Advances in Information Storage Systems

The following sections are included:

• Preface

Alternate disk substrates are being developed for gigabit recording devices. Primary interest for alternate substrates stems from the concern of deformation related damage caused by head-disk contact (head slap) from external mechanical shock in portable computers. Glass and glass-ceramics are being considered because of their high rigidity, dent resistance, smoothness and high maximum use temperature. Chemically-strengthened glass substrates are now used in 65-mm diameter disk drives for portable computers. Glass-ceramics are seriously being considered for future applications. Comprehensive micromechanical data of various substrates does not exist. Paper by Bhushan et al. presents results of micromechanical characterization of currently used Ni-P and chemically-strengthened glass and of two different types of glass-ceramics which are potential candidates for future disks.

Chemically-strengthened glass and glass-ceramics are beginning to be used as substrate materials for magnetic thin-film rigid disks because of their superior rigidity, dent resistance, smoothness and high maximum use temperature with respect to commonly used Ni-P coated aluminum-magnesium (Al-Mg) substrates. Elastic-plastic deformation behavior, hardness and Young's modulus of elasticity of various glass and glass-ceramic substrates were measured using a depth-sensing nanoindenter. The damage mechanisms were studied during microscratching using a scratching option in the nanoindenter and during Vickers indentations at relatively high loads using a microindenter. Based on the length of the cracks produced during Vickers indentation, fracture toughness was obtained. Hardness, Young's modulus, scratch resistance and fracture toughness of glass-ceramics are found to be a function of composition, phases and grain size. The mechanical properties of glass-ceramics are superior to those of chemically-strengthened glasses.

Gigabit recording, i.e. areal densities equal to or greater than 1 Gb/in², in rigid disk drives will require advances in both disk and head design and an upgrading of overall system performance. Flying heights (the mean separation between magnetic head slider and magnetic disk) used in current disk drives range from 50 to 100 nm. Flying heights will have to be reduced. As the flying height becomes smaller, the glide height (a flying height at which no physical contact between the asperities of head and disk surfaces occurs) becomes a large fraction of the desired flying height. Therefore, at low flying heights, a reasonable value of “clearance” with no physical contact (equal to the flying height minus the glide height) becomes critical for a durable interface. Goals for future drives with areal density of 10 Gbit/in² are to have flying height of about 15 nm and disk glide height of 10 nm (estimated clearance approximately 5 nm), half magnetic-coating thickness of about 6 nm, lubricant film thickness of about 0.5-0.7 nm and hard overcoat thickness of about 5 nm, and pole-tip recession of about 2 nm. The rms roughness of the disks also need to be decreased from 4-6 nm to about 1 nm. To minimize friction and wear, the slider size needs to be reduced from 2.0 × 1.6 mm² to about 100 × 100 μm² with a reduction in suspension load from 3 g to about 100 mg. To achieve adequate durability and low stiction with supersmooth and marginally lubricated disks with ultra-thin overcoats is not an easy task. This provides many opportunities and challenges for mechanics and tribology researchers. Papers in the Part Two address important issues relevant to mechanics and tribology of magnetic disk drives as well as magnetic tape drives…

With the current evolution of computer technology, the demand for higher data storage capacity pushes for significant contact recording improvements in the design and development of new rigid disk drives. A challenge to this approach is the assurance of durability at the head disk interface. In this study, we will focus on the opportunities and challenges of near contact recording and their influence on media and head designs. This review starts with a summary of surface techniques that aim at the reduction of stiction on smooth disk surfaces. Included among them are zone textures made either by mechanical texture processes or by photolithography techniques, and sputter-induced texture on smooth alternate substrates. The application of these techniques and their correlation are illustrated in glide height and durability (ambient and environmental) performance and contamination studies from seek tests. Reliability testing results with recently proposed proximity recording inductive heads, tripad and negative pressure designs, and the sensitivity to the issue of contamination due to sliding contacts of each head design are presented. Finally, results of carbon coated heads leading to order of magnitude improvement in durability are presented. Emphasis will also be made to the added advantages of carbon overcoat to allow heads flying lower for improving magnetic recording performance in near contact recording.

Beam-type contact heads, as a simplified model for investigating the head behavior during contact, were fabricated out of silicon by an anisotropic etching technique. An experimental setup with an actuator, which could control the normal load with an accuracy of 10 μN, was also developed. While lowering the head toward the disk, the instant of touching (the reference point of zero-load) can be detected by the disappearance of the first resonant peak of the beam. As long as the normal load is less than the hydrodynamic force, the head follows the perpendicular runout of the disk without contact. Once the load exceeds the hydrodynamic force, the head starts sliding on the disk. The sliding can be detected by observing higher resonant peaks of the beam excited by the sliding. Negative hydrodynamic force effect was also investigated.

