Abrasion-resistant deep black decorative coating

Abstract

A deep black coating layer system having excellent scratch resistance with controllable electrical conductivity for high precision component applications (medical, decorative, general engineering) is described. The coating layer system comprises a base layer of diamond-like carbon (DLC) and a gradient layer having decreasing refractive index. The base layer is essentially hydrogen free, and the gradient layer comprises hydrogen at concentrations of 1-35 at%. A method for producing the coating layer system is disclosed using HiPIMS operated with a carbon containing target for the base layer and a carbon containing target simultaneously with introduction of a hydrocarbon gas for the deposition of the gradient layer.

Description

Abrasion-resistant Deep black decorative coating

[0001] The present invention relates to a method for producing wear-resistant coatings with a deep black appearance and adjustable electrical conductivity for decorative and medical applications.

[0002] Decorative deep black appearance also known as deep black continues to be a highly popular coating color for decorative purposes especially in the field of consumer articles such as jewellery, watches, mobile phones, sanitary fixtures and medical applications. According to the CIE 1976 L*a*b* color space, surfaces are considered as deep-black surfaces if they exhibit L* < 40, most preferably L*=< 35 with a / b values near 0.

[0003] In addition to the aesthetic appeal, decorative coatings feature additional functional properties such as high hardness, enhanced wear resistance of the coated components against scratches and abrasive wear or even of the resistance against chemicals and to corrosion.

[0004] The German application DE3639469A1 describes a hard material layer with a decorative black appearance, which simultaneously has a high wear resistance. This hard material layer includes a first layer composed of an element from the groups IVa and Va of the periodic table of elements, a second layer that includes a nitride of this element of the first layer, a third layer that contains a carbide of the element, and a covering layer composed of a hard carbon layer, with carbide crystallites of the same element being embedded in this covering layer.

[0005] It is also known to use DLC layers. These have a black appearance and a high degree of hardness. However conventional DLC layers have neutral gray values (L* = lightness) that lie in the range above 40.

[0006] The US application 20160002792A1 describes a method for producing a wear-resistant layer for decorative application with a deep-black appearance. This hard material layer comprising a diamond-like carbon (DLC) layer with a hardness of at least 10 GPa and a refractive index nDLC of nDLC > 2.1; and a separate gradient layer on top of the DLC layer, wherein the gradient layer is at least 300 nm thick and is produced on the DLC layer as a gradient layer with a decreasing density and therefore a decreasing refractive index, resulting in a refraction index gradient, wherein an average along 30 nm adjacent to an interface with the DLC layer, is greater than or equal to 2.0 and wherein an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an outer surface opposite the DLC layer, does not exceed 1.85. By means of the refraction index progression that this produces in the gradient layer, the gradient layer function as a reflection-reducing layer. The chemical composition of the gradient layer differs from a chemical composition of the DLC layer essentially only with regard to hydrogen content. The DLC layer and gradient layer disclosed in US 20160002792A1 comprise hydrogen in the range from approximately 25 at.% to 55 at.%.

[0007] The US application 20160053371A1 describes a decorative article comprising a base and a black hard coating film which is formed on the base, and which comprises diamond-like carbon; wherein a hydrogen content of a surface, reverse to a surface close to the base, of the black hard coating film is more than a hydrogen content of the sur-face, closer to the base, of the black hard coating film; and the hydrogen content of the surface, revere to the surface close to the base, of the black hard coating film is 30.0 to 70 at.% and wherein the article comprising the decorative article is a time-piece, a necklace, a pendant, a brooch, or glasses. The black hard coating film comprises a gradient layer comprising diamond-like carbon; a hydrogen content in the gradient layer is increased with increasing a distance from the base. Similarly to the USapplication 20160002792A1, by means of the gradient of hydrogen content, the gradient layer function as a reflection-reducing layer.

[0008] DLC is naturally an electrical insulator, which is normally desired for most of the surgical medical instruments, aerospace components, tools and other relevant high-end deco applications.

[0009] However, in certain of these above-mentioned applications, components coated with electrically conductive and wear-resistant functional coating can be desirable, especially in applications where the electrical contact across sliding elements should remain high. This includes anti-static, sensors, bipolar plates for fuel cell applications.

[0010] It is well-known that AlTiN-based coatings can exhibit a deep black appearance and are electrically conductive. However, the wear resistance is generally lowered than for DLC coatings. The coefficient of friction of the AlTiN coatings is also well-known to be higher than DLC coatings.

[0011] Consequently, there exists a need for an alternative wear-resistant and hard layer with a deep-black appearance and a controllable electrical conductive property.

[0012] It is therefore an object of the present invention to disclose a hard surface coating that produces a Deep-Black visual appearance and adjustable electrical conductive property by means of HiPIMS. A further objective is to provide a coating with improved mechanical properties and simultaneously a very good sliding in comparison to the state of the art.

[0013] It is a further objective of the present invention to provide an alternative industrial-suited coating method with high degree of process reliability and uniformity for producing tools or components coated with the aforesaid conductive deep black coatings.

[0014] The objective of the present invention can be at least partially attained by providing a coating layer system comprising a base layer and a gradient layer. The base layer has a refraction index n DLC of n DLC >2.1. The gradient layer may have a decreasing density and therefore a decreasing refractive index, resulting in a refraction index gradient. In particular, an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an interface with the base layer may be greater than or equal to 2.0, and an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an outer surface opposite the base layer may does not exceed 1.85

[0015] In a preferred embodiment, the base layer is closer to the surface of the component than the gradient layer. In a further preferred embodiment, the gradient layer is disposed on top of the base layer.

