2696
J. Phys. Chem. C 2007, 111, 2696-2701
Friction of Octadecyltrichlorosilane Monolayer Self-Assembled on Silicon Wafer in 0% Relative Humidity Om P. Khatri and Sanjay K. Biswas* Department of Mechanical Engineering, Indian Institute of Science, Bangalore, 560012, India ReceiVed: NoVember 1, 2006
Self-assembled octadecyltrichlorosilane monolayers on silicon wafer were slid against a smooth steel ball at a 350 to 750 MPa contact pressure and in 0.02 to 1 cm/s velocity ranges in a nominal 0% relative humidity environment. The friction was found to increase monotonically over sliding time stretching to 20 h. Within a framework that the friction is related to the generation of strains and defects at the molecular scale we explore the dissipation process in some detail by varying the normal load, velocity, and a priori defect population in the monolayer. The results indicate that the rate of friction change is significantly influenced by the relative magnitude of the time allowed to the molecules to relax in intermittent contact and the characteristic relaxation time that reflects the collective response of a large number of molecules in contact at a given time. We propose a simple model to account for the observations, where the friction at the commencement of sliding is governed by backbone stiffness. This process for all practical purposes is independent of sliding time. The kinetic friction on the other hand is related to time-dependent defect generation and accumulation at the terminal end and is found to consist of reversible and irreversible components. The latter provides the monolayer with a memory of its previous loading history.
Introduction Organic molecules of lengths often limited to within 2-3 nm have been found to be remarkably protective, when selfassembled on solid substrates. These molecules organize themselves into crystalline or near crystalline order, with a terminal end that is often hydrophobic. They transmit power efficiently as they generally exhibit moderate frictional dissipation in supporting large loads.1,2 They thus find extensive use as additives in boundary lubrication,3-5 microelectromechanical systems (MEMS),6,7 corrosion inhibitors,8 hard disks,9 optical devices,10 and biosensors.11,12 The frictional dissipation mechanisms of these self-assembled monolayers (SAMs) have come under much scrutiny. Deformation at the terminal end,13 generation of gauche defects at the terminal end,13-15 percolation of such defects to the backbone,14,16 discrete and collective tilt,17,18 stretching of bonds,19,20 and steric interactions21 are the energy dissipation mechanisms suggested on the basis of molecular dynamics simulation studies and experimental works principally using the atomic force microscope (AFM) and sumfrequency generation (SFG) spectroscopy. Frictional studies using surface force apparatus have generally tended to use viscoelastic concepts to account for dissipation.22,23 Studies of relevant structural and chemical parameters such as a priori monolayer defects,24 hydrophobicity of terminal groups,25 chain length,24,26,27 size of the terminal groups,28 and external parameters such as normal load17,19,29 and humidity30,31 have contributed to an understanding of the dissipation mechanisms. Our interest in the area stems from a desire to understand the boundary lubrication mechanism of additive molecules which should enable the selection/design of molecules for efficient lubrication in practical engineering systems. In these applications time scales and contact areas are very large * Address correspondence to this author. E-mail: skbis@mecheng. iisc.ernet.in. Phone: +91-80-2293-2512, 2351. Fax: +91-80-2360-0648.
