Molecular dynamics study of sliding friction of ordered organic monolayers

Abstract

Molecular dynamics is used to model the friction between two ordered monolayers of alkane chains six units long bound at their ends to two rigid substrates. Depending on the interfacial interaction strength, energy dissipation occurs by a discontinuous "plucking" mechanism and a continuous "viscous" mechanism. The plucking mechanism follows a simple thermal activation model, while for the viscous mechanism, friction force is enhanced at the rotator transition temperature of the films.PACS numbers: 68.35.Gy, 46.30.Pa, 68.35.Md, 81.40.Pq Interfacial friction and wear between two sliding surfaces can be minimized by binding to each surface films which interact with each other only weakly. We have used molecular dynamics (MD) to study this boundary lubrication process between films of close-packed hydrocarbon chains six units long. Two types of energy dissipation are found: a "plucking" mechanism associated with sudden release of shear strain, and a more continuous "viscous" behavior arising from continuous collisions of atoms of opposite films. The influence of interaction strength, temperature, and velocity on these mechanisms is described. The surface force apparatus [1-4], the force microscope [5-9], and a new crystal oscillator technique [10] have recently been used to understand sliding friction at the atomic level. MD has been used to simulate friction of a variety of types of materials [11], and the dynamics of organized alkane chain monolayers systems have been modeled [12][13][14][15], but MD has not been applied previously to friction between organic monolayers.We model the films by a 6 x 6 array of alkane chains of six monomers at rigid bond lengths (Fig. 1), extended with periodic boundary conditions parallel to the substrates [16]. The CH3 and CH2 groups are treated as single spherical united atoms. For the eff'ective interaction potential between nearby atoms on the same chain, we sum the bond-bending potential of Ref. [17] with the torsion potential of Ref. [18]. For pairs of atoms on the same chain more distant than third nearest neighbors and for all interactions between chains, we use the LennardJones potential (7LJ('') =4e[(cj/r)*^ -(cr/r)^], where 20.43 A FIG. 1. Top and side view of the computational cell, showing the initial configuration and sliding direction. The JC axis is the sliding direction. (T = 3.965 A and r is the atom pair separation. The interaction strength s is eo^'lOA K [19], except for interactions between atoms across the interface, where it is designated s\. We vary s\ to simulate changing the end group and to elucidate frictional mechanisms. For smoothing, the nonbonded interactions are truncated at r =rc =2.25o-by multiplying ^LJC'*) by [1 -{r/rc)^] ^ for r < r^ and by 0 otherwise. For all atoms, the total potential includes a substrate interaction (5.3x10^ K)/z^^ -(2.1x10"* K)/z^ depending on the distance z (in A) of the atom from the substrate. An isotropic harmonic potential, with force constant 50 K/A^, pins one of the end atoms of each chain to a 2D array with spacin...

Predictions and Observations of Multiple Slip Modes in Atomic-Scale Friction

“…This result was presented previously [2,7]. However, here we demonstrate that it is an integral part of a more general solution (6) condition to observe multiple slips, since the dynamics and the damping will determine the observed slip multiplicity. Figure 4 shows a ''stick-slip opportunity map.''…”

“…Idealized models have been extensively used by researchers to study stick-slip friction behavior [2,[5][6][7]. It has long been established that the one-dimensional Tomlinson model for sliding of an elastically compliant system in a periodic potential [8] predicts an analytical condition for transition between smooth sliding and singleslip regimes [7].…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Friction during the plucking mechanism followed the simple stress-augmented thermal activation model proposed by Briscoe and Evans [225] from experimental friction data for Langmuir-Blodgett films. Friction during the viscous mechanism was generally much lower but increased at the rotator transition temperature of the monolayers [236] (T at which molecules become rotationally ordered in a herringbone-like pattern).…”

“…In 1993, Glosli et al [236] became the first to use NEMD to investigate the friction of OFM-like systems. They studied the friction between two close-packed monolayers (head group area 17 Å 2 ) bound at their ends to two rigid substrates (similar to OFM molecules absorbed on solid surfaces).…”

The characteristics of phosphorylated 3-aminopropyltriethoxysilane self-assembled nanometre thin films

“…[16]. By following the friction force calibration procedures developed by Glosli [17], voltages corresponding to friction forces can be converted to force units. A coefficient of friction is obtained from the slope of friction force data measured as a function of normal loads.…”