Silicon and oxygen-doped amorphous carbon coating composition and uses thereof

Abstract

The present disclosure relates to a silicon and oxygen-doped amorphous carbon coating composition and uses thereof. The present disclosure comprises a non-hydrogenated composite coating composition comprising wherein said coating composition is a silicon and oxygen-doped amorphous carbon comprising: 40 to 70 at. % carbon, silicon as a dopant, and 5 to 20 at. % oxygen as stabilizer.

Description

SI LICON AND OXYGEN-DOPED AMORPHOUS CARBON COATI NG COMPOSITION AND USES TH EREOFTECHNICAL FIELD

[0001] The present disclosure relates to a silicon and oxygen-doped amorphous carbon coating composition and uses thereof.BACKGROUND

[0002] Silicon and oxygen-doped hydrogenated amorphous carbon (a-C:H:Si:O) coatings conveniently also known as diamond-like nanocomposite (DLN), is one of the most industrially acclaimed carbon-based coating materials. A unique combination of high hardness, low intrinsic stress, low friction, high thermal stability, improved biocompatibility, increased corrosion resistance, low surface energy, enhanced dielectric performance, and excellent UV absorption as compared to conventional amorphous carbon (a-C) or diamond-like carbon (DLC) coating, makes DLN as an ideal coating material for a variety of advanced applications. With a range of commercially available variants, DLN coatings have been widely used in automotive, tooling, aerospace, biomedical, and advanced electronics.

[0003] From transportation to the manufacturing sectors, modern technologies are pushing the boundaries of the service temperature limits of materials. To keep up with this exponentially increasing demand, coatings with high thermal stability are of huge importance. Albeit the improved thermal stability of DLNs as compared to DLCs, their maximum usage temperature is restricted below 600°C, depending on the specific composition and operating atmosphere.

[0004] Diamond-like nanocomposite coatings and their method of production were initially disclosed in the US patent 5352493. Substantive disclosures about the same materials are encompassed in US patents 546643, 5718976, 5728465 and 5786068. While these patents describe a broad range of deposition processes, possible coating compositions and the associated coating properties, it is not obvious how thedeposition process, coating structure, or composition can be altered to achieve specific properties, including an increase in thermal stability in air or other atmospheric conditions. Hence, subsequent disclosures about the same coating material were made, focussing on improving the composition to enhance low friction properties, or non-sticking properties, or even the structure and arrangement of multilayer stacks as disclosed in US patents 546643, 5718976, 5728465, 5786068 and 20200370161, to name a few.

[0005] It was claimed in US patent 5352493 that the DLN coatings deposited by PECVD, as the preferred method, are thermally stable up to 1250°C. However, subsequent investigations have revealed that DLN coatings deposited by the conventionally used PECVD method can undergo significant graphitization above 400°C under aerobic conditions, resulting in a loss of the hardness and mechanical performance. Moreover, although it has been mentioned that DLNs can be prepared without hydrogen, it is not obvious what changes in coating structure or properties may result from such variations. The prior art relating to DLNs pertains exclusively to hydrogenated silicon and oxygen-doped amorphous carbon.

[0006] Another variation of silicon and oxygen-doped amorphous carbon materials, termed as strongly bonded quasi-amorphous "QUASAM" was disclosed in US patent 6080470. This material was defined to comprise sp2-bonded graphite-like layers connected together by a sp3diamond-like network, where the whole carbon structure is stabilized by a combination of alloying elements, preferably silicon, and oxygen. While the structure of the QUASAM is inherently different from that of DLNs, further differences lie in the deposition method and the conditions used. The QUASAM is deposited in a temperature range of 300 to 600°C by PECVD using a liquid siloxane- based precursor, and is characterized by a lower density i.e. 1.35 to 1.75 g / cm3as opposed to 2.1 to 2.3 g / cm3for DLN. Further, QUASAM is reported to have a thermal stability of 200 K higher than DLN, with consistent mechanical properties up to a temperature of 500 to 550°C.

[0007] The thermal stability of hydrogenated amorphous carbon (a-C:H) based coatings is generally restricted due to the diffusion of hydrogen from the coatings which leads to significant graphitization of the film beginning at 300°C. The oxidativeatmosphere only enhances this problem further due to the volatilization of carbon atoms. In the case of DLNs, silicon, and oxygen dopants can significantly impede sp3to sp2transformation of carbon atoms by forming Si-C and Si-O-C based linkages, hence increasing the thermal stability of the coating. However, diffusion of unbounded hydrogen still occurs at temperatures above 150°C followed by scission of C-H bonds, which not only accelerates hydrogen diffusion but also promotes sp3to sp2transformation of carbon, leading to graphitization of the coating and loss of mechanical properties. In an oxidative atmosphere, such as air, the volatilization of carbon atoms can still occur at temperatures even below 400°C. Therefore, while hydrogen incorporation in DLN is generally associated with the stabilization of the carbon network and termination of dangling carbon bonds, at high temperatures however, hydrogen diffuses out of the coating structure, thereby compromising the integrity of the coating.

