Method of preparation of solid multi-layer lubricant coatings

Abstract

The present invention relates to a method for obtaining a multi-layer lubricant coating by magnetron sputtering, wherein the deposition of each layer of steps b) and c) is performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique. Additionally, the present invention relates to a multi-layer lubricant coating formed on a substrate obtainable by said method. Finally, the present invention is directed to the use of the multi-layer lubricant coating obtainable by the method of the present invention in automotive, aeronautical, space, metal-mechanic and cryogenic sectors.

Description

[0001] METHOD OF PREPARATION OF SOLID MULTI-LAYER LUBRICANT COATINGS

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method for preparing solid multi-layer lubricant coatings by magnetron sputtering. The present invention also relates to the solid multilayer lubricant coating obtainable by the said method and its uses.

[0004] STATE OF THE ART

[0005] The development of solid lubricants has mainly proceeded for intended use in environments wherein lubricating oil could not be easily employed, such as high temperature, extreme low temperature, ultra-high pressure, vacuum and the like. Among such solid lubricants, there are those which can be applied to parts having a complex shape by being deposited as a coating onto the object to be treated by a process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Materials such as molybdenum disulphide and diamond-like carbon (DLC) are already being put to practical use.

[0006] Of these, molybdenum disulphide is being applied to many sliding parts to improve seize resistance, due to its superior sliding properties and the fact that it can maintain a low friction coefficient of up to several hundred °C. In particular, by depositing molybdenum disulphide onto a hard layer such as a nitride or a carbonitride which have excellent wear resistance, a multi-layer coating having excellent sliding properties can be produced providing wear resistance and seize resistance at the same time.

[0007] However, molybdenum disulphide has the drawback of being an extremely soft substance, which when formed into a coating does not always possess sufficient adhesion with the substrate to be treated. For this reason, if a molybdenum disulphide coating is applied as is onto a sliding part, there is the serious problem that the coating is susceptible at an initial stage of use to peel-off and thereby lost without exhibiting sufficient performance. In addition, molybdenum disulphide also has the drawback that it is highly hygroscopic, which causes lubricating performance to deteriorate in humid air.

[0008] For example, document GB2565320A discloses a method of forming a coating on a substrate, wherein the coating comprises a metal bonding layer of chromium, a hard layer of chromium carbide, and a lubricant layer of graphite-like carbon (GLC) or molybdenum disulphide or diamond-like carbon (DLC) applied by magnetron sputtering. Patent EP1712654B1 discloses a method for forming a multilayer coating comprising metal nitrides and / or carbonitrides and a lubricating layer consisting of a metal element and molybdenum disulfide. US6423419B1 discloses a method for preparing a molybdenum-sulphur coating doped with metals such as Ti, W, Zr or V to improve its hardness.

[0009] Despite the above-mentioned coatings, there is a need for developing new methods for preparing solid multi-layer lubricant coatings that avoid the above- mentioned problems of currently known methods.

[0010] The present invention overcomes the drawbacks mentioned above by providing a method to prepare a multi-layer lubricant coating having improved adhesion properties as well as enhanced humidity resistance and endurance.

[0011] BRIEF DESCRIPTION OF THE INVENTION

[0012] The authors of the present invention have developed a method for obtaining a solid multi-layer lubricant coating having improved properties, which is carried out by using magnetron sputtering for all the method steps, and in particular, by using the High- Power Impulse Magnetron Sputtering (HIPIMS) technique at least for some of the method steps. Thus, by combining the application of the HIPIMS technique with an optimized underlayer composition having a progressive transition between different materials, a multilayer coating having improved adhesion to the substrate as well as higher tribological properties, specifically higher endurance at different humidities.

[0013] Therefore, a first aspect of the present invention is directed to a method for obtaining a multi-layer lubricant coating by magnetron sputtering, wherein the method comprises the following steps: a) placing a substrate for coating, a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr, a sputtering source of WC, wherein the sputtering source of WC can additionally comprise Co, and a sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 in a vacuum chamber; b) introducing an inert gas in said vacuum chamber and forming a layer of a metal on the surface of said substrate by deposition using the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr; c) introducing an inert gas or a reactive gas in said vacuum chamber and forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr on the surface of the layer of the metal resulting from step b) by deposition using the sputtering source of the metal selected from the group consisting of Cr, Ti and Zr; d) forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr resulting from step c) by deposition using the sputtering source of WC while the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr is being deposited according to step c); e) forming a layer of WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co resulting from step d) by deposition using the sputtering source of WC; and f) forming a top layer of a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, on the surface of the layer of WC or WC-Co resulting from step e) by deposition using the sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 while WC or WC-Co is optionally being deposited according to step e); wherein the deposition of each layer of steps b) and c) is performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique.

[0014] The method of preparation for obtaining a multi-layer lubricant coating described herein allows obtaining improved adhesion (to the substrate) as well as longer service life (higher endurance) and humidity resistance.

[0015] Thus, a second aspect of the present invention is directed to a multi-layer lubricant coating formed on a substrate obtainable by the method as defined above, wherein the coating comprises: i) an underlayer which is a multi-layer comprising sequentially

[0016] - a layer of a metal selected from the group consisting of Cr, Ti and Zr,

[0017] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr adjacent to the layer of a metal selected from the group consisting of Cr, Ti and Zr,

[0018] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and

[0019] - a layer of WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr and WC or WC-Co; and ii) a top layer comprising a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, and adjacent to the layer of WC or WC-Co, wherein the underlayer is interposed between the substrate and the top layer.

