Method for coating a ceramic matrix composite part with an environmental barrier coating

Abstract

The present invention relates to a method for electrophoretically producing an environmental barrier coating (12) on a ceramic matrix composite component (10), the method comprising the following steps: -E10: electrophoretically applying a liquid suspension comprising a composition of at least one rare earth silicate powder and a conductive filler to the surface (S) of the component (10); -E20: drying the applied suspension; and -E30: subjecting the rare earth silicate powder to a sintering heat treatment, characterized in that, as a conductive filler, at least one nitrate selected from the group consisting of nitrates of aluminum, yttrium and ytterbium is used, the amount of the at least one nitrate being between 0.2 and 1 millimole per liter of liquid suspension.

Description

Technical Field

[0001] This invention relates to a method for electrophoretically coating components made of ceramic matrix composites (CMCs) with an environmental barrier, particularly in the aerospace field. Background Technology

[0002] CMC technology has been developed for over 20 years, with the goal of applying these technologies to future generations of civilian turbojet engines, in which case they are used to produce turbine components and rear fuselage components.

[0003] CMC materials have good mechanical properties that make them suitable for constructing structural components, and they also retain these properties advantageously at high temperatures.

[0004] However, under the operating conditions of aero-turbines (especially high-temperature and corrosive environments), CMC materials are very sensitive to corrosion, so it is necessary to protect them from premature degradation.

[0005] Therefore, when CMC components include a silicon carbide (SiC) matrix, corrosion of the CMC causes the SiC to oxidize into silicon dioxide, which then volatilizes as Si(OH)4 hydroxide in the presence of water vapor. This corrosion leads to premature degradation of the CMC.

[0006] Therefore, in order to ensure the service life of CMC, it is necessary to use an environmental barrier (EBC) to protect CMC from wet corrosion.

[0007] Currently used EBCs typically include:

[0008] - Silicon layer, providing antioxidant protection;

[0009] - A layer of rare earth disilicate (usually Y or Yb) with the chemical formula RE2Si2O7 (where RE refers to rare earth) allows it to act as a diffusion barrier against oxidizing substances and protects CMC from corrosion at high temperatures.

[0010] - and a layer of yttrium monosilicate Y2SiO5 (YMS) to ensure resistance to degradation and CMAS.

[0011] In the aerospace field, the target components for this application are turbine components, such as blades or distributors. For these components, it is necessary to deposit a coating on the leading and trailing edges of the blades to protect the components from corrosion. However, this deposition must be as thin as possible to avoid compromising the aerodynamic performance of the coated components.

[0012] They can also be parts with holes, reliefs, or more generally, parts with variations in shape, regardless of how complex those variations are.

[0013] Therefore, the strong constraints on these complex components result in uniform and thin coatings (typically less than 100 μm for all three layers), and more specifically:

[0014] - Thickness of the silicon sublayer: typically less than 30μm;

[0015] The thickness of the RE2Si2O7 layer is approximately 30 μm;

[0016] The thickness of the YMS layer is approximately 10 μm.

[0017] Silicon layers are typically dry-deposited, either by thermal spraying or chemical vapor deposition (CVD), while rare-earth silicate layers can be dry-deposited or wet-deposited.

[0018] Therefore, in the context of wet deposition, electrophoresis is known, which is a wet method of immersing components in a liquid suspension of rare-earth silicate powder under an electric field. The components, depending on their polarity, are located at the anode or cathode and are gradually covered by the deposit by the mobility of the powder under the influence of the electric field.

[0019] Electrophoresis is typically chosen because it can produce thin coatings on complex parts, unlike thermal spraying, which is a directional method and therefore unsuitable for protecting complex parts.

[0020] Furthermore, because thermal spraying uses powder with a size of approximately 60 μm, it is reasonable to produce a coating with a minimum thickness of approximately 100 μm. Therefore, this method cannot be used to produce coatings with a maximum thickness of 100 μm on the leading / trailing edges.

[0021] Finally, regarding the vapor deposition method for rare earth silicate mixed oxides, it should be noted that the chemical properties of chemical vapor deposition are uncontrolled, and the thickness required for physical vapor deposition (PVD) is too large.

[0022] However, electrophoresis cannot uniformly cover the unique features of a component, even just the edges of the specimen. More precisely, the electric field concentration effect leads to localized excessive thickness or material shortage. This results in material shortage at the top of the component and, conversely, material retention in the hollow parts of the component. This is known as the "geometric effect."

[0023] This is particularly evident in the weft yarns of the CMC piece, which result in alternations of hollow and raised sections.

