Method for depositing an aluminium oxide coating
Patent Information
- Application Number
- EP2023772301
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-08-30
- Filing Date
- 2023-08-29
- Publication Date
- 2025-07-09
Smart Images
Figure 1.1
Abstract
Description
Process for depositing an aluminum oxide coating Technical Field
[0001] The present invention relates to the general field of aluminum oxide coatings (also called alumina) and more particularly to coatings for metal alloys, and even more particularly it relates to a device and a method for depositing aluminum oxide coatings by chemical deposition under pressure and temperature. Prior art
[0002] Various metal alloys such as titanium alloys, TiAI, or nickel-based alloys require protection against oxidation and / or corrosion to maintain their performance at higher operating temperatures.
[0003] Among the many solutions that can be considered, an alpha alumina layer is the best possible solution in the majority of cases. Indeed, alpha alumina exhibits excellent resistance to oxidation and corrosion. In addition, the oxygen diffusion coefficient in its alpha crystalline form is low, making it relatively impermeable to oxygen. It is also the most stable form of alumina at high temperatures.
[0004] However, the temperature range usually required to grow alpha alumina (homogeneous and heterogeneous nucleations) is around 900°C and above. Indeed, alpha aluminum oxide is classically produced by chemical vapor deposition (CVD), physical vapor deposition (PVD) or sol-gel. Both CVD and sol-gel methods use temperatures above 1000°C for stabilization of the alpha phase of aluminum oxide. This temperature is not compatible with most metal alloys. As for PVD, this technique allows stabilization of the desired phase at lower temperatures (from 480°C to 580°C), even if, generally, the coating has mixed phases of metastable aluminum oxide such as the gamma phase. The disadvantage of PVD is its appearance directional which does not offer the possibility of coating substrates with complex geometries such as turbine blades, unlike chemical deposition techniques. In addition, these methods generally suffer from a relatively low deposition speed.
[0005] Various studies have been conducted to attempt to obtain alpha alumina from alumina of a different phase. However, converting alumina of any phase into alpha alumina results in a significant volume change that impacts the mechanical performance of the coating, or even the coated part.
[0006] It is known to use lower temperatures to obtain alpha alumina, but only in the context of homogeneous nucleation, which allows the synthesis of materials in powder form and not in the form of a coating. Alpha aluminum oxide is thus stabilized in water at relatively low pressure (from 1 MPa) and at a temperature between 374 °C and 500 °C, according to the binary diagram AI2O3-H2O.
[0007] By hydrothermal synthesis, aluminum oxide is produced in two successive stages. The first consists of the hydrolysis by water of an aluminum precursor in order to form a hydrated aluminum oxide, boehmite Y- AIO(OH): AI(NO3)3+ 2H2O — > y-AIOOH + 3HNO3(Eq. 1)
[0008] The second step consists of the dehydration of this intermediate phase, which is boehmite, in order to obtain alpha aluminum oxide: 2y-AIOOH — >O-AI2O3+ H2O (Eq.2)
[0009] According to the literature, dehydration is the work of successive hydroxylation / dehydroxylation phenomena causing a reorganization of the solid state structure of boehmite towards alpha aluminum oxide.
[0010] However, as previously indicated, hydrothermal synthesis under pressure allows the production of alpha aluminum oxide but in the form of powders and not coatings.
[0011] The only solution that currently allows the growth of an alumina layer on the substrate, particularly in the case of TiAI, is the use of the effect halogen, as described for example in patent application WO2020 / 229747, which in fact requires the use of halogenated gases, which can pose toxicity problems.
[0012] Thus there is a need to find a new process allowing the deposition of a layer of alpha alumina on a substrate of complex geometry, such as a turbine blade, at a temperature compatible with most metal alloys and in particular below 850°C, without the use of halogenated gases and having sufficiently high deposition rates. Statement of the invention
[0013] The inventors surprisingly discovered that it was possible to achieve such a deposition using solvothermal synthesis assisted by induction heating, which thus allows chemical deposition under pressure and temperature assisted by induction heating. The inventors discovered that such a process made it possible to obtain a thick deposit (greater than 1 pm, advantageously greater than 10 pm) of alpha alumina and was also suitable for the deposition of other types of aluminum oxide such as metastable aluminum oxides, on any metal substrate. It also avoids the use of halogenated gases and has a high deposition speed.
