J. Phys. Chem. C 2007, 111, 16339-16344
16339
Frictional Response of a Silane Monolayer to Sliding in a Humid Environment Om P. Khatri,† Souvik Math,† Colin D. Bain,‡ and Sanjay K. Biswas*,† Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India, and Department of Chemistry, Durham UniVersity, South Road, Durham DH1 3LE, U.K. ReceiVed: May 20, 2007; In Final Form: July 14, 2007
The effect of humidity has been studied on the friction coefficient of a steel ball sliding in reciprocating mode on a self-assembled monolayer of octadecyltrichlorosilane on silicon. At all relative humidities (RH < 70%), the friction coefficient first increases with sliding time, as defects build up in the monolayer, and then subsequently decreases. The deliberate introduction of defects by heat treatment of the monolayer to 130 °C leads to a substantial increase in the friction coefficient in a dry environment but a decrease in a moist environment. It is proposed that the interaction of water adsorbed to the steel probe with the monolayer results in a softening of the monolayer and a decrease in the relaxation times of the processes responsible for energy dissipation in the monolayer in a dry environment. The friction coefficient of a monolayer in a moist environment can reach values of 0.02: lower than the value of 0.04 observed for a pristine monolayer.
Introduction Silane monolayers self-assembled on a solid surface are contenders for the lubrication of engines,1-3 microelectromechanical devices, and machines.4-6 These applications are principally in the ambient and have thus provoked a number of lateral force microscopy (LFM) studies,7-13 where the effect of humidity on friction has been recorded. The silane molecules of interest have a hydrophobic terminal group attached to a hydrocarbon or fluorocarbon backbone with the number of carbon atoms varying between 8 and 18. Compared to the friction of hydrophilic surfaces, such as silicon oxide on a Si (100) wafer, the coefficient of friction of silane monolayers against a Si3N4 AFM tip in a humid environment is found to be low.7,12,13 Some authors7 have found that the friction in the lower-humidity (2-70%) range, like adhesion,12 is insensitive to relative humidity (RH), leading to the conclusion that no capillary junctions are formed. At higher humidities (>70%), the friction drops with increasing humidity.13,11,7 One author7 suggests that a thin layer of water condenses on the hydrophobic SAM surface at these humidities. The condensation volume is not sufficient to form a capillary junction but may provide sufficient coverage to act as a lubricant. Doshi et al.14 have discussed the genesis of low friction at hydrophobic interfaces as due to the formation of nano bubbles and nano bridges and, in general, due to the lowering of water viscosity in a 1-nm gap between the SAM and the bulk water in which the density of water is depleted. Zhang et al.15 have also brought up the issue of dewetting of hydrophobic surfaces. Scherge et al.,9 however, have found evidence of solvated water at the hydrophobic interface in high vacuum, which led them to comment on the difficulty of the mechanical removal of water from solid surfaces in friction experiments. In a previous paper,16 we studied the variation in the friction with time of a self-assembled monolayer (SAM) of an alkyltrichlorosilane on an oxidized silicon wafer in a dry environment * Corresponding author. Phone no: + 0091 80 2293 2589. Fax: + 91 80 2360 0648. E-mail:
[email protected]. † Indian Institute of Science. ‡ Durham University.
(0% RH). We argued that in long-duration nanotribological experiments there is an accumulation of molecular defects with each cyclic contact between the probe and the sample. The rate of accumulation of the defects is dictated by the time between two successive contacts in relation to the relaxation time of defects within the monolayer. Furthermore, the defect population eventually becomes saturated after which time the friction remains constant. At low loads, the response is reversible: on unloading the original friction is recovered. In this paper, we address the influence of water vapor on friction in the same system. 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 (ODTS), purchased from Sigma-Aldrich, was used as received. Silicon samples were cleaned thoroughly using piranha solution. Silicon substrates were immersed in freshly prepared ODTS solution (1 mM) for 1 h deposition time. These samples were then annealed for 2 h at room temperature in vacuum just prior to tribological and FTIR experiments. Detailed procedures for the cleaning of silicon wafer and ODTS monolayer preparation are given in an earlier paper.16 2. Instrumentation. Friction measurements were carried out using 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 and tangential force constant: 0.0585 ( 0.0002 mN/µm) associated with two optical sensors (perpendicular to each other in the X and Z directions) is used for measuring the normal and lateral force deflection. In the Z direction, a piezo is used to adjust 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 the horizontal and vertical planes by two high-precision displacement sensors using the optical signals reflected from mirrors of X and Z force
10.1021/jp073892z CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007
16340 J. Phys. Chem. C, Vol. 111, No. 44, 2007 sensors. A 2-mm-diameter steel ball (EN 31) of 1.4 ( 0.2 nm rms roughness (over 2 × 2 µm2 area), is attached to the end of the cantilever. To obtain a 370 MPa Hertzian pressure, the normal load required (taking the elastic deflection of the slider and the substrate into account) in a nanotribometer is about 100 mN. This order of pressure is commonly found in metal-cutting and cold-rolling operations. In internal combustion engines, the order of pressure near the top dead center can reach about 100 MPa. Friction measurements were carried out in different relative humidities from 0% to 70%. Nominal 0% relative humidity environment was maintained in the nanotribometer enclosure by continuous purging with ultrapure nitrogen gas (
