Carbon deposition process on a substrate

Abstract

The invention relates to a method for depositing a carbon-based material from a target onto a metallic substrate by sputtering, in the presence of ion assistance. According to the invention, the ratio between the flux of ions directed towards the substrate and the flux of neutral carbon atoms directed towards the substrate is adjusted between 1.7 and 3.5, and a bias voltage of between -35 V and -100 V is applied to the substrate. Figure for the abstract: Fig. 3

Description

Title of the invention: Method for depositing carbon onto a substrate technical field

[0001] The invention relates to the technical field of vacuum surface treatment, and in particular to the physical deposition of carbon in vapor phase on a substrate. Previous art

[0002] The invention relates to electrochemical systems such as fuel cells and batteries, and in particular to proton exchange membrane fuel cells, or "PEMFC" according to the English acronym for "proton exchange membrane fuel cells".

[0003] The operation of these electrochemical systems involves an acidic or basic environment, oxidizing at the cathode, a temperature ranging from 60 to 160 °C, and the possible presence of halides. This environment promotes corrosion of the system's components, such as the interconnecting plates, also referred to as electrodes, interconnectors, or bipolar or monopolar plates.

[0004] In particular, the bipolar plates are critical components for the durability of these systems: they are made of metal sheets approximately 100 µm thick. They must be protected by a coating to maintain good electrical conductivity on the surface and to prevent corrosion of the metal sheets in the aggressive environment of the fuel cell.

[0005] The surface conduction of a bipolar plate made of metallic material, including in corrosive environments, is generally achieved by depositing a carbon- or gold-based functional layer on the outermost surface of a substrate. The prior deposition of an underlayer on the substrate can improve the adhesion of the functional layer and ensure good mechanical strength of the stack.

[0006] In general, the adhesion between the layer and the substrate, as well as the mechanical strength of the functional layer, which results in an absence of damage by cracking or delamination, are important parameters.

[0007] Above all, the barrier function of this layer must not degrade and the functional layer must remain hermetic to reactive species present in the medium (for example O2, H+, halides) over the duration of operation of the electrochemical system, in order to protect the metallic substrate from oxidation, in order to prevent the emission into the medium of the cell of metallic cations from the substrate, even in small quantities.

[0008] In the case of use for powering an electric vehicle, the battery must present A long lifespan, on the order of 10,000 hours, while maintaining acceptable performance. During such extended use, accidental phenomena can occur, such as reagent depletion or localized drying or flooding of the fuel cell. These accidental phenomena can lead to local and transient increases in temperature, potential, or current density. Furthermore, independently of accidental phenomena, the start-up and shutdown conditions of the electrochemical system, which are generally transient, can lead to overpotentials at the electrodes.

[0009] These local and generally temporary variations in the conditions of the electrochemical system put increased stress on the functional layer, within which defects such as gaps, cracks, holes, intercolumnar spaces, can cause rapid degradation of the substrate, in particular by galvanic coupling with the functional layer.

[0010] Furthermore, when the cell membrane is made of fluoropolymer, it can release F- ions which, among other things, promote pitting corrosion of stainless steel substrates. This can then lead to a rapid and catastrophic failure of the entire cell.

[0011] In order to protect a battery from such failures, while meeting the objectives of mechanical strength and electrical conductivity described above, it is known in the prior art to deposit a functional layer, in particular carbon-based, by providing additional energy during deposition.

[0012] Document WO2020019693Al describes the deposition of a functional layer based Carbon deposition involves heating the substrate to a high temperature, between 400 and 500 °C, primarily to densify the deposited layer. These high temperatures can generate significant residual stresses in the stack after processing. This can be detrimental to the coating's adhesion, and particularly its resistance to deformation if the plate is formed after deposition.

[0013] Other deposition methods exist, but have the following disadvantages: - Arc evaporation deposition technologies generate growth defects in the layer in the form of droplets, which are detrimental to the durability of the deposit over long operating times, especially when the layer is thick, i.e. on the order of 100 nm or more. - High-power pulse magnetron sputtering or filtered arc deposition technologies, known as "HIPIMS" (High-power impulse magnetron sputtering), involve low deposition rates. - Magnetron sputtering deposition technologies in unbalanced configurations, possibly in closed fields, can hardly be used for efficient industrial-scale production of large numbers of parts. Ion beam bombardment-assisted deposition cannot efficiently bombard large parts, as the ion beam must scan the entire surface of the part. The flux density is therefore insufficient and incompatible with the high deposition rates required for industrial productivity.

[0014] The processes described in the prior art do not allow obtaining an electrochemical system whose performance associated with efficiency remains sufficiently high over a long period of life of such a system, and particularly for electric vehicles.

[0015] One object of the invention is therefore to overcome the disadvantages of the prior art described above.

[0016] The invention aims in particular to provide a method for depositing a material comprising carbon on a substrate, making it possible to form a layer of carbon material ensuring good coverage of the substrate, exhibiting few defects and therefore few mechanical weaknesses, while exhibiting good electrical conductivity.

[0017] The invention also aims to provide such a process, which is efficient and inexpensive to implement, taking into account the improved properties of the deposited layer described that the process aims to obtain. Description of the invention

[0018] To this end, a method for depositing a carbon-based material from a target onto a metallic substrate using cathodic sputtering has been developed with ionic assistance.

[0019] According to the invention, the ratio between the ion flux directed towards the substrate and the neutral carbon atom flux directed towards the substrate is adjusted between 1.7 and 3.5, and a bias voltage of between -35 V and -100 V is applied to the substrate.

[0020] Adjusting the ratio between the ion flux and the neutral carbon atom flux between 1.7 and 3.5, and the substrate bias voltage between -35 volts and -100 volts—which amounts to adjusting the energy of the ionic assistance ions between 35 eV and 100 eV (electronvolts)—makes it possible to obtain a carbon-based layer with good adhesion, ensuring good substrate coverage, while exhibiting few defects compared to layers deposited using known processes. This reduction in defects within the layer, particularly cracks or delaminated regions, results in a dense layer that optimally protects the underlying substrate, thus preventing the risk of contamination of the electrochemical environment and improving the lifespan of the electrochemical system.

[0021] This combination of specific values ​​of the ratio between the ion flux and the flux of neutral carbon atoms, and the bias voltage of the substrate, leads to a carbon-based layer which also exhibits good electrical conductivity as well as intrinsic corrosion resistance.

