DLC nanocomposite films fabricated by unbalanced magnetron sputtering

Applied Surface Science 284 (2013) 165–170

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Preparation and properties of Ag/DLC nanocomposite films fabricated by unbalanced magnetron sputtering Yanxia Wu a,b , Jianmin Chen a,∗ , Hongxuan Li a,∗∗ , Li Ji a , Yinping Ye a , Huidi Zhou a a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 10 April 2013 Received in revised form 9 June 2013 Accepted 15 July 2013 Available online 22 July 2013 Keywords: Diamond-like carbon (DLC) Silver incorporation Microstructure Mechanical properties Tribological properties

a b s t r a c t Silver (Ag)/diamond-like carbon (DLC) nanocomposite films with different Ag concentrations ranging from 0 to 11.02 at.% were prepared by medium frequency unbalanced magnetron sputtering, in which the mixed Ar/CH4 of different volume ratios were used as the source gases. The doping effects of Ag concentration on microstructure, mechanical and vacuum tribological properties of the DLC films were investigated. It is found that the Ag concentration increased with the increasing Ar/CH4 ratios, accompanied with the increasing number and size of Ag crystalline. With moderate incorporation of Ag at 3.55 at.%, the film (deposited at the Ar/CH4 = 65/45) maintained a low internal stress without considerable decrease of hardness and thus improved the adhesion strength. Moreover, the film showed low friction coefficient and the longest sliding lifetime in vacuum. The significant improvement in tribological properties of Ag/DLC nanocomposite films with moderate Ag concentration can be attributed to the low shear strength of Ag clusters on the surface, as well as the diffusion of Ag from the bulk to the surface and wear track. © 2013 Elsevier B.V. All rights reserved.

1. Introduction During the last two decades, diamond-like carbon (DLC) films have attracted significant research attention for their wide applications as protective layers because of their high strength, low roughness and excellent tribological properties [1]. The incorporation of DLC films with metals (Ti, Mo, Cr, W, Au, Ag, Al, Si, etc.), which can reduce the internal stress to improve the adhesion and tribological properties have gained increasing attention as a new area of DLC research [2]. Amongst them, the Ag-incorporated DLC films demonstrate promising candidate for protective layers because of their low friction coefficient, good wear resistance, hemo-compatibility and antibacterial properties [2–5]. The excellent tribological properties of the Ag-incorporated DLC films can be attributed to the following factors: For one thing, because of the face-centered cubic structured, Ag nanoparticle has a significantly smaller elastic modulus than DLC, and it may absorb compressive stress from the DLC matrix. Meanwhile, the toughness of composite film may be increased and the formation of bonds between the nanocrystallite and the matrix may be diminished. For another, the soft metallic Ag with low shear strength film onto a hard metallic surface can modify the contact interface and decrease friction

∗ Corresponding author. Tel.: +86 931 4968422; fax: +86 931 8277088. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Chen), [email protected] (H. Li). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.074

coefficient due to plastic flow in the process of relative slip [6]. However, researches on the Ag-incorporated DLC film are mainly focused on the mechanical and air friction tribological properties up till now. The tribological behaviors of such composite films in high vacuum correlated with the corresponding microstructures and mechanical properties have seldom been reported. Additionally, various techniques have been used to synthesize the Ag/DLC films [2,7–10], but the magnetron sputtering with Ar and CH4 as precursor combined the advantages of both magnetron sputtering and PECVD is merely reported. For this method, on one hand, the ionization of the precursor was induced by the glow discharge of the graphite and silver targets. On the other hand, the hydrocarbon particles adsorbed on targets would cause target poisoning and thus decrease the sputtering yields of the graphite and silver targets [11]. Therefore, the concentration of the Ag in the film can be controlled by varying the volume ratios of the source gases. Furthermore, the DLC based films prepared by this method can maintain low internal stress, high load capacity, low friction and wear properties as reported previously [12]. In this work, we fabricated the Ag/DLC composite films with Ag concentrations ranging from 0 to 11.02 at.% by medium frequency unbalanced magnetron sputtering technique, and the mixed Ar/CH4 gases of different volume ratios as the source gases. The surface morphology, microstructure, mechanical and vacuum tribological properties of the film were investigated. The results revealed that the film deposited at the Ar/CH4 = 65/45 (sccm/sccm) with moderate Ag concentration of 3.05 at.% shows the highest

