Comparison of Friction Measurements Using the Atomic Force Microscope and the Surface Forces Apparatus: the Issue of Scale

Author(s): McGuiggan, Patricia M., Jun Zhang, Stephen M. Hsu

Journal: Tribol. Lett. (2001) 10(4): 217-223.

Abstract:
Results are presented of lateral force measurements using the atomic force microscope (AFM) and the surface forces apparatus (SFA). Two different probes are used in the AFM measurements; a sharp silicon nitride tip (radius R ≈ 20 nm) and a glass ball (R ≈ 15 µm). The lateral force is measured between the (silicon nitride or glass) probe and a mica surface which has been coated by a thin lubricant film. In the SFA, a thin lubricant film separates two molecularly smooth mica surfaces (R ≈ 1 cm) which are slid relative to each other. Perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) were used as the lubricant films. In the SFA where the contact diameter is largest, the PFPE film shows much lower friction than PDMS. As the size of the probe decreases, the difference in the measured friction decreases. For sharp AFM tips, no clear distinction between the tribological properties of the films can be made. Hence, the measured coefficient of friction varies according to the length scale probed, at least for small dimensions.