[0014] Thus, the use of a more efficient process (high speed) according to the invention is also advantageous for reducing the environmental footprint of the Applicant. Indeed, - It allows to increase and optimize manufacturing, production and / or repair capacity and, consequently, to significantly reduce the associated greenhouse gas emissions. This optimization also allows to reduce the consumption of raw materials; - It allows to extend the life of the components and, consequently, to reduce the number of replacements with new parts; and - It significantly reduces the number of discarded parts that may be difficult to recycle.
[0015] In addition, the solution also has the advantage of reducing its energy input (water, electricity, etc.) and / or the use of any products chemicals contrary to current environmental standards and regulations.
[0016] The present invention therefore relates to a method for depositing a continuous coating of aluminum oxide on a metal substrate by chemical deposition under pressure and temperature assisted by induction heating, comprising a solvothermal synthesis step from an aluminum oxide precursor dissolved in a mixture of water and co-solvent heated by induction at a temperature of between 400°C and 700°C and at a pressure of between 1 MPa and 25 MPa.
[0017] In this application, the expressions "between ... and ..." must be understood to include the limits unless explicitly stated otherwise.
[0018] The metal substrate according to the invention is in particular a metal substrate comprising titanium, more particularly titanium alloy, even more particularly titanium-aluminium alloy, for example based on titanium aluminide, such as a gamma-TiAI alloy.
[0019] The metal substrate according to the invention may constitute a turbomachine part, and for example an aeronautical turbomachine part. The substrate is advantageously intended to be used in an oxidizing atmosphere and at a temperature greater than or equal to 800°C. The substrate may for example be a turbine part. It may for example be a turbine blade or a turbine ring sector. It may thus be a part with complex geometry, i.e. non-planar, in particular 3D. But the method may also be implemented on a substrate with planar geometry.
[0020] In the context of the present invention, a pressurized fluid is a fluid whose pressure is higher than atmospheric pressure, in particular at a pressure of between 1 MPa and 25 MPa, advantageously between 6 MPa and 10 MPa.
[0021] In the context of the present invention, a chemical deposition under pressure and temperature is any deposition by chemical means at a pressure higher than atmospheric pressure, in particular at a pressure between 1 MPa and 25 MPa, advantageously between 6 MPa and 10 MPa, and at a temperature above room temperature, advantageously below 850°C, in particular at a temperature between 400°C and 700°C.
[0022] The precursor of aluminum oxide according to the invention is any water-soluble precursor, such as aluminum nitrate AI(NO3)3 or aluminum salts (such as AI2(SO4)3). Advantageously, it is an aluminum nitrate AI(NO3)3.
[0023] The co-solvent according to the invention is chosen from alcohols, in particular ethanol, nitrogen, carbon dioxide, argon and mixtures thereof, advantageously it is nitrogen. The co-solvent makes it possible to conduct heat better and therefore to heat the substrate more easily.
[0024] In the context of the process according to the invention, it is water under pressure and at temperature which will allow the production of alumina from the precursor which is dissolved there and therefore its deposition on the substrate.
[0025] Advantageously, the water / co-solvent molar ratio, in particular water / nitrogen, is between 0.1 and 50%. In particular, the water flow rate is advantageously between 0.1 and 10 mL / min, more particularly it is 1.3 mL / min. In one embodiment, the flow rate of the co-solvent, in particular nitrogen, is between 0.1 and 10 mL / min, more particularly it is 2 mL / min.
[0026] The temperature of the solvothermal synthesis step is between 400°C and 700°C, advantageously between 500°C and 7002°C, more advantageously between 550°C and 680°C, even more advantageously between 600°C and 650°C, in particular it is 630°C.
[0027] 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 it is 10 MPa.
