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

Abstract

The invention relates to a substrate coating method comprising a step of supplying a substrate, in a chamber, having an electrical resistivity less than or equal to 1 Ω·cm, a step of supplying a coating target made of an austenitic alloy into the chamber, a step of evacuating the chamber and degassing the chamber under vacuum, a step of ion etching the substrate, a step of injecting a gas comprising at least argon and nitrogen into the chamber to form a chamber atmosphere, and then a step of reactive magnetron sputtering of the coating target into the chamber atmosphere, forming a coating comprising at least nickel and chromium, and doped with nitrogen. The invention also relates to the part obtained by the method. Figure for the abstract: Fig. 12

Description

Title of the invention: Method for coating a substrate by reactive magnetron sputtering and part comprising such a coated substrate. Technical field of the invention

[0001] The present invention relates to a method of coating a substrate, in particular an electrically conductive substrate, by reactive magnetron sputtering.

[0002] It also relates to a part obtained by such a process, comprising such a coated substrate. State of the art

[0003] Mechanical components operating at high temperatures, i.e. above 280°C, or even at least 400°C, must have their surface protected against corrosion and oxidation, in particular by dry means, i.e. in a gas containing at least one oxidizing gaseous species, such as oxygen, or even water vapor.

[0004] Certain alloys are used to withstand high temperatures.

[0005] However, these alloys have rather low hardnesses, i.e. generally less than 500 Hv, which gives them low resistance to wear.

[0006] For example, in the aeronautical field, blade roots are subjected to vibrations that induce fretting wear, that is, a type of wear occurring at high sliding speeds between the surfaces involved and with small displacements. Fretting is often observed between two parts fixed together and set in vibration.

[0007] Thus, it is necessary to change these parts regularly, resulting in significant maintenance costs.

[0008] It would therefore be very interesting to be able to extend the operating life of certain parts subjected to high temperature and mechanical wear.

[0009] An object of the present invention is, for example, to propose a coating material with metallic characteristics, which is more resistant to wear than the metallic alloys traditionally used, and which itself exhibits high resistance to oxidation.

[0010] For resistance to hot wear, nitride-type coatings exist, such as titanium aluminum nitride (denoted AlTiN or TiAIN), or chromium aluminum nitride (denoted AlCrN or CrAIN). These are metallic-type nitride ceramic materials. Their hardness is particularly high (i.e., a Vickers hardness of at least approximately 2500 Hv). These coatings are primarily applied to Hard metallic substrates, such as tool steels or even cemented carbides (WC-Co), exhibit high resistance to dry oxidation due to their aluminum content, which forms a barrier layer of alumina on the surface. However, the elastic moduli of metal nitrides are particularly high (at least 300 GPa), while those of the metallic substrates of interest are generally lower 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.

[0011] 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 type substrates to produce a surface-hardened material.

[0012] The problem arises for other metallic alloys for which none of the technical solutions set out above apply.

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

[0014] 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 10³ Q.cm, the substrate being for example metal, for example a metal alloy, for example TA6V4, or inconel, for example inconel 690; - 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 - A step of vacuum-sealing the chamber and degassing the chamber under vacuum; then - An ionic stripping step of the substrate; then - A step involving the injection of a gas containing at least argon and nitrogen within the enclosure to create an atmosphere within the enclosure; then - 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.

[0015] 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 the doping (nitriding) is carried out on the bulk material by adding nitrogen, which then produces a doping gradient in the final treatment.

[0016] A reactive deposition process with such a target makes it possible to coat a wide variety of substrates.

[0017] 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, TiA16V4, 3.7165 or Grade 5 Titanium; typically with a composition comprising C<0.08, 5.50 <Al<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.

[0018] It is thus 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.

[0019] For example, it is possible to use a target (cathode) made of NiCr alloy, in particular NiCr 60 / 40, or a target made of Inconel.

[0020] In one particular embodiment, the alloy of the coating target used is an Inconel 690, due to the absence of cobalt in its composition, which makes it compatible with certain applications in which cobalt is prohibited.

[0021] For other applications, inconels of other compositions can be used, even those containing cobalt.