[0020] Therefore, the multi-layer lubricant coating obtainable by the method as defined above comprises an underlayer and a top layer. The underlayer obtained according to the method of the present invention is a multilayer comprises a progressive transition between different materials, which improved the adhesion to the substrate. The top layer comprises standard metal compounds selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, for higher resistance to humidity.

[0021] A final aspect of the present invention is directed to use of the multi-layer lubricant coating obtainable by the method as defined above in automotive, aeronautical, space, metal-mechanic and cryogenic sectors.

[0022] Additional advantages and features of the invention will become apparent from the detailed description that follows and will be particularly pointed out in the appended claims.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] To complete the description and provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as examples of how the invention can be carried out. The drawings comprise the following figures:

[0025] Figure 1 shows a schematic structure of a multi-layer coating obtained according to the method of the present invention and having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique.

[0026] Figure 2 shows a SEM-BES image of a cross-section of the coating of Figure 1 , wherein the optimized etching described in this invention was previously applied.

[0027] Figure 3 shows images of adherence results for four coatings, wherein: (A) is a coating having the layer composition WC-Co / MoSx+WC-Co, wherein WC- Co is the underlayer and MoSx-WC-Co is the top layer, and wherein each layer was deposited by DC technique;

[0028] (B) is a coating as defined for coating (A), and, wherein, additionally, the optimized etching described in this invention was previously applied;

[0029] (C) is a coating having the layer composition Cr / CrN / CrN+WC-Co / WC- Co / MoSx+WC-Co obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC- Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique; and

[0030] (D) is a coating as defined for coating (C) and, wherein, additionally, the optimized etching described in this invention was previously applied.

[0031] Figure 4 shows a graphical representation of adherence results for the four coatings (A), (B), (C) and (D) of Figure 3.

[0032] Figure 5 shows adherence results of four coatings after being exposed to different relative humidities (%), wherein:

[0033] (A) is a coating having the layer composition WC-C0 / M0S2, wherein WC-Co is the underlayer and M0S2 is the top layer, and wherein each layer was deposited by DC technique;

[0034] (B) is a coating having the layer composition WC-Co / MoSx+ WC-Co, wherein WC- Co is the underlayer and MoSx+WC-Co is the top layer, and wherein each layer was deposited by DC technique;

[0035] (C) is a coating having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoS2 obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co was progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer M0S2 were deposited by DC technique; and

[0036] (D) is a coating having the layer composition Cr / CrN / CrN+WC / WC / MoSx+WC-Co obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co was progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique and, wherein, additionally, the optimized etching described in this invention was previously applied.

[0037] Figure 6 shows endurance of tribo-tests variations for four coatings, wherein:

[0038] (A) is a coating having the layer composition WC-Co / MoSx-WC-Co, wherein WC- Co is the underlayer and MoSx-WC-Co is the top layer, and wherein each layer was deposited by DC technique;

[0039] (B) is a coating as defined for coating (A), and, wherein, additionally, the optimized etching described in this invention was previously applied;

[0040] (C) is a coating having the layer composition Cr / CrN / CrN+WC-Co / WC- Co / MoSx+WC-Co obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC- Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique; and

[0041] (D) is a coating as defined for coating (C), wherein, additionally, the optimized etching described in this invention was previously applied.

[0042] Figure 7 shows endurance of tribo-tests variations for four coatings, wherein:

[0043] (A) is a coating having the layer composition WC-C0 / M0S2, wherein WC-Co is the underlayer and M0S2 is the top layer, and wherein each layer was deposited by DC technique;

[0044] (B) is a coating as defined for coating (A), wherein, additionally the optimized etching described in this invention was previously applied;

[0045] (C) is a coating having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoS2 obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer M0S2 were deposited by DC technique; and

[0046] (D) is a coating as defined for coating (C), wherein, additionally, the optimized etching described in this invention was previously applied.

[0047] Figure 8 shows comparison of endurance of tribo-tests variations (durability and coefficient of friction (CoF)) at different relative humidities (%) for two coatings, wherein 1 , 2 and 3 are three repetitions of the tribo-tests and the graphs presented are: (above) a coating having the layer composition WC-Co / MoSx+WC-Co, wherein WC-Co is the underlayer and MoSx+WC-Co is the top layer, and wherein each layer was deposited by DC technique;

[0048] (below) a coating having the layer composition Cr / CrN / CrN+WC-Co / WC- Co / MoSx+WC-Co obtained by the method of the present invention, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC- Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique, and additionally, the optimized etching described in this invention was previously applied.

[0049] Figure 9 shows endurance of tribo-tests (1 , 2 and 3 are three repetitions) for a coating having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co obtained according to the method of the present invention and having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co, wherein, in particular, each of the layers Cr, CrN and CrN+WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co were progressively deposited by HIPIMS technique and the layer WC-Co of the underlayer Cr / CrN / CrN+WC-Co / WC-Co and the top layer MoSx+WC-Co were deposited by DC technique, wherein the optimized etching described in this invention was previously applied, and the coating having a thickness of around 6.5 pm and applied on different substrates, i.e., stainless steel (specifically, AISI 440C substrate) and a titanium alloy (specifically, Ti6AI4V substrate).

[0050] DESCRIPTION OF THE INVENTION

[0051] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs.

[0052] The method of the present invention intends to solve the shortcomings of priorart methods for obtaining solid lubricant coatings, which is carried out by combining the High-Power Impulse Magnetron Sputtering (HIPIMS) technique with an optimized underlayer composition having a progressive transition between different materials.

[0053] Thus, it is an object of the present invention to provide a method for obtaining solid multi-layer lubricant coatings by magnetron sputtering.

