FR3113261A1 - SURFACE PREPARATION PROCESS COMPATIBLE WITH THE Y/Y' COATING AND THE SPS DEPOSIT PROCESS - Google Patents

Abstract

Surface preparation process compatible with the γ/γ' coating and the SPS deposition process. The invention relates to a coating of a substrate with a thermal barrier comprising at least the following steps: - a step of creating a first roughness on the surface of the substrate by at least one chemical or electrochemical attack, the first roughness being characterized by a maximum roughness value Rz1 and an arithmetic average roughness along the profile Ra1; - a step of depositing an undercoat; - a step of creating a second roughness on the surface of the underlayer by at least sandblasting or electrochemical attack, the second roughness being characterized by a maximum roughness value Rz2 and an arithmetic average roughness along the profile Ra2, the Rz2 and Ra2 values being lower than the roughness values Rz1 and Ra1; - a step of depositing a thermal barrier by SPS. Figure for abstract: Fig. 2

Description

Surface preparation process compatible with the γ/γ’ coating and the SPS deposition process.

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

The provision of a thermal barrier on the surface of a substrate or an underlayer makes it possible to protect them during use at high temperature.

Thermal barriers are generally deposited on a first coating of the substrate, called undercoat, which can have two roles: protect the substrate against oxidation and promote the grip of the thermal barrier.

Such a sub-layer is generally chosen to form an alumina layer on its surface, the alumina layer fulfilling the two aforementioned roles very well, in particular when the alumina is in its coarse-grained α form.

In the case of deposition of thermal barriers by spraying by plasma suspension, it has been observed that the roughness of the underlayer plays an important role for the adhesion of the thermal barrier.

The spray deposition process by plasma suspension is abbreviated in the rest of the application by the abbreviation “SPS” (for the acronym in English “Suspension plasma spraying”).

Thus, it is generally proposed to texturize the underlayer before the deposition of the thermal barrier. This texturing, i.e. a step that seeks to obtain a particular roughness on the surface of the undercoat, is carried out by sandblasting. The purpose of such sandblasting is to strip unstable oxides and impurities, create roughness in the underlayer, and promote the formation of alumina.

For nickel-based or cobalt-based superalloy substrates, γ-Ni/γ’-Ni3Al sub-layers are currently proposed which make it possible to form an alumina layer on their surface. However, it is observed that sandblasting as usually practiced strips a significant part of the undercoat. Thus, not only does sandblasting reduce the thickness of the undercoat, which reduces its resistance to corrosion and oxidation, but in addition, the reduction in thickness of the undercoat is not uniform over the whole surface of the part, especially when the part has a complex geometry.

These two drawbacks therefore make the deposition of a thermal barrier by SPS incompatible with the process for preparing the underlayers of the prior art.

It is therefore necessary to have a method devoid of the abovementioned disadvantages in order to enable superalloy substrates to be covered.

The invention aims precisely to respond to this problem.

Thus, according to a first of its aspects, the invention relates to a process for coating a substrate with a thermal barrier comprising at least the following steps:

- a step of creating a first roughness on the surface of the substrate by at least one chemical and/or electrochemical attack, the first roughness being characterized by a maximum roughness value R _z 1 and an arithmetic average roughness along the profile R _has 1;

- a step of depositing an undercoat;

- a step of creating a second roughness on the surface of the underlayer by at least sandblasting or electrochemical attack, the second roughness being characterized by a maximum roughness value R _z 2 and an arithmetic average roughness along of the profile R _a 2, the values R _z 2 and R _a 2 being lower than the roughness values R _z 1 and R _a 1;

- a step of depositing a thermal barrier by SPS.

The inventors have observed that a first high roughness makes it possible to obtain a very good lifespan of the thermal barrier.

In one embodiment, the first roughness is characterized by a maximum roughness value R _z 1 comprised between 13 and 16 μm and an arithmetic mean of roughness along the profile R _a 1 comprised between 2.5 and 3.0 μm.

By “roughness” within the meaning of the application and unless explicitly mentioned to the contrary, is meant a pair of values comprising a maximum roughness value R _z and an arithmetic mean value of roughness along the profile R _a . Moreover, one roughness will be said to be “lower” than another, if the two values R _z and R _a of the first roughness are respectively lower than the values R _z and R _a of the second.

For example, the roughness of a surface can be measured using a roughness meter or a 3D microscope.

The inventors have found that a method according to the invention allows excellent adhesion of the thermal barrier to the undercoat, very good resistance of the undercoat to corrosion and oxidation, and also to obtain 'a uniform γ/γ' sub-layer, whatever the geometry of the substrate.

These advantages make it possible to form a thermal barrier on a substrate by SPS, unlike a method comprising a sandblasting step in accordance with those of the prior art.

In one embodiment, the step of creating a first roughness may include an acid etching step. For example, the substrate can be exposed to an acidic solution, for example by immersion in a bath of solution.

