FR3141473A1 - Coating comprising hafnium - Google Patents

Abstract

Coating comprising hafnium The invention relates to a method of forming a coating (50) on a surface of a metal substrate (10), the method (2000) comprising at least the following steps: a) depositing 'a hafnium layer (20) whose thickness e1 is between 0.2 µm and 10 µm; b) deposition of a layer of platinum (30) whose thickness is between 2.0 µm and 10 µm; c) aluminization of the part obtained after steps a) and b). Figure for abstract: Fig. 3

Description

Coating comprising hafnium

The invention relates to the field of metal parts coated with a thermal barrier and more precisely to bonding layers to increase adhesion between the thermal barrier and the metal parts.

Generally speaking, the performance of an aeronautical turbomachine depends on the temperatures reached.

However, the high temperatures and the particular chemical environment of turbomachines subject the parts of the turbomachine to significant stresses.

Conventionally, materials used for parts in such environments are coated with a thermal barrier and/or an environmental barrier in order to improve their lifespan.

To improve the adhesion of the thermal barrier to a substrate, it is generally proposed to interpose an adhesion layer, also called an “underlayer” between the substrate and the thermal barrier, the adhesion layer fulfilling two main roles : protect the substrate against oxidation and corrosion and promote adhesion of the thermal barrier.

The adhesion of the adhesion layer to the substrate and to the thermal barrier is important because it increases the lifespan of the thermal barrier and, consequently, of the entire part.

To improve the adhesion of the part, it is sometimes proposed to boost the adhesion layer with hafnium. Two solutions exist for this, but neither offers complete satisfaction.

According to a first option, it is possible to add a large quantity of hafnium to the composition of the substrate, so that the quantity of hafnium migrating by diffusion in the adhesion layer, during its production, is more important.

This option nevertheless requires modifying the composition of the metal substrate, which is rarely desirable. In addition, this requires working with substrates comprising a greater hafnium content which makes it more difficult to dissolve the hafnium in said substrate.

According to a second option, it is possible to add a small quantity of hafnium to the primer layer directly during its deposition.

Unfortunately, we see when trying to put this option into practice that the quantity of hafnium and the homogeneity of the distribution of hafnium in the thickness of the adhesion layer is not perfectly controlled. There is therefore a risk of degrading the oxidation life and the performance of the thermal barrier.

There therefore remains a need for a process for manufacturing an adhesion layer making it possible to control the quantity of hafnium and the homogeneity of its distribution in the adhesion layer.

The invention aims precisely to meet this need and proposes, according to a first of its aspects, a method of forming a coating on a surface of a metal substrate, the method comprising at least the following steps:

a) the deposition of a layer of hafnium whose thickness is between 0.2 µm and 10 µm;

(b) the deposition of a layer of platinum the thickness of which is between 2.0 µm and 10 µm;

c) aluminization of the part obtained after steps a) and b).

The process is advantageously better controlled and more repeatable than the processes of the prior art. In addition, it makes it possible to obtain a coating which ensures an improvement in the lifespan of the final part, after a thermal barrier is deposited on the coating, compared to the parts of the prior art.

In one embodiment, the coating is an adhesion layer for a thermal barrier, for example an adhesion layer comprising nickel, platinum and aluminum which is doped with hafnium.

Indeed, the coating described can be used as an adhesion layer for a thermal barrier on a metal substrate. Such an adhesion layer will have all the advantages of the presence of hafnium described above and also presenting good homogeneity of the hafnium in the adhesion layer, without the need to modify the composition of the metal substrate.

In one embodiment, steps a) and b) can be carried out by chemical vapor deposition (CVD) processes or physical vapor deposition (CVD) processes. Vapor Deposition” or PVD). In one embodiment, the deposition of the platinum layer in step b) is carried out by electrolytic deposition.

In one embodiment, step b) may or may not be followed by a diffusion heat treatment.

For example, such a diffusion heat treatment can be carried out at a temperature of between 1050°C and 1100°C and for a time of between 30 minutes and 2 hours and at a pressure of between 10 ^-5 and 10 ^-6 mbar.

However, such diffusion heat treatment is not necessary, because it was observed that the subsequent aluminization step c) already allowed good diffusion of the species between the layers deposited during steps a) and b).

The process is not limited by the relative organization of steps a) and b).

In one embodiment, step b) is carried out after step a).

In one embodiment, step b) is carried out before step a).

The presence of the hafnium layer makes it possible to reinforce the grain boundaries in the coating, which blocks the diffusion of metal cations and also slows down the diffusion of oxygen.

