into non-toxic gases.

[0025] A cord according to the invention may further include one or more of the following optional and preferred features: the median size D 50 of the inorganic particle set is less than 5 micrometers, preferably less than 4 micrometers, preferably less than 3 micrometers, preferably less than 1 micrometer, in particular to constitute a columnar structure; the viscosity of the core polymer binder and / or the sheath polymer and / or the core constituent material and / or the sheath constituent material is / are between 30 and 300 mPa.s, or between 30 and 300 centipoise at 20°C, said viscosity being measured, with a Höppler viscometer, on a mixture comprising 2% by mass of a dry powder of the core polymer binder and / or the sheath polymer and / or the core constituent material and / or the sheath constituent material, respectively, in demineralized water;the sheath and / or matrix is / are made of a cellulose derivative, i.e. a constituent comprising cellulose molecules, preferably methylhydroxyethylcellulose, a cellulose derivative having a viscosity and low associated ash content particularly well suited to thermal spraying; inorganic particles represent more than 45%, preferably more than 50%, and / or preferably less than 70%, preferably less than 75%, as a percentage by volume based on the volume of the core of the cord, excluding any solvent;the inorganic particles are: particles preferably of alumina, zirconia, titanium oxide, chromium oxide, yttrium oxide, or a combination of several of these oxides, for example in mullite or spinel, and / or carbide-based Cermet particles, the carbides being chromium and / or tungsten and / or titanium and / or tantalum and / or zirconium and / or niobium carbides, said carbides being associated with a metallic phase, and / or particles of a ceramic chosen from nitrides, borides and carbonitrides, possibly associated with a metallic phase in the form of Cermets; particles of a special metal chosen from metallic amorphous materials, quasicrystals or approximants, and more broadly non-drawingable metallic alloys;the inorganic particles are selected from particles in a ceramic, particles in an intermetallic alloy, particles in a metallic amorphous material, and particles in a quasicrystal or an approximate phase; the thickness of the sheath is greater than 100 micrometers, preferably greater than 150 micrometers, preferably less than 400 micrometers; for a bead of 2.5 to 3.5 mm outside diameter, the thickness of the sheath is between 200 and 400 micrometers; for a bead of 1 to 2.5 mm outside diameter, the thickness of the sheath is between 100 and 250 micrometers; the ash content of said bead is preferably less than 2%, preferably less than 1%, preferably less than 0.7%, preferably less than 0.5% as a mass percentage on the basis of the dry mass of said bead;The polymer binder and the optional matrix lubricant, identical or different from that of the sheath, together represent more than 95%, preferably more than 97%, preferably more than 99%, preferably substantially 100%, as a percentage by volume based on the volume of the matrix, without taking into account any solvent residues; the polymer binder, preferably a cellulose derivative, more preferably a methylhydroxyethylcellulose, represents more than 5%, preferably more than 10% and / or less than 25%, preferably less than 20%, or even less than 15% as a percentage by volume, based on the volume of the core of the cord, without taking into account any solvent residues;the matrix preferably represents more than 25%, preferably more than 30%, preferably more than 40%, or even more than 45%, and / or preferably less than 70%, preferably less than 60%, preferably less than 55%, preferably less than 50%, as a percentage by volume based on the core volume of the cord, without taking into account any solvent residues; in one embodiment, the matrix comprises a matrix lubricant, the content of said matrix lubricant being greater than 5%, greater than 10% and / or less than 25%, or less than 20%, as a percentage by volume based on the core volume of the cord, without taking into account any solvent residues; the matrix lubricant is selected from polyols, glycerides, in particular glycerol and its derivatives, stearates, amino alcohols, preferably from glycerin and triethanolamine, more preferably glycerin;the matrix solvent is water or a denatured alcohol, preferably water; the residual solvent content, preferably water, in the core is less than 5%, by mass percentage on the basis of the dry mass of the cord core; the matrix consists of more than 99%, preferably substantially 100%, of an organic material, by volume percentage; the sheath polymer and the optional sheath lubricant together represent more than 95%, preferably more than 97%, preferably more than 99%, preferably substantially 100%, by volume percentage on the basis of the sheath volume, without taking into account any solvent residues;the sheath contains a sheath lubricant, identical or different from the optional matrix lubricant, the content of which is greater than 10%, preferably greater than 20%, preferably greater than 30%, and / or preferably less than 50%, preferably less than 40%, as a percentage by volume based on the volume of the sheath, without taking into account any solvent residues; the content of the sheath polymer is greater than 45%, preferably greater than 55%, preferably greater than 60%, and / or preferably less than 80%, preferably less than 75%, preferably less than 70%, as a percentage by volume based on the volume of the sheath, without taking into account any solvent residues; the polymer binder and the sheath polymer contain an identical polymer, preferably contain, as polymer(s), only identical polymers, preferably in the same proportions;The impurities of the matrix and / or the sheath consist of more than 90%, preferably more than 95%, preferably substantially 100%, of organic impurities and / or impurities containing the element hydrogen (H), and / or metallic impurities; the impurities of the matrix and / or the sheath represent less than 5%, preferably less than 3%, preferably less than 1%, by volume based on the volume of the matrix and / or the sheath, respectively, without taking into account any solvent residues; the sheath contains a coloring pigment, which may be any conventionally used coloring, representing less than 1%, preferably less than 0.5%, preferably less than 0.4%, preferably less than 0.1%, or even less than 0.05% of the volume of the sheath; the solvent of the sheath is preferably water or a denatured alcohol, preferably water;the residual solvent content, preferably water, in the sheath, is less than 10%, preferably less than 5%, as a mass percentage based on the mass of the cord sheath; the sheath is composed of more than 99%, preferably substantially 100%, of an organic material, as a volume percentage; the cord is wound on itself, in the form of a roll or a coil, preferably wound on a mandrel with a diameter greater than 50 mm, preferably greater than 100 mm, preferably greater than 150 mm, or even greater than 200 mm, and / or less than 1000 mm, preferably less than 500 mm, preferably less than 400 mm, preferably less than 300 mm.