[0008] Another limitation of conventional DLN coating is that the deposition techniques are restricted, generally to plasma enhanced chemical vapor deposition (PECVD) based processes, employing specialized silanes, siloxanes, or organosilicon based liquid precursors, used independently or alongside hydrocarbons. While the original disclosure in US patent 5352493 mentions the sputtering of graphite and silicon targets and the usage of hydrogen or hydrocarbon gas mixed with oxygen gas, as a possible alternative method, yet, PECVD using liquid siloxane precursor remains the preferred method of the prior art. As the subsequent studies in the field almost exclusively used PECVD as the deposition method for DLN coatings. The traditional precursors used for DLN coatings, have a high environmental footprint. Over the years, serious concerns have been raised regarding the ecological impact of low molecular- weight siloxanes. Recent environmental studies suggest that volatile methyl siloxanes should be classified as emerging environmental contaminants, prompting scrutiny of their widespread use. Moreover, using the liquid precursors further adds to the complexity of the deposition process by requiring a vaporization step before introducing the precursors into the deposition chamber. Hence, the significant environmental and economic footprint of these precursors coupled with the limited thermal stability of DLN coatings, warrants further improvements, not only in coatingproperties but also in the development of cost-effective and sustainable deposition processes.

[0009] Since the properties of DLN coatings greatly depend upon the composition of the coating, a precise control of dopant concentration permits the required coating performance. However, due to the very nature of the chemical precursors used in PECVD processes, the fixed stoichiometry of the precursors makes it quite challenging to achieve a specific coating composition. To enable this control, either various precursors are used in combination or specialized precursors are required to be synthesized, leading to additional energy and economic costs.

[0010] Some recent studies have employed physical vapor deposition (PVD) as an alternative deposition method for silicon and oxygen doped amorphous carbon coatings, however, any improvement in coating performance over the prior art, is still to be seen. Some authors studied the friction performance of silicon and oxygen-co- doped non-hydrogenated amorphous carbon coatings but their thermal stability has not been tested. Others have recently reported the deposition of silicon and oxygen- doped hydrogenated amorphous carbon coatings, however, hardness and frictional performance lags behind that of conventional DLNs. Therefore, variations of the PECVD-based process remain the primary techniques for the deposition of DLN coatings. The deposition of thermally stable DLN coatings, by deposition techniques other than PECVD and specifically, using a purely PVD process is still unexplored.

[0011] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.GENERAL DESCRIPTION

[0012] The present disclosure relates to a silicon and oxygen-doped amorphous carbon coating composition and uses thereof.

[0013] The disclosed technology relates to a silicon and oxygen-doped nonhydrogenated amorphous carbon coating composition, also known as nonhydrogenated diamond-like nanocomposite, with improved high-temperature resistance, without observing significant changes in coating structure and properties.

[0014] This technology relates to an improved silicon and oxygen-doped amorphous carbon coating composition, or non-hydrogenated diamond-like nanocomposite, comprising 40 to 70 at. % carbon, 20 to 35 at. % silicon, and 5 to 20 at. % oxygen, and the coating deposition method thereof, wherein the coating composition is substantially free of hydrogen, and wherein the coating exhibits structural and chemical stability when exposed to high temperatures in the range of 600°C to 1100°C, without undergoing significant degradation or alteration in mechanical, thermal, or tribological properties. While conventional hydrogen-containing diamond-like nanocomposite coatings suffer from graphitization, and carbon volatilization at temperatures as low as 400°C in air, the improved non-hydrogenated diamond-like nanocomposite can withstand significantly harsh environment with improved structural stability and can withstand temperature exposure of up to 800 to 1100°C in air; without observing significant changes in coating structure and properties. The coatings herein can be used as protective and functional coatings for application in harsh environments, requiring high thermal stability.

[0015] The present disclosure comprises a non-hydrogenated composite coating composition comprising wherein said coating composition is a silicon and oxygen- doped amorphous carbon comprising: 40 to 70 at. % carbon, silicon as a dopant, and 5 to 20 at. % oxygen as stabilizer.

[0016] In an embodiment, the coating composition comprises 20 to 35 at. % of silicon.

[0017] In an embodiment, the coating composition comprises less than 1 at. % of hydrogen.

[0018] In an embodiment, the coating composition and the coating obtained is stable in air or inert or vacuum atmosphere, at temperatures above 600 °C.

[0019] In an embodiment, the coating composition comprises 40 to 70 at. % carbon, 20 to 30 at. % silicon as a dopant, and 10 to 20 at. % oxygen as stabilizer.

[0020] In an embodiment, the coating composition comprises 50 to 70 at. % carbon, 25 to 30 at. % silicon as a dopant, and 10 to 20 at. % oxygen as stabilizer.

[0021] In an embodiment, the coating composition may further comprise an additional dopant with at least one element selected from a list consisting of: Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Mo, Pd, Ag, Hf, Ta, W, Pt, Au, Al, Sn, Pb, F or N.

