Ni(100) Interfaces - American Chemical

Friction Anisotropy at Ni(100)/Ni(100) Interfaces. J. S. Ko and A. J. Gellman*. Department of Chemical Engineering, Carnegie Mellon University,. Pitts...
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Langmuir 2000, 16, 8343-8351

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Friction Anisotropy at Ni(100)/Ni(100) Interfaces J. S. Ko and A. J. Gellman* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received February 5, 2000. In Final Form: July 31, 2000 The combined use of an ultrahigh vacuum tribometer and a number of surface science techniques has enabled us to explore the tribological properties of interfaces between single-crystal metal surfaces and to address the fundamental issue of frictional anisotropy. Friction measurements have been made between a pair of Ni(100) surfaces which were prepared to be truly clean or modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. Measurements made with systematic variation of the relative crystallographic orientations of the two Ni(100) surfaces have revealed that the friction coefficient is anisotropic with respect to lattice orientation. When aligned (θ ∼ 0°) and sliding along the 〈110〉 direction, the friction coefficient between the two clean Ni(100) surfaces was µs ) 8.6 ( 2.5. The minimum static friction coefficient occurred when the two clean Ni(100) surfaces were misoriented by θ ∼ 45° (µs ) 2.5 ( 1). This orientational anisotropy is consistently observed even in the presence of adsorbed atomic sulfur and up to 4 monolayers of adsorbed ethanol, although these modified surfaces no longer have the same surface lattice periodicity as the Ni(100) substrate. The effect of lattice orientation is damped out at the point that the surfaces are separated by >20 monolayers of adsorbed ethanol. The friction anisotropy observed between Ni(100) surfaces suggests that surface lattice commensurability is not the only cause of friction anisotropy in this system.

1. Introduction Several investigations of the tribological properties of various crystalline materials have shown that friction forces between single crystals are anisotropic with respect to either the shearing direction or the lattice orientation.1-14 The obvious question that arises is the source of this anisotropy. Does slip occur more easily in particular crystallographic directions of the surface lattice, or does slip occur via motion along slip planes in the bulk or subsurface region? The most well-defined interface for study of frictional anisotropy is probably that between mica surfaces often used in the surface forces apparatus. McGuiggan and Israelachivili1,2 have shown that the friction and adhesion forces between two mica surfaces in distilled water and in aqueous KCl depend on the orientation of the surface lattices. Their results showed that the adhesion forces peak at specific angles corresponding to crystallographic alignment of the mica surfaces (θ ) 0°, 60°, 120°, and 180°). The magnitude of the adhesion anisotropy observed in McGuiggan and Israelachivili’s work was a function of * To whom correspondence should be addressed. (1) McGuiggan, P. M.; Israelachvili, J. N. J. Mater. Res. 1990, 5 (10), 2232. (2) Israelachvili, J. N.; Berman A. D. Handbook of Micro/Nanotribology; Bhushan, B., ed.; CRC Press: New York, 1999; pp 415-416. (3) Hirano, M.; Shinjo, K.; Kaneko, R.; Murata, Y. Phys. Rev. Lett. 1991, 67 (19), 2642. (4) Sheehan, P. E.; Lieber, C. M. Science 1996, 272, 1158. (5) Enonmoto, Y.; Tabor, D. Proc. R. Soc. London A 1981, 373, 405. (6) Buckley, D. H. ASLE Trans. 1968, 11, 89. (7) Buckley, D. H.; Johnson, R. L. ASLE Trans. 1966, 9, 121. (8) Tsuya, Y. Wear 1969, 14, 309. (9) Miyoshi, K.; Buckley, D. H. Appl. Surf. Sci. 1982, 10, 357. (10) McFadden, C. F. Ultrahigh Vacuum Studies of the Boundary Lubrication of Metals. Ph.D. Thesis, University of Illinois, 1996. (11) McFadden, C. F.; Gellman, A. J. Surf. Sci., 1997, 391, 287. (12) Overney, R. M.; Takano, H.; Fujihira, M. Phys. Rev. Lett. 1994, 72 (22), 3546. (13) Germann, G. J.; Cohen, S. R.; Neubauer, G.; McClelland, G. M.; Seki, H.; Coulman, D. J. Appl. Phys. 1993, 73, 163. (14) Morita, S.; Fujisawa, S.; Sugawara, Y. Surf. Sci. Rept. 1996, 23, 1.

the spacing between the mica surfaces, and the anisotropy was observed to extend to a gap thickness equivalent to four molecular layers of liquid. The static and dynamic friction forces between single-crystal mica surfaces have also been measured by Hirano et al.3 who found that the friction force is anisotropic and depends strongly on the “lattice misfit angle”. The maximum friction force occurred when the crystallographic axes of the two basal planes were aligned, and the minimum force was measured at 30° lattice misfit angle. At an interface formed of two different materials Sheenan and Lieber4 studied the “lattice-directed sliding” of well-characterized nanocrystals of MoO3 on single crystalline MoS2 substrates. They reported that MoO3 slides only along certain preferred directions with respect to the unit-cell axes of the substrate. As a final example of friction anisotropy, molecular dynamics simulations by He et al.15 and Rajasekaran et al.16 suggested that static friction vanishes between incommensurate surfaces and is maximum when surfaces are commensurate. In those cases incommensurability can be achieved by rotational misalignment of the lattices. All of these studies have suggested that it is the commensurability of the contacting surface lattices that is the primary cause of frictional anisotropy. In contrast to the results described above, several investigations have provided evidence that the cause of frictional anisotropy can be more complicated than just surface lattice commensurability. Enomoto and Tabor5 studied the frictional anisotropy of diamond sliding in air and observed that when a diamond pin slides over the diamond (100) face, the friction force is greater along the 〈100〉 direction than along the 〈110〉 direction. The friction anisotropy disappeared at small loads, i.e., when the contact was elastic and no deformation was observed. This suggests that friction anisotropy at high loads is due largely to subsurface damage produced during sliding and (15) He, G.; Muser, M. H.; Robbins, M. O. Science 1999, 284, 1650. (16) Rajasekaran, E.; Zeng, X. C.; Diestler, D. J. Micro/Nanotribology and Its Applications; Bhushan, B., Ed., Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997, pp 371-377.

