Coating coating method and turbine blade having coating coated according to method

Abstract

The invention relates to a method for applying a coating to a substrate (100) made of a nickel-based superalloy and to a turbine blade made of a nickel-based superalloy, having a coating applied according to the method. The coating method comprises at least a first chemical vapor deposition step for depositing chromium and / or cobalt on the surface of the substrate (100) and a second chemical vapor deposition step for depositing aluminum on the underlayer (102) of the substrate (100) after at least partially diffusing the chromium and / or cobalt deposited in the first chemical vapor deposition step into the underlayer (102) of the substrate (100).

Description

Technical Field

[0001] The invention relates to a method for applying a coating to a substrate made of a nickel-based superalloy, and a turbine blade made of a nickel-based superalloy having a coating applied according to the method. A nickel-based superalloy is understood to mean a superalloy in which nickel predominates by mass. Thus, nickel is understood to be the element having the highest mass percentage. Background Art

[0002] Nickel-based superalloys are known for use in hot parts of gas turbines of aircraft or helicopter engines, in particular for the production of stationary or mobile single-crystal blades.

[0003] The main advantage of these materials is that they combine high creep resistance at high temperatures with oxidation and corrosion resistance.

[0004] Over time, the nickel-based superalloys used for single-crystal blades have undergone significant changes in their chemical composition, in particular with the aim of improving their high-temperature creep characteristics while maintaining resistance to the highly aggressive environments in which these superalloys are used. These superalloys contain an austenitic matrix of face-centered cubic structure, a nickel-based solid solution, called the gamma phase ("γ"), precipitates of the gamma phase ("γ") hardening phase with an ordered cubic structure L12 of the Ni3Al type. The whole (matrix and precipitates) is therefore described as a gamma / γ' superalloy. Typically, at room temperature, these superalloys contain a high proportion of gamma' phase precipitates, of the order of 60 to 70% by mass.

[0005] Furthermore, metallic coatings suitable for these superalloys have been developed to improve their resistance to the aggressive environments in which they are used, particularly oxidation and corrosion resistance. In addition, ceramic coatings of low thermal conductivity that act as thermal barriers can be added to reduce the temperature at the metal surface.

[0006] Typically, a complete protection system comprises at least two layers. The first layer, also referred to as a primer or bonding layer, is deposited directly on the nickel-based superalloy component to be protected, also referred to as a substrate, such as a blade. The deposition step is followed by a step of diffusing the primer into the superalloy. Deposition and diffusion can also be performed in a single step. The materials commonly used to produce such primers include aluminum-forming metal alloys of the MCrAlY type (M = Ni (nickel) or Co (cobalt)) or mixtures of Ni and Co, Cr = chromium, Al = aluminum and Y = yttrium, or nickel aluminide alloys (NixAly), some also containing platinum (NixAlyPtz). The second layer is a ceramic coating, usually referred to as a thermal barrier or "TBC" for "thermal barrier coating", which contains, for example, yttria-stabilized zirconia, also referred to as "YSZ" according to the acronym "yttria-stabilized zirconia", or "YPSZ" according to the acronym "yttria partially stabilized zirconia", and has a porous structure. This layer can be deposited by different methods such as electron beam evaporation (“EB-PVD”, from the acronym “Electron Beam Physical Vapor Deposition”), thermal spraying (“APS”, from the acronym “Atmospheric Pressure Plasma Spray”, or “SPS”, from the acronym “Suspension Plasma Spray”), or any other method that allows obtaining a porous ceramic coating having a low thermal conductivity.

[0007] Due to the use of these materials at high temperatures, for example from 650°C to 1250°C, interdiffusion phenomena occur on a microscopic scale between the nickel-based superalloy of the substrate and the metal alloy of the sublayer. These interdiffusion phenomena associated with the oxidation of the sublayer particularly change the chemical composition, microstructure and thus the mechanical properties of the sublayer from the coating manufacturing, and then during the use of the blade in the turbine. These interdiffusion phenomena also change the chemical composition, microstructure and thus the mechanical properties of the superalloy of the substrate under the coating. Therefore, in superalloys with particularly high additions of refractory elements, especially rhenium, a second reaction zone (SRZ) can be formed in the superalloy at a depth of tens or even hundreds of microns below the sublayer. The mechanical characteristics of this SRZ are significantly inferior to those of the substrate superalloy. The formation of the SRZ is undesirable because it leads to a significant reduction in the mechanical strength of the superalloy by promoting the initiation of cracks at grain boundaries and interfaces, which is even more critical in the thin walls of turbine blades.

