EP4192635A1

EP4192635A1 - Protection against oxidation or corrosion of a hollow part made of a superalloy

Info

Publication number: EP4192635A1
Application number: EP21762511.0A
Authority: EP
Inventors: Jérémy RAME; Amar Saboundji; Mirna Bechelany
Original assignee: Safran SA
Current assignee: Safran SA
Priority date: 2020-08-06
Filing date: 2021-08-05
Publication date: 2023-06-14
Also published as: FR3113254B1; FR3113254A1; CN115989102A; US20230304409A1; WO2022029388A1

Abstract

The present document relates to a protection method, for protecting at least one hollow internal region (13, 14) of a turbomachine part (2), made of a superalloy, from oxidation and/or corrosion, said at least one hollow internal region having been formed using at least one core made of ceramic material bounded by a surrounding external surface (26), characterized in that, prior to adding the superalloy around the ceramic core, said external surface is coated with a material comprising at least: – a nanometre-scale layer of hafnium (Hf) and/or – a micrometer-scale layer of platinum (Pt), or – a mixture of at least hafnium and platinum.

Description

DESCRIPTION

Title: Protection against oxidation or corrosion of a hollow superalloy part

Technical field of the invention

The invention relates to the protection against oxidation and/or corrosion of at least one hollow internal zone of a turbomachine part made of a superalloy.

A method of protection is thus concerned, as well as a monocrystalline part of a gas turbine engine for an aircraft made of a superalloy, and a foundry core which can be used to provide the surface of said hollow internal zone of the part with the material necessary for protection against oxidation and/or corrosion.

State of the prior art

Throughout this text, as conventionally, a superalloy, or high performance alloy, is an alloy which exhibits several high characteristics in mechanical strength, resistance to thermal creep deformation, surface stability and resistance to corrosion. or oxidation.

Its crystalline structure is typically face-centered cubic austenitic.

A superalloy includes:

- mainly (more than 50% by mass) a gamma austenitic matrix in which Ni can be substituted by Co, Cr, Mo, W, as well as by Nb, Al, Ti, Ta, Fe

- intermetallic precipitates ordered gamma': Ni3(Ti,Al) or gamma » Ni3Nb which occupy 30 to 70% of the volume and whose dimensions vary between 10 nm and a few micrometers, and

- primary (MC type) and secondary (M23C6 type) carbides, preferentially precipitated at the grain boundaries.

Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene, Incoloy, MP98T, TMS alloys and single crystal CMSX alloys.

FIG. 1 illustrates a curve of stress (MPa) as a function of temperature (°C) for various materials which can be used on a gas turbine engine for aircraft, including superalloys.

Nickel-based (Ni) superalloys are particularly targeted in this text.

These are gamma/gamma alloys. A nickel-based alloy is defined as an alloy in which the mass percentage of nickel is predominant.

In particular, a nickel-based superalloy comprising, in mass percentages, 5.0 to 6.0% aluminum, 6.0 to 9.5% tantalum, 0 to 1.50% titanium, 8.0 10.0% cobalt, 6.0-7.0% chromium, 0.30-0.90% molybdenum, 5.5-6.5% tungsten, 0-2.50% rhenium, 0.05 to 0.15% hafnium, 0.70 to 4.30% platinum, 0 to 0.15% silicon, the balance consisting of nickel and inevitable impurities. Unavoidable impurities are defined as elements which are not intentionally added to the composition and which are added with other elements.

Among the unavoidable impurities, mention may in particular be made of carbon (C) or sulfur (S). Another example of a nickel-based superalloy includes, in mass percentages, 6 to 8% aluminum, 12 to 15% cobalt, 4 to 8% chromium, 0 to 0.2% hafnium, 0.5 4% molybdenum, 3.5-6% rhenium, 4-6% tantalum, 1-3% titanium, 0-2% tungsten, 0-0.1% silicon, balance consisting of nickel and inevitable impurities.

Yet another example is a nickel-based superalloy comprising, in mass percentages, 4.0 to 5.5% rhenium, 3.5 to 12.5% cobalt, 0.30 to 1.50% molybdenum, 3.5-5.5% chromium, 3.5-5.5% tungsten, 4.5-6.0% aluminum, 0.35-1.50% titanium, 8.0-10 .5% tantalum, 0.15 to 0.30% hafnium, preferably 0.17 to 0.30% hafnium, 0.05 to 0.15% silicon, the remainder consisting of nickel and unavoidable impurities.