[0022] These optimized properties are essential particularly in the context of the manufacture of a monopolar or bipolar plate comprising a metallic substrate, especially within a battery, which is also an object of the present invention.

[0023] In addition to the properties described above, and as a consequence thereof, the part obtained by the process of the invention, which has an external surface comprising the metallic substrate coated with a layer of the carbon-based material, is principally characterized in that the layer of carbon-based material comprises less than 1 at.% oxygen. This oxygen content is a ratio of the number of oxygen atoms to the number of carbon atoms within the layer of carbon-based material. Such a part, obtainable by the process of the invention as described above, constitutes another object of the invention.

[0024] This oxygen ratio of less than 1 at.% oxygen indicates low contamination of the carbon-based layer by oxygen. This ratio is characteristic of the invention. Indeed, the correct implementation of the invention makes it possible to obtain this ratio and thereby overcome the recurring difficulty of removing oxygen from carbon-based deposits, since oxygen can come from residual voids or from the carbon target, which is always porous (porosity can reach 10% of the target volume).

[0025] It is specified that the layer of carbon-based material is preferably deposited by magnetron sputtering in continuous regime, as opposed to a pulsed regime.

[0026] It is recalled that ion assistance is characterized by the quantity of ions directed towards the growing material layer, as well as by the energy of these ions. The ion flow is directed towards the substrate, and the polarization of the latter accelerates the ion flow. These interactions between the ions and the substrate take place in close proximity to the substrate.

[0027] The ions that bombard the growing layer come from the magnetron cathode (for example in the case of an unbalanced magneton sputtering), and when a complementary plasma source is present, from the magnetron cathode and said complementary plasma source.

[0028] The ion flux therefore includes ions from the gaseous mixture in which the plasma is generated, such as argon ions, for example, and possibly ions from the target. Regardless of the nature of the ions, they bombard the growing layer, thereby densifying it.

[0029] Ion assistance is not necessarily simultaneous with cathodic sputtering. They can operate alternately, so that: - the substrate receives an initial quantity of carbon-based material by spraying, - then ionic assistance is implemented to densify the deposited material.

[0030] The substrate thus passes in front of the carbon source and then in front of the ion source, repeatedly.

[0031] The alternating sequences are chosen according to the sputtering method and the design of the installation implementing the deposition process. In practice, the sputtering and ion-assisted systems can operate continuously, while the part to be coated passes successively in front of said systems. It is evident that when ion-assisted coating is implemented, the associated ion flux is always greater than zero; that is, the ion flux is non-zero, otherwise the ion-assisted coating could not fulfill its function.

[0032] The flow of neutral carbon atoms is directed from the target towards the substrate. It mainly comprises carbon atoms, constituting the layer of material to be deposited, coming from the target.

[0033] The ion flux and neutral carbon atom flux values ​​are temporal and spatial averages, calculated from measurements. Indeed, it is understood that in practice the substrates to be coated are mobile within the installation, while the magnetron cathode and the plasma source are fixed. The substrates do not receive the same quantities of ions and carbon atoms depending on their position at a given time.

[0034] The substrate bias voltage, or more simply the substrate bias, is defined as the potential difference applied between the substrates and the ground of the device implementing the method. This bias can be continuous or pulsed. In the latter case, the bias voltage is the average value of the voltage applied to the substrates. The bias current is the (average) current measured across the biased substrate.

[0035] The (kinetic) energy of the ions is imparted to them by acceleration in the electric field surrounding the substrates. It is related to the bias voltage and is calculated by multiplying the absolute value of the potential difference between the substrate and the plasma by the electric charge of the particle or species in question. In general, the plasma potential relative to the mass is considered negligible compared to the potential difference between the mass and the components. This is equivalent to considering that the energy of the single-charged ions in eV corresponds to the voltage delivered by the bias generator in volts.

[0036] In order to simplify the implementation of the process and in particular the taking of measurements or the evaluation of quantities, the following characteristics may be taken alone or according to their technically possible combinations: - the ion flux is determined from the polarization current of the substrate, and the neutral carbon atom flux is determined from the deposition rate of the material on the metallic substrate; - the (single-charged) ion flux can be determined by dividing the polarization current by the surface area of ​​the substrates exposed to the plasma, in order to obtain an average polarization current density, and then dividing said polarization current density by the elementary charge. - the flux of neutral carbon atoms is determined by multiplying the deposition rate of the material on the metallic substrate by the density of the material, and then dividing by the molar mass of the material, and then multiplying the result obtained by Avogadro's number.

[0037] In order to further improve the properties of the deposited carbon-based layer, particularly with regard to mechanical strength, electrical conductivity, and corrosion resistance, the ratio between the flow of gaseous ions and the flow of neutral carbon atoms is preferably between 2 and 3.1.

[0038] In these cases, a bias voltage between -50 V and -75 V may preferably be chosen for the purpose of optimizing the process.

[0039] In this case, it is also possible and preferably, for reasons of process optimization, to carry out the deposition in an enclosure under a controlled atmosphere and whose working pressure is between 1 x 10⁶ bar and 4 x 10⁶ bar, preferably between 2.0 x 10⁶ bar and 2.6 x 10⁶ bar.

[0040] In order for the protection of the substrate to be sufficient in the event of operating incidents or local variations in operating parameters, in particular in the event of overpotentials linked to transient or accidental operating regimes of the electrochemical system, the material deposited on the substrate forms a layer called a thin film, having a thickness greater than or equal to 20 nm, preferably between 20 nm and 500 nm, more preferably between 50 nm and 250 nm, even more preferably between 80 nm and 150 nm, and even more preferably between 80 nm and 120 nm.

[0041] In a particular embodiment adapted to the field of fuel cells, the substrate comprises stainless steel, titanium, a titanium alloy, or a nickel-chromium-iron alloy, which is preferably an Inconel®.

[0042] Preferably, the substrate is a plate with a thickness between 10 pm and 1000 pm.

[0043] In a first embodiment, the ion flux (of the ion assistance) is generated by a magnetron cathode, for example when the process consists of an unbalanced magneton sputtering.

[0044] In a second embodiment, the ion flow is generated by a system com- complementary to the magnetron cathode, preferably by microwave plasma.

[0045] For the purpose of productivity and rationalization of equipment, and optionally, the substrate passes within an installation in front of a magnetron sputtering station and then in front of a plasma generation station, preferably cyclically.