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hardness and lower internal stress, as well as the excellent tribological properties in vacuum. 2. Experimental details

of the substrate. The curvature radii of the composite film were measured using a MicroXAM surface mapping microscope (ADE shift, America) and the internal stress was calculated based on the Stoney equation [14]: Es 6(1 − s )



ts 2 tf



1 1 − R2 R1



2.1. Deposition of the Ag/DLC composite films

=

Ag/DLC composite films with thickness about 1 ␮m were deposited on the stainless steel (used for friction and wear tests) and Si p(1 1 1) (used for characterization) substrates. The deposition parameters of the film with Ag concentration of 0 at.% (pure DLC film) were reported as previous [13]. For the deposition of the other films, Ar (purity 99.9%) and CH4 (purity 99.9%) were used as sputtering gases of one graphite target (size 94 mm × 300 mm, purity 99.9%) and one target consisted of a stack of Ag and graphite bars. The number of Ag bar to that graphite one was equivalent to the target area ratio, was set to 1/8. Prior to film deposition, the substrates were cleaned ultrasonically in an acetone bath and dried in air. Then the substrates were assembled in the substrate holder which is 20 cm away from the sputtering targets, followed by plasma etching with Ar+ ions in a vacuum chamber to remove the native oxide on the substrates’ surface. The substrate holder kept revolving (5 rev/min) during the deposition process, which would be beneficial to improve the uniformity of the composite films. The chamber temperature during the deposition was in the range of 60–80 ◦ C monitored by a thermocouple inserted into the substrate holder. A Si target (99.99%) was connected to medium frequency pulsed DC power supply to deposit a Si interlayer. Prior to the film deposition, the silicon interlayer, about 200 nm thick, was deposited on substrates by magnetron sputtering (a target current of 8 A) to improve the adhesion between the substrate and film. Then film deposition was performed under substrate bias voltage of −200 V with duty cycle of 20% for 3 h. Typical deposition parameters were listed in Table 1. Special samples for TEM analysis, with the thickness about 30 nm, were grown directly on the freshly cleaved single-crystal NaCl wafers.

where  is the internal stress, R2 is the curvature of the film deposited on Si wafer. The s and Es are Poisson ratio and Young modulus of the substrate, ts and tf as the thickness of the substrate and film, respectively. The curvature of the Si flat wafers (R1 ) before deposition of the films is assumed to tend to infinite. Young’s modulus of 131 GPa and the Poisson ratio of 0.278 are adopted for Si p(1 1 1) substrates, respectively. The thickness of the film (tf ) could be measured by MicroXAM-3D surface profiler.

2.2. Characterization of the Ag/DLC composite films The composition of the film was determined using a multifunctional X-ray photoelectron spectroscope (XPS, operating with Al-K␣ radiation and detecting chamber pressure of below 10−8 Torr). The film was etched 3 min by Ar+ ion to ensure that the tested material represented bulk coating. The atom concentrations of the sample were calculated from XPS signals corresponding to Ag (3d), Au (1s) and C (1s) core levels using the standard sensitivity factors of the instrument. The bonding structure information of the Ag/DLC film was obtained by Jobin-Yvon HR-800 Raman spectrometer at an argon-ion laser source power density of 0.3 mW/m2 , having wavelength of 532 nm. The crystallographic structure of the as-deposited film was studied by conventional Bragg–Brentano X-ray diffraction (XRD) using a Philips X’Pert-MRD type diffractometer with a Cu tube operated at 40 kV and 60 mA. A JEOL 2010 transmission electron microscope (TEM) was performed at an accelerating voltage of 200 kV to record high resolution TEM images (HRTEM) of Ag/DLC samples which were firstly deposited on NaCl substrate and then collected for TEM analysis after NaCl substrate were dissolved in distilled water. The surface morphology of the film was examined using a JSM-6701F cold field Scanning electron microscope (FESEM). The wear surface was characterized by JSM-5600LV scanning electron microscope (SEM). The hardness value of the composite film was determined using a NanoTest 600 nanomechanical system (MicroMaterials Ltd., UK), where the maximum indentation depth was controlled to be about 90 nm (less than 10% of film thickness) so as to minimize the effect

