diamond-like carbon composite thin films with super anti-corrosion properties

Accepted Manuscript Magnetron-sputtered copper/diamond-like composite thin films with super anti-corrosion properties

Sara Khamseh, Eiman Alibakhshi, Mohammad Mahdavian, Mohammad Reza Saeb, Henri Vahabi, Ninel Kokanyan, Pascal Laheurte PII: DOI: Reference:

S0257-8972(17)31148-9 doi:10.1016/j.surfcoat.2017.11.012 SCT 22860

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

22 August 2017 31 October 2017 4 November 2017

Please cite this article as: Sara Khamseh, Eiman Alibakhshi, Mohammad Mahdavian, Mohammad Reza Saeb, Henri Vahabi, Ninel Kokanyan, Pascal Laheurte , Magnetronsputtered copper/diamond-like composite thin films with super anti-corrosion properties. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2017.11.012

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ACCEPTED MANUSCRIPT Magnetron-sputtered Copper/Diamond-like Composite Thin Films with Super Anti-corrosion Properties Sara Khamseh1*, Eiman Alibakhshi2,3, Mohammad Mahdavian3, Mohammad Reza Saeb4, Henri Vahabi 5,

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Ninel Kokanyan 5, 6, Pascal Laheurte 7

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Department of Nanomaterial and Nanocoatings, Institute for Color Science and Technology, Tehran, Iran

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Department of Chemical Engineering, Payame Noor University, Tehran, Iran

3

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Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran

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Department of Resins and Additives, Institute for Color Science and Technology, P.O. Box 16765-654,

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Tehran, Iran 5

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Universite de Lorraine, Laboratoire MOPS E.A. 4423, Metz, F-57070, France

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CentraleSupelec, Laboratoire Matériaux Optiques, Photonique et Systèmes, 2 rue E. Belin, 57070 Metz,

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France 7

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Universite de Lorraine, Laboratoire LEM3 UMR 7239, Metz, F-57045, France

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Corresponding author: Tel: +98-2122969777; E-mail: [email protected]

ACCEPTED MANUSCRIPT Abstract Super anti-corrosive copper/diamond-like carbon (Cu/DLC) composite films are applied on mild steel utilizing magnetron sputtering in a mixed atmosphere of Ar and CH4. Mechanical, contact angle, and corrosion performance of the resulting Cu/DLC thin films are probed and discussed in

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terms of Ar/CH4 and Cu/C ratios. Overall, Cu/C ratio has augmented by Ar/CH4 ratio. Raman

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spectra of films revealed typical features of G and D bands indicating formation of DLC phase.

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The Cu/DLC thin films with higher Cu content exhibited a higher degree of sp2 carbon clustering, but lower diamond-like sp3 bonding. Internal stress values of Cu/DLC thin films

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decreased with increasing Cu/C ratio. Addition of a few amount of Cu to DLC resulted in a rise

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in plastic hardness and H3/E2 ratio of Cu/DLC composite thin films, but optimum value was observed for composite films having an intermediate Cu concentration. The contact angle of iron

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substrate increased when coated with Cu/DLC thin films, but Cu content of films played a minor

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role. The Cu/DLC thin films formed via magnetron sputtering revealed super anti-corrosion performance, the term which is defined, conceptualized, and quantified in the current study.

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corrosion Performance

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Keywords: Diamond-like Carbon; Magnetron Sputtering; Mechanical properties; Super Anti-

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ACCEPTED MANUSCRIPT 1- Introduction Diamond-like carbon (DLC) is a general term for a class of amorphous carbon films composed of a mixed structure of diamond with sp3 bond and graphite with sp2 bond [1-3]. The ratio of sp3/sp2 to a large extent determines DLC structure and properties [4-6]. DLC is known

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for its chemical inertness, corrosion resistance, biocompatibility, and good mechanical and

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tribological properties [7-13]. Such properties give the DLC potential to be noticed as protective

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coating. However, DLC coatings suffer from inadequate adhesion to metal substrates. There are some reports that underlined the fact that adhesion of DLC to the metal depends on sp3 bond

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fraction, residual stress, and roughness of the substrate [9,14,15].