[0022] In an embodiment, the amount of the additional dopant element in the coating composition is not more than 10 at .%.

[0023] In an embodiment, the hardness of the coating composition ranges from 15 to 35 GPa, preferably from 15 to 28 GPa.

[0024] In an embodiment, the coating composition comprises a coefficient of friction less than 0.08 against steel or ceramic counter bodies during dry or lubricated conditions, at room temperature.

[0025] In an embodiment, the coating composition comprises a water contact angle of less than 65°.

[0026] In an embodiment, the coating composition has a sheet resistance greater than 10 MQ.sq1.

[0027] In an embodiment, the coating composition comprises a thickness ranging from 10 nm to 5 pm.

[0028] It is also disclosed a thin film stack comprising the non-hydrogenated composite coating composition described above.

[0029] In an embodiment, the thin film stack comprises alternating layers of the nonhydrogenated composite coating, in particular the thin film stack comprising a silicon and oxygen-doped amorphous carbon coating layer deposited over the substrate.

[0030] In an embodiment, the thin film stack comprises an interlayer deposited over the substrate and a silicon and oxygen-doped amorphous carbon coating layer deposited over the interlayer.

[0031] In an embodiment, the material of the interlayer can be selected from a list consisting of: C, Cr, Si or Ti, either in elemental or compound form as in oxides, carbides and nitrides, and their combinations.

[0032] In an embodiment, the interlayer can comprise a single or multiple layers of which may be deposited as distinct layers, a homogeneous or heterogeneous mixture,or gradient layers wherein the composition transitions continuously or stepwise between different elements, compounds, or mixtures thereof.

[0033] In an embodiment, the silicon and oxygen-doped amorphous carbon coating layer is also deposited directly on the substrate without an interlayer.

[0034] It is also disclosed the use of the non-hydrogenated composite coating composition in protective applications, anti-fogging applications, dielectrics, optical applications, cathode for batteries, diffusion control layers, memristors, triboelectric nanogenerators and biocompatible layers for implants.

[0035] It is also disclosed a method for depositing the non-hydrogenated composite coating composition, comprising the following steps: providing a suitable substrate; depositing a non-hydrogenated composite coating composition according to the above mentioned.

[0036] In an embodiment, the method comprises the following steps: providing a vacuum chamber; evacuating the chamber to a base pressure of less than 3 x IO-4Pa; introducing the inert gas into the chamber; establishing a plasma discharge; sputtercleaning the surface of substrates and targets; depositing the interlayer over a substrate; introducing reactive gas into the chamber and sputtering of targets for deposition of silicon and oxygen-doped amorphous carbon coating layer according to the description in the text.

[0037] In an embodiment, the vacuum chamber used in the method is part of a magnetron sputtering unit.

[0038] In an embodiment, the magnetron sputtering unit used in the method comprises at least four targets aligned and positioned at 90° relative to each other, wherein said targets include: a carbon (C) or chromium (Cr) or titanium (Ti) or silicon (Si) target for interlayer deposition, two carbon (C) targets, and one silicon (Si) target.

[0039] In an embodiment, the inert gas used in the method is selected from a list consisting of: Ar, Ne, or a mixture of both.

[0040] In an embodiment, the method comprises the gas flow rate of inert gas ranges from 20 to 60 seem, preferably from 30 to 50 seem.

[0041] In an embodiment, the reactive gas used in the method is oxygen.

[0042] In an embodiment, the method comprises the gas flow rate of the reactive gas ranges from 1 to 20 seem of the reactive gas, preferably from 5 to 15 seem.

[0043] In an embodiment, the substrate is biased using a pulsed DC voltage from 0 to 200 V during the deposition of silicon and oxygen-doped amorphous carbon coating layer.

[0044] In an embodiment, the silicon and oxygen-doped amorphous carbon layer is deposited using direct magnetron sputtering method, in which the targets are connected to DC power supplies.

[0045] In an embodiment, the deposition temperature of the method is maintained below 200°C.

[0046] In an embodiment, the C, Si, O and metallic dopant species used in the method are generated by sputtering elemental, carbide, oxide, oxycarbide or silicide targets, or any combination thereof.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.

[0048] Figure 1: Schematic representation of an embodiment of a chamber used for the deposition of silicon and oxygen-doped nonhydrogenated amorphous carbon coatings, comprising a deposition chamber (a), evacuated with vacuum pumps (b), connected to a gas supply (c), with four magnetrons (d) equipped with sputter targets (e), a rotating substrate holder (f), substrates (g) and DC / pulsed DC power supplies for the substrates and the targets (h).

[0049] Figure 2: Graphic representation of a Raman spectra of as-deposited coatings.

[0050] Figure 3: Graphic representation of results of a C-K edge near edge x-ray absorption fine structure spectra of as deposited coatings (a), and calculated sp3fraction (b).

[0051] Figure 4: Graphic representation of thermogravimetric analysis (TGA) of as- deposited coating on AI2O3 substrates, in synthetic air (80% N2 and 20% O2) from room temperature to 1100°C acquired at a heating rate of 10°C / min.

