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Structural, mechanical and tribological behavior of fullerene-like carbon film
Thin Solid Films 518 (2010) 5938–5943
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Structural, mechanical and tribological behavior of fullerene-like carbon film Peng Wang ⁎, Xia Wang, Bin Zhang, Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2009 Received in revised form 5 May 2010 Accepted 21 May 2010 Available online 31 May 2010 Keywords: Structural properties Mechanical properties Carbon films Fullerene-like Tribological properties Magnetron sputtering Transmission electron microscopy
a b s t r a c t Hydrogenated carbon films containing fullerene-like structures (FL-C:H) were synthesized using magnetron sputtering of a graphite target in methane and argon atmospheres. The results indicate clearly that a substantial part of the film is made up of fragments of such fullerene-like structures mixed in a hydrogenated amorphous carbon matrix. The film exhibits excellent mechanical properties with a hardness of 27.4 GPa and almost complete elastic recovery (as high as 95%). The tribological properties of the FL-C:H film were tested and compared with a-C:H and a-C films. The results show that the lowest friction coefficient of the FL-C:H film, about 0.02, is recorded in oxygen environment. The ranges of dispersion of the friction coefficient for the FL-C:H film are merely 0.02 in different testing environments, much lower than that of a-C:H and a-C films, which indicates the friction-coefficient-insensitivity of the FL-C:H film to different atmospheres. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon-based thin films have been the subject of extensive research over the past two decades due to their excellent properties, such as low friction coefficient, chemical inertness, infrared transparency, and high hardness [1]. High hardness of such films has been attributed to the presence of a high percentage of sp3 (diamond-like) bonds, whereas a high concentration of sp2 (graphitic) bonds is regarded as leading to the formation of soft films. However, the discovery of C60 fullerene molecule and carbon nanotubes which are sp2 bonded, opens up the possibility of obtaining three-dimensional structures, which exploit the extremely strong planar bonds of graphite (stronger than diamond) in a class of hard thin-film materials [2–5]. These materials have been named as fullerene-like materials due to the presence of curved and cross-linked basal planes, resembling the structure of fullerene molecules. Over the past decades many studies have been published on the production of carbon-based films with fullerene-like structures. Such a structure was first discovered in carbon nitride (CNx) film [6]. Although the research in CNx films has been mainly driven by the search of superhard β-C3N4 structure, most of the attempts resulted in sp2-dominated materials [6–9]. Among these, the most promising sp2dominated CNx are referred to as fullerene-like CNx, owing the notation to their microstructure composed of a 3D arrangement of extended, bent and cross-linked N-containing graphite basal planes. Fullerene-like CNx exhibits high hardness with extremely large elastic recovery, making it a promising candidate for tribological
⁎ Corresponding author. E-mail address: [email protected] (P. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.096
applications. So far, the formation of fullerene-like CNx films has been studied using different growth techniques such as magnetron sputtering [10], pulsed laser deposition [11], ion assisted deposition [12] and anodic jet carbon arc [13]. Considering the conditions for fullerene-like CNx nucleation and subsequent growth, low-energy ion bombardment (b100 eV) and moderate substrate temperatures (250–500 °C) have been shown to be essential for the synthesis of fullerene-like structures [14]. These growth conditions are necessary to increase the mobility and reactivity at the surface, and to enable selective etching of less-favorable bonding environments without detrimental ion-induced damage for the evolution of the microstructure [15]. It is well known that the friction of carbon films mainly arises from the chemical interactions caused by the strong free σ-bond at the sliding interfaces. Therefore, the elimination of free σ-bonds on sliding surfaces of carbon films by hydrogen atoms is an essential requirement for achieving low friction coefficients. However, the critical requirements of growth fullerene-like CNx structures limited the wider applications of the films, and the carbon-based films without hydrogen incorporation exhibited poor friction behavior compared with highly hydrogenated carbon-based films [16]. Recently, much effort has been devoted to synthesizing the hydrogenated fullerene-like carbon films using chemical vapor deposition techniques [17–19]. In comparison to fullerene-like CNx films, the hydrogenated fullerene-like carbon films deposited by chemical vapor deposition has less order in the graphitic planes, but a considerably higher hardness and lower friction. In our previous work, we reported hydrogenated carbon films containing fullerene-like structures (FL-C:H) prepared in our laboratory by magnetron sputtering of a titanium target in a hydrocarbon gas [19]. By investigating the film growth process, it was concluded that
P. Wang et al. / Thin Solid Films 518 (2010) 5938–5943
sputtering hydrocarbon layer deposited on titanium target may be essential to the formation of fullerene-like structure. Inspired by this conclusion, we tried to prepare FL-C:H film by using magnetron sputtering of a graphite target using Ar-hydrocarbon mixtures. Here, the fullerene-like structure was also obtained and the film even exhibited higher hardness and elastic recovery. Furthermore, the FL-C: H film showed low friction coefficients in a relatively wide range of environmental conditions, which promises a valuable material for applications requiring of insensitivity to the environment.
