The effects of deposition conditions on hydrogenation, hardness and elastic modulus of W-C:H coatings

Journal Pre-proof The effects of deposition conditions on hydrogenation, hardness and elastic modulus of W-C:H coatings ´ a, ´ Lenka Kvetkova, ´ Jozef Frantiˇsek Lofaj, Margita Kabatov Dobrovodsk´y

PII:

S0955-2219(19)30906-9

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.12.062

Reference:

JECS 12975

To appear in:

Journal of the European Ceramic Society

Received Date:

28 August 2019

Revised Date:

27 December 2019

Accepted Date:

30 December 2019

´ a´ M, Kvetkova´ L, Dobrovodsk´y J, The effects of Please cite this article as: Lofaj F, Kabatov deposition conditions on hydrogenation, hardness and elastic modulus of W-C:H coatings, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.12.062

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The effects of deposition conditions on hydrogenation, hardness and elastic modulus of W-C:H coatings František Lofaja* [email protected], Margita Kabátováa, Lenka Kvetkováa, Jozef Dobrovodskýb,

a

Institute of Materials Research of the Slovak Academy of Sciences, Watsonova 47, 040 01

Košice, Slovakia b

Advanced Technologies Research Institute, Faculty of Materials Science and Technology in

Corresponding author; Tel: +421-55-7922407; Fax: +421-55-7922408;

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*

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Trnava, Slovak University of Technology in Bratislava, J. Bottu 25, 91724 Trnava, Slovakia

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ABSTRACT

The additions of C2H2, CH4 and H2 in hybrid PVD-PECVD of W-C:H coatings deposited using High Power Impulse Magnetron Sputtering (HiPIMS) and High Target Utilization

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Sputtering (HiTUS) were investigated to determine their effects on the content and chemical composition of the amorphous carbon-based boundary phase and mechanical properties of the

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coatings. Substantial differences were observed: CH4 always produced higher concentrations

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of hydrogen and lower concentrations of carbon than C2H2 and HiPIMS resulted in higher contents of amorphous carbon-based boundary phase and higher levels of its hydrogenation than HiTUS. The detrimental effects of higher carbon and hydrogen contents in the boundary phase on hardness and indentation modulus were attributed to the consumption of C-C bonds by C-H during hydrogenation and reduction of cross-linking of the polymeric network in the boundary phase. The HiPIMS W-C:H coatings deposited with acetylene and hydrogen exhibited medium (~20 GPa) hardness and elastic modulus (200 - 220 GPa) with HIT/EIT > 1

0.1 suggesting improved toughness and wear resistance. These properties were attributed to the optimum combination of hydrogenation, hybridization and cross linking in the carbonbased boundary phase.

Keywords: W-C:H coatings, Hybrid PVD-PECVD, High Power Impulse Magnetron Sputtering (HiPIMS), High Target Utilization Sputtering (HiTUS), Hydrogenation, Hardness, Indentation modulus.

Introduction

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1.

The main requirements for PVD coatings for engineering applications during the last two decades gradually evolved from high hardness toward better oxidation, corrosion and

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wear resistance at higher temperatures and more recently also towards increased toughness [15]. The approaches to achieve these properties involve the use of

intrinsically hard and more refractory systems (nitrides, carbides and borides of refractory

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metals and their multicomponent combinations); -

more complex coating structures (e.g. nanocomposites, nanolaminates, etc.);

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improved deposition techniques [6-12]. In magnetron sputtering, highly ionized pulsed

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methods, e.g. High Power Impulse Magnetron Sputtering (HiPIMS) or High Target

10].

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Utilization Sputtering (HiTUS) can be used for better control of the resulting properties [7-

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The current study combines all three approaches. Diamond-like amorphous carbon (DLC) coatings are well-known for low friction applications. However, their alloying with transition metals is often preferred due to lower residual stresses, better adhesion, higher toughness and better tolerance to overloading under contact pressure [3, 13 - 28]. Among the metal-doped DLC coatings, W seems to provide the best compromise of hardness, friction and wear resistance [14, 20, 22]. The W-C:H coatings are often produced by hybrid PVD-PECVD magnetron sputtering from WC (or W and C targets) in a hydrocarbon precursor containing an 2

Ar atmosphere. The most frequently used reactive gas is acetylene (C2H2) [13-14, 17-19, 2730], but the other precursor gases with a higher H : C ratio (e.g. CH4) may also be used [3132]. During WC target sputtering, Ar plasma strongly interacts with the hydrocarbon gas and drives its decomposition and recombination reactions. The products of these reactions involve various CxHy (x, y = 0, 1, 2..) radicals and fragments, which are deposited on the substrate simultaneously with the sputtered WC molecules. The hydrogenation of carbon matrix is a consequence of the attachment of CxHy fragments to the dangling carbon bonds on the growing boundary carbon phase [33 - 39].

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Depending on hydrocarbon gas additions, a wide range of coating compositions can be obtained. When the additions of hydrocarbon precursor gas are high and doping by WC is low, WC grains cannot form and amorphous a-C(:H:W) coatings are obtained. Without the

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addition of the hydrocarbon precursor, nanocrystalline WC1-x coatings may be produced.

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From the viewpoint of hardness increase, the most interesting coatings involve those with nanocomposite structure [40]. The nanocomposite W-C:H coatings consist of nanocrystalline

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WC1-x grains bound together by a boundary phase from amorphous hydrogenated carbon [13 24, 26 - 30]. They form in the range of small hydrocarbon additions to WC sputtering because

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the content of the amorphous boundary phase has to be small compared to the nanocrystalline carbide phase [40]. When the content of the amorphous carbon phase exceeds that for “true”

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nanocomposite, it becomes the weakest link in the coating structure and effectively controls the resulting mechanical properties. Therefore, the content of boundary carbon phase is

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among the most important structural parameters of the nanocomposite structures. However, the properties of the amorphous carbon phase depend also on the content of hydrogen in it, on the level of its sp3 hybridization, branching and cross-linking [27-28, 37-39]. The content of the amorphous carbon phase and its hydrogenation are influenced by the type and amount of precursor gas in the sputtering atmosphere [33-34, 37]. With the same hydrocarbon gas flow, CH4 should be more effective source of hydrogen than C2H2, which has a considerably lower 3

H:C ratio [41]. Hydrogenation also affects the level of hybridization: hydrogen in the polymeric carbon chains separates sp3 bonds into C-C and C-H bonds. Thus, the ratio between the contents of sp3 and sp2 clusters can be also controlled via hydrogenation [27, 37, 42]. For engineering applications involving sliding contacts under mechanical stress, high hardness combined with low Young’s modulus of the coating are desirable to generate favorable stress distributions resulting in low wear rates and higher toughness [1, 5, 43]. Thus, the optimization of the contents of carbon and hydrogen via additions of different amounts of hydrocarbons may be used as a tool for the control of mechanical and tribological properties

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of the W-C:H coatings [15, 17-19, 22, 25-27, 38-39, 41]. However, the early studies were often performed on non-hydrogenated W-C coatings produced by co-deposition [21-24]. An inverted correlation between hardness and coefficients of friction (CoF) vs. carbon content

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was reported [23-24]. The relationships between hardness, HIT, reduced Young’s modulus,

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EIT, (EIT = E/(1-2),  – Poisson’s ratio), their combination HIT/EIT, CoF, wear rate and carbon contents were described in the work of Abad et al. [23]. The ratio HIT/EIT remained within a

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narrow range around 0.07 within the studied range of carbon contents from 10 % to 31 at. %. It should be noted that these contents corresponded to the amorphous carbon in the boundary

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(also called “matrix”) phase

The difficulties with the measurement of hydrogen concentrations in thin

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hydrogenated W-C:H coatings caused that their properties were related only to the total carbon content and the influence of hydrogen was omitted [17-20, 31-32, 44]. The studies

