Slider-disk Interface Research Articles

Investigation on Temperature Rise of a Micro-Thermal Sensor in Helium–Air Gas Mixtures Considering Near-Field Radiation

The thermal response of a micro-thermal sensor at the slider–disk interface filled with helium–air gas mixtures is investigated. The heat transfer due to both the heat conduction in gas films and the near-field radiation is included. The effects of the helium fraction on the sensor temperature are discussed. The results show that due to the evanescent photo tunneling at a small spacing, the near-field radiation generates a large heat flux, resulting in decreases in the sensor temperature. However, at a large spacing over 10 nm, the heat transfer is dominated by the heat conduction in gas films. An increasing helium fraction causes an increase in the heat conduction and a consequent decrease in the sensor temperature.

Modeling, design and analysis of a micro flying slider in near-field optical storage system

A micro flying slider is developed to satisfy the demands for large capacity, high data density, and fast transfer rate of mass storage systems. The air-bearing surface of the micro flying slider is designed based on micro fluid theory of hydrodynamics to obtain positive and negative pressures, thereby enhancing flying height stability. The lubrication model is investigated based on the generalized Reynolds equation. The finite volume method is utilized to solve the revised Reynolds equation that governs the air-bearing pressure and flying system behavior. The dynamic model for the slider–disk interface of the micro flying slider is developed, and the air-bearing system is modeled as a 3-degrees-of-freedom, lumped-parameter model to simulate flying characteristics. The air-bearing pressures with flying attitudes in the loading/unloading processes are characterized and calculated using the proposed model, with flying height and pitch angle as the key variables. The sample data obtained via accurate triggering and superposed averaging are processed to acquire modal frequencies and damping ratios based on the parameter identification method of modal analysis technique. System experiments are presented to illustrate the effectiveness of the proposed micro flying slider and its dynamic model for flying-height control. Theoretical analysis and experimental results show that a stable storage working process can be obtained at 1600 rpm and a pitch angle of 1.5°. Moreover, the micro flying slider exhibits satisfactory flying stability, with a near-field working distance of less than 75 nm. Good correlation between the experimental and simulation results is achieved.

Effect of Disk Asperity Size on Contact-Induced Head Scratches and Reader Temperature of a Thermal Flying Height Control Slider–Disk Interface

The mechanical and thermal response of a thermal flying height control slider during contacts with a disk asperity is numerically studied. The effect of asperity height and asperity diameter on the scratch dimensions in the read/write shields is investigated, and the maximum temperature at the reader location is determined. The effect of the number of contact cycles is studied.

Effect of Interfacial Roughness on Slider-Disk Interactions at Near-Contact Regime

One of the major efforts in reducing fly height (FH) in order to achieve higher recording areal density for hard disk drive, is to reduce slider-disk interfacial roughness. However, as roughness improves to the atomically smooth level, various interaction forces such as van der Waals force, electrostatic force, and intermittent contact force affect slider fly stability. In this paper, an electrostatic force manipulation methodology was employed to investigate roughness effect on slider-disk interactions at near-contact regime. In such methodology, an AC excitation with pre-determined frequency was applied across the slider-disk interface. First harmonic vibration response of the excitation was monitored. Such vibration response is sensitive to various interaction forces, as FH is reduced to near-contact regime. The phenomena observed were verified with static and dynamic simulations, using self-developed ABsolution simulation software. It was found that disk with higher roughness has larger adhesive force gradient and hence larger first harmonic vibration at near-contact. First harmonic vibration dip was observed near touchdown point. Simulation result suggests that the phenomenon is due to time-averaging effect of slider intermittently “snap” to the disk surface. It was found that magnitude of the vibration dip is surface roughness dependent. At contact regime, nonlinear contact stiffness-based contact detection is highly sensitive for smooth surface. Abrupt transition of low to high vibration state of smooth surface during contact is attributed to larger adhesion hysteresis effect.

Low Molecular Weight Z-Tetraol Boundary Lubricant Films in Hard Disk Drives

Lower molecular weight Z-Tetraol films exhibit increased mechanical spacing in the slider-disk interface due to a lower z-profile. An increased resistance to lubricant disturbance on the disk surface (e.g., lube moguls) with decreasing film thickness is attributed to an increasing contribution from the polar component of the disjoining pressure. Evaporative loss at temperatures typically encountered in a hard-disk drive also increases with decreasing molecular weight but is strongly dependent on the initial bonded fraction.

