Doped dlc for tribological applications

Abstract

The invention relates to a non-hydrogenated transition metal doped diamond-like carbon (DLC), wherein the non-hydrogenated DLC comprises at least one transition metal selected from the group consisting of group 4d, group 5d and group 6d of the periodic table of elements. A part of the at least one transition metal is present in the non-hydrogenated DLC as a matrix in the form of a carbide of the at least one transition metal and another part is present as metal droplets. The non-hydrogenated transition metal doped DLC has an indentation hardness of ≥ 35 GPa, preferably ≥ 40 GPa. Due to the presence of the metal droplets, the doped DLC very effectively improves the wear resistance and / or reduces the friction of a surface when a coating of the material is applied on the surface. Further, the invention provides a cathodic arc discharge deposition method for depositing a non-hydrogenated transition metal doped DLC coating according to the invention.

Description

Technical Field

[0001] This invention relates to non-hydrogenated transition metal-doped diamond-like carbon (DLC), its use in reducing surface friction, and layer systems including coatings thereof disposed on substrates. Furthermore, this invention relates to a method for depositing a non-hydrogenated transition metal-doped DLC coating according to the invention, the method being a cathodic arc discharge deposition method. Background Technology

[0002] Diamond-like carbon (DLC) is a metastable form of amorphous carbon. Widespread applications of DLC films have been discovered in science and technology. J. Robertson provides a comprehensive overview of the material and its applications in Materials Science and Engineering R 37, 2002, pp. 129-281. As Robertson reports, a distinction needs to be made between hydrogenated and non-hydrogenated forms of DLC. Another criterion for classifying DLC ​​is sp. 3 Fraction of sp 3 (bonding). For non-hydrogenated materials, sp 3 When the bonding fraction reaches a high level, the material is usually represented as tetrahedral amorphous carbon, abbreviated as ta-C.

[0003] S. Xu et al. reported in Philosophical Magazine Part B, 76:3, 351-361, on the deposition of ta-C films on silicon at room temperature using a filtered cathode vacuum arc technique. High sp[s] were obtained. 3 Bonding fraction (approximately 80% or higher). Compressive stress was found in the range of 7.5 GPa to 12 GPa, and hardness in the range of 20 GPa to 55 GPa. Maximum hardness and highest sp(t) concentration were found as determined by electron-energy-loss spectroscopy (EELS). 3 The scores are the same.

[0004] Doped carbon coatings have also been developed. A. Deu Gharam et al. investigated the high-temperature tribological properties of W-DLC on aluminum in Surface and Coatings Technology 206 (2011), 1905-1912. W-DLC coatings were deposited using a physical vapor deposition (PVD) system. Adding W to hydrogenated DLC resulted in a decrease in the coefficient of friction over a temperature range of 400°C to 500°C. The problem is that doping of hydrogenated DLC (e.g., with W) occurs at the atomic level via sputtering. Non-hydrogenated DLC (ta-C) exhibits better wettability to many lubricants and better temperature stability than hydrogenated DLC.

[0005] Z. Wang et al., in their paper *International Journal of Hydrogen Energy* 42 (2016), 5783-5792, deposited W-doped carbon on a stainless steel substrate. They used a closed-field unbalanced magnetron sputtering ion plating (CFUBMSIP) system with a bias voltage of -60 V. The CFUBMSIP system was equipped with one tungsten (W) target, two graphite targets, and was targeted at thin Cr seed layers and thin intermediate MC layers. x The deposition of layers (M being Cr and W) is equipped with a chromium (Cr) target.

[0006] K. Hou et al. deposited niobium (Nb)-doped amorphous carbon (aC) films on stainless steel substrates. As in the aforementioned scientific article by Z. Wang et al., they used the CFUBMSIP system. The bias voltage was -100V. The CFUBMSIP system was equipped with one Nb target, two graphite targets, and an additional titanium target. Niobium carbide was required to be embedded in the aC matrix, and the presence of pure niobium was reported. Simulations using first-principles calculations assuming one Nb atom in a supercell showed that sp... 3 The score is approximately 58%. A high score of 54% was obtained from fitting the XPS data. 3 Score. However, for the membrane to achieve a maximum sp of 54%, 3 In the fraction, the cathode current of the Nb cathode remains zero, so no Nb is sputtered from the target. However, Raman results of the Nb-doped aC film show that sp... 3 The score was low. Specifically, when considering ACFerrari's teachings in Diamond and Related Materials 11 (2002), 1053-1061, the score was approximately 2.5 out of 10. D / IG The ratios demonstrate this. No hardness data was reported in K. Hou et al.'s article.

[0007] D. Zhang et al. (Carbon 145 (2019), 333-344) used the CFUBMSIP system to dope amorphous carbon films with silver (Ag) or with films co-doped with silver and chromium. They found that higher percentages of dopant resulted in lower hardness and graphitization. Their reported simulated hardness showed fairly high values. For pure C, their simulations showed a simulated hardness of 56 GPa. However, their measurements showed that the compressive stress for all C-doped coatings ranged from 2.40 GPa to 3.37 GPa.

[0008] M. Andersson et al. (Vacuum 86 (2012), 1408-1416) deposited and characterized amorphous Cr-C films by magnetron sputtering. They used non-reactive DC magnetron sputtering from an elemental target. They found these films to be X-ray amorphous without crystallites. They reported a hardness of 6.9 GPa for approximately 15 at.% (i.e., atomic%) Cr dopant levels, increasing to 10.6 GPa for approximately 75 at.% Cr.

[0009] Y. Lin and S. Zhang investigated the effect of Cr addition on the properties of graphite-like carbon (GLC) films in J. Nanosci. Nanotechnol. 16 (2016), 12720-12725. The films were deposited by nonequilibrium magnetron sputtering. They reported a hardness of 10.4 GPa for pure C and a hardness as high as 17.4 GPa for Cr-doped GLC films. Although the dopant level was not explicitly mentioned, relatively low dopant levels can be expected with power increases from 0.1 kW to 0.3 kW for the Cr target and from 0 kW to 5 kW for the carbon target.

[0010] A. Amanov et al. (Tribology International 62 (2013), 49-57) deposited Cr-doped and undoped DLC films using unbalanced magnetron sputtering (UBMS). They reported a hardness of 22.47 GPa for Cr-doped DLC and 10.75 GPa for undoped DLC.

[0011] A.Ya. Kolpakov et al. (Nanotechnologies in Russia, 5 (2010), 160-164) deposited ta-C coatings doped with nitrogen, tungsten, or aluminum using a pulsed vacuum arc method. The level of the dopant (e.g., tungsten) was not mentioned, nor was the amount of W in the composite graphite cathode. The pulse repetition frequency in the pulsed vacuum arc method was 2.5 Hz. The doped carbon coating was described as having an amorphous structure and no crystalline impurities. For the W-doped film, its microhardness (HV) was found to be as high as less than 20 GPa. No evidence of pure W droplets was found. RH Horsfall's Proc. Soc. Vacuum Coaters (1998), 60-85, also describes the generation of ta-C using arc discharge. VN Inkin et al.'s Diamond and Related Materials 13 (2004), 1474-1479, also describes pulsed discharge to generate ta-C.

[0012] In summary, the hardness of pure C is quite low under unbalanced magnetron sputtering (excluding filters used to filter uncharged particles), with hardness reported to date in the range of 20 GPa. However, the addition of large amounts of dopants (typically in the range of 10 at.% or higher) shows an increase in hardness, which is due to the higher percentage of carbides reaching a hardness of about 22.5 GPa, as described in the aforementioned scientific article by A. Amanov et al.

[0013] As mentioned above, various applications of DLC membranes have been discovered in science and technology. Some studies have also investigated amorphous carbon membranes as protective coatings for bipolar plates (BBPs) in proton exchange membrane fuel cells (PEMFCs), which use polymer electrolyte membranes (PEMs) or proton exchange membranes (PEMs). BBPs play a crucial role in PEMFCs and PEMFC stacks. For example, they separate individual cells within a PEMFC stack, distribute and separate fuel gases, act as current collectors, facilitate the removal of heat and water, provide mechanical support for other components, and serve as the backbone of the FC stack.

[0014] For example, in the aforementioned scientific article, Z. Wang et al. deposited W-doped carbon on an austenitic stainless steel substrate using CFUBMSIP and investigated the interface contact resistance (ICR) and corrosion resistance. They observed that the W dopant level increased corrosion resistance between 2.54 at.% and 24.41 at.%. They obtained the best results at 2.54 at.% and 8.56 at.%. They attributed this corrosion resistance to the formation of a tungsten oxide layer.