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involving the measurements of hydrogen concentrations are still rather limited. Makowka et al. [27] determined hydrogen content in DC magnetron sputtered W-C:H coatings prepared with different additions of CH4 and H2 by Secondary Ion Mass Spectroscopy and investigated its influence on CoF and wear rates. It was found out that a high degree of hydrogenation is required to achieve low CoF values. The role of hybridization on hardness was not investigated. Nouvellon et al. [28] investigated W-C:H coatings produced with C2H2 additions 4

using XPS. Although the correlations between acetylene addition, hydrogenation and sp3 hybridization were discussed, no clear correlations to hardness and wear rates were reported. Our previous works on hybrid PVD-PECVD W-C:H coatings prepared with C2H2 [29-30, 38] and CH4 [39] and driven by DC magnetron sputtering (DCMS), HiPIMS and HiTUS investigated the corresponding deposition mechanisms, their mechanical and tribological properties. The properties were correlated with the amount of hydrocarbon gas added into the sputtering atmosphere and with the carbon and hydrogen contents measured by a combination of Rutherford Backscattering (RBS) and Elastic Recoil Detection Analysis (ERDA). Even the

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first attempts to correlate the level of hybridization with the hydrogen concentration and

hardness were made in HiPIMS W-C:H coatings [38]. The conclusions from these studies were that the carbon content is coupled to the level of sp3 hybridization and to its

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hydrogenation because both processes are equivalent to the formation of single C-H and C-C

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bonds. The detrimental effects of the increased contents of carbon phase, hydrogenation and hybridization levels on hardness were attributed to the consumption of  bonds for C-H at the

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expense of cross-linking among C-C chains [38-39]. However, these parameters depended not only on the precursor gas, but also on the method of sputtering: HiPIMS usually produced

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harder coatings than HiTUS and DCMS [30, 38-39, 45]. The limitation of these works was that the accuracy of ERDA/RBS analyses was only around 5 at. % and that only hardness vs.

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coefficient of friction were analyzed. It is known that the accuracy of ERDA/RBS can be increased to well below 1 at. % [46]. Therefore, the aim of the current work is to investigate

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the relationships between the level of hydrogenation based on improved ERDA/RBS analyses and hardness as well as reduced Young’s modulus in W-C:H coatings prepared with the additions of acetylene and methane by hybrid PVD-PECVD processes powered by HiPIMS and HiTUS.

2.

Materials and Methods 5

All studied coatings were deposited in Cryofox Discovery 500 (Polyteknik, Denmark) deposition system with two unbalanced magnetrons and two power sources (DC and HiPIMS – HIP3, Solvix) on hardened polished 100Cr6 steel discs (diameter - 25 mm, thickness - 3 mm) and on (111) Si wafer fragments. The substrates were ultrasonicated in acetone and ethanol prior to their plasma cleaning (35 min at -450 V on the substrate) followed by a deposition of a ~200 nm thick Cr bond layer using DC mode. The base pressure was 5 x 10-4 Pa and the initial working pressure at a constant (25 standard cubic cm per minute (sccm)) Ar) flow was 0.5 Pa. The subsequent HiPIMS depositions of W-C layers were performed with

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constant average power of 350 W (frequency of 150 Hz, impulse length of 175 µs and a duty cycle of 2.62 %) applied to the WC target with a diameter of 76.2 mm. The substrates were kept at floating bias without rotation and their temperature gradually increased during

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constant deposition time (45 min) from room temperature to < 200oC. The only variables

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were the additions of the reactive gases. The acetylene added to the constant Ar flow in 2 sccm increments was in the range from 0 sccm up to 8 sccm and methane up to 12 sccm. In

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addition to that, three different additions of hydrogen were examined at three levels of hydrocarbon additions - 0 sccm, 10 sccm and 20 sccm hydrogen. These additions resulted in

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the increase of the working pressure in the chamber from 0.5 up to around 0.8 Pa. The HiTUS W-C coatings were deposited in the S500 (Plasma Quest Ltd., United

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Kingdom) deposition system using identical targets and substrates. The RF power applied to the remote plasma source was 1800 W, the sputtering from the WC target was powered by RF

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500W and an RF bias of 5 W was applied to the substrate. Despite the larger vacuum chamber, the base pressure and initial working pressure were identical to those in the HiPIMS coatings via adjustment of the Ar flow (120 sccm). The deposition time was increased up to 90 min due to larger sputtering distance. The substrates were set to rotate (5 rpm) and their temperature gradually increased up to < 200oC during the deposition. The additions of C2H2, CH4 and H2 were in the same range as in HiPIMS. 6

Ion beam analyses of W-C:H coatings by RBS (Rutherford Backscattering Spectroscopy) and ERDA (Elastic Recoil Detection Analysis) were performed at the ion beam analysis end station of a 6 MV Tandetron ion accelerator at MTF STU in Trnava. ERDA was used to measure H depth concentration profiles, while RBS was used to measure W and C depth profiles. Both analyses were performed with enhanced energy of the primary He+ beam, the value of which was gradually optimized. In the first set of measurements, He+ ions with the energy of 3.1 MeV (for ERDA) and 4.5 MeV (for RBS), following 4.7 MeV (for both RBS and ERDA) were used. In the subsequent two sets of measurements, the ion beam energy was

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increased to 5.8 MeV (RBS and ERDA). The reasons for choosing higher energy of the

primary ions were related to the relatively large thicknesses of the studied coatings (1 - 3 µm) and to increase the sensitivity of the carbon measurements. Although the Rutherford

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backscattering refers to W atoms at these energies, the non-Rutherford cross sections apply in

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the case of C and H. Consequently, the C signal is enhanced in comparison with the standard RBS. Thus, similar sensitivities for both C and W can be achieved. These energies also make

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it possible to measure the thickness of the WC layers as well as the hydrogen concentration throughout the whole coating thicknesses by ERDA. The backscattering angle of RBS was

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170°, the scattering angle for ERDA was 30° (incident angle 15°, exit angle 15°). Depending on the energy used, a 12, 24 or 36 µm thick absorption Kapton foils were located in front of

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the ERDA charge particle detector. The RBS and ERDA spectra were evaluated using SIMNRA ver. 7.02 simulation software [http://home.mpcdf.mpg.de/~mam/]. The hydrogen

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concentrations were originally calculated based on the reference HDPE polyethylene (C2H4) standard, the electronic stopping power data by Ziegler, Biersack and Littmark [47] and the scattering cross sections data for carbon and hydrogen SigmaCalc 2.0 [https://wwwnds.iaea.org/exfor/ibandl.htm]. The RBS and ERDA measurements are absolute and do not need the reference materials, only the application of the most appropriate database for for SIMNRA simulation program is required. The measured W and C concentrations on the 7

stoichiometric WC reference sample using the above database differed by about 5 % from the stoichiometric composition. Subsequently, it may result in systematic deviations in W, C, H data reported in our earlier studies [38-39]. However, near stoichiometric composition was generated when the SRIM 2013 [http://www.srim.org/] electronic stopping data were used in SIMNRA simulations. Therefore, all the measured data was systematically recalculated using the updated database. The uncertainties of the concentrations of W, C and H as well as their thicknesses in at/cm2 in the studied W-C:H coatings were estimated at about 1 %. The nanoindentation tests were performed with a nanoindenter (model G200, Agilent,

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USA) on a set of 16 indents (matrix 4 x 4 indents 25 µm apart) in continuous stiffness mode (CSM) using a diamond Berkovich tip. The amplitude of the sinusoidal signal was 2 nm and its frequency was 45 Hz. The strain rate was set to 0.05 s-1 and the load increased up to

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constant indentation depth of 800 nm or 1000 nm. The maximum and/or plateau on the

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average hardness and indentation modulus profiles in the depth range from 100 nm to around 200-300 nm were used to determine the corresponding properties of the coatings without the

Results

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3.

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influence of substrate.

Since the deposition rates, phase and chemical compositions, structure, hardness and

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CoF-s of the studied W-C:H coatings deposited by DC, HiPIMS and HiTUS were already described in our previous works [29-30, 38-39, 45], the focus of the current work is on the

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improved ERDA/RBS and the correlations of hydrogenation with the hardness and elastic moduli of these coatings.

