Wear 259 (2005) 1424–1431

The effect of contact area on nano/micro-scale friction

Eui-Sung Yoon∗, R. Arvind Singh, Hyun-Jin Oh, Hosung KongTribology 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 2005Available 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 tochange 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 thenano-scale, the friction between ball and surface was measured with the applied normal load using an atomic force microscope (AFM), and atthe micro-scale it was measured using a ball-on-flat type micro-tribotester. All experiments were conducted on silicon wafer and diamond-likecarbon (DLC) coated silicon samples, at controlled conditions of temperature of 24± 1 ◦C and relative humidity of 45± 5%. Friction wasm .R es. Similarb ecreasedw rence int mainly byp icro-scale,s situation,w on frictionw©

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easured 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-scaleesults at the nano-scale showed that the friction increased with the applied normal load and tip size, for both kinds of samplehavior of friction with the applied normal load and ball size was observed for silicon at the micro-scale. However, for DLC friction dith the ball size. This distinct difference in the behavior of friction in DLC at the nano- and micro-scale was attributed to the diffe

he operating mechanisms. At nano-scale, friction in DLC was affected by adhesion, whereas at the micro-scale it was affectedlowing. Evidences of the operating mechanisms at micro-scale were obtained using scanning electron microscope (SEM). At molid–solid adhesion was dominant in silicon, while DLC showed plowing. Contrary to the nano-scale that is almost a ‘wear-less’ear was prominent at the micro-scale. At both the nano- and micro-scales, the effect of applied normal load and the tip/ball sizeas discussed as the influence of contact area on these parameters.2005 Elsevier B.V. All rights reserved.

eywords:Nano; Micro; Friction; Contact area; Wear; Tribology; AFM

. Introduction

Nano- and micro-scale tribology plays a prominent role inany emerging fields, such as microelectromechanical sys-

ems (MEMS)[1] and high-density magnetic recording me-ia [2]. These systems are comprised of elements that aremall in size and operate at nano/micro-scale loads. At thesecales of size, the ratio of surface area to volume is highnd the surface forces, such as adhesion and friction assumerimary importance in defining the tribology at the contact.t these scales, the high surface forces decrease the perfor-ance and consequently reduce the operating lifetimes ofano/micro-scale devices[1–3]. It is, therefore, important to

∗ Corresponding author. Tel.: +82 29585651; fax: +82 29585659.E-mail address:esyoon@kist.re.kr (E.-S. Yoon).

understand the effect of these forces, while studying thbology at nano/micro-scales.

In MEMS, silicon is a widely used material and henmost of the investigations have been directed towards ustanding its tribological performance at nano/micro-sc[1]. In the past, investigations brought forth variousproaches towards reducing the surface forces in silicoundertaking topographical[3] and chemical modificationsits surface[4,5], and thereby, enhancing its performanceder tribological contact.

In the field of tribology, concerning high-density manetic recording media, hard amorphous carbon coatingsmally referred to as diamond-like carbon (DLC) coatihave found their application[6,7]. These coatings are chacterized by relatively high hardness and exhibit goodresistance. Owing to these attributes, DLC coatings (co

043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2005.01.033

E.-S. Yoon et al. / Wear 259 (2005) 1424–1431 1425

Fig. 1. Optical micrograph of borosilicate ball with radius 1.25�m used asAFM tip, mounted on commercial triangular cantilever.

with a molecularly thin film of lubricant[8]) are used to pro-tect against the occasional contact that occurs between theslider and disk surface in hard disk drives in most computersystems. Many tribological studies have been conducted onthese two materials namely, silicon and DLC, but only a fewof them include the study of the factors that affect the surfaceforces, such as the contact size and the environment[9–12].

In the present work, the effect of tip/ball size (contactarea) on surface forces, mainly the friction force in Si-wafer(1 0 0) and DLC, has been investigated experimentally, bothat the nano- and micro-scales. To understand this effect at thenano-scale, a pseudo-single asperity contact has been simulated using tips of various radii in AFM, and at micro-scaleexperiments were conducted using glass balls of various sizesagainst Si-wafer (1 0 0) and DLC in a ball-on-flat type micro-tribotester. At both the nano- and micro-scales, the effect ofapplied normal load and the tip/ball size on friction was dis-cussed 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 n-t inals ct ofc -ct dii0 f thee ere

conducted on Si-wafer ((1 0 0), produced by LG Siltron) andDLC film coated on Si-wafers (1 0 0), at ambient temperature(24± 1◦C) and humidity (45± 5%). Si-wafer (1 0 0) samplesof 10 mm× 10 mm were cut from the as-received discs usinga diamond tip cutter and were cleaned with blowing air usinga hand-blower. The DLC films were deposited by a radio fre-quency plasma-assisted chemical vapor deposition method(r.f.PACVD) using benzene (C6H6) for the reaction gas. De-tails of the experimental set up were described elsewhere[13]. The deposition time was adjusted to obtain about 1�mthick film. The film thickness was measured by an Alpha-step(Tencor P-1). The structure and mechanical properties of thedeposited DLC films were reported previously[13].

