Method for coating a substrate by reactive magnetron sputtering and part comprising such a coated substrate

Abstract

The invention relates to a method for coating a substrate comprising a step of providing a substrate, in a chamber, having an electrical resistivity of less than or equal to 1 Ω.cm, a step of providing a coating target made of an austenitic alloy, in the chamber, a step of evacuating the chamber and a degassing of the vacuum chamber, a step of ion etching of the substrate, a step of injecting a gas comprising at least argon and nitrogen into the chamber to form an atmosphere of the chamber, then a step of reactive magnetron sputtering of the coating target in the atmosphere of the chamber, forming a coating comprising at least nickel and chromium, and doped with nitrogen. The invention also relates to the part obtained by the method.

Description

[0001] Method of coating a substrate by reactive magnetron sputtering and part comprising such a coated substrate.

[0002] Technical field of the invention

[0003] The present invention relates to a method for coating a substrate, in particular an electrically conductive substrate, by reactive magnetron sputtering. It also relates to a part obtained by such a method, comprising such a coated substrate.

[0004] State of the art

[0005] Mechanical components operating at high temperatures, i.e., above 280°C, or even at least 400°C, must have their surfaces protected against corrosion and oxidation, particularly by dry methods, meaning in a gas containing at least one oxidizing gas, such as oxygen, or even water vapor. Certain alloys are used to withstand high temperatures.

[0006] However, these alloys have rather low hardnesses, i.e. generally less than 500 Hv, which gives them low resistance to wear.

[0007] For example, in the aeronautical field, blade roots are subjected to vibrations that induce fretting wear, a type of wear that occurs at high sliding speeds between the surfaces involved and with small movement. Fretting is often observed between two parts fixed together and set in vibration.

[0008] Therefore, it is necessary to change these parts regularly, resulting in significant maintenance costs.

[0009] It would therefore be very interesting to be able to extend the operating life of certain parts subjected to high temperature and mechanical wear.

[0010] An object of the present invention is, for example, to propose a coating material with metallic characteristics, which is more resistant to wear than traditionally used metallic alloys, and which itself exhibits high resistance to oxidation.

[0011] For resistance to hot wear, nitride-type coatings exist, such as titanium aluminum nitride (AITiN or TiAIN) or chromium aluminum nitride (AlCrN or CrAIN). These are metallic-coated nitride ceramic materials. Their hardness is particularly high (i.e., a Vickers hardness of at least approximately 2500 Hv). These coatings are typically applied to hard metallic substrates, such as tool steels or even cemented carbides (WC-Co). These coatings exhibit high resistance to dry oxidation due to their aluminum content, which forms an alumina barrier layer on the surface. However, the elastic moduli of metallic nitrides are particularly high (at least approximately 300 GPa), while those of the metallic substrates of interest are generally less than 220 GPa, and typically around 100 GPa.This significant difference in elastic properties between the substrate and the coating is detrimental to the mechanical behavior of coated substrates because applying elastic deformation to the coated substrate generates intense shear at the interface. Furthermore, metal nitrides are ceramic-type materials; therefore, they exhibit virtually no plasticity and tend to crack when subjected to intense deformation.

[0012] Some of the materials used, such as austenitic structural materials, can be surface-nitrided. Nitriding must then be carried out at less than 450°C to limit the risk of CrN precipitation. Under these conditions, nitrogen inserts itself into the face-centered cubic (fcc) lattice of the austenite and can produce significant hardening of the material to a depth on the order of tens of micrometers. This can be achieved by plasma, as the temperature must be able to remain below 450°C. It should be noted that among plasma processes, those not using hydrogen (or ammonia) are preferable, to avoid the risk of hydrogen embrittlement. Molten or gaseous salt nitriding is traditionally excluded because, when operating above 450°C, nitrogen combines with chromium to precipitate as CrN (chromium nitride), which depletes the austenitic chromium phase and reduces the corrosion resistance of the main austenite-type phase (i.e.(the matrix), a well-known phenomenon for stainless steels. Nitriding processes therefore require the use of Inconel or stainless steel substrates to produce a surface-hardened material. The problem arises for other metal alloys for which none of the technical solutions described above apply.

[0013] The present invention thus aims to remedy at least in part the aforementioned disadvantages, leading in addition to other advantages.

[0014] Description of the invention

[0015] To this end, a substrate coating process is proposed, according to a first aspect, comprising at least the following steps: A step of supplying a substrate into an enclosure, the substrate having an electrical resistivity less than or equal to 1 Q.cm, or even 10⁻¹⁰. 3 Q.cm, the substrate being for example metal, for example a metal alloy, for example TA6V4, or inconel, for example inconel 690;

[0016] A step of supplying, within the enclosure, a coating target made of an austenitic alloy, for example comprising at least nickel and chromium and an iron content between 0 wt.% and 50 wt.%, or even, for example, less than 48 wt.%, or even 45 wt.%, or even 40 wt.%, or even 30 wt.%, for example a nickel-chromium alloy (NiCr) or an Inconel; then

[0017] A step of vacuum-sealing the chamber and degassing the chamber while it is empty; then

[0018] An ionic stripping step of the substrate; then

[0019] A step involving the injection of a gas containing at least argon and nitrogen into the enclosure to create an atmosphere within the enclosure; then

[0020] A reactive magnetron sputtering step of the coating target in the chamber atmosphere comprising at least argon and nitrogen, forming a coating on the substrate, the coating comprising at least nickel and chromium, the nickel (Ni) of the coating having a content between 20 at.% and 60 at.% and the chromium (Cr) of the coating having a content between 10 at.% and 40 at.%, and the coating being nitrogen-doped.

