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micro-scale friction
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The effect of contact area on nano/micro-scale friction Eui-Sung Yoon ∗ , R. Arvind Singh, Hyun-Jin Oh, Hosung Kong Tribology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Received 2 August 2004; received in revised form 13 January 2005; accepted 18 January 2005 Available online 5 March 2005
Abstract The effect of contact area on nano/micro-scale friction was experimentally studied. Glass balls with various radii were used in order to change the contact area. Borosilicate glass balls (radii—0.32 m, 0.5 m, 1.25 m and 2.5 m) attached on the top of AFM tip (NPS, DI) were applied for nano-scale contact and Soda Lime balls with radii 0.25 mm, 0.5 mm and 1 mm were applied for micro-scale contact. At the nano-scale, the friction between ball and surface was measured with the applied normal load using an atomic force microscope (AFM), and at the micro-scale it was measured using a ball-on-flat type micro-tribotester. All experiments were conducted on silicon wafer and diamond-like carbon (DLC) coated silicon samples, at controlled conditions of temperature of 24 ± 1 ◦ C and relative humidity of 45 ± 5%. Friction was measured with the applied load in the range of 0–160 nN at the nano-scale and at 1000 N, 1500 N, 3000 N and 4800 N at the micro-scale. Results at the nano-scale showed that the friction increased with the applied normal load and tip size, for both kinds of samples. Similar behavior of friction with the applied normal load and ball size was observed for silicon at the micro-scale. However, for DLC friction decreased with the ball size. This distinct difference in the behavior of friction in DLC at the nano- and micro-scale was attributed to the difference in the operating mechanisms. At nano-scale, friction in DLC was affected by adhesion, whereas at the micro-scale it was affected mainly by plowing. Evidences of the operating mechanisms at micro-scale were obtained using scanning electron microscope (SEM). At micro-scale, solid–solid adhesion was dominant in silicon, while DLC showed plowing. Contrary to the nano-scale that is almost a ‘wear-less’ situation, wear was prominent at the micro-scale. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters. © 2005 Elsevier B.V. All rights reserved. Keywords: Nano; Micro; Friction; Contact area; Wear; Tribology; AFM
1. Introduction Nano- and micro-scale tribology plays a prominent role in many emerging fields, such as microelectromechanical systems (MEMS) [1] and high-density magnetic recording media [2]. These systems are comprised of elements that are small in size and operate at nano/micro-scale loads. At these scales of size, the ratio of surface area to volume is high and the surface forces, such as adhesion and friction assume primary importance in defining the tribology at the contact. At these scales, the high surface forces decrease the performance and consequently reduce the operating lifetimes of nano/micro-scale devices [1–3]. It is, therefore, important to ∗
Corresponding author. Tel.: +82 29585651; fax: +82 29585659. E-mail address: [email protected] (E.-S. Yoon).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.033
understand the effect of these forces, while studying the tribology at nano/micro-scales. In MEMS, silicon is a widely used material and hence, most of the investigations have been directed towards understanding its tribological performance at nano/micro-scales [1]. In the past, investigations brought forth various approaches towards reducing the surface forces in silicon by undertaking topographical [3] and chemical modifications of its surface [4,5], and thereby, enhancing its performance under tribological contact. In the field of tribology, concerning high-density magnetic recording media, hard amorphous carbon coatings, normally referred to as diamond-like carbon (DLC) coatings have found their application [6,7]. These coatings are characterized by relatively high hardness and exhibit good wear resistance. Owing to these attributes, DLC coatings (coated
E.-S. Yoon et al. / Wear 259 (2005) 1424–1431
Fig. 1. Optical micrograph of borosilicate ball with radius 1.25 m used as AFM tip, mounted on commercial triangular cantilever.
with a molecularly thin film of lubricant [8]) are used to protect against the occasional contact that occurs between the slider and disk surface in hard disk drives in most computer systems. Many tribological studies have been conducted on these two materials namely, silicon and DLC, but only a few of them include the study of the factors that affect the surface forces, such as the contact size and the environment [9–12]. In the present work, the effect of tip/ball size (contact area) on surface forces, mainly the friction force in Si-wafer (1 0 0) and DLC, has been investigated experimentally, both at the nano- and micro-scales. To understand this effect at the nano-scale, a pseudo-single asperity contact has been simulated using tips of various radii in AFM, and at micro-scale experiments were conducted using glass balls of various sizes against Si-wafer (1 0 0) and DLC in a ball-on-flat type microtribotester. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters.
