Ni(100) Interfaces

J. S. Ko and A. J. Gellman* ... us to explore the tribological properties of interfaces between Ni(100) ... or sulfided Ni(100) surfaces without adsor...
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J. Phys. Chem. B 2001, 105, 5186-5195

Molecular Layering Effects on Friction at Ni(100)/Ni(100) Interfaces J. S. Ko and A. J. Gellman* Department of Chemical Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213 ReceiVed: September 13, 2000; In Final Form: March 19, 2001

The combined use of an ultrahigh vacuum tribometer and a variety of surface science techniques has enabled us to explore the tribological properties of interfaces between Ni(100) surfaces and to observe phenomena attributable to molecular layering. Friction measurements have been made between a pair of clean Ni(100) surfaces, modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. Friction measurements made with ethanol coverages ranging from 0 to 10 monolayers (ML) on each Ni(100) surface reveal that the friction coefficient is discontinuous in coverage and can be correlated to the coverage dependence of the ethanol desorption energy. During shearing, sliding never commences between clean Ni(100) surfaces or sulfided Ni(100) surfaces without adsorbed ethanol. In the submonolayer coverage regime of either atomic sulfur or adsorbed ethanol, the behavior is characterized by a high friction coefficient (µs > 5.5) accompanied by high adhesive forces (µad ) 1.5 ( 0.7). An abrupt decrease in both the friction coefficient and adhesion coefficient occurs at a coverage of 1 ML of ethanol on each surface. The friction coefficient drops to µs ) 3.1 ( 1, while the adhesion coefficient is lowered to µad ≈ 0.25. At coverages between 1.0 and 2.5 ML of ethanol on each Ni(100) surface, the static friction coefficient decreases in a stepwise manner that is correlated with discontinuities in the ethanol desorption energy. This stepwise decrease in both the friction coefficient and the desorption energy may be due to molecular layering of the ethanol.

1. Introduction When a liquid is confined between two surfaces or within a confined space with dimensions of less than 10 molecular diameters, its molecules can become increasingly ordered as the dimensions of the confining volume decrease. With increased ordering, the molecules behave more like those of a liquid crystal or a solid than of a liquid.1-13 Between flat surfaces, this solidlike state is characterized by ordering of the molecules into discrete layers. Under stress, such ultrathin films exhibit yield points2-6,8,12 commonly associated with fracture in solids, and their effective viscosity, molecular diffusion rates, and relaxation times can be orders of magnitude higher than those of the bulk liquid.1-5,13 Monte Carlo and molecular dynamics simulations have revealed that molecules are expected to order into discrete layers in films having thicknesses of less than 6-10 molecular diameters.10,11,13 Experimentally, this has been observed by Israelachvili and co-workers, who measured the force between two surfaces separated by various liquids and by distances of 1000 K and cooled to ∼95 K through mechanical contact with a liquid nitrogen reservoir. The temperature was measured with a chromelalumel thermocouple junction spot-welded to the back of the crystal. The Ni(100) crystal on the manipulator was polished with a 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 allows the simultaneous measurement of both shear and normal forces between the two samples when they are brought into contact and sheared relative to one another.21 The flat Ni(100) sample on the tribometer was spotwelded to two Ta wires that are attached to a copper frame. The frame is clamped to a copper-beryllium sheet spring that deflects during friction measurements. Strain gauges bonded to the sides of the CuBe sheet spring are used to transduce the forces applied to the sample surface. A thoriated tungsten filament located behind the sample on the tribometer allows heating by electron bombardment to T > 900 K, and a liquid nitrogen reservoir permits 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. The pin slides in one direction over the flat surface which is stationary. A detailed description and schematic of the UHV tribometer has been published elsewhere.21 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,

J. Phys. Chem. B, Vol. 105, No. 22, 2001 5187 the samples were aligned optically to ensure that their surface normals were parallel and that the sliding motion of the manipulator crystal (pin) was parallel to the surface of the flat tribometer sample. Once the samples were aligned, they were brought into contact under the desired normal force (FN ≈ 3050 mN) and kept in contact at rest for a period of 6-10 s. The samples were 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 a sliding distance of 400 to 600 µm. The usual shearing conditions were: FN ≈ 40 mN, Vshear ≈ 20 µm/s, and T ≈ 300 K unless otherwise specified. 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 performed at different contact points between the surfaces. Between each single-pass measurement, the curved sample on the manipulator was rotated by (1.5° from normal and moved vertically to ensure that contact occurred at different points on the surfaces. This procedure allowed us to obtain good statistics for the measured friction coefficients between the surfaces. The lattice orientations of the Ni(100) samples were determined from LEED patterns of their surfaces. For this investigation, the tribometer sample surface was oriented such that sliding was along its 〈110〉 direction. The manipulator crystal and the tribometer Ni(100) crystal were misoriented by θ ) 75°. The effect of lattice misorientation on friction at the Ni(100)/Ni(100) interfaces has been investigated previously.22 2.2. Surface Preparation. Three types of Ni(100) surface conditions were used for friction measurements: clean, c(2 × 2)-S, and CH3CH2OH/c(2 × 2)-S. After any exposure to atmospheric conditions, the Ni(100) surfaces were cleaned in a vacuum using multiple cycles of 1.5 keV Ar+ bombardment, followed by annealing at 1000 K for 10 min. Auger spectra were used to determine surface cleanliness. The cleaning cycles were continued until the coverages of sulfur (152 eV), carbon (272 eV), and oxygen (510 eV) contaminants were reduced to the noise level of the AES spectra. A sharp LEED pattern was observed after the samples had been cleaned and annealed properly. The effect of sulfiding the Ni(100) surfaces through repeated adsorption and decomposition of H2S has been studied in great detail.23,24 Sulfiding the surface passivates the substrate and prevents the decomposition of ethanol upon adsorption. Background exposure to H2S followed by annealing to 800 K was repeated until the Auger sulfur 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 ) 0.5 ML. Ethanol was adsorbed on the sulfided Ni(100) surfaces at T e 120 K by backfilling the chamber with ethanol vapor introduced through a leak valve. Exposures are reported in units of Langmuir (1 L ) 10-6 Torr‚s), with the pressure uncorrected for ion gauge sensitivity. The ethanol used in this study was purified using freeze-pump-thaw cycles until no air or other high vapor pressure impurities were detected by mass spectrometry. Temperature-programmed desorption (TPD) was used to determine the surface coverages and ethanol desorption energy at different coverages. Following ethanol adsorption, the manipulator crystal was positioned in front of an aperture to the quadrupole mass spectrometer. The sample was then heated resistively at a constant rate of 2 K/s from 120 to 450 K. During heating, the mass spectrometer was tuned to m/q ) 31 in order to measure the rate of ethanol desorption from the surface. The

