Effect of bias voltage on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering

Surface & Coatings Technology 205 (2010) 793–800

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Effect of bias voltage on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering Yongxin Wang a,c, Liping Wang a,b,⁎, Guangan Zhang a, S.C. Wang b, R.J.K. Wood b, Qunji Xue a a b c

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China National Centre for Advanced Tribology at Southampton (nCATS), School of Engineering Sciences, University of Southampton, SO17 1BJ, UK Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e

i n f o

Article history: Received 23 September 2009 Accepted in revised form 29 July 2010 Available online 11 August 2010 Keywords: Graphite-like carbon (GLC) film Bias voltage Microstructure Water environment Tribological property

a b s t r a c t Ti-doped graphite-like carbon (GLC) films with different microstructures and compositions were fabricated using magnetron sputtering technique. The influence of bias voltages on microstructure, hardness, internal stress, adhesion strength and tribological properties of the as-deposited GLC films were systemically investigated. The results showed that with increasing bias voltage, the graphite-like structure component (sp2 bond) in the GLC films increased, and the films gradually became much smoother and denser. The nanohardness and compressive internal stress increased significantly with the increase of bias voltage up to −300 V and were constant after −400 V. GLC films deposited with bias voltages in the range of -300–-400 V exhibited optimum adhesion strength with the substrates. Both the friction coefficients and the wear rates of GLC films in ambient air and water decreased with increasing voltages in the lower bias range (0–-300 V), however, they were constant for higher bias values (beyond −300 V) . In addition, the wear rate of GLC films under water-lubricated condition was significantly higher for voltages below −300 V but lower at high voltage than that under dry friction condition. The excellent tribological performance of Ti-doped GLC films prepared at higher bias voltages of −300–-400 V are attributed to their high hardness, tribo-induced lubricating top-layers and planar (2D) graphite-like structure. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Traditional man-made oil hydraulic systems have been widely used for driving many different systems in the industry. But the most important disadvantage of using oil is the potential of leakage which could lead to costly and environmental damages, as well as the negative effects on working environment and risk of fire [1]. To solve these problems, water hydraulic systems have attracted major interests due to their unique characteristics such as environmental compatibility, energy efficiency and user friendliness [2]. However, water hydraulic systems have problems in controllability, reliability and corrosion. Furthermore, water has a poor lubricating effect owing to its low viscosity which results in high friction, severe wear and occurrence of catastrophic seizures for tribo-pairs working with in water lubricated system [3]. Thus, it is necessary to develop new processes to improve tribological performance of tribo-materials used in water. One of the most efficient methods is to deposit different protectively high-performance films or coatings on the working surfaces of tribo-systems [4]. Among different kinds of protective and ⁎ Corresponding author. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. Tel.: + 86 931 4968080; fax: + 86 931 4968163. E-mail address: [email protected] (L. Wang).

lubricating films, carbon-based films are the most promising candidates due to their beneficial chemical and physical properties such as high chemical inertness, high hardness and favorable tribological properties in various working environments [5–10]. Due to the high hardness and excellent tribological properties (low friction and high wear resistance), amorphous carbon (a-C) films that comprise a hybrid of sp2- and sp3-bonded microstructures are attractive as a solid-lubricant for water hydraulic systems [11,12]. One of the famous a-C films is diamond-like carbon (DLC) film which mainly have the diamond-like structure (sp3 bond). Unfortunately, most DLC films fail easily during rubbing in water. Many attempts have been taken to improve the adhesion or mechanical and tribological characteristics of DLC films, for instance, using interlayers, ion implantation and doping elements (Si, Ti, Cr, etc.) [13–15]. An example of such metal-containing DLC films are the Ti-DLC films which have shown reduced internal stress and improved adhesion strength on substrates [16,17]. On the other hand, the a-C films which are predominantly composed of graphite-like structure (sp2 bonds) rather than diamond-like structures (sp3 bonds). These graphite-like carbon (GLC) films are currently arousing great interest because of their environmentally self-adopted tribological properties [18–20]. Zhou et al. reported that graphite-like a-CNx film deposited using ion beam assisted deposition (IBAD) could enhance the wear resistance of SiC tribo-materials and shorten the running-in period of SiC/SiC tribo-