2. Experimental details 2.1. Test specimens Glass (Borosilicate) balls of radii 0.32 m, 0.5 m, 1.25 m and 2.5 m mounted on triangular cantilevers (contact mode type—Nitride Probe Sharpened (NPS); nominal spring constant 0.58 N/m) were used to study the effect of contact area on nano-scale friction. Fig. 1 shows an optical micrograph of a tip specimen taken at 400× magnification. Soda Lime balls (Duke Scientific Corporation) with radii 0.25 mm, 0.5 mm and 1 mm were used for the study of the effect of contact area at micro-scale. All experiments were
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conducted on Si-wafer ((1 0 0), produced by LG Siltron) and DLC film coated on Si-wafers (1 0 0), at ambient temperature (24 ± 1 ◦ C) and humidity (45 ± 5%). Si-wafer (1 0 0) samples of 10 mm × 10 mm were cut from the as-received discs using a diamond tip cutter and were cleaned with blowing air using a hand-blower. The DLC films were deposited by a radio frequency plasma-assisted chemical vapor deposition method (r.f.PACVD) using benzene (C6 H6 ) for the reaction gas. Details of the experimental set up were described elsewhere [13]. The deposition time was adjusted to obtain about 1 m thick film. The film thickness was measured by an Alpha-step (Tencor P-1). The structure and mechanical properties of the deposited DLC films were reported previously [13]. Table 1 shows the properties of the tip/ball materials and test specimens, Si-wafer (1 0 0) and DLC films used in the present study. These data are referred from various sources [13–17]. 2.2. Test apparatus and methods 2.2.1. Friction measurements at nano-scale Nano-scale friction tests were conducted using a commercial Atomic Force Microscope (Multimode SPM, Nanoscope IIIa, Digital Instruments). The friction force was measured in LFM (Lateral Force Microscope) mode. Friction measurements were conducted under applied normal loads in the range of 0–160 nN and at the scanning speed of 5 m/s (scan rate of 0.5 Hz) for the scan size of 5 m × 5 m. Each test was repeated for 15 times and the average values were plotted. 2.2.2. Friction measurements at micro-scale Micro-scale friction tests were performed with a ball-onflat type micro-tribotester (shown in Fig. 2) under reciprocating motion. Friction was measured with the applied normal loads of 1000 N, 1500 N, 3000 N and 4800 N. The sliding speed and the scan length were kept constant at 1 mm/s and 3 mm, respectively. Each test was conducted for about 15 min. Tests were repeated more than three times and the average values were plotted.
3. Results and discussion 3.1. Friction at nano-scale using AFM Figs. 3 and 4 show the variation of the friction force for Siwafer and DLC against tips of various sizes, with the applied normal load. In these figures, it is worth noting that the friction
Table 1 Properties of the tip/ball materials and the test specimens Material
Youngs modulus (GPa)
Poisson’s ratio
Water contact Angle (◦ )
Interfacial energy (mN/m)
Borosilicate glass Soda Lime glass Si (1 0 0) DLC
63 68 165 120
0.2 0.16 0.28 0.26
– – 22 66
– – 72 41.3
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Fig. 4. Friction force measured using AFM in DLC with the applied normal load (tip radii—0.32 m, 0.5 m, 1.25 m and 2.5 m).
environment is the origin of the capillary force that leads to the formation of meniscus bridge between the tip and the sample [11]. Eqs. (1) and (2) [11,20] give the expressions for the adhesive force generated due to the formation of meniscus bridge and the attractive van der Waal forces, respectively. Fm = 4πRγ cos θ Fig. 2. A close-up view of the ball-on-flat type micro-tribotester.
force exists even at the zero applied normal load. This is mainly attributed to the influence of the intrinsic adhesive force on the friction force [18]. The adhesive force arises due to the contribution of various attractive forces, such as capillary, electrostatic, van der Waal and chemical bonding under different circumstances [4]. In Si-wafer, it is considered that the capillary force contributes to a major extent owing to its hydrophilic nature [11,19]. van der Waal forces also contribute [12], but their magnitude is less when compared to that of the capillary force [4]. In contrast to Si-wafer, DLC is hydrophobic in nature (Table 1), and hence, capillary force decreases to a large extent. Condensation of water from the
where R is the tip radius, γ the surface tension of water and θ the contact angle of water on the mating surface. Fvdw =
AR 6D2
(2)
where A is the Hamaker constant, R the tip radius and D the separation distance between the two surfaces. As shown in the two equations, the magnitude of both these forces is directly dependent on the tip size (R). This explains the fact that the contribution of the inherent adhesive force to the friction force increases with the tip size, for both Siwafer and DLC. The contribution of adhesive force Si-wafer is much greater than that in DLC because of the high capillary force and interfacial energy (Table 1). It is observed that the friction force increases linearly with the applied normal load, for both Si-wafer and DLC (Figs. 3 and 4). It is also evident that the friction force increases with the tip size. These results could be explained by considering the fundamental law of friction given by Bowden and Tabor [21]. According to this law, the friction force is directly dependent on the real area of contact, for a single asperity contact. Eq. (3) gives the expression for the friction force. Ff = τAr
Fig. 3. Friction force measured using AFM in Si-wafer with the applied normal load (tip radii—0.32 m, 0.5 m, 1.25 m and 2.5 m).
(1)
(3)
where τ is the shear strength, an interfacial property and Ar the real area of contact. The behavior of the friction force in both Si-wafer and DLC is very much consistent with this law of friction. Fig. 5 shows the estimated contact area for Si-wafer and DLC with the applied normal load for various tip sizes. The contact area was estimated using the JKR model [22], assuming that the
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Fig. 5. Estimated contact area for Si-wafer and DLC with the applied normal load for various tip sizes.
contact at the nano-scale is a pseudo-single asperity contact. From this figure, it can be observed that the contact area increases with the applied load and the tip size. This explains well the increase in friction force with the applied normal load and the tip size for both Si-wafer and DLC, which is due to the increase in the real area of contact. Fig. 6 shows friction force as a function of contact area in Si-wafer for two different tip sizes (0.32 m and 2.5 m). Bhushan and Sundararajan [11] and Bhushan and Dandavate [12] have also observed similar relationship between the friction force and the contact area, for both materials. It is also observed that Si-wafer shows higher friction force than DLC (Figs. 3 and 4). This is because Si-wafer has higher adhesive force and contact area than DLC. The larger contact area of Si-wafer is due to its higher interfacial energy when compared to that of DLC (Table 1).
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Fig. 7. Coefficient of friction for Si-wafer and DLC with the tip size.
