METHOD FOR PREPARING THE SURFACE OF A SUBSTRATE MADE OF POROUS CERAMIC MATRIX COMPOSITE MATERIAL FOR THE PURPOSE OF A DENSIFICATION TREATMENT OF SAID MATERIAL

Abstract

The invention relates to a method for preparing the surface of a composite substrate based on reinforcing fibers distributed in a porous ceramic matrix, for subsequent densification by silicification of this material. This method comprises the formation of a protective film on a portion of the substrate surface by applying a pre-vulcanized liquid latex composition to this surface portion, followed by a drying step. It then comprises the formation of an anti-wetting agent coating on the substrate surface, including the portion of the surface covered by the protective film.

Description

Title of the invention: METHOD FOR SURFACE PREPARATION OF A SUBSTRATE MADE OF POROUS CERAMIC MATRIX COMPOSITE MATERIAL FOR THE PURPOSE OF A DENSIFICATION TREATMENT OF SAID MATERIAL

[0001] The present invention falls within the field of manufacturing parts made of composite material based on reinforcing fibers distributed in a ceramic matrix, called CMC material, this material being densified by silicification.

[0002] More particularly, the present invention relates to a surface preparation method for a composite substrate based on reinforcing fibers distributed in a porous ceramic matrix, for subsequent densification by silicification of this ceramic matrix. The invention also relates to a substrate that can be obtained as a result of this surface preparation method. Another object of the invention is a more general method for preparing a part made of densified composite material, starting from a composite substrate based on reinforcing fibers distributed in a porous ceramic matrix, implementing a surface preparation method for a substrate according to the invention prior to a densification step by silicification of the substrate material.

[0003] Thermo-structural composite materials are frequently used in many industrial fields, particularly in the aeronautical field and in many other fields that take advantage of their beneficial mechanical properties and their ability to retain these properties at high temperatures. Among such materials are ceramic matrix composites, or CMCs, comprising a ceramic matrix, for example made of silicon carbide (SiC), in which a fibrous reinforcement of refractory fibers, such as carbon fibers or ceramic fibers, for example made of silicon carbide, is distributed.

[0004] The manufacture of parts made of CMC material typically comprises the development of a fibrous structure, called a fibrous preform, having a shape similar to the intended part, and the filling of this preform with a ceramic matrix. This filling can be carried out by liquid or gaseous means. The liquid method may, for example, consist of impregnating the preform with a liquid composition containing a precursor of the matrix. The transformation of the precursor is carried out by heat treatment. The gaseous method consists of carrying out chemical vapor infiltration. For this purpose, the preform is placed in a furnace into which a reaction gaseous phase is introduced. The pressure and temperature conditions in the furnace are adjusted to allow diffusion of the gaseous phase within of the fibrous preform and the formation of a matrix deposit on the fibers by decomposition of a constituent of the gaseous phase.

[0005] Regardless of the manufacturing process used, the CMC materials obtained at the end of this filling step exhibit residual porosity, formed of pores of varying sizes, which are intercommunicating.

[0006] In order to fill these pores and densify such CMC materials, a silicification treatment is generally implemented, consisting of infiltrating molten silicon into the pores of the material. Silicon is indeed very fluid in its molten state and has strong wetting properties, particularly with respect to silicon carbide surfaces. When a CMC material is impregnated with liquid silicon, the latter progresses spontaneously through the material's porosity network, following the surface of the pores until they are filled. Silicification is thus carried out by immersing the part to be treated in a bath of liquid silicon alloy at a very high temperature, above approximately 1400 °C, and then cooling it.However, during this cooling process, the volume of the treated part increases, causing droplets of the silicon alloy to form on its surface. These droplets then transform into silicon nodules upon further cooling. This is detrimental to the surface finish and aerodynamic performance of the resulting part, necessitating time-consuming and costly downstream processes to address the issue.

[0007] To avoid such a phenomenon, it is common practice to coat the surface of the parts with an anti-wetting agent coating prior to the silicification treatment. This coating must be such that it can be removed from the part at the very end of the process. Such a step is described, for example, in document EP 3348536, which proposes forming, on a CMC material, before the melt infiltration step, a non-wetting coating based on a non-wetting / anti-wetting agent with which the molten silicon has a contact angle (0c) of at least 45 degrees, such as boron nitride.

[0008] In such a context, however, it is necessary that certain areas of the part's surface remain free of such a coating, in order to allow the penetration, by capillary action, of the molten silicon intended to densify the ceramic matrix. Generally, such areas are located at the ends of the part.

