EP3619338A1

EP3619338A1 - Superalloy turbine part and associated method for manufacturing by bombardment with charged particles

Info

Publication number: EP3619338A1
Application number: EP18728684.4A
Authority: EP
Inventors: Amar Saboundji; Virginie Jaquet
Original assignee: Safran SA
Current assignee: Safran SA
Priority date: 2017-05-05
Filing date: 2018-05-07
Publication date: 2020-03-11
Also published as: US20200191002A1; CN110709536A; FR3065968B1; FR3065968A1; WO2018202964A1

Abstract

The invention relates to a turbine part, such as a turbine blade or a distributor fin, for example, comprising a substrate made of a monocrystalline nickel superalloy, a metal sublayer covering the substrate, and a protective layer of metal oxide covering the sublayer, characterised in that the metal sublayer has one surface in contact with the protective layer and the surface has a mean roughness of less than 1 μm.

Description

SUPERALLIATION TURBINE PIECE AND PROCESS FOR MANUFACTURING THE SAME BY BOMBING FILLED PARTICLES

FIELD OF THE INVENTION

The invention relates to a turbine part, such as a turbine blade or a distributor blade, for example, used in aeronautics.

STATE OF THE ART

In a turbojet, the exhaust gases generated by the combustion chamber can reach high temperatures, higher than 1200 ° C or 1600 ° C. Parts of the turbojet, in contact with these exhaust gases, such as turbine blades, for example, must be able to maintain their mechanical properties at these high temperatures.

For this purpose, it is known to manufacture some parts of the turbojet engine "superalloy". Superalloys are a family of high strength metal alloys that can work at temperatures relatively close to their melting temperatures, typically 0.7 to 0.8 times their melting temperatures.

In order to reinforce the thermal resistance of these superalloys and to protect them against oxidation and corrosion, it is known to cover them with a coating acting as a thermal barrier.

Figure 1 schematically illustrates a section of a turbine part 1, for example a turbine blade or a distributor blade. The part 1 comprises a substrate 2 of monocrystalline metal superalloy covered with a thermal barrier 10.

FIG. 2 is a photomicrograph illustrating a section of a portion of the thermal barrier 10 of the turbine part 1, covering the substrate 2. The black rectangle of FIG. 2 is a scale bar corresponding to a length of 50 μιτι. . The thermal barrier 10 comprises a metal sub-layer 3, a protective layer 4 and a thermally insulating layer 5. The metal sub-layer 3 covers the substrate 2 in metallic superalloy. The metal sub-layer 3 is covered with the protective layer 4, formed by thermal oxidation of the metal underlayer 3 (the protective layer is designated by TGO, acronym for Thermally Grown Oxide). The protective layer 4 protects the superalloy substrate from corrosion and / or oxidation. The thermally insulating layer 5 covers the protective layer 4. The thermally insulating layer 5 may be ceramic, for example made of yttriated zirconia. The metal sub-layer 3 provides a connection between the surface of the superalloy substrate and the protective layer.

During the manufacture of the thermal barrier, it is known to etch the oxides formed on the surface of the underlayer after the deposition of the underlayer. These oxides are formed in contact with the ambient atmosphere and are unstable or metastable when using the turbine part.

For this purpose, it is known to sand the outer surface of the metal underlayer. Sandblasting is used to etch oxides formed on the surface of the underlayer after deposition of the underlayer.