[0046] In order to improve the adhesion of the carbon-based material deposited on the substrate, and to protect the substrate from possible oxidation, the process may include a preliminary step of depositing on the substrate a metallic underlayer intended to be between the substrate and the carbon-based material, in contact with said substrate, the material of the metallic underlayer being chosen from one or more of the following materials: chromium, titanium, zirconium, tantalum, or their alloys as well as their nitrides and carbides, and preferably titanium or tantalum, or their alloys (alloys comprising titanium and / or tantalum) as well as their nitrides and carbides.

[0047] In order to have a favorable compromise between the deposition time of the metallic undercoat and the improvement in adhesion that it confers, its thickness is between 5 nm and 100 nm, preferably between 20 nm and 40 nm.

[0048] In order to improve the adhesion of the carbon-based material deposited on the substrate, and to improve corrosion resistance, the process may include a preliminary step of depositing on the substrate a carbon-based underlayer intended to be between the substrate and the carbon-based material described above, said underlayer being in contact with said carbon-based material.

[0049] The carbon-based sub-layer is preferably made of the same material as the overlying carbon-based layer. Choosing carbon as the sub-layer material allows only one sputtering target to be used within the magnetron, thus simplifying the implementation of the process.

[0050] To deposit the carbon-based sublayer, the ratio between the ion flux directed towards the substrate and the neutral carbon atom flux directed towards the substrate is adjusted to a value less than 1, and preferably less than 0.5, the ion flux being non-zero. The bias voltage applied to the substrate is between -35 V and -100 V, preferably -50 V and -75 V.

[0051] In order to have a favorable compromise between the deposition time of the carbon-based undercoat and the improvement in adhesion that it confers, its thickness is between 2 nm and 40 nm, preferably between 10 nm and 30 nm.

[0052] Implementing the process according to the aforementioned characteristics and comprising a step of depositing a carbon-based material from a target onto a metallic substrate by magnetron sputtering thus enables the functionalization of a monopolar or bipolar plate comprising said metallic substrate coated with a layer comprising said carbon-based material, for example by providing protection durable against corrosion of a bipolar plate while maintaining a high level of surface electrical conductivity over time.

[0053] The invention therefore also relates to a method for manufacturing a monopolar or bipolar plate comprising a metallic substrate coated with a layer comprising a carbon-based material. This method includes a step of depositing said carbon-based material from a target onto said metallic substrate, by magnetron sputtering, according to the deposition method described above.

[0054] The invention further relates to a part obtainable by a process of depositing a carbon-based material from a target onto a metallic substrate by sputtering in the presence of ion assistance, as described above. Said part has an external surface comprising said metallic substrate coated with a layer of the carbon-based material. The layer of carbon-based material comprises less than 1 at.% oxygen, calculated as the number of oxygen atoms relative to the number of carbon atoms within the layer of carbon-based material. Brief description of the drawings

[0055] [Fig-1] is a schematic representation, top view, of an installation for the implementation of the process according to the invention.

[0056] [Fig.2] is a schematic representation, top view, of another installation for implementing the process according to the invention.

[0057] [Fig.3] is a graph illustrating the corrosion current density of carbon-based material layers deposited on substrates obtained during several series of tests, as a function of the ratio between the ion flux and the neutral carbon atom flux during their deposition.

[0058] [Fig.4] is a micrograph illustrating the exposure after corrosion test of a substrate coated with a deposit not in accordance with the invention.

[0059] [Fig.5] is a cyclic voltammetry graph in a chlorinated environment carried out on layers of carbon-based material deposited on stainless steel substrates as a function of the ratio between the ion flux and the neutral carbon atom flux, obtained during several series of tests.

[0060] [Fig.6] is a detailed view of the graph of [Fig.5].

[0061] [Fig.7] is a graph illustrating the interfacial contact resistance of 100nm layers of carbon-based material deposited on substrates as a function of the ratio between the ion flux and the neutral carbon atom flux, as well as the corrosion current density, obtained during several series of tests.

[0062] [Fig.8] is an observation of a section obtained by electron scanning microscopy of a substrate treated according to the process of the invention.

[0063] [Fig.9] is a graph illustrating the evolution of the corrosion current density of carbon-based material layers deposited on substrates as a function of time, obtained during several series of tests.

[0064] [Fig. 10] is a graph illustrating the oxygen content (measured by nuclear reaction analysis, or "NRA") of carbon-based material layers obtained during several tests, deposited on substrates as a function of the ratio between the ion flux and the neutral carbon atom flux.

[0065] [Fig. 11] is a photograph of a monopolar plate.

[0066] [Fig. 12] is a partial representation of a cross-section of such a plate. Detailed description of the invention

[0067] In the field of surface treatment, several types of technologies exist, each with its own advantages and disadvantages. In the context of processing parts, and in particular monopolar or bipolar plates for fuel cells, the Applicant sought to optimize known deposition processes.

[0068] Based on the known and industrializable technology of magnetron sputtering deposition with ion assistance, the Applicant carried out various series of tests and interpretations aimed at obtaining a deposit of a carbon-based material (M), forming a layer on a substrate (S), and exhibiting good properties in particular of mechanical strength, corrosion resistance, adhesion, and electrical conductivity.

[0069] With reference to Figures 1 and 2, the installation (1) used to implement a preferred embodiment of the process comprises a secondary vacuum chamber (10), equipped with a pumping system (20), a conventional magnetron sputtering source (30) (balanced or unbalanced), a complementary plasma source (40) generating a plasma (P) of gaseous ions, and a substrate holder (50) on which the substrate(s) (S) to be treated are mounted.

[0070] The pumping system (20) makes it possible to obtain a secondary vacuum in the chamber (10), that is to say, a pressure on the order of magnitude between 10⁸ mbar and 10³ mbar. The pumping system (20), or another independent system, is capable of introducing a gas (a noble gas) into the vacuum chamber (10). The gas is intended to be ionized; it is preferably argon.

[0071] The magnetron sputtering source (30) is a conventional magnetron (30) powered in continuous mode. In this mode, the flux ions (q>i) are generated by a plasma source (40) complementary to the magnetron cathode (30). The plasma source (40) is of any suitable type, but the plasma (P) is preferably generated by microwaves.

[0072] In other embodiments, the ions of the flux ( <pi) sont générés par la cathode magnétron (30), notamment dans les cas où le magnétron est déséquilibré. Un magnétron déséquilibré possède une structure magnétique déséquilibrée, qui permet d’envoyer une partie des ions générés par le plasma de la cathode vers les pièces.

