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Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring
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Applied Surface Science 254 (2008) 2364–2368 www.elsevier.com/locate/apsusc
Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring Naoki Yasumaru a,*, Kenzo Miyazaki b, Junsuke Kiuchi c a
Department of Mechanical Engineering, Fukui National College of Technology, Sabae 916-8507, Japan b Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan c Eyetec Co. Ltd., Sabae 916-0016, Japan Received 7 July 2007; received in revised form 13 September 2007; accepted 13 September 2007 Available online 18 September 2007
Abstract This paper reports tribological properties of diamond-like carbon (DLC) films nanostructured by femtosecond (fs) laser ablation. The nanostructure was formed in an area of more than 15 mm 15 mm on the DLC surface, using a precise target-scan system developed for the fslaser processing. The frictional properties of the DLC film are greatly improved by coating a MoS2 layer on the nanostructured surface, while the friction coefficient can be increased by surface texturing of the nanostructured zone in a net-like patterning. The results demonstrate that the tribological properties of a DLC surface can be controlled using fs-laser-induced nanostructuring. # 2007 Elsevier B.V. All rights reserved. PACS : 61.80.Ba; 79.20.Ds; 42.62.Cf Keywords: Femtosecond-laser ablation; Nanostructure; Tribology; Diamond-like carbon
1. Introduction Diamond-like carbon (DLC) has been extensively used as a material of protective film for hard disks and magnetic heads, because of its hardness, chemical inertness, and insulation properties close to those of diamond [1]. Furthermore, much attention has recently been focused on the excellent surface smoothness and low friction coefficient of DLC films for a variety of applications in tribological technology. Several authors have used lasers for the purposes of texturing thin DLC film surfaces and optimizing the tribological properties, and the effect of such treatment on the friction coefficient has been reported [2,3]. For example, Dumitru et al. observed that laser texturing consisting of shallow dimples (25 mm in diameter and 15–20 mm deep) greatly increased the lifetime of the DLC film [2]. Although laser texturing has been studied so far for improving the frictional properties of a variety of materials [4], the size of
* Corresponding author. Tel.: +81 778 62 1111; fax: +81 778 62 3306. E-mail address: [email protected] (N. Yasumaru). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.037
laser texturing has been limited to a micrometer level, and no study has been reported of the effect of nanometer-size texturing on the tribological properties. The difficulty of laserinduced nanoscale texturing is due to the diffraction limit of light that restricts the surface pattern to a size larger than the laser wavelength l, while such nanoscale texturing of thin films is often required to maintain effective protection of substrates used for micro-machines and electronic devices. Our previous studies have demonstrated that a DLC film surface, as well as TiN, can be structured on a nanometer level when femtosecond (fs) laser pulses are irradiated at a fluence around the ablation threshold [5–8]. The linearly polarized fslaser pulse produces arrays of slender ablation traces on the surface, while the circularly polarized light forms dot-like traces, as described in detail in Ref. [5]. The mean spacing or periodicity in the nanostructure was observed to decrease down to l/10, and a nanostructure was produced with a spacing of approximately 30 nm by utilizing ultraviolet fs-laser pulses at 267 nm [5]. The nanostructure is much smaller than the wellknown ripple structures often observed on laser-ablated material surfaces [9,10]. A recent study suggests that the generation of a local field plays a major role in this nanostructure formation [11].