The transient flying characteristics of complex shaped positive and negative pressure sliders over rough surfaces were simulated. A numerical finite-element algorithm incorporating a generalized form of the Reynolds equation, which is based upon the linearized Boltzmann equation, was used to analyze slider behavior over arbitrarily shaped complex surface topography. Spectral analysis was introduced to explore the transient behavior of sliders in the frequency domain. Fast Fourier transformations were generated for the dynamic response of trailing edge flying height, pitch, and roll of the sliders while flying over three-dimensional surface defects and asperities. A dynamic stability analysis of the flying characteristics was also performed for several slider sizes, air bearing surface shapes, and air inflow velocities.

A scaling theory, based on an order of magnitude analysis of the load bearing capacity, is presented and used to compare differently shaped slider designs and the miniaturization of these designs. Shaped rail bow-tie sliders are introduced to determine the effect of slider geometry on the scaling equation. An effective area concept is investigated in order to determine upper and lower bounds for slider miniaturization for the bow-tie sliders. The effective area is the portion of the total air bearing surface area which contributes significantly to the load bearing capacity. A scaling equation has also been developed for negative pressure sliders. The equation accurately replicates simulation results for a reverse-step slider.

The stiffness and damping characteristics of pocket-slider bearings are investigated numerically and experimentally in order to maximize the bearing load and estimate the dependence of the bearing damping coefficient on spacing. Using a numerical solution of the 2-D Reynolds lubrication equation, the optimum slider geometry was determined for both a trapezoidal and rectangular pocket. The predictions were verified experimentally. The nonlinear damping characteristics of a trapezoidal pocket slider are estimated numerically for a range of film thicknesses, with measurement being difficult for low spacing.

To analyze the interface phenomena of magnetic recording, a modified Reynolds equation is usually used as a lubrication equation. The lubrication equation is coupled with a deformation equation for the magnetic media. When we use Reynolds equation, we assume that a lubricant does not separate from the surface of a magnetic media, head, and drum. If a gap between the magnetic media and drum increases steeply, the lubricant separates from the boundary. In an inlet and outlet region where the tape is loaded and unloaded on the drum, the floating height of the tape increases steeply. Thus we should consider the separated condition of flow in analyzing the interface phenomena of VCR.

In this study, we extended our numerical method to analyze the interface phenomena of the magnetic tape to be able to consider the separated condition of the air film in the inlet and outlet regions. Therefore, the calculated result was improved.

Currently, many computer hard-disk drive companies and spindle motor manufacturers are looking for a substitute for ball bearings to fulfill the dramatic progress in the increasing capacity of data-storage system. Herringbone and/or spiral groove liquid-lubricated bearings (SGB) have attracted considerable attention. However, the bearing friction of the SGB is much higher than the ball bearing. In this paper bearing torque of both SGB and ball bearing spindle motors were measured at different environmental temperatures, rotating speeds, and static loads. In addition, the bearing cooling path design was discussed for the SGB spindle motor.

We describe a numerical method for the analysis of dynamic characteristics of coupled herringbone-type journal and thrust hydrodynamic bearings. The non-dimensional generalized Reynolds equation is discretized on a non-orthogonal grid which is mapped into a square. The computational domain conforms to the herringbone grooves to improve the accuracy of the solution. The journal and thrust regions are mapped separately and connected through internal flux boundary conditions. The discretized pressure field is solved iteratively using the rapidly convergent ADI method. The stiffness and damping coefficients are obtained by perturbing the equilibrium solution of the Reynolds equation and solving the perturbation equations. The accuracy of the present calculation is confirmed by comparing with previously existing data. Analyses are performed for self-contained coupled hydrodynamic bearing systems which can be used to support the spindle motor of a magnetic hard-disk drive.

Disk flatness has become an incresingly important parameter as lower flying height of sliders is required to achieve higher recording density. In this paper, the role of disk axial runout and disk curvature was examined. It was found that gravitational sag was significant in the measurement of disk axial runout, especially for thin disks. The disk axial runout was found to have poor correlation with flying height fluctuation. On the contrary, disk curvature has significant influence on flying height fluctuation. The disk curvature was defined as a two-dimensional tensor and calculated from disk-height information based on actual disk drive configuration. Effect of clamping distortion on disk curvature was studied. Good correlation between disk crown and flying height fluctuation was experimentally verified.

