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

Abstract

Method for obtaining an oxidation-protective coating for a titanium-based alloy part. The invention relates to a method for obtaining an oxidation-protective coating for 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 metal-organic sol-gel precursor of a metallic element E1 or a salt of this metallic element E1, said metallic element E1 being selected 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 selected 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 deposited liquid composition and their condensation to form the protective coating against oxidation, which comprises an interconnected mixed oxide network of elements E1 and E2. Figure for the abstract: no figure.

Description

Title of the invention: Method for obtaining a protective coating against oxidation of a titanium-based alloy part. Technical field

[0001] The present exposition relates to obtaining a protective coating against oxidation of titanium-based alloys, for example for applications in turbomachinery, in particular aeronautical turbomachinery. Previous technique

[0002] Titanium-based alloys offer a particularly attractive combination of properties up to operating temperatures of 550°C for applications in turbomachinery such as turbojet engines. Indeed, titanium-based alloys exhibit low density, good damage tolerances, and good fatigue resistance.

[0003] However, reducing pollutant emissions remains a major strategic challenge for the aeronautical industry. Two main approaches exist for reducing pollutant emissions: improving engine efficiency, which involves increasing the engine's operating temperature, and / or reducing the aircraft's total mass. The mechanical properties of titanium-based alloys tend to decline significantly above 550°C. One of the main sources of degradation in the properties of titanium-based alloys with increasing temperature is related to oxidation. For titanium-based alloys, oxidation manifests itself through two distinct mechanisms: firstly, the growth of the titanium oxide layer on the surface of the part, and secondly, significant oxygen diffusion within the alloy underlying the oxide layer. This second phenomenon is linked to the high solubility of oxygen in titanium.The depths affected by oxygen diffusion in the alloy can reach several hundred microns in this temperature range after 100 hours of operation at service temperatures.

[0004] The incorporation of oxygen into the crystal lattice of the metallic phase leads, at 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. Applying coatings to these alloys can limit these phenomena and increase the exposure temperatures of these alloys.

[0005] Today, titanium-based alloys are not protected against high-temperature oxidation. The type of alloy for a part is chosen according to the Maximum operating temperatures and expected lifespan depend on the reduction in mechanical properties resulting from oxygen diffusion within the titanium alloy. Titanium parts made of thin materials (plates, honeycomb structures) are more susceptible to oxidation as their thickness decreases.

[0006] To address this issue, the literature presents tests for the development of coatings, mainly oxides, such as silica, alumina, zirconia, by different development processes by dry method (chemical vapor deposition, physical vapor deposition, plasma spraying...) or by wet method.

[0007] All of these works nevertheless lead either to the penetration of oxygen into the substrate after a few tens of hours of oxidation, or to a loss of adhesion or cracking of the deposit under thermal cycling conditions due to the strong difference in thermal expansion with the substrate which leads to stresses within the film on the order of GPa, leading to the oxidation of the substrate. Description of the invention

[0008] 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 El or a salt of that metallic element El, said metallic element El being selected from aluminium or zirconium, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element E2 or a salt of that element E2, said element E2 being selected from aluminium, zirconium, titanium, tin, zinc, phosphorus or rare earths and being different from the metallic element El, 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 the elements El and E2.

[0009] The invention makes it possible to produce a multifunctional coating that protects the substrate from oxidation at temperatures up to 700°C and thus increases the service life of parts in operation compared to existing coatings. The invention relates to the field of wet surface treatments for metallic substrates, and in particular to the titanium alloys Ti6242 and Ti321S. It can be applied to all titanium parts, for example, compressor discs in aircraft engines, exhaust nozzles of nacelles, or impellers in helicopter engines. The invention is based on the choice of specific precursors, as described above, which, in the presence of water, are capable of forming an oxide network. The mixed oxide network consists of interconnected elements E1 and E2 with E1-O-E2 bonds, providing protection against high-temperature oxidation, particularly up to 700°C. Depending on the choice of precursors, hydrolysis and condensation leading to network formation can occur without heat input at ambient temperature (20°C), or may require heat treatment, for example, at a temperature of 150°C or higher, specifically between 150°C and 700°C, to accelerate network formation and enable industrial processing. The duration of any heat treatment varies depending on the precursors used and can typically be 10 minutes or higher, for example, between 10 minutes and 5 hours, specifically between 10 minutes and 2 hours. The water in the liquid composition provides at least some of the oxygen for the mixed oxide network.Water can be added in addition to the precursors, either as a solvent for the latter, or it can be supplied by using hydrated first and second precursors, the liquid medium of the composition then being non-aqueous, for example alcoholic. Generally, the first and second precursors can be dissolved in the liquid composition.

