AVS 49th International Symposium
    Surface Science Thursday Sessions
       Session SS-ThA

Paper SS-ThA3
A Quantitative Study of the Mechanical and Adhesive Origin of Molecular-Level Friction

Thursday, November 7, 2002, 2:40 pm, Room C-110

Session: Tribology at Surfaces
Presenter: J.E. Houston, Sandia National Laboratories
Authors: J.E. Houston, Sandia National Laboratories
H.I. Kim, Aerospace Corporation
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In earlier work, we explored the origins of molecular-level friction using the interfacial force microscope and Au tips and substrates terminated by self-assembled monolayer films having various functional end groups. Here we continue that effort by applying contact-mechanics models to quantitatively analyze the mechanical properties of the films, their interfacial energies and the friction shear-stress values for each functional-group combination. Methyl end-group combinations represent only van der Waals bonding and purely mechanical friction, whereas carboxylic acid groups show significant hydrogen bonding. The results indicate that both the composite elastic modulus and the mechanical portion of the friction shear stress scale inversely with the total molecular length of the tip/substrate films. The adhesive energy shows an odd/even effect changing from inter-film for an even number of methylene units to purely intra-film bonding for odd-numbered films--the intra-film adhesion only appears in the lateral friction force. Very short methyl-terminated combinations indicate a significant non-contact component to the friction force. These results will be discussed in terms of the possible use of these films as molecular-level lubricants. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin company, for the DOE under Contract DE-AC04-94AL85000.