0257-8972/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.112

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copules in water lubrication [21]. Stallard et al. fabricated a C/Cr graphite-like carbon films named Graphit-iC, using DC magnetron sputtering technique [22]. These C/Cr graphite-like carbon films had a hardness of 14 GPa and exhibited low friction and high wear resistance at a high applied load in water. The addition of thin layers of Cr to the carbon films softened and toughened the as-deposited GLC films and allowed them to survive the application of very high loads. Therefore, the excellent tribological properties of GLC films in water make it a good candidate material used in water hydraulic systems. However, the research on the Ti-doped GLC composite films, in particular the relationship between the deposition conditions, the mechanical and tribological behaviors is not yet systemically reported. Among various deposition conditions for each system, the substrate bias voltage is an important one, as sputtered ion energy is proportional to it [23]: E∝

Vb ; p1 = 2

where E, Vb and p are the ion energy, bias voltage and process pressure, respectively. In this paper, Ti-doped GLC films were fabricated using DC magnetron sputtering. The influences of different substrate bias voltage on the microstructure, composition, mechanical and tribological properties of the as-deposited Ti-GLC films have been investigated. 2. Experimental details

the films were performed on a Perkin-Elmer PHI-5702 multifunctional X-ray photoelectron spectroscope, using Al Kα radiation (photon energy 1476.6 eV) as the excitation source. The samples were cleaned by Ar + before acquiring XPS spectra in order to liberate the surface from adventitious contamination. The films on NaCl substrates were left in distilled water, and were collected on carbon Cu grids for HRTEM observation on a JEOL 3010 TEM operated at 300 kV. Surface and cross sectional morphologies of GLC films were investigated using a JSM-6701 scanning electron microscope (SEM).

2.3. Mechanical properties of the Ti-GLC films Nanohardness of GLC films were measured by a Nanotest600 nanoindenter apparatus (Micro Materials Ltd., UK) with a Berkovich indenter at a load of 50 mN. The maximum indenting depth was 100 nm, 10–12% of the film thickness. For this depth penetration of indenter, it was considered that the substrate could not have any pronounced influence on the measured value according to reference [24]. After measuring the bending of the coated substrate with a surface profilometer, the film internal stress was calculated by Stoney's equation [25]:

2

σs =

Es ts 6ð1−νs Þ tf

!

 1 1 − ; R2 R1

2.1. Deposition method The Ti-GLC nanocomposite films were deposited on Si (100) and stainless steel substrates synchronously by unbalance magnetron sputtering technique. Twin magnetron Ti targets were fitted on the concave surface of the vacuum chamber. One magnetron graphite target was located in the middle of Ti targets. All of the three targets focused on the sample holder. Prior to deposition, the substrates were cleaned ultrasonically in ethanol and acetone baths in succession and dried with a hot air blower. A base pressure of 1.0 × 10− 3 Pa was attained in the chamber with a turbomolecular pumping system. As the pressure of the vacuum chamber was reached, the substrate surfaces were first cleaned for 10 min by Ar + ion sputtering, then a Ti metal interlayer about 100 nm was deposited using the midfrequency power to improve the adhesion between GLC films and the substrates. On top of the Ti interlayer, carbon films were deposited using a DC power with the incorporation of Ti using a mid-frequency power. Pulse bias was applied on substrates during each deposition. The particular deposition conditions of GLC films were as follows: (1) applied DC current of 1.2 A on the graphite target and mid-frequency current of 0.4 A on the Ti target; (2) deposition bias voltage 0–-500 V (duty cycle 50%); (3) gas flow rate of Ar 50 sccm; (4) deposition pressure 1 Pa; (5) deposition time 100 minutes. Then films with thicknesses varying from 1050 to 510 nm were prepared on Si(100) and stainless steel substrates. The films on silicon wafers were used to investigate the microstructure, composition, hardness and internal stress. The films on stainless steel were used to measure the adhesion strengths and tribological properties. Films of thickness around 30 nm were deposited on NaCl substrates using the same parameters to prepare samples for high-resolution transmission electron microscopy (HRTEM) observation. 2.2. Microstructure of the Ti-GLC films Raman spectra of the as-deposited films were obtained by a HR800 Raman spectroscopic measurement using an Ar + laser of 532 nm with a resolution of 1 cm− 1. The typical data acquisition time was in the range of 60 s and the spectrum was recorded in the range of 800– 2400 cm− 1 in order to allow reliable fitting. The XPS measurements of