3.1. Hydrogenation in W-C:H coatings by ERDA/RBS Fig. 1 shows a typical experimental RBS spectrum obtained on the W-C:H coating deposited with the additions of 8 sccm CH4 and 10 sccm H2 using 5.8 MeV He+ beam. The 8

energy of the backscattered He at a given angle depend on the mass of the element nuclei on which the scattering occurred. The backscattering of He from W in the studied W-C:H coating occurred in the energy range 5400 – 4450 keV, from carbon in the range 1500 – 600 keV. These ranges are related to the thickness of the coating while the amplitude of each signal is proportional to the concentration of the corresponding element. The depth profiles of individual elements (including Cr bond layer at ~3500 keV and from Fe in steel substrate), were iteratively simulated (in SIMNRA) adjusting thicknesses and concentrations to obtain the best fit between experimental and modelling curves. An analogous depth profile for

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hydrogen obtained from ERDA is shown in Fig. 2. Only the intensities of hydrogen atoms forward recoiled by He in the range from 2250 keV to 750 keV can be seen because the He+ ions scattered on C and W atoms of the sample were absorbed by Kapton foil. The intensities

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decreased with the depth increase mainly due to a decrease in the hydrogen atoms recoil cross

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section with a decrease He energy (at larger depths). The difference visible at the falling edge (at the energy ~750 keV) resulted from reduced resolution in larger depths. The absolute

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hydrogen concentrations and coating thicknesses from ERDA were determined using the

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simulations in the same way as in RBS.

HiTUS W-C:H coating/Cr /steel substrate 8 sccm CH4 + 10 sccm H2

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Intensity, arb. units

experimental data coating

simulation

W-C:H surface W

C

W-C:H thickness W-C:H thickness

Fe substrate

0

1000

Cr bond layer

2000

3000

4000

Energy, keV

9

5000

6000

Fig. 1. The experimental and simulated RBS spectra of the HiTUS W-C:H coating deposited with the additions of 8 sccm CH4 and 10 sccm H2 obtained using 5.8 MeV He+ beam. The summary simulated spectrum and the contributions of the individual elements in the coating, bond layer and in the steel substrate are also indicated.

HiTUS W-C:H coating 8 CH4 + 10 H2

coating

Intensity, arb. units

Cr/steel substrate

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experimental data

W-C:H coating thickness

0

500

1000

1500

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simulation

top surface

2000

3000

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Energy, keV

2500

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Fig. 2. The experimental and simulated ERDA spectra of recoiled hydrogen atoms taken from

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the HiTUS W-C:H coating (with 8 sccm CH4 + 10 sccm H2).

The coating thicknesses from the ERDA spectra of hydrogen agreed well with the

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thicknesses obtained from the W and C signals in the corresponding RBS spectra. It confirmed that the ERDA and RBS simulations were performed correctly. The results of the

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RBS and ERDA simulations in terms of W, C and H concentrations at different acetylene and methane additions into the sputtering atmosphere in HiPIMS and HiTUS W-C:H coatings are summarized in Fig. 3 and Fig. 4, respectively. The comparison with our earlier data reported in [39] suggests that the current concentrations of carbon in the HiPIMS W-C:H coatings with C2H2 additions became higher and hydrogen content lower only by about 4 at. %. In the coatings with CH4 additions, carbon contents remained roughly the same, but hydrogen 10

content increased by 4 at. % at the expense of tungsten. Thus, despite the absolute

HiPIMS W-C:H 80

C2H2 additions

C

70

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60 50 40 30

H

20 10

W

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ERDA/RBS atomic concentration, at. %

concentrations are slightly different, the earlier conclusions would not change.

0 0

1

2

3

4

5

6

C

60

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CH4 additions

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HiPIMS W-C:H

70

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50 40 30

H

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20

W

10

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ERDA/RBS atomic concentration,at. %

C2H2 flow, sccm

7

0

0

2

4

6

8

10

12

CH4 flow, sccm

Fig. 3. The dependencies of the atomic concentrations of H, C and W in the HiPIMS WC:H coatings deposited with a) - C2H2) and b) - CH4 additions from the ERDA/RBS measurements.

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ERDA/RBS atomic concentration, at. %

HiTUS W-C:H

C2H2 additions

70

C 60 50 40 30

W

20 10

H

0 0

2

4

6

8

10

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HiTUS W-C:H

CH4 additions

0.7

C

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0.6 0.5

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0.4 0.3

W

0.2

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ERDA/RBS atomic concentration,at. %

C2H2 flow, sccm

H

0.1 0.0 0

2

4

6

8

10

12

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CH4 flow, sccm

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Fig. 4. The summary of atomic concentrations of H, C and W in the HiTUS W-C:H coatings deposited with the additions of a) - C2H2 and b) - CH4 into Ar atmosphere during

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hybrid PVD-PECVD.

The plots in Fig. 3 and Fig. 4 indicate that the concentrations of W and C in HiPIMS and HiTUS W-C coatings without any hydrocarbon precursor remained in the 40 : 60 ratio despite opposite ratio W : C was expected from the presence of substoichiometric WC1-x (x ~ 0.2) reported in the literature [23-24, 27-28] and from our earlier X-ray diffraction studies [38-39]. 12

The additions of both hydrocarbon gases resulted in similar changes of concentrations of the corresponding elements in all sputtering method and hydrocarbon gas combinations: hydrogen concentrations gradually and approximately linearly increased at small additions and in some cases, saturation appeared at higher flows. Hydrogen concentration increase occurred at the expense of tungsten concentration (Fig. 3 and Fig. 4), whereas carbon concentrations remained approximately constant after the first hydrocarbon additions. The strong inverse correlations between tungsten and hydrogen concentrations seem to be the consequence of binding W and H to the same carbon and normalization of the sum of their

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concentrations to 100 %. The differences between HiPIMS and HiTUS and hydrocarbons were in the absolute concentrations (Fig. 5): at the same flow, the HiPIMS deposited coatings contained more than twice the amount of hydrogen than the HiTUS coatings, but the

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differences between acetylene and methane addition were insignificant. The absolute

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maximum hydrogen concentrations depended on the added gas flow, but they were in the range 20 - 30 at % in HiPIMS and 12 – 16 at. % in HiTUS W-C:H coatings.

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Fig. 6a and Fig. 6b show how the hydrocarbon concentrations increased when hydrogen was added into sputtering atmosphere. The additions of only hydrogen resulted in zero (within

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the error of measurement) hydrogen concentrations in the coatings. The differences appeared after hydrocarbon precursors were added. In HiTUS case, the additions of 10 sccm hydrogen

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increased its concentrations both with C2H2 and CH4, but at 20 sccm H2, a decrease (or saturation) of the resulting hydrogen concentrations was observed. Surprisingly, the limited

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data on methane and hydrogen additions indicates that the hydrogen concentrations in the coatings made with CH4 and 10 sccm H2 were slightly lower compared to those with C2H2. In HiPIMS, the absolute values of hydrogen concentrations at comparable hydrocarbon additions were approximately twice higher than in HiTUS and only a linear increase (with the exception of saturation at 8 CH4 + 20 sccm H2) up to 20 sccm H2 can be seen with acetylene as well as with methane additions. 13

Hydrogen concentration, at. %

HiPIMS vs. HiTUS W-C:H coatings 30

CH4

HiPIMS C2H2

20

C2H2

10

CH4

HiTUS 0 2

4

6

8

10

12

Hydrocarbon (C2H2 or CH4) flow, sccm

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Fig. 5. The comparison of the concentrations of hydrogen in the hybrid PVD-PECVD W-

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C:H coatings as a function of hydrocarbon additions in HiPIMS and HiTUS coatings

deposited with C2H2 and CH4 reactive gases (the full symbols correspond to C2H2, open

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HiPIMS W-C:H coatings

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35

+8 sccm CH4

25

+4 CH4

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30

+6 sccm C2H2

20

+2 sccm CH4

+4 C2H2

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ERDA/RBS hydrogen concentration, at. %

symbols to CH4).