[0024] However, these thickness differences are detrimental to the coating because:

[0025] -In areas lacking materials: there is not enough coating to provide its corrosion protection;

[0026] - In areas with additional thickness, the coating may crack due to the high thermomechanical stress generated during the final sintering step caused by the high thickness.

[0027] To overcome these difficulties, document FR3084377 proposes introducing conductive fillers into the electrophoresis bath, which typically allows for the acquisition of uniform coatings of uniform thickness.

[0028] However, the applicant has found that the technology described in that document can still be improved. In fact, the use of carbonaceous fillers results in porous coatings, which do not guarantee optimal protection against oxidative corrosion. Furthermore, the use of metal salts, such as I₂, reacts with disilicates, thereby reducing their oxidative corrosion protection performance.

[0029] The use of rare earth disilicate solutions in combination with boron (for densifying the environmental barrier) and iron oxide (as a sintering agent) is also known. However, the same problems as described above have been observed.

[0030] The purpose of this invention is to provide a solution to this problem. Summary of the Invention

[0031] Therefore, the present invention relates to a method for producing an environmental barrier coating on a ceramic matrix composite component by electrophoresis, the method comprising the following steps:

[0032] -E10: A liquid suspension comprising at least one rare earth silicate powder and a conductive filler is applied to the surface of a component by electrophoresis.

[0033] -E20: Drying the applied suspension; and

[0034] -E30: Sintering heat treatment of rare earth silicate powder.

[0035] The invention is characterized in that, as a conductive filler, at least one nitrate selected from the group consisting of aluminum, yttrium and ytterbium nitrates is used, wherein the amount of the at least one nitrate is between 0.2 and 1 millimole per liter of liquid suspension.

[0036] The applicant was surprised to discover that by selecting salts from the aforementioned nitrates, a particularly uniform coating with a uniform thickness could be obtained at salt concentrations as low as those described above.

[0037] Therefore, in the case of a conductive suspension, the potential drop of the suspension is greater, thus weakening the driving force for coating formation, which reduces the concentration area of ​​electric field lines. This adjustment of conductivity allows for a more uniform coating and avoids "edge effects" and / or "geometric effects".

[0038] Furthermore, such a concentration allows for a very significant increase in the conductivity of the suspension without compromising its stability.

[0039] In addition, the addition of these salts will form alumina, yttrium oxide or ytterbium oxide after heat treatment, which helps to densify the coating.

[0040] Other advantageous and non-limiting features of the method, whether used alone or in any technically compatible combination of at least two of them:

[0041] - The composition of the suspension includes a sintering agent, wherein the sintering agent is present in the composition at a mass ratio of about 0% to 5%;

[0042] - The mass content of the rare earth silicate powder is between about 0.5% and 30%, or between about 1% and 10%;

[0043] - The average particle size of rare earth silicate powder is less than or equal to about 5 μm, or less than or equal to 1 μm;

[0044] - The composition of the suspension includes a solvent, the solvent being present in a mass content between about 70% and 99%, or about 85% and 95%;

[0045] - The sintering agent is an oxide filler of Fe2O3, Al2O3, MgO, CaO or RE2O3 type (RE = rare earth), or a precursor sol of these oxides, or a precursor sol of rare earth silicates;

[0046] - To obtain a given thickness of the environmental barrier, repeat steps E10 to E30 several times until the desired thickness is obtained;

[0047] - The component in question is a turbine component. Attached Figure Description

[0048] Other features and advantages of the invention will become apparent from the description now given with reference to the accompanying drawings, which illustrate possible embodiments of the invention in a non-limiting manner.

[0049] In these diagrams:

[0050] - Figure 1 The steps implemented according to the method of the present invention for forming an environmental barrier are illustrated schematically;

[0051] - Figure 2 The diagram shows a component comprising a ceramic material and an environmental barrier formed according to the present invention;

[0052] - Figure 3 These are scanning electron microscope images of the edges of a sample coated by electrophoresis, produced using existing technology.

[0053] - Figure 4The image is a scanning electron microscope image of the edge of a sample coated by electrophoresis, according to the method of the present invention. Detailed Implementation

[0054] The following detailed description considers forming an environmental barrier on the surface of a component made of a silicon-containing CMC material. However, it should be noted that the invention is applicable to components made of silicon-containing ceramic matrix composites, and more generally to components in which at least one adjacent component is coated with a silicon layer.

[0055] Components made from silicon-containing CMC materials include fiber reinforcements, which can be made of carbon fiber (C) or ceramic fiber (such as SiC fiber).

[0056] Now refer to Figure 1 and combination Figure 2 To describe the formation of environmental barriers on CMC components, Figure 1 Different steps of an example coating method are shown. Figure 2 A component made of CMC material 10 with a coating produced by this method is shown.