In this study, we propose to use a new parameter, the OD rolloff point, as a disk OD rolloff measurement, as compared with the conventional OD rolloff parameter, the dub-off value. Four types of disks with typical OD geometry were used to test glide performance. Experimental results showed that the conventional dub-off values of the four types of disks had poor correlation with glide performance of disk surfaces at OD region. On the contrary, good correlation was obtained between the glide performance and the newly defined rolloff point. It was found furthermore that the changes in minimum flying height of glide heads are directly proportional to the slope change of the disk surface at OD region. Finally implications to flying height budget design of head/disk interface at OD region were discussed based on the changes in gap flying height and changes in the minimum flying height of the heads as a function of the new parameter.

Cyclic phosphazenes are being developed as advanced lubricants for both rigid and flexible thin film magnetic media. Contact Start-Stop (CSS) test results with X-1P, a developmental cyclic phosphazene lubricant from The Dow Chemical Company, show it to have similar tribological performance to a standard perfluoropolyalkyl ether (PFPE), Fomblin® Z-DOL, as a topical lubricant on carbon-overcoated thin film rigid disks under ambient conditions. However, X-1P significantly outperforms Z-DOL in CSS testing under hot/wet (30°C, 80% RH) conditions. In addition to its CSS test performance, X-1P was found to be stable when heated in the presence of either water or a standard slider material, Al₂O₃-TiC. Results are presented which show the correlation between X-1P film thickness, obtained using x-ray photoelectron spectroscopy, and infrared absorbance.

In this study, the role of CaTiO₃ sliders in head-disk sliding contact was examined with contact start-stop tests. The sliders were characterized with respect to composition, hardness, and surface texture. Examination of the stiction data, the sliders, and the disks after the CSS test showed that disk wear takes place in two stages. Stage one was gradual two-body abrasive wear of the disk's carbon overcoat. Here there was friction buildup and transfer of carbon from the disk to the slider. Stage two was rapid three-body wear of the magnetic layer after wear out of the carbon overcoat. In this case, there was erratic friction and generation of many free particles. Stiction in stage one was dependent on the slope of surface features protruding from the slider. A simple model for wear-induced stiction agreed with this data.

This research addresses a nondestructive, noncontact technique to measure electric field signals associated with the surface wear of magnetic hard disk owing to the inadvertent Contact with the head. The vibrating capacitor (Kelvin probe) measures the Contact Potential Difference (CPD) which is used as a parameter for detecting wear damage on a magnetic hard disk subjected to a pin-on-disk sliding test. Three modes of acquiring wear-induced CPD changes were conducted. In Mode 1, a CPD scan along the wear track position, before and after the tribological test, was performed by rotating the disk at 1 rpm. Mode 2 also involved a pre-wear and a post-wear CPD scan, except that the disk was rotated such that the track moved with a speed of 600 cm/s. Mode 3 involved measurement of the CPD during the wear test as the disk rotated at 600 cm/s. Modes 1 and 2 produced identical ΔCPD for a given wear track width, ranging from 10 to 30 mV. Mode 3 resulted in a higher ΔCPD (85 mV) which is attributed to triboelectrification effect during sliding contact.

The role of adhesive outgassing on disk drive performance was studied because of its importance in pseudocontact and contact recording. Here, disk drive CSS tests showed that disk wear increased linearly with the amount of uncured adhesive in the drive. To identify outgassing species, purge and trap techniques used in conjunction with GC/MS were investigated to collect outgassed materials from both disk drives and their components. When the purge and trap techniques were used with a DB-FFAP column, there was better separation between adhesive outgassing materials and interfering grease components. The GC/MS analysis of disk drive outgassing materials clearly differentiated between the volatile and the potentially more harmful semi-volatile species. Adhesive components were easily detected in outgas tests on both motors and actuators.

Linear actuator bearings using ball rollers must be optimized for minimal wear and yet not be so compliant as to contribute to nonlinearities in the disk drive's servo system. A matrix experiment was completed which determined how actuator bearing parameters of axial play and free angel affect wear and airborne particle count. These two parameters have the ability to accommodate the actuator's manufacturing and assembly inaccuracies known as toe and camber angles.

Fifteen linear actuators were assembled with bearings of each type and life tested 150 million seeks. During seeking, airborne particle counts were continuously measured. At completion of the test, the bearing wear scar width and rail scar depth were measured.