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15

10

0

5

10

15

20

Hydrogen flow, sccm

14

25

20

+10 C2H2

15

+5 C2H2

10

+8 CH4 +3 C2H2

5

+4 CH4 +0 CH4

+0 C2H2

0 0

5

10

15

20

25

Hydrogen flow, sccm

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ERDA/RBS hydrogen concentration, at. %

HiTUS W-C:H coatings

Fig. 6. The summary of the effect of the additions of hydrogen into Ar and Ar+ hydrocarbon gas (C2H2 or CH4) sputtering atmosphere on the hydrogen concentration in the hybrid PVD-

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correspond to C2H2, open symbols to CH4).

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PECVD W-C:H coatings deposited using a – HiPIMS, and b - HiTUS (the full symbols

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3.2. Hardness and indentation modulus of W-C:H coatings The results of the nanoindentation measurements on the studied WC:H coatings are

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summarized in Fig. 7 and Fig. 8. The addition of only 2 sccm C2H2 in HiPIMS coatings caused a decrease of hardness from initial ~30 GPa to around 20 GPa but then hardness

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remained approximately constant up to 8 sccm (Fig. 7a). In the case of CH4, the degradation of hardness occurred continuously from more than 30 GPa down to 13 GPa at 12 sccm.

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Indentation modulus also exhibited strong decrease from ~330 GPa to 212 - 220 GPa after 2 sccm C2H2, and 260 GPa at 2 CH4. Further increase of the hydrocarbon flows caused continuous degradation of the moduli but it was more pronounced in the case of methane than acetone additions. Except for 2 sccm, the hardness and indentation modulus values were equal or higher in the cases with C2H2 additions than with CH4 additions.

15

35

HiPIMS W-C:H coatings 100Cr6 substrate

Hardness, HIT [GPa]

30

25

C2 H2

Al 2024 substrate

20

CH4 15

10 0

2

4

6

8

10

12

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Hydrocarbon (C2H2, CH4) flow, sccm

350 100Cr6 substrate

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300

+ C2H2

200

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250

Al 2024

substrate

150

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Indentation modulus, EIT [GPa]

HiPIMS W-C:H coatings

+ CH4

100 0

2

4

6

8

10

12

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Hydrocarbon flow, sccm

Fig. 7. The comparison of a) - hardness and b) - indentation moduli values in the HiPIMS W-

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C:H coatings deposited by hybrid PVD-PECVD with the additions of C2H2 and CH4.

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35

HiTUS W-C:H coatings 100Cr6 substrate

Hardness, HIT [GPa]

30

25

+CH4

20

15

+C2H2

10 0

2

4

6

8

10

12

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Hydrocarbon (C2H2, CH4) flow, sccm

350

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100Cr6 substrate

300

CH4

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250

200

150

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Indentation modulus, EIT [GPa]

HiTUS W-C:H coatings

C2H 2

100 0

2

4

6

8

10

12

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Hydrocarbon flow, sccm

Fig. 8. The dependencies of a) - hardness and b) - indentation moduli in the HiTUS W-C:H

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coatings deposited by hybrid PVD-PECVD with the additions of C2H2 and CH4.

In the coatings made by HiTUS, the hardness and elastic modulus values of the

coatings deposited without hydrocarbon and hydrogen additions were approximately equal to those in HiPIMS (Fig. 8). Hydrocarbon gases resulted in continuous degradation of both properties. Contrary to HiPIMS, the hardness and indentation modulus values with CH4 additions were higher than in the C2H2 case (Fig. 8a and 8b). The addition of hydrogen into the hydrocarbon containing sputtering atmospheres caused additional degradation of 17

mechanical properties. The data on hardness and indentation modulus as a function of hydrogen concentration and regardless of its origin (from hydrocarbon gas only or from hydrocarbon and hydrogen additions) are summarized in Fig. 9a and Fig. 9b, respectively. Despite certain scatter it can be seen that in all cases, the degradation of mechanical properties occurred approximately linearly with the increase of hydrogen concentration. Main differences between HiPIMS and HiTUS were in the slopes of these dependencies. The hardness and elastic modulus values in HiPIMS coatings were much higher than in HiTUS

compared to the influence of the deposition method.

40 0 sccm CxHy + b H2

W-C:H coatings

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35 30 25

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20

HiPIMS - CH4

HiPIMS - C2H2

15 HiTUS - CH4

10 5

HiTUS - C2H2

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Hardness, HIT [GPa]

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coatings despite higher hydrogenation. The influence of precursor type was relatively small

+ a CxHy+ b H2 0 0

5

10

15

20

25

30

35

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Hydrogen concentration, at. %

ur

450

Jo

Indentation modulus, EIT [GPa]

400 350

W-C:H coatings

+ a CxHy+ b H2

300 250 200

HiPIMS - CH4

150

HiPIMS - C2H2

HiTUS - CH4

100 50

0 sccm CxHy + b H2

HiTUS - C2H2

0 0

5

10

15

20

25

30

Hydrogen concentration, at. %

18

35

Fig. 9. The dependencies of a) - hardness and b) - indentation moduli on the content of hydrogen in the HiPIMS and HiTUS W-C:H coatings deposited by hybrid PVD-PECVD with the additions of C2H2, CH4 and H2 (the full symbols correspond to the additions of acetylene

Jo

ur

na

lP

re

-p

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and the open symbols to the additions of methane; the lines are guides for eyes).

19

3.3. HIT/EIT ratio in W-C:H coatings Despite large number of methods developed for the measurement of fracture toughness of thin coatings [48-52], the simplest the estimation of the resistance of a wide range of brittle coatings to cracking is to use the ratio of hardness to indentation modulus, HIT/EIT (and/or HIT3/EIT2), which provides reasonable correlations with the wear resistance and plasticity [1, 5, 43, 53-57]. In terms of absolute HIT/EIT ratios, the value of 0.1 was defined as a limit above which suppression of a crack formation was observed [53]. The data from Fig. 9a and Fig. 9b were therefore replotted as a function of HIT/EIT ratio vs. hydrogen content in Fig. 10. Despite

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certain scatter, HIT/EIT ratios in all coatings increased non-linearly with the increase of

hydrogen content and the HiPIMS W-C:H coatings exhibited different dependencies than HiTUS coatings. The additions of hydrogen into Ar + x C2H2 atmosphere caused noticeable

-p

HIT/EIT increase, however, without the increase of hydrogenation level. In principle, the

re

increase of HIT/EIT could be obtained either by HIT increase and/or by EIT decrease. The comparison of the relative changes of both properties indicates that hydrogenation results in

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faster degradation of EIT than that of HIT (Fig. 9), which agrees with the conclusions in earlier

0.14 0.13

HiTUS-C2H2

0.10

6-20 4-20

HiPIMS-C2H2

0.11

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HIT / EIT

4-20

W-C:H coatings

ur

0.12

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works [5, 53, 56-57].

10-0

6-0 6-0

4-0

10-0 8-20 12-0 8-10 4-20

HiPIMS-CH4

0.09

HiTUS-CH4

0.08 0.07

0

5

10

15

20

25

Hydrogen concentration, at. %

20

30

35

Fig. 10. The dependencies of the HIT/EIT ratio in the studied W-C:H coatings on hydrogen concentration for C2H2 and CH4 used in the hybrid PVD-PECVD by HiPIMS and HiTUS (the full symbols correspond to the additions of C2H2, the open symbols to the additions of CH4; the first number at data points corresponds to hydrocarbon flow and the second number to hydrogen flow; the full lines connect the data in the coatings deposited without hydrogen additions).

4.

Discussion

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4.1. Hydrogenation in W-C:H coatings

Fig. 7 and Fig. 8 indicate continuous degradation of HIT and EIT with the increase of hydrocarbon flows but the dependencies in Fig. 9 and Fig. 10 refer only to the hydrogen

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concentrations. However, the effects of hydrogen principally cannot be separated from those

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of carbon in hybrid PVD-PECVD processes because the amount of hydrogen incorporated into amorphous carbon-based phase is directly proportional to the amount of carbon itself.

