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Atomic Scale Origin of Adhesion and Friction
Viscoelastic Effects in Model Lubricant Monolayers
Abstract We discuss the origin of energy dissipation in friction with model lubricant monolayers of alkyl-silanes and thiols. Atomic Force Microscopy (AFM), Surface Forces Apparatus (SFA) and Sum Frequency Generation vibrational spcctrocopy (SFG) were used to study the structure of the films, (ordering, thickness, molecular conformation), as a function of applied load. We show that energy dissipation can be described in terms of excitation of specific molecular rearrangements. This include generation of terminal gauche defects at loads of 20 to 80 MPa and collective tilts of the alkyl chains at certain threshold loads in the GPa regime. The rigid molecular tilts are accompanied by stepwise increases in friction. Only a discrete number of tilt angles and corresponding film heights are observed. The results are explained by a simple close packing and chain interlocking model driven by the van der Waals attractive energy. Friction energy is spent in overcoming an activation energy barrier for tilting. The activation consists in a slight increase in the separation (∼0.2 Å) between chains to allow the interlocked alkyl chains to slide past each other. The increase in separation causes a loss of cohesive energy of the film.
Atomic Scale Origin of Adhesion and Friction
Viscoelastic Effects in Model Lubricant Monolayers
Abstract We discuss the origin of energy dissipation in friction with model lubricant monolayers of alkyl-silanes and thiols. Atomic Force Microscopy (AFM), Surface Forces Apparatus (SFA) and Sum Frequency Generation vibrational spcctrocopy (SFG) were used to study the structure of the films, (ordering, thickness, molecular conformation), as a function of applied load. We show that energy dissipation can be described in terms of excitation of specific molecular rearrangements. This include generation of terminal gauche defects at loads of 20 to 80 MPa and collective tilts of the alkyl chains at certain threshold loads in the GPa regime. The rigid molecular tilts are accompanied by stepwise increases in friction. Only a discrete number of tilt angles and corresponding film heights are observed. The results are explained by a simple close packing and chain interlocking model driven by the van der Waals attractive energy. Friction energy is spent in overcoming an activation energy barrier for tilting. The activation consists in a slight increase in the separation (∼0.2 Å) between chains to allow the interlocked alkyl chains to slide past each other. The increase in separation causes a loss of cohesive energy of the film.
Atomic Scale Origin of Adhesion and Friction
Viscoelastic Effects in Model Lubricant Monolayers
Salmeron, Miquel (author) / Kopta, Susanne (author) / Barrena, Esther (author) / Ocal, Carmen (author)
2001-01-01
12 pages
Article/Chapter (Book)
Electronic Resource
English
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