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formula (dmax − dmin) / dmax, where dmax and dmin are the maximum and minimum displacements during unloading, respectively. The friction behaviors of the films sliding against Si3N4 balls were measured on a ball-on-disc tribometer equipped with an environmental chamber with which the relative humidity and gaseous environment could be controlled. The friction experiments were performed with a normal load of 2 N, a sliding velocity of about 0.52 m/s, to a maximum sliding time of 60 min. The worn surfaces of the films and the counterpart balls were characterized using a JSM5600LV SEM.
2. Experimental details In this study, all carbon films with different structures were deposited in a custom-designed multifunctional deposition system. The system combines different physical and chemical deposition methods in one chamber, so carbon films with different structure can be prepared in one system, more details of the deposition system were given elsewhere [20]. FL-C:H films with the thickness of 1 μm were deposited on silicon substrates by magnetron sputtering of a graphite target. The film was deposited at a pressure of 5.0 × 10− 1 Pa using methane and argon gases as the precursors with the flow rates of 80 and 120 sccm, respectively. Prior to deposition, the silicon substrates were subjected to ion bombardment in an argon glow discharge under a negative bias voltage of 1000 V for 10 min to remove the native oxide layer. During deposition, the sputter current was limited at 2.5 A at discharge voltage of ∼500 V. The negative bias voltage was generated by a pulse power supply (4 kHz, 15% duty cycle) at a constant value of 1200 V. Although there was no deliberate substrate heating during deposition, local temperature could rise up to a maximum of 120 °C, as measured by a thermocouple clamped near the substrate holder. In addition, magnetron sputtering and radio frequency PECVD, two components mounted in the multifunctional deposition system, were used to deposit a-C and a-C:H films, and the deposition conditions are detailed in Table 1. X-ray photoelectron spectroscopy (XPS) was performed using a PHI-5702 system. The Al-Kα radiation was used as the excitation source. The XPS was collected in a constant analyzer energy mode, at a chamber pressure of 10− 8 Pa and the binding energy of the Au 4f7/2 core level electron (Au 4f7/2:84.0 eV) as the reference, at a pass energy of 29.35 eV. Fourier transformation infrared (FTIR) spectra were recorded on a Biorad FTS165 spectrometer to detect the bonding in the film. The film microstructure was characterized by JEM 2010 high-resolution transmission electron microscopy (HRTEM) with 0.19 nm resolution. Plan-view samples were obtained by floating off ∼ 20 nm thick films deposited on NaCl crystal in deionized water, the films were then collected directly onto Cu microscopy grids. Mechanical properties of the film were obtained by using a nanoindentater with a three-sided pyramidal Berkovich diamond tip and continuous stiffness option. Five replicate indentations were made for each sample, the maximum indentation depth was set below 10% of the film thickness (1 μm) in order to minimize substrate influence, and the hardness and elastic modulus were calculated from the loading–unloading curves. The elastic recovery is calculated using the
3. Results The XPS C1s spectrum of the FL-C:H film is shown in Fig. 1, for comparison, also the spectra of a-C and a-C:H film. Usually, the C 1s spectra of the amorphous carbon films can be fitted into three components around 284.3, 285.3 and 286.6 eV, respectively. The one around 284.3 eV corresponds to sp2 carbon atoms, and that around 285.3 eV corresponds to sp3 carbon atoms, a third peak of near 286.6 eV is assigned to some C–O contamination formed on the film surface due to air exposure. According to the work of Javier et al. [21], the content of the sp3 carbon atoms in the amorphous carbon films can be determined as the ratio of the corresponding sp3 peak area over the total C 1s peak area. Using method mentioned above, we can calculate that the sp3 content of the FL-C:H films is about 33%, the sp3 content of the a-C and a-C:H films is 20% and 42%, respectively. In order to get the bonding configuration in the film, the film was analyzed by FTIR. Fig. 2 shows the IR spectra of the FL-C:H, a-C and a-C:H film. Three bands can be observed in the spectra of the films. The absorption peak at 1380 cm− 1 represents sp3 CH3 or (CH3)n stretching mode [1]. The appearance of a peak centered around 1450 and 2920 cm− 1 indicates that hydrogen is predominantly bonded to saturated (sp3) carbon atoms [22,23]. The strong peak around 1600 cm− 1 can also be observed, its originated from the Raman active G band (graphite) at ∼1570 cm− 1 and D band (disorder) at ∼1360 cm− 1, which can be attributed to olefinic or aromatic vibrations of CC bonds in the film [1,22,23]. In the spectra of a-C film, the features are the C–H stretching band around 1380 cm− 1 and the CC vibration around 1640 cm− 1, and a weak band at 2920 cm− 1. In a-C:H film, CH stretching mode around 1450 cm− 1emerges, at the same time, the band intensity around 2920 cm− 1 increases remarkably and the band around 1640 cm− 1 weaken, indicating the decrease of CC vibration and the increase of CH stretching mode in a-C:H film. The result of FTIR analysis indicates that the FL-C:H film is hydrogenated and has complex configuration consisting of sp2 olefinic or aromatic CC bonds and sp3 C–H bonds.
Table 1 Summary of a-C and a-C:H film deposition conditions. Item
Target Ar gas flow rate (sccm) Methane gas flow rate (sccm) Deposition pressure (Pa) Power (W) DC bias (V) Thickness (μm)
Parameter a-C
a-C:H
FL-C:H
Graphite 150 0 0.5 1000 − 200 1
/ 150 60 5 700 − 200 1
Graphite 120 80 0.5 1250 − 1200 1
Fig. 1. The C1s spectra of FL-C:H, a-C and a-C:H films.