Wear was found to correlate strongly bearing free angel and axial play. Particle count correlated strongly to free angle, toe angle, and wear scar width. This analysis reveals a ball bearing design with 20 to 28 μm of radial play and a 60% raceway curvature is ideally suited for minimizing contamination form wear in linear actuators.

Corrosion of hardened martensitic stainless steels used in magnetic disk drives is problematic because the rusting surface becomes a source of particulate contamination that can compromise the operational performance and reliability of the file. Corrosion of the stainless steels can be initiated by either atmospheric moisture or during aqueous based cleaning of the parts that typically occurs during their manufacturing steps or prior to file build. Aqueous based cleaning is commonplace since CFC cleaning solvents are losing favor because they are expensive and are discouraged by the EPA for their environmentally “unfriendly” nature.

Passivation of the stainless stees can be accomplished by thermal treatment or chemical means, using nitric acid. Four techniques (1 thermal and 3 chemical) were evaluated in this study on two base materials, 440C and X105CrMo17. Natural passivation initiated by abrasive polishing was also tested. Corrosion tests were performed on specimens made of 440C and X105CrMo17 rails, the martensitic type, using electropotential in a basic electrolyte and electropotential in an acidic electrolyte.

To understand the importance of metallurgical, physical and chemical variables and how they could influence the rail's corrosion resistance, three measurements were taken:

1. Grain size and carbide distribution using EDS.

2. Surface morphology.

3. Auger electron spectroscopy to determine the iron/chrome ratio and the chrome oxide layer thickness.

When properly done, thermal and chemical passivation are excellent methods to make martensitic stainless steels corrosion resistant and water cleanable. Natural passivation when initiated by abrasive polishing does not perform well. Passivation is imparted to the stainless steel by the high chromium/low iron content at the surface and by a thin, chrome oxide layer at the surface.

Three types of 12.7-mm wide metal particle tapes were studied. One of the tapes was calendered, whereas the other two tapes were additionally burnished one or two times using a proprietary process. Each type of tape was studied after 100 passes in the BetaCam SP drive and also in the virgin (0 pass) state. It was reported that in the case of the unburnished tape, head wear was high in the first pass and decreased during use. However, head wear for the double burnished tape was low for the first pass and increased during use. Whereas in the single burnished tape, head wear was low in the first pass and remained low. The objective of this study is to understand the mechanisms for loss and growth of head wear by correlating the surface characteristics of the tapes to head wear rate and to determine the changes in the surface characteristics occurring from 0 to 100 passes in the drive. It was found that summit density, mean and rms summit height, and mean and rms summit curvature correlate well to the head wear data. During manufacture and use in the drive, as the summit density, mean and rms summit height, and mean and rms summit curvature decrease, the head wear rate decreases. The mechanism for head wear is the initial ploughing of dense, sharp and high tape asperities into the surface of the head material resulting in a high head wear rate and a high coefficient of friction. Double burnishing during manufacturing removes high asperities, thus making the tape very smooth and possibly results in high adhesion and head wear growth with use. Chemical changes of the tape surface during double burnishing (not part of this study) also may be responsible for head wear growth during use.

The astounding trend of increasing track densities for hard disk drives has continued since the last publication of AISS. In 1996, several disk drive companies will be announcing products with track densities in excess of 6500 tracks per inch (260 tracks per millimeter). As track densities continue to increase, the dynamics and mechanics issues, which have traditionally vexed engineers, will continue to challenge them. Furthermore, dynamics which could formerly be neglected must now be accounted for. This volume of AISS contains papers addressing analysis, control, and design related to both old and new problems in dynamics and control…

Excessive rocking motion of the spindle motor system can cause track misregistration resulting in poor throughput or even drive failure. The chance of excessive disk stack rocking increases as a result of decreasing torsional stiffness of spindle motor bearing system due to the market demand for low profile hard drives. As the track density increases and the vibration specification becomes increasingly stringent, rocking motion of a spindle motor system deserves even more attention and has become a primary challenge for a spindle motor system designer. Lack of understanding of the rocking phenomenon combined with misleading paradox has presented a great difficulty in the effort of avoiding the rocking motion in the hard-disk drive industry. This paper aims to provide fundamental understanding of the rocking phenomenon of a rotating spindle motor system, to clarify the paradox in disk-drive industry and to provide a design guide to an optimized spindle system. This paper, theoretically and experimentally, covers a few important areas of industrial interest including the prediction of rocking natural frequencies and mode shape of a rotating spindle, free vibration, and frequency response under common forcing functions such as rotating and fixed-plane forcing functions. The theory presented here meets with agreeable experimental observation.