[0028] The aluminum oxide of the continuous coating obtained on the surface of the metal substrate by the process according to the invention will depend on the temperature and pressure used, the co-solvent and the quantity of water used during the reaction (such as the water / co-solvent ratio). Indeed, hydrothermal dehydration (or solvothermal synthesis) leads to alpha alumina while dehydration in air leads to other phases of alumina. Advantageously, the aluminum oxide of the continuous coating is a metastable aluminum oxide. (such as kappa aluminum oxide or theta aluminum oxide or gamma aluminum oxide), an alpha aluminum oxide or a mixture of these oxides (a mixed oxide), advantageously it is an alpha aluminum oxide
[0029] The synthesis using the precursor AI(NO3)3 to obtain a continuous coating of alpha aluminum oxide is advantageously carried out at a pressure of 100 Pa, a temperature of 630°C with a water / nitrogen mixture (nitrogen being the co-solvent), a water flow rate of 1.3 ml / min and a nitrogen flow rate of 2 ml / min.
[0030] In an advantageous embodiment, the continuous coating of aluminum oxide obtained is thick, i.e. of a thickness greater than 1 μm, in particular of a thickness of at least 2 μm, more particularly of a thickness of between 1 μm and 75 μm, even more particularly of a thickness greater than 10 μm, advantageously of a thickness of between 50 μm and 72 μm.
[0031] In an advantageous embodiment, the deposition speed of the coating is between 100 and 500 nm / min, advantageously it is 300 nm / min.
[0032] In an advantageous embodiment, the method according to the invention is implemented within a chemical deposition reactor under pressure and at temperature assisted by induction heating.
[0033] Advantageously, the chemical deposition reactor under pressure and at temperature assisted by induction heating which can be used in the process according to the invention is as described in application FR31 12972, more particularly in the case where the pressure is between 1 MPa and 10 MPa.
[0034] In another advantageous embodiment, the pressure and temperature-assisted chemical deposition reactor assisted by induction heating that can be used in the method according to the invention is the device 100 for depositing an aluminum oxide on a metal substrate 104 as described below.
[0035] In an advantageous embodiment, the method according to the present invention comprises the following steps: a- introduction of the metal substrate to be coated into a chemical deposition reactor under pressure and at temperature assisted by induction heating; b- introduction into the reactor of the precursor of aluminum oxide previously dissolved in water and of the co-solvent; c- implementation of the solvothermal synthesis from said precursor of aluminum oxide by induction heating 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, of the precursor of aluminum oxide dissolved in the mixture of water and co-solvent; d- recovery of the substrate coated with a continuous coating of aluminum oxide.
[0036] In an advantageous embodiment, the water and the co-solvent are introduced separately into the reactor during step b).
[0037] In a particular embodiment, the method comprises an intermediate step a1), located between steps a) and b), of preheating the reactor to a temperature between 400°C and 700°C and pressurizing to a pressure between 1 MPa and 25 MPa.
[0038] In an advantageous embodiment, the method is a semi-continuous or discontinuous process (or closed mode), advantageously semi-continuous.
[0039] In the context of a semi-continuous process, the precursor of the aluminum oxide previously dissolved in water and the co-solvent are introduced continuously, in particular during step b) of the process according to the invention, circulate continuously, in particular within the reactor according to the present invention previously heated and pressurized, and are purged continuously. On the other hand, the substrate is fixed. It is therefore introduced into the reactor, advantageously in step a), prior to the continuous introduction of the precursor of the aluminum oxide previously dissolved in water and the co-solvent, advantageously during step b) and the pressurization and heating of the reactor. The coated substrate is then recovered, once the process has been carried out and the reactor has been cooled to room temperature and depressurized. During the semi-continuous process, the deposit grows as the precursor, water and co-solvent are added and react.This semi-continuous mode allows for precise control and adjustment of the deposition and the quantities of precursor, water and co-solvent introduced for better control of the. kinetics of formation and growth of aluminum oxide on the surface of the metal substrate.