The adhesive force inherently contributes to the friction force. It acts as an additional normal load and increases the friction force [23–25]. Estimating the coefficient of friction by taking the slope from the plots of friction force data versus the applied normal load would eliminate the contribution of the adhesive force. Fig. 7 shows the coefficient of friction for Si-wafer and DLC with the tip size. The coefficient of friction was calculated as the slope from the plots of friction force data versus the applied normal load (Figs. 3 and 4). Result shows that the coefficient of friction increases with a tip size and Si-wafer exibits higher coefficient of friction than DLC. As mentioned previously, for the same tip size, Siwafer exhibits a higher contact area than DLC because of its higher interfacial energy. This leads to an increased friction force, which in turn increases the coefficient of friction in Si-wafer. 3.2. Friction at micro-scale using micro-tribotester
Fig. 6. Friction force with the contact area in Si-wafer for two different tip sizes. (a) 0.32 m and (b) 2.5 m.
Fig. 8(a) and (b) shows the variation of friction force with time both for Si-wafer and DLC (normal load: 4800 N). The arrow marks indicate the steady state, from which the coefficient of friction was read in the present investigation. The friction characteristics for Si-wafer and DLC are distinctly different. In the case of Si-wafer, during the initial stage of sliding, a peak occurs. This shows a characteristic of material that has dominant adhesive component of friction [26]. The friction of DLC does not show any peak. The friction of Si-wafer was affected mainly by solid–solid adhesion, since it has a high interfacial energy that supports adhesion. In the past, experiments conducted by Gardos [27] showed that Si-wafer exhibits a high adhesive friction followed by shearinduced microcracking in the wake of the sliding contact. Fig. 9(a)–(c) show the wear tracks of Si-wafer tested against glass balls of radii 0.25 mm, 0.5 mm and 1 mm at 3000 N, respectively. Wear debris are seen on these tracks. The width of the track increases with the ball size. Fig. 9(d) is a high magnification micrograph that shows debris smeared at the center of the wear track seen in Fig. 9(c). Fig. 10(a) shows the surface of the glass ball of radii 0.5 mm that was tested
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Fig. 8. Friction force with the time for (a) Si-wafer and (b) DLC (normal load of 4800 N). The arrow mark indicates the steady state.
against Si-wafer at 3000 N normal load. Fig. 10(b) is a high magnification micrograph that shows debris sticking at the tip of the ball surface shown in Fig. 10(a). These evidences clearly indicate that adhesion was prominent in Si-wafer. Fig. 11(a) is a high magnification micrograph of the wear track in DLC that was tested against the glass ball of 0.25 mm radii at 3000 N normal load. The morphology of the wear track shows evidences of plowing and plastic deformation. There is no wear debris on the wear track. Fig. 11(b) shows an AFM image of wear track in DLC. The surface profile of the wear track (Fig. 11(c)) shows two peaks that correspond to the ridges (material flow) formed along the wear track due to the plowing effect. Fig. 12(a) shows the surface of glass ball of radii 0.25 mm that was tested against DLC at 3000 N normal load. Fig. 12(b) is a SEM micrograph at higher magnification that shows the counterface material stacked at the tip of the ball surface. The mechanism of transfer film formation followed by interfilm sliding has been frequently observed earlier in DLC coatings [28,29]. Under such circumstances, the material removal occurs at the third body layer (tribochemically mixed and compacted transfer layer) in the form of rolled debris. These rolled debris are usually seen on both the sides of the wear track along its length [29]. In the present work, the absence of wear debris at the wear track (Fig. 11)
Fig. 9. SEM images of wear tracks in Si-wafer (3000 N normal load). (a), (b) and (c) are the wear tracks from tests against ball of radii of 0.25 mm, 0.5 mm and 1 mm, respectively, (d) debris smeared at the center of the wear track.
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and the uneven stacking of material observed (absence of compaction) on the tip of the ball (Fig. 12) indicate that there is no formation of tribolayer. These evidences rather indicate ‘plowing’ to be the dominant operating mechanism that largely influences the friction in DLC. Figs. 13 and 14 show the variation of the coefficient of friction with the applied normal load against glass balls of various sizes, for both Si-wafer and DLC, respectively. From Fig. 13, it is observed that the coefficient of friction in Si-wafer increases with the applied load and ball size. In this case, the contact area directly affects the adhesive force (Fig. 9) and increases the friction force, thereby, increasing the coefficient of friction. Contact area increases with the applied load and the tip size as shown in Fig. 15, which shows the estimated contact area for Si-wafer and DLC with the ball size for various normal loads used in the present investigation. The estimation of the contact area for Si-wafer was done using the JKR model [22] as it includes the contribution of adhesion, whereas for DLC, the Hertzian model was used. In the case of DLC, the coefficient of friction decreases with the ball size (Fig. 14), which is due to the larger contribution of plowing when compared to adhesion. Considering the size effect, the plowing component of friction force (Fp ) has a direct, but inverse relation with the size of the slider, as seen from Eq. (4) [30]. This explains for the decrease in the coefficient of Fig. 10. (a) SEM image of the ball surface (radius of 0.5 mm) tested against Si-wafer at 3000 N normal load, (b) debris sticking at the tip of the ball surface.
Fig. 11. (a) SEM images of the wear track in DLC against the glass ball of 0.25 mm radius at 3000 N normal load, (b) AFM image of the wear track in DLC (Fig. 10) and (c) cross-sectional view of the depth profile of the wear track.