However, when a TGO is formed on the underlayer after a sandblasting step according to a known method:

impurities are transported to the surfaces of the underlayer. These impurities are incorporated into the protective layer during the formation of the protective layer by oxidation;

- the grain size of the TGO is heterogeneous. The protective layer has in particular small grains (for example of a size less than 1 μπι), known to reduce the resistance to corrosion and oxidation of thermal barriers, as well as the adhesion of the protective layer to the undercoat ;

- Different allotropic phases can coexist in the protective layer. In the case of a TGO in alumina, it is known that under conditions of use of the room, at high temperature, the different phases of the phase a transform into phase a by changing volume. This variation in volume causes tensile stresses and cracks in the TGO, favoring its peeling. Thus, the lifetime of the thermal barrier is significantly reduced;

- The kinetics of growth of TGO is different on different parts of the metal underlayer. This disparity in the growth kinetics of TGO causes mechanical stress in the TGO when using the thermal barrier and decreasing its life. SUMMARY OF THE INVENTION

An object of the invention is to provide a solution for effectively protecting a superalloy turbine part from oxidation and corrosion while having a longer service life than with known thermal barriers.

This object is achieved in the context of the present invention by means of a method of manufacturing a turbine part comprising:

a monocrystalline nickel base superalloy substrate,

a metal sub-layer covering the substrate, and

a protective metal oxide layer covering the underlayer, the method comprising steps of:

a) bombardment of charged particles on a surface of the metal sub-layer, then

b) formation of the protective layer on the bombarded surface during step a).

Since the underlayer is bombarded with charged particles, it is possible to obtain an etched surface of the metal sub-layer in contact with the protective layer having a roughness less than the roughnesses generally obtained by conventional mechanical sand blasting techniques. . In addition, the roughness obtained has a better homogeneity. This results in the protective layer growing at homogeneous kinetics during its formation, which makes it possible to avoid mechanical stresses during the use of the part, causing the protective layer to peel. The invention is advantageously completed by the following characteristics, taken individually or in any of their technically possible combinations:

the step of bombarding charged particles is carried out by a plasma;

the method comprises a step of depositing the metal sub-layer in the vapor phase on the substrate before step a);

the workpiece is heated under vacuum at a temperature above 1000 ° C between steps a) and b); the part is heated between 800 ° C. and 1200 ° C. between the deposition of the metal underlayer and step a).

the workpiece is rotated during step a);

the piece is kept under vacuum between steps a) and b).

the part is heated to a temperature greater than 1000 ° C. during step b);

step a) is implemented in a first vacuum chamber, step b) is implemented in a second vacuum chamber, and the piece is transported, between steps a) and b), of the first speaker to the second speaker in a passage, kept under vacuum, connecting the two speakers.

The invention also relates to a turbine part comprising: a monocrystalline nickel base superalloy substrate,

a metal sub-layer covering the substrate, and

a protective metal oxide layer covering the underlayer, characterized in that the metal underlayer has a surface in contact with the protective layer and in that the surface has an average roughness of between 100 nm and 1 μηη.

The invention is advantageously completed by the following characteristics, taken individually or according to any of their technically possible combinations:

the standard deviation of the roughness of the surface is less than 20% of the average roughness of the surface;

the protective layer comprises a layer of alumina in phase a.

PRESENTATION OF THE DRAWINGS

Other features and advantages will emerge from the description which follows, which is purely illustrative and nonlimiting, and should be read in conjunction with the appended figures, among which:

- Figure 1 schematically illustrates a section of a turbine part, for example a turbine blade or a fin distributor; FIG. 2 is a photomicrograph illustrating a section of a portion of the thermal barrier of the turbine part;

- Figure 3 illustrates a method of manufacturing a turbine part;

- Figure 4 schematically illustrates the section of a turbine part of part;

FIG. 5 is a photomicrograph illustrating the surface of the metal underlayer in contact with the protective layer;

FIG. 6 illustrates a device for depositing the metal underlayer;

FIG. 7 illustrates a device for bombarding charged particles on the metal sub-layer;

- Figure 8 illustrates a device for keeping the turbine part under vacuum between a step of etching the metal underlayer and a step of forming the protective layer. DEFINITIONS

The term "superalloy" refers to a complex alloy having, at high temperature and at high pressure, very good resistance to oxidation, corrosion, creep and cyclic stresses (particularly mechanical or thermal). Superalloys find particular application in the manufacture of parts used in aeronautics, for example turbine blades, because they are a family of high-strength alloys that can work at temperatures relatively close to their melting points (typically 0). , 7 to 0.8 times their melting temperatures).