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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 sample surfaces had the same coverages of ethanol. This was done by moving the manipulator sample to the position of the tribometer sample before simultaneously exposing them to ethanol and performing the desorption or friction experiment. The desorption spectra could only be obtained from the surface of the sample on the manipulator. Although the surface was exposed to the ethanol while in the same position used for the friction measurements, it was positioned in front of the mass spectrometer while obtaining the TPD spectra. 3. Results 3.1. Adsorption of Ethanol on the c(2 × 2)-S/Ni(100) Surface. The preliminary goals of this investigation were to determine and define the monolayer coverage of ethanol on the c(2 × 2)-S/Ni(100) surface. Adsorbed sulfur passivates the Ni(100) surface and thereby minimizes ethanol decomposition during adsorption and desorption.23,24 Defining the monolayer coverage and measuring the desorption energy (∆Edes) versus coverage is necessary in order to correlate the coefficients of friction between the sulfided Ni(100) surfaces with the coverage and the properties of adsorbed films of ethanol. A series of TPD spectra were obtained following increasing exposures of ethanol to the c(2 × 2)-S/Ni(100) surface 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 backfilling the chamber with ethanol vapor. The TPD spectra are presented in Figure 1, which depicts desorption rate as a function of temperature for several coverages. The signal at m/q ) 31 (CH2OH+) was monitored during desorption, as it was the most intense signal in the fragmentation pattern of ethanol. At the lowest-exposure (1.2 L) desorption occurred during heating at T ) 184 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 2.3 L, the monolayer saturates, and a second peak begins to evolve near T ) 168 K. A third peak emerges at T ) 162 K after exposures of 3.0 L. Finally, at an exposure of ∼5 L a low-temperature peak appears at T ) 150 K, which has the characteristics of multilayer desorption in that it exhibits zero-order desorption kinetics and does not apparently saturate with increasing exposure. The various desorption peaks are highlighted by dash lines in Figure 1. The thermal desorption spectra illustrated here are similar to desorption spectra obtained previously on the same surface.21 One objective of performing the TPD experiments has been to determine the monolayer coverage. Since the TPD spectra exhibit several peaks, it is not clear when the monolayer saturates. To address this issue, TPD of ethanol adsorbed on a Cu(111) surface was performed in the same UHV chamber. The differentiation of monolayer and multilayer desorption features is quite clear on the Cu(111) surface.25,26 Those desorption spectra reveal two peaks: a high-temperature peak (Tp ) 180 K) attributed to desorption from the first monolayer with the ethanol directly bonded to the Cu(111) surface, and a multilayer desorption peak appearing at a lower temperature (Tp ) 155 K). On the Cu(111) surface, the monolayer peak is saturated at an exposure of approximately 2.5 L. Assuming similar packing densities and sticking coefficients, it appears that defining the monolayer as the saturation of the 180 K peak (at 2 L exposure) is correct for the ethanol/c(2 × 2)-S/Ni(100) system. Our definition of the monolayer coverage is illustrated by the data in the inset of Figure 1. These show several of the low

Figure 1. Temperature-programmed desorption spectra taken following increasing background exposures of ethanol to the c(2 × 2)-S/Ni(100) surface at T e 120 K. At the lowest exposure (1.2 L), desorption from the monolayer occurred during heating at T ) 184 K. As the ethanol coverage is increased, distinct peaks are observed at 180, 168, and 162 K and then at the highest coverage at 150 K. The peak at T ≈ 150 K does not saturate with increasing exposure and is due to multilayer desorption. The inset shows expanded versions of the spectra at low coverages. The spectrum from an exposure of 2.0 L was multiplied by a factor of 1.6 to generate a spectrum that saturates the 180 K desorption peak. The area under this scaled peak was used as that of the ethanol monolayer. Heating rate ) 2 K/s, m/q ) 31 amu (CH2OH+).