[0021] The contents determined as atomic percentages (“at.%”) in the coatings were measured with a measurement standard deviation of approximately 2%.

[0022] Such a process allows the coating to be applied from the target, forming a cathode, at the same time as it is doped, unlike other prior art processes in which doping (nitriding) is carried out on the bulk material by adding nitrogen, which then produces a doping gradient in the final treatment.

[0023] A reactive deposition process with such a target allows for the coating of a wide variety of substrates.

[0024] In order to provide better surface oxidation resistance to a wider range of materials, for example metal alloys, possibly used in corrosive conditions (hot and / or humid), for example zircaloy, TA6V4 (i.e. a titanium alloy also designated TA6V, TiAl6V4, 3.7165 or Grade 5 Titanium; typically with a composition containing C<0.08, 5.50 <AI<6.75, 3.50<V<4.50, Fe<0.30, O<0.20, H<0.015, Y<0.005, N<0.05, Ti solde), inconel, ou autres, tout en en améliorant les propriétés de frottement et d’usure, l’invention propose ainsi de déposer, par pulvérisation cathodique magnétron réactive, un alliage comportant au moins du nickel et du chrome, et de le doper avec de l’azote.

[0025] It is therefore possible to obtain a coating which is deposited relatively quickly, hardens to more than 1000 Hv (Vickers hardness), and retains good dry corrosion resistance properties.

[0026] For example, it is possible to use a target (cathode) made of NiCr alloy, in particular NiCr 60 / 40, or an Inconel target.

[0027] In one particular implementation example, the target coating alloy used is Inconel 690, due to the absence of cobalt in its composition, making it compatible with certain applications where cobalt is prohibited. For other applications, Inconels of different compositions can be used, even those containing cobalt.

[0028] The presence of nitrogen gives the coating a significant improvement in resistance to galling and increases its hardness, for example producing a hardening greater than 800 kg / mm². -2The coating is thus able to maintain a relatively high level of hardness, notably up to 700°C.

[0029] The presence of nitrogen also confers an unexpected improvement in oxidation resistance by forming a barrier oxide offering protection up to 700°C, instead of 500°C without nitrogen or with little nitrogen in the coating.

[0030] The paper by Saker et al. (Reactive magnetron sputtering of Inconel 690 by Ar-N2 plasma) describes this type of material produced using magnetron sputtering, with the aim of understanding the results obtained by a plasma nitriding process. This paper focuses on the microstructural properties of the coatings, including their composition and some mechanical characteristics such as hardness and modulus of elasticity. However, functional properties such as tribological properties or resistance to high-temperature oxidation are not addressed. In one implementation example, the sputtering step is configured to produce a nitrogen content in the coating between 9 at.% and 22 at.%, for example, between 12 at.% and 16 at.%.

[0031] For example, nitrogen content is measured by EDX (energy dispersive X-rays) under 10 keV.

[0032] For example, nickel content is measured by EDX at 10 keV. For example, chromium content is measured by EDX at 10 keV. For example, oxygen content is measured by EDX at 10 keV, which is a satisfactory method for measuring oxygen content when the composition of the sputtering target is known.

[0033] In another example, oxygen content is measured using GDOS (Glow Discharge Optical Spectroscopy), also known as GDOES (Glow Discharge Optical Emission Spectroscopy).

[0034] In one example of the embodiment, the Ni content is greater than the Cr content.

[0035] For example, a ratio between Ni content and Cr content is at least equal to 1.5, for example between 1.5 and 3.

[0036] For example, the ratio can be between 2.5 and 3 for Inconel 718, for example worth about 2.7, or even more.

[0037] In one example of implementation, the process also includes a step of setting the substrate in motion, for example in rotation.

[0038] In one implementation example, the spraying step is implemented during the substrate movement step.

[0039] In one implementation example, the spraying step includes a step of applying an electrical voltage to the substrate between -100 V and -50 V, for example about -75 V.

[0040] In one implementation example, the reactive magnetron sputtering step is implemented at a temperature less than or equal to 400°C, for example between 150°C and 400°C.

[0041] In one implementation example, the process also includes a step of depositing an underlayer between the substrate and the coating.

[0042] Such a step is, for example, implemented before the magnetron reactive sputtering step of the coating target forming the coating. In one example of implementation, the process further includes a step of evacuating the chamber and a step of injecting a gas containing at least argon and a reactive gas, for example nitrogen.

[0043] In one example implementation, the sublayer is configured to have a diffusion barrier function.

[0044] The sublayer is for example formed by at least one nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN).

