Sliding varnish comprising graphene and its use for the manufacture of turbomachines

Abstract

The present invention relates to a sliding varnish formulation comprising a water-based carrier suspension, an inorganic binder and graphene. This varnish formulation is used to protect the parts of a turbomachine from contact wear in assemblies, in particular in the blading of the hot section of an engine.

Description

Graphene-based sliding varnish and its use in the manufacture of turbomachinery Technical Field

[0001] The invention relates to the field of manufacturing and maintenance of aircraft turbomachinery. More particularly, it relates to a sliding varnish composition that can be applied to a part subjected to friction induced by the relative movement between two parts of an assembly during the operation of the turbomachine, such as the root of a compressor or turbine blade. Previous technique

[0002] A turbomachine includes a fan that draws in an airflow, which is typically circulated in two independent paths to form a primary and a secondary airflow. The air in the primary flow is compressed in a low-pressure compressor and then a high-pressure compressor, before being mixed with fuel and burned in a combustion chamber. The exhaust gases from the combustion chamber pass through a high-pressure turbine and then a low-pressure turbine before being accelerated through a nozzle.

[0003] The rotating elements of compressors and turbines, called rotors, comprise an annular series of blades mounted on a disc, or one-piece bladed discs, which drive a rotating shaft under the effect of the passing airflow. Each blade consists of a blade and a root that is housed in a disc cavity to hold the blade in place during the operation of the turbomachine. Despite this assembly, the blade root undergoes very small displacements relative to the disc, causing contact wear (fretting) that can damage the blade bearing surfaces and / or the disc cavity bearing surfaces.

[0004] The friction that occurs at the point of contact between the blade and the disc can lead to deterioration of the blade's root structure and compromise the integrity of the blade body. Wear-resistant protections designed to limit friction have been proposed in the prior art. These may consist of metallic coatings covered with a film of sliding varnish.

[0005] Slip coatings are generally composed of lubricating particles distributed within a thermoplastic or thermosetting organic matrix, such as epoxy polymers. The lubricating particles themselves are typically made of molybdenum disulfide, graphite, polytetrafluoroethylene (PTFE), or boron nitride. This matrix can also be composed of a matrix inorganic for high temperature applications, typically above 400°C.

[0006] The need remains to improve the performance of sliding varnishes to prevent contact wear, which occurs between parts in an assembly subjected to very low-amplitude, high-frequency oscillatory movements and high temperatures. There is also a need to develop a bio-based varnish composition whose raw materials are produced using simplified processes, and whose preparation and application processes are streamlined. Finally, it would be desirable to develop a formulation whose application results in a protective film resistant to the temperatures reached in the hot part of the reactor. Brief description of the drawings

[0007] Figure 1 schematically represents a partial perspective view of a movable turbine blade according to the invention comprising a blade connected to a foot, one surface of which is covered with a sliding varnish film.

[0008] Figure 2 is a perspective diagram of a portion of a turbine disk comprising two cells.

[0009] Figure 3 is a partial cross-sectional view of a turbine along a plane perpendicular to its axis of rotation in which the movable blades according to the invention have been inserted into the recesses of the disc.

[0010] Figure 4 is a scanning electron microscopy image of graphene particles used in the composition of sliding varnish according to the invention.

[0011] Figure 5 is an image obtained by transmission electron microscopy of graphene particles which are part of the composition of the sliding varnish according to the invention. Description of the invention

[0012] The invention addresses at least one of the needs mentioned above and proposes a new formulation of aqueous-based sliding varnish containing graphene and an inorganic binder capable of meeting a functional need at temperatures ranging from 400°C to 750°C.

[0013] The sliding varnish of the invention has the advantage of reducing the coefficients of friction between two opposing parts undergoing repeated relative displacements, and also has the advantage of delaying the onset of structural damage caused by contact wear, by reducing contact wear phenomena through a lowering of the coefficient of friction, particularly when the parts are exposed to high temperatures. The sliding varnish is characterized by better thermal stability, improved tribological properties, and a simplified preparation process.

[0014] The sliding varnish of the invention is also advantageous for reducing the environmental footprint for at least the following reasons. First, it increases and optimizes manufacturing, production, and / or repair capacity and, consequently, significantly reduces associated greenhouse gas emissions. This optimization also decreases raw material consumption. Second, it extends the lifespan of components and, therefore, reduces the number of refurbishments. Finally, it significantly reduces the frequency of repairs performed by removing the worn varnish and applying a new layer.

