Effect of Thermal Bonding on Frictional Properties of Monolayer Lubricant Films Coated on Magnetic Disk Surfaces

Monolayer films of functional perfluoropolyethers (PFPEs) that are partially bonded to disk surfaces are widely used to lubricate magnetic disks. The bonding ratio (i.e., ratio of bonded thickness to total film thickness) possibly impacts the tribological behavior of the lubricant films. In this work, under lightly loaded (0-1 mN) and slowly rotated (1 or 2 r/min) conditions that prevent disturbance of the lubricant bonding state, we measured the frictional properties of 2-nm-thick PFPE Zdol2000 and Zdol4000 films as a function of bonding ratio adjusted up to 100% by anneal treatment. The friction forces of both Zdol2000 and Zdol4000 increased with increasing bonding ratio, but the increase was less noticeable for Zdol4000. Additionally, the friction of the nonannealed (5%-bonded) Zdol2000 films exhibited a linear increase with external load as described by Amontons' law, and it increased with sliding speed. In contrast, the friction of the 100%-bonded Zdol2000 films increased nonlinearly with external load as predicted by the Johnson-Kendall-Roberts (JKR) model, and it showed no obvious speed dependency. These results suggest that mobile molecules show liquid-like frictional properties dominated by viscosity resistance, but bonded molecules exhibit elastomer-like frictional properties determined by energy dissipation during molecule deformation.