The nickel-based superalloys targeted here are thus in particular those intended for the manufacture of single-crystal gas turbine components, such as fixed or moving blades.

Other types (phases) of superalloys include cobalt (Co) based superalloys.

For superalloys operating at elevated temperatures and exposed to corrosive environments, oxidation and/or corrosion behavior is a concern. Indeed, this involves chemical reactions of the alloying elements with oxygen to form new oxide phases, generally at the surface of the metal. If left unmitigated, oxidation and/or corrosion can degrade the alloy over time in a number of ways, including:

- sequential oxidation, cracking and spalling of the surface, leading to erosion of the alloy over time,

- embrittlement of the surface by the introduction of oxide phases favoring the formation of cracks and fatigue failure,

- depletion of essential alloying elements, affecting the mechanical properties of the superalloy and possibly compromising its performance.

Presentation of the invention

The object of the invention is to provide a solution to this problem of behavior to oxidation and/or corrosion and of protection to be provided, in particular when the area to be protected is difficult to access.

In particular, a protection solution is offered which combines:

- the use of a manufacturing technique using a material core comprising a ceramic, metal, or a hybrid ceramic/metal material allowing the manufacture of hollow parts,

- and the use of a protective coating based on hafnium (Hf) and/or platinum (Pt), or even chromium, and/or silicon and/or Yttrium, and/or their mixtures.

Summary of the invention

More specifically, a protection method is proposed here, to protect from oxidation and/or corrosion at least one hollow internal zone of a turbomachine part made of a superalloy, said at least one hollow internal zone having been formed, by means of at least one core in a material comprising a ceramic or metal or a hybrid metal and ceramic material, and limited by an outer surface which surrounds it, characterized in that before bringing the superalloy around the core, said outer surface is coated with a coating material comprising hafnium (Hf), and/or platinum (Pt), and/or chromium (Cr) and/or silicon (Si) and / or Yttrium (Y), or a mixture thereof.

Combining the material with a technique of coating the core and then, through it, adding it to the part has proven to be effective in terms of efficiency.

In this regard, it is also proposed that the coating material with which said outer surface is coated should favorably include:

- an at least nanometric layer containing hafnium, or else hafnium is finally present between 0.3 and 5% m, or even 15% m, at the surface of the hollow internal zone, in the superalloy, and/or

- an at least micrometric layer containing platinum, or that platinum is finally present between 10 and 80% m at the surface of the hollow internal zone, in the superalloy, or

- a mixture of at least hafnium and platinum, over a thickness of at least micrometric, which can usefully be between 1 μm and 100 μm, and/or

- at least one layer containing Cr and/or Si and/or Y, over a thickness at least nanometric, which can usefully be between 20 nm and 100 μm, or that chromium is present between 2 and 30% m at the surface of the superalloy of the hollow internal zone of the final part, and/or that silicon is present between 0.2 and 10% m at the surface of the superalloy of the hollow internal zone of the final part, or that Yttrium is finally present between 0.3 and 15% m on the surface of the superalloy of the hollow internal zone of the final part.

For all purposes, it is specified that in this text "%m surface of the superalloy" will indicate in this case the percentage by mass of the element in the total mass of superalloy thus loaded, after diffusion of the core towards the part of any or part of the reactive elements considered (Hf, Pt, Cr, Si, Y) or their at least partial mixture. Preferably, the hafnium will be in the majority in %m in the possible at least nanometric layer containing hafnium, the same for platinum in the possible at least micrometric layer containing platinum, and for Cr and/or Si and/or Y in the or their layer.

In the present text "majority" means that it is the main constituent in %m in the layer. There may be more than 50%.

In the coating material with which said outer surface of the core is coated:

- if a Cr layer is provided, its thickness may be less than 10 pm, or even less than 2 pm, and/or

- If a layer of Si is provided, its thickness may be limited between 50 and 500 nm, and more preferably between 100 and 200 nm.

