WO2010092298A1

WO2010092298A1 - Process for depositing a protective layer on a part

Info

Publication number: WO2010092298A1
Application number: PCT/FR2010/050221
Authority: WO
Inventors: Yannick Cadoret; Claude Estournes; Daniel Monceau; Djar Oquab
Original assignee: Snecma; Institut National Polytechnique De Toulouse; Universite Paul Sabatier - Toulouse Iii; Centre National De La Recherche Scientifique
Priority date: 2009-02-10
Filing date: 2010-02-10
Publication date: 2010-08-19
Also published as: FR2941966B1; FR2941966A1

Abstract

Process for depositing a protective layer on a part (10), in which: a flash sintering apparatus is used, this apparatus having a compression chamber (8) through which an electric current flows during the flash sintering; at least one coating forming a precursor of said protective layer is deposited on the part (10); the part is placed in the compression chamber (8); the part and its coating are buried in a refractory powder (20), this refractory powder remaining fluid in the temperature range used for the flash sintering; the part and its coating immersed in the refractory powder are subjected to a flash sintering operation so as to obtain cohesion between the part and its coating and to transform this coating into a protective layer. Application to the deposition of a protective layer acting as environmental and/or thermal barrier on a turbine blade.

Description

METHOD FOR DEPOSITING A PROTECTIVE LAYER ON A WORKPIECE

FIELD OF THE INVENTION

The invention relates to a method for depositing a protective layer on a part. It is intended for any type of part, more particularly metal parts and even more particularly superalloy parts such as aeronautical or land-based turbine engine parts, subjected to high temperatures in operation. In particular, said part may be a dawn or a turbojet engine turbine or turboprop turbine distributor.

STATE OF THE PRIOR ART

In the field of aeronautics, certain parts and in particular the blades (mobile or fixed) of the high-pressure turbine of a turbomachine evolve in an aggressive environment composed of gas at very high temperature (more than 1000 ⁰ C), ejected at high speed. In this aggressive environment, the blades must retain their mechanical properties and resist corrosion. These vanes are often made of a superalloy resistant to high temperature, usually a superalloy based nickel (Ni), ideally monocrystalline. However, even the most efficient superalloys currently have, in such an environment, insufficient mechanical performance and life. For this reason, it is necessary to cover these superalloys with a protective layer, commonly called "thermal barrier".

The thermal barriers currently used are made by depositing a layer of ceramic on the part. This ceramic layer is typically based on zirconia (zirconium oxide Zrt> ₂ ). It insures the thermal insulation of the room and keeps the room at temperatures where its mechanical performance and its life are acceptable. In order to ensure the anchoring of this ceramic layer, a metal sub-layer is generally interposed between the piece and the ceramic layer. This underlayer provides adhesion between the part and the ceramic layer, knowing that the adhesion between the underlayer and the part is by inter-diffusion, and that the adhesion between the underlayer and the layer ceramics is done by mechanical anchoring and by the propensity of the layer to be developed at high temperature, at the ceramic / undercoat interface, a thin oxide layer which ensures chemical contact with the ceramic. In addition, this metal underlayer provides protection of the part against corrosion phenomena. Typically, this metal underlayer is a platinum modified nickel aluminide (Ni, Pt) Al.

Today, it is known to deposit on a Ni-based superalloy part, a thermal barrier which comprises a metal sub-layer (Ni, Pt) AI covering the part and a ZrO 2 -based ceramic layer covering said undercoat, according to a process comprising the following steps: the preparation of the surface of the part by chemical etching and sanding; depositing on the part, by electrolysis, a platinum coating (Pt); the possible heat treatment of the assembly to diffuse Pt in the room; aluminum deposition (Al) by chemical vapor deposition (CVD) or physical vapor deposition (PVD); the possible heat treatment of the assembly to diffuse Pt and Al in the room; the preparation of the surface of the formed metallic underlayer; and electron beam evaporation (EB-PVD) deposition of a ceramic coating. This example of known method has the advantage of being used on a piece of relatively complex shape such as a turbine blade. However, it has the disadvantage of being long and complex to implement, especially because of its many steps.

