Coated nickel superalloy

Abstract

Coated nickel superalloy. The invention relates to a part (100) for an aeronautical turbomachine comprising: - a substrate (10) made of a nickel superalloy consisting of a γ phase and a γ' phase; - a coating (20) covering the substrate, said coating comprising a γ' phase for more than 95% by volume, and in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the γ' phase of the superalloy of the substrate. Figure for the abstract: Fig. 2

Description

Title of the invention: coated nickel superalloy. Technical field

[0001] This presentation concerns the aeronautical field and more precisely the metallic alloys used in this field and particularly the protective coatings of such alloys. Prior art

[0002] The efficiency of aeronautical turbomachines depends on their operating temperatures.

[0003] The operating temperature of turbomachines has been able to increase in recent decades, in particular thanks to the use of nickel superalloys.

[0004] These superalloys in fact have good resistance to high temperature fining together with resistance to corrosion and oxidation compatible with use in an aeronautical turbomachine, at higher operating temperatures.

[0005] One material used for this application is the alloy commercially available under the name AMI.

[0006] Since the success of this superalloy, new superalloy compositions have been proposed to further increase their mechanical properties at high temperature and thus envisage a further increase in the temperature of turbomachines.

[0007] In doing so, the composition of the superalloys has seen a significant decrease in chromium content compared to AMI, which has jointly led to an increased sensitivity of the new alloys to the environment of an aeronautical turbomachine.

[0008] It has been proposed to compensate for this sensitivity by the development of new protective coatings, improving the resistance to oxidation or corrosion of the underlying substrate, and allowing excellent resistance of the thermal barrier, usually placed on the outside of the turbomachine parts to protect them from the temperature.

[0009] Nevertheless, at the temperatures involved in the turbomachine, diffusion allows significant mobility of the atoms so that phase transformations are often observed in the coating.

[0010] These phase transformations result in the creation of surface undulations between the substrate and the coating, which promotes flaking of the thermal barrier.

[0011] We then speak of “rumpling”, according to the established English term, to characterize the appearance of these surface undulations between the substrate and the coating causing a loss of adhesion of the thermal barrier placed on the coating.

[0012] For the most recent coated superalloys, richer in rhenium and poorer in chromium than AMl, in addition to rumpling, the appearance of secondary reaction zones (or "SRZ" for the acronym in English "Secondary Reaction Zone") has been observed, which reduce the service life of the entire part.

[0013] Such areas are caused by the interdiffusion of the elements of the coating and the substrate. More specifically, some of the elements of the coating migrate to the substrate and some of the elements of the substrate migrate to the coating, which creates between the substrate and the coating an area of ​​a phase distinct from both the substrate and the coating. Such an area significantly decreases the mechanical characteristics of the coated superalloy.

[0014] Indeed, these zones are associated with degradations of the microstructure and consequently a reduction in the mechanical resistance properties, in particular less good resistance to cracks, ultimately causing poor performance of the thermal barrier and therefore premature wear of the parts in operation.

[0015] Thus, there remains a need for a material having mechanical properties and resistance to oxidation allowing the operating temperatures of aeronautical turbomachines to be increased compared to those permitted by AM1. Statement of the invention

[0016] The invention aims to propose a solution to the problems set out above.

[0017] For this, it relates according to a first of its aspects to a part for an aeronautical turbomachine comprising: - a substrate made of a nickel superalloy consisting of a y phase and a y' phase; - a coating covering the substrate, said coating comprising a y' phase for more than 95% by volume, and in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the superalloy of the substrate.

[0018] The embodiment proposed above makes it possible to overcome the drawbacks of the coatings of the prior art and to propose a more advantageous solution than coated AM1 for turbomachine parts.

[0019] The coating layer comprises for more than 95% by volume a y' phase which allows a limitation of diffusion. The limitation of diffusion is particularly advantageous because it makes it possible to limit the appearance of secondary reaction zones which allows the coating layer to maintain excellent integrity and a very stable microstructure even at high temperature.

[0020] In particular, the appearance of surface undulations (rumpling) between the coating and the substrate is avoided by limiting diffusion. The absence of these surface undulations allows excellent resistance of the thermal barrier, and thus an extended service life for the part as a whole.

[0021] Thus, the coating makes it possible to meet not only environmental and mechanical requirements, by offering on the one hand excellent protection against corrosion and oxidation and on the other hand mechanical characteristics compatible with the desired application.