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Then, approximately constant concentrations of carbon at growing hydrogen concentrations in Fig. 3 and Fig. 4 have to be explained. One shall consider that these concentrations

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correspond to normalized total concentrations in the carbide and amorphous boundary phases. Another reason may be related to a shift of the ratio between carbon bound in the crystalline

ur

carbide phase and carbon in the amorphous boundary phase. Although the concentrations of carbon in the boundary phase cannot be separated from its total amount by RBS, its minimum

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amount can be estimated assuming that all W atoms in the coating are bound to carbon. Then, the minimum amount of carbon-based boundary phase would be defined as a difference between the total concentrations of carbon and tungsten [39]. The obtained amounts would be underestimated due to the possible presence of tungsten in the amorphous phase and due to the deviations from stoichiometry in the carbide phase. The minimum amounts of carbonbased amorphous phase calculated in such way in the coatings deposited without the addition 21

of hydrocarbon precursors were around 13 % (with the exclusion of HiPIMS W-C prepared with CH4 additions, where 22 % was obtained) (see Fig. 11). This is surprising because in unhydrogenated W-C coatings, only crystalline WC1-x (and sometimes W2C) phases were identified by X-ray diffraction [38-39, 45] and the amorphous carbon-based phase was not detected neither by high resolution transmission electron microscopy nor by Raman spectroscopy [29-30, 38-39, 45]. Evidently, RBS results need to be confronted with the results of an independent method for the quantitative measurement of amorphous carbon content. After hydrocarbons were added, different dependencies of estimated boundary phase

HiPIMS vs. HiTUS W-C:H coatings C2H2 HiPIMS

50

HiTUS

re

40

CH4

C2H2

60

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70

CH4

30 20 10 0 0

2

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a-C(:H) concentration, at. %

80

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were obtained for HiPIMS and HiTUS and methane vs. acetylene (Fig. 11).

4

6

8

10

12

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Hydrocarbon (C2H2 or CH4) flow, sccm

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Fig. 11. The estimated minimum amounts of carbon-based amorphous phase in the hybrid PVD-PECVD W-C:H coatings as a function of hydrocarbon additions in HiPIMS and HiTUS

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coatings deposited with C2H2 and CH4 reactive gases (the full lines correspond to HiPIMS and the dotted lines to HiTUS).

The highest concentrations of amorphous carbon of 70 – 76 at. % were found out in HiPIMS deposited with C2H2 additions. It is even possible that a saturation limit of ~76 at. % was achieved at 4 sccm flow. The dependence of CH4 additions with HiPIMS overlapped with 22

that of HiTUS with C2H2 and saturated at around 60 at. % above 10 sccm additions. In the case of HiTUS and CH4, the highest concentrations in the range 40 – 45 at. % were achieved at >10 sccm but the saturation was not clear. Thus, HiPIMS and C2H2 always produced higher contents of amorphous carbon phase than HiTUS and CH4 combinations. The hydrocarbons additions can be then interpreted in terms of a suppression of crystallization of carbide phase and a shift from nanocrystalline WC1-x structure at zero additions via nanocomposite structure consisting of nanocrystalline and amorphous phases at small additions toward fully amorphous polymeric a-C:H:W structures at higher additions. At the same flows, C2H2 should

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be more effective in the supply of carbon and less effective in hydrogenation than CH4 due to the H:C ratio. However, this argument is contrary to relatively low concentrations of

hydrogen found in methane case in Fig. 5. Because hydrogen can be bound only to the carbon

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in the amorphous phase and the concentrations of hydrogen in Fig. 5 are absolute, the true

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hydrogenation should involve hydrogen concentrations relative to the amount of amorphous carbon phase. Using the data from Fig. 11, Fig. 12 shows that the highest ratio of relative

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concentration of hydrogen in the amorphous boundary phase of around 50 % was achieved in the HiPIMS with CH4 combination whereas the additions of C2H2 resulted only in ~26 %.

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Thus, the above controversy was only apparent. Moreover, the relative hydrogenation of around 50 % complies with the hydrogenation limit of 50 % predicted from the random

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covalent network (RCN) model for a-C:H coatings [58]. In HiTUS made W-C:H coatings, the corresponding ratios were ~ 30 % in CH4 and 23 % in the C2H2 case. They are also below the

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RCN limit and around the range 35 - 38 at. % reported for CH4 and C2H2 additions in PECVD a-C:H coatings [37]. Thus, methane additions result in higher hydrogenations and acetylene in higher carbon contents, which fully complies with the H : C ratio in these precursors.

23

H/a-C(:H)* concentration ratio

1.0 2-20

0.9 0.8 0.7

4-20

2-10

0.6 4-10

0.5 0.4 0.3

3-24

C2H2

3-10

8-10 8-20

CH4

4-20 5-24 5-10

HiPIMS

10-10

6-20

10-24

CH4 8-10

0.2

4-10

HiTUS

C2H2

0.1 0.0 0

1

2

3

4

5

6

7

8

9

10 11 12 13

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x C2H2 or CH4 + y H2 flow, sccm

Fig. 12. The comparison of the dependencies of relative concentration of hydrogen in the carbon-based boundary phase on hydrocarbon and hydrogen additions in the studied

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hybrid PVD-PECVD W-C:H coatings deposited using HiPIMS vs. HiTUS coatings (the full

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lines correspond to HiPIMS and the dotted lines to HiTUS).

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Fig. 12 also shows that hydrogen additions into the sputtering atmosphere with both hydrocarbon precursors resulted in an increase of the relative hydrogen concentrations. In

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most of the combinations, the changes were below 10 % in comparison with the corresponding reference. However, in the case of HiPIMS with acetylene and hydrogen

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additions, the levels above 70 % and 90 % were achieved. These values do not fit with RCN model and emphasize the deviations between real amounts of amorphous phase in the

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nanocomposite coatings and the estimated minimum amounts of amorphous carbon used in the calculations. The systematic differences between HiPIMS and HiTUS in the concentrations of

hydrogen and carbon seem to be related to the energy involved in the plasma hybridization reactions [30, 38-39]. In the case without reactive gases, the energy supplied to the system would be consumed only on WC target sputtering (including Ar gas ionization and target 24

heating). The plasma polymerization reactions triggered after the addition of hydrocarbon precursor would require a part of the supplied energy on the formation of CxHy radicals [3337]. At constant power, it must occur at the expense of WC sputtering and result in an inverse relationship between tungsten and hydrogen concentrations. If the amount of energy is sufficient, carbon and hydrogen concentrations should be directly proportional to the supply of hydrocarbon gas. It agrees with the increase of the corresponding concentrations at small hydrocarbon additions (see Fig. 3 and Fig. 4). When the supply of hydrocarbons exceeds the energy available for the polymerization reactions, only a corresponding part of the precursor

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gas may be utilized. An un-reacted part of precursor gas would be pumped away and a saturation of hydrogen and carbon in the boundary phase should occur.