[0054] In particular, the method of the present invention comprises the steps of: a) placing a substrate for coating, a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr, a sputtering source of WC, wherein the sputtering source of WC can additionally comprise Co, and a sputtering source selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 in a vacuum chamber; b) introducing an inert gas in said vacuum chamber and forming a layer of a metal on the surface of said substrate by deposition using the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr; c) introducing inert gas and reactive gas in said vacuum chamber and forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr on the surface of the layer of the metal resulting from step b) by deposition using the sputtering source of the metal selected from the group consisting of Cr, Ti and Zr; d) forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr resulting from step c) by deposition using the sputtering source of WC while the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr is being deposited according to step c); e) forming a layer of WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co resulting from step d) by deposition using the sputtering source of WC; and f) forming a top layer of a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2 and optionally WC or WC-Co on the surface of the layer of WC or WC-Co resulting from step e) by deposition using the sputtering source of a metal compound selected from M0S2, WSe2, WS2 and MoSe2 while WC or WC-Co is optionally being deposited according to step e); wherein the deposition of each layer of steps b) and c) is performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique.

[0055] In the context of the present invention, the term “Magnetron Sputtering” refers to a vacuum coating process that falls under the category of physical vapor deposition (PVD) and is mainly used for depositing metals, alloys, oxides and compound materials with a thickness from some nanometers up to some microns with controlled microstructure. Magnetron sputtering is a plasma-based coating method that generates a magnetically confined plasma near the surface of a target (or “sputtering source”). Then, positively charged energetic ions from the plasma collide with the negatively charged target material and atoms from the target are ejected or “sputtered”, which then deposit on a substrate growing as a coating.

[0056] The term “Magnetron Sputtering” encompasses many types of techniques, such as direct current (DC), radio frequency (RF), mid frequency (MF), DC Pulsed (DCp) and high-power impulse magnetron sputtering (HiPIMS).

[0057] In the context of the present invention, the term “High-Power Impulse Magnetron Sputtering technique” or “HIPIMS” or “HiPIMS”, also known as “High-Power Pulsed Magnetron Sputtering” or “HPPMS”, is a method for physical vapor deposition of thin films which is based on magnetron sputter deposition. This technique employs high- power densities of the order of kW-cm-2in short pulses (impulses) of tens of microseconds.

[0058] Thus, each steps b) to f) of the method of the present invention are performed using Magnetron Sputtering, and in particular, at least steps b) and c) of the method of the present invention are performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique.

[0059] In another particular embodiment, at least one of the steps d) to f), apart from steps b) and c), are also performed using HIPIMS technique, i.e., all steps d) to f) are also performed using the HIPIMS technique; only step d) is also performed using the HIPIMS technique; or only step e) is also performed using the HIPIMS technique; or only step f) is also performed using the HIPIMS technique; or only steps d) and e) are also performed using the HIPIMS technique; or only steps d) and f) are also performed using the HIPIMS technique; or only steps e) and f) are also performed using the HIPIMS technique.

[0060] The steps of the method of the present invention not performed by HIPIMS technique are preferably performed by Direct Current (DC) technique.

[0061] As previously mentioned, the method of the present invention comprises a step a) of placing a substrate for coating, a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr, a sputtering source of WC, wherein the sputtering source of WC can additionally comprise Co, and a sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 in a vacuum chamber.

[0062] In the context of the present invention, the term “substrate for coating” is used to refer to a solid material which its surface is to be totally or partially coated. Non-limiting examples of substrates for coating suitable for the method of the present invention are metal and metal alloys substrates such as stainless steel such as AISI 440C and titanium alloys such as Ti6AIV4 as well as ceramic materials.

[0063] The term “sputtering source” refers to the target substance generating material which is sputtered during the deposition process performed by Magnetron Sputtering. The sputtering source are usually used as solid bulk materials.

[0064] In the method of the present invention, the term “a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr” may refer to a sputtering source of the pure metal selected from the group consisting of Cr, Ti and Zr or to a sputtering source of a metal nitride, a metal carbide or a metal boride of a metal selected from the group consisting of Cr, Ti and Zr. Therefore, a sputtering source of the pure metal selected from the group consisting of Cr, Ti and Zr can be employed to generate a metal layer of Cr, Ti or Zr or a layer of a nitride, a carbide or a boride of a metal selected from Cr, Ti and Zr. For example, in the method of the present invention, by using a source of a pure metal selected from the group consisting of Cr, Ti and Zr simultaneously with nitrogen gas as reactive gas, a layer of chromium nitride (CrN) can be generated. Alternatively, in the method of the present invention, a sputtering source of a metal nitride, a metal carbide or a metal boride of a metal selected from the group consisting of Cr, Ti and Zr with argon as inert gas can be used to produce a metal nitride, a metal carbide or a metal boride of a metal selected from the group consisting of Cr, Ti and Zr.

[0065] In addition, in the method of the present invention, a sputtering source of tungsten carbide (WC), wherein the sputtering source of WC can additionally comprise Co, is employed to generate a layer of pure WC, a layer of WC and Co (WC-Co) or a layer of another element / compound and WC or WC-Co. For example, in the method of the present invention, by using the source of WC simultaneously with a sputtering source of M0S2, a layer of M0S2 and WC (referred herein as “M0S2+WC”) or a layer of M0S2 and WC-Co (referred herein as “M0S2+WC-C0”) can be generated. Therefore, in the context of the present invention, the term “sputtering source of WC” refers to sputtering sources (for examples, commercial ones) made mainly of tungsten carbide that can additionally comprises elements such as cobalt, preferably in an amount from 0 to 15 wt.%, more preferably from 0 to 10 wt.%.

