FR3138152A1 - Process for obtaining a protective coating against oxidation of a titanium-based alloy part - Google Patents

Abstract

Method for obtaining a protective coating against the oxidation of a part made of a titanium-based alloy. The invention relates to a method for obtaining a protective coating against the oxidation of a part made of a titanium-based alloy. titanium base, comprising: - the deposition of a liquid composition on the part comprising at least (i) a first precursor which is a metallo-organic sol-gel precursor of a metallic element E1 or a salt of this metallic element E1 , said metallic element E1 being chosen from aluminum or zirconium, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element E2 or a salt of this element E2, said element E2 being chosen from aluminum, zirconium, titanium, tin, zinc, phosphorus or rare earths and being different from the metallic element E1, and (iii) water, and - the hydrolysis of the first and second precursors of the liquid composition thus deposited and their condensation so as to form the protective coating against oxidation which comprises an interconnected mixed oxide network of elements E1 and E2. Figure for abstract: no figure.

Description

Process for obtaining a protective coating against oxidation of a titanium-based alloy part

This presentation concerns obtaining a protective coating against the oxidation of titanium-based alloys, for example for applications in turbomachines, in particular aeronautical turbomachines.

Titanium-based alloys offer a particularly interesting combination of properties up to operating temperatures of 550°C for applications in turbomachines such as turbojets. Indeed, titanium-based alloys have low density, good damage tolerances and good fatigue resistance.

However, reducing polluting emissions remains a major strategic issue for the aeronautical industry. Two main ways exist to reduce pollutant emissions: improving engine efficiency which involves increasing the operating temperature of the engine and/or reducing the total mass of the aircraft. The mechanical properties of titanium-based alloys tend to decrease cripplingly above 550°C. One of the main sources of degradation of the properties of titanium-based alloys with increasing temperature is linked to oxidation phenomena. For titanium-based alloys, oxidation manifests itself by two distinct mechanisms: on the one hand, the growth of the titanium oxide layer on the surface of the part and on the other hand a significant diffusion of oxygen within the alloy underlying the oxide layer. This second phenomenon is linked to the high solubility of oxygen in titanium. The depths affected by the diffusion of oxygen in the alloy can reach a few hundred microns in this temperature range after 100 hours of operation at service temperatures.

The incorporation of oxygen into the crystal lattice of the metallic phase leads, on the atomic scale, to more covalent bonds. This results in a loss of ductility of the alloy underlying the oxide layer, in the oxygen-enriched zone, which significantly lowers the macroscopic mechanical properties. The application of coatings on these alloys can limit these phenomena and increase the exposure temperatures of these alloys.

Today, titanium-based alloys are not protected against high temperature oxidation. The nature of the alloy for a part is chosen according to the maximum operating temperatures and the expected lifespan according to the reduction in mechanical properties resulting from the diffusion of oxygen within the titanium-based alloy. . Titanium parts made of thin thicknesses (plates, honeycomb) are all the more sensitive to oxidation phenomena as their thickness is reduced.

To respond to this problem, the literature presents tests for the development of coatings, mainly oxides, such as silica, alumina, zirconia, by different dry production processes (chemical vapor deposition, physical vapor deposition). steam, plasma projection, etc.) or by wet method.

All of this work nevertheless leads either to the penetration of oxygen into the substrate after a few dozen hours of oxidation, or to a loss of adhesion or cracking of the deposit under thermal cycle conditions due to the strong difference in thermal expansion with the substrate which leads to stresses within the film of the order of GPa, leading to oxidation of the substrate.

The invention relates to a method for obtaining a protective coating against oxidation of a titanium-based alloy part, comprising:
- the deposition of a liquid composition on the part comprising at least (i) a first precursor which is a metallo-organic sol-gel precursor of a metallic element E1 or a salt of this metallic element E1, said metallic element E1 being chosen from aluminum or zirconium, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element E2 or a salt of this element E2, said element E2 being chosen from aluminum, zirconium , titanium, tin, zinc, phosphorus or rare earths and being different from the metallic element E1, and (iii) water, and
- the hydrolysis of the first and second precursors of the liquid composition thus deposited and their condensation so as to form the protective coating against oxidation which comprises an interconnected mixed oxide network of elements E1 and E2.