[0073] The plasma source (40) is therefore optional and its presence depends on the type of magnetron sputtering implemented and the quantity of ions available to generate an ion flux ( <pi) suffisant.

[0074] In all cases, it is possible to add several magnetron cathodes (30) to deposit the material (M) on the substrate (S) more quickly, in which case each cathode is powered by its own generator.

[0075] The substrate holder (50) is polarized, that is, a negative voltage or potential difference is applied across its terminals, in order to accelerate the gaseous ions of the plasma and thus create an ion flow (q>i) towards the substrate holder (50). This acceleration of the gaseous ions occurs in the vicinity of the substrates (S), since the electric field resulting from the polarization of the parts extends over a short distance, on the order of 1 mm to 3 mm.

[0076] Regardless of the sputtering method used, ions are attracted to the magnetron's target material (M) in order to sputter it and emit the atoms that form the deposit on the substrate (S). It is not these ions that are of interest to the Applicant in the present invention. Indeed, it is the ions attracted to the substrate (S) where the material (M) deposit grows that define the ionic assistance and are important for the quality of the deposited layer. In the context of this application, the ions are preferably gaseous species such as argon.

[0077] The role of these ions is to bombard the growing material (M) deposit on the substrate to compact it and eliminate species that do not form sufficiently stable bonds with the atoms of the material. This increases the density of the growing material (M) layer and eliminates oxygen from said growing material (M) layer. However, care must be taken not to eject the material (M) already deposited on the substrate (S), so as not to slow down the deposition or degrade the quality of the ongoing deposition.

[0078] In general, the ions in plasmas from magnetron cathodes or auxiliary plasma sources such as microwave plasma are "slow." They therefore have no power to compact a growing layer of material (M) or to remove oxygen from that layer. Thus, as previously stated, a negative voltage is applied to the substrates (S) to be coated, which attracts and accelerates the positive ions towards said substrates (S). The bias voltage is between -35V and -100V, and preferably between -50V and -75V.

[0079] In the case of the polarization of a substrate (S) in a plasma (P), the voltage of po A magnetic field is applied between the substrates (S) and the mass of the installation (1). A potential difference is established between the substrates (S) and the plasma (P). It is in this potential drop zone, over approximately 1 to 3 mm of the surface of the substrates (S), that the ions are accelerated.

[0080] The kinetic energy of the ions can be approximated by the potential difference between the plasma (P) and the substrates (S). In most plasmas, the plasma potential is not known, but it is generally a few volts, for example +5V to +10V. In practice, the plasma potential (P) is considered to be 0V when the voltage applied to the substrates (S) reaches a few tens of volts in absolute value.

[0081] This approximation is valid at low pressure, because the ions are not slowed down by collisions in the acceleration phase near the substrates (S).

[0082] Since the acceleration of these ions is proportional to their charge and the potential difference, the bias voltage is approximated by the energy imparted to the ions during deposition, by multiplying this bias voltage by the charge of an electron. Indeed, in the technical field considered, the ions are generally single-charged.

[0083] In the installation (1) illustrated [Fig. 1], the substrate carrier (50) is of the trolley type, i.e., it includes a linear actuator to move a substrate (S) in translation and alternately in front of the magnetron (30) to receive material (M), and then in a position (S') in front of the plasma source (40) so that the impacts of gas ions compact the deposited layer of material (M). In this case, the installation is arranged lengthwise.

[0084] In the installation (1) illustrated [Fig. 2], the substrate carrier (50) is of the carousel type, that is to say, it comprises a platform (51) on which one or more substrates (S) are arranged, and this platform (51) is driven in a rotation (rl). In this way, each substrate (S) passes alternately in front of a magnetron sputtering station and then in front of a plasma generation station (P).

[0085] Depending on the exact implementation and the size of the substrates, additional rotations can of course be superimposed on the rotation (rl) of the plate.

[0086] In each of these embodiments, it is advantageous to arrange several magnetron cathodes (30) alternately with several plasma sources (40). In this way, the movement of the substrate (S) is continuous, and it passes alternately in front of a magnetron sputtering station and then in front of a plasma generation station (P). Adding magnetron cathodes (30) alternately with plasma sources (40) increases the productivity of the installation (1).

[0087] In all cases, the substrate carrier (50) can be of any type suitable depending on the substrates (S) to be treated or the construction of the installation (1), the latter can also be arranged vertically or horizontally or adapted in shape and dimensions.

[0088] In order to be able to evaluate the performance of the layer of material (M) deposited on the substrate (S), the following measurements are carried out.

[0089] The lifespan of the deposited material layer is evaluated by subjecting it to a corrosion test.

[0090] The electrochemical tests are carried out in an acidic solution with a pH of 3 (H₂SO₄), at 80°C and with 0.1 ppm of fluoride ions. These parameters are defined by the DOE (Department of Energy) in the United States of America to simulate the operating environment of a PEMFC. The potential is set at +0.8 V on the working electrode on which the material to be tested is mounted, relative to the Ag / AgCl reference electrode. The addition of air bubbling allows for the simulation of the cathode compartment of a fuel cell.

[0091] The corrosion current is a measure of the rate of degradation of a part comprising a substrate (S) coated with a layer of material (M). Indeed, the higher the corrosion current, the more the part is oxidizing, meaning that the layer of material (M) is not fulfilling its protective role effectively. In practice, a corrosion current density of less than 300 nA / cm² after 24 hours at a potential of 0.8 V is considered acceptable.

[0092] The surface conductivity of the coating is evaluated by measuring its interfacial contact resistance, or "ICR". A coating with good surface conductivity has a low ICR, for example less than 10 mQ.cm2.

[0093] The RCI measurement is carried out on a stack composed of a Copper block - Carbon Foil (GDL, acronym for "Gas diffusion layer" in English) - Deposition on substrate - Nickel paint (back face of the substrate) - Copper block, on which a current of 100 mA is applied for an area of ​​1 cm2, then the resistance of the assembly is calculated from the measured voltage.

[0094] This stack is representative of the coated bipolar plate / GDL contact. A pressure of 138N / cm2 is applied to it by a lever arm system with weights, this pressure being representative of that applied to an electrochemical cell during its assembly.