[0008] In order to avoid this problem, various methods have been proposed, in particular in French patent applications FR 3 201643 A1 and FR 3 113 260 A1, for applying protective coatings more compatible with high rhenium contents to nickel-based superalloys by physical vapor deposition. However, it may be difficult to apply these methods to apply such protective coatings in small internal cavities, such as cooling channels of turbine blades, which may also be exposed to corrosion and oxidation. Summary of the invention

[0009] A first aspect of the invention relates to a coating application method which can be used even in small cavities and which allows substrates made of nickel-based superalloys to be protected from oxidation and corrosion even in aggressive environments, in particular at very high temperatures, without mechanically weakening the substrate.

[0010] To this end, the method according to this first aspect comprises at least a first chemical vapor deposition step for depositing chromium and / or cobalt on the surface of the substrate, and a second chemical vapor deposition step for introducing aluminum into the underlying layer of the substrate after at least part of the chromium and / or cobalt deposited in the first chemical vapor deposition step has been diffused into the underlying layer of the substrate.

[0011] The diffusion of chromium and / or cobalt into the bottom layer of the substrate after its chemical vapor deposition allows to modify the microstructure of the superalloy in this lower layer and in particular to replace the rhenium that may form the second reaction zone. The subsequent deposition of aluminum allows to increase the concentration of aluminum in this lower layer, which may reach between 5% and 12% by mass in the second chemical vapor deposition step, and thus form a coating of aluminum with a phase structure β or a combination of phases β, γ and γ'. This coating allows to provide a good environmental protection for the superalloy substrate, even in channels of limited size, such as cooling channels with a width of about 100 microns. Chemical vapor deposition also allows to reach difficult-to-access gaps by this treatment.

[0012] The nickel-based superalloy subjected to the coating application method may contain at least 3% by mass of rhenium. Rhenium in fact allows to slow down the diffusion of chemical species in the superalloy during high temperature use and limits the agglomeration of γ' phase precipitates, thereby improving the resistance of the superalloy to high temperature creep, but has the following disadvantages: using conventional coatings, it contributes to the formation of a second reaction zone with the coating in the lower layer. With the coating deposition method according to the first aspect, this disadvantage of rhenium in nickel-based superalloys can be avoided.

[0013] The first chemical vapor deposition step may form a surface layer having a thickness between 5 μm and 20 μm. This thickness provides satisfactory protection against corrosion and oxidation without unduly blocking narrow cooling channels.

[0014] In a second chemical deposition stage, the aluminum may be doped with silicon and / or at least one rare earth, such as hafnium, yttrium or zirconium, in order to further increase its oxidation resistance.

[0015] For the first and / or second chemical vapor deposition step, it is particularly conceivable to use any of the methods of plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), ultra high vacuum chemical vapor deposition (UHVCVD) and atomic layer chemical vapor deposition (ALCVD). In particular, the first and / or second chemical vapor deposition step can be a low pressure chemical vapor deposition, and is therefore carried out at a temperature between 900° C. and 1150° C., for example between 1000° C. and 1100° C., and at a pressure between 2 kPa and 50 kPa. More specifically, the first chemical vapor deposition step can be carried out at a temperature between 1040° C. and 1060° C., and at a pressure between 5 kPa and 20 kPa or between 15 kPa and 25 kPa, preferably between 18 kPa and 22 kPa, while the second chemical vapor deposition step is at a temperature between 1050° C. and 1070° C., and at a pressure of about 20 kPa.

[0016] A second aspect of the invention relates to a turbine blade made of a nickel-based superalloy having a coating applied according to the method of the first aspect. In particular, the blade may include one or more cooling channels, and the coating may be applied within the cooling channels, which is facilitated by using a chemical vapor deposition step.

[0017] A third aspect of the invention relates to a turbomachine comprising a turbine blade according to the second aspect. The turbomachine may be a turbofan engine, or other types of turbomachines, such as a turboprop engine, a turboshaft engine or a turbojet engine. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above mentioned characteristics and advantages as well as other characteristics and advantages will become apparent on reading the following description of an embodiment given as a non-limiting example with reference to the attached drawings.

[0019] Figure 1 is a schematic longitudinal cross-sectional view of a turbine.

[0020] Figure 2 Schematic diagram of a cooling channel Figure 1 Turbine blades of a turbine.

[0021] Figure 3 A substrate made of a nickel-based superalloy is schematically shown with a metal layer deposited on the substrate surface in a first chemical vapor deposition step.