[0022] The presence of nitrogen gives the coating a notable improvement in resistance to galling, and increases its hardness, for example producing a hardening greater than 800 kg.mm2. The coating is thus able to maintain a relatively high level of hardness, in particular up to 700°C.

[0023] 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.

[0024] The document Saker et al. (Reactive magnetron sputtering of inconel 690 by Ar-N2 plasma) describes this type of material produced by magnetron sputtering, with the aim of understanding the results obtained by a plasma nitriding process. This document 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.

[0025] In one example of implementation, the spraying step is configured to produce a nitrogen content in the coating of between 9 at.% and 22 at.%, for example between 12 at.% and 16 at.%, the nitrogen content being measured by EDX under 10 keV.

[0026] In one example of implementation, the process further includes a step of setting in motion, for example in rotation, the substrate.

[0027] In one example of implementation, the spraying step is carried out during the substrate movement step.

[0028] In one example of implementation, the spraying step includes a step of applying an electrical voltage to the substrate of between -100 V and -50 V, for example of about -75 V.

[0029] In one example of implementation, the reactive magnetron sputtering step is carried out at a temperature less than or equal to 400°C, for example between 150°C and 400°C.

[0030] In one example of implementation, the process further includes a step of depositing an underlayer between the substrate and the coating.

[0031] Such a step is for example implemented before the magnetron reactive sputtering step of the coating target forming the coating.

[0032] In one example of implementation, the process further includes a step of vacuuming the enclosure and a step of injecting a gas comprising at least argon and a reactive gas, for example nitrogen.

[0033] In an example implementation, the sublayer is configured to have a diffusion barrier function.

[0034] The sublayer is for example formed by at least one nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN).

[0035] For applications above 700°C, the coating can be advantageously combined with a nitride-type undercoat to form a diffusion barrier.

[0036] For example, nitride is chosen because it has the highest absolute value of formation energy, such as zirconium nitride (ZrN) to limit or even avoid any redox phenomenon at the interfaces of this layer.

[0037] Indeed, if necessary, and especially for use at high temperatures (i.e. Typically at at least 500°C, or even 700°C, it is advantageous to first deposit a sub-layer, particularly zirconium nitride, which has the benefit of forming a diffusion barrier between the substrate and the coating. This limits, for example, the risk of forming brittle intermetallics or low-melting-point eutectics from the inter-diffusion of elements between the substrate and the coating. Such a sub-layer can therefore prevent redox reactions with elements of the coating or substrate 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 elements whose properties may be less desirable.

[0038] 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 galling 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.

[0039] In one example of implementation, the substrate used is made of a material which does not resist dry oxidation above 400°C, or even from 300°C.

[0040] In one example of an implementation, the sub-layer deposition step includes a step for supplying a sub-layer target comprising at least zirconium (Zr), the sub-layer deposition step being configured to form the sub-layer containing zirconium nitride, the underlayer having a thickness between 0.1 pm and 5 pm.

[0041] A second aspect also proposes a coated part, the part comprising: - A substrate having an electrical resistivity less than or equal to 1 Q.cm, or even 103 Q.cm, for example in metal, or in a metallic alloy, for example in inconel, for example in inconel 690; - 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.

[0042] In one embodiment, the part can be obtained by a process as described above.

[0043] Such a part can however also be obtained by other processes, for example by filtered arc deposition.

[0044] 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, in particular at high temperature (i.e. at least 500°C), possibly in dry oxidation.

[0045] The coating can be deposited on any type of substrate that is electrically conductive.

[0046] In one embodiment, the substrate comprises at least one metal or metal alloy.

[0047] In one embodiment, the substrate is made of a material which does not resist dry oxidation above 400°C, or even from 300°C.

[0048] The contents determined as atomic percentage in the coatings were measured with a measurement standard deviation of approximately 2%.

[0049] Such a coating makes it possible to improve resistance to galling and wear on substrates having 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.

[0050] 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.

[0051] For example, the coating has a nitrogen content between 9 at.% and 22 at.%, for example between 12 at.% and 16 at.%, the content being measured by EDX under 10 keV.