[0015] The solution proposed by the invention also has the advantage of reducing the energy input required for its implementation in certain embodiments, and / or adapting the choice of products used to current environmental standards and regulations. Description of the implementation methods

[0016] The invention therefore has as its first object a liquid composition of sliding varnish for a turbomachine part, comprising an aqueous base, an inorganic binder capable of leading to an inorganic matrix enabling it to meet a functional need ranging from 400°C to 750°C, and graphene used as solid lubricant particles.

[0017] In some embodiments, the proportion of graphene in the composition is between 1% and 50% or between 2% and 50%. According to the embodiment of the invention, the proportion of graphene ranges from 1% to 40%, from 1% to 30%, from 2% to 25%, or from 1% to 5%, the percentages being expressed by mass relative to the mass of the composition.

[0018] The inorganic binder content can be between 1% and 40%, preferably between 20% and 40% or between 25% and 30%, the percentages being expressed by mass relative to the mass of the composition.

[0019] The term "graphene" refers to a polycyclic aromatic carbon particulate material comprising at least one planar arrangement of sp2 carbon atoms linked by covalent bonds, called a "layer" or "sheet." The aromatic rings can consist of five, six, or seven carbon atoms.

[0020] Graphene particles can be selected from single-sheet graphene particles, multi-sheet graphene particles, and mixtures thereof. A population of single-sheet graphene particles can be referred to as "single-sheet graphene," while a collection of multi-sheet graphene particles can be referred to as "multi-sheet graphene."

[0021] A multi-sheet graphene particle consists of at least two stacked planes or sheets, for example, from 2 to 300 planes, which are linked together by Van der Waals bonds. A particular multi-sheet graphene comprises particles with a number of planes ranging from 2 to 30, for example, from 2 to 10. In a particular embodiment of the invention, the number of planes in the multi-sheet graphene ranges from 2 to 5.

[0022] The graphene used in the composition of the sliding varnish of the invention may comprise a mixture of single-sheet graphene and multi-sheet graphene, the multi-sheet graphene comprising particles having up to 10 planes.

[0023] According to one embodiment, the population of graphene particles entering into the composition of the sliding varnish formulation of the invention comprises several different fractions, each fraction being able to be characterized by a number of sheets and a thickness.

[0024] Graphene is preferably dispersed in an aqueous base. In this case, the composition comprises an "aqueous dispersion" of graphene. The terms "aqueous dispersion" and "aqueous suspension" will be used interchangeably in this description.

[0025] The thickness of the graphene particles is preferably less than 150 nm, and may be between 0.5 nm and 100 nm. In one particular embodiment, the thickness of the graphene particles is between 0.5 nm and 20 nm, preferably between 0.9 nm and 5 nm. The particle thickness can be measured by any method known to those skilled in the art, in particular by microscopy.

[0026] Graphene particles can be further characterized by a largest dimension ranging from 0.1 micrometers to 150 micrometers, for example, between 5 and 6 micrometers. The value of the largest dimension of graphene particles can be measured by any method known to those skilled in the art, particularly by microscopy. The largest dimension can characterize a single particle, the median of several particles, or the average of several particles.

[0027] Graphene can be obtained by various synthesis routes known to those skilled in the art, such as liquid-phase exfoliation or solid-phase exfoliation.

[0028] In a particular embodiment of the invention, powdered graphene obtained by solid-state exfoliation, pyrolysis, epitaxy, or chemical vapor deposition is used. The use of powdered graphene results in a more stable varnish and avoids unwanted chemical interactions that can occur between certain graphene surface products used in aqueous exfoliation and the organic binder used in the final varnish formulation. Powdered graphene also offers the advantage of allowing larger quantities of graphene to be incorporated into the final varnish formulation. Finally, the use of powdered graphene enables the use of a wider range of graphene dimensions in terms of thickness and lateral size.

[0029] According to a particular embodiment of the invention, graphene can be obtained by exfoliating graphite in an aqueous-based solvent, using an organic material of natural origin.