[0095] The total resistance Rtotai obtained is the sum of (equation 1): • The resistance of the Cu-Cu system (ROffset) • Once the interfacial contact resistance of Copper Carbon RCu / c • Once the resistance of the RC Carbon felt is zero • The resistance of the 316L R316L steel insert (zero) • The linear resistance of the deposit Rdeposit • Interfacial contact resistance between the deposit and the RC / deposit carbon • Once the interfacial contact resistance Nickel Copper RNi / Cu

[0096] [Math.l] htotal ^-offset ^Cu / C + ^C / depot + ^depot"^ (1)

[0097] The RCI is determined using equation (2).

[0098] [Math.2] ^C / deposit + ^deposit — ^total ^offset ^Cu / C (2)

[0099] The RCI can be measured before or after a corrosion test, in which case the latter simulates accelerated aging of the treated parts.

[0100] If necessary, it is also possible to make sections of the samples, for example by focused ion beam (FIB), in order to observe the morphology of the deposited layer. It is also possible to perform other chemical characterization tests of the deposited layer, for example by NRA.

[0101] The principle of the NRA method is based on the study of nuclear reactions between the nuclei of a high-energy incident ion flux and the atoms of the target at rest. The sample is placed in the analysis chamber under a vacuum of 2E6 torrs, or 3 x 10⁹ bar, with the area to be studied facing the incident particle beam. This beam consists of a ²H⁺ ion flux with an energy of 930 keV, and generates an incident current of 250 nA on the target for an analysis area of ​​a few mm². The backscattered particles resulting from the ¹⁶O(d,p)¹⁷O nuclear reaction are detected at 150° to the initial direction and, after processing by the acquisition chain, form spectra. The detector is shielded by a 1 µm thick sheet of Mylar. Comparison with a reference standard of alumina (O: 720E15at / cm2) makes it possible to determine, for a given total integrated charge, the amount of oxygen present in each sample.Knowing the amount of carbon present in the volume of material analyzed, based on the density of the material, the ratio between the atomic amount of oxygen and carbon can then be obtained.

[0102] Within the installation (1), several series of tests were carried out. The substrates (S) used are 316L stainless steel test specimens intended to be coated on both sides, in order to simulate the coating of bipolar plates.

[0103] The substrate (S) is positioned on a fixture, cleaned and blown to remove any contaminants and dust that may be present on its surface. It is then introduced into a vacuum deposition unit (1).

[0104] The pumping system (20) is activated so that the pressure in the enclosure (10) is less than 5x109 bar, and the enclosure (10) is heated to remove the water adsorbed on its walls.

[0105] The surfaces of the substrate (S) to be coated are heated and bombarded to remove the water adsorbed on the surfaces and to strip the layer of chromium oxide present in surface.

[0106] The pumping system (20) then introduces argon into the enclosure (10) so that there is an argon pressure of 2.5x106 bar.

[0107] The magnetron cathode (30) is supplied with a power of 3.2 kW in order to spray a carbon graphite target, and a potential of -55V with respect to the ground of the installation (1) is applied in pulsed mode to the substrate (S). The substrate (S) is thus coated for 5 minutes.

[0108] Then an auxiliary plasma source (40) is switched on to generate a sufficient ion flux (q>i). The plasma source (40) is maintained at a power of 500 W (flux ratio of 2.7), so that the current density on the substrate holder (50) reaches 2.5 A / m². The remainder of the deposition is carried out by alternating sputtering the target onto the magnetron cathode (30) and ion bombardment by the plasma source (40), over a total duration of 25 min.

[0109] With this first example, a 100nm layer of carbon is thus obtained on both faces of the substrate (S). The enclosure (10) is then returned to the air and the substrate (S) is recovered.

[0110] A first series of preliminary tests is carried out by modifying: - the deposition times, which provides a deposited layer of varying thickness; - the power of ionic assistance; and - the presence or absence of an underlayer (SC) to improve the adhesion of the material layer (M) to the substrate (S).

[0111] The power applied to the magnetron cathode (30), and therefore the deposition rate, remains constant. Test compliance is assessed by measuring the RCI and by the corrosion resistance of the deposited layers.

[0112] In order to use quantitative measurements and to enable scaling up of the process, the cathode power quantities of the magnetron (30) and the ion assist are translated: - in flow ( <pn) d’atomes neutres de carbone pour la puissance cathodique du magnétron (30) ; et - in flow ( <pi) d’ions pour l’assistance ionique.

[0113] In this case, the flow ( <pn) d’atomes neutres de carbone reçu par le substrat est déterminé à partir de la vitesse de dépôt de la couche considérée exprimée en cm / s, multipliée par la masse volumique de la couche de carbone (2,1 g-cm3), divisée par la masse molaire du carbone (12 g / mole) et puis multipliée par le nombre d’Avogadro, ce qui donne un nombre d’atomes de carbone par cm2 et par s.

[0114] The density of the deposited carbon was verified by electron energy loss spectroscopy, in order to validate that the carbon density data available in the literature did indeed correspond to the deposited carbon.

[0115] Calculating the flow ( <pn) d’atomes neutres de carbone est une moyenne : en divisant l’épaisseur du dépôt par la durée de dépôt on détermine une vitesse de dépôt moyenne, malgré le fait que le dépôt se forme uniquement pendant le passage des substrats (S) devant la cathode magnétron (30). Cependant, il y a bien toute la surface des substrats (S) qui est revêtue pendant la durée totale du dépôt, et c’est donc comme si toute la surface recevait en permanence le flux d’atomes neutres de carbone (<pn) ainsi calculé.

[0116] The calculation of the ion flux is carried out in a similar manner ( <pi) : le courant total de polarisation en A est divisé par la surface totale polarisée en cm2, ce qui donne une densité moyenne de courant sur les substrats (S) en A / cm2. En la divisant par la charge élémentaire, on obtient un flux d’ions par cm2 et par s.

[0117] Although the plasma (P) is localized at the plasma source (40) and the bombardment of the substrates (S) takes place near it, the total current collected by the substrates (S) is the same as in the case where the entire surface constantly receives an average ion bombardment, therefore an average current density.

[0118] The ratio between the ion flux ( <pi) et le flux (<pn) d’atomes neutres de carbone dirigés vers le substrat (S) est donc sans unité.

[0119] The results obtained are shown in the table below.

