Measured transitions between sticking and slipping at lubricated line contacts

Sets of unsteady friction experiments are presented. The data allow an interpretation to be made of the principal physical phenomena that occur during the transitions that take place during impending slip or stick in interrupted sliding. The test geometry is that of a lubricated line contact, to which different patterns of velocity oscillation are applied. During acceleration from a rest or near-rest condition, a tangential contact stiffness and a microslip regime are observed before macroscopic sliding takes place. We also note a region of `microstick,' somewhat analogous to microslip, which makes it difficult to unambiguously identify a particular point where sticking actually begins. High frequency vibrations were also monitored, and while interesting, do not appreciably affect the main friction behavior. The friction behavior is considerably more complicated than for dry contacts. Implications to the modeling of unsteady lubricated friction are discussed.