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Typically, two routes are proposed to overcome such inadequate adhesion: (i) applying an interlayer on the surface of metal substrate before DLC film deposition [9, 15–17], which assists

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in reduction of the difference between thermal expansion coefficients of metal substrate and

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DLC film [9, 15–17]. For instance, Azzi et al. prepared DLC films with interlayers of amorphous hydrogenated silicon-based materials and revealed that a-SiNx interlayer significantly improves

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the adhesion and corrosion resistance of DLC-based coating [9]; (ii) addition of metal or Si to

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DLC structure [18–25]. Metal incorporation into the carbon matrix results in thin films suitable for a variety of applications [22, 23]. Metal incorporation greatly enhances graphitization of

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carbon films and reduces surface tension [21, 22]. It has been shown that titanium incorporation into a-C films gives a tribocorrosion response to the film, increases the corrosion resistance and improves biocompatibility of Ti alloy [19,20]. In the light of above, composite films based on DLC could be promising materials for achieving high performance coatings. There are some reports on the effect of Cu addition on the physical and mechanical properties of DLC coatings [23,26,27]. For instance, it was found that

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ACCEPTED MANUSCRIPT a-C films possessing higher Cu content are richer in sp2-bonded carbon and show superior antibacterial activity and high biocompatibility [23, 26]. Elsewhere, increase of Cu content resulted in improvement of hardness and adhesion of DLC film to the metal substrates [27,28]. Moreover, reports emphasize that there is a specific range for Cu content and nanograin size

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sputtered Cu/DLC films applied on magnesium substrate [29].

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essential for promoting the mechanical performance and blood compatibility of magnetron-

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There is no denying that coated substrates are expected to be exposed to the aggressive environments. Therefore, the physicochemical behavior of the top layer cannot be neglected

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anymore. Copper is a corrosion resistant metal, particularly when formed as nanostructured thin

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films [30,31]. The corrosion properties of DLC coatings on metallic substrates have been widely reported in the literature, but protection properties depend on the adhesion to the substrate [7–

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10]. To the best of the authors’ knowledge, there is no report on corrosion resistance of Cu/DLC

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thin films. The present work reports on the effects of Cu incorporation on the microstructure and corrosion protection of Cu/DLC thin films prepared in a magnetron sputtering system under a

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mixed atmosphere of Ar and CH4. Surface and bulk characters of the films are analyzed by

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microscopy. Interpretations based on contact angle, hardness, and Raman spectroscopic measurements provided useful insights into the situation of surface and bulk interactions between

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elements present in the coating. Super anti-corrosion performance is defined, conceptualized and quantified for the first time in this study.

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ACCEPTED MANUSCRIPT 2- Experimental Cu/DLC thin films were prepared in a planar type magnetron sputtering apparatus, (Yarenikane saleh-DRS320). Figure 1 illustrates magnetron sputtering systems used in this

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study, as successfully employed for coatings of different type in previous investigations [32-34].

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Fig. 1. Schematic illustration of sputtering system used in this study.

The system was evacuated to a vacuum down to 3×10-5 Torr prior to deposition. The copper

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target was sputtered under a mixture of argon (working gas) and methane (reactive gas) by altering the CH4/Ar ratio between 0.2 and 1. A mirror-polished iron wafers and microscopic glass slides 20 mm square was used as the substrate. All substrates were ultrasonically cleaned with acetone, ethanol, and then 2-propanol prior to sputtering deposition. The target-to-substrate distance was fixed at 110 mm. The substrate temperature increased to 150 oC during deposition due to particle bombardment of the substrate even without bias application and substrate heating.