Effect of Laser Heating Duration on Lubricant Depletion in Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording to further increase the areal recording density of hard disk drive media. However, HAMR brings about serious problems to slider-disk interface, such as lubricant depletion on disk surface at the high temperature in HAMR. Experimental studies of lubricant depletion under laser irradiation are still very limited so far. It is essential to do experimental studies under real HAMR conditions or under equivalent conditions when a standalone laser is used to emulate HAMR system. Laser heating duration in one heating and cooling process in HAMR is as short as 1 ns. It is believed that lubricant depletion under such short heating duration should be different from that under long time continuous heating. In this work, the effect of laser heating duration in one heating and cooling process on lubricant depletion is studied experimentally on a self-developed HAMR tester. Laser power, laser heating rate and laser heating duration of the tester are tunable and comparable with that of a HAMR head laser. The tester also supports macroscopic study. A method to control laser focus point on disk surface is also developed, which is the prerequisite to control laser heating temperature. From the experimental studies, it is found that longer laser heating duration results in more severe lubricant depletion. It is also found that lubricant depletion under laser heating is caused by both lubricant flow on disk surface and lubricant loss via evaporation/decomposition.

Transient Thermomechanical Contact of an Impacting Sphere on a Moving Flat

Contact between a slider and a magnetic recording disk is modeled as transient contact of a sphere on a moving flat. The sphere is assumed to be rigid, and the flat is treated as an elastic-plastic body with isotropic hardening. Heat generation is related to friction at the contact interface. Dimensionless solutions are obtained for maximum temperature rise, maximum contact force, maximum contact area, and maximum penetration as a function of dimensionless vertical initial velocity of the sphere. It is observed that transient thermomechanical contact with elastic-plastic deformation deviates from “classical theories” for dynamic elastic and quasi-static elastic-plastic contacts as the dimensionless vertical initial velocity of the sphere increases. The results are applied to optimize the slider-disk interface in a hard disk drive with respect to slider-disk contacts.

Numerical Solution of the Boltzmann Based Lubrication Equation for the Air-Bearing Interface Between a Skewed Slider and a Disk With Discrete Data Tracks

Discrete track recording (DTR) is a method for increasing the recording density of a data storage disk by use of a pattern arrangement of discrete tracks. The DTR track structure consists of a pattern of very narrow concentric raised areas and recessed areas underneath a magnetic recording layer. In order to design the air-bearing slider platform that houses the magnetic transducer for DTR application at very low fly heights, the influence of the disk surface topography as a surface roughness effect must be taken into account. This paper is focused on the numerical solution of the roughness averaged lubrication equation reported recently in the work of White (2010, “A Gas Lubrication Equation for High Knudsen Number Flows and Striated Rough Surfaces,” ASME J. Tribol., 132, p. 021701) and is specialized for the influence of discrete disk data tracks on the recording head slider-disk air-bearing interface subject to a nonzero skew angle formed between the slider longitudinal axis and the direction of disk motion. The generalized lubrication equation for a smooth surface bearing and appropriate for high Knudsen number analysis is quite nonlinear. And including the averaging process required for treatment of a nonsmooth disk surface, as well as the rotational transformation required to allow for a nonzero skew angle, increases further the nonlinearity and general complexity of the lubrication equation. Emphasis is placed on development of a numerical algorithm that is fast, accurate, and robust for air-bearing analysis of complex slider surfaces. The numerical solution procedure developed utilizes a time integration of the lubrication equation for both steady-state and dynamic analyses. The factored-implicit scheme, a form of the more general alternating-direction-implicit numerical approach, was chosen to deal with the two-dimensional and highly nonlinear aspects of the problem. Factoring produces tightly banded coefficient matrices and results in an algorithm that is second-order accurate in time while requiring only the solution of tridiagonal systems of linear equations in advancing the computation from one time level to the next. Numerical solutions are presented that demonstrate the performance of the computational scheme and illustrate the influence of some discrete track parameters on skewed air-bearing performance as compared with a flat surface data storage disk.