[0057] This component can be a static or rotating part of the turbine. Turbine components can be, for example, components present in the hot parts of the turbine, such as the turbine itself, and constitute, for example, a part of a turbine blade, a turbine ring, etc.

[0058] In the first step E10 of the method, the suspension is applied to the surface S of the CMC component 10.

[0059] The suspension used during electrophoresis is applied to a bonding layer 11 containing silicon and pre-existing on the surface S of component 10. In this example, the suspension is applied directly to the bonding layer 11 (i.e., in contact with it). The bonding layer can be a silicon or metal silicide layer.

[0060] However, applying the suspension directly to the surface of the CMC component (in the absence of the bonding layer 11, in direct contact with the component) does not exceed the scope of the present invention.

[0061] The bonding sublayer 11 is known in itself, which enables good adhesion of the environmental barrier coating 12 to the component 10. More generally, this bonding sublayer 11 ensures good mechanical compatibility between the first environmental barrier coating 12 and the surface S, and in particular compensates for the differential thermal expansion that may exist between the materials of the coating 12 and the CMC component 10.

[0062] When component 10 comprises a ceramic matrix composite material, the bonding sublayer 11 may comprise silicon or a metal silicide. Generally, the material of the bonding sublayer 11 is adjusted according to the materials forming component 10 and coating 12.

[0063] The previously known steps first include, for example, depositing a bonding sublayer 11 on the surface S of the component 10 by thermal spraying or vapor deposition (CVD or PVD).

[0064] An environmental barrier coating 12 is then deposited on the bonding sublayer 11 (step E10). The coating composition is applied by electrophoretic deposition.

[0065] The suspension includes a liquid medium in which at least one rare earth silicate RESiO powder is present, the rare earth silicate powder being selected from rare earth disilicates or monosilicates, wherein the rare earth is, for example, yttrium, ytterbium, lutetium or erbium.

[0066] In one embodiment, the rare earth silicate powder is selected from the following silicate forms: Y2Si2O7, Yb2Si2O7, Y2SiO5, Yb2SiO5 and Yb2Si2O7.

[0067] The mass content of rare earth silicate powder can be between about 0.5% and 30%, preferably between about 1% and 10%. The average particle size of rare earth silicate powder can be less than or equal to about 5 μm, preferably less than or equal to about 1 μm.

[0068] The liquid medium can be, for example, water or an alcohol such as ethanol, isopropanol, 1-propanol, or a mixture of at least two of these alcohols. The mass content of the solvent in the liquid medium can be between about 70% and 99%, preferably between about 85% and 95%.

[0069] According to the present invention, the liquid medium further includes at least one salt soluble in the medium, said salt being selected from aluminum, yttrium and ytterbium nitrates (Al(NO3)3, Yb(NO3)3 and Y(NO3)3), said salt being in an amount between 0.2 and 1 millimole per liter of liquid.

[0070] The liquid medium also includes a sintering agent at a mass content of about 0% to 5%. The sintering agent is, for example, an oxide filler of the type Fe2O3, Al2O3, MgO, CaO or RE2O3 (RE = rare earth), or a precursor sol of the above oxides, or a precursor sol of rare earth silicates. The addition of the sintering agent in very small amounts significantly increases the density of the coating thus produced.

[0071] In electrophoresis, component 10 constitutes the electrode of the electrophoresis system, and a counter electrode exists opposite this electrode. The counter electrode is, for example, made of platinum.

[0072] A potential difference is applied between component 10 and the counter electrode by a generator. The generator can be DC or pulsed. Component 10 is polarized to carry a charge opposite to that of the particles suspended in the liquid medium. Due to the electric field applied between component 10 and the counter electrode, the particles move and deposit on component 10 to form a ceramic coating.

[0073] Preferably, the voltage applied by the generator is between about 50V and 200V. The duration of electrophoretic deposition is between about 1 minute and 30 minutes.

[0074] Adding nitrate in the above amounts can increase the conductivity of the suspension (between 1 μS / cm and 5 μS / cm, more particularly between 2 μS / cm and 2.5 μS / cm) without compromising its stability.

[0075] Following the step of preparing the coating by electrophoresis, in step E20, the CMC component 10 coated with the suspension composition is subjected to a drying process to remove the deposit. In this drying step, all or part of the liquid medium is evaporated.

[0076] Drying can be carried out, for example, at a temperature between about 50°C and 200°C for a period of time, for example, about 5 minutes to 2 hours.

[0077] Then, in step E30, the CMC component 10 undergoes a sintering heat treatment to further densify the coating.

[0078] During the sintering heat treatment, organic compounds are pyrolyzed and sintering agents react with rare earth silicates to heal the coating and achieve better densification during the sintering heat treatment.