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ACCEPTED MANUSCRIPT The film thickness was controlled between 850 to 900 nm by monitoring the sputtering time. Details on film deposition are summarized in Table 1.

Table 1. Details on deposition parameters and thickness of the films.

C-2

1.5 × 10

C-3

1.5 × 10

C-4

1.5 × 10

1

510

−2

1.5

485

−2

2.3

486

−2

4

480

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−2

Sputtering current (A)

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1.5 × 10

Sputtering voltage (V)

Film's thickness (nm)

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850

18

850

18

850

18

850

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C-1

Ar/CH4 Ratio

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Sputtering pressure (Torr)

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Sample no

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Composition of the films was evaluated by electron probe microanalysis (EPMA)

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measurement (JEOL, JXA-8530F). The structure of the films was characterized using a Raman spectrometer (Horiba Jobin-Yvon, LabRam HR EVOLUTION).

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The crystal structure of the films was assigned using X-ray diffractometry (X'Pert Pro MPD-

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PANalytical) with thin film method. Scans were made in the grazing angle mode (Seeman– Bohlin mode) with an incident beam angle of 5o.

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The evaluation of the film internal stress was carried out using a surface profilometer with Stoney’s equation. The mechanical properties of different coating systems were characterized using an Ultra Nanoindentation Tester (UNHT) commercialized by Anton-Paar. Atomic-force microscopy (AFM) was conducted on an AFM, Park Scientific Instrument (PC) to monitor surface roughness of the prepared films. Contact angle measurements were performed by a contact angle measuring system model OCA 15 plus as well.

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ACCEPTED MANUSCRIPT The bulk morphology of Cu/DLC thin films was observed by scanning electron microscopy (SEM) technique provided by a Mira (field emission-scanning electron microscope (FE-SEMmodel Tescan) apparatus.

The corrosion properties of the samples were quantified by electrochemical impedance

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spectroscopy (EIS) test. The EIS was conducted on an Ivium Compactstat in 3.5 wt.% NaCl

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solution utilizing a three electrode cell including Ag/AgCl (3 M KCl), graphite and the steel specimens as reference, counter and working electrodes, respectively. The EIS measurements

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were performed at open circuit potential (OCP), 10 mV perturbation and in the frequency range

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of 10 kHz-10 mHz.

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3- Results and discussions

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Figure 2a shows chemical composition of the deposited Cu/DLC thin films as a function of Ar/CH4 ratio. It can be seen that copper content of the films increased slightly with the increase

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of Ar/CH4 ratio from 56 at. % for Ar/CH4 ratio of 1 to 88 at. % for Ar/CH4 ratio of 4. By contrast, carbon concentration of the films decreased from 44 at. % for Ar/CH4 ratio of 1 to 12

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at. % for Ar/CH4 ratio of 4. It is also apparent that Cu/C ratio of the films increased continuously

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with increasing Ar/CH4 ratio (Fig. 2b). There is a clue that with increasing the CH4 content, surface of Cu target is covered by organic compounds and sputter yield of the copper target decreases accordingly [35].

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Fig. 2. Evolution of (a) chemical composition and (b) Cu/C ratio as a function of Ar/CH4 ratio.

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Fig. 3. X-ray diffraction patterns of C-1 and C-4 films measured by thin-film method.

XRD patterns in the grazing angle mode for C-1 and C-4 films are shown in Fig. 3. The XRD

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results show that an amorphous- like structure with broadened peaks is formed in both samples. Since Cu is immiscible in carbon, two phase structures are easily formed in Cu/DLC thin films [23,36]. Accordingly, formation of an amorphous- like structure in the films might be attributed to the formation of Cu nano particles embedded in amorphous carbon matrix in the film which cannot be detected by XRD. Raman spectroscopy is a very useful tool in characterization of carbon nanomaterials. Raman spectra of the Cu/DLC thin films are shown in Fig. 4. The Raman spectra of all samples (Fig. 9