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Fig. 12. (a) SEM image of the ball surface (radius 0.25 mm) tested against DLC at 3000 N normal load and (b) counterface material stacked at the tip of the ball surface.
friction with the ball size in DLC. Fp =
d3P 12R
(4)
where d is the track width, P the mean pressure required to displace the material in the surface and R the radius of curvature of the slider. In Fig. 14, the coefficient of friction in the smallest ball size viz. 0.25 mm at low loads (1000 N and 1500 N) does not show a good trend at low applied normal
Fig. 15. Estimated contact area for Si-wafer and DLC with the ball size (applied normal loads—1000 N, 1500 N, 3000 N and 4800 N).
Fig. 13. Coefficient of friction of Si-wafer with the applied normal load, against glass balls of radii 0.25 mm, 0.5 mm and 1 mm.
load, because the contact of smallest ball may be unstable at the micro-tribotester used in this work. Furthermore, for surfaces making contact at a number of asperities (multiple asperity) the plowing term reduces with the increase in the number of points of contact, for the same load [30]. In the present work, at the micro-scale, the contact is not a single asperity contact but a multiple asperity contact. This also explains for the decrease in the coefficient of friction in the case of DLC with the ball size, and its reduction at higher normal loads.
4. Conclusions The effect of contact area on friction in nano/micro-scale was studied with various tip/ball radii against Si-wafers and DLC film using AFM and micro-tribotester. The test results can be summarized as follows:
Fig. 14. Coefficient of friction of DLC with the applied normal load, against glass balls of radii 0.25 mm, 0.5 mm and 1 mm.
1. The friction force at the nano-scale increases with the applied normal load and the tip size because of the increase in the contact area. 2. The lower frictional property of DLC at the nano-scale compared to that of Si-wafer is attributed to the smaller contact area and the lower adhesive force, which are affected by the lower interfacial energy.
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3. The coefficient of friction of Si-wafer at the micro-scale increases with the ball size, while that of DLC decreases due to the difference in the friction mechanisms at the surface. The friction mechanisms at the micro-scale are solid–solid adhesion in Si-wafer and plowing in DLC. 4. The friction at the nano-scale was governed mainly by its adhesive characteristics, but the friction at the micro-scale was greatly affected by the wear behavior. Acknowledgements This research was supported by a Grant (04K1401-01010) from Center for Nanoscale Mechatronics Manufacturing of 21st Century Frontier Research Program and the National Research Laboratory Program. The authors would like to thank Dr. Kwang-Ryeol Lee for the supply of DLC specimen. References [1] B. Bhushan, Tribology on the macroscale to nanoscale of microelectromechanical system materials, Proc. Instn. Eng. ImechE J 215 (2001) 1–18. [2] K. Komvopoulos, Head-disk interface contact mechanics for Ultrahigh density magnetic recording, Wear 238 (2000) 1–11. [3] K. Komvopoulos, Surface engineering and microtribology for microelectromechanical systems, Wear 200 (1996) 305–327. [4] R. Maboudian, R.T. Howe, Critical review: Adhesion in surface micromechanical structure, J. Vac. Sci. Technol. B 15 (1) (1997) 1–20. [5] B. Bhushan, A.V. Kulkarni, V.N. Koinkar, Microtribological characterization of self-assembled and langmuir-blodgett monolayers by atomic and friction force microscopy, Langmuir 11 (1995) 3189–3198. [6] R. Memming, H.J. Tolle, P.E. Wierenga, Properties of polymeric layers of hydrogenated amorphous carbon produced by a plasmaactivated chemical vapour deposition process II: tribological and mechanical properties, Thin Solid Films 143 (1986) 31–41. [7] A. Grill, Review of the tribology of diamond-like carbon, Wear 168 (1993) 143–153. [8] Z. Zhao, B. Bhushan, Tribological performance of PFPE and X-1P lubricants at head-disk interface. Part I. Experimental results, Tribol. Lett. 6 (1999) 129–139. [9] Y. Ando, J. Ino, Friction and pull-off forces on submicron-size aperities, Wear 216 (1998) 115–122. [10] Y. Ando, The effect of relative humidity on friction and pull-off forces measured on submicron-size asperity arrays, Wear 238 (2000) 12–19.
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[11] B. Bhushan, S. Sundararajan, Micro/nanoscale friction and wear mechanisms of thin films using atomic force and friction force microscopy, Acta Mater. 46 (11) (1998) 3793–3804. [12] B. Bhushan, C. Dandavate, Thin-film friction and adhesion studies using atomic force microscopy, J. Appl. Phys. 87 (3) (2000) 1201–1210. [13] K.-R. Lee, K.Y. Eun, J.S. Rhee, Structure and mechanical properties of nitrogen incorporated diamond-like carbon films, Mater. Res. Soc. Symp. Proc. 356 (1995) 233–238. [14] M. Grischke, A. Hieke, F. Morgenweck, H. Dimigen, Variation of the wettability of DLC-Coatings by network modification using silicon and oxygen, Diamond Relat. Mater. 7 (1998) 454–458. [15] S. Sundararajan, B. Bhushan, Develpoment of a continous microscratch technique in an atomic force microscope and its application to study scratch resistance of Ultrathin hard amorphous carbon coatings, J. Mater. Res. 16 (2) (2001) 437–445. [16] Matweb—Online material data sheet http://www.matweb.com. [17] M. Scherge, S.N. Gorb, Biological Micro- and Nanotribology, Springer-Verlag, Berlin Heidelberg, 2001, pp. 7–76. [18] B. Bhushan, A.V. Kulkarni, Effect of normal load on microscale friction measurements, Thin Solid Films 278 (1996) 49– 56. [19] H. Liu, I.-U. Ahmed, M. Scherge, Microtribological properties of silicon and silicon coated with diamond like carbon, octadecyltrichlorosilane and steric acid cadmiun salt films: a comparitive study, Thin Solid Films 381 (2001) 135–142. [20] C.M. Mate, Nanotribological studies of carbon surfaces by force microscopy, Wear 168 (1993) 17–20. [21] F.P. Bowden, D. Tabor, Friction and Lubrication, Methuen, London, 1956. [22] K.L. Johnson, K. Kendall, A.D. Roberts, Surface energy and contact of elastic solid, Proc. R. Soc. Lond. A 324 (1971) 301–313. [23] J. Skinner, N. Gane, Sliding friction under negative load, J. Phys. D5 (1972) 2087–2094. [24] E.-S. Yoon, S.H. Yang, H.-G. Han, H. Kong, An experimental study on the adhesion at a nano-contact, Wear 254 (2003) 974– 980. [25] E.-S. Yoon, S.H. Yang, H. Kong, K.-H. Kim, The effect of topography on water wetting and micro/nano tribological characteristics of polymeric surfaces, Tribol. Lett. 15 (2) (2003) 145–154. [26] B. Bhushan, Principles and Applications of Tribology, John Wiley & Sons Inc., 1998, pp. 344–430. [27] M.N. Gardos, Surface chemistry-controlled tribological behaviour of Si and diamond, Tribol. Lett. 2 (1996) 173. [28] B. Bhushan, Modern Tribology Handbook, vol. 2, CRC Press, 2001, pp. 871–908. [29] D.S. Kim, T.E. Fischer, B. Gallois, The effects of oxygen and humidity on friction and wear of diamond-like carbon films, Surf. Coat. Technol. 49 (1991) 537–542. [30] F.P. Bowden, D. Tabor, The friction and lubrication of solids, Clarendon Press, Oxford, 1950, pp. 90–121.
