The present invention relates to the general field of protective coatings against erosion. The invention relates more particularly to an aircraft engine part coated with a protective coating against erosion.

In operation, aircraft engines or helicopter suck large amounts of air. The intake air is then compressed to supply the oxidizer combustion chamber. This air may be loaded more or less hard particles that may offend fixed or moving parts within the engine. Thus, the parts impacted by these high speed particles can see their geometry modified by erosion, which can cause engine failure.

Discloses the use of sand filters, particularly in helicopter engines, to reduce the presence of particles in the engine. However, these filters pass a small amount of particles sufficient to cause damage to engine parts over the long term.

Also known is the use of anti-erosion coatings on engine parts prone to erosion. For example, it is known to deposit on a workpiece of metal nitrides (TiN type TiAIN, AlCrN, etc.) by a physical vapor deposition (PVD) or chemical vapor deposition (CVD). However, these coatings are generally not sufficiently resistant to temperatures necessary for production of the part, and methods for forming such coatings do not allow for coating complex shape parts.

Also known is the deposition by electroplating of a chromium coating on hexavalent metal parts. Such a method of coating parts with complex geometries. However, such coatings generally do not exhibit sufficient hardness for the application held in the aforementioned erosion.

Document GB2180558 discloses a method of manufacturing a chromium coating comprising a plasma spray deposition step on one piece of a metal alloy comprising chromium and chromium carbide Cr 3 C 2 , and a step of heat treating the coating for a period exceeding 200 hours. This method does not allow to treat parts with complex geometries and present the drawback of being long to achieve, partly because of the long duration of the heat treatment.

There is therefore a need for an aircraft engine part coated with a protective coating against erosion which does not present the aforementioned drawbacks, as well as a method for manufacturing such a workpiece.

OBJECT AND SUMMARY OF THE INVENTION

The present invention therefore has the main purpose to overcome such drawbacks by proposing an aircraft engine part coated with a protective coating against erosion which exhibits satisfactory erosion resistance under the conditions of operation of a aircraft engine, and which can be manufactured simply even when it has a complex geometry.

This object is achieved with an aircraft engine part comprising at least a metal substrate and a protective coating against erosion present on the substrate, the coating comprising at least one phase containing at least chromium in an upper atomic content or equal to 45% and carbon in an atomic content of between 5% and 20%, said phase consisting of chromium carbide Cr 7 C 3 and Cr 23 C 6 .

The presence of chromium carbide Cr 7 C 3 and Cr 23 C 6 in the coating advantageously enables to significantly improve the resistance to erosion of the coated part. As will be detailed below, these chromium carbides are formed after heat treatment of a composition of chromium and carbon coating formed on the substrate. Thus, in the invention, the application of a heat treatment strengthens the coating formed by precipitating Cr carbides 7 C 3 and Cr 23Co. Such an effect is unexpected insofar as conventionally, with the chromium-based coatings of the prior art, applying a heat treatment after deposition on the workpiece has the effect of significantly reducing the hardness of the coating.

Thus, in the invention, the choice to produce on the substrate a deposit chromium / carbon having a particular carbon content advantageously possible to form, after heat treatment, a coating comprising chromium carbide Cr 7 C 3 and Cr 2 3C6 having a good resistance to erosion in the environment of an aircraft engine as well as good adhesion to the underlying substrate. The presence of the carbide Cr 7 C 3 and Cr 23 C 6 is detectable in particular by X-ray diffraction (XRD). The carbon atom content in the coating must be greater than 5% to form sufficiently carbide Cr 7 C 3 and Cr23 C6 and improve significantly the hardness of the coating. The carbon content must however be less than 20% in order not to affect the mechanical strength of the coating.

The workpiece coating according to the invention may advantageously have a high hardness, for example greater than 1500 HV (Vickers hardness). Note that the coating may additionally comprise oxygen, in proportions which depend on the coating of the manufacturing process.

Preferably, the phase of coating comprises chromium in an atomic content of between 48% and 58% and carbon in an atomic content of between 8% and 18%.