[0026] A bead according to the invention is not intended for welding. Preferably, it does not contain a fluxing agent (“ flow "), such a fluxing agent is classically used to clean and deoxidize the weld area or to form a protective slag.

[0027] Preferably, a cord according to the invention does not include a fluxing agent selected from fluorspar or calcium fluoride, cryolite which is an alumina and sodium fluoride, and borates.

[0028] The invention also relates to a thermal projection device comprising: a torch comprising a plasma or flame generator and an injection device; and a cord according to the invention arranged so as to be injectable, by the injection device, into the plasma or flame generated by said generator, the torch being capable of melting at least partially the inorganic particles of the bead and of projecting the at least partially melted inorganic particles at more than 150 m / s.

[0029] Preferably, the torch is capable of projecting inorganic particles, at least partially molten, at speeds exceeding 150 m / s and / or below 1000 m / s, with the projection velocity conventionally measured at the torch outlet. In one embodiment, the particles are projected at speeds exceeding 300 m / s, preferably exceeding 500 m / s, preferably exceeding 600 m / s, and preferably exceeding 700 m / s. In another embodiment, the particles are projected at speeds exceeding 150 m / s and below 300 m / s.

[0030] The injection device is preferably arranged to inject the bead along an injection axis extending in a radial plane, i.e., passing through the X-axis of the plasma or flame flow, and forming with a plane P transverse to the X-axis an angle θ, in absolute value, greater than 60°, greater than 70°, greater than 80°, preferably greater than 85°, the injection axis I being preferably substantially parallel to the X-axis, the injection or "feed" being described as "axial" injection or feed. The torch is preferably axially fed.

[0031] An angle θ close to 90° advantageously promotes homogeneous combustion of the cladding and matrix, and therefore uniform dispersion of the inorganic particles released into the plasma stream or high-speed flame. This ensures a centered and optimal trajectory of the particles in the torch nozzle, and reduces the risk of nozzle fouling, and thus the risk of process malfunctions and coating defects.

[0032] Preferably, the injection is performed upstream of the nozzle from which the plasma stream, or "jet," flows, or the flame in the case of a flame torch, flows. Preferably, for a flame torch, the bead is drawn to the combustion chamber. Preferably, for a plasma torch, preferably a multi-cathode torch with axial injection of the material to be projected, the bead is drawn to the confluence zone of the elementary plasma streams from the cathodes, upstream of the nozzle from which the plasma jet resulting from the fusion of these elementary plasma streams flows flows.

[0033] Remarkably, a cord according to the invention has sufficient flexibility to be wound and unwound while being rigid enough to allow axial injection, preferably using a conventional drive device located at the rear (upstream) of the torch.

[0034] The injection device preferably opens inside the torch.

[0035] As illustrated on the figure 1 Preferably, the injection device is arranged so as to introduce the cord into the plasma or combustion chamber 17, preferably into a part 2 of the plasma flow or flame which extends less than 10 cm, preferably less than 5 cm, and / or more than 2 cm, preferably more than 3 cm, from the orifice or orifices 7 through which the plasma flow or flame exits the generator.

[0036] The torch can be in particular a multi-cathode axial injection plasma torch or a high-speed flame torch of type HVOF or HVAF or a conventional flame-wire type torch, or of type HVOF-Wire or HVAF-Wire.

[0037] A further method is described for coating a substrate surface with a coating, in which a cord according to the invention is injected into a plasma stream or into a torch flame so as to project, onto said surface, inorganic particles of the cord at least partly fused in the plasma stream or the flame.

[0038] Finally, a thermal spraying process is described using a thermal spraying device according to the invention, a process in which the torch is fed with a cord according to the invention so as to create a coating on the surface of a substrate.

[0039] The substrate is preferably a substrate made of a metal, a ceramic, a cermet, a polymer, an organic material or a composite material, in particular with a ceramic matrix.

[0040] The substrate can have various shapes, for example planar or rotational geometries, including cylindrical ones, or complex geometries; the only limitation is its accessibility to the jet of inorganic particles, at least partially molten. Axial feeding upstream of the spray nozzle or flow nozzle significantly improves this accessibility.

[0041] In one embodiment, the coating provides a surface functionality to the substrate, preferably improving abrasion resistance, modifying the coefficient of friction, or creating a thermal barrier or electrical insulation.

[0042] The coating can be used in particular for thermal, chemical or mechanical protection of parts, especially in a reactor, for example for the creation of columnar thermal barriers on parts of aeronautical turbines or stationary turbines, environmental barriers on ceramic-ceramic composite parts, or for the creation of functional layers in solid electrolyte fuel cell devices, these applications being cited only as examples.

[0043] The thermal spray cord and device according to the present invention are particularly advantageous for producing columnar and nanostructured coatings, thanks to the projection of submicron particles contained within said cord. They also avoid the drawbacks of "SPS" or "SPPS" processes. The "columnar structure" is described in particular in Benjamin Bernard's doctoral thesis on Thermal Barriers by Plasma Spraying of Suspensions, available on the website: http: / / docnum.univ-lorraine.fr / public / DDOC T 2016 0212 BERNARD.pdf.