In addition, the presence of hafnium in the coating also makes it possible to block the diffusion of sulfur, whether it comes from the substrate or from the electrolytic platinum deposit.

The presence of the platinum layer makes it possible to provide the platinum necessary for the formation of the desired coating.

Also, the thicknesses of the platinum and hafnium layers ensure that the content of these elements in the final part complies with what is desired.

Aluminization step c) is a step known as such. Applied to the process of the invention, it allows on the one hand an enrichment of the aluminum coating, and also the diffusion of the species present in the hafnium and platinum layers or even of the species present in the substrate to form, at the surface of the substrate a single coating layer, for example an adhesion layer.

Thanks to the aluminization step, a surface layer of the coating is formed which includes a significant content of aluminum, platinum and nickel which also includes hafnium, due to the diffusion of these species from the substrate or the layers deposited in steps a) and b).

Indeed, after aluminization, the layers of platinum and hafnium formed during steps a) and b) are no longer discrete because the diffusion caused by the aluminization has allowed the elements of these two layers to mix in the layer formed.

In one embodiment, the aluminization can be carried out at a temperature between 980°C and 1150°C, preferably between 1040°C and 1150°C for a period of between 2 and 8 hours, for example between 2 and 3 hours.

After the aluminization treatment of step c), the coating is no longer present in the form of distinct layers but in the form of a single layer comprising all the species having diffused during the aluminization.

Such a coating notably comprises a high and homogeneous hafnium content, compared to the coatings obtained by processes of the prior art.

In one embodiment, the aluminization step is carried out without additional addition of hafnium.

Indeed, the presence of the hafnium layer deposited during step a) is sufficient to guarantee the beneficial effects of the presence of hafnium without it being necessary to add more.

The aluminization step without additional addition of hafnium is much easier and does not require adaptation of existing aluminization processes compared to an aluminization stage during which we would also like to add hafnium.

Hafnium having a great affinity for sulfur, the latter remains blocked in the presence of hafnium and therefore no longer migrates towards the layer formed by aluminization.

This results in better adhesion of the thermal barrier layer to the adhesion layer, sulfur being known to cause the adhesion layer to detach, and therefore harm the performance of the thermal barrier.

In addition, the provision of hafnium in the form of a thin layer rather than a deposit carried out jointly with the formation of the adhesion layer ensures excellent homogeneity of the distribution of hafnium in the layer. hook.

According to another of its aspects, the invention also relates to a method of manufacturing a metal part coated with a thermal barrier comprising at least the following steps:

- the formation of a coating on a surface of a metal substrate by a process as just described;

- the deposition of a thermal barrier on said coating.

The invention is not limited by the nature of the thermal barrier, or the method of depositing it on the coating.

As has just been described, this embodiment makes it possible to have bonding layers guaranteeing better adhesion, and better resistance to oxidation of thermal barriers than the bonding layers of the prior art.

Thus, the parts according to the invention have better resistance over time than those of the prior art, whose underlayers would be different.

In one embodiment, the thermal barrier may comprise zirconia stabilized with yttrium oxide (often called YSZ for the acronym in English “Yttria-stabilized zirconia”) or gadolinium zirconate, of formula Gd ₂ Zr ₂ O ₇ (often called GdZ).

In one embodiment, the thermal barrier can be deposited by plasma suspension (or SPS for the acronym in English "Suspension Plasma Spraying"), or by electron beam physical vapor deposition processes (or EBPVD, for the acronym in English “Electron Beam Physical Vapor Deposition”). For example, the thermal barrier may comprise a single layer of zirconia stabilized with yttrium oxide or a layer of zirconia stabilized with yttrium oxide and a layer of gadolinium zirconate.

According to another of its aspects, the invention relates to a coated metal part comprising:

- a metal substrate;

- a layer of hafnium whose thickness is between 0.2 µm and 10 µm, and placed on one side of the substrate; And

- a platinum layer whose thickness is between 2.0 µm and 10 µm, and placed on the hafnium layer, on the side of the hafnium layer opposite the substrate.

According to another of its aspects, the invention relates to a coated metal part comprising:

- a metal substrate;

- a layer of platinum whose thickness is between 2.0 µm and 10 µm, and placed on one side of the substrate; And

- a hafnium layer whose thickness is between 0.2 µm and 10 µm and placed on the platinum layer, on the side of the platinum layer opposite the substrate.

These two stacks make it possible, after an aluminization step c), as described above, to obtain a coating comprising a high and homogeneous hafnium content.

For example, the metal substrate can be chosen from nickel superalloys known under the names AM1, N5, AM21, MCNG or CMSX10.