[0052] Figure 5: Scanning electron microscope images acquired from surface of the samples after exposure to 1100°C in air, and elemental composition as measured by EDS.

[0053] Figure 6: Graphical representation of hardness comparison of different coatings, as deposited and after exposure to 1100°C in air.

[0054] Figure 7: Graphic representation of a Raman spectra of a-C:Si:O coating, as deposited and after annealing at 800°C in air for 1 hour.DETAILED DESCRIPTION

[0055] This technology relates to a thermally stable silicon and oxygen-doped nonhydrogenated amorphous carbon coating composition, with improved thermal stability compared to the prior art of DLNs.

[0056] The present disclosure relates to a non-hydrogenated composite coating composition, specifically an enhanced non-hydrogenated diamond-like nanocomposite. The coating composition is a silicon- and oxygen-doped amorphous carbon, comprising 40 to 70 atomic percent carbon, with silicon as a dopant and 5 to 20 atomic percent oxygen as a stabilizer. Surprisingly, the composition is substantially free of hydrogen. The coating demonstrates excellent structural and chemical stability when exposed to high temperatures ranging from 600°C to 1100°C, without significant degradation or changes in its structural, mechanical, thermal, or functional performance.

[0057] In an embodiment, the coating composition comprises 20 to 35 at. % of silicon, for better results.

[0058] In an embodiment, the coating composition comprises less than 1 at. % of hydrogen, for better results.

[0059] In an embodiment, the coating composition and the coating obtained is stable in air or inert or vacuum atmosphere even at temperatures above 600°C, for better results.

[0060] In an embodiment, the coating composition comprises 50 to 70 at. % carbon, 25 to 30 at. % silicon as a dopant, and 10 to 20 at. % oxygen as stabilizer, for better results.

[0061] In an embodiment, the coating composition may further comprise an additional dopant with at least one element selected from a list consisting of: Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Mo, Pd, Ag, Hf, Ta, W, Pt, Au, Al, Sn, Pb, F or N, for better results.

[0062] In an embodiment, the amount of the additional dopant element in the coating composition is not more than 10 at .% , for better results.

[0063] In an embodiment, the hardness of the coating composition ranges from 15 to 35 GPa, preferably from 15 to 28 GPa, for better results.

[0064] In an embodiment, the coating composition comprises a coefficient of friction less than 0.08 against steel or ceramic counter bodies during dry or lubricated conditions, for better results.

[0065] In an embodiment, the coating composition comprises a water contact angle of less than 65°, for better results.

[0066] In an embodiment, the coating composition comprises a sheet resistance greater than 10 MQ.sq-1, for better results.

[0067] In an embodiment, the coating composition comprises a thickness ranging from 10 nm to 5 pm, for better results.

[0068] It is also disclosed a thin film stack comprising the non-hydrogenated composite coating composition described above.

[0069] In an embodiment, the thin film stack comprises alternating layers of the nonhydrogenated composite coating comprising of varying carbon, silicon and oxygen content, for better results.

[0070] In an embodiment, the thin film stack comprises an interlayer deposited over the substrate and a silicon and oxygen-doped amorphous carbon coating layer deposited over the interlayer, for better results.

[0071] In an embodiment, the interlayer can be selected from a list consisting of: C, Cr, Si or Ti, either in elemental or compound form as in oxides, carbides and nitrides, for better results.

[0072] In an embodiment, the interlayer can consist of a single or multiple layers of which may be deposited as distinct layers, a homogeneous or heterogeneous mixture, or gradient layers wherein the composition transitions continuously or stepwise between different elements, compounds, or mixtures thereof, for better results.

[0073] In an embodiment, the silicon and oxygen-doped amorphous carbon coating layer is also deposited directly on the substrate without an interlayer, for better results

[0074] It is also disclosed the use of the non-hydrogenated composite coating composition in protective applications, anti-fogging applications, dielectrics, optical applications, cathode for batteries, diffusion control layers, memristors, triboelectrictric nanogenerators and biocompatible layers for implants.

[0075] It is also disclosed a method for depositing the non-hydrogenated composite coating composition, comprising the following steps: providing a suitable substrate; depositing a non-hydrogenated composite coating composition according to the above mentioned, wherein the coating composition is controlled by adjusting the power to the silicon and carbon targets and the oxygen gas flow.

[0076] In an embodiment, the method comprises the following steps: providing a vacuum chamber; evacuating the chamber to a base pressure of less than 3 x IO-4Pa; introducing the inert gas into the chamber; establishing a plasma discharge, sputtercleaning the surface of substrates and targets; depositing the interlayer over a substrate; introducing reactive gas into the chamber and sputtering of targets for deposition of silicon and oxygen-doped amorphous carbon coating layer according to the description in the text, for better results.

[0077] In an embodiment, the vacuum chamber used in the method is part of a magnetron sputtering unit, for better results.