[0016] The base layer may comprise or consist of a diamond-like carbon (DLC) layer. The gradient layer may comprise or consist of a diamond-like carbon (DLC) layer. In a preferred embodiment, the base layer and the gradient layer consist of diamond-like carbon (DLC) layers.

[0017] In a preferred embodiment, the base layer consists of diamond-like carbon (DLC) layer which is essentially hydrogen free. In particular, the base layer may be an a-C layer with hydrogen concentration less than 1 at%.

[0018] The gradient layer may comprise hydrogen at a concentration in the range from 1 at% to 35 at%. In particular, the gradient layer may be an a-C:H layer with H concentration from 1 at% to 35 at%.

[0019] The inventors have noted that an essentially hydrogen free DLC base layer brings unprecedented opportunities for the design of improved damage-resistant transition layer deep black coatings with advantageous mechanical properties (high hardness and elastic modulus) and wear damages reduction which is of essential for medical or decorative applications. While the inventors do not wish to be held to any particular theory, it is believed that the additional of a hydrogen free DLC base layer can contribute to an effective improvement of the load-bearing capacity of the overall coating stacks due to an effective fracture toughness enhancements as more energy is required to initiate and propagate cracks of various sizes due to the high mechanical properties of the hard carbon layer. It is also important to mention that the deposited hydrogen-free base layer possesses very high electrical conductivity which help improving the electrical conductivity of the inventive deep black layer.

[0020] The combination of an essentially hydrogen free base layer with a hydrogen containing gradient layer is advantageous because it offers the possibility to deposit a deep black layer with an improved load-bearing capacity and ultimately an improved protection against abrasive and adhesive wear. Ultimately, the durability and stabilization of the deep black color with be significantly improved in comparison to state of the art deep black solutions.

[0021] Essentially hydrogen free in the context of the present invention denotes hydrogen content not beyond trace levels or contaminations. Essentially hydrogen free may in particular be understood as a hydrogen content of less than 1.0 at.%.

[0022] The coating layer system may comprise further layers. For example, an adhesion layer may be present between base layer and the substrate (tool or component to be coated). In a further example, a transition layer may be preset between the base layer and the substrate. In yet another example, an adhesion layer and a transition layer may be present between the base layer and the substrate, where preferably the adhesion layer is disposed closer to the substrate than the transition layer and most preferably the adhesion layer is disposed directly on the substrate. The transition layer may comprise or consist of a metal carbide and may further display a graded composition across the thickness of the layer. For example, the transition layer may have a graded design where the carbon concentration is increasing along the thickness of the layer.

[0023] In some embodiments of the present invention, the gradient layer is a wear-resistant, conductive metal-doped Deep-Black hard carbon layer, such as a-C:H:Cr deposited by means of co-sputtering HiPIMS in a mixed gas environment. The gradient layer contains a gradient coating design with an adjusted hydrogen and metalconcentration resulting in a better control of the optical and electrical conductive property.

[0024] The inventors have discovered that it is surprisingly possible to produce a wear-resistant coatings of hard material made of amorphous carbon with a decorative Deep-Black appearance, and, at the same time, an electrically conductive property if the gradient layer is applied by means of co-sputtering HiPIMS, in which the process parameters are selected in such a way of inducing on one hand a refractive index gradient whose film density decreases in the direction toward the outer surface to provide the reflection-reducing function (in another word to improve the absorption capacity of light in the visible wavelength resulting in the deep black appearance) and on the other hand a metal doping with a fluence or concentration high enough to form a conductive network which greatly increases the electrical conductivity of the coating layer system.

[0025] The present inventors have considered introducing metal doping through the implantation of different metal M (M = Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr) within the functional decorative gradient layer.

[0026] In embodiments of the invention, the gradient layer is implanted with at least one metal element selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, to a concentration of at least 1 at.%. The presence of one or more metals to a concentration of at least 1 at.% is described as doping.

[0027] In a preferred embodiment, the gradient layer is doped with at least one metal element and the base layer does not contain metal doping.

[0028] A method for producing a coating layer system, i.e. a coating method, according to the invention comprises the steps of

[0029] - loading a coating chamber with substrates that are to be coated- pumping-out the coating chamber and introducing an inert process gas, preferably argon,- depositing a base layer utilizing plasma while applying bias to the substrates that are to be coated,

[0030] - depositing a gradient layer utilizing plasma while applying bias to the substrates that are to be coated

[0031] It is preferred that the step of depositing a gradient layer is conducted immediately following the step of depositing a base layer. As such, no interruption of the process or further deposition steps would be conducted between the deposition of the base layer and the gradient layer.

[0032] A coating chamber is suited for vacuum processes and is equipped with means for creating vacuum, means for introducing gases, means for generating plasma, means for heating and controlling temperature, and means for holding and handling substrates to be coated.

[0033] Suitable inert processes may be selected from the noble gases, particularly Ne, Ar, Kr, and Xe. Preferred inert process gas is Ar due to generally higher availability and lower cost. The inert process gas may also be a combination of different elements, such as Ar combined with another noble gas element.

[0034] The step of depositing the base layer and the step of depositing the gradient layer are preferably conducted using a HiPIMS method. Plasma used for deposition of the layers are thus created through a HiPIMS discharge.

