EP4025719A1

EP4025719A1 - Method for forming a layer of alumina at the surface of a metallic substrate

Info

Publication number: EP4025719A1
Application number: EP20732998.8A
Authority: EP
Inventors: Marjorie Christine CAVARROC; Stephane Knittel; Jolenta SAPIEHA; Ludvik Martinu; Florence BERGERON; Simon LOQUAI
Original assignee: Safran SA
Current assignee: Safran SA
Priority date: 2019-05-15
Filing date: 2020-04-24
Publication date: 2022-07-13
Also published as: WO2020229747A1; US20220259717A1; CA3043564A1; CN113825857A; CN113825857B

Abstract

The invention relates to a method for forming a layer of alumina (203) at the service of a metallic substrate (201) made of aluminium-containing alloy, the method comprising at least: - the deposition of a first layer of aluminium on a surface of the metallic substrate, - the deposition of a second layer by vapour deposition on the first layer, the second layer comprising aluminium, a halogen and oxygen, and - the heat treatment of the substrate coated with the first and second layers under an oxidising atmosphere in order to form the layer of alumina at the surface of the metallic substrate.

Description

Title of the invention: Process for forming an alumina layer on the surface of a metal substrate

Technical area

The present invention relates to a method of forming an alumina layer on the surface of a metal substrate so as to provide said substrate with protection against oxidation and corrosion at high temperature. The invention relates in particular to the protection of a substrate intended for use in an aeronautical turbomachine.

Prior art

The use of nickel-based superalloys in aeronautical turbomachines is known. With a view to reducing the weight of turbomachines, alloys comprising less heavy metals such as aluminum have been developed. By way of example, mention may be made of alloys based on titanium aluminide (also called TiAI alloys) which have good mechanical characteristics and which are significantly lighter than nickel-based superalloys.

It is however desirable to improve the resistance of these alloys to oxidation and corrosion at high temperature, that is to say at temperatures above 800 ° C. Solutions for forming a protective alumina layer on the surface of this type of alloy have been developed. In particular, the publication Hornauer et al. (Surface and Coatings Technology, 2003, 174, pp. 1182-1186) has proposed a solution in which there is first implantation of chlorine by a plasma process on the surface of the part then in which the protective layer of alumina is formed. However, this solution has the drawback of leading locally, near the surface of the part, to a depletion of aluminum during the formation of the alumina layer. This aluminum depletion can result in an undesirable deterioration of the properties of the substrate at high temperature, in particular due to the formation of brittle intermetallic phases which decrease the fatigue life of the material. Disclosure of the invention

The present invention relates to a method of forming an alumina layer on the surface of a metal substrate made of an alloy comprising aluminum, the method comprising at least:

- the deposition of a first layer of aluminum on a surface of the metal substrate,

- the deposition of a second layer by vapor deposition on the first layer, the second layer comprising aluminum, a halogen and oxygen, and

the heat treatment of the substrate coated with the first and second layers in an oxidizing atmosphere in order to form the layer of alumina on the surface of the metal substrate.

The invention provides a solution in which an intermediate layer of aluminum (first layer) is deposited on the substrate before deposition of the second layer comprising aluminum, halogen and oxygen. This first layer provides the aluminum to form the layer of alumina during the heat treatment in an oxidizing atmosphere. As a result of this contribution, the phenomenon of local aluminum depletion of the underlying metal substrate encountered in the prior art is reduced, or even eliminated.

In an exemplary embodiment, the thickness of the first layer is greater than or equal to 20 nm.

Such a thickness value advantageously makes it possible to further reduce the phenomenon of aluminum depletion of the underlying substrate during the formation of the alumina layer.

In an exemplary embodiment, the thickness of the first layer is less than or equal to 1000 nm, for example 500 nm.

The fact of limiting the thickness of the first layer makes it possible to avoid any risk of the aluminum diffusing from the first layer towards the underlying substrate and inducing a structural modification therein.

In particular, the thickness of the first layer can be between 20 nm and 1000 nm, for example between 20 nm and 500 nm. In an exemplary embodiment, the second layer is deposited by a process chosen from among physical vapor deposition, chemical vapor deposition, plasma assisted chemical vapor deposition and case cementation.

The second layer comprises a compound of the formula AlO _n X _m in which n and m are each strictly positive and X represents halogen.