The "base" of the superalloy refers to the main metal component of the matrix. In the majority of cases, the superalloys comprise an iron, cobalt or nickel base, but also sometimes a titanium or aluminum base.

"Nickel-based superalloys" have the advantage of offering a good compromise between oxidation resistance, high temperature rupture strength and weight, which justifies their use in the hottest parts of turbojet engines.

The term "vacuum" denotes a primary, medium or high vacuum, that is to say characterized by a pressure of between 10 ^-3 and 5 mbar, such a vacuum can be adapted to a bombardment of charged particles by example by the forming a plasma at room temperature. The plasma can be an argon plasma.

The term "alumina" refers to an allotropic variety of alumina corresponding to corundum, of rhombohedral crystalline structure. A layer of alumina may be formed by several grains of α-alumina, each of the grains delimiting a crystalline phase a.

Roughness is generally understood to mean a measurement of the surface state representative of the deviations in the normal direction of an average plane locally tangent to the surface considered. By average roughness, R _a , the arithmetic average of the standard deviations of a surface from the mean surface, will be referred to as:

where Vi is a measure of a deviation of the surface from the mean surface.

Homogeneity of the roughness is defined as a roughness dispersion smaller than a reference dispersion, characterized and / or measured by a standard deviation of the roughness of a surface less than 20% of the average roughness. DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 3, the manufacturing method 100 of a turbine part comprises the following steps.

During a first step 101 of the manufacturing process of the part 1, a metal sub-layer 3 is deposited on a substrate 2 base nickel monocrystalline. For example, one or more metal layers comprising nickel and / or aluminum may be deposited by physical vapor deposition (PVD). Such a deposit can be made by sputtering, and / or by any other known method of PVD.

In a second step 102 of the method, the substrate provided with the metal underlayer is heated to a temperature T of between 800 ° C. and 1200 ° C. This heat treatment causes the diffusion of the metal ions of the underlayer 3 into the substrate 2 so as to form an interdiffusion zone, allowing better resistance to oxidation during the use of the part. In a third step 103 of the method, a surface of the metal sub-layer 3 is bombarded with charged particles. These particles may be ions, such as argon ions, and / or electrons. For example, it is possible to etch a surface of the metal sub-layer 3 with the plasma 7, that is to say by using a plasma 7. The substrate provided with a metal underlayer can be placed in an enclosure kept under vacuum , in which a continuous flow of one or more gases supplying the chemical element (s) composing the plasma is controlled. In general, one or more gases are used for metal etching. Advantageously, argon or oxygen is used. This charged particle bombardment step is used to etch the metastable oxides formed natively on the surface 16 of the underlayer 3.

Thus, the surface roughness 16 may be smaller than using known methods of the prior art, such as sanding and electrochemical etching. The surface 16 of the metal sub-layer 3 has for example an average roughness R _a less than 1 μιτι, preferably less than 500 nm and preferably between 00 nm and 300 nm.

The use of a bombardment of charged particles also makes it possible to etch the entire surface 16 of the part in a homogeneous manner. This effect is particularly suitable for parts 1 whose geometry is complex. For example, the standard deviation of the roughness on the surface 16 of the plasma etched sub-layer 3 is less than 500 nm, preferably less than 300 nm and preferably less than 100 nm.

In general, the bombardment of charged particles of step 103 may be carried out by any ionic and / or electronic bombardment method for etching a metal surface with a roughness R _{a of} less than 1 μm. It can also be performed using a femtosecond laser.

Advantageously, the part 1 is rotated during the step 103 of bombardment of charged particles. For this purpose, the piece 1 can be arranged in a drum in the enclosure or on a rotary support. The rotation of the part makes it possible to increase the homogeneity of the roughness of the surface 16 of the underlayer 3. As the bombardment of charged particles does not cause any mechanical contact during the etching, the transport of impurities on the surface 16 of the underlayer 3 is avoided.

