A comparative study of surface morphology, mechanical and tribological properties of DLC films deposited on Cr and Ni nanolayers

Journal Pre-proof A comparative study of surface morphology, mechanical and tribological properties of DLC films deposited on Cr and Ni nanolayers Fatemeh Shahsavari, Maryam Ehteshamzadeh, Mohamad Hassan Amin, Anders J. Barlow PII:

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Please cite this article as: F. Shahsavari, M. Ehteshamzadeh, M.H. Amin, A.J. Barlow, A comparative study of surface morphology, mechanical and tribological properties of DLC films deposited on Cr and Ni nanolayers, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.251. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

A comparative study of surface morphology, mechanical and tribological properties of DLC films deposited on Cr and Ni nanolayers Fatemeh Shahsavari1, Maryam Ehteshamzadeh2, Mohamad Hassan Amin3, Anders J. Barlow4 1

Young Researchers Society, Mineral Industries Research Center, Shahid Bahonar University of Kerman, Kerman, Iran. 2 Department of Materials Engineering and Metallurgy, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran. 3 School of Science, RMIT University, 124 La Trobe St Melbourne, Victoria 3000, Australia 4 Centre for Materials and Surface Science and Department of Chemistry and Physics, School of Molecular Sciences, La Trobe University, Melbourne, VIC 3086, Australia

Abstract: Applying metallic interlayer between diamond-like carbon (DLC) and substrate is a promising method to improve the adhesion and efficiency of DLC films. Understanding how the interlayer parameters affect the properties of DLC coatings leads to production of long-time excellent performance of them. In this study, DLC films deposited by PECVD and 10, 20 and 40 nanometers layers of Ni and Cr were sputtered on silicon substrates to use as the adhesion layers. The role of chemical structure and physical parameters (surface roughness) of interlayers on the final properties of DLC films were investigated using atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), nanoscratch and nanoindentation techniques. It was found that a lower surface roughness with a sharp and homogenous distribution of particles sizes (height) results in formation of a smooth DLC film with lower friction coefficient, more stress reduction and better adhesion strength. It was discussed that the chemical structure of the interlayer was important in the solubility and diffusion of carbon atoms and consequently in instability of sp3 and sp2 bonding. Results demonstrated that 40 nm Cr interlayer, not only improved the adhesion of the DLC films but also generated the lowest friction coefficient, better wear resistance and the highest hardness (27 GPa).

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Corresponding author : E-mail address: [email protected] ; Tel: +98 9128130286

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Keywords: Diamond-like carbon; surface roughness; Atomic force microscopy; Raman spectroscopy; Nanoindentation test; Nanoscratch test

1. Introduction Diamond-like carbon (DLC) is an amorphous form of carbon which has many excellent properties of diamond-like high hardness, wear resistance, chemical inertness, and low coefficient of friction [1-4]. DLC films consist of a mixture of sp2 and sp3 carbon bonds in which the ratio of sp2/sp3 contents affects the properties of DLC film. [5, 6]. There are various techniques to synthesize DLC films [7-12], among them plasma-enhanced chemical vapor deposition (PECVD) is an important method [13-16] because it can deposit the films at low temperatures and on complex shaped items[17]. The main drawback in performance of DLC coating is high residual stress which causes poor adhesion to the substrate and thus decreases in its durability [2, 6]. To overcome this limitation, various strategies have been investigated such as element doping to the films [18-21], applying metallic and graded layers between DLC film and substrate [16, 22-24], surface implantation [25, 26] and surface thermal treatments [27]. Although, incorporating metal into DLC film is a simple and promising method for improving the adhesion, but it seems to reduce the wear resistance and increase friction coefficient of the coating [28, 29]. During thermal annealing, a gradually increasing Raman G band can generally be observed which corresponds to a graphitization process which decreases sp3 bonds and thus hardness [30-32] . Therefore, applying interlayers seems to have more benefits and thus it is essential to understand the important parameters and mechanisms of the buffer layers on the performance of DLC films. Wei et al. investigated the metal interlayers of Cr, Ti and Al on adhesion of DLC films on silicon, glass and steel substrates [16, 29]. They referred to the thermal coefficient of expansion of the interlayer and substrate as the important parameter on the stress reduction of the films.