[0052] In one embodiment, the part further comprises an underlayer between the substrate and the coating.

[0053] For example, the sublayer comprises at least nitride, for example zirconium nitride (ZrN) or titanium nitride (TiN).

[0054] For example, the underlayer has a thickness between 0.1 pm and 5 pm.

[0055] In one embodiment, 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.

[0056] In an interesting embodiment, such a part forms, for example, a mechanical component of an aircraft engine, a turbocharger for internal combustion vehicles, or any other part subject to wear at high temperatures. Brief description of the figures

[0057] The invention, according to an exemplary embodiment, will be better understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and in no way limiting, with reference to the accompanying drawings in which:

[0058] [Fig.1] shows results of capotest carried out on coatings, and, qualitatively, their respective propensity to seizing;

[0059] [Fig.2] illustrates an evolution of nitrogen pressure as a function of nitrogen flow rate;

[0060] Figure 3 illustrates the evolution of the Vickers hardness Hv and the reduced modulus of elasticity E' (in GPa) as a function of the nitrogen content in the coating (in atomic percentage, noted "at.%");

[0061] [Fig.4] shows cap test results illustrating damage to the seizing of a coating as a function of its nitrogen content;

[0062] [Fig.5] then illustrates adhesion characterizations of coatings as a function of nitrogen content in the coating;

[0063] [Fig.6] shows an influence of temperature on hardness;

[0064] [Fig.7] shows measurements of the modulus of elasticity as a function of temperature;

[0065] [Fig.8] represents the evolution of the oxygen content on the surface of inconel coatings for different nitrogen contents in the coating, as a function of temperature in degrees Celsius (°C);

[0066] [Fig.9] represents the evolution of the oxygen content on the surface of different types of coatings as a function of temperature in degrees Celsius (°C);

[0067] [Fig. 10] illustrates an inter-diffusion phenomenon by SEM (scanning electron microscope) imaging;

[0068] [Fig.11] illustrates the phenomenon of inter-diffusion of [Fig.9], through mapping of chemical elements, respectively of chromium (Cr), [Fig.11] a), iron (Fe), [Fig.11] b), carbon (C), [Fig.11] c), and nitrogen (N), [Fig.11] d);

[0069] [Fig. 12] shows a test cap made on a sample according to an example of an embodiment of the invention;

[0070] [Fig. 13] shows a cross-sectional SEM image, before oxidation ([Fig. 13] a)) and after oxidation ([Fig. 13] b));

[0071] [Fig. 14] represents element maps to demonstrate the effectiveness of the diffusion barrier;

[0072] Fig. 15 shows a small cap test carried out on the coating after an oxidation test; and

[0073] Figure 16 shows the evolution of Vickers hardness (Hv, in kg / mm²) on the surface of different types of coatings as a function of temperature in degrees Celsius (°C). Detailed description

[0074] According to the invention, the coatings of interest can be obtained by reactive magnetron sputtering.

[0075] To do 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 arranged.

[0076] The magnetron cathode is thus equipped with a target made of an austenitic structure material.

[0077] Target materials are primarily composed of nickel and chromium and may optionally contain iron. However, the iron content is preferably limited; for example, the iron content is preferably less than 50 wt.%, or even less than 48 wt.%, or even 45 wt.%, or even 40 wt.%. The composition of these materials is such that they form 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, whose structures are ferritic or martensitic, cannot generally be easily processed by magnetron sputtering.

[0078] Within the scope of the present invention, interesting target materials are, 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.

[0079] All the characterizations described below, for illustrative purposes only, are obtained here from an Inconel 690 target. Alternative coatings were then developed from NiCr 60 / 40 alloy targets and X8CrNi25-21 type stainless steel.

[0080] Examples of coatings are obtained with deposition parameters close to those used for the series of tests from an Inconel 690 target.

[0081] The substrates used here are AISI M2 tool steel substrates (Z85WDCV 6.5.4.2 according to AFNOR).