[0022] In addition, the coating comprises a composition very close to the y' phase of a nickel superalloy, in order to reduce interdiffusion. This composition also ensures that the coating comprises more than 95% by volume of the y' phase.

[0023] The proposed solution ensures excellent chemical compatibility between the substrate and its coating. In addition, the coating plays a protective role against oxidation.

[0024] The expression "the mass content of an element is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the substrate" is intended to mean that the mass content of the element is between (X-2)% and (X+2)%, where X is, in percentage, the mass content of the element in the y' phase of the substrate.

[0025] The above formulation is intended to mean that the coating comprises the same elements as the y' phase of the substrate and that each element present except chromium and aluminum is included in the same content within 2.0% as it was in the y' phase of the substrate.

[0026] It is understood that this formulation covers cases where certain elements were present in the y' phase of the substrate in a mass content less than or equal to 1.5% and are not present in the coating. On the other hand, preferably, no element present in the coating is absent from the substrate. In other words, the coating only comprises elements present in the y' phase of the substrate.

[0027] Preferably, the mass content of each element except chromium and aluminum in the coating is equal to plus or minus 1.0%, or even to 0.7%, or even to 0.3%, better still to 0.15% of the mass content of the element in the y' phase of the superalloy of the substrate.

[0028] These embodiments are even more preferred, because the closer the coating has a composition to the y' phase of the substrate, the greater the chemical compatibility between the substrate and the coating.

[0029] In one embodiment the chromium and aluminum content is equal to plus or minus 2.0% of the mass content of the element in the y phase of the substrate superalloy.

[0030] The possible variation of the content of chromium and aluminum, present in greater proportions in the alloy than the other elements, ensures that it is possible to vary the other elements as much as desired, while guaranteeing the coating excellent oxidation resistance properties and excellent mechanical properties.

[0031] In one embodiment, it is even preferable to reduce the content of elements such as molybdenum and rhenium, which do not play a key role in protecting the substrate from oxidation and which are initially only present in small quantities.

[0032] In one embodiment, it is even preferable to reduce the content of elements such as ruthenium for cost reasons.

[0033] In one embodiment, it is preferable to decrease the titanium content in the coating relative to the titanium content in the y' phase of the substrate as this improves the lifetime of the thermal barrier.

[0034] In one embodiment, it is preferable to increase the tantalum content in the coating relative to the tantalum content in the y' phase of the substrate.

[0035] In one embodiment, it is important to increase the chromium content in the coating relative to the chromium content in the γ' phase of the substrate to ensure good protection against oxidation and corrosion.

[0036] In one embodiment, the substrate comprises a chromium content of less than or equal to 7% and a rhenium content of greater than or equal to 2.5% or even greater than or equal to 4.0%. Such contents are characteristic of the latest generation nickel superalloys which have less chromium and more rhenium than the alloys of the AM1 generation.

[0037] In these new generation alloys, the appearance of secondary reaction zones with the coatings of the prior art is very marked, and the effects of a coating such as described above are all the more interesting.

[0038] Indeed, these secondary reaction zones can be avoided thanks to the coatings described which, in addition to the advantages already described, significantly improves the mechanical properties of the resulting part.

[0039] In one embodiment, the substrate may be chosen from nickel superalloys commercially available under the names CMSX-4 PLUS, MCNG, CMSX-10.

[0040] In one embodiment, the coating comprises a titanium content of less than or equal to 0.1%.

[0041] Indeed, the inventors identified that titanium was not desirable in a coating of the invention, because it can have a deleterious role in the maintenance of the thermal barrier.

[0042] In one embodiment, the coating comprises a sulfur, carbon and oxygen content of less than or equal to less than 50 ppm.

[0043] In one embodiment, the coating is disposed directly in contact with the substrate.

[0044] In one embodiment, the part may further comprise a thermal barrier layer, for example arranged on the coating.

[0045] In one embodiment, the turbomachine part does not comprise any layers other than the substrate, the coating, which is arranged directly in contact with the substrate and possibly the thermal barrier layer, which is preferably arranged directly in contact with the coating.

[0046] This embodiment is particularly advantageous, because it makes it possible to simply obtain a turbomachine part with a very simple constitution.

[0047] In one embodiment, the coating may comprise a platinum mass content of less than or equal to 0.1%.

[0048] In one embodiment, the coating may comprise a cobalt mass content of less than or equal to 0.1%.