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Fig. 2. Infrared spectra of FL-C:H and reference a-C and a-C:H films for comparison.
Plan-view HRTEM images from a-C, a-C:H and FL-C:H films were used for structure investigation. The HRTEM images of the three kind films are shown in Fig. 3. Obviously, there were no detectable graphite plane fragments or curved graphite planes in the HRTEM images of a-C (Fig. 3(a)) and a-C:H film (Fig. 3(b)) due to a completely amorphous character of the film. For FL-C:H film (Fig. 3(c)), it clearly shows ordered domains of several nanometers in size. These domains consist of curved and closed planes which have small radii of curvature and present fullerene-like arrangements with 2–3 nm in diameter. In addition, no obvious cross-linking between these planes can be detected, these curved graphene planes are just homogeneously embedded in and separated by an amorphous carbon matrix. The HRTEM and IR results indicate clearly that a substantial part of the film is made up of fragments of such fullerene-like nanostructures mixed in a random forming co-continuous matrix with the hydrogenated amorphous carbon component. It should be mentioned that the quantity and the dimension of the fullerene-like structure in our film are less compared with previous reported well-structure fullerene-like films, we ascribe it to the incorporation of hydrogen into the film. It is well known that methane can be decomposed into some hydrocarbon radicals and atomic or ionic hydrogen [24,25]. The hydrogen atoms prefer to stay at the edges rather than stay inside the clusters thereby inhibiting the 3D network growth and cross-linking [26]. As a result, isolated rather
cross-linking fullerene-like structure was formed. Correspondingly, the size of the fullerene-like structures in our films is smaller than that of previous reported films [5,7,11]. One outstanding feature reported for fullerene-like films is its extreme elasticity in combination with relatively high elastic modulus and hardness. These properties were determined by nanoindentation in this study. Fig. 4 shows the load–displacement curves of the FL-C:H film compared to that of a-C film and a-C:H film. The plots clearly demonstrate that the FL-C:H film exhibits a high elasticity combined with a considerable hardness. The hardness of the FL-C:H film is 27.4 GPa, much higher than that of a-C film and a-C:H film, 12.4 and 14.5 GPa, respectively. Noticeably, the FL-C:H film shows almost complete elastic recovery (as high as 95%) after removing load. For comparison, the reference carbon films all exhibit residual deformation. Moreover, the FL-C:H film shows a strong curvature of the unloading portion and a no or very little tendency to plastic deformation, which indicates that most indentation energy is stored elastically and is released upon unload. These processes can be obtained by reversible bond rotation and bond angle deflection rather than slip and bond breaking [10,11]. Hydrogenated carbon films are usually used as wear-resistance and friction-reducing coatings in many fields. Although typical a-C:H films exhibit extremely low friction coefficients in dry nitrogen or ultra-high vacuum [27–30], their tribological properties remain poor at humid or oxygen-dominant atmosphere [30,31], which seriously limit their technological applications. It has been a longtime dream for tribologists or material scientists to fabricate a lubricating film with a low friction coefficient in different testing environments. Fig. 5 shows the friction coefficients of the films against Si3N4 balls as a function of sliding time under different atmosphere. It is clear that the friction behaviors of the a-C and a-C:H films are strongly dependent on the relative humidity. For a-C film, the friction coefficient as high as ∼0.18 was obtained in dry air (RH=5%) and it significantly decreased to ∼0.12 in humid air (RH=50%). Oppositely, the a-C:H film exhibited very low and stable friction coefficient of ∼0.07 in dry air, while the friction was quite unstable and the friction coefficient was increased to about 0.12 as the relative humidity increased to 50%. With respect to the FL-C:H film, low and stable friction coefficients were obtained in both dry and humid air, merely varying from ∼0.026 in dry air to ∼0.042 in humid air. Noticeably, the lowest friction coefficient, about 0.02, is recorded for the FL-C:H film tested in oxygen atmosphere. To get more insights into the friction and wear mechanism of the three kinds of films sliding against the Si3N4 balls, the worn surfaces of
Fig. 3. HRTEM images of (a): a-C film, (b): a-C:H film and (c) FL-C:H film.
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coefficient increases to 0.11 when humid air is introduced into the test chamber. The ranges of dispersion of the friction coefficient for the a-C:H film in different atmospheres reached up to 0.1. Noticeably, the FL-C:H film presented low friction coefficient on the whole and the maximum friction coefficient difference in different atmospheres is as small as 0.02, which indicates the friction-coefficient-insensitivity of the FL-C:H film to different atmospheres.