[0044] Another advantage of the present invention is the possibility of producing, by thermal spraying, hybrid coatings, that is to say coatings comprising different materials, from a cord comprising particles of different physico-chemical natures.

[0045] The manufacture of a hybrid coating can be achieved using known thermal spraying techniques.

[0046] A hybrid coating can be obtained from a cord comprising particles of a ceramic oxide material and metallic particles.

[0047] The invention allows, in particular, the combination of different materials that would be difficult to combine using other methods, due to their different densities or sizes. This is especially problematic for the thermal spraying of suspensions or dry powders to be co-injected by thermal spraying. Brief description of the figures

[0048] Other features and advantages of the invention will become apparent upon reading the detailed description that follows and examining the attached drawing in which: [ Fig 1 ] there figure 1 schematically illustrates a thermal projection device according to the invention; Fig 2 ] there figure 2 schematically illustrates the cross-section of a cord according to the invention; [ Fig 3 ] there figure 3 schematically illustrates a device used for the examples to evaluate flexibility.

[0049] In the different figures, identical references are used to designate identical or similar organs. Definitions

[0050] The "equivalent outside diameter" of a cord is the diameter of a disk with the same surface area as its cross-section at mid-length of the cord.

[0051] The 10th percentile (D10), 50th percentile (D50), and 90th percentile (D90) of a set of particles are defined as the particle sizes corresponding to the percentages equal to 10%, 50%, and 90%, respectively, by number, on the cumulative particle size distribution curve of the set of particles, with these particle sizes being ranked in ascending order. According to this definition, 10% by number of particles in the set of particles have a size less than D10, and 90% by number have a size greater than or equal to D10. The particle size distribution curve can be generated using a laser particle size analyzer. The SYSMEX FPIA 3000 device is particularly well-suited for obtaining such curves.

[0052] The "median size" of a set of particles is called the 50th percentile, D 50. The median size therefore divides the particles of the set of particles into first and second populations equal in number, these first and second populations consisting only of particles with a size greater than or equal to, or less than respectively, the median size.

[0053] The percentiles for the sizes of inorganic particles in a bead are those measured on the inorganic particle powder used to manufacture the bead. They can be estimated from the bead by debinding it through calcination to remove the organic constituents and recover the inorganic particles. If the inorganic particles are oxidizable and susceptible to degradation by the debinding temperature, debinding is preferably carried out under a neutral atmosphere, for example, under argon. The size distribution of the inorganic particles extracted by debinding can then be measured by volume, for example, by laser granulometry. The volumetric particle distribution can be easily calculated relative to the volume of the bead, core, or cladding, the dimensions of which can be measured, for example, using a micrometer or calipers before and after removing the cladding from the bead.

[0054] The measurement of a percentage based on the "dry mass" of the cord can be carried out on a 100g sample of the cord, after drying at 110°C for one hour.

[0055] When a volume percentage is calculated on the basis of the cord, core or sheath, the volume of the cord, core or sheath is that delimited by the outer surface of the cord, core or sheath.

[0056] The lubricant volume content can be estimated from the amount of lubricant introduced into the initial batch during manufacturing. The lubricant can be liquid or solid (Graphite, BN, etc.).

[0057] The concept of "color pigment" is well known to those skilled in the art. A pigment is a powder that, during the cord manufacturing process, imparts color. A coloring pigment typically takes the form of a powder with a median particle size of less than 1 micrometer. A coloring pigment can, in particular, be an "oxide pigment," that is, one composed of oxides.

[0058] By "inorganic" particles, we mean particles made of a non-organic material, that is, one that does not contain carboxyl chains as a major component. This family of materials includes metals, glasses, ceramics, and composites made of metal, glass, or ceramics. Preferably, inorganic particles do not contain carboxyl chains.

[0059] A material that is neither metallic nor organic, for example, one chosen from among oxides, nitrides, carbides, and borides, is called a "ceramic." Ceramic materials include, in particular, glasses, cermets, and glass-ceramics. For the purposes of this invention, diamond, graphite, graphene, and metal or metalloid carbides are considered ceramic materials.

[0060] The term “cermet” refers to a material comprising at least two phases, at least one phase being ceramic and at least one other phase being metallic.

[0061] A material is considered "brittle" if its plastic deformation range under load before failure represents less than 5%, preferably less than 1%, of its elastic deformation range, and is preferably practically zero. In other words, the width of the stress range leading to plastic deformation without failure represents less than 5%, preferably less than 1%, of the width of the stress range leading to elastic deformation.

[0062] The term "impurities" refers to the components of the cord whose presence is undesirable, i.e., components other than inorganic particles, the polymer binder, the sheath polymer, and the optional lubricant(s). Solvent residues are not considered impurities. Impurities can include impurities present in the raw material sources, as well as residues of additives used during cord manufacturing, such as plasticizer residues.

[0063] The "ash content" of the bead corresponds to the residue left by the combustion of the sheath and core matrix of the bead. It can be determined by calcination according to standard NF T30-012, by measuring the mass difference between the mass resulting from calcination at a temperature of 450°C and the mass resulting from calcination at a temperature of 950°C. The temperature of 450°C allows for the decomposition of all organic constituents, while the temperature of 950°C allows for the vaporization of residues that could interfere with the melting of inorganic particles. The calcination must therefore be sufficient to extract substantially all the organic constituents from the bead. It is preferably carried out for a duration sufficient to ensure that this extraction is substantially complete. The calcination time is thus adapted to the dimensions of the bead sample being analyzed.

[0064] The "mineral content by volume" is measured by dividing the volume of mineral matter by the volume of the core of the bead. According to techniques well known to those skilled in the art, the volume of the core of the bead can be measured geometrically. The volume of mineral matter is determined according to Archimedes' method, by weighing the mineral matter extracted from the core of the bead after unbinding.