ACCEPTED MANUSCRIPT 4a) consists of two typical features of G (for graphite) and D (for disorder) bands, indicating formation of the DLC phase [6, 20]. The peak position, the integral intensity ratio of D and G peaks and their full width at half maximum (FWHM) values give useful information about the structure of carbon based films. It can be seen that the position of the D peak, ID/IG and FWHM

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values of the films are all governed by Cu content (Figs. 4a, 4b and 4c). ID/IG ratio was evaluated

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using the integral-area ratio of the D and G bands from the Gaussian curve fittings, as shown in

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Fig. 4b. Accordingly, ID/IG ratio increases with increasing Cu/C ratio (increasing Ar/CH4 ratio), which is a signature of the higher proportion of sp2 bonded carbon and an increased graphitic

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domain size. Thus, Cu/DLC thin films with higher Cu content exhibit a higher degree of sp2

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carbon clustering and lower value of diamond-like characteristic (sp3 bonding). It has been demonstrated that metal incorporation assists in formation of sp2 bonded carbon sites [37,38].

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The higher fraction of sp2 bonded carbon results in metal doped DLC films with lower internal

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stress and higher adhesion [39]. FWHM value of D and G peaks give further information about the crystallinity of carbon-based films [40]. When most of the carbon film is amorphous, broad

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and diffused peaks appear and sharp peaks appear in crystalline material. FWHM value of D and

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G peaks decreased with increasing Cu content of Cu/DLC thin films (Fig. 4c) suggesting higher

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crystallinity of the films.

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Fig. 4. (a) Raman spectra (b) ID/IG ratio as a function of Cu/C ratio and (c) FWHM value for D

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and G peaks as a function of Cu/C ratio, of Cu/DLC thin films

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AFM and SEM micrographs were employed to probe changes in the morphology and surface

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roughness of Cu/DLC thin films. The morphology and surface roughness of the films prepared at low (C-1) and high (C-4) Ar/CH4 ratio are compared based on AFM and SEM images (Fig. 5).

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Fig. 5. Plane-view FE-SEM and AFM images of C-1 and C-4 films (a, d) C-1 film with Cu/C

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ratio of 1 (b, c, e) and C-4 film with Cu/C ratio of 4. (which one?).

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Plane view of SEM image of C-1 film possessing the lowest Cu content among studied thin films (Fig. 5a) depicts development of a pebble-like structure with open grain boundaries. On the

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other hand, in the Cu/DLC thin film with the highest Cu content (C-4) some round lumpy

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nanosize clusters are appeared in the matrix (Fig. 5b). The cross-sectional micrograph of C-4 film is shown in Fig. 5c. A smooth and fine grained structure is formed in this film. This film seems to consist of very fine grains as can be seen in an amorphous or nanocrystalline films. This kind of morphology has previously reported for the immiscible metal/carbon composite thin films [23,27]. It is well known that Cu and carbon are immiscible because the copper-carbon bonding is weak [23,28,36]. Hence, two phase structures can be easily formed in Cu/DLC thin films [23,28,36]. On the other hand, Cu clusters segregate from carbon phase and two phase 12

ACCEPTED MANUSCRIPT structures form. Quantitative analysis of surface roughness of Cu/DLC thin films due to AFM images showed that the surface roughness of C-1 film (Rms=9.4 nm) with the lowest Cu/C ratio is higher than that of C-4 film (Rms=1 nm) with the highest Cu/C ratio. The pebble-like and rough structure of C-1 film can increase its surface roughness.

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Figure 6 shows variation of the internal stress values of the Cu/DLC thin films with Cu/C

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ratio. All stress values in the films are compressive. Internal stress values of Cu/DLC thin films

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are decreased with increasing Cu/C ratio. It has been shown that metal incorporation in carbon

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matrix leads to a fall in the film's internal stress value and better film adhesion [41,42].