[0009] Currently, the solutions implemented to protect localized surface areas of a CMC material part from the presence of an anti-wetting agent coating are mainly of two types. One of these solutions consists of applying a masking adhesive to these areas to be protected. However, on the one hand, implementing this solution is time-consuming and cannot be automated, and on the other hand, this prevents the application of the anti-wetting agent to the surface of the part by the The first method involves application by immersion. The second solution consists of using a more or less flexible mask, for example made of silicone, custom-made according to the specific geometry of the part and allowing for selective coverage of the areas to be protected. However, such a solution offers little tolerance for geometric and / or dimensional variations between different parts in a given production batch. It requires prior contouring of individual parts to adapt them to the precise shape of the mask, which generates a significant additional cost and a risk of damage to the parts, such as delamination, cracking, etc. Furthermore, this solution also prevents the application of the anti-wetting agent to the surface of the parts by immersion.

[0010] In another area, it has been proposed by the prior art, notably illustrated by documents CA 2,241,275 and US 4,315,957, to implement, in order to protect a surface, in particular concrete or metal, a removable coating formed from hydrocarbon wax.

[0011] However, such a solution is by no means satisfactory for surface protection of a porous ceramic matrix CMC material intended to undergo a liquid silicon infiltration step into the material's pores. Indeed, the removal of coatings based on such waxes must be carried out prior to the silicification step in order to free up the areas concerned and allow silicon penetration. However, these coatings are difficult to remove from the surface of the substrates to which they are applied, particularly when the surface is uneven, which complicates and lengthens the processes involved. Furthermore, there is a significant risk that wax residues will remain on the surface, and these residues are likely to interfere with the silicification step.

[0012] The present invention aims to remedy the disadvantages of the processes proposed by the prior art for adequately preparing the surface of a porous matrix CMC material, for a densification treatment by silicification, and in particular to remedy the disadvantages set out above, by proposing such a process which is easy and quick to implement, and in particular automatable, which is not detrimental to the proper execution of the subsequent silicification step, which allows any method of depositing the non-wetting coating on the surface of the material, in particular the deposit method by immersion which is particularly advantageous for parts of complex shape, and which is more tolerant of the geometric / dimensional variations of the parts to be treated than the processes proposed by the prior art.

[0013] It has now been discovered by the present inventors that these objectives are achieved by implementing a latex-based film for surface protection of areas of a part to be treated intended to constitute silicon supply areas for the part during a subsequent silicification step.

[0014] Thus, according to a first aspect, the present invention proposes a method for preparing the surface of a composite substrate based on reinforcing fibers distributed in a porous ceramic matrix (CMC), for subsequent densification by silicification of this material. This method comprises the formation of an anti-wetting agent coating on the substrate surface. It further comprises, prior to this step, the formation, on a portion, for example on several distinct portions, of this surface, of a protective film, interposed between the substrate and the anti-wetting agent coating, by: - application to this or these surface portion(s) of a pre-vulcanized liquid latex composition, - then drying of this liquid composition of pre-vulcanized latex applied to this or these portion(s) of surface.

[0015] Such a process is advantageously easily automated, particularly in that it allows for the application of the pre-vulcanized liquid latex composition and the anti-wetting agent to the substrate in any manner. Furthermore, it allows for easy adaptation to the geometric and / or dimensional variations of each individual substrate within a given production batch.

[0016] The protective film formed according to the invention effectively protects the surface area of ​​the substrate that it covers against the anti-wetting agent, which does not infiltrate between this protective film and the surface of the substrate during the subsequent formation of the anti-wetting agent coating on this surface.

[0017] Furthermore, the protective film can be very easily and quickly removed from the substrate prior to silicification treatment, leaving little or no residue that could clog the pores of the material. Indeed, the inventors have observed that the liquid composition of pre-vulcanized latex does not penetrate the pores of the CMC materials to which it is applied.