In particular, the second layer can comprise a mixture of aluminum halide and aluminum oxyhalide, optionally with alumina.

In an exemplary embodiment, the halogen is fluorine. In the latter case, the second layer can comprise a compound of formula AlO _n F _m in which n and m are each strictly positive. In particular, the second layer can comprise a mixture of aluminum fluoride and aluminum oxyfluoride, optionally with alumina. Alternatively, the halogen is chlorine.

In an exemplary embodiment, the thickness of the second layer is greater than or equal to 10 μm, preferably between 10 μm and 50 μm. Such a thickness makes it possible to promote the growth of the alumina layer on the one hand, and to minimize the internal stresses in the layer on the other hand, which allows the layer to avoid any risk of adhesion defect on the layer. the substrate.

In an exemplary embodiment, the second layer is deposited from a vapor phase comprising a halogen gas, oxygen and aluminum.

For example, the second layer is deposited from a vapor phase comprising fluorine, oxygen and aluminum. When the halogen is fluorine, the vapor phase can comprise CF ₄ , C2F6, SiF _4, SF ₆ or a mixture of these compounds, as well as oxygen and aluminum.

As a variant, the second layer is deposited from a vapor phase comprising chlorine, oxygen and aluminum. When the halogen is chlorine, the vapor phase can comprise SiCl ₄ , CI2 or a mixture of these compounds, as well as oxygen and aluminum.

In an exemplary embodiment, the second layer is deposited by sputtering a target comprising aluminum in an atmosphere comprising oxygen and halogen gas.

In an exemplary embodiment, the heat treatment is carried out at a temperature greater than or equal to 800 ° C. In an exemplary embodiment, the metal substrate is made of an alloy based on titanium aluminide.

In an exemplary embodiment, the metal substrate is a part of a turbomachine, for example a part of an aeronautical turbomachine.

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

- a metal alloy substrate comprising aluminum,

- a first layer of aluminum on a surface of the metal substrate, and

a second layer comprising aluminum, a halogen and oxygen, the second layer covering the first layer.

The coated metal substrate introduced above corresponds to the intermediate product obtained during the implementation of the process described above before the heat treatment in an oxidizing atmosphere.

In an exemplary embodiment, the first layer has a thickness greater than or equal to 20 nm. The thickness of the first layer may be less than or equal to 1000 nm, for example 500 nm. In particular, the thickness of the first layer can be between 20 nm and 1000 nm, for example between 20 nm and 500 nm. In one embodiment, the halogen of the second layer is fluorine.

In particular, the second layer can comprise a compound of formula AlO _n F _m in which n and m are each strictly positive. In particular, the second layer can comprise a mixture of aluminum fluoride and aluminum oxyfluoride, optionally with alumina.

Description of figures

[Fig. 1] FIG. 1 illustrates the substrate covered with the first layer of aluminum after implementation of a first step of an example of the method according to the invention.

[Fig. 2] FIG. 2 illustrates the substrate covered with the first and second layers after implementation of a second step of an exemplary process according to the invention.

[Fig. 3] FIG. 3 illustrates the substrate covered with a surface alumina layer obtained after heat treatment in an oxidizing atmosphere. [Fig. 4] FIG. 4 schematically describes a device for the vapor deposition of layers on a substrate which can be implemented within the framework of the invention.

[Fig. 5] FIG. 5 represents a photograph obtained by scanning electron microscopy of the result obtained after implementation of a method according to the invention.

[Fig. 6] FIG. 6 represents a photograph obtained by scanning electron microscopy of the result obtained after implementation of a method outside the invention in which the first aluminum layer is not deposited.

Description of embodiments

The present invention relates to the coating of a metal substrate 11 made of an alloy comprising aluminum. The treated substrate can be an alloy based on titanium aluminide, such as a gamma-TiAI alloy.

The treated substrate can constitute a part of a turbomachine, and for example a part of an aeronautical turbomachine. The substrate is intended for use in an oxidizing atmosphere and at a temperature greater than or equal to 800 ° C. The substrate can for example be a turbine part. It may for example be a turbine blade or a turbine ring sector.