exposure ethanol TPD spectra with aligned baselines. The hightemperature monolayer peak at 180 K is clearly saturated at exposures of 2.6 and 3.0 L. By multiplying the peak for an exposure of 2.0 L by a factor of 1.6, we have generated the spectrum shown as the dotted line in the inset that has the same amplitude for the monolayer peak as that of the higher-coverage spectra. The area under this curve has been defined as that of the monolayer and used as the reference to define the coverages of all other desorption spectra. 3.2. Ethanol Desorption Energy as a Function Coverage. The choice of ethanol as a model lubricant was made because it desorbs molecularly from the sulfided Ni(100) surface rather than decomposing during heating. In other words, the activation barrier to decomposition is higher than the barrier to desorption. As a result, it is possible to use the TPD spectra to estimate the barrier to desorption of ethanol as a function of coverage. If one assumes that the adsorption of ethanol is unactivated, then the barrier to desorption is equal to the desorption energy (∆Edes) and roughly equal but opposite in sign to the heat of adsorption (∆Hads). The simplest analysis of the TPD spectra is performed using Redhead’s equation which relates ∆Edes to the desorption peak temperature (Tp)

∆Edes RT2p

)

(

)

-∆Edes ν exp β RTp

(1)

where ν is the desorption pre-exponent (s-1), β is the heating rate (K/s), and R is the universal gas constant.27 The simplest

Ni(100)/Ni(100) Interfaces

Figure 2. Differential desorption energy, defined as the desorption energy of additional ethanol on a surface already modified by the presence of some coverage of ethanol, can be determined by subtracting one TPD spectrum from another. The desorption spectrum from a 3.3 ML coverage differs from the spectrum for 2.7 ML by the presence of a shoulder at the leading edge of the spectrum. This area of the spectrum is due to desorption of the additional 0.6 ML of ethanol adsorbed between exposures. The peak temperature for both spectra is Tp ≈ 168 K, which implies a desorption energy of ∆Edes = 43 kJ/mol. However, this does not truly represent the incremental desorption energy at 3.3 ML. Subtracting the spectra of the two different ethanol coverages yields the resulting difference spectrum (bottom spectrum of Figure 2) with a peak temperature of 160 K. This implies that the differential desorption energy of the additional ethanol is ∆Edes = 41 kJ/mol.

approximation used in determining ∆Edes assumes that the preexponential factor is ν ) 1013 s-1. The differential desorption energy, defined as the desorption energy of ethanol from a surface already modified by the presence of some coverage of ethanol, can be determined by subtracting one TPD spectrum from another. Figure 2 illustrates TPD spectra corresponding to 3.4 and 4.0 L exposures of the c(2 × 2)-S/Ni(100) surface to ethanol. These represent coverages of 2.7 and 3.3 ML, respectively. The TPD spectrum from 3.3 ML of ethanol is exactly the same as the spectrum for 2.7 ML, except for the additional area at the leading edge of the spectrum. This area under the TPD spectrum is due to desorption of the additional 0.6 ML of ethanol that makes up the difference between the two coverages. The peak desorption temperature for both spectra is Tp ≈ 168 K, which would have corresponded to a desorption energy of ∆Edes = 43 kJ/mol. However, this does not truly represent the differential desorption energy at 3.3 ML. When the spectra of the two different coverages of ethanol are subtracted, the resulting spectrum (bottom of Figure 2) displays a peak temperature at 160 K. This represents, more accurately, the desorption peak of the additional 0.6 ML of ethanol. Assuming that this desorbs in a first-order process, the differential desorption energy is therefore found to be ∆Edes = 41 kJ/mol at a coverage of 3 ML. By plotting the differential desorption energy as a function of ethanol coverage, one observes that it is a discontinuous function of ethanol coverage as shown in Figure 3. The incremental changes in the desorption energy with increasing ethanol coverage reveal a discontinuous change in the properties of the film as the coverage increases. One

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Figure 3. Differential desorption energy of ethanol is discontinuous in ethanol coverage. The desorption energy is highest (∼47 kJ/mol) in the submonolayer regime. A step change in the desorption energy can be seen at 1 ML ethanol coverage. The desorption energy is ∼43 kJ/ mol and remains constant until the ethanol coverage reaches 1.5 ML. With an ethanol coverage of 1.5-2.5 ML, the desorption energy decreased to ∼40 kJ/mol. For ethanol coverages of greater than 2.5 ML the desorption energy reached a limit of 37 kJ/mol, indicating multilayer desorption.

possibility, of course, is that they arise from the differences in discrete layers that form as the ethanol coverage increases. On the basis of our comparison with the desorption from the Cu(111) surface, we believe that the monolayer is saturated at an exposure of 2.0L. Additional ethanol, therefore, must adsorb into second, third, and high layers. Properties such as the desorption energies of these layers might be expected to vary monotonically, and perhaps discontinuously, between that of the monolayer and that of the condensed multilayer. Molecules interact most strongly with the sulfided Ni(100) surface, and the adsorbate-surface interaction strength decreases with addition of each molecular layer until multilayer coverage is reached. The coverage dependence of the desorption energy manifests itself in Figure 1 as a shift in the peak temperature from Tp ≈ 184 K to Tp ≈ 150 K with increasing coverages. One of the less than satisfying aspect of the data in Figure 3 is that the amounts of ethanol in each of the layers do not appear to be equal. If anything, the discontinuities in the desorption energy appear to occur at coverages of 1, 1.5, and 2.5 ML. There are several possible causes for this, one of which is the possible errors associated with the difficulty in identifying the saturation of the monolayer in the TPD spectra. Assuming that the model of molecular layering is correct, it is possible that the packing densities in the first, second, and third layers are not identical. Alternately, our picture of molecular layer may not be the cause of the observed discontinuities in the desorption energy. 3.3. Friction Measurements between Clean Ni(100) Surfaces. To determine the effect of lubricant coverage on friction, we made measurements between a pair of Ni(100) surfaces, modified by a c(2 × 2) overlayer of adsorbed atomic sulfur, with varying coverages of adsorbed ethanol. The first set of friction measurements was made using two clean Ni(100) surfaces. Auger electron spectroscopy was used to determine

