Process for coating a substrate with aluminum oxide

Abstract

Method for coating a substrate with aluminum oxide. The present invention relates to a method for coating a substrate (12) with alpha aluminum oxide by the high-power pulsed magnetron sputtering technique, in which an aluminum target (11) is used and in which the substrate coating is carried out under an atmosphere containing a mixture of argon and oxygen, at a pressure less than or equal to 10 Pa, the polarization of the target being controlled during the coating by imposing on it the superposition of a continuous polarization at a potential (Vc) between -300 V and 0 V and a pulsed polarization whose pulses have a potential (Vp) between -1200 V and -400 V and a frequency between 50 Hz and 5000 Hz. Figure for the abstract: Fig. 2.

Description

Title of the invention: Method for coating a substrate with aluminum oxide. Technical field

[0001] The present invention relates to the general field of aluminium oxide (also called alumina) coatings and more particularly to coatings for metallic alloys, and even more particularly it relates to a process of coating a substrate with aluminium oxide. Previous technique

[0002] Various metal alloys such as titanium alloys, TiAl, 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 most cases. Indeed, alpha alumina exhibits excellent resistance to oxidation and corrosion. Furthermore, 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 typically required for growing alpha alumina (homogeneous and heterogeneous nucleation) is on the order of 900°C and above. Indeed, alpha aluminum oxide is conventionally produced by chemical vapor deposition (CVD), physical vapor deposition (PVD), or sol-gel deposition. Both CVD and sol-gel methods use temperatures above 1000°C for stabilizing the alpha phase of aluminum oxide. This temperature is incompatible with most metallic alloys and therefore severely limits the choice of substrates.

[0005] PVD encompasses various techniques that allow the stabilization of the desired alpha phase at substrate heating temperatures below 1000°C (from 480°C to 580°C), even though the coating generally contains mixed phases of metastable aluminum oxide such as the gamma phase. This is the case, for example, with reactive or non-reactive magnetron sputtering (temperatures from 480°C to 580°C), reactive pulsed magnetron sputtering (temperature of 760°C), or high-power pulsed magnetron sputtering (HIPIMS) on a cermet substrate (temperature of 650°C). Co-sputtering has also been used to promote The growth of the alpha phase and the limitation of gamma phase growth can be achieved by adding a dopant such as chromium, while simultaneously reducing the temperature used (500°C). It has also been proposed to use a chromium oxide sublayer, taking advantage of the small variation in lattice parameters between the two phases (temperature 450°C). Generally, to deposit well-crystallized alumina, temperatures above 300°C are required for metastable phases, and temperatures above 500°C for alpha alumina, or alternatively, post-heat treatments may be necessary. This temperature remains too high for certain alloys, such as titanium and TiAl alloys, and depending on the deposition technique used, does not guarantee a well-crystallized alpha alumina layer.In summary, existing solutions based on the use of magnetron sputtering in reaction or high-power pulsed magnetron sputtering (HIPIMS) do not allow the production of alpha aluminium oxide at temperatures below that which degrades the metallic substrate (< 850 °C for the TiAl alloy).

[0006] The only solution which today allows the growth of an alumina layer on the substrate, in particular in the case of TiAl, is the use of the halogen effect, as described for example in patent application WO2020 / 229747, which in fact requires the use of halogenated gases, which can pose toxicity problems.

[0007] Thus there is a need to find a new process allowing the deposition of an alpha alumina layer on a substrate of complex geometry, such as a turbine blade, at a temperature compatible with most metallic alloys and in particular below 850°C and even below 500°C, without the use of halogenated gases and having sufficiently high deposition rates.

[0008] The invention is based on the use of the high-power pulsed magnetron sputtering (HiPIMS) technique to deposit alpha alumina through precise control of the process parameters. Among other advantages, the HiPIMS technique allows for increased coating density, improved adhesion to the substrate, and, through precise control of the process parameters, the ability to control the structure and microstructure of the deposited phases. Furthermore, it avoids the use of halogenated gases and offers a high deposition rate. It also has a significantly lower environmental impact than so-called "wet" methods.