[0018] In embodiments of the invention in which the process includes a final step of removing the protective film from the surface portion of the substrate it covers, this final removal step can advantageously be carried out in two different ways: - by mechanical means, for example by peeling, particularly at room temperature; such a removal method cannot be used satisfactorily when applying a hydrocarbon wax as proposed by the prior art, particularly in configurations where the substrate surface is uneven; - and / or by thermal means, by exposing the substrate to a temperature of 500 °C or higher for at least 10 minutes, for example, for a period of between 10 minutes and 1 hour. Such exposure can, in particular, and quite advantageously, be carried out directly in the oven used for The subsequent silicification process occurs during the temperature increase necessary for pre-sintering the ceramic particles impregnating the substrate and for melting the silicon. Indeed, unlike waxes used in the prior art, when exposed to the high temperatures typically used in the silicification furnace before the substrate comes into contact with the molten silicon, latex decomposes rapidly, and its decomposition products vaporize directly, without passing through a liquid state. They can then be easily removed from the furnace, for example by suction, without risk of falling into the molten silicon bath and contaminating it.When the protective film has been mechanically removed, any latex residue that may have remained on the surface of the substrate is also spontaneously eliminated in the same way in the silicification furnace, so that it does not harm the silicification operation.

[0019] In particularly preferred embodiments of the invention, the pre-vulcanized latex liquid composition used in the process according to the invention is free of wax, in particular hydrocarbon wax such as paraffin wax.

[0020] In the present description, a CMC material, that is to say a composite material based on reinforcing fibers distributed in a ceramic matrix, is defined in a completely classical way in itself, and as described above.

[0021] The substrate formed in such CMC material, to which the process according to the invention is applied, may have been obtained in any conventional way in itself, in particular from a fibrous preform.

[0022] This fibrous preform, which will act as fibrous reinforcement in the substrate, may in particular have been obtained by weaving using a three-dimensional weaving technique. "Three-dimensional weaving" or "3D weaving" refers here to a weaving method in which at least some of the warp yarns interlock with weft yarns over several weft layers, such as an "interlock weave." "Interlock weaving" refers to a 3D weave structure in which each warp layer interlocks with several weft layers, with all yarns in the same warp column having the same movement within the plane of the weave.

[0023] The fibrous preform may, for example, comprise silicon carbide fibers or any other ceramic material, and / or carbon fibers.

[0024] Once the fibrous preform is completed and shaped, it may have undergone an interphase deposition step, known per se, for example, of the chemical vapor deposition (CVD) type, of an interphase material, for example, a mixture of boron nitride (BN) and silicon carbide (SiC). A sheath of BN and SiC then forms around the fibers of the preform, which consolidates and fixes it in the shape given to it during shaping. The consolidated preform obtained at the end of this interphase deposition step, whose fibers are coated with an interphase sheath, remains very porous.

[0025] The consolidated preform can then be transferred into a mold of a shape adapted to conform to the preform's form, for the injection of a ceramic slip. Such a slip may, for example, comprise, in addition to a ceramic particle powder, such as silicon carbide (SiC), a solvent, such as water, carbon, and / or boron. It may also comprise one or more organic additives, for example, an organic binder, such as polyvinyl alcohol, a dispersant, etc. By way of example, the concentration of the ceramic particle powder, particularly SiC, in the slip may be approximately 20% by volume. The concentration of the organic binder may, for its part, represent approximately 1 to 15% by mass relative to the mass of ceramic particle powder, particularly SiC, in the slip.A drying step may then be carried out to remove the solvent from the slip, for example in an oven, either in the mold or outside the mold, at a temperature, for example, between 60 and 110 °C. During such a drying step, the ceramic particles of the slip present within the preform settle and are deposited onto the preform fibers as the solvent is removed, thus filling some of the preform's pores. This results in a porous matrix CMC material substrate, on which it is highly advantageous to apply the surface preparation process according to the invention, prior to a densification treatment of the material by silicification.

[0026] Thus, in particular embodiments of the invention, in the porous matrix CMC material to which the process according to the invention is applied: - the reinforcing fibers are made of carbon and / or ceramic material, in particular silicon carbide, or alumina, in particular aluminosilicate; - and / or the matrix is ​​formed from silicon carbide, possibly with added boron nitride, carbon and / or other ceramic substances.

[0027] The substrate to which the process according to the invention is applied can have any shape and any dimensions. In particular, it can have a complex geometric shape.

