Nanotribological properties of elastic thin films.

摘要

A considerable number of fundamental and applied studies have focused on the nano-tribological properties of confined, molecularly thin films. Despite these efforts, the molecular-level mechanisms of friction and the corresponding functions of actual lubricant additives and surfactants at solid surfaces are only partially understood. To address some of these issues, thiol self-assembled monolayers (SAMs) were studied in single-asperity contacts, as model systems for the natural boundary lubrication of solid surfaces by the aromatics content in mineral oil-based fuels and lubricants. Friction force microscopy, a technique based on atomic force microscopy (AFM), and the Surface Forces Apparatus (SFA) were used to investigate the effects of molecular packing, adhesion strength, and sliding speed on the friction of self-assembled monolayers of the aromatic compounds thiophenol, p-phenylthiophenol, p-terphenylthiol, 2-naphthalene-thiol, benzyl mercaptan, and phenyl-terminated alkanethiols Ph(CH2)nSH (n = 13-16) on template-stripped gold or silver surfaces. The adhesion between a monolayer-covered tip and substrate was controlled by immersing the sliding contacts in ethanol or in dry N2 gas.;At low loads L, low adhesion (obtained in ethanol) resulted in a linear dependence of the friction force F on load, F = muL, whereas in N2, the higher adhesion between the same monolayers gave a non-linear, apparently area-dependent friction. This nonlinear friction in the adhesive systems was well described by F = ScA, with the contact area, A, calculated for a thin, linearly elastic film confined between rigid substrates using the extended Thin-Coating Contact Mechanics (TCCM) model, and S c being the critical shear stress, a constant for each monolayer system. A systematic change was found in the friction coefficient, mu, obtained in ethanol and the critical shear stress, Sc, obtained in N2 with increasing molecular packing for the aromatic thiols, and with the odd or even number of methylene units for the phenyl-terminated alkanethiols. The investigations have also been extended to preliminary experiments on thin films of polyolefins with and without stretching-induced orientation to study the dependence of friction on the polymer chain orientation.