[0030] This source of graphene is particularly advantageous for at least one of the following reasons: the exfoliation yield is high, and the reagents can be bio-based. Furthermore, the graphene suspension from the exfoliation step can be diluted in water without risk of destabilization, which simplifies the preparation of the sliding varnish. This is because it eliminates the need to functionalize the suspension or the graphene particles before the dilution step to obtain varnish compositions with high water content, for example, around 75%, which is highly beneficial from both a health and environmental perspective.

[0031] The aqueous phase exfoliation process also has the advantage of limiting the defects that are generally observed on sheets obtained by chemical exfoliation using corrosive reagents.

[0032] The term "graphite" refers to a three-dimensional crystal with a hexagonal structure composed of parallel sheets of sp2-hybridized carbon atoms. Graphite can be natural, meaning essentially in its naturally occurring crystalline form of geological origin, or synthetic, such as expanded or unexpanded graphite. Examples of natural graphite include so-called amorphous (nanocrystal) graphite, flake graphite, and vein graphite. Examples of Synthetic graphite includes pyrolytic graphite, highly oriented pyrolytic graphite (HOPG) and synthetic graphite flakes.

[0033] The term "expanded graphite" refers to graphite that has undergone an expansion process to increase the interlayer space. Expanded graphite is synthesized from an intermediate called "expandable graphite," for example, by treating graphite flakes with an acid (nitric, sulfuric, or acetic acid) that intercalates between the layers without altering the interlayer distance. The introduction of acid(s) between the layers can be facilitated by an oxidizing treatment or an electrochemical treatment. The acid-treated graphite can then be neutralized, washed, and dried. When expandable graphite is subjected to high-temperature heat treatment, the intercalated molecules are removed, and the graphite layers are separated.

[0034] For the purposes of this invention, "aqueous base" means a liquid base comprising water and optionally a water-miscible co-solvent. The co-solvent advantageously improves the stability of the varnish composition comprising the graphene particles and / or facilitates the application of the varnish composition by spraying. The co-solvent is preferably a polar solvent, for example, acetone, a glycol ester, or acetic acid.

[0035] In one embodiment, the aqueous base consists primarily of water. Advantageously, it consists mainly of purified water, which avoids the potential presence of traces of organic solvent in the suspension that could not be removed, and limits the use of volatile organic compounds (VOCs) for environmental reasons. Water can represent between 65% and 75% by mass of the composition.

[0036] An example of a process for preparing a graphene dispersion by aqueous exfoliation involves inserting an organic compound between the graphite sheets in the presence of water and possibly a co-solvent as described above, and then subjecting the mixture to a mechanical dispersion means.

[0037] An example of a process for synthesizing a colloidal aqueous suspension of graphene includes an aqueous intercalation step of an organic molecule in graphite. The natural organic molecule must comprise at least one ring and at least one polar group. This is followed by a sonication exfoliation step. The suspension obtained after sonication can be filtered by centrifugation to remove unexfoliated particles. This process yields a stable graphene suspension with a high exfoliation yield.

[0038] An organic molecule is natural in the sense that it exists in nature. It can be extracted from plant matter, for example. A natural organic molecule is capable of interacting with graphite sheets to intercalate between them.

[0039] In a particular embodiment, the molecule is chosen from aromatic or cyclic molecules comprising at least one alcohol, aldehyde or ketone functional group, and possibly an amine functional group.

[0040] Examples of such molecules are alkaloids including piperine (also called 1-piperoylpiperidine, CAS number 94-62-2) and capsaicin (8-methyl-N-vanillyl-trans-6-nonenamide, CAS number 404-86-4); curcuminoids including curcumin (also called 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiane-3,5-dione or diferuloyl methane, CAS number 458-37-7); terpenes, and in particular monoterpenoids including 1,8-cineole (1,3,3-trimethyl-2-oxabicyclo(2,2,2)-octane, CAS number 470-82-6).

[0041] The pH of the aqueous phase used to prepare the graphene dispersion by aqueous exfoliation is preferably greater than 7. In a particular embodiment, the pH is greater than 10.

[0042] Exfoliation can be performed in the presence of a surfactant. Surfactants are preferably chosen from among non-ionic, anionic, and zwitterionic surfactants.

[0043] For an alkaline environment, nonionic and anionic surfactants are generally the best choices due to their stability at high pH. Zwitterionic surfactants may be considered in certain specific cases.

