Micro-distortion of polymer surfaces by friction

A point-contact microscope (PCM) with a very sharp diamond tip works with loads as light as 10 nN, which allows high-resolution topographies to be obtained. This PCM can also be used to perform indentation, scratch and wear mark tests. Nanometre-level wear mark tests were performed on polycarbonate and epoxy surfaces using loads ranging from submicronewtons to micronewtons. During these tests, the scanning-scratched surfaces of polycarbonate formed projections, and no depressions or wear particles were observed. This phenomenon is novel compared with the depressed wear marks formed on the scratched surfaces of ordinary materials such as metals or ceramics. The projections on the surface involve an increase in volume and they are softer than the non-scratched surface. It seems that this distortion was caused by the frictional force. In contrast, an epoxy surface did not show any growth of projections, and wear particles were produced from the surface.