4. Discussion
Fig. 4. Load–displacement curves obtained from the FL-C:H film in comparison to a-C and a-C:H films.
the films and counterpart balls were studied by SEM. Fig. 6 shows the SEM pictures of the worn surface of the films and counterpart balls tested in dry air. As seen in Fig. 6(a), the friction surface of the FL-C:H film was smooth in appearance and film remained intact, despite after the 60,000 revolutions performed, the wear track on the film is barely visible, except for few debris scattering at the edge of the wear track. The friction surface on the counterpart Si3N4 ball was covered by a compact and homogeneous transfer layer with few wear debris around (Fig. 6(b)). In the case of a-C:H film, obvious flake-like desquamation and wear debris was observed on the wear track of the film (Fig. 6(c)). Although a compact transfer layer was also observed on the contact region of the Si3N4 ball, the size of the transfer layer is smaller and more wear debris accumulated around the transfer layer (Fig. 6(d)), compared to the case of the FL-C:H film. In the case of a-C film (Fig. 6 (e,f)), the friction surface of the Si3N4 ball was larger and much rougher, a discontinuous layer is formed in the middle of the worn surface of the film, indicating a wear failure of the film. The friction behaviors of the three kinds of films in different atmospheres are summarized in Fig. 7. It can be seen that the highest friction coefficient as well as the widest range of friction coefficient differences was obtained for a-C film. Although the a-C:H film exhibited the lowest friction coefficient of ∼0.013 in dry nitrogen, its friction
Fig. 5. The friction behaviors of the FL-C:H films against Si3N4 balls as a function of sliding revolutions under different atmospheres. For comparison, also the friction coefficients of the a-C and a-C:H films are shown.
Combining the above-mentioned experimental results, it can be concluded that it is possible to improve the mechanical and tribological properties of hydrogenated carbon films by the incorporation of fullerene-like graphene planes. We argue that the excellent mechanical properties of the fullerene-like films, especially toughness and elasticity, resulted from the existence of strong interlocking between fullerene-like graphene planes and hydrogenated amorphous carbon matrix, which effectively inhibits interplanar slip of the predominantly sp2-coordinated graphene sheets [5,6,10]. That is, at the edges of these fullerene-like graphene planes unsaturated bonds through which they may bond either to the sp3 bonded chain-like matrix or to hydrogen atoms [5]. As the bond between graphene and sp3 bonded chain-like matrix is covalent bond, which is stronger than van der Waals-type bonding in graphite, the amorphous matrix, especially high sp3 bond content, can constrict the gliding of the graphene plane, so the film containing this stiff network will exhibit high hardness. Simultaneously, fullerene-like structure just like a “molecule spring” disperses in film [19], reserves the elastic energy during distortion through reversible bond rotation and bond angle deflection, when sets of graphene planes are compressed their built-in curvature causes them to recover a shape close to their initial state after deformation, which is the origin of the apparently high elastic recovery in the films. As a result, a material combining stiffness with flexibility can be formed. The results also demonstrated that the FL-C:H film has low friction coefficient in different testing environments. Considering the low friction coefficient of FL-C:H film, mechanism such as super-low friction coefficient in a-C:H film [28] can still occur in FL-C:H film. With respect to the friction-coefficient-insensitivity of the FL-C:H film to different testing environments, following features of the film should be considered. The FL-C:H film consists of curved graphene planes and hydrogenated carbon matrix. The curved graphene planes mainly consist of sp2-bonds, at the inner of these fullerene-like structures, no hydrogen atoms bond on carbon sites and some free-bond exist in these graphene planes. During the film sliding in humid air/or oxygen environments, the free-bonds in graphene planes can be terminated and passivated by adsorbing water/or oxygen molecules, just like in a-C films [27–29], and hence the FL-C:H films exhibit low friction coefficient in humid air and oxygen environments. Considering the rest of the FL-C: H film, hydrogenated carbon matrix contains high hydrogen content, which is similar to the structure of a-C:H films, most of dangling bonds on the hydrogenated carbon matrix surface are eliminated by hydrogen, which significantly eliminates the energetic dangling bonds at the edge of the growing structure as discussed above, thereby the elimination of free σ-bonds on sliding surfaces by hydrogen atoms can provide a high degree of chemical passivation and thus low friction [29,31]. As a result, the unique structure of embedded fullerene-like graphene planes in hydrogenated carbon matrix can enhance the stability of the FL-C:H film in humid environments and oxygen-dominant atmosphere, and prevent it from being oxidized during friction in different environments [32]. Furthermore, good wear-resistance of the FL-C:H film was owed to high hardness and excellent elastic recovery of the film, which assure the good load-bearing and immediate recovery of friction-induced deformation during friction. Consequently, a stiff as well as flexible material with low friction coefficients in a relatively wide range of environment can be formed.
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Fig. 6. SEM pictures of the wear tracks on the (a) FL-C:H, (c) a-C:H and (e) a-C films, and corresponding wear scars on the counterpart balls (b, d, f) obtained in air.
5. Conclusions A hydrogenated carbon film containing a fullerene-like structure was synthesized using magnetron sputtering of a graphite target in methane atmosphere. Compared with amorphous carbon and hydrogenated amorphous carbon films, the film not only exhibited higher hardness and elastic recovery but also showed low friction coefficients in a wider range of environment, promising a valuable material with excellent mechanical properties in addition to friction-insensitivity to environment. Acknowledgements The authors gratefully acknowledged the financial support from the National Natural Science Foundation of China (Grant No: 50902133 and 50905177), 973 projects (Grant No. 2007CB607601) of the Ministry of Science and Technology of China. References Fig. 7. Friction coefficients of FL-C:H, a-C:H and a-C films for sliding under different atmospheres.