[0045] For applications above 700°C, the coating can be advantageously combined with a nitride-type underlayer to form a diffusion barrier. For example, nitride is chosen because it has the highest absolute formation energy, such as zirconium nitride (ZrN), to limit or even prevent any redox reactions at the interfaces of this layer. Indeed, if necessary, and especially for high-temperature use (i.e., typically at least 500°C, or even 700°C), it is advantageous to first deposit an underlayer, particularly zirconium nitride, which has the benefit of forming a diffusion barrier between the substrate and the coating, thus limiting, for example, the risk of formation of brittle intermetallics or low-melting-point eutectics from the interdiffusion of elements between the substrate and the coating.Such an undercoat can therefore prevent redox reactions with the coating or substrate components during thermal stress on the coated part. It can also prevent the coating from dissolving into the substrate or from forming a new material with substrate components whose properties may be less desirable.

[0046] It is thus possible to deposit on a wide variety of electrically conductive substrates, in particular metallic, a relatively hard coating (for example with a hardness between 1000 Hv and 1200 Hv, or even 1400 Hv), which allows better resistance to seizing or wear than the substrates traditionally used and also gives them an improvement in corrosion resistance in environments such as steam turbines, aircraft turbojets etc., while maintaining a ductile character of the coating.

[0047] In one example of implementation, the substrate used is a material that does not resist dry oxidation above 400°C, or even from 300°C.

[0048] In one example implementation, the sub-layer deposition step includes a sub-layer target supply step containing at least zirconium (Zr), the sub-layer deposition step being configured to form the sub-layer containing zirconium nitride, the sub-layer having a thickness between 0.1 pm and 5 pm.

[0049] A second option is also offered: a coated piece, the piece comprising:

[0050] A substrate with an electrical resistivity less than or equal to 1 Q.cm, or even 10⁻¹⁰ 3 Q.cm, for example in metal, or in a metal alloy, for example in inconel, for example in inconel 690;

[0051] A coating, applied to the substrate, the coating comprising at least nickel and chromium, the nickel (Ni) of the coating having a content between 20 at.% and 60 at.% and the chromium (Cr) of the coating having a content between 10 at.% and 40 at.%, and the coating being nitrogen-doped.

[0052] In one example of implementation, the part can be obtained by a process as described above.

[0053] However, such a part can also be obtained by other processes, for example by filtered arc deposition.

[0054] A coating according to the invention thus forms a hard coating, i.e. with a hardness for example between 1000 Hv and 1400 Hv, with a metallic character, allowing satisfactory corrosion resistance, particularly at high temperature (i.e. at least 500°C), possibly in dry oxidation.

[0055] The coating can be deposited on any type of electrically conductive substrate. In one example, the substrate comprises at least one metal or metal alloy.

[0056] In one example of implementation, the substrate is made of a material that does not resist dry oxidation above 400°C, or even from 300°C.

[0057] The contents determined as atomic percentage in the coatings were measured with a measurement standard deviation of approximately 2%.

[0058] Such a coating makes it possible to improve resistance to galling and wear on substrates with either high chemical stability at high temperatures (i.e. at least 500°C) but low hardness (i.e. less than 500 Hv), such as zircaloy, austenitic stainless steel, inconel, etc., or on a hard substrate (i.e. at least 500 Hv) with low resistance to oxidation at high temperatures (i.e. less than 500°C), such as 17-4PH steel, or maraging 250, among others.

[0059] For example, the coating has a thickness between 0.5 and 50 pm, for example between 0.5 pm and 20 pm, for example between 5 pm and 15 pm.

[0060] For example, the coating has a nitrogen content between 9 at.% and 22 at.%, for example between 12 at.% and 16 at.%

[0061] For example, nitrogen content is measured by EDX under 10 keV.

[0062] For example, nickel content is measured by EDX under 10 keV.

[0063] For example, chromium content is measured by EDX under 10 keV.

[0064] For example, oxygen content is measured by EDX under 10 keV, which is a satisfactory method for measuring oxygen content when the composition of the spray target is known.

[0065] In another example, the oxygen content is measured using GDOS (Glow Discharge Optical Spectroscopy). In one embodiment, the Ni content is higher than the Cr content.

[0066] For example, a ratio between Ni content and Cr content is at least equal to 1.5, for example between 1.5 and 3.

[0067] For example, the ratio can be between 2.5 and 3 for Inconel 718, for example worth about 2.7, or even more.

[0068] In one example of the embodiment, the part also includes an underlayer between the substrate and the coating.

[0069] For example, the underlayer contains at least nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN).

[0070] For example, the underlayer has a thickness between 0.1 pm and 5 pm.

[0071] In one example of implementation, the modulus of elasticity of the coating, for example its Young's modulus, is between 200 GPa and 300 GPa, or even between 200 GPa and 250 GPa.

[0072] In an interesting implementation example, such a part forms, for example, a mechanical component of an aircraft engine, a turbocharger of internal combustion vehicles, or any other part subject to wear at a high temperature.