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Figure 4. Three randomly selected plots of friction measurements made between clean Ni(100) crystal surfaces. The upper trace (dotted line) is a plot of the normal force (FN), while the lower trace (solid line) is a plot of the shear force (FS). As one surface is sheared with respect to the other, the shear force continues to rise to the point that it approaches the upper limit for which the tribometer is calibrated (∼250 mN) and shearing has to be stopped. Sliding never commenced as the two surfaces adhered to one another. When shearing never commenced, the shear yield force is deemed to be greater than the value of the shear force (FS) at which the shearing was stopped, and thus, the static friction coefficient is denoted µs > 5.5. The negative value of the normal force upon separation of the surfaces is evidence of a strong intermetallic adhesion force (Fad). Sliding conditions: FN ≈ 40 mN, Vshear ) 20 µm/ s, Tc ≈ 6-10 s, and T ≈ 300 K. The misorientation angle between the two Ni(100) surface lattices is θ ) 75°.

surface composition during cleaning and following surface modification. Auger 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 spectrum of the crystal surface after extensive cleaning showed that the signals from oxygen and other airborne contaminants were reduced to below the noise level leaving only the peaks assigned to Ni (61 eV, 105 eV). Initially, a set of 12 single-pass friction measurements were made in UHV using a pair of clean Ni(100) crystal surfaces. Figure 4 shows three randomly selected plots of normal force (FN) and shear force (FS) measured as a function of time during sliding. The procedure used to make the friction measurement involves several steps. By examining Figure 4, one can observe the response of the tribometer signal to these various steps. At point A, the two sample surfaces were out of contact, and both FN and FS were zero. At point B, the manipulator sample was brought into contact with the tribometer sample, and a load was applied between the two as indicated by the increase in FN. 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 FS increased linearly with time. In this case, the two clean Ni(100) crystals adhered to one another, and thus, sliding never occurred. 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 shearing was observed as a negative normal force (Fad) between the two as the clean Ni(100) surfaces were being separated. Adhesion after shearing is commonly observed between clean metallic surfaces.22,28-30 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

Ko and Gellman

Figure 5. Three randomly selected pots of friction measurements made between sulfided Ni(100) crystal surfaces modified by 1.2 ML of adsorbed ethanol on each surface. The upper trace is a plot of the normal force (FN), while the lower trace is a plot of the shear force (FS). The sliding behavior can be generally characterized as stick-slip behavior with a high static friction coefficient (µs ≈ 3). Sliding conditions: FN ≈ 40 mN, Vshear ) 20 µm/s, Tc ≈ 6-10 s, T ≈ 120 K, and θ ) 75°.

in Figure 4. In all of the friction measurements made between the clean Ni(100) surfaces, the friction force continued to rise to the point where the shear force was approaching the upper limit for which the tribometer was calibrated (∼250 mN). At this point, experience has shown that there is a danger of having the two surfaces cold-weld irreversibly, and the friction measurement was thus stopped. In the case where the sliding never commences, the shear yield force (FS) is deemed to be greater than the value of the shear force at which the shearing had to be stopped, and thus, the static friction coefficient is denoted µs > 5.5. Other definitions of µ such as the dynamic friction coefficient (µd), defined as the ratio of the steady-state shear force to the normal force, would not be meaningful to report since steady-state sliding friction was never achieved in this system. Therefore, in this paper, values of µs will be reported for comparison of friction measurements between clean and lubricated Ni(100) surfaces. 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 4 again, the adhesion force is indicated by the negative normal force when the two surfaces are separated. Adhesion coefficients are usually large between clean Ni(100) surfaces. Under sliding conditions of FN ≈ 40 mN, Vshear ) 20 µm/s, and T ≈ 300 K, the results obtained from 12 single-pass friction measurements using clean Ni(100) crystal surfaces indicate that the static friction coefficient is µs > 5, with an adhesion coefficient of µad ) 1.5 ( 0.7. 3.4. Friction between Lubricated Ni(100) Surfaces. Friction measurements were made between pairs of sulfided Ni(100) surfaces with 0-14 ML of ethanol adsorbed on each surface. The results of three sets of friction measurements with various coverages of ethanol will be reported here. It is important to note that all the friction measurements were made between the lubricated surfaces immediately following adsorption of ethanol at 120 K and the samples were held at this temperature throughout the friction measurements. Three randomly selected plots of friction measurements made between sulfided Ni(100) surfaces with 1.2 ML of adsorbed ethanol on each surface are presented in Figure 5. The frictional behavior between these surfaces lubricated with a total of 2.4

Ni(100)/Ni(100) Interfaces

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Figure 6. Three randomly selected plots of friction measurements made between sulfided Ni(100) crystal surfaces modified by 1.5 ML of adsorbed ethanol on each surface. The upper trace is a plot of the normal force (FN), while the lower trace is a plot of the shear force (FS). The sliding behavior can be generally characterized as stick-slip behavior with little or no adhesion measured. Sliding conditions: FN ≈ 40 mN, Vshear ) 20 µm/s, Tc ≈ 6-10 s, T ≈ 120 K, and θ ) 75°.