[0015] In the scientific paper cited above, D. Zhang et al. found that amorphous carbon (aC) films doped with Ag and Cr simultaneously achieved low ICR. Furthermore, their tests showed that when used as a coating on metallic BPP with dopant levels of 4.89 at.% Ag and 12.37 at.% Cr, it improved lifetime, reduced corrosion, and less outward diffusion.

[0016] P. Yi et al. provided a review of carbon-based coatings for metallic BPPs in PEMFCs in the International Journal of Hydrogen Energy 44 (2019), 6813-6843. This review also mentions transition metal carbide (TMC) coatings.

[0017] Applications of DLC coatings in industry have also been discovered to reduce friction and improve wear resistance. Typically, hydrogenated diamond-like carbon (aC:H) has been applied for this purpose. The limitations of using hydrogenated diamond-like carbon lie in its hardness range of 20-35 GPa and its application temperature limitation of approximately 300°C. Another limitation is that low friction depends on the presence of water. To improve temperature stability, 4d, 5d, or 6d transition metal elements (such as W) are added to hydrogenated DLC. An additional effect of adding easily oxidized transition elements (such as W, Ta, V) to hydrogenated DLC is that the metal reacts with oxygen and increases lubricity at higher temperatures. The scientific article by A. Abou Gharam et al. cited above shows that adding W to hydrogenated DLC results in a reduction in the coefficient of friction over a temperature range of 400-500°C. The problem is that the addition of W-like dopants to hydrogenated DLC occurs only at the atomic level via sputtering.

[0018] Physical vapor deposition (PVD) and chemical vapor deposition (CVD, including plasma-assisted chemical vapor deposition (PACVD)) and their derivatives are used to enhance the properties of substrates. Performance improvements can involve, for example, improved wear resistance or reduced friction. To adjust coating properties, coatings can be doped with additional elements, altering the coating composition, texture, and internal stress levels. Commonly doped coatings have been widely used over the past 20 years. Extensive experimental work has been conducted on the properties of transition metal-doped hydride DLCs (aC:H:Me), summarized in S. Yazawa et al., Lubricants 2 (2014), 90-112. A beneficial effect of adding transition metals is that elements typically similar to W and Mo form sulfides with good lubricating properties. This can be particularly effective for elements that come into contact with both fuels and lubricants, as these elements may contain sulfur. Furthermore, W has been shown to have beneficial effects on the coefficient of friction of contacts lubricated with molybdenum dithiocarbamate (MODTC). B. Vengudusamy et al., in Tribology International 54 (2012), 68-76, provide a reference on the beneficial effects of tungsten incorporated into DLC layers for different lubricants. In WO 2014 / 000991A1, a method for introducing W into hydrogenated diamond-like carbon coatings using a metal-organic precursor is described. The primary reason for applying this method is to increase the deposition rate in the process.

[0019] In all the methods described above, the transition metal elements are in a plasma phase at the atomic level, and the atoms will arrive at the surface primarily in single-atom form. Due to the parallel flow of carbon in the above experiments, the transition metal will be atomically incorporated into the aC:H coating, thereby bonding the transition metal with carbon to form carbides. WJMeng et al., in J. of Appl. Phys. _88, (2000), 2415-2422, give a detailed description of Ti, for example, incorporated into hydrogenated DLC (AC:H).

[0020] In view of the above, there is a need for tribological coatings (coatings that reduce friction and improve wear resistance), which have improved temperature stability and better wettability to a variety of lubricants.

[0021] In summary, the industry is very interested in providing a coating material that exhibits excellent performance as a coating for BPP in fuel cells, and also possesses excellent tribological properties. Summary of the Invention

[0022] Non-hydrogenated transition metal doped DLC

[0023] This application satisfies these needs by providing a non-hydrogenated transition metal-doped diamond-like carbon (DLC) as defined in claim 1. Therefore, the non-hydrogenated DLC comprises at least one transition metal selected from Groups 4d, 5d, and 6d of the periodic table, and a portion of the at least one transition metal exists in the non-hydrogenated DLC as a matrix in the form of a carbide of the at least one transition metal. The non-hydrogenated transition metal-doped DLC is characterized in that it has a hardness of ≥35 GPa, preferably ≥40 GPa, wherein the hardness is measured on a film of the non-hydrogenated transition metal-doped DLC deposited on a polished substrate, and the indentation depth is less than 10% of the film thickness. Furthermore, the non-hydrogenated transition metal-doped DLC is characterized in that another portion of the at least one transition metal exists in the form of a droplet of the transition metal.

[0024] In this application, the non-hydrogenated transition metal-doped DLC according to claim 1 will occasionally be simply referred to as "doped DLC according to the invention". The transition metals selected from Group 4d, Group 5d and Group 6d present in the doped DLC according to the invention are sometimes simply referred to herein as "transition metals" or abbreviated as "TM".

[0025] The preamble of claim 1 is based on scientific articles by Z. Wang et al. and K. Hou et al., which observed amorphous WC and NbC phases, respectively. However, the hardness of the W-doped carbon film by Z. Wang et al. and the Nb-doped carbon film by K. Hou et al. is less than 35 GPa.

[0026] When the hardness is below 35 GPa, the coating is too soft.

[0027] For example, the hardness of the doped DLC according to the present invention can be in the range of 40 GPa to 60 GPa. More preferably, the hardness of the doped DLC according to the present invention is ≥45 GPa.

[0028] The hardness of the doped DLC according to the present invention is expressed in GPa. The hardness is measured by nanoindentation of a Vickers pyramidal indenter on a non-hydrogenated transition metal doped DLC film deposited on a flat, polished hardened substrate, with an indentation depth less than 10% of the film thickness. The surface roughness Ra of the flat, polished hardened substrate is 0.01 μm, and Rz is 0.25 μm. The flat, polished hardened substrate used has a hardness of 83.6 HRa (Rockwell hardness A, HRA), 62.1 HRc (Rockwell hardness C, HRC), and 747 HV10 (Vickers hardness at a 10 kgf load). Details of the hardness measurement can be found in the "Detailed Description of Preferred Embodiments" section of this application.

[0029] The doped DLC according to the present invention has a much higher hardness than BPP coatings known in the literature (e.g., Z. Wang et al. and K. Hou et al.), which is due to the higher sp... 3 Caused by the bond fraction, this sp 3 Bond fraction is typically higher than 60%. In non-hydrogenated DLC, hardness is related to sp. 3 Fractional correlation, i.e., based on sp, sp 2 and sp 3 In hybrid materials, carbon atoms (i.e., sp, sp) 2 and sp 3 The total number of bonded carbon atoms, expressed as sp. 3 The fraction of carbon atoms present in the bonded state. Doped DLCs according to the invention typically have ≥60%, preferably ≥70%, more preferably ≥80%, and most preferably ≥85% sp. 3 Fraction.

[0030] Due to the high sp of the doped DLC according to the present invention 3 The fraction, according to the present invention, the doped DLC can be represented as ta-C, i.e. tetrahedral amorphous carbon.

[0031] Due to the high hardness of the doped DLC according to the invention, and because it is a non-hydrogenated DLC, the doped DLC according to the invention is excellent as a coating to reduce friction and / or improve wear resistance of a surface (on which a coating is applied).

[0032] Raman spectroscopy can be used to evaluate the sp(s) of carbon atoms. 3 The presence of fractional and transition metal carbides, i.e., the presence of TM-C bonds in the doped DLC according to the present invention. For example, when TM is W, the presence of TM-C bonds can be detected in Raman spectroscopy by the presence of 80-150 cm⁻¹ at an excitation wavelength of 532 nm. -1 Detection is performed using peaks within a given range. Regarding sp... 3The fraction is determined by the position of the G peak in the Raman spectrum of the excitation wavelength used, and the D peak (I) D The intensity of ) and the G peak (I G The ratio of the intensity of ) (i.e., I) D / I G ), which can estimate sp 3 Score. This is described by ACFerrari in its aforementioned scientific article, and in the article's... Figure 2 As shown in the figure. The Raman excitation wavelength used in this paper is 532 nm. For the doped DLC sample according to the present invention, the position of the G peak is typically 1,605 cm⁻¹. -1 For this excitation wavelength, it points to the sp atoms of more than 60% of the carbon atoms. 3 Fraction.

[0033] In this invention, the compositional analysis of the doped DLC and material coating according to the invention is preferably performed by electron probe microanalysis (EPMA). In particular, the content of at least one transition metal in the doped DLC according to the invention can be determined by EPMA.