[0028] In this description, the term "anti-wetting agent," also referred to as "non-wetting agent," conventionally means an agent capable of reducing the ability of a liquid to wet a given surface. In the context of the invention, the anti-wetting agent used is chosen to be compatible with a silicification treatment and, when applied as a coating to the surface of a porous matrix CMC substrate, to prevent the molten silicon from wetting this surface. Thus, the anti-wetting agent used to form the coating is Specifically chosen for its high thermal resistance, and in particular for its high melting point, exceeding 1500 °C. Typically, the anti-wetting agent used in the process according to the invention is chosen to form a contact angle (0c) of at least 45 degrees with the molten silicon. It is notably chosen from ceramic materials, particularly metal or metalloid nitrides, and especially boron nitride, aluminum nitride, and silicon nitride, or any mixture thereof, with boron nitride being particularly preferred in the context of the invention. Alternatively, the anti-wetting agent may, for example, be yttrium oxide, yttrium disilicate, silica, alumina, or any other anti-wetting agent commonly used in the field of the invention.

[0029] The step of forming an anti-wetting agent coating on the surface of the substrate of the process according to the invention can be carried out in any conventional way in itself, such a step being well known to the person skilled in the art.

[0030] In particular embodiments of the invention, the formation of an anti-wetting agent coating on the surface of the substrate comprises: - the application on this surface of a composition containing a suspension of particles of one or more anti-wetting agent(s) in a liquid vehicle, - then the drying of the composition thus applied on the surface of the substrate.

[0031] The application to the substrate surface of a composition containing a suspension of particles of one or more anti-wetting agent(s) in a liquid vehicle can be carried out in any conventional manner, for example by coating, for example by roller or brush, spraying, or immersion. Preferably, it is carried out by spraying. Alternatively, it can be carried out by immersing the substrate in the composition, for example for a period of between 1 and 300 seconds, in particular approximately 5 seconds.

[0032] The vehicle for the composition containing a suspension of particles of the anti-wetting agent(s) may be aqueous or organic. It may, in particular, be water, or a mixture of water and alcohol. This composition also preferably contains a binder, preferably of an inorganic type, such as alumina, aluminum phosphate, magnesium silicate, etc.

[0033] The total concentration of particles of one or more anti-wetting agent(s), in particular boron nitride particles, in this composition may in particular be between 20 and 60%, for example be about 30%, by weight relative to the volume of the composition.

[0034] The drying step of the composition based on particles of anti-wetting agent(s) thus applied to the surface of the substrate is preferably carried out under conditions allowing the substantial removal of all the liquid phase, of to obtain a dry coating. It can be carried out at room temperature, for example in the open air. Preferably, it is carried out at a temperature between 20 and 100°C, in particular around 60°C, and / or for 1 to 60 minutes, in particular for 1 to 30 minutes, in particular for around 5 minutes.

[0035] The anti-wetting agent coating formed on the surface of the substrate can have any thickness, for example a thickness between 1 and 200 pm, in particular between 1 and 25 pm, for example between 1 and 5 pm.

[0036] This coating preferentially covers the entire surface of the substrate.

[0037] Prior to the formation of an anti-wetting agent coating on the substrate surface, the process according to the invention includes a step of forming a protective film on one or more localized areas of this surface. These areas, also referred to herein as "surface portions," are preferably those that will constitute the infiltration zones of molten silicon into the substrate during a subsequent silicification step.

[0038] This protective film is formed from pre-vulcanized latex.

[0039] In the present description, the term latex is understood classically in itself as a colloidal dispersion of natural or synthetic rubber polymer particles in an aqueous medium.

[0040] In preferred embodiments of the invention, the latex is a natural rubber latex. Such a latex, corresponding to CAS No. 9006-04-6, is a natural product derived from certain plants, trees, or vines of the equatorial forest, particularly the Hevea brasiliensis plant. This product consists mainly of polyisoprene, in particular cis-1,4-polyisoprene, suspended in water, as well as small amounts of proteins, resins, sugars, and minerals. During harvesting, it is generally treated with ammonia or potash, typically at a concentration of less than 0.4% by weight relative to the total weight of the product, to facilitate its preservation.

[0041] In alternative embodiments of the invention, the latex is a synthetic rubber latex, that is to say, an aqueous dispersion of polymers produced by chemical polymerization, optionally in the presence of surfactant(s), from monomers such as styrene, butadiene, racrylonitrile, chloroprene, isoprene, etc., or any mixture thereof. The synthetic rubber latex implemented according to the invention may, for example, be of the styrene-butadiene (SBR), nitrile butadiene (NBR), chloroprene, polyvinyl chloride (PVC), acrylate, etc. type.

[0042] By way of example, the pre-vulcanized latex liquid composition implemented according to the invention may contain from 20 to 80%, for example about 60%, by weight of solids relative to the total volume of the composition. As described above, the vehicle for this liquid composition is aqueous.