where σs is the internal stress, R1 and R2 are the curvatures of the substrate before and after deposition, νs is the Poisson's ratio of the substrate, Es is the Young's modulus of the substrate, and ts and tf are the thickness of the substrate and film, respectively. The scratch test was performed at various bias voltages using a scratch tester. A diamond tip, 500 μm in radius, was used as the scratching stylus with increasing normal load from 0 to 10 N at a fixed loading rate of 10 N/min and a scratching speed of 5 mm/min under ambient condition. The microstructure of the scratch track was examined by a JSM-5600 SEM.

2.4. Tribological performance The tribological performance of Ti-GLC films deposited with different bias voltages was tested on a ball-on-disc reciprocating sliding tribometer (Lanzhou Institute of Chemical Physics, ACS., China) in both ambient air of a relatively humidity 35 ± 5% and distilled water environment at room temperature. Mating balls of uncoated Si3N4 with a diameter of 3 mm were used for the tests. All frictional tests were performed under a load of 2 N with the amplitude of 5 mm and the frequency of 5 Hz. The friction coefficients and sliding time were recorded automatically and precisely during the test. Four values of the friction coefficient for the same film were obtained by averaging the friction coefficient in steady-state period of friction curves. Then the statistic of the four average values of the friction coefficients was calculated. Wear track profiles were analysed using a non-contact 3D surface profiler (model MicroMAXTM, made by ADE Phase Shift, Tucson, AZ, USA) after the wear test. The wear volume was determined from the wear track profiles. The wear rate of all the films was calculated using the equation [26]:K = V / SF where V is the wear volume in m3, S is the total sliding distance in metres and F is the normal load in newtons. The resulted wear rate of each deposit was obtained by averaging four wear tests under the same wear conditions. The contact mating surfaces were analysed by a JSM-5600 SEM and a HR800 Raman spectroscopic measurement using an Ar+ laser of 532 nm and a resolution of 1 cm− 1.

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3. Results and discussion 3.1. Microstructure evolution of the Ti-GLC films Owing to the high sensitivity to π bonds of Raman spectrum [27], the conventional visible Raman spectrum of amorphous carbon is dominated by the G peak at about 1560 cm− 1 and the D peak around 1350 cm− 1, both of which are attributed to sp2-bonded carbon. However, the G mode is actually the stretching vibration of all pair of sp2 atoms in both rings and chains, while the D mode is the breathing mode of those sp2 sites only in rings but not in chains [28,29]. Due to the different degree resonances of the G (graphitic) and the D (disorder induced) bands [30], the increase of D band was usually considered as the sign of increasing sp2 sites [31]. From Fig. 1, all of the as-deposited GLC films with different bias voltages were consistent with the typical feature of amorphous carbon, showing the G and D bands clearly. Meanwhile, it was clear that the intensities of D bands increased as the bias voltages changed from 0 to -500 V, and the value of ID/IG derived from peak fitting increased from 2.2 to 4.5 in the same time as shown in Fig. 2, all of which indicated the increasing percentage of sp2 sites as the ion energy increase. It was further sustained by the variation of C1s spectrum in XPS analysis as shown in Fig. 3. Because each element has a unique set of binding energies, XPS may offer a reliable analysis to the structure and bonding state of the films [32]. These chemical shifts can be used to identify the chemical state of the materials. As seen in Fig. 3, the C1s peaks of GLC films deposited with different bias voltages were broad bands containing sp2 peak at 284.4 eV and sp3 peak at 285.2 eV [33]. And it was clear that the band shifted towards lower binding energy with the increase of bias voltages, which indicated the increase of sp2 bonds ratio. As the intensity of the binding energy is linearly proportional to the fraction of sp2 and sp3 bonds [34], the percentage of the sp2 can be calculated after peak fitting as shown in Fig. 2. The result showed that the concentration of sp2 bonds increased from 55% to 75% as the bias voltages increased from 0 to −500 V. However, all of the C1s spectra did not exhibit the peak feature of TiC binding energy around 282 eV [35], which might be related to the low concentration of Ti as shown in Fig. 4 [36–38]. The data showed only a slight change of the film compositions with different bias voltages. All the films deposited at bias voltages of 0–-500 V had a concentration of around 10 at.% calculated from XPS analysis for the incorporated Ti. While less than 2 at.% O was also demonstrated in the

Fig. 1. Raman spectra of GLC films with different bias voltages.

