(86c) Molecular Dynamics Simulation of Nano-Scale Lubricant Films in Humidity

Humidity effects on the performance of lubrication at head-disk interface (HDI) have gained much attention during the past decade. These effects become more pronounced when harsher environmental conditions are imposed for the application of disk drives in portable customer electronics and automotive devices. From the experiments, it was found that adsorbed water in the media interface improved the mobility of perfluoropolyether (PFPE) molecules, decreased functional endgroup chemisorption through competition of surface active sites, reduced the desorption energy between backbone chain and overcoat, increased the amplitude of capillary waves, and expedited the relaxation of PFPE films. However, the understanding of molecular structure and dynamics of PFPE nano-films in humidity at the molecular level is still limited.

In this study, molecular dynamics simulation with a bead-spring model was employed to examine the molecular structure and dynamics of PFPE nano-films in the humid environment. The distribution functions of water, functional end bead, backbone bead were examined to reveal the nanostructure of PFPE nano-films in humidity. The conformation of PFPE molecules were also studied by calculating the radius of gyration. It was found that water coupled with functional end groups and wall forming characteristic cluster structure. The normalized autocorrelation function of normal modes for entire PFPE chain was examined to extract the characteristic relaxation time. The relaxation times become shorter in the higher humidity, which is consistent with experimental findings. The self-diffusion coefficients of water and PFPE were also calculated to investigate the dynamic property of PFPE films.

The molecular dynamics simulation results provide the fundamental understanding of nanostructure and dynamics of PFPE lubricant films, which is critical to optimize the lubricant/overcoat system in HDI.