PROTECTION AGAINST OXIDATION OR CORROSION OF A HOLLOW PART MADE OF A SUPERALLOY

Abstract

A protection method protects at least one hollow internal area of a turbine engine part made of a superalloy from oxidation and/or corrosion, wherein the at least one hollow inner area has been formed by means of at least one core made of a ceramic material limited by an external surface that surrounds it. Before bringing the superalloy around the core made of a ceramic material, the external surface is coated with a material that includes a nanometric layer of hafnium (Hf), and/or a micrometric layer of platinum (Pt), or

a mixture at least of hafnium and platinum.

Claims

1. A protection method, for protecting at least one hollow internal area of a turbine engine part made of a superalloy from oxidation and/or corrosion, said at least one hollow inner area having been formed by means of at least one core: made of a material comprising a ceramic and/or metal or a metal and ceramic hybrid material, and limited by an external surface which surrounds the at least one core, the method comprising the steps of: before bringing the superalloy around the core, coating said external surface 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; before bringing the superalloy around the core, diffusing said coating material in the core between 800° C. and 1,250° C., under a pressure lower than 1 Pa; and after coating said external surface of the core with the coating material, bringing the molten superalloy in contact with said coated external surface.

2. The protection method according to claim 1, wherein the coating material with which said external surface is coated comprises: a layer at least nanometric containing hafnium (Hf), or wherein hafnium is present between 0.3 and 15 w % at the surface of the hollow inner area, in the superalloy.

3. The protection method according to claim 2, wherein: the layer at least nanometric of hafnium with which said external surface is coated with the coating material has a thickness between 50 nm and 800 nm, or hafnium is present between 0.3 and 5 w % at the surface of the hollow inner area, in the superalloy.

4. (canceled)

5. The protection method according to claim 1, wherein bringing the superalloy around the core comprises dissolving the superalloy, wherein a diffusion of the coating material in the core is initiated when dissolving the superalloy.

6. The protection method according to claim 1, wherein the superalloy is nickel-based.

7. The protection method according to claim 1, wherein the coating material with which said external surface is coated comprises: a layer at least micrometric containing platinum (Pt), or wherein platinum is present between 10 and 80 w % at the surface of the hollow inner area, in the superalloy.

8. The protection method according to claim 1, wherein the coating material with which said external surface is coated comprises: a mixture of at least hafnium (Hf) and platinum (Pt), across a thickness at least micrometric.

9. The protection method according to claim 1, wherein the coating material with which said external surface is coated comprises: at least one layer containing Cr and/or Si and/or Y across a thickness at least nanometric, or chromium is present between 2 and 30 w % at the surface of the superalloy of the hollow inner area of the final part, or silicon is present between 0.2 and 10 w % at the surface of the superalloy of the hollow inner area of the final part, or Yttrium is present between 0.3 and 15 w % at the surface of the superalloy of the hollow inner area of the final part.

10. The protection method according to claim 2, wherein: the layer at least micrometric of platinum has a thickness between 1 μm and 5 μm at the external surface of the core, or platinum is present between 15 and 60 w % at the surface of the hollow inner area, in the superalloy, said at least one layer containing Cr and/or Si and/or Y and having a thickness comprised between 30 nm and 10 μm, or chromium is present between 4 and 10 w % at the surface of the superalloy of the hollow inner area of the final part, or silicon is present between 0.2 and 2 w % at the surface of the superalloy of the hollow inner area of the final part.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0088] FIG. 1 represents a curve of stress (MPa) as a function of temperature (° C.) for different materials, including superalloys,

[0089] FIG. 2 schematically represents, in a very local section, the diffusion of the coating applied over the core, which has therefore partially diffused in the area (the closest to the core/part interface) of the part made of superalloy

[0090] FIG. 3 schematically represents a portion of a hollow blade of an aircraft turbine engine,

[0091] FIG. 4 represents a section according to IV-IV of FIG. 3,

[0092] FIG. 5 schematically represents a portion of the core for the aforementioned hollow blade, and

[0093] FIG. 6 schematically represents a variant of the enlarged area of the surface of the core illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

[0094] The following description, provided as a non-limiting example, relates to a fixed or movable blade of a turbine engine turbine for an aircraft.

[0095] As explained in EP1754555, such a blade can be obtained by casting a molten alloy in a mold according to the lost wax casting technique.

[0096] In particular, to make inside the blade at least one inner cavity for the circulation of a cooling fluid (typically air), the inner core (around which the material of the blade will be cast afterwards) will comprise a ceramic material and/or metal or a metal and ceramic hybrid material.

[0097] Thus, the core can have a porous structure and be made from a mixture consisting of a refractory charge in the form of particles and a more or less complex organic fraction forming a binder. Examples of compositions are given in the patents EP 328 452, FR 2 371 257 or FR 2 785 836.

[0098] As an example of a ceramic composition of the core, mention may be made of a composition advantageously derived from a mixture of silica powder, such as molten or vitreous silica, zircon and others, such as favorably cristobalite, alumina or zirconia. Examples of ceramic compositions can be found in the U.S. Pat. No. 5,043,014. In particular, it consists of a mixture of silica, zircon and cristobalite, particularly in respective proportions of 70-80/15-25/1-5 in % by weight, even more particularly respective proportions in % by weight of 77/20/3. The silica powder may have different grain-size distributions.