Fig. 2. ID/IG derived from fitting Raman spectra and percentages of sp2 bond derived from fitting C1s XPS spectra at different bias voltages.

as-deposited GLC films. The presence of O might be closely related to the unavoidable oxygen contamination during the film deposition process in the limited vacuum. The XPS spectra of Ti2p shown in Fig. 5 were used to analyse the chemical state of Ti in the as-deposited GLC films. As seen from Fig. 5, the peak position of Ti2p was affected slightly as the bias voltage increases from 0 to −500 V. According to references [35,39], the strong peak at ~455.0 eV in XPS spectra of Ti2p was assigned to Ti-C bonding. However, TiO may also attribute to the peak around 455 their close binding energy (455.1 eV for TiO [40]). It has been reported that Ti atoms were preferentially bound to oxygen rather than carbon [41]. Nevertheless, there is no argument that the peak at ~454 eV to the Ti in metallic state [42]. Moreover, no considerable influence of bias voltage on film composition was observed as evidenced from the peak densities in Fig. 5 [43]. Thus it was proposed that three chemical states of Ti may exist inside the carbon matrix, including TiC, TiO and elemental Ti. The graphite-like (high sp2 bonds content) amorphous carbon has been characterized by Raman spectrum and XPS spectrum, nanocomposite microstructure was imaged in HRTEM as shown in Fig. 6. Clearly, some nano-clusters with size ranging from 3 to 8 nm were inhomogenously embedded in the amorphous carbon matrix (Fig. 6a). The lattice fringe has a spacing of 0.21 nm (Fig. 6b), which was consistent to the spacing of (111) planes in diamond [44]. It indicated that nano-diamond particles might exist in GLC films. Therefore, the as-deposited GLC films exhibited a typical nano-cystallites/

Fig. 3. C1s spectra in XPS analysis of GLC films.

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Fig. 4. Contents of Ti and O calculated from XPS analysis in GLC films.

amorphous matrix structure. However, the presence of TiC, TiO or Ti was not confirmed, implying that amorphous TiC or C-O-Ti solid solution may be generated inside the nano-cystallites/amorphous GLC film [38]. The surface and cross-sectional morphologies of the GLC films deposited with different bias voltages were observed using SEM shown in Fig. 7. A debris structure could be seen clearly on the surface of films deposited without bias voltage (Fig. 7a), while the surface of films would become smoother as the bias voltages increased (Fig. 7b, c and d). A columnar structure was observed in the cross-sectional films without deposition bias voltage (Fig. 7e) or with low bias voltage (Fig. 7f), while the cross-sectional films deposited with high bias voltage (Fig. 7g and h) showed more homogeneous and denser images. All these indicated the dense films require a high deposition bias voltage, i.e. high ion energy. It can be concluded that the bombardment or impingement of energetic ions resulted in a compaction effect on the GLC films [10]. In case the ion energy is not high enough (low or no bias voltage), diffusion in surface layers takes place as the ions cannot penetrate into the interior structure. The incoming ions will be trapped on the growing surface and tend to generate cross-linked structure, which result in a loose and rough surface. If the energy exceeds the critical value for atomic displacement in the structure, the ions can penetrate deep into the interior of

Fig. 6. HRTEM images of GLC films: (a) nanocrystallite/amorphous microstructure; (b) lattice fringe of the nanocluster.

the structure. Then the ion energy tends to be dissipated into the volume nearby. Consequently, dense, smooth films are formed when the bias voltage increased to a proper value (-300–-400 V). The thickness of GLC layer degreases gradually from 1050 to 810 nm as the bias voltage increases from 0 to −500 V, which is attributed to the enhanced compaction effect result from the bombardment or impingement of energetic ions at high bias voltage range. 3.2. Mechanical properties

Fig. 5. Ti2p spectra in XPS analysis of GLC films.