[0082] The following table summarizes the nitrogen content of the coatings obtained as well as the main characteristics obtained, as described below. Example / Test Target Nature Nitrogen Content (% at N) Hardness Life (Hv) in kg.mm2 Galvanizing Resistance Oxidation Resistance at 700°C Conformity 1 NiCr 60 / 40 0 730 no no no 2 NiCr 60 / 40 19 1300 yes yes yes 3 X8CrNi25-21 0 740 no no no 4 X8CrNi25-21 21 1090 yes no no 5 Inconel 690 0 650 no no no 6 Inconel 690 15 1350 yes yes yes

[0083] Fig. 1 shows results of cap tests carried out on these coatings and their respective propensity to seizing.

[0084] It should be noted that the "calotest" 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 with a diamond suspension, with a particle size of one-quarter micrometer (µm) in this case. During these measurements, it was possible to observe a significant difference in galling behavior.

[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 respectively of NiCr 60 / 40 alloy, X8CrNi25-21 stainless steel and Inconel 690.

[0087] Hardness levels exceed 1000 kg.mm2 with nitrogen contents within the range of interest.

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

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

[0090] It should be noted that the undoped NiCr alloy undergoes strong seizing, thus demonstrating the improvement obtained through nitrogen doping.

[0091] These coatings are intended for hot tribological loading. 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, for example, to obtain the nitrogen contents that allow the compositions of interest to be obtained as described below.

[0093] Figure [Fig.2] illustrates an evolution of nitrogen pressure as a function of nitrogen flow rate in the deposition chamber.

[0094] To define an optimal amount of nitrogen in the coating, we first identify the nitrogen flow rate that saturates the coating.

[0095] To do this, for example, the nitrogen flow rate is increased gradually by recording the pressure increase in the deposition chamber.

[0096] This experiment is carried out a first time without plasma and a second time by turning on the plasma of the magnetron cathode at the power which will be used for the deposition of coatings on the substrates to be coated.

[0097] When the pressure increase curve, cathode on, becomes parallel to the pressure increase line, cathode off, a critical nitrogen flow rate can then be determined which saturates the coating.

[0098] This critical flow rate depends in particular on the magnetron cathode, its size, the power applied to it, the number of cathodes and the sizing of the pumping.

[0099] Regardless of the operating conditions, the coating material is saturated with nitrogen at the critical flow rate.

[0100] The curve in [Fig.2] shows that from a flow rate of approximately 150 sccm (“standard cubic centimeter per minute”, or cm3 / min at standard pressure and temperature), the pressure evolution with the cathode lit has the same slope as without discharge, therefore the additional nitrogen introduced is no longer incorporated into the coating.

[0101] In the context of this example, the critical flow rate is therefore approximately 150 sccm.

[0102] Subsequently, coating deposition at lower nitrogen flow rates was carried out.

[0103] As already mentioned, it is not the flow rates that will 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 the following may also be taken into account: the size of the cathodes, their number, machine pumping sizing, etc. as indicated above).

[0104] In order 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 substantially constant. In the present case, the light emission from the 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.

[0105] Depending on the desired nitrogen content in the coating, the light emission intensity setpoint which controls the nitrogen flow rate is adjusted.

[0106] Fig. 3 illustrates the evolution of Vickers hardness Hv (curve "H") and reduced modulus of elasticity E' (in GPa; curve "E") as a function of nitrogen content in the coating obtained from an Inconel 690 target (in atomic percentage, noted "at.%").

[0107] Each pair of points corresponds to doubled deposition tests, which allows estimation of a measurement accuracy of composition, hardness and reduced modulus of elasticity of the coating considered.

[0108] The nitrogen content of the coatings is measured by EDX (energy dispersive X-ray) analysis under 10 keV.

[0109] The hardness of the coatings is measured by Vickers instrumented micro-indentation under 100 mN.

[0110] Coatings of at least 9 pm thickness are sufficiently thick that the analysis does not incorporate the underlying substrate.

[0111] 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.

[0112] The graph in [Fig.3] shows an increase in coating hardness with increasing nitrogen content of the coating up to a content of about 8-10 at.%; beyond that, tests have shown that the hardness is almost constant.

