| We have investigated the relationship between the chemical structure of perfluoropolyether (PFPE) lubricants and their physical and tribological properties on both hydrogenated (CHx) and nitrogenated (CNx) amorphous carbon surfaces. Our work has focused on ultrathin lubricant films in the thickness range of 0∼40 Å that exist in the boundary or molecular lubrication regime. Solution-phase adsorption measurements performed using a quartz crystal microbalance suggest that after initial adsorption, the molecules reorganize on the surface, with the lubricant end group preferentially orienting itself toward the surface. This produces a first monolayer that has unique tribological properties and a strong resistance to displacement. Atomic force microscopy (AFM) friction and adhesive force measurements show a semi quantitative correlation between zero normal load friction and surface energy; however, a strong correlation between adhesive forces and surface energy, which would be expected based on adhesion theory, is not observed. We suggest that this is due to kinetic constraints imposed on the lubricant due to the molecular-scale confinement that it experiences as a thin film. Combined AFM and quartz oscillator measurements extend the velocity of our AFM measurements, providing a velocity range of 10−7 to 1.0 m/s. The data exhibit low friction at velocities below 5 × 10−6 m/s, an increase to a plateau at velocities from 5 × 10−6 to 5 × 10 −4 m/s, and a decrease at velocities above 2 × 10 −2 m/s. The adhesive forces are independent of velocity below 5 × 10−2 m/s and decrease dramatically at higher velocities. We interpret these results in terms of a frictional that is dominated by adhesive forces and interaction thermodynamics. Chemical modification of ZDOL through the addition of a series of hydrocarbon end groups shows that many technologically important properties such as substrate affinity, lubricant volatility, surface energy, and friction coefficients vary systematically with end group structure. A ZDOL derivative that contains reactive vinyl end groups exhibits extremely unique characteristics, forming a crosslinked network on the CNx surface that leads to distinct material properties such as solidification, an exceptionally low surface energy, and a high resistance to displacement. |