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Another research by Gayathri et al. was about the effects of Cr, Ag, Ti and Ni bilayers and multilayers on reducing the stress in DLC films using pulsed laser deposition technique [6]. They observed a significant reduction of the internal stress in the presence of metal interlayers in which the metal bilayer showed stronger effect than those multilayers. They reported that the chemical nature of these metal as the important parameter by changing the formation and stabilization of sp2 cluster size. It seems that the effect of surface roughness parameters such as RMS roughness, average height, etc. on adhesion strength, morphology and mechanical behavior of DLC films was not properly noticed. In this study, the surface parameters of interlayers were investigated by atomic force microscopy device and an image analyzer software (enable detailed analyzing of surface parameters of interlayer), nanoscratch and nanoindentation tests (for an accurate investigation of adhesion and mechanical properties of DLC films) and X-ray photoelectron microscopy (for a close survey of chemical bonding of the films). Two different metal interlayer, Cr and Ni which have been recommended interlayers, were chosen to deposit on the substrates in nanometric thicknesses. Comparison between surface roughness parameters of these interlayers and morphology, adhesion, wear resistance and mechanical properties of the final coating showed that both chemical structure and surface parameters of interlayers were effective.

2. Experimental details The p-type silicon wafers were used as the substrates and at first, each on them were ultrasonically cleaned using acetone, ethanol, and deionized water for 10 minutes each. Ni and Cr nano layers were coated on the substrates by high vacuum DC&RF magnetron sputtering system (Model: MSS160, ACECR (Sharif university branch)). The sputtering

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system was equipped to a quartz crystal which monitored and controlled the thickness of the deposited layer. The details of experimental conditions are summarized in Table.1. After metallic deposition, atomic force microscopy (AFM, Autoprobe CP, in contact mode) was applied for topographical and surface roughness measurement of metallic nanolayers. RMS roughness, average roughness, mean height and particle size distribution was obtained from data of AFM processed by WSxM software [33]. For the growth of DLC films, a homemade DC plasma-enhanced chemical vapor (DC-PECVD) deposition system was used which was fully described elsewhere [14]. After loading the samples in the CVD chamber, methane gas as a hydrocarbon source with flow of 20 sccm was fed in the stainless steel CVD chamber to enhance the nucleation by pre-carburizing the substrates which is mentioned in the Table. 1. as a pre-implantation step [25]. Then, for synthesizing the DLC films, a mixture of methane and argon gases with flow of 40 sccm was entered into the growth chamber. Samples were named according to type and thickness of interlayers in which Cr10, Cr20 and Cr40 belonged to samples with 10, 20 and 40 nm thickness of Cr interlayers and Ni10, Ni 20 and Ni40 were assigned to 10, 20 and 40 nm thickness of Ni interlayers, respectively. The morphology and thicknesses of DLC films were studied using Ziess Sigma VP Field Emission Scanning Electron Microscopes (FE-SEM). Raman spectroscopy (Nd:YAG laser with wavelength of 532 nm) was used to measure the atomic bonds of DLC films. The spectra were fitted with Gaussian peaks to measure the position and intensity of the peaks. X-ray photoelectron spectroscopy was performed on an AXIS Nova (Kratos Analytical, Manchester, UK) utilizing a monochromatic Al Kα X-ray source operated at 225 W (15 kV, 15 mA). Survey spectra were collected at 160 eV pass energy while highresolution spectra were collected at 20 eV pass energy. Nanoscratch and nanoindentation tests were used to measure the friction coefficient, adhesion, hardness and elastic modules of

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the films. A Hysitron Inc. TriboScope® Nanomechanical Test Instrument with Berkovich indenter was used. The AFM (NanoScope E from Digital Instruments, USA) was a part of the instrument. X-ray diffraction (XRD) with Cu-Ka radiation (k = 1.54060 A˚), using an STOE– XRD diffractometer was used to analyze the residual stress of the DLC films according to the peak shifts of the Si substrate.