[0078] In an embodiment, the magnetron sputtering unit used in the method comprises at least four targets aligned and positioned at 90° relative to each other, wherein said targets include: a carbon (C) or chromium (Cr) or titanium (Ti) or silicon (Si) target for interlayer deposition, two carbon (C) targets, and one silicon (Si) target, for better results.

[0079] In an embodiment, the inert gas used in the method is selected from a list consisting of: Ar, Ne, or a mixture of both, for better results.

[0080] In an embodiment, the reactive gas used in the method is oxygen, for better results.

[0081] In an embodiment, the method comprises the use of 20 to 60 seem of the inert gas, preferably from 30 to 50 seem, for better results.

[0082] In an embodiment, the method comprises the use of 1 to 20 seem of reactive gas, preferably from 5 to 15 seem, for better results.

[0083] In an embodiment, the substrate is biased using a pulsed DC voltage from 0 to 200 V, for better results.

[0084] In an embodiment, the silicon and oxygen-doped amorphous carbon layer is deposited using direct magnetron sputtering method, during which the targets are connected to DC power supplies.

[0085] In an embodiment, the deposition temperature of the method is maintained below 200°C and no external heating is applied, for better results.

[0086] In an embodiment, the C, Si, O and metallic dopant species are generated by sputtering elemental, carbide, oxide, oxycarbide, silicide targets, or any combination thereof, for better results.

[0087] In an embodiment, additional metallic dopants can be seamlessly incorporated into the coating by sputtering using the same deposition chamber, for better results.

[0088] In an embodiment, non-metallic dopants such as nitrogen (N) or fluorine (F) can be incorporated into the coating through reactive deposition, utilizing their respective gaseous precursors, for better results.

[0089] In an embodiment, a DC or RF bias may be applied to the substrate. Also, DC or RF sputtering can be used for deposition of the coatings, for better results.

[0090] In an embodiment, the C, Si, O and metallic dopant species used in the method are generated by sputtering elemental, carbide, oxide, oxycarbide or silicide targets, or any combination thereof, for better results.

[0091] The improved thermal stability requires eliminating the use of hydrogen dopant, thereby avoiding the diffusion of hydrogen from the coating and resulting graphitization at higher temperatures. This improved stability can be achieved within a preferred composition range of 50 to 70 at. % carbon and using a high concentration of Si dopants, preferably between 25 and 30 at. %, to promote sp3type C-C and C-Si bonds further stabilized by O atoms with a preferable concentration of 10 to 20 at. %.

[0092] Thermal stability of the silicon and oxygen-doped non-hydrogenated amorphous carbon (a-C:Si:O) coating developed herein, has been significantly improved compared with prior art, demonstrating the ability to withstand temperatures of up to 800 to 1100°C in air, without observing significant changes in coating structure and properties.

[0093] In an embodiment, the coatings are deposited using a closed field unbalanced magnetron sputtering semi-industrial unit Teer UDP650-4 (Teer Coatings ltd. UK), equipped with four targets (380 x 175 x 8 mm) vertically aligned and positioned at 90° relative to each other (figure 1). One Cr target (99.9% purity) for the interlayer, alongside two graphite targets (99.99% purity) and one Si target (99.99 % purity) is used for the deposition of coatings. The reactive sputtering is carried out using an inert gas i.e. Ar (99.99 % purity) as the carrier gas, and O2 (99.99 % purity) as the reactive gas. In order to draw a clear distinction between hydrogenated and non-hydrogenated coatings and improvements over the prior art, silicon and oxygen-doped hydrogenated amorphous carbon coatings were also deposited (a-C:H:Si:O). For hydrogenated coatings, CH4 (99.99 % purity) is used as the hydrogen source. Prior to the deposition, the substrates are ultrasonically cleaned in analytical-grade acetone and ethanol for 15 minutes each and subsequently blow-dried in hot air. The chamber is evacuated to a base pressure of less than 3 x 10-4Pa followed by sputter-cleaning of targets and substrates for a total time of 40 minutes. During the deposition, the targets areoperated in a DC power-controlled mode, whereas a pulsed DC bias of -50 V is applied to the rotating substrates using power supplies (Advanced Energy Pinnacle, US). A Cr interlayer of approximately 390 nm is deposited to improve adhesion, followed by the final coating itself. All the coatings are deposited without external heating and the deposition time is fixed to achieve a coating thickness of around 1.1 ± 0.1 pm. Power on carbon targets is kept constant during all the depositions, while the doping content is controlled by adjusting the power on the silicon and carbon targets, and the flow of the reactive gas using FC7700CD mass flow controllers (Proterial ltd. previously known as Hitachi Metals ltd., JP), calibrated for the respective gas. The reactive gasses are introduced only after the deposition of the gradient layer and the working pressure is maintained < 0.5 Pa. The deposition conditions are summarized in table 1.Table 1: Summary of deposition parameters

[0094] Using the above-described process parameters and apparatus features, undoped non-hydrogenated amorphous carbon (a-C), silicon and oxygen-doped nonhydrogenated amorphous carbon (a-C:Si:O) and silicon and oxygen-doped hydrogenated amorphous carbon (a-C:H:Si:O) coatings were deposited. It must be noted here that this invention relates only to a-C:Si:O, as an improved nonhydrogenated diamond-like nanocomposite coatings, and the results for a-C and a- C:H:Si:O coatings are included only, as reference for comparison purposes, so as topresent distinct features of this invention by highlighting changes in structure and properties with and without the inclusion of various dopants.