Also known is the publication entitled "Resilient Oxidation Aluminized MCrAlY Coating Prepared by Spark Plasma Sintering (SPS)" by Djar Oquab, Claude Estournes and Daniel Monceau (published in 2007 in the journal Advanced Engineering Materials 2007, 9, No. 5) which discloses another example of a method of depositing a protective layer on a workpiece, this part being made of superalloy based on Ni and having the shape of a pellet, more precisely of a cylinder of revolution. According to this known method, a metal coating is deposited in MCrAIY (where M = Co, Ni or Co / Ni) in the form of a dry powder, on the flat faces of said wafer (ie on the bases of said cylinder of revolution) but not on the slice. The assembly (pellet and coating) is then wrapped in a graphite sheet before being placed in the compression chamber of a flash sintering apparatus or SPS apparatus. The pellet is placed in the compression chamber so that that the pistons of the device exert on the flat faces of Ia pellet and coating a uniaxial pressure oriented parallel to the axis of revolution of this pellet and therefore perpendicular to these planar faces. After flash sintering, the graphite sheet is removed by oxidation of carbon by heating in air at 800 ^° C. for 10 minutes. In the method disclosed in this publication, the simple shape of the part, the flatness of the areas of the part to be covered and the positioning of the part in the compression chamber are adapted to the uniaxial compression exerted. However, this publication does not give any information on how to deposit a protective layer on areas of the part that are neither flat nor perpendicular to the direction of uniaxial compression exerted during flash sintering.

PRESENTATION OF THE INVENTION

The object of the invention is to propose a deposition method for depositing a protective layer on several zones of the same piece of complex shape, these zones being oriented differently and / or being curved (i.e. non-planar).

This process is such that;

a flash sintering apparatus is used, this apparatus having a compression chamber traversed by an electric current during flash sintering, and a refractory powder remaining fluid in the temperature range used for the flash sintering (this refractory powder therefore does not sinter). in the temperature range used);

at least one precursor coating of said protective layer is deposited on the part (i.e. on said zones of the part), so that a coated part is obtained;

the refractory powder and the coated part are placed in the compression chamber, so that the coated part is drowned in the refractory powder;

the coated part embedded in the refractory powder is subjected to flash sintering so as to obtain a cohesion between the part and its coating and the transformation of this coating into a protective layer.

According to one embodiment, said deposited precursor coating is in the form of a sheet or several superimposed sheets, or in the form of a powder suspended in a liquid or a gel. The coating then has the advantage of being easy to deposit on the areas to be protect. In addition, in this form, it is easy to control the amount of material deposited and the thickness of the desired protective layer.

In general, the method of the invention has the advantage of being fast and reproducible. In addition, during flash sintering, the refractory powder surrounding the coated part transmits the pressure exerted by the flash sintering apparatus on the quasi-isostatically coated part. Thus, all areas of the part covered by the coating (even those that are not flat or not oriented perpendicular to the direction of the uniaxial compression exerted by the device) are subjected to a pressure allowing the attachment of the coating on these areas and at the end of the flash sintering, these areas are covered by the desired protective layer.

The process described above can therefore be used to deposit a protective layer on a piece of complex shape, this protective layer may, in particular, serve as an environmental barrier and / or thermal.

Note that all areas of the room are not necessarily covered by the coating and therefore by the protective layer. In this case, during the flash sintering, the uncovered areas are in direct contact with the refractory powder. As the powder does not sinter in the temperature range concerned, there is no adhesion of the refractory powder on these uncovered areas.

It will also be noted that said refractory powder may be electrically conductive or not. In the case where this refractory powder is electrically conductive, it is heated by Joule effect during flash sintering. If the coated part is also electrically conductive, it is also heated by Joule effect. If the coated part is not electrically conductive, it is heated by heat conduction of the powder to the workpiece. In the case where the powder is not electrically conductive, sintering is possible because the powder is heated by heat conduction from the flash sintering apparatus to the powder and the coated part is heated by heat conduction of the powder to the room.

According to one embodiment, in order to dispose the coated part in the compression chamber, a first layer of refractory powder is first placed at the bottom of the compression chamber, then coated part on this first layer and, finally, covering the coated part and the first layer of a second layer of refractory powder.

Said coating may be in the form of a sheet or several superimposed sheets. For example, it may be a metal sheet provided that, in the present application, a part (ie part, coating, sheet, etc.) is said to be metallic when it is metal, alloy and / or in intermetallic compound. The coating then has the advantage of being easy to deposit on the surfaces to be protected. In addition, in this form, it is easy to control the amount of material deposited and the thickness of the desired protective layer by controlling the initial thickness of the sheet (s) used. When several metal sheets are superimposed, they can be of different compositions.