Fig. 6. Influence of Cu/C ratio on internal stress value of Cu/DLC thin films. The dependence of hardness and Young’s modulus of Cu/DLC thin films on Cu/C ratio is shown in Fig. 7a. The addition of a few amount of Cu to DLC film (C-1 sample), has improved mechanical properties of iron substrate, and similarly increased hardness and Young’s modulus. By contrast, the hardness values of Cu/DLC thin films with higher Cu content dropped suddenly, 13

ACCEPTED MANUSCRIPT while Young’s modulus values of the films are almost constant and do not show a big change with increasing Cu content. As discussed above, internal stress values of Cu/DLC thin films decreased with increasing Cu/C ratio. Reports suggest that metal incorporation into the carbon matrix leads to a reduction in film's internal stress value and the hardness value of carbon film

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decreases accordingly [41,42]. Moreover, Cu is a soft element and Cu/DLC thin films with

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higher Cu contents are expected to represent lower hardness values. In order to estimate the

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tribological properties of Cu/DLC thin films, variation of H3/E2 ratio with Cu/C ratio is measured and plotted in Fig. 7b. The H3/E2 ratio reflects the resistance of a film to plastic deformation. It

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can be seen that H3/E2 ratio of C-1 film with the lowest Cu content takes a higher value than that

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of iron substrate. However, the H3/E2 ratios of Cu/DLC thin films with higher Cu contents are decreased, even down to that of iron substrate. Higher hardness, lower Young’s modulus and

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higher H3/E2 ratio of C-1 film are typical properties of nanocomposite films [43,44]. The

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nanocomposite films have dense microstructure and contain nanograins of one phase that are fully embedded in a continuous phase [26,27]. Since Cu is immiscible in carbon, two phase

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structures are easily formed in Cu/DLC thin films [23,36]. Accordingly, it can be concluded that

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a nanocomposite microstructure has probably formed in C-1 film.

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Fig. 7. Variation of (a) plastic hardness and Young's modulus and (b) H3/E2 ratio of Cu/DLC composite thin films. 15

ACCEPTED MANUSCRIPT The contact angle of a surface determines its stability in wet conditions. The variation of contact angle of Cu/DLC thin films is shown in Fig. 8. It can be seen that the contact angle of iron substrate increases when coated with Cu/DLC thin films because of hydrophobic nature of carbon films [12,45]. However, Cu content of Cu/DLC thin films was not adequate for a big

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change in contact angle of the samples. The hydrophobic nature of Cu/DLC thin films is an

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important factor determining the corrosion protection behavior of the parts and tools in humid

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environments [12].

Fig. 8. Influence of Cu/C ratio on contact angle of a-CNx/Cu composite thin films.

The corrosion resistance performance of the coatings was investigated by EIS test. The EIS analysis was performed on the blank (uncoated) and different coated samples (C-1, C-2, C-3 and

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ACCEPTED MANUSCRIPT C-4) dipped in 3.5 wt.% NaCl solution for 1, 4 and 24 h. The Nyquist and Bode plots of the

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prepared samples are given in Figs 9 and 10.

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Fig.9. Nyquist plots of the coatings; (a) blank, (b) C-1, (c) C-2, (d) C-3 and (e) C-4 immersed in the 3.5 wt.% NaCl solution for different immersion times; solid lines and marker points are attributed to the fitted and experimental data, respectively.

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ACCEPTED MANUSCRIPT Fig. 10. Bode plots of the coatings; (a) blank, (b) C-1, (c) C-2, (d) C-3 and (e) C-4 immersed in the 3.5 wt.% NaCl solution for different immersion times; solid lines and marker points are attributed to the fitted and experimental data, respectively. As in Fig. 9, the Cu/DLC coatings had a significant impact on the diameter of semicircles, which could be related to the charge transfer reaction and the electrical double layer formed on the

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surface. It is generally known that a bigger diameter of semicircles in Nyquist plot stand for a

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better corrosion resistance property of the substrate. It can be seen from the Nyquist plots that the

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C-4 coated sample shows the largest semicircle. For all the coated samples, the diameter of the semicircles decreases with time, probably due to increase in the corrosion rate through the

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generation of further pores and defects in the bulk of coating.

