A model for lubricant flow from disk to slider

A model is presented that calculates the equilibrium lubricant thickness on a slider, resulting from a steady state where net inflow from disk evaporation equals net outflow from evaporation back to the disk, and flow to the slider back end from air shear. Based on experimental vapor pressure and viscosity data, and using surface viscosity enhancement factor and disjoining pressure values available in the literature, this model predicts a transition from a flooded regime for molecular weight M/sub n/ below 1.5 kDaltons to a starved regime where slider lubricant thickness drops to a value close to zero. At the same time, lubricant accumulation to the slider back end is expected to decrease exponentially.

I. Introduction

As HEAD/MEDIA magnetic spacing in disk drives is fast approaching the 10 nm mark, the need to understand the spacing contribution of the disk lubricant becomes more pressing. Earlier studies have attempted to indirectly measure the effect of lubricant thickness on the actual head/media spacing, using the readback signal [1]. A moderately quantitative correlation was observed. In addition to its effect on physical spacing, disk lubricant has been shown to exhibit slider-assisted redistribution [2]. In the more distant past, issues such as fly/stiction [3], [4] have been studied, and a reasonable assumption for the presence of lubricant on the slider is that the low molecular weight part of the perfluoropolyether (PFPE) from the disk somehow transfers to the slider through an evaporation/condensation process. In this paper, a model of lubricant transfer between disk and slider surface is presented that takes into account evaporation/condensation driven by thin-film vapor pressure, as well as shear toward the back of the slider. Fig. 1. Lubricant head/disk mass transfer model.