The coating may also be in the form of a powder suspended in a liquid or a gel (i.e. a slip). For example, it may be a metal powder or a ceramic powder. In this form, the coating can be easily deposited, for example by spraying or with the aid of a brush. In addition, the composition of the powders and, therefore, deposited coatings is easily controlled.

Of course, when several coatings are deposited, some may be in the form of powder suspended in a liquid or a gel and the others in sheet form.

To deposit the coating or coatings, it is therefore not necessary to resort to complex deposition techniques such as physical or chemical vapor deposition (CVD or PVD), HVOF projection deposition (for "High Velocity Oxy -Fuel "), electrolytic deposition, etc., which are often long to implement and with which it is often difficult to control the composition of the deposit made.

Said refractory powder used to coat the part and its coating (s) is, for example, a boron nitride powder, an alumina powder, a graphite powder, or a mixture thereof. In general, the choice of the refractory powder depends on the type of coating to be deposited on the part (composition and sintering temperature).

According to one embodiment, after the flash sintering step, the piece and the protective layer are subjected to heat treatment in order to obtain a particular microstructure in the protective layer. Generally, this heat treatment step allows the chemical elements of the part and the protective layer to inter-diffuse. This additional step can be carried out in the flash sintering apparatus.

According to one mode of implementation, the piece is metallic and the protective layer consists of at least one metal layer covering the part.

According to another embodiment, the part is metallic and the protective layer comprises a ceramic layer directly covering the part.

According to another embodiment, the piece is metallic and the protective layer comprises at least one metal underlayer covering the piece and a ceramic layer covering the metal underlayer.

In the latter case, according to one embodiment, the method comprises the following steps: a first precursor metal coating of said metal sub-layer is deposited on the part, and

a second ceramic coating, a precursor of said ceramic layer, is deposited on the first coating.

Such a protective layer can act as a thermal barrier. The flash sintering step allows the chemical elements of the workpiece and the first inter-diffuse coating to form said underlayer and provides a bond between the workpiece and the first coating and between the first and second coatings. In addition, the flash sintering step makes it possible to densify the coatings which are in the form of powder. All this is done in a single step that usually lasts only a few minutes, which is of great practical interest.

In addition, the simultaneous flash sintering of the coatings makes it possible to obtain different porosities in the metallic underlayer and in the ceramic layer formed, namely a very low or even zero porosity in the metal underlayer and a high porosity. in the ceramic layer. This has the advantage that the lower the porosity in the metal underlayer, the more the underlayer protects against corrosion and oxidation. Conversely, a high porosity in the formed ceramic layer contributes to a low thermal conductivity of this layer and therefore to a good thermal protection of the part. According to one embodiment, said piece is a Ni-based superalloy and said first coating comprises aluminum associated with at least one element chosen from: Pt, Pd, Ir, Rh and Ru. This gives a sub-layer of modified nickel aluminide. Thus, if M denotes said element selected from Pt, Pd, Ir, Rh and Ru, a modified nickel aluminide (Ni, M) Al is obtained. When the first coating is in the form of superposed sheets, it is possible, for example use an Al sheet and a Pt sheet. In this case, the composition of the underlayer is controlled by the ratio between the thicknesses of the sheets used. According to another embodiment, said part is a Ni-based superalloy and said first coating comprises an element chosen from: Pt, Pd, Ir, Rh and Ru. This produces a diffused noble element coating.

Furthermore, to obtain a particular microstructure in this underlayer, after the sintering step, the workpiece and its thermal barrier can be subjected to a heat treatment so as to influence the inter-diffusion between the workpiece and the first coating. . In particular, when the underlayer is platinum modified nickel aluminide (Ni, Pt) AI, this heat treatment makes it possible to obtain the desired phase or phases (in particular a combination of the γ-Ni, γ'-Ni ₃ Al phases. , β-NiAI, α-NiPtAI, PtAb) among the phases of the ternary Ni-Pt-Al diagram.

According to another embodiment, the first coating is Ni-M-Al-Cr alloy where M is at least one element selected from: Pt, Pd, Ir, Rh and Ru. According to one mode of implementation, the second coating is based on a ceramic having a low thermal conductivity so as to provide thermal protection of the room. This is, for example, a zirconia stabilized with at least one oxide of a member selected from the group consisting of rare earths, preferably in the subgroup: Y, Dy, Er, Eu, Gd, Sm , Yb, or with a combination of a tantalum oxide (Ta) and at least one rare earth oxide, or with a combination of a niobium oxide (Nb) and at least one rare earth oxide .