The first aluminum layer 12 is first of all deposited on an external surface S of the substrate. The first layer 12 can be deposited in contact with the external surface S of the substrate. The first layer 12 formed of elementary aluminum (Al) can be deposited by implementing a technique known per se. In particular, it can be deposited by physical vapor deposition, for example by vacuum evaporation or by spraying. The first aluminum layer 12 can be deposited by other deposition methods such as for example an electrochemical deposition, a chemical vapor deposition (CVD for the English acronym Chemical Vapor Deposition), a chemical vapor deposition assisted by plasma (PECVD for the English acronym Plasma Enhanced Chemical Vapor Déposition) or by case cementation. The temperature imposed during the deposition of the first layer 12 may be between 20 ° C and 600 ° C, for example between 20 ° C and 400 ° C. In an exemplary embodiment, the first aluminum layer 12 can be deposited by magnetron radiofrequency sputtering, for example with a power equal to 200 W, under a reduced pressure, for example at 0.66 bar, using an argon flow rate. , for example 60 standard cubic centimeters per minute, and in temperature, for example at 400 ° C.

The first layer 12 may have a thickness ei of between 20 nm and 1000 nm.

The metal substrate 11 covered with the first layer 12 is illustrated in FIG.

1.

The second layer 13 comprising aluminum, a halogen and oxygen is then formed on the first layer 12. The second layer 13 can be deposited in contact with the first layer 12. The first layer 12 is interposed between the substrate. 11 and the second layer 13. This second layer 13 is formed by vapor deposition.

As indicated above, the second layer 13 comprises a compound of formula AlOnXm in which n and m are each strictly positive and X represents halogen. The second layer 13 can comprise a mixture of aluminum halide and aluminum oxyhalide, optionally with alumina. The halogen can be fluorine or chlorine as indicated above.

The second layer 13 can comprise in atomic percentages:

- 3% to 70% halogen, for example 55% to 65% halogen,

- from 5% to 40% aluminum, for example 10% to 30% aluminum, and

- del% to 20% oxygen, for example from 3% to 15% oxygen.

The second layer 13 can have an atomic ratio between halogen and oxygen of between 2: 1 and 8: 1.

The inventors have observed that halogen is an activator of the reaction for the formation of alumina by oxidation of the aluminum present with oxygen.

The second layer 13 is produced by vapor deposition, and in particular by magnetron sputtering. The temperature imposed during the deposition of the second layer 13 may be between 20 ° C and 800 ° C, for example between 20 ° C and 400 ° C. FIG. 4 schematically represents a device making it possible to carry out deposition by magnetron sputtering.

In a chamber 101, a gas is introduced through the inlet 106 and a plasma is generated between the target 105 and the substrate 111 to be covered. Under the effect of an electric field, obtained by imposing a voltage between target 105 and substrate 111, electrons are generated by the target and can ionize the constituent atoms of the plasma by collision. The presence of a magnetic field generated by a magnet 104 disposed near the target 105 confines the electrons generated near the target and increases the probability that the collision between an electron and an atom in the plasma will occur there. When such a collision takes place, a high energy species is generated, and the latter can bombard the target 105 and tear off, by elastic shock, particles from the target 105. The particles from the target 105 thus torn off can then be separated. deposit on the substrate 111 to form the deposit.

The deposition of the second layer 13 can be carried out under vacuum, for example at a pressure less than or equal to 10 Pa (75 mTorr), for example between 0.67 Pa (5 mTorr) and 10 Pa (75 mTorr). During the deposit, we can impose:

- a flow rate of oxygen injected into chamber 101 of between 1 cm ³ standard per minute (sccm) and 100 cm ³ standard per minute, and

- a flow rate of halogen gas in chamber 101 of between 2.5 cm ³ standards per minute and 100 cm ³ standards per minute,

- And optionally, a flow rate of inert gas injected into chamber 101 of between 1 standard cm ³ per minute and 100 standard cm ³ per minute.

The inert gas can for example be argon.

In the case where the halogen is fluorine, the halogen gas can be chosen from CF ₄ , C ₂ F ₆ , SiF _4, SF ₆ or a mixture of these compounds.

In the case where the halogen is chlorine, the halogen gas can be chosen from SiCl ₄ , Cl ₂ or a mixture of these compounds.

The second layer 13 may have a thickness e ₂ greater than or equal to 0.1 μm, for example 10 μm, for example between 10 μm and 100 μm.