For example, the acid solution may comprise fluonitric acid, nitric acid, hydrochloric acid, ferric chloride, hydrogen peroxide or a mixture of the aforementioned acids.

Preferably, the solution is chosen from a solution of fluonitric acid, a solution of hydrochloric acid and ferric chloride, a solution of nitric acid, hydrochloric acid and ferric chloride, a solution of hydrochloric acid and hydrogen peroxide or a solution of nitric acid and hydrochloric acid.

In one embodiment, the creation of the first roughness includes only acidic or electrochemical etching.

In one embodiment of the invention, the step of creating the first roughness comprises an acid attack and an electrochemical attack.

For example, the substrate can be exposed to a solution comprising acetic acid, nitric acid, sulfuric acid, phosphoric acid or a mixture of these acids.

In one embodiment, the chemical attack or attacks can be carried out by exposure for 3 to 30 minutes to a solution of hydrochloric acid having a concentration between 30 and 50 g/L, and of ferric chloride in a concentration of 400 to 500 g/L at a temperature between 50 and 70°C.

In one embodiment, the electrochemical attack or attacks can be carried out by exposure for 2 to 10 minutes under a current of 3 to 20 A, and in a solution of phosphoric acid at 70% by volume, for example with 300 to 800 mL of phosphoric acid in 200 to 700 mL of water.

Preferably, the solution is chosen from a solution of nitric acid and acetic acid, a solution of sulfuric acid, or a solution of phosphoric acid.

In one embodiment, the creation of the first roughness includes only an electrochemical etching step.

In one embodiment, it is possible to create the first roughness by the joint or successive application of an acid chemical attack and an electrochemical attack.

Creating the first roughness by exposing the substrate to a solution, whether by chemical or electrochemical attack or the joint action of the two makes it possible to create a first homogeneous roughness on the surface of the substrate. Indeed, whatever the complexity of the geometry of the substrate, such a step ensures that the entire surface of the substrate is exposed to equivalent conditions, thus allowing the creation of a homogeneous roughness.

Using a chemical attack followed by an electrochemical attack makes it possible to precisely choose the conditions of each of the attacks according to the substrate.

In one embodiment, the roughness created at the surface of the substrate can be checked by macrography before the deposition of the underlayer. This step ensures that the texturing obtained is sufficient to guarantee good adhesion of the undercoat to the substrate.

In one embodiment, the substrate is chosen from single crystals of nickel, a nickel-based or cobalt-based superalloy, for example chosen from nickel-based alloys with trade names R125, R77, INCO78, DS200, CMSX4 SLS, CMSX4 PLUS or among cobalt-based alloys such as MARM509.

In one embodiment, after the creation of the first roughness, the deposition of the underlayer can be carried out by spraying, by physical vapor deposition or by chemical vapor deposition, or even by HVOF spraying (for the acronym in English High Velocity Oxy Fuel) or by electrolytic deposition.

Preferably, the deposition of the underlayer is carried out by physical vapor deposition.

Physical vapor deposition ensures the deposition of an underlayer that matches the roughness of the substrate.

The sub-layer is preferably chosen from among a sub-layer composed of γ-Ni/γ′-Ni ₃ Al, of NiAl, of NiAlPt, of CoCrAlY or of NiCrAlY.

In a preferred embodiment, the underlayer is composed of γ-Ni/γ'-Ni ₃ Al.

In a method of the invention, after the deposition of the underlayer, a second roughness is created on the surface of the underlayer. The creation of this roughness, lower than the first roughness present between the substrate and the underlayer, makes it possible to improve the resistance of the thermal barrier to the underlayer, while preserving a sufficient thickness of the underlayer, guaranteeing thus its function of resistance to oxidation and corrosion.

The creation of the second roughness allows a fairly light stripping of the undercoat, over only a few micrometers, and thus makes it possible to maintain a good thickness of the undercoat which allows it to retain excellent resistance to oxidation and corrosion.

In one embodiment, the second roughness is characterized by a maximum roughness value R _z 2 of between 2.5 and 3.5 μm.

In one embodiment, the creation of the second roughness is carried out by sandblasting.

In another embodiment, the creation of the second roughness can be carried out by an electrochemical etching step.

For example, the creation of the second roughness can be created by exposing the substrate coated with the underlayer to a solution comprising acetic acid, nitric acid, sulfuric acid, phosphoric acid or a mixture of these acids.

Preferably, the solution is chosen from, a solution of nitric acid and acetic acid, a solution of sulfuric acid, and a solution of phosphoric acid.

Another step of the method of the invention is the deposition of the thermal barrier layer by SPS.

The deposition of the thermal barrier by SPS makes it possible to obtain a barrier of low thermal conductivity, and also makes it possible to reduce the costs of the deposition compared to other processes of the prior art.

There is a micrograph obtained by scanning electron microscopy of a substrate covered with an underlayer in accordance with the first steps of a method according to the invention.