[0044] Non-ionic surfactants have no charge, and their stability is primarily linked to their chemical structure, such as ethoxylated or propoxylated chains. Examples include alkyl polyglucosides and alcohol ethoxylates.

[0045] Zwitterionic surfactants can also be compatible with alkaline media. They have both positive and negative charges, which can make them stable over a wide pH range. Examples include betaines and amine oxides.

[0046] Anionic surfactants are often compatible with alkaline media because their functional groups are stable at high pH. Examples include alkyl sulfates, sulfonates, and carboxylates.

[0047] The inorganic binder is preferably chosen from among transition metal silicates, alkali metal silicates, alkaline earth metal silicates, alkali metal phosphates, alkaline earth metal phosphates, alkali metal chromates, alkaline earth metal chromates, transition metal chromates, and mixtures thereof. In a particular embodiment, an inorganic binder soluble in water at 25°C, or soluble in a mixture of water and a co-solvent miscible with water at 25°C, is chosen.

[0048] More specifically, the inorganic binder is chosen from sodium silicate, lithium silicate, potassium silicate, calcium silicate, magnesium silicate, sodium phosphate, potassium phosphate, calcium phosphate and magnesium phosphate, and mixtures thereof.

[0049] The inorganic binder is intended to form the matrix in which the graphene is dispersed. It is available in liquid or solid form. A process for forming a sliding varnish film according to the invention comprises solubilizing the inorganic binder in the aqueous base, then the binder undergoes a curing cycle for the purpose of its polymerization, the formation of an inorganic matrix, and the evaporation of the aqueous base of the varnish composition.

[0050] The varnish composition of the invention may include at least one additive to facilitate its application to the part to be protected from contact wear. Such an additive may, for example, improve the sprayability of the varnish composition or ensure better wettability of the varnish after spraying onto the substrate surface. Additives may also affect the rheology of the varnish to ensure sufficient flow and allow for homogeneous coating of complex shapes such as sharp edges.

[0051] The varnish composition may also include additives that ensure the stability of the suspension during the storage of the varnish composition by limiting, for example, the reactivity between the aqueous base and the graphene, but also by limiting the sedimentation of the graphene.

[0052] Finally, the varnish composition may include additives that improve the thermal stability of the varnish film formed by polymerization of the varnish composition once deposited on the turbomachine part.

[0053] The said at least additive is chosen for example from among wetting agents, dispersing additives, corrosion inhibitors, rheological agents and stabilizing agents.

[0054] A person skilled in the art will be able to choose the optimal quantity of the additive used to fulfill the desired function. In a particular embodiment of the invention, the varnish composition contains 1% to 3% of a dispersing additive.

[0055] The sliding film is advantageously a ceramic material, in particular a ceramic material other than a cermet selected from the cermets of the inorganic binder in a low-melting-point metal selected from the group consisting of tin, lead, zinc, and indium. In a particular embodiment, the liquid varnish composition does not include a low-melting-point metal, in particular a post-transition metal.

[0056] A first example of a process for preparing the varnish composition according to the invention comprises: a step of preparing an aqueous suspension of graphene, and a step of mixing the aqueous suspension of graphene, water, the inorganic binder, and optionally at least one additive selected from those described above. The aqueous suspension of graphene can be prepared according to an aqueous exfoliation synthesis process described above.

[0057] A second example of a process for preparing the varnish composition according to the invention includes a step of mixing water, binder, powdered graphene, and optionally at least one additive chosen from the additives described above.

[0058] The sliding varnish formulation described above is a dry lubrication method which advantageously reduces contact wear between two parts of an assembly subjected to repeated vibrations or relative movements, but also to high thermal stresses.

[0059] The invention is applicable to the manufacture and maintenance of any turbomachine component subject to repeated friction during turbomachine operation, particularly at temperatures between 650°C and 750°C. The components concerned include interfaces requiring tribological applications to reduce the coefficient of friction, whether hot or cold. In addition to blades, high-pressure compressor discs may be cited.

[0060] Thus, the invention has as its second object a method of manufacturing or repairing a turbomachine blade, which consists of applying, on a surface of a blade foot attachment, a layer of liquid sliding varnish composition conforming to the previous description, and baking the varnish composition to cover said surface with a solid lubricating sliding film.