FIG. 2 represents a substrate 11 covered with a first layer of aluminum 12 and with a second layer 13 as described above. A heat treatment is then carried out in an oxidizing atmosphere so as to form the alumina layer 14 on the surface S of the metal substrate.

The heat treatment can be carried out at a temperature greater than or equal to 800 ° C, for example between 800 ° C and 1000 ° C, for example between 850 ° C and 900 ° C.

The heat treatment can be carried out in air. The heat treatment can be annealing.

As described above, due to the presence of the aluminum layer 12, the growth of the alumina layer 14 during the heat treatment takes place by limiting or even eliminating the depletion of aluminum near the surface. S of the substrate.

At the end of the heat treatment, and as shown schematically in FIG. 3, an alumina layer 14 is formed on the surface S of the metal substrate 11.

The heat treatment can also lead to the formation on the alumina layer 14 of a layer 15 comprising titanium, aluminum and oxygen.

In an exemplary embodiment, the alumina layer 14 has a thickness e3 of between 10 nm and 50,000 nm.

FIG. 5 is a photograph of a gamma-TiAI 201 substrate coated by the implementation of a method as described above. The substrate 201 is covered with a layer of alumina 203 and having on its surface a layer 204 comprising titanium, aluminum and oxygen.

By way of comparison, FIG. 6 is a photograph of a gamma-TiAI substrate 301 coated with an alumina layer 303 by a process outside the invention, not comprising the initial deposition of the aluminum layer. This substrate also has on its surface a layer 304 comprising titanium, aluminum and oxygen.

It can be observed that the implementation of the method described above makes it possible to significantly reduce the zone depleted in aluminum (202 in FIG. 5 and 302 in FIG. 6) present on the surface of the substrate during the coating of a metal substrate. . In the examples presented in FIGS. 5 and 6, it is in fact observed that the zone depleted in aluminum near the coated surface of the substrate is 2 μm thick when the layer of alumina is prepared by a process of the invention ( zone 202 FIG. 5), against 4 μm when the alumina layer is prepared by a process outside the invention (zone 302 FIG. 6).

In the present application, the expression “comprised between ... and ...” should be understood as limits included, unless explicitly stated otherwise.

Claims

[Claim 1] A method of forming an alumina layer (14) on the surface of a metal substrate (11) made of an alloy comprising aluminum, the method comprising at least:

- depositing a first layer of aluminum (12) on a surface of the metal substrate,

- the deposition of a second layer (13) by vapor deposition on the first layer, the second layer comprising aluminum, a halogen and oxygen, and

[Claim 2] A method according to claim 1, wherein the thickness ei of the first aluminum layer (12) is between 20nm and 1000nm.

[Claim 3] A method according to one of claims 1 or 2, wherein the halogen is fluorine.

[Claim 4] A method according to any one of claims 1 to 3, wherein the second layer is deposited by a method selected from physical vapor deposition, chemical vapor deposition, chemical vapor deposition assisted by plasma and case-hardening.

[Claim 5] A method according to any one of claims 1 to 4, wherein the thickness of the second layer (13) is greater than or equal to 10 µm.

[Claim 6] A method according to any of claims 1 to 5, wherein the second layer (13) is deposited from a gas phase comprising oxygen, a halogen gas and aluminum.

[Claim 7] The method of claim 6, wherein the second layer (13) is deposited by sputtering a target (105) comprising aluminum in an atmosphere comprising oxygen and halogen gas.

[Claim 8] A method according to any one of claims 1 to 7, wherein the heat treatment is carried out at a temperature greater than or equal to 800 ° C.

[Claim 9] A method according to any one of claims 1 to 8, wherein the metal substrate (11) is made of a titanium aluminide base alloy.

[Claim 10] A method according to any one of claims 1 to 9, in which the metal substrate (11) is a part of a turbomachine.

[Claim 11] A coated metal substrate comprising:

- a metal substrate (11) made of an alloy comprising aluminum,

- a first layer (12) of aluminum on a surface of the metal substrate, and

- a second layer (13) comprising aluminum, a halogen and oxygen, the second layer covering the first layer.

[Claim 12] A metallic substrate according to claim 11, wherein the first layer (12) has a thickness ei of between 20nm and 1000nm.

[Claim 13] A metallic substrate according to any one of claims 11 or 12, wherein the second layer (13) comprises a compound of the formula AIO _n F _m in which n and m are each strictly positive.