[0034] When the doped DLC according to the invention is designated as "non-hydrogenated," this means that no hydrogen is intentionally added during deposition, particularly no large amounts of hydrogen are added during the deposition of the material. However, small amounts of hydrogen from water vapor in the system can be present and incorporated into the doped DLC according to the invention. Therefore, due to such a hydrogen source, the "non-hydrogenated" doped DLC according to the invention can have a hydrogen content of <1 at.% particularly from water vapor in the system, for example when the material is deposited at a high productivity and the deposition chamber pumping and heating time is short.

[0035] For the same reasons described above for hydrogen, trace amounts of oxygen from water vapor and trace amounts of air in the system, and trace amounts of argon from trace amounts of air and an inert gas atmosphere in the system, can be present and incorporated into the doped DLC according to the invention. Therefore, due to this source of oxygen and argon, the non-hydrogenated doped DLC according to the invention can have an oxygen content of <1 at.% and an argon content of <1 at.% in particular from water vapor, trace amounts of air, and an inert gas atmosphere in the system (e.g., when the material is deposited at a high productivity and the deposition chamber pumping and heating time is short). Preferably, the oxygen content is <0.5 at.% and the argon content is <0.5 at.%. Most preferably, the oxygen content is <0.1 at.% and the argon content is <0.1 at.%.

[0036] The doped DLC according to the present invention is doped with at least one transition metal. The transition metal is selected from Groups 4d, 5d, and 6d of the periodic table. Therefore, the at least one transition metal is selected from the group consisting of: titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), and tungsten (W). All of the aforementioned transition metals used in the present invention can form carbides. Therefore, they can all exist in the form of carbides in the non-hydrogenated DLC used as the matrix as required by this application.

[0037] The content of at least one transition metal is not particularly limited, but according to a preferred embodiment, the content of the at least one transition metal, calculated based on a non-hydrogenated transition metal-doped DLC, is in the range of 0.1 at.% to 5 at.%. According to a preferred embodiment, the content of at least one transition metal in the doped DLC according to the invention is 0.2 at.% to 2.5 at.%, more preferably 0.3 at.% to 2.0 at.%, most preferably 0.5 at.% to 1.5 at.%, or the content is 1 at.% to 5 at.%, more preferably 2 at.% to 4 at.%.

[0038] When the doped DLC according to the invention is used as a tribological coating (i.e., for reducing surface friction and / or improving wear resistance, particularly for reducing surface friction), the content is advantageously in the range of 1 at.% to 5 at.%, more preferably 2 at.% to 4 at.%. This is because it has been found that for such a high dopant level of at least one transition metal, in the doped DLC according to the invention, the proportion of metal droplets relative to the transition metal as a carbide can be higher, and the metal droplets have a lubricating effect.

[0039] The non-hydrogenated transition metal-doped DLC according to the present invention is preferably a homogeneous material. This is because the doped DLC according to the present invention can be obtained in the cathode arc discharge deposition method of the present invention, and a homogeneous coating can be deposited. This is due to the fact that a carbon target doped with at least one transition metal is used as the target in the cathode arc discharge in the present invention. Examples using the literature of CFUBMSIP (e.g., Z. Wang and K. Hou in their above-mentioned scientific articles) have used cathodes with pure C and pure metals, wherein the cathodes are opposite each other in the deposition chamber (for a 2-cathode system) or at 90 degrees (for a 4-cathode system). Due to the rotation of the substrate stage, the concentration of the transition metal in these cases shows a layer-by-layer adjustment of the transition metal content on the order of 2 nm to 30 nm, depending on the rotation speed, deposition rate, and device layout. This is because when, for example, one TM target and two graphite targets (such as those used by Z. Wang and K. Hou in their above-mentioned scientific articles) are used in the CFUBMSIP method, the TM content of the coating is higher when facing the TM target during rotation and lower when facing the graphite target. In the doped DLC coating according to the invention, the aforementioned nanoscale adjustment of the transition metal content in the coating thickness direction, particularly in the 2 nm to 30 nm dimension, can be avoided. This means that in the doped DLC according to the invention, at least one transition metal (also present as a carbide or as a metal droplet) can be uniformly distributed throughout the coating, particularly uniformly distributed in the coating thickness direction.

[0040] In this invention, at least one transition metal in carbide form is preferably uniformly present (i.e., distributed) in the non-hydrogenated DLC serving as the matrix. The size of the TM carbide phase or structural domain is not particularly limited. The carbide may be present at the lower end as WC units distributed in atomic form and at the upper end as islands (particularly those in the nanometer range, i.e., “nanoscale islands”). As used herein, the nanometer range (i.e., nanoscale) is defined as including sizes from 0.1 nm to 100 nm. According to a preferred embodiment, the TM carbide is distributed in atomic form until nanoscale islands are formed, more preferably until nanoscale islands with a size of at most 2 nm are formed. More preferably, the carbide of at least one transition metal is present in the form of nanoscale islands with a size of about 0.5 nm to 2 nm. This proves advantageous for meeting the stringent requirements of a BPP coating, particularly for ensuring the presence of covalently bonded transition metals in the doped DLC according to the invention, thereby obtaining the desired low ICR. Typically, very hard ta-C coatings have high ICR. In this invention, the size of the transition metal carbide islands can be determined by transmission electron microscopy (TEM), particularly by bright field TEM (BFTEM) and high angle annular dark field with spot for scanning TEM (HAADF-STEM).

[0041] According to the invention, another portion (preferably other portions) of at least one transition metal exists in the form of metal droplets. The diameter of the droplets is less than 1 μm, preferably from 0.1 nm to 100 nm, and more preferably from 0.5 nm to 40 nm. These small TM droplets, distributed in the matrix of the doped DLC according to the invention and thus embedded in the coating of the material, can form a particularly effective source of free transition metal. Moreover, when a coating of the doped DLC according to the invention comprising such droplets is applied to the surface, it has been shown that droplets of this size most effectively reduce surface friction.

[0042] To distinguish between transition metals present as carbides and transition metals present as droplets in the non-hydrogenated DLC matrix, transmission electron microscopy (TEM) studies using bright-field TEM (BFTEM) and high-angle annular dark-field scanning TEM with a light spot (HAADF-STEM) were performed in this invention. The combination of BFTEM and HAADF-STEM with Raman spectroscopy allows for the differentiation of the two forms of transition metals present in the non-hydrogenated DLC matrix of the material according to this invention. Furthermore, the size of the metal droplets can be determined by TEM, particularly by the BFTEM and HAADF-STEM of this invention.

[0043] Furthermore, as will be shown in the specific embodiments below, W, having a melting point higher than 3400°C, can form metal droplets in the cathodic arc discharge deposition method according to the invention, these metal droplets being present in a non-hydrogenated DLC matrix according to a preferred embodiment of the invention. Therefore, this is also possible for other aforementioned transition metals with lower melting points used in the invention.

[0044] According to a preferred embodiment, based on the total content of transition metals in the material, a portion of at least one transition metal present in the doped DLC according to the invention in the form of carbides may be present in an amount of 60 at.% or less, more preferably 50 at.% or less, even more preferably 40 at.% or less, further preferably 30 at.% or less, and most preferably 20 at.% or less. As the inventors have discovered, there is almost no “free” transition metal (or segregated transition metal) in the doped DLC according to the invention, because it is neither in the form of carbides (e.g., in the form of carbide islands) nor in the form of metal droplets. That is, according to a preferred embodiment of the doped DLC according to the invention, at least 85 at.% of the total amount, preferably ≥90 at.% and more preferably ≥95 at.% of the at least one transition metal exists in the matrix of the non-hydrogenated DLC in the form of carbides (preferably as carbide islands) and / or in the form of metal droplets. Therefore, it is found that the remainder of at least one transition metal (i.e., the difference between the percentage of the transition metal present in carbide form and 100%) preferably exists in the form of metal droplets of transition metal. Thus, in a preferred embodiment of the invention, the aforementioned percentage of the transition metal in carbide form and the aforementioned percentage of the transition metal in the metal droplets total 100% of the transition metal present in the doped DLC of the present invention.

[0045] Layer system

[0046] The doped DLC according to the present invention can form a layer system comprising at least one layer disposed on a substrate. The thickness of the coating of the doped DLC according to the present invention and the thickness of one or more layers of the doped DLC according to the present invention in the layer system can be measured by SEM according to the present invention.