The phase of the coating can comprise chromium in an atomic content of between 45% and 80% and carbon in an atomic content of between 5% and 20%. The phase of the coating may optionally comprise oxygen in an atomic content of between 15% and 40%, and optionally other elements in a lower atomic content not exceeding 4%, or even between 0.5% and 4% .

In an exemplary embodiment, the phase of the coating may comprise carbon in an atomic content of between 12% and 18%.

In an exemplary embodiment, the phase of the coating can comprise chromium in an atomic content of between 45% and 55%, e.g. between 48% and 52%.

In an embodiment, chromium is the only metallic element present in the phase of the coating.

Preferably the coating further comprises metallic particles and / or ceramic particles. In addition, the volume content in the coating of the metallic particles and / or ceramic particles may be less than 20%, or between 5% and 15%. Such metallic particles may for example be particles of tungsten or nickel, tungsten being preferred because it is heavier and more tenacious. Such ceramic particles may for example be particles of alumina or zirconia. The particles are preferably dispersed in the phase of the coating containing chromium and carbon, said phase then playing the role of matrix for the particles. These particles can further increase the resistance to erosion, especially in the face of higher size particles ingested.

The metallic particles and / or ceramic particles may have a size greater than or equal to lpm. The metallic particles and / or ceramic particles may have a size less than or equal to 30 .mu.m, for example between 1 pm and 30 pm. By size is meant the average size D 50 of the particles.

Preferably, a thickness of the coating is between 5 pm and 100 pm.

The substrate can be steel, aluminum based alloy, an alloy based on titanium, nickel-based alloy or other metallic material that can be used in a gas turbine.

The part may be an aircraft engine part selected from the following: at least a portion of a diffuser, at least part of an axial or centrifugal compressor, at least a portion of a dispenser, or any other piece used in a turbine and capable of being subjected to an airflow.

The invention also provides, in another aspect, a method of manufacturing a component such as that shown above, the method comprising at least the following steps:

- depositing on the substrate a coating composition comprising at least chromium in an atomic content greater than or equal to 45% and carbon in an atomic content of between 5% and 20%, and

- heat treatment of the part coated with the said composition at a temperature between 250 ° C and 700 ° C to obtain the coating.

The heat treatment makes it possible, as explained above, to obtain improved hardness to coating after precipitation and coalescence of chromium carbides. Before the heat treatment, the chromium carbides Cr C 3 and Cr 23 C 6 are not detectable by X-ray diffraction (XRD) in the coating composition. The latter can be free of these carbides. Alternatively, the coating composition may already include the carbides Cr 7 C 3 and Cr 2 3C6 but they have an average size (D50) less than or equal to 10 nm, not allowing their detection by XRD.

The heat treatment temperature may preferably be between 300 ° C and 600 ° C, or even more preferably between 300 ° C and 500 ° C, for example between 400 ° C and 500 ° C. The duration of thermal treatment can be adapted in particular depending on the desired hardness for the coating. The duration of this heat treatment may be greater than 10 minutes, for example greater than 15 minutes or greater than 30 minutes. The heat treatment time may be for example between 15 minutes and 280 minutes. The duration of thermal treatment can be adapted depending on the temperature chosen and the desired hardness for the coating.

In addition, the coating composition is deposited on the substrate by electrodeposition from an electrolyte bath comprising at least trivalent chromium and an organic compound. In this case, the part to be coated may constitute the cathode of the electroplating device, so that the reduction of chromium ions occurs on the workpiece. Such electrodeposition coating step is advantageous because it allows to deposit the coating composition on complex geometry parts easily while providing more control of the formation of the coating. Indeed, it is easy to change the configuration of electroplating, such as voltage, current, or the electrode surface, for example to finely control the thickness of the coating, its rate of formation and the microstructure of the coating. In addition, the presence of trivalent chromium which is usually complexed with organic ligands in the electrolytic bath, it possible to deposit simultaneously the chromium and carbon coating. It is noted that oxygen can also be incorporated into the coating on the piece by this method.

