Microscopic Friction Studies on Metal Surfaces

Tribol Lett (2010) 39:19 24 DOI 10.1007/s11249-009-9508-5 ORIGINAL PAPER Microscopic Friction Studies on Metal Surfaces Nitya Nand Gosvami Æ Tobin Filleter Æ Philip Egberts Æ Roland Bennewitz Received: 1 July 2009 / Accepted: 23 August 2009 / Published online: 9 September 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Atomically flat and clean metal surfaces exhibit a regime of ultra-low friction at low normal loads. Atomic force microscopy, performed in ultra-high vacuum on Cu(100) and Au(111) surfaces, reveals a clear stick-slip modulation in the lateral force but almost zero dissipation. Significant friction is observed only for higher loads (*4 6 nn above the pull-off force) together with the onset of wear. We discuss the minor role of thermal activation in the low friction regime and suggest that a compliant metallic neck between tip and surface is formed which brings upon the low, load-independent shear stress. Keywords Nanotribology Stick-slip Atomic force microscopy (AFM) 1 Introduction Friction between two sliding surfaces is a phenomenon relevant to a variety of technological applications and scientific curiosity [1, 2]. Friction at the nanometer scale or even in a single asperity contact carries importance to understand and minimize wear, frictional losses, and to enhance the reliability of the future micro- and nano-scale devices involving sliding contacts [3 6]. Small-scale devices are often coated with a thin layer of metal to N. N. Gosvami (&) P. Egberts R. Bennewitz INM Leibniz Institute for New Materials, Saarbruecken, Germany e-mail: nityanand.gosvami@inm-gmbh.de T. Filleter P. Egberts R. Bennewitz Department of Physics, McGill University, Montreal, QC, Canada enhance various physical properties [7]. For such applications, friction and wear properties are crucial to understand. At macroscopic scales, high friction is always observed for sliding between clean metal surfaces. Despite the significant literature that exists for macroscopic friction and wear properties of metal surfaces [8], an understanding of friction and wear behavior at the microscopic level is still limited [9 12]. At the nanometer scale, theoretical [13] and experimental studies [14] have reported observation of atomic stick-slip instabilities on metal surfaces; however, several key questions of interest remain to be answered. These include: How do metal surfaces behave when a single asperity slides under pressures on the order of GPa, which is significantly higher than the bulk elastic modulus of metals? How does the friction change with load? What are the microscopic mechanisms of friction on metals? These questions provide us the motivation to perform a systematic study of frictional behavior of metal surfaces at small scales. An ultra-high vacuum atomic force microscopy (UHV- AFM) is a powerful tool that can be used to measure friction on clean metal surfaces, while simultaneously recording images of the surface in real space. In this article, we have explored the atomic scale frictional properties of clean Cu(100) and Au(111) surfaces using UHV-AFM. The frictional response of both the metal surfaces are compared as a function of load. The possible mechanisms for the observed frictional behavior are discussed in the context of various mechanical models of friction. 2 Experimental Methods The AFM used for all experiments is a home-built instrument with the optical beam deflection method of force

20 Tribol Lett (2010) 39:19 24 detection, operated in contact mode [15]. All measurements were conducted at room temperature under UHV conditions, where the pressure was lower then 2 9 10-10 mbar. Clean Cu(100) surfaces were prepared by repeated cycles of argon ion sputtering (1 kev) and annealing (725 K) of the (100) face of a polished copper single crystal, resulting in atomically flat terraces ranging between 50 and 200 nm. Au(111) surfaces were prepared by thermally evaporating 300-nm thick polycrystalline films onto mica substrates in a vacuum evaporator (Oerlikon Leybold Vacuum GmbH, Univex 300, Cologne, Germany) under high vacuum conditions (1 9 10-8 mbar). Following the deposition process, the gold films were annealed with a butane flame under atmospheric conditions to obtain * 200 nm large terraces with the (111) orientation. The gold samples were then mounted and introduced into the UHV system immediately and were heated at 120 C for 1 h to remove contaminants. Silicon cantilevers with an integrated tip (Nanosensors, PPP-CONT) were used as force sensors. The stiffness of the cantilevers in both lateral twisting and normal bending was determined by the beam geometry method [3, 16], using the first normal resonant frequency to determine the thickness of the cantilever. The spring constant of these cantilevers in normal bending was 0.05 0.09 N/m and in torsion was 30 60 N/m. The calibration from volts, as detected by the photodiode into nanonewtons, was determined by recording force distance curves for small z-displacements of the scanning piezotube. The zero force was defined where the cantilever deflection in contact was equal to the free cantilever deflection. Throughout this article we will refer to the recorded signal of the cantilever