[0035] The step of depositing a base layer is preferably conducted operating a carbon containing target, in particular a graphite target.

[0036] The step of depositing a gradient layer is preferably conducted operating a carbon containing target simultaneous with the introduction of a hydrocarbon gas. The hydrocarbon gas is preferably selected from the group of CH4, C2H2, and C7H8, most preferred Acetylene C2H2. The combination of a carbon containing target and a hydrocarbon gas enables control of the hydrogen content in the gradient layer and the density of the gradient layer in combination with applied bias. These properties are important to control the refractive index.

[0037] Hydrocarbon gases should be avoided during the deposition of the base layer, to not introduce hydrogen. It is therefore preferred that the step of depositing the base layer is conducted without introducing hydrocarbon gas.

[0038] The flow of hydrocarbon gas is preferably increased during the deposition of the gradient layer, such that the value of hydrocarbon gas flow is lower at the beginning of the step and higher at the end of the step. The increase is preferably continuous throughout the step, in particular increasing according to a gradient. It is preferrable to conduct the increase of the hydrocarbon gas simultaneously with a reduction of the magnitude of the bias voltage to obtain a coating layer with decreasing density. The reduction of the magnitude of the bias voltage is preferably reduced continuously to obtain a smooth profile of density. A plurality of reduction steps may be alternatively used to approximate a continuous reduction.

[0039] In some embodiments of the invention, the gradient layer comprises metal doping at a concentration in the range from 1 at% to 20 at%, preferably in the range from 1.5 at% to 10 at% and especially preferably in the range from 5 at% to 6 at%.

[0040] The method for producing the coating layer system may in one embodiment be adapted to introduce metal doping in that the step of depositing the gradient layer includes co-sputtering of at least one carbon containing target with at least one metal target to incorporate a metal content between 1 at% and 20 at% into the gradient layer, wherein the at least one metal target comprises at least one metal element selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr.

[0041] In another embodiment, the method for producing the coating layer system may be adapted to introduce metal doping in that during the step of depositing the gradient layer, a carbon containing target is used which further comprise at least one metal at a concentration in the range from 1 at% to 10 at%, to incorporate a metal content between 1 at% and 20 at% into the gradient layer, wherein the at least one metal comprised in the carbon containing target is selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr

[0042] In preferred embodiments, as will be discussed below, co-sputtering of at least chromium and carbon with argon and acetylene as reactive gas is disclosed. Co-sputtering may be used in particular for synthesis of the gradient layer.

[0043] According to some embodiments the present invention, the gradient layer may comprise at least a chromium and hydrogen doped amorphous carbon (a-C:H:Cr) by means of co-sputtering reactive HiPIMS, wherein, to deposit the gradient layer, at least one target containing Cr, for example a Cr target, is used as the Cr source and at least one target containing carbon, for example a graphite target, is used as the primary source of carbon (atoms or ions), said targets being used for sputtering in the coating chamber and operated with pulsed HiPIMS power within a reactive atmosphere having at least one inert gas, preferably argon, and at least one hydrocarbon gas (CH4, C2H2, C7H8, …), preferably acetylene (C2H2), to provide an incoming flux of hydrogen during the deposition.

[0044] With the above-described sputtering method, hydrogen and metal doping can be reliably performed in such a way that, for example, the hydrogen content y in(C1-xCrx)1yHyis controlled by increasing the acetylene flow in the coating chamber, while the metal content x in (C1-xCrx)1-yHyis controlled by increasing the average power (PAv) to the chromium targets. Meanwhile, graphite targets are operated with a constant average power (PAv) during the coating process to control the carbon content.

[0045] The electrical HiPIMS power supplied to the carbon containing target, e.g. graphite target, is preferentially delivered in pulses with lengths (tpulse) as less than 10 ms, preferably less than 1 ms, particularly preferably less than 0.1 ms, with peak power density and duty cycle of individual pulses preferably above 100 W●cm-2and 1% respectively, for achieving a sufficient highly ionized Ar plasma during the HiPIMS pulses suitable to promote the growth of dense and hard a-C:H:Cr but not energetic enough to induce arcing events at the surface of the carbon containing target, resulting in the deposition of a smooth gradient layer with deep black appearance and few surface droplets.

[0046] The hydrogen concentration within the gradient layer is preferably lower than 50 at.%, as for example 30 at.%. Preferably, the gradient layer is a hydrogen-doped amorphous carbon layer is applied as a gradient layer, wherein the concentration of hydrogen increases toward the outer surface of the gradient layer.

[0047] The inventors found surprisingly that the inventive coating layer system with desirable wear-resistant and conductive Deep-Black properties can be advantageously produced by controlling the metal content within the gradient layer, in such a way that the content of metal in the said gradient layer is preferably lower than 10 at.%, more preferably lower than 5 at .%. Indeed, the inventors have observed that above 10 at.% of metal content, the mechanical properties of the gradient layer are significantly reduced most likely due to a lower densification of the gradient layer. Also the inventors have observed at the same time a modification of the color appearance of the coating shifting from deep Black to greyish which is not ideal for the application. However, the metal content should preferably be at least 1 at.%. Preferred range for the metal content is thus in the range from 1 at.% to 10 at.%, further preferred from 1 at.% to 5 at.%.