2.3. Ball-on-disk friction and wear test The tribological properties test of the film was evaluated with a ball-on-disk tribometer equipped with a vacuum chamber. The photograph and schematic diagram of the apparatus have been described elsewhere [15]. The steel ball (GCr15, ˚ 6 mm) was used as the counter body. Briefly, sliding tests of the film against steel ball counterpart was run at a normal load of 5 N, a sliding speed of 300 rev/min, a room temperature of about 20 ◦ C and a maximum sliding duration of 3600 s while the wear track radius was fixed at 6 mm. As to the friction and wear tests in vacuum environment, a pressure of about 5.0 × 10−3 Pa was attained in the chamber with a turbomolecular pumping system. Before each friction and wear test, the frictional pair was ultrasonically cleaned with acetone. Friction experiments were conducted thrice under each experimental condition. 3. Results and discussion 3.1. Composition and bonding structure The chemical compositions of the samples have been analyzed by XPS, and the relative atomic concentrations of Ag were shown in Table 1. It can be seen that the concentration of Ag in the films increased from 3.55 to 11.02 at.% with decreasing volume fraction of CH4 in the Ar/CH4 gas mixture, which is attributed to the decreased target poisoning and thus the increased sputtering yield of the graphite and silver targets. With the highest volume fraction of CH4 in the gas mixture (Ar/CH4 = 65/45), a silver concentration of 3.55 at.% was found. Similar results were reported before [8,16]. However, the deposition rate of the film was mainly attributed to the strong etching effect of Ar+ on the film. With increasing volume fraction of Ar in the Ar/CH4 gas mixture, the deposition rate decreased (Table 1). Fig. 1(a) shows the Ag 3d5/2 XPS spectra of the composite films, the peak located at 368.4 eV indicated that the Ag specie was distributed in carbon network in the form of metallic phase. This proved the existence of Ag nanoclusters in the a-C:H matrix. As shown in Fig. 1(b), the increased sp2 C concentration with the increasing Ag concentration was attributed to the formed Ag nanocluster could absorb the compressive stress from the hydrogenated amorphous carbon matrix and thus reduce the carbon densification [17]. Raman spectra (see Fig. 2) obtained from the composite films show two prominent features of DLC films: the D (disorder) line around 1350 cm−1 and the G (graphite) line around 1580 cm−1 [18]. Table 2 summarizes the characteristics of Raman spectra for the asdeposited film. It can be seen that the incorporation of Ag resulted in the increase of the intensity ratio of D peak and G peak, I(D)/I(G) and the narrowing of G peak, which can be attributed to an increase

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Table 1 The deposition conditions and chemical composition of the as-deposited films with different Ag concentrations. Ar/CH4 ratio (sccm/sccm)

Deposition pressure (Pa)

Targets current (A)

Deposition rate (nm/min)

[Ag] (at.%)

[C] (at.%)

65/45 65/45 75/35 85/25

0.53 0.53 0.53 0.53

14 (C) 14 (C+Ag) 14 (C+Ag) 14 (C+Ag)

5.71 4.47 4 3.27

0 3.55 7.05 11.02

100 96.45 92.95 88.98

Fig. 1. XPS spectra of the as-deposited films with different Ag concentrations: (a) Ag3d and (b) C1s.

Table 2 The Raman parameters of the as-deposited films with different Ag concentrations. Parameter

Ag concentration (at.%)

−1

G peak position (cm ID /IG ratio D FWHM (cm−1 ) G FWHM (cm−1 )

)