10.1021/la000161g CCC: $19.00 © 2000 American Chemical Society Published on Web 09/29/2000

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is minimized by sliding along preferred directions. In another experiment, clean surfaces of single-crystal cobalt sliding against polycrystalline cobalt showed friction anisotropy in which the friction force on the (0001) planes sliding in the 〈1120〉 direction was 50% lower than that on the (1010) planes sliding in the 〈1120〉 direction.6,7 Some other materials such as copper8 and ceramics9 have also shown friction anisotropy that is dependent on the sliding direction. Finally, friction measurements made between clean Cu(111) surfaces in our laboratory have given very different values of the static friction coefficients after the crystals have been rotated and remounted.10,11 All of these systems exhibit friction anisotropy but under conditions of plastic deformation rather than elastic contact. Under conditions of bulk plastic deformation it is not clear that the surface lattice commensurability should be important in determining friction forces. It is clear from these previous studies that friction anisotropy is a complicated issue which involves several mechanisms. To address the issue of friction anisotropy, this paper reports the results of systematic studies performed in a highly controlled ultrahigh vacuum (UHV) environment in which crystallographic orientation effects on friction at crystalline metallic interfaces were measured under conditions of plastic deformation. Friction measurements were made between a pair of Ni(100) surfaces which were prepared to be truly clean or modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. The adsorbed layer of sulfur forms a c(2 × 2) surface lattice which is well-ordered but rotated by 45° with respect to the Ni(100) substrate lattice. The ethanol films have no long-range order. The unique aspect of adsorbing atomic sulfur and/or ethanol on the Ni(100) surfaces is that they modify the surface lattice without changing the bulk lattice orientation. We have observed friction anisotropy which is not related to surface lattice incommensurability. 2. Experimental Section 2.1. UHV Chamber and Tribometer Capabilities. All experiments were performed in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure below 1.0 × 10-10 Torr. The chamber is equipped with a four-grid retarding field analyzer for low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), a quadrupole mass spectrometer is used for the desorption measurements, an Ar+ ion sputter gun is used for cleaning of the crystal surfaces, leak valves, with or without capillary arrays, are used to introduce gases into the chamber, and an UHV tribometer is used to measure shear and normal forces between pairs of single-crystal surfaces in sliding contact. Two Ni(100) single crystals of ∼1-cm diameter were mechanically polished to a mirror finish with diamond and alumina paste. These were used to perform friction measurements in ambient or UHV conditions. One sample was spot-welded between two tantalum wires on the end of an UHV manipulator. The manipulator allows a wide range of motion in the x, y, and z (vertical) directions as well as rotation about the vertical axis of the chamber. The sample could be heated resistively to T > 1000 K and cooled to ∼95 K through mechanical contact of the sample with a liquid nitrogen reservoir. The temperature of the crystal was measured with a chromel-alumel thermocouple junction spot-welded to the back of the crystal. The Ni(100) crystal on the manipulator was polished with slight spherical curvature (radius ∼ 15 cm) to avoid edge contact with the second (flat) Ni(100) crystal during friction measurements. The second sample used in the friction measurements was a flat Ni(100) crystal mounted on an UHV tribometer (force transducer). This tribometer allowed the simultaneous measurement of both shear and normal forces when the two samples were brought into contact and sheared relative to one another.17 The flat Ni(100) sample on the tribometer was spot-welded to two Ta wires that are attached to a copper frame. The frame is

Ko and Gellman clamped to a copper-beryllium sheet spring that deflects during friction measurements. A thoriated tungsten filament located behind the sample on the tribometer allowed heating by electron bombardment to T > 900 K, and a liquid nitrogen reservoir allowed cooling to ∼120 K. The temperature was measured with a chromel-alumel thermocouple junction spot-welded to the side of the tribometer crystal. The contact between the curved sample on the manipulator and the flat surface on the tribometer creates a pin-on-flat geometry for measurement of friction forces. The pin slides in one direction over the flat surface. A detailed description and schematic of the UHV tribometer has been published elsewhere.17 The response of the tribometer was calibrated while it was outside the UHV chamber by using objects ranging in weight (force under gravity ) 2-250 mN). Both normal and shear responses (measured separately) were observed to be linear in the applied force over the calibration range. Within the chamber, the samples were aligned optically to ensure that their normals were parallel and that the sliding motion of the manipulator crystal (pin) was parallel to the surface of the tribometer sample (flat). Once the samples were aligned, they were brought into contact under the desired normal force (FN ≈ 20-50 mN), kept in contact for a period of 6-10 s, and then sheared relative to one another at a constant sliding speed (vshear ) 20 µm/s) using motorized micrometers. Both the normal and shear forces were measured simultaneously over the usual sliding distance of 400600 µm. Unless otherwise specified, the usual shearing conditions were load of FN ≈ 40 mN, sliding speed of vshear ≈ 20 µm/s, and temperature of T ≈ 300 K. For measurements with adsorbed ethanol, the surfaces were held at T e 120 K. For any given set of experimental conditions, a set of at least 12 single-pass friction measurements was made at different contact points between the surfaces. Between each single-pass measurement, the curved sample on the manipulator was rotated ((∼1.5° from normal) and moved vertically to ensure that contact occurred at different points on the surfaces. 2.2. Surface Preparation. Three types of surface conditions were used for friction measurements: clean, c(2 × 2)-S, and CH3CH2OH/c(2 × 2)-S. After any exposure to atmosphere, the Ni(100) surfaces were cleaned in a vacuum by multiple cycles of 1.5 keV Ar+ bombardment followed by annealing at 1000 K for 10 min until the coverages of sulfur (152 eV), carbon (272 eV), and oxygen (510 eV) contaminants were reduced to the noise level of AES. A sharp LEED pattern was observed after the samples had been cleaned and annealed properly. The effect of sulfiding the Ni(100) surfaces with repeated adsorption and decomposition of H2S has been studied in great detail.18,19 Sulfiding the surface passivates it and prevents the decomposition of adsorbed ethanol. Background exposure to H2S followed by annealing to 800 K was repeated until the sulfur Auger signal (152 eV) was maximized and LEED indicated that the sulfur overlayer formed a c(2 × 2) lattice. Under these conditions, the sulfur coverage is known to be θs ) 1/2 monolayer (ML). Ethanol was adsorbed on the sulfided Ni(100) surfaces at T e 120 K by backfilling the chamber with ethanol vapor through a leak valve. Exposures are reported in units of langmuir (1 langmuir ) 10-6 Torr s) with the pressure uncorrected for ion gauge sensitivity. The ethanol used in this study was purified by freeze-pump-thaw cycles until no air or other high vapor pressure impurity was detected by mass spectrometry. Temperature programmed desorption (TPD) was used to determine the coverages of ethanol. Following adsorption, the manipulator crystal was positioned in front of the aperture to the quadrupole mass spectrometer and heated resistively at a constant rate of 2 K/s. The details of the desorption spectra and our definition of the monolayer will be discussed in section 3. Before making friction measurements, care was taken to be certain that both crystal samples had the same coverages of ethanol for a given exposure. This was done by moving the manipulator sample to the tribometer sample position before exposure to ethanol. With (17) Gellman, A. J. J. Vac. Sci. Technol. A, 1992, 10 (1), 180. (18) Johnson, S.; Madix, R. J. Surf. Sci. 1981, 103, 361. (19) McLaren, J. M.; Pendry, J. B.; Rous, P. J.; Saldin, D. K.; Somorjai, G. A.; Van Hove, M. A.; Vvedensky, D. D. A Handbook of Surface Structures, Reidel: Dordrecht, 1987, pp 141-148.