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Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Structural, mechanical and tribological behavior of fullerene-like carbon film Peng Wang ⁎, Xia Wang, Bin Zhang, Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2009 Received in revised form 5 May 2010 Accepted 21 May 2010 Available online 31 May 2010 Keywords: Structural properties Mechanical properties Carbon films Fullerene-like Tribological properties Magnetron sputtering Transmission electron microscopy
a b s t r a c t Hydrogenated carbon films containing fullerene-like structures (FL-C:H) were synthesized using magnetron sputtering of a graphite target in methane and argon atmospheres. The results indicate clearly that a substantial part of the film is made up of fragments of such fullerene-like structures mixed in a hydrogenated amorphous carbon matrix. The film exhibits excellent mechanical properties with a hardness of 27.4 GPa and almost complete elastic recovery (as high as 95%). The tribological properties of the FL-C:H film were tested and compared with a-C:H and a-C films. The results show that the lowest friction coefficient of the FL-C:H film, about 0.02, is recorded in oxygen environment. The ranges of dispersion of the friction coefficient for the FL-C:H film are merely 0.02 in different testing environments, much lower than that of a-C:H and a-C films, which indicates the friction-coefficient-insensitivity of the FL-C:H film to different atmospheres. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon-based thin films have been the subject of extensive research over the past two decades due to their excellent properties, such as low friction coefficient, chemical inertness, infrared transparency, and high hardness [1]. High hardness of such films has been attributed to the presence of a high percentage of sp3 (diamond-like) bonds, whereas a high concentration of sp2 (graphitic) bonds is regarded as leading to the formation of soft films. However, the discovery of C60 fullerene molecule and carbon nanotubes which are sp2 bonded, opens up the possibility of obtaining three-dimensional structures, which exploit the extremely strong planar bonds of graphite (stronger than diamond) in a class of hard thin-film materials [2–5]. These materials have been named as fullerene-like materials due to the presence of curved and cross-linked basal planes, resembling the structure of fullerene molecules. Over the past decades many studies have been published on the production of carbon-based films with fullerene-like structures. Such a structure was first discovered in carbon nitride (CNx) film [6]. Although the research in CNx films has been mainly driven by the search of superhard β-C3N4 structure, most of the attempts resulted in sp2-dominated materials [6–9]. Among these, the most promising sp2dominated CNx are referred to as fullerene-like CNx, owing the notation to their microstructure composed of a 3D arrangement of extended, bent and cross-linked N-containing graphite basal planes. Fullerene-like CNx exhibits high hardness with extremely large elastic recovery, making it a promising candidate for tribological
⁎ Corresponding author. E-mail address: [email protected] (P. Wang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.096
applications. So far, the formation of fullerene-like CNx films has been studied using different growth techniques such as magnetron sputtering [10], pulsed laser deposition [11], ion assisted deposition [12] and anodic jet carbon arc [13]. Considering the conditions for fullerene-like CNx nucleation and subsequent growth, low-energy ion bombardment (b100 eV) and moderate substrate temperatures (250–500 °C) have been shown to be essential for the synthesis of fullerene-like structures [14]. These growth conditions are necessary to increase the mobility and reactivity at the surface, and to enable selective etching of less-favorable bonding environments without detrimental ion-induced damage for the evolution of the microstructure [15]. It is well known that the friction of carbon films mainly arises from the chemical interactions caused by the strong free σ-bond at the sliding interfaces. Therefore, the elimination of free σ-bonds on sliding surfaces of carbon films by hydrogen atoms is an essential requirement for achieving low friction coefficients. However, the critical requirements of growth fullerene-like CNx structures limited the wider applications of the films, and the carbon-based films without hydrogen incorporation exhibited poor friction behavior compared with highly hydrogenated carbon-based films [16]. Recently, much effort has been devoted to synthesizing the hydrogenated fullerene-like carbon films using chemical vapor deposition techniques [17–19]. In comparison to fullerene-like CNx films, the hydrogenated fullerene-like carbon films deposited by chemical vapor deposition has less order in the graphitic planes, but a considerably higher hardness and lower friction. In our previous work, we reported hydrogenated carbon films containing fullerene-like structures (FL-C:H) prepared in our laboratory by magnetron sputtering of a titanium target in a hydrocarbon gas [19]. By investigating the film growth process, it was concluded that
P. Wang et al. / Thin Solid Films 518 (2010) 5938–5943
sputtering hydrocarbon layer deposited on titanium target may be essential to the formation of fullerene-like structure. Inspired by this conclusion, we tried to prepare FL-C:H film by using magnetron sputtering of a graphite target using Ar-hydrocarbon mixtures. Here, the fullerene-like structure was also obtained and the film even exhibited higher hardness and elastic recovery. Furthermore, the FL-C: H film showed low friction coefficients in a relatively wide range of environmental conditions, which promises a valuable material for applications requiring of insensitivity to the environment.
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formula (dmax − dmin) / dmax, where dmax and dmin are the maximum and minimum displacements during unloading, respectively. The friction behaviors of the films sliding against Si3N4 balls were measured on a ball-on-disc tribometer equipped with an environmental chamber with which the relative humidity and gaseous environment could be controlled. The friction experiments were performed with a normal load of 2 N, a sliding velocity of about 0.52 m/s, to a maximum sliding time of 60 min. The worn surfaces of the films and the counterpart balls were characterized using a JSM5600LV SEM.