compared to those undertaken in the studies quoted above, and the substrates are generally rough and the environment is humid. In a previous paper24 we have reported tribological studies in unidirectional sliding of alkylsilane monolayers self-assembled on aluminum and silicon surfaces, the former with a view to the use of this test molecule as an additive, in lubrication of aluminum based internal combustion engines. When the substrate used was aluminum we attributed the sharp rise in coefficient of friction24 with sliding time to a progressive reduction of monolayer packing density brought about by microplasticity of the substrate. In the same paper we reported for comparison the frictional data of the same monolayer but assembled on a much harder silicon substrate. Topographical imaging of the silicon substrate after the experiment revealed the absence of any microplastic grooving. We found the frictional coefficient in the latter case to still rise with sliding time, but at a rate much reduced from what was observed when the substrate was aluminum. Mikulski and Harrison19,20 have used molecular dynamic simulation to model sliding friction between a flat hydrogen terminated diamond counterface and C18 alkane monolayer in a 6-60 GPa pressure range at a sliding velocity of 100 m/s. They observe that frictional stress develops in consonance with the stretching of bonds in the molecules over the entire backbone. Salmeron and co-workers1,13 on the basis of their in situ SFG (Sum Frequency Generation) spectroscopy of SAMs slid against a counterface under moderate pressure attribute friction to the work done to generate the gauche defect at the terminal end. Both groups observe complete relaxation of the molecules after they have been slid by the rigid probe. Mikulski and Harrison’s20 work shows a relaxation time of the order of 10-12 s. The relaxation times of OMCTS probed in a SFA,32 hexadecane thiol probed in an AFM,33 and a 1H,1H,2H,2Hperfluorooctyltrichlorosilane monolayer34,35 probed in a contact force apparatus were found to be 101-103, 8 × 10-2, and 10-1-
10.1021/jp067206s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007
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J. Phys. Chem. C, Vol. 111, No. 6, 2007 2697
TABLE 1: Relaxation Time τs and Time Allowed for Relaxation tr as a Function of Experimental Load and Velocitya velocity ) 0.02 cm/s
velocity ) 1.05 cm/s
load, N V
D, 10-6 m
pm, MPa
tr, s
τs, s
(D/l)[1 - (tr/τs)]
D, 10-6 m
pm, MPa
tr, s
τs, s
(D/1)[1 - (tr/τs)]
0.1 0.8
18.7 37.5
363 726
0.9 0.8
7.91 15.97
0.082 0.17
18.7 37.5
363 726
0.018 0.016
7.91 15.97
0.093 0.1875
a τs is measured using a contact force apparatus as a function of mean contact pressure pm. pm and D are estimated from a contact mechanical analysis of bilayer systems.38
102 s, respectively. Jeffrey et al.36 have determined the relaxation times of water molecules to be in the 10-3-10-4 s range. It has also been shown34,36 that there is a liquid to solid-type phase transition when a large number of molecules (≈105)35 are compressed by a probe, and at contact pressures above that which mark this transition, there is a large increase in the relaxation time in response to a small increase in applied pressure. In any practical use of organic self-assembled monolayers as molecular lubricant the monolayers at any time are compressed under micron (10-6 m) order contact dimensions in a 100 MPa- 1 GPa contact pressure range. In such situations the monolayers are likely to respond to tribological traction with relaxation times which are many orders greater than those (10-12 s) which happen at a single molecular level. In tribology, relaxation time is an important parameter as contact is intermittent, time elapses between any two successive contacts of a point on a surface by the counterface. If this time is significantly less than the relaxation time, at the time of the “next” contact partially stretched or partially disordered molecules are encountered. We explore the impact of this on the history of friction in sliding. This is done by varying the normal load, which alters the relaxation time and speed which with normal load influences the intermittency of contact. In this work we perform sliding tests on octadecyltrichlorosilane (ODTS) monolayer self-assembled on silicon substrate, but in accordance with our interest in internal combustion engines we conduct these tests in the reciprocating mode. We use a smooth steel ball to slide against the ODTS monolayer in a mean contact pressure range of 350 to 750 MPa and velocity range of 0.02 to 1.0 cm/s. Experimental Section 1. Materials and Sample Preparation. p-type silicon (100) wafers with 0.2-0.3 nm rms roughness (over a 2 × 2 µm2 area) were used as substrates. Octadecyltrichlorosilane purchased from Sigma-Aldrich, USA, was used as received. Isooctane (99.5%, dry) obtained from Sd-fine Chem, India, was used as a solvent. Acetone, sulfuric acid, hydrogen peroxide (Sd-fine Chem, India), ethanol (Lee Alcohols, Ontario), and HPLC grade water (Merck, India) were used for silicon surface cleaning. Silicon wafers were cut into 1 × 1 cm2 squares and rinsed thoroughly with acetone and methanol, respectively, in an ultrasonic bath. After rinsing, silicon substrates were immersed in a mixture of H2SO4 and H2O2 (3:1 v/v) solution for 30 min and then washed four times with water and dried in a stream of dry nitrogen gas. Before monolayer deposition, silicon substrates were kept in a UV cleaning chamber (Bioforce Nanosciences, USA) for 30 min to burn off all carbonaceous contaminants. Silicon substrates were immersed in freshly prepared ODTS solutions (1 mM) for 1 h deposition time, then taken out, rinsed, and washed with isooctane (twice) to remove excess and physisorbed ODTS molecules. Finally samples were annealed for 2 h at room temperature in vacuum prior to tribological experiments.