[0010] More specifically, the specific choice of precursors made in the invention makes it possible to produce a multifunctional protective coating which simultaneously presents the following functions: - a modification of the surface to limit the amount of oxygen penetrating the substrate, - an accommodation of thermomechanical stresses during operation thanks to a coefficient of thermal expansion relatively close to that of the part (limiting cracking and increasing the durability of the coating's adhesion during operation), and - a thermal barrier property to allow lowering the room temperature, thus reducing oxygen penetration.

[0011] The coating obtained limits in particular the amount of oxygen diffusing within the substrate over the first 10 to 20 microns for oxidations up to 700°C to levels much lower than the levels found for the uncoated alloy oxidized under the same conditions.

[0012] In one embodiment, the metallic element El is aluminium and the element E2 is a rare earth, in particular element E2 may be yttrium.

[0013] Alternatively, the metallic element El is aluminium and the element E2 is phosphorus.

[0014] Various types of precursors can be implemented within the framework of the invention. In particular, in the case where the first precursor is a metal-organic sol-gel precursor, it can have the general formula R'^E^OR2^ where v is the valence of El, x is an integer between 0 and v, and R1 and R2 are organic groups chosen independently from among: alkyls, branched alkyls, methacrylates, carbamates, epoxides, cycloepoxides, isocyanates, amino groups, alkylamino groups, vinyl groups, and imide groups. When several R1 and / or R2 groups are present, they may be identical or different. Generally, each R1 and R2 group can consist of 1 to 4 carbon atoms.

[0015] Similar considerations apply to the second precursor when it is an organic-inorganic sol-gel precursor of element E2. Thus, in this case, the second precursor can have the general formula R3W yE2(OR4)y, where w is the valence of E2, y is an integer between 0 and w, and R3 and R4 are organic groups chosen independently from among: alkyls, branched alkyls, methacrylates, carbamates, epoxides, cycloepoxides, isocyanates, amino groups, alkylamino groups, vinyl groups, and imide groups. When several R3 and / or R4 groups are present, they can be identical or different. Generally, each R3 and R4 group can comprise from 1 to 4 carbon atoms.

[0016] In one 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.

[0017] Such a choice of precursor further helps to limit cracking of the coating during operation. This case corresponds to at least one of x and y being non-zero and at least one of R2 and R4 corresponding to an alkyl or a C4-Ci branched alkyl in the precursor formulas above. Advantageously, the first and second precursors are such organoalkoxide sol-gel precursors.

[0018] As indicated above, the precursors can alternatively be in the form of metallic salts, having a general formula ElCIh or E2CI2W depending on whether it is the first or second precursor, with Cil and CI2 denoting 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.

[0019] In one embodiment, the liquid composition further comprises reactive particles, distinct from the first and second precursors, capable of reacting with oxygen or trapping it.

[0020] Such a characteristic makes it possible to further improve the protection against oxidation conferred by the coating by having particles capable of interacting chemically with oxygen.

[0021] By way of example of such reactive particles, one can cite particles formed from the following compounds: carbides, borides, nitrides, silicides or metallic, by example, Si, SiC, TiC, VB2, TiSi, TiB2, TiSi2, MgO, Ti, Ag, Cu.

[0022] 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 oxygen diffusion. The fillers are inert with respect to oxygen.

[0023] Such a feature makes it possible to further improve the protection against oxidation conferred by the coating by having particles capable of physically blocking the path of access of oxygen through the interconnected network.

[0024] As an example of such charges, one can cite the particles formed from the following compounds: oxides or metallic, for example, SiO2, TiO2, ZrO2, Al2O3, or BN.

[0025] In general, reactive particles or charges can have an average size D50 between 10 nm and 500 nm.