Table 1shows the properties of the tip/ball materials andtest specimens, Si-wafer (1 0 0) and DLC films used in thepresent study. These data are referred from various sources[13–17].

2.2. Test apparatus and methods

2.2.1. Friction measurements at nano-scaleNano-scale friction tests were conducted using a commer-

cial Atomic Force Microscope (Multimode SPM, NanoscopeIIIa, Digital Instruments). The friction force was measuredin LFM (Lateral Force Microscope) mode. Friction measure-ments were conducted under applied normal loads in therr sr ted.

2on-

fl t-i malli mm/sa bout1 d thea

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.25�m and 2.5�m mounted on triangular cantilevers (coact mode type—Nitride Probe Sharpened (NPS); nompring constant 0.58 N/m) were used to study the effeontact area on nano-scale friction.Fig. 1 shows an optial micrograph of a tip specimen taken at 400× magnifica-ion. Soda Lime balls (Duke Scientific Corporation) with ra.25 mm, 0.5 mm and 1 mm were used for the study offect of contact area at micro-scale. All experiments w

able 1roperties of the tip/ball materials and the test specimens

aterial Youngs modulus (GPa) Poiss

orosilicate glass 63 0.2oda Lime glass 68 0.16i (1 0 0) 165 0.28LC 120 0.26

-

.

ange of 0–160 nN and at the scanning speed of 5�m/s (scanate of 0.5 Hz) for the scan size of 5�m× 5�m. Each test waepeated for 15 times and the average values were plot

.2.2. Friction measurements at micro-scaleMicro-scale friction tests were performed with a ball-

at type micro-tribotester (shown inFig. 2) under reciprocang motion. Friction was measured with the applied noroads of 1000�N, 1500�N, 3000�N and 4800�N. The slid-ng speed and the scan length were kept constant at 1nd 3 mm, respectively. Each test was conducted for a5 min. Tests were repeated more than three times anverage values were plotted.

. Results and discussion

.1. Friction at nano-scale using AFM

Figs. 3 and 4show the variation of the friction force for Safer and DLC against tips of various sizes, with the appormal load. In these figures, it is worth noting that the fric

tio Water contact Angle (◦) Interfacial energy (mN/m

– –– –

22 7266 41.3

1426 E.-S. Yoon et al. / Wear 259 (2005) 1424–1431

Fig. 2. A close-up view of the ball-on-flat type micro-tribotester.

force exists even at the zero applied normal load. This ismainly attributed to the influence of the intrinsic adhesiveforce on the friction force[18]. The adhesive force arisesdue to the contribution of various attractive forces, such ascapillary, electrostatic, van der Waal and chemical bondingunder different circumstances[4]. In Si-wafer, it is consideredthat the capillary force contributes to a major extent owingto its hydrophilic nature[11,19]. van der Waal forces alsocontribute[12], but their magnitude is less when comparedto that of the capillary force[4]. In contrast to Si-wafer, DLCis hydrophobic in nature (Table 1), and hence, capillary forcedecreases to a large extent. Condensation of water from the

F liedn

Fig. 4. Friction force measured using AFM in DLC with the applied normalload (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 tothe formation of meniscus bridge between the tip and thesample[11]. Eqs.(1) and (2) [11,20]give the expressions forthe adhesive force generated due to the formation of meniscusbridge and the attractive van der Waal forces, respectively.

Fm = 4πRγ cosθ (1)

whereR is the tip radius,γ the surface tension of water andθ the contact angle of water on the mating surface.

Fvdw = AR

6D2(2)

whereA is the Hamaker constant,R the tip radius andD theseparation distance between the two surfaces.

As shown in the two equations, the magnitude of both theseforces is directly dependent on the tip size (R). This explainsthe fact that the contribution of the inherent adhesive forceto the friction force increases with the tip size, for both Si-wafer and DLC. The contribution of adhesive force Si-waferis much greater than that in DLC because of the high capillaryforce and interfacial energy (Table 1).

It is observed that the friction force increases linearlywith the applied normal load, for both Si-wafer and DLC(Figs. 3 and 4). It is also evident that the friction force in-creases with the tip size. These results could be explained byc w-d ei inglea nf

F

w dt

ndDs witht areaw e

ig. 3. Friction force measured using AFM in Si-wafer with the appormal load (tip radii—0.32�m, 0.5�m, 1.25�m and 2.5�m).

onsidering the fundamental law of friction given by Boen and Tabor[21]. According to this law, the friction forc

s directly dependent on the real area of contact, for a ssperity contact. Eq.(3) gives the expression for the frictio

orce.

f = τAr (3)

hereτ is the shear strength, an interfacial property anArhe real area of contact.