Fig. 8 shows the nanohardness and the internal stress of asdeposited GLC films as a function of various bias voltages. The nanohardness increased from 14 to 23 GPa with the deposition bias voltage increasing from 0 to –300 V, and kept constant when the bias voltage increased further. Combining this with the Raman spectra (Fig. 1) and XPS (Fig. 2) analysis results, it seems the hardness enhancement of GLC films was not the percentage of sp2- or sp3 hybridized carbon. The amorphous carbon matrix could be strengthened efficiently by the fine super-hard nanoparticles such as nanocrystalline diamond, but the film compositions and nanocrystallite/amorphous structures of the as-deposited GLC films were similar to each other and not greatly affected by the bias voltages. Seen from the surface and cross-sectional morphologies evolution (Fig. 7), another important factor contributing to the hardness might be the density of the whole films. Since denser films carry much higher load, it is reasonable that the hardness increasing with the increase of bias voltages is closely related to the increase of films density in the process. Furthermore, stresses were generally induced via energetic particle bombardment as shown in Fig. 8, and the negative character indicated a kind of compressive internal stress. From Fig. 8, the

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Fig. 7. SEM images of GLC films: (a), (b), (c) and (d) are surface images of GLC films deposited with bias voltage 0 V, −100 V, −300 V and −500 V, respectively; (e), (f), (g) and (h) are cross-sectional images of GLC films deposited with bias voltage 0 V, −100 V, −300 V and −500 V, respectively.

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Fig. 8. Nanohardness and internal stresses of GLC films with various bias voltages.

compressive internal stress increased from 0.5 GPa for as-deposited GLC films without bias voltage to 2.5 GPa for GLC films deposited at a bias of –400 V, and the stress did not increase any more till a bias voltage of –500 V. With the increase of bias voltages, it was reasonable that the compressive internal stress increased since the energetic particle bombardment was enhanced at high bias voltages. But the heating effect of ion bombardment lead to a diffusing and recombining effect of defects at the same time. Then a balance between above two effects appeared after the bias voltage of –400 V, which was consistent with the variation trend of film's density discussed above. The increased internal compressive stress could be considered as another factor contributed to hardness, as the compressive stress increases the contacts between clusters and limits the sliding along the cluster boundaries. The adhesion strengths between the GLC films and stainless steel substrates were evaluated by the scratch test. Fig. 9 shows the scratch tracks of the GLC films deposited on stainless steel substrates with different bias voltages. There was a clear difference in the fracture behavior of the film depending on the bias voltage. The GLC film deposited without bias voltage spalled easily at low scratching force. With the increase of bias voltage, the adhesion between film and substrate was enhanced. Few delaminations can be seen in the scratch track when bias voltage increased to −300 or −400 V. However, film deposited with bias voltage −500 V had large area delamination. The scratch behavior might be closely related to the film density and internal stress. Low film density at low bias voltages must result in the weak adhesion, while film deposited with high bias voltage was possibly destroyed due to the high film internal stress. In this work, it seemed that film deposited with bias voltages -300–-400 exhibited the optimum adhesion strength.

Fig. 9. Scratch tracks of the GLC films with various bias voltages.

Fig. 10. Friction coefficients of the GLC films with various bias voltages.

3.3. Tribological performance The evolution of microstructures with various ion energies, as well as the mechanical properties, affected the tribological properties. As seen in Fig. 10, either in ambient air or distilled water, the coefficients of friction decreased rapidly as the deposition bias voltages increased in the lower range, then the coefficients of friction fluctuated slightly when bias voltages further increased upon –300 V. At the same time, the friction coefficients in water were always lower than those in ambient air, especially at low bias voltages. However, the GLC films deposited without bias voltage and under the bias voltage of –100 V were seriously damaged and worn out after a few cycles when sliding in water environment. The GLC films deposited at high bias voltages were found much more wear resistance when sliding in both ambient air and distilled water under the same testing conditions. Their friction coefficients were very low, especially in the distilled water. According to other studies [45,46], the low friction of a-C films can be attributed to the graphitization effect or tribolayers formed on the contact surfaces during sliding. It was sustained by Fig. 11 which

Fig. 11. SEM images of contact surfaces of Si3N4 balls against GLC film: (a) in ambient air; (b) in distilled water.