[0073] Brief description of the figs

[0074] The invention, according to an exemplary embodiment, will be well understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and not limitation, with reference to the accompanying drawings in which: Figure 1 shows cap test results carried out on coatings, and, qualitatively, their respective propensity to seizing; Figure 2 illustrates an evolution of nitrogen pressure as a function of nitrogen flow rate; Figure 3 illustrates the evolution of Vickers hardness Hv and reduced modulus of elasticity E' (in GPa) as a function of the nitrogen content in the coating (in atomic percentage, noted "at.%"); Figure 4 shows cap test results illustrating seizing damage to a coating as a function of its nitrogen content; Figure 5 then illustrates adhesion characterizations of the coatings as a function of the nitrogen content in the coating;Figure 6 shows the influence of temperature on hardness; Figure 7 shows measurements of the modulus of elasticity as a function of temperature; Figure 8 represents the evolution of oxygen content on the surface of Inconel coatings for different nitrogen contents in the coating, as a function of temperature in degrees Celsius (°C); Figure 9 represents the evolution of oxygen content on the surface of different types of coatings as a function of temperature in degrees Celsius (°C); Figure 10 illustrates an inter-diffusion phenomenon using SEM (scanning electron microscopy) imaging; Figure 11 illustrates the inter-diffusion phenomenon of Figure 9, through chemical element mapping, respectively of chromium (Cr), Figure 11a), iron (Fe), Figure 11b), carbon (C), Figure 11c), and nitrogen (N), Figure 11d); Figure 12 shows a cap test carried out on a sample according to an example of an embodiment of the invention;Figure 13 shows a cross-sectional SEM image, before oxidation (Figure 13a)) and after oxidation (Figure 13b)); Figure 14 shows element maps to demonstrate the effectiveness of the diffusion barrier; Figure 15 shows a small-scale cap test carried out on the coating after oxidation testing; and Figure 16 shows the evolution of Vickers hardness (Hv, in kg / mm; 2 ) on the surface of different types of coatings depending on the temperature in degrees Celsius (°C).

[0075] Detailed description

[0076] According to the invention, the coatings of interest can be obtained by reactive magnetron sputtering.

[0077] To achieve this, the process is for example implemented in a magnetron sputtering device which typically includes an enclosure in which the substrate to be coated and at least one target, also referred to as a "source", forming a cathode are placed.

[0078] The magnetron cathode is thus equipped with a target made of an austenitic structure material.

[0079] Target materials are primarily composed of nickel and chromium and may contain iron. However, the iron content is preferably limited, for example, less than 50 wt.%, or even less than 48 wt.%, 45 wt.%, or 40 wt.%. These materials are composed of austenitic structures. These targets are, for example, non-ferromagnetic, which allows for their easy use as magnetron sputtering targets. Materials whose composition makes them ferromagnetic, with ferritic or martensitic structures, are generally not easily processed by magnetron sputtering.

[0080] Within the scope of the present invention, interesting target materials include, for example, Inconel 690 (containing nickel (Ni) at least approximately 58 wt.%, chromium (Cr) from approximately 27 wt.% to 31 wt.%, and iron (Fe) from approximately 7 wt.% to 11 wt.%), or a NiCr alloy (Ni 60%, Cr 40%). These materials contain at least 25 at.% chromium (Cr), the remainder being nickel (Ni) and iron (Fe). They all exhibit non-magnetic properties.

[0081] All the characterizations described below, for illustrative purposes only, were obtained here using an Inconel 690 target. Alternative coatings were then developed using NiCr 60 / 40 alloy targets and X8CrNi25-21 stainless steel. Examples of coatings were obtained with deposition parameters close to those used for the series of tests on an Inconel 690 target.

[0082] The substrates used here are AISI M2 tool steel substrates (Z85WDCV 6.5.4.2 according to AFNOR).

[0083] The following table summarizes the nitrogen content of the coatings obtained as well as the main characteristics obtained, as described below.

[0084] Figure 1 shows the results of cap tests performed on these coatings and their respective propensity for galling. As a reminder, the "cap test" consists of rubbing a steel ball on the surface of the coating in question (thin layer), and the thin layer is then worn away by abrasion using a diamond suspension, with a particle size of one-quarter micrometer (µm) in this case. During these measurements, a significant difference in galling behavior was observed.

[0085] Coatings 1, 3 and 5 are produced without the introduction of nitrogen (N), from targets of NiCr 60 / 40 alloy, X8CrNi25-21 stainless steel and Inconel 690 respectively.

[0086] Coatings 2, 4 and 6 are prepared with the introduction of nitrogen, from targets of NiCr 60 / 40 alloy, X8CrNi25-21 stainless steel and Inconel 690 respectively.

[0087] The hardness levels exceed 1000 kg. mm -2 with nitrogen levels within the range of interest.

[0088] The cap tests also show a significant reduction in the propensity to seize.

[0089] Nitrogen-doped NiCr coating appears, however, slightly less resistant to galling, as evidenced by some scratches, compared to coatings made from X8CrNi25-21 stainless steel or Inconel 690, as illustrated in Figure 4.

[0090] It is noted, however, that the undoped NiCr alloy suffers from strong seizing, thus demonstrating the improvement obtained through nitrogen doping.

[0091] These coatings are designed for hot tribological applications. Generally, a thin tribological coating cannot replace the core characteristics of the substrate. In particular, the substrate must not undergo plastic deformation through the coating when subjected to tribological loading. This implies that the substrate must have a minimum hardness. Although this hardness depends on the tribological loading, it is preferable to apply the coating to a sufficiently hard substrate, for example, at least 250 Hv and preferably at least 400 Hv.