In the above hypothesis of contribution of at least some of the aforementioned reactive elements (Hf, Pt, Cr, Si, Y), or their at least partial mixture, it is even proposed, to further optimize the efficiency of the solution:

- that the at least nanometric layer of hafnium, with which said outer surface is coated with the coating material, is present, on the surface of the core, over a thickness of between 50 nm and 800 nm, or that hafnium is finally present between 0.3 and 15% m on the surface of the hollow internal zone of the part, in the superalloy, and/or

- that the at least micrometric layer of platinum has a thickness of between 1 μm and 5 μm on the outer surface of the core, or that platinum is present between 15 and 60% m on the surface of the superalloy of the hollow internal zone of the final part , and or

- that said at least one layer containing Cr and/or Si and/or Y has a thickness of between 30 nm and 10 μm, or else that chromium is present between 4 and 10% m at the surface of the superalloy of the hollow internal zone of the final part, or that silicon is present between 0.2 and 2% m on the surface of the superalloy of the hollow internal zone of the final part, or that Yttrium is present between 0.3 and 15% m on the surface of the superalloy of the hollow internal zone of the final part.

After having coated said outer surface of the core with the selected coating material, the molten superalloy will advantageously be brought into contact with said coated outer surface.

Thus, part of the invention consists in using a core as mentioned above as a source of local modification of the chemistry of the alloy of the part, during a hollow part manufacturing process, advantageously according to the technique of molten wax (or lost wax), for example for forming cooling channels in a turbine engine blade for an aircraft.

Among the advantages of using such a core as a substrate for producing a protective coating for the internal cavity of a hollow part, in particular blade cavities hollow, we can raise

- that the cavity has a modified surface chemistry to increase the resistance to oxidation and corrosion of the material,

- that the coating can be uniform inside the cavity,

- that the traditional lost wax casting process is not necessarily modified,

- that no post-casting deposition process is absolutely necessary to produce this coating.

An objective here is thus to adapt the chemical composition of the superalloy on the surface in order to increase the resistance to the environment of an internal part of a hollow part, such as an internal cavity of a hollow turbine blade.

The solution proposed here allows this, it being specified that: nanometric (nm; 10' ⁹ m) has the meaning: thickness 10nm < e <1000nm, micrometric or micronic (pm; 10' ⁶ m) has the meaning: thickness 0.5pm < e < 1000 pm. To promote the attachment of said layer(s) to the relevant surface of the core, it is further proposed that after having coated said outer surface of the core with the coating material, but before bringing the superalloy around this core, the coating of said outer surface is diffused between 800° C. and 1250° C., under high vacuum.

A secondary vacuum is defined as a space where there is a pressure of less than 1 Pa, for example a pressure of approximately 10 ⁻¹ Pa, to within 10%.

Another difficulty encountered concerned the reactivity between the liquid metal of the part to be manufactured and the elements deposited on the surface of the core. A suitable deposition method, and/or a diffusion heat treatment should make it possible to overcome this difficulty. It is therefore further proposed that, the addition of the superalloy around the core comprising a solution treatment of the superalloy, a diffusion of the coating material into the core is initiated by heat treatment during said solution treatment of the superalloy.

The solution treatment will consist of heating the alloy to an appropriate temperature, maintaining this temperature long enough to cause the transformation of one or more constituents into a solid solution and cooling it fast enough to maintain these constituents in the solution. Possible heat treatments by subsequent precipitation already make it possible to control the release of these constituents in a natural (ambient temperature) or artificial (higher temperatures) state.

The heating temperature of the superalloy for the solution treatment may be between 1100°C and 1375°C, depending on the alloy.

As advantages, it may be noted that these solution heat treatment, or solution heat treatment and hardening by precipitation ageing, should make it possible to improve the characteristics such as the mechanical strength at ambient and/or high temperature ( over 600°C), corrosion resistance and oxidation resistance. In a preferred application linked to vanes (fixed vanes, also called distributors or rectifiers, or moving vanes, in particular monocrystalline) of an aeronautical turbine or compressor, it is moreover proposed that the superalloy be nickel-based.

With such a material, one expects the advantage of combining high creep resistance at high temperature as well as resistance to oxidation and corrosion.

Nickel-based superalloys are indeed materials with an austenitic nickel-based matrix y (cubic with centered faces, therefore rather ductile) reinforced by hardening precipitates y' (of structure also CFC, but of ordered atomic nature) coherent with the matrix, that is to say having an atomic unit very close to this one.