[0022] Figure 4 A substrate made of a nickel-based superalloy is schematically shown after diffusion of the metal deposited in a first chemical vapor deposition step into an underlying layer of the substrate.

[0023] Figure 5 A substrate made of a nickel-based superalloy is schematically shown with an aluminum layer deposited on the substrate surface in a second chemical vapor deposition step. DETAILED DESCRIPTION

[0024] Nickel-based single-crystal superalloys consist of γ'-Ni3(Al, Ti, Ta) precipitates, a nickel-based solid solution, dispersed in a γ matrix of face-centered cubic structure and have mechanical properties, especially at high temperatures, which make them interesting candidates for the manufacture of single-crystal components for the hot part of turbojet engines. These superalloys may in particular have a rhenium concentration equal to or greater than 3% by mass to slow down the diffusion of chemical species within the superalloy and limit the agglomeration of γ' phase precipitates during high-temperature use, a phenomenon that leads to a reduction in mechanical strength. Thus, rhenium allows improving the high-temperature creep resistance of nickel-based superalloys. Nickel-based superalloys also enriched with rhenium include CMSX-4Plus Mod CMSX- and MC-NG. These alloys have the following composition, with the balance consisting of 100% nickel and unavoidable impurities:

[0025] Table 1

[0026]

[0027] In particular, these nickel-based superalloys can be used to manufacture single-crystal blades by solidification methods directed by thermal gradients. The use of single-crystal seeds or grain selectors at the beginning of solidification allows obtaining such a single-crystal structure, oriented, for example, in the crystallographic direction, <001> , which is the orientation that generally gives superalloys the best mechanical properties.

[0028] Figure 1 The turbofan engine 10 is shown in section along a vertical plane passing through its main axis A. Depending on the cycle of the air flow, such a turbofan engine 10 may comprise, from upstream to downstream, a fan 12 , a low-pressure compressor 14 , a high-pressure compressor 16 , a combustion chamber 18 , a high-pressure turbine 20 and a low-pressure turbine 22 .

[0029] The high pressure turbine 20 may include a plurality of blades 20A rotating with the rotor and stationary blades 20B mounted on a stator, also referred to as a rectifier. The stator of the turbine 20 may include a plurality of stator rings 24 arranged opposite to the rotating blades 20A of the turbine 20.

[0030] Thus, it is possible to manufacture blades 20A, 20B for turbines comprising nickel-based superalloys. However, despite the good resistance of these superalloys to high temperatures, those that predominate in the hot part of the turbine may require cooling of the blades 20A, 20B. To ensure this cooling, such blades 20A, 20B may include internal cavities, in particular in the form of cooling channels 21, to allow the circulation of a cooling fluid, for example, air taken from the compressors 14, 16. However, the width of the cooling channels 21 is limited by the shape and size of these blades 20A, 20B and may be of the order of one tenth of a millimeter, that is to say a hundred micrometers.

[0031] In order to protect the superalloy surface around these cavities from oxidation and corrosion, a coating can be applied to the surface according to a method comprising two consecutive chemical vapor deposition steps. These two consecutive steps can be carried out in the same chemical vapor deposition apparatus. Figure 3 As shown, in a first chemical vapor deposition step, chromium can be deposited on the surface of the substrate made of a nickel-based superalloy 100 at a temperature between 1040° C. and 1060° C., for example, and at a pressure between 15 kPa and 25 kPa, preferably between 18 kPa and 22 kPa, to form a surface layer 101, which is superimposed on the substrate 100 and has a thickness e1 of, for example, between 5 μm and 20 μm. However, in this first chemical vapor deposition step, cobalt can be deposited alternatively or in addition to chromium.

[0032] The chromium and / or cobalt deposited in the first chemical vapor deposition step can then diffuse into the substrate 100 to form a lower layer 102 that is rich in chromium and / or cobalt and poor in rhenium. After a first chemical vapor deposition step of chromium on a substrate 100 made of a superalloy and the subsequent diffusion of this chromium into the underlying layer 102, as Figure 4 As shown, the thickness e2 of this lower layer 102 is generally less than the initial thickness e1 of the surface layer 101 before diffusion, and this lower layer 102 may have a composition by mass percentage: 40% to 55% nickel, 10% to 30% chromium, 7% to 8% cobalt, 1% to 2% aluminum, and practically no rhenium, the latter having been replaced by chromium from this lower layer 102. This diffusion can be promoted in particular by heat treatment, for example at a temperature greater than 1000° C. and 10 -3 and 10 -4The heat treatment may be carried out at a pressure between 100 and 200 Pa for up to 4 hours, or at a partial pressure of a neutral gas (e.g., argon) between 0.1 Pa and 1 Pa for 2 to 4 hours. During this heat treatment, although during this heat treatment, elements of the lower layer 102 of the substrate 100 may also rise toward the surface layer 101 to replace the copper and / or cobalt diffused toward the surface layer, the thickness of the surface layer 101 may be significantly reduced.