The effect of contact area on nano/micro-scale friction Eui-Sung Yoon ∗ , R. Arvind Singh, Hyun-Jin Oh, Hosung Kong Tribology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea Received 2 August 2004; received in revised form 13 January 2005; accepted 18 January 2005 Available online 5 March 2005
Abstract The effect of contact area on nano/micro-scale friction was experimentally studied. Glass balls with various radii were used in order to change the contact area. Borosilicate glass balls (radii—0.32 m, 0.5 m, 1.25 m and 2.5 m) attached on the top of AFM tip (NPS, DI) were applied for nano-scale contact and Soda Lime balls with radii 0.25 mm, 0.5 mm and 1 mm were applied for micro-scale contact. At the nano-scale, the friction between ball and surface was measured with the applied normal load using an atomic force microscope (AFM), and at the micro-scale it was measured using a ball-on-flat type micro-tribotester. All experiments were conducted on silicon wafer and diamond-like carbon (DLC) coated silicon samples, at controlled conditions of temperature of 24 ± 1 ◦ C and relative humidity of 45 ± 5%. Friction was measured with the applied load in the range of 0–160 nN at the nano-scale and at 1000 N, 1500 N, 3000 N and 4800 N at the micro-scale. Results at the nano-scale showed that the friction increased with the applied normal load and tip size, for both kinds of samples. Similar behavior of friction with the applied normal load and ball size was observed for silicon at the micro-scale. However, for DLC friction decreased with the ball size. This distinct difference in the behavior of friction in DLC at the nano- and micro-scale was attributed to the difference in the operating mechanisms. At nano-scale, friction in DLC was affected by adhesion, whereas at the micro-scale it was affected mainly by plowing. Evidences of the operating mechanisms at micro-scale were obtained using scanning electron microscope (SEM). At micro-scale, solid–solid adhesion was dominant in silicon, while DLC showed plowing. Contrary to the nano-scale that is almost a ‘wear-less’ situation, wear was prominent at the micro-scale. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters. © 2005 Elsevier B.V. All rights reserved. Keywords: Nano; Micro; Friction; Contact area; Wear; Tribology; AFM
1. Introduction Nano- and micro-scale tribology plays a prominent role in many emerging fields, such as microelectromechanical systems (MEMS) [1] and high-density magnetic recording media [2]. These systems are comprised of elements that are small in size and operate at nano/micro-scale loads. At these scales of size, the ratio of surface area to volume is high and the surface forces, such as adhesion and friction assume primary importance in defining the tribology at the contact. At these scales, the high surface forces decrease the performance and consequently reduce the operating lifetimes of nano/micro-scale devices [1–3]. It is, therefore, important to ∗
Corresponding author. Tel.: +82 29585651; fax: +82 29585659. E-mail address: [email protected] (E.-S. Yoon).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.01.033
understand the effect of these forces, while studying the tribology at nano/micro-scales. In MEMS, silicon is a widely used material and hence, most of the investigations have been directed towards understanding its tribological performance at nano/micro-scales [1]. In the past, investigations brought forth various approaches towards reducing the surface forces in silicon by undertaking topographical [3] and chemical modifications of its surface [4,5], and thereby, enhancing its performance under tribological contact. In the field of tribology, concerning high-density magnetic recording media, hard amorphous carbon coatings, normally referred to as diamond-like carbon (DLC) coatings have found their application [6,7]. These coatings are characterized by relatively high hardness and exhibit good wear resistance. Owing to these attributes, DLC coatings (coated
E.-S. Yoon et al. / Wear 259 (2005) 1424–1431
Fig. 1. Optical micrograph of borosilicate ball with radius 1.25 m used as AFM tip, mounted on commercial triangular cantilever.
with a molecularly thin film of lubricant [8]) are used to protect against the occasional contact that occurs between the slider and disk surface in hard disk drives in most computer systems. Many tribological studies have been conducted on these two materials namely, silicon and DLC, but only a few of them include the study of the factors that affect the surface forces, such as the contact size and the environment [9–12]. In the present work, the effect of tip/ball size (contact area) on surface forces, mainly the friction force in Si-wafer (1 0 0) and DLC, has been investigated experimentally, both at the nano- and micro-scales. To understand this effect at the nano-scale, a pseudo-single asperity contact has been simulated using tips of various radii in AFM, and at micro-scale experiments were conducted using glass balls of various sizes against Si-wafer (1 0 0) and DLC in a ball-on-flat type microtribotester. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball size on friction was discussed as the influence of contact area on these parameters.