2. Nanotribometer. Friction measurements were carried out with a nanotribometer (CSM Instruments, Switzerland), consisting of X- and Y-axis stepper motors linked to a reciprocating module and a Z-axis stepper motor linked to the measuring head. A removable cantilever is mounted on the measuring head. The cantilever (normal force constant: 0.1493 ( 0.0001 mN/µm; tangential force constant: 0.0585 ( 0.0002 mN/µm) associated with two optical sensors (perpendicular to each other in the Xand Z-directions) is used for measuring the normal and the lateral force deflections. The adjustment of range and the position of optical sensors in relation to the mirrors are carried out with a combination of mechanical and software (TriboX 2.9, CSM Instruments, Switzerland) tuning. In the Z-direction a piezo is used to adjust the applied normal force. The coefficient of friction is determined during sliding by measurement of the deflection in the elastic arm of the cantilever in both horizontal and vertical planes by two-high precision displacement sensors, using the optical signals reflected from mirrors of X and Z force sensors. A 2 mm diameter chrome steel ball (Cr 1.3-1.6%, C 1%, Si 0.15-0.3%, Mn 0.25-0.45%, Fe rest) of 1.4 ( 0.2 nm rms roughness (over 2 × 2 µm2 area) is attached to the end of the cantilever. Before attachment, the steel ball was sonicated with acetone to remove dust particles. All measurements were carried out in a nominal 0% relative humidity environment maintained in the nanotribometer enclosure. Prior to the experiment the sealed perpex chamber enclosing the nanotribometer was flushed with ultrapure dry ( tr. The experimental work of Salmeron and co-workers40-42 shows that when an AFM tip slides over an organic monolayer gauche defects are generated at the terminal end of a molecule. When the molecule is released by the tip the defect relaxes back releasing frictional energy. Harrison and co-workers19,20 using
Figure 1. Coefficient of friction of ODTS monolayer self-assembled on silicon: load 100 mN, sliding velocity 0.02 cm/s, and 0% relative humidity. The monolayer is unloaded after different sliding times and rested for 10 min before being loaded again and the test recommenced. The figure shows the existence of a static friction obtained immediately on recommencement and a kinetic friction that changes perceptibly with sliding time. It is to be noted that friction-time slope (θ) at interruption and recommencement of a test after prolonged stoppage (unloaded) are the same.
molecular dynamics show that in a single pass when a probe drags a molecule in the sliding direction bonds stretch to increase the friction stress. When the molecule is released, it relaxes back to its original state. This is also supported by the sum frequency generation spectroscopy1,13 of the mechanically compressed monolayer, which on unloading shows recovery but also some irreversible changes. In general most previous works based on very short sliding distance experiments and simulation suggest complete recovery of the original molecular structure, on unloading. The complete recovery suggests that a characteristic relaxation time τs e tr. In all these reported cases the friction rises to a peak between two contacts and falls back to the initial level before the next contact is made. The peak friction thus remains invariant over the complete duration of the sliding experiment. In the present experiments a large number of molecules are in contact with the probe at any given time. As τs > tr, we may expect that the molecules do not have sufficient time to relax back before they come next into contact with the probe. They are thus likely to be able to relax the imposed strain only partially before the next contact. Thus the probe at the next contact meets residually strained molecules, and with repeated contact the residual strain level can be expected to increase. If the state of strain by bond stretching or gauche defect generation is directly related to the friction force as has been suggested, the friction force, as strains accumulate with repeated contact, can be expected to rise with time, as long as τs > tr. Figure 1, shows the change in coefficient of friction with sliding time, measured in 0% RH environment at 0.02 cm/s velocity and 363 MPa mean contact pressure. Table 1 shows that under these experimental conditions the value of τs is significantly greater than tr. The friction is seen to increase with time. At the commencement of sliding, the friction force jumps to a certain value, which we designate static friction, almost instantaneously. Static friction force needs to be reached before a kinetic friction regime is initiated where the friction increases gradually with time. The rate of rise of friction decreases with time until at large time the friction becomes insensitive to sliding time. If the kinetic friction is assumed to be related to defect generation in sliding, it has been shown43 that as defects
Self-Assembled Octadecyltrichlorosilane Monolayers
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2699
Figure 2. Vibrational spectra of ODTS SAM on silicon showing the (heat treatment) temperature-dependent shifts of methylene antisymmetric and symmetric stretch mode.
