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

Paper SS2-MoA3
Direct Observation of Superlubricity

Monday, November 3, 2003, 2:40 pm, Room 327

Session: Tribology, Adhesion, and Friction
Presenter: M. Dienwiebel, IAVF AG, Germany
Authors: J.W.M. Frenken, Leiden University, The Netherlands
M. Dienwiebel, IAVF AG, Germany
N. Pradeep, Leiden University, The Netherlands
K.B. Jinesh, Leiden University, The Netherlands
G.S. Verhoeven, Leiden University, The Netherlands
J.A. Heimberg, Corvis Corporation
Correspondent: Click to Email
We have constructed a frictional force microscope (FFM) that is able to quantitatively track the forces between a tip and sample in three dimensions. At the heart of the FFM is a silicon sensor, the 'Tribolever'. The 3D motion of a metal tip, which is held by this sensor, is detected with four interferometers. The two lateral spring constants of the Tribolevers are typically 1 N/m, which is up to two orders of magnitude lower than the torsional force constants of conventional AFM cantilevers. The spring constant perpendicular to the surface is typically an order of magnitude higher than in the lateral directions. The friction force resolution is as low as 15 pN, even under normal loads up to several tens of nN. We present experimens with W-tips on highly oriented, pyrolytic graphite (HOPG) surfaces. We observe the familiar, atomic-scale stick-slip behavior, in which the tip performs a 'least-resistance', zig-zag path over the corrugated graphite surface. To our surprise, the amplitude of the friction forces depends strongly on the relative orientation of the tip and the graphite surface. When we rotate the graphite around an axis normal to the surface, the average friction force shows characteristic variations between a high value and a near-zero value, close to the detection limit of our FFM. These observations, combined with additional, circumstantial evidence, support a simple interpretation, in which a small graphite flake intervenes between the W tip and the HOPG substrate. Thus, the FFM actually records the lateral forces between two parallel graphite lattices. By rotating the substrate with respect to the tip, we periodically go through fully aligned and completely misoriented configurations. When the misalignment is sufficiently severe, the lateral forces on the C-atoms in the flake cancel, thereby dramatically reducing the total friction force. This phenomenon has been predicted more than ten years ago, and is referred to as superlubricity.