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Impedance magnitudes at low frequency for coated samples are higher than that of the uncoated sample. Also, it can be seen from the impedance data that the corrosion reactions of the blank

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(uncoated) sample is under control of charge transfer and only one relaxation time can be seen up

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to 24 h immersion. However, there are two time constants for coated samples in all of the immersion times, implying the presence of a film on the surface. The electrochemical impedance

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response of the samples was simulated using the equivalent electrical circuits provided in Fig.

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11. In these circuits, Rs represents solution resistance, Rf is film resistance, CPEf is constant phase element of film, Rct is charge transfer resistance and CPEdl is constant phase element of double

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layer. Double layer and film capacitance values were calculated according to Eq. 1 [46–48].

C  (Q.R1n )1 / n

(1)

where C shows capacitance of double layer and/or film, Q is the magnitude of admittance of the CPE, R is charge transfer and/or film resistance and n is the empirical exponent. The impedance data extracted from Fig. 11 are presented in Table 2 at various immersion times.

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Table 2. Electrochemical parameters extracted from the impedance plots for uncoated and coated iron samples immersed in the 3.5 wt.% NaCl solutions at various times; the values are the

Cf

log │Z│

(μF.cm-2)

(Ω.cm2)

-

-

3.16±0.01

-

-

-

2.99±0.01

-

-

-

-

2.5±0.01

46.2±10.6

160.9±14.1

0.82±0.01

54.9

3.36±0.02

140.5

34.1±11.9

168.9±16.3

0.83±0.02

58.7

3.32±0.01

177.3

20.8±3.3

285.2±30

0.77±0.02

61.7

3.22±0.01

14.8

123. 4±23.1

64.3± 1.3

0.67±0.02

5.9

3.72±0.03

0.71±0.03

81.7

66.2±17.7

171.5±12.1

0.58±0.01

6.7

3.49±0.02

0.7±0.02

93.9

58.3±16.8

342.6±11.6

0.51±0.01

32.9

3.39±0.02

0.86±0.01

64.1

210.1±22.9

28.5±3.1

0.65±0.01

1.8

3.73±0.03

101.4±14.2

0.7±0.01

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30.7±14.3

191.3±18.7

0. 56±0.02

3.4

3.57±.02

144.3±24.3

0.62±0.02

72.2

25.1±2.6

285.8±20.3

0. 54±0.02

4.3

3.39±0.02

0.8±0.01

0.67±0.02

0.2

13950±754

1.4±0.05

0.62±0.01

0.1

5.05±0.07

5886±313.1

44.7±2.2

0.74±0.01

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378.2±26.2

7.5±0.6

0.6±0.01

0.2

3.83±0.03

3716±217.8

92.3±9.9

0.69±0.1

57.1

100.7±14.7

70.5± 5.2

0.65±0.01

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3.61±0.02

Blank

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1458±50.2

502.1±53.7

(without

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940.2±40.6

coating)

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C-1

(μF.cm-2)

(Ω.cm2)

(μsn.Ω-1.cm-2)

0.76±0.01

455

-

-

611.3±48.1

0.77±0.01

518.1

-

360.4±71.6

770.1±114

0.78±0.01

536.5

1

1956±118

140.7±18

0.93±0.02

127.7

4

1645±124.6

165.1±37.6

0.89±0.02

24

1480±155

272±27.7

0.68±0.01

1

4821±295

29.4±2.1

0.74±0.02

4

3614±355

116.4±27.7

24

2177±124.6

151.2±17.6

1

4651±214

75.9±6.4

4

3627±156

24

2237±87.6

1

105270±926

4 24

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Y0,f

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(μsn.Ω-1.cm-2)

C-4

Rf

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(Ω.cm2)

C-3

Cdl

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(h)

C-2

ndl

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Y0,dl

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Rct

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Time

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Sample

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mean of three replicates and (±) corresponds to the standard deviations.