N. Yasumaru et al. / Applied Surface Science 254 (2008) 2364–2368
In addition, the Raman spectrum of the nanostructured DLC surface indicated that the DLC was modified into a glassy carbon (GC) layer under almost the same experimental condition as that for nanostructuring [6]. The GC layer, which has higher conductivity and thermal resistance, provides additional physical and chemical properties to the thin DLC film [12], in addition to possible improvement of the tribological properties. This paper reports tribological properties of the nanostructured DLC film, on the basis of our recent studies of the fs-laser-induced nanostructure formation on hard thin films [5–8]. In the present study we developed a precise targetscanning apparatus to produce a broad nanostructured area on the target surface, and measured the frictional properties with a ball-on-disc friction test machine. The results show that the DLC film has excellent frictional performance when the nanostructured surface is coated with molybdenum disulfide (MoS2). Control of the tribological properties was also studied by means of surface texturing the nanostructured area, and it was found that the friction coefficient of the surface can be controlled with a net-like patterning of the nanostructured zone. 2. Experimental procedures DLC film was deposited on commercial titanium plates of 2 mm thickness using an unbalanced magnetron sputtering system with a carbon target at a bias voltage of 150 V in a mixed atmosphere of Ar and 5% CH4. The coated DLC film was 1.4 mm thick, with a Vickers hardness HVof 3000 and a surface roughness of 9 nm. This DLC film was irradiated in air with linearly polarized, 800 nm, 100 fs-laser pulses from a Ti:sapphire chirped-pulse amplification system operated at a repetition frequency of 1 kHz. The fs-laser pulse energy of E = 110 mJ was focused on the DLC surface using a 50 cm focal-length parabolic mirror to ablate the film. The laser fluence F for E = 110 mJ corresponds to F = 0.14 J/cm2, which is just above the ablation threshold of F = 0.11 J/cm2 reported in Ref. [6]. The target plate coated with DLC was mounted on a precise X–Y stage and continuously translated at a constant speed to ablate the film surface over an area of 15 mm 15 mm, while the laser was operated at 1 kHz. The scan speed of the focal point was fixed at 1.6 mm/pulse for the nanostructure formation. A single scan produced an ablated line zone with a width D 240 mm. In the present experiment, two types of laser scan were used, as schematically shown in Fig. 1. Fig. 1(a) illustrates the parallel scan of the focal spot with a periodicity of L = 120 mm, used to produce a uniformly nanostructured surface. It was estimated from the scan conditions that fs-laser pulses of approximately 150 shots were effectively superimposed on the target point during the parallel scan. In addition to the nanostructured surface, the tribological properties of a nanostructured surface coated with a 0.5–1 mm MoS2 layer were investigated, where a different magnetron sputtering system was used for this coating. On the other hand, as shown in Fig. 1(b), a crossed scan along a net-like pattern with a periodicity of H = 480 mm was used to form a partially
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Fig. 1. Schematic diagram of the laser scan for (a) uniformly nanostructured surface and (b) net-like patterning of the nanostructured zone.
nanostructured area on the surface, and the effect of nonuniform nanostructuring on the tribological properties was investigated. An optical microscope, a field-emission scanning electron microscope (SEM), and a scanning probe microscope (SPM) were used to examine the morphological changes of the DLC surface. The bonding structure of the ablated DLC film was analyzed using Raman spectroscopy with a 514.5 nm Ar ion laser with a focal spot size of 1 mm. The friction coefficient of the DLC surface was measured in air with a ball-on-disc friction test machine (Rhesca Co. Ltd., FPR-2000). For this measurement, two kinds of 6 mm diameter balls were used; one made of hardened bearing steel with an HV of ca. 600, and one of a hard metal (WC–Co) with an HV of ca. 1600. The ball was rotated with a radius of 3–5 mm at a constant velocity of 0.03 m/s, maintaining a load of W = 2 N or 10 N applied to the DLC surface, and the friction coefficient was measured at the revolution R, up to R = 104. 3. Results and discussion The friction coefficient m was measured for five kinds of DLC surfaces. Those test pieces are the non-irradiated DLC films without coating and with a MoS2 coating, the uniformly nanostructured DLC without coating and with a MoS2 coating, and the DLC partially nanostructured with a net-like pattern having no coating. The relevant tendencies of friction coefficients obtained for these films are compared in Table 1, and details of the results are presented and discussed below. 3.1. Nanostructuring and surface modification Fig. 2 shows SEM images of DLC film surfaces ablated using (a) the parallel scan with a periodicity of L = 120 mm and