[0040] In a batch process (or closed mode), the quantity of fluids (precursors + co-solvent + water) and the substrate are fixed and the fluids and the substrate are introduced beforehand into the reactor according to the invention. The reactor is then heated and pressurized in order to carry out the solvothermal synthesis. In general, the quantity of co-solvent and water governs the maximum achievable pressure depending on the applied temperature. No precursor, water or co-solvent is introduced during the growth of the aluminum oxide on the surface of the metal substrate. Once the deposition is carried out, the reactor is cooled to room temperature and depressurized in order to recover the coated substrate. The fluids are also purged.
[0041] Advantageously, the duration of the process according to the invention is between 30 min and 180 min.
[0042] The present invention further relates to a device for depositing an aluminum oxide (in particular as described above) on a metal substrate by means of chemical deposition under pressure and temperature comprising: - an enclosure delimited by walls forming a closed volume (V), intended to contain a fluid under pressure and temperature (in particular under the temperature and pressure conditions as described above) and the wall material of which is transparent to electromagnetic radiation; - a support also transparent to electromagnetic radiation intended to support the metallic substrate located inside the enclosure; - an induction heating device surrounding the exterior of the enclosure so as to be able to heat the metal substrate placed on the support; - an inlet located in the upper part of the enclosure and configured to allow the introduction of the precursor material previously dissolved in water, in particular as described above, inside the enclosure; - an inlet located in the lower part of the enclosure and configured to allow the introduction of a fluid (or co-solvent in particular as described above) inside the enclosure; - at least one outlet configured to purge the closed volume (V); - a sapphire window placed on the upper part of the enclosure, allowing the temperature of the metal substrate to be controlled using a bichromatic pyrometer placed outside the enclosure; - a set of polymer seals; - a metal assembly screwed and constrained by metal columns in which a fluid circulates, temperature controlled by a cryostat.
[0043] The device of the invention makes it possible to form a continuous coating, in particular thick (greater than 1 |im), more particularly as described above, of aluminum oxide on flat metal substrates or on metal substrates with complex geometry, in particular as described above.
[0044] The induction heating device according to the invention, advantageously consisting of an induction generator and an induction loop, makes it possible to heat only the metal substrate while maintaining a temperature on the walls lower than the temperature of the metal substrate. Indeed, having walls transparent to electromagnetic radiation makes it possible to avoid inductive couplings with these walls and to keep them at a temperature colder than that of the metal substrate in order to control convection movements within the enclosure.
[0045] Induction heating also provides better performance than resistive heaters because it allows the entire surface of the metal substrate with complex geometry to be heated more quickly and more evenly, or by limiting the highest heating to a thickness close to the outermost surface of the substrate.
[0046] According to a particular feature of the invention, the material of the walls of the enclosure is a ceramic. The majority of ceramics are transparent to electromagnetic radiation, therefore ceramics are excellent candidates for forming the walls. Advantageously, it is a ceramic made of silicon nitride Si3N4.
[0047] This ceramic is pressure resistant thanks to a set of polymer seals (such as PEEK (PolyEtherEtherKetone), Viton® (Fluorinated Carbon Rubber (FKM) marketed by the DuPont company), EPDM (ethylene- propylene-diene monomer) and / or Kalrez® (Perfluorinated rubber FFKM FFPM. marketed by the DuPont company) and a metal assembly, in particular cylindrical, screwed and constrained by metal columns, in particular 6 in number distributed equidistant from each other, in which circulates a fluid such as ethylene glycol, temperature controlled at 20°C by a cryostat.
[0048] Advantageously, the device according to the invention does not comprise a double wall.
[0049] Advantageously, the inlets of the device are equipped with a pump, in particular HPLC for the water inlet and Isco for the fluid inlet. In the case where the fluid is 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.
[0050] Advantageously, the inlet located in the lower part of the enclosure and configured to allow the introduction of a fluid is an inlet for the cosolvent as described above within the framework of the process, fluid which will serve, in mixture with water, as supercritical fluid.