The differences in the concentrations and saturations limits between HiPIMS and HiTUS

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in Fig. 5 and Fig. 12 may reflect the differences in the effectivity of the polymerization

re

reactions driven by pulsed and RF energy in HiPIMS and HiTUS, respectively. Despite the average power applied to the target in HiPIMS (350 W) was lower than in HiTUS (RF 500

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W), the peak power in HiPIMS was higher by almost two orders of magnitude. Since such high peak energies in HiPIMS result in substantially higher ionization of the sputtered

na

material [7], more effective decomposition and recombination reactions during plasma polymerization may also be expected. Higher concentrations of hydrogen and carbon in

ur

HiPIMS compared to HiTUS would be a consequence of the effectivity difference. The above ideas can be partially applied to describe the variations among the

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dependencies in Fig. 6 and Fig. 12. The additions of only hydrogen into Ar atmosphere in both HiPIMS and HiTUS did not introduced hydrogen into the coating because of lack of carbon available for hydrogen to bond in the carbide phase. The additions of 20 sccm hydrogen to acetylene in HiTUS (Fig. 6b) resulted in the reduction of the absolute hydrogen concentrations whereas only an increase of the relative concentrations was indicated in Fig. 12. Simultaneously, excessive relative hydrogen concentrations in HiPIMS and methane 25

combination in this plot are contrary to moderate increase of absolute concentrations in Fig. 6a. The dissociation of the molecular hydrogen in the plasma would consume additional energy and therefore, less energy would be available for the plasma polymerization reactions on hydrocarbons and for WC sputtering. In HiPIMS at small (2 and 4 sccm) CH4 additions, high power density in the power pulses would dissociate methane and hydrogen resulting in hydrogen “oversaturation” compared to the amount of carbon. Apparently, the minimum amount of carbon calculated using the assumption of full bonding of tungsten to carbon was underestimated and subsequent values of relative hydrogen concentration overestimated and

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above the limit set by RCN model. At high methane and hydrogen additions, the excess of relative hydrogenation is reduced which may be related to the limited power in the plasma. Obviously, an independent measurement of the content of amorphous phase in these coatings

re

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is necessary to unambiguously resolve these discrepancies.

4.2. Mechanical properties of W-C:H coatings vs. hydrogenation, hybridization and

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cross-linking of the amorphous boundary phase

The additions of hydrocarbons into sputtering atmosphere in Fig. 7 and Fig. 8 clearly

na

show deleterious influence on hardness and indentation modulus in all cases but the combination of HiPIMS with C2H2. These trends were transformed into approximately linear

ur

decrease with the increase of hydrogen concentration with clear stratification between

Jo

HiPIMS and HiTUS methods while the differences between C2H2 and CH4 were much smaller (Fig. 9). However, because of strong coupling between hydrogen and carbon in the boundary phase (see 4.1), the influence of the content of amorphous carbon on mechanical properties cannot be omitted. Fig. 13 summarizes the hardness and indentation moduli dependencies on the estimated content of the amorphous carbon-based boundary phase in the coatings made with C2H2 and CH4 additions.

26

45

W-C:H coatings

40 0 sccm CxHy

Hardness, HIT [GPa]

35

0-0 0-0 0-0 0-0

30 25

HiPIMS - CH4 HiPIMS - C2H2

20 15

HiTUS - CH4

10

HiTUS - C2H2

+ a CxHy

5 0 0

10

20

30

40

50

60

70

80

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a-C(:H)* content, at. %

450

W-C:H coatings

0 sccm CxHy

350

-p

300 HiPIMS - CH4

200 150

HiTUS - CH4

100 + a CxHy

50 0 0

10

20

re

HiPIMS - C2H2 HiTUS - C2H2

250

lP

Indentation modulus, EIT [GPa]

400

30

40

50

60

70

80

na

a-C(:H)* content, at. %

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Fig. 13. The dependencies of a) - hardness and b) - indentation moduli on the estimated content of the amorphous carbon-based boundary phase in the HiPIMS and HiTUS W-C:H

Jo

coatings deposited with the additions of C2H2 and CH4 (the full symbols correspond to the additions of acetylene and the open symbols to the additions of methane).

The corresponding properties decreased approximately linearly in HiPIMS and HiTUS coatings with the exception of HiPIMS coatings and C2H2 additions discussed later. Despite the stratification between HiPIMS and HiTUS was not so pronounced, the dependencies 27

follow the same behavior as in Fig. 9. Thus, close correlations between mechanical properties and a-C:H(:W) phase contents were confirmed. The degradation seems to be a natural consequence of the increase of the content of soft polymeric carbon-based phase. The comparison of Fig. 9 and Fig. 13 offers an additional possibility for further exploration of the role of hydrogen in stratification of the coating properties. The difference in mechanical properties between HiPIMS and HiTUS at the same hydrogen contents imply the differences in the structure of these coatings. In our previous study of hybrid PVD-PECVD HiPIMS W-C:H coatings it was shown that 2 - 4 sccm additions of C2H2 caused not only an

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increase of carbon and hydrogen contents, but also that of sp3 [38]. In a-C:H coatings, the increase of sp3 content at a given hydrogen content was expected to result in an increase of hardness due to a higher number of  bonds [59]. However, Thiry et al. [37] reported higher

-p

hardness values at lower sp3 contents of a-C:H coatings with similar hydrogen concentrations.

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Their work emphasized the importance of cross-linking of the polymeric carbon structural network for the increase of hardness. In the HiPIMS W-C:H coatings containing a-C:H(:W)

lP

phase, small hardness degradation was also observed with the increase of sp3 hybridization of the boundary phase [38]. The crucial point to understand these results is that the sp3

na

hybridization corresponds to the total number of  bonds. These bonds involve C-C bonds in the polymeric chains, C-C bonds among the neighboring chains determining cross-linking and

ur

C-H bonds determining hydrogenation. Thus, at the same sp3 hybridization level, different combinations of hydrogenation and cross-linking are possible. It means that sp3 hybridization

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is a parameter which is not sufficient to fully characterize the structure of the amorphous hydrogenated carbon-based phases. Cross-linking among hydrogenated carbon chains may occur also via C-C bonds. Higher cross-linking would not change the total number of C-C bonds (unless double C=C bonds are broken and/or hydrogenation extraction is involved) but it would increase the density, stiffness and hardness of the polymeric network of the boundary phase. Thus, also the resulting properties of the coatings would be controlled by the balance 28

between the number of regular C-C (and C=C) bonds, C-C bonds involved in cross-linking and C-H bonds. It can be also concluded that coupling occurs not only between carbon and hydrogen but also among hydrogenation, hybridization and cross-linking. The mechanisms of cross-linking via opening of new  bonds between carbon atoms in the neighboring chains can be analogous to that in the growth of polymeric carbon chains via attachments of CxHy radicals to the active sites - free (“dangling”) carbon bonds. Continuous generation of such bonds involves breaking of double/triple carbon bonds and/or partial

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dehydrogenation (= extraction of the existing hydrogen from the surface. Since the breaking C-H bond requires only 3 eV and 25 eV would be necessary for the breaking of C-C bonds [60], hydrogen extraction would be preferential mechanism for cross-linking. Subsequently, an inverse relationship between the degree of cross-linking and hydrogenation level can be

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expected. Thus, despite no experimental data on cross-linking, degradation of hardness and

re

indentation modulus in Fig. 9 can be qualitatively explained. Vice versa, the suppression of hardness degradation would require strongly cross-linked amorphous carbon structure with

lP

high C-C sp3 hybridization and reduced hydrogenation. Apparently, this is the case of HiPIMS and C2H2 combination resulting in medium (~20 GPa) hardness and elastic modulus

na

(200- 220 GPa) values and HIT/EIT > 0.1 in relatively wide range of acetylene additions. Finally, the obtained results can be confronted with the “lost memory” effect proposed by

ur

Wild et al. [61]. The meaning of the effect is that the final properties of the a-C:H coatings do not depend on the precursor gas type because the same primary CxHy fragments would be

Jo

generated by the plasma polymerization reactions from each hydrocarbon precursor. The current work clearly demonstrates that not only composition (Fig. 3, Fig. 4 and Fig. 6) but also hardness, indentation modulus and their ratio are highly sensitive to the type of precursor gas and to the deposition method. Thus, despite the properties of the studied W-C:H coatings are controlled by the a-C:H(:W) phase, the “lost memory” effect is not applicable.

29

5.

