Computational Nanotribology: SAMs for MEMS
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Computational Nanotribology: SAMs for MEMS Rajiv J. Berry,1 Nicole L. Wintrich,1 Rishikesh K. Bharadwaj1 and Martin Schwartz1,2 Materials Directorate, Air Force Research Laboratory, Wright-Patterson AFB OH 45433. 2 Department of Chemistry, University of North Texas, Denton TX 76203. 1
ABSTRACT Self-assembled monolayers (SAMs) consisting of hydrocarbon chains attached to silica walls were evaluated computationally for their high temperature stability, life cycle and performance in microelectromechanical systems (MEMS). Ab initio calculations at sufficiently high level of theory were conducted on model compounds to predict the bond strengths holding the monolayer tethered to the MEMS device and relate them to its thermal stability. Non-equilibrium molecular dynamics (NEMD) simulations under sliding periodic boundary conditions were employed to compute the frictional force as a function of applied load. The NEMD trajectories were analyzed for the structure and chain dynamics of the SAMs and compared with NEMD and equilibrium MD results for the fluid. The significance of monolayer penetration depth, monolayer gauche fraction, wall thermostat characteristics and the size of the simulation box on the computed results were investigated. INTRODUCTION Recent advances in device miniaturization have led to numerous new aerospace applications of microelectromechanical systems (MEMS). However, the device life of MEMS with moving, rubbing or impacting components (such as relays, pumps, optical switches, shutters, scanners, etc.) is often limited due to friction and wear. Efforts directed towards designing stable reduced friction devices have led to the consideration of alkylsilane based SAMs which are chemisorbed onto the surfaces of MEMS [1, 2]. Experimental efforts to produce and characterize these modified surfaces are challenged by concerns of reproducibility due to ambient molecular level contamination (including humidity fluctuations), surface asperities, cross-linking and instrument sensitivity requirements. A goal of this effort is to investigate these surfaces computationally in an “unadulterated” form and, eventually, to study the impact of various contaminants/surfaceimperfections on an individual basis.
COMPUTATIONAL DETAILS NEMD simulations were conducted using the confined shear protocol [3] and the COMPASS materials force field [4, 5] from Molecular Simulations, Inc. Atoms in the simulation box were grouped into three subsets: BOTTOMWALL, MID (middle layer containing the SAM or fluid atoms) and TOPWALL. The wall subsets were constructed from an approx. 13Å thick slab of cristobalite of dimensions (x,y) = (28,28Å). An overall box height of z = 100Å was maintained to allow for an adequate layer of vacuum above the topwall in order to eliminate interbox interactions along the z-direction as illustrated in figure 1. Each exposed wall-surface silicon atom was end-capped with two hydroxyl groups. The MID subset contained C10 hydrocarbon chains with dimethyl silane termini (figure 2). The terminal groups were covalently
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