[0065] A percentage is determined "solvent-free" when the calculation basis for that percentage does not take into account any solvent that may be present. In particular, the solvent is preferably water, and volume percentages are then measured, notably for inorganic particles, polymer binder, matrix lubricant, sheath polymer, or sheath lubricant, by dividing the volume of the component in question by the volume of a dry base, that is, without considering any water present. The base may be, in particular the core, for inorganic particles, the matrix, for the polymer binder and matrix lubricant, the sheath, for the sheath polymer and sheath binder.

[0066] To avoid taking water into account, the measurement of these percentages can be carried out after complete drying.

[0067] In this description, the terms "upstream" and "downstream" are used with reference to the direction of flow, along a "flow axis" of the plasma-generating gas flow or flame gases.

[0068] By "based on", it is classically understood that the corresponding quantity is greater than 50% by mass.

[0069] A "transverse plane" is a plane perpendicular to the X-axis.

[0070] A "radial plane" is a plane containing the X-axis.

[0071] For clarity, a distinction is made between the "polymer binder" in the core of the cord and the "sheath polymer" in the sheath. The polymer binder and the sheath polymer may be the same or different.

[0072] For clarity, a distinction is also made between the "matrix" binder and the "sheath" binder. These binders may be identical or different.

[0073] “Contain” or “understand” or “present” should be interpreted in a non-limiting manner. Detailed description Projection device

[0074] There figure 1 schematically illustrates a thermal projection device 10 according to the invention, comprising a torch 12 and a cord 15 according to the invention supplying said torch with inorganic particles.

[0075] The torch can be in particular a multi-cathode type torch allowing axial injection, a flame torch, preferably high velocity or high speed of the air-oxygen (or HVOF) or air-gas (or HVAF) type, or a classic flame-wire type torch, or of the HVOF-Wire or HVAF-Wire type.

[0076] The torch 12 classically comprises one or more plasma or combustion gas generators 13 in the case of a flame torch, and an injection device 14 to inject, through an injection orifice 4 and along an injection axis I, the bead 15 into the plasma flow 16 or the flame produced in the chamber 17, upstream of the spray nozzle or flow nozzle 21 of the torch.

[0077] The axis of the plasma or flame flow is called the "X-axis".

[0078] In a radial plane containing the X-axis and passing through the center of the injection orifice, the projection of the injection axis I forms an angle θ with the X-axis. The angle θ is preferably greater than 60°, greater than 70°, greater than 80°, and preferably greater than 85°. Preferably, the X-axis is contained within said radial plane, and preferably coincides perfectly with the X-axis. Cord

[0079] There figure 2 schematically illustrates the cross-section of a cord 15 according to the invention. In particular, the core 18 and the sheath 20, surrounding the core, are distinguished.

[0080] The cord preferably has a constant cross-section along its entire length. It preferably has a circular cross-section, and preferably has an equivalent outside diameter greater than 1.5 mm, preferably greater than 2 mm, and / or less than 3.3 mm, preferably less than 3.2 mm, with an equivalent outside diameter of 3 mm being preferred.

[0081] The cord is preferably wound on a mandrel, preferably in the form of an easily handled and unwindable reel for supplying the projection device.

[0082] Preferably, the cord does not contain metallic salts or hydroxides; for example, it does not contain aluminum hydroxide (boehmite), which forms a gel during paste preparation, nor ammonium acetate. The inventors have observed that these constituents, mentioned in the prior art, can have a negative impact. In particular, the formation of an inorganic gel, for example, due to the use of aluminum hydroxides, leads to the agglomeration of inorganic particles during spraying, preventing the release of all individual fine particles and thus hindering the production of finely structured layers. These constituents also result in reduced flexibility for cords whose inorganic particles have a median size of less than 10 micrometers. Heart

[0083] The core 18, in the form of a wire, preferably of constant cross-section along the entire length of the cord, preferably has a circular cross-section. Its equivalent outside diameter is preferably greater than 2 mm, and / or preferably less than 3.2 mm.

[0084] The core contains a set of 22 inorganic particles intended to be melted into droplets in the plasma stream or flame, then projected onto a substrate to form a coating on the substrate.

[0085] Inorganic particles preferably constitute more than 40%, more than 50%, more than 55%, and / or less than 80%, less than 75%, less than 70%, or less than 65% of the cord core volume. An inorganic particle volume content below 40% increases the energy required during spraying to break down the organic components of the binder matrix and cladding. An inorganic particle volume content above 80% is detrimental to cord flexibility.

[0086] The median size D50 of all inorganic particles is less than 10 micrometers. This very small median size is intended to allow for a coating with a very fine structure.

[0087] The median size D 50 of all inorganic particles is preferably less than 5 micrometers and preferably greater than 0.1 micrometer.

[0088] The median size of all inorganic particles is preferably between 1 and 5 micrometers to obtain a dense coating intended for mechanical and / or chemical protection.

[0089] The median size of all inorganic particles is preferably between 0.2 and 0.5 micrometers to obtain a coating formed of columnar or "feathery" layers with a more thermally insulating microstructure, in particular to obtain a thermal barrier.

[0090] A median size greater than 0.1 micrometer advantageously reduces safety problems during cord manufacturing.

[0091] Preferably, the ratio R of the equivalent outside diameter d of the bead, in micrometers, to the median size D 50 of the inorganic particles, in micrometers, is between 200 and 20,000, preferably greater than 500, preferably greater than 1,000 and / or less than 10,000, preferably less than 5,000, preferably less than 2,000. A ratio R between 200 and 1,600 is particularly advantageous, especially for beads intended for high-velocity plasma or flame torches with axial material injection.