[0009] Although widely documented, this technique has never been developed for depositing aluminum oxide coatings, and its applicability to this application was not obvious. Indeed, the reactive gas used is very different from that described in the prior art, which necessitates a significant modification of the process parameters used. Thus, the reactive gas used in the present invention is oxygen, which has Oxygen's unique characteristic is that it is an electronegative gas, whereas the main plasma-forming gases (such as nitrogen) are electropositive. This means that oxygen introduces negative ions into the discharge. This drastically alters the management of coupled energy in the discharge, as it is necessary to consider two ion distributions, one positive and the other negative. Therefore, the transposition of the prior art technique to the process according to the invention is far from straightforward. This technique makes it possible to obtain alpha alumina, which is very difficult to produce at low temperatures. Description of the invention

[0010] The present invention therefore relates to a method of coating a substrate with aluminium oxide by high-power pulsed magnetron sputtering technique, in which an aluminium target is used and in which the coating of the substrate is carried out under an atmosphere containing a mixture of argon and oxygen, at a pressure less than or equal to 10 Pa, the polarization of the target being controlled during the coating by imposing on it the superposition of a continuous polarization at a potential between -300 V and 0 V, advantageously between -250 V and -50 V, more advantageously between -250 V and -100 V, in particular between -250 V and -150 V, more particularly a potential of -200 V, and a pulsed polarization whose pulses have a potential between -1200 V and -400 V and a frequency between 50 Hz and 5000 Hz.

[0011] In this application, the expressions "between ... and ..." shall be understood to include the boundaries unless explicitly stated otherwise.

[0012] The invention is based on the use of a high-power pulsed magnetron sputtering technique (referred to by the acronym "HiPIMS" in the English-language literature for "High-Power Impulse Magnetron Sputtering") in which the target is polarized in a specific way to form an aluminum oxide coating, particularly alpha aluminum oxide. Continuous polarization maintains a residual plasma when pulses are not applied, thus limiting the additional energy required to initiate the plasma for coating formation. Pulsed polarization then allows for a significant increase in plasma reactivity and thus the creation of charged species in the plasma in a much greater number than that obtained without continuous polarization.The presence of a residual plasma maintained by continuous polarization also helps to reduce electrical instabilities that may appear during the application of the pumps. This allows for precise control of the microstructure of the resulting aluminum oxide coating.

[0013] Advantageously, the aluminum oxide of the coating is a metastable aluminum oxide (such as kappa aluminum oxide or theta aluminum oxide or gamma aluminum oxide, in particular gamma aluminum oxide), an alpha aluminum oxide or a mixture of these oxides (a mixed oxide), advantageously it is an alpha aluminum oxide.

[0014] The process can allow the coating of a substrate by at least one continuous layer of aluminium oxide, in particular a thin layer (on the order of a micron), depending on the operating conditions chosen.

[0015] It is also possible to deposit several layers of aluminum oxide with stable or unstable structures, for example, a gamma aluminum oxide layer and an alpha aluminum oxide layer, or to deposit a mixed layer in order to create specific architectures by alternating alpha-Al₂O₃-rich areas with gamma-Al₂O₃-rich areas, or to create layers with compositional gradients. Advantageously, the deposited layer is predominantly (i.e., more than 50% by mass) composed of alpha aluminum oxide; in particular, it is essentially composed of alpha aluminum oxide, and more specifically, it is composed exclusively of alpha aluminum oxide.

[0016] In one embodiment, the substrate is coated with at least one layer of alpha aluminum oxide (i.e. exclusively alpha aluminum oxide, without any trace of other aluminum oxides), in particular by imposing on the target the superposition of a continuous bias at a potential between -300 V and 0 V, in particular between -250 V and -150 V and a pulsed bias whose pulses have a potential between -1200 V and -400 V, in particular between -900 V and -700 V, and a frequency between 50 Hz and 5000 Hz, in particular between 100 Hz and 300 Hz.

[0017] Such an alpha aluminum oxide coating is particularly advantageous because it exhibits excellent resistance to oxidation and corrosion. Furthermore, alpha aluminum oxide has very low oxygen uptake, making it relatively impermeable to oxygen. It is also the most stable form of alumina at high temperatures.