[0033] The diffusion of chromium and / or cobalt into the lower layer 102 may be followed by a second chemical vapor deposition step. In this second chemical vapor deposition step, which may be in the form of a "flash" type aluminization and which may be carried out, for example, at a temperature between 900° C. and 1150° C., in particular between 1050° C. and 1070° C., at a pressure between 2 kPa and 50 kPa, for example at about 20 kPa, and / or for a duration between 30 minutes and 8 hours, in particular between 30 minutes and 60 minutes, aluminum may be introduced into the lower layer 102, as Figure 5 As shown, aluminum can reach a concentration of 5% to 12% by mass. In the surface layer 101, the concentration of aluminum can even reach 20%. The aluminum in the surface layer 101 and the lower layer 102 can then form aluminum oxide in use, thereby helping to protect other alloying elements. In the second physical vapor deposition step, aluminum can be deposited purely or doped with silicon and / or one or more rare earths, such as hafnium, yttrium or zirconium, so that a concentration of, for example, 0.5% to 2% by mass can be reached in the surface layer 101 and the lower layer 102, respectively.

[0034] It is conceivable that, before the second chemical vapor deposition step, the part forming the substrate 100 is extracted from the enclosure in which the first chemical vapor deposition step has been carried out, in particular if the first and second chemical vapor deposition steps must be carried out in two different enclosures in order to meet industrial constraints. In this case, the heat treatment for the diffusion of the chromium and / or cobalt deposited in the first chemical vapor deposition step towards the underlying layer 102 can be carried out in the enclosure used for the second chemical vapor deposition step, during its temperature increase before the second chemical vapor deposition step is carried out.

[0035] Alternatively, however, the first and second chemical vapor deposition steps may be performed in a single enclosure, maintained at the required temperature and pressure for a suitable time for the thermal treatment of the diffusion, in the interval between the first and second chemical vapor deposition steps.

[0036] Although the present disclosure has been described with reference to specific exemplary embodiments, it is apparent that various modifications and variations may be made to these embodiments without departing from the general scope of the invention as defined by the claims. In addition, individual features of the various embodiments mentioned may be combined in further embodiments. Therefore, the description and drawings should be considered in an illustrative rather than a restrictive sense.

Claims

1. A method for applying a coating to a substrate (100) made of a nickel-based superalloy, the method comprising at least: a first chemical vapor deposition step for depositing chromium and / or cobalt on the surface of the substrate (100), and A second chemical vapor deposition step is used to introduce aluminum into the lower layer (102) of the substrate (100) after the chromium and / or cobalt deposited in the first chemical vapor deposition step has at least partially diffused into the lower layer (102) of the substrate (100).

2. The method according to claim 1, wherein: The nickel-based superalloy contains at least 3% by mass of rhenium.

3. The method according to claim 1 or 2, wherein: The first chemical vapor deposition step forms a surface layer (101) having a thickness (e1) between 5 μm and 20 μm.

4. The method according to any one of claims 1 to 3, wherein: In a second chemical vapor deposition step, aluminum reaches a concentration of between 5 and 12% by mass in the lower layer (102).

5. The method according to any one of claims 1 to 4, wherein: In a second chemical deposition stage, the aluminum is doped with silicon and / or at least one rare earth.

6. The method according to any one of claims 1 to 5, wherein: The first and / or second chemical vapor deposition step is performed at a temperature between 900° C. and 1150° C., and at a pressure between 2 kPa and 50 kPa.

7. The method according to any one of claims 1 to 6, wherein: The first chemical vapor deposition step is carried out at a temperature between 1040° C. and 1060° C., and a pressure between 15 kPa and 25 kPa, preferably between 18 kPa and 22 kPa.

8. The method according to any one of claims 1 to 7, wherein: A second chemical vapor deposition step is performed at a temperature between 1050° C. and 1070° C. and a pressure of about 20 kPa.

9. A turbine blade (20A, 20B) made of a nickel-based superalloy, having a coating applied according to the method of any one of claims 1 to 8.

10. The turbine blade (20A, 20B) according to claim 9, comprising one or more cooling channels (21), wherein: The coating is applied inside the cooling channel (21).

11. A turbine, comprising the turbine blade (20A, 20B) according to claim 9 or 10.

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