FUNDAMENTAL MECHANISMS OF INTERFACIAL FRICTION .2. STICK-SLIP FRICTION OF SPHERICAL AND CHAIN MOLECULES

The friction and rheological properties of molecularly thin films of hexadecane between two shearing mica surfaces were measured under different conditions of load or applied pressure, film thickness, sliding velocity, temperature, and time or previous history. The aim was to investigate the tribological and rheological properties of ultrathin liquid films composed of chain molecules and to compare these properties with those of spherical molecules. The differences were found to be more quantitative than qualitative, and the results therefore provide insights into the general properties of liquids in thin films. The results show that during steady-state sliding between two solid crystalline surfaces, a liquid film may be considered to be in a one-phase or two-phase dynamic regime. In the two-phase regime, the film undergoes periodic ''stick-slip'' transitions between a solid like and a liquid like state. However, especially in the case of chain molecules such as hexadecane, the liquidlike state is very different from the bulk liquid. The frequency of transitions between the two dynamic states determines the static and dynamic friction forces and other dynamic properties of a shearing film. The effects produced by changing the load, temperature, sliding velocity, stopping time, etc., were found to be highly correlated, enabling us to test new equations for the properties and existence of stick-slip motion. Our results provide new insights into the role of molecular shape on the structure and dynamics of molecules in ultrathin films, and more specifically into discontinuous transitions and instability effects in thin films that result in stick-slip motion.