[0079] Furthermore, during this sintering heat treatment process, the aforementioned nitrates will be oxidized to form aluminum oxide, yttrium oxide, or ytterbium oxide, thereby participating in the densification of the coating.

[0080] The sintering heat treatment is preferably carried out at a temperature between about 1200°C and 1400°C for a period of about 1 hour to 50 hours.

[0081] At the end of steps E10 to E30, the environmental barrier 12 can be obtained by performing each step of steps E10 to E30 only once. As a variation, steps E10 to E30 can be repeated to obtain the environmental barrier 12. Therefore, to achieve the desired thickness, the environmental barrier 12 can be produced, for example, by performing this series of steps E10 to E30 at least twice, or even at least four times.

[0082] The thickness of the environmental barrier obtained by electrophoresis can be particularly between about 25 μm and 100 μm.

[0083] Therefore, the described method enables the acquisition of uniform coatings of uniform thickness even on components with complex shapes. The conductive silicon-bonded sublayer facilitates the creation of an environmental barrier via electrophoresis. Indeed, by using the nitrates according to the invention in combination with the addition of a sintering agent, excellent deposit regularity can be achieved due to the increased conductivity of the rare-earth silicate suspension, and the densification of the resulting environmental barrier can be enhanced by using the sintering agent.

[0084] Figure 3 Scanning electron microscope images of the edges of a sample coated by electrophoresis according to existing techniques are shown. The edge effect of the coating R on the substrate ST is readily observable, i.e., the lack of regularity of the coating R at the corners of the substrate.

[0085] On the contrary, according to Figure 4 According to the method of the present invention, the extremely regularity of the coating R is observed by scanning electron micrograph of the edge of the electrophoretically coated sample, and the coating is visually distinguishable from the underlying binding sublayer SC.

[0086] The following describes an example of the coating on the component in detail.

[0087] In this case, the component is a complex-shaped object composed of turbine blades previously obtained through 3D printing.

[0088] According to the present invention, an environmental barrier coating is formed using a suspension composed of the following:

[0089] Solvent: Prop-2-ol, containing:

[0090] -5% by mass of YbDS (ytterbium disilicate) / solvent;

[0091] -1% by mass Fe2O3 / YbDS;

[0092] -1% by mass boron / YbDS;

[0093] -0.25% by mass of (NO3)3Al·9H2O / YbDS.

[0094] Deposition parameters: 3 minutes at 100V.

[0095] This produces a uniform coating across the entire surface of the blade, even on the platform including the blade. This also applies after heat treatment at 1350°C for 5 hours under airflow.

Claims

1. A method for producing an environmental barrier coating (12) on a ceramic matrix composite component (10) by electrophoresis, the method comprising the steps of: - E10: A liquid suspension comprising at least one rare earth silicate powder and a conductive filler is applied to the surface of the component (10) by electrophoresis. - E20: Drying the applied suspension; and - E30: Sintering heat treatment of rare earth silicate powder, characterized in that, as a conductive filler, at least one nitrate selected from the group consisting of nitrates of aluminum, yttrium and ytterbium is used, wherein the amount of the at least one nitrate is between 0.2 and 1 millimole per liter of liquid suspension.

2. The method according to claim 1, characterized in that, The composition of the suspension includes a sintering agent, wherein the sintering agent is present in the composition at a mass ratio of 0% to 5%.

3. The method according to claim 1 or 2, characterized in that, The mass content of the rare earth silicate powder is between 0.5% and 30%.

4. The method according to claim 1 or 2, characterized in that, The mass content of the rare earth silicate powder is between 1% and 10%.

5. The method according to claim 1 or 2, characterized in that, The average particle size of the rare earth silicate powder is less than or equal to 5µm.

6. The method according to claim 1 or 2, characterized in that, The average particle size of the rare earth silicate powder is less than or equal to 1µm.

7. The method according to claim 1 or 2, characterized in that, The suspension composition includes a solvent, the solvent comprising between 70% and 99% by mass in the composition.

8. The method according to claim 1 or 2, characterized in that, The suspension composition includes a solvent, the solvent comprising 85% to 95% by mass in the composition.

9. The method according to claim 2, wherein, The sintering agent is Fe2O3, Al2O3, MgO, CaO or RE2O3, wherein RE = rare earth oxide filler, or precursor sol of these oxides, or precursor sol of rare earth silicates.

10. The method according to claim 1 or 2, characterized in that, To obtain a given thickness of the environmental barrier, repeat steps E10 to E30 several times until the desired thickness is achieved.

11. The method according to claim 1 or 2, wherein, Component (10) is a turbine component.

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