[0018] The substrate according to the invention is in particular a metallic substrate such as, for example, steel. Advantageously, it is a metallic substrate comprising titanium and / or aluminum, such as Inconel 718, more particularly a titanium and / or aluminum alloy, even more particularly a titanium-aluminum alloy, for example based on titanium aluminide, such as a gamma-TiAl alloy.

[0019] The metallic substrate according to the invention can constitute a turbomachine component, and for example, an aeronautical turbomachine component. It can thus be an aircraft component. The substrate is advantageously designed for use in an oxidizing atmosphere and at a temperature greater than or equal to 800°C. The substrate For example, it could be a turbine component. This could be a turbine blade or a turbine ring sector. It could therefore be a component with a complex geometry, i.e., non-planar, particularly 3D. However, the process can also be implemented on a substrate with a planar geometry.

[0020] In the context of the present invention, the substrate can be positively or negatively polarized, advantageously negatively, more advantageously at a potential of -100 V. Applying such a potential makes it possible to accelerate (positive polarization) or slow down (negative polarization) the electrically charged species. In an advantageous embodiment, the substrate is not polarized.

[0021] The substrate may be heated during coating, for example to a temperature below 500°C, advantageously below or equal to 450°C, in particular 400°C. Alternatively, the substrate may not be heated during coating and thus be at ambient temperature (20°C). The substrate temperature may therefore, for example, be above or equal to 20°C during coating, for example between 20°C and 499°C, or even between 30°C and 450°C, in particular between 350°C and 450°C.

[0022] Temperature allows thermal energy to be supplied to the substrate, and thus allows a certain mobility of atoms promoting the recombination of atoms deposited on the surface of the substrate.

[0023] In one embodiment, the volumetric content of oxygen in the atmosphere is between 10% and 90% and the volumetric content of argon in the atmosphere is between 90% and 10%. Advantageously, the volumetric content of oxygen in the atmosphere is between 10% and 50% and the volumetric content of argon in the atmosphere is between 90% and 50%, more advantageously the volumetric content of oxygen in the atmosphere is between 10% and 20% and the volumetric content of argon in the atmosphere is between 90% and 80%, particularly advantageously, the volumetric content of oxygen in the atmosphere is 10% and the volumetric content of argon in the atmosphere is 80%.

[0024] The pulsed polarization of the method according to the invention is characterized by the potential of its pulses Vp, the duration of the pulses Tl, and the duration T2 during which no pulse is applied, corresponding to the time between two consecutive pulses. The pulsed polarization comprises a succession of pulses alternating with phases of non-application of pulses. Each pulse has a plateau at a potential Vp between -1200 V and -400 V, advantageously between -1000 V and -500 V, in particular between -900 V and -700 V, more particularly Vp = -800 V. The duration Tl of each pulse can be between 1 ps and 500 ps, ​​advantageously between 5 ps and 100 ps, ​​more advantageously between 10 ps and 30 ps, ​​and even more advantageously Tl = 20 ps. The pulsed polarization of high-power pulsed magnetron sputtering is periodic with a The pulse frequency is between 50 Hz and 5000 Hz, advantageously between 50 Hz and 600 Hz, more advantageously between 100 Hz and 300 Hz, particularly 200 Hz. The duty cycle of the pulsed polarization, corresponding to the ratio between the pulse duration and the pulsed polarization period, i.e., in this case T1 / (T1+T2), can be between 0.1% and 50%, advantageously between 0.2% and 10%, more advantageously between 0.5% and 0.8%, particularly between 0.6% and 0.7%. The pulses can have a square wave shape or a peak shape with a peak at a potential between -1200 V and -400 V, advantageously a square wave shape.

[0025] The process is carried out at a pressure less than or equal to 10 Pa, for example between 0.5 Pa and 10 Pa, advantageously between 4 Pa ​​and 6 Pa. In particular, the pressure is equal to 5 Pa.