[0095] The inert gas may comprise Ar or Ne or a mixture of them in varying ratios. Further, the interlayer may or may not be present. The interlayer may comprise carbon (C), chromium (Cr), titanium (Ti), or silicon (Si) in their elemental form or in the form of compounds, including but not limited to oxides, carbides, nitrides, or other derivatives of the respective elements. The interlayer may be configured as distinct layers, a homogeneous or heterogeneous mixture, or a gradient layer wherein the composition transitions continuously or stepwise between different elements, compounds, or mixtures thereof. Moreover, the coatings can be deposited on rotating or stationary substrate with or without substrate bias in a range of 0 to -200 V.

[0096] Coatings with thicknesses ranging from several nanometres to a few microns can be deposited on polymeric, metallic, ceramic, or composite substrate, be it rigid or flexible. Moreover, the same deposition method can be employed to deposit stacks comprising alternating layers of silicon and oxygen-doped non-hydrogenated amorphous carbon coating separated by non-hydrogenated amorphous carbon layers, and the number of stacks can vary from 1 to 20.

[0097] The reactive magnetron sputtering process as the preferred deposition method for silicon and oxygen-doped non-hydrogenated amorphous carbon (or non- hydrogenated diamond-like nanocomposite) coatings, has considerably lower energy, economic, and environmental footprint, as compared to PECVD. This is due to the lower cost of precursors used i.e. solid graphite and carbon targets, and oxygen gas as opposed to the silanes, siloxanes, or organosilicon precursors. The exclusion of the vaporization stage necessary for using conventional liquid precursors in PECVD contributes to energy savings. Moreover, the solid graphite and silicon targets and oxygen gas as sources of C, Si, and O species have inherently lower environmental footprint when compared with the conventional precursors for PECVD. With the growing demand for environmentally friendly manufacturing technologies and a cradle-to-grave philosophy for sustainable materials, there exists an imminent need for alternative deposition pathways for protective coatings, which this technology aims to address.

[0098] Another aspect of the present deposition method is the usage of solid graphite and silicon targets along with oxygen gas, which enables a precise and straightforward control of coating composition. Thus, as an improvement over prior art, a range of coating compositions can be deposited without making any changes to the precursors, as normally required in PECVD.

[0099] In another aspect of this invention, the method disclosed herein can be used to improve the electrical or optical properties of the coatings by incorporating one or more metallic dopants similar to what is described in US 5352493, for hydrogenated DLNs. The dopants can include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Mo, Pd, Ag, Hf, Ta, W, Pt, Au, Al, Sn or Pb. In some cases, the surface energy of the coatings can be modified by doping elements such as F or N, which can be incorporated using a gas source. In the case of the doped non-hydrogenated diamond-like nanocomposite, the coating may contain up to 10 % of the dopant concentration.

[0100] The Raman spectra of non-hydrogenated DLN i.e. silicon and oxygen doped non-hydrogenated amorphous carbon (a-C:Si:O) is compared with that of conventional DLN i.e. silicon and oxygen doped hydrogenated amorphous carbon (a- C:H:Si:O) and undoped amorphous carbon (a-C) is compared in figure 2. Raman spectra of a-C:Si:O is similar to the traditional spectra for amorphous carbon with a broad peak (850 - 1850 cm'1) constituting individual breathing modes of Aigand E2g Raman active modes of graphite. A blue shift in the peak position coupled with a reduction of fullwidth half maxima (FWHM), indicates a possible increase in sp3content with silicon and oxygen doping.

[0101] The carbon hybridization state and its local atomic environment were investigated using NEXAFS of C K-edge region (Figure 3(a)). Disordered carbon presents a pre-edge resonance around 284.8 eV which corresponds to the transition from Cis to unoccupied n*orbital, mainly originating from sp2(C=C) and some contribution from sp (C C). On the other hand, a broad region from 290 to 320 eV corresponds to different transitions from Cis to o* bonds, mainly relating to the sp3(C-C) bonds but can also have some degree of contribution from sp and sp2bonds. As shown in Figure 3a, not only does the intensity of sp2(C=C) reduce monotonically with successive doping of Si, O, and H in amorphous carbon but the peak becomes narrower indicatinga reduction in the sp2character of the films and limiting the bond length distribution. NEXAFS spectra shows a reduction in intensity for Cis to o* transition above 293.5 eV for doped coatings as compared to the undoped one, indicating a possible replacement of C-C sp3bonds with C-Si sp3bonds. The blue shift of NEXAFS spectra towards the C-Si region (290 eV) further indicates the presence of linkages in the amorphous structure. In addition to these, some relatively weak features can also be observed at 286.6 eV, 288.2 eV and 288.9 eV, which can arise not only due to the air exposure and adsorption of contaminants but also due to the presence of C=H bonds as evidenced by highest intensity of a-C:H:Si:O in this region. Overall, NEXAFS being an established technique for quantification of carbon hybridization state, established a clear increase in sp3fraction of the coatings with successive doping of Si, O and H (figure 3(b)). Although the highest sp3fraction i.e. ~66 % is observed in case a-C:H:Si:O (similar to conventional DLN), however, it must be noted that this increase originates from the presence of C-H bonds, and even in case of a-C:Si:O coatings the sp3fraction is ~60 % which is significantly higher than that of a-C. NEXAFS analysis thus indicates a pronounced sp3character of a-C:Si:O coatings and the presence of interconnected amorphous networks, analogous to conventional hydrogenated DLN.