[0120] [Tables 1] Tests Metallic sublayer C layer thickness Power on the auxiliary plasma source Flux ratio <pi / <pn pendant le dépôt Performance Exemple 1 Non 100 nm 500 W 2,7 Excellente Exemple 2 Ti, 30nm 100 nm 500 W 2,7 Excellente Exemple 3 Non 100 nm ow 0,3 Insuffisante Exemple 4 Non 100 nm 1200 W 4,1 Insuffisante Exemple 5 Non 20 nm 500 W 2,7 Correcte Exemple 6 Ti, 30rtm 20 nm 500 W 2,7 Correcte Exemple 7 Gradient Ti / TiC, 30m 20 nm 500 W 2,7 Correcte Exemple 8 Non 100 nm 0 W, puis 500W après dépôt 0,3 Insuffisante Exemple 9 Ti, 30nm 100 nm 1200 W 4,1 Insuffisante

[0121] The performance is obtained by validating the criteria of good corrosion resistance, good mechanical strength, low RCI, and low oxygen content of the deposited layer, as described in detail below in relation to figures 3 to 10.

[0122] This table shows that the ratio between the ion flux (q>i) and the flux ( <pn) d’atomes neutres de carbone est un paramètre essentiel. La puissance, liée à ce rapport, ne doit évidemment pas être nulle. La présence d’une sous-couche métallique ainsi que l’épaisseur de la couche déposée sont des paramètres qu’il est possible d’ajuster pour optimize the mechanical and physical properties of said deposited layer.

[0123] With reference to [Fig.3], the Applicant carried out several series of deposition tests, varying the following parameters: - presence or absence of a metallic sub-layer (SC) between the material layer (M) and the substrate (S); - thickness of the material layer (M); - ratio between the ion flux ( <pi) et le flux (<pn) d’atomes neutres de carbone dirigés vers le substrat.

[0124] The corrosion current density is evaluated in order to make an initial selection among the results obtained. It should be noted that the corrosion current density illustrates the corrosion resistance of the deposited layer: a low corrosion current indicates good corrosion resistance in the test environment.

[0125] The corrosion current density is measured at the end of a 24h potentiostatic test at potential +0.8V / ref(Ag / AgCl).

[0126] The results of [Fig.3] show a dispersion of the points in each series, however the corrosion current density passes through a minimum towards a flux ratio range (<pi) / (q> n) between 2.2 and 3.1. There is therefore a preferred range for this flow ratio (<pi) / (q> n) To achieve good corrosion resistance: minimal bombardment is necessary to ensure good corrosion resistance of the layer, but excessive bombardment is also detrimental as it causes layer degradation and a significant increase in corrosion. This applies to different thicknesses of deposited carbon layers, whether or not a metallic underlayer (SC) is previously deposited.

[0127] With reference to [Fig. 4], scanning electron microscopy observations are made on a substrate (S) having received a titanium sublayer and then a 20nm thick carbon layer deposited by being subjected to excessive ion-assisted bombardment (flux ratio (<pi) / (q> n) equal to 4.4). These observations show that after a corrosion test at 0.8 V, coating defects appeared on the surface of the sample, where the carbon layer is no longer present and the titanium sublayer is exposed in the light-colored areas. Despite the presence of a titanium sublayer (SC), a 20 nm thick layer of carbon-based material (M) is not sufficient for the coated substrate (S) to exhibit optimal properties: the carbon-based material layer (M) must be deposited according to the criteria defined by the invention, namely that the flux ratio (q>i) / (q>n) must be adequate.

[0128] Another way to assess the quality of the coating and the protection provided by the deposit to the substrate is a corrosion test in a saline environment. The sample is immersed in a 35 g / L sodium chloride solution, similar to seawater, at a temperature The sample is kept at room temperature for 3 hours. A potential is applied to the sample from the equilibrium potential EO up to +0.8 V, then the potential decreases to -0.4 V before returning to EO (versus Ag / AgCl reference) at a sweep rate of 1 mV / s. The current is measured for 2 cycles.

[0129] Figures 5 and 6 illustrate voltammetry graphs of the corrosion current density measured in a saline environment. The sample is immersed in a 35 g / L NaCl solution (analogous to seawater) at room temperature. The potential applied to the sample is then cycled twice between -0.4 V and 0.8 V (versus an Ag / AgCl reference), and the current is measured.

[0130] The three samples tested are 316L substrates (S) on which a layer of carbon-based material (M) measuring 100nm in thickness has been deposited: - the deposit of the first sample does not correspond to the invention, with a flux ratio (q>i) / (q>n) of 0.3; - the deposition of the second sample corresponds to the invention, with a flux ratio (q>i) / (q>n) of 2.3; - the deposit of the third sample does not correspond to the invention, with a flux ratio (q>i) / (q>n) of 4.1.

[0131] In [Fig.5], we see that: - the first sample shows a moderate corrosion current; - the second sample corresponding to the invention shows a very low corrosion current; - The third sample shows a sudden surge in current when the potential exceeds 0.5V. This corresponds to pitting corrosion of stainless steel in a NaCl solution, a well-known phenomenon. At the end of the test, the stainless steel slab is pierced in several places.

[0132] In [Fig.6], which is a detailed view of [Fig.5], it can be seen that: - the first sample shows an increase in current, admittedly limited, but which also corresponds to the pitting of the stainless steel; - the second sample corresponding to the invention exhibits a very low current, since the anodic current is less than 1 pA / cm2.

[0133] It can therefore be deduced that carbon layers deposited with a flux ratio (q>i) / (q>n) that is too low or too high do not effectively protect the substrate (S) from corrosion, whereas the carbon layer deposited with a flux ratio (q>i) / (q>n) in the range corresponding to the invention protects the substrate (S) optimally.

[0134] In particular, these corrosion resistance tests show that an over-bombarded layer of carbon-based material (M) does not effectively protect the substrate (S) or the sub-layer (SC): local defects (gaps) in the layer appear in a corrosive environment. The subsequent exposure of the sub-layer (SC) or the substrate (S) In a corrosive environment, this leads to corrosion, or at the very least, the release of metallic cations into solution. In a battery, these cations are detrimental to the durability of the membrane-electrode assembly, and therefore to the battery itself.

[0135] In addition to corrosion resistance, it is relevant to consider the surface conductivity of the coated substrates (S). Indeed, a substrate (S) coated with a metallic underlayer (SC) followed by a carbon layer can exhibit good corrosion resistance, which can be explained in some cases by passivation of the underlayer (SC) material in the event of carbon layer degradation. However, this passivated material is not sufficiently conductive at the surface, which means that a bipolar plate functionalized with such a coating protects a fuel cell from accidental degradation, but the performance of this fuel cell would be reduced (low efficiency due to significant ohmic losses).