Compound y' of Nis(Al,Ti) formulation also has, due to its ordered nature, the remarkable property of having a mechanical resistance which increases with temperature up to approximately 800°C. The very strong coherence between y/y' confers a very high hot mechanical strength of nickel-based superalloys, which itself depends on the rate of hardening precipitates, which has led to:

- alloys with high resistance up to 700°C, but whose resistance decreases sharply above 800°C, which makes them suitable for hot forging (above 1000°C),

- alloys with intermediate resistance up to 700°C and good mechanical resistance to very high temperatures (up to 1100°C). These alloys are used in precision casting.

However, the efficiency of an (aeronautical) gas turbine is highly dependent on its operating temperature, this temperature being limited by the hot resistance of the materials of which it is composed. Nickel-based superalloys are currently the materials of choice for the hot parts of gas turbines, located in particular at the outlet of the combustion chamber. These materials have the advantages of combining both high creep resistance at high temperature and satisfactory resistance to oxidation and corrosion. Certain grades of nickel-based superalloys are thus used for the manufacture of fixed monocrystalline parts (such as distributors, ring segments) or mobile parts (such as turbine blades). The development of new superalloy grades with the aim of improving the mechanical properties at high temperature has led, over the years, to a significant reduction in the chromium content. Thus, for example, the first generation AM1 alloy contained 7.5 wt% Cr, the second generation CMSX-4 contained 6.5 wt%, and the corresponding third generation alloy, called CMSX-10, contained 2% m of Cr. The decrease in the concentration of this element, ensuring the resistance to oxidation and corrosion of superalloys, has led to greater sensitivity of superalloys to the environment, requiring increased use of a protective coating. Coatings can thus be used in order to improve the resistance to the oxidizing and/or corrosive environment of the combustion gases and to act as a thermal insulator in order to reduce the temperature seen by the superalloy substrate. This is particularly the case for the protection of the external parts of high-pressure turbine blades subjected to high stresses and temperatures.

Coatings are usually made up of two layers. The first layer, deposited on the surface of the alloy then diffused, generally called bonding layer or sub-layer, is composed of an aluminoforming alloy, for example an alloy of the MCrAlY type (M = Ni and/or Co) or indeed a platinum-modified nickel aluminide. This layer can have two essential roles. The first is to protect the superalloy from oxidation and corrosion when using this coating alone. The second can be to ensure the adhesion of a second layer, generally called a thermal barrier, in the case of the use of a porous coating made up of a ceramic (for example of yttria zirconia).

The aforementioned blades of aeronautical turbomachines can be hollow in order to be able to be cooled via the use of internal channels.

As already indicated, the cooling channels can be obtained during the process for producing such a blade by the use of cores as proposed here, therefore containing at least one ceramic or metal or a hybrid material (composite) metal and ceramic, and having for example the shape of the cooling channels that one wishes to obtain.

As a core containing ceramic, mention may be made of a core consisting mainly of amorphous silica (−80% by mass, to within 10%) and of cristobalite (−20% by mass, to within 10%). Different elements can be added depending on the desired properties such as alumina, zirconia, oxides or alkaline ions (CaCOs or MgO ₂ ).

Core heat treatment cycles can be performed such as debinding and sintering (T~1200°C, within 10%).

Once the core is ready, the metal of the part to be produced (here the selected superalloy) can then be poured into a mould, called a shell, so as to surround the core. The core is then dissolved, resulting in the expected part, such as a hollow blade structure. In the case of such a blade, its hollow parts are therefore exposed to the environment, and can be all the more sensitive to this environment if the alloy used for the manufacture of the blade is a latest generation alloy. containing a small amount of chromium.

It may therefore be necessary, as mentioned above, to deposit a coating inside the cavity(ies) of the blade in order to protect the latter from this environment. However, if depositing a protective coating on a blade, and more generally on a superalloy turbomachine part, is known, this is only so to protect the outer part of the part, in this case the blade. The implementation method is in fact not suited to coating the inside of a hollow part, in particular of a blade, which may have a width of a few hundred microns. The use of the usual deposition processes, such as physical vapor deposition (PVD), electrodeposition or chemical vapor deposition (CVD) proves difficult for obtaining a suitable protective coating.

Also, a part having the following characteristics is not a priori known.

