Method for depositing aluminum oxide coating

Abstract

The invention relates to a method for depositing a continuous aluminum oxide coating on a metal substrate by means of induction heating-assisted pressurized temperature-controlled chemical deposition, comprising a thermosolvation step based on an aluminum oxide precursor dissolved in a water-co-solvent mixture, the alumina precursor dissolved in the water-co-solvent mixture is heated by induction to a temperature between 400 DEG C and 700 DEG C and a pressure between 1 MPa and 25 MPa. The invention also relates to a device (100) for depositing aluminum oxide on a metal substrate (104) by means of pressurized temperature-controlled chemical deposition, said device comprising:-a chamber (102) delimited by walls forming a closed volume (V), said chamber being intended to contain a pressurized and heated fluid, the material of the chamber walls being permeable to electromagnetic radiation; a support (106) likewise permeable to electromagnetic radiation for supporting a metal substrate (104) located in the chamber; -induction heating means (109) surrounding the outside of the chamber (102) so as to be able to heat a metal substrate (104) positioned on the support (106); -an inlet (116) located at the upper part of the chamber (102) and configured to allow a precursor material previously dissolved in water to be added to the chamber (102); -an inlet (120) located at a lower portion of the chamber (102) and configured to allow a fluid to be added to the chamber (102); -at least one outlet (124) configured to empty the volume (V); -a sapphire window (112) arranged at the upper part of the chamber, allowing the temperature of the metal substrate (104) to be controlled by a two-tone pyrometer (114) arranged outside the chamber; -a set of polymeric seals (200); -a metal component (202) rigidly screwed together by a metal post (204), containing a circulating fluid that maintains a temperature at 20 DEG C by the cryostat (206).

Description

Technical Field

[0001] The present invention generally relates to the field of aluminum oxide (also known as alumina) coatings, more particularly to coatings for metal alloys, and even more particularly to an apparatus and method for depositing aluminum oxide coatings by means of pressurized temperature-controlled chemical deposition. Background Art

[0002] Various metal alloys, such as titanium alloys, TiAl or nickel-based alloys, need to be protected against oxidation and / or corrosion in order to maintain their performance at higher operating temperatures.

[0003] Of the many possible solutions, an alpha alumina layer is the best solution in most cases. In fact, alpha alumina has excellent resistance to oxidation and corrosion. In addition, its alpha crystalline form has a low oxygen diffusion coefficient, which makes it relatively impermeable to oxygen. It is also the most stable form of alumina at high temperatures.

[0004] However, the temperatures required to grow alpha alumina (homogeneous and heterogeneous nucleation) are generally in the range of 900°C and above. In fact, alpha alumina is traditionally produced by chemical vapor deposition (CVD), physical vapor deposition (PVD) or sol-gel methods. Both CVD and sol-gel methods use temperatures above 1000°C to stabilize the alpha phase of alumina. This temperature is incompatible with most metal alloys. As for PVD, this technology allows the desired phase to be stabilized at lower temperatures (480°C to 580°C), even though the coating often has mixed phases of metastable alumina, such as the γ phase. The disadvantage of PVD is its directionality, which, unlike chemical deposition techniques, does not allow the coating of substrates with complex geometries, such as turbine blades. In addition, these methods generally have relatively slow deposition rates.

[0005] Various studies have been conducted to try to obtain alpha alumina from other phases of alumina. However, the conversion of any phase of alumina to alpha alumina results in significant volume changes that affect the mechanical properties of the coating and even the coated part.

[0006] It is known that alpha alumina can be obtained using lower temperatures, but only in the context of homogeneous nucleation, which allows the synthesis of materials in powder form rather than in coating form. Thus, according to the binary diagram Al2O3-H2O, alpha alumina is stable in water at relatively low pressures (from 1 MPa) and temperatures between 374°C and 500°C.

[0007] Alumina is formed by hydrothermal synthesis in two consecutive steps. The first step is to hydrolyze the aluminum precursor with water to form hydrated aluminum oxide, boehmite Al(NO3)3+2H2O---->γ-AlOOH+3HNO3 (Equation 1)

[0008] The second step involves dehydrating the intermediate phase (i.e., boehmite) to obtain α-alumina: 2γ-AlOOH---->α-Al2O3+H2O (Eq. 2)

[0009] According to the literature, dehydration is the effect of a continuous hydroxylation / dehydroxylation phenomenon, resulting in the solid-state structure of boehmite being reorganized into alpha alumina.