[0066] In addition, in the method of the present invention, a sputtering source of WC is employed to generate a layer of WC, wherein the layer of WC can additionally comprise Co (layer of WC-Co), preferably in an amount between 0 and 10 wt.%, more preferably in from 0 to 5 wt.%. Moreover, in the method of the present invention, a sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 is employed to generate a layer of a pure metal compound selected from MoSx, WSe2, WS2 and MoSe2 or a layer of metal compound selected from MoSx, WSe2, WS2 and MoSe2 with WC. For example, in the method of the present invention, by using the source of a metal compound selected from M0S2, WSe2, WS2 and MoSe2 simultaneously with a sputtering source of WC, a layer of a metal compound selected from MoSx, WSe2, WS2 and MoSe2 and WC or WC-Co can be generated.

[0067] The term “MoSx” refers to a non-stoichiometric molybdenum sulphide.

[0068] Therefore, in the method of the present invention, a substrate for coating, a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr, a sputtering source of WC, and a sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 are placed inside a vacuum chamber, preferably, in a specific sample holder.

[0069] The term “vacuum chamber” refers to a rigid enclosure from which air and other gases are removed by a vacuum pump. This results in a low-pressure environment within the chamber, commonly referred to as a vacuum. In the method of the present invention, a high vacuum level is needed. Thus, in a preferred embodiment, the vacuum chamber is at a pressure equal or lower than 0.5 mPa.

[0070] As previously mentioned, the method of the present invention comprises a step b) of introducing an inert gas in said vacuum chamber and forming a layer of a metal selected from the group consisting of Cr, Ti and Zr on the surface of said substrate by deposition using the sputtering source of a pure metal selected from the group consisting of Cr, Ti and Zr.

[0071] The term “inert gas” refers to a gas that has extremely low reactivity with other substances. Non-limiting examples of inert gases suitable in the method of the present invention are helium, argon, neon, xenon, krypton, radon, and element 118 (lluo) in their elemental form (Group 18 of the periodic table). In a preferred embodiment, the inert gas of step b) is argon.

[0072] In step b) of the method of the present invention, a layer of a metal selected from the group consisting of Cr, Ti and Zr is formed on the surface of said substrate by deposition using the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr.

[0073] In a particular embodiment, step b) comprises introducing an inert gas in the vacuum chamber, preferably argon, to reach a pressure between 400 mPa and 800 mPa, preferably 600 mPa.

[0074] In another particular embodiment, step b) comprises applying to the sputtering source of a metal selected from Cr, Ti and Zr an electrical power between 0.5 kW and 1.5 kW, preferably 1.0 kW.

[0075] In a particular embodiment, step b) comprises introducing an inert gas in the vacuum chamber, preferably argon, to reach a pressure between 400 mPa and 800 mPa, preferably 600 mPa, and applying to the sputtering source of a metal selected from Cr, Ti and Zr an electrical power between 0.5 kW and 1.5 kW, preferably 1 .0 kW.

[0076] In another particular embodiment, step b) comprises applying a HiPIMS frequency signal between 100 Hz and 4000 Hz, preferably 500 Hz having a pulse duration between 50 ps and 200 ps, preferably 70 ps.

[0077] In a particular embodiment, step b) is performed between 5 minutes and 30 minutes.

[0078] Thus, in step b) of the method of the present invention, a layer of a metal selected from the group consisting of Cr, Ti and Zr is formed. In a particular embodiment, the layer of a metal selected from the group consisting of Cr, Ti and Zr obtained in step b) has a thickness ranging from 100 nm to 300 nm, wherein the thickness is measured by scanning electron microscopy (SEM) or contact profilometry, preferably by SEM using a backscattered electron (SEM-BES).

[0079] The method of the present invention further comprises a step c) of introducing an inert gas or a reactive gas in said vacuum chamber and forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr on the surface of the layer of the metal resulting from step b) by deposition using the sputtering source of the metal selected from the group consisting of Cr, Ti and Zr.

[0080] The term “reactive gas” refers to a gas that chemically reacts with other substances. A non-limiting example of a reactive gas suitable in the method of the present invention is nitrogen. In a preferred embodiment, the inert gas of step c) is nitrogen.

[0081] Thus, in step c), a second layer is deposited on top of the first layer obtained in step b). To this end, nitrogen gas as a reactive gas is introduced in the vacuum chamber in order to react with the sputtering source of a pure metal selected from the group consisting of Cr, Ti and Zr while is being deposited, and, consequently, forming a layer of a metal nitride selected from chromium nitride (CrN), titanium nitride (TiN) and zirconium nitride (ZrN).

[0082] Alternatively, in step c) of the method of the present invention, the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr can also be produced also by using a sputtering source of the specific nitride, carbide or boride of a metal selected from the group consisting of Cr, Ti and Zr with argon as inert gas.

[0083] In step c), a carbide of a metal selected from Cr, Ti and Zr can be formed, i.e., Cr3C2, TiC and ZrC.

[0084] In step c), a boride of a metal selected from Cr, Ti and Zr can be formed, i.e., CrB, Cr2B, Cr5B3, Cr3B4, CrB2, CrB4, TiB2and ZrB2.

[0085] In a particular embodiment, the nitrogen flow in step c) is between 200 mln and 600 mln, preferably 270 mln.

[0086] In another particular embodiment, the inert gas flow, preferably argon, of step b) is maintained in step c) to a pressure between 400 mPa and 800 mPa, preferably 600 mPa.

[0087] After step b), electrical power is increased. In particular, step c) comprises applying to the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr an electrical power between 4 kW and 5 kW, preferably 4.5 kW.

[0088] In another particular embodiment, step c) comprises applying a HiPIMS frequency signal between 100 Hz and 4000 Hz, preferably 500 Hz having a pulse duration between 50 ps and 200 ps, preferably 70 ps.