[0113] This graph also shows a maximum hardness level of approximately 1300 Hv.

[0114] 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.

[0115] Fig. 4 shows cap test results illustrating galling damage of a coating obtained from an Inconel 690 target as a function of its nitrogen content.

[0116] The effect of hardening on the tribological behavior of coatings is immediately apparent when thickness measurements are taken by capo test.

[0117] Without nitrogen (coating no. 5 in the table above), although the contact pressure becomes very low as the coating wears, fairly pronounced galling damage is observed. It is a known property of stainless steels and Inconel alloys to have a strong propensity for galling. It is observed that a coating with approximately 5 at.% nitrogen still exhibits a tendency to galling, but this disappears for a coating with approximately 8-9 at.% nitrogen.

[0118] For this reason (resistance to galling), a coating having a nitrogen content of less than 9 at.% is considered here to be non-compliant.

[0119] Fig. 5 then illustrates adhesion characterizations of these coatings, which also gives indications of the brittle behavior and resistance to cracking of the materials used for the coating.

[0120] Considering a criterion of resistance to galling, a nitrogen content of 8-9% and above is very satisfactory, but if this criterion is combined with resistance to cracking, a content of 8-9% exhibits brittleness, as evidenced by the long cracks visible in [Fig. 5]. It may therefore be advantageous to have a nitrogen content of at least 12 at.%. A content between 12 at.% and 16 at.% shows short cracks, which is a characteristic of hard materials (which generally tend to crack).

[0121] 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.

[0122] Hardness and modulus of elasticity characterizations are carried out by instrumented microindentation under 30 mN.

[0123] The hardness values ​​result from the average of ten measurements.

[0124] After characterization, the samples are returned to the oven for a new isothermal temperature 100°C higher than the previous one.

[0125] The isotherms obtained are therefore 400°C, 500°C, 600°C, 700°C and 800°C.

[0126] Fig. 6 shows the Vickers Hv hardness measurement results as a function of the isotherm temperature that preceded the hardness measurement.

[0127] This figure shows that the trends in the evolution of the hardness curves of coatings containing different nitrogen contents undergo similar variations.

[0128] The hardnesses of the coatings immediately after deposition are reported for a temperature of 20°C.

[0129] After the first isotherm at 400°C, the mechanical properties of the coatings can be considered unchanged.

[0130] The small variations are probably due to the experimental dispersion of hardness measurements estimated at + / - 15%.

[0131] A first notable drop in hardness occurs after the 500°C isotherm.

[0132] However, coatings containing 12 at.% and 15 at.% of nitrogen retain a hardness between 900 and 1000 kg.mm2.

[0133] Hardness measurements appear to be stable for isotherms from 500°C to 700°C.

[0134] The coatings richest in nitrogen retain a hardness of the order of 800 kg.mm2.

[0135] Finally, all coatings undergo a notable drop in hardness after the isotherm at 800°C.

[0136] Fig. 7 then shows measurements of the modulus of elasticity as a function of temperature.

[0137] These measurements corroborate the variations in properties observed on the hardness measurements ([Fig.6]), namely that the coatings undergo a slight modification of properties after heating to 500°C, regardless of the nitrogen content, including in the absence of nitrogen.

[0138] These properties are maintained up to and including approximately 700°C.

[0139] The properties seem to start to change sharply after the 800°C isotherm.

[0140] After each isotherm, EDX analyses of the coatings are carried out under 10 keV.

[0141] 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 an oxide layer alone 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 and a greater or lesser proportion of the underlying metal depending on the oxide thickness.

[0142] Fig. 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).

[0143] After deposition, the oxygen of a passivation layer is not detectable.

[0144] The passivation layer is an oxide layer that forms spontaneously on a metal in contact with air (or water) when the coating obtained under vacuum is exposed to air (or water) to be removed from the machine. It thickens over time and all the more rapidly as the temperature is high.

[0145] After 24h at 400°C, the coatings show an oxygen content of around 5 at.%. The coatings then exhibit a yellow colour, related to optical interferences produced by the thin oxide layer.