3. Results and discussion 3.1. Morphology of DLC films The evolution of morphology of DLC films which were synthesized on different metallic and different thickness of nano layers is shown in Fig. 1. For better comparison and observation of top-view and cross-section images of samples, they were loaded in the FESEM chamber by a sample holder with an angle of 60º. On Cr nanolayers (Fig. 1 (b, d and f)) a dense and smooth coating were formed, in which the thickness of the coating increased by increasing the thickness of Cr nanolayer. Mean thicknesses of coatings which were estimated by Digimizer image analysis software were 45nm, 80nm and 100 nm for Cr10, Cr20 and Cr40, respectively. On the other hand, the surface morphology on Ni nanolayers is rather rough. For Ni10, FESEM images showed scattered carbon grains all over the samples which were grown and became bigger in Ni20. In Ni40, the uniform grains were observed. Actually, As the grains grow, they got into more sphere and compacted ones. The mean thicknesses of the coatings formed in Ni10, Ni20 and Ni40, estimated by Digimizer software, were around 280, 230 and 165 nm, respectively. Thus, on the Ni nanolayers, there was a decrease in coating thickness by increasing the nanolayer thickness which was the inverse of Cr interlayers. This is due to changes in the morphology of the coating in Ni samples. These results indicated that the morphology of DLC films was affected by both material and thickness of interlayer. According to nucleation and the growth mechanisms of 5

CVD diamond films on various substrates; metals have different behavior during these processes which is related to C diffusion rate and the uniformity and thickness of C on the surface [34-36]. It was reported that in metals like Cr which form stable carbides, the nucleation starts after the surface is carburized. For substrates which do not form stable carbides but can dissolve C (like Ni), C diffusion occurs only after the entire substrate is saturated [34, 37]. It seems that both the chemical (atomic structure) and physical (thickness and surface roughness) properties of interlayers play an important role in the morphology, microstructure and mechanical properties of the final coating. For better comparison, Ni40 and Cr40 were selected and closely analyzed

3.2. AFM study of interlayers (Surface parameters of interlayers) At the first, surface parameters of Ni and Cr nanolayers with a thickness of 40 nm before loading in CVD chamber were analyzed by AFM. Fig. 1 represented the 3D AFM images on a 3µm × 3µm area of Ni nanolayer (Fig.1, a) and Cr nanolayer (Fig.1, b). Clearly, Cr nanolayer showed lower surface roughness than Ni nanolyer with lower Z value. Kinetically, Lower surface roughness causes more sites for nucleation and growth of carbon atoms. This means more carbon nucleation and better formation of the film. Root mean square (RMS) roughness is the most important roughness parameter and usually the only considered surface parameter [16, 38, 39] . However, advance studies showed that the determination of distribution of heights (particle sizes) is also important for describing surface asymmetry and flatness features [40, 41]. When the height points are uniformly distributed (a homogeneous distribution), which happens when RMS and average roughness values were the same. If these values were different, the height points distribution is not following a normal distribution [42, 43]. In this study, the particles size distributions

