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, a relatively well-defined tribological configuration, 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. The early stages of gross slip are characterized by an increase in the friction force as shear of the lubricant within the contact adds to the existing solid friction, even though, at this point, the fluid carries virtually no load. As the sliding speed increases further, the surfaces move apart and the solid friction component decreases significantly, eventually disappearing altogether. However, in the mixed lubrication regime, the friction force during acceleration remains higher than it would be under an equilibrium sliding condition at the same velocity. During both acceleration and deceleration, a finite time is needed for the normal separation between the sliding surfaces to change and the oscillations in friction force lag the sliding velocity. As a stick condition is approached, the fluid shear decreases more rapidly than the solid friction increases. The net effect is that the total friction force begins to decrease before the sliding velocity reaches zero. We also note a region of `microstick,' somewhat analogous to microslip, that 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.