[0010] However, as mentioned previously, pressurized hydrothermal synthesis can form alpha alumina, but in the form of a powder rather than a coating.

[0011] The only solution currently allowing the growth of aluminum oxide layers on substrates, in particular in the case of TiAl, is to exploit the halogen effect, as described, for example, in patent application WO2020 / 229747, which actually requires the use of halogenated gases, which may pose toxicity problems.

[0012] Therefore, there is a need to find a new method that allows the deposition of a layer of alpha alumina on substrates with complex geometries (such as turbine blades) at deposition temperatures compatible with most metal alloys, in particular below 850°C, without the use of halogenated gases and with sufficiently high deposition rates. Summary of the invention

[0013] The inventors unexpectedly discovered that such deposition can be achieved using solvothermic induction heating assisted synthesis, thereby allowing induction heating assisted (type) pressurized, temperature controlled chemical deposition. The inventors found that this method can obtain thick alpha alumina deposits (greater than 1 μm, advantageously greater than 10 μm), and is also suitable for depositing other types of aluminum oxides, such as metastable aluminum oxide, on any metal substrate. It also avoids the use of halogenated gases and has a high deposition rate.

[0014] Therefore, the use of a more efficient method (high speed) according to the present invention also helps reduce the environmental pollution of the applicant. In fact,

[0015] - It allows to increase and optimize manufacturing, production and / or repair capacities, thus significantly reducing the associated greenhouse gas emissions. This optimization can also reduce the consumption of raw materials;

[0016] -It can extend the service life of parts and reduce the number of times of replacing new parts;

[0017] -It can significantly reduce the amount of scrapped parts that are difficult to recycle.

[0018] Additionally, this solution has the advantage of reducing its energy input (water, electricity, etc.) and / or reducing the use of any chemical products that violate current environmental standards and regulations.

[0019] The present invention therefore relates to a method for depositing a continuous coating of aluminium oxide on a metal substrate by means of pressurised temperature-controlled chemical deposition assisted by induction heating, the method comprising a solvothermal synthesis step based on an aluminium oxide precursor dissolved in a water-co-solvent mixture, heated by induction to a temperature between 400° C. and 700° C. and a pressure between 1 MPa and 25 MPa.

[0020] In the present application, unless explicitly stated otherwise, the expression "between ... and ..." must be understood as including the endpoints.

[0021] The metal substrate according to the invention is in particular a metal substrate comprising titanium, more in particular a titanium alloy, even more in particular a titanium aluminum alloy, for example based on titanium aluminide, such as a γ-TiAl alloy.

[0022] The metal substrate according to the invention can constitute a turbine component, for example an aerospace turbine component. Advantageously, the substrate is intended for use in an oxidizing atmosphere and at temperatures greater than or equal to 800° C. The substrate can, for example, be a turbine component. For example, it can be a turbine blade or a turbine ring sector. Thus, it can be a component with a complex geometry, that is to say a non-planar, in particular a 3D component. However, the method of the invention can also be implemented on substrates with a planar geometry.

[0023] In the context of the present invention, a pressurized fluid is a fluid having a pressure higher than atmospheric pressure, in particular a fluid having a pressure between 1 MPa and 25 MPa, advantageously between 6 MPa and 10 MPa.

[0024] In the context of the present invention, pressurized temperature-controlled chemical deposition is any deposition carried out by chemical means at a pressure above atmospheric pressure, in particular at a pressure between 1 MPa and 25 MPa, advantageously at a pressure between 6 MPa and 10 MPa, and at a temperature above ambient temperature, advantageously below 850°C, in particular at a temperature between 400°C and 700°C.

[0025] The alumina precursor according to the invention is any water-soluble precursor, such as aluminum nitrate Al(NO3)3 or an aluminum salt such as Al2(SO4)3. Advantageously, it is aluminum nitrate Al(NO3)3.

[0026] The cosolvent according to the invention is selected from alcohols, in particular ethanol, nitrogen, carbon dioxide, argon and mixtures thereof, advantageously nitrogen. The cosolvent can conduct heat better and thus make it easier to heat the substrate.