[0136] With reference to the graph in [Fig. 7], the Applicant focused on a series of tests for depositing 100 nm thick layers. This graph summarizes, as a function of the flux ratio (<pi) / (q> n): - corrosion current densities, with the scale on the left; - the RCI obtained after the substrates (S) have been subjected to the corrosion test, which simulates accelerated aging of said substrates (S), with the scale on the right.

[0137] This graph shows that in the case of 100nm carbon layer deposits without a metallic sublayer, and with a flux ratio range (<pi) / (q> n) between 2.2 and 3.1, not only is good corrosion resistance obtained, but also good RCI since the values ​​are always less than 10 mQ.cm2. The measurement of RCI alone does not allow the selection of a particular flux ratio, but confirms that the flux ratio range (<pi) / (q> n) selected is relevant for the particular application of fuel cells because the RCI is low.

[0138] Further, more severe corrosion tests were carried out, again with the aim of improving the lifetime of electrochemical systems incorporating the functionalized substrates (S). In these tests, the lifetime was increased to Ih and the potential to 1.4 V and 1.6 V (versus ref(Ag / AgCl)). These severe tests were performed on various substrates (S) coated with carbon layers of different thicknesses and with or without a sublayer (SC).

[0139] Carbon consumption under these corrosion conditions is progressive: - the 20nm and 50nm carbon layers are completely consumed, and the metallic substrate (S) or sub-layer (SC) is exposed over almost the entire surface of the substrates (S); - The 100nm, 160nm, or 300nm layers are not completely consumed: the tested surface retains a black appearance; a certain thickness of carbon deposit is still present on the surface, which helps to maintain the good surface conduction properties of the coating with a low RCI.

[0140] It is therefore advantageous to deposit a sufficiently large carbon thickness to ensure good resistance of the treatment to accidental overpotentials that may occur during the use of a fuel cell, i.e. with dynamic operating conditions (with potential cycling), or even a cycle of starting and stopping the fuel cell (which leads to a higher cathodic potential, or the presence of air in contact with dihydrogen in the anodic medium at start-up), etc.

[0141] Of course, the maximum thickness of the carbon layer is limited by the cost of the treatment, which is linked to the required deposition time.

[0142] The Applicant then became interested in the structure and chemical composition of the conforming deposits.

[0143] With reference to [Fig.8], a conforming deposit obtained according to the following parameters can be observed: - deposition on the substrate (S) of a carbon sublayer (SC), obtained by performing magnetron sputtering with weak ion assistance, i.e. the flux ratio (<pi) / (q> n) is only worth 0.3; - deposition of a dense layer (M) of carbon, obtained by performing magnetron sputtering with ion assistance according to the invention, i.e. the flux ratio (<pi) / (q> n) equals 2.5.

[0144] In order to be able to carry out ion beam cutting, a layer of platinum (Pt) in different forms is deposited on the part to protect it during cutting and is visible in [Fig.8], but this layer is not part of the process.

[0145] On this sample: - the carbon sublayer (SC) in contact with the substrate (S) measures approximately 17 nm in thickness; - the dense carbon layer (M) deposited on the sublayer (SC) measures approximately 98 nm in thickness; - the total thickness of the deposit therefore measures approximately 115 nm in thickness.

[0146] With reference to [Fig. 9], the Applicant compared the 24-hour evolution of the corrosion current density measured in a potentiostatic test at 0.8V (vs. Ag / AgCl reference) for two samples that received a 100nm carbon layer deposited with a flux ratio of 2.3 corresponding to the invention. The first sample had previously received a carbon underlayer (deposited with a flux ratio of 0.3), and the second sample had previously received a titanium underlayer.

[0147] This test, the duration of which is prolonged, is an aging test similar to the test in [Fig.3], except that in this case the evolution of the current density as a function of time is represented more specifically.

[0148] It is noted that: - the first sample with carbon underlayer shows a low corrosion current, and especially that it decreases over time; - The second sample exhibits a slightly higher corrosion rate, which, more importantly, tends to increase over time. This result suggests that the lifespan of the second sample will be shorter than that of the first sample.

[0149] This test demonstrates that to obtain an even longer service life, a carbon underlayer is preferable to a metallic underlayer.

[0150] However, a metallic underlayer may be advantageous depending on the type of substrate used: - if the substrate (S) is stainless steel, a metallic underlayer made of titanium allows, in the event of degradation of the carbon-based material layer (M), the creation of a passivation layer which ensures that the stainless steel of the substrate (S) will not emit metallic cations into the electrochemical system; - if the substrate (S) is titanium, a metallic underlayer also made of titanium can improve the adhesion of the coatings subsequently deposited.

[0151] A particular embodiment may therefore include: - the substrate (S); - a first metallic underlayer (SC) deposited on the substrate (S); - a second carbon-based sublayer (SC) deposited on the first metallic sublayer (SC); - a layer of carbon-based material (M) deposited on the second carbon-based sublayer (SC).

[0152] With reference to [Fig. 10], chemical characterizations by nuclear reaction analysis (NRA) of material layers (M) deposited on substrates as a function of the flux ratio (<pi) / (q> n) were then carried out. The advantage of these characterizations is that they allow us to determine, from a plate functionalized by deposition, whether the deposition was carried out according to parameters conforming to the invention. Other techniques besides NRA exist for characterizing the oxygen content of a deposit: - for example, X-ray photoelectron spectroscopy, or "XPS" (X-ray photoelectron spectroscopy). This technique is not precise enough to measure very small quantities of oxygen, and the results can be biased by ion abrasion; - or secondary-ion mass spectrometry, or "SIMS" according to the acronym for "secondary-ion mass spectrometry" in English, but this technique is not quantitative. For these reasons, the Applicant selected NRA as a reliable and quantitative oxygen measurement technique in the carbon layer.

[0153] As expected, the carbon-based material layer (M) consists mainly of carbon, since the target being sprayed is carbon-based. However, the residual oxygen content varies according to the flux ratio (q>i) / (q>n): - If the ion-assisted bombardment is insufficient (ratio less than 1.7), oxygen is not expelled from the growing deposit layer. The residual oxygen content is therefore greater than or equal to approximately 1 at.%. - when the flux ratio (q>i) / (q>n) increases, the oxygen content decreases and appears to pass through a minimum within the flux ratio range corresponding to the invention. - Next, when the ion-assisted bombardment is too high (flux ratio (q>i) / (q>n) greater than 3.5), the oxygen content increases again. This can be explained by the appearance of local defects and carbon-oxygen bonds, and / or by local exposure of the substrate (deposition defect, or higher stress in the over-bombarded deposit leading to a tendency for local delamination of the substrate). In this latter case, the substrate becomes passivated, and oxygen ends up in the passivation layer.