[0092] Preferably, (D 90 -D 10 ) / D 50 is greater than 1 and / or less than 1.8.

[0093] In one embodiment, for a median size of inorganic particles between 0.2 and 0.5 micrometers, the 10th percentile (D10) of the inorganic particle set is preferably greater than 50 nm, preferably greater than 100 nm, preferably greater than 150 nm, and the 90th percentile (D90) of the inorganic particle set is preferably less than 1000 nm, preferably less than 900 nm, preferably less than 850 nm.

[0094] In one embodiment, for a median size of inorganic particles between 1 and 5 micrometers, the 10th percentile (D10) of the set of inorganic particles is preferably greater than 0.1 micrometer, preferably greater than 0.5 micrometer, and the 90th percentile (D90) of the set of inorganic particles is preferably less than 10 micrometers, preferably less than 8 micrometers.

[0095] The nature of the inorganic particles is determined according to the nature of the desired coating.

[0096] Preferably, the inorganic particles are made of a material consisting of more than 80%, preferably more than 90%, preferably more than 95%, or even substantially 100% by mass, of one or more of the following oxides, alone or in solid solution: Al 2 O 3 , SiO 2 , ZrO 2 , Cr 2 O 3 , and TiO 2 .

[0097] The inorganic particles can be in a non-oxide material, specifically chosen from: metallic oxides, preferably alumina, zirconia, titanium oxide, chromium oxide, yttrium oxide, or a combination of several of these oxides, for example mullite or spinel, and / or carbide-based cermets, the carbides being, for example, chromium, tungsten, titanium, tantalum, zirconium carbides, said carbides being associated with a metallic phase, and / or SiC-YAG composites, "YAG" meaning "Yttrium-Aluminum Garnet", allowing the thermal spraying of a SiC-based compound, and / or ceramics such as nitrides, borides and carbonitrides, possibly associated with a metallic phase in the form of cermets, and / or refractory metals or refractory metal alloys preferably having a melting point above 2500 K, and / or special metal alloys such as amorphous metallic, quasicrystals or approximants,and more broadly, non-wire-drawing metallic alloys.

[0098] Special metallic alloys are classically intermetallic alloys, and in particular alloys with brittle mechanical fracture such as metallic amorphous or metallic glasses, quasicrystals or approximants (i.e. approximating phases of quasicrystals), as described for example in https: / / www.universalis.fr / encyclopedie / quasi-cristaux / 4-phases-approximantes-et-defauts / .

[0099] Preferably, inorganic particles are chosen from ceramic particles, intermetallic alloy particles, metallic amorphous particles, and quasicrystal or approximating phase particles.

[0100] Inorganic carbide particles are particularly well suited to HVOF technology.

[0101] The inorganic particles are embedded, preferably dispersed in a substantially uniform manner, in a matrix 24 binding said inorganic particles.

[0102] The matrix is ​​essentially made of an organic material, so that it is reduced to ash during projection.

[0103] The plasticizer content by volume may be greater than 1%, preferably greater than 4%, and / or less than 10%, preferably less than 8%, expressed as a percentage by volume based on the volume of the core or matrix, excluding solvent. The plasticizer may be any known plasticizer, for example, a phthalate, in particular BBP (Butylbenzyl phthalate), or polyvinyl alcohol (or "PVA" for short).

[0104] In a preferred embodiment, the remaining 100% of the polymer binder in the matrix preferably consists, excluding impurities and solvent residues, of a lubricant, preferably glycerin. The lubricant may, in particular, constitute 10% to 20% of the core volume. Sheath

[0105] The 20mm sheath contributes to the bead's flexibility by enhancing its ability to withstand bends without degradation. Specifically, it allows the bead to be wound without visible damage, particularly without cracking or separation of its components. The sheath also contributes to the bead's toughness, necessary due to its small equivalent outside diameter, and provides a surface finish that promotes smooth sliding. This facilitates bead advancement through the torch's injection port and reduces wear on the torch components with which the bead is in contact, especially the injection port.

[0106] The sheath surrounds the core along the entire length of the cord. It preferably has a constant thickness in a plane perpendicular to the direction of the cord's length, or preferably in any plane perpendicular to the direction of the cord's length.

[0107] Preferably, the ratio of the sheath thickness, in micrometers, to the equivalent outside diameter of the cord, in micrometers, is greater than 0.03 and less than 0.6, preferably greater than 0.05, or even greater than 0.1 and / or less than 0.5, or even less than 0.3, or even less than 0.2. A ratio between 0.05 and 0.5 is particularly suitable when the set of inorganic particles has a median size of less than 5 micrometers.

[0108] Preferably, the main constituent of the sheath is a polymer of the same family, or even of the same chemical composition, or even of the same molecular formula, as the polymer binder of the core matrix of the cord. The monomer crosslinked to form the sheath polymer is preferably the same as that of the polymer binder.

[0109] The sheath polymer preferably represents between 55% and 75%, preferably about 65% of the sheath volume, excluding solvent.

[0110] Preferably, the cladding contains a lubricant, known as "cladding lubricant," preferably glycerin, in a concentration preferably greater than 25% by volume on the cladding base, excluding solvent. The cladding lubricant facilitates co-extrusion during manufacturing, reduces wear on the torch components over which the bead slides, and, by facilitating sliding, limits the risk of bead buckling during injection into the plasma stream or flame, thus contributing to the quality of the manufactured coating.