Anisotropy at Ni(100)/Ni(100) Interfaces

Figure 1. Schematic diagram showing the lattice mismatch of the Ni(100) sample surfaces on the tribometer and manipulator, the shearing direction, and the lattice misorientation angle, θ. The tribometer crystal (light circles) orientation is held constant while the manipulator crystal (dark circles) is rotated to vary the lattice misorientation angle. both samples in the same position during dosing with ethanol their surfaces will be exposed to equal fluxes of ethanol before performing the desorption or friction experiments. Both Ni(100) samples used in this study were purchased from Monocrystals, Inc. H2S (Matheson, CP grade) and ethanol (McCormick Distilling Co, dehydrated) were used to modify the interface and provide independent control of surface lattice and substrate lattice. 2.3. Determination of Lattice Orientation. The lattice orientations were determined by the LEED patterns of the Ni(100) surfaces. For this investigation, the tribometer sample surface was oriented such that the sliding direction was along its 〈110〉 direction. The manipulator crystal was rotated about the surface normal to study the effect of lattice misorientation angle, θ, on friction between the two surfaces. When θ ) 0°, the 〈110〉 directions are aligned parallel to the sliding direction, and the lattices are commensurate or in registry with one another. To change orientation, the manipulator sample was removed, rotated, and remounted. The contact geometry can be seen schematically in Figure 1. After each remounting both surfaces were cleaned by Ar+ ion bombardment.

3. Results 3.1. Methods of Studying Orientation Effects on Friction. Two general approaches have been employed in previous work studying the effect of lattice orientation on friction. To probe the issue of commensurability,1-3 lattice misorientation angles are varied by rotating one of the two surface lattices while keeping the shearing direction fixed. The other approach keeps the lattice misorientation angle constant while changing the direction of sliding.5-9 Due to the limitations of our tribometer, we chose to study the friction anisotropy of Ni(100) surfaces by keeping the shearing direction constant and rotating the manipulator sample about its surface normal (Figure 1). The interface can be described as a twist boundary of the (100) face with rotation about the surface normal. The following sections describe the results of friction measurements between Ni(100) surfaces in three states: clean, modified by 1/2 monolayer (ML) of sulfur, and lubricated by adsorbed ethanol. 3.2. Friction Measurements between Clean Ni(100) Surfaces. The first set of friction measurements was made with clean Ni(100) surfaces. Auger electron spectra were used to determine surface composition during cleaning

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Figure 2. Three randomly selected plots of friction measurements made between clean Ni(100) crystal surfaces with a lattice misorientation angle of θ ∼ 90°(the manipulator sample has been rotated 90° from its original position). The upper trace (solid line) is a plot of the normal force (FN) and the lower trace (dashed line) is a plot of the shear force (FS). Sliding conditions were FN ≈ 20 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K. The negative value of the normal force upon separation of the surfaces is a measure of the strong intermetallic adhesion force (Fad). The sliding behavior can be generally characterized as stick-slip behavior with very high static friction coefficient (µs ∼ 9).

and following surface modification. Spectra of the Ni(100) crystal samples prior to cleaning showed that the major contaminants were C (272 eV), O (510 eV), S (152 eV), and Cl (181 eV). The Auger spectra of the crystal surfaces after extensive cleaning showed that the signals from oxygen and other airborne contaminants were reduced to below the noise level. Only the peaks assigned to Ni (61 eV, 105 eV) were detected, indicating that the Ni(100) surfaces were clean. Both surfaces gave sharp LEED patterns, indicating that they were clean and ordered. Numerous friction measurements were made with the Ni(100) crystals in UHV after their surfaces had been cleaned. Friction measurements were made as a function of the lattice misorientation angle (θ ) 0°, 30°, 45°, 55°, 75°, 90°, and 135° (( 2°)). Since the Ni(100) lattice has 4-fold rotational symmetry, the 0°/90° and 45°/135° orientations and friction coefficients should, in principle, be identical. Two of the seven sets (θ ) 45° and 90°) of measurements will be described in detail. The first set of friction measurements were made between two commensurate clean Ni(100) surfaces with θ ) 90° ()0°). Figure 2 shows three randomly selected plots of shear and normal forces measured as a function of time during sliding. The procedure used to make the friction measurement involves several steps. Referring to Figure 2, one can observe the response of the tribometer signal to these various steps. At point A, the two sample surfaces were out of contact. At point B, the manipulator sample was brought into contact with the tribometer sample as indicated by the increase in the normal force. The two samples were held in contact for a brief period (6-10 s) before shearing began at point C. At a constant load and shearing velocity, the shear force increased as shearing began and then dropped precipitously indicating the onset of stick-slip sliding of one surface over the other. At point D, the shearing was stopped, and the samples were then separated at point E with the normal and shear forces returning to zero. Intermetallic adhesion between the surfaces following sliding was observed as a negative normal force (Fad) between the two as they were being separated. Adhesion