Consequently, in addition to the process that has just been presented, the invention also relates to a single-crystal part of a gas turbine engine for aircraft made of a superalloy, the part having:

- at least one hollow internal zone and,

- on at least part of the surface which delimits said hollow internal zone, a coating limited to a non-zero depth less than or equal to 1 mm and comprising a concentration of hafnium, and/or platinum, and/or chromium and/ or silicon and/or yttrium, or a mixture thereof.

Given the above and the quality and efficiency of the manufacturing process (diffusion via a core, as mentioned above), the expected protection against oxidation and/or corrosion of this hollow internal zone may even be such that, in the final part obtained, the surface concentration of hafnium, and/or platinum, and/or chromium and/or silicon and/or Yttrium, or one of their mixtures in the superalloy, at the location of its outer coating, is limited at a non-zero depth, less than or equal to 0.5mm.

For further optimized efficiency of the compositions of the protective elements on the surface of the final part, it is recommended that the concentration, on the surface, in the superalloy be:

- between 0.3 and 10 or even 15%m, and preferably between 0.4 and 4.5%m, for hafnium, and/or

- between 10 and 90% m, and preferably between 15 and 60% m, for platinum, and/or

- between 2 and 30% m for chromium, and/or

- between 0.2 and 10%m for silicon, and/or

- between 0.3 and 10 or even 15% m for Yttrium.

In terms of application and again taking into account the above, it may be useful to provide that the part obtained:

- defines a fixed or mobile turbine blade of the turbomachine in which said at least one hollow internal zone is an internal channel of the blade communicating with the outside and adapted to receive a fluid in order to cool the blade internally, and /Where

- that its superalloy is nickel-based. Brief description of figures

[Fig. 1] represents a curve of stress (MPa) as a function of temperature (°C) for various materials, including superalloys,

[Fig. 2] schematically represents, in a very local section, the diffusion of the coating applied to the core, which has therefore partially diffused in the zone (closest to the core/part interface) of the superalloy part

[Fig. 3] schematically represents part of a hollow aircraft turbine engine blade, [Fig. 4] represents a section along IV-IV of Figure 3,

[Fig. 5] schematically represents a part of the core for the aforementioned hollow blade, and [Fig. 6] schematically represents a variant of the enlarged surface area of the core illustrated in FIG. 5.

Detailed description of the invention

The following description, provided by way of non-limiting example, relates to a fixed or moving blade of a turbomachine turbine for an aircraft.

As explained in EP1754555, such a blade can be obtained by casting a molten alloy in a mold using the lost wax casting technique.

To achieve in particular, inside the blade, at least one internal cavity for the circulation of a cooling fluid (typically air), the internal core (around which the material of the blade) will comprise a ceramic material or metal or a hybrid metal and ceramic material.

The core can thus have a porous structure and be made from a mixture consisting of a refractory filler in the form of particles and a more or less complex organic fraction forming a binder. Examples of compositions are given in patents EP 328 452, FR 2 371 257 or FR 2 785 836.

As an example of a ceramic composition of the core, mention may be made of a composition advantageously resulting from a mixture of silica powder, such as fused or vitreous silica, zircon and others, such as favorably cristobalite, alumina or zirconia. Examples of ceramic compositions can be found in US patent 5043014. In particular, it is a mixture of silica, zircon and cristobalite, particularly in respective proportions of 70-80/15-25/1-5 in % mass, even more particularly respective proportions in mass % of 77/20/3. Silica powder can have different grain sizes.

As an example of a metal composition, mention may be made of a foundry core made of a refractory metal alloy, which may typically be a molybdenum alloy. Such a refractory metal degrading easily under an oxidizing atmosphere and being soluble in the superalloy, it may therefore be necessary to protect the metal against oxidation and erosion. This protection will be favorably ensured by a metallic and/or ceramic multilayer coating with specific properties: antioxidant, anti-erosion, diffusion barrier... or other. In general, as material, marked 28 in figure 6, for protection against the oxidation of molybdenum and its alloys, silicides are recommended here (MoSi2, up to 1600°C or MoSi2 + Cr, Cr-B, Cr-B-Al, Sn-Al) and silicide complexes (SiCrFe, up to 1500°C). There are others based on aluminides, ceramics (Al2O3, ZrO2 + HfO2/ Y2O3, Al-Cr, Al-Si, Sn-Al) and metals (Cr, Ni, noble metals, alloys.. .) produced by various techniques (CVD - Chemical Vapor Deposition -, PVD - Physical Vapor Deposition -, Plasma, etc.).