Transient slider-disk contacts in the presence of spherical contamination particles

Transient thermal–mechanical contacts between a slider and a rotating disk in the presence of contamination particles are investigated using finite element analysis. The maximum temperature rise due to frictional heating, the maximum plastically deformed area, and the maximum residual penetration into the disk surface are determined as a function of particle properties for aluminum and glass disk substrate. It is found that contact with soft and deformable particles can cause a large temperature rise, sufficient for thermal erasure, even though physical damage of the media is not observed. Contact with hard and rigid particles can lead to severe plastic deformation as well as high temperature rise of the media, representing the worst case for tribological integrity of the slider-disk interface.

A Gas Lubrication Equation for High Knudsen Number Flows and Striated Rough Surfaces

This paper describes the derivation and numerical solution of a lubrication equation appropriate for high Knudsen number flows and certain types of striated rough surfaces. The derivation begins with the compressible form of the lubrication equation together with the nonlinear series form of the Poiseuille flow reported by Fukui and Kaneko (1990, “A Database for Interpolation of Poiseuille Flow Rates for High Knudsen Number Lubrication Problems,” ASME J. Tribol., 112, pp. 78–83.). A multiple-scale analysis is performed on the lubrication equation for a finite-width time-dependent bearing and is limited to either stationary-transverse or longitudinal striated surface roughness of very short length scale. The rough surface averaging that takes place within the multiple-scale analysis includes a fully coupled treatment of the Poiseuille flow. What results is an especially nonlinear lubrication equation with averaged surface roughness effects that is appropriate for high Knudsen number analysis. A rotational transformation is also introduced to provide the roughness averaged lubrication equation in a form that allows analysis of the skewed orientation of a recording head slider with roughness defined relative to the direction of disk motion but with the lubrication equation conveniently expressed in the coordinate system of the slider. A factored-implicit numerical algorithm is described that provides the solution of the roughness averaged lubrication equation. Even though the lubrication equation is highly nonlinear, the numerical scheme is crafted to be fully second-order, time-accurate, and noniterative for tracking the solution in time either to an asymptotic steady-state or in response to a dynamic event. Numerical solutions of several simple geometry bearings are presented that utilize parameters that are typical of the slider-disk interface of current hard disk drives. It is anticipated that the primary benefit of this work may be the ability to accurately and efficiently include the influence of discrete disk data tracks in the air bearing design of very low clearance recording head sliders.

Numerical Simulation of the Head/Disk Interface for Patterned Media

The use of patterned media is a new approach proposed to extend the recording densities of hard disk drives beyond 1 Tb/in.2. Bit-patterned media (BPM) overcome the thermal stability problems of conventional media by using single-domain islands for each bit of recorded information, thereby eliminating the magnetic transition noise (Albrecht et al., Magnetic Recording on Patterned Media, 2003). Considering steady state conditions, we have transferred the pattern from the disk surface onto the slider surface and have investigated the pressure generation due to the bit pattern. To reduce the numerical complexity, we have generated the bit pattern only in the areas of the slider near the trailing edge, where the spacing is small. Cylindrical protrusions were modeled using very small mesh size on the order of nanometers to obtain the flying characteristics for the entire slider air bearing surface (ABS) using the “CMRR” finite element Reynolds equation simulator (Duwensee et al., Microsyst Technol, 2006; Wahl et al., STLE Tribol Trans, 39(1), 1996). The effect of pattern height, pattern diameter, slider skew angle, and slider pitch angle on flying height of a typical slider is investigated. Numerical results show that the flying height decreases for a patterned slider and the change in flying height is a function of the pattern height and ratio of the pattern diameter to the pattern pitch. In comparison to discrete track media, the flying height loss is larger for a patterned slider disk interface for the same recessed area of pattern.

