Frictional Properties of Self-Assembled Alkylsilane Chains on Silica
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Frictional Properties of Self-Assembled Alkylsilane Chains on Silica M. CHANDROSS, B. PARK, M. STEVENS, AND G.S. GREST Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT We present the results of molecular dynamics simulations of pairs of alkylsilane monolayers on silica surfaces under shear. In particular, we investigate the effects of shear velocity on the friction for chains of 6, 8, 12, and 18 carbon atoms covalently bonded to a crystalline surface. Our studies are performed at loads close to 0.2 and 2 GPa for relative velocities of 0.2, 2.0, and 20.0 m/s. We find that for perfect (defect-free) monolayers, the effects of chain length and velocity are weak, indicating that the experimentally measured dependence of friction on these properties is primarily due to defects in the monolayer. We have investigated possible finite size effects by varying our system dimensions from 43 ˚ A × 50 ˚ A to 174 ˚ A × 201 ˚ A. We find that√increasing the surface area by a factor of N reduces the noise in the shear stress by √ a factor of N , and has a comparable effect to averaging the smaller system data over bins of N points. This indicates that finite size effects are negligible in our simulations. INTRODUCTION
The possibility of using self-assembled monolayers (SAMs) to alter and control the chemical nature of surfaces has led to their widespread use in industry. Alkylsilane SAMs, in particular, are able to bond to oxide surfaces and are currently used as adhesion modifying coatings and lubricants for SiO2 based microelectromechanical systems (MEMS) [1]. While the utility of SAMs in such applications is clear from empirical evidence, the microscopic mechanisms of the frictional properties are far from understood. The adhesion and frictional forces at SAM coated surfaces can be measured by surface probes such as the atomic force microscope (AFM) [2], but these techniques only yield atomic scale force data. Without the accompanying structural information it is impossible to answer fundamental questions regarding the molecular mechanisms of the frictional properties of SAMs. Molecular dynamics (MD) simulations are able to produce both the atomic scale force data as well as the underlying structural information, and are thus ideal for probing the properties of SAM coated surfaces. We present here the results of MD simulations of the adhesion and lubrication of SAM coated SiO2 surfaces. In particular, we study the effects of chain length, system size, and shear velocity between pairs of fully packed, well-ordered SAMs covalently bonded to crystalline surfaces. Future work will deal with the effects of defects in the monolayers, as well as the comparison between alkylsilane chains on silica and alkanethiols on gold. MODEL AND METHOD
To simulate the SAMs, a pair of monolayers on tridymite surfaces were constructed as shown in Fig. 1. Alkylsilane chains with backbones containing n = 6, 8, 12 and 18 carbons were constructed in all trans configurations, and covalently bonded to the O atoms at the upward pointing tetraheda of the tridy
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