In a fourth step 104 of the process, the part is heated, advantageously under vacuum, to a temperature greater than 1000 ° C. Thus, atoms of the plasma, such as argon atoms, possibly adsorbed on the surface 16 of the metal underlayer 3, are removed or transported away from the room.

In a fifth step 105 of the process, the protective layer 4 is formed on the bombarded surface 16 of the metal sub-layer 3. The surface 16 may be a plasma-etched surface during the step 103 of the process. The protective layer 4 is advantageously only composed of α-alumina. For this purpose, the part is heated in an atmosphere comprising oxygen at a temperature above 1000 ° C., so as to form a protective layer 4 by thermal oxidation. Preferably, the temperature of 1000 ° C. is reached in less than ten minutes and preferably in less than five minutes, so as to avoid the formation of metastable oxide on the metal sub-layer 3.

The roughness R _a of the surface 16 of the metal sub-layer 3, small with respect to the usual roughness values, makes it possible to form a protective layer 4 comprising α-alumina grains whose size is greater than the grains of α-alumina. protective layers made according to known methods. The protective layer 4 may for example comprise a layer of alumina in phase a. This layer may be formed of grains having an average size in a plane locally tangent to the surface 16, greater than 50 ^μι η. Increasing the grain size of alumina a makes it possible to increase the lifetime of the thermal barrier. The protective layer 4 may also comprise a layer of alumina exclusively in phase a.

In addition, the homogeneity of the roughness of the surface 16 of the metal sub-layer 3 bombarded by charged particles makes it possible to form the protective layer 4 at constant kinetics on the surface 16 of the metal sub-layer 3. Thus, the protective layer 4 formed has substantially constant mechanical properties and thickness on the surface 16 of the sub-surface. metal layer 3, which avoids mechanical stresses during the use of the workpiece, causing peeling of the protective layer 4.

All the steps of the process can advantageously be carried out under vacuum, or generally, without exposing the room to the ambient atmosphere. In particular, the part can be kept under vacuum between the steps 103 and 105 of the process. Thus, the formation of unstable and / or metastable oxide on the surface 16 is avoided.

FIG. 4 schematically illustrates the section of a turbine part part 1 obtained by a method according to the method of FIG. 3. The turbine part 1 is for example a turbine blade, a distributor blade or any other element, part or piece of turbine. It comprises a monocrystalline nickel base superalloy substrate 2, a metal sub-layer 3 covering the substrate 2 and a protective metal oxide layer 4 covering the underlayer 3. A thermally insulating layer 5 may for example cover the protective layer 4. The thermal barrier 10 comprises the metal sub-layer 3, the protective layer 4 and the thermally insulating layer 5. The metal sub-layer 3 has a surface 16 in contact with the protective layer 4 having a roughness of less than 1 μm, preferably less than 1 μm. at 500 nm and preferably between 100 and 300 nm.

Figure 5 is a photomicrograph of a detail of a turbine part 1. The black rectangle of Figure 5 is a scale bar corresponding to 5 μm. The part comprises a protective metal oxide layer 4 covering a metal sub-layer 3. In this embodiment of the invention, the metal sub-layer 3 has been etched with plasma, then a protective layer 4 has been formed on the metal underlayer 3.

With reference to FIG. 6, the PVD deposit corresponding to step 101 may be made inside an enclosure 12 containing part 1 and one or more target (s) 8 corresponding to (x) material (s). to place. The part 1 illustrated in FIG. 6 can be a turbine blade 6, a distributor blade, or any other element, part or part of a turbine. The superalloy substrate 2 can be biased by an electrical connection 15 connected to a potential generator electric. Under the application of a positive potential difference between the target (s) 8 and the substrate 2, an argon plasma 7 can be formed, the positive species of which are attracted to the cathode (target 8) and collide with it. The atoms of the target (s) 8 are pulverized and then condense on said part so as to form the sub-layer (s) 3 metal. Preferably, the deposit conditions are as follows:

heating during the deposition: from 100 to 900 ° C .;

pressure: from 0.1 Pa to 1 Pa;

- power density: 2 to 15 W / cm ² ;

- polarization: from 0 to 400 V.