The behavior of the friction force in both Si-wafer aLC is very much consistent with this law of friction.Fig. 5hows the estimated contact area for Si-wafer and DLChe applied normal load for various tip sizes. The contactas estimated using the JKR model[22], assuming that th

E.-S. Yoon et al. / Wear 259 (2005) 1424–1431 1427

Fig. 5. Estimated contact area for Si-wafer and DLC with the applied normalload 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 areaincreases with the applied load and the tip size. This explainswell the increase in friction force with the applied normalload and the tip size for both Si-wafer and DLC, which isdue to the increase in the real area of contact.Fig. 6 showsfriction force as a function of contact area in Si-wafer fortwo different tip sizes (0.32�m and 2.5�m). Bhushan andSundararajan[11] and Bhushan and Dandavate[12] have alsoobserved similar relationship between the friction force andthe contact area, for both materials.

It is also observed that Si-wafer shows higher friction forcethan DLC (Figs. 3 and 4). This is because Si-wafer has higheradhesive force and contact area than DLC. The larger contactarea of Si-wafer is due to its higher interfacial energy whencompared to that of DLC (Table 1).

F t tips

Fig. 7. Coefficient of friction for Si-wafer and DLC with the tip size.

The adhesive force inherently contributes to the frictionforce. It acts as an additional normal load and increases thefriction force [23–25]. Estimating the coefficient of frictionby taking the slope from the plots of friction force data versusthe applied normal load would eliminate the contribution ofthe adhesive force.Fig. 7 shows the coefficient of frictionfor Si-wafer and DLC with the tip size. The coefficient offriction was calculated as the slope from the plots of frictionforce data versus the applied normal load (Figs. 3 and 4).Result shows that the coefficient of friction increases witha tip size and Si-wafer exibits higher coefficient of frictionthan DLC. As mentioned previously, for the same tip size, Si-wafer exhibits a higher contact area than DLC because of itshigher interfacial energy. This leads to an increased frictionforce, which in turn increases the coefficient of friction inSi-wafer.

3.2. Friction at micro-scale using micro-tribotester

Fig. 8(a) and (b) shows the variation of friction force withtime both for Si-wafer and DLC (normal load: 4800�N). Thearrow marks indicate the steady state, from which the coef-ficient of friction was read in the present investigation. Thefriction characteristics for Si-wafer and DLC are distinctlydifferent. In the case of Si-wafer, during the initial stage ofs ate-rT ofS incei n. Int tS ar-i act.F instgr widthom t thect sted

ig. 6. Friction force with the contact area in Si-wafer for two differenizes. (a) 0.32�m and (b) 2.5�m.

liding, a peak occurs. This shows a characteristic of mial that has dominant adhesive component of friction[26].he friction of DLC does not show any peak. The frictioni-wafer was affected mainly by solid–solid adhesion, s

t has a high interfacial energy that supports adhesiohe past, experiments conducted by Gardos[27] showed thai-wafer exhibits a high adhesive friction followed by she

nduced microcracking in the wake of the sliding contig. 9(a)–(c) show the wear tracks of Si-wafer tested agalass balls of radii 0.25 mm, 0.5 mm and 1 mm at 3000�N,espectively. Wear debris are seen on these tracks. Thef the track increases with the ball size.Fig. 9(d) is a highagnification micrograph that shows debris smeared a

enter of the wear track seen inFig. 9(c). Fig. 10(a) showshe surface of the glass ball of radii 0.5 mm that was te

1428 E.-S. Yoon et al. / Wear 259 (2005) 1424–1431

Fig. 8. Friction force with the time for (a) Si-wafer and (b) DLC (normalload of 4800�N). The arrow mark indicates the steady state.

against Si-wafer at 3000�N normal load.Fig. 10(b) is a highmagnification micrograph that shows debris sticking at thetip of the ball surface shown inFig. 10(a). These evidencesclearly indicate that adhesion was prominent in Si-wafer.