Figure 7. Three randomly selected plots of friction measurements made between sulfided Ni(100) crystal surfaces modified by 2.6 ML of adsorbed ethanol on each surface. The upper trace is a plot of the normal force (FN), while the lower trace is a plot of the shear force (FS). The sliding behavior can be characterized as either stick-slip (first two measurements) or slip (third measurement), with no adhesion measured during separation of the surfaces. Sliding conditions: FN ≈ 40 mN, Vshear ) 20 µm/s, Tc ≈ 6-10 s, T ≈ 120 K, and θ ) 75°.

ML of ethanol between them can be characterized as stickslip with little or no adhesion observed. The friction is clearly lower than that observed between clean Ni(100) surfaces. For a set of 12 single-pass friction measurements, the average static friction coefficient was µs ) 3.0 ( 0.4. The results seen in Figure 5 are similar to the results of friction measurements made between sulfided Ni(100) surfaces, with a total of either 2 ML of ethanol (µs ) 3.3 ( 1) or 2.8 ML (µs ) 3.2 ( 0.9) present at the interface. With a slight increase in ethanol coverage to 1.5 ML on each surface, the static friction coefficient decreased significantly to µs ) 2.4 ( 0.5, as illustrated in Figure 6. The frictional behavior between lubricated surfaces modified by the presence of 1.5 ML of ethanol is also stick-slip, with little or no adhesion observed upon separation of surfaces. The stick-slip behavior during friction measurements with a coverage of 1.5 ML of ethanol on each surface (Figure 6) is not as erratic as that observed during friction measurements with ethanol coverages of 1.2 ML (Figure 5). The static friction coefficients measured between sulfided Ni(100) surfaces modified by adsorption of 1.7 ML (µs ) 2.1 ( 0.7), 2.0 ML (µs ) 2.4 ( 0.8), and 2.4 ML of ethanol (µs ) 2.1 ( 0.7) on each surface were similar to the friction plots presented in Figure 6. In this coverage range of 1.7-2.3 ML, the friction coefficient does not change appreciably. A drop in the static friction coefficient occurred when the coverage of ethanol on the two sulfided Ni(100) surfaces was raised to > 5 ML. Figure 7 presents typical results of friction measurements for this lubrication regime where a transition from stick-slip to slip behavior can be seen. At this coverage, the static friction coefficient is lowered to µs ) 1.2 ( 0.4. The frictional behavior between sulfided Ni(100) surfaces modified by the presence of 2.6 ML of ethanol can generally be characterized as stick-slip, with little or no adhesion observed upon separation of surfaces. However, more cases of friction measurements exhibiting slip behavior, as seen in the third trace of Figure 7, were observed in this coverage regime than at lower coverages. Similarly, static friction coefficients measured between sulfided Ni(100) surfaces modified by adsorption of 2.4 ML (µs ) 1.1 ( 0.5), 2.5 ML (µs ) 1.25 ( 0.4), and 3.8

ML (µs ) 1.0 ( 0.3) of ethanol were not significantly different from the friction measurements made between sulfided Ni(100) surfaces with ethanol coverages of 2.6 ML. The primary experimental objective of this work has been to study friction as a function of ethanol coverage on two identical and well-defined surfaces. The static friction coefficients are presented in Figure 8 as a function of ethanol coverage. When ethanol is confined between the sulfided Ni(100) surfaces, the friction coefficient between them appears to be a discontinuous function of ethanol coverage. The error bars associated with each set of friction measurements have been removed from Figure 8 for the purpose of clarity. In general, the standard deviation is approximately 20-35% of the average static friction coefficient measured in each set of friction measurements. The higher deviations occur for the surfaces with lower ethanol coverages and thus higher friction. Shearing in the submonolayer ethanol coverage regime is characterized by very high static friction coefficients (µs > 5) with high adhesion observed upon separation of the surfaces. The arrows above the low coverage data points in Figure 8 indicate that these are lower limits on the static friction coefficient. Submonolayer coverages of either atomic sulfur or molecular ethanol adsorbed on the Ni(100) surfaces are not sufficient to lubricate the interface between them. At least one monolayer (ML) of ethanol on each surface is needed to provide lubrication. The friction coefficient is significantly reduced at an ethanol coverage of 1 ML (µs ) 3.1 ( 1) on each surface and decreases in a stepwise fashion until coverages greater than 2.5 ML of ethanol are adsorbed on each sulfided surface. With a total ethanol coverage > 18 ML at the interface, the frictional behavior can be characterized as slip behavior with the static friction coefficient lowered to µs ) 0.3 ( 0.08. This reduction in friction between surfaces with such adsorbate coverages is consistent with observations made in prior work.21,26 At coverages of ethanol between 1 ML and ∼2.5 ML on each sulfided Ni(100) surface, a regime is observed where the effects attributable to molecular layering are evident. The peaks in the TPD spectra suggest that distinct layers form in this coverage range with discrete values of the desorption energy. The friction

