Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus

Friction and adhesion hysteresis experiments were carried out on fluorocarbon surfactant monolayer-coated surfaces using the surface forces apparatus. Measurements were made as a function of temperature, load, sliding velocity, and relaxation time, and the resulting properties are contrasted with those of hydrocarbon monolayers and also with bulk fluorocarbon surfaces (e.g., Teflon, PTFE). The dynamic adhesion measurements show that the adhesion hysteresis and friction of fluorocarbon monolayer-coated surfaces are related: large friction forces being associated with large adhesion hysteresis. The results also show that the overall tribological properties of fluorocarbon surfactants follow the same generic "friction phase diagram" behavior as do hydrocarbon surfactants. However, the friction phase diagram for fluorocarbon surfactant has at least two peaks-one well above and the other well below room temperature-indicating that two different molecular relaxation processes are involved in friction and adhesion energy dissipation. Apparently, chain interdigitation, which is the most important molecular relaxation mechanism in the friction and adhesion hysteresis of hydrocarbon materials, does not play a major role with fluorocarbon surfaces. Instead, the surface topography and its change at the molecular and submolecular levels during shear is the most important factor determining the friction of these surfaces, but only so long as the monolayers remain molecularly smooth or "undamaged". Reasons for the beneficial tribological properties of fluorocarbon surfaces are discussed.