[0146] This content indicates that the EDX analysis incorporates the thin oxide layer as well as the underlying material for the bulk of the analysis.

[0147] After 24 hours of isotherm at 500°C, the increase in oxygen content indicates that the oxide has thickened, but that, with regard to the level between 10% and 15%, the oxide remains very thin in view of the depth of analysis under 10 keV.

[0148] The layers then exhibit a blue interference coloration.

[0149] After the 24h isotherms at 600°C and 24h at 700°C, two populations of coatings can be distinguished: Coatings with at least 10 at.% nitrogen show 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 oxygen level of 15 at.% also indicates that the oxide thickness remains less than the EDX analysis depth. 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.

[0150] Finally, after an additional 24 hours at 800°C, the oxygen content of all the coatings increased significantly, 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.

[0151] 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 from about 10 at.%. However, the improvement in oxidation resistance by nitrogen doping of the coating was unexpected and counterintuitive.

[0152] 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 runs rampant from 500°C or 600°C.

[0153] The thickness of the oxides was evaluated by the calotest method.

[0154] The oxide thicknesses are estimated to be between 0.3 pm and 0.4 pm after a stay of 24h at the different temperatures of 400°C to 800°C, which remains very low in absolute terms (as demonstrated by [Fig. 14] described later for example).

[0155] Despite the poor thickness measurement accuracy of this technique, which is below one micrometer, it can be concluded that the hardness measurements taken after the 800°C isotherm incorporate some of the oxide, since the indented depths are on the order of 0.5 µm. It can also be noted that the indented depths are much smaller than 10% of the coating thickness, which is approximately 9 µm.

[0156] Part of the significant changes in mechanical properties at 800°C could be induced by inter-diffusion phenomena of the coating with the substrate.

[0157] Indeed, from 800°C onwards, the phenomena of diffusion of metallic elements begin to be significant.

[0158] Fig. 9 represents the evolution of the oxygen content as a function of temperature in degrees Celsius (°C) on the surface of coatings respectively of NiCr doped with 18 at.% nitrogen, of 25 / 21 stainless steel doped with 20 at.% nitrogen, and of Inconel 690 doped with 15 at.% nitrogen.

[0159] Although doped 25 / 21 stainless steel seems to meet the criteria of interest for coatings in terms of hardness, or improvement of resistance to seizing, 24h isotherms in air show that the resistance to oxidation of doped 25 / 21 stainless steel is lower than that of doped Ni / Cr 60 / 40 alloy, or even doped Inconel 690.

[0160] According to [Fig.9], doped stainless steel exhibits massive oxidation from 500°C.

[0161] Changes in oxygen content as a function of isotherm temperature, Inconel 690 with 15 at.% nitrogen and NiCr 60 / 40 alloy with 18 at.% nitrogen are similar.

[0162] It then 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 stainless steel 25 / 21 is harmful.

[0163] 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).

[0164] Although the nitrogen-doped stainless steel deposit here has a chromium content similar to that of the other coatings, the oxidation behavior is different.

[0165] This difference appears to be due to the replacement of a high iron content by a high nickel content which benefits the resistance to oxidation, itself appearing to be further improved by nitrogen doping.

[0166] Hardness measurements after isotherms, as illustrated [Fig. 16], show that the hardness evolutions as a function of temperature are similar between Inconel 690 at 15 at.% nitrogen and those obtained for the NiCr 60 / 40 alloy at 18 at.% nitrogen.

[0167] However, for doped stainless steel, an increase in hardness appears from 500°C, which seems to reflect massive oxidation of the deposit. It would then no longer be the hardness of the deposit that is measured, but that of the oxide layer.

[0168] Hardness measurement is no longer possible after the 800°C isotherm, because the oxide becomes rough and crumbles.

[0169] 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.

[0170] Figures 10 and 11 illustrate these inter-diffusion phenomena, through micrographic sections, SEM (scanning electron microscopy) imaging ([Fig. 10]) and chemical element mapping, respectively of chromium (Cr), iron (Fe), carbon (C) and nitrogen (N) ([Fig. 11], a) to d) respectively).