[0048] The electrical power supplied to the chromium target is preferentially delivered in pulses with a HIPIMS power supply, pulsed DC, Bipolar or continuous with a direct current power (DC) supply.

[0049] The inventors believe that using HiPIMS could be an advantage versus state of the art sputtering methods (Bipolar, pulsed DC or continuous) by providing higher degree of process reliability and uniformity especially in reactive sputtering process involving a reactive gas such as acetylene. It is well-known that when coating by means of reactive sputter processes with at least one reactive gas, often the reactive gas reacts at least partially with the material from the target surface in such a manner that areas of the target surface become covered by a compound layer, for instance CrC:H. Theformation of the compound layer can result in drop of the deposition rate, instability of the process and unreliability of the metallic content within the film due to the change of the target chemistry. The degree of target covering of the compound depends from the process parameters e.g. significantly from the reactive gas glow and the sputter power applied on the target. This phenomenon is usually called target poisoning and generally leads to process instability and poor reproducibility due to the appearance of hysteresis behavior.

[0050] For the reasons cited above, it can be seen that slight changes to the process parameters result in a deviation from the desired operating point, a situation that can, in turn, have a negative impact on the desired layer quality and homogeneity. Due to the instantaneous high density plasma produced during the HiPIMS pulsed process, it is well known that HiPIMS plasma can suppress the poisoning of the target racetrack even at low average power. The absence of racetrack poisoning offers much higher degree of process reproducibility in terms of metal content within the coatings.

[0051] The electrical HiPIMS power supplied to the metal target (e.g. Cr) is preferentially delivered in pulses with lengths (tpulse) above 0.05 ms, preferably above 0.1 ms, with peak power density and duty cycle of individual pulses preferably above 100 W●cm-2and 1% respectively to achieve efficient in-situ target cleaning by Argon bombardment during process and produce enough metal ion for the implantation.

[0052] The process is usually carried out at a total pressure of about 0.1 to 0.6 Pa.

[0053] A negative bias voltage can be applied to the substrates to be coated. The negative bias voltage can be continuous, or pulsed, or synchronized with the HiPIMS pulses applied to the chromium targets, wherein the bias voltage is lower than -200 V, preferably lower than -100 V and further preferentially lower than -50 V.

[0054] The inventors found surprisingly that the inventive coating layer system with desired wear-resistant and conductive Deep-Black properties can be advantageously produced by controlling the mass density of the coating via the control of the ion bombardment during the film growth, in particular during synthesis of the gradient layer. Preferably, during synthesis of the gradient layer the bias voltage is ramped down in magnitude during the film growth, starting from a high value (for example -100V) down to a value close to the floating potential (for example -25V or lower). The end bias value will strongly determine the mass density of the films hence the surface reflectivity and therefore color appearance and the mechanical properties such as hardness. The inventors observed that without proper control of the of the ion bombardment during the film growth, the density of the gradient layer is significantly reduced during the film growth synthesis which ultimately lead to a significant drop of the mechanical properties and wear resistance performance.

[0055] During the deposition process for synthesis of the coating layer system, the temperature of the substrate may be maintained at a value between 100°C and 250°C, preferably between 150 and 200 °C or more preferably between 100°C and 150°C. The process may be conducted without external heating. In other words, no heating other than the thermal energy generated from the deposition process itself. Heating elements, resistive heaters or the like are therefore not included or not operated during the deposition process.

[0056] According to another preferred embodiment of the present invention, the gradient layer has a minimum preferred hardness that is not less than 10 GPa, preferably not less than 15 GPa, and even more preferably not less than 18 GPa.

[0057] According to another preferred embodiment of the present invention, the hardness of the hydrogen-free base layer is preferably higher than 30 GPa. The preferred range for the hardness of the amorphous carbon layer is between 30 GPa and 40 GPa.

[0058] Preferably, the electrical resistivity of the said gradient layer is lower than 103Ω●cm-1, preferably lower than 101Ω●cm-1.

[0059] Preferably, the electrical resistivity of the hydrogen-free base layer carbon layer is lower than 10-3Ω cm-1, preferably lower than 10-4Ω.cm-1

[0060] Preferably, the gradient layer has a Deep Black value L* between 30 and 40 (according to the CIE 1976 L* a* b* color space based on a D65 standard illumination).

[0061] Preferably, the total thickness in the said gradient layer is lower than 1.0 μm, preferably lower than 0.5 μm,.

[0062] The invention will now be explained in detail and by way of example with reference to a process description and figures

[0063] Brief description of the figures

[0064] : Cross section SEM of inventive deep black coating layer system.

[0065] : Reflectivity spectrum of two coating products: state of the art anthracite hydrogen free carbon and inventive deep black coating layer system.

[0066] : Wear behavior of two conductive deep black coating solutions vs abrasion cycle : 1) state of the art AlTiN deep black and 2) the inventive deep black coating layer system under “Crockmaster” abrasion testing device.

[0067] Dry Pin on Disc (PoD) friction coefficient vs sliding distance of two selected deep black coatings : (a) Inventive insulating Deep Black - 1 (black) and (b) inventive conductive Deep black - 2 (light gray).

[0068] Coating layer system (10) according to the invention comprising a base layer (30) and a gradient layer (40) on a substrate (20).

[0069] Coating layer system (10) according to the invention comprising a base layer (30), a gradient layer (40), an adhesion layer (50) and a transition layer (60) on a substrate (20).