[0092] In preparation for the development of a nitrogen-doped coating, the nitrogen flow rate to be introduced into the deposition chamber is determined to obtain the nitrogen concentrations required for the desired compositions, as described below. Figure 2 illustrates the evolution of nitrogen pressure as a function of the nitrogen flow rate in the deposition chamber.

[0093] To define an optimal amount of nitrogen in the coating, we first identify the nitrogen flow rate that saturates the coating.

[0094] To achieve this, for example, the nitrogen flow rate is gradually increased while recording the pressure increase in the deposition chamber. This experiment is performed first without plasma and then again with the magnetron cathode plasma activated at the power that will be used for coating the substrates.

[0095] When the pressure increase curve, cathode on, becomes parallel to the pressure increase line, cathode off, a critical nitrogen flow rate that saturates the coating can then be determined.

[0096] This critical flow rate depends in particular on the magnetron cathode, its size, the power applied to it, the number of cathodes and the pumping sizing.

[0097] Regardless of the operating conditions, the coating material is saturated with nitrogen at the critical flow rate.

[0098] The curve in Figure 2 shows that from a flow rate of approximately 150 sccm (standard cubic centimeter per minute, or cm³) 3 / min at standard pressure and temperature), the pressure evolution with cathode lit has the same slope as without discharge, therefore the additional nitrogen introduced is no longer incorporated into the coating.

[0099] In this example, the critical flow rate is therefore approximately 150 sccm. Subsequently, coating deposition at lower nitrogen flow rates was carried out. As already mentioned, it is not the flow rates that describe the properties of the coatings, but their nitrogen content, because the nitrogen flow rate to be applied during the coating application step depends, in particular, on the power applied to the cathode (but can also be taken into account: the size of the cathodes, their number, the sizing of the machine pumping, etc., as indicated above).

[0100] To obtain a coating with a controlled composition, the optical emission of the plasma is monitored during deposition, and the nitrogen flow rate is controlled so that the optical emission remains essentially constant. In this case, the light emission from chromium atoms at a wavelength of 520 nm is monitored, and the nitrogen flow rate is controlled according to the light emitted by the sprayed chromium atoms.

[0101] Depending on the desired nitrogen content in the coating, the light emission intensity setpoint which controls the nitrogen flow rate is adjusted.

[0102] Figure 3 illustrates the evolution of Vickers hardness Hv (curve "H") and reduced modulus of elasticity E' (in GPa; curve "E") as a function of the nitrogen content in the coating obtained from an Inconel 690 target (in atomic percentage, noted "at.%"). Each pair of points corresponds to doubled deposition tests, which allows for the estimation of the measurement accuracy of the composition, hardness, and reduced modulus of elasticity of the coating considered.

[0103] The nitrogen content of the coatings is measured by EDX (energy dispersive X-ray) analysis under 10 keV.

[0104] The hardness of the coatings is measured by Vickers instrumented micro-indentation under 100 mN.

[0105] Coatings at least 9 pm thick are thick enough that the analysis does not incorporate the underlying substrate.

[0106] The thickness of the coating ensures that the indentation depth remains at most 10% of the coating thickness (9 pm for example) and does not integrate the substrate.

[0107] The graph in Figure 3 shows an increase in coating hardness with increasing nitrogen content of the coating up to a content of approximately 8-10 at.%; beyond this, tests have shown that the hardness is almost constant.

[0108] This graph also shows a maximum hardness level of approximately 1300 Hv.

[0109] With a process according to an example of implementation of the invention, the coating for a nitrogen content of about 15 at.% (coating no. 6 of the table above) can thus have a hardness of about 1300 Hv.

[0110] Figure 4 shows cap test results illustrating galling damage of a coating obtained from an Inconel 690 target as a function of its nitrogen content.

[0111] The effect of hardening on the tribological behavior of coatings is immediately apparent when thickness measurements are taken using a cap test.

[0112] Without nitrogen (coating #5 in the table above), although the contact pressure becomes very low as the coating wears, significant galling damage is observed. It is a known property of stainless steels and Inconel alloys to have a strong propensity for galling. A coating with approximately 5 at.% nitrogen still exhibits a tendency to galling, but this disappears with a coating containing approximately 8-9 at.% nitrogen.

[0113] For this reason (resistance to seizing), a coating with a nitrogen content of less than 9 at.% is considered non-compliant here.

[0114] Figure 5 then illustrates adhesion characteristics of these coatings, which also provides information on the brittle behavior and crack resistance of the materials used for the coating. Considering galling resistance as a criterion, a nitrogen content of 8-9% and above is very satisfactory, but if this criterion is combined with crack resistance, a content of 8-9% exhibits brittleness, as evidenced by the long cracks visible in Figure 5. It may therefore be advantageous to have a nitrogen content of at least 12 at.%. Contents between 12 at.% and 16 at.% show short cracks, which is a characteristic of hard materials (which generally tend to crack).

[0115] With reference to figures 6 and 7, to show an influence of temperature on hardness and elastic modulus, AISI M2 tool steel substrates (Z85WDCV 6.5.4.2 according to AFNOR) coated with a coating obtained from an Inconel 690 target according to an embodiment of the invention as described above are subjected to 24h isotherms in open air.

[0116] Hardness and modulus of elasticity characterizations are performed by instrumented microindentation under 30 mN.