[0061] The amount of varnish composition applied to the foot is preferably chosen so that the varnish film after baking has a thickness of preferably between 5 micrometers and 65 micrometers, preferably between 25 micrometers and 35 micrometers, and preferably again between 5 micrometers and 15 micrometers.

[0062] In a standard embodiment, the varnish is applied by spraying with a paint gun manually or on automated means.

[0063] In other embodiments, it can be applied by brush for local applications on small surfaces, or by dipping for parts processed in large quantities such as screws and bolts.

[0064] The varnish can be applied to a surface that has been previously sandblasted, for example with a white corundum abrasive, to create roughness and promote mechanical adhesion of the varnish to the surface of the treated part. A surface roughness between 1 and 5 micrometers is desirable, preferably between 2 and 4 micrometers.

[0065] The varnish can alternatively be applied over a plasma coating with a roughness ranging from 5 to 10 micrometers, again to improve the varnish's mechanical adhesion. The plasma coating can be a CuAl or NiFeCrSi coating. Its thickness can range from 100 to 200 micrometers, preferably from 130 to 180 micrometers.

[0066] A turbine or compressor blade comprises a blade set in rotation by the flow of gas streams, and a foot used to hold the blade attached to a disc when the turbomachine is in operation.

[0067] The disk has, on its outer edge, a series of disk cells arranged at regular intervals, their longitudinal direction parallel to the disk's axis of rotation. The blade roots of each stage are engaged in the disk cells, and a platform connecting the root to the blade covers the opening of the cells on the outer edge of the disk to hold the blades in place during disk rotation. The shape of the root is configured to allow insertion along the longitudinal axis of the disk cell. In this description, "root" means an element comprising a portion of the attachment that bears against the disk cell supports during rotation and a portion called the strut that emerges from the cell and is connected to the platform. The shape of the attachment portion is configured to exert radial resistance on the walls of the disk cell. It may be shaped like a Christmas tree or a dovetail.

[0068] Contact wear occurs at the interface between the toe attachment portion and the disc. More specifically, deterioration occurs on the areas of the attachment surface that are in contact with the walls of the well, which are subject to friction. The materials in contact can be similar, such as nickel-based, titanium-based, or stainless steel. The assemblies can also be dissimilar, for example, in the case of nickel-based / titanium, steel / titanium, or nickel-steel contact.

[0069] Contact wear results from the degradation of at least one of the two components and stems from localized friction between the two contacting surfaces as they slide against each other. For example, friction can result from the movement of the attachment within the disk's recess, particularly during changes in engine speed. Centrifugal force exerts a tangential force that pushes the blades towards the rotor blade. Contact wear can also result from blade vibrations that are transmitted to the attachment.

[0070] Contact wear, as defined in the invention, is a physicochemical or structural modification of a material in a turbomachine component in contact with another component in an assembly. This wear is caused by very small relative displacements compared to the contact surface area. Contact wear occurs at the interface of two assembled elements and can manifest as deformation, cracking, friction, oxidation, corrosion, or a combination of these phenomena in either of the two elements. Contact wear can lead to cracks in the blade root or at the disk cavity.

[0071] A method for manufacturing a blade according to the invention includes a step of depositing the sliding varnish composition described above onto at least one area of ​​the foot surface, this area being located on the attachment, on the Léchasse, or on the rear of the blade foot. In one embodiment, the varnish composition is applied at least to the sides of the attachment portion.

[0072] The formation of the sliding varnish can be achieved by implementing the process comprising the following steps: 1 / surface preparation by degreasing the area to be treated, then sandblasting or plasma deposition as described previously; 2 / optionally, and depending on the composition of the varnish and the viscosity of the mixture, it may be necessary to preheat the part to around one hundred degrees to ensure adhesion of the first layer and to prevent runs when increasing the thickness; 3 / application of the varnish by spraying with a paint gun, by brush, or by dipping; 4 / a desolvation flash of approximately 30 minutes at room temperature; 5 / curing of the varnish composition; 6 / checking the thickness, adhesion and appearance.

[0073] A skilled professional will know how to adjust the duration and temperature of the firing according to the nature of the binder. For example, in the case of a phosphate-based binder, the duration will be on the order of one to several hours and the temperature will be between 300°C and 400°C.