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after shearing is commonly seen between clean metallic surfaces.20-23 Throughout this paper we have reported values of the static friction coefficient (µs), defined as the shear force (FS) needed to initiate sliding divided by the normal force (FN) as indicated in Figure 2. The choice of this static friction force is consistently defined as the force needed to induce the first major slip between the surfaces. The first major slip is defined relative to the succeeding slip events. When the following slip is smaller in magnitude, then the preceding slip is the first major slip. Taking the first trace of Figure 2 as an example, we could have chosen the static friction force as 170 mN; however, it does not represent the first major slip. Thus, the static friction force for that particular measurement was determined to be 250 mN. Other definitions of µ such as the dynamic friction coefficient (µd), defined as the ratio of the shear force to the normal force during steady-state sliding, were also obtained and could have been reported but would not change the overall conclusions of this work regarding the orientation dependence of friction. In this paper, values of µs will be reported for comparison of friction measurements between clean and modified Ni(100) surfaces with varying lattice orientations. Another quantity reported in this paper is the adhesion coefficient, defined as the negative pull-off force or adhesion force (Fad) divided by the normal force. Referring to Figure 2 again, the adhesion force is indicated at the end of the first sliding event. Adhesion coefficients are usually large between clean Ni(100) surfaces and might be a function of the orientation angle. However, the variations in the adhesion coefficient were too great to allow meaningful interpretation. Under sliding conditions of: FN ≈ 20 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K, the results of 12 single-pass friction measurements using clean Ni(100) crystal surfaces indicate that the average static friction coefficient between two commensurate surfaces (θ ) 90°) is µs ) 8.6 ( 2.5. The average adhesion coefficient is µad ) 3.8 ( 3.5. Friction measurements were made between the Ni(100) surfaces after the manipulator sample was rotated by 45° from its aligned orientation (θ ) 0° or 90°). Under sliding conditions similar to those used at θ ) 90° (except that FN ≈ 40 mN), a set of 12 single-pass friction measurements were made between the Ni(100) surfaces. Figure 3 shows three randomly selected plots of shear and normal forces measured as a function of time during sliding with 45° lattice misorientation between the two clean Ni(100) surfaces. Differences in the static friction coefficient and sliding behavior can be observed between the two sets of friction measurements (θ ) 90°, Figure 2, vs θ ) 45°, Figure 3). The average static friction coefficient between two clean Ni(100) crystal surfaces with a lattice misorientation angle of 45° is µs ) 2.5 ( 1.0 with an average adhesion coefficient of µad ) 0.52 ( 0.5. The other interesting feature is that while the sliding behavior between the commensurate surfaces (θ ) 90°, Figure 2) generally indicated stick-slip behavior, the sliding behavior between the clean Ni(100) surfaces with a lattice misorientation angle of θ ) 45° (Figure 3) generally indicated slip. The randomly selected traces reveal one case, the third trace of Figure 3, in which the friction force (20) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, 1986. (21) McFadden, C. F.; Gellman, A. J. Langmuir 1995, 11, 273 (22) Dekoven, B. M.; Hagans, P. L. J. Vac. Sci. Technol. A 1990, 8 (3), 2393 (23) Buckley, D. H. Surface Effects in Adhesion, Friction, Lubrication, and Wear; Elsevier: Amsterdam, 1981.

Ko and Gellman

Figure 3. Three randomly selected plots of friction measurements made between clean Ni(100) crystal surfaces with a lattice misorientation angle of θ ∼ 45°. The upper trace (solid line) is a plot of the normal force (FN) and the lower trace (dashed line) is a plot of the shear force (FS). Sliding conditions were FN ≈ 40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K. The negative value of the normal force (Fad) upon separation of the surfaces indicates intermetallic adhesion. The sliding behavior can be generally characterized as slip behavior with high static friction coefficient (µs ∼ 3).

continues to rise to the point that the shear force is approaching the upper limit for which the tribometer is calibrated (∼250 mN). At this point, experience has shown that there is a danger of having the two surfaces coldweld irreversibly and the friction measurement is thus stopped. It is also evident that a large adhesion force is seen upon separation of the two clean Ni(100) surfaces. In the third trace of Figure 3, the static coefficient of friction is deemed to be µs > 5.5. It is clear from Figure 3 that there can be a great deal of variance among the friction measurements made between the perfectly clean surfaces. The primary difference between the conditions used for the two sets of friction measurements illustrated in Figures 2 and 3 is that in one the crystal lattices are commensurate (θ ) 90°), while in the other they are not (θ ) 45°). Although the normal forces are different, past experience has shown that the friction coefficient is independent of load.10,11,21,24 The lower load (FN) 20 mN) for θ ) 90°was used to prevent the shear force from exceeding that of the safe operating limit of the tribometer. This allowed us to obtain an estimate of the friction coefficient for the aligned surfaces. Clearly, these two sets of measurements reveal friction anisotropy. Friction measurements were made at five lattice misorientation angles in addition to those at 45° and 90° described above. If commensurability is the dominant factor in friction anisotropy, one would expect that the static friction coefficient would decrease significantly with the slightest lattice mismatch. This very high sensitivity to lattice misorientation has indeed been seen by McGuiggan and Israelachvili1,2 and Hirano et al.3 in measurements of friction and adhesion forces between mica surfaces. It has also been observed in molecular simulations by Rajasekaran et al.16 of AFM measurements with a single atom tip sliding over a hexagonally closed pack surface. However, as seen in Figure 4 where the static friction coefficients between Ni(100) surfaces are plotted as a function of lattice misorientation angle (θ), the minimum static friction coefficient occurs when the two surfaces are misoriented by θ ) 45° (µs ) 2.5 ( 1.0) or, equivalently, by θ ) 135° (µs ) 3.2 ( 1.2). All the other lattice (24) Ko, J. S.; Gellman, A. J.; Lograsso, T. A.; Jenks, C. J.; Thiel, P. A. Surf. Sci. 1999, 423 243