[0117] The hardness values ​​are the average of ten measurements.

[0118] After characterization, the samples are put back in the oven for a new isotherm 100°C higher than the previous one.

[0119] The isotherms obtained are therefore 400°C, 500°C, 600°C, 700°C and 800°C.

[0120] Figure 6 shows the Vickers Hv hardness measurement results as a function of the isotherm temperature that preceded the hardness measurement.

[0121] This figure shows that the trends in the evolution of the hardness curves of coatings with different nitrogen contents undergo similar variations.

[0122] The hardness of the coatings immediately after deposition is reported for a temperature of 20°C.

[0123] After the first isotherm at 400°C, the mechanical properties of the coatings can be considered unchanged.

[0124] The small variations are probably due to the experimental dispersion of hardness measurements estimated at + / - 15%.

[0125] A first notable drop in hardness occurs after the 500°C isotherm.

[0126] However, coatings containing 12 at.% and 15 at.% nitrogen maintain a hardness between 900 and 1000 kg / mm².-2 .

[0127] Hardness measurements appear stable for isotherms between 500°C and 700°C. Coatings with the highest nitrogen content maintain a hardness of around 800 kg / mm². -2 Finally, all coatings undergo a significant drop in hardness after the 800°C isotherm.

[0128] Figure 7 then shows measurements of the modulus of elasticity as a function of temperature.

[0129] These measurements corroborate the variations in properties observed on the hardness measurements (figure 6), namely that the coatings undergo a slight change in properties after heating to 500°C, regardless of the nitrogen content, including in the absence of nitrogen.

[0130] These properties are maintained up to and including approximately 700°C.

[0131] The properties appear to change significantly after the 800°C isotherm. After each isotherm, EDX analyses of the coatings are performed at 10 keV. An accelerating voltage of 10 keV allows for a more precise separation of the contribution of chromium to the deposit from that of oxygen, unlike analyses at 5 keV, which would be more limited in depth to a single oxide layer if one forms. A voltage of 10 keV is a good compromise for distinguishing oxygen (O) from chromium (Cr) and also provides an indirect method for observing the evolution of the thickness of the oxide layer that may form, since these analyses incorporate the entire oxide layer and a greater or lesser proportion of the underlying metal, depending on the oxide thickness.

[0132] Figure 8 represents the evolution of the oxygen content on the surface of the samples, in particular the inconel coatings, as a function of temperature in degrees Celsius (°C).

[0133] After deposition, oxygen in a passivation layer is not detectable.

[0134] The passivation layer is an oxide layer that forms spontaneously on a metal upon contact with air (or water) when the vacuum-applied coating is exposed to air (or water) to be removed from the machine. It thickens over time, and the faster the thickening occurs at higher temperatures.

[0135] After 24 hours at 400°C, the coatings show an oxygen content of around 5 at.%. The coatings then exhibit a yellow color, related to optical interference produced by the thin oxide layer.

[0136] This content indicates that the EDX analysis incorporates the thin oxide layer as well as the underlying material for the bulk of the analysis.

[0137] After 24 hours of isothermal exposure at 500°C, the increase in oxygen content indicates that the oxide has thickened, but compared to the level between 10% and 15%, the oxide remains very thin compared to the depth of analysis under 10 keV. The layers then exhibit a blue interference coloration.

[0138] After the 24h isotherms at 600°C and 24h at 700°C, two populations of coatings stand out:

[0139] Coatings with at least 10 at.% nitrogen exhibit a consistent oxygen content of around 15 at.%. This indicates that the oxide formed protects the coating by preventing oxygen diffusion. This is confirmed by visual examination of the samples, which remain blue. The 15 at.% oxygen level also indicates that the oxide thickness remains below the EDX analysis depth.

[0140] Conversely, samples with less than 10 at.% nitrogen (i.e., here with 5 at.% or no nitrogen) show a very significant increase in surface oxygen content, indicating that the oxide layer is growing thicker. With oxygen atomic content levels of 20 at.% to 40 at.%, we can deduce that the oxide thickness begins to make a significant contribution to the depth of analysis. Correspondingly, the surface appearance of the samples becomes very dark, indicating that the oxide has thickened to such an extent that it no longer produces optical interference in white light.

[0141] Finally, after an additional 24 hours at 800°C, all coatings showed a significant increase in oxygen content, reaching values ​​between 40 at.% and 66 at.%. At these oxygen content levels, the EDX analysis depth is on the order of magnitude of the oxide thickness, or only slightly greater.

[0142] These results unexpectedly indicate that resistance to dry oxidation, particularly for an Inconel coating, appears to be improved by the introduction of nitrogen into the coating, starting at approximately 10 at.%. However, the improvement in oxidation resistance by nitrogen doping of the coating was unexpected and counterintuitive.

[0143] Oxidation resistance is maintained up to and including 700°C, whereas for coatings not doped with nitrogen or weakly doped (for example here at 5 at.%), oxidation accelerates rapidly from 500°C or 600°C.

[0144] The thickness of the oxides was evaluated using the cap test method.

[0145] The oxide thicknesses are estimated to be between 0.3 pm and 0.4 pm after a 24h stay at different temperatures from 400°C to 800°C, which remains very low in absolute terms (as demonstrated by Figure 14 described later, for example).