[0089] In another particular embodiment, step c) comprises introducing nitrogen gas inside the vacuum chamber, preferably a nitrogen flow of 270 mln, and applying to the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr an electrical power between 4 kW and 5 kW, preferably 4.5 kW.

[0090] In a particular embodiment, step c) is performed between 30 minutes and 120 minutes, preferably during 1 hour.

[0091] Thus, in step c) of the method of the present invention, a layer of a nitride, a carbide or a boride of a metal selected the group consisting of from Cr, Ti and Zr is formed on top of the first layer of a metal selected from the group consisting of Cr, Ti and Zr. In a particular embodiment, the layer of a nitride, a carbide or a boride of a metal selected from Cr, Ti and Zr obtained in step c) has a thickness ranging from 100 nm to 1000 nm, wherein the thickness is measured by scanning electron microscopy (SEM) or contact profilometry, preferably by SEM using a backscattered electron (SEM-BES).

[0092] The method of the present invention further comprises a step d) of forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co on the surface of the layer resulting from step c) by deposition using the sputtering source of WC while the layer of a nitride, a carbide or a boride of a metal selected from Cr, Ti and Zr is being deposited according to step c).

[0093] In a particular embodiment, the nitrogen flow in step d) is between 200 mln and 600 mln, preferably 270 mln.

[0094] In another particular embodiment, the inert gas flow, preferably argon, of steps b) and c) is maintained in step d) to a pressure between 400 mPa and 800 mPa, preferably 600 mPa.

[0095] In a particular embodiment, step d) comprises applying to the sputtering source of WC an electrical power between 2 kW and 3 kW, preferably 2.5 kW, while a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr is being deposited according to step c).

[0096] In a particular embodiment, step d) is performed between 5 minutes and 30 minutes, preferably during 900 seconds.

[0097] Thus, in step d) of the method of the present invention, a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co is formed on top of the second layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr. In a particular embodiment, the layer obtained in step d) has a thickness ranging from 50 nm to 150 nm, wherein the thickness is measured by scanning electron microscopy (SEM) or contact profilometry, preferably by SEM using a backscattered electron (SEM-BES).

[0098] As mentioned above, the method of the present invention further comprises a step e) of forming a layer of WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co resulting from step d) by deposition using the sputtering source of WC.

[0099] In a particular embodiment, the nitrogen flow in step e) is between 200 mln and 600 mln, preferably 270 mln.

[0100] In another particular embodiment, the inert gas flow, preferably argon, of steps b) and c) is maintained in step e) to a pressure between 400 mPa and 800 mPa, preferably 600 mPa. In a particular embodiment, step e) comprises applying to the sputtering source of WC an electrical power between 2 kW and 3 kW, preferably 2.5 kW.

[0101] In a particular embodiment, step e) is performed between 5 minutes and 1 hour.

[0102] Thus, in step e) of the method of the present invention, a layer of WC or WC-Co is formed on top of the third layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co. In a particular embodiment, the layer obtained in step e) has a thickness ranging from 500 nm to 3000 nm, wherein the thickness is measured by scanning electron microscopy (SEM) or contact profilometry, preferably by SEM using a backscattered electron (SEM-BES).

[0103] In another particular embodiment, each of step d) to f) are performed by Direct Current (DC) technique, more particularly, a Direct Current (DC) bias between 50 V and 200 V, preferably 95 V, and a heating power between 1 kW and 2 kW, preferably 1.5 kW, are applied to the sputtering sources during each of step d) to f).

[0104] In yet another particular embodiment, the temperature of the vacuum chamber is maintained at a temperature between 200 °C and 300 °C, preferably below 250 °C, during each of step b) to f) of the method of the present invention by using, for example, resistive heaters.

[0105] Because of performing steps b) to e) of the method of the present invention, an underlayer which is a multilayer composed by sequential and progressive transitions between the following materials is formed:

[0106] - a layer of a metal selected from the group consisting of Cr, Ti and Zr,

[0107] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr adjacent to the layer of a metal selected from Cr, Ti and Zr,

[0108] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and

[0109] - a layer of WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co.

[0110] An example of an underlayer composition that can be obtained according to steps b) to e) of the method of the present invention is Cr / CrN / CrN+WC-Co / WC-Co.

[0111] The authors of the present invention have observed that the multilayer underlayer composed by progressive transitions of the above-described layers present a better adhesion to the substrate to be coated.

[0112] After steps a) to e) of the method of the present invention, a top layer (or lubricant layer) is formed. In particular, the method of the present invention further comprises a step f) of forming a top layer of a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, on the surface of the layer of WC or WC-Co resulting from step e) by deposition using the sputtering source of a metal compound selected from M0S2, WSe2, WS2 and MoSe2 while WC or WC-Co is optionally being deposited according to step e).

[0113] In a particular embodiment, a top layer (lubricant layer) of MoSx+WC or MoSx+WC-Co is formed. In a more particular embodiment, the obtained MoSx+WC or MoSx+WC-Co layer has a composition, measured by X-ray Photoelectron Spectroscopy (XPS) as follows:

[0114] Mo from 30 wt.% to 75 wt.%;

[0115] S from 10 wt.% to 40 wt.%

[0116] W from 0 wt.% to 45 wt.%;

[0117] Co from 0 wt.% to 10 wt.%; and

[0118] C from 0 wt.% to 15 wt.%.

[0119] Even in a more particular embodiment, the obtained MoSx+WC or MoSx+WC-Co layer has a composition as follows:

[0120] Mo from 39 wt.% to 65 wt.%;

[0121] S from 18 wt. % to 29 wt. %

[0122] W from 0 wt. % to 34 wt. % ;

[0123] Co from 0 wt.% to 5 wt.%; and

[0124] C from 4 wt.% to 8 wt.%.