According to one embodiment, at least one third coating is inserted between the piece and the first coating, or between the first and second coating. The nature, function and form of this coating can be various. Regarding its shape, the third coating may be in the form of one or more superimposed metal sheets, and / or in the form of a powder, optionally suspended in a liquid or a gel, which has the aforementioned advantages and, in particular, the fact of being able to easily dose the chemical elements brought by this third coating.

Different examples of third coating are given below. It should be noted that these examples are not incompatible and can therefore be combined with one another. According to one example, the third coating comprises at least one of: Zr, Y, Si, Hf, Ce, La, Sr, Ti, Ta, and / or at least one platinoid element from: R, Pd, Ir, Os , Re, Rh, Ru and / or at least one precious or semi-precious metal among: Au, Ag, this third coating may be interposed between the workpiece and the first coating, or between the first and second coating. This makes it possible to introduce into the sub-layer elements with varied properties. In particular, Zr, Si, Y, Re, Ru are beneficial to the resistance to oxidation. Such elements must generally be added in perfectly controlled quantities (typically of the order of a few hundred ppm). To easily control the added amounts, the third coating is advantageously in the form of a powder, optionally suspended in a liquid or a gel. For example, the powder can be sprayed on the part or on the first coating.

According to another example, the third coating may be deposited to produce a diffusion barrier at the substrate / undercoat metal interface, this diffusion barrier possibly being of alumina, Re, Hf-Ni, Hf-Pt or Ir-Ta type. .

According to another example, the third coating makes it possible to achieve a concentration gradient at the interface between the metal underlayer and the ceramic layer in order to reduce the abrupt variation in coefficient of expansion between the underlayer and the layer. of ceramic, and to limit, thus, flaking occurring usually at this interface. In this case, the third coating is disposed between the first and the second coating, and is a mixture comprising a ceramic powder and a metal powder. Advantageously, this ceramic powder and this metal powder have, respectively, the same composition, or a composition close to those of the first and second coatings. For example, when the second ceramic coating is zirconia-based, the third coating is a mixture of a zirconia powder and a metal powder.

According to one embodiment, a fourth coating based on a hard phase (for example based on SiC) is deposited on the second coating. This fourth coating makes it possible to form an outer layer on the surface of the ceramic layer and thus protect the latter against erosion and surface damage. Said outer layer may possibly be formed by reaction between the elements of the second coating and those of the fourth coating.

According to one mode of implementation, the second coating and the fourth coating are in the form of powders, the powder of the fourth coating being dispersed in the powder of the second coating, on the surface of the second coating. For example, the powder of the second coating is a zirconia powder and the powder of the fourth coating is an SiC powder.

Whatever the number or nature of the coatings deposited on the part, they are of course deposited before drowning the piece in said refractory powder.

The method described above is well suited to the deposition of a protective layer acting as a thermal barrier on a turbine part and, more particularly, on a blade or a turbomachine turbine distributor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages will be better understood on reading the detailed description which follows.

This description refers to the appended Figures 1 and 2. Figure 1 schematically shows the main members of an exemplary flash sintering apparatus that can be used to implement the method of the invention.

Figure 2 is a detail view, in section, showing the compression chamber of the apparatus of Figure 1, a blade embedded in a refractory powder being inside this chamber. DETAILED DESCRIPTION EXAMPLE ¹ (S) FOR CARRYING OUT

The flash sintering apparatus, or SPS apparatus, typically comprises a pulsed DC generator (eg 3.3 ms duration of the draw), a hydraulic pressure system, a vacuum chamber or a controlled atmosphere, a compression tool with pistons upper and lower, electrodes connected to said pistons, a graphite jacket surrounding said pistons, and a control system for measuring and controlling, in particular, the temperature and pressure inside the vacuum chamber or in a controlled atmosphere, and that the vertical displacement of one of the pistons, so as to exert pressure on the sintered part and to follow the withdrawal (ie the decrease in thickness) thereof.

An example of a flash sintering apparatus is shown in FIG. 1. It comprises two opposite upper 6A and lower 6B electrodes, respectively connected to graphite upper 3A and lower 3B pistons; a jacket 2, also in graphite, traversed vertically by a hole 1 inside which the pistons 3A, 3B, penetrate; and a vacuum chamber 7 surrounding a portion of the electrodes 6A, 6B, the pistons 3A, 3B, and the graphite jacket 2. The upper and lower pistons 3A, 3B are aligned along a main vertical axis A and at least one of the pistons is movable in translation along this axis A. The end faces of the pistons 3A, 3B are arranged in a vertical direction. screw and define with the inner wall 4 of the jacket 2 delimiting the hole 1, a compression chamber 8.