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Figure 8. Static friction coefficient plotted as a function of ethanol coverage between two clean Ni(100) surfaces and two c(2 × 2)-S/Ni(100) surfaces with ethanol coverages in the range of 0-5 ML. The static friction coefficient was found to be a discontinuous function of ethanol coverage. In the submonolayer regime, the frictional behavior is characterized by very high friction coefficients (µs > 5.5), with high adhesion observed upon separation of surfaces. The arrows in these cases indicate that the data point is a lower limit on the friction coefficient. Submonolayer coverages of either atomic sulfur or molecular ethanol on each surface do not lubricate the Ni(100) interface. At least 1 ML of ethanol on each surface is needed to provide lubrication. The error bars associated with each set of friction measurements have been removed from the plot for the purpose of clarity. In general, the standard deviation is approximately 20-35% of the average static friction coefficient from each set of friction measurements.

coefficient decreases discontinuously with ethanol coverage, and its coverage dependence is correlated with the coverage dependence of the ethanol desorption energy. It is important to note that throughout the course of these friction measurements, care has been taken to vary the ethanol coverage randomly rather than monotonically. TPD was performed immediately following each set of friction measurements to ensure that the desired coverage of ethanol had been achieved. In other words, the static friction coefficients reported in this paper and seen in Figure 8 are not the result of any systematic change in the surfaces during the course of the experiments. 3.5. Adhesion versus Ethanol Coverage. The work of Bowden and Tabor concluded that adhesion between surfaces was an important factor in dictating friction.28 Adhesion forces are often observed between clean metallic surfaces in contact with one another.22,28-31 The contacting surface asperities form interatomic bonds and cold-weld. Interatomic forces across the junction lead to macroscopic adhesion forces that are observed during separation of the surfaces. The same interatomic forces lead to friction during shearing. As a result, very high friction coefficients are usually measured between clean metallic surfaces.22,25,28,30,31 The presence of materials at the interface reduces the formation of intermetallic junctions and thus reduces friction and lubricates the interface. Previous work with Cu(111) surfaces in a vacuum has shown that the friction between pairs of surfaces is reduced dramatically once the coverages of adsorbed molecules on both surfaces reach 1 ML.29,36 In general, most materials have lower friction coefficients in air than in a

Ko and Gellman

Figure 9. Adhesion coefficient plotted as a function of ethanol coverage between two clean Ni(100) surfaces and two c(2 × 2)-S/Ni(100) surfaces with ethanol coverages in the range of 0-5 ML. The highest adhesion coefficient, µad ) 1.5 ( 0.7, was observed between the two clean Ni(100) surfaces. After adsorbing 0.5 ML of atomic sulfur, the adhesion coefficient was reduced to µad ) 0.65 ( 0.4. The adhesion coefficient remained constant at µad ∼ 0.7 with adsorption of submonolayer coverages of ethanol on each surface. Submonolayer coverages of adsorbed ethanol do not prevent metal-metal contacts. At least 1 ML of ethanol on each surface is needed to prevent strong intermetallic bonding. The adhesion coefficient is significantly reduced at an ethanol coverage of ∼1 ML on each surface (µad ) 0.27 ( 0.2) and gradually decreases to µad ) 0 at coverages greater than 2.5 ML.

vacuum as a result of the presence of adsorbed molecules and airborne contaminants on their surfaces.25,26,28-35 What adsorbate coverage is needed to prevent intermetallic bonding and lower adhesion? If adhesion occurs, does it influence friction? Figure 9 summarizes the measurements of adhesion coefficients observed after shearing of the c(2 × 2)S/Ni(100) surfaces, each modified by adsorbed ethanol coverages in the range 0-5 ML. The highest adhesion coefficient, µad ) 1.5 ( 0.7, was observed between the two clean Ni(100) surfaces. After adsorbing 0.5 ML of atomic sulfur, the adhesion coefficient was reduced to µad ) 0.65 ( 0.4. For these two surfaces, the friction coefficients were too high to be measured in this study. However, in previous studies of atomic adsorbates on Cu(111) surfaces, we have observed very little difference in the friction between clean surfaces and surfaces covered with atomic adsorbates.37 The adhesion coefficient remained constant at µad ≈ 0.7 for ethanol coverages in the range of 0-1 ML on each surface. Submonolayer coverages of molecular ethanol do not prevent metal-metal contacts. As the ethanol coverage on each surface is increased past 1 ML, the adhesion coefficient appears to drop to (µad ) 0.27 ( 0.2). The adhesion coefficient then drops to zero for ethanol coverages greater than 2.5 ML on each surface. It is important to note that the deviations in the adhesion coefficients are much greater than those in the friction measurements and that as a result it is much more difficult to observe structure in their coverage dependence. Although the discontinuities in the adhesion coefficients are not as pronounced as the discontinuities in the friction coefficient, they are certainly dependent on the ethanol coverage and appear

Ni(100)/Ni(100) Interfaces

J. Phys. Chem. B, Vol. 105, No. 22, 2001 5193

to display some plateau in the coverage range between 1 and 2.5 ML of ethanol on each surface. 4. Discussion 4.1. Adsorbate Coverage Dependence of Friction. Several previous papers have reported the friction coefficient between Cu(111) surfaces modified by the presence of adsorbed atoms and molecules.27,29,37 In general, it has been found that the friction coefficients between the clean surfaces are very high (µs > 5) and that the adsorption of atomic species such as Cl, S, and I does little to reduce friction. The maximum coverages of the atomic species were determined by various factors related to the mechanisms by which they were deposited on the surface and were limited to coverages of 1ML. The role of the work reported in this paper is to examine the dependence of the friction coefficients between two single crystal surfaces on the coverage of adsorbed molecules in the range >1 ML. It is in this range that the friction between the two surfaces is easily measurable using our apparatus and is dropping toward the value for the lubricated surfaces. In a previous paper studying the friction between Cu(111) surfaces modified by varying coverages of trifluoroethanol, the friction coefficient dropped markedly after completion of the monolayer on each surface and then appeared to plateau and only drop again once 2 ML of trifluoroethanol were adsorbed on each surface.37 This paper explores such coverage dependence in greater detail. To maximize the chances of observing some systematic changes in the friction as the adsorbate coverage increases, we have chosen to use ethanol on the c(2 × 2)-S/ Ni(100) surface. This is a system in which there are observable changes in the properties of the adsorbed layers as the coverage increases between 1ML and the condensed multilayer. In a previous study of this system, we observed distinct structure to the ethanol desorption spectra as the coverage was increased,