[0043] The latex used in the process according to the invention is advantageously pre-vulcanized. This means, in a conventional sense, that it is in the form of a network in which the polymer chains are at least partially cross-linked, this network then exhibiting increased stability and elastic properties. Pre-vulcanized latex is thus defined as latex in which the rubber / elastomer polymers: are chemically cross-linked, thanks to the action of a so-called vulcanizing agent, which is mixed with the latex, and a heat treatment; and remain in a fluid state in the presence of ammonia. After drying, a vulcanized protective film can thus advantageously be obtained without additional heating. This film exhibits improved temperature behavior compared to films obtained from non-pre-vulcanized latex.

[0044] The pre-vulcanized latex used according to the invention may have been obtained by any method known to those skilled in the art, in particular by mixing the latex with a vulcanizing agent capable of causing the formation of cross-linked chemical bonds between the polymer chains of the latex, and heat treatment at a temperature below 100 °C. Examples of such vulcanizing agents include sulfur chloride, metal oxides such as zinc oxide, selenium, tellurium, tetramethylthiuram sulfide, etc., or crosslinking accelerators such as dithiocarbamates; these substances may be used alone or in mixtures.

[0045] The method according to the invention may also meet one or more of the characteristics described below, implemented in isolation or in each of their technically operative combinations.

[0046] In particular embodiments of the invention, the application of the pre-vulcanized latex liquid composition to the targeted surface area(s) of the substrate is carried out by coating, in particular by brush, or by spraying.

[0047] Alternatively, the application of the pre-vulcanized latex liquid composition to the targeted surface area(s) of the substrate can be carried out by immersing this surface area(s) in a bath of this liquid composition. This immersion is preferably carried out for a few seconds, in particular for 1 to 300 seconds, for example for 5 seconds.

[0048] The drying of the pre-vulcanized latex liquid composition applied to the surface portion of the substrate is preferably carried out under conditions that substantially remove all of the liquid phase, so as to obtain a dry film. This drying is preferably carried out at a temperature between 20 and 100 °C, in particular around 60 °C, preferably for 1 to 30 minutes, in particular around 5 minutes.

[0049] At the end of this drying step, a protective latex film is obtained on the surface area(s) of the substrate concerned. This protective latex film may to present any desired thickness, for example a thickness between 1 µm and 1 mm. After the step of forming a coating of anti-wetting agent on the surface of the substrate, this protective film is interposed between the substrate and the coating of anti-wetting agent at the level of the surface portions concerned.

[0050] The surface portion(s) onto which the pre-laccanized liquid latex composition is applied, and the protective film is formed, may be located in any desired area(s) of the substrate surface. They are preferably located in one or more end portions of this surface.

[0051] As described above, the process according to the invention may include a final step of removing the protective latex film (the latter being coated with the anti-wetting agent) from the surface portion(s) covered by it. This removal step automatically eliminates the anti-wetting agent coating that was located on the protective film from the relevant surface portions of the substrate. It may be carried out manually or automatically.

[0052] This removal step can, for example, be carried out mechanically, in particular by peeling or using rotating brushes. Advantageously, it is then particularly simple and quick to perform, at room temperature, due to the specific elastic and mechanical properties of the protective latex film formed according to the invention.

[0053] This removal step can otherwise be carried out thermally, by exposing the substrate to 500 °C or more for at least 10 minutes. In this respect, in the context of surface preparation of a substrate for densification by silicification, the present invention proves particularly advantageous. Indeed, as explained above, the protective latex film, and consequently the anti-wetting agent coating covering it, is spontaneously removed inside the furnace used for the silicification treatment, under the effect of the high temperature prevailing in this furnace, before the substrate comes into contact with the molten silicon, and even before the silicon melts.

[0054] In both cases, after the removal of the protective latex film, advantageously no residue remains on the surface of the surface portions that were covered with it, and the pores of the material are in no way blocked.

[0055] Another aspect of the invention relates to a substrate that can be obtained, in particular having been obtained, by a surface preparation process according to the invention, this process meeting one or more of the above characteristics, and being devoid of a final step of removing the protective latex film from the portion(s) of surface that it covers.

[0056] This substrate is formed from a composite material based on reinforcing fibers distributed in a porous ceramic matrix, and its surface is coated with a coating of anti-wetting agent. This substrate is further covered, on a portion of its surface, or on several such distinct portions, with a protective film of pre-vulcanized latex interposed between the substrate and the anti-wetting agent coating.

