AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoA

Paper SS2-MoA8
A Comparative Study of the Adhesion, Friction, and Mechanical Properties of CF@sub3@ and CH@sub 3@ Terminated Alkanethiol Monolayers

Monday, November 3, 2003, 4:20 pm, Room 327

Session: Tribology, Adhesion, and Friction
Presenter: C.M. Doelling, Princeton University
Authors: C.M. Doelling, Princeton University
H. Ying, Princeton University
J.E. Houston, Sandia National Laboratories
T.K. Vanderlick, Princeton University
G. Scoles, Princeton University
T.R. Lee, University of Houston
Correspondent: Click to Email
Considerable interest has been given to the potential use of self-assembled monolayer films as lubricants for applications such as MEMS. Of particular interest are fluorinated films because they are inert and show high thermal stability. Research performed directly compares the adhesion, friction, mechanical properties and contact potentials of two alkanethiol SAM films, differing only in the composition of their terminal head group (-CH@sub 3@ vs. -CF@sub 3@). Quantitative data of both friction and normal force was collected using the Sandia-developed interfacial force microscope (IFM). In comparison with the CH@sub 3@-terminated monolayer, we find that the CF@sub 3@-terminated film exhibits a longer-ranged attraction to an approaching metal probe tip, and the work required to separate the surfaces is greater despite the maximum attractive forces being similar. The friction is also larger, as is the force-displacement hysteresis upon loading/unloading cycles. Close inspection of the frictional response shows the presence of "non-contact" friction, namely, significant energy dissipation before the probe and monolayer develop a component of repulsive-contact force. From these findings we conclude that the mechanical behavior of the film is influenced by the strong dipole inherent to the -CF@sub 3@ terminal group. Finally, atomic force microscopy measurements of friction were also undertaken, taking advantage of nanografting to directly compare both types of films using the same probe tip. At small compressive loads, the friction of the fluorinated film is about two times that of the hydrocarbon film, in agreement with the IFM results. @FootnoteText@ The work at Sandia was supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin company, for the DOE under Contract DE-AC04-94AL85000.