[0027] In the context of the method according to the invention, it is the pressurized and heated water that allows the precursors dissolved therein to generate aluminum oxide and thus deposit it on the substrate.

[0028] Advantageously, the molar ratio of water / cosolvent, in particular the molar ratio of water / nitrogen, is between 0.1 and 50%. In particular, the flow rate of water is advantageously between 0.1 and 10 mL / min, more particularly 1.3 mL / min. In one embodiment, the flow rate of the cosolvent, in particular the flow rate of nitrogen, is between 0.1 and 10 mL / min, more particularly 2 mL / min.

[0029] The temperature of the solvothermal synthesis step is between 400°C and 700°C, advantageously between 500°C and 700°C, more advantageously between 550°C and 680°C, even more advantageously between 600°C and 650°C, in particular 630°C.

[0030] The pressure of the hydrothermal synthesis step is between 1 MPa and 25 MPa, advantageously between 5 MPa and 20 MPa, more advantageously between 7 MPa and 15 MPa, in particular 10 MPa.

[0031] The aluminum oxide of the continuous coating obtained on the surface of the metal substrate by the method according to the invention will depend on the temperature and pressure used, the amount of co-solvent and water used during the reaction (e.g. the water / co-solvent ratio). In practice, hydrothermal dehydration (or solvothermal synthesis) produces alpha aluminum oxide, while dehydration in air produces other phases of aluminum oxide. Advantageously, the aluminum oxide of the continuous coating is a metastable aluminum oxide (e.g. kappa aluminum oxide or theta aluminum oxide or gamma aluminum oxide), an alpha aluminum oxide or a mixture of these oxides (mixed oxide), advantageously an alpha aluminum oxide.

[0032] The synthesis of a continuous coating of alpha alumina using the precursor Al(NO3)3 is advantageously carried out at a pressure of 10 MPa, a temperature of 630°C using a water / nitrogen mixture (nitrogen as a co-solvent), a water flow rate of 1.3 ml / min and a nitrogen flow rate of 2 ml / min.

[0033] In an advantageous embodiment, the obtained aluminum oxide continuous coating is thick, that is, thicker than 1 μm, in particular at least 2 μm thick, more particularly between 1 μm and 75 μm thick, even more particularly thicker than 10 μm, advantageously between 50 μm and 72 μm thick.

[0034] In an advantageous embodiment, the coating deposition rate is between 100 and 500 nm / min, advantageously 300 nm / min.

[0035] In an advantageous embodiment, the method according to the invention is carried out in a pressurized, temperature-controlled chemical deposition reactor assisted by induction heating.

[0036] Advantageously, the pressurized and temperature-controlled chemical deposition reactor assisted by induction heating that can be used in the method according to the invention is that described in application FR 3 112 972, more particularly at a pressure between 1 MPa and 10 MPa.

[0037] In another advantageous embodiment, the induction heating assisted pressurized temperature controlled chemical deposition reactor that can be used in the method according to the present invention is an apparatus 100 for depositing aluminum oxide on a metal substrate 104 as described below.

[0038] In an advantageous embodiment, the method according to the invention comprises the following steps:

[0039] a- Adding the metal substrate to be coated to the induction heating assisted pressurized temperature controlled chemical deposition reactor;

[0040] b- adding the alumina precursor previously dissolved in water and co-solvent to the reactor;

[0041] c- solvothermal synthesis based on an alumina precursor dissolved in a water-cosolvent mixture by inductive heating of the alumina precursor at a temperature between 400° C. and 700° C. and at a pressure between 1 MPa and 25 MPa, advantageously between 6 MPa and 10 MPa;

[0042] d-Recovery of the substrate coated with the continuous alumina coating.

[0043] In an advantageous embodiment, water and the co-solvent are added separately to the reactor in step b).

[0044] In a specific embodiment, the method comprises an intermediate step a1) between step a) and step b): preheating the reactor to a temperature between 400° C. and 700° C. and pressurizing it to a pressure between 1 MPa and 25 MPa.

[0045] In an advantageous embodiment, the process is a semi-continuous or discontinuous process (or closed mode), advantageously semi-continuous.

