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In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.

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IntroductionA gap flying height of less than 5 nm is required for ultrahigh density recording. For such ultralow flying sliders, the changes in FH during track-seeking motion not only cause signal loss, but also significantly increase the risk of head-disk contact. Moreover, the presence of nanoscale adhesion forces, such as intermolecular and electrostatic forces, adversely increases the FH drop and even causes head-disk impact. In order for a reliable head-disk interface to be maintained, the FH change and contact between the slider and disk must be avoided. Different ABS designs can perform quite differently during the track-seeking process. Therefore, the dynamic track-seeking performance of air bearing sliders is becoming of increasing importance. A better understanding of factors that cause FH change should help improve the ABS design to achieve better track-seeking performance. In this paper, we propose a quasi-static approximation of track-seeking motion. The track-seeking performances of four different air bearing surface (ABS) designs are numerically investigated by the quasi-static approximation and the CML Dynamic Simulator. Results and DiscussionIn order to reduce the computation efforts of track-seeking simulations and quantitatively study the contribution of various factors on the FH change during track-seeking, we carried out a quasi-static approximation of track-seeking motion. Instead of simultaneously solving the generalized Reynolds equation and the equations of motion of a slider at each time step, we solve the Reynolds equation under static suspension loading with consideration of the seeking velocity and HGA inertia at different radial positions during track-seeking. We studied four ABS designs with subambient pressure regions (Fig. 1). ABS I was designed using an optimization algorithm for a nearly uniform 5-nm FH across the disk. The second design labeled ABS II was obtained from a commerical product. The third and fourth designs, named Scorpion III and Scorpion IV, were designed for controlled-FH sliders with thermal and piezoelectric nanoactuators [1],[2],respectively. For a controlled-FH slider, the FH is about 10 nm in the off duty cycle and is reduced to ~2 nm during reading and writing by applying either a current or a voltage to an active element, such as a resistive heating element or piezoelectric material. Fig. 1 shows comparisons of FHs during the corresponding full-stroke seeking loop between dynamic simulations and quasi-static approximations. It is seen that for all the designs the quasi-static method demonstrated good agreements with dynamic simulations. Scorpion III and IV exhibit extremely small FH variations, which can significantly reduce the risk of head-disk contact . Fig. 3 shows the effects of various factors on the FH changes of ABS I seeking from ID to OD. The skew angle change is the dominant factor that causes FH change during track-seeking and the inertia effect accounts for ~12% of the FH change. In order to study the effect of intermolecular and electrostatic...