In one embodiment, the platinum layer comprises platinum at more than 90% by mass, or even more than 99% by mass. For example, the platinum layer does not include any element other than platinum and inevitable impurities.

In one embodiment, the hafnium layer comprises hafnium at more than 90% by mass, or even more than 99% by mass. For example, the hafnium layer does not include any element other than hafnium and inevitable impurities.

The invention is not limited by the relative arrangement of the hafnium layer and the platinum layer.

In one embodiment, the hafnium layer is located between the metal substrate and the platinum layer and preferably directly in contact with them.

Such an embodiment makes it possible to reinforce the grain boundaries of the sub-layer, thus blocking the diffusion of metal cations and slowing down the diffusion of oxygen in the latter and therefore the kinetics of oxidation of the sub-layer and neutralizing the sulfur. in consideration of the substrate.

In one embodiment, the hafnium layer is located on the platinum layer and preferably directly in contact with it.

Such an embodiment makes it possible to reinforce the grain boundaries of the underlayer, thus blocking the diffusion of metal cations and slowing down the diffusion of oxygen in the latter and therefore the kinetics of oxidation of the underlayer.

In addition, this makes it possible to neutralize sulfur coming from the substrate and also platinum deposits, which ensures an even better lifespan for the thermal barrier. Sulfur is in fact known to cause the bonding layer to peel off.

In one embodiment, the metal substrate is a turbomachine blade or a turbomachine distributor.

There schematically represents a method according to one embodiment of the invention.

There schematically represents a method according to another embodiment of the invention.

There schematically represents a part according to one embodiment of the invention.

There schematically represents a part according to another embodiment of the invention.

There schematically represents a part according to another embodiment of the invention.

There represents test results comparing two parts according to the invention to two parts of the prior art.

There is a micrograph obtained by scanning electron microscopy, identifying measurement points of the atomic composition of an adhesion layer obtained by a method of the invention.

The invention is now described by means of figures, presented for descriptive purposes to illustrate certain embodiments of the invention and which should not be interpreted as limiting the latter. In particular and unless otherwise stated, the figures are represented without scales, and the relationships between the distances are not intended to be realistic.

As described, the invention relates to a method 2000, comprising the following steps:

a) the deposition of a layer of hafnium whose thickness is between 0.2 µm and 10 µm;

(b) the deposition of a layer of platinum the thickness of which is between 2.0 µm and 10 µm;

c) aluminization of the part obtained after steps a) and b).

According to a first embodiment of the invention, described in , step a) is carried out before step b).

According to another embodiment of the invention, described in , step b) is carried out before step a).

Indeed, it is not necessary, to obtain the technical effect of the invention, for the deposition of the platinum layer to be carried out specifically before or after the deposition of the hafnium layer, provided that these two layers are deposited before the aluminization step.

In one embodiment, the aluminization step can be carried out in a manner known per se.

Generally speaking, aluminization processes are carried out by bringing the superalloy into contact with gaseous aluminum precursors, for example FAlCl ₃ , under pressure and temperature conditions which allow the formation of gaseous aluminum. The aluminum thus formed reacts with the nickel of the substrate which diffuses towards the surface of the substrate, with the help of the temperature to form a layer comprising nickel and aluminum on the surface of the substrate, for example Ni ₂ Al ₃ .

Furthermore, in the process of the invention, the aluminization being carried out after the deposition of the platinum and hafnium layers, the layer formed by aluminization therefore also includes these elements.

Of course, other elements present in the substrate can migrate by diffusion into the layer formed by aluminization.

For example, the coating includes the elements of the substrate, as well as a majority of nickel, platinum and aluminum as well as hafnium. Such a layer is often called NiPtAl, and is here doped with hafnium.

In one embodiment, the coating does not include other elements present in a content of more than 5% by mass than nickel, aluminum and platinum.

In one embodiment, the coating further comprises up to 5% by weight of chromium and/or cobalt.

In one embodiment, the coating comprises between 0.5 and 3% by weight of hafnium.

For example, an aluminization treatment can be carried out at a temperature between 1040°C and 1080°C, for a duration between 200 and 500 minutes, and with a flow rate of aluminum precursor, for example FAlCl₃ between 0.23 and 1.0 L.min^-1.

Unlike the processes of the prior art, the presence of the hafnium layer allowing the production of the adhesion layer by diffusion ensures on the one hand the presence, and on the other hand a homogeneous distribution of the hafnium in the adhesion layer, without requiring modification of the composition of the underlying substrate.

This ensures that the thermal barrier which will be deposited on the adhesion layer has improved adhesion properties, which will improve the lifespan of the part, as will be described in the example, in connection with the .

