Wear of polytetrafluoroethylene: some crystallographic observations

Abstract

When polytetrafluoroethylene (PTFE) discs are slid against EN24 steel, a transition in wear rate occurs after approximately 0.1 km of sliding. In the post-transition stage, the wear rate increases steadily up to a sliding distance of approximately 3 km after which it stabilizes. Structural characteristics of the worn surface derived from X-ray diffraction studies indicate that in the post-transition stage, a close correlation exists between the volumetric shrinkage of the unit cell and the wear rate. The shrinkage along the crystal-lographic a-axis, which can promote subsequent easy sliding of the polymer chains against each other, is found to be nearly twice the shrinkage along the crystallographic c-axis. Increase in the average crystallite size initiated by the sliding process does not, however, exhibit further progressive increase with sliding. The wear debris is less dense than the bulk PTFE. X-ray data from the debris indicate a lack of long range order, a reduction in crystallite size and an enlargement of the unit cell. Scanning electron micrographs show the simultaneous presence of crystalline and less crystalline fractions in the debris.