[0046] In a semi-continuous process, an aluminum oxide precursor previously dissolved in water and a co-solvent is continuously introduced, in particular continuously introduced in step b) of the method according to the invention, in particular continuously circulated in a reactor previously heated and pressurized according to the invention, and is continuously removed / exhausted. On the other hand, the substrate is fixed. Therefore, the substrate is advantageously added to the reactor in step a), and then the aluminum oxide precursor previously dissolved in water and a co-solvent is advantageously continuously added and the reactor is pressurized and heated in step b). Once the method has been implemented and the reactor has been cooled to room temperature and decompressed, the coated substrate is recovered. In a semi-continuous process, the deposit grows continuously as the precursors, water and co-solvents are added and reacted. This semi-continuous mode allows precise control and adjustment of the deposit and the amount of the introduced precursors, water and co-solvents to better control the formation and growth kinetics of aluminum oxide on the surface of the metal substrate.

[0047] In a discontinuous method (or closed mode), the amount of fluid (precursor+cosolvent+water) and substrate are fixed, and the fluid and substrate are added to a reactor according to the present invention in advance. The reactor is then heated and pressurized to implement solvothermal synthesis. Generally speaking, the amount of cosolvent and water determines the maximum pressure that can be achieved according to the applied temperature. Precursors, water or cosolvents are not introduced during the growth of aluminum oxide on the surface of the metal substrate. Once deposition is achieved, the reactor is cooled to room temperature and decompressed so that the coating substrate can be recovered. The fluid is also removed / exhausted.

[0048] Advantageously, the duration of the method according to the invention is between 30 and 180 minutes.

[0049] The present invention also relates to a device for depositing aluminum oxide (especially the aluminum oxide as described above) on a metal substrate by pressure-controlled temperature chemical deposition, the device comprising:

[0050] - a chamber delimited by walls forming a closed volume (V), the chamber being intended to contain a pressurized and heated fluid (in particular under the temperature and pressure conditions as described above), the material of the chamber walls being transparent to electromagnetic radiation;

[0051] - a support, also transparent to electromagnetic radiation, for supporting the metal substrate within the chamber;

[0052] - induction heating means surrounding the outside of the chamber so as to be able to heat the metal substrate situated on the support;

[0053] an inlet located in the upper part of the chamber and configured to allow the addition of a precursor material previously dissolved in water, in particular a precursor material as described above, to the chamber;

[0054] an inlet located in the lower part of the chamber and configured to allow the addition of a fluid (or a co-solvent, in particular a co-solvent as described above) to the chamber;

[0055] - at least one outlet configured to purge / exhaust the enclosed volume (V);

[0056] - a sapphire window, arranged in the upper part of the chamber, allowing the temperature of the metal substrate to be controlled by a two-color pyrometer arranged outside the chamber;

[0057] - a set of polymer seals;

[0058] - Metal components containing a circulating fluid, the temperature of which is controlled by a cryostat, screwed together rigidly by metal posts.

[0059] The device according to the invention can form a continuous coating made of aluminum oxide, which is particularly thick (greater than 1 μm), particularly a coating as described above, on a flat metal substrate or on a metal substrate with complex geometry, particularly a metal substrate as described above.

[0060] The induction heating device according to the invention, advantageously consisting of an induction generator and an induction loop, allows heating only the metal substrate while keeping the temperature on the walls lower than that of the metal substrate. In fact, making the walls transparent to electromagnetic radiation makes it possible to avoid inductive coupling with these walls and keep them at a lower temperature than that of the metal substrate, in order to control the convective movements in the chamber.

[0061] Induction heating also offers better performance than resistive heaters because it can heat the entire surface of a metal substrate with complex geometry more quickly, more evenly, or by confining the highest heating to the thickness near the outermost surface of the substrate.

[0062] According to a particular feature of the invention, the material of the chamber wall is ceramic. Most ceramics are transparent to electromagnetic radiation and are therefore an excellent candidate for forming the wall. Advantageously, it is a ceramic made of silicon nitride Si3N4.

[0063] The ceramic is pressure-resistant thanks to a group of polymers (such as PEEK (polyetheretherketone), (fluorocarbon rubber (FKM) produced by DuPont), EPDM (ethylene-propylene-diene monomer) and / or A seal made of (perfluororubber FFKMFFPM produced by DuPont)) and a metal component (especially cylindrical) are rigidly screwed together by metal columns, especially 6 metal columns distributed equally, in which a fluid such as ethylene glycol circulates, and the temperature is controlled at 20°C by a low-temperature thermostat.