[0051] Advantageously, the outlet of the device is equipped with a pressure regulator.
[0052] According to another particular characteristic of the invention, covers may be present at the ends of the enclosure in order to close them. Advantageously, only one of the two covers is movable. The covers can, for example, be made of steel, and more particularly of 316L steel.
[0053] The present invention finally relates to the use of the device according to the invention for implementing the method according to the invention.
[0054] It further relates to a method according to the invention in which the pressure and temperature chemical deposition reactor assisted by induction heating is the device according to the invention.
[0055] Thus, in a particular embodiment of the method according to the invention, the metal substrate to be coated is placed in the enclosure of the device (or reactor) on the support (step a) of the method according to the invention).
[0056] Then in a next step, the metal substrate is heated by induction using the induction heating means (step a1) of the method according to the invention).
[0057] Then, as soon as the temperature of the metal substrate is at a temperature between 400°C and 700°C, the aluminum oxide precursor (or precursor material) previously dissolved in water and the co-solvent (step b) of the process according to the invention) is introduced into the enclosure. The water in which the precursor is dissolved and the introduced co-solvent will then be subjected to an increase in pressure and temperature until the desired pressure and temperature are reached.
[0058] During the introduction of the aluminum oxide precursor (or precursor material) previously dissolved in water and the co-solvent, the metal substrate is continued to be heated by induction. This allows the conditions necessary for the formation of aluminum oxide on the surface of the metal substrate to be reached in the vicinity of the metal substrate.
[0059] The aluminum oxide precursor, water and co-solvent introduced therefore react under solvothermal conditions to form aluminum oxide on the surface of the substrate (step c) of the method according to the invention). Throughout the formation and growth of the aluminum oxide on the surface of the substrate, the aluminum oxide precursor previously dissolved in water and the co-solvent are introduced into the enclosure. The formation of the aluminum oxide on the surface of the substrate thus takes place in semi-continuous mode. This makes it possible to adjust the quantities of aluminum oxide precursor, water and co-solvent as the aluminum oxide layer grows.
[0060] When the thickness of the continuous layer of aluminum oxide is sufficient, the introduction of the precursor of the aluminum oxide previously dissolved in water and of the co-solvent is stopped, the enclosure is cooled before depressurizing it to recover the coated substrate (step d) of the process according to the invention).
[0061] In another particular embodiment of the method according to the invention, the metal substrate to be coated is placed in the enclosure of the device (or reactor) on the support (step a) of the method according to the invention).
[0062] Then in a next step, the metal substrate is heated by induction using the induction heating means (step a1) of the method according to the invention).
[0063] As soon as the temperature and pressure of the water / co-solvent mixture within the enclosure (T, P) vreach the desired conditions, the precursor of aluminum oxide (or precursor material) previously dissolved in water and the co-solvent is introduced into the enclosure, the water and the co-solvent thus being placed under pressure and temperature (step b) of the process according to the invention).
[0064] The aluminum oxide precursor, water and co-solvent introduced therefore react under pressure and temperature conditions to form, by solvothermal synthesis, a continuous layer of aluminum oxide on the surface of the substrate which will grow throughout the duration of the reaction (step c) of the process according to the invention). During 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 takes place in a closed mode or batch process.
[0065] When the growth of the continuous layer of aluminum oxide is complete, that is to say when all the precursor material (or precursor of the aluminum oxide) has reacted, the enclosure is cooled before depressurizing it to recover the coated substrate (step d) of the process according to the invention).
[0066] The present invention will be better understood in light of the description of the figures and examples which follow. The examples are given for informational purposes only and are not limiting. Brief description of the drawings
[0067] [Fig. 1] Figure 1 represents the partial diagram of a device according to the invention, without the metal assembly or the joints.
[0068] [Fig.2] Figure 2 schematically and partially represents a sectional view of a device according to the invention with the metal assembly and the joints.