Figure 10. Comparison of the friction coefficients and desorption energies between two c(2 × 2)-S/Ni(100) surfaces modified by ethanol coverages in the range 0-5 ML. Both the friction and the desorption energy drop monotonically with increasing ethanol coverage. It appears that there are discontinuities in both quantities and that the coverages at which these discontinuities are observed are correlated to one another. They occur at coverages of 1., 1.5, and 2.5 ML of ethanol on each surface. In the case of the friction measurements this implies coverages of 2, 3, and 5 ML of ethanol at the interface between the two single crystals.

indicating that there are discrete changes in the properties of the ethanol layers as their coverage increases. The discontinuous changes in the properties of ethanol adsorbed on the c(2 × 2)S/Ni(100) surface were observed again by the careful measurements of ethanol desorption illustrated in Figure 1 and analyzed in terms of the coverage dependence of the desorption energy in Figure 2. The friction coefficient between the c(2 × 2)-S/Ni(100) surfaces clearly depends on the coverage of adsorbed ethanol. At submonolayer coverages, it is very high and then begins to decrease as the coverage is increased past 1 ML on each surface. What is more interesting is that the drop in friction appears to be a discontinuous function of coverage and that these discontinuities can be roughly correlated to the discontinuities in the coverage dependence of the desorption energy. This is illustrated in Figure 10, which plots both µs and ∆Edes against the same ethanol coverage scale. It should be noted that the scale is the coverage of ethanol on each of the two surfaces. Thus, for the desorption energy, the discontinuous drops appear to occur at coverages of 1, 1.5, and 2.5 ML on the surface. The drops in friction though occur at coverages of 2, 3, and 5 ML of ethanol at the interface. It is important to point out that the real correlation that has been observed in this work is that between µs and ∆Edes. Our scaling of this correlation with adsorbate coverage is subject to our definition of the monolayer coverage. In prior work, the monolayer coverage of ethanol on the surface was believed to be the coverage at which the desorption peak at 162 K was saturated.21 In this paper, we now suggest that in fact the coverage at this point is 2.5 ML, where the monolayer is defined as the coverage at which the desorption peak at 180 K becomes saturated. Two things suggest that the previous definition was

5194 J. Phys. Chem. B, Vol. 105, No. 22, 2001 incorrect. One is the comparison with the ethanol desorption spectra from the Cu(111) surface on which the monolayer desorption feature is much better defined and occurs at an ethanol exposure of 2.5 L. Saturation of the peak desorbing at 180 K on the c(2 × 2)-S/Ni(100) surface also occurs at a similar ethanol exposure. The second reason to believe that the coverage definition used in this work is correct is that the initial drop in friction is now observed at an ethanol coverage of 1 ML, which is consistent with all prior results on the Cu(111) surface.26,29 The discontinuities in the ∆Edes could easily be interpreted in terms of the formation of discrete layers with increasing coverage and thus decreases in the influence of the substrate on the ∆Edes. The only odd feature of this interpretation is that the discontinuities do not appear to occur at integer multiples of the coverage. It may be that the density in each layer is not identical or that there are some subtle features to the ordering or structure of these layers. If in fact, the interpretation of the ∆Edes in terms of layering is correct, then it suggests that the discontinuities in the friction coefficient are also reflections of layering of the adsorbed ethanol at the interface. Effects of adsorbate layering on friction would be consistent with previous observations using the surface forces apparatus to measure friction between mica surfaces. 4.2. Effects on Friction of Adsorbate Layering at Interfaces. One suggested reason for the discontinuities observed in the friction coefficients between the c(2 × 2)-S/Ni(100) surface as a function of ethanol coverage is that discrete layers of ethanol are formed in the confined region between the two surfaces forming the interface. The fact that confined liquids form layered structures has been demonstrated on numerous occasions using the surface forces apparatus (SFA) to confine liquids between the perfectly flat surfaces of mica.1-5,7,8,12 In those experiments, the layering is often observed as oscillations as a function of separation distance in the load forces needed to compress two mica surfaces together. The oscillations have periods equal to the molecular diameter of the confined liquid. This layering can also have an effect on the friction measured between two mica surfaces during shearing.2,3,14,15 The discrete friction coefficients that are observed between the c(2 × 2)-S/Ni(100) surfaces covered by ethanol are suggestive of the formation of discrete layers at the interface. If so, this is quite an interesting result because there are substantial differences between our experiment and that of the SFA. For one, the mica surfaces used in the SFA are as close to perfectly flat as can be realized experimentally. They have no atomic steps across the contact region, and the corrugation of the surfaces in the contact region is simply that of the atoms in the perfect basal plane of the mica structure. In our experiment, the surfaces of the Ni(100) surfaces are polished to the best of our ability, and although they give very good low-energy electron diffraction patterns, they are not perfectly flat. As with all such surfaces, they must have some roughness imparted by atomic height steps, and as a result, when they are brought into contact, it is really only at the peaks of asperities that the contact is formed. Although the Ni(100) contacts are not the perfectly flat confining volumes of the mica-mica interface, the presence of the thin films of ethanol is sufficient to reduce fiction and apparently to retain some of the structure and/or properties of ethanol adsorbed on the c(2 × 2)-S/Ni(100) surface. A second difference between the mica-mica interfaces and the Ni(100)-Ni(100) interface that is used in our experiments is the nature of the contact mechanics. In the SFA experiment, the mica deforms elastically as a result of the compliance of