[0047] The substrate can be cleaned by ion bombardment to remove native oxides from the substrate surface. Ion bombardment of the substrate promotes adhesion of another layer to the top of the substrate before depositing another layer on top of the substrate. For example, the substrate can be cleaned by Ar etching (i.e., argon ion bombardment). In a layer system, an adhesion layer can be directly formed on the substrate, on which at least one layer of doped DLC according to the invention can be formed. The adhesion layer can be, for example, a layer of metallic Cr or metallic Ti. The layer system can also include multiple layers, wherein the multiple layers include at least one layer of doped DLC according to the invention. Preferably, the multiple layers are multiple layers of at least one layer of non-hydrogenated transition metal-doped DLC, wherein, based on the layer, the content of at least one transition metal is X at.%, and in the at least one layer of non-hydrogenated transition metal-doped DLC, based on the layer and / or at least one ta-C layer, the content of at least one transition metal is greater than 0 to 0.8 times X at.%. According to a preferred embodiment, there are therefore at least two layers in the multiple layers, one layer having a higher transition metal content and one layer having a lower transition metal content (or no transition metal at all, which is the case for the ta-C layer). In a multilayer, there may be more than two alternating layers of high transition metal and low / no transition metal. By ramping down the adhesive metal layer and ramping up the doped DLC or ta-C layer according to the invention, the layer system may also include a transition layer between the adhesive layer and a single layer of the doped DLC according to the invention, or a transition layer between the adhesive layer and a multilayer of the doped DLC according to the invention as described above, or a transition layer between the adhesive layer and a multilayer containing the doped DLC according to the invention as described above.

[0048] When the layer system according to the invention comprises a single layer of the doped DLC according to the invention, the thickness of the layer is preferably in the range of 50 nm to 3 μm, more preferably in the range of 80 nm to 1 μm. This also applies to each layer of the doped DLC according to the invention when it exists as a multilayer or in the form of multiple layers. The thickness of the multilayer is preferably 0.1 μm to 30 μm, more preferably 0.2 μm to 10 μm. This does not include the thickness of optional adhesion layers and optional transition layers present on the substrate, nor does it include the thickness of the substrate itself.

[0049] Tribological applications

[0050] When used as a tribological coating, the doped DLC according to the invention has been shown to exhibit better wettability and higher temperature stability to a variety of lubricants compared to hydrogenated DLC, as described by A. Abou-Gharam et al. in the aforementioned scientific article. Non-hydrogenated DLC has the additional advantage of exhibiting better wettability to many lubricants than hydrogenated DLC, see M. Kano, Tribology International 39 (2006), 1682-1685. The doped DLC according to the invention already exhibits low frictional stepping at 300°C and also improves wear resistance. Therefore, the invention also relates to the use of the doped DLC according to the invention to reduce surface friction and / or improve wear resistance by coating surfaces with the doped DLC.

[0051] The friction reduction effect is particularly noticeable because a portion of at least one transition metal exists in the doped DLC according to the invention as metal droplets. Especially for higher contents of at least one transition metal (e.g., in the range of 1 at.% to 5 at.% according to the doped DLC according to the invention), and for a higher proportion of at least one transition metal present as metal droplets compared to the portion present as carbides, the metal droplets further reduce friction. This is because the transition metal droplets are embedded within the matrix of the material and can form a free transition metal source with a lubricating effect.

[0052] It is understood that the required layer system, particularly a system with a single layer of DLC according to the invention with a thickness in the range of 0.5 μm to 3 μm (with an optional adhesive layer between the substrate and the DLC layer according to the invention), has proven to be very useful for improving the abrasion resistance of the substrate surface and / or reducing friction on the substrate surface.

[0053] Deposition methods

[0054] As described in claim 1, non-hydrogenated DLC having a hardness of up to ≥35 GPa, preferably ≥40 GPa, can be designated as "ta-C", and can typically produce an sp content greater than 60% only when a high plasma density (in other words, a high degree of ionization) is present. 3 Fractional non-hydrogenated DLC. Therefore, the doped DLC according to the present invention can be obtained without utilizing standard unbalanced magnetron sputtering methods such as those employed by Z. Wang et al. and K. Hou et al. Unlike high-power pulsed magnetron sputtering (HIPIMS), the degree of ionization is too low for standard unbalanced magnetron sputtering.

[0055] This invention also provides a method for depositing a non-hydrogenated DLC coating comprising at least one transition metal selected from Groups 4d, 5d, and 6d of the periodic table. The method is a cathodic arc discharge deposition method. In the cathodic arc discharge, a direct current (DC) is superimposed with a pulsed current. The pulsed current has a pulse frequency in the range of 10 kHz to 100 kHz. A carbon target doped with at least one transition metal is used as the target in the cathodic arc discharge. The target is directly connected to the cathode. As measured on the cathode, each pulse of the pulsed current causes a voltage rise at a rate greater than 5 V / μs. Each pulse of the pulsed current has an effective pulse width of less than 30 μs. In this method, the ionization degree of the evaporated target material is close to 100%.

[0056] The dopant level of the target can be in the range of 0.5 at.% to 10.0 at.%, preferably 1.0 at.% to 8 at.%, more preferably 1.0 at.% to 6 at.%, and even more preferably 0.5 at.% to 2.5 at.%, of at least one transition metal selected from Groups 4d, 5d, and 6d of the periodic table. The at least one transition metal can be a transition metal as described above, such as tungsten (W). Using electron probe microanalysis (EPMA), the inventors found that approximately 60% of the transition metal is present as a dopant in the target in the doped DLC according to the invention. For example, 8 at.% of the W dopant in the target is 5 at.% in the coating of the doped DLC according to the invention, while 2 at.% of the W in the target is 1.2 at.% in the coating.

[0057] Cathodic arc discharge deposition is a physical vapor deposition (PVD) technique in which an electric arc is generated between an anode and a target that acts as a cathode or is connected to a cathode. Within the region where the arc exists, the arc evaporates the material on the target surface. The evaporated target material is deposited onto a substrate to form a coating of the target material on the substrate. A. Anders describes cathodic arc discharge deposition in detail in his textbook "Cathodic ARCs," Springer, 2008, ISBN 978-0-387-79107-4.

[0058] The difference between cathodic arc sputtering and standard unbalanced magnetron sputtering lies in the fact that the ionization of atoms to be deposited is much higher in cathodic arc discharge than in unbalanced magnetron sputtering. A. Anders, in “Cathodic ARCs”, Springer, 2008, ISBN 978-0-387-79107-4, pp. 194-195, paragraph 4.3, describes the ionization of carbon (C) in a DC arc, applicable also to an arc current of 200 A, where the carbon (C) is 100% individually ionized and even partially double-ionized at higher currents. The ionization of carbon by magnetron sputtering is much lower; typical values ​​for C sputtering are in the range of 5%. Lifschitz et al. (Physical Review B, Vol. 41, No. 15, pp. 10468-10480) have described higher sp... 3 The fractional ta-C is due to a sub-plantation process. Carbon atoms arrive at sufficiently high energy to penetrate the surface of the growth film, typically to a depth of three atomic layers. At this depth, the implanted atoms experience high pressure, and spc is formed due to this pressure. 3 Bond. In the case of standard (non-equilibrium) magnetron (non-HIPIMS), due to the low degree of ionization and the low average energy, most C atoms cannot undergo subimplantation.

[0059] In the cathodic arc discharge deposition method of the present invention, a cathodic arc discharge is generated or supplied by supplying a direct current superimposed on a pulsed current. By employing this current superposition, the generation of large particles of the target material (i.e., large particles of carbon doped with at least one transition metal) during the cathodic arc discharge process can be significantly reduced. Furthermore, the formation of relatively deep pits with sharp edges on the target surface can be minimized. In particular, the pulsed current superimposed on the direct current causes the arc to split into multiple arcs, jumping out of the pit of the first arc, and in the process, the edges are rounded by evaporation, thereby avoiding the formation of pits with sharp edges, especially deep pits. Such sharp edges can be released from the target as large particles of the target material, thereby affecting the quality of the coating to be formed. In this method, the pulse of the pulsed current has a high rise rate. Therefore, the splitting into multiple arcs occurs within a particularly short time period.

[0060] Therefore, high-quality coatings can be achieved. Furthermore, since the formation of these large particles can be suppressed, there is no need to use large-particle filters in the deposition method, thus allowing for a significant simplification of the process.

[0061] The target is directly connected to the cathode, meaning there is no intermediate layer or structure between the target and the cathode.

[0062] The pulse frequency of the pulsed current is in the range of 10 kHz to 100 kHz, preferably in the range of 20 kHz to 90 kHz, more preferably in the range of 30 kHz to 80 kHz, and even more preferably in the range of 40 kHz to 70 kHz. By selecting a pulse frequency in the range of 10 kHz to 100 kHz, the formation of large particles in the target material can be suppressed particularly effectively and reliably.

[0063] For production applications, the cathodic arc discharge deposition process can be performed in a deposition chamber (especially a vacuum chamber) equipped with multiple cathodes. For example, if there are simultaneously deposited cathode groups, the arc pulses can be synchronized with the delay sets between different arc sources, so that only one pulse occurs at a time, thereby avoiding overload of the bias voltage power supply.