2. Experimental details In this study, all carbon films with different structures were deposited in a custom-designed multifunctional deposition system. The system combines different physical and chemical deposition methods in one chamber, so carbon films with different structure can be prepared in one system, more details of the deposition system were given elsewhere [20]. FL-C:H films with the thickness of 1 μm were deposited on silicon substrates by magnetron sputtering of a graphite target. The film was deposited at a pressure of 5.0 × 10− 1 Pa using methane and argon gases as the precursors with the flow rates of 80 and 120 sccm, respectively. Prior to deposition, the silicon substrates were subjected to ion bombardment in an argon glow discharge under a negative bias voltage of 1000 V for 10 min to remove the native oxide layer. During deposition, the sputter current was limited at 2.5 A at discharge voltage of ∼500 V. The negative bias voltage was generated by a pulse power supply (4 kHz, 15% duty cycle) at a constant value of 1200 V. Although there was no deliberate substrate heating during deposition, local temperature could rise up to a maximum of 120 °C, as measured by a thermocouple clamped near the substrate holder. In addition, magnetron sputtering and radio frequency PECVD, two components mounted in the multifunctional deposition system, were used to deposit a-C and a-C:H films, and the deposition conditions are detailed in Table 1. X-ray photoelectron spectroscopy (XPS) was performed using a PHI-5702 system. The Al-Kα radiation was used as the excitation source. The XPS was collected in a constant analyzer energy mode, at a chamber pressure of 10− 8 Pa and the binding energy of the Au 4f7/2 core level electron (Au 4f7/2:84.0 eV) as the reference, at a pass energy of 29.35 eV. Fourier transformation infrared (FTIR) spectra were recorded on a Biorad FTS165 spectrometer to detect the bonding in the film. The film microstructure was characterized by JEM 2010 high-resolution transmission electron microscopy (HRTEM) with 0.19 nm resolution. Plan-view samples were obtained by floating off ∼ 20 nm thick films deposited on NaCl crystal in deionized water, the films were then collected directly onto Cu microscopy grids. Mechanical properties of the film were obtained by using a nanoindentater with a three-sided pyramidal Berkovich diamond tip and continuous stiffness option. Five replicate indentations were made for each sample, the maximum indentation depth was set below 10% of the film thickness (1 μm) in order to minimize substrate influence, and the hardness and elastic modulus were calculated from the loading–unloading curves. The elastic recovery is calculated using the
3. Results The XPS C1s spectrum of the FL-C:H film is shown in Fig. 1, for comparison, also the spectra of a-C and a-C:H film. Usually, the C 1s spectra of the amorphous carbon films can be fitted into three components around 284.3, 285.3 and 286.6 eV, respectively. The one around 284.3 eV corresponds to sp2 carbon atoms, and that around 285.3 eV corresponds to sp3 carbon atoms, a third peak of near 286.6 eV is assigned to some C–O contamination formed on the film surface due to air exposure. According to the work of Javier et al. [21], the content of the sp3 carbon atoms in the amorphous carbon films can be determined as the ratio of the corresponding sp3 peak area over the total C 1s peak area. Using method mentioned above, we can calculate that the sp3 content of the FL-C:H films is about 33%, the sp3 content of the a-C and a-C:H films is 20% and 42%, respectively. In order to get the bonding configuration in the film, the film was analyzed by FTIR. Fig. 2 shows the IR spectra of the FL-C:H, a-C and a-C:H film. Three bands can be observed in the spectra of the films. The absorption peak at 1380 cm− 1 represents sp3 CH3 or (CH3)n stretching mode [1]. The appearance of a peak centered around 1450 and 2920 cm− 1 indicates that hydrogen is predominantly bonded to saturated (sp3) carbon atoms [22,23]. The strong peak around 1600 cm− 1 can also be observed, its originated from the Raman active G band (graphite) at ∼1570 cm− 1 and D band (disorder) at ∼1360 cm− 1, which can be attributed to olefinic or aromatic vibrations of CC bonds in the film [1,22,23]. In the spectra of a-C film, the features are the C–H stretching band around 1380 cm− 1 and the CC vibration around 1640 cm− 1, and a weak band at 2920 cm− 1. In a-C:H film, CH stretching mode around 1450 cm− 1emerges, at the same time, the band intensity around 2920 cm− 1 increases remarkably and the band around 1640 cm− 1 weaken, indicating the decrease of CC vibration and the increase of CH stretching mode in a-C:H film. The result of FTIR analysis indicates that the FL-C:H film is hydrogenated and has complex configuration consisting of sp2 olefinic or aromatic CC bonds and sp3 C–H bonds.
Table 1 Summary of a-C and a-C:H film deposition conditions. Item
Target Ar gas flow rate (sccm) Methane gas flow rate (sccm) Deposition pressure (Pa) Power (W) DC bias (V) Thickness (μm)
Parameter a-C
a-C:H
FL-C:H
Graphite 150 0 0.5 1000 − 200 1
/ 150 60 5 700 − 200 1
Graphite 120 80 0.5 1250 − 1200 1
Fig. 1. The C1s spectra of FL-C:H, a-C and a-C:H films.