[0111] The complement to 100% of the sheath polymer and sheath lubricant is preferably made up of organic impurities, in particular resulting from organic additives such as a plasticizer used to shape the cord sheath during its manufacture.

[0112] Generally, the matrix and cladding compositions are determined to achieve a low ash content in the bead. The ash content, resulting from the presence of the polymer binder, matrix lubricant, cladding polymer, cladding lubricant, and plasticizer used, is less than 2.5%, preferably less than 2%, or even less than 1%, as a mass percentage based on the bead mass. Those skilled in the art know how to adjust the composition to reduce the ash content, possibly by performing a few simple tests.

[0113] An ash content exceeding 3% indicates a high level of organic components and significant torch contamination. This results in substantial process disruptions and a source of defects in the thermal spray coating.

[0114] In particular, preferably, the sheath polymer and / or the polymer binder of the matrix, preferably the organic filler consisting of the sheath polymer and the polymer binder of the matrix, is / are composed of more than 80%, more than 90%, more than 95%, preferably substantially 100% by mass, of a cellulosic derivative, as a percentage by volume based on the sheath or the polymer binder, respectively, excluding solvent. A cellulosic derivative advantageously allows for a very low ash content, typically less than 1%, as a mass percentage based on the mass of the cord.

[0115] Preferably, the cellulosic derivative is chosen from among the cellulose ethers, most preferably from methylcellulose (MC), ethylcellulose (EC), methylethylcellulose (MEC), hydroxymethylcellulose (HMC), hydroxyethylcellulose (HEC), methylhydroxyethylcellulose (MHEC), hydroxymethylethylcellulose (HMEC), hydroxypropylcellulose (HPC), hydroxypropylmethylcellulose (HPMC), hydroxypropylethylcellulose (HEPC), carboxymethylcellulose (CMC), and mixtures thereof. Preferably, the cellulosic derivative is chosen from among the hydroxyethylcelluloses, in particular methylhydroxyethylcellulose, as this subfamily of cellulose exhibits rheological properties well-suited to the manufacture of a cord with a small diameter and an overall inorganic particle size of less than 10 micrometers.

[0116] The alkali content, particularly sodium, in the cellulosic derivative is preferably less than 1% by mass percentage based on the mass of said cellulosic derivative. This characteristic advantageously limits the ash content and long-term corrosion of the torches. Preferably, the cellulosic derivative contains little or no cellulose fibers.

[0117] A cord according to the invention can be manufactured by any conventional process, in particular by co-spinning a first paste intended to form the core of the cord and a second paste intended to form the sheath. A process of this type is described in particular in FR 1 443 142.

[0118] A process such as that described in GB 1 151 091A is particularly suitable.

[0119] The flexibility of the cord can be easily adjusted by adjusting the amount of polymer binder in the core as well as its viscosity and / or the viscosity of the sheath polymer. Applications

[0120] A cord according to the invention is particularly well suited for plasma torches, and in particular for axial injection plasma torches, in particular for multi-cathode plasma torches, also referred to as "multi-chamber".

[0121] The flexibility of the cord according to the invention allows it to be wound around a mandrel for unwinding during the coating application. This enables a virtually continuous spray pattern. The sheath also ensures smooth contact with the injection orifice, thus minimizing wear.

[0122] The rigidity of the bead is sufficient to allow for a nearly axial injection, preferably using a remote drive system upstream of the torch. This axial injection promotes homogeneous heating of the inorganic particles, homogenizes the distribution of the inorganic particle jet in the flow nozzle, thus limiting fouling, and improves the reproducibility and quality of the coating.

[0123] The limited quantity of liquid phase, and in particular solvent, limits the amount of energy consumed and maximizes the energy available to pyrolyze the cladding and matrix and melt the inorganic particles.

[0124] The size of the inorganic particles is chosen according to the projection device and the desired microstructure for the coating.

[0125] From the moment the cord enters chamber 17 of the torch, the following sequence of events unfolds: pyrolysis of the cladding and polymer binder; release of individual inorganic particles without agglomeration effect between them; rapid entrainment in the jet (flame or plasma) and total or partial melting in flight of the inorganic particles; impact of the at least partially melted inorganic particles on the substrate, then solidification to create the coating.

[0126] In particular, the cord can be used to make: a mechanical or chemical protective coating, in particular against corrosion by chemical species, vapors, etching plasma; an environmental barrier or a thermal barrier; a tribological coating, an anti-wear coating, an electrically insulating coating, or an electrically conductive coating.

[0127] Preferably, the coating has a thickness between 10 and 500 micrometers. Examples

[0128] The following non-limiting examples are given for the purpose of illustrating the invention.

[0129] Example 1 was carried out according to the teaching of JP2016156058, by spinning the first dough.

[0130] For the other examples, a first paste was made, in accordance with the teaching of GB 1 515 091, from of a powder of alumina particles obtained by melt-solidification, with a median size of 7.5 micrometers and a purity greater than 99.5% and of methylhydroxyethylcellulose (polymer binder) having a viscosity of 50 mPa.s.

[0131] The viscosity of methylhydroxyethylcellulose was measured by a Höppler viscometer at 20°C on the basis of a powder of methylhydroxyethylcellulose mixed with demineralized water at a mass load of 2%.

[0132] The mass proportions of the constituents of this first paste are shown in Table 1 below.

[0133] A second paste is then prepared to form the sheath. For this purpose, the same methylhydroxyethylcellulose as that used for the preparation of the first paste is kneaded with a quantity of water, glycerin and colouring pigment according to the mass proportions indicated in Table 1 below.