[0064] Advantageously, the device according to the invention does not comprise a double wall.

[0065] Advantageously, the inlets of the device are equipped with pumps, in particular HPLC for the water inlet and Isco for the fluid inlet. In the case of CO2, the Isco pump is replaced by a pump dedicated to the injection of liquid carbon dioxide, which must therefore be cooled to 1°C using a cryostat added to the pump.

[0066] Advantageously, the inlet situated in the lower part of the chamber and configured to allow the addition of a fluid is the inlet of a co-solvent as described above within the framework of the method, which fluid, after mixing with water, will serve as supercritical fluid.

[0067] Advantageously, the outlet of the device is equipped with a pressure regulator.

[0068] According to another particular feature of the invention, a lid can be provided at the end of the chamber to close it. Advantageously, only one of the two lids is removable. The lid can be made of steel, more particularly 316L steel, for example.

[0069] Finally, the invention relates to the use of the device according to the invention for carrying out the method according to the invention.

[0070] It also relates to a method according to the invention, wherein the device according to the invention is a pressurized temperature-controlled chemical deposition reactor assisted by induction heating.

[0071] Thus, in a particular embodiment of the method according to the invention, the metal substrate to be coated is positioned in a chamber (or reactor) of the apparatus, on a support (step a of the method according to the invention).

[0072] Then, in a subsequent step, the metal substrate is induction heated using an induction heating device (step a1 of the method of the present invention).

[0073] Then, once the temperature of the metal substrate reaches between 400° C. and 700° C., the aluminum oxide precursor (or precursor material) and the co-solvent previously dissolved in water are added to the chamber (step b of the method of the present invention). The added water with the dissolved precursor and the co-solvent will then be subjected to an increase in pressure and temperature until the desired pressure and temperature are reached.

[0074] When the aluminum oxide precursor (or precursor material) and the co-solvent previously dissolved in water are added, the metal substrate is continuously heated by induction. This allows the conditions required to form aluminum oxide on the surface of the metal substrate to be achieved in the vicinity of the metal substrate.

[0075] Thus, the introduced aluminum oxide precursor, water and co-solvent react under solvothermic conditions to form aluminum oxide on the substrate surface (step c of the method according to the invention). The aluminum oxide precursor and co-solvent previously dissolved in water are introduced into the chamber throughout the formation and growth of aluminum oxide on the substrate surface. Thus, the formation of aluminum oxide on the substrate surface is carried out in a semi-continuous mode. This allows the amount of aluminum oxide precursor, water and co-solvent to be adjusted as the aluminum oxide layer grows.

[0076] When the thickness of the aluminium oxide layer is sufficient, the addition of the aluminium oxide precursor and the co-solvent previously dissolved in water is stopped, the chamber is cooled and then depressurised to recover the coated substrate (step d of the process according to the invention).

[0077] In another particular embodiment of the method according to the invention, the metal substrate to be coated is positioned in a chamber (or reactor) of the apparatus, on a support (step a of the method according to the invention).

[0078] Then, in a subsequent step, the metal substrate is induction heated using an induction heating device (step a1 of the method according to the present invention).

[0079] Once the temperature and pressure (T,P) of the water / cosolvent mixture in the chamber V Once the desired conditions have been reached, the aluminium oxide precursor (or precursor material) and the co-solvent, previously dissolved in water, are introduced into the chamber, the water and the co-solvent thus being under pressure and temperature (step b of the method according to the invention).

[0080] Thus, the aluminum oxide precursor, water and co-solvent react under pressure and temperature control to form a continuous aluminum oxide layer on the substrate surface by solvothermal synthesis, which layer will grow over the entire reaction duration (step c of the method according to the invention). In this reaction step, no precursor material (or aluminum oxide precursor) or fluid is added. The formation of aluminum oxide on the surface of the metal substrate is carried out in a closed mode or discontinuous process.

[0081] When the growth of the aluminium oxide layer is complete, that is to say when all the precursor material (or aluminium oxide precursor) has reacted, the chamber is cooled and then depressurised to recover the coated substrate (step d of the method according to the invention).