[0064] In "Production of highly ionized species in high-current pulsed cathodic arcs" (R. Sanginés, AMIsrael, ISFalconer, DR McKenzie and MM M Bilek, Applied Physics Letters 96, 221501

[2010] ) Figure 2 As shown, in Al deposition by a pulsed arc with a relatively long pulse of 600 μs and a peak current of 800 A, double-ionized Al is formed, but reaches its maximum after 40 μs. It then rapidly decreases, leaving only individually ionized Al. The arc splits continuously at a rate of approximately 60 A increments. The arcs repel each other. The fact that double ionization decreases significantly after 100 μs implies that the plasma resembles a DC arc after this period.

[0065] To maintain a state of high ionization without recombination with neutral particles returning to standard DC arc conditions, this method uses a high pulse voltage rise rate, ensuring that all arcs remain close to each other and that the effective pulse width of each pulse of the pulsed current remains relatively short. The maximum time the arc voltage continues to increase is less than 30 μs.

[0066] Each pulse of the pulsed current superimposed on the direct current causes the voltage measured at the cathode (i.e., the arc discharge voltage) to rise at a rate exceeding 5 V / μs. The voltage can be measured between the cathode and the anode. The anode can be at ground potential.

[0067] Using pulses that induce such a high voltage rise rate at the cathode (the target is directly connected to the cathode) enables the generation of plasma with exceptionally high plasma intensity, density, and temperature. By generating such a high-density plasma, molten transition metal droplets form on the surface of the target. Furthermore, the high-density plasma ensures that larger transition metal droplets released from the target surface are broken down into smaller droplets during their transfer from the target to the substrate to be coated, thus reducing droplet size. Therefore, the quality of the coating can be further improved. In particular, incorporating these smaller droplets into the coating can significantly enhance its conductivity, which is particularly beneficial in coatings on bipolar plates, such as those in fuel cells or electrolyzers. Furthermore, the droplets can improve the coating's friction reduction properties. Because the high-density plasma reduces the droplet size during transfer from the target to the substrate, the incorporation of large droplets into the coating can be avoided, resulting in a coating with a flat and uniform surface structure.

[0068] Preferably, the rate of voltage rise caused by the pulse measured at the cathode is greater than 8V / μs, more preferably greater than 10V / μs, even more preferably greater than 12V / μs, even more preferably greater than 14V / μs, and even more preferably greater than 16V / μs.

[0069] Each of the pulses of the pulsed current superimposed on the DC current has an effective pulse width of less than 30 μs. The effective pulse width is defined as the time before the arc current induced by the pulse decays. The arc current induced by the pulse is measured at the cathode. When the pulse power supply to the pulse is cut off, the arc current induced by the pulse may begin to decay. By cutting off the pulse after a relatively short time of less than 30 μs, arc separation into a state where the distance between the arcs is large and the plasma density decreases to approximately the DC arc plasma density can be reliably avoided.

[0070] The effective pulse width of the pulse current can be 1 μs or greater and less than 30 μs, preferably in the range of 2 μs to 20 μs, and more preferably in the range of 4 μs to 10 μs. Particularly preferably, the effective pulse width of the pulse current can be 5 μs.

[0071] The direct current can be in the range of 50A to 1000A, preferably in the range of 100A to 800A, more preferably in the range of 200A to 600A, and even more preferably in the range of 250A to 500A. Selecting a direct current in the range of 50A to 1000A can further enhance the formation of droplets of molten transition metal at the target surface.

[0072] The peak current of the DC current superimposed with the pulsed current can exceed 200 A. The DC current superimposed with the pulsed current is measured at the cathode. By selecting a peak current greater than 200 A, the droplet size can be reduced in a particularly effective manner using high-density plasma during droplet transfer from the target to the substrate.

[0073] The pulse interval of the pulse current can be in the range of 20 μs to 200 μs, preferably in the range of 40 μs to 150 μs, and more preferably in the range of 60 μs to 120 μs. Particularly preferably, the pulse interval of the pulse current can be 80 μs.

[0074] The method of the present invention can be carried out at a deposition temperature in the range of 50°C to 180°C, preferably in the range of 70°C to 150°C. The deposition temperature is measured at the substrate to be coated. Particularly preferably, the deposition temperature is maintained between 70°C and 150°C. By controlling the deposition temperature to not exceed 150°C, it can be ensured particularly reliably that the Young's modulus and hardness of the coating will not decrease significantly.

[0075] In the cathodic arc discharge deposition process, a bias voltage can be applied to the substrate. The bias voltage can be in the range of 10V to 100V, preferably in the range of 20V to 80V, and more preferably in the range of 40V to 60V. Particularly preferably, the bias voltage can be 50V.

[0076] The background pressure in the cathode arc discharge deposition process can be 1×10 -5 mbar to 5×10 -4 Within the range of mbar, preferably within 2×10 -5 mbar to 1×10 -4 Within the range of mbar, more preferably within 4×10 -5 mbar to 6×10 -5 Within the range of mbar. Particularly preferably, the background pressure in the cathode arc discharge deposition process can be 5 × 10⁻⁶ mbar. -5 mbar.

[0077] The cathode arc discharge deposition process can be performed in an atmosphere containing an inert gas. More specifically, the cathode arc discharge deposition process can be performed in an atmosphere containing argon (Ar) or nitrogen (N2). In particular, the cathode arc discharge deposition process can be performed in a deposition chamber in which such a gas is introduced. The background pressure of the inert gas (especially Ar) can be 1 × 10⁻⁶. -4 mbar to 9×10 -4 Within the range of mbar, preferably within 2×10 -4 mbar to 8×10 -4 Within the range of mbar, more preferably within 4×10 -4mbar to 6×10 -4 Within the range of mbar. Particularly preferably, the background pressure of the inert gas (especially Ar) can be 5 × 10⁻⁶ mbar. -4 mbar. By performing the cathodic arc discharge deposition process in such an atmosphere, the ignition of the arc can be promoted.

[0078] Before depositing the coating, the substrate to be coated can be cleaned, for example, by ion etching using Ar or metal ions.

[0079] The coating can be deposited directly on the surface of the substrate, i.e., without any intermediate layer between the surface and the coating. Alternatively, an initial adhesion layer can be provided on the surface of the substrate to be coated before the coating is deposited onto the substrate. The initial adhesion layer can be, for example, a chromium (Cr) layer or a titanium (Ti) layer. There are no restrictions on the method for applying the initial adhesion layer, and any CVD or PVD method, including sputtering, can be used.

[0080] During the cathodic arc discharge deposition process, deposition parameters (e.g., characteristics of DC current and pulse current, deposition temperature, substrate bias, gas atmosphere, and pressure) can be kept at least substantially constant.

[0081] In some implementations, the pulse frequency of the pulsed current can be in the range of 20 kHz to 90 kHz. The dopant level of the target can be in the range of 0.5 at.% to 10.0 at.%. As measured on the cathode, each pulse of the pulsed current can cause a voltage rise at a rate greater than 8 V / μs. The effective pulse width of each pulse of the pulsed current can be in the range of 2 μs to 20 μs. The peak current can be higher than 200 A. The pulse interval of the pulsed current can be in the range of 20 μs to 200 μs. The method can be carried out at deposition temperatures in the range of 50 °C to 180 °C. In the cathode arc discharge deposition process, a bias voltage in the range of 10 V to 100 V can be applied to the substrate. The background pressure in the cathode arc discharge deposition process can be 1 × 10⁻⁶. -5 mbar to 5×10 -4 Within the range of mbar.

[0082] The coating deposited by the method of the present invention is a coating of doped DLC according to the present invention. The coating deposited by the method of the present invention may have the above-described features, properties and characteristics.

[0083] The inventors have discovered that a higher content of transition metal (e.g., tungsten) in the target results in a relatively greater number of metal droplets in the doped DLC coating according to the invention. This is readily understood, as large particles of molten transition metal separate along the arc trajectory on the target surface under the influence of the electric arc. When the arc strikes such transition metal particles, the droplets are ejected and broken into smaller droplets due to the high-density plasma caused by a high voltage rise rate greater than 10 V / μs, while the metal droplets travel from the target to the substrate to be coated, thereby reducing the droplet size. The percentage of transition metal (e.g., W) that separates as metal droplets on the target surface is in a more than linear proportion to the dopant level of the target. In this way, the relative proportions of transition metal present in the form of carbides and transition metal present in the form of metal droplets in the doped DLC according to the invention can be adjusted. Figure 8 An example of a target with 8 at.% W exposed to an electric arc is shown. Small pits can be seen on the target surface. The white droplets are W.