[0049] The inventors have further verified that these compositions allow excellent resistance of the thermal barrier on the turbomachine part.

[0050] The coating in fact makes it possible to avoid the appearance of phenomena causing delamination of the thermal barrier while guaranteeing excellent protection of the substrate against corrosion and oxidation, and this with substrates which are by nature more sensitive to corrosion and oxidation than AML.

[0051] In one embodiment, the substrate comprises a volume content of y phase of between 25% and 35% and a complementary volume content of y' phase.

[0052] It is known that the content of the phases varies depending on the temperature. The values ​​given are understood to be at the actual operating temperature of the part, for example between 1000°C and 1200°C.

[0053] By "in a complementary volume content" it is understood that the sum of the volume contents in phase y and in phase y' is equal to 100%, as the substrate is made up of a phase y and in phase y'.

[0054] The coating comprises a y' phase for more than 95% by volume, which allows it to ensure that the coating is not subject to microstructural modifications, and this throughout its lifetime.

[0055] Indeed, the evolution of the phase y' towards y does not create any surface undulation (rumpling) and is also extremely slow, because we are close to equilibrium with the substrate in the conditions encountered in the turbomachine.

[0056] By ensuring that the y' phase initially accounts for more than 95% by volume of the coating, it is guaranteed that the microstructural evolution has very little impact on the structure of the coating.

[0057] In one embodiment, the coating comprises a y' phase for more than 99% by volume, or even consists of a y' phase.

[0058] As indicated above, the more the y' phase is present, the less microstructural evolution of the coating is to be feared.

[0059] Furthermore, the y' phase of the coating makes it possible to ensure better resistance to finishing than a coating comprising other phases, a coefficient of expansion very close to that of the substrate and finally a chemical compatibility with the substrate sufficiently good to avoid the appearance of secondary reaction zones.

[0060] In one embodiment, the thickness of the coating is between 5.0 μm and 100 μm, or even between 20 μm and 50 μm.

[0061] Such a coating thickness is lower than that of prior art coatings for comparable properties, which allows a weight saving for the parts.

[0062] In one embodiment, the average composition of the coating is constant throughout the thickness of the coating.

[0063] In one embodiment, the turbomachine part is a turbomachine blade, and preferably a turbomachine hot part blade.

[0064] Indeed, the invention is very advantageous for such parts in particular because the invention makes it possible to reduce the thickness of the coatings for equivalent resistance properties, and the blades of turbomachines, the geometry of which is constrained for aerodynamic reasons, particularly benefit from this advantage.

[0065] According to another of its aspects, the invention also relates to a method for obtaining an aeronautical turbomachine part as described above.

[0066] Such a method comprises: - a step of coating the external surface of a substrate in a nickel superalloy consisting of a y phase and a y' phase, with a coating covering the substrate, said coating comprising a y' phase for more than 95% by volume, and in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the substrate superalloy.

[0067] In one embodiment, the method may further comprise, after the coating step, a heat treatment for homogenization and improvement of the adhesion of the substrate, between 1000°C and 1200°C under vacuum or under an argon or oxygen atmosphere for 1 hour and 6 hours.

[0068] In one embodiment, the coating step can be carried out by physical vapor deposition (PVD), for example under an electron beam (EB-PVD), by arc or by magnetron, by high-speed flame spraying (HVOF) or by a plasma process.

[0069] All of these processes make it possible to manufacture the coating directly consisting of more than 95% by volume of a y' phase, which ensures a limitation of interdiffusion.

[0070] The very low interdiffusion between the substrate and the coating ensures the absence of secondary reaction zones which ensures obtaining the technical effects described above for the coating, in particular better stability over time of the thermal barrier placed on the coating than that which can be observed for coatings of the prior art.

[0071] Furthermore, these methods ensure that the coating is deposited and is not itself obtained by diffusion of the elements of the substrate towards the coating, which ensures the absence of secondary reaction zones.

[0072] The homogenization treatment, if present, ensures excellent reproducibility of the part obtained. Such a step is not strictly necessary but can be carried out as a precaution to ensure that all the parts produced by the process have an identical thermal history, and that they thus have a homogeneity of composition.

[0073] Optionally, the method further comprises a step of depositing a thermal barrier. The thermal barrier may be deposited on the coating by methods known per se and preferably identical to those used for depositing the coating.

[0074] The method proposed here then makes it possible, unlike the methods of the prior art, to obtain the coating in a single deposition step, carried out in a single machine.