Conclusions The study of the relationships among deposition conditions of hybrid PVD-PECVD

HiPIMS and HiTUS W-C:H coatings, their hydrogenation and mechanical properties assessed by nanoindentation revealed that: 

the additions of hydrocarbon precursor, hydrogen and sputtering methods (HiPIMS and HiTUS) can be effectively used for the control of the ratio between nanocrystalline W-C phase and amorphous carbon-based boundary phase, its



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hydrogenation and mechanical properties; CH4 precursor resulted in higher concentrations of hydrogen and lower concentrations of carbon than C2H2 in agreement with their H : C ratios; 

HiPIMS resulted in higher contents of amorphous carbon-based boundary phase and

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higher levels of its hydrogenation than HiTUS, most probably due to higher effectivity



re

of plasma polymerization reactions at high peak power density in HiPIMS pulses; be the additional parameters controlling the structure and mechanical properties of the

lP

studied W-C:H coatings seem to be the levels of sp3 hybridization and cross-linking in the amorphous carbon-based boundary phase;

na

 strong correlations (coupling) anticipated among hydrogenation, hybridization and cross-linking in the a-C:H(:W) phase were related to the balance between  C-C and C-

the detrimental effects of the amorphous carbon-based boundary phase and its

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

ur

H bonds;

hydrogenation on hardness and indentation modulus of W-C:H coatings may be attributed to the consumption of C-C bonds by C-H during hydrogenation and reduction of cross-linking of the polymeric network in the boundary phase;



HiPIMS W-C:H coatings deposited with acetylene and hydrogen, exhibit medium (~20 GPa) hardness and elastic modulus (200 - 220 GPa) values with HIT/EIT > 0.1

30

suggesting improved toughness and wear resistance in relatively wide range of acetylene additions.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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ACKNOWLEDGEMENTS

The support provided by the projects APVV 15-0168, APVV-17-0320, APVV-17-0049 and VEGA 2/0017/19 are acknowledged. The equipment used in the work was acquired from

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the projects “Research Centre of Advanced Materials and Technologies for Recent and Future Applications” PROMATECH, ITMS: 26220220186 and “University Scientific Park Campus

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MTF STU – CAMBO” ITMS: 2622022079. The contribution of D. Vaňa and M. Beňo to

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ur

na

lP

ERDA/RBS measurements are appreciated.

31

REFERENCES [1] J. Musil, Hard and superhard nanocomposite coatings, Surf. Coat. Technol., 125 (2000) 322 -330. [2] L. Hultman, J. E. Sundgren, Structure/property relationships for hard coatings, in R.F. Bunshah, (Ed.), Handbook of hard coatings Deposition technologies, properties and applications, Noyes Publ. William Andrew Publ., Norwich, USA, 2001, pp. 108 – 180. [3] J. Vetter, 60 years of DLC coatings: historical highlights and technical review of cathodic

ro of

arc processes to synthesize various DLC types, and their evolution for industrial applications Surf. Coat. Technol., 257 (2014) 213-240.

[4] Y. X Wang, S. Zhang, Present status of hard-yet-tough ceramic coatings, in S. Zhang

-p

(Ed.), Thin films and coatings: Toughening and toughness characterization, CRC Press, Boca

re

Raton, USA, 2016, pp. 1- 45

[5] J. Musil, Hard nanocomposite coatings: Thermal stability, oxidation resistance and

lP

toughness, Surf. Coat. Technol., 205 (2012) 50-65.

[6] J.E. Greene, Tracing the recorded history of thin-film sputter deposition: From the 1800s

na

to 2017, J. Vac. Sci. Technol. A: Vacuum, Surfaces and Films, 35 (2017) 05C204. [7] V. Kouznetsov, K. Macák, J.M. Schneider, U. Helmersson, I. Petrov, A novel pulsed

ur

magnetron sputter technique utilizing very high target power densities, Surf. Coat. Technol., 122 (1999) 290-293.

Jo

[8] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Ionized physical vapor deposition (IPVD): A review of technology and applications, Thin Solid Films, 513 (2006) 1-24. [9] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: A review on scientific and engineering state of the art, Surf. Coat. Technol., 204 (2010) 16611684. 32

[10] D. Lundin, K. Sarakinos, An introduction to thin film processing using high-power impulse magnetron sputtering, J. Mater. Res., 27 (2012) 780-792. [11] M.J. Thwaites, UK Patent GB 2 343 992 B: High Density Plasmas (2001). [12] S.J. Wakeham, M.J. Thwaites, B.W. Holton, C. Tsakonas, W.M. Cranton, D.C. Koutsogeorgis, R. Ranson, Low temperature remote plasma sputtering of indium tin oxide for flexible display applications, Thin Solid Film, 518, (2009) 1355-1358. [13] K. Bewilogua, H. Dimigen, Preparation of W-C:H coatings by reactive magnetron sputtering, Surf. Coat. Technol., 61 (1993) 144-150.

ro of

[14] M. Grischke K. Bewilogua, H. Dimigen, Preparation, properties and structure of metal containing amorphous hydrogenated carbon films, Mat. Manuf. Processes, 8 (1993) 407-417. [15] A.A. Voevodin, J.P. O’Neill, J.S. Zabinski Tribological performance and tribochemistry

-p

of nanocrystalline WC/amorphous diamond-like carbon composites, Thin Solid Films, 342

re

(1999) 194-200.

[16] A.A. Voevodin, J.P. Neil, S.V. Prasad, J.S. Zabinski, Nanocrystalline WC and WC/a-C

lP

composite coatings produced from intersected plasma fluxes at low deposition temperatures, J. Vac. Sci. Technol., A 17 (1999) 986-992.

na

[17] A. Czyzniewski, Deposition and some properties of nanocrystalline WC and nanocomposite WC/a-C:H coatings, Thin Solkid Films, 433 (2003) 180-185.

ur

[18] C. Strondl, N.M. Carvalho, J.Th.M. De Hosson, G.J. van der Kolk, Investigation of the formation of tungsten carbide in tungsten-containing diamond-like carbon coatings, Surf.

Jo

Coat Technol., 162 (2003) 288-293. [19] C. Strondl, N.M. Carvalho, J.Th.M. De Hosson, T.G. Krug, Influence of energetic ion bombardment on W-C:H coatings deposited with W and WC targets, Surf. Coat Technol., 200 (2005) 1142-46.

33

[20] C. Corbella, E. Bertran, M.C. Polo, E. Pascual, J.L. Andújar, Structural effects of nanocomposite films of amorphous carbon and metal deposited by pulsed-DC reactive magnetron sputtering, Diam. Relat. Mater., 16, 2007, 1828-1834. [21] J. C. Sánchez-López, A. Fernández, Doping and alloying effects on DLC coatings, in B. Bhushan (Ed.), Handbook of Modern Tribology, CRC Press, Boca Raton, 2008, pp. 311-338. [22] J. C. Sánchez-López, D. Martínez-Martínez, M.D. Abad, A. Fernández, Metal carbide/amorphous C-based nanocomposite coatings for tribological applications, Suf. Coat Technol., 204 (2009) 947-954.

ro of

[23] M.D. Abad, M.A. Muñoz-Márquez, S. El Mrabet, A. Justo, J.C. Sánchez-López, Tailored synthesis of nanostructured WC/a-C coating by dual magnetron sputtering. Surf. Coat. Technol., 204 (2010) 3490 – 3500.

-p

[24] S. El Mrabet, M.D. Abad, J.C. Sánchez-López, Thermal evolution of WC/C

re

nanostructured coatings by Raman and in situ XRD analysis, Surf. Coat. Technol., 206 (2011) 1913–1920.

lP

[25] H. Hetzner, Ch. Smid, S. Tremmel, K. Durst, S. Wartzack, Empirical-statistical study on the relationship between deposition parameters, process variables, deposition rate and

na

mechanical properties of a-C:H:W coatings, Coatings, 4 (2014) 772-95. [26] W. Wang, V.O. Pelenovich, M.I. Yousaf, S. Yan, H. Bin, Z. Wang, A.B. Tolstogouzov,

ur

P. Kumar, B. Yang, D.J. Fu, Microstructure, mechanical and tribological properties of WC/aC:H coatings deposited by cathodic arc ion-plating, Vacuum, 132 (2016) 31-39.