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parameter (height points distribution) was determined by WSXM software and were illustrated in Fig. 2 (d) [33]. As it was respected from AFM images (Fig. 2 (a,b)), the RMS roughness and average height of Cr layer, with amounts of 0.29 and 2.2 nm, respectively, were lower than Ni layer which had 2.47 and 8.93 nm, respectively. In Fig. 2 (d), the histograms of particles height distribution on each metallic layer are represented. On Cr nanolayer, a sharp and homogenous distribution with a peak centered around 2.2 nm existed while on Ni nanolayer, it was a wide and asymmetric distribution which its maximum events occurring in 5.75 nm. The different values of average height and the peak of height distribution is related to heterogeneous distribution of Ni particles. Heterogeneous distribution of heights describes surface asymmetry and flatness features [43]. These features seem to be effective in the nucleation, growth and final structure of the film. A normal distribution of height points expresses uniform valleys or nucleation sites. More nucleation sites (which demands a lower roughness) in small range of sizes (shaper distribution) could lead to a better uniform film formation. This event is in agreement with the final morphology of DLC film on Cr layer. The surface of Cr interlayer had lower roughness than Ni surface layer which was rather rough with small grains. These roughness parameters are important and should be considered because the morphology of DLC films effects on some of their final mechanical properties [44-46]. Therefore, physical parameters of the interlayers like surface roughness beside their chemical nature play roles in final properties of DLC films.

3.3.Microstructure of DLC films The microstructure of the synthesis films was studied using Raman spectroscopy. Raman spectroscopy is an important and non-destructive technique for characterization of carbon materials. In Raman spectra, a peak around 1350 cm-1 is related breathing modes of sp2 rings (D-band) and a peak around 1550 cm-1 is related to the sp2 bond stretching mode 7

both in rings and chains (G-band) [6, 47, 48]. The typical Raman spectra of samples shown in Fig.3 and the results of Gaussian fitted data were listed in Table. 2. The peaks position, the full width at half maximum of the G-band (FWHM (G)) and the relative intensity of D and G peaks are (ID/IG) important factor determining the properties of DLC films [17]. The lower the intensity ratio ID/IG in Cr40 indicate that the sp2 sites are arranged in chains than in rings [49]. Decreasing the ID/IG ratio with shifting the position of G peak to the lower wavelength correlates increasing of sp3 content. Thus, decreasing of the ID/IG from 0.95 in Ni40 to 0.82 for Cr 40 indicates the increasing of sp3 content in Cr40 [50-52]. Another important parameter is FWHM (G) which is proportional to the mechanical properties of the film [49, 53]. This recent fit parameter is related to the structural disorder in DLC films and is in direct relation with sp3 bonding content [53]. FWHM (G) in Cr40 was higher than Ni 40 (86 to 83) which is in agreement in indicating of the increasing of sp3 bindings [54]. Thus, Cr interlayer obtained better DLC film quality because of a lower value of sp2

carbon clustering than the Ni interlayer (Ni40). It has been reported that the chemical nature of the substrate plays an important role in the stabilization of sp2 or sp3 bonded phases [34]. Cr ([19] 3d5 4s1) and Ni ([19] 3d9 4s1) belongs to the same period but different group of periodic tables. Goldschmidt was noted that the 3d shell gradually fills up across the period while the stability of the carbides decreases [55]. In Cr, six electrons, which obtained from the 3d and 4s orbitals are utilized in bonding, which is two bonding electrons in Ni. The number of bonding electrons affects the stability of carbides in the transmittal metals. Since the C-C bond strength increases in the series from Mn to Zn, graphite is more stable in Ni rather than in Cr [34, 56]. Therefore, more stable sp2 bonding forms on Ni than Cr which is due to the chemical nature of these elements and causes formation of softer coating on Ni. This result is in agreement with others which reported more sp3 binding and harder carbon coatings on Cr substrate or interlayer than Ni [6, 57].

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For investigating the results of Raman spectroscopy and a better comparison of surface bonding states of the samples, XPS spectroscopy was employed. The XPS C1s spectra in Fig.4 show asymmetrical peaks which are related to the various bonding states of carbon atoms [58, 59]. These peaks were deconvoluted into five peaks included 283.5, 285.2, 286.5, 287.5 and 289 eV which are respectively assigned to metal carbide, C-C sp2, C-C sp3, C-O and C=O [18, 60-62]. The ratio of C2 peak area to total C1s for Cr40 (12.4%) and Ni40 (7.6%) which is used to estimate sp3 hybridized carbon atom, confirmed the higher amount in Cr40, this result is in agreement with Raman results. Carbide and oxides peaks were observed in both samples which others studies showed they are effective in final properties of DLC films [29, 63].