[0075] Conventionally, the deposition methods of the prior art comprise an initial step of electrolytic platinum deposition followed by an aluminization step carried out by chemical vapor deposition which requires changing machines between the two stages.

[0076] This embodiment allows for time savings and improved simplicity of deposition compared to the methods of the prior art.

[0077] The characteristics of the coating, both the composition and the microstructure, ensure excellent resistance over time of the coating and of the thermal barrier placed on the coating.

[0078] According to another of its aspects, the invention relates to an aeronautical turbomachine comprising a part as described above and in which said part is a blade.

[0079] Such a turbomachine can be used at higher temperatures than those of the prior art, which allows it to have better efficiency.

[0080] Indeed, the coated blades as described above offer better resistance to oxidation and better temperature resistance than the blades of the prior art, which overall allows the turbomachine to operate at a higher temperature. Brief description of the drawings

[0081] [Fig.l] [Fig.l] is a schematic representation of a turbomachine.

[0082] [Fig.2] [Fig.2] represents a coated turbomachine blade according to an embodiment of the invention. Description of the embodiments

[0083] The invention is now described by means of figures, present for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter.

[0084] [Fig.l] represents, in section along a vertical plane passing through its main axis A, a double-flow turbojet 1. It comprises from upstream to downstream according to the circulation of the air flow, a fan 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6, and a low-pressure turbine 7.

[0085] In the present application, the relative terms of positioning, for example “upstream”, “downstream”, “internal” and “external”, will be understood in relation to the horizontal axis A of the casing defining the axial direction, traveled in the direction of flow of the main and secondary air flows of the turbomachine.

[0086] Thus, a so-called “upstream” element will be crossed before a so-called “downstream” element and a so-called “internal” element will be closer to axis A than an “external” element.

[0087] [Fig.2] represents a moving blade, which can belong to a low pressure compressor 3, to a high pressure compressor 4, to a high pressure turbine 6, or to a low pressure turbine.

[0088] More specifically, the blade 100 comprises: - a substrate 10 made of a nickel superalloy consisting of a y phase and a y' phase; and - a coating 20 covering the substrate 10, said coating comprising a y' phase for more than 95% by volume, and in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the superalloy of the substrate; and - a thermal barrier layer 30.

[0089] In the embodiment described in [Fig.l] and although this is not necessary, the blade 100 further comprises a thermal barrier 30.

[0090] As described above, such a blade offers better temperature resistance and better grip of the thermal barrier resulting in a longer service life of the part compared to the turbomachine parts of the prior art.

[0091] [Fig.2] further illustrates what is meant by the “thickness” ei of the coating 20.

[0092] This word takes here and in the application its classical meaning, namely the smallest distance that it is necessary to travel to cross the coating from one side to the other.

[0093] In the present application, the y and y' phases of a nickel superalloy will have the usual meaning in the field.

[0094] Nickel-based superalloys can consist of a y-Ni face-centered cubic austenitic phase (or matrix), possibly containing α-substitution solid solution additives (Co, Cr, W, Mo), and a y'-Ni3X type y'-Ni3X phase (or precipitates), with X = Al, Ti or Ta. The y' phase has an ordered Li2 structure, derived from the face-centered cubic structure, consistent with the matrix, i.e. having an atomic mesh very close to it.

[0095] The determination of a phase quantity in a given substrate or coating can be made by methods known to those skilled in the art, for example by X-ray diffraction or by the succession of an analysis step under an EDS microscope to determine the chemical composition and a simulation step carried out on the basis of the exact chemical composition determined by the EDS analysis.

[0096] The simulation is carried out by tools known as such, for example by means of the THERMOCALC ® software.

[0097] In one embodiment, the substrate may be chosen from nickel superalloys commercially available under the names CMSX-4 PLUS, MCNG, CMSX-10.

[0098] As described above, the coating 20 is a nickel alloy and comprises for more than 95% by volume a y' phase.

[0099] In the described mode, the blade 100 further comprises a thermal barrier layer 30 arranged on the coating.

[0100] Preferably, the thermal barrier layer 30 is an external surface of the blade 100.

[0101] In one embodiment, the thermal barrier layer may comprise a layer of yttria-containing zirconia.

[0102] In one embodiment, the thermal barrier layer may comprise a layer of gadolinium zirconate (Gd2Zr2O7).

[0103] In one embodiment the thermal barrier layer may comprise a layer of yttria zirconia and a layer of gadolinium zirconate disposed directly in contact with each other.