[0074] The process of the invention makes it possible to manufacture a turbomachine blade, such as a turbine blade or a compressor blade comprising a foot, one surface of which is covered with a sliding film obtained by baking a deposit of sliding varnish composition described above.

[0075] This varnish film provides dry lubrication and helps to limit contact wear on the foot. The thickness of the sliding film is advantageously between 5 micrometers and 65 micrometers, preferably between 25 micrometers and 35 micrometers, and even more preferably between 5 micrometers and 15 micrometers.

[0076] The blade can be a low-pressure turbine rotor blade or a high-pressure compressor moving blade.

[0077] The invention is illustrated by the figures and examples that follow.

[0078] Figure 1 is a simplified diagram of a cross-sectional view of a turbomachine blade, along a plane perpendicular to its axis of rotation. The blade, which may be part of a compressor or turbine stage, comprises blades 10 mounted on a disk 20.

[0079] Figure 2 shows a blade 10 positioned opposite a recess 21 of a disk 20 for insertion. The blade comprises a blade 11 and a foot 12 connected by a platform 30. The foot includes a bracket 13 (shown here in the shape of a dovetail) with flanks 16, and a strut 14. The bracket 13 retains the blade 10 during engine operation, while the strut 14 prevents contact between the platform 30 and the disk 20.

[0080] Figure 3 is a partial cross-sectional view along a plane perpendicular to the axis of rotation of the blade, showing two blades 10 each positioned in a recess of the disk 20. The surface of the attachment 13 faces the surface of the walls and the bottom of the recess 21. When the turbomachine is in operation, the centrifugal forces and vibrations experienced by the blades 11 are transmitted to the attachments 13 and cause friction between the bearing surfaces of the blade root and the disk recess 21 at the contact areas, resulting in wear of the parts. The contact area is generally located on the flanks 16 of the attachment 13. Assembly A in Figure 3 represents the prior art, and the friction produced on the flanks 16 of the attachment caused a crack 22 in the disk. In assembly B, a sliding film 40, obtained by drying a deposit of sliding varnish composition, is present. According to the invention, it covers the surface of the sides 16 of the fastener 13, and helps to limit contact wear and deterioration of materials.

[0081] The sliding film reduces friction between the contact areas between the fasteners and the disc cells, thus limiting contact wear of the blade and the disc.

[0082] This description includes the following items:

[0083] l. A liquid composition of sliding varnish for a turbomachine part comprising an aqueous base, an inorganic binder capable of leading to an inorganic matrix enabling it to meet a functional requirement from 400°C to 750°C, and graphene used as solid lubricant particles.

[0084] 2. A varnish composition according to object 1, characterized in that the inorganic binder is selected from transition metal silicates, alkali metal silicates, alkaline earth metal silicates, alkali metal phosphates, alkaline earth metal phosphates, alkali metal chromates, alkaline earth metal chromates, transition metal chromates and mixtures thereof.

[0085] 3. A varnish composition according to object 1 or 2, characterized in that the proportion of graphene in the composition is between 2% and 25%, in that the content of inorganic binder is between 1% and 40%, the percentages being expressed by mass relative to the mass of the composition.

[0086] 4. A varnish composition according to one of the preceding objects, characterized in that the graphene is in dispersion in the aqueous base.

[0087] 5. A varnish composition according to any one of the preceding claims, characterized in that it comprises at least one additive selected from wetting agents, dispersing additives, corrosion inhibitors, rheological agents and stabilizing agents.

[0088] 6. A method for manufacturing or repairing a turbomachine blade which consists of applying, on a surface of a blade foot (13) attachment (12), a layer of liquid sliding varnish composition conforming to one of the objects 1 to 5, and baking the varnish composition to coat said surface with a solid lubricating sliding film (40).