[0125] Therefore, the underlayer formed in steps b) to e) is interposed between the substrate and the top layer obtained in step f).

[0126] In a particular embodiment, the inert gas flow, preferably argon, of steps b) to e) is maintained in step f) to a pressure between 400 mPa and 800 mPa, preferably 600 mPa.

[0127] In a particular embodiment, step f) comprises applying to the sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 an electrical power between 500 W and 1500 W, preferably 800 W, while an electrical power between 0.2 kW and 1 kW, preferably 0.45 kW, is applied to the sputtering source of WC. In a particular embodiment, step f) is performed between 30 minutes and 1300 minutes, preferably during 135 minutes.

[0128] Thus, in step f) of the method of the present invention, a top layer of a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, is formed. In a particular embodiment, the layer obtained in step f) has a thickness ranging from 500 nm to 10000 nm, wherein the thickness is measured by scanning electron microscopy (SEM) or contact profilometry, preferably by SEM using a backscattered electron (SEM-BES).

[0129] The authors of the present invention have observed that combining a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2 with WC or WC-Co improves the resistance to humidity of the lubricant layer.

[0130] In another particular embodiment, the method of the present invention further comprises a pretreatment comprising ion etching the sputtering samples used in steps b) to f) before their deposition.

[0131] In a preferred embodiment, the ion etching pretreatment is performed in two stages comprising first applying a pulsed voltage, and second applying a high current of ionized plasma.

[0132] In a preferred embodiment, the first step of applying a pulsed voltage is performed at 650V, preferably during 900s, using preferably a signal frequency of 240kHz, preferably at a pulse duration of 1600 ns.

[0133] In a preferred embodiment, the second step of applying a high current of ionized plasma is performed preferably applying 20 A of current intensity to boosters and 50 V of DC bias to the samples during 900 s.

[0134] In another preferred embodiment, both steps, pulsed voltage and high current of ionized plasma, are applied twice alternatively.

[0135] The authors of the present invention have observed that this ion etching pretreatment step (also referred as “optimized etching”) further improves the final properties of the coating obtainable by the method of the present invention.

[0136] In another particular embodiment, the pretreatment further comprises a cleaning step before ion etching the sputtering samples. This cleaning process is performed in order to remove impurities such as oils and dust. This cleaning step can be performed for example by using cloths that leaves no residue and acetone. Additionally, an ultrasound bath can be used in this cleaning step. Afterwards, sputtering samples are dried, preferably by compressed dry air, and placed inside the vacuum chamber in a specific sample holder. During all this process, sample contamination must be avoided.

[0137] According to a further aspect of the present invention, it is provided a multi-layer lubricant coating formed on a substrate obtainable by the method of the present invention, wherein the coating comprises: i) an underlayer which is a multi-layer comprising sequentially

[0138] - a layer of a metal selected from the group consisting of Cr, Ti and Zr,

[0139] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr adjacent to the layer of a metal selected from the group consisting of Cr, Ti and Zr,

[0140] - a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and

[0141] - a layer of WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co; and ii) a top layer comprising a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, and adjacent to the layer of WC or WC-Co, wherein the underlayer is interposed between the substrate and the top layer.

[0142] The inventors have surprisingly found that the method of the present invention as defined above allows obtaining a solid lubricant coating having improved humidity resistance with longer endurance and enhanced adhesion compared to solid lubricant coatings obtained by other methods of the prior art as well compared to other coating not having the composition of the underlayer of the coating obtainable by the method as defined above, both having poor mechanical properties, low adhesion to the substrate and low resistance to humidity.

[0143] Finally, another aspect of the present invention refers to the use of the multi-layer lubricant coating obtainable by the method as defined above as coating for metal pieces used in automotive, aeronautical, space, metal-mechanic and cryogenic sectors.

[0144] In the context of the present invention, it is to be understood as designating any value lying within the range defined by the number ±5 %, more preferably a range defined by the number ±2 %. For example, “10” should be construed as “within the range of 9.5 to 10.5”, preferably “within the range of 9.8 to 10.2”. Through the description and the claims, the word “comprises” and variations thereof are not intended to exclude other technical features, components or steps. Additional advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention without undue burden.

[0145] EXAMPLES

[0146] In the following, the invention will be further illustrated by means of Examples. The Examples should in no case be interpreted as limiting the scope of the invention, but only as an illustration of the invention.

[0147] The following four coating compositions were prepared and used in the examples below:

[0148] - WC-C0 / M0S2;

[0149] - WC-Co / MoSx+WC-Co,

[0150] - Cr / CrN / CrN+WC-Co / WC-Co / MoS2; and

[0151] - Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co.

[0152] These coating compositions were deposited on stainless steel (EN 1.4301 ; AISI 440C) mirror polished samples (50 mm diameter and 5 mm thick) and TiAI6V4 samples. The coatings were deposited simultaneously on silicon wafers samples (100) for the thickness characterization.

[0153] The synthesis of the coatings was carried out by reactive magnetron sputtering in a commercial synthesis unit (Cemecon CC800HiPIMS) with, 1 rectangular chromium (99.9%) target, 1 WC target and 2 M0S2 (99,5%) targets (500x88mm). The samples were placed inside the vacuum chamber in a double rotation planetary holder table.

[0154] The thicknesses of the as-deposited coatings were determined with a contact profilometer (Dektak 150). The morphology of the samples was studied by scanning electron microscopy (SEM) (JEOL JSM-IT700), and critical loads of the as deposited coatings were assessed in scratch tests respectively (MST3 Step 700, Anton Paar). Wear and coefficient of friction was analysed by means of tribological tests were performed in MT 60 HT Microtest tribometer.