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Table 1 Summary of the friction coefficient m measured and compared with m0 of the non-irradiated DLC DLC surface
Coating
m (compared with m0)
Non-irradiated
None MoS2
m0 Decrease
Uniformly nanostructured
None MoS2
Small decrease or almost unchanged Large decrease
Partially nanostructured
None
Large increase
(b) the crossed scan with H = 480 mm. The surface undulation created by the laser ablation was measured to be less than 10 nm with the SPM. Fig. 2(c) shows a typical SEM image of the ablated area observed at a larger magnification than those in Fig. 2(a) and (b), where the fabricated nanostructure is almost oriented in the direction perpendicular to the laser polarization. The mean spacing in the periodic nanostructures was measured to be approximately 120 nm along the polarization direction, similar to our previous observations, which is much smaller than the laser wavelength used [5–8]. The nanostructure formation was observed on the whole surface area irradiated with the fs-laser pulses. Raman spectra confirmed that the laser-irradiated area of the DLC surface was modified into GC. Fig. 3 shows (a) the typical spectrum observed for the irradiated DLC surface, which represents the GC including two peaks at 1355 and 1590 cm1 [6,7,13] and (b) a Raman spectrum of the non-irradiated DLC film for comparison. 3.2. Tribological properties of nanostructured DLC Fig. 4 represents the friction coefficient m, of the uniformly nanostructured DLC surface, which was measured at a load of W = 10 N with (a) the steel ball at a revolution of R = 2000 and with (b) the WC–Co ball at R = 104, with the results for the nonirradiated DLC shown for comparison. Compared with the friction coefficient m0 of the non-irradiated DLC surface, the friction coefficient m of the nanostructured surface was observed to decrease down to m (4/5)m0 for the steel ball, whereas m m0 was observed for the WC–Co ball. The decrease in m suggests that the nanostructured layer acts as an additional thin layer that reduces friction between the DLC surface and the steel ball, which has a relatively high adhesive property, as discussed below regarding the MoS2 coating on DLC. As shown in Fig. 4, the friction coefficient measured with the WC–Co ball is much smaller than that with the steel ball,
Fig. 2. SEM images of the DLC surfaces irradiated with fs pulses of (a) a parallel scan, (b) a crossed scan along a net-like pattern, and (c) an image of the nanostructured surface. The image magnifications are 30 for (a) and (b), and 20,000 for (c). The arrow indicates the polarization direction of the fs-laser pulses.
Fig. 3. Raman spectra of (a) the nanostructured DLC film and (b) the nonirradiated film.
N. Yasumaru et al. / Applied Surface Science 254 (2008) 2364–2368
Fig. 4. Friction coefficients of the nanostructured DLC film (shaded bar) and the non-irradiated DLC film (white bar) measured under load W = 10 N with (a) a steel ball at R = 2000 and (b) a WC–Co ball at R = 104.
and the nanostructure appears to have no effect on the frictional property. This results from the high hardness and low adhesive property of the WC–Co ball, compared with that of the steel ball. We have found that the frictional property of DLC films can be greatly improved by coating a MoS2 layer on the nanostructured surface. Fig. 5 shows the results of m for the nanostructured surface coated with a MoS2 layer, together with m measured for the non-irradiated DLC films with and without the MoS2 coating, where the measurements were made with (a) a steel ball at W = 2 N and R = 104, (b) a WC–Co ball at W = 10 N and R = 4000, and with (c) a WC–Co ball at W = 10 N and R = 104. The results obtained for the different conditions clearly demonstrate the pronounced effect of the MoS2 layer on the frictional property of the DLC films. For example, in group (a) with the MoS2 coating on the nanostructured DLC, m is decreased to m (2/5)m0, while the friction coefficient m0 of the non-irradiated DLC film with the MoS2 coating is also decreased to m0 (3/4)m0. Such a large decrease in m due to the MoS2 coating is also seen in groups (b) and (c), representing the results measured with the WC–Co ball. The most pronounced decrease to m (1/4)m0 is observed for group (b), together with the smallest friction coefficient of m 0.02. This large reduction of m with the additional coating can be attributed to the following two major reasons. The surface nanostructure acts as a lubricant reservoir to store the MoS2 layer, and the GC layer produced improves the bonding interface between the DLC and MoS2 layers.
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Fig. 6. Friction coefficients of the DLC film partially nanostructured with a netlike pattern (shaded bar) and the non-irradiated DLC film (white bar), measured at W = 10 N with (a) a steel ball and (b) a WC–Co ball. In (a) and (b), revolutions of R = 187 and 52 were used, respectively, for the partially nanostructured DLC film, and R = 104 was used for the non-irradiated DLC film.