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Figure 4. Static friction coefficient as a function of the lattice misorientation angle between two clean Ni(100) surfaces. The minimum static friction coefficient is observed at 45° (and 135°) lattice misorientation (µs ∼ 3). The friction coefficient at other lattice misorientation angles was too high to measure (µs > 5.5) at loads of FN ) 40 mN and the lower limits on the friction are indicated by points with upward arrows. At 90° misorientation the friction force was measured with a lower load of FN ) 20 mN and the friction coefficient was measurable (µs ) 8.6 ( 2.5). Sliding conditions were FN ≈ 20 or 40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K.

Figure 5. Static friction coefficient as a function of the lattice misfit angle between two fully sulfided Ni(100) surfaces (1/2 ML of S). The minimum static friction coefficient is observed at 45° (and 135°) lattice misorientation (µs ∼ 2.5). The friction coefficient at other lattice misorientation angles was to high to measure (µs > 4.5) at loads of FN ) 40 mN and the lower limits on the friction are indicated by points with upward arrows. At 90° misorientation the friction force was measured with a lower load of FN ) 20 mN and the friction coefficient was measurable (µs ) 6.2 ( 0.8). Sliding conditions were FN ≈ 20 or 40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K.

misorientation angles produced high static friction coefficients (µs > 5.5). In fact, most of the friction measurements are similar to the third trace of Figure 3 where the friction force continues to rise to the point that the shear force is approaching the upper limit for which the tribometer is calibrated (∼250 mN). Thus, for most of the lattice misorientation angles (θ ) 0°, 30°, 55°, 75°) where the load was FN ) 40 mN, the lower limit of the static friction coefficient is plotted in Figure 4 with an arrow. Only by lowering the load to FN ) 20 mN could the static friction be measured at angles other than (θ ) 45°/135°) as illustrated by the data point at θ ) 90° in Figure 2. At the load of FN ) 20 mN, the friction coefficient was µs ) 8.6 ( 2.5. The two independent sets of friction measurements made with lattice misorientation angles of θ ) 45° (the point plotted at 45° with an open circle is actually data obtained at θ )135°) both clearly show the reproducibility of the friction measurements and the friction anisotropy. Lower adhesion coefficients were measured between Ni(100) surfaces with lattice misorientation angles of θ ) 45° and 135° (µad ) 0.52 ( 0.5 and 0.85 ( 0.75, respectively) than those observed at other lattice misorientation angles (µad > 1.5). Since the deviations of the measurements of adhesion coefficients are quite large, adhesion anisotropy at the Ni(100) interfaces is not observed as clearly as friction anisotropy. 3.3. Friction between Sulfided Ni(100) Surfaces. To discriminate between surface lattice and bulk lattice effects on friction anisotropy, sulfur was adsorbed on the Ni(100) surfaces. Saturation of the Ni(100) surface with sulfur produces a c(2 × 2) overlayer structure and, thus, rotates the surface lattice by 45° with respect to the substrate bulk lattice.18,19 Friction measurements were made between fully sulfided Ni(100) surfaces to examine the effect of this surface modification. The sliding behavior

was similar to friction measurements made between clean Ni(100) surfaces. Stick-slip behavior, very high static friction coefficients, and high adhesion were observed. In summary, Figure 5 illustrates the static friction coefficient as a function of the lattice misorientation angle between sulfided Ni(100) surfaces. For the lattice misorientation angle of 90°, the static friction coefficient (measured at FN ) 20 mN) between sulfided Ni(100) surfaces was µs ) 6.2 ( 0.8, while between clean Ni(100) surfaces it was µs ) 8.6 ( 2.5. The minimum static friction coefficient (measured at FN ) 40 mN) still occurred when the two surfaces were misoriented by θ ) 45° (µs ) 2.0 ( 0.5) or θ ) 135° (µs ) 3.0 ( 1.0). All the other lattice misorientation angles produced high static friction coefficients (µs > 4.5). Although the surface lattice is rotated with respect to that of the clean Ni(100) surface, these results clearly demonstrate that friction anisotropy in this system is the same as that observed with the clean Ni(100) surfaces and is not dictated by the surface lattice misorientation. 3.4. Friction between Lubricated Ni(100) Surfaces. Ethanol was adsorbed on Ni(100)/c(2 × 2)-S surfaces to study the friction anisotropy in the presence of adsorbed layers. Sulfiding the Ni(100) surface passivates it and minimizes the ethanol decomposition during adsorption. Furthermore, since the ethanol is not observed to form an ordered overlayer, its presence eliminates long-range order of the surface lattice. A series of TPD spectra were obtained following increasing exposure of ethanol to the Ni(100)/c(2 × 2)-S surfaces at T e 120 K. Ethanol exposures were calculated using pressures indicated by the ion gauge with the pressure uncorrected for ion gauge sensitivity. All exposures were performed by back filling the chamber with ethanol vapor and care was taken to ensure that equal coverages of ethanol were adsorbed on both sample surfaces at any given exposure. The TPD spectra are

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Figure 6. Thermally programmed desorption spectra taken following increasing background exposures of the Ni(100)/c(2 × 2)-S surface to ethanol at T e 120 K. Several desorption peaks are observed with the monolayer corresponding to the peak at 182 K that saturates with an exposure of 2 langmuirs. The peak at T ∼ 160 K does not saturate with increasing exposure. All ethanol coverages are calculated relative to the area under the saturated monolayer curve at an exposure of 2 langmuirs. Heating rate ) 2 K/s. Mass/charge monitored ) 31 (CH2OH+).

