Sripathi Vangipuram Canchi

Slider Dynamics in the Lubricant-Contact Regime

Thermal fly-height control sliders that are capable of subnanometer level actuation are used to investigate the vertical, down-track, and off-track slider dynamics in slider-disk lubricant contact. Slider-lubricant contact introduces significant excitation in all three directions, and the slider dynamics is dependent on the degree of lubricant-contact. The lubricant surface has a significant role in determining the physical clearance and slider fly-height. While slider-lubricant contact may be successfully achieved by carefully controlling the heater power, improved slider designs and associated heater induced protrusion profiles are necessary to successfully mitigate contact induced vibrations and meet the challenges for future hard disk drives.

Thermal Fly-Height Control Slider Instability and Dynamics at Touchdown: Explanations Using Nonlinear Systems Theory

Thermal fly-height control sliders are widely used in current hard disk drives to control and maintain subnanometer level clearance between the read-write head and the disk. The peculiar dynamics observed during touchdown/contact tests for certain slider designs is investigated through experiments and analytical modeling. Nonlinear systems theory is used to highlight slider instabilities arising from an unfavorable coupling of system vibration modes through an internal resonance condition, as well as the favorable suppression of instabilities through a jump condition. Excitation frequencies that may lead to large amplitude slider vibrations and the dominant frequencies at which slider response occurs are also predicted from theory. Using parameters representative of the slider used in experiments, the theoretically predicted frequencies are shown to be in excellent agreement with experimental results. This analytical study highlights some important air bearing surface design considerations that can help prevent slider instability as well as help mitigate unwanted slider vibrations, thereby ensuring the reliability of the head-disk interface at extremely low head-disk clearances.

Nanoscale heat transfer in the head-disk interface for heat assisted magnetic recording

Laser heating has been introduced in heat-assisted magnetic recording in order to reduce the magnetic coercivity and enable data writing. However, the heat flow inside a couple of nanometers head-disk gap is still not well understood. An experimental stage was built for studying heat transfer in the head-disk interface (HDI) and the heat-induced instability of the HDI. A laser heating system is included to produce a heated spot on the disk at the position of the slider. A floating air bearing slider is implemented in the stage for sensing the temperature change of the slider due to the heat transfer from the disk by the use of an embedded contact sensor, and the gap between the two surfaces is controlled by the use of a thermal fly-height control actuator. By using this system, we explore the dependency of the heat transfer on the gap spacing as well as the disk temperature.

Slider-Lubricant Interactions and Lubricant Distribution for Contact and Near Contact Recording Conditions

Lubricant distribution and recovery for near contact and contact recording conditions are experimentally investigated using thermal fly-height control (TFC) sliders. Contact between the protruded center pad and the disk lubricant causes a thickness modulation (rippling) together with lubricant depletion that grows with contact duration. Slider dynamics and lubricant rippling evolve quickly in the first few revolutions of contact and rippling frequencies are strongly correlated with the slider air bearing frequencies. Peculiar cases where suppressed “stable” slider dynamics occur for TFC heater power beyond the touchdown power correspond to negligible lubricant rippling and this condition may be sustained for fairly long durations in some tests. Experiments with different lubricant types (ZTMD, Z-tetraol+A20H, Z-dol+A20H) and different lubricant thicknesses show that a larger depletion is observed for the thicker lubricant of each type at a given TFC heater power. Lubricants with lower bonded fraction show shorter recovery time after the slider is unloaded. From the current experiments it is extrapolated that Z-dol+A20H (30% bonded) lubricant recovers in a time scale of a few hours, while Z-tetraol+A20H (60% bonded) lubricant of comparable thickness takes a few days, and ZTMD (75% bonded) takes weeks to fully recover.

Parametric Investigations at the Head-Disk Interface of Thermal Fly-Height Control Sliders in Contact

Accurate touchdown power detection is a prerequisite for read-write head-to-disk spacing calibration and control in current hard disk drives, which use the thermal fly-height control slider technology. The slider air bearing surface and head gimbal assembly design have a significant influence on the touchdown behavior, and this paper reports experimental findings to help understand the touchdown process. The dominant modes/frequencies of excitation at touchdown can be significantly different leading to very different touchdown signatures. The pressure under the slider at touchdown and hence the thermal fly-height control efficiency as well as the propensity for lubricant pickup show correlation with touchdown behavior which may be used as metrics for designing sliders with good touchdown behavior. Experiments are devised to measure friction at the head-disk interface of a thermal fly-height control slider actuated into contact. Parametric investigations on the effect of disk roughness, disk lubricant parameters, and air bearing surface design on the friction at the head-disk interface and slider burnishing/wear are conducted and reported.

Electrostatically Tunable Adhesion in a High Speed Sliding Interface

Contact hysteresis between sliding interfaces is a widely observed phenomenon from macro- to nanoscale sliding interfaces. Most such studies are done using an atomic force microscope (AFM) where the sliding speed is a few μm/s. Here, we present a unique study on stiction between the head-disk interface of commercially available hard disk drives, wherein the vertical clearance between the head and the disk is of the same order as in various AFM-based fundamental studies but with a sliding speed that is nearly 6 orders of magnitude higher. We demonstrate that, although the electrostatic force (dc or ac voltage) is an attractive force, the ac-voltage-induced out-of-plane oscillation of the head with respect to the disk is able to completely suppress the contact hysteresis.

Dynamic Friction Study on Voltage Assisted Overcoat Wearing Head-Disk Interface

Voltage assisted wear at the head and the disk interface is investigated with the motive of understanding the head overcoat wear processes. In this work, we report the quantitative analysis of voltage assisted wear on head carbon overcoat at high sliding speed interfaces. We found that voltage assisted TFC head wear acts asymmetrically at the interface with positive voltage leading to high wear. We quantitatively analyzed the interface using a strain gauge based friction measurement.Copyright

Compensation for the Write Start Transient in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) uses a laser to heat the magnetic media above its Curie temperature to enable magnetic writing when the write field is limited. However, there are thermal transients caused by the thermal–mechanical response of the head and media when HAMR writing starts. Understanding this transient and compensating for it are critical to successfully implement the HAMR system. In this letter, some possible contributions to the write start transient are discussed. Methods are demonstrated for achieving more uniform recording.

Overcoat Wear and Dynamic Friction Study at the Head-Disk Interface

The dynamic friction and wear at the head and the disk interface is investigated with the motive of understanding the head overcoat wear process associated with physical contact between the head and the disk. In this work, the results from systematic experiments under overpush conditions are presented. Various regimes of head wear are identified based on the individual wear rate of the participating overcoat layers. A strain gauge based friction measurement is used to extract the friction coefficient and the adhering shear strength between the head and the disk.Copyright

Parametric Instability of a Rotating Circular Ring With Moving, Time-Varying Springs

Parametric instabilities of in-plane bending vibrations of a rotating ring coupled to multiple, discrete, rotating, time-varying stiffness spring-sets of general geometric description are investigated in this work. Instability boundaries are identified analytically using perturbation analysis and given as closed-form expressions in the system parameters. Ring rotation and time-varying stiffness significantly affect instability regions. Different configurations with a rotating and non-rotating ring, and rotating spring-sets are examined. Simple relations governing the occurrence and suppression of instabilities are discussed for special cases with symmetric circumferential spacing of spring-sets. These results are applied to identify possible conditions of ring gear instability in example planetary gears.Copyright