3.3. Patterning of nanostructure In order to control the tribological properties of DLC films, we produced partially nanostructured DLC surfaces with a crossed scan, as shown in Fig. 1(b), and measured m with the same methods as the above. Fig. 6 shows the result measured at W = 10 N with (a) the steel ball and (b) the WC–Co ball, where m0 of the non-irradiated DLC is shown for comparison. For this measurement, the friction test machine had to be stopped at R = 187 and 52 for the steel ball and the WC–Co ball, respectively, because m was so large as to induce a rapid increase in mechanical vibration at the contact surface of the sample. As seen in Fig. 6, a notable increase from m0 = 0.08 to m = 0.45 was observed for the hard WC–Co ball. This large increase in m is ascribed to the spatial modulation of surface hardness that was introduced by the patterning of nanostructured zone, as discussed below. In the present experiment, the laser fluence used was so small that the surface roughness was hardly influenced. In fact, the ablation depth measured with SPM was in the order of 10 nm, which was almost comparable to the surface roughness of the initial nonablated surfaces. This indicates that the change in surface roughness is not the origin of the large m for the patterned surface. On the other hand, the fs-laser-induced nanostructuring does lead to a decrease in the hardness of the surface, due to the structural change of DLC into GC. The net-like patterning of the ablated zone created by the crossed scan could produce spatial modulation of the surface hardness with a periodicity of H = 240 mm. This spatial modulation of hardness could increase m of the DLC film. We have observed that m can be changed by controlling H and D in the patterning of the nanostructured zone. 4. Conclusions
Fig. 5. Friction coefficients of the nanostructured DLC film with a MoS2 coating (black bar), the non-irradiated DLC film with a MoS2 coating (hatched bar), and the non-irradiated DLC film (white bar). Each group represents the results measured with (a) a steel ball at W = 2 N and R = 104, (b) a WC–Co ball at W = 10 N and R = 4000, and (c) a WC–Co ball at W = 10 N and R = 104.
We have studied the tribological properties of DLC surfaces nanostructured with fs-laser pulses. A thin MoS2 layer coated on the nanostructured surface was found to be extremely effective for the purpose of decreasing the friction coefficient m of the DLC surface, where the smallest value of m = 0.02 was observed for the WC–Co ball. We have also developed an effective method for increasing m by means of patterning the
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nanostructured zone on the DLC surface, while maintaining the initial surface flatness. The present results demonstrate a useful and effective method to control the tribological properties of DLC films to a considerable extent using fs-laser pulses. Acknowledgements The authors would like to express their thanks to K. Akari of Kobe Steel, Ltd. for the DLC coatings, and H. Magara of the Industrial Technical Center of Fukui Prefecture for measurement of the Raman spectra. References [1] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, T. Akamatsu, J. Appl. Phys. 64 (1988) 6464.
[2] G. Dumitru, V. Romano, H.P. Weber, S. Pimenov, T. Kononenko, J. Hermann, S. Bruneau, Y. Gerbig, M. Shupegin, Diamond Rel. Mater. 12 (2003) 1034. [3] A. Erdemir, Tribol. Int. 38 (2005) 249. [4] I. Etsion, Trans. ASME J. Tribol. 127 (2005) 248. [5] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 76 (2003) 983. [6] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 79 (2004) 425. [7] K. Miyazaki, N. Maekawa, W. Kobayashi, M. Kaku, N. Yasumaru, J. Kiuchi, Appl. Phys. A 80 (2005) 17. [8] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 81 (2005) 933. [9] See, for example, D. Ba¨uerle, Laser Processing and Chemistry, Springer, Berlin 1996 (Chapter 28). [10] J. Reif, F. Costache, M. Henyk, S.V. Pandelov, Appl. Surf. Sci. 197/198 (2002) 891, and references therein. [11] G. Miyaji, K. Miyazaki, Appl. Phys. Lett. 89 (2006) 191902. [12] A. Dekanski, J. Stevanovic, R. Stevanovic, B.Z. Nikolic, V.M. Jovanovic, Carbon 39 (2001) 1195, and references therein. [13] M. Yoshikawa, N. Nagai, M. Matsuki, H. Fukuda, G. Katagiri, H. Ishida, A. Ishitani, I. Nagai, Phys. Rev. B 46 (1992) 7169.