Fig. 11(a) is a high magnification micrograph of the weartrack in DLC that was tested against the glass ball of 0.25 mmradii at 3000�N normal load. The morphology of the weartrack shows evidences of plowing and plastic deformation.There is no wear debris on the wear track.Fig. 11(b) showsan AFM image of wear track in DLC. The surface profile ofthe wear track (Fig. 11(c)) shows two peaks that correspondto the ridges (material flow) formed along the wear track dueto the plowing effect.Fig. 12(a) shows the surface of glassball of radii 0.25 mm that was tested against DLC at 3000�Nnormal load.Fig. 12(b) is a SEM micrograph at higher magni-fication that shows the counterface material stacked at the tipof the ball surface. The mechanism of transfer film formationfollowed by interfilm sliding has been frequently observedearlier in DLC coatings[28,29]. Under such circumstances,the material removal occurs at the third body layer (tribo-chemically mixed and compacted transfer layer) in the formof rolled debris. These rolled debris are usually seen on boththe sides of the wear track along its length[29]. In the presentwork, 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 weartrack.

E.-S. Yoon et al. / Wear 259 (2005) 1424–1431 1429

Fig. 10. (a) SEM image of the ball surface (radius of 0.5 mm) tested againstSi-wafer at 3000�N normal load, (b) debris sticking at the tip of the ballsurface.

and the uneven stacking of material observed (absence ofcompaction) on the tip of the ball (Fig. 12) indicate that thereis no formation of tribolayer. These evidences rather indi-cate ‘plowing’ to be the dominant operating mechanism thatlargely influences the friction in DLC.

Figs. 13 and 14show the variation of the coefficient of fric-tion with the applied normal load against glass balls of varioussizes, for both Si-wafer and DLC, respectively. FromFig. 13,it is observed that the coefficient of friction in Si-wafer in-creases with the applied load and ball size. In this case, thecontact area directly affects the adhesive force (Fig. 9) andincreases the friction force, thereby, increasing the coefficientof friction. Contact area increases with the applied load andthe tip size as shown inFig. 15, which shows the estimatedcontact area for Si-wafer and DLC with the ball size for var-ious normal loads used in the present investigation. The es-timation of the contact area for Si-wafer was done using theJKR model[22] as it includes the contribution of adhesion,whereas for DLC, the Hertzian model was used. In the caseof DLC, the coefficient of friction decreases with the ball size(Fig. 14), which is due to the larger contribution of plowingwhen compared to adhesion. Considering the size effect, theplowing component of friction force (Fp) has a direct, butinverse relation with the size of the slider, as seen from Eq.(4) [30]. This explains for the decrease in the coefficient of

Fig. 11. (a) SEM images of the wear track in DLC against the glass ball of 0.(Fig. 10) and (c) cross-sectional view of the depth profile of the wear track.

25 mm radius at 3000�N normal load, (b) AFM image of the wear track in DLC

1430 E.-S. Yoon et al. / Wear 259 (2005) 1424–1431

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 theball surface.

friction with the ball size in DLC.

Fp = d3P

12R(4)

whered is the track width,P the mean pressure requiredto displace the material in the surface andR the radius ofcurvature of the slider. InFig. 14, the coefficient of friction inthe smallest ball size viz. 0.25 mm at low loads (1000�N and1500�N) does not show a good trend at low applied normal

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.

Fg

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).

load, because the contact of smallest ball may be unstable atthe micro-tribotester used in this work.

Furthermore, for surfaces making contact at a number ofasperities (multiple asperity) the plowing term reduces withthe increase in the number of points of contact, for the sameload[30]. In the present work, at the micro-scale, the contactis not a single asperity contact but a multiple asperity contact.This also explains for the decrease in the coefficient of frictionin the case of DLC with the ball size, and its reduction athigher normal loads.

4. Conclusions

The effect of contact area on friction in nano/micro-scalewas studied with various tip/ball radii against Si-wafers andDLC film using AFM and micro-tribotester. The test resultscan be summarized as follows:

ig. 14. Coefficient of friction of DLC with the applied normal load, againstlass balls of radii 0.25 mm, 0.5 mm and 1 mm.

1. The friction force at the nano-scale increases with the ap-plied normal load and the tip size because of the increasein the contact area.

2. The lower frictional property of DLC at the nano-scalecompared to that of Si-wafer is attributed to the smallercontact area and the lower adhesive force, which are af-fected by the lower interfacial energy.

E.-S. Yoon et al. / Wear 259 (2005) 1424–1431 1431

3. The coefficient of friction of Si-wafer at the micro-scaleincreases with the ball size, while that of DLC decreasesdue to the difference in the friction mechanisms at thesurface. The friction mechanisms at the micro-scale aresolid–solid adhesion in Si-wafer and plowing in DLC.

4. The friction at the nano-scale was governed mainly by itsadhesive characteristics, but the friction at the micro-scalewas greatly affected by the wear behavior.

Acknowledgements

This research was supported by a Grant (04K1401-01010)from Center for Nanoscale Mechatronics Manufacturing of21st Century Frontier Research Program and the National Re-search Laboratory Program. The authors would like to thankDr. Kwang-Ryeol Lee for the supply of DLC specimen.

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