The aforementioned coating proposed by the invention and referenced 22 below will, in the case of such a core (with a metal core), be added either over the protective material mentioned above, or directly on the (core) du) metal core itself, if it has not been previously coated with such a protective material.

As an example of a bi-material hybrid core, mention may be made of a core consisting of a first material mainly based on silica/zircon (more precisely the core of the core) for example obtained by injection, machining or additive manufacturing and a second material containing the reactive elements (surface of the core) and which can be obtained by overinjection or additive manufacturing (projection of drops of material or fusion of wire through a heating nozzle).

Whatever the choice of core retained (coated core or not), once it has been manufactured, it will, according to the invention, be covered with the proposed anti-oxidation and/or anti-corrosion protective coating; after which the superalloy can be molded on the core covered with the protective coating of the invention, and thus protect the internal parts of certain aeronautical turbomachine parts made of a superalloy from oxidation and/or corrosion, such as in particular dawns.

In one aspect, the invention therefore consists in having used a core coated with reactive elements as a source of local modification of the chemistry of the superalloy, the objective having been to adapt the chemical composition of the superalloy on the surface in order to increase the resistance to the environment of the internal part of the part concerned: the internal cavity(ies) of a blade, in the preferred example selected.

To produce these "reactive elements", we will therefore have, before bringing the superalloy around the core, coated the outer surface of this core, marked 26 figures 5,6 and which is adapted to the shape of the final part 7 to be manufactured. , with a coating material 22 comprising at least hafnium (Hf), and/or platinum (Pt), and/or chromium and/or silicon and/or yttrium, or a mixture thereof .

Regarding the core 20 itself, its core 24 therefore contains a ceramic or metal or a metal/ceramic hybrid material. Examples of ceramic composition, metallic composition and hybrid (or bi-material) ceramic/metallic composition of the core 24 of the core 20 have been presented above and are among the most appropriate. As already explained, around this core 24, possibly already protected by a first protective coating 28, a notable increase in the surface resistance to the environment of the final part 2 (see FIGS. 2-4), therefore of the chemistry surface of the superalloy 40 which constitutes it (essentially; see FIG. 2), was noted by coating the outer surface of the core 20 with, as outer coating 24, a material comprising (possibly with a mixture of the elements below) :

- at least one at least nanometric layer of hafnium (Hf), and/or

- at least one at least micron layer of platinum (Pt), or

- at least one at least nanometric layer containing Cr and/or Si and/or Y, the preferred thicknesses or %m on the surface of the superalloy of the hollow internal zone of the final part having already been specified, further in the text.

To combine mechanical performance and optimization of the quantities and the process of implementation, it may however be preferred, as also already specified:

- that the thickness e1 of the Hf layer is 20 nm < e1 < 900 nm , and even either 50 nm < e1 < 500 nm, and/or

- that the thickness e2 of the Pt layer be 1 pm < e2 < 15pm, and even be 1 pm < e2 < 5pm. Depositing between 1 μm and 5 μm platinum and 0.5 μm of Hf (within 10%) has proven to be a particularly relevant solution, given the intended goals.

Given the nature of the aforementioned coating to be deposited (reference 1 in the figures), the elements mentioned can be deposited by one or more processes, as follows:

- the Pt and/or: Hf, possibly Cr, Si, Y layers or elements (alone or in a mixture) can be produced in the same deposition machine and be deposited by one physical vapor phase (PVD) processes such as: EBPVD, Joule evaporation, pulsed laser ablation or cathode sputtering,

- the Pt layer or element can be deposited by electrolytic deposition on condition that the composition of the core is doped with electrically conductive elements, such as metal or carbon.

- The Hf and/or Cr, Si or Y layers or elements can be deposited by Chemical vapor deposition, in French chemical vapor deposition; CVD (PECVD, LPCVD, UHVCVD, APCVD, ALCVD, UHVCV...).

After the coating deposition(s) carried out on the surface of the core, a diffusion treatment can be carried out in order to cause its aforementioned coating material(s) to diffuse into the core, and thus promote the supply profitable of all or part of these elements. Provision can be made for this diffusion treatment in the core to be carried out during the dissolution of the superalloy, which can take place during a heat treatment.