[0102] The thermal stability of the as-deposited coatings on AI2O3 substrate was tested using thermogravimetric analysis (TGA) by heating them in synthetic air (80% N2 and 20% O2) up to 1100°C (figure 4) and recording changes in mass. The onset of oxidation and residual mass were recorded in this order: a-C < a-C:H:Si:O < a-C:Si:O. While a-C:H:Si:O clearly demonstrated increased resistance to oxidation and delayed onset for mass loss curve as compared to undoped a-C, a-C:Si:O observed no significant mass loss / gain even until 1100°C, indicating an excellent resistance to oxidation. These results establish the superior thermal stability of a-C:Si:O (as nonhydrogenated DLN) as compared to a-C:H:Si:O (conventional hydrogenated DLN) as a salient feature of this technology.

[0103] The surface morphology and elemental composition of coatings after exposure to 1100°C in air was recorded using field emission scanning electron microscopy (FESEM) and Energy dispersive spectroscopy (EDS) (figure 5). The analysis indicates that the carbon in a-C and a-C:H:Si:O was almost completely volatilized, evenleading to oxidation of the Cr interlayer. However, a-C:Si:O coating still retained most of its carbon content, thereby indicating significantly improved thermo-oxidative stability of a-C:Si:O as non-hydrogenated DLN.

[0104] The degradation in mechanical properties of the coatings after thermal exposure was characterized by hardness measurements before and after heating (figure 6). The a-C:Si:O coating demonstrated a minimal change in hardness, unlike a-C and a-C:H:Si:O, thereby establishing the integrity of the coating and retention of its mechanical properties even after high-temperature exposure.

[0105] The improved thermal stability of a-C:Si:O (or non-hydrogenated DLN) is linked to improved structural stability of the coating. In the absence of hydrogen, the preferred formation of C-Si bonds impede the sp3to sp2transformation and C-Si-0 bonds further passivate the film surface thereby restricting volatilization of carbon, as demonstrated by substantially similar Raman spectra of the coating before after high temperature exposure (figure 7).

[0106] In some embodiment of the present invention, as-deposited silicon and oxygen-doped nonhydrogenated amorphous carbon coatings have a density of 2.6 to 2.8 g / cm3, which is significantly higher than QUASAM (1.35 to 1.75 g / cm3) and convention hydrogenated DLN (2.1 to 2.23 g / cm3), which is related to the dense structure achieved in the absence of hydrogen. The hardness of a-C:Si:O coatings range from 15 to 28 GPa, and reduced modulus from 140 to 250 GPa, with residual stress not higher than 400 MPa. As deposited a-C:Si:O coatings demonstrate a hydrophilic behaviour with a water contact angle less than 65°. The a-C:Si:O coatings demonstrate a sheet resistance high than 10 MQ.sq-1. Moreover, the coatings show a reasonably low coefficient of friction i.e. less than 0.08, when sliding against metallic or ceramic counterparts (100Cr6 or AI2O3 or SisN^, under dry or lurbicated conditions. Thereby, indicating that the absence of hydrogen from the a-C:Si:O (as non-hydrogenated DLN) structure, has no significant detrimental effect on structural or functional performance of the coating.

[0107] The improved non-hydrogenated diamond-like nanocomposite coatings disclosed herein are promising to be used as protective hard coatings in harsh environments, specifically high-temperature and oxidizing atmospheres. The improvedthermal stability and high hardness coupled with low friction performance can be exploited by using these coatings in heat-assisted magnetic recording for hard drives, glass and ceramic manufacturing molds, exhaust systems for automotive engines, metal processing tools, heat exchangers, wafer handling components, high- temperature bearings, high temperature protective applications, memristors, triboelectric nanogenerators, and spacecraft components.

[0108] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0109] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above-described embodiments are combinable.

[0110] The following dependent claims further set out particular embodiments of the disclosure.

Claims

C L A I M S1. A non-hydrogenated composite coating composition, wherein said coating composition is a silicon and oxygen-doped amorphous carbon, comprising:40 to 70 at. % carbon, silicon as a dopant, and5 to 20 at. % oxygen as a stabilizer.