[0069] The device 100 makes it possible to deposit an aluminum oxide coating on a metal substrate 104. The device 100 comprises a cylindrical enclosure 102 delimited by walls forming a closed volume V. The enclosure 102 is adapted to receive a fluid under pressure and at temperature thanks to a set of polymer seals 200, in particular located at the sapphire window 112 and the junction between the metal assembly 202 and the enclosure 102, a metal assembly 202, in particular cylindrical, screwed and constrained by metal columns 204, in particular 6 in number distributed equidistant from each other, in which circulates a fluid controlled in temperature at 20 °C by a cryostat 206, in particular the fluid being located above and below the cylindrical enclosure, more particularly on either side of the sapphire window 112 and the inlet 120 and outlet 124. The metal assembly 202, the metal columns 204 and the screws being in particular made of 306L steel.
[0070] The device 100 also comprises an inlet 120 located in the lower part of the enclosure 102 to be able to introduce into the volume V a fluid which will be the co-solvent. It also comprises an inlet 116 located in the upper part of the enclosure 102 to be able to introduce water and precursor materials previously dissolved in water into this same volume V. The inlets 116 and 120 can be equipped with a pump 118 and 122.
[0071] An outlet 124 is also present in the device 100 to purge the volume V, and thus allow semi-continuous operation of the deposition device 100. The outlet 124 can be equipped with a pressure regulator 126.
[0072] A support 106 is placed in the enclosure 120 to support the metal substrate 104 on which the coating is deposited. Preferably, the support 106 is placed in the enclosure 120 so that the metal substrate 104 is held in the center of the inductor forming the induction heater 109. Preferably, the support 106 has a shape making it possible to support the metal substrate 104 with a minimum of contact points in order to coat the largest possible surface area of the metal substrate with the deposited aluminum oxide coating and to limit disturbances in the induction convection flow. The support 106 is composed of a material transparent to electromagnetic radiation. It is for example composed of a thermally and electrically non-conductive material such as alumina.
[0073] An induction heater 109 consisting of an induction generator 108 and an induction loop 110 surrounds the enclosure 102. The induction heater makes it possible to heat the metal substrate 104 while limiting the heating of the precursor materials present in the volume V.
[0074] In order not to disturb the induction heating of the metal substrate 104, the walls of the enclosure 102 are transparent to electromagnetic radiation. They are for example made of ceramic. The ceramics used may be boron nitride, aluminum nitride, alumina or silicon nitride, more particularly silicon nitride. These examples of dense and non-porous ceramics allow the walls of the enclosure 102 to have excellent mechanical strength and thus to withstand the pressures present in the volume V.
[0075] A sapphire window 112 is arranged on the upper part of the enclosure 102 and allows the temperature of the metal substrate 104 to be controlled using a bichromatic pyrometer 114 arranged outside the enclosure 102. EXAMPLE
[0076] The reactor used in the examples and as described above with reference to figures 1 and 2 and consists of a cylindrical ceramic enclosure made of silicon nitride Si3N4 with an internal volume of approximately 300 mL which contains the pressurized fluid and the metal substrate held by an alumina support. This ceramic being insensitive to magnetic fields, it is surrounded by an induction loop, itself connected to an induction generator with a maximum power of 7 kW. This makes it possible to preferentially heat the metal substrate located in the center of the pressurized fluid. This ceramic is held under pressure by a set of polymer seals (Peek, Viton®, EPDM and / or Kalrez®) and a cylindrical metal assembly screwed and constrained by 6 metal columns equidistant from each other made of 316 L steel.To prevent excessive deformation due to the increase in temperature of this metal assembly, a fluid (ethylene glycol) controlled at 20°C by a cryostat circulates within it. This holding assembly offers the possibility of pressurizing a fluid up to 25 MPa. Another improvement. concerns the addition of tappings on the upper metal part allowing the injection of fluids from the top while maintaining the location of a sapphire window. The latter makes it possible to control the temperature of the metal substrate using a bichromatic pyrometer while keeping the injection of fluids and precursor from the top of the reactor. The fluids and precursors are injected at a controlled flow rate using an HPLC pump for water and an Isco pump for the co-solvent (here nitrogen). The pressure is maintained by an outlet pressure regulator. This reactor operates in semi-continuous mode with a fixed substrate and continuously circulating fluids.