twisting as the lateral force. This signal is periodically modulated by the crystalline structure of the surface and subject to thermal and instrumental noise. We will refer to the average lateral force as the friction force. The averaging is done over many pairs of forward and backward scan across several unit cells of the crystal structure. The error in the friction force is determined as the standard deviation in the averaging. Each pair of forward and backward scan is referred to as lateral force loop. 3 Results 3.1 Friction Measurements on Cu(100) Figure 1 shows the friction force versus load as measured on a Cu(100) surface. Below an applied normal load of -5.0 nn, the friction force is extremely low; zero within the error of the measurement, and does not vary with load. The lower limit of the applied load is given by the adhesion force below which the tip would jump off the surface. Fig. 1 Friction force versus normal load curve obtained for a Cu(100) surface. The measurements started at load close to the pull-off force, which would break the adhesive contact Above a load of -5.0 nn, the friction force slightly increases with load. Finally, there is a sharp increase in friction force above -3.5 nn, the error in the friction force increases dramatically, and the friction does not anymore vary systematically with load. We assign this behavior as the onset of wear in the tip-sample contact. Figure 2 shows lateral force maps recorded on the Cu(100) surface together with typical line profiles of the lateral force for different loads. At low loads (-7.7 nn, Fig. 2a), the lattice of Cu(100) can be clearly resolved. The line profile (Fig. 2b) indicates that the tip follows the atomic structure partially in smooth sliding, partially in a stick-slip motion with negligible friction force. At higher load (-5.6 nn, Fig. 2c, d), the lattice becomes visible in a clear atomic stick-slip motion with the characteristic sawtooth appearance of the lateral force curve, as recently observed by Filleter et al. [17]. However, the friction force is still negligible. At high loads (-2.1 nn, Fig. 2e, f), stick-slip is still observed; however, significant irregularity is observed in the lateral force map and line profile, which also shows the increased friction force. These results support our interpretation that the sharp increase in friction force at -3.5 nn coincides with the onset of wear. They also indicate that earlier experimental studies [14] where the regular atomic stick-slip on Cu(100) surface could not be observed have been carried out in the wear regime. 3.2 Friction Measurements on Au(111) Figure 3 shows lateral force maps of a Au(111) surface. The well-known herringbone reconstruction [18] is clearly observed on a flat terrace. It is worth noting that details in

Tribol Lett (2010) 39:19 24 21 Fig. 3 Lateral force maps of a clean and flat Au(111) surface showing the herringbone reconstruction (frame size a 90 nm and b 47 nm) Fig. 2 Lateral force map acquired on a Cu(100) surface at an applied load of a -7.7 nn, c -5.6 nn, and e -2.1 nn (frame size 3 nm). Corresponding lateral force loops showing b mixture of smooth sliding and stick-slip, d clear atomic stick-slip, and f irregular stickslip behavior (darker black lines forward scan, lighter red lines backward scan). (Color figure online) the elbows of this structure are imaged with a resolution of better than 3 nm, which provides us with an upper limit for the contact diameter. Furthermore, it can be observed that wear of the surface starts at characteristic sites of the herringbone structure which seem to be less stable. Figure 4 contains the friction versus load results for a flat Au(111) terrace. Similar to the Cu(100) surface, the measured friction force is almost zero and independent of the applied load over a range of several nanonewtons up to 3.8 nn. Above this force, there is a sharp increase in friction force and the error in measured friction force increases dramatically as the wear of the surface takes place. This is illustrated by the lateral force maps and typical lateral force line profiles in Fig. 5. For the lower load Fig. 4 Friction versus normal load curve obtained for an Au(111) surface. The measurements started at load close to the pull-off force, which would break the adhesive contact (Fig. 5a), the atomic structure of the Au(111) surface is clearly resolved albeit modulated by the herringbone reconstruction. The lateral force profile (Fig. 5b) reveals clear atomic stick-slip with negligible friction. For the high-load regime (Fig. 5c), the atomic periodicity is lost in the lateral force map and the lateral force profile indicates highly irregular stick-slip patterns with significant friction (Fig. 5d). No surface damage has been observed in low-load contact mode measurements following the high-load experiments. This is expected as wear occurred on the nanometer scale and may be limited to the contact only. Furthermore, it is known that the high-surface mobility of atoms will restore Au(111) and Cu(100) surfaces quickly at room temperature [19, 20]. The quick recovery of the surface is also demonstrated by our observation of atomic stick-slip patterns when reducing the load after measuring in the wear regime.