[0089] The invention is illustrated by the following examples, which are given by way of non-limiting example. Unless otherwise indicated, the temperature is equal to ambient temperature and the pressure is equal to atmospheric pressure. EXAMPLE 1: A varnish according to the invention Preparation of a colloidal graphene suspension An aqueous dispersion of graphene was prepared by exfoliation of graphite in aqueous phase in a sonicator. The dispersion included a surfactant, such as betaine. Graphene has been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the SEM image in Figure 4, we can observe the morphology of a multi-layered particle whose largest dimension is approximately 5 micrometers. In the TEM image of the same graphene reproduced in Figure 4, we find a value of the largest median dimension equal to 5 micrometers, and the surface of the sheets is regular. Preparation of the sliding varnish composition A varnish composition according to the invention was prepared by mixing water, 25% sodium silicate and 5% of the previously prepared colloidal aqueous suspension of graphene, and optionally a few percent of additives and / or other rheology agents, the percentages being expressed by mass relative to the mass of the composition. EXAMPLE 2: Composition B of sliding varnish according to the invention A varnish composition according to the invention was prepared with water, 35% sodium silicate, 20% graphene powder (graphene nanoplatelets, lateral size 20 pm, thickness less than 50 sheets or about 17 nm), and a dispersing additive in a content of 1% to 3%, the percentages being expressed by mass relative to the mass of the composition. The formulation obtained for application has a viscosity close to 150 cP. EXAMPLE 3: Composition C of sliding varnish according to the invention A varnish composition according to the invention was prepared with an aqueous solution, 35% sodium silicate, 30% graphene powder (graphene nanoplatelets, lateral size 150 pm, thickness less than 200 sheets or about 70 nm), and a dispersing additive in a content of 1% to 3%, the percentages being expressed by mass relative to the mass of the composition. The formulation obtained for application has a viscosity close to 90 cP.

Claims

Demands [Claim 1] A method for manufacturing or repairing a turbomachine blade that consists of applying, on a surface of a blade foot (13) attachment (12), a layer of liquid sliding varnish composition and baking the varnish composition to coat said surface with a solid lubricating sliding film (40), said liquid sliding varnish composition comprising an aqueous base, an inorganic binder, and graphene, the inorganic binder being selected from transition metal silicates, alkali metal silicates, alkaline earth metal silicates, alkali metal phosphates, alkaline earth metal phosphates, alkali metal chromates, alkaline earth metal chromates, transition metal chromates, and mixtures thereof. [Claim 2] A method for manufacturing or repairing a turbomachine blade according to claim 1, characterized in that the proportion of graphene in the composition is between 2% and 25%, in that the inorganic binder content is between 1% and 40%, the percentages being expressed by mass relative to the mass of the composition. [Claim 3] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the graphene is in dispersion in the aqueous base. [Claim 4] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the liquid composition of sliding varnish comprises at least one additive selected from wetting agents, dispersing additives, corrosion inhibitors, rheological agents and stabilizing agents. [Claim 5] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the inorganic binder is selected from alkali metal silicates, alkali metal phosphates and alkali metal chromates. [Claim 6] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the inorganic binder is selected from sodium phosphate, potassium phosphate, calcium phosphate and magnesium phosphate. [Claim 7] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the inorganic binder is soluble in the aqueous base at 25°C. [Claim 8] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the liquid varnish composition which is applied to the blade foot (12) in sufficient quantity so that, after curing, the sliding film (40) has a thickness of between 5 micrometers and 35 micrometers, preferably between 5 micrometers and 25 micrometers, and preferably still between 5 micrometers and 15 micrometers. [Claim 9] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the method includes a step of supplying a graphene dispersion obtained by exfoliating graphite in an aqueous-based solvent. [Claim 10] Method for manufacturing or repairing a turbomachine blade of claim 9, characterized in that the supply of the graphene dispersion consists of the synthesis of a colloidal aqueous suspension of graphene comprising an aqueous phase intercalation step of an organic molecule in graphite, and a sonication exfoliation step. [Claim 11] Method of manufacturing or repairing a turbomachine blade of claim 10, characterized in that the organic molecule is a natural organic molecule comprising at least one ring and at least one polar group. [Claim 12] A method for manufacturing or repairing a turbomachine blade according to any one of claims 1 to 8, characterized in that the method comprises a supply step of graphene powder obtained by solid-phase exfoliation, pyrolysis, epitaxy or chemical vapor deposition. [Claim 13] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the graphene is a multi-sheet graphene comprising particles whose number of planes ranges from 2 to 30, for example from 2 to 10. [Claim 14] A method for manufacturing or repairing a turbomachine blade according to any one of the preceding claims, characterized in that the graphene particles have a larger dimension between 0.1 micrometers and 150 micrometers, for example between 5 micrometers and 6 micrometers.

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