Jo

[27] M. Makowka, W. Pawlak, P. Konarski, B. Wendler, Hydrogen content influence on tribological properties of nc-WC/a-C:H coatings, Diamond. Relat. Mat., 67 (2016) 16-25. [28] C. Nouvellon, R. Belchi, L. Libralesso, O. Douhéret, R. Lazzaroni, R. Snyders, D. Thiry, WC/C:H films synthesized by an hybrid reactive magnetron sputtering/Plasma Enhanced Chemical Vapor Deposition process: An alternative to Cr(VI) based chromium plating, Thin Solid Films, 630 (2017) 79-85. 34

[29] F. Lofaj, L. Kvetková, P. Hviščová, M. Gregor, M. Ferdinandy, Reactive processes in the high target utilization sputtering (HiTUS) W-C based coatings, J. Eur. Ceram. Soc., 36 (2016) 3029-3040. [30] F. Lofaj, M. Kabátová, M. Klich, D. Medveď, V. Girman, Tribological behavior of hydrogenated W-C/a-C:H coatings deposited by three different sputtering techniques, Cerâmica, 65 (2019) 58-69. [31] J. Esteve, G. Zambrano, C. Rincon, E. Martinez, H. Galindo, P. Prieto, Mechanical and tribological properties of tungsten carbide sputtered coatings, Thin Solid Films, 373 (2000)

ro of

282-86.

[32] S. J. Park, K-R. Lee, D-H. Ko, K.Y. Eun, Microstructure and mechanical properties of WC-C nanocomposite films, Diam. Relat. Mat., 11 (2002) 1747-52.

-p

[33] W. Jacob, Surface reactions during growth and erosion of hydrocarbon films, Thin Solid

re

Films, 326 (1998) 1-42.

[34] T. Schwarz-Selinger, A. von Keudell, W. Jacob, Plasma chemical vapor deposition of

lP

hydrocarbon films: The influence of hydrocarbon source gas on the film properties, J. Appl. Phys., 86 (1999) 3988 -3996.

na

[34] A. von Keudell, Formation of polymer-like hydrocarbon films from radical beams of methyl and atomic hydrogen, Thin Solid Films, 402 (2002) 1-37.

ur

[36] A.von Keudell, M. Meier, C. Hopf, Growth mechanism of amorphous hydrogenated carbon, Diam. Relat. Mat., 11 (2009) 969-975.

Jo

[37] D. Thiry, A. De Vreese, F. Renaux, J.L. Colaux, S. Lucas, Y. Guinet, L. Paccou, E. Bousser, R. Snyders, Toward better understanding of the influence of the hydrocarbon precursor on the mechanical properties of a a/C:H coatings synthesized by a hybrid PECVD/PVD method, Plasma Process Polym., 13 (2016) 316-323.

35

[38] F. Lofaj, M. Kabátová, L. Kvetková, J. Dobrovodský, V Girman, Hybrid PVD-PECVD W-C:H coatings prepared by different sputtering techniques: The comparison of deposition processes, composition and properties, Surf. Coat. Technol., 375 (2019) 839-853. [39] F. Lofaj, M. Kabátová, J. Dobrovodský, Hydrogenation and hybridization in hard W-C:H coatings prepared by hybrid PVD-PECVD method with methane and acetylene, Int. J. Refractory Metals Hard Mat., submitted, June 2019. [40] S. Veprek, Recent search for new superhard materials: Go nano, J. Vac. Sci. Technol., A 31 (2013) 050822-1 – 33.

ro of

[41] A. Erdemir, O.L. Eryilmaz, I.B. Nilufer, G.R. Fenske, Effect of source gas chemistry on tribological performance of diamond-like carbon films, Diam. Relat. Mat., 9 (2000) 632-637. [42] W.G. Cui, Q.B. Lai, F.M. Wang, Quantitative measurements of sp3 content in DLC films

-p

with Raman Spectroscopy, Surf. Coat. Technol., 205 (2010) 1995-1999.

re

[43] A. Leyland, A. Matthews, On the significance of H/E ratio in wear control: A nanocomposite coating approach to optimized tribological behavior, Wear, 246 (2000) 1-11.

lP

[44] B. R. Pujada, G. C. A. M. Janssen. Density, hardness and reduced Young’s modulus of W-C:H coatings, Surf. Coat. Technol., 201 (2006) 4284-88.

na

[45] F. Lofaj, M.Kabátová, M. Klich, D. Vaňa, J. Dobrovodský, The comparison of structure and properties in DC magnetron sputtered and HiPIMS W-C:H coatings with different

ur

hydrogen content, Ceram. Int., 45 (2019) 9502-14. [46] M.V. Moro, R. Holeňák, L. Zendejas Medina, U. Jansson, D. Primetzhofer, Accurate

Jo

high-resolution depth profiling of magnetron sputtered transition metal alloy film containing light species: A multi-method approach, Thin Solid Films, 686, (2019) 137416-1 137416-8. [47] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids. Vol. 1. The Stopping and Ranges of Ions in Matter. Pergamon Press, New York, 1985. [48] X. Zhang, X Zhang, Toughness evaluation of hard coatings and thin films, Thin Solid Films, 520 (2012) 2375 – 89. 36

[49] Ch. Wang, K. Shi, C. Gross, J.M. Pureza, M. Mesquita Lacerda, Y-W. Chung, Toughness enhancement of nanostructured hard coatings: Design strategies and toughness measurement techniques, Surf. Coat. Technol., 257 (2014) 206-212. [50] M. Sebastiani, K.E. Johanns, E.G. Herbert, G.M. Pharr, Measurement of fracture toughness by nanoindentation methods: recent advances and future challenges, Current Opinion Solid State Mater. Sci., 19 (2015) 324-33. [51] M. Tkadletz, N. Schalk, R. Daniel, J. Keckes, Ch. Czettl, Ch. Mitterer, Advanced characterization methods for wear resistant hard coatings: A review on recent progress, Surf.

ro of

Coat Technol., 285 (2016) 31-46.

[52] G. Dehm, B.N. Jaya, R. Raghavan, C. Kirchlechner, Overview on micro- and

nanomechanical testing: New insight in interface plasticity and fracture at small scales, Acta

-p

Mater., 142 (2018) 248-282.

Technol., 201 (2007) 5148-52.

re

[53] J. Musil, M. Jirout, Toughness of hard nanostructured ceramic thin films, Surf. Coat.

lP

[54] Y.T. Pei, D. Galvan, J.T.M.D. Hosson, C. Strondl Advanced TiC/a-C:H nanocomposite coatings deposited by magnetron sputtering, J. Eur. Ceram. Soc., 26 (2006), 565-570.

na

[55] W.Q. Bai, J.B. Cai, X.L. Wang, D.H. Wang, C.D. Gu, J.P. Tu, Mechanical and tribological properties of a-C/a-C:Ti multilayer films with various bilayer periods, Thin Solid

ur

Films, 558 (2014), 176-183.

[56] M. Mikula, M.Truchlý, D. G. Sangiovanni, D. Plašienka, T. Roch, M. Gregor, P. Ďurina,

Jo

M. Janík, P. Kúš, Experimental and computational studies on toughness enhancement in TiAl-Ta-N quaternaries, J. Vac. Sci. Technol. A. Vacuum, surfaces, and films, 35 (2017) 060602-1 – 060602-6. [57] A. N. Hidenobu Marunoa, Adhesion and durability of multi-interlayered diamond-like carbon films deposited on aluminum alloy, Surf. Coat. Technol., 354 (2018) 134-144.

37

[58] J.C. Angus, F. Jansen, A dense “diamondlike” hydrocarbons as random covalent networks, J. Vac. Sci. Technol., 6 (1988) 1778-82. [59] S.S. Tzeng, Y.J. Wu, J.S. Hsu, The effects of plasma pre-treatment and post-treatment on diamond-like carbon films synthesized by RF plasma enhanced chemical vapor deposition, Vacuum, 83 (2008) 618-21. [60] W. Möller, Hydrogen trapping and transport in carbon, J. Nucl. Mater., 162 (1989) 13850. [61] C. Wild, J. Wagner, P. Koidl, Process monitoring of a-C:H plasma deposition. J. Vac.

Jo

ur

na

lP

re

-p

ro of

Sci. Technol. A, 5 (1987) 2227-30.

38