0.00

3.55

7.05

11.02

1549 0.30 127.00 170.00

1556 0.33 318.78 140.93

1563 0.86 379.18 124.95

1573 1.02 406.75 107.30

of the average crystalline size of sp2 -bonded clusters [19]. In addition, the position of G peak shifted over a considerable range from 1549 to 1573 cm−1 as the Ag concentration increased from 0 to 11.02 at.%. All these results confirmed the increase of the graphitelike bonds in hydrogenated amorphous carbon matrix as convinced by XPS [20], The effect of Ag concentration on the microstructure of the as-deposited films can be characterized by the X-ray diffraction patterns, as shown in Fig. 3. The pure DLC film exhibited a broad hump like shape obtained in the range of 30◦ –45◦ due to the amorphous mature of carbon. The Ag doped films exhibited crystalline phase of Ag with a predominant (1 1 1) orientation. Since Ag was not capable of forming a carbide as convinced by XPS, there were Ag crystal clusters within the carbon matrix [8]. The relative intensity of the Ag peaks increased with the increasing Ag concentrations. The film with Ag concentration of 3.55 at.% was showed a broad peak in the (1 1 1) direction of the silver phase, which demonstrated the crystalline had a smaller particle size. Furthermore, the films

Fig. 2. Raman spectra of the as-deposited films with different Ag concentrations.

with Ag concentrations higher than 3.55 at.% were showed sharply defined peaks indicating Ag phase of highly polycrystalline. In order to gain more details of the crystallization and grain size, the HRTEM was performed. Fig. 4 shows the HRTEM images and selected area diffraction (SAD) patterns of the as-deposited films with various Ag concentrations. For the pure DLC film, a uniform amorphous structure was observed as shown in Fig. 4a. The Ag clusters were found in the a-C:H matrix with the increasing Ag concentration. The darker spots correspond to the metallic Ag region, while the brighter region corresponds to the DLC matrix, respectively. Incorporation of 3.55 at.% Ag (Fig. 4b), Ag crystallines with a size of from 1 to 2 nm dispersed uniformly in the film. As the Ag concentration up to 11.02 at.% (Fig. 4c), the size of crystalline Ag is uneven, and large agglomerates with a diameter of approximately 5 nm were observed. These results were in agreement with the XRD analysis. Additionally, the Ag in the form of single crystal and polycrystalline dispersed asymmetrically in the composite films with Ag concentration of 11.02 at.% which were revealed by the ring patterns in the inset of Fig. 4c. Typical surface morphology images of as-deposited Ag/DLC films are shown in Fig. 5. The difference in brightness of the FESEM image allows us to distinguish between the particles (Ag) and the a-C:H matrix. With the increasing concentration of Ag in the film, the increments in the number and size of the lighter colored clusters (Ag clusters) on the surface were more notable, indicating an increasing roughness of the film. Furthermore, the film with

Fig. 3. XRD pattern of the as-deposited films with different Ag concentrations.

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Fig. 4. HRTEM micrographs and the corresponding SADE patterns indexing of the as-deposited films: (a) 0 at.% Ag; (b) 3.55 at.% Ag; and (c) 11.02 at.% Ag.

Fig. 5. FESEM surface images of the as-deposited films with different Ag concentrations: (a) 0 at.% Ag; (b) 3.55 at.% Ag; (c) 7.05 at.%; and (d)11.02 at.% Ag.

Ag concentration of 3.55 at.% contained many granular structure with similar sizes, while the films with Ag concentration higher than 3.55 at.% were mainly composed of islands with big sizes distributed on the surface. It was attributed to that the Ag particles had a tendency to form clusters on or near the surface during the deposition [21]. 3.2. Mechanical properties Fig. 6 shows the effect of the Ag concentration on the residual compressive stress and hardness of the as-deposited films. Ag doping led to the decreases in both residual compressive stress and hardness. However, the film with 3.55 at.% Ag incorporation was showed a significant reduction in the residual stress without a considerable degradation of hardness, implying that the DLC films with low concentration of Ag was effective from the perspective of mechanical properties as reported before [2]. The drastic decrease of the magnitude in residual compressive stress was mainly ascribed to the formation of metallic Ag amorphous phases (as shown in Fig. 4) acting as efficient buffer sites to absorb the stress in the DLC matrix. Further incorporation of Ag up to 11.02 at.% decreased the residual stress gradually down to 0.3 GPa, which can be considered as an analogy of the percolation structural transition of a transition metal incorporated DLC structure [16]. The decrease of hardness with the increasing Ag concentration can be regarded

Fig. 6. The internal stress and hardness of the as-deposited films deposited with different Ag concentrations.