presented in Figure 6 which depicts desorption rate as a function of temperature for several initial coverages. The signal at m/q ) 31 (CH2OH+) was monitored as it was the most intense signal in the fragmentation pattern of ethanol. The TPD spectra in Figure 6 have been used only as a means of calibrating the ethanol coverage on the surface and are not intended to provide any description of the surface chemistry of ethanol on Ni(100)/c(2 × 2)-S surfaces. At the lowest exposure (1.2 langmuirs), desorption occurred during heating at T ) 182 K. This peak represents desorption from the first monolayer of ethanol, which is bonded directly to the sulfided Ni(100) surface. At an exposure of 2 langmuirs, the monolayer is saturated and a second peak begins to evolve near T ) 170 K. This feature is better illustrated by the TPD obtained following an exposure of 3 langmuirs. At the highest exposure (20 langmuirs), the desorption peak is observed at T ∼ 160 K and does not saturate with increasing exposure, indicating multilayer desorption. Since the TPD spectra exhibit several peaks, it is not clear where the monolayer saturates. To address this issue, ethanol TPD on a Cu(111) surface was performed in the same system. The differentiation of monolayer and multilayer desorption features is quite clear on the Cu(111) surface.10,25 The desorption spectra from the Cu(111) surface revealed two peaks: a higher temperature peak due to desorption from the monolayer in which the ethanol was directly bonded to the Cu(111) surface and a multilayer desorption peak at the lower temperature. The monolayer peak is saturated at an exposure of approximately 2.5 langmuirs. Assuming similar packing densities and sticking coefficients, it appears that for the CH3CH2OH/Ni(100)/c(2 × 2)-S system defining the monolayer as the saturation of the 182 K peak (at 2-langmuirs exposure) is correct. All ethanol coverages are calculated relative to the area under the saturated monolayer curve at an exposure of 2 langmuirs.

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Figure 7. Three randomly selected plots of friction measurements made between Ni(100)/c(2 × 2)-S surfaces with 2 ML of CH3CH2OH at a lattice misorientation angle of θ ∼ 45° (manipulator sample has been rotated 135° from the original position). The upper trace (solid line) is a plot of the normal force (FN) while the lower trace (dashed line) is a plot of the shear force (FS). Sliding conditions were FN ≈ 40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 120 K. The sliding behavior can be generally characterized as stick-slip behavior without any adhesion measured.

Friction measurements were made between sulfided Ni(100) surfaces with 2 ML of ethanol adsorbed on each surface. This provides a total of 4 ML of ethanol present at the interface. It is important to note that all the friction measurements made between the lubricated surfaces were done immediately following adsorption at 120 K and without heating. Three randomly selected plots of friction measurements made with this interface (4 ML of CH3CH2OH between Ni(100)/c(2 × 2)-S surfaces) at a lattice misorientation angle of 45°are presented in Figure 7. The friction behavior between lubricated surfaces modified by the presence of 4 ML of ethanol can be characterized as stick-slip behavior with no adhesion observed. The friction is clearly lower than that observed for clean or sulfided Ni(100) surfaces. For a set of 12 single-pass friction measurements, the average static friction coefficient was µs ) 0.85 ( 0.2. Friction measurements were also made with a thick film of CH3CH2OH at the interface. At an even higher coverage of 28 ML of ethanol present at the interface, the friction behavior can be characterized as slip with the static friction coefficient lowered to µs ) 0.3 ( 0.08. The reduction in friction for surfaces with such adsorbate coverages is consistent with prior work.17,25 To study the propagation of friction anisotropy through adsorbed layers, friction measurements have been made at various lattice orientation angles using Ni(100)-c(2 × 2)/S surfaces with 4 ML and ∼28 ML of adsorbed ethanol. The results for the lubricated surfaces are presented in Figure 8. With a total of 4 ML of ethanol present, a minimum in the static friction coefficient (µs ∼ 1) occurred when the lattice misorientation angle was 45° or 135°, while a static friction coefficient of µs ∼2 was measured at other lattice mismatch angles. The friction anisotropy disappeared at θ ) 28 ML, and the friction coefficient at all angles was µs ) 0.3 ( 0.08. Another way of reporting the results is to plot the static friction coefficient as a function of ethanol coverage. As illustrated in Figure 9, static friction coefficients can be plotted as a function of ethanol coverage between two Ni(100) surfaces with and without molecular and atomic lubricants. Submonolayer coverages of ethanol do not lubricate the Ni(100)/Ni(100) interface at any misorientation angle. At least one (25) McFadden, C. F.; Gellman, A. J. Surf. Sci. 1998, 409, 171

Anisotropy at Ni(100)/Ni(100) Interfaces

Figure 8. Static friction coefficient as a function of the lattice misfit angle between two sulfided Ni(100) surface lubricated with ethanol. The minimum static friction coefficient is consistently observed at 45° (or 135°) lattice misorientation. This orientation effect is observed even in the presence of 4 ML of ethanol at the interface. Only at higher coverages (28 ML) of ethanol does the friction anisotropy vanish. Sliding conditions were FN ≈ 40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 120 K.