3.4.Adhesion and friction coefficient of DLC films Adhesion strengths of DLC films in Cr40 and Ni40 were compared by the nanoscratch test. Each scratch test was performed by moving the probe laterally a distance of 4 µm while concurrently ramping the normal force from 0 to 500, 1000 and 2000µN for the duration of 30 seconds. In order to investigate the damage incurred from the test, the sample surfaces were probed using atomic force microscopy [64]. Variation of the coefficient of friction (COF) of the samples was measured by changing the ramp force and were used to investigate the wear behavior and adhesion of DLC films to the substrates. Fig. 5 illustrates the changing of COF by increasing the normal force up to 1000 µN and AFM images after applied force to DLC films on Ni interlayer (Ni40) and on Cr interlayer (Cr40). The coefficient of friction of Ni40 had significant deviations during the scratching but there was a miner fluctuation in Cr40 which represented better wear behavior of the coating on Cr40. For comparing the friction coefficient of DLC films of samples, the average COF were considered in the scratch distance of one to three micrometers. This value was 0.20 and

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0.14 for Ni40 and Cr40, respectively. It seems that lower surface roughness parameters of Cr interlayer caused the lower friction coefficient. The change in the friction coefficient with different surface roughness was reported in other works as well [46, 65, 66]. Bai et al. found when the surface roughness of the DLC film increases, its abrasive wear is highly promoted and causes the friction force increases [66]. Shioga et al. applied different nitride layers between DLC and substrate which resulted in different surface roughness of DLC film. Roughness was observed to affect the tribological performance of DLC films in which lower RMS led to the lower friction coefficient [65]. The in-situ AFM image of nanoscratch test showed delamination in Ni40 after 3.5 µm of starting the scratch and at the normal load of 800 µN which is known as the critical load (LC) [5, 60, 67]. The value of LC is the load in which the initial critical event to film delamination occurs. Determination of LC is a technique to compare the adhesive strength of the films [5, 68]. Therefore, the results of nanoscratch test confirmed better adhesion of DLC films on Cr interlayer. The better adhesion result of DLC films of Cr40 could be explained by both surface roughness and chemical nature of interlayers. The two important mechanisms of adhesion are mechanical interlocking and chemical bonding in which According to adhesion mechanisms, chemical bonding is one of the important and strong mechanism in the adhesion of a coating [13, 69, 70]. Here, the formation of carbides indicated the presence of the strong chemical bonding between the coating and interlayers which was reported also by others [29, 71]. The surface area of C5 peaks in XPS results (Fig.4) are related to metal carbides revealed higher amount for Ni40. However, oxide peaks should be considered. Studies showed that the oxidization of metal interlayer weakens the adhesion strength and leads to the peeling of the films [29, 63]. C3 and C4 are related to oxidation of metal layers which showed higher amount in Ni40. Therefore, the main reason, here for weaker adhesion of Ni40 was related to existence of more oxidation on its interlayer.

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Wei at al. also reported that the thermal stress in the DLC film is linearly affected by the roughness. A rougher surface induces higher stress which is detrimental for deposition [22, 72]. Therefore, a Cr interlayer with lower roughness has lower stress, which causes a better adhesion [40]. According to the later model about surface roughness, the height points distribution could affect the stress reduction of DLC films and improve the adhesion strength. A narrow and homogeneous distribution of nucleation sites in the interfacial of interlayers with the coating causes better stress reduction. Cr interlayer had a sharper height points distribution and lower RMS roughness than Ni interlayer which led to better stress reduction and stronger adhesion strength.