[0026] The thickness of the resulting aluminum oxide coating can be between 20 nm and 9 pm, advantageously between 0.5 pm and 1.5 pm, in particular 1 pm. The coating time can be between 5 minutes and 5 hours, advantageously between 1 hour and 3 hours. In particular, the duration is 2 hours.

[0027] In an advantageous embodiment, the coating deposition speed is between 1 nm / min and 50 nm / min, advantageously between 9 nm / min and 11 nm / min, in particular it is 10 nm / min.

[0028] The present invention will be better understood in light of the description in the figures and examples that follow. The examples are given by way of illustration, not limitation. Brief description of the drawings

[0029] [Fig. 1] The [Fig. 1] schematically describes a device enabling the realization of an example of a coating process according to the invention.

[0030] [Fig.2] Fig.2 schematically describes the polarization of the target during the time during a coating process according to the invention.

[0031] Fig. 1 schematically represents a device for carrying out a high-power pulsed magnetron sputtering deposition according to an embodiment of the invention.

[0032] The device includes a chamber 101 for receiving the plasma-generating gas consisting of a mixture of argon and oxygen. The device further includes a plasma-generating gas source (not shown) in communication with the chamber 101.

[0033] The chamber 101 comprises an aluminum target 11 constituting the cathode and a coating substrate 12 constituting the anode. In one embodiment, the target 11 comprises aluminum in a proportion of more than 99% by atomic percentage, and preferably in a proportion of more than 99.9% by atomic percentage. The substrate 12 to be coated is electrically conductive and its nature can vary depending on the application sought, as indicated above.

[0034] During coating, the target 11 is polarized as described above by superimposing a continuous polarization and a pulsed polarization. The polarization of the target 11 is imposed by the power supply assembly 102. The power supply assembly 102 comprises a voltage spike generator 13 and a continuous voltage generator 14 electrically connected to the target 11. The voltage spike generator 13 imposes the pulsed polarization on the target 11. The continuous voltage generator 14 imposes the continuous polarization on the target 11. This assembly 102 may also include a current measuring element 16 connected to a current-cutting device 15. During operation, if the current value measured by the current measuring element 16 exceeds a threshold value, the current-cutting device 15 is configured to cut off the power supply to the target 11 and thus stop the process.The combination of the current measuring element 16 and the current cutting device 15 prevents any risk of damage to the coating or the device due to the creation of an electric arc in chamber 101. Such a combination of elements 15 and 16 is known per se.

[0035] In chamber 101, applying a voltage between target 11 and substrate 12 in the presence of a nitrogen-containing atmosphere creates a plasma. Electrons are generated by target 11 and can ionize the constituent atoms of the plasma by collision. One or more permanent magnets (not shown) can be introduced into chamber 101, near target 11. The magnetic field of these magnets confines the generated electrons close to target 11 and increases the probability of a collision between an electron and a plasma atom. When such a collision occurs, a high-energy wave is generated, which can bombard target 11 and, by elastic collision, eject particles from it. These ejected particles can then be deposited on substrate 12 to form the coating.

[0036] The substrate 12 can be heated during coating by a heating element (not shown). Alternatively, the substrate 12 may not be heated during coating.

[0037] Figure 2 represents the polarization 203 imposed on the target over time. As shown, the polarization 203 imposed on the target corresponds to the superposition of a continuous polarization 201 of value Vc, and a pulsed polarization 202 having peaks at potential Vp.

[0038] The pulsed polarization 202 is characterized by the potential of its pulses Vp, the duration of the pulses T1, and the duration T2 during which no pulse is applied, corresponding to the time between two consecutive pulses. The pulsed polarization 202 comprises a succession of pulses alternating with phases of non-application of the pulses. Each pulse presents a plateau at a potential Vp between -1200 V and -400 V. The pulsed bias 202 can be periodic. The duty cycle of the pulsed bias, corresponding to the ratio between the duration of the pulses and the period of the pulsed bias, is in this case T1 / (T1+T2).