Applied Surface Science 254 (2008) 2364–2368 www.elsevier.com/locate/apsusc
Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring Naoki Yasumaru a,*, Kenzo Miyazaki b, Junsuke Kiuchi c a
Department of Mechanical Engineering, Fukui National College of Technology, Sabae 916-8507, Japan b Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan c Eyetec Co. Ltd., Sabae 916-0016, Japan Received 7 July 2007; received in revised form 13 September 2007; accepted 13 September 2007 Available online 18 September 2007
Abstract This paper reports tribological properties of diamond-like carbon (DLC) films nanostructured by femtosecond (fs) laser ablation. The nanostructure was formed in an area of more than 15 mm 15 mm on the DLC surface, using a precise target-scan system developed for the fslaser processing. The frictional properties of the DLC film are greatly improved by coating a MoS2 layer on the nanostructured surface, while the friction coefficient can be increased by surface texturing of the nanostructured zone in a net-like patterning. The results demonstrate that the tribological properties of a DLC surface can be controlled using fs-laser-induced nanostructuring. # 2007 Elsevier B.V. All rights reserved. PACS : 61.80.Ba; 79.20.Ds; 42.62.Cf Keywords: Femtosecond-laser ablation; Nanostructure; Tribology; Diamond-like carbon
1. Introduction Diamond-like carbon (DLC) has been extensively used as a material of protective film for hard disks and magnetic heads, because of its hardness, chemical inertness, and insulation properties close to those of diamond [1]. Furthermore, much attention has recently been focused on the excellent surface smoothness and low friction coefficient of DLC films for a variety of applications in tribological technology. Several authors have used lasers for the purposes of texturing thin DLC film surfaces and optimizing the tribological properties, and the effect of such treatment on the friction coefficient has been reported [2,3]. For example, Dumitru et al. observed that laser texturing consisting of shallow dimples (25 mm in diameter and 15–20 mm deep) greatly increased the lifetime of the DLC film [2]. Although laser texturing has been studied so far for improving the frictional properties of a variety of materials [4], the size of
* Corresponding author. Tel.: +81 778 62 1111; fax: +81 778 62 3306. E-mail address: [email protected] (N. Yasumaru). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.037
laser texturing has been limited to a micrometer level, and no study has been reported of the effect of nanometer-size texturing on the tribological properties. The difficulty of laserinduced nanoscale texturing is due to the diffraction limit of light that restricts the surface pattern to a size larger than the laser wavelength l, while such nanoscale texturing of thin films is often required to maintain effective protection of substrates used for micro-machines and electronic devices. Our previous studies have demonstrated that a DLC film surface, as well as TiN, can be structured on a nanometer level when femtosecond (fs) laser pulses are irradiated at a fluence around the ablation threshold [5–8]. The linearly polarized fslaser pulse produces arrays of slender ablation traces on the surface, while the circularly polarized light forms dot-like traces, as described in detail in Ref. [5]. The mean spacing or periodicity in the nanostructure was observed to decrease down to l/10, and a nanostructure was produced with a spacing of approximately 30 nm by utilizing ultraviolet fs-laser pulses at 267 nm [5]. The nanostructure is much smaller than the wellknown ripple structures often observed on laser-ablated material surfaces [9,10]. A recent study suggests that the generation of a local field plays a major role in this nanostructure formation [11].