as follows. Firstly, the soften effect of Ag deteriorated the mechanical properties. Secondly, the doped Ag as a locally independent unit, which is isolated by the amorphous carbon matrix, was bad for the overall connectivity of the carbon network and hence low hardness is obtained [22]. Thirdly, Ag doping in hydrogenated amorphous carbon matrix slightly degraded the fraction of the bridging sp3 hybrid carbon concentration in the films, as shown in Raman spectra, which led to the poor connectivity of the separate graphitic

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Fig. 7. The vacuum tribological performances of the as-deposited films.

clusters in the film structure and consequently low hardness was achieved [23,24].

3.3. Tribological performances Frictional characteristics tested by a ball-on-disk tribometer in vacuum condition were shown in Fig. 7. The test was performed at a normal load of 5 N and sliding speed of 0.19 m s−1 in vacuum (10−3 Pa). As the Ag concentration increases, there were little differences in friction coefficient (0.005) of the as-deposited films. However, doping Ag of low concentration in the film prolonged the vacuum sliding lifetime. Further increased Ag concentrations, the sliding lifetime of the film decreased. The film with 3.55 at.% Ag exhibited the best tribological properties in vacuum, which maintained the longest sliding lifetime of more than 3600 s, and low friction coefficient of 0.005 for the first 1000 s and then increased dramatically from 0.02 to 0.04. Then SEM was used to inspected the wear tracks of the 3.55 at.% Ag composite film after a sliding time of 200 s. As shown in Fig. 8(a), there were more bright particles identified as Ag in the sides of wear track than that outside of the track as reported before [25]. Elongated steaks of Ag were also observed in the direction of the wear track where its presence was required, allowing lubrication between the steel ball and the film. These morphologies were due to the high plasticity of silver and the high stiffness modulus of DLC. Fig. 8(b) presents the SEM image of the worn surface on mating ball. A distinct transfer film on the contact surface of the mating ball can be observed, and some loose particle-like wear debris with different sizes scattered around the transfer film. The transfer film on mating balls was relatively rich in Ag by EDS analysis, which was more effective to support load and reduce wear [26,27]. It indicated that the Ag is more likely to diffuse from the bulk to the worn area and form the transfer film. The transfer film acting as a solid lubricant which prevented the direct contact between the counterparts and

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hence significantly reduced wear, may be the result of long sliding lifetime in this stage. The friction evolution of Ag/DLC films with 3.55 at.% Ag has demonstrated that a friction coefficient of 0.005 and a long sliding lifetime of more than 3600 s can be achieved in vacuum. The excellent tribological performances were attributed to the lubrication combined the high stiffness modulus of DLC film with the high plasticity of Ag. During the early stage of frictional sliding, Ag clusters on the surface as well as the diffusion of Ag from the bulk to the worn area improved the anti-wear properties. As the tests conducted further, the Ag concentration on the worn surface decreased while the wear debris increased with the frictional losses. The wear debris trapped at the sliding contact interface and underwent severe physical grinding action increasing the thickness of transfer film on the mating ball [1]. Poor adhesion between the transfer film and the mating ball, arising from the thickening and non-carbide formers (Ag), may result in higher and fluctuated friction coefficient. The poor tribological properties of the films with higher Ag concentrations were caused by the increased graphitization and surface roughness, as well as the decreased mechanical properties. It was reported that the graphitization of DLC film may cause sever wear in vacuum [28,29]. In addition, the roughness of the film can affect the formation of transfer film [30] and the adhesion of the film [31], hence the vacuum triboligical properties of the film. On one hand, the rougher films with high Ag concentrations can cause abrasive of the steel ball during the running-in period, which impedes the formation of a stable transfer film and leads to great wear loss. On the other hand, the rougher surface offers a higher number of asperities, which may facilitate severe adhesion interactions between the sliding surface of the film and the steel counterpart. 4. Conclusions DLC films with various Ag concentrations were deposited by medium frequency unbalanced magnetron sputtering with mixed Ar/CH4 of different volume ratios as the source gases. A comparative study was carried out to investigate the Ag doping effect on the surface, mechanical and tribological characteristics. The results were showed that the number and size of the crystalline Ag phase dispersed in hydrogenated amorphous carbon films increased with the increasing Ag concentration. Furthermore, the sp2 /sp3 ratio in the films increased and thus the hardness and internal stress decreased. Importantly, the film with Ag concentration of 3.55 at.% was showed a significant reduction in the residual stress without a considerable degradation of the hardness compared with the pure DLC film. The sliding lifetime of the film in vacuum was sensitive to Ag concentration. A properly Ag concentration of 3.55 at.% in the film showed a long sliding lifetime of more than 3600 s in vacuum. The excellent tribological properties of the film were attributed