accumulate it becomes more difficult to generate more defects. When the monolayer becomes saturated with defects it is not possible to generate any further defect by sliding. When a test is interrupted, unloaded, stopped for a prolonged period of time, and recommenced (loaded) the level of static friction of an unslid (t ) 0) monolayer is recovered initially, and with increasing numbers of interruptions the recommenced static friction decreases slightly. The gradient of friction with time at recommencement, however, remains more or less the same as that at interruption. This is an important finding as it implies that it is possible to maintain a low level of friction with frequent interruptions. We do two further experiments to establish that (1) defect population directly influences friction and (2) the relative magnitude of relaxation time (τs) and the time allowed (tr) in an experiment to the molecules to relax influences friction. (1) A monolayer is assembled on the substrate and heat treated. Heat treatment increases the defect population of the monolayer. Here, we report the friction characteristics with time of a heat-treated monolayer. The conformational order of the ODTS monolayer self-assembled on silicon was characterized by infrared spectroscopy. We have taken the vibrational spectra of the ODTS monolayer, freshly prepared and after heat treatment to 130 °C, at room temperature (22 °C). Figure 2 shows the methylene antisymmetric and symmetric stretches (d- and d+, respectively) of the ODTS monolayer. The peak frequencies of d- and d+ modes are used44,45 to comment upon the conformational order of the monolayer. A freshly prepared ODTS monolayer is observed as highly ordered with all-trans conformation. ODTS heat treated to 130 °C becomes disordered, as shown by the d-/d+ peak shift toward higher frequency as well as an increase in peak intensity (Table 2). We have reported earlier46 that at 130 °C most of the defects are limited to the near surface part of the monolayer. The bandwidth (fwhm) of
Figure 3. Coefficient of friction of the ODTS monolayer. The figure shows the effect of heat treating the monolayer a priori to 130 °C peak temperature, before the commencement of sliding. Normal load: 100 mN. Sliding velocity: 0.02 cm/s.
the methylene stretch mode is an important parameter, which directly correlates with the packing density of the alkyl chain in the monolayer.47,48 We have observed low values (13-15 cm-1) of fwhm (∆ν1/2) for the d- mode of a freshly prepared ODTS monolayer, which indicates a crystalline densely packed order of molecules. With heat treatment to a peak temperature of 130 °C, the peak becomes wide with a high value of fwhm as shown in Table 2, which indicates untightening of the closely packed alkyl chain, allowing rotational and transitional motion. Figure 3 shows that compared with the response of the freshly prepared SAM, the friction of the heat-treated SAM jumps to a very high value at the commencement of the test and maintains a value higher than that observed for the more ordered SAM, over a long sliding time. (2) Changing normal load changes the contact diameter “D” and relaxation time τs. For a fixed stroke length “l” by changing normal load and velocity it is possible to change (tr/τs). If the rate of rise in friction df/dt is proportional to the rate of rise in defect population dg/dt it can be shown, for the case when the defects relax linearly with time, that the rate of rise in friction in a cycle of contact is the following:
(
) ( )
tr l-D D df dg D ) 1∝ ∝ 1dt dt l τsV l τs
We estimate the rhs of the above equation and show in Table 1 that this value (proportional to df/dt) increases with load and velocity. Figure 4 shows the enhancement in df/dt obtained experimentally by increasing load and velocity. The corroboration of trends given in Table 1 by experimental result indicates a strong influence of tr/τs on the rate of rise in friction with time.
TABLE 2: Peak Frequency, Peak Intensity, and fwhm of Methylene Stretch, from Vibrational Spectra of ODTS Monolayer Self-Assembled on Silicon peak frequency, cm-1
peak intensity, au
ODTS monolayer
d- stretch
d+ stretch
d- stretch
d+ stretch
fwhm, cm-1 d- stretch
freshly prepared heat-treated (130 °C)
2917.5 ( 0.5 2922.7 ( 0.5
2850.4 ( 0.3 2852.2 ( 0.3
0.0660 0.0685
0.0256 0.0284
14.7 ( 0.3 21.2 ( 0.7
2700 J. Phys. Chem. C, Vol. 111, No. 6, 2007
Figure 4. Coefficient of friction of the ODTS monolayer obtained under different loads and velocities. The figure shows the aggregate rise in friction with time in the very early stages (