[0155] The adherence tests were performed according to the international standard ASTM C1624-05 (standard test method for adhesion, strength and mechanical failure modes of ceramic coatings by quantitative single point scratch testing) by using the scratch test technique with a Rockwell tip of 100nm radius and applying a load from 0 to 2N and over a length of 20mm. The tribological tests and friction tests were performed following the ASTM G 99- 95a (standard test method for wear testing with a pin-on-disk apparatus).

[0156] 1. Preparation of coating according to the method of the present invention having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co

[0157] In order to remove the impurities (such as oil and rust) from the starting materials a cloth that leaves no residue and acetone were used. Then a deeper cleaning was performed using an ultrasound ethanol during at least 15 minutes. Finally, the materials were dried by compressed dry air and placed inside the vacuum chamber in a specific sample holder. During all this process, the operator was wearing the appropriate gloves in order to avoid sample contamination.

[0158] Once the doors of the vacuum chamber were closed, the process was launched, and the vacuum system started to evacuate the atmosphere inside the chamber. In order to minimize the presence of oxygen inside the chamber, a high vacuum level was needed (<0.5 mPa).

[0159] Once the desired vacuum was reached (<0.5mPa), the chamber was heated by resistive heaters at a temperature in the range of 200 °C to 300 °C for at least 3.5 hours. After this period, and maintaining the chamber temperature, a degassing check was performed, and the process was stopped while the internal pressure was above 0.5 mPa.

[0160] Before coating deposition and after the heating, the samples and the turntable were etched using ion bombardment (using argon). During the plasma etching, electrical power was applied on the heaters in order to maintain the temperature of the samples high enough (total heating power 1500W, below 250°C) and using a mixture of gases argon (200 mln) and krypton (50 mln). The etching process was performed in two steps. In the first step, a pulsed voltage (650 V, frequency 240 kHz and toff 1600 nanoseconds) was applied to the samples during 900s. In the second step, a high current of ionized plasma between the boosters and cathodes (electron Booster technology from CemeCon) was applied having 20 A of current intensity to the boosters and 50 V of DC bias to the samples during 900 seconds. In this stage, a cooling stop (during 3600 seconds) was performed before the electron booster. Both steps were applied twice alternatively.

[0161] Then, the deposition of the underlayer was performed as follows using argon (695 mln). In this phase, first, a small gathering was performed in order to reduce the amount of oxygen inside the chamber, for that, two titanium targets were switched on during 900 seconds (argon pressure of 600 mPa). After the gathering, an underlayer between the substrate and the lubricant coating (top layer) was deposited, the Cr was switched on (argon pressure of 600 mPa) and an electrical power was applied (1 kW) for 15 minutes (signal frequency of 500 Hz, pulse duration of 70 microseconds). After that, the electrical power of the chromium target was increased to 4.5 kW (argon pressure of 600 mPa). Once the 4.5kWwere reached, nitrogen was introduced inside the vacuum chamber and chromium nitride was deposited (argon pressure 600 mPa, HiPIMS frequency 500 Hz, nitrogen flow 270 mln) during 1 hour. At this point, a second magnetron with a tungsten carbide target was switched on (power ramp to 2.5 kW, 500Hz, during 70 microseconds) while chromium target was depositing chromium nitride (4.5 kW, 500 Hz, during 70 microseconds, argon pressure 600 mPa and nitrogen flow of 270 mln) during 900 seconds. During all these stages, a DC bias of 95 V was applied to the substrates and a heating power of 1.5 kWwas applied.

[0162] Then, the deposition of the lubricant layer (top layer) was performed as follows. Once the underlayer was deposited, the heaters, the nitrogen mass flow controller and the substrate bias were switched off. The power applied to the tungsten carbide target was slowly decreased to 0.45 kW and at the same time, a power ramp was applied to the magnetrons with the M0S2 targets installed (250 W to 800 W, argon pressure of 600 mPa). Once the power of the M0S2 targets reach 850 W, the process was maintained for 135 minutes.

[0163] Finally, once the coating was applied, all the electrical power to the magnetrons was switched off and the system was waiting until the chamber temperature is below 100 °C. After that, the vacuum chamber was vented, and the samples extracted and conveniently stored.

[0164] Figure 1 shows a schematic structure of the coating obtained as described above having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co. Figure 2 shows SEM-BES image of a cross-section view of the obtained the coating.

[0165] 2. Adherence tests

[0166] Figure 3 shows images of the adherence results in terms of adhesion over a AISI 440C mirror polished substrate. Coatings C and D (comprising the underlayer composition Cr / CrN / CrN+WC-Co / WC-Co according to the method of the present invention, and for coating D, additionally, the optimized etching described in this invention was previously applied) presented better adhesion to the substrate than coatings A and B (comprising standard underlayer composition WC-Co, and for coating B, additionally, the optimized etching described in this invention was previously applied) as it can be confirmed in Figure 4. Adherence performance was also tested for the coatings after being exposed to different environmental relative humidities (%). Figure 5 shows adherence results for 4 different coatings over a AISI 440C mirror polished substrate. Adhesion improvement was observed when the coatings according to the present invention were subjected to high humidity (80%).

[0167] 3. Tribological tests

[0168] Figure 6 shows endurance of tribotests for the four different coatings over a AISI 440C mirror polished substrate. Relevant improvement in terms of tribological properties was observed. Endurance of the coatings was significantly increased as it can be seen in Figure 6.