There describes a part 1000 obtained at the end of step c) of aluminization of a process 2000 carried out in accordance with the embodiment of the .

Such a part 1000 includes:

- a metal substrate 10;

- a layer of hafnium 20 with a thickness e ₁ of between 0.2 µm and 10 µm;

- a layer of platinum 30 with a thickness e ₂ of between 2.0 µm and 10 µm.

As indicated, the two layers 20, 30 are intended to form the adhesion layer 50 by aluminization.

In one embodiment and as shown, the hafnium layer 20 can be directly in contact with the substrate 10.

In one embodiment and as shown, the platinum layer 30 can be directly in contact with the hafnium layer 20, on the side opposite the substrate 10.

There describes a part 1000 obtained at the end of step c) of a process 2000 carried out in accordance with the embodiment of the .

Indeed, the inversion of steps a) and b) has the effect of obtaining on the surface of the metal substrate 10 the platinum layer 30 and the hafnium layer 20 on the surface of the platinum layer 30.

In one embodiment and as shown, the platinum layer 30 can be directly in contact with the substrate 10.

In one embodiment and as shown, the hafnium layer 20 can be directly in contact with the platinum layer 30, on the side opposite the substrate 10.

There describes a part 1000 obtained after aluminization of a part as described in the or 4 indifferently.

The part 1000 no longer comprises two distinct layers 20, 30 but a single layer 50, which also comprises aluminum provided by aluminization.

There illustrates comparative results of oxidation resistance for two parts according to the invention 101, 102 and two parts outside the invention 201, 202.

The four parts are similar and include a metal substrate, an adhesion layer and a thermal barrier placed on the adhesion layer.

However, the bonding layers of the parts outside the invention 201, 202 were obtained according to a process of the prior art, and do not include hafnium, while the parts according to the invention include hafnium and were obtained according to a process of the invention.

There represents on the abscissa 100 the number of oxidation cycles, and on the ordinate 200 the mass gain in mg.cm ^-2 observed for the part. The experiments are carried out at 1150°C.

It can be read on the that the loss of mass (on the , the mass gain 200 is in fact negative) is greater from 2000 cycles for the parts of the prior art than for the parts of the invention.

There aims to characterize the homogeneity of the distribution of hafnium in a primer layer 50 of the invention.

For this, nine samples were taken, spaced in three columns across the width of the sample (1, 2 and 3 on the ) and in three layers 50a, 50b and 50c in the thickness, the sampling points being marked by squares.

The mass composition in 6 elements for six of the identified points is reported in Table 1.

[Tab. 1]

Table 1 clearly illustrates that the process of the invention makes it possible to obtain low variations in the hafnium content in the adhesion layer 50, the hafnium content remaining everywhere between 1.2% and 1.6% by weight. .

Claims (9)

A method of forming a coating (50) on a surface of a metal substrate (10), the method (2000) comprising at least the following steps:
a) the deposition of a layer of hafnium (20) whose thickness (e ₁ ) is between 0.2 µm and 10 µm;
b) the deposition of a layer of platinum (30) whose thickness (e ₂ ) is between 2.0 µm and 10 µm;
c) aluminization of the part obtained after steps a) and b). Method according to claim 1, in which step b) is carried out after step a). Method according to claim 1, in which step b) is carried out before step a). Method according to any one of claims 1 to 3, in which step b) is followed by a diffusion heat treatment. Process for manufacturing a metal part coated with a thermal barrier comprising at least the following steps:
- the formation of a coating (50) on a surface of a metal substrate (10) by a method of any one of claims 1 to 4; And
- the deposition of a thermal barrier on said coating. Coated metal part (1000) comprising:
- a metal substrate (10);
- a hafnium layer (20) whose thickness (e ₁ ) is between 0.2 µm and 10 µm, and placed on one face of the substrate; And
- a platinum layer (30) whose thickness (e ₂ ) is between 2.0 µm and 10 µm and placed on the hafnium layer, on the side of the hafnium layer opposite the substrate. Coated metal part (1000) comprising:
- a metal substrate (10);
- a platinum layer (30) whose thickness (e ₂ ) is between 2.0 µm and 10 µm and placed on one face of the substrate; And
- a hafnium layer (20) whose thickness (e ₁ ) is between 0.2 µm and 10 µm and placed on the platinum layer, on the side of the platinum layer opposite the substrate. Coated metal part according to claim 6 or 7, in which the metal substrate is chosen from nickel superalloys known under the names AM1, N5, AM21, MCNG or CMSX10. Coated metal part according to any one of claims 6 to 8, in which the metal substrate is a turbomachine blade or a turbomachine distributor.