During flash sintering, a current of high intensity is applied to the electrodes 6A, 6B, transmitted to the pistons 3A, 3B and passes through the compression chamber 8. In addition, a uniaxial compression, oriented along the axis A, is exerted on the contents of this enclosure 8 by displacement of the piston 3B. In FIG. 2, the arrows P symbolize the pressure exerted by the piston 3B on the contents of the enclosure 8 and the counter-pressure exerted by the piston 3A. The passage of current in the pistons 3A, 3B, the jacket 2 and, as the case may be, the contents of the enclosure 8, makes it possible to heat the assembly by Joule effect, so as to obtain inside the enclosure 8 the desired temperature.

FIG. 2 is a detailed view of the compression chamber 8, in section along a section plane containing the axis A in FIG. 2. In the compression chamber 8, a turbine blade 10 has been disposed aircraft turbojet engine. The cross section of this dawn 10 appears on Ia FIG. 2 shows the intrados face 11, the extrados face 12, the leading edge 13 and the trailing edge 14 of the blade 10.

A turbine blade such as blade 10 is typically a Ni-based superalloy. More particularly, it may be an "AMl" type superalloy, that is to say a Ni-based superalloy having the following composition, in percentages by weight: 5 to 8% Co; 6.5 to 10% Cr; 0.5 to 2.5%

Mo; 5 to 9% W; 6-9% Ta; 4.5 to 5.8% Al; 1 to 2% Ti; 0 to 1.5% Nb; C, Zr,

B each less than 0.01%; the 100% complement being constituted by Ni.

In the example, an "AM1" type superalloy blade having the following composition was used in percentages by weight: 6.5% Co; 7.5% Cr; 2% Mo; 5.5% W; 8% Ta; 5.3% Al; 1.2% Ti; 64% Ni. The surface of the blade 10 has been cleaned, degreased. A first metal coating formed of two metal sheets, each 10 μm thick, was deposited manually on the blade 10: the first sheet, the closest to the blade, being in Pt, and the second sheet being in Al .

Then, a second ceramic coating consisting of a powder sold under the name "TZ8Y", that is to say a yttria-containing zirconia powder comprising, as an atomic percentage, 8% of Y 2 O 3, was deposited on the first coating. The "TZ8Y" powder was deposited on the surface of the dawn / Pt sheet / AI sheet assembly (i.e., to say on the surface of the AI sheet) and was compacted to cold with a slight pressure in a mold at the edges of the room used. The powder mass "TZ8Y" was calculated to give after sintering a ceramic layer of 100 microns thick. The thickness of the coating on the surface of the workpiece can be controlled by the thickness of the layers deposited on the surface of the workpiece.

At the end of these steps, a blade 10 coated with the first and second coatings, hereinafter referred to as "coated blade", was obtained. In the example, only the areas of the blade exposed in service at very high temperatures were covered with said coatings. Thus, the areas above the platform of dawn (ie the blade of dawn) have been covered, while the areas under the platform of dawn (ie the foot of the dawn dawn) were not covered so as not to degrade the mechanical behavior of the dawn foot. Then, this chamber 8 was filled with a first layer of powder 20, then the blade coated horizontally on this first layer of powder and finally covered the coated blade and the first layer of a second layer of powder 20.

The powder 20 constitutes a refractory powder within the meaning of the invention. It surrounds the areas of the dawn on which the first and second coatings have been deposited.

In the example, a hexagonal boron nitride powder, denoted h-BN, was chosen.

As the blade 10 has cooling channels opening on its surface, in the case of a deposit coating on the blade by "liquid" (ie use of a coating powder in suspension in a liquid or in a gel), a refractory fluid powder having grains of sufficiently small size can be used to allow their infiltration into these channels. These channels generally having a diameter greater than 1 mm, the grain size retained was less than 1 micron (1 micron).

Next, the blade / first coat / second coating / refractory powder was subjected to flash sintering. Flash sintering makes it possible to densify the second ceramic coating and to join the first and second coatings together and the first coating to the substrate in a single operation.