[0070] Hydrogen profile of an exemplary embodiment the inventive hydrogen-free a-C base layer measured by Rutherford Backscattering Spectrometry (RBS) by 2 MeV He ions. The scale “Channel” on the horizontal axis refers to depth into the sample. A reference standard sample with 9.5 at% H is included for calibration of the measurement. The inventive example of the base layer shows a content of H lower than 1 at% below the sample surface. A small peak with higher H fraction is detectable approximately channel 180 – 200, which is due to contamination of the sample surface.

[0071] Example 1

[0072] In order to produce an exemplary embodiment of the coating layer system according to the present invention, workpieces (substrates) made of steel with hardness of 62 HRC were placed in an industrial Oerlikon Balzers INGENIA s3p vacuum processing chamber equipped at least with three targets of chromium and at least three targets of graphite, whereupon the vacuum chamber was pumped down to a pressure of about 10-5mbar.

[0073] In order to demonstrate the effectiveness of improved electrical conductivity of the deep black coating layer system according to the present invention, three samples were deposited with identical parameters for all process steps up to and including the step of depositing the base layer. The deposition process included the deposition of a metallic adhesion-promoting layer (adhesion layer), a metal carbide transition layer and a hard carbon layer as base layer. In the step for deposition of the gradient layer, the parameters have been slightly changed to deposit samples with three different gradient layers with different metal content in it.

[0074] At a first part of the deposition process, a plasma heating and ion etching is carried out for 30 minutes. In this pretreatment step, an Ar hydrogen plasma is ignited by means of a Plasma ARC between the ionization chamber and an auxiliary anode.

[0075] Following the pretreatment step, an Ar ion plasma etching process is carried out for 20 minutes duration, is initiated by activating the low voltage arc ionization chamber and an auxiliary anode. The Ar ions are drawn from the Plasma by means of a negative bias voltage onto the substrates to be cleaned with the primarily goal to remove impurities such as native oxides or also organic impurities via ballistic removal (i.e. native oxides and impurities are sputtered etch by the intense Ar+ion bombardment) to insure a good layer adhesion of the adhesive metal layer that takes place after the ion cleaning.

[0076] As the next step, a 300 nm-thick adhesion promoting layer (adhesion layer) of Cr is deposited by means of HiPIMS method onto the surface of the substrate to be coated using the following process parameters: an average power Pav of 75 W●cm-2with a power density of individual pulses of 700 W●cm-2and a tpulseof 5.0 ms, an Ar total pressure of 0.3 Pa and a constant bias voltage of -50V at a coating temperature lower than 180°C.

[0077] Then, immediately after the adhesion promoting layer, a 200 nm-thick graded CrC transition layer was deposited by co-sputtering method wherein the chromium targets and the graphite targets are co-sputtered using the following process parameters: An Ar total pressure of 0.3 Pa, the three graphite targets were operated with an average power Pav starting from 80 W●cm-2and increasing to 161 W●cm-2during the deposition of the CrC transition layer in order to gradually increase the C content, wherein the chromium targets were operated with a constant average power Pavof 20 W●cm-2. The power density of the individual pulses supplied to both graphite and chromium targets was selected at 500 W●cm-2to provide suitable metal-ion irradiation during the film growth.

[0078] Immediately afterwards, a 0.7 μm-thick base layer consisting of a wear-resistant hydrogen-free a-C layer with a coating hardness of 40 GPa and an elastic modulus of 290 GPa (measured with a load of 10 mN on a Fischerscope Instruments) was deposited on top of transition layer by means of HiPIMS method according to the present invention using the following parameters: an average power Pav of 100 W●cm-2, a power density of individual pulses of 500 W●cm-2, with a tpulseof 0.05 ms, at a total Ar pressure of 0.3 Pa and a bias voltage above 50V at a coating temperature lower than 180 °C.

[0079] The hydrogen profile of the deposited base layer of hydrogen-free a-C was evaluated by Rutherford Backscattering Spectrometry (RBS) and is presented inThe measurement indicated that the hydrogen concentration inside the a-C layer is below 1 at.% and confirm that the a-C layer is hydrogen free.

[0080] Finally, a 0.4 μm-thick gradient layer consisting of a wear resistant Deep black a-C:H layer was deposited by means of reactive HiPIMS method according to the present invention using the following parameters: An Ar total pressure of 0.3 Pa, the three graphite targets were operated with an average power Pav of 110 W.cm-2, power density of the individual pulses of360 W●cm-2and with a tpulse of 0.2 ms. Regarding the three chromium targets, for each of the three samples, the chromium targets were operated with different average power for deposition of samples with different gradient layers : (1) Sample 1 (Inventive DLC deep black- 1) no power was supplied to the chromium targets so 0 W●cm-2, (2) Sample 2 (Inventive DLC deep black - 2) the average power was set to 2.5 W●cm-2and (3) Sample 3 (Inventive DLC deep black – 3) the average power was set to 5 W●cm-2. For Sample 2 and 3, the power density of the individual pulses supplied to the chromium targets was selected at 363 W●cm-2. To produce the deep black appearance, the mass density of the gradient layer has to be continuously decreased and the hydrogen concentration of the gradient layer has to be continuously increased in order to reduce the refractive index and hence increase the probability of light absorption. To achieve that, the acetylene flow and substrate voltage continuously increase during the entire film growth. The flow of acetylene starts from 20 sccm and increase during the deposition of the gradient layer up to 50 sccm while the magnitude of the bias voltage is decreased (in absolute value) from -100 V to -25 V. This way, a Deep black and Hard a-C:H layer as gradient layer can be produced.