3.5.Hardness and elastic modulus of DLC films To determine hardness and elastic modulus of the samples, nanoindentation test was accomplished by forcing a diamond pyramidal probe into the film to a specified force, holding the force for five seconds, and then withdrawing the probe [64, 68]. The loaddisplacement curves as can be seen in Fig. 6 were used to determine the hardness (H) and elastic modulus (E). The force which was used in these samples was 500µN based on penetration depth of indenter that must be less than around 10% of the total thickness of the film to avoid substrate effects [30, 50, 73] . The higher indentation depth in sample Ni40 indicated lower hardness of this film. The nanoindentation results were listed in Table. 3. The improvement in hardness and elastic modules in Cr40 properties are related to increasing of sp3 content in DLC film [60, 74, 75] . The results of Raman spectroscopy and XPS revealed the higher sp3 fraction in Cr40 than Ni40. DLC films with a higher proportion of sp3 fraction lead to higher H and E values and consequently have better mechanical properties [4, 76, 77]. knowing E and H, the wear behavior of DLC films can be evaluated. Better behavior obtained from the high value of H3/E2. A low value of this ratio is an 11

indicator of a more presentation of plastic behavior of the investigated film. Many of the mechanisms of film failure begin with or directly involve the plastic deformation. According to Table. 3, Cr40 with higher ratio of H3/E2 exhibited more elastic behavior in compare with Ni40. which lead to better performance when used for wear-resistant applications. These better mechanical properties of Cr 40 were correlated with FWHM (G) [49, 50, 78-80]. It was found in this study that the lower surface roughness of the DLC film not only caused the lower friction coefficient but also improved the wear resistance of the tribosystem. This is most likely due to the lower degree of the contact area of the surface [81]. Some researchers reported a relationship between the surface roughness of the DLC film and wear friction force in hard coating materials [65, 66, 82]. However, these parameters are not material constants of the bodies in contact and a simple relationship between them should not be expected, especially in the soft coating materials, which wear can be affected by transfer effect [83].

3.6 Residual stress measurement Residual stresses can be determined from the XRD data by calculating the strain from the diffraction peak positions. Uniform strain shifts the diffraction peak while a non-uniform strain can alter both the peak shape and position. Here, DLC films were deposited on single crystal Si substrates. The shift of silicon diffraction peak position (the diffraction angle of around 69° was selected as the “strain gauge”) to higher or lower diffraction angles depending on the nature of the residual stresses (compressive or tensile stresses) [84-86]. Fig. 7 shows the XRD patterns of a raw Si (400) substrate, DLC coating on Cr (Cr40) and Ni (Ni40) nano interlayers. There is a shift to higher diffraction angles for both samples which is higher amount for Ni 40, demands more compressive stress in this sample. It is obvious that the diffraction peak of Ni40 is also wider than of raw Si or Cr40, which reveals a non-

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uniform strain. This is in agreement with the discussed point in the section 3.4 about better stress reduction in a sharp and homogeneous height points distribution of Cr40 than Ni40. The magnitude of residual stresses (σ) is calculated by [87, 88]: −

= − Where , ,



are Young’s modulus (obtained by Table. 3), Poisson ratio, the lattice

spacing with stress, and the lattice spacing without stress (the reference), respectively. As is listed in Table. 4, the residual stress in Cr40 had the lower value than that of Ni40.

4. Conclusions In this work, DLC films were deposited on silicon substrates using nickel and chromium nano layers as a buffer layer. A study between the effect of chemical structure and surface parameters of the interlayers on the morphology, coefficient of friction, hardness, wear resistance and adhesion strength of DLC film were carried out. The following conclusions were drawn from this work: 1- The morphology of DLC films on Cr interlayers was smooth while on Ni interlayers grain structures were grown. Increasing the thickness of interlayer, increased the thickness of DLC films on Cr interlayers which for Ni interlayers, grain sizes grown. 2- Both the chemical (atomic structure) and physical (thickness and surface roughness) properties of interlayers played a role on final morphology DLC film. 3- The AFM results of interlayers showed the lowest surface roughness with a sharp and homogenous particle size distribution on Cr nanolayer which lead to formation of smooth DLC film. 4- The results of Raman spectroscopy and XPS analysis indicated more sp3 bonding on Cr nanolayer.