[0099] 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 might therefore be necessary to protect the metal against oxidation and erosion. This protection will be favorably ensured by a metal and/or ceramic multilayer coating with specific properties: antioxidant, anti-erosion, diffusion barrier . . . inter alia. In general, as a material, marked 28 FIG. 6, for protection against oxidation of molybdenum and its alloys, silicides are recommended herein (MoSi2, up to 1,600° C. or MoSi2+Cr, Cr—B, Cr—B—Al, Sn—Al) and silicide complexes (SiCrFe, up to 1,500° 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 . . . ) made by various techniques (CVD—Chemical Vapor Deposition—, PVD—Physical Vapor Deposition—, Plasma . . . ).

[0100] The aforementioned coating suggested by the invention and referenced 22 hereinafter will be added, in the case of such a metal core (at the heart), either on top of the above-mentioned protective material, or directly over (at the heart of) the metal core itself, if it has not been coated beforehand with such a protective material.

[0101] As an example of a bi-material hybrid core, mention may be made of a core consisting of a first material predominantly based on silica/zircon (more specifically the heart of the core) obtained for example by injection, machining or additive manufacturing and of a second material containing reactive elements (at the surface of the core) and which can be obtained by over-injection or additive manufacturing (projection of drops of material or melting of wire throughout a heating nozzle).

[0102] Regardless of the retained choice of the core (with a coated or uncoated heart), once the latter has been manufactured, according to the invention, it will be covered with the suggested anti-oxidation and/or anti-corrosion protective coating; after which it is possible to mold the superalloy over the core covered with the protective coating of the invention, and thus protect the inner portions of some aeronautical turbine engine parts made of a superalloy, such as vanes in particular, from oxidation and/or corrosion.

[0103] According to 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 in order to increase the resistance to the environment of the inner portion of the considered part: the inner cavity(ies) of a blade, in the retained preferred example.

[0104] Hence, to make these “reactive elements”, one will therefore have, before bringing the superalloy around the core, coated the external surface of this core, marked 26 FIGS. 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 (Hf), and/or chromium and/or silicon and/or yttrium, or a mixture thereof.

[0105] As regards the core 20 itself, its heart 24 therefore contains a ceramic or metal or a metal/ceramic hybrid material. Examples of a ceramic composition, of a metal composition or of a hybrid (or bi-material) ceramic/metal composition of the heart 24 of the core 20 have been disclosed hereinbefore and are amongst the most suitable ones.

[0106] As already explained, around this heart 24, possibly already protected by a first protective coating 28, a substantial increase of the resistance of the surface to the environment of the final part 2 (cf. FIGS. 2-4), therefore of the surface chemistry of the superalloy 40 that forms it (essentially; cf. FIG. 2), has been noticed when coating the external surface of the core with, as an external coating 24, a material comprising (possibly with a mixture of the elements hereinafter): [0107] at least one layer at least nanometric of hafnium (Hf), and/or [0108] at least one layer at least micronic of platinum (Pt), or [0109] at least one layer at least nanometric containing Cr and/or Si and/or Y,
the preferred thicknesses or w % at the surface of the superalloy of the hollow inner area of the final part having already been specified, before in the text.

[0110] Nonetheless, to combine mechanical performance and optimizations of the amounts and of the implementation process, one could prefer, as also already specified: [0111] that the thickness e1 of the Hf layer is 20 nm≤e1≤900 nm, and even 50 nm≤e1≤500 nm, and/or [0112] that the thickness e2 of the Pt layer is 1 μm23e2≤15 μm, and even 1 μm≤e2≤5 μm.

[0113] In particular, depositing between 1 μm and 5 μm of platinum and 0.5 μm of Hf (within a 10% margin) has turned out to be a relevant solution, considering the targeted aims.

[0114] Given the nature of the aforementioned coating to be deposited (marked 1 in the figures), the mentioned elements can be deposited by one or several process(es), as follows: [0115] the layer(s) or elements Pt and/or: Hf, possibly Cr, Si, Y (alone or mixed) can be made in the same deposition machine and be deposited by one amongst the physical vapor deposition (PVD) processes such as: EBPVD, Joule evaporation, pulsed laser ablation or cathodic spraying, [0116] the layer or element Pt can be deposited by electrolytic deposition provided that the composition of the core is doped with electrically-conductive elements, such as metal or carbon, [0117] the layer(s) or elements Hf and/or Cr, Si or Y can be deposited by chemical vapor deposition CVD (PECVD, LPCVD, UHVCVD, APCVD, ALCVD, UHVCV . . . ).

[0118] After the coating deposition(s) performed at the surface of the core, a diffusion treatment may be carried out in order to make its aforementioned coating material(s) diffuse in the core, and thus promote the profitable supply of all or part of these elements.

[0119] It is possible to provide for this diffusion treatment in the core to be carried out when dissolving the superalloy, which can be done during a heat treatment.