[0084] The present invention also provides a coating of non-hydrogenated transition metal-doped DLC that can be obtained by the method of the present invention.

[0085] Implementation Plan

[0086] Specific embodiments of the present invention will be outlined below.

[0087] (1) A non-hydrogenated transition metal-doped diamond-like carbon (DLC), wherein the non-hydrogenated metal-doped DLC comprises at least one transition metal selected from Groups 4d, 5d, and 6d of the periodic table, and a portion of the at least one transition metal exists in the non-hydrogenated DLC as a matrix in the form of a carbide of the at least one transition metal, and wherein the non-hydrogenated transition metal-doped DLC has a hardness ≥35 GPa, preferably ≥40 GPa. The hardness can be measured on a film of the non-hydrogenated transition metal-doped DLC deposited on a polished substrate, the film of the non-hydrogenated transition metal-doped DLC having an indentation depth less than 10% of the film thickness.

[0088] (2) The non-hydrogenated transition metal doped DLC according to item (1), wherein at least a portion of the carbide of at least one transition metal exists as an island in the non-hydrogenated DLC as a matrix.

[0089] (3) The non-hydrogenated transition metal doped DLC according to item (2), wherein the island has a size of up to 2 nm.

[0090] (4) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (3), wherein the non-hydrogenated DLC is tetrahedral amorphous carbon, i.e., ta-C.

[0091] (5) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (4), wherein another portion of at least one transition metal exists in the form of a droplet of metal as a transition metal.

[0092] (6) The non-hydrogenated transition metal doped DLC according to item (5), wherein the diameter of the transition metal droplets is less than 1 μm, preferably from 0.1 nm to 100 nm, and more preferably from 0.5 nm to 40 nm.

[0093] (7) The non-hydrogenated transition metal-doped DLC according to item (5) or (6), wherein at least 85 at.% and preferably at least 90 at.% of a total transition metal are present in the matrix of the non-hydrogenated DLC in the form of a carbide, preferably as islands of carbides and / or in the form of metal droplets.

[0094] (8) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (7), wherein the carbon atoms of the non-hydrogenated transition metal-doped DLC have sp... 3 The fraction is ≥60%, preferably ≥70%, more preferably ≥80%, and most preferably ≥85%.

[0095] (9) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (8), wherein the transition metal is selected from the group consisting of chromium, molybdenum and tungsten, and is preferably tungsten.

[0096] (10) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (9), wherein, based on the non-hydrogenated transition metal-doped DLC, the content of at least one transition metal is in the range of 0.1 at.% to 5 at.%, preferably in the range of 0.2 at.% to 2.5 at.%, more preferably in the range of 0.3 at.% to 2.0 at.%, and most preferably in the range of 0.5 at.% to 1.5 at.%.

[0097] (11) The non-hydrogenated transition metal-doped DLC according to any one of items (1) to (10), wherein the hardness of the non-hydrogenated transition metal-doped DLC is in the range of 40 GPa to 60 GPa.

[0098] (12) A layer system comprising at least one layer of non-hydrogenated transition metal doped DLC disposed on a substrate according to any one of items (1) to (11).

[0099] (13) The layer system according to item (12), wherein the layer is a homogeneous layer.

[0100] (14) The layer system according to item (12) or (13), wherein the thickness of the layer is in the range of 50 nm to 3 mm.

[0101] (15) A layer system according to any one of items (12) to (14), the layer system comprising the following multiple layers:

[0102] At least one layer of non-hydrogenated transition metal-doped DLC, wherein, based on the layer, the content of at least one transition metal is X at.%, and

[0103] At least one layer of non-hydrogenated transition metal-doped DLC, wherein, based on the layer and / or at least one ta-C layer, the content of at least one transition metal is greater than 0 to 0.8 times X at.%.

[0104] (16) The layer system according to item (15), wherein the thickness of the multilayer is in the range of 0.1 μm to 30 μm, preferably in the range of 0.2 μm to 10 μm.

[0105] (17) The layer system according to any one of items (12) to (16), wherein an adhesion layer is directly disposed on the substrate, and at least one non-hydrogenated transition metal doped DLC layer is formed on top of the adhesion layer.

[0106] (18) The layer system according to any one of (12) to (17), wherein the substrate is a metal substrate.

[0107] (19) The layer system according to any one of (12) to (18), wherein the metal substrate is a stainless steel substrate, a titanium substrate or an aluminum substrate.

[0108] (20) The layer system according to item (19), wherein the metal substrate is a stainless steel substrate.

[0109] (21) The layer system according to item (19), wherein the metal substrate is a titanium substrate.

[0110] (22) Use of non-hydrogenated transition metal doped DLC according to any one of items (1) to (11) in improving the wear resistance of a surface and / or reducing the friction of a surface by applying a coating of non-hydrogenated transition metal doped DLC to the surface.

[0111] (23) The use according to item (22), wherein the non-hydrogenated transition metal doped DLC has at least one transition metal in a content ranging from 1 at.% to 5 at.%.

[0112] (24) The use according to item (22) or (23), wherein the coating has a thickness in the range of 0.5 μm to 3 μm.

[0113] (25) A method for depositing a coating of a non-hydrogenated DLC comprising at least one transition metal selected from Groups 4d, 5d and 6d of the periodic table, wherein the method is a cathodic arc discharge deposition method, wherein a direct current is superimposed with a pulsed current in the cathodic arc discharge, wherein the pulse frequency of the pulsed current is in the range of 10 kHz to 100 kHz, wherein a carbon target doped with at least one transition metal is used as a target in the cathodic arc discharge, the target being directly connected to the cathode, wherein each pulse of the pulsating current causes a voltage, as measured on the cathode, to rise at a rate greater than 5 V / μs, and wherein the effective pulse width of each pulse of the pulsed current is less than 30 μs.

[0114] (26) The method according to item (25), wherein the peak current is higher than 200A.

[0115] (27) The method according to item (25) or (26), wherein the coating is a coating of the non-hydrogenated transition metal doped DLC as defined in any one of items (1) to (11).

[0116] (28) A coating of non-hydrogenated transition metal doped DLC as defined in any of items (1) to (11), the coating being obtained by a method as defined in item (25) or (26). Attached Figure Description

[0117] Figure 1 A cross-sectional view is shown for performing a deposition method using a cathode arc discharge deposition apparatus according to an embodiment of the present invention.

[0118] Figure 2 Raman spectra of specific embodiments of the doped DLC coating according to the present invention are shown. The samples have different dopant levels, i.e., the content of transition metals (i.e., W), as follows: B1: 0 at.% (for reference); B2: 0.3 at.%; B3: 0.6 at.%; B4: 1.4 at.%; B5: 1.4 at.%. The Raman excitation wavelength is 532 nm, with a 50 cm⁻¹... -1 Edge filter.

[0119] Figure 3 A scanning electron microscope (SEM) image of W-doped ta-C at a W concentration of 1.4 at.% at 80 nm, according to a specific embodiment of the doped DLC of the present invention, is shown. A wafer window with a Si3N4 film is visible, with doped ta-C deposited on top of the wafer window. The dark spheres are W.

[0120] Figure 4High-resolution transmission electron microscopy (HRTEM) image of ta-C doped with 1.4 at.% W in HAADF-STEM mode, according to an embodiment of the doped DLC of the present invention. Due to focusing, only 20 nm of the plate is observed as a projection.

[0121] Figure 5 In a specific embodiment of the doped DLC according to the present invention, an HR-TEM bright field of 5 at.% W-doped ta-C at 80 nm is formed on top of a 20 nm Si3N4 substrate. The dark spheres are W droplets.

[0122] Figure 6 This is a TEM micrograph of ta-C doped with 5 at.% W at 80 nm in dark field mode, according to a specific embodiment of the doped DLC of the present invention. The bright spots are pure W droplets embedded in the ta-C matrix.

[0123] Figure 7 HAADF-STEM image of ta-C doped with 5 at.% W, according to a specific embodiment of the doped DLC of the present invention. W is shown in white because the photomicrograph was taken in dark field mode.

[0124] Figure 8 SEM image of a graphite target containing 8 at.% W after deposition using the cathode arc discharge deposition method according to the present invention. White droplets are W.

[0125] Detailed description of preferred embodiments

[0126] Deposition methods

[0127] In the following text, reference will be made to Figure 1 A method for depositing a non-hydrogenated DLC coating according to an embodiment of the present invention is described, the non-hydrogenated DLC coating comprising at least one transition metal selected from Group 4d, Group 5d and Group 6d of the periodic table.