2. Experimental details 2.1. Test specimens Glass (Borosilicate) balls of radii 0.32 m, 0.5 m, 1.25 m and 2.5 m mounted on triangular cantilevers (contact mode type—Nitride Probe Sharpened (NPS); nominal spring constant 0.58 N/m) were used to study the effect of contact area on nano-scale friction. Fig. 1 shows an optical micrograph of a tip specimen taken at 400× magnification. Soda Lime balls (Duke Scientific Corporation) with radii 0.25 mm, 0.5 mm and 1 mm were used for the study of the effect of contact area at micro-scale. All experiments were
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conducted on Si-wafer ((1 0 0), produced by LG Siltron) and DLC film coated on Si-wafers (1 0 0), at ambient temperature (24 ± 1 ◦ C) and humidity (45 ± 5%). Si-wafer (1 0 0) samples of 10 mm × 10 mm were cut from the as-received discs using a diamond tip cutter and were cleaned with blowing air using a hand-blower. The DLC films were deposited by a radio frequency plasma-assisted chemical vapor deposition method (r.f.PACVD) using benzene (C6 H6 ) for the reaction gas. Details of the experimental set up were described elsewhere [13]. The deposition time was adjusted to obtain about 1 m thick film. The film thickness was measured by an Alpha-step (Tencor P-1). The structure and mechanical properties of the deposited DLC films were reported previously [13]. Table 1 shows the properties of the tip/ball materials and test specimens, Si-wafer (1 0 0) and DLC films used in the present study. These data are referred from various sources [13–17]. 2.2. Test apparatus and methods 2.2.1. Friction measurements at nano-scale Nano-scale friction tests were conducted using a commercial Atomic Force Microscope (Multimode SPM, Nanoscope IIIa, Digital Instruments). The friction force was measured in LFM (Lateral Force Microscope) mode. Friction measurements were conducted under applied normal loads in the range of 0–160 nN and at the scanning speed of 5 m/s (scan rate of 0.5 Hz) for the scan size of 5 m × 5 m. Each test was repeated for 15 times and the average values were plotted. 2.2.2. Friction measurements at micro-scale Micro-scale friction tests were performed with a ball-onflat type micro-tribotester (shown in Fig. 2) under reciprocating motion. Friction was measured with the applied normal loads of 1000 N, 1500 N, 3000 N and 4800 N. The sliding speed and the scan length were kept constant at 1 mm/s and 3 mm, respectively. Each test was conducted for about 15 min. Tests were repeated more than three times and the average values were plotted.
3. Results and discussion 3.1. Friction at nano-scale using AFM Figs. 3 and 4 show the variation of the friction force for Siwafer and DLC against tips of various sizes, with the applied normal load. In these figures, it is worth noting that the friction
Table 1 Properties of the tip/ball materials and the test specimens Material
Youngs modulus (GPa)
Poisson’s ratio
Water contact Angle (◦ )
Interfacial energy (mN/m)
Borosilicate glass Soda Lime glass Si (1 0 0) DLC
63 68 165 120
0.2 0.16 0.28 0.26
– – 22 66
– – 72 41.3
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Fig. 4. Friction force measured using AFM in DLC with the applied normal load (tip radii—0.32 m, 0.5 m, 1.25 m and 2.5 m).
environment is the origin of the capillary force that leads to the formation of meniscus bridge between the tip and the sample [11]. Eqs. (1) and (2) [11,20] give the expressions for the adhesive force generated due to the formation of meniscus bridge and the attractive van der Waal forces, respectively. Fm = 4πRγ cos θ Fig. 2. A close-up view of the ball-on-flat type micro-tribotester.
force exists even at the zero applied normal load. This is mainly attributed to the influence of the intrinsic adhesive force on the friction force [18]. The adhesive force arises due to the contribution of various attractive forces, such as capillary, electrostatic, van der Waal and chemical bonding under different circumstances [4]. In Si-wafer, it is considered that the capillary force contributes to a major extent owing to its hydrophilic nature [11,19]. van der Waal forces also contribute [12], but their magnitude is less when compared to that of the capillary force [4]. In contrast to Si-wafer, DLC is hydrophobic in nature (Table 1), and hence, capillary force decreases to a large extent. Condensation of water from the
where R is the tip radius, γ the surface tension of water and θ the contact angle of water on the mating surface. Fvdw =
AR 6D2
(2)
where A is the Hamaker constant, R the tip radius and D the separation distance between the two surfaces. As shown in the two equations, the magnitude of both these forces is directly dependent on the tip size (R). This explains the fact that the contribution of the inherent adhesive force to the friction force increases with the tip size, for both Siwafer and DLC. The contribution of adhesive force Si-wafer is much greater than that in DLC because of the high capillary force and interfacial energy (Table 1). It is observed that the friction force increases linearly with the applied normal load, for both Si-wafer and DLC (Figs. 3 and 4). It is also evident that the friction force increases with the tip size. These results could be explained by considering the fundamental law of friction given by Bowden and Tabor [21]. According to this law, the friction force is directly dependent on the real area of contact, for a single asperity contact. Eq. (3) gives the expression for the friction force. Ff = τAr
Fig. 3. Friction force measured using AFM in Si-wafer with the applied normal load (tip radii—0.32 m, 0.5 m, 1.25 m and 2.5 m).
