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

Abstract

Method for Obtaining an Oxidation Protection Coating for a Titanium-Based Alloy Part. The present invention relates to a method for obtaining an oxidation protection coating for a titanium-based alloy part, comprising: - the deposition of a liquid composition on the part comprising at least (i) a first alkoxysilane sol-gel precursor, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element E1 or a salt of this element E1, said element E1 being selected from silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus, or rare earth elements, (iii) a third 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 silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus, or rare earth elements, with E1 and E2 not both being silicon, and (iv) water,and the hydrolysis of the first, second, and third precursors of the deposited liquid composition and their condensation to form the protective coating against oxidation, which comprises an interconnected mixed oxide network of silicon 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. The incorporation of oxygen into the crystalline lattice of the metallic phase leads, at the atomic scale, to stronger covalent bonds. This results in a loss of ductility in the alloy underlying the oxide layer, in the oxygen-enriched zone, which significantly reduces the macroscopic mechanical properties. Applying coatings to these alloys can mitigate these phenomena and increase the exposure temperatures of these alloys.

[0004] Today, titanium-based alloys are not protected against high-temperature oxidation. The type of alloy for a part is chosen according to 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.

[0005] 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.

[0006] 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

[0007] 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 alkoxysilane sol-gel precursor, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element El or a salt of that element El, said element El being selected from silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus or rare earths, (iii) a third 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 silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus or rare earths, with El and E2 not both being silicon, and (iv) water, and - the hydrolysis of the first, second and third 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 silicon, El and E2.

[0008] The invention makes it possible to produce a multifunctional coating which makes it possible to protect the substrate from oxidation at temperatures up to 700°C and thus increase the service life of parts in operation compared to existing ones.

[0009] 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 interconnected mixed oxide network of silicon, El, and E2 elements, notably with E1-O-E2 bonds providing protection against high-temperature oxidation, particularly up to 700°C. Depending on the choice of precursors, the hydrolysis and condensation leading to the formation of the network can take place without heat input at ambient temperature (20°C), or require heat treatment, for example at a temperature greater than or equal to 150°C, particularly between 150°C and 700°C, to accelerate the kinetics of obtaining this network and be compatible with an industrial processing rate.The application time of any heat treatment varies depending on the precursors used and can typically be 10 minutes or more, for example, between 10 minutes and 5 hours, particularly 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, as a solvent for them, or can be provided by using hydrated precursors, in which case the liquid medium of the composition is non-aqueous, for example, alcoholic. Generally, the first, second, and third 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 performs 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 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, El is silicon and E2 is aluminum.

[0013] Various types of precursors can be implemented within the framework of the invention.

[0014] The first alkoxysilane precursor can have the general formula R'4 xSi(OR2)x where x is an integer between 1 and 4, R1 is an organic grouping chosen from: Alkyls, branched alkyls, methacrylates, carbamates, epoxides, cycloepoxides, isocyanates, amino groups, alkylamino groups, vinyl groups, and imide groups, and R2 is an alkyl or branched alkyl group. When multiple 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] In the case where the second precursor is an organic-inorganic sol-gel precursor, it can have the general formula R3W yEl(OR4)y, where w is the valence of El, 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 the case where the third precursor is an organic-inorganic sol-gel precursor, it can have the general formula R5a ZE2(OR6)Z, where a is the valence of E2, z is an integer between 0 and a, and R5 and R6 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 R5 and / or R6 groups are present, they can be identical or different. Generally, each R5 and R6 group can comprise from 1 to 4 carbon atoms.

[0017] In one embodiment, the first precursor comprises 1 to 4 carbon atoms for each alkoxy group present, and at least one of the second and third precursors is an organoalkoxide sol-gel precursor with 1 to 4 carbon atoms for each alkoxy group present.

[0018] Such a choice of precursor further helps to limit cracking of the coating during operation. This case corresponds to R2, which is an alkyl or a C4-Ci branched alkyl in the formula of the first precursor, and at least one of R4 and R6, which are alkyl or a C4-Ci branched alkyl in the formulas of the second and third precursors above. Advantageously, the second and third precursors are such organoalkoxide sol-gel precursors.