[0120] The temperatures to promote the diffusion of the aforementioned reactive elements Pt and/or Hf, Cr, Si, Y will favorably be comprised between 800° C. and 1,250° C., under a secondary vacuum, typically 10.sup.−6X 10.sup.5 Pa, within a 10% margin.

[0121] Whether there has been a step of diffusion, towards the inside of the core, of the aforementioned layer(s) or elements, or not, it is during casting of the superalloy of the part to be manufactured around the core enriched at the 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.

[0122] This casting of the superalloy of the part to be manufactured around the core can be favorably followed by a heat treatment in order to best promote the diffusion of the coating component(s) of the core, schematized in 20 FIGS. 5-6, towards the superalloy of the part, marked 2 in the illustrative figures.

[0123] The conditions may be the same as before: between 800° C. and 1,250° C., within a 10% margin, under a secondary vacuum, typically 10.sup.−1 Pa, within a 10% margin.

[0124] FIG. 2 schematically illustrates the effect of enrichment/diffusion on the aforementioned coating 1, which coating has therefore, in the figure, partially diffused into the upper area (the closest to the inner surface 2a) of the part 2 made of a superalloy.

[0125] The limit or the interface that could be considered to exist between the superalloy 40 itself and the coating 1 has been identified in 3, assuming that there would be no heat treatment of diffusion.

[0126] Hence, if there has been enrichment with diffusion, one will find, across the thickness of the part 2, and starting from its inner surface 2a: [0127] first a first layer 4 of the coating 1, not or relatively barely diffused, predominantly consisting of the added or enrichment element(s) in Pt and/or Hf, Cr, Si, Y transmitted to the part 2 during casting of the superalloy 40 over the core 20 entirely or partially coated with these same elements (generally marked 22 FIG. 5 or 6), [0128] then, deeper down, 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 anchored portion of the actual coating 1 for protection against oxidation and/or corrosion, [0129] then an area 6 (which sinks into the part) formed from the actual mass of superalloy 40.

[0130] FIG. 5 or 6, the coating 22 at the external surface 26 of the core, or model (of a blade in the example), 20 has been identified as well dissociated from the base material 24, or core, of this core, while the separation is not really so neat.

[0131] As regards the heat treatment of dissolving the superalloy, it should be noted that the solidifying nickel-based superalloys can be heat-treated to obtain the desired distribution and size of the different phases. The first heat treatment (T) can be a microstructure homogenization treatment which aims to dissolve the γ′ phase precipitates and eliminate the γ/γ′ eutectic phases or significantly reduce their volume fraction. This treatment is carried out at a temperature higher than the solvus temperature of the γ′ phase and lower than the starting melting temperature of the superalloy (Tsolidus). Afterwards, quenching can be carried out at the end of this first heat treatment to obtain a fine and homogeneous dispersion of the γ′ precipitates.

[0132] Afterwards, quenching heat treatments may be carried out in two steps, at temperatures lower than the solvus temperature of the γ′ phase: During a first step (R1), to enlarge the γ′ precipitates and obtain the desired size, then during a second step (R2), to increase the volume fraction of this phase up to about 70% at room temperature.

[0133] Example of heat treatments:

[0134] Superalloy AM1:

[0135] Treatment at 1,300° C. for 3 hours under partial pressure of argon or under vacuum followed by gas quenching (argon).

[0136] R1: 1,100° C. for 5h in air,

[0137] R2: 870° C. for 16h in air

[0138] Superalloy CMSX-4:

[0139] Treatment in stages from 1,277° C. to 1,321° C. in 16h and a 2h stage at 1,321° C. under partial pressure of argon or under vacuum followed by gas quenching (argon).

[0140] R1: 1,100° C. for 4h in air

[0141] R2: 870° C. for 20h in air.

[0142] 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. On the other hand, it consists of a hollow blade 2. In the figure, one can identify the root 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 (cf. section FIG. 4) and comprises the bathtub 7 at its top opposite to the platform. The bathtub is delimited laterally by the wall of the blade and the bottom is formed by a bathtub bottom wall 11, perpendicular to the radial axis of the blade. This bottom wall 11 which can be seen in section in FIG. 4 is crossed by orifices 12 which communicate with the inner cavities 13, 14 of the blade to evacuate a portion of the cooling fluid from the latter. In turn, this fluid is evacuated into the hot gas flow path through the clearance existing between the top and the annular surface of the stator located radially opposite thereto.

[0143] Hence, the solution of the invention will have allowed protecting the inner 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 inner surface 2a of the superalloy 40 in which the blade 2, and in this instance at least the hollow blade 10, is made.

[0144] Finally, it should be noted that the invention has allowed: [0145] defining the elements and amounts to be deposited over the final part, in particular in the case of turbine engine blade channels to protect them from oxidation/corrosion, [0146] using a suitable deposition method to deposit these desired elements at the surface of intermediate feed cores, [0147] carrying out a suitable heat treatment for the diffusion of the desired elements from the core towards the surface of the metal of the part to be enriched at the surface to protect it.