[0081] Cross-section scanning electron microscope image of the inventive undoped Deep Black coating layer system (inventive DLC deep black sample 1), shown in, showings the very dense and compact microstructure of the inventive solution.

[0082] In order to confirm the deep black appearance, the coating color characteristic of the inventive coating layer system was determined with the spectrophotometer Konica-Minolta CN-2600d and compared with with a state of the art hydrogen free a-C coating as reference. The color characteristic of the inventive coating layer system in form of sample 1 “Inventive DLC deep black - 1” was L* = 34.5, a* = 0.1 and b* = 0.2 characteristic of a deep black appearance according to the CIE 1976 L*a*b* color space. The state of the art a-C reference sample exhibit a color characteristic L* = 50, a* = 0.6 and b* = 3.0 characteristic of an anthracite color appearance.The spectrophotometer can also provide the reflection behavior of the two selected coatings as depicted by the reflectivity spectrum vs wavelength in theIt can be seen that the percentage(%) of the light reflection with the deep black coating is very low compared to the anthracite coating which is also expected from a deep black appeared surface. Another point to consider is the flatness of the spectrum for the deep black color of the inventive coating layer system which tends to demonstrate an absence of light interference in the coatingdue to a smooth gradient of reflective index within the film induced by the smooth gradient of process parameters such as Acetylene flow and bias voltage. Absence of light interference is an important esthetic aspect to take into consideration, it means that the color appearance is independent of the angle of observation.

[0083] An overview of the main characteristic (color, mechanical and electrical properties) of three selected inventive deep black coatings coating layer system are listed in the table 1. It can be observed that the state of the art anthracite hydrogen free a-C is ultra-hard (HIT = 40 GPa) and electrically conductive with a two-probe contact resistance of 3.0 Ω, which is a characteristic of a good electrical conductor. The undoped inventive Deep Black coating sample 1 (inventive DLC deep black -1) ) where no power was supplied to the chromium targets shown a deep black appearance with a L* of 34.5, a lower hardness value of 12.0 GPa and an insulating electrical behavior characterized by an out of range resistance contact under our two probe contact equipment. On the other hand, sample 2 and 3 (inventive DLC deep black sample 2 & 3, respectively) are metal doped with a chromium content increasing with increasing applied power supplied to the chromium targets.

[0084] Table 1 - Overview of the main characteristic of few selected products including the inventive deep black coating layer system.Optical propertiesMechanical propertiesElectrical propertiesCr-doping [at.%]L*a*b*HIT [GPa]EIT [GPa]Two-probe contact [Ω]Anthracite Hydrogen-free DLC (State of the art)0500.63.0402853.0Inventive DLC deep black – 1034.50.10.212.0120Out of range (too insulating)Inventive DLC deep black – 21.5635.00.10.512.51225.0x105Inventive DLC deep black – 35.6835.50.20.812.81253x102

[0085] The incorporation of metal to the gradient layer (a-C:H) in samples 2 and 3 (inventive DLC deep black sample 2 & 3, respectively) did not modify the color appearance and mechanical properties. Both samples 2 and 3 have a deep black appearance with a hardness value close to 13 GPa. On the other hand, the metal incorporation has a clear influence on the electrical properties of the deep black coatings, where a clear reduction of the contact resistance could be observed with our two probe contact measurements. This measurement gives a clear indication that our deep black inventive coating layer system becomes more and more conductive with the incorporation of metal doping.

[0086] However, the authors have noticed that a metal content above 20 at% in the coatings tends to change the color appearance of the solution to greyish color.

[0087] While the inventors do not wish to be held to any particular theory, it is believed that the change of color appearance might be due to the presence of color center and / or an increase of the refractive index leading to an increasing of the reflection of the light and hence shift the color appearance from Deep black to black and finally a greyish color.

[0088] It is therefore desired to keep the metal doping below 10 at% for optimum performance.

[0089] The wear resistance of the undoped inventive deep black coating layer system (Inventive DLC deep black - 1) was examined using an applied testing method that is based on the “Crockmaster” method from Jame Heal. In this method, a coated sample was abraded with a 1 cm x 1 cm piece of abrasive paper (3M 281Q Wetordry, with 9 µm Al2O3 particles). The device moves the abrasive paper back and forth with a frequency of 1 Hz on the coated surface. A 9 N load is exerted on the abrasive paper and the abrasive paper is replaced after 500 back-and-forth cycles. After 5000 cycles, no significant changes in the color values (L*, a* and b*) and no traces of scratching, were discernable for the sample 1, sample 2 and sample 3, as depicted in theIn comparison to this, state of the art deep black (L* 35) AlTiN PVD layers tested under the same conditions already showed significant wear after only 500 cycles.

