Thermal Fly-height Control Slider Dynamics and Slider-Lubricant Interactions in Hard Disk Drives
- Author(s): Vangipuram Canchi, Sripathi
- Advisor(s): Bogy, David B
- et al.
The storage industry's density target of 10 Tb/sq.in. in hard disk drives within the next decade requires a significant change in head-disk interface (HDI) architecture, and it likely involves a combination of new technologies such as Heat Assisted Magnetic Recording and Bit Patterned Media Recording to mention a few. Independent of the actual recording technology, it is necessary to reduce the magnetic spacing to within 2nm, which implies a physical spacing as little as 0.25nm at the read-write transducer location. At such a small spacing intermittent contact between the slider and the lubricant layer or hard overcoat surface on the disk becomes inevitable. A continuous lubricant-contact HDI may in fact be necessary to meet future magnetic spacing needs. While the new recording technologies impose a significantly tighter budget on the slider dynamics in all three directions (vertical, down-track and off-track), the contacting HDI must be reliable, ensuring no degradation of lubricant or disk overcoats even after prolonged operation.
The current slider technology uses Thermal Fly-height Control (TFC) to bring the read-write portion of the slider closer to the disk by resistive heating induced thermal deformation/protrusion. While subnanometer level clearance can be achieved using the TFC, slider stability and HDI reliability at very small spacing remains to be understood. In order to further reduce the magnetic spacing using the TFC architecture, a recording strategy with a small portion of the thermal protrusion in intermittent or continuous contact with the lubricant layer of the disk has been proposed, but there is limited theoretical and experimental work to verify the feasibility of this technique. The focus of this work is to advance the understanding of TFC slider dynamics and slider-lubricant interactions at a HDI with contact through experiments and modeling.
Slider-lubricant contact is experimentally established by carefully controlling the TFC heater power, and the three dimensional slider dynamics under lubricant-contact is investigated. The degree of slider-lubricant contact is shown to influence the slider's vibration modes. A simple two degree of freedom model that accounts for nonlinearities at the HDI through quadratic and cubic approximations is used to analytically investigate the interesting features of this problem. It is shown that the thermal protrusion induced by the heater power can cause the system modes to couple unfavorably for certain heater power ranges, and this condition can manifest itself as large amplitude slider vibrations. Experiments are conducted to understand the interplay between slider dynamics and disk lubricant evolution under the thermal protrusion for contact and near contact conditions. Slider dynamics and lubricant rippling are shown to be well correlated and a mechanism of lubricant transfer from the slider to the disk at the onset of contact is demonstrated. Parametric investigations are conducted to understand the effect of lubricant type and thickness on lubricant distribution, lubricant depletion and subsequent lubricant recovery behavior at a contacting HDI.