Deposition by electrodeposition of the coating composition reduces the duration of heat treatment performed after the deposition. Indeed, electrodeposition provides a homogeneous distribution of carbon in the deposited coating composition, which is generally not the case with other deposition techniques such as plasma spraying. This homogeneous distribution of the carbon is then used to advantageously reduce the duration of the heat treatment required to form or develop the chromium carbide Cr 7 C 3 and Cr 2 3 C 6 to increase the hardness of the coating.

Also preferably, the electrolytic bath further comprises metallic particles and / or ceramic particles. During the electrodeposition of the coating composition, the suspended particles are incorporated into the coating composition evenly and continuously. Thus, no additional steps are needed to integrate the particles in the coating.

DC or pulsed current can be used in electroplating. The choice of a continuous or pulsed current generally depends on the microstructure and desired coating thickness.

The method may also comprise a surface of the substrate degreasing step and a step of preparing the substrate surface, prior to the step of depositing the coating composition on the substrate. The surface preparation step can for example be made by etching (e.g., acid etching), by sandblasting, etc.

Brief Description of Drawings

Other features and advantages of the present invention will be apparent from the description given below, with reference to the accompanying drawings. In the figures:

- Figure 1 is a schematic sectional view of an aircraft engine component coated with a protective coating against erosion, - Figure 2 is a flowchart showing the main steps of a manufacturing process of a part coated according to the invention,

- Figure 3 is a schematic sectional view of a device for electrodepositing a coating composition according to the invention,

- Figure 4A shows the influence of the heat treatment temperature on the hardness of the coating of a workpiece according to the invention, and

- Figure 4B shows the influence of the temperature of a heat treatment after deposition of a chromium coating of the prior art.

Detailed Description of the Invention

Throughout the presentation, the expression "between ... and ..." should be understood as including the terminals.

1 shows schematically a sectional view at the surface of a piece 1 of an aircraft engine according to the invention may for example be constituted by a turbine diffuser.

Such piece 1 comprises a metal substrate 2, for example steel, aluminum, titanium, an alloy based on aluminum, titanium based alloy, or a nickel based alloy. The substrate 2 is coated with a protective coating 3 against erosion. The protective coating against erosion 3 is here in direct contact with the substrate 2 and covers it. Preferably, the coating 3 has a thickness e of between 5 pm and 100 pm.

According to the invention, coating 3 comprises a phase 4, the majority by weight in the coating comprising a chromium and carbon basis. More specifically, the phase 4 comprises chromium in an atomic content greater than or equal to 45% and carbon in an atomic content of between 5% and 20%. Chromium and carbon within phase 4 of coating 3 are in particular present in the form of chromium carbides of the type Cr 7 C 3 and Cr 2 3C 6 .

The coating 1 may also comprise a dispersed phase 5 in phase 4 of chromium and carbon, comprising metallic particles and / or ceramic. Such metal particles may be for example, particles of tungsten or nickel. Such ceramic particles may for example be particles of alumina or zirconia. The volume content in the coating of the metallic particles and / or ceramic particles is preferably less than 20%, or even more preferably, between 5% and 15%. The metallic particles and / or ceramic particles may have a size between 1 and 30 pm μιη.

Thus, the coating may be formed of phase 4 comprising chromium and carbon in which are dispersed metal particles and / or ceramic particles 5. In a variant not illustrated the coating 3 may be formed solely of phase 4 comprising chromium and carbon.

A method of manufacturing an aircraft engine room 1 according to the invention will now be described with reference to the flowchart of Figure 2 and the diagram illustrating an electroplating device 6 of Figure 3. The process which will be described comprises a step of electrodepositing a coating composition 3 '. Of course, the invention is not limited to the deposition of the electrodeposition coating composition, other techniques are available to obtain the coated part of the invention. They include for example physical deposition techniques vapor of chemical vapor deposition, or cementation.