[0171] SEM images were obtained under 20 keV of an inconel coating with 15 at.% nitrogen after all successive 24h isotherms, up to and including 800°C.

[0172] As a reminder, the coatings are deposited on an AISI M2 tool steel substrate.

[0173] In this example, [Fig. 10] shows clear signs of inter-diffusion between the coating and the substrate, as the interface has become diffuse and precipitates are observed there.

[0174] On [Fig. 11], the EDX maps show the presence of substrate elements in the coating, and vice versa.

[0175] Interdiffusion can be an undesirable phenomenon for very high temperature applications (i.e., here at least 500°C, or even at least 600°C). Among the disadvantages, one can cite, for example, that interdiffusion depends on the nature of the substrate, that it can in some cases cause the precipitation of brittle intermetallics, or even the formation of low-melting-point eutectics.

[0176] An effective remedy consists of interposing a layer acting as a diffusion barrier.

[0177] In the following example, a first layer of zirconium nitride (ZrN), here referred to as "underlayer", is deposited before the coating.

[0178] Zirconium nitride is one of the most thermodynamically stable nitrides, which avoids, for example, its reduction by other metallic elements, for example, of the substrate or coating.

[0179] Even though the oxidation resistance of ZrN is low, this material as an underlayer of a nitrogen-doped coating becomes protected against oxidation.

[0180] Figure 12 shows a test cap made through a stack according to an embodiment of the invention, comprising an Inconel coating doped at 15 at% nitrogen, then a 1.7 pm thick ZrN underlayer, then an M2 tool steel substrate.

[0181] After 24h at 800°C in air, the sample is cut and passed in SEM to show the barrier effect provided by the ZrN layer.

[0182] Fig. 13 shows a cross-sectional SEM image, before and after oxidation (respectively figures 13a and 13b).

[0183] Fig. 13 a), at left, shows, at top, the inconel coating doped with 15 at.% nitrogen as it comes out of the deposition machine.

[0184] The coating then has a thickness of approximately 7.8 pm. The sample then exhibits a lighter-colored ZrN sublayer with a thickness of approximately 1.7 pm, and below this, the M2 tool steel substrate. The white grains in the steel correspond here to tungsten carbide precipitates.

[0185] Fig. 13 b), on the right, shows the same coating after exposure to air at 800°C for 24h.

[0186] Dark CrN precipitates are observed in the nitrogen-doped Inconel coating.

[0187] On the free surface of the coating, a very thin layer of oxide, approximately 0.3 pm-0.4 pm, is identified, which is distinguishable in places as a dark line.

[0188] The ZrN sublayer appears unchanged, which demonstrates its effectiveness as a diffusion barrier.

[0189] In the substrate, secondary precipitation of tungsten carbides, induced by the heat treatment, is visible.

[0190] Fig. 14 represents element maps to demonstrate the effectiveness of the ZrN diffusion barrier.

[0191] Fig. 14 (respectively Fig. 14 a) to e)) shows, from left to right, the mapping of chromium (Cr), iron (Fe), carbon (C), nitrogen (N), and zirconium (Zr).

[0192] 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 hand.

[0193] This barrier therefore makes it possible to limit, or even avoid, risks of eutectic formation between the nitrogen-doped Inconel and the coated substrate, or also risks of formation of potentially brittle intermetallic phases.

[0194] In the example presented here, the formation of chromium carbides by inter-diffusion of the coating and the substrate is no longer observed, in the absence of a barrier (underlayer).

[0195] Finally, [Fig. 15] shows a small-dimension cap test made on the coating after oxidation test.

[0196] This test allows the thickness of the oxide layer to be estimated at approximately 0.4 pm.

[0197] Thus, thanks to a process according to an example of an 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.%.

[0198] Such a homogeneous nitrogen content in the coating is not achievable with standard prior art processes.

[0199] 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 a chamber, 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 into the chamber; then - A step of evacuating the chamber and degassing the chamber under vacuum; then - A step of ion-etching 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 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) content 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 wherein 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 less than or equal to 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, prior to 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 pm.

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