[0154] A functionalized plate according to the invention therefore comprises within its functional layer an oxygen content of less than 1% at, and preferably less than 0.7% at, calculated as the number of oxygen atoms relative to the number of carbon atoms within said functional layer.

[0155] The functional layer may also include argon from ion assist (or another noble gas if a gas other than argon is used).

[0156] Fig. 11 illustrates a non-functionalized monopolar plate (60), on which the gas delivery and water vapor evacuation channels can be distinguished, which have been shaped prior to the deposition of a functional layer.

[0157] Figure 12 is a diagram illustrating a partial cross-sectional view of such a bipolar plate (60). This diagram shows that the thickness of the substrate (Es) is less than the thickness (Epb) of the bipolar plate (60). Indeed, the thickness of the final plate (60) depends on how it is shaped.

[0158] The process according to the invention does indeed make it possible to functionalize substrates (S): - by providing low RCI values ​​not only before but also after corrosion testing; - by providing good protection against corrosion, even over long periods and high potentials; - the deposited layer exhibiting good structural quality, since it does not present defects such as droplets; - the process does not require high temperatures, which eliminates potential risks to the adhesion or diffusion of the deposited material as well as to the de- plate formation; - the process being compatible with different types of parts such as sheets, monopolar or bipolar plates (possibly already welded and assembled), and made of different materials based on stainless steel, titanium, Inconel® type alloy, i.e. an alloy based on nickel, chromium and iron.

[0159] In the case of using a complementary plasma source (40), it is possible to adjust the ion bombardment independently of the spray source and thus adapt the bombardment from one treatment to another, and possibly within a stack.

[0160] Moreover, the process can be carried out differently from the examples given without departing from the scope of the invention, which is defined by the claims.

[0161] In an alternative not shown, the plasma (P) of the ion assistance system is not generated by microwaves. Indeed, it is not the power consumed by the plasma source (40) that is important, but the quantity of ions available at the substrates (S), hence the Applicant's proposed interpretation of the ion flux (q>i). Other ion sources can therefore be used.

[0162] Unbalanced magnetron sputtering in a closed field is also possible. These variants may require proper adjustment of the magnetron imbalance and the field line looping between cathodes to achieve the desired flux ratio range.

[0163] Furthermore, the technical characteristics of the various embodiments and variants mentioned above can be combined, in whole or in part. For example, it is possible to produce only a carbon-based sublayer (SC), only a metallic sublayer (SC), or both a carbon-based sublayer (SC) and a metallic sublayer (SC). Thus, the process and the installation (1) can be adapted in terms of cost, functionality, and performance.

Claims

Demands

1. A method for ion-assisted deposition of a carbon-based material (M) from a target onto a metallic substrate (S) by sputtering, characterized in that the ratio between the ion flux (q>i) directed towards the substrate (S) and the flux ( <pn) d’atomes neutres de carbone dirigé vers le substrat (S) entre 1,7 et 3,5, et on applique une tension de polarisation comprise entre -35V et -100V au substrat (S).

2. A method according to claim 1, characterized in that the ratio between the flow ( <pi) d’ions gazeux et le flux (<pn) d’atomes neutres de carbone est compris entre 2 et 3,1.

3. A method according to claim 1 or claim 2, characterized in that the material (M) deposited on the substrate (S) forms a layer called a thin film, having a thickness greater than or equal to 20 nm, preferably between 20 nm and 500 nm, more preferably between 50 nm and 250 nm, even more preferably between 80 nm and 150 nm, and even more preferably between 80 nm and 120 nm.

4. A method according to any one of the preceding claims, characterized in that the substrate (S) comprises stainless steel, titanium, a titanium alloy, or a nickel-chromium-iron alloy.

5. A method according to any one of the preceding claims, characterized in that the ion flow is generated by a magnetron cathode.

6. A method according to claim 5, characterized in that the ion flux is generated by a system complementary to the magnetron cathode, preferably by microwave plasma.

7. A method according to claim 6, characterized in that the substrate (S) passes within an installation in front of a magnetron sputtering station and then in front of a plasma generation station (P), preferably in a cyclic manner.

8. A method according to any one of the preceding claims, characterized in that the substrate (S) is a plate of thickness between 10 pm and 1000 pm.

9. A method according to any one of the preceding claims, characterized in that A comprises a preliminary step of depositing onto the substrate (S) a carbon-based underlayer (SC) intended to find between the substrate (S) and the carbon-based material (M), in contact with said carbon-based material (M), and that during this step the ratio between the ion flux ( <pi) dirigé vers le substrat (S) et le flux (<pn) d’atomes neutres de carbone dirigé vers le substrat à une valeur inférieure à 1, de préférence inférieure à 0,5, le flux d’ions étant non nul.

10. A method according to claim 9, characterized in that the thickness of the carbon-based sublayer (SC) is between 2 and 40 nm, preferably between 10 nm and 30 nm.

11. A method according to any one of the preceding claims, characterized in that A comprises a preliminary step of depositing on the substrate (S) a metallic underlayer (SC) intended to be located between the substrate (S) and the carbon-based material (M), in contact with said substrate (S), the material of the metallic underlayer (SC) being selected from one or more of the following materials: chromium, titanium, zirconium, tantalum, or their alloys as well as their nitrides and carbides.

12. A method according to claim 11, characterized in that the thickness of the metallic underlayer (SC) is between 5 and 100 nm, preferably between 20 nm and 40 nm.

13. A method according to any one of the preceding claims, characterized in that the bias voltage is between -50 V and -75 V.

14. A method for manufacturing a monopolar or bipolar plate comprising a metallic substrate (S) covered with a layer comprising a carbon-based material (M), characterized in that it comprises a step of depositing said carbon-based material (M) from a target onto said metallic substrate (S), by magnetron sputtering, by implementing a deposition process according to any one of the preceding claims.

15. Part obtainable by a process of depositing a carbon-based material (M) from a target onto a metallic substrate (S), by sputtering in the presence of ion assistance, according to any one of claims 1 to 13, said part having an external surface comprising said metallic substrate (S) coated with a layer of the carbon-based material (M), and in which the layer of carbon-based material (M) comprises less than 1 at% oxygen, calculated as the number of oxygen atoms relative to the number of carbon atoms within the layer of carbon-based material (M).

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