[0057] This substrate may have one or more of the characteristics described above with reference to the surface preparation process according to the invention, and concerning the composite material itself, the anti-wetting agent coating and the protective film, as well as the portion or portions of the surface that it covers.

[0058] A further object of the invention relates to a method for treating a substrate made of composite material based on reinforcing fibers distributed in a porous ceramic matrix, in order to densify this material. This method comprises: - applying a surface preparation process according to the invention to the substrate, - followed by a densification treatment of the substrate material by silicification.

[0059] The substrate on which this treatment process is implemented may have one or more of the characteristics described above with reference to the surface preparation process according to the invention, in particular with regard to the composite material from which it is formed.

[0060] The surface preparation process may either include a final step of removing the protective film prior to introducing the substrate into the silicification furnace, or it may not include such a step. In the latter case, the protective film is removed spontaneously in the furnace used for silicification, under the effect of the temperature prevailing therein.

[0061] The thermal profile applied in the silicification furnace for the densification treatment by silicification is conventional in itself, and it is within the competence of those skilled in the art to determine its exact operating parameters, depending on the different substances present. It notably includes a hold at a pre-sintering temperature for the ceramic particles contained in the substrate, and a hold at a temperature greater than or equal to the melting temperature of silicon, during which the substrate is brought into contact with the molten silicon. Prior to the hold at the pre-sintering temperature, the thermal profile applied in the silicification furnace may include a hold at 500 °C for a duration of 10 to 60 minutes, ensuring the removal of the protective latex film.However, such a plateau is not necessary when the initial temperature rise and the pre-sintering plateau are such that the substrate is subjected to temperatures greater than or equal to 500 °C for at least 10 min, prior to the substrate being brought into contact with the molten silicon.

[0062] Exposing the substrate to a temperature of 500 °C or higher for at least 10 minutes in the silicification oven leads to the degradation of the film latex protector, and to the vaporization of the degradation products formed, thus leaving free and intact the portion(s) of surface of the substrate which was previously covered with the protective latex film, this or these portion(s) of surface, free of anti-wetting agent coating, can then be used as area(s) for feeding the silicon substrate in the subsequent step of contacting the molten silicon.

[0063] The contact between the substrate and the molten silicon can be carried out in any conventional manner. Schematically, it aims to impregnate the substrate, at the surface portions free of antiwetting agent coating, with a molten silicon phase, which penetrates the porosity of the material and progresses through it by capillary action. This molten silicon phase can be obtained by heating silicon, alone or in the form of one of its alloys, to a temperature greater than or equal to its melting point, preferably at least 1380 °C, in particular to a temperature between 1380 and 1500 °C (a plateau at a temperature greater than or equal to the melting point of silicon mentioned above).

[0064] The impregnation of the substrate can be carried out by immersing at least its portion(s) of surface free of anti-wetting agent coating in the bath of molten silicon or silicon alloy, for example for 5 minutes to 5 hours, depending on the size of the substrate and the initial degree of porosity of the material.

[0065] The silicon bath can contain any conventional additive in itself in the field.

[0066] The densification treatment of the substrate material by silicification is preferably carried out under an inert atmosphere, essentially devoid of oxygen. Such an inert atmosphere can be obtained, in particular, by purging an inert gas through the silicification furnace. Alternatively, a secondary vacuum, i.e., between 0.1 and 10⁷ Pa, preferably dynamic, can be applied in this furnace.

[0067] After this treatment, once cooled, the densified substrate material can optionally be cleaned to remove the anti-wetting agent coating from its surface. This cleaning can be carried out mechanically, for example using brushes, and / or by immersion in an aqueous bath and ultrasonic treatment. An adjustment can also be made to remove any residual silicon nodules that may be present on the substrate surface.

[0068] The present invention proves particularly advantageous for the manufacture of parts for the aeronautical, space, automotive, naval, etc. industries, in particular parts of complex geometric shape.

[0069] Thus, by way of example, the fibrous preform used to form the CMC material substrate to which the processes according to the invention are applied can be chosen Among the fibrous preforms commonly used for manufacturing aeronautical parts, such as a turbomachine blade or a distributor section, are fibers. Such parts generally include cavities designed to form complex cooling circuits, for which an improved surface finish is advantageous.