[0134] A co-spinning of the first and second pastes in a press makes it possible to obtain a precursor of flexible cord of 3 mm, of substantially circular cross-section, of outer diameter of 3 mm and having a sheath having a thickness of substantially 350 micrometers.

[0135] Drying the cord precursors results in cords with a residual water content of less than 5%. Residual moisture measurements indicate a value of approximately 3%. The cords are wound onto a 70 mm diameter mandrel. Diameters and thicknesses

[0136] The outer diameter of the cord and the thickness of the sheath were measured using a Tesa Micromaster ®< 0-30 mm digital micrometer. Flexibility test

[0137] For each example: A sample of at least 0.5 meters of the cord to be tested is wound around a cylindrical rod with a diameter of 25 mm, so as to form close-set turns, as shown in the figure 3 The cord must not break during this operation; a sample of at least 0.5 meters of the cord to be tested is wound around a cylindrical rod with a diameter of 70 mm, so as to form close-wound turns, as shown in the diagram. figure 3The cord sheath must not show any externally visible cracks, or any internal cracks visible externally, by a color change resulting from the semi-transparency of the sheath. A sample of at least 0.5 meters of cord is subjected to a drive test consisting of rolling a 26 mm diameter, 2 kg roller over the sample at a speed of 1 m / s. The cord may deform slightly and exhibit a diameter variation of less than 15%, but it must not show any externally visible cracks.

[0138] If the cord passes these three tests, it exhibits acceptable flexibility. If it fails at least one of these three tests, it exhibits unacceptable flexibility. Ash content

[0139] The ash content is determined by (m 0 - m 1 ) / m 0 , m 0 and m 1 being respectively the masses after calcination of a 3 cm long sample of the cord, in a furnace at 450°C and 950°C under air, respectively, for a period of 1 h. Report R

[0140] The R ratio is the ratio of the equivalent outside diameter of the cord, in micrometers, to the median size (D 50 ) of the inorganic particles, in micrometers. Enthalpy variation

[0141] The enthalpy change associated with the decomposition of the organic constituents of the bead at 2300 K was calculated using Factsage® software for each example based on its formulation, considering: an injection of the bead upstream of the flame or plasma stream, between the orifice through which the plasma stream or flame exits the generator and the spray nozzle (or flow nozzle); in the case of a high velocity plasma torch, a neutral atmosphere, i.e. without oxygen, hydrogen, and bead decomposition products of type C x H y; in the case of a high velocity flame torch (type HVOF), an oxidizing atmosphere and direct decomposition in the form of CO 2 and H 2 O; a residual water measurement in the bead of 3% after drying.

[0142] The contribution of organic compounds due to the presence of a possible sheath was taken into account for the determination of the quantity and nature of the organic products of the cord.

[0143] In Table 1 below, the enthalpy change is given in kJ / mol of alumina.

[0144] The enthalpy variation in a neutral atmosphere is considered particularly advantageous when it is as low as possible, especially less than or equal, in absolute value, to 1000 kJ / mol of alumina.

[0145] The enthalpy change in an oxidizing atmosphere is considered particularly advantageous when it is less than +1000 kJ / mol of alumina.

[0146] The quantity of inorganic particles is provided as a volume percentage based on the core volume of the cord, without taking the solvent into account.

[0147] Example 1 (comparative), carried out according to the teachings of JP2016156058, does not include a sheath. Example 3 (comparative) is carried out according to the teachings of GB 1 151 091, but with finer inorganic particles. Examples 4* and 5* are according to the invention. [Table 1] Examples Invention Preferences 1 2 3 4* 5* Features of the cord Equivalent outside diameter (micrometers) 1000 - 3500 3000 3000 3000 3000 3000 Ash content <5% <5% <5% <5% <5% <5% Formulation of the first paste for core formation (mass %) Quantity of inorganic particles (alumina) 75,7 37 80 65,8 MHEC Polymer 1.5 MHEC 35 MHEC 5 MHEC 9.9 MHEC Plasticizer (PVA) 1,5 0 0 0 Aluminum hydroxide 3,8 0 0 0 Lubricant (glycerin) 0,8 5 5 5,3 % water 16,7 23 10 19 Formulation of the second paste for sheath formation (mass %) ) MHEC Polymer N / A 24.7 MHEC Lubricant (glycerin) N / A 11,5 Dye N / A 0,3 % water N / A 63,5 Features of the heart (vol % based on the heart, excluding solvent) Inorganic particles Alumina Alumina Alumina Alumina Alumina Alumina Quantity of inorganic particles > 40% 6,3% 81,1% 25,5% 73,4% 61,0% D 50 of inorganic particles (micrometers) < 10 micrometers 1.2 micrometers 7.5 micrometers 7.5 micrometers 7.5 micrometers 7.5 micrometers Matrix polymer binder 93.7% Cellulose fibers 4.2% of MHEC 63.6% of MHEC 12.1% of MHEC 23.8% of MHEC Plasticizer (PVA) 0% 5,3% 0% 0% 0% Aluminum hydroxide 0% 6,7% 0% 0% 0% Lubricant (glycerin) 2,7% 10,9% 14,5% 15,2% Features of the sheath (flight % based on the sheath, (solvent-free) Sheath polymer Cellulose derivatives - 65% of MHEC 65% of MHEC 65% of MHEC 65% of MHEC Lubricant (glycerin) - 35% 35% 35% 35% Sheath thickness (micrometers) 100 à 500 0 350 350 350 350 Report R Preferably 200 to 20000 2500 > 60 400 400 400 Results Flexibility test Acceptable Acceptable Unacceptable Acceptable Acceptable Acceptable Enthalpy change in kJ / mol of inorganic particle material Neutral atmosphere ≥ -1000 < -1000 -475 -1530 -825 -710 Oxidizing atmosphere < 1000 > 2000 480 2980 1250 920 MHEC: methylhydroxyethyl cellulose; ND: not determined; NA: not applicable

[0148] As is now clear, the invention provides a bead which, thanks to its flexibility, can advantageously be introduced continuously, substantially along the axis of the torch, into the heart of the flame or plasma. The inorganic particles are well dispersed upon injection. The thermal decomposition properties of the bead, in particular its very low enthalpy variation at 2300 K, make it possible to obtain a coating which advantageously exhibits controlled and defect-free roughness, with limited energy consumption.