Fig. 8. SEM images of wear track and scar of the composite film with 3.55 at.% Ag.

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to the presence of low shear Ag clusters on the surface and the diffusion of the Ag from the bulk to the worn area. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51275509 and 51175491, the 973 Project of China (No. 2013CB632300), and the Chinese Academy of Science for financial support. References [1] A. Erdemir, C. Donnet, Tribology of diamond-like carbon films: recent progress and future prospects, Journal of Physics D: Applied Physics 39 (2006) R311–R327. [2] H.W. Choi, J.H. Choi, K.R. Lee, J.P. Ahn, K.H. Oh, Structure and mechanical properties of Ag-incorporated DLC films prepared by a hybrid ion beam deposition system, Thin Solid Films 516 (2007) 248–251. [3] F.R. Marciano, D.A. Lima Oliveira, R.S. Pessoa, E.C. Almeida, N.S. Da-Silva, E.J. Corat, V.J. Trava-Airoldi, Recent advances in nanoparticle-incorporated diamond-like carbon films, Diamond and Related Materials 18 (2009) 1010. [4] H.W. Choi, R.H. Dauskardt, S.C. Lee, K.R. Lee, K.H. Oh, Characteristic of silver doped DLC films on surface properties and protein adsorption, Diamond and Related Materials 17 (2008) 252–257. [5] K. Baba, R. Hatada, S. Flege, W. Ensinger, Y. Shibata, J. Nakashima, T. Sawase, T. Morimura, Preparation and antibacterial properties of Ag-containing diamondlike carbon films prepared by a combination of magnetron sputtering and plasma source ion implantation, Vacuum 89 (2013) 179–184. [6] Q. Wang, J. Tu, S. Zhang, D. Lai, S. Peng, B. Gu, Effect of Ag content on microstructure and tribological performance of WS2 –Ag composite films, Surface and Coatings Technology 201 (2006) 1666–1670. [7] C.P. Lungu, Nanostructure influence on DLC-Ag tribological coatings, Surface and Coatings Technology 200 (2005) 198–202. [8] K. Baba, R. Hatada, S. Flege, W. Ensinger, Preparation, Properties of Agcontaining diamond-like carbon films by magnetron plasma source ion implantation, Advances in Materials Science and Engineering 2012 (2012) 1–5. [9] H.-S. Zhang, Comparative surface and nano-tribological characteristics of nanocomposite diamond-like carbon thin films doped by silver, Applied Surface Science 255 (2008) 2551–2556. [10] C.P. Lungu, I. Mustata, G. Musa, V. Zaroschi, A. Mihaela Lungu, K. Iwasaki, Low friction silver-DLC coatings prepared by thermionic vacuum arc method, Vacuum 76 (2004) 127–130. [11] Y. Wang, Y. Ye, H. Li, L. Ji, Y. Wang, X. Liu, J. Chen, H. Zhou, Microstructure and tribological properties of the a-C:H films deposited by magnetron sputtering with CH4 /Ar mixture, Surface and Coatings Technology 205 (2011) 4577–4581. [12] Y. Wang, Y. Ye, H. Li, L. Ji, J. Chen, H. Zhou, A magnetron sputtering technique to prepare a-C:H films: effect of substrate bias, Applied Surface Science 257 (2010) 1990–1995. [13] Y. Wu, H. Li, L. Ji, L. Liu, Y. Ye, J. Chen, H. Zhou, Effect of vacuum annealing on the microstructure and tribological behavior of hydrogenated amorphous carbon films prepared by magnetron sputtering, in: Proceedings of the Institution

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