[0169] In addition, improvement in terms of tribological properties was observed for coatings comprising M0S2 obtained according to the method of the present invention in comparison with M0S2 pure (see Figure 7).

[0170] Durability improvement was also observed even for coatings obtained according to the method of the present invention at high humidity (Figure 8).

[0171] 4. Comparison of different substrates to be coated.

[0172] Tribological properties were measured for a coating obtained according to the method of the present invention (etching pretreatment and coating deposition) having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoSx+WC-Co when applied to different substrates. In particular, similar results were obtained when the coating was applied to AISI 440C and to TiAI6V4 substrates (Figure 9).

[0173] 5. Friction coefficient tests

[0174] Tribological tests were performed for a coating obtained according to the method of the present invention (etching pretreatment and coating deposition) having the layer composition Cr / CrN / CrN+WC-Co / WC-Co / MoS2+WC-Co on a stainless steel 316L substrate. Both configurations, pin-on-disk and reciprocating mode, were tested. Low coefficient of friction was observed for tests performed in both configurations (Tables 1 and 2, below).

[0175] Table 1. Results of friction coefficient tests in reciprocating mode under different conditions.

[0176]

[0177] Table 2. Results of tribological tests in pin-on-disc mode under different conditions.

Claims

CLAIMS1. A method for obtaining a multi-layer lubricant coating by magnetron sputtering, wherein the method comprises the following steps: a) placing a substrate for coating, a sputtering source of a metal selected from the group consisting of Cr, Ti and Zr, a sputtering source of WC, wherein the sputtering source of WC can additionally comprise Co, and a sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 in a vacuum chamber; b) introducing an inert gas in said vacuum chamber and forming a layer of a metal on the surface of said substrate by deposition using the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr; c) introducing an inert gas or a reactive gas in said vacuum chamber and forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr on the surface of the layer of the metal resulting from step b) by deposition using the sputtering source of the metal selected from the group consisting of Cr, Ti and Zr; d) forming a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr resulting from step c) by deposition using the sputtering source of WC while the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr is being deposited according to step c); e) forming a layer of WC or WC-Co on the surface of the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co resulting from step d) by deposition using the sputtering source of WC; and f) forming a top layer of a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, on the surface of the layer of WC or WC-Co resulting from step e) by deposition using the sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 while WC or WC-Co is optionally being deposited according to step e); wherein the deposition of each layer of steps b) and c) is performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique.

2. The method of claim 1 , wherein the method further comprises ion etching the sputtering samples used in steps b) to f) before their deposition.

3. The method according to claim 2, wherein the ion etching is performed in two stages comprising first applying a pulsed voltage, and second applying a high current of ionized plasma.

4. The method of any one of claims 1 to 3, wherein the vacuum chamber of step a) is at a pressure equal or lower than 0.5 mPa.

5. The method of any one of claims 1 to 4, wherein step b) comprises introducing an inert gas in the vacuum chamber to reach a pressure between 400 mPa and 800 mPa, and applying to the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr an electrical power between 0.5 kW and 1.5 kW.

6. The method of any one of claims 1 to 5, wherein step c) comprises introducing nitrogen gas inside the vacuum chamber, and applying to the sputtering source of a metal selected from the group consisting of Cr, Ti and Zr an electrical power between 4 kW and 5 kW.

7. The method of any one of claims 1 to 6, wherein the step d) comprises applying to the sputtering source of WC an electrical power between 2 kW and 3 kW while a nitride, a carbide or a boride of a metal selected from Cr, Ti and Zr is being deposited according to step c).

8. The method of any one of claims 1 to 7, wherein the step e) comprises applying to the sputtering source of WC an electrical power between 2 kW and 3 kW while the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr is stopped being deposited.

9. The method of any one of claims 1 to 8, wherein step f) comprises applying to the sputtering source of a metal compound selected from the group consisting of M0S2, WSe2, WS2 and MoSe2 an electrical power between 6.5 kW and 10.5 kW.

10. The method of any one of claims 1 to 9, wherein a Direct Current (DC) bias between 50 V and 200 V and a heating power between 1 kW and 2 kW are applied to the sputtering sources during each of step d) to f).

11. The method of any one of claims 1 to 9, wherein the deposition of the layers of at least one of the steps d) to f) is also performed using the High-Power Impulse Magnetron Sputtering (HIPIMS) technique.

12. The method according to any one of claims 1 to 11 , wherein the High-Power Impulse Magnetron Sputtering (HIPIMS) technique is performed at a signal frequency between 100 Hz and 2000 Hz, preferably 500 Hz, and a pulse duration between 50 ps and 200 ps, preferably 70 ps.

13. The method according to any one of claims 1 to 12, wherein the temperature of the vacuum chamber is maintained at a temperature between 200 °C and 300 °C during each of step b) to f).

14. A multi-layer lubricant coating formed on a substrate obtainable by the method according to any one of claims 1 to 13, wherein the coating comprises: i) an underlayer which is a multi-layer comprising sequentially- a layer of a metal selected from the group consisting of Cr, Ti and Zr,- a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr adjacent to the layer of a metal selected from the group consisting of Cr, Ti and Zr,- a layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co, adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and- a layer of WC or WC-Co adjacent to the layer of a nitride, a carbide or a boride of a metal selected from the group consisting of Cr, Ti and Zr, and WC or WC-Co; and ii) a top layer comprising a metal compound selected from the group consisting of MoSx, WSe2, WS2 and MoSe2, and optionally WC or WC-Co, and adjacent to the layer of WC or WC-Co, wherein the underlayer is interposed between the substrate and the top layer.

15. Use of the multi-layer lubricant coating according to claim 14 in automotive, aeronautical, space, metal-mechanic and cryogenic sectors.

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