A first process step (step E) may be to degrease the substrate surface on which is deposited a coating composition, for example by means of a degreasing aqueous solution. the substrate surface can then be prepared (step E2) to ensure that the plating will be uniform over the substrate 2 and to increase its efficiency. To prepare the surface can, in a known way, the sand, the etching (e.g., with an acid solution), etc.

an electrolytic bath may then be prepared 7 which comprises at least one of chromium III ions (trivalent chromium), and a complexing organic compound to the chromium ions. for example may be used with known aqueous solutions comprising chromium chloride III and a carboxylic acid as complexing agent. The electrolytic bath 7 can optionally be heated during the electrodeposition step. Furthermore, the electrolytic bath may comprise metallic particles and / or ceramic slurry, of the type presented above, so that they are incorporated into the coating composition during electroplating.

The coin 1 whose surface has been prepared can then be connected to the negative terminal (serving as a cathode) of an 8-current generator and immersed in the electrolytic bath 7 prepared above. In the device 6 of Figure 3, two electrodes 9 serving as the anode are connected to the positive terminal of the generator 8 and immersed in the bath 7, so that the coin 1 is located between the two electrodes 9 the bath. The ratio of the anode surface (corresponding to the useful area of ​​the two electrodes 9) on the cathode surface (corresponding to the surface of the substrate 2 to be coated of the workpiece) is preferably of the order of 4. The electrodes 9 forming the anodes are remote preference the surface of the workpiece by a distance of between 1 cm and 20 cm.

The generator 8 is then turned on to initiate the electrodeposition of the coating composition 3 'on the workpiece (step E3). During this stage, the chromium III is reduced on the substrate 2 of the workpiece 1 to form the coating composition 3 'comprising chromium, carbon (from the organic compound present in the bath) and metal particles and / or ceramic which were suspended in the bath. Can be adapted such parameters as the surface current, the bath temperature and the duration of the electrodeposition depending in particular on the thickness of coating that is desired. In addition, it is possible to perform electroplating using a direct current or a pulsed current.

Once the part coated with the coating composition 3 ', it is rinsed and dried and then placed in an oven. One then proceeds to the heat treatment (step E4) of the workpiece 1 provided with the coating composition 3 to a temperature preferably between 250 ° C and 700 ° C, or more preferably between 300 ° C and 600 ° C, or even more preferably between 400 ° C and 500 ° C. The heat treatment may be performed under inert atmosphere. The duration of this heat treatment may be greater than 10 minutes, for example greater than 15 minutes or greater than 30 minutes. The heat treatment time can for example be between 15

minutes and 280 minutes. The duration of thermal treatment can be adapted depending on the temperature chosen and the desired hardness for the coating.

The coating 3 of the piece 1 according to the invention obtained after the thermal treatment step may have a hardness of more than 1500 HV (Vickers hardness) and has a sufficient erosion resistance for use in a motor 'aircraft.

The part 1 may for example form at least a portion of a turboprop diffuser, at least part of an axial or centrifugal compressor, for example an impeller, at least a portion of a dispenser, or any other part of a turbomachine that may be subjected to an air flow.

example 1

In the following example, a steel turboprop diffuser part is coated with a coating by a process according to the invention. The surfaces to be coated have been degreased and prepared.

The deposition of the coating composition is carried out by electroplating in an electroplating bath. The electrolytic bath used is an aqueous solution comprising:

- 0.39 mol / L of chromium III chloride hexa hydrate (CRCI 3 ; 6H 2 0),

- 3.72 mol / L ammonium formate (NH 4 COOH), and

- 0.81 mol / L of potassium chloride (KCI).

The bath is heated to about 35 ° C to effect electroplating. The part is immersed in the bath and connected to the negative terminal of the current generator. The anode electrodes are immersed in the bath and connected to the generator, as described above. The ratio of the anode surface at the cathode surface is equal to 4.

A continuous current of 40 A / dm 2 is imposed for 180 minutes to form on the substrate the coating composition. Once electrodeposition performed, the workpiece is rinsed and dried.