[0070] The features and advantages of the invention will become more apparent in the light of the following implementation examples, provided by way of illustration only and in no way limiting the invention, with the support of Figures 1 to 3, in which:

[0071] [Fig-1] Figure [Fig.1] shows photographs illustrating different successive stages of a surface preparation process according to the invention implemented on a flat-shaped CMC material substrate.

[0072] [Fig.2] Fig.2 shows a photograph of a complex-shaped CMC material substrate in the final stage of a surface preparation process according to the invention, removing the protective latex film.

[0073] [Fig.3] Fig.3 shows a tomography image of a substrate that has been subjected to a treatment process, for obtaining a part in densified CMC material, according to the invention, after the silicification step; an area that had initially been covered with a protective latex film in accordance with the invention is highlighted by a white frame on this figure.

[0074] 1 / Example 1 - sub strat plan

[0075] A flat, rectangular, fibrous preform is used for this example. This preform has a three-dimensional weave of silicon carbide fibers. Prior to implementing the process according to the invention, the preform was subjected to the following successive steps, which are carried out in a conventional manner: - an interphase deposition step, by chemical vapor deposition, the deposited interphase material being formed of boron nitride (BN) and silicon carbide (SiC); a sheath of BN and SiC then formed around the fibers of the preform; - a step of injecting a ceramic slip, into a mold of appropriate shape, this slip comprising more precisely water, silicon carbide (SiC) powder at a concentration of 20% v / v, and polyvinyl alcohol at a concentration of 1% by mass relative to the mass of SiC powder; - a drying stage in an oven, at a temperature of 60 °C, for 1 night.

[0076] The substrate thus obtained is subjected to a surface preparation process according to the invention. Figure 1 shows photographs of this substrate after each step of this process.

[0077] In the first step, an area (portion of surface) of the substrate which is to ultimately be free of anti-wetting agent coating, more precisely a rectangular area occupying the entire lower part of the substrate, it is immersed for 5 seconds in a bath of pre-vulcanized natural rubber latex marketed by Ceradel (CAS No. 9006-04-6, latex with added ammonia, zinc oxide, and zinc dibutylthiocarbamate). Prior to this, the total decomposition temperature of this latex was determined at 450 °C by thermogravimetric analysis (TGA) under an inert argon atmosphere, using a SETARAM Labsys Evo TG-DSC instrument, ramp 2 and 100°C / min up to 1540 °C.

[0078] The substrate is then dried in an oven at 60 °C for 5 min, so as to obtain a protective latex film on the area concerned of the substrate.

[0079] At the end of this step, the substrate shown in A / on [Fig.1] is obtained. On this figure, the area of ​​the substrate covered with the protective latex film is identified by a bracket (lower end area of ​​the substrate).

[0080] The substrate, including its area covered with the protective film, is then immersed for 5 s in an anti-wetting agent composition, formed from a suspension of boron nitride particles at 30% w / v in water.

[0081] After drying in an oven for 5 min at 60 °C, the substrate shown in B / in [Fig. 1] is obtained. The surface of the substrate coated with an anti-wetting agent is clearly visible in white in this photograph. Here again, the area of ​​the substrate covered with the protective latex film (itself coated with the anti-wetting agent) is marked by a bracket (lower end zone of the substrate).

[0082] In the particular embodiment of the method according to the invention described in this example, the method includes a subsequent step of removing the protective latex film. This step, illustrated in C / in [Fig. 1], is carried out manually by peeling off this film using tweezers (not visible in the figure), in the direction indicated by an arrow in the figure. This operation is easy to perform in one continuous motion, as the protective film can be completely removed in one piece.

[0083] Following this removal step, as shown in D / in the figure, the substrate has a surface coated with an anti-wetting agent (white surface) and, at its lower end, a portion of the surface not coated with such an agent (indicated by a bracket in the figure), corresponding to the area that was previously covered with the protective latex film. As can be clearly seen, this area is clean; it shows no trace of anti-wetting agent infiltration, nor any residue of the protective latex film.

[0084] 2 / Example 2 - substrate of complex shape

[0085] A complex-shaped substrate, formed from the same materials and in a similar manner to the substrate of Example 1, is subjected to a surface preparation process according to the invention. This substrate has undeburred edges with a rough surface, on which the protective latex film is deposited.

[0086] The surface preparation process is implemented as described in Example 1, with the exception of the formation of the anti-wetting agent coating: this coating is formed here by spraying an aerosol of a composition comprising 30% w / v of boron nitride particles in a mixture of acetone and butanone.

