Frictional properties of self-assembled alkylsilane chains on silica

M. Chandross, B. Park, M. Stevens, G.S. Grest, Materials Research Symposium Proceedings, 687, 179-184 (2002).

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 Angstrom x 50 Angstrom to 174 Angstrom x 201 Angstrom. We find that increasing the surface area by a factor of N reduces the noise in the shear stress by a factor of rootN, and has a comparable effect to averaging the smaller system data over bins of rootN points. This indicates that finite size effects are negligible in our simulations.

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