N. Yasumaru et al. / Applied Surface Science 254 (2008) 2364–2368
In addition, the Raman spectrum of the nanostructured DLC surface indicated that the DLC was modified into a glassy carbon (GC) layer under almost the same experimental condition as that for nanostructuring [6]. The GC layer, which has higher conductivity and thermal resistance, provides additional physical and chemical properties to the thin DLC film [12], in addition to possible improvement of the tribological properties. This paper reports tribological properties of the nanostructured DLC film, on the basis of our recent studies of the fs-laser-induced nanostructure formation on hard thin films [5–8]. In the present study we developed a precise targetscanning apparatus to produce a broad nanostructured area on the target surface, and measured the frictional properties with a ball-on-disc friction test machine. The results show that the DLC film has excellent frictional performance when the nanostructured surface is coated with molybdenum disulfide (MoS2). Control of the tribological properties was also studied by means of surface texturing the nanostructured area, and it was found that the friction coefficient of the surface can be controlled with a net-like patterning of the nanostructured zone. 2. Experimental procedures DLC film was deposited on commercial titanium plates of 2 mm thickness using an unbalanced magnetron sputtering system with a carbon target at a bias voltage of 150 V in a mixed atmosphere of Ar and 5% CH4. The coated DLC film was 1.4 mm thick, with a Vickers hardness HVof 3000 and a surface roughness of 9 nm. This DLC film was irradiated in air with linearly polarized, 800 nm, 100 fs-laser pulses from a Ti:sapphire chirped-pulse amplification system operated at a repetition frequency of 1 kHz. The fs-laser pulse energy of E = 110 mJ was focused on the DLC surface using a 50 cm focal-length parabolic mirror to ablate the film. The laser fluence F for E = 110 mJ corresponds to F = 0.14 J/cm2, which is just above the ablation threshold of F = 0.11 J/cm2 reported in Ref. [6]. The target plate coated with DLC was mounted on a precise X–Y stage and continuously translated at a constant speed to ablate the film surface over an area of 15 mm 15 mm, while the laser was operated at 1 kHz. The scan speed of the focal point was fixed at 1.6 mm/pulse for the nanostructure formation. A single scan produced an ablated line zone with a width D 240 mm. In the present experiment, two types of laser scan were used, as schematically shown in Fig. 1. Fig. 1(a) illustrates the parallel scan of the focal spot with a periodicity of L = 120 mm, used to produce a uniformly nanostructured surface. It was estimated from the scan conditions that fs-laser pulses of approximately 150 shots were effectively superimposed on the target point during the parallel scan. In addition to the nanostructured surface, the tribological properties of a nanostructured surface coated with a 0.5–1 mm MoS2 layer were investigated, where a different magnetron sputtering system was used for this coating. On the other hand, as shown in Fig. 1(b), a crossed scan along a net-like pattern with a periodicity of H = 480 mm was used to form a partially
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Fig. 1. Schematic diagram of the laser scan for (a) uniformly nanostructured surface and (b) net-like patterning of the nanostructured zone.
nanostructured area on the surface, and the effect of nonuniform nanostructuring on the tribological properties was investigated. An optical microscope, a field-emission scanning electron microscope (SEM), and a scanning probe microscope (SPM) were used to examine the morphological changes of the DLC surface. The bonding structure of the ablated DLC film was analyzed using Raman spectroscopy with a 514.5 nm Ar ion laser with a focal spot size of 1 mm. The friction coefficient of the DLC surface was measured in air with a ball-on-disc friction test machine (Rhesca Co. Ltd., FPR-2000). For this measurement, two kinds of 6 mm diameter balls were used; one made of hardened bearing steel with an HV of ca. 600, and one of a hard metal (WC–Co) with an HV of ca. 1600. The ball was rotated with a radius of 3–5 mm at a constant velocity of 0.03 m/s, maintaining a load of W = 2 N or 10 N applied to the DLC surface, and the friction coefficient was measured at the revolution R, up to R = 104. 3. Results and discussion The friction coefficient m was measured for five kinds of DLC surfaces. Those test pieces are the non-irradiated DLC films without coating and with a MoS2 coating, the uniformly nanostructured DLC without coating and with a MoS2 coating, and the DLC partially nanostructured with a net-like pattern having no coating. The relevant tendencies of friction coefficients obtained for these films are compared in Table 1, and details of the results are presented and discussed below. 3.1. Nanostructuring and surface modification Fig. 2 shows SEM images of DLC film surfaces ablated using (a) the parallel scan with a periodicity of L = 120 mm and