There is a micrograph obtained by scanning electron microscopy of a substrate covered with an undercoat and a thermal barrier according to one embodiment of a method of the invention.

There is a micrograph obtained by scanning electron microscopy of a substrate covered with an underlayer without a first roughness having been created between the two.

There is a micrograph obtained by scanning electron microscopy of a substrate covered with an underlayer and a thermal barrier according to a method of the prior art.

The invention will now be described by means of figures having an illustrative purpose and should not be interpreted as limiting the invention.

There is a micrograph obtained by scanning electron microscopy of a substrate 11 covered with an underlayer 12 after creation of a first roughness in accordance with the first steps of a method of the invention.

For the substrate 11 shown in , the creation of the first roughness was carried out by exposing the substrate 11 to 3 to 30 minutes to a solution of hydrochloric acid having a concentration between 30 and 50 g/L, and of ferric chloride in a concentration of 400 to 500 g /L at a temperature between 50 and 70°C.

The roughness thus created can be characterized by its values R _z and R _a respectively comprised between 13 and 16 μm and 2.5 and 3 μm for the embodiment shown in .

Following the creation of this first roughness, the underlayer 12 is deposited by physical vapor deposition.

The deposits are made by physical vapor deposition, preferably by magnetron sputtering or by evaporation.

The proposed filing conditions are as follows:

- ion bombardment at a voltage varying from -200 V to 400 V for 10 to 30 minutes;

- a deposit at a power density of between 3 and 10 W/cm ² ;

- heating during deposition of between 200 and 700°C;

- a bias between -150 V and -300V.

- a very low pressure in the chamber, between 0.1 and 2 Pa in the case of magnetron sputtering and between 10 ^-4 and 10 ^-6 Pa in the case of evaporation.

It is observed that the thickness of the underlayer 12 is uniform over the entire surface of the substrate 11.

There is a micrograph obtained by scanning electron microscopy of the substrate 11 covered with an underlayer 12, illustrated in , on which a second roughness was created, then deposited a thermal barrier, according to one embodiment of a method of the invention.

It can be observed on the that the thickness of the underlayer 12 remains homogeneous over the entire surface of the substrate 11, even after the deposition of the thermal barrier 13.

There is a micrograph obtained by scanning electron microscopy and represents a substrate 31 coated with an undercoat 32, without a first roughness in accordance with that of the invention having been prepared between the substrate 31 and the undercoat 32.

The underlayer was deposited by physical vapor deposition with the same parameters as above.

The thickness of the undercoat is homogeneous over the entire surface of the substrate.

There is a micrograph obtained by scanning electron microscopy and represents the substrate 31 coated with the underlayer 32, illustrated in on which a thermal barrier 33 has been deposited with a method of the prior art, that is to say after sandblasting carried out to create a roughness on the surface of the underlayer 32 allowing the attachment of the thermal barrier 33.

It can be observed that the sandblasting of the underlayer 32 creates significant thickness differences in the thickness of the underlayer 32 which is detrimental to the properties of resistance to oxidation and corrosion of the underlayer. 32.

Thus, FIGS. 1 to 4 illustrate that a method of the invention effectively makes it possible to obtain a thermal barrier at the surface of a substrate covered with an underlayer, by ensuring good cohesion of the thermal barrier to the sub-layer and ensuring that the sub-layer has a uniform thickness over the whole of the substrate, unlike a process where only a significant roughness is created on the sub-layer by sanding.

Claims (7)

Process for coating a substrate with an underlayer and a thermal barrier comprising at least the following steps:
- a step of creating a first roughness on the surface of the substrate by at least one chemical and/or electrochemical attack, the first roughness being characterized by a maximum roughness value R _z 1 and an arithmetic mean of roughness along the profile R _has 1;
- a step of depositing an underlayer;
- a step of creating a second roughness on the surface of the underlayer by at least sandblasting or electrochemical attack, the second roughness being characterized by a maximum roughness value R _z 2 and an arithmetic average roughness along of the profile R _a 2, the values R _z 2 and R _a 2 being lower than the roughness values R _z 1 and R _a 1;
- a step of depositing a thermal barrier by SPS. A method according to claim 1, wherein the step of creating the first roughness comprises acid etching. A method according to claim 1 or 2, wherein the step of creating the first roughness comprises electrochemical etching. Process according to any one of Claims 1 to 3, in which the substrate is chosen from nickel-based or cobalt-based superalloys. Process according to any one of Claims 1 to 4, in which the substrate has, after the step creating the first roughness, an average roughness R _a of 2.5 to 3.0 µm and a maximum roughness R _z of 13 to 16 µm. A method according to any of claims 1 to 5, wherein the underlayer is composed of γ-Ni/γ'-Ni ₃ Al, NiAl, NiAlPt, or MCrAlY where M is Cobalt Co or nickel Neither. A method according to any of claims 1 to 6, wherein the underlayer is deposited by physical vapor deposition.