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(b)

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(a)

Fig. 11. Equivalent circuits used to model measured data on EIS diagrams; (a) one time-constant

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and (b) two time-constant equivalent circuits.

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Looking at Table 2 the lowest charge transfer resistance belongs to the blank sample during

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immersion time. EIS results show that the charge transfer resistance values of Cu/DLC

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composite thin films is higher than that of blank sample during immersion time. This means that Cu/DLC composite thin films could provide proper corrosion resistance performance on iron

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surface. However, a decreasing trend was observed for all of the coatings. This could be related

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to the electrolyte diffuse into the coatings. The highest charge transfer values were found in the case of C-4 sample during the immersion times, indicating the best corrosion resistance for this

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sample. This could be attributed to its higher sp2 carbon clustering, as shown in Fig. 4. In other words, the lower sp3 structure of C-4 sample results in its higher corrosion resistance compared to other samples.

Notably, in the case of C-4 coated sample, the Nyquist and Bode plots exhibit different behavior compared to other coatings after 1 h immersion. The charge transfer value was ca. 105000 Ωcm2, which was higher than that for the other coatings taking values in the range of 2000-5000 Ωcm2. Moreover, the charge transfer value for this sample was much higher than that reported for Cu 21

ACCEPTED MANUSCRIPT doped DLC coating (around 25000 Ωcm2) [26]. This suggests that the higher values of copper existed in the DLC thin film results in a super anti-corrosion performance on the iron surface. A coating with super anti-corrosion performance could be able to completely block the pathway of dissolved oxygen and chloride ion intrusion through defects into the underlying surface.

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From Table 2, the double layer capacitance values of the Cu/DLC composite thin films was

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lower than the blank sample due to the decrease of dielectric constant or increase of double layer

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thickness [49,50]. The lowest film capacitance and the highest film resistance was recorded for C-4 sample, which could be related homogenous film structure observed in Fig. 5 [50–53].

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|Z|10 mHz values can present the general corrosion resistance performance, which can be directly

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read from the Bode diagrams without need for fitting and consequently without including the errors resulted from the fitting process [54–57]. The |Z|10 mHz was obtained from the Bode plots

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and the results are shown in Table 2. These values show the same trend as of the charge transfer

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resistance. As can be seen, the variation of |Z|10

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over immersion time is insignificant for the blank

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sample. With application of Cu/DLC composite thin films on iron surface, |Z|10

mHz

increases

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sharply within 1 h immersion time. So the good corrosion resistance performance of these films drastically decreases after 24 h as a result of electrolyte diffusion to the iron surface and film

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deterioration. Nevertheless, the |Z|10

mHz

values of coated samples is higher than the blank in

whole of immersion periods. Again, the C-4 sample shows the highest |Z|10 at all immersion times revealing that this sample provides better corrosion resistance performance than other samples. As discussed above Cu/DLC composite thin film provides good corrosion resistant properties to iron substrate. The corrosion resistance of Cu/DLC composite thin film increased with increasing Cu content. The increase in corrosion resistance of Cu/DLC composite thin film with higher Cu

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ACCEPTED MANUSCRIPT contents can be explained by two facts. The first is higher sp2 fraction and lower internal stress of Cu/DLC composite thin films with higher Cu contents. This leads to the film's better adhesion to the iron substrate and increases corrosion resistance [41,42]. The second is films morphology. As shown in Fig. 5, the surface of Cu/DLC composite thin film with the lowest Cu content shows

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rough and round grains with open grain boundaries. This defective structure and open grain

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boundaries make several paths for electrolyte to penetrate into the coating and decrease its

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corrosion resistant. In contrast the surface of Cu/DLC composite thin film with the highest Cu content (C-4) contains round lumpy nanosized clusters embedded in a smooth matrix