[0090] In addition to that, the friction of the inventive metal-doped deep black sample 2 (Inventive DLC deep black - 2) was compared to the undoped inventive deep black sample 1 (Inventive DLC deep black - 1) using the pin-on-disk test (Unidirectional sliding tribometer, CSC Instruments). The test was performed in air under dry condition at a temperature of 22°C and 43 % relative humidity. The sample was abraded against an uncoated 100Cr6 steel ball with a diameter of 3 mm. The steel ball served as a static friction partner and the coated sample was turned underneath it (radius 5 mm, speed 0.3 m / s). A 30 N load was applied on the ball. This corresponds to an instantaneous contact pressure of 2.2 GPa applied onto the surface of the hard carbon layer. The measurement of the inventive metal doped deep black coating sample 2 (Inventive DLC deep black - 2) was compared to the undoped deep black coating sample 1 (Inventive DLC deep black - 1). Representative friction coefficient after 2000 m of distance of dry sliding for these two coatings are plotted in

[0091] Surprisingly, the steady-state friction coefficient of the inventive metal-doped deep black coating sample 2 (Inventive DLC deep black - 2) is at a low level, COF ~ 0.05, much lower than the COF value for the undoped deep black coating sample 1 (Inventive DLC deep black - 1), demonstrating a superior friction behavior of the inventive metal doped deep black coating layer system.

[0092] Very surprising was the fact that the inspection of the abraded surfaces after the test shows significantly less wear of the coating layer and slightly less counterpart component wear for the inventive metal-doped deep black layer sample 2 (Inventive DLC deep black - 2) compared than for undoped inventive deep black solution the sample 1 (Inventive DLC deep black - 1 ): width of the abrasion part of the coating 200 µm vs 250 µm, diameter of the abraded area of the ball 210 µm vs 290 µm for metal-doped inventive deep black sample 2 vs undoped inventive deep black sample 1 , demonstrating a suitable combination of enhanced wear-resistant properties, low friction and improved electrical properties of the inventive metal-doped deep black coating layer system compared to the inventive undoped deep black. One possible explanation for this surprising lower abrasion wear on the uncoated counter-body ball could be due to the metal incorporation within the gradient layer (e.g. a-C:H) provided by some embodiments of the inventive conductive deep black deposited according to the inventive method.

[0093] Disclosed is a coating layer system, on a component comprising:

[0094] - a base layer of diamond-like carbon (DLC) with a refraction index n DLC of n DLC >2.1; and

[0095] - a separate gradient layer on top of the base layer, wherein the gradient layer is at least 300 nm thick and is produced on the base layer as a gradient layer with a decreasing density and therefore a decreasing refractive index, resulting in a refraction index gradient, wherein an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an interface with the base layer, is, greater than or equal to 2.0, and wherein an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an outer surface opposite the base layer, does not exceed 1.85,

[0096] wherein the base layer is essentially hydrogen free, preferably an a-C layer with hydrogen concentration less than 1.0 at%, and the gradient layer comprises hydrogen at a concentration in the range from 1.0 at% to 35 at%, preferably an a-C:H layer.

[0097] The decreasing density in direction to the surface may be realized by an increasing hydrogen content in the gradient layer.

[0098] The gradient layer may comprise a metal doping with a percentage in the range of 1at% to 20at%, preferably in the range of 1.5at% to 10at% and especially preferred between 5at% and 6at%.

[0099] The base layer may be essentially free of metal doping.

[0100] The concentration of metal doping may be constant across the thickness of the gradient layer.

[0101] The refraction index gradient may yield a steadily decreasing refraction index in the gradient layer, in direction from the base layer toward the outer surface.

[0102] The metal doping in the gradient layer may comprise chromium or may consist of chromium.

[0103] Disclosed is a method for manufacturing a coating layer system on a component, where the method comprises the steps of:

[0104] - loading a coating chamber with substrates that are to be coated- pumping-out the coating chamber and introducing an inert process gas, preferably argon,- depositing a base layer utilizing plasma while applying bias to the substrates that are to be coated,

[0105] - depositing a gradient layer utilizing plasma while applying bias to the substrates that are to be coated,wherein

[0106] - the step of depositing the base layer and the step of depositing the gradient layer is conducted using a HiPIMS method, and

[0107] - at least one carbon containing target, preferably a graphite target, is operated, during the step of depositing the base layer and at least one carbon containing target, preferably a graphite target, is operated during the step of depositing the gradient layer, and

[0108] - the at least one carbon containing target is operated in HiPIMS simultaneously with introduction of a hydrocarbon gas during the step of depositing the gradient layer, wherein the hydrocarbon gas is preferably selected from the group of CH4, C2H2, and C7H8, most preferred C2H2, and

[0109] - the step of depositing the base layer is conducted without introduction of a hydrocarbon gas to form an essentially hydrogen-free base layer,

[0110] - the flow of hydrocarbon gas is increased, preferably increased continuously, and the magnitude of the bias voltage is reduced continuously or with a plurality of reduction steps during the step of depositing the gradient layer.

[0111] During the step of depositing the base layer the HiPIMS method may be operated with power density of individual pulses of at least 500 W●cm-2and with pulse time (tpulse) in the range of 0.01-0.05 ms.

[0112] During the step of depositing the gradient layer the HiPIMS method may be operated with power density of individual pulses which is the same or lower as the power density during the step of depositing the base layer, and wherein the pulse time is higher than the pulse time during the step of depositing the base layer, preferably the pulse time during the step of depositing the gradient layer is at least 0.2 ms.

[0113] The magnitude of the bias voltage during the step of depositing the base layer may be in the range from 50 V to 200 V, preferred in the range from 50 V to 150 V. The magnitude of the bias voltage at the end of the step of depositing the gradient layer may be 25 V or lower.

