AFM study of polymer lubricants on hard disk surfaces

Abstract

Thin liquid films of PFPE (perfluoropolyether) lu- bricants dip-coated on hard disk surfaces were imaged with non-contact mode AFM. Demnum lubricants with phosp- hazene additives exhibited strong interactions with a silicon tip due to the formation of liquid bridges between the lubri- cants and the tip, as indicated by a remarkable hysteresis loop between approach and retraction curves in force vs. distance measurements. Features resulting from capillary forces due to tip tapping to the lubricants were revealed, which demon- strated that the capillary forces could be used to lock the non-contacting tip at a certain separation from the substrate surface to obtain AFM images. Force vs. distance curves for Fomblin Z-dol lubricants showed negligible hysteresis effects and features corresponding to lateral distortion of the tip by the lubricants only were observed. In both cases, only when the tip was positioned far above the surfaces could the natural distributions of the lubricants be imaged. Capillary forces are common phenomena at tip-sample in- terfaces in AFM experiments when liquid layers are present on sample surfaces (1-3). This effect can be observed by means of force vs. distance curve measurements using AFM. When a tip approaches the sample surface and touches the liquid layers, a liquid bridge or meniscus will form be- tween the tip and the sample surface, attracting the tip to jump into contact with the surface. When the tip is re- tracted from the surface the liquid bridge can be elongated, resulting in a hysteresis loop of approach and retraction curves. In a study of the spread and distribution of perflu- oropolyether (PFPE) lubricants on hard disk surfaces with AFM force vs. distance curve measurements, remarkable dif- ferences were found for chemically bonded and unbonded lubricants, i.e. liquid behavior for physisorbed liquid PFPE and rigid, non-plastic behavior for a chemically bonded sam- ple (4). However, no detailed explanation was given for this phenomenon. Corresponding author In our efforts to map the distribution of PFPE lubricants with non-contact mode AFM on hard disk surfaces, we also observed this interesting occurrence for different lubricants. We will show in this paper, by combining the force vs. dis- tance measurements with our non-contact AFM images of the lubricants, that the liquid behavior is related to the for- mation of a liquid bridge, whereas the non-plastic behavior can be explained by the dipping of the tip into the lubricant films, where no liquid bridges form. We will also show that, as suspected by Hartmann (5), capillary forces may be used to modulate the probe oscillation at a certain separation from the sample surface to form AFM images. Such images can give us information about how the lubricants distribute in the texture topography of the disk surface.