Ko and Gellman the glue that is used to mount it. In the case of the Ni(100) surfaces, the contact is plastic in nature, and the surfaces at the contact regions deform irreversibly. After performing numerous measurements on the samples, we are able to remove them from the vacuum system and observe wear tracks of the order of 2-5 µm wide on the surfaces. The details of how these are formed or the dependence of their formation on ethanol coverage or sliding conditions cannot be determined. The fact that ethanol remains at the interface between the Ni(100) surfaces although they undergo plastic deformation is quite remarkable. The fact that it apparently retains some of the properties of the ethanol at the surface initially and may even remain in a layered structure is even more remarkable. One possible mechanism is that the bulk deformation of the Ni in the contact regions occurs by a well-defined mechanism that can retain the crystallinity of the surface region. Bulk deformation of metals under stress occurs by motion over well-defined slip planes. In the case of Ni, which is an fcc metal, these would be the (111) slip planes. If, during contact, asperities covered with ethanol are deformed during compression by motion of material slipping along these planes, then the contacting interface will be flatter than the original surface and may well retain the single crystalline character of the initial surfaces. As such, ethanol at the surfaces could feel itself confined essentially between two flat c(2 × 2)-S/Ni(100) surfaces and thus could retain the layered structure and properties that it has on the surface in a vacuum. 4.3. Desorption Energy Effects on Friction. The causeeffect relationship behind the correlation of friction coefficients and desorption energies that we have observed in this work is extremely difficult to determine. The discussion of the possible role of adsorbate layering in the previous section suggests that the relationship is based on changes in the structural characteristics of the film as the ethanol coverage increases. This is implicit in many of the previous observations based on the use of mica-mica interfaces. In those experiments one measures mica-mica separation or film thickness directly and observes the formation of discrete layers with thicknesses that are integer multiples of the molecular dimensions of the intervening fluid. In our experiments, the measured property of the ethanol films is their desorption energies, which take on discrete values as a function of coverage that are suggestive of the formation of discrete layers. The observation of discontinuities in the friction between the surfaces as a function of the ethanol coverage could be a reflection of the fact that film properties other than structure are changing discontinuously as a function of coverage. Just as the friction between surfaces separated by bulk fluids is dependent on the bulk fluid viscosity, one could imagine that the viscous properties of the ultrathin films used in this work might vary with thickness. The fact that the desorption energy varies discontinuously with thickness indicates that the interaction of the ethanol with the surface is varying discontinuously. One could easily imagine that film properties such as mobility or viscosity could be related to the strength of interaction of the molecules with the surface and that these therefore might vary discontinuously with coverage and yield the observed effects on friction. 5. Conclusions The friction between pairs of Ni(100) surface modified by the presence of c(2 × 2) layers of sulfur and varying coverages of ethanol decreases monotonically but discontinuously with ethanol coverage. The coverages at which the discontinuities in the friction occur are also coverages at which the ethanol

Ni(100)/Ni(100) Interfaces desorption energy changes in a discontinuous fashion. The correlation between friction and the discontinuities in the desorption energy is suggestive of the influence of ethanol layering at the interface between the two sulfided Ni(100) surfaces and is very similar to effects of adsorbate layer on friction between mica surfaces measured with the surface forces apparatus. Acknowledgment. This work was supported by the Air Force Office of Scientific Research under Grant No. F4962098-100218. References and Notes (1) Granick, S. Science 1991, 253, 1374. (2) Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N. J. Chem. Phys. 1990, 93, 1895. (3) Israellachvili, J. N.; Berman, A. D. In Handbook of Micro/ Nanotribology; Bhushan, B., Ed.; CRC Press: New York, 1999. (4) Hu, H.-W.; Granick, S. Science 1992, 258, 1339. (5) Christenson, H. K. J. Phys. Chem. 1986, 90, 4. (6) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: London, 1985. (7) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (8) Yoshizawa, H.; Chen, Y. L.; Israelachivili, J. N. J. Phys. Chem. 1993, 97, 4128. (9) Persson, B. N. J.; Tosatti, E. Phys. ReV. B 1994, 50 (8), 5590. (10) Gao, J.; Luedtke, W. D.; Landman, U. In Physics of Sliding; Persson, B. N. J., Tosatti, E., Eds.; Kluwer: Dordrecht, The Netherlands, 1996. (11) Schoen, M.; Diestler, D. J.; Cushman, J. J. Chem. Phys. 1987, 87, 5464. (12) Chan, D. Y. C.; Horn, R. G. J. Chem Phys. 1985, 83, 5311. (13) Bitsanis, I.; Magda, J. J.; Tirrell, M.; Davis, H. T. J. J. Chem. Phys. 1987, 87, 733.

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