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Fig. 2. Infrared spectra of FL-C:H and reference a-C and a-C:H films for comparison.
Plan-view HRTEM images from a-C, a-C:H and FL-C:H films were used for structure investigation. The HRTEM images of the three kind films are shown in Fig. 3. Obviously, there were no detectable graphite plane fragments or curved graphite planes in the HRTEM images of a-C (Fig. 3(a)) and a-C:H film (Fig. 3(b)) due to a completely amorphous character of the film. For FL-C:H film (Fig. 3(c)), it clearly shows ordered domains of several nanometers in size. These domains consist of curved and closed planes which have small radii of curvature and present fullerene-like arrangements with 2–3 nm in diameter. In addition, no obvious cross-linking between these planes can be detected, these curved graphene planes are just homogeneously embedded in and separated by an amorphous carbon matrix. The HRTEM and IR results indicate clearly that a substantial part of the film is made up of fragments of such fullerene-like nanostructures mixed in a random forming co-continuous matrix with the hydrogenated amorphous carbon component. It should be mentioned that the quantity and the dimension of the fullerene-like structure in our film are less compared with previous reported well-structure fullerene-like films, we ascribe it to the incorporation of hydrogen into the film. It is well known that methane can be decomposed into some hydrocarbon radicals and atomic or ionic hydrogen [24,25]. The hydrogen atoms prefer to stay at the edges rather than stay inside the clusters thereby inhibiting the 3D network growth and cross-linking [26]. As a result, isolated rather
cross-linking fullerene-like structure was formed. Correspondingly, the size of the fullerene-like structures in our films is smaller than that of previous reported films [5,7,11]. One outstanding feature reported for fullerene-like films is its extreme elasticity in combination with relatively high elastic modulus and hardness. These properties were determined by nanoindentation in this study. Fig. 4 shows the load–displacement curves of the FL-C:H film compared to that of a-C film and a-C:H film. The plots clearly demonstrate that the FL-C:H film exhibits a high elasticity combined with a considerable hardness. The hardness of the FL-C:H film is 27.4 GPa, much higher than that of a-C film and a-C:H film, 12.4 and 14.5 GPa, respectively. Noticeably, the FL-C:H film shows almost complete elastic recovery (as high as 95%) after removing load. For comparison, the reference carbon films all exhibit residual deformation. Moreover, the FL-C:H film shows a strong curvature of the unloading portion and a no or very little tendency to plastic deformation, which indicates that most indentation energy is stored elastically and is released upon unload. These processes can be obtained by reversible bond rotation and bond angle deflection rather than slip and bond breaking [10,11]. Hydrogenated carbon films are usually used as wear-resistance and friction-reducing coatings in many fields. Although typical a-C:H films exhibit extremely low friction coefficients in dry nitrogen or ultra-high vacuum [27–30], their tribological properties remain poor at humid or oxygen-dominant atmosphere [30,31], which seriously limit their technological applications. It has been a longtime dream for tribologists or material scientists to fabricate a lubricating film with a low friction coefficient in different testing environments. Fig. 5 shows the friction coefficients of the films against Si3N4 balls as a function of sliding time under different atmosphere. It is clear that the friction behaviors of the a-C and a-C:H films are strongly dependent on the relative humidity. For a-C film, the friction coefficient as high as ∼0.18 was obtained in dry air (RH=5%) and it significantly decreased to ∼0.12 in humid air (RH=50%). Oppositely, the a-C:H film exhibited very low and stable friction coefficient of ∼0.07 in dry air, while the friction was quite unstable and the friction coefficient was increased to about 0.12 as the relative humidity increased to 50%. With respect to the FL-C:H film, low and stable friction coefficients were obtained in both dry and humid air, merely varying from ∼0.026 in dry air to ∼0.042 in humid air. Noticeably, the lowest friction coefficient, about 0.02, is recorded for the FL-C:H film tested in oxygen atmosphere. To get more insights into the friction and wear mechanism of the three kinds of films sliding against the Si3N4 balls, the worn surfaces of
Fig. 3. HRTEM images of (a): a-C film, (b): a-C:H film and (c) FL-C:H film.
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coefficient increases to 0.11 when humid air is introduced into the test chamber. The ranges of dispersion of the friction coefficient for the a-C:H film in different atmospheres reached up to 0.1. Noticeably, the FL-C:H film presented low friction coefficient on the whole and the maximum friction coefficient difference in different atmospheres is as small as 0.02, which indicates the friction-coefficient-insensitivity of the FL-C:H film to different atmospheres.