2. The coating composition according to the previous claim comprising 20 to 35 at. % of silicon.

3. The coating composition according to the previous claim, wherein the coating composition comprises less than 1 at. % of hydrogen.

4. The coating composition according to any of the previous claims, wherein the coating is stable in air or inert or vacuum atmosphere, at temperatures above 600 °C.

5. The coating composition according to any of the previous claims, comprising40 to 70 at. % carbon,20 to 30 at. % silicon as a dopant, and10 to 20 at. % oxygen as stabilizer.

6. The coating composition according to any of the previous claims 1 to 4, comprising50 to 70 at. % carbon,25 to 30 at. % silicon as a dopant, and10 to 20 at. % oxygen as stabilizer.

7. The coating composition according to any of the previous claims, further comprising an additional dopant with at least one element selected from a list consisting of: Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Mo, Pd, Ag, Hf, Ta, W, Pt, Au, Al, Sn, Pb, F or N.

8. The coating composition according to the previous claim, wherein the amount of the additional dopant element is not more than 10 at .%.

9. The coating composition according to any of the previous claims, wherein the coating comprises a hardness from 15 to 35 GPa, preferably from 15 to 28 GPa.

10. The coating composition according to any of the previous claims, wherein the coating comprises a coefficient of friction less than 0.08 against steel or ceramic counter bodies during dry or lubricated conditions, at room temperature.

11. The coating composition according to any of the previous claims, wherein the coating comprises a water contact angle of less than 65°.

12. The coating composition according to any of the previous claims, wherein the coating comprises a sheet resistance greater than 10 MQ.sq-1.

13. The coating composition according to any of the previous claims, wherein the coating comprises a thickness ranging from 10 nm to 5 pm.

14. A thin film stack comprising the non-hydrogenated composite coating composition according to any of the claims 1 to 13.

15. The thin film stack according to the previous claim comprising alternating layers of the non-hydrogenated composite coating composition.

16. The thin film stack according to any of the previous claim 14-15 comprising a silicon and oxygen-doped amorphous carbon coating layer deposited over the substrate.

17. The thin film stack according to any of the previous claims 14-16 comprising an interlayer deposited over the substrate and a silicon and oxygen-doped amorphous carbon coating layer deposited over the interlayer.

18. The thin film stack according to any of the previous claims 14-17, wherein the material of the interlayer is selected from a list consisting of: C, Cr, Si or Ti, either in elemental or compound form as in oxides, carbides, and nitrides, and their combinations.

19. The thin film stack according to any of the previous claims 14-18, wherein the interlayer comprises a single or multiple layers, which may be deposited as distinct layers, a homogenous or heterogenous mixture, or gradient layers wherein the composition transitions continuously or stepwise between different elements, compounds, or mixtures thereof.

20. Use of the non-hydrogenated composite coating composition described in any of the claims 1 to 13 in protective applications, anti-fogging applications, dielectrics, optical applications, cathode for batteries, diffusion control layers, memristors, triboelectrictric nanogenerators and biocompatible layers for implants.

21. A method for depositing the non-hydrogenated composite coating composition described in any of the claims 1 to 13, comprising the following steps: providing a suitable substrate; depositing a non-hydrogenated composite coating composition according to claims 1-13.

22. The method according to the previous claim, comprising the following steps: providing a vacuum chamber; evacuating the chamber to a base pressure of less than 3 x IO-4Pa; introducing the inert gas in the chamber; establishing a plasma discharge; sputter-cleaning the surface of substrates and targets; depositing the interlayer over a substrate; introducing reactive gas in the chamber and sputtering of targets for depositing the silicon and oxygen-doped amorphous carbon coating layer according to claims23. The method according to any the previous claims 21 to 22, wherein the vacuum chamber is part a magnetron sputtering unit.

24. The method according to any of the previous claims 21 to 23, wherein the magnetron sputtering unit comprises at least four targets aligned and positioned at 90° relative to each other, wherein said targets include: a carbon (C) or chromium (Cr) or titanium (Ti), or silicon (Si) target for interlayer deposition, two carbon (C) targets, and one silicon (Si) target.

25. The method according to any of the previous claims 21 to 24, wherein the inert gas is selected from a list consisting of: Ar, Ne, or a mixture of both.

26. The method according to any of the previous claims 21 to 25, wherein the gas flow rate of inert gas ranges from 20 to 60 seem, preferably from 30 to 50 seem.

27. The method according to any of the previous claims 21 to 26, wherein the reactive gas is oxygen.

28. The method according to any of the previous claims 21 to 27, wherein gas flow rate of the reactive gas ranges from 1 to 20 seem, preferably from 5 to 15 seem.

29. The method according to any of the previous claims 21 to 28, wherein the substrate is biased using pulsed DC voltage from 0 to 200 V.

30. The method according to any of the previous claims 21 to 29, wherein the deposition temperature is less than 200°C.

31. The method according to any of the previous claims 21 to 30, wherein the C, Si, O and metallic dopant species are generated by sputtering elemental, carbide, oxide, oxycarbide or silicide targets, or any combination thereof.

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