22 Tribol Lett (2010) 39:19 24 Fig. 5 Lateral force maps of an Au(111) surface a frame size 4.5 nm, load -0.45 nn and c frame size 5 nm, load 7.29 nn. Corresponding lateral force loops recorded showing b regular atomic stick-slip modulated by the herringbone structure and d irregular stick-slip behavior (darker black lines forward scan, lighter red lines backward scan). (Color figure online) 4 Discussion Understanding the load dependence of kinetic friction is of significant interest in tribology. Kinetic friction of single asperities at the nanometer scale has been observed to show a variety of load dependencies. For non-adhesive systems, often a linear load dependence is observed, whereas adhesive systems often exhibit a sub-linear characteristic [21]. In the studies described in this article, we find a quite different load dependence of friction on atomically flat surfaces of metal single crystals. The friction versus load behavior of both Cu(100) and Au(111) surfaces exhibits extremely low frictional dissipation and no increase with load until the onset of wear. We have systematically observed such low friction even though the lateral force shows clear atomic stick-slip instabilities. In the following, we will discuss these observations in comparison with experiments and atomistic simulations of single-asperity kinetic friction. Classically, two contributions to the increase in friction with load are considered: the increase in contact area with load and the increase in shear stress with load, friction being the product of the two. A standard picture assumes that the shear stress is constant and that the friction grows sub-linearly in proportion to the contact area predicted by contact mechanics. This dominant role of the contact area for friction has to be questioned for cases with a linear dependence throughout the load range [21]. In a recent atomistic simulation, Mo et al. [22] have studied the load dependence of kinetic friction in a small contact and refined the picture with respect to the definition of the contact area. Taking the contact area as the number of atoms in the range of chemical interaction, the authors were able to reproduce the different load dependencies for adhesive and non-adhesive contacts. For their system of an amorphous carbon tip sliding over diamond, they found no change of shear stress with load. On the other hand, an atomistic simulation of Cu tips sliding over Cu singlecrystal surfaces reported by Sørensen et al. [13] showed that friction increases with load for a constant number of atoms forming the contact area, i.e., that the shear stress increases with load. In a first-principles calculation, Zhong and Tománek [23] also found that the shear stress between Pd and graphite varies with load. An increase in shear stress with load has also been assumed in the interpretation of experimental results on NaCl(100), which show a load dependence very similar to the one reported here [24]: a regime of ultra-low friction and no increase with load was observed in lowest load range. The atomic scale sliding friction was explained based on the Tomlinson model [25], where the tip with the effective lateral stiffness k is moving over a sinusoidal potential (amplitude E o and periodicity a), whose amplitude is considered to increase with load. In the Tomlinson model, smooth sliding with ultra low dissipation is observed below a threshold value of E o, whereas above the threshold load stick-slip appears and the friction increases with load. This transition from smooth sliding to a stickslip regime can be identified by defining the parameter g ¼ 2p2 E o ka 2 ð1þ Stick-slip instabilities and friction set in when g [ 1. The parameter g can be determined from two experimental results, the slope of the sticking part of the lateral force loop k exp and the maximum lateral force required for slip to occur F max L : g ¼ 2pFmax L k exp a 1 ð2þ The calculation of the parameter g for the Cu(100) and Au(111) results shown in Figs. 2 and 5 gives values of 2.8 and 1.6, respectively, over the full load range where no increase in friction is observed. Such large values of the g parameter (g [1) are in agreement with the observation of clear atomic stick-slip for both the surfaces. Although the stick-slip instabilities are less clear in the lateral force loop for Cu(100) near the pull-off force, the calculated value of