In the example, a flash sintering apparatus having the following references was used: Dr. Sinter model SPS-2080 SPS Syntex INC Japan. This flash sintering apparatus was programmed to have in the compression chamber a rise in temperature for 10 min at a speed of 100 ^o C / min, then a hold at 950 ⁰ C for 1 min. With regard to the uniaxial compression exerted on the contents of the compression chamber 8, a force of 0.1 kN was applied by the piston 3B to the powder 20, for 2 min, then a force of 7.8 kN was applied for 10 min. More generally, the uniaxial compression exerted is preferably between a few hundred N / m ² and 10 000 N / m ² , the sintering temperature is preferably between 500 ⁰ C and 1200 ⁰ C, and the sintering time is preferably between 1 minute and 1 hour. The sintering is preferably carried out under vacuum and under a reducing atmosphere (H ₂ , Ar ... in the presence of C of the tools). In the above-mentioned temperature and compression ranges, the powder 20 of h-BN does not sinter and remains fluid, so that it transmits the pressure P exerted by the piston 3B and the backpressure P exerted by the piston 3A to the dawn is coated, in a quasi-isostatic way. Thus, even areas of the blade 10 which are not perpendicular to the axis A (ie which are not horizontal in the example) are subjected to pressures. These pressures are represented diagrammatically by the arrows P ¹ in FIG. 2. Under the effect of pressure, heat and current, the coatings used adhere to the surface of the blade 10 while transforming to form the layer desired protection. In the end, we obtain a blade 10 having its lower face surfaces 11 and extrados 12, and its leading edges 13 and leakage 14, covered by said protective layer. To recover this blade, the refractory powder 20 is removed and / or the blade 10 is removed from the powder, which poses no problem since the refractory powder 20 remained fluid.

Claims

A method of depositing a protective layer on a plurality of zones of the same piece (10) of complex shape, these zones being oriented differently, in which:

a flash sintering apparatus is used, this apparatus having a compression chamber (8) traversed by an electric current during flash sintering, and a refractory powder (20) remaining fluid in the temperature range used for flash sintering; at least one precursor coating of said protective layer is deposited on said zones, so that a coated part is obtained, said deposited precursor coating being in the form of a sheet or of several superimposed sheets, or in the form of a powder suspended in a liquid or a gel; the refractory powder and the coated part are placed in the compression chamber (8), so that the coated part is drowned in the refractory powder (20);

the coated part embedded in the refractory powder (20) is subjected to flash sintering so as to obtain a cohesion between the part (10) and its coating and the transformation of this coating into a protective layer.

The method of claim 1, wherein said areas are not planar.

3. The method of claim 1 or 2, wherein said piece is metallic and said protective layer comprises at least one metal underlayer covering the workpiece and a ceramic layer covering the underlayer, and wherein:

a first precursor metal coating of said metal underlayer is deposited on the part, and

- Is deposited on the first coating, a second ceramic coating, precursor of said ceramic layer.

The method of claim 3, wherein said part is a Ni-based superalloy and wherein either said first coating comprises aluminum associated with at least one element chosen from: Pt, Pd, Ir, Rh and Ru, or said first coating comprises an element chosen from: Pt, Pd, Ir, Rh and Ru, or said first coating is in Ni-M-Al-Cr alloy where M is at least one element selected from: Pt, Pd, Ir, Rh and Ru.

5. The method of claim 3 or 4, wherein the second coating is based on a zirconia stabilized with at least one oxide of a member selected from the group consisting of rare earths, preferably in the subgroup: Y , Dy, Er, Eu, Gd, Sm, Yb, or with a combination of a tantalum oxide (Ta) and at least one rare earth oxide, or with a combination of a niobium oxide (Nb) and at least one rare earth oxide.

6. Method according to any one of claims 3 to 5, wherein a third coating is interposed between the workpiece and the first coating, or between the first and second coating, the third coating comprising at least one reactive element among: Zr , Y, Si, Hf, Ce, La, Sr, Ti ₇ Ta, and / or at least one platinoid element from among: Pt, Pd, Ir, Os, Re, Rh, Ru and / or at least one precious or semi-precious metal -precious among: Au, Ag.

7. Method according to any one of claims 3 to 6, wherein is deposited on the second coating a fourth coating based on a hard phase, for example based on SiC, the fourth coating forming an outer layer to Ia surface of the ceramic layer, said outer layer being formed by reaction between the elements of the second coating and those of the fourth coating.

The method of claim 7, wherein the second coating and the fourth coating are in the form of powders, the powder of the fourth coating being dispersed in the powder of the second coating, at the surface of the second coating.

The method of any one of claims 1 to 8, wherein said part belongs to a turbine.

The method of any one of claims 1 to 9, wherein said part is a blade.