4. Discussion
Fig. 4. Load–displacement curves obtained from the FL-C:H film in comparison to a-C and a-C:H films.
the films and counterpart balls were studied by SEM. Fig. 6 shows the SEM pictures of the worn surface of the films and counterpart balls tested in dry air. As seen in Fig. 6(a), the friction surface of the FL-C:H film was smooth in appearance and film remained intact, despite after the 60,000 revolutions performed, the wear track on the film is barely visible, except for few debris scattering at the edge of the wear track. The friction surface on the counterpart Si3N4 ball was covered by a compact and homogeneous transfer layer with few wear debris around (Fig. 6(b)). In the case of a-C:H film, obvious flake-like desquamation and wear debris was observed on the wear track of the film (Fig. 6(c)). Although a compact transfer layer was also observed on the contact region of the Si3N4 ball, the size of the transfer layer is smaller and more wear debris accumulated around the transfer layer (Fig. 6(d)), compared to the case of the FL-C:H film. In the case of a-C film (Fig. 6 (e,f)), the friction surface of the Si3N4 ball was larger and much rougher, a discontinuous layer is formed in the middle of the worn surface of the film, indicating a wear failure of the film. The friction behaviors of the three kinds of films in different atmospheres are summarized in Fig. 7. It can be seen that the highest friction coefficient as well as the widest range of friction coefficient differences was obtained for a-C film. Although the a-C:H film exhibited the lowest friction coefficient of ∼0.013 in dry nitrogen, its friction
Fig. 5. The friction behaviors of the FL-C:H films against Si3N4 balls as a function of sliding revolutions under different atmospheres. For comparison, also the friction coefficients of the a-C and a-C:H films are shown.
Combining the above-mentioned experimental results, it can be concluded that it is possible to improve the mechanical and tribological properties of hydrogenated carbon films by the incorporation of fullerene-like graphene planes. We argue that the excellent mechanical properties of the fullerene-like films, especially toughness and elasticity, resulted from the existence of strong interlocking between fullerene-like graphene planes and hydrogenated amorphous carbon matrix, which effectively inhibits interplanar slip of the predominantly sp2-coordinated graphene sheets [5,6,10]. That is, at the edges of these fullerene-like graphene planes unsaturated bonds through which they may bond either to the sp3 bonded chain-like matrix or to hydrogen atoms [5]. As the bond between graphene and sp3 bonded chain-like matrix is covalent bond, which is stronger than van der Waals-type bonding in graphite, the amorphous matrix, especially high sp3 bond content, can constrict the gliding of the graphene plane, so the film containing this stiff network will exhibit high hardness. Simultaneously, fullerene-like structure just like a “molecule spring” disperses in film [19], reserves the elastic energy during distortion through reversible bond rotation and bond angle deflection, when sets of graphene planes are compressed their built-in curvature causes them to recover a shape close to their initial state after deformation, which is the origin of the apparently high elastic recovery in the films. As a result, a material combining stiffness with flexibility can be formed. The results also demonstrated that the FL-C:H film has low friction coefficient in different testing environments. Considering the low friction coefficient of FL-C:H film, mechanism such as super-low friction coefficient in a-C:H film [28] can still occur in FL-C:H film. With respect to the friction-coefficient-insensitivity of the FL-C:H film to different testing environments, following features of the film should be considered. The FL-C:H film consists of curved graphene planes and hydrogenated carbon matrix. The curved graphene planes mainly consist of sp2-bonds, at the inner of these fullerene-like structures, no hydrogen atoms bond on carbon sites and some free-bond exist in these graphene planes. During the film sliding in humid air/or oxygen environments, the free-bonds in graphene planes can be terminated and passivated by adsorbing water/or oxygen molecules, just like in a-C films [27–29], and hence the FL-C:H films exhibit low friction coefficient in humid air and oxygen environments. Considering the rest of the FL-C: H film, hydrogenated carbon matrix contains high hydrogen content, which is similar to the structure of a-C:H films, most of dangling bonds on the hydrogenated carbon matrix surface are eliminated by hydrogen, which significantly eliminates the energetic dangling bonds at the edge of the growing structure as discussed above, thereby the elimination of free σ-bonds on sliding surfaces by hydrogen atoms can provide a high degree of chemical passivation and thus low friction [29,31]. As a result, the unique structure of embedded fullerene-like graphene planes in hydrogenated carbon matrix can enhance the stability of the FL-C:H film in humid environments and oxygen-dominant atmosphere, and prevent it from being oxidized during friction in different environments [32]. Furthermore, good wear-resistance of the FL-C:H film was owed to high hardness and excellent elastic recovery of the film, which assure the good load-bearing and immediate recovery of friction-induced deformation during friction. Consequently, a stiff as well as flexible material with low friction coefficients in a relatively wide range of environment can be formed.
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Fig. 6. SEM pictures of the wear tracks on the (a) FL-C:H, (c) a-C:H and (e) a-C films, and corresponding wear scars on the counterpart balls (b, d, f) obtained in air.
5. Conclusions A hydrogenated carbon film containing a fullerene-like structure was synthesized using magnetron sputtering of a graphite target in methane atmosphere. Compared with amorphous carbon and hydrogenated amorphous carbon films, the film not only exhibited higher hardness and elastic recovery but also showed low friction coefficients in a wider range of environment, promising a valuable material with excellent mechanical properties in addition to friction-insensitivity to environment. Acknowledgements The authors gratefully acknowledged the financial support from the National Natural Science Foundation of China (Grant No: 50902133 and 50905177), 973 projects (Grant No. 2007CB607601) of the Ministry of Science and Technology of China. References Fig. 7. Friction coefficients of FL-C:H, a-C:H and a-C films for sliding under different atmospheres.
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