[0019] As indicated above, the second and third precursors may alternatively be in the form of metallic salts, having a general formula ElCl'w or E2Cl2a depending on whether it is the second or third precursor, with Cil and Cl2 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 w and a being as defined above.

[0020] In one embodiment, the liquid composition further comprises reactive particles, distinct from the first, second and third precursors, capable of reacting with oxygen or trapping it.

[0021] 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.

[0022] As an example of such reactive particles, one can cite the particles formed from the following compounds: carbides, borides, nitrides, silicides or metallic, for example, Si, SiC, TiC, VB2, TiSi, TiB2, TiSi2, MgO, Ti, Ag, Cu.

[0023] Alternatively or in combination, the liquid composition further comprises fillers, distinct from the first, second, and third precursors, suitable for filling a porosity in the interconnected mixed oxide network so as to impede oxygen diffusion. The fillers are inert with respect to oxygen.

[0024] 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.

[0025] 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.

[0026] In general, reactive particles or charges can have an average size D50 between 10 nm and 500 nm.

[0027] 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%.

[0028] 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).

[0029] 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.

[0030] Example: mullite system 3 Al2O3 / 2 SiO2

[0031] 17.9 mL of tetraethoxysilane and 4.02 mL of methyltriethoxysilane are added to 63.65 mL of isopropanol (stirred for 1 hour) at room temperature. Then, 7.2 mL of water acidified with 0.1M hydrochloric acid and 7.20 mL of water are added to the previous mixture and left to stir for 1 hour at room temperature.

[0032] 38.18 grams of a 64% wt% Aluminium tri-sec-butoxide solution in Isopropanol and 25.30 mL of isopropanol, along with 25.27 mL of ethyl acetoacetate, are stirred for 1 hour at room temperature. Then, 3.6 mL of water acidified with hydrochloric acid (1 / 36 mL) + 3.6 mL of water are added to the previous mixture and left to stir for 1 hour at room temperature.

[0033] 2 volumes of the silica solution are mixed with 6 volumes of the solution of aluminium. The mixture is left under stirring for 24 hours before coating.

[0034] The mixture thus obtained was applied to a titanium alloy part and the coating underwent heat treatment. This resulted in improved protection against oxidation.

[0035] The expression "between ... and ..." should be understood as including the boundaries.

Claims

Demands

1. A method for obtaining an oxidation-protective coating for a titanium-based alloy part, comprising: - deposition of a liquid composition on the part comprising at least (i) a first alkoxysilane sol-gel precursor, (ii) a second precursor which is an organic-inorganic sol-gel precursor of an element El or a salt of this element El, said element El being selected from silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus, or rare earth elements, (iii) a third 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 silicon, aluminum, zirconium, titanium, tin, zinc, phosphorus, or rare earth elements, with El and E2 not both being silicon, and (iv) water, and - hydrolysis of the first,second and third 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 silicon, El and E2, in which the liquid composition further comprises reactive particles, distinct from the first, second and third precursors, capable of reacting with or trapping oxygen and / or in which the liquid composition further comprises fillers, distinct from the first, second and third precursors, capable of filling a porosity of the interconnected mixed oxide network so as to hinder the diffusion of oxygen.

2. Method according to claim 1, wherein El is silicon and E2 is aluminum.

3. A process according to claim 1 or 2, wherein the first precursor comprises 1 to 4 carbon atoms for each alkoxy group present, and at least one of the second and third precursors is an organoalkoxide sol-gel precursor with 1 to 4 carbon atoms for each alkoxy group present.

4. A method according to any one of claims 1 to 3, wherein the liquid composition comprises the reactive particles or fillers in a mass content of between 2% and 50%, or the liquid composition comprises the reactive particles as well as the 8 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%.

5. A method according to any one of claims 1 to 4, wherein the part is an aircraft part.

6. Method according to claim 5, wherein the part is a compressor part, an exhaust nozzle or part of such a nozzle, or a helicopter impeller.