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[23,28,36,41]. Since Cu is immiscible in carbon, a two-phase structure can be easily formed in

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Cu/ DLC thin films [23,28,36,41]. In this two-phase structure, a tissue of amorphous phase (here a-C) covers whole surfaces of nano grains (here Cu nano grains) and there is no grain boundary

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[58]. When a perfect two phase structure forms in the films, there is no grain boundary acting as

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a defect for electrolyte to penetrate into the coating. Therefore, due to the lower internal stress and defect free structure the Cu/DLC composite thin films with higher Cu contents can provide

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superior corrosion resistance. However, C-4 sample performs as a super anti-corrosion coating

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on the iron surface. A mechanistic description of such terminology is schematically provided in Fig. 12. In a perfect nanocomposite structure Cu nanoparticles are embedded in DLC amorphous

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matrix. This metal nanograins make a barrier against penetration of corrosive media. As shown in Fig.2, C-4 sample contains the highest Cu content which leads to higher density of Cu nanograins embedded in DLC matrix. This leads to the highest prevention, against penetration of corrosive media towards substrate and its super anti-corrosion property.

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Poor Anti-corrosion Performance

Good Anti-corrosion Performance

Super Anti-corrosion Performance

(Partially Blocked Diffusion)

(Completely Blocked Diffusion)

(Unblocked Diffusion)

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(DLC) with Cu/C Ratio=0 (DLC+Cu) with Cu/C Ratio=1

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Cu Nanograin

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C

O2, H2O, Cl-, Na+

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O2, H2O, Cl-, Na+

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O2, H2O, Cl-, Na+

(DLC+Cu) with Cu/C Ratio=5

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Fig. 12. Mechanistic description of anti-corrosion properties for unblocked, partially blocked and

performance.

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completely blocked diffusion paths, respectively assigned to poor, good, and super anti-corrosion

Conclusions

Cu/DLC composite thin films with different Cu contents were applied by a planar type reactive sputtering system on mild steel substrates and their anti-corrosion characteristics were analyzed both quantitatively and qualitatively. The effects of Ar/CH4 and Cu/C ratio on the microstructure and corrosion resistance of Cu/DLC composite thin films were investigated as well. The Cu/C ratio of the films increased continuously with increasing Ar/CH4 ratio. The

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ACCEPTED MANUSCRIPT Raman spectra of the samples indicated formation of the DLC phase. The Cu/DLC thin films with higher Cu content exhibited a higher degree of sp2 carbon clustering and lower value of diamond-like characteristic (sp3 bonding). Moreover, internal stress values of Cu/ DLC thin films decreased with increasing Cu/C ratio. The addition of a few amount of Cu to DLC has

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augmented plastic hardness and H3/E2 ratio of the Cu/DLC composite thin films. However,

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Cu/DLC composite thin films with higher Cu content did not show good mechanical properties.

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The contact angle of iron substrate increased when coated with Cu/ DLC thin films, but Cu content of Cu/DLC thin films did not change markedly contact angle of the samples. Cu/DLC

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thin films revealed excellent corrosion resistance to iron substrate, so that super anti-corrosion

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performance is defined, schematically explained, and quantified in this work. There was a close relation between Cu content of the films, microstructure, mechanical and corrosion resistance of

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the samples. The results of current study indicated that it is possible to prepare Cu/DLC

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composite thin films with controlled microstructure, mechanical properties and supper anti-

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CE

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corrosion performance using magnetron sputtering system.

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ACCEPTED MANUSCRIPT Highlights Magnetron sputtering used to prepare Cu/diamond-like carbon (DLC) thin films

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Cu/DLC composite films with different Cu/C ratios applied for corrosion protection

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SEM, AFM, Raman, hardness, contact angle, and EIS analyses performed

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Composite thin films of Cu/DLC exhibited super anti-corrosion performance

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Super anti-corrosion performance was defined, conceptualized, and quantified

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