[0039] A shape of concave power sources has been represented, but these could alternatively have a peak shape with a summit at a potential between -1200 V and -400 V. EXAMPLE

[0040] Example 1: Deposition of alpha alumina oxide on an Inconel 718 substrate

[0041] The process according to the invention implemented has the following characteristics: - Vc: -200 V; - Vp: -800 V; - substrate temperature: ambient temperature (20°C); - substrate polarization: 0 V; - Oxygen and argon volume content: 10% and 90%; - duration of the Tl draws: 20 ps; - duration between two T2 pumps: 10 ps; - shape of the wells: crenellated...; - frequency of the pumps: 200 Hz; - pressure: 5 Pa; - substrate material: Inconel 718; - duration of the process: 120 min; - coating thickness: 1 µm; - coating type: alpha aluminium oxide only; - Target purity: 99.999% aluminum

[0042] The results of X-ray diffraction (grazing angles (0.1 - 0.2 and 0.3°) Bruker® D8 diffractometer with Bragg-Brentano 0-20 geometry equipped with an X'Celerator detector using a standard Cu Kal beam (operating voltage 40 kV, operating current 40 mA)) on Inconel 718 after deposition at room temperature therefore show the presence only of a monolayer of alpha aluminum oxide. There is no trace of any other crystalline phase of alpha aluminum oxide.

[0043] Example 2: Deposition of alpha alumina oxide on a TiAl substrate

[0044] The method according to the invention implemented has the same characteristics as in Example 1, with the following exceptions: - Substrate temperature: 400°C; - Substrate material: TiAl

[0045] The results of X-ray Diffraction (grazing angles; (0.1 - 0.2 and 0.3°) Bruker® D8 diffractometer with Bragg-Brentano geometry 0-20 equipped with a detector X'Celerator, using a standard Cu Kal beam (working voltage 40 kV, working current 40 mA) on TiAl after deposition at a temperature of 400 °C, therefore shows the presence only of a monolayer of alpha aluminum oxide. There is no trace of any other crystalline phase of alpha and alpha' aluminum oxide.

[0046] In conclusion, this process makes it possible to produce thin films (on the order of microns) of predominantly alpha or pure alpha aluminum oxides (homogeneous or with a controlled gradient) at low temperature (<500 °C), with a simple (monolayer) or complex (multilayer) architecture, of low roughness (on the order of a few nanometers) on metallic substrates of all kinds.

Claims

Demands

1. A method for coating a substrate (12) with aluminium oxide by high-power pulsed magnetron sputtering, wherein an aluminium target (11) is used and wherein the coating of the substrate is carried out under an atmosphere containing a mixture of argon and oxygen, at a pressure less than or equal to 10 Pa, the polarization of the target being controlled during the coating by imposing on it the superposition of a continuous polarization at a potential (Vc) between -300 V and -50 V and a pulsed polarization whose pulses have a potential (Vp) between -1200 V and -400 V and a frequency between 50 Hz and 5000 Hz, characterized in that the substrate (12) is heated to a temperature less than 500 °C.

2. Coating method according to claim 1, characterized in that the duration of the dips is between 1 ps and 500 ps.

3. Coating method according to claim 1 or 2, characterized in that the substrate (12) is heated to a temperature less than or equal to 450 °C, in particular 400 °C.

4. A method according to claim 1 or 2, characterized in that the substrate (12) is at room temperature.

5. A method according to any one of claims 1 to 4, characterized in that the substrate (12) is metallic.

6. A method according to any one of claims 1 to 5, characterized in that the substrate (12) is made of titanium alloy, advantageously of titanium aluminide-based alloy.

7. A method according to any one of claims 1 to 6, characterized in that the substrate (12) is an aircraft part, advantageously a turbine part.

8. A process according to any one of claims 1 to 7, characterized in that the aluminum oxide is a metastable aluminum oxide, an alpha aluminum oxide, or a mixture of these oxides, advantageously an alpha aluminum oxide.

9. A method according to any one of claims 1 to 8 characterized in that the coating deposition rate is between 1m / min and 50 nm / min, advantageously it is 10 nm / min.

10. A method according to any one of claims 1 to 9, characterized in that the volumetric content of oxygen in the atmosphere is between 10% and 90% and the volumetric content of argon in the atmosphere is between 90% and 10%.

Images