The ion bombardment is carried out for 10 to 30 minutes.

With reference to FIG. 7, the bombardment of charged particles, for example implemented by a plasma 7, corresponding to step 103, can be carried out inside an enclosure 12 containing part 1 and one or more targets 8 corresponding to the material (s) to be deposited. The enclosure may be the enclosure used during step 101 illustrated in FIG. 6. The superalloy substrate 2 may be polarized by an electrical connection 15 connected to an electric potential generator. Under the application of a negative potential difference between the target (s) 8 and the substrate 2, an argon plasma 7 may be formed, the positive species of which are attracted to the cathode (turbine part). and collide with it. Thus, the surface 16 of the metal sub-layer 3 can be etched. Preferably, the deposit conditions are as follows:

pressure: from 0.1 Pa to 1 Pa;

- power density: 2 to 15 W / cm ² ;

- polarization: from 0 to - 400 V.

With reference to FIG. 8, step 103 of manufacture of the part can be carried out in a first enclosure 13. The part can be transported from the first enclosure to a second enclosure 14, in which is implemented the step 105, in a passage 9 maintained under vacuum, connecting the two enclosures 13, 14. The passage may be delimited by a channel, a conduit and / or a pipe. Thus, the workpiece can be kept under vacuum between steps 103 and 105 so as to avoid the formation of metastable or unstable oxide before formation of the protective layer 4 in step 105. The passage may comprise a valve 11, for controlling the vacuum in only one of the first or second speakers, depending on the step of manufacturing the workpiece. The opening of the valve 11 is adapted to transport the turbine part, from the first chamber to the second chamber.

Claims

1. A method of manufacturing a turbine part (1) comprising:

a substrate (2) made of monocrystalline nickel base superalloy,

a metal sub-layer (3) covering the substrate, and

a protective metal oxide layer (4) covering the underlayer, the method comprising steps of:

a) bombarding charged particles (7) on a surface (16) of the metal underlayer, then

b) formation of the protective layer (4) on the bombarded surface during step a).

2. The method of claim 1, wherein the bombardment of charged particles is implemented by a plasma.

3. Method according to claim 1 or 2, comprising a step of depositing the metal undercoat in the vapor phase on the substrate before step a) of the method.

4. Method according to one of claims 1 to 3, comprising a step of heating the room, under vacuum, at a temperature above 1000 ° C, between steps a) and b).

5. Method according to one of claims 1 to 4, wherein the part is heated between 800 ° C and 1200 ° C between the deposition of the metal underlayer and step a).

6. Method according to one of claims 1 to 5, wherein the part is rotated during step a).

7. Method according to one of claims 1 to 6, wherein the piece is kept under vacuum between steps a) and b).

8. Method according to one of claims 1 to 7, wherein the room is heated to a temperature above 1000 ° C during step b).

9. Method according to one of claims 1 to 8, wherein step a) is implemented in a first chamber (13) under vacuum, step b) is implemented in a second chamber (14) under vacuum, and wherein the part is transported, between steps a) and b), from the first chamber to the second chamber in a passage (9), maintained under vacuum, connecting the two enclosures.

A turbine part (1) comprising:

a substrate (2) made of monocrystalline nickel base superalloy,

a metal underlayer (3) covering the substrate, and

a protective metal oxide layer (-4) covering the underlayer, characterized in that the metal underlayer has a surface (16) in contact with the protective layer and the surface has an average roughness of between 100 nm and 1 \\ m.

1 1. The turbine part of claim 10 wherein the standard deviation of the roughness of the surface is less than 20% of the average roughness of the surface (16).

Turbine part according to claim 10 or 11, wherein the protective layer comprises a layer of alumina in phase a.