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Fig. 12. Raman spectra of contact surfaces of Si3N4 balls against GLC film.

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Fig. 13. Specific wear rates of the GLC films with various bias voltages.

4. Conclusions showed the dense tribolayers formed on the mating ball surfaces sliding in both ambient air and distilled water, and the graphite character of each tribo-layer was identified by the Raman spectrum as shown in Fig. 12. The planar 2D graphite-like structure of sp2hybrized carbon in the surface will reduce dangling σ bonds on the clean surface, thus the strong adhesive interaction between contact surfaces can be avoided. The increase of bias voltages leads to the increase of sp2-hybrized carbon in the as-deposited GLC films, thus the planar 2D graphite structure on surfaces of the as-deposited GLC films would be enhanced with the increase of bias voltages. The graphitization effect could attribute the decreasing trend of friction. For sliding in water, it can be observed that there were less transfer materials on the contact surface of the mating ball in Figs. 11 and 12. The relatively lower friction could be attributed to the hydrodynamic lubrication effect. There might be a hydrodynamic lubricating film being formed between the contact surfaces during sliding in water. The water hydrodynamic lubrication effect would be enhanced at low bias voltages since the water lubricating film could be easily formed on these rough contact surfaces, The water hydrodynamic lubrication effect gradually decreased with the increase of the bias voltages, due to the formation of smoother surfaces on GLC films, although the friction coefficients of GLC films in water were still lower than those in ambient air. The specific wear rates were calculated after scanning the wear track with various bias voltages as shown in Fig. 13. As a whole, the specific wear rates of GLC films both in ambient and distilled water decrease with increasing bias voltages. The wear rate remarkably decreased from 0 to −300 V at low bias voltages range, and then slightly fluctuated with the further increase of bias voltage. The wear rates of GLC films in water were always significantly higher than those in ambient air conditions in the lower bias range, but the opposite was true at high bias voltages range. Thus the GLC nanocomposite films exhibited a high wear resistance at appropriately higher depositing bias voltages, especially in the water environment. Generally, the wear rates of GLC films in water lubrication should always be lower than that in ambient air, however, the wear rates in water for GLC films deposited at low bias voltages were much higher than that in ambient air, and the films were penetrated under some conditions. So it can be concluded that film density of GLC films also plays crucial role in the tribological behavior of GLC films deposited at low bias voltages when sliding in water. As discussed in the above the GLC films deposited at low bias voltages were a relatively loosely adhered and hence water molecules would easily infiltrate into the loose surface layer and consequently accelerated the wear of the films. If the GLC films were dense enough, it was very difficult to infiltrate into the films for water molecules.

Very hard Ti-doped GLC films were fabricated using magnetron sputtering technique. The graphite structure (sp2 bonds) increased with the increase of bias voltages. The hardness, density and compressive internal stresses increased markedly as the bias voltages increased in the low range and kept constant when bias voltages increased further. The friction coefficients in both ambient air and water environment decrease with the increase of bias voltages in the low range, while they will fluctuate slightly when bias voltages increase beyond the bias voltage of −300 V. The specific wear rates in both ambient air and water environment decrease significantly as the bias voltages increase in the low range, and keep at low values at high bias voltages. The friction coefficients in water are lower than that in ambient air. Whereas, the specific wear rates in water are higher than those in ambient air for films deposited with low bias voltages, but much lower than those in ambient air for films deposited with appropriately high bias voltages. GLC film prepared at higher bias voltages of -300–-400 V possess optimum mechanical and tribological properties in both ambient air and water environment. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 50905178 & 50823008) and the 863 Program of Chinese Ministry of Science and Technology (No. 2009AA03Z105). The authors gratefully acknowledge Yunfeng Wang, Shanhong Wan and Jun Wang for their partial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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