Tribo-Charge of Diamond-Like Carbon (DLC) Contact at Slider Disk Interface and Tribo Current Flow Through Disk DLC

This paper reports an experimental investigation on tribo-charging of diamond-like carbon (DLC) at the slider-disk interface and tribo-current flow through disk DLC. Measurements of the tribo-current are in three regimes-slider flying and touch down during disk deceleration, slider dragging on disk surface, and slider takeoff and flying during disk acceleration. Comparisons are made for tribo-charge currents at different disk initial linear velocities. It is observed that the tribo-current appears earlier with larger amplitude for a disk with larger initial velocities. During disk constant velocity phase, tribo-current ripple was observed while the slider is dragging on the disk surface due to intermittent slider/disk interactions. Despite having the same constant velocity, the equilibrium level of the ripple is different and depends on the disk initial velocity. Additional tribo-voltage and acoustic emission measurements were performed and compared with the tribo-current measurement to provide more insight to the tribo-charging process.

Further Study of the Effect of Water Vapor on Slider Air Bearing

It has been proved that water vapor in moist air contributes to slider air-bearing pressure in a totally different way from dry air. It is a very good first order approximation to take a constant water vapor pressure inside and outside the interface. The simulated slider flying height and attitude in moist air are different from that in dry air and the higher the relative humidity the bigger the difference. This work is a further study of the effect of water vapor on slider air bearing. In this paper, the effect of slider air-bearing pressure on the saturated water vapor pressure is explained. The adsorbed water film thickness on slider and disk surfaces as a function of relative humidity at different temperatures is calculated. The effects of slider air-bearing pressure, temperature, slider-disk spacing, and relative humidity on the water vapor pressure in slider-disk interface are studied. An empirical equation is developed for the calculation of accurate water vapor pressure distribution in slider-disk interface.

A Theoretical Study of Vapor Lubrication for Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising technique to overcome the superparamagnetic limit to further increase the areal recording density of hard disk drives. However, HAMR brings about serious problems to the slider-disk interface, such as lubricant depletion on disk surface caused by laser heating. It is proposed to overcome the lubricant depletion problem by using vapor lubrication. The lubricant film formation process on disk surface in vapor lubrication is studied theoretically based on fundamental adsorption and desorption theories. The controlling parameters of lubricant film thickness and film formation time are identified. It is found that the lubricant film thickness is controlled mainly by lubricant vapor pressure and molecular weight. The film formation time can be shortened by using low molecular weight lubricant and high temperature lubricant vapor.

Lube Depletion Caused by Thermal-Desorption in Heat Assisted Magnetic Recording

Heat assisted magnetic recording is a promising technique to overcome the superparamagnetic limit. However, it brings about serious problems to the slider-disk interface. One of the problems is lube depletion on disk surface caused by laser heating. In this work, the process of lube depletion caused by lube desorption during laser heating is explained. The effects of lube molecular weight, laser heating temperature, slider air-bearing pressure, etc. on the lube depletion are studied. It is found that lube thinning rate at disk surface decreases dramatically with the increase in lube molecular weight. It is a possible solution to overcome the lube depletion problem by using high molecular weight lube. Lube thinning rate can also be decreased significantly by lowering laser heating temperature, lowering slider air-bearing pressure over the laser heating area, and lowering lube vapor pressure.

Air Bearing Interface Characteristics of Opposed Asymmetric Recording Head Sliders Flying on a 1 in. Titanium Foil Disk

The current effort was motivated by the increasing appearance of data storage devices in small portable and mobile product formats and the need for these devices to deliver high storage capacity, low power requirements, and increased ruggedness. In order to address these requirements, this work considered the storage device to utilize a 1 in. titanium foil disk and a pair of opposed femtosized zero-load recording head sliders with asymmetrically configured air bearing surfaces. A titanium foil disk, due to its reduced thickness and relatively low mass density, requires less operational energy than a hard disk while providing storage densities and data transfer rates typical of a hard disk. The zero-load sliders were chosen in order to make negligible the air bearing interface normal force acting on the disk surface that can lead to high speed disk instability. The asymmetry of the slider air bearing surfaces, together with the disk dynamic flexibility, greatly improves the ability of the slider-disk interface to absorb substantial mechanical shock and other dynamic effects without the associated contact and impact typically observed with a hard disk. The current project evaluated the characteristics of this slider-disk air bearing interface for both static and unsteady operating conditions. Time dependent studies included a numerical simulation of the dynamic load process and the response to mechanical shock. A comparison with the performance of a hard disk interface was also included.