[0082] The present invention will be better understood according to the following description of the drawings and embodiments, which are provided for reference only and are not intended to be limiting. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] [ Figure 1 ] Figure 1 A partial view of a device according to the invention is shown without metal components or seals.

[0084] [ Figure 2] Figure 2 A schematic, partially cross-sectional view of a device according to the invention is shown with metal components and seals. DETAILED DESCRIPTION

[0085] The device 100 allows the deposition of an aluminum oxide coating on a metal substrate 104. The device 100 comprises a cylindrical chamber 102, which is delimited by walls forming a closed volume V. The chamber 102 is suitable for receiving a pressurized heated fluid through a set of polymer seals 200, a metal assembly 202, which is particularly located at the sapphire window 112, and the joint between the metal assembly 202 and the chamber 102, in particular the cylindrical metal assembly 202 is rigidly screwed together by metal posts 204, in particular there are 6 metal posts 204, which are equidistant from each other, and a fluid circulates in the metal assembly at a temperature of 20° C. controlled by a cryostat 206, in particular the fluid is located above and below the cylindrical chamber, more particularly on both sides of the sapphire window 112 and the inlet 120 and the outlet 124. The metal assembly 202, the metal posts 204 and the screws are particularly made of 306L steel.

[0086] The device 100 also comprises an inlet 120 located in the lower part of the chamber 102 so as to be able to introduce a fluid as a co-solvent into the volume V. It also comprises an inlet 116 located in the upper part of the chamber 102 so as to be able to add water and a precursor material previously dissolved in water into the same volume V. The inlets 116 and 120 may be equipped with pumps 118 and 122.

[0087] There is also an outlet 124 in the device 100 for purging / evacuating the volume V, thereby allowing the deposition device 100 to operate semi-continuously. The outlet 124 may be equipped with a pressure regulator 126.

[0088] The support 106 is located in the chamber 120 for supporting the metal substrate 104 on which the coating is deposited. Preferably, the support 106 is located in the chamber 120 so that the metal substrate 104 is held in the center of the inductor, which forms the induction heater 109. Preferably, the support 106 has a shape that allows the metal substrate 104 to be supported with a minimum of contact points, so as to coat as large a surface of the metal substrate as possible with the deposited aluminum oxide coating while limiting disturbances of the induction convection. The support 106 is composed of a material that is transparent to electromagnetic radiation. For example, it is composed of a material that is non-thermally and electrically non-conductive, such as aluminum oxide.

[0089] An induction heater 109, comprised of an induction generator 108 and an induction loop 110, surrounds the chamber 102. The induction heater can heat the metal substrate 104 while limiting heating to the precursor material present in the volume V.

[0090] In order not to interfere with the inductive heating of the metal substrate 104, the walls of the chamber 102 are transparent to electromagnetic radiation. They are made of ceramic, for example. The ceramic used may be boron nitride, aluminum nitride, aluminum oxide or silicon nitride, more particularly silicon nitride. These dense and non-porous ceramic examples give the walls of the chamber 102 excellent mechanical strength, thus being able to withstand the pressure present in the volume V.

[0091] A sapphire window 112 is arranged at the upper portion of the chamber 102 and allows the temperature of the metal substrate 104 to be controlled by a two-color pyrometer 114 arranged outside the chamber 102 .

[0092] Example

[0093] The reactor used in the embodiment is as described above. Figure 1 and 2 The described, consists of a cylindrical ceramic chamber made of silicon nitride Si3N4, with an internal volume of about 300mL, containing a pressurized fluid and a metal substrate held by an alumina support. The ceramic is insensitive to magnetic fields and is surrounded by an induction loop, which itself is connected to an induction generator with a maximum power of 7kW. This allows preferential heating of the metal substrate located in the center of the pressurized fluid. The ceramic is surrounded by a set of polymer seals (Peek, EPDM and / or ) and a cylindrical metal assembly that is rigidly screwed together by 6 metal columns equidistant from each other, made of 316L steel. In order to avoid excessive deformation of the metal assembly due to temperature increase, a fluid (ethylene glycol) is circulated in it, the temperature of which is controlled by a low-temperature thermostat at 20°C. The holding assembly can pressurize the fluid up to 25MPa. Another improvement is the addition of a tap on the upper part of the metal assembly, which allows the fluid to be injected from the top while maintaining the position of the sapphire window. The latter allows the temperature of the metal substrate to be controlled using a two-color pyrometer while maintaining the injection of fluids and precursors from the top of the reactor. Fluids and precursors are injected at a controlled flow rate using an HPLC pump (for water) and an Isco pump (for a co-solvent (here nitrogen)). The pressure is maintained by an outlet pressure regulator. The reactor is operated in semi-continuous mode with a fixed substrate and continuous circulation of the fluid.

