Frictional behavior of highly oriented pyrolytic graphite

Abstract

Friction of a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface and a roughened graphite surface against a silicon nitride tip is measured using a friction force microscope. The cleaved graphite surface is an atomically smooth surface of (0001) plane with a small fraction of line-shaped regions. It is observed that the coefficient of friction is extremely small (<0.006) for a freshly cleaved HOPG surface of (0001) plane. However, the coefficient of friction is more than an order of magnitude larger in some line-shaped regions on the surface compared with that of smooth regions of (0001) planes. Transmission electron microscopy analysis indicates that the line-shaped regions consist of graphite planes of different orientations [other than (0001)] as well as amorphous carbon. This result suggests that (0001) graphite planes have the lowest coefficient of friction in comparison with other surface orientations or amorphous carbon. The coefficient of friction of a roughened graphite surface is found to be much larger than that of cleaved graphite. These observations may explain the large differences between the atomic-scale coefficient of friction of graphite (usually cleaved) (about 0.006 or even less) and its macroscopic counterpart (about 0.1).