Dominant Factors in Lubricant Transfer and Accumulation in Slider-Disk Interface

Lubricant transfer from disk to slider and lubricant accumulation on slider are very important in designing a stable slider-disk interface of ultra-low spacing. In this article, the effects of different parameters on the lubricant transfer and accumulation are studied and the reasons behind the effects are explained. Furthermore, the time for the lubricant transfer to reach steady state is estimated. It is found that lubricant molecular weight plays a dominant role in the lubricant transfer and accumulation. Lubricant transfer and accumulation decrease dramatically with the increase in lubricant molecular weight. Lubricant transfer also strongly depends on lubricant thickness and bonding ratio on disk surface. A thinner lubricant and higher lubricant bonding ratio on disk surface reduce lubricant transfer obviously, which results in less lubricant accumulation. A diamond-like-carbon (DLC) overcoat of low adsorption area density on slider surface can reduce lubricant transfer and accumulation, especially for lubricant of low molecular weight. Lubricant accumulation increases with disk velocity and increases slightly with the decrease in slider flying height. Lubricant accumulation can be reduced by minimizing the area of slider pad. Lubricant transfer and accumulation become worse at higher ambient temperature. It takes seconds for lubricant of low molecular weight to reach steady transferred thickness and hours for lubricant of high molecular weight to reach the steady state.

Contribution of water vapor to slider air-bearing pressure in hard disk drives

Water vapor in humid air contributes to slider air-bearing pressure in a totally different way from that of dry air. Water vapor pressure keeps constant in slider-disk interface and is identical to the water vapor pressure outside the interface. The simulated slider flying height and attitude in humid air are different from that in dry air and the higher the relative humidity the bigger the differences.

Evaluation on Applicability of Reynolds Equation for Squared Transverse Roughness Compared to CFD

A discrete probability distribution function is used to represent the squared transverse roughness effect in a modified Reynolds equation, and the Reynolds equation is used to calculate the hydrodynamic lubrication in a slider-disk interface compared to the CFD method. When the roughness height is below 1% of the film thickness, the results acquired by the two methods are the same and the surface roughness does not show obvious effect on the lubrication results compared to that on the smooth surface. The load capacity and friction force increase as the roughness height increases when the roughness height exceeds 1% of the film thickness. Moreover, the forces acquired by Reynolds equations are smaller than those acquired by CFD, and the difference between them exceeds 10% when the roughness height is higher than 10% of the film thickness. Sidewall effect is considered to be the main reason for the difference, and the Reynolds equation is believed not suitable for calculating the effect of the squared transverse roughness in the hydrodynamic lubrication.

Air Bearing Slider-Disk Interface for Single-Sided High Speed Recording on a Metal Foil Disk

There are disk-drive data storage applications best served by single-sided recording configurations. These include situations where (i) storage requirements can be achieved on a single side of a disk and (ii) dimensional constraints on the disk drive prohibit the presence of a recording head and its associated mounting device on each side of the disk. Even if dimensional requirements are not a concern, the most cost-effective and operationally efficient slider-disk air-bearing interface for single-sided recording is one that does not include an air-bearing slider, pressure pad, or other air-bearing structure on the nondata side of the disk. A metal foil disk offers some of the best characteristics of both the hard disk and floppy disk for digital data storage. It offers hard disk recording densities, increased shock resistance, reduced manufacturing cost, and requires less operational energy than a hard disk. However, use of a conventional recording head slider assembly without opposing air-bearing support for single-sided recording on a high-speed metal foil disk presents a fundamental problem because the air-bearing surface of the slider produces a net transverse force to the disk. This force causes the disk to deflect and can result in flying height and stability problems at the slider-disk interface. The current work describes an air-bearing interface for low flying height single-sided recording on a high-speed metal foil disk that minimizes disk deflection and instability without the presence of air-bearing components on opposing sides of the disk. The new interface utilizes a vacuum cavity-type air-bearing with little or no preload. Examples will be presented and discussed for the new interface that illustrate the flying characteristics of a picosized slider on a 1.8in. stainless steel disk with thickness of 25.4μm.