Finally, it has the coated part of the coating composition in an oven, and a heat treatment at 500 ° C for 1 hours is imposed.

The coating has a thickness of about 35 pm.

The hardness of the coating thus formed is about 2050 HV (Vickers hardness).

The chemical composition of the coating thus formed (atomic concentrations), evaluated by photoelectron spectrometry X ( "XPS") is given in Table 1 below.

Table 1 - Contents of atomic elements in the coating

Analysis of the coating by X-ray diffraction (XRD) also showed the presence of chromium carbides of the type Cr 7 C 3 and Cr ^ Gs.

example 2

eleven steel substrates were coated in the same conditions as in Example 1 by varying the heat treatment parameters (temperature and duration). The results are shown in the graph of FIG 4A.

Was taken up in Figure 4B a graph showing the evolution of the hardness of a chromium coating deposited by electrodeposition from a solution of chromium VI to a substrate, depending on the temperature of a heat treatment performed after deposition. The electrolytic bath used is a standard solution chromic acid having a concentration of about 250 g / L CrO 3 , the solution further comprising 2.5 g / L of sulfuric acid H 2 SO 4 . To perform deposition, a current 40 A / dm 2has been used. These data come from the work of F. Durut "Search microstructural mechanisms that govern the macroscopic properties of chromium deposits: influence of preparation parameters," Engineering Science [physics], Ecole des Mines de Saint-Etienne, 1999.

It is observed that, for the chromium-based coating of the prior art (Figure 4B), a heat treatment performed after deposition does not increase the hardness of the coating. Specifically, it is observed that the hardness does not change significantly or not to heat treatment temperatures of 400 ° C, then it decreases.

Conversely, it is observed that, for the coating of a workpiece according to the invention comprising chromium and carbon (Figure 4A), the coating hardness increases with the temperature of heat treatment performed after deposition. This figure also shows that the duration of heat treatment did not significantly affect the hardness of the coating for heat treatment durations longer than 10 hours.

CLAIMS

1. A method of manufacturing a workpiece (1) for aircraft engine comprising at least a metal substrate (2) and a protective coating (3) against erosion present on the substrate, the coating comprising at least one phase (4) comprising at least chromium in an upper atomic content not exceeding 45% and carbon in an atomic content of between 5% and 20%, said phase consisting of chromium carbide Cr 7 C 3 and C ^ C O , the method comprising at least the steps of:

- deposition on the substrate (2) a coating composition (3 including at least chromium in an atomic content greater than or equal to 45% and carbon in an atomic content of between 5% and 20%, the coating composition (30 being deposited on the substrate by electrodeposition from an electrolyte bath (7) comprising at least trivalent chromium and an organic compound, and

- heat treatment of the part coated with the said composition at a temperature between 250 ° C and 700 ° C to obtain the coating (3).

2. The method of claim 1, wherein the heat treatment temperature is between 300 ° C and 600 ° C.

3. A method according to any one of claims 1 and 2, wherein the electrolytic bath (7) further comprises metallic particles and / or ceramic particles in suspension, the resulting coating further comprising the metallic particles and / or ceramic particles (5).

4. A method according to any one of claims 1 to 3, wherein a thickness (e) of the coating is between 5 pm and 100 μιτι.

5. A method according to any one of claims 1 to 4, wherein the substrate (2) is steel, alloy based on aluminum, an alloy based on titanium, or a nickel-based alloy.

6. A method according to any one of claims 1 to 5, wherein the workpiece (1) is an aircraft engine part selected from the following: at least a portion of a diffuser, at least a portion of a axial or centrifugal compressor, at least a portion of a dispenser.

7. A method according to any one of claims 1 to 6, wherein the heat treatment time is between 15 minutes and 280 minutes.

8. A method according to any one of claims 1 to 7, wherein the step of coating includes carbon in an atomic content of between 12% and 18%.

9. A method according to any one of claims 1 to 8, wherein the step of coating includes chromium in an atomic content of between 45% and 55%.