[0114] The step of depositing the gradient layer may include co-sputtering the at least one carbon containing target with at least one metal target to incorporate a metal content between 1 at% and 20 at% into the gradient layer, wherein the at least one metal target may comprise at least one metal element selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr.

[0115] During the step of depositing the gradient layer, at least one carbon containing target which further comprise at least one metal at a concentration in the range from 1 at% to 10 at% may be used, to incorporate a metal content between 1 at% and 20 at% into the gradient layer. The at least one metal comprised in the carbon containing target may be selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr.

[0116] The metal of the metal target and / or the metal comprised in the carbon containing target may comprise chromium or may be chromium.

[0117] The step of depositing the base layer may be directly followed by the step of depositing the gradient layer.

Claims

A coating layer system, on a component comprising:- a base layer of diamond-like carbon (DLC) with a refraction index n DLC of n DLC >2.1; and- a separate gradient layer on top of the base layer, wherein the gradient layer is at least 300 nm thick and is produced on the base layer as a gradient layer with a decreasing density and therefore a decreasing refractive index, resulting in a refraction index gradient, wherein an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an interface with the base layer, is, greater than or equal to 2.0, and wherein an averaged refraction index of the gradient layer determined as an average along 30 nm adjacent to an outer surface opposite the base layer, does not exceed 1.85characterized in thatthe base layer is essentially hydrogen free, preferably an a-C layer with hydrogen concentration less than 1.0 at%, and the gradient layer comprises hydrogen at a concentration in the range from 1.0 at% to 35 at%, preferably an a-C:H layer.Coating layer system according to claim 1, characterized in that the decreasing density in direction to the surface is realized by an increasing hydrogen content in the gradient layer.The coating layer system according to claim 1 or 2, wherein the gradient layer comprises a metal doping with a percentage in the range of 1at% to 20at%, preferably in the range of 1.5at% to 10at% and especially preferred between 5at% and 6at%.The coating layer system according to claim 3, characterized in that the base layer is essentially free of metal doping.The coating layer system according to claim 3 or 4, characterized in that the concentration of metal doping is constant across the thickness of the gradient layer.The coating layer system according to any one of the previous claims, wherein the refraction index gradient yields a steadily decreasing refraction index in the gradient layer, in direction from the base layer toward the outer surface.The coating layer system according to any one of claims 3 – 6, characterized in that the metal doping in the gradient layer comprises chromium and preferably is chromium.A method for manufacturing a coating layer system on a component, preferably according to any one of the preceding claims, the method comprising the steps of:- loading a coating chamber with substrates that are to be coated- pumping-out the coating chamber and introducing an inert process gas, preferably argon,- depositing a base layer utilizing plasma while applying bias to the substrates that are to be coated,- depositing a gradient layer utilizing plasma while applying bias to the substrates that are to be coated,characterized in that- the step of depositing the base layer and the step of depositing the gradient layer is conducted using a HiPIMS method, and- at least one carbon containing target, preferably a graphite target, is operated, during the step of depositing the base layer and at least one carbon containing target, preferably a graphite target, is operated during the step of depositing the gradient layer, and- the at least one carbon containing target is operated in HiPIMS simultaneously with introduction of a hydrocarbon gas during the step of depositing the gradient layer, wherein the hydrocarbon gas is preferably selected from the group of CH4, C2H2, and C7H8, most preferred C2H2, and- the step of depositing the base layer is conducted without introduction of a hydrocarbon gas to form an essentially hydrogen-free base layer,- the flow of hydrocarbon gas is increased, preferably increased continuously, and the magnitude of the bias voltage is reduced continuously or with a plurality of reduction steps during the step of depositing the gradient layer.Method according to claim 8, wherein during the step of depositing the base layer the HiPIMS method is operated with power density of individual pulses of at least 500 W●cm-2and with pulse time (tpulse) in the range of 0.01-0.05 ms.

10. Method according to claim 9, wherein during the step of depositing the gradient layer the HiPIMS method is operated with power density of individual pulses which is the same or lower as the power density during the step of depositing the base layer, and wherein the pulse time is higher than the pulse time during the step of depositing the base layer, preferably the pulse time during the step of depositing the gradient layer is at least 0.2 ms.Method according to any one of claims 8-10, wherein the magnitude of the bias voltage during the step of depositing the base layer is in the range from 50 V to 200 V, preferred in the range from 50 V to 150 V, and wherein the magnitude of the bias voltage at the end of the step of depositing the gradient layer is 25 V or lower.Method according to any one of claims 8-11, wherein the step of depositing the gradient layer includes co-sputtering the at least one carbon containing target with at least one metal target to incorporate a metal content between 1 at% and 20 at% into the gradient layer, wherein the at least one metal target comprises at least one metal element selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr.Method according to any one of claims 8-11, wherein during the step of depositing the gradient layer, at least one carbon containing target is used which further comprise at least one metal at a concentration in the range from 1 at% to 10 at%, to incorporate a metal content between 1 at% and 20 at% into the gradient layer, wherein the at least one metal comprised in the carbon containing target is selected from the group of Cr, Ti, Cu, Ni, Fe, Ag, Au, Y, W, Al and Zr, preferably Cr.Method according to claim 12 or 13, characterized in that the metal of the metal target and / or the metal comprised in the carbon containing target comprises chromium and preferably is chromium.Method according to any one of claims 8-13, characterized in that the step of depositing the base layer is directly followed by the step of depositing the gradient layer.

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