Effects of molecularly thin liquid lubricant films on slider hysteresis behavior in hard disk drives

In order to achieve a magnetic recording density of I Tb/in(2), the spacing is expected to be less than 2-3 tim. However a critical issue in achieving such an ultralow spacing is the dynamic instability qf the head disk interface (HDI). That is, the experimentalli, observed hysteresis of fly sliders. The phenomenon of slider hysteresis has two features: slider touchdown and slider takeoff. The goal of this research is to experimentally clarify, the effects of the lubricant bonded ratio as well as the lubricantfilm thickness on slider hysteresis behavior in detail. It also aims to determine the contributing factors. In this study, the difference in the touchdown and takeoff velocities was monitored by varying the lubricant bonded ratio and lubricantfilm thickness of the disks. Furthermore, the correlation between the observed phenomenon and the variation in the experimental parameters was investigated. The results showed that the touchdown velocities lvere, almost independent of the lubricant bonded ratio, while the takeoff velocities were greaterfor a. lubricant with a higher bonded ratio. These results were ob tained.for a constant lubricant filin thickness of around one monolayer Therefore, the slider hysteresis was greaterfor a lubricant with a higher bonded ratio. With regard to the effect of lubricantfilm thickness, 1. t was observed that the touchdown and takeoff velocities were greaterfor thinner lubricants. These results for the effect of lubricant film thickness are very similar to those obtained by Ambekar, Gupta, and Bogy (2005, ASME J. Tribol., 127(3), pp. 530-536). However, the slider hysteresis was greaterfor thicker lubricants. Considering these experimental results as well as the experimental datafor the effect of the surface roughness of a disk on the slider hysteresis obtained by (Tani et al. (2006, J. Appl. Phys., 99(8), pp. 08N]04-1-08N]04-3), it was suggested that the variation in the touchdown velocity is due to a variation in the intermolecularfiorces. Furthermore, it was suggested that the variation in the takeoft'velocity is caused by a variation in the friction forces between the slider and disk suijace. This occurs because the takeoff velocity was greaterfor a lubricant with a higher bonded ratio or a thinner lubricant, which only has a sinallfraction of free mobile lubricant. The results predicted by the simulations are consistent with those observed experimentally. In addition, a design guideline,for next-generation HDI, with small touchdown and takeoff velocities, resulting in small slider hysteresis, is discussed in detail in this paper.