(1)
(3)
where τ is the shear strength, an interfacial property and Ar the real area of contact. The behavior of the friction force in both Si-wafer and DLC is very much consistent with this law of friction. Fig. 5 shows the estimated contact area for Si-wafer and DLC with the applied normal load for various tip sizes. The contact area was estimated using the JKR model [22], assuming that the
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Fig. 5. Estimated contact area for Si-wafer and DLC with the applied normal load for various tip sizes.
contact at the nano-scale is a pseudo-single asperity contact. From this figure, it can be observed that the contact area increases with the applied load and the tip size. This explains well the increase in friction force with the applied normal load and the tip size for both Si-wafer and DLC, which is due to the increase in the real area of contact. Fig. 6 shows friction force as a function of contact area in Si-wafer for two different tip sizes (0.32 m and 2.5 m). Bhushan and Sundararajan [11] and Bhushan and Dandavate [12] have also observed similar relationship between the friction force and the contact area, for both materials. It is also observed that Si-wafer shows higher friction force than DLC (Figs. 3 and 4). This is because Si-wafer has higher adhesive force and contact area than DLC. The larger contact area of Si-wafer is due to its higher interfacial energy when compared to that of DLC (Table 1).
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Fig. 7. Coefficient of friction for Si-wafer and DLC with the tip size.
The adhesive force inherently contributes to the friction force. It acts as an additional normal load and increases the friction force [23–25]. Estimating the coefficient of friction by taking the slope from the plots of friction force data versus the applied normal load would eliminate the contribution of the adhesive force. Fig. 7 shows the coefficient of friction for Si-wafer and DLC with the tip size. The coefficient of friction was calculated as the slope from the plots of friction force data versus the applied normal load (Figs. 3 and 4). Result shows that the coefficient of friction increases with a tip size and Si-wafer exibits higher coefficient of friction than DLC. As mentioned previously, for the same tip size, Siwafer exhibits a higher contact area than DLC because of its higher interfacial energy. This leads to an increased friction force, which in turn increases the coefficient of friction in Si-wafer. 3.2. Friction at micro-scale using micro-tribotester
Fig. 6. Friction force with the contact area in Si-wafer for two different tip sizes. (a) 0.32 m and (b) 2.5 m.
Fig. 8(a) and (b) shows the variation of friction force with time both for Si-wafer and DLC (normal load: 4800 N). The arrow marks indicate the steady state, from which the coefficient of friction was read in the present investigation. The friction characteristics for Si-wafer and DLC are distinctly different. In the case of Si-wafer, during the initial stage of sliding, a peak occurs. This shows a characteristic of material that has dominant adhesive component of friction [26]. The friction of DLC does not show any peak. The friction of Si-wafer was affected mainly by solid–solid adhesion, since it has a high interfacial energy that supports adhesion. In the past, experiments conducted by Gardos [27] showed that Si-wafer exhibits a high adhesive friction followed by shearinduced microcracking in the wake of the sliding contact. Fig. 9(a)–(c) show the wear tracks of Si-wafer tested against glass balls of radii 0.25 mm, 0.5 mm and 1 mm at 3000 N, respectively. Wear debris are seen on these tracks. The width of the track increases with the ball size. Fig. 9(d) is a high magnification micrograph that shows debris smeared at the center of the wear track seen in Fig. 9(c). Fig. 10(a) shows the surface of the glass ball of radii 0.5 mm that was tested
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Fig. 8. Friction force with the time for (a) Si-wafer and (b) DLC (normal load of 4800 N). The arrow mark indicates the steady state.
against Si-wafer at 3000 N normal load. Fig. 10(b) is a high magnification micrograph that shows debris sticking at the tip of the ball surface shown in Fig. 10(a). These evidences clearly indicate that adhesion was prominent in Si-wafer. Fig. 11(a) is a high magnification micrograph of the wear track in DLC that was tested against the glass ball of 0.25 mm radii at 3000 N normal load. The morphology of the wear track shows evidences of plowing and plastic deformation. There is no wear debris on the wear track. Fig. 11(b) shows an AFM image of wear track in DLC. The surface profile of the wear track (Fig. 11(c)) shows two peaks that correspond to the ridges (material flow) formed along the wear track due to the plowing effect. Fig. 12(a) shows the surface of glass ball of radii 0.25 mm that was tested against DLC at 3000 N normal load. Fig. 12(b) is a SEM micrograph at higher magnification that shows the counterface material stacked at the tip of the ball surface. The mechanism of transfer film formation followed by interfilm sliding has been frequently observed earlier in DLC coatings [28,29]. Under such circumstances, the material removal occurs at the third body layer (tribochemically mixed and compacted transfer layer) in the form of rolled debris. These rolled debris are usually seen on both the sides of the wear track along its length [29]. In the present work, the absence of wear debris at the wear track (Fig. 11)
Fig. 9. SEM images of wear tracks in Si-wafer (3000 N normal load). (a), (b) and (c) are the wear tracks from tests against ball of radii of 0.25 mm, 0.5 mm and 1 mm, respectively, (d) debris smeared at the center of the wear track.