[0128] In this implementation scheme, the following is used: Figure 1 The cathode arc discharge deposition apparatus 2 shown is used to perform this method. Apparatus 2 includes a deposition chamber 11, an anode 4, a target 6, a cathode 7, and a power supply 8 electrically connected to the anode 4 and cathode 7. The cathode 7 can be made of a metal such as copper (Cu). For example, the cathode 7 can be a metal plate, such as a Cu plate. The cathode 7 can be cooled, for example, by water cooling. The anode 4 is integral with the wall of the deposition chamber 11. The anode 4 can form part of the wall of the deposition chamber 11. The anode 4 can be made of the same material as the wall of the deposition chamber 11, such as a metal such as steel. The anode 4 has a substantially annular shape, which... Figure 1The image is shown in cross-section. For example, the anode 4 can be a substantially annular metal plate, such as a substantially annular steel plate. In particular, the anode 4 can be a substantially annular flat plate, such as a substantially annular flat metal plate. The anode 4 and the cathode 7 are arranged in a plane substantially perpendicular to the direction from the cathode 7 toward the target 6 (see [link to image]). Figure 1 The cathode 7 is arranged within the central opening of the substantially annular anode 4.

[0129] The target 6 is directly connected to the cathode 7, meaning there is no intermediate layer or structure between the target 6 and the cathode 7. Specifically, the surface of the target 6 (e.g., the back surface) can contact (e.g., in full contact) the cathode 7 (e.g., the body of the cathode 7). The target 6 can be mounted to the cathode 7, for example, by bolt connection (not shown). The target 6 is directly electrically connected to the cathode 7, for example, through direct contact between the back surface of the target 6 and the cathode body.

[0130] Power supply 8 is configured to supply a direct current superimposed on a pulsed current to generate an electric arc between target 6 and anode 4. In the region where the arc exists, the arc evaporates the material at surface 10 of target 6. The evaporated target material 12 (see...) Figure 1 The target material is transferred from the target 6 to the substrate 14 and deposited on the substrate 14 to form a coating of the target material on the substrate.

[0131] Anode 4, target 6, cathode 7, and substrate 14 are arranged within a space formed inside deposition chamber 11. Deposition chamber 11 can be a vacuum chamber, such as an ultrahigh vacuum (UHV) chamber. Anode 4 is integral with and connected (i.e., electrically connected) to the wall of deposition chamber 11. Power supply 8 is electrically connected to anode 4 via the wall of deposition chamber 11 (see [link to power supply]). Figure 1 Specifically, the walls of the deposition chamber 11 can be made of a conductive material such as metal, thereby establishing an electrical connection between the power source 8 and the anode 4. In other embodiments, a plurality of cathodes 7 can be arranged in the deposition chamber 11.

[0132] The background pressure in sedimentation chamber 11 can be 5 × 10⁻⁶. -5 mbar. The cathode arc discharge deposition process of this embodiment can be carried out in an atmosphere containing an inert gas (particularly Ar) in the deposition chamber 11. For example, the pressure of Ar in the deposition chamber 11 can be 5 × 10 mbar. -4 mbar.

[0133] In this embodiment, target 6 is a carbon target doped with tungsten (W). Therefore, a non-hydrogenated DLC coating containing W is formed on substrate 14 by a deposition process. The dopant level of target 6 ranges from 0.5 at% to 8.0 at% W.

[0134] The superimposed current supplied by power supply 8 has the following properties: A 50A DC current is superimposed with a pulsed current having a pulse frequency in the range of 10kHz to 100kHz. The pulsed current has an effective pulse width of 5μs and a pulse interval of 80μs. The peak current of the DC current superimposed with the pulsed current is higher than 200A. The DC current and the pulsed current are measured at cathode 7.

[0135] Each pulse of the pulsed current causes the voltage measured at cathode 7 (i.e., the arc discharge voltage) to rise at a rate greater than 5 V / μs. The voltage is measured between cathode 7 and anode 4. Anode 4 is at ground potential.

[0136] By employing this current superposition, the generation of large particles in the target material during the cathode arc discharge process can be significantly reduced. Furthermore, the occurrence of pits on the target surface can be minimized. In addition, it ensures that small-sized molten W droplets are bound within the coating, thereby enhancing the coating's conductivity and friction-reducing properties. Therefore, a particularly high-quality coating can be provided on the substrate 14.

[0137] The pulsed current superimposed on the direct current causes the arc to split into multiple arcs, as described in detail above. In this embodiment, the current (i.e., the arc current) of each individual arc obtained through this split can be, for example, about 60 A. At the peak current, the total number of arcs can be, for example, five or six. If the arc current remains at a high level, the arcs obtained by the arc split caused by the pulse repel each other, and the plasma characteristics become similar to those of a DC arc. Therefore, the effective pulse width remains short, i.e., less than 30 μs. In this embodiment, the effective pulse width is 5 μs, as described in detail above.

[0138] In this embodiment, the cathode arc discharge deposition process is performed at a deposition temperature ranging from 70°C to 150°C. The deposition temperature is measured at substrate 14. Furthermore, a bias voltage of 50V is applied to substrate 14.

[0139] During the cathode arc discharge deposition process of this embodiment, deposition parameters (e.g., characteristics of DC current and pulse current, deposition temperature, substrate bias, gas atmosphere and pressure) remain substantially constant.

[0140] Before depositing the coating, the substrate 14 can be cleaned, for example, by Ar etching. The coating can be deposited directly on the surface of the substrate 14. Alternatively, an initial adhesion layer can be formed on the surface of the substrate 14 before depositing the coating. The initial adhesion layer can be, for example, a chromium (Cr) layer or a titanium (Ti) layer.

[0141] From the initial adhesion layer to the doped carbon layer, the metal deposition rate may change abruptly or gradually decrease, while the doped carbon deposition rate will increase rapidly. Example

[0142] The invention will be further illustrated by means of embodiments, which should not be construed as limiting.

[0143] The coating was deposited by running a cathodic arc discharge on a W-doped carbon target. The arc discharge was a DC arc superimposed with pulses. Samples were prepared using a 50 A DC arc current superimposed with pulses of 5 μs width, 80 μs interval, and a peak arc current exceeding 200 A. W dopant levels ranging from 0.5 at.% to 5 at.% were applied. The deposition temperature was maintained between 70 °C and 150 °C. A bias energy voltage of 50 V was applied to the substrate. The background pressure was typically 5 × 10⁻⁶. -5 mbar. To achieve proper arc ignition, a small amount of Ar is permitted in the chamber, typically up to 5 × 10⁻⁶. -4 Ar pressure in mbars.

[0144] Prior to deposition, the product to be coated (i.e., the substrate) is cleaned by Ar etching (i.e., argon ion bombardment). Adding an initial adhesion layer (such as metallic Cr or metallic Ti) has been applied to many samples, but coatings without an adhesion layer have also been produced.

[0145] During the deposition step, the ta-C coating is deposited without changing any parameters. This means that conditions such as pressure, gas atmosphere, bias voltage, and substrate temperature remain constant during the deposition process.

[0146] analyze

[0147] Compositional analysis

[0148] The compositional analysis of the coatings was performed using an electron probe microanalysis (EPMA) system with an accelerating voltage of 5 keV, a current of 200 nA, and 10 test probes per sample. Detailed compositional analysis is shown in Table 1.

[0149]

[0150] Table 1: Overview of Coating Research

[0151] hardness

[0152] The hardness is indentation hardness (HIT) and has been measured by nanoindentation using a diamond indenter on a flat, polished, hardened substrate according to ISO 14577 (i.e., the English versions of ISO 14577-1:2015, ISO 14577-2:2015, and ISO 14577-4:2016, dated November 1, 2016). The diamond indenter used has Vickers hardness. That is, the indenter is a drill bit with a pyramidal shape based on a square, with the plane at 220° relative to the horizontal plane; or in other words, the indenter is shaped as an orthogonal pyramid (Vickers hardness pyramid) with a square base and an angle of 68° between the axis of the diamond cone and one of the faces. Hardness is expressed in GPa. The substrate roughness Ra is less than 0.06 μm. Specifically, the surface roughness Ra of the substrate (test plate) was 0.01 μm, and Rz was 0.25 μm. The flat, polished, hardened substrate used had a hardness of 83.6 HRa (Rockwell hardness A, HRA), 62.1 HRc (Rockwell hardness C, HRC), and 747 HV10 (Vickers hardness at a 10 kgf load). The test plate (i.e., the flat, polished, hardened substrate) was 15 × 6 mm in size. The indenter load and the coating thickness deposited on the test plate were selected such that the indentation depth was less than 10% of the coating thickness. Ten points were measured for each sample (uniformly distributed on the surface of the coating to be measured), and the average of the ten hardness indentation values ​​HIT measured therefrom was used (in GPa). The relationship between the indentation hardness HIT in GPa and the Vickers hardness (Hv) is:

[0153] Hv = 94,53 HIT.