[0146] Despite the poor thickness measurement accuracy of this technique (below a micrometer), it can be concluded that the hardness measurements taken after the 800°C isotherm incorporate some of the oxide, since the indentation depths are on the order of 0.5 µm. It can also be noted that the indentation depths are significantly smaller than 10% of the coating thickness, which is approximately 9 µm. Part of the significant changes in mechanical properties at 800°C could be attributed to inter-diffusion phenomena between the coating and the substrate.

[0147] Indeed, from 800°C onwards, the diffusion phenomena of metallic elements begin to be significant.

[0148] Figure 9 represents the evolution of oxygen content as a function of temperature in degrees Celsius (°C) on the surface of coatings respectively of NiCr doped with 18 at.% nitrogen, stainless steel 25 / 21 doped with 20 at.% nitrogen, and Inconel 690 doped with 15 at.% nitrogen.

[0149] Although doped 25 / 21 stainless steel seems to meet the criteria for interesting coatings in terms of hardness, or improved resistance to seizing, 24h isotherms in air show that the oxidation resistance of doped 25 / 21 stainless steel is lower than that of doped Ni / Cr 60 / 40 alloy, or even doped Inconel 690.

[0150] According to figure 9, doped stainless steel exhibits massive oxidation from 500°C.

[0151] The changes in oxygen content, as a function of isotherm temperature, of Inconel 690 at 15 at.% nitrogen and of NiCr 60 / 40 alloy at 18 at.% nitrogen are similar.

[0152] It therefore appears that the 10 wt.% mass of iron in Inconel 690 would not impact the oxidation behavior of the coating, whereas the 54 wt.% mass of iron in 25 / 21 stainless steel is harmful.

[0153] The iron content of the target alloy is therefore preferably chosen to be less than 50 wt.%, as for example in Inconel 800, or even less than 48 wt.%, or even 45 wt.%, or even 40 wt.%, for example in Inconel 825, or even less than 30 wt.% (e.g. Inconel 690, Inconel 718).

[0154] Although the nitrogen-doped stainless steel deposit here has a similar chromium content to other coatings, the oxidation behavior is different.

[0155] This difference appears to be due to the replacement of a high iron content with a high nickel content, which benefits oxidation resistance, itself further improved by nitrogen doping.

[0156] Hardness measurements after isotherms, as illustrated in Figure 16, show that the hardness changes with temperature are similar between Inconel 690 at 15 at.% nitrogen and those obtained for NiCr 60 / 40 alloy at 18 at.% nitrogen. However, for the doped stainless steel, an increase in hardness appears starting at 500°C, which seems to reflect the massive oxidation of the deposit. In this case, it would no longer be the hardness of the deposit itself that is being measured, but rather that of the oxide layer.

[0157] Hardness measurement is no longer possible after the 800°C isotherm, because the oxide becomes rough and crumbles.

[0158] It is also observed that at 500°C, the nitrogen-doped stainless steel layer sees its hardness collapse while the oxide has not yet developed much.

[0159] Figures 10 and 11 illustrate these inter-diffusion phenomena, through micrographic sections, SEM (scanning electron microscopy) imaging (Figure 10) and chemical element mapping, respectively of chromium (Cr), iron (Fe), carbon (C) and nitrogen (N) (Figure 11, a) to d) respectively).

[0160] The SEM images were obtained at 20 keV of an Inconel coating with 15 at.% nitrogen after all successive 24-hour isotherms, up to and including 800°C. As a reminder, the coatings are deposited on an AISI M2 tool steel substrate. In this example, Figure 10 shows clear signs of interdiffusion between the coating and the substrate, as the interface has become diffuse and precipitates are observed.

[0161] In Figure 11, the EDX maps show the presence of substrate elements in the coating, and vice versa.

[0162] Interdiffusion can be an undesirable phenomenon for very high-temperature applications (i.e., at least 500°C, or even at least 600°C). Among the disadvantages, interdiffusion depends on the nature of the substrate, can in some cases lead to the precipitation of brittle intermetallics, or the formation of low-melting-point eutectics.

[0163] An effective remedy is to interpose a layer that acts as a diffusion barrier.

[0164] In the following example, a first layer of zirconium nitride (ZrN), here called "underlayer", is deposited before the coating.

[0165] Zirconium nitride is one of the most thermodynamically stable nitrides, which avoids, for example, its reduction by other metallic elements, such as the substrate or coating.

[0166] Even though the oxidation resistance of ZrN is low, this material as an underlayer of a nitrogen-doped coating becomes protected against oxidation.

[0167] Figure 12 shows a cap test carried out through a stack according to an example of an embodiment of the invention, comprising an inconel coating doped with 15 at% nitrogen, then a ZrN underlayer 1.7 pm thick, then an M2 tool steel substrate.

[0168] After 24 hours at 800°C in air, the sample is cut and passed through SEM to show the barrier effect provided by the ZrN layer.

[0169] Figure 13 shows a cross-sectional SEM image, before and after oxidation (respectively figures 13 a) and 13 b)).

[0170] Figure 13 a), on the left, shows, at the top, the inconel coating doped with 15 at. % nitrogen as it comes out of the deposition machine.