The temperatures to promote the diffusion of the aforementioned reactive elements Pt and/or Hf, Cr, Si, Y will be favorably between 800° C. and 1250° C., under secondary vacuum, typically 10' ⁶ X 10 ⁵ Pa, at 10% close.

That there was a diffusion step, towards the inside of the core, of the aforementioned layer(s) or elements, is during the casting of the superalloy of the part to be manufactured around the core enriched in surface by its said coating that the superalloy will be able to react with the aforementioned components Hf, and/or Pt, and/or Cr and/or Si and/or Y.

This casting of the superalloy of the part to be manufactured around the core could be favorably followed by a heat treatment in order to best promote the diffusion of the coating component(s) of the core, shown schematically in FIGS. 5-6, towards the superalloy of the piece, marked 2 in the illustration figures.

The conditions may be the same as above: between 800° C. and 1250° C., to within 10%, under secondary vacuum, typically 10 ⁻¹ Pa, to within 10%.

FIG. 2 schematically illustrates the effect of the enrichment/diffusion on the aforementioned coating 1, which coating has therefore, in the figure, partially diffused in the upper zone (closest to the internal surface 2a) of the part 2 in superalloy.

We have identified in 3, the limit or the interface that could be considered to exist between the superalloy 40 itself and the coating 1 in the event that there would be no diffusion heat treatment.

If there has therefore been enrichment with diffusion, we will find, in the direction of the thickness e of part 2, and starting from its internal surface 2a:

- first a first layer 4 of the coating 1, not or relatively little diffused, consisting very mainly of the contribution or enrichment element(s) in Pt and/or Hf, Cr, Si, Y transmitted to the part 2 during the casting of the superalloy 40 on the core 20 coated in whole or in part with these same elements (identified globally 22 in FIG. 5 or 6),

- then, more deeply, a layer 5 of said added or enrichment elements, intimately mixed with the superalloy 40; this is the layer where said elements have (further) diffused into the superalloy, forming the most deeply embedded part of the coating 1 itself for protection against oxidation and/or corrosion,

- then a zone 6 (which sinks into the part) formed of the actual mass of superalloy 40.

Figure 5 or 6, the coating 22 on the outer surface 26 of the core, or model (of the blade in the example), 20 has been identified as clearly dissociated from the base material 24, or core, of this core, whereas the separation is not actually so clear cut. Concerning the heat treatment of the superalloy in solution, it will be noted that the as-solidified nickel-based superalloys can be heat-treated to obtain the desired distribution and size of the different phases. The first heat treatment (T) may be a treatment for homogenizing the microstructure, the purpose of which is to dissolve the y′ phase precipitates and to eliminate the y/y′ eutectic phases or to significantly reduce their volume fraction. This treatment is carried out at a temperature above the solvus temperature of the y' phase and below the starting melting temperature of the superalloy (Tsolidus). Quenching can then be carried out at the end of this first heat treatment to obtain a fine and homogeneous dispersion of the precipitates y′.

Tempering heat treatments can then be carried out in two stages, at temperatures below the solvus temperature of the y' phase: During a first stage (R1 ), to enlarge the y' precipitates and obtain the desired size , then during a second step (R2), to increase the volume fraction of this phase to about 70% at room temperature.

Example of heat treatments:

Superalloy AM1:

Treatment at 1300°C for 3 hours under partial pressure of argon or under vacuum followed by gas quenching (argon).

R1: 1100°C for 5 h in air, R2: 870°C for 16 h in air Superalloy CMSX-4:

Treatment in stages from 1277°C to 1321°C in 16 hours and stage of 2 hours at 1321°C under partial pressure of argon or under vacuum followed by gas quenching (argon).