[0154] Raman test

[0155] Raman spectroscopy was performed on samples before and after the etching step using different dopant concentrations. The Raman excitation wavelength was 532 nm. The G peak was located at 1605 cm⁻¹. -1 At this location, the excitation wavelength points to sp 3 The score exceeds 60%. (Between 80-150cm) -1 Between these values, a peak that is becoming visible indicates a WC bond. This peak is not observed in pure ta-C samples or samples with 0.3 at% W. The peak is visible for 0.6 at% W and 1.4 at% W, and is stronger for higher W contents. Raman spectroscopy does not provide information about whether the WC bond points to WC crystallites or to bonds between individual W atoms and C. In coating tests following the etching step, no significant difference was observed in the Raman spectra before and after oxidation.

[0156] TEM study of the sample

[0157] TEM studies were performed using bright-field TEM (BFTEM) with a beam voltage of 200 kV and high-angle annular dark-field scanning TEM (HAADF-STEM) with a light spot. The sample consisted of a silicon (Si) wafer with a 20 nm thick Si3N4 layer on top, containing a 1 × 1 mm... 2 The Si was etched away. This is how the window exists. An 80 nm layer of ta-C, doped with 0.3, 0.6, and 1.4 at% W, was deposited on top of the Si3N4 foil. Most of the Si3N4 foil fractured under the compressive stress of the doped ta-C film, but undisturbed portions of the foil could be found at the corners of the window. This was observed from the side of the doped DLC coating according to the invention deposited on the Si3N4 foil.

[0158] The combination of BFTEM and HAADF-STEM imaging allows for the differentiation of W droplets and carbon clusters.

[0159] W droplets were identified in an 80 nm film. Figure 3 The coating doped with 1.4 at% W is shown as observed using BFTEM. The small black dots are W droplets confirmed by EDX analysis. Figure 3 In the samples tested, the droplet size ranged from 2 nm to 40 nm. In other samples, droplets up to 100 nm in size were observed.

[0160] exist Figure 4 The image shown is a HAADF-STEM image of the same sample doped with 1.4 at.% W. Subnm clusters of W are identifiable (shown as white in HAADF-STEM). It should be recognized that, due to the focusing of the HRTEM, a total film thickness of 80 nm was not observed; instead, only grooves with a thickness of approximately 20 nm were observed. While it is impossible to precisely measure such particles in a thick coating, a coarse measurement yields a cluster size of ~0.5 nm. W exhibits a bcc structure with a lattice spacing of 0.34 nm. WC typically has a hexagonal distribution with lattice parameters of 0.29 nm and 0.28 nm. Therefore, these clusters consist of a very limited number (maximum 10) of W atoms.

[0161] A semi-quantitative analysis was performed on the amount of W visible in the metal droplets and the WC islands, assuming that the islands and droplets were spherical. For the doped DLC coating with 0.6 at% W according to the invention, it was found that 50% of the W could be attributed to the WC islands and 50% of the W was in the metal droplets. For the film doped with 1.4 at% W, approximately 30% of the W could be attributed to the WC islands, and 70% of the W was in the metal droplets. This is consistent with the expectation that the amount of droplets increases more than linearly with increasing percentage of W in the target.

[0162] For only 1.4 at% W, the presence of WC in the Raman spectrum is very evident, clearly indicating that the smaller crystallites are WC. Quantitative analysis shows that there is almost no "free" W.

[0163] In order to study the tribological properties of the doped DLC coating according to the present invention, the deposited coating as described above was achieved this time with a W content (as an example of a transition metal) of 5 at.% (based on W-doped DLC).

[0164] HR-TEM and TEM confirmed the presence of W droplets in the sample doped with 5 at.% W. Typical TEM micrographs of the sample are shown below. Figure 5 and Figure 6 In. Figure 5 In bright-field HR-TEM images, pure W grains appear black, and in dark-field images... Figure 5 In the TEM micrographs, pure W particles appear as bright spots. The largest particle size observed in this sample is less than 250 nm. EDX analysis of the particles indicates that they are W droplets.

[0165] exist Figure 7 The image shown is a HAADF-STEM image of a sample doped with 5 at.% W. Sub-nm clusters of W are identifiable (shown as white in HAADF-STEM). It should be noted that due to the focusing of the HRTEM, the total film thickness of 80 nm was not observed; instead, only grooves with a thickness of approximately 20 nm were observed. Furthermore, the presence of WC in the Raman spectrum was confirmed as described above. Therefore, it is clear that the sub-nano clusters of W are WC.

[0166] Therefore, it has been shown that the doped DLC according to the invention, when used in tribological applications, has a much higher hardness than the transition metal-doped hydrogenated DLC used in the prior art (e.g., A. Abou-Gharam et al.). Furthermore, the transition metal droplets are shown to have a small size and are incorporated into the non-hydrogenated DLC as a matrix, thereby enabling the acquisition of a coating with a smooth and uniform surface structure, which further comprises transition metal droplets with lubricating effects.

Claims

1. A non-hydrogenated transition metal-doped diamond-like carbon, wherein, The non-hydrogenated transition metal-doped diamond-like carbon includes at least one transition metal selected from Groups 4d, 5d, and 6d of the periodic table, and a portion of the at least one transition metal exists in the non-hydrogenated diamond-like carbon matrix in the form of a carbide of the at least one transition metal. Its features are, The non-hydrogenated transition metal-doped diamond-like carbon has a hardness of ≥35 GPa, wherein the hardness is measured on a film of the non-hydrogenated transition metal-doped diamond-like carbon deposited on a polished and hardened substrate, and the film of the non-hydrogenated transition metal-doped diamond-like carbon has an indentation depth less than 10% of the thickness of the film. Another portion of the at least one transition metal exists in the form of a droplet of the transition metal; The diameter of the metal droplets of the transition metal is from 0.1 nm to 40 nm.

2. The non-hydrogenated transition metal-doped diamond-like carbon according to claim 1, wherein, A portion of the carbide of the at least one transition metal exists as islands in the non-hydrogenated diamond-like carbon that serves as the matrix.

3. The non-hydrogenated transition metal-doped diamond-like carbon according to claim 2, wherein, The island has a size of up to 2 nm.

4. The non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 3, wherein, The content of the at least one transition metal is in the range of 0.1 at.% to 5 at.%.

5. The non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 3, wherein the hardness of the non-hydrogenated transition metal-doped diamond-like carbon is in the range of 40 GPa to 60 GPa.

6. The non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 3, wherein the carbon atoms of the non-hydrogenated transition metal-doped diamond-like carbon have a sp... 3 A score of ≥60% 7. The non-hydrogenated transition metal-doped diamond-like carbon according to claim 6, wherein, The sp of carbon atoms in the non-hydrogenated transition metal-doped diamond-like carbon 3 A score of ≥80% 8. The non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 3, wherein the content of the at least one transition metal in the non-hydrogenated transition metal-doped diamond-like carbon is in the range of 1 at.% to 5 at.%.

9. The non-hydrogenated transition metal-doped diamond-like carbon according to claim 1, wherein, The non-hydrogenated transition metal-doped diamond-like carbon has a hardness of ≥40 GPa.

10. A layer system comprising at least one layer of non-hydrogenated transition metal-doped diamond-like carbon disposed on a substrate according to any one of claims 1 to 9.

11. The layer system according to claim 10, wherein, The thickness of the layer of the non-hydrogenated transition metal-doped diamond-like carbon is in the range of 50 nm to 3 µm.

12. The use of a non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 9 for improving the wear resistance and / or reducing the friction of a surface by applying a coating of the non-hydrogenated transition metal-doped diamond-like carbon to the surface.

13. The use according to claim 12, wherein, The thickness of the coating is in the range of 0.5 µm to 3 µm.

14. A method for depositing a coating of non-hydrogenated transition metal-doped diamond-like carbon according to any one of claims 1 to 9, wherein the method is a cathodic arc discharge deposition method, wherein a direct current and a pulsed current are superimposed in the cathodic arc discharge, wherein the pulse frequency of the pulsed current is in the range of 10 kHz to 100 kHz, wherein a carbon target doped with the at least one transition metal is used as a target in the cathodic arc discharge, the target being directly connected to the cathode, wherein each pulse of the pulsed current causes a voltage measured on the cathode to increase at a rate greater than 5 V / µm, wherein the effective pulse width of each pulse of the pulsed current is less than 30 µs.

15. The method of claim 14, wherein, Peak current is higher than 200 A.

16. A coating that is obtainable by the method according to claim 14 or 15.

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