[0171] The coating then has a thickness of approximately 7.8 µm. Next, the sample exhibits a lighter-colored ZrN sublayer, approximately 1.7 µm thick, and below it, the M2 tool steel substrate. The white grains in the steel correspond to tungsten carbide precipitates.

[0172] Figure 13 b), on the right, shows the same coating after exposure to air at 800°C for 24h.

[0173] The appearance of dark CrN precipitates is noted in the nitrogen-doped Inconel coating.

[0174] On the free surface of the coating, a very thin layer of oxide, approximately 0.3 pm-0.4 pm, can be identified, which is distinguishable in places as a dark line.

[0175] The ZrN underlayer appears unchanged, which demonstrates its effectiveness as a diffusion barrier.

[0176] In the substrate, secondary precipitation of tungsten carbides, induced by the heat treatment, is visible.

[0177] Figure 14 represents element maps to demonstrate the effectiveness of the ZrN diffusion barrier.

[0178] Figure 14 (respectively Figure 14 a) to e)) shows, from left to right, the mapping of chromium (Cr), iron (Fe), carbon (C), nitrogen (N), and zirconium (Zr).

[0179] These analyses confirm the absence of diffusion between ZrN and the steel of the substrate on the one hand, and between ZrN and the nitrogen-doped Inconel of the coating on the other.

[0180] This barrier therefore makes it possible to limit, or even avoid, the risks of eutectic formation between the nitrogen-doped Inconel and the coated substrate, or also the risks of formation of potentially fragile intermetallic phases.

[0181] In the example presented here, the formation of chromium carbides by inter-diffusion between the coating and the substrate is no longer observed, in the absence of a barrier (underlayer). Finally, Figure 15 shows a small-scale cap test performed on the coating after the oxidation test.

[0182] This test allows us to estimate the thickness of the oxide layer to be approximately 0.4 pm.

[0183] Thus, by means of a process according to an example of implementation of the invention, the part obtained can have a nitrogen-doped coating in a more homogeneous manner, in particular with a nitrogen content between about 9 at. % and 22 at. %.

[0184] Achieving such a homogeneous nitrogen content in the coating is not feasible with standard prior art processes.

[0185] In addition, such a part exhibits better resistance to oxidation, and less risk of seizing, at temperatures of at least 500°C.

Claims

DEMANDS 1. A method for coating a substrate comprising at least the following steps: A step of supplying a substrate into an enclosure, the substrate having an electrical resistivity less than or equal to 1 Q.cm; A step of supplying a coating target made of an austenitic alloy, within the enclosure; then A step of vacuum-sealing the chamber and degassing the chamber while it is empty; then An ionic stripping step of the substrate; then A step of injecting a gas comprising at least argon and nitrogen into the chamber to form a chamber atmosphere; then a step of reactive magnetron sputtering of the coating target into the chamber atmosphere comprising at least argon and nitrogen, forming a coating on the substrate, the coating comprising at least nickel and chromium, the nickel (Ni) of the coating having a content between 20 at.% and 60 at.% and the chromium (Cr) of the coating having a content between 10 at.% and 40 at.%, and the coating being nitrogen-doped.

2. A method according to claim 1, wherein the spraying step is configured to produce a nitrogen content in the coating of between 9 at.% and 22 at.%, the nitrogen content being measured by EDX under 10 keV.

3. A method according to any one of claims 1 to 3, further comprising a step of moving the substrate, and in which the spraying step is carried out during the step of moving the substrate.

4. A method according to any one of claims 1 to 4, wherein the spraying step comprises a step of applying an electrical voltage to the substrate of between -100 V and -50 V.

5. A method according to any one of claims 1 to 5, wherein the reactive magnetron sputtering step is carried out at a temperature between 150°C and 400°C.

6. A method according to any one of claims 1 to 6, comprising a step of depositing a sub-layer between the substrate and the coating, before the step of reactive magnetron sputtering of the coating target forming the coating, and a step of evacuating the chamber and a step of injecting a gas comprising at least argon and a reactive gas, for example nitrogen, the sub-layer being configured to have a diffusion barrier function, the sub-layer being for example formed by at least one nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN).

7. A method according to claim 6, wherein the sub-layer deposition step comprises a sub-layer target supply step comprising at least zirconium (Zr), the sub-layer deposition step being configured to form the sub-layer comprising zirconium nitride, the sub-layer having a thickness of between 0.1 pm and 5 pm.

8. Coated part comprising: A substrate having an electrical resistivity less than or equal to 1 Q.cm; A coating, applied to the substrate, the coating comprising at least nickel and chromium, the nickel (Ni) of the coating having a content between 20 at.% and 60 at.% and the chromium (Cr) of the coating having a content between 10 at.% and 40 at.%, and the coating being nitrogen-doped.

9. Part according to claim 8, wherein the coating has a thickness between 0.5 and 50 µm.

10. Part according to any one of claims 8 or 9, wherein the coating has a nitrogen content between 9 at.% and 22 at.%, the content being measured by EDX under 10 keV.

11. Part according to any one of claims 8 to 10, comprising an underlayer between the substrate and the coating, the underlayer comprising at least nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN), the underlayer having a thickness between 0.1 pm and 5 pm.

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