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and the uneven stacking of material observed (absence of compaction) on the tip of the ball (Fig. 12) indicate that there is no formation of tribolayer. These evidences rather indicate ‘plowing’ to be the dominant operating mechanism that largely influences the friction in DLC. Figs. 13 and 14 show the variation of the coefficient of friction with the applied normal load against glass balls of various sizes, for both Si-wafer and DLC, respectively. From Fig. 13, it is observed that the coefficient of friction in Si-wafer increases with the applied load and ball size. In this case, the contact area directly affects the adhesive force (Fig. 9) and increases the friction force, thereby, increasing the coefficient of friction. Contact area increases with the applied load and the tip size as shown in Fig. 15, which shows the estimated contact area for Si-wafer and DLC with the ball size for various normal loads used in the present investigation. The estimation of the contact area for Si-wafer was done using the JKR model [22] as it includes the contribution of adhesion, whereas for DLC, the Hertzian model was used. In the case of DLC, the coefficient of friction decreases with the ball size (Fig. 14), which is due to the larger contribution of plowing when compared to adhesion. Considering the size effect, the plowing component of friction force (Fp ) has a direct, but inverse relation with the size of the slider, as seen from Eq. (4) [30]. This explains for the decrease in the coefficient of Fig. 10. (a) SEM image of the ball surface (radius of 0.5 mm) tested against Si-wafer at 3000 N normal load, (b) debris sticking at the tip of the ball surface.
Fig. 11. (a) SEM images of the wear track in DLC against the glass ball of 0.25 mm radius at 3000 N normal load, (b) AFM image of the wear track in DLC (Fig. 10) and (c) cross-sectional view of the depth profile of the wear track.
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Fig. 12. (a) SEM image of the ball surface (radius 0.25 mm) tested against DLC at 3000 N normal load and (b) counterface material stacked at the tip of the ball surface.
friction with the ball size in DLC. Fp =
d3P 12R
(4)
where d is the track width, P the mean pressure required to displace the material in the surface and R the radius of curvature of the slider. In Fig. 14, the coefficient of friction in the smallest ball size viz. 0.25 mm at low loads (1000 N and 1500 N) does not show a good trend at low applied normal
Fig. 15. Estimated contact area for Si-wafer and DLC with the ball size (applied normal loads—1000 N, 1500 N, 3000 N and 4800 N).
Fig. 13. Coefficient of friction of Si-wafer with the applied normal load, against glass balls of radii 0.25 mm, 0.5 mm and 1 mm.
load, because the contact of smallest ball may be unstable at the micro-tribotester used in this work. Furthermore, for surfaces making contact at a number of asperities (multiple asperity) the plowing term reduces with the increase in the number of points of contact, for the same load [30]. In the present work, at the micro-scale, the contact is not a single asperity contact but a multiple asperity contact. This also explains for the decrease in the coefficient of friction in the case of DLC with the ball size, and its reduction at higher normal loads.
4. Conclusions The effect of contact area on friction in nano/micro-scale was studied with various tip/ball radii against Si-wafers and DLC film using AFM and micro-tribotester. The test results can be summarized as follows:
Fig. 14. Coefficient of friction of DLC with the applied normal load, against glass balls of radii 0.25 mm, 0.5 mm and 1 mm.
1. The friction force at the nano-scale increases with the applied normal load and the tip size because of the increase in the contact area. 2. The lower frictional property of DLC at the nano-scale compared to that of Si-wafer is attributed to the smaller contact area and the lower adhesive force, which are affected by the lower interfacial energy.
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3. The coefficient of friction of Si-wafer at the micro-scale increases with the ball size, while that of DLC decreases due to the difference in the friction mechanisms at the surface. The friction mechanisms at the micro-scale are solid–solid adhesion in Si-wafer and plowing in DLC. 4. The friction at the nano-scale was governed mainly by its adhesive characteristics, but the friction at the micro-scale was greatly affected by the wear behavior. Acknowledgements This research was supported by a Grant (04K1401-01010) from Center for Nanoscale Mechatronics Manufacturing of 21st Century Frontier Research Program and the National Research Laboratory Program. The authors would like to thank Dr. Kwang-Ryeol Lee for the supply of DLC specimen. References [1] B. Bhushan, Tribology on the macroscale to nanoscale of microelectromechanical system materials, Proc. Instn. Eng. ImechE J 215 (2001) 1–18. [2] K. Komvopoulos, Head-disk interface contact mechanics for Ultrahigh density magnetic recording, Wear 238 (2000) 1–11. [3] K. Komvopoulos, Surface engineering and microtribology for microelectromechanical systems, Wear 200 (1996) 305–327. [4] R. Maboudian, R.T. Howe, Critical review: Adhesion in surface micromechanical structure, J. Vac. Sci. Technol. B 15 (1) (1997) 1–20. [5] B. Bhushan, A.V. Kulkarni, V.N. Koinkar, Microtribological characterization of self-assembled and langmuir-blodgett monolayers by atomic and friction force microscopy, Langmuir 11 (1995) 3189–3198. [6] R. Memming, H.J. Tolle, P.E. Wierenga, Properties of polymeric layers of hydrogenated amorphous carbon produced by a plasmaactivated chemical vapour deposition process II: tribological and mechanical properties, Thin Solid Films 143 (1986) 31–41. [7] A. Grill, Review of the tribology of diamond-like carbon, Wear 168 (1993) 143–153. [8] Z. Zhao, B. Bhushan, Tribological performance of PFPE and X-1P lubricants at head-disk interface. Part I. Experimental results, Tribol. Lett. 6 (1999) 129–139. [9] Y. Ando, J. Ino, Friction and pull-off forces on submicron-size aperities, Wear 216 (1998) 115–122. [10] Y. Ando, The effect of relative humidity on friction and pull-off forces measured on submicron-size asperity arrays, Wear 238 (2000) 12–19.
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