[0104] For illustration purposes, the thermal barrier layer is here represented by a single layer 30.

[0105] [Fig.2] illustrates the thickness e2 of the thermal barrier 30.

[0106] In one embodiment, the thickness e2 of the thermal barrier 30 may be between 100 μm and 200 μm.

[0107] As stated above, it is to the credit of the inventors to have understood that a coating whose composition is very close to the y' phase of a nickel superalloy would make it possible to protect it from oxidation without creating surface undulations or secondary reaction zones in the particular case of new generation alloys.

[0108] This ensures stability to the coating throughout its lifetime, because interdiffusion is accelerated when the phases are different between the substrate and the coating, which is not the case here. The only, relatively small, difference in chemical composition between the substrate and the coating is not a sufficient driving force for interdiffusion to be penalizing on the scale of the lifetime of the part considered. Examples

[0109] For the purpose of explaining an embodiment of the invention, composition values ​​of the substrate and the coating are proposed below in a particular embodiment and for a given superalloy.

[0110] It is understood that the invention is not limited to this single superalloy as a substrate and that the approach described can be applied to other superalloys to determine the chemical composition of the y' phase of the substrate to determine the acceptable composition for the coating.

[0111] Starting from an MC-NG substrate, the composition of the y' phase is determined via a first step of determining the general composition of the alloy followed by a numerical simulation step carried out by the THERMOCALC software.

[0112] Starting from the composition of phase y', the inventors proposed depositing a coating whose composition approaches the composition of phase y' while retaining content limits that deviate at most from 1.5% around the composition of phase y'.

[0113] [Table 1] below indicates the composition determined for the entire substrate (line 1) and for the y' phase alone of this superalloy (line 2) and the composition proposed for the coating (line 3).

[0114] In the table below % is intended to indicate that nickel forms the complement to 100 of the composition and — is intended to mean that the element is not present more than in trace form.

[0115] [Tables 1]

[0116] The inventors then found that the coating with the proposed composition was indeed more than 95% made up of y' phase and allowed an increase in the resistance of the alloy to high temperatures.

Claims

Claims

1. Part (100) for an aeronautical turbomachine comprising: - a substrate (10) made of a nickel superalloy consisting of a y phase and a y' phase; - a coating (20) covering the substrate, said coating comprising a y' phase for more than 95% by volume, and in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the superalloy of the substrate.

2. Part (100) for an aeronautical turbomachine according to claim 1, further comprising a thermal barrier (30) arranged on the coating (20).

3. Part (100) for an aeronautical turbomachine according to claim 1 or 2 which only comprises the substrate (10), the coating (20), which is arranged directly in contact with the substrate and where appropriate a thermal barrier (30), arranged directly in contact with the coating.

4. Part (100) for an aeronautical turbomachine according to one of claims 1 to 3, in which the coating (20) comprises a mass content of platinum less than or equal to 0.1%.

5. Part (100) for an aeronautical turbomachine according to one of claims 1 to 4, in which the coating (20) comprises a mass content of cobalt less than or equal to 0.1%.

6. Part (100) for an aeronautical turbomachine according to one of claims 1 to 5, in which the substrate (10) comprises a volume content of phase y of between 25% and 35% and a complementary volume content of phase y'.

7. Part (100) for an aeronautical turbomachine according to one of claims 1 to 6, in which the substrate (10) comprises a chromium content less than or equal to 7% and a rhenium content greater than or equal to 2.5%.

8. Method for obtaining an aeronautical turbomachine part according to one of claims 1 to 7, which comprises: - a step of coating the external surface of a substrate (10) in a nickel superalloy consisting of a y phase and a y' phase, by a coating (20) covering the substrate, said coating comprising a y' phase for more than 95% by volume, and in in which the mass content of each element except chromium and aluminum is equal to plus or minus 2.0% of the mass content of the element in the y' phase of the superalloy of the substrate; and - possibly a heat treatment for homogenization and improvement of the adhesion of the substrate between 1000°C and 1200°C under vacuum or under an argon or oxygen atmosphere for 1 hour and 6 hours.

9. A method according to claim 8, wherein the coating step is carried out by physical vapor deposition (PVD) optionally under an electron beam (EB-PVD), by high velocity oxygen fuel (HVOF) or by a plasma process.

10. Aeronautical turbomachine comprising a part (100) according to one of claims 1 to 7, said part being a blade.

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