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Table 1 Summary of the friction coefficient m measured and compared with m0 of the non-irradiated DLC DLC surface
Coating
m (compared with m0)
Non-irradiated
None MoS2
m0 Decrease
Uniformly nanostructured
None MoS2
Small decrease or almost unchanged Large decrease
Partially nanostructured
None
Large increase
(b) the crossed scan with H = 480 mm. The surface undulation created by the laser ablation was measured to be less than 10 nm with the SPM. Fig. 2(c) shows a typical SEM image of the ablated area observed at a larger magnification than those in Fig. 2(a) and (b), where the fabricated nanostructure is almost oriented in the direction perpendicular to the laser polarization. The mean spacing in the periodic nanostructures was measured to be approximately 120 nm along the polarization direction, similar to our previous observations, which is much smaller than the laser wavelength used [5–8]. The nanostructure formation was observed on the whole surface area irradiated with the fs-laser pulses. Raman spectra confirmed that the laser-irradiated area of the DLC surface was modified into GC. Fig. 3 shows (a) the typical spectrum observed for the irradiated DLC surface, which represents the GC including two peaks at 1355 and 1590 cm1 [6,7,13] and (b) a Raman spectrum of the non-irradiated DLC film for comparison. 3.2. Tribological properties of nanostructured DLC Fig. 4 represents the friction coefficient m, of the uniformly nanostructured DLC surface, which was measured at a load of W = 10 N with (a) the steel ball at a revolution of R = 2000 and with (b) the WC–Co ball at R = 104, with the results for the nonirradiated DLC shown for comparison. Compared with the friction coefficient m0 of the non-irradiated DLC surface, the friction coefficient m of the nanostructured surface was observed to decrease down to m (4/5)m0 for the steel ball, whereas m m0 was observed for the WC–Co ball. The decrease in m suggests that the nanostructured layer acts as an additional thin layer that reduces friction between the DLC surface and the steel ball, which has a relatively high adhesive property, as discussed below regarding the MoS2 coating on DLC. As shown in Fig. 4, the friction coefficient measured with the WC–Co ball is much smaller than that with the steel ball,
Fig. 2. SEM images of the DLC surfaces irradiated with fs pulses of (a) a parallel scan, (b) a crossed scan along a net-like pattern, and (c) an image of the nanostructured surface. The image magnifications are 30 for (a) and (b), and 20,000 for (c). The arrow indicates the polarization direction of the fs-laser pulses.
Fig. 3. Raman spectra of (a) the nanostructured DLC film and (b) the nonirradiated film.
N. Yasumaru et al. / Applied Surface Science 254 (2008) 2364–2368
Fig. 4. Friction coefficients of the nanostructured DLC film (shaded bar) and the non-irradiated DLC film (white bar) measured under load W = 10 N with (a) a steel ball at R = 2000 and (b) a WC–Co ball at R = 104.
and the nanostructure appears to have no effect on the frictional property. This results from the high hardness and low adhesive property of the WC–Co ball, compared with that of the steel ball. We have found that the frictional property of DLC films can be greatly improved by coating a MoS2 layer on the nanostructured surface. Fig. 5 shows the results of m for the nanostructured surface coated with a MoS2 layer, together with m measured for the non-irradiated DLC films with and without the MoS2 coating, where the measurements were made with (a) a steel ball at W = 2 N and R = 104, (b) a WC–Co ball at W = 10 N and R = 4000, and with (c) a WC–Co ball at W = 10 N and R = 104. The results obtained for the different conditions clearly demonstrate the pronounced effect of the MoS2 layer on the frictional property of the DLC films. For example, in group (a) with the MoS2 coating on the nanostructured DLC, m is decreased to m (2/5)m0, while the friction coefficient m0 of the non-irradiated DLC film with the MoS2 coating is also decreased to m0 (3/4)m0. Such a large decrease in m due to the MoS2 coating is also seen in groups (b) and (c), representing the results measured with the WC–Co ball. The most pronounced decrease to m (1/4)m0 is observed for group (b), together with the smallest friction coefficient of m 0.02. This large reduction of m with the additional coating can be attributed to the following two major reasons. The surface nanostructure acts as a lubricant reservoir to store the MoS2 layer, and the GC layer produced improves the bonding interface between the DLC and MoS2 layers.
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Fig. 6. Friction coefficients of the DLC film partially nanostructured with a netlike pattern (shaded bar) and the non-irradiated DLC film (white bar), measured at W = 10 N with (a) a steel ball and (b) a WC–Co ball. In (a) and (b), revolutions of R = 187 and 52 were used, respectively, for the partially nanostructured DLC film, and R = 104 was used for the non-irradiated DLC film.
