Tribological Durability of Silane Monolayers on Silicon

Abstract

We report the frictional performance and long-term tribological stability of various alkyl silane monolayer films on silicon by using pin-on-disk tribometry at ambient conditions. We show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor, which we relate to the cohesive energy of these monolayers through molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage consisted of the loss of molecular components that could be partially replaced upon exposure to a solution containing perfluorinated silane precursors. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with hydroxyl surfaces exhibited reduced stabilities due to stronger tip-surface interactions. Monolayers with the capability for cross-linking exhibited much greater stabilities than monolayers where cross-linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersional interactions and cross-linking and minimizing tip-surface interactions.

Reactive Molecular Dynamics Simulations of the Silanization of Silica Substrates by Methoxysilanes and Hydroxysilanes

“…This ad-hoc method allows new silane reactant to enter the system and reach reasonable surface coverage while maintaining low concentration of free silane. Both the density (1.0 g/cm 3 ) and the concentration of silane (about 0.24 M) of the mixture inserted into the source layer are far from experimental conditions. By estimation, assuming an ideal mixture, the density of the solution should be near 0.71 g/cm 3 .…”

“…Both the density (1.0 g/cm 3 ) and the concentration of silane (about 0.24 M) of the mixture inserted into the source layer are far from experimental conditions. By estimation, assuming an ideal mixture, the density of the solution should be near 0.71 g/cm 3 . The high concentration of silane was used here in order to make simulations tractable and achieve reasonably high grafting densities.…”

Characterization of Alkylsilane Self-Assembled Monolayers by Molecular Simulation

“…It is also possible to consider the thickness of the hydrocarbon chain of the molecules, or tail thickness. The tail thickness is equal to the average distance normal to the surface between the been demonstrated experimentally 29 . It has been shown that, at a given coverage, the monolayer thickness increases linearly with the alkylsilane chain length 4,41 .…”

Tunable transition from hydration to monomer-supported lubrication in zwitterionic monolayers revealed by molecular dynamics simulation

“…In the high shear stress regime, monomer-monomer interactions dominate the shear response which thus resembles the response of previously studied dense alkylsilane monolayers without water. [38][39][40][41][42] The low shear regime, where friction coefficients compare to those observed in experimental brush systems, 4,7-9 is characterized by a thin layer of water separating monomers in opposing substrates. Estimates of friction coefficients in these regimes approximately agree with other simulation and experimental studies of systems with respectively comparable environments.…”