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5- Nanoscratch test showed better adhesion and wear resistance of DLC film on Cr nanolayer which was due to the lower surface roughness and then better stress reduction of this nanolayer. 6- It was found from nanoindentation test that the hardness of DLC films on Cr nanolayer with amount of 27GPa was better than on Ni interlayer with amount of 11GPa which was in agreement with Raman and XPS results

5. Acknowledgement The authors would like to thank professor S. K. Sadrnezhad for their valuable assistance and cooperation .This work was performed in part at the Australian National Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy, through the La Trobe University Centre for Materials and Surface Science.

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Table. 1: Details of deposition conditions of each experiment step.

Step Conditions

Cr Interlayers

Ni Interlayers

Pre-Implantation

DLC Deposition

Method

DC magnetron sputtering

RF magnetron sputtering

DC Plasma Enhancement CVD

DC Plasma Enhancement CVD

Initial Pressure

8 × 10-5 mbar

3 × 10-5 mbar

3 × 10-4 mbar

3 × 10-4 mbar

Working Pressure

10 mbar

10 mbar

1 mbar

3 mbar

Temperature

67-80 °C

23-30 °C

150 °C

300 °C

Voltage/Current Or Power

450 v 125-170 mA

69w

600 v 70 mA

Gas/Gases

Ar

Ar

CH4

500 v 20-21 mA CH4+Ar (=20% vol +80% vol)

deposition Time

Depends on thickness

Depends on thickness

10 min

-2

-2

30 min

Table 2. Position and FWHM of D and G bands and ID/IG ratio of deposited samples. D band -1

Ni 40 Cr40

G band -1

-1

Position (cm )

FWHM (cm )

Position (cm )

FWHM (cm-1)

1335 1343

211 206

1583 1582

83 86

0.95 0.81

Table 3. Variation of hardness (H), modules (E) and the coefficient of friction (COF) of samples. Sample Ni40 Cr40

H (GPa)

E (GPa)

COF

H3/E2

11.11 27.15

139.3 377.0

0.20 0.14

0.07 0.14

Table 4. XRD data of samples of a raw Silicon wafer, DLC coating on 40nm Cr (Cr40) and Ni (Ni40) nanolayers Sample Si (reference) Cr 40 Ni 40

Position (°°2Th.)

69.2469 69.2600 69.4670

d-spacing (Å)

FWHM (°°2Th.)

σ (GPa.)

1.35685 1.35663 1.35197

0.3434 0.3478 0.6720

0.29 1.62

20

Fig. 1. Top-view and cross-section FE-SEM images of DLC films growth on a) Ni10 with 10 nm Ni interlayer, b) Cr 10 with 10 nm Cr interlayer, c) Ni20, d) Cr20, e) Ni40 and f) Cr40 with 40 nm Cr nanolayer.

21

Fig. 2. 3D AFM images on a 3µm × 3µm area of Ni nanolayer (a) and Cr nanolayer (b). Comparing the surface roughness and average height of Ni (Ni 40) and Cr (Cr40) films(c). The distributions of metal nano particles sizes on the samples inserted in the figure(d).

Fig. 3. Raman spectra of DLC films on Ni40 and Cr40

22

Fig. 4. Deconvoluted spectra of C1s peaks for (a) Cr40 and (b) Ni40

Fig. 5. AFM image after nano scratch test on (a) Ni40, (b) Cr40 and (c) Changing the friction coefficient by increasing ramped force from 0 to 1000µN.

23

Fig. 6. Load–depth curves obtained with the applied load of 500µN for Ni40 and Cr40

Fig. 7. XRD spectra of reference sample (Si) and DLC coated samples

24

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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

none