3.3. Patterning of nanostructure In order to control the tribological properties of DLC films, we produced partially nanostructured DLC surfaces with a crossed scan, as shown in Fig. 1(b), and measured m with the same methods as the above. Fig. 6 shows the result measured at W = 10 N with (a) the steel ball and (b) the WC–Co ball, where m0 of the non-irradiated DLC is shown for comparison. For this measurement, the friction test machine had to be stopped at R = 187 and 52 for the steel ball and the WC–Co ball, respectively, because m was so large as to induce a rapid increase in mechanical vibration at the contact surface of the sample. As seen in Fig. 6, a notable increase from m0 = 0.08 to m = 0.45 was observed for the hard WC–Co ball. This large increase in m is ascribed to the spatial modulation of surface hardness that was introduced by the patterning of nanostructured zone, as discussed below. In the present experiment, the laser fluence used was so small that the surface roughness was hardly influenced. In fact, the ablation depth measured with SPM was in the order of 10 nm, which was almost comparable to the surface roughness of the initial nonablated surfaces. This indicates that the change in surface roughness is not the origin of the large m for the patterned surface. On the other hand, the fs-laser-induced nanostructuring does lead to a decrease in the hardness of the surface, due to the structural change of DLC into GC. The net-like patterning of the ablated zone created by the crossed scan could produce spatial modulation of the surface hardness with a periodicity of H = 240 mm. This spatial modulation of hardness could increase m of the DLC film. We have observed that m can be changed by controlling H and D in the patterning of the nanostructured zone. 4. Conclusions
Fig. 5. Friction coefficients of the nanostructured DLC film with a MoS2 coating (black bar), the non-irradiated DLC film with a MoS2 coating (hatched bar), and the non-irradiated DLC film (white bar). Each group represents the results measured with (a) a steel ball at W = 2 N and R = 104, (b) a WC–Co ball at W = 10 N and R = 4000, and (c) a WC–Co ball at W = 10 N and R = 104.
We have studied the tribological properties of DLC surfaces nanostructured with fs-laser pulses. A thin MoS2 layer coated on the nanostructured surface was found to be extremely effective for the purpose of decreasing the friction coefficient m of the DLC surface, where the smallest value of m = 0.02 was observed for the WC–Co ball. We have also developed an effective method for increasing m by means of patterning the
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nanostructured zone on the DLC surface, while maintaining the initial surface flatness. The present results demonstrate a useful and effective method to control the tribological properties of DLC films to a considerable extent using fs-laser pulses. Acknowledgements The authors would like to express their thanks to K. Akari of Kobe Steel, Ltd. for the DLC coatings, and H. Magara of the Industrial Technical Center of Fukui Prefecture for measurement of the Raman spectra. References [1] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, T. Akamatsu, J. Appl. Phys. 64 (1988) 6464.
[2] G. Dumitru, V. Romano, H.P. Weber, S. Pimenov, T. Kononenko, J. Hermann, S. Bruneau, Y. Gerbig, M. Shupegin, Diamond Rel. Mater. 12 (2003) 1034. [3] A. Erdemir, Tribol. Int. 38 (2005) 249. [4] I. Etsion, Trans. ASME J. Tribol. 127 (2005) 248. [5] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 76 (2003) 983. [6] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 79 (2004) 425. [7] K. Miyazaki, N. Maekawa, W. Kobayashi, M. Kaku, N. Yasumaru, J. Kiuchi, Appl. Phys. A 80 (2005) 17. [8] N. Yasumaru, K. Miyazaki, J. Kiuchi, Appl. Phys. A 81 (2005) 933. [9] See, for example, D. Ba¨uerle, Laser Processing and Chemistry, Springer, Berlin 1996 (Chapter 28). [10] J. Reif, F. Costache, M. Henyk, S.V. Pandelov, Appl. Surf. Sci. 197/198 (2002) 891, and references therein. [11] G. Miyaji, K. Miyazaki, Appl. Phys. Lett. 89 (2006) 191902. [12] A. Dekanski, J. Stevanovic, R. Stevanovic, B.Z. Nikolic, V.M. Jovanovic, Carbon 39 (2001) 1195, and references therein. [13] M. Yoshikawa, N. Nagai, M. Matsuki, H. Fukuda, G. Katagiri, H. Ishida, A. Ishitani, I. Nagai, Phys. Rev. B 46 (1992) 7169.