The invention makes it possible to produce a multifunctional coating which makes it possible to protect the substrate from oxidation at temperatures of up to 700°C and therefore increase the lifespan of the parts in operation compared to the existing one. The invention relates to the field of wet surface treatments for metal substrates and in particular titanium alloys Ti6242 and Tiβ21S. It can be applied to all titanium parts, for example, compressor disks in aircraft engines, nacelle exhaust nozzles or impellers in helicopter engines. The invention is based on the choice of particular precursors as described above which, in the presence of water, are capable of forming a mixed oxide network interconnected by elements E1 and E2 with E1-O-E2 bonds providing protection against oxidation at high temperatures, notably up to 700°C. Depending on the choice made for the precursors, the hydrolysis and condensation leading to the formation of the network can take place without adding heat at room temperature (20°C), or require heat treatment for example at a temperature greater than or equal to 150°C, in particular between 150°C and 700°C, to accelerate the kinetics of obtaining this network and to be compatible with an industrial processing rate. The duration of application of the possible heat treatment varies depending on the precursors used and can typically be greater than or equal to 10 minutes, for example between 10 minutes and 5 hours, in particular between 10 minutes and 2 hours. The water in the liquid composition provides at least part of the oxygen from the mixed oxide network. Water can be added in addition to the precursors, as a solvent for the latter, or can be provided by the use of first and second hydrated precursors, the liquid medium of the composition then being non-aqueous, for example alcoholic. Generally speaking, the first and second precursors can be dissolved in the liquid composition.

More precisely, the specific choice of precursors made in the invention makes it possible to produce a multifunctional protective coating which simultaneously has the following functions:
- a modification of the surface making it possible to limit the quantity of oxygen penetrating within the substrate,
- accommodation of thermomechanical constraints in operation thanks to a thermal expansion coefficient relatively close to that of the part (limiting cracking and increasing the durability of the grip of the coating in operation), and
- a thermal barrier property to help lower the temperature of the room, thus reducing the penetration of oxygen.

The coating obtained notably limits the quantity of oxygen diffusing within the substrate over the first 10 to 20 microns for oxidations up to 700°C at contents much lower than the contents encountered for the uncoated alloy oxidized in the same terms.

In an exemplary embodiment, the metallic element E1 is aluminum and the element E2 is a rare earth, in particular the element E2 can be yttrium.

Alternatively, the metallic element E1 is aluminum and the element E2 is phosphorus.

Various types of precursors can be used in the context of the invention. In particular, in the case where the first precursor is a metallo-organic sol-gel precursor, it can be of general formula R ¹ _vx E1(OR ² ) _x where v is the valence of E1, x is an integer between 0 and v and R ¹ and R ² are organic groups chosen, independently of each other, from: alkyls, branched alkyls, methacrylates, carbamates, epoxides, cycloepoxides, isocyanates, amino groups , alkylamino groups, vinyl groups, and imide groups. When several R ¹ and/or R ² groups are present, they may be identical or different. Generally speaking, each R ¹ and R ² group can comprise from 1 to 4 carbon atoms.

Similar considerations apply to the second precursor when it is an organic-inorganic sol-gel precursor of element E2. So in this case, the second precursor can be of general formula R ³ _wy E2(OR ⁴ ) _y where w is the valence of E2, y is an integer between 0 and w and R ³ and R ⁴ are selected organic groups , independently of each other, from: alkyls, branched alkyls, methacrylates, carbamates, epoxides, cycloepoxides, isocyanates, amino groups, alkylamino groups, vinyl groups, and imide groups . When several R ³ and/or R ⁴ groups are present, they may be identical or different. Generally speaking, each R ³ and R ⁴ group can comprise from 1 to 4 carbon atoms.

In an exemplary embodiment, at least one of the first and second precursors is an organoalkoxide sol-gel precursor with 1 to 4 carbon atoms for each alkoxy group present.

Such a choice of precursor helps to further limit cracking of the coating in operation. This case corresponds to at least one of x and y not zero and at least one of R ² and R ⁴ corresponding to an alkyl or a branched C ₁ to C ₄ alkyl in the precursor formulas above. Advantageously, the first and second precursors are such organoalkoxide sol-gel precursors.

As indicated above, the precursors can alternatively be in the form of metal salts, having a general formula E1CI ¹ _v or E2CI ² _w depending on whether it is the first or the second precursor, with CI1 and CI2 designating a counter -ion chosen from: the nitrate ion, the acetate ion, a halide ion, for example the chloride ion, or a carbamate ion, and v and w being as defined above.

In an exemplary embodiment, the liquid composition further comprises reactive particles, distinct from the first and second precursors, capable of reacting with oxygen or trapping it.

Such a characteristic makes it possible to further improve the protection against oxidation provided by the coating by having particles capable of interacting chemically with oxygen.

As an example of such reactive particles, mention may be made of particles formed from the following compounds: carbides, borides, nitrides, silicides or metals, for example, Si, SiC, TiC, VB ₂ , TiSi, TiB ₂ , TiSi ₂ , MgO, Ti, Ag, Cu.

Alternatively or in combination, the liquid composition further comprises fillers, distinct from the first and second precursors, capable of filling part of the porosity of the interconnected mixed oxide network so as to hinder the diffusion of oxygen. The charges are inert with respect to oxygen.

Such a characteristic makes it possible to further improve the protection against oxidation conferred by the coating by having particles capable of physically blocking the access path of oxygen through the interconnected network.

As an example of such fillers, mention may be made of particles formed from the following compounds: oxides or metals, for example, SiO ₂ , TiO ₂ , ZrO ₂ , Al ₂ O ₃ , or BN.