Tribol Lett (2010) 39:19 24 23 g remains unchanged. Within error, there is no change in the amplitude of the stick-slip pattern or the slope of the sticking region with varying load in the non-wear regime for both the surfaces. Within the picture of the Tomlinson model, this suggests that the corrugation of the surface potential E o as well as the lateral stiffness k do not change with applied normal load. These observations are in contrast to the observations on NaCl(100) [24], where the corrugation of the surface potential, and hence g parameter, increases with the applied normal load in the stick-slip regime. The observed constancy of E o, k, and g for varying applied normal load suggests that neither the atomic configuration between the tip and the substrate nor the shear stress change noticeably within the range of applied loads. Recently, the importance of thermal effects on nanoscale friction has moved into the focus of attention. Krylov and Frenken [26] found that the thermal fluctuations of the localized contact can result in a variety of friction regimes not predicted by the Tomlinson model when neglecting thermal effects. In particular, the range of ultra-low friction at low loads could be extended toward higher loads by thermal effects (see Fig. 3 in Ref. [26]). Krylov s calculations suggest that thermal delocalization of the contact could lead to extremely low friction independent of normal load, despite the observation of atomic stick-slip in the lateral force signal. In the stick-slip regime (g [ 1), the value of a parameter b indicates the number of thermally activated jumps between adjacent atomic positions in one macroscopically observed stick-slip event. b ¼ a rffiffiffiffi k exp E o ð3þ V m k B T In our case, using the respective lattice constants a, scan velocity V = 10 20 ms -1, k = 1.3 Nm -1 for Au(111), and k = 1.5 Nm -1 for Cu(100), m = 1 9 10-20 kg as suggested by Krylov and Frenken, and T = 300 K results in b = 6 9 10 4 for Cu(100) and 5 9 10 6 for Au(111). The choice of the small mass is motivated by the idea that a small cluster at the tip apex dominates the elastic properties of the contact. The large values of b indicate that the sliding process is thermally activated. For large b values, a further parameter C in the description compares the energy corrugation E o of the effective tip-sample interaction averaged over rapid thermal motion of the tip apex with the energy stored in the cantilever spring K at the deflection of one lattice spacing a: C ¼ 2p2 E o Ka 2 with E o ¼ 1 8 ka2 ð4þ The calculated values of C are 0.07 for Cu(100) and 0.1 for Au(111), far less than 1. Krylov and Frenken predict for our case, i.e., large b values and C \ 1 a friction regime called thermolubricity. The tip is expected to be completely delocalized due to thermal activation. The lateral force should exhibit no structure on the scale of the lattice constant and friction should be extremely low. This prediction is in stark contrast to our observation of clear stick-slip patterns. In order to obtain a b parameter close to 1 we would have to assume a 10 8 times higher effective mass of the tip or a nine times higher lateral potential, both physically unreasonable values. We conclude that an explanation of the observed low-friction regime through thermal lubrication by Krylov s model is not possible. 5 Conclusion In summary, we have observed very low, almost zero friction on clean and atomically flat metal surfaces for low loads. The friction is independent of the applied normal load over a range of several nanonewtons. However, friction increases sharply with the onset of wear as detected by irregular lateral force. Clear atomic stick-slip events were observed in the non-wear regime with low friction. The interpretation of our results within the standard models of single asperity tribology is difficult. One explanation for ultra-low friction on atomic scale, namely thermolubricity caused by contact delocalization through thermal effects, can be excluded since we observe clear stick-slip patterns in the lateral force even at lowest loads. In conclusion, we have to consider a sliding contact with an extremely lowshear strength which does not increase with load until the onset of wear. The most likely scenario is a crystalline metallic neck forming between the tip and the surface. Spontaneous neck formation has been predicted for nanoscale contacts by atomistic simulations [13, 27] and has recently been observed in transmission electron microscopy [28]. The neck would form a commensurate contact with the sample surface. Note that the range of loads for observation of ultra-low friction is mostly in the adhesive regime, i.e., at negative applied load. When pulling the tip out of contact, we observe jumps in the adhesive force, which are typical for plastic deformation in the neck before final rupture [29]. Only when applying more positive loads, the neck will break down and the irregular behavior indicating wear will be observed. Given that the shear stress is almost zero, atomic-scale changes in the contact area between neck and surface would not even be detectable. In this scenario, the significant contrast between atomic scale friction behavior on metals and ionic surfaces would be an effect of different atomic bonding favoring neck formation for metals and stable surfaces for ionic crystals. While the increasing normal pressure between tip and the surface of a ionic surface yields an increase in the shear stress, the normal

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