[0077] A parallelepiped metallic substrate of gamma-based titanium-aluminium with dimensions of 1.5x1.5x0.5 cm is therefore introduced into this reactor.
[0078] Deposition of a continuous alpha aluminum oxide coating with water and nitrogen (as co-solvent) flow rates of 1.3 mL / min and 2 mL / min respectively, a pressure of 10 MPa and temperatures of 510 °C, 630 °C and 700 °C were carried out on this substrate using aluminum nitrate AI(NO3)3 as precursor material.
[0079] Using a grazing incidence X-ray diffractometer at an angle of 1° (or GIXRD for Grazing incidence angle XRD) on the coated substrate and analyzing the diffractogram obtained using EVA software, we observe that at a temperature of 510°C, the deposit contains a mixture of metastable aluminum oxide, the kappa phase, and above all, alpha aluminum oxide.
[0080] At a temperature of 630°C, the diffraction lines of alpha aluminum oxide at 25.51° and 43.23° are more easily observed, still with this metastable phase, kappa aluminum oxide.
[0081] At 700°C, it is difficult to detect the presence of alpha aluminum oxide. The diffraction lines of a new metastable phase, theta aluminum oxide, are clearly distinguished.
[0082] By scanning electron microscope observation of the cross-section produced by metallographic preparation of the coated substrate, we see that the morphology of the deposits produced in a water / nitrogen mixture is homogeneous and consists of aggregates of hexagonal grains of a few hundred nanometers.
[0083] We also observe that the cross-section of this coating produced at 600 °C provides information on the morphology of the deposit in depth and also on its thickness. The deposit seems to have two types of structures going from the substrate towards the outside, a relatively dense one over 2 pm (closest to the substrate) and a relatively porous one over 60 pm. In addition, the average thickness is approximately 61 ± 11 pm which allows us to establish a deposition speed of approximately 300 nm.min' 1 These two areas consist of aluminum oxides.
[0084] In conclusion, this induction heating-assisted pressure and temperature chemical deposition process has made it possible to develop alpha and mixed aluminum oxide coatings at temperatures well below 850 °C at a pressure between 1 MPa and 25 MPa on TiAI metal substrates of complex geometries.
Claims
Claims
1. Method for depositing on a metal substrate a continuous coating of aluminum oxide by chemical deposition under pressure and temperature assisted by induction heating comprising the step of solvothermal synthesis from a precursor of aluminum oxide dissolved in a mixture of water and co-solvent heated by induction at a temperature between 400°C and 700°C and at a pressure between 1 MPa and 25 MPa.
2. Method according to claim 1, characterized in that it comprises the following steps: a- introduction of the metal substrate to be coated into a chemical deposition reactor under pressure and at temperature assisted by induction heating; b- introduction into the reactor of the precursor of aluminum oxide previously dissolved in water and of the co-solvent; c- implementation of the solvothermal synthesis from said precursor of aluminum oxide by induction heating 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, of the precursor of aluminum oxide dissolved in the mixture of water and co-solvent; d- recovery of the substrate coated with a continuous coating of aluminum oxide.
3. Method according to claim 1 or 2, characterized in that the metal substrate is made of titanium alloy, advantageously of titanium aluminide-based alloy.
4. Process according to any one of claims 1 to 3, characterized in that the co-solvent is chosen from alcohols, in particular ethanol, nitrogen, carbon dioxide, argon and their mixtures, advantageously it is nitrogen.
5. A method according to any one of claims 1 to 4, characterized in that the method is a semi-continuous method.
6. A method according to any one of claims 1 to 5, characterized in that the aluminum oxide of the continuous coating is a metastable aluminum oxide, an alpha aluminum oxide or a mixture of these oxides, advantageously an alpha aluminum oxide.
7. Method according to any one of claims 1 to 6 characterized in that the deposition speed is between 100 and 500nm / min, advantageously it is 300nm / min.