R1: 1100°C for 4 h in air R2: 870°C for 20 h in air. FIG. 3 schematizes an example of a hollow blade, of the “bathtub” type, but the presence or absence of such a “bathtub” (cavity at the top of the blade open radially outwards) is irrelevant. It is on the other hand a hollow blade 2. In the figure, one can identify the foot 8 of the blade by which it is mounted on a turbine rotor, the platform 9 and the blade 10. The blade 10 is hollow (see section in FIG. 4) and comprises at its top opposite to the platform, the bath 7. The bath is delimited laterally by the wall of the blade and the bottom is formed by a wall 11 of the bottom of the bath , perpendicular to the radial axis of the blade. This bottom wall 11 which can be seen in section in FIG. 4 is traversed by orifices 12 which communicate with the internal cavities 13, 14 of the blade in order to evacuate part of the cooling fluid from the latter. This fluid is itself discharged into the stream of hot gas by the gap existing between the top and the annular surface of the stator located radially opposite.

The solution of the invention will therefore have made it possible to protect the internal surfaces 2a of these cavities 13, 14 by having locally enriched in Pt and/or Hf, possibly Cr, and/or Si, and/or Y, the internal surface 2a of the superalloy 40 in which the blade 2, and in this case at least the hollow blade 10, is made.

Finally, it will be noted that the invention has enabled:

- to define the elements and quantities to be deposited on the final part, in particular in the case of turbomachine blade channels to protect them from oxidation/corrosion, - to use a suitable deposition method to deposit these desired elements at the surface of intermediate supply cores,

- to carry out a heat treatment suitable for the diffusion of the desired elements from the core to the surface of the metal of the part to be surface-enriched in order to protect it.

Claims

1 . Protection method, for protecting against oxidation and/or corrosion at least one hollow internal zone (13, 14) of a turbomachine part (2) made of a superalloy, said at least one hollow internal zone having been formed, via at least one core (20):

- in a material (24) comprising a ceramic or metal or a hybrid metal and ceramic material, and

- limited by an outer surface (26) which surrounds it, characterized in that before bringing the superalloy around the core, said outer surface is coated with a coating material (22) comprising hafnium (Hf) , and/or platinum (Pt), and/or chromium (Cr) and/or silicon (Si) and/or Yttrium (Y), or a mixture thereof and before bringing the superalloy around of the core (20,24), said coating material is caused to diffuse into the core between 800°C and 1250°C, under a pressure of less than 1 Pa.

2. Method of protection according to claim 1, wherein the coating material (22) with which said outer surface is coated comprises:

- an at least nanoscale layer containing hafnium (Hf), or else hafnium is present between 0.3 and 15 %m at the surface (2a) of the hollow internal zone, in the superalloy, and/or

- an at least micrometric layer containing platinum (Pt), or else platinum is present between 10 and 80% m at the surface (2a) of the hollow internal zone, in the superalloy, and/or

- a mixture of at least hafnium (Hf) and platinum (Pt), over a thickness of at least micrometric, and/or

- at least one layer containing Cr and/or Si and/or Y over a thickness of at least nanometric, or chromium is present between 2 and 30% m at the surface of the superalloy of the hollow internal zone of the final part, or either silicon is present between 0.2 and 10% m at the surface of the superalloy of the hollow internal zone of the final part, or else yttrium is present between 0.3 and 15% m at the surface of the superalloy of the zone internal hollow of the final part.

3. Method of protection according to claim 2, in which:

- the at least nanometric layer of hafnium with which said outer surface is coated with the coating material (22) has a thickness of between 50 nm and 800 nm, or hafnium is present between 0.3 and 5% m on the surface (2a) of the hollow internal zone, in the superalloy, and/or - the at least micrometric layer of platinum has a thickness of between 1 μm and 5 μm on the outer surface (26) of the core, or platinum is present between 15 and 60% m on the surface (2a) of the hollow internal zone, in the superalloy

- said at least one layer containing Cr and/or Si and/or Y has a thickness of between 30 nm and 10 μm, or chromium is present between 4 and 10% m at the surface of the superalloy of the hollow internal zone of the part final part, or else silicon is present between 0.2 and 2% m at the surface of the superalloy of the hollow internal zone of the final part.

4. Method of protection according to any one of the preceding claims, in which, after having coated said outer surface (26) of the core with the coating material, the molten superalloy (40) is brought into contact with said coated outer surface. .

5. Method of protection according to any one of the preceding claims, in which, the addition of the superalloy around the core (20,24) comprising a dissolution of the superalloy, a diffusion of the coating material (22) is initiated in the core (20) during said dissolution of the superalloy (4).

6. Protection method according to any one of the preceding claims, in which the superalloy is nickel-based.