Molecular dynamics simulations of friction between alkylsilane monolayers

Abstract

Alkylsilane monolayer films are used in the fabrication of microelectromechanical systems (MEMS) as anti-stiction coatings. Recently, these films are also being considered as anti-friction coating for MEMS parts that operate under conditions of friction, such as gears, motors, etc. Study of frictional behaviour of these films is of significant interest from the performance point of view. In this work we use the method of molecular dynamics (MD) to study the friction between two rigid silica substrates coated with alkylsilane monolayers. The friction coefficient, friction force and normal force on the films are obtained as a function of separation between the substrates, temperature of the films and velocity of the substrates. The results of simulations are compared with the thermal activation model of Briscoe and Evans (1982 Proc. R. Soc. Lond. A 380 389). The frictional behaviour of the films as a function of separation between the substrates follows the thermal activation model. The MD simulations in the present work show that the normal force (or pressure) on the films is dependent on the temperature and the substrate velocity. This is in contrast to the thermal activation model in which the pressure on the films is assumed to be independent of these variables. The simulation results are seen to be in agreement with a modified thermal activation model which takes into account the dependence of pressure on the temperature and substrate velocity.

Molecular Dynamics Simulation of Conformational Transition and Frictional Performance Modulation of Densely Packed Self-Assembled Monolayers Based on Electrostatic Stimulation

“…The scale of COF (0.1−0.7) in our simulation is comparable to reported computational results of COF on alkyl monolayers and other organic monolayers. 25,27,31,39,49 For shallow indentation and sliding of densely packed PEGterminated SAMs with either cylindrical or spherical indenter, COF increases with respect to the greater normal load. This is because as indentation load rises, i.e., greater indentation depth is achieved, more SAM chains are brought into contact with the indenter and deform during sliding.…”

Molecular dynamics simulation of atomic friction: A review and guide

“…MD-based temperature dependence studies have been performed on a variety of substrate materials including metals (Cu and Au), diamond, and alkysilane monolayers. 37,40,56,111,139 All previous MD studies have reported a decrease of friction with temperature.…”

Investigation of the Impact of Cross-Polymerization on the Structural and Frictional Properties of Alkylsilane Monolayers Using Molecular Simulation

“…Computational methods, including molecular dynamics (MD) simulations, have emerged as an important tool to probe the molecular-level behavior of nanotribological systems, and recent improvements in computational speed and modeling methods have made simulations that closely mimic experimental systems possible. For example, MD simulations have been applied to improve our understanding of alkylsilane monolayers on oxidized surfaces [14,16,23,24,51,52,53,54,55,56,57,58,59,60,61], providing insight into the effects of various monolayer properties on frictional behavior, including monomer structure (e.g., backbone [14,55] and terminal group [13,16]) and surface structure (e.g., roughness and density of surface-bound hydroxyl groups) [53,54,55,57,60]. However, the simulations of alkylsilane monolayers performed to date have examined systems without siloxane cross-linkages.…”