[0149] Of course, the invention is not limited to the examples and embodiments described, which are provided by way of illustrative and non-limiting examples.

Claims

1. Cord intended to serve as feedstock for a thermal spray torch in order to create a coating, said cord having an equivalent outside diameter (d) of between 1 mm and 3.5 mm and consisting of a core (18) in the form of a wire, and a sheath (20) covering the core along its entire length, the core consisting - of a collection of inorganic particles (22) of which the median size (D50) is less than 10 micrometres, the inorganic particles representing more than 40% and less than 80% of the volume of the core, said inorganic particles being particles of one or more metal oxides and / or particles of carbide-based cermet, and / or inorganic particles of SiC-YAG and / or particles containing or made of a ceramic material and / or particles made of one or more metals or metal alloys having a melting point higher than 2500 K and / or particles made of a special metal alloy; and - of a matrix (24) binding said inorganic particles, the matrix comprising: - a polymer binder consisting, in respect of over 80% of its mass, of a cellulose derivative, and - optionally a matrix lubricant, the polymer binder and the matrix lubricant together representing more than 90% of the volume of the matrix, the sheath (20) having a thickness of between 50 micrometres and 500 micrometres and comprising: - a polymer referred to as "sheath polymer" consisting, in respect of over 80% of its mass, of a cellulose derivative, and - preferably a lubricant referred to as "sheath lubricant", the sheath polymer and the sheath lubricant together representing more than 90% of the volume of the sheath, the volume percentages being determined without accounting for the possible presence of solvent residue, the median size of a collection of particles being the 50th percentile of said collection of particles corresponding to a percentage of 50% by number, on the cumulative particle-size distribution curve, of the sizes of particles of said collection of particles, determined using a laser granulometer, said particle sizes being ranked in increasing order, the ash content of said cord being less than 2.5%, the ash content being determined by (m0 - m1) / m0, m0 and m1 being respectively the masses, after calcination, of a 3 cm sample length of said cord in a furnace at 450°C and 950°C, in air, respectively, for a duration of 1 hour.

2. Cord according to the preceding claim, wherein said ash content is less than 1%.

3. Cord according to either one of the preceding claims, wherein the ratio (R) of the equivalent outside diameter (d) of the cord, in micrometres, to the median size (D50) of the collection of inorganic particles, in micrometres, is comprised between 200 and 20 000.

4. Cord according to the immediately preceding claim, wherein said ratio (R) is comprised between 200 and 2000.

5. Cord according to any one of the preceding claims, wherein the median size D50 of the collection of inorganic particles is less than 5 micrometres.

6. Cord according to any one of the preceding claims, wherein the viscosity of the polymer binder of the core and / or of the sheath polymer is comprised between 30 and 300 mPa.s at 20°C.

7. Cord according to any one of the preceding claims, wherein the sheath contains a sheath lubricant, the content of said sheath lubricant being greater than 10% and less than 50%, as a percentage by volume on the basis of the volume of the sheath, not accounting for the possible presence of a solvent.

8. Cord according to any one of the preceding claims, wherein the inorganic particles are selected from - particles of alumina, of zirconia, of titanium oxide, of chromium oxide, of yttrium oxide, or of a combination of several of these oxides, and / or - particles of cermet containing more than 50% by mass of a carbide selected from the carbides of chromium and / or of tungsten and / or of titanium and / or of tantalum and / or of zirconium, and / or of niobium, and / or - particles of a ceramic material in the form of a nitride, of a boride, or of a carbo-nitride, said ceramic material being optionally associated with a metallic phase in the form of cermet, and / or - particles of a brittle material, and / or - particles of an amorphous metal alloy, or of a quasi-crystal or approximant, or of a metal alloy that cannot be wire-drawn.

9. Cord according to any one of the preceding claims, wherein the thickness of the sheath is greater than 200 micrometres and less than 400 micrometres.

10. Cord according to any one of the preceding claims, wherein the polymer binder and the sheath polymer contain an identical polymer.

11. Assembly comprising a spool of a diameter less than 500 mm and a cord according to any one of the preceding claims, wound on said spool.

12. Thermal spray device comprising - a torch (12) comprising a plasma-stream or flame generator (13) and an injection device (14); and - a cord (15) according to any one of Claims 1 to 10 arranged in such a way as to be able to be injected, by the injection device, into said plasma stream or said flame generated by said generator, the torch being able to at least partially melt the inorganic particles and to spray the at least partially molten inorganic particles at over 150 m / s.

13. Thermal spray device according to the immediately preceding claim, wherein the injection device is arranged in such a way as to inject the cord along an injection axis (I) extending in a plane passing through the axis (X) of the plasma stream or of the flame and making with a plane perpendicular to said axis (X) an angle θ which, in terms of absolute value, is greater than 60° and preferably greater than 80°.

14. Thermal spray device according to either one of the two immediately preceding claims, wherein the injection device is arranged in such a way as to inject the cord upstream of a spray nozzle of the torch or of a jet nozzle of the torch.