[0026] Generally, the reactive particles or fillers may be in solid form in the liquid composition as well as in the resulting coating. The liquid composition may comprise the reactive particles or fillers in a mass content of between 2% and 50%, or the liquid composition may comprise the reactive particles and the fillers in a total mass content, corresponding to the sum of the mass contents of the reactive particles and the fillers, of between 2% and 50%.

[0027] A person skilled in the art will recognize that the process just described can be preceded by cleaning and activating the surface of the part to be coated, using techniques known per se that do not need to be described further in this document. The liquid composition can be deposited in contact with the titanium alloy part (without any interposed layer between this composition and the titanium alloy).

[0028] In one 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.

[0029] Examples

[0030] Example 1: Y3A150i2 (YAG) system

[0031] 38.301 grams of Y(NO3)3.6H2O are introduced into a 100mL volumetric flask. Absolute ethanol is added to the flask (approximately 35 mL). The solution is stirred at 40-50°C for 30 to 60 minutes until the salt is completely dissolved. The flask is then filled to the required 100 mL with more ethanol, and the mixture is left to stir for 24 hours at room temperature.

[0032] In a second 100 mL flask, 37.513 grams of Al(NO3)3.9H2O are introduced. Absolute ethanol is added to the flask (approximately 35 mL). The solution is stirred at 40-50°C for 30 to 60 minutes until complete dissolution of the salt. The flask is then filled to 100mL with ethanol and the mixture is left to shake for 24 hours at room temperature.

[0033] After 24 hours, 3 volumes of the yttrium solution are mixed with 5 volumes of the aluminum solution. The mixture is ready for coating.

[0034] The mixture thus obtained was applied to a TI6242 type titanium alloy part and the coating underwent a heat treatment for 60 minutes at a temperature of 700°C. This resulted in improved protection against oxidation.

[0035] Example 2: A12POX system

[0036] 24.00 grams of triethylphosphite (P(OCH2CH3)3) are introduced into a vial of 100 mL and made up to the mark with absolute ethanol. The solution is left under stirring for a minimum of 12 hours. 56.27 grams of Al(NO3)3.9H2O are introduced into a 100 mL volumetric flask made up to the mark with absolute ethanol. The solution is left under gentle heating (40-50°C) until the nitrate is completely dissolved, then left under stirring at room temperature for a minimum of 12 hours.

[0037] The two solutions are then mixed: 2 volumes of the aluminum solution and 1 volume of the phosphorus-based solution. The mixture is left to stir for 24 hours before coating.

[0038] The mixture thus obtained was applied to a part made of Ti6242 type titanium alloy and the coating underwent a heat treatment for 5 hours at a temperature of 600°C. An improvement in oxidation protection was thus obtained.

[0039] The expression "between ... and ..." should be understood as including the boundaries.

Claims

Demands

1. A method for obtaining a coating suitable for protecting a titanium-based alloy part from oxidation at temperatures up to 700°C, comprising: - the deposition of a liquid composition on the part comprising at least (i) a first precursor which is a metal-organic sol-gel precursor of a metallic element El or a salt of this metallic element El, said metallic element El being selected from aluminium 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 selected from aluminium, zirconium, titanium, tin, zinc, phosphorus or rare earths and being different from the metallic element El, 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 the elements El and E2.

2. A method according to claim 1, wherein the metallic element El is aluminium and the element E2 is a rare earth.

3.

4. A method according to claim 2, wherein the element E2 is yttrium. A method according to claim 1, wherein the metallic element El is aluminum and the element E2 is phosphorus.

5. A process according to any one of claims 1 to 4, wherein 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.

6. A method according to any one of claims 1 to 5, wherein the liquid composition further comprises reactive particles, distinct from the first and second precursors, capable of reacting with or trapping oxygen.

7. A method according to any one of claims 1 to 6, wherein 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.

8. A method according to claim 6 or 7, wherein the liquid composition comprises the reactive particles or fillers in a content mass content between 2% and 50%, or the liquid composition includes the reactive particles as well as the charges present in a total mass content, corresponding to the sum of the mass contents of the reactive particles and the charges, between 2% and 50%.

9. A method according to any one of claims 1 to 8, wherein the part is an aircraft part.

10. A method according to claim 9, wherein the part is a compressor part, an exhaust nozzle or part of such a nozzle, or a helicopter impeller.