Experimental Comparisons of Spreading and Replenishment Flows of Molecularly Thin Lubricant Films Coated on Magnetic Disks

To ensure the reliability and durability of the head-disk interface of magnetic disk drives, the lubricant film flow coated on the disk surface has become a significant concern because it is related to recovery performance when the film is damaged. In this study, the replenishment characteristics of perfluoropolyether (PFPE) polar lubricant films after lubricant depletion induced by a glass ball sliding on the disk were clarified by measuring the profile changes of the lubricant film. Lightly loaded and slowly rotated test conditions were selected to avoid strongly disturbing the molecular layering structure. We found that the replenishment process dramatically changed with increasing film thickness, and recoverable and unrecoverable features appeared depending on the coated and residual film thicknesses. The replenishment process was compared with the step-boundary flow, i.e., the flow from the step-shaped lubricant boundary and the disjoining pressure distributions. The diffusion coefficients calculated from the step-boundary flow were found to be ineffective for replenishment flow, and the disjoining pressure differences between the coated and residual film thicknesses bifurcate the replenishment flow into recoverable and unrecoverable features.