The friction and transfer of polytetrafluoroethylene

Abstract

Earlier work has shown that the friction of polytetrafluoroethylene ( PTFE ) increases with increasing velocity and decreases with rise of temperature as though a relaxation process were involved. A study by microscopy, interference microscopy and electron microscopy of the friction tracks formed when PTFE slides on itself or on ‘clean’ glass shows that there are basically two friction regimes. At high speeds or low temperatures the friction is high ( μ , = 0.07 to 0.3) and there is fairly massive transfer and movement of polymer. The details depend on the sliding conditions but in general the transfer is in the form of lumps, ribbons or sheets, the thickness generally exceeding a few tenths of a micrometre. At low speeds and moderate temperatures a very different behaviour is observed: the friction is low ( μ < 0.07) and a thin film of PTFE is laid down or drawn over the surfaces. This film may show strong adhesion to the surfaces if they are clean. It is very fibrous and has a thickness varying between about 100 and 400 Å; in addition it has a highly oriented crystal structure. The low friction under these conditions is not due to poor adhesion but to easy shear of relevant units of the PTFE crystal. As the speed of sliding is increased or the temperature diminished the viscous force to shear the film increases until a stage is reached where the shear force exceeds the strength of the boundaries between crystals or grains. The higher friction is then accompanied by the transfer of relatively large fragments of PTFE . These two régimes in the frictional behaviour may thus be interpreted in terms of a relaxation time for intra-crystalline flow. The small change in friction at higher speeds suggests that the shear of larger units within the polymer is not appreciably rate dependent.