Generally speaking, the reactive particles or fillers can have an average size D50 of between 10 nm and 500 nm.

Generally speaking, the reactive particles or fillers can be in the solid state in the liquid composition as well as in the coating obtained. The liquid composition may comprise the reactive particles or the fillers in a mass content of between 2% and 50%, or the liquid composition may comprise the reactive particles as well as the fillers present in a total mass content, corresponding to the sum of the mass contents. reactive particles and fillers, between 2% and 50%.

Those skilled in the art will recognize that the process which has just been described can be preceded by cleaning and activation of the surface of the part to be coated, using techniques known per se which are not necessary to be described further in this presentation. The liquid composition can be deposited in contact with the titanium alloy part (without a layer interposed between this composition and the titanium alloy).

In an exemplary embodiment, the part is an aircraft part, in particular a compressor part, an exhaust nozzle or part of such a nozzle, or a helicopter impeller.

Examples

Example 1: system Y ₃ Al ₅ O ₁₂ (YAG)

38.301 grams of Y(NO ₃ ) _{3.6H 2} O are introduced into a 100mL volumetric flask. Absolute ethanol is added to the flask (approximately 35 mL). The solution is left stirring at 40-50°C for 30 to 60 minutes until the salt is completely dissolved. The flask is then topped up to 100 mL again with ethanol and the whole is left stirring for 24 hours at room temperature.

In a second 100mL flask, 37.513 grams of Al(NO ₃ ) _{3.9H 2} O are introduced. Absolute ethanol is added to the flask (approximately 35 mL). The solution is left stirring at 40-50°C for 30 to 60 minutes until the salt is completely dissolved. The flask is then topped up to 100 mL again with ethanol and the whole is left stirring for 24 hours at room temperature.

At the end of 24 hours, 3 volumes of the yttrium solution are mixed with 5 volumes of the aluminum solution. The mixture is ready for coating.

The mixture thus obtained was applied to a part made of titanium alloy type TI6242 and the coating underwent a heat treatment for 60 minutes at a temperature of 700°C. An improvement in protection against oxidation was thus obtained.

Example 2: Al ₂ PO _x system

24.00 grams of triethylphosphite (P(OCH ₂ CH ₃ ) ₃ ) are introduced into a 100mL flask and supplemented with absolute ethanol. The solution is left stirring for a minimum of 12 hours. 56.27 grams of Al(NO ₃ ) _{3.9H 2} O are introduced into a 100 mL volumetric flask filled with absolute ethanol. The solution is left under slight heating (40-50°C) until the nitrate is completely dissolved, then left stirring at room temperature for at least 12 hours.

The two solutions are then mixed: 2 volumes of the aluminum solution and 1 volume of the phosphorus-based solution. The mixture is left stirring for 24 hours before coating.

The mixture thus obtained was applied to a Ti6242 type titanium alloy part and the coating underwent a heat treatment for 5 hours at a temperature of 600°C. An improvement in protection against oxidation was thus obtained.

The expression “between… and…” must be understood as including the limits.

Claims (10)

Process for obtaining a protective coating against oxidation of a titanium-based alloy part, comprising:
- the deposition of a liquid composition on the part comprising at least (i) a first precursor which is a metallo-organic sol-gel precursor of a metallic element E1 or a salt of this metallic element E1, said metallic element E1 being chosen from aluminum or zirconium, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element E2 or a salt of this element E2, said element E2 being chosen from aluminum, zirconium , titanium, tin, zinc, phosphorus or rare earths and being different from the metallic element E1, and (iii) water, and
- the hydrolysis of the first and second precursors of the liquid composition thus deposited and their condensation so as to form the protective coating against oxidation which comprises an interconnected mixed oxide network of elements E1 and E2. Method according to claim 1, in which the metallic element E1 is aluminum and the element E2 is a rare earth. Method according to claim 2, in which the element E2 is yttrium. Method according to claim 1, in which the metallic element E1 is aluminum and the element E2 is phosphorus. Method according to any one of claims 1 to 4, in which at least one of the first and second precursors is an organoalkoxide sol-gel precursor with from 1 to 4 carbon atoms for each alkoxy group present. Method according to any one of claims 1 to 5, in which the liquid composition further comprises reactive particles, distinct from the first and second precursors, capable of reacting with oxygen or trapping it. Method according to any one of claims 1 to 6, in which the liquid composition further comprises fillers, distinct from the first and second precursors, capable of filling part of the porosity of the interconnected mixed oxide network so as to hinder the diffusion of oxygen. Method according to claim 6 or 7, in which the liquid composition comprises the reactive particles or the fillers in a mass content of between 2% and 50%, or the liquid composition comprises the reactive particles as well as the fillers present in a total mass content , corresponding to the sum of the mass contents of the reactive particles and the fillers, between 2% and 50%. Method according to any one of claims 1 to 8, in which the part is an aircraft part. Method according to claim 9, in which the part is a compressor part, an exhaust nozzle or part of such a nozzle, or a helicopter impeller.