[0087] After the anti-wetting agent coating has been applied and dried, the protective latex film is removed by peeling, in the direction indicated by the arrow in [Fig. 2]. Again, the protective film can be easily removed in one piece. As can be seen in this figure, the area on which the protective latex film was initially deposited, and from which it has been removed (visible in dark tones in the figure), bears no residue of either anti-wetting agent or latex. This demonstrates the performance of the protective latex film formed according to the invention on the substrate surface, in terms of ease and cleanliness of removal, even when the surface is uneven.

[0088] 3 / Example 3 - Silicification process

[0089] This example is implemented on the complex-shaped substrate obtained in Example 2, after the removal of the protective latex film by peeling.

[0090] The substrate is introduced into a furnace for the densification step by silicification of the material. It is held above a crucible containing silicon powder. A dynamic secondary vacuum of 10⁶ Pa is applied in the furnace. A conventional thermal profile is also applied in the furnace to successively pre-sinter the ceramic particles of the substrate and melt the silicon. The substrate is then brought into contact with the molten silicon, at the level of its surface portions that had previously been coated with the protective latex film and are now free of it, by immersion in the silicon bath for a period of between 5 minutes and 5 hours. The furnace temperature is maintained at a value between 1380 °C and 1500 °C during this immersion. This allows the molten silicon to infiltrate, by capillary action, into the substrate material, resulting in densification of the material.

[0091] At the end of this heat treatment, the part is unloaded from the oven, left to cool to room temperature, then cleaned by dry means using brushes and then treated by ultrasound for 20 min in aqueous medium, so as to remove the anti-wetting coating.

[0092] The resulting substrate is analyzed by tomography using an UltraTom tomograph. A tomographic image thus obtained is shown in [Fig. 3]. In this figure, the area that was initially covered with the protective latex film is indicated by a white outline. No defects in the filling of this area by the silicon are observed in this image. This demonstrates that no penetration of the latex into the pores occurred during the implementation of the process. of the material, penetration which would have interfered with the densification process by silicification. Such interference is not observed for the process according to the invention.

Claims

Demands

1. A method for preparing the surface of a composite material substrate based on reinforcing fibers distributed in a porous ceramic matrix, for the purpose of subsequent densification treatment by silicification of said material, said method comprising the formation on the surface of said substrate of a coating of anti-wetting agent, and being characterized in that it comprises, prior to the formation on a portion of said surface of a protective film, interposed between said substrate and said coating, by: - ​​application on said portion of surface of a pre-vulcanized liquid latex composition, - then drying of said pre-vulcanized liquid latex composition applied on said portion of surface.

2. Method according to claim 1, wherein said application is carried out by coating or spraying, or by immersing said portion of surface in a bath of said liquid composition.

3. A method according to claim 1 or 2, wherein the drying of said pre-vulcanized latex liquid composition applied to said portion of surface is carried out at a temperature between 20 and 100 °C, preferably for 1 to 30 minutes.

4. A method according to any one of claims 1 to 3, wherein said latex is a natural rubber latex.

5. A method according to any one of claims 1 to 4, wherein said anti-wetting agent is a metal or metalloid nitride.

6. A method according to any one of claims 1 to 5, wherein the formation on the surface of the substrate of an anti-wetting agent coating comprises: - applying to said surface a composition containing a suspension of particles of an anti-wetting agent in a liquid vehicle, - then drying said composition thus applied to said surface of the substrate, preferably at a temperature between 20 and 100 °C for 1 to 60 minutes.

7. A method according to any one of claims 1 to 6, comprising a final step of removing said protective film from said portion of surface.

8. A method according to claim 7, wherein said final removal step is carried out mechanically and / or by exposing said substrate to a temperature greater than or equal to 500 °C for at least 10 minutes.

9. Substrate obtainable by a surface preparation process according to any one of claims 1 to 6, said substrate being formed of composite material based on reinforcing fibers distributed in a porous ceramic matrix, and being coated on its surface with an anti-wetting agent coating, said substrate being characterized in that it is covered, on a portion of said surface, with a pre-vulcanized latex protective film intercalated between said substrate and said coating.

10. A process for treating a composite material substrate based on reinforcing fibers distributed in a porous ceramic matrix, characterized in that it comprises: - the implementation on said substrate of a surface preparation process according to any one of claims 1 to 8, - then a densification treatment by silicification of said material.

Images