monolayer of ethanol on each surface is needed to provide lubrication. This is consistent with the results of previous friction measurements on Cu(111) surfaces.10,21,25 The minimum static friction coefficient is consistently observed at 45° lattice misorientation (solid and hollow squares in Figure 9). The frictional anisotropy is most pronounced for the clean Ni(100) and the Ni(100)/c(2 × 2)-S surfaces. The anisotropy is still observed with 4ML of ethanol at the interface. Only at coverages of ∼28 ML of ethanol does the effect vanish. The absorbed ethanol layer at 4 ML thickness displays no long-range order. The fact that friction anisotropy is still observed in the presence of 4 ML of ethanol without long-range order indicates that the surface lattice is not an important factor in determining friction anisotropy in this system. 4. Discussion 4.1. Friction Behavior between Clean, Sulfided, and Lubricated Ni(100) Surfaces. The results of our measurements indicate that the friction between Ni(100) surfaces is anisotropic and depends on the misorientation angle of the bulk lattice structure. Friction anisotropy is also observed when the surface lattices have been altered by the adsorption of either atomic sulfur in a c(2 × 2) array or a total of 4 ML of ethanol. Although the c(2 × 2) layer is ordered and commensurate with the Ni(100) surface, there is no evidence of commensurate ordering of the ethanol at the interface. This implies that the primary energy dissipation mechanism leading to anisotropy is a process occurring in the bulk rather than at the interface. The friction behavior between clean and sulfided Ni(100) surfaces can generally be characterized as stick-slip behavior accompanied by high static friction coefficients and high adhesion coefficients. This behavior is observed for all lattice misorientation angles except those near θ ) 45°. Slip behavior rather than stick-slip

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Figure 9. Static friction coefficient as a function of ethanol coverage between two Ni(100) surfaces with/without adsorbates (molecular/atomic) present. The minimum static friction coefficient is consistently observed at 45° (or 135°) lattice misorientation (solid and hollow squares). The frictional anisotropy is most pronounced for the clean Ni(100) and Ni(100)-c(2 × 2)-S surfaces and is revealed by the wide spread of the values of the friction coefficients. This orientation effect is still observed in the presence of 4 ML of ethanol at the Ni(100)-c(2 × 2)-S interface. Only at high multilayer coverage (28 ML) of ethanol does the effect vanish. Also, submonolayer coverages of sulfur or ethanol do not lubricate the Ni(100)/Ni(100) interface. At least one monolayer of ethanol on each surface is needed to provide lubrication. Sliding conditions were FN ≈ 20-40 mN, vshear ) 20 µm/s, tc ≈ 6-10 s, and T ≈ 300 K (clean and fully sulfided), and T ≈ 120 K (with adsorbed ethanol).

was observed at θ ) 45°, and more importantly, lower static friction coefficients were measured (µs ∼ 3 versus µs > 5.5). Friction anisotropy and stick-slip behavior were still observed between the sulfided Ni(100) surfaces with 4 ML of ethanol at the interface. Only at thick multilayer coverages (28 ML) of ethanol does the anisotropy vanish and friction behavior changes from stick-slip to slip. The observation of the friction anisotropy in the presence of 4 ML of ethanol suggests that plastic deformation of the subsurface region is occurring even with the presence of the adsorbate. Many molecules confined between two surfaces separated by less than 5-10 molecular diameters become ordered into layers.26-31 Such ultrathin films can be thought of as liquid crystalline or solidlike rather than liquidlike. Stick-slip behavior between surfaces with adsorbed layers can occur by a different mechanism than stick-slip seen between clean surfaces. Phase transitions between melting and freezing of confined lubricants have been proposed as the cause of this stick-slip behavior.28-30 As a result of the presence of these solidlike films between surfaces, shearing can result in plastic deformation or wear of surfaces. Molecular dynamics simulations have (26) Granick, S. Science 1991, 253, 1374. (27) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834. (28) Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N. J. Chem. Phys. 1990, 93, 1895. (29) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (30) Yoshizawa, H.; Chen, Y. L.; Israelachvili, J. N. J. Phys. Chem. 1993, 97, 4128. (31) Persson, B. N. J.; Tosatti, E. Phys. Rev. B 1994, 50 (8), 5590.

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Figure 10. Fcc lattice unit cell with the (100) surface exposed at the top. The four (111) slip planes are shown in the bulk of the unit cell and project out to the (100) surface. The 〈110〉 shearing direction is marked as 0°. When the two surfaces were aligned, shearing occurred along the 0° direction for both and friction was high. When the two surfaces were misaligned by 45° the friction was lowest. Under these conditions the shearing occurs along the 0° direction for one crystal and along the 45° direction for the other.

shown that with as many as five layers of hexadecane present between Au(111) surfaces, bulk plastic deformation still occurs.29,32 Although the rheological properties of 4 ML of ethanol at the Ni(100)/Ni(100) interface are not known, there is ample evidence that such thin films can have solidlike properties and thus can transmit sufficient stresses between the surfaces to allow plastic deformation of their bulk lattices. There is no evidence from LEED for the ordering of ethanol films on sulfided Ni(100) surfaces and we assume that this is still true at the Ni(100)/Ni(100) interface. The fact that friction anisotropy is observed in the presence of 4 ML of ethanol without long-range order indicates that the surface lattice orientation is not an important factor in this system. Hence, we suggest that the friction between the two surfaces is due predominantly to plastic work being done in the bulk of the Ni(100) surfaces. At ethanol coverages of 28 ML there is no evidence of friction anisotropy between the Ni(100) surfaces. We do not know whether there is still plastic deformation of the surfaces during sliding contact. The fact that there is no anisotropy, however, suggests that the bulk of the frictional work goes into shearing of the ethanol film rather than plastic deformation of the Ni(100) bulk. 4.2. The Origin of Friction Anisotropy. Whether the friction coefficient is highly dependent on the slipplane orientation of the bulk lattice or on the surface lattice orientation remains to be discussed. The slip planes are usually the most densely packed atomic planes and are also the most widely separated.6,31 For face-centered-cubic (fcc) structures such as nickel, the slip planes are the (111) planes and the slip directions are the 〈110〉 directions. For fcc metals, there are four slip planes and three slip directions or 12 possible slip systems. The greater the number of slip systems the easier it is to initiate plastic deformation since the probability for some of the slip systems to be favorably oriented with respect to the shear stress also increases. For our experiment the (111) slip planes and the relative motion of the surfaces with respect to the bulk lattices are illustrated in Figure 10. In the case of the fixed Ni(100) surface shearing was always along the 〈110〉 direction denoted as 0° in Figure 10. In the case of the Ni(100) surface that was rotated during our experiment, shearing moved from the 〈110〉 direction, (32) Gao, J.; Luedtke, W. D.; Landman, U. Physics of Sliding; Persson, B. N. J., Tosatti, E., Eds.; Kluwer: Dordrecht, 1996. (33) Carter, G. F. Principles of Physical and Chemical Metallurgy, American Society for Metal: Metals Park, OH, 1979, pp 177-180.