[0094] A gamma-based titanium aluminum parallelepiped metal substrate with dimensions of 1.5×1.5×0.5 cm was introduced into the reactor.

[0095] Using aluminum nitrate Al(NO3)3 as the precursor material, a continuous α-alumina coating was deposited on the substrate at temperatures of 510°C, 630°C and 700°C, respectively. The flow rates of water and nitrogen (as a co-solvent) were 1.3 mL / min and 2 mL / min, respectively, and the pressure was 10 MPa.

[0096] The coated substrate was analyzed using a 1° grazing incidence X-ray diffractometer (or GIXRD, for grazing incidence XRD) and the diffraction patterns obtained were analyzed using EVA software, and it was observed that at a temperature of 510°C, the deposit contained a mixture of metastable alumina, kappa phase and, most importantly, alpha alumina.

[0097] At a temperature of 630°C, the diffraction lines of α alumina are more easily observed at 25.51° and 43.23°, still with the metastable state, ie, κ alumina.

[0098] At 700°C, the presence of α-alumina is hardly noticeable. The diffraction lines of the new metastable θ-alumina are clearly visible.

[0099] Scanning electron microscopy of cross sections produced by metallographic preparation of the coated substrates showed that the morphology of the deposits produced in the water / nitrogen mixture was homogeneous and consisted of aggregates of hexagonal grains of a few hundred nanometers in size.

[0100] It was also observed that the cross-section of the coating prepared at 600 °C provided morphological information about the depth and thickness of the deposition. The deposit appeared to have two structures from the substrate to the outside, a relatively dense structure with a thickness of more than 2 μm (closest to the substrate) and a relatively porous structure with a thickness of more than 60 μm. In addition, the average thickness was about 61 ± 11 μm, which gave a deposition rate of about 300 nm.min -1 . These two regions consist of aluminum oxide.

[0101] In summary, this induction heating-assisted pressure-controlled temperature chemical deposition method can form α and mixed alumina coatings on TiAl metal substrates with complex geometries at pressures ranging from 1 MPa to 25 MPa and at temperatures significantly lower than 850 °C.

Claims

1. A method for depositing a continuous aluminum oxide coating on a metal substrate by pressurized temperature-controlled chemical deposition assisted by induction heating, comprising a solvothermal synthesis step based on an aluminum oxide precursor dissolved in a water-cosolvent mixture, wherein the aluminum oxide precursor dissolved in the water-cosolvent mixture is inductively heated to a temperature between 400°C and 700°C and a pressure between 1 MPa and 25 MPa.

2. The method according to claim 1, characterized in that The method comprises the following steps: a- Adding the metal substrate to be coated into an induction heating-assisted pressurized temperature-controlled chemical deposition reactor; b- adding an alumina precursor and a co-solvent previously dissolved in water to the reactor; c- solvothermal synthesis based on an alumina precursor dissolved in a water-cosolvent mixture by inductive heating of the alumina precursor at a temperature between 400° C. and 700° C. and at a pressure between 1 MPa and 25 MPa, advantageously between 6 MPa and 10 MPa; d-Recovery of the substrate coated with the continuous alumina coating.

3. The method according to claim 1 or 2, characterized in that: The metal substrate is made of a titanium alloy, advantageously an alloy based on titanium aluminide.

4. The method according to any one of claims 1 to 3, characterized in that The co-solvent is selected from alcohols, in particular ethanol, nitrogen, carbon dioxide, argon and mixtures thereof, advantageously nitrogen.

5. The method according to any one of claims 1 to 4, characterized in that: The process is a semi-continuous process.

6. The method according to any one of claims 1 to 5, characterized in that The alumina of the continuous coating is metastable alumina, alpha alumina or a mixture of these oxides, advantageously alpha alumina.

7. The method according to any one of claims 1 to 6, characterized in that The deposition rate is between 100 and 500 nm / min, advantageously 300 nm / min.

Images