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which was aligned with the fixed surface lattice, and gave high friction to the 〈100〉 direction, which is marked as 45° in Figure 10, and gave low friction. At this point we do not have a quantitative model for the origin of low friction at 45° misorientation between the two surfaces. Several discussions with persons more knowledgeable than ourselves in the bulk mechanical properties of crystalline materials suggest that this is not an trivial problem. For the time being we will have to rest our discussion on a qualitative examination of Figure 10. Since the shearing direction with respect to the fixed surface is always along the 〈110〉 (0°) direction, we assume that the friction anisotropy is not due to work done in the deformation of its bulk. Instead it seems reasonable to suggest that the anisotropy arises from work done to the bulk of the rotated crystal since the shearing vector is rotated with respect to its bulk during our experiments. During sliding of the aligned lattices the shearing vector is along the direction marked as 0° and is perpendicular to two of the slip planes as they project onto the surface. When the lattice is rotated such that sliding occurs along the direction marked 45°, the shear vector bisects the four slip planes as they project onto the surface. It is under these conditions that the minimum is observed in the friction coefficient between the two Ni(100) surfaces. Friction measurements between single crystals of copper sliding on copper in air and vacuum were made by Buckley.6,23 The surface lattices across the interface were aligned. Buckley’s results showed that the lowest friction, either in air or with clean surfaces in a vacuum, was obtained using surfaces representing the preferred slip planes for fcc metals (µ111 < µ110 < µ100). In another study of frictional anisotropy of fcc metals, Tsuya8 made friction measurements between two Cu(100) single crystals in air while the two lattices were aligned (θ ∼ 0°). The results reported showed that µ was lower when sliding in the 〈011〉 direction than in the 〈001〉 direction. Furthermore, the wear tracks showed smooth edges when sliding in the 〈011〉 direction and jagged edges when sliding in the 〈001〉 direction. This friction anisotropy was explained in terms of deformation of the bulk by motion of the subsurface crystallographic lattice slip planes. It is worth noting that if slip occurs via slip systems one would expect the friction coefficient to be a smooth continuous function of misorientation angle. The fact that we observe what appears to be a discontinuous function where the minimum static friction coefficient occurs at θ ∼ 45° suggests that the anisotropy observed in our system is not due to slip systems alone. Without making measurements as a continuous function of misorientation angle, it is not possible to address the functional form of the anisotropy. A detailed examination of the slip lines and the depth of the wear scars might reveal more information about the relationship of the frictional anisotropy and the slip planes. It might also be worthwhile to study the role of adhesion in this system. Several studies have suggested that commensurability of the contacting surface lattices is the primary cause of frictional anisotropy.1-4,15,16 However, those are experiments or simulations performed on interfaces which are in elastic contact. Under those conditions, a small misalignment of the surface lattices causes a significant decrease in static friction coefficient. Instead, under our conditions of plastic contact between the Ni(100) surfaces we have shown that the static friction coefficient remains high (µs ∼ 5.5-9) over a wide range of misorientation angles and that the minimum static friction coefficient occurs near a misorientation angle of θ ∼ 45°. The most important aspect of our experiment is that we have been

Anisotropy at Ni(100)/Ni(100) Interfaces

able to control independently the relative orientations of the surface lattice and the bulk lattices by the adsorption of species onto the surface. In the case of the adsorbed sulfur the surface is well ordered and its lattice vectors are rotated by 45° with respect to the bulk. Although they are still commensurate with one another at θ ∼ 0°, the direction of the sliding is now rotated by 45° with respect to the surface lattice. The orientation of the sliding direction with respect to the bulk lattice remains unchanged. When ethanol is adsorbed on the surface there is no long-range order to the overlayer. The fact that it influences the friction coefficient at 4 ML coverage though indicates that it is still present at the interface during sliding. The fact that friction anisotropy is observed in the presence of atomic sulfur and 4 ML of ethanol indicates that the surface lattice is not an important factor in this system. Friction anisotropy is due largely to subsurface damage produced in preferred slip planes. 5. Conclusions To address the issue of the origins of frictional anisotropy, this paper reports the results of systematic studies performed in a highly controlled environment that investigate crystallographic orientation effects on friction at crystalline metallic interfaces under conditions of plastic contact. Friction measurements were made between a pair of Ni(100) crystals with surfaces prepared to be truly clean or modified by the presence of adsorbed atomic sulfur with or without adsorbed ethanol. The unique aspect of adsorbing atomic sulfur or ethanol is that they modify the

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surface lattice orientation without changing the bulk lattice orientation. Measurements made by systematic variation of the crystallographic orientation of the Ni(100) surfaces have revealed that the friction coefficient is anisotropic with respect to lattice orientation. When aligned (θ ∼ 0°), the friction coefficient between the two clean Ni(100) surfaces was µs ) 8.6 ( 2.5. The minimum static friction coefficient occurred when the two clean Ni(100) surfaces were misoriented by θ ∼ 45 o (µs ) 2.5 ( 1). This effect of misorientation is consistently observed even with the presence of atomic sulfur and up to 4 ML of adsorbed ethanol, although these adsorbate modified surfaces no longer have the same periodicity or relative orientations as the Ni(100) substrate. The effect of lattice orientation is damped at the point that the surfaces are separated by >28 ML of adsorbed ethanol. Previous studies using the surface force apparatus and molecular dynamics simulations have suggested that commensurability of surface lattices in elastic contact is the dominant factor in frictional anisotropy.1-4,15,16 However, the friction anisotropy observed between Ni(100) surfaces suggests that surface lattice commensurability cannot be the only cause of friction anisotropy when plastic deformation occurs. Acknowledgment. This work was supported by the Air Force Office of Scientific Research under Grant No. F49620-98-100218. LA000161G