The Restoring-Force surface Method (RFM) has been applied to characterize the nonlinear track-follow dynamics of the ball bearing pivot within the rotary actuator of a rigid disk drive. RFM allows rapid data acquistion and visualization for comparing the performance of alternative pivots. By non-dimensionalizing data, pivot designs can be compared within actuators having different physical characteristics (e.g. size, inertia, and control torque). Previous testing using swept-sine measurements showed the existence of an amplitude-dependent “low-frequency pole”, a phenomenon observed as the pivot's ball bearings undergo small, pre-rolling motion. RFM and dimensional analysis are useful tools for representing the low-frequency pole in both phase space and the frequency domain. Use of these tools has improved the ability to evaluate effects of physical properties and design tradeoffs (e.g. pivot geometry, bearing pre-load, lubricant, and actuator inertia).

This paper presents experimental results on repeatable runout compensation of PERM (Pre-Embossed Rigid Magnetic) media. PERM disks have a permanently embossed embedded servo sectors and discrete magnetic tracks on plastic-based media. The disc drive based on PERM disks has the potential to improve storage capacity, simplify the manufacturing process, and reduce overall manufacturing cost. However, these disks provide new technical challenges. One of the prominent problems is Repeatable RunOuts (RRO) which are mainly due to inherent eccentricity caused by the discrepancy between the true center of a PERM disk and the center of a spindle motor. In order to compensate these RRO, a harmonic signal generator and a piezoelectric actuator were built and installed on a spin-stand. It has been demonstrated that the primary components of RRO were effectively removed.

Access speed is an important factor in the head positioning of a rigid disk drive, and so it is necessary to shorten the track-seeking operation time and the settling motion time. We investigate a new servo method that we call Unity Mode Access Control. Since it does not change the control modes during track-seeking and track-following operations, the settling motion is smooth. Furthermore, the feedforward-type two-degree-of-freedom control is applied, and the target trajectories are adjusted to the seek stroke. Therefore, an actuator performs fully for any stroke, and in particular, the proximate maximum acceleration is available for long-stroke seeking. Access time is shorter by more than 1.2 ms by experiment. We also discuss the design and implementation of the controller.

A method for velocity estimation and control for systems with only incremental position measurements (e.g. motor/encoder systems) is presented that is appropriate for a wide velocity range. In order to maintain good performance at very low velocities, where Coulomb friction becomes more significant, a multirate estimation and control scheme, which incorporates sliding mode control, is proposed. The method will be compared to and further combined with other schemes such as one for high velocities based on counting encoder pulses between sampling instants and one for low velocities based on measuring time between encoder pulses. Superior velocity tracking control over a wide operating range is demonstrated through implementation of the algorithm on the capstan motor of a digital tape drive. Since the algorithm relies significantly on the system model, the algorithm is shown to be robust to parameter uncertainty with the inclusion of an adaptive element.

The voice coil motor for the actuator in disk drives is one of the main components controlling the dimensions and performance of the drive. Therefore, optimum design of the VCM is of significant importance in achieving maximum performance within the tight-space restrictions of modern small drives. Recent software developments allow practical and accurate formal optimization of the highly nonlinear problems associated with VCM magnetics. This paper presents a case study of the application of optimization to the design of a 2 1/2” drive. The optimization is performed in two separate parts, first for the magnetic circuit, and second for the coil itself. For the former, the 3D finite-element magnetic analysis capabilities of the ANSYS program is used. These capabilities are flexible enough to allow optimizing for objective functions that can be calculated by analysis, and subject to any geometric or response constraints. In this case, the design is optimized to maximize the flux density in the air gap, while maintaining flux leakage at the drive envelope to acceptable specification limits. Results presented include magnetic flux plots in both poles, as well as computed torque constant drop-off variation for different pivot positions, and comparisons to test measurements. In the second part, the coil parameters are optimized by varying the wire diameter and number of turns, while maintaining pivot balance and accounting for the change in inertia. Results show the relationships between wire gage, power consumption, and seek time.

An improved means of mounting computer hard drives is outlined based on a simple, cost-effective design that utilizes a conformal surface to provide the high thermal performance of direct conduction in a package that can conform to different printed circuit-board shapes and sizes without the mechanical complexity that has been previously necessary. By combining this approach with advanced vibration analysis and ergonometric design, the mounting facilitates ease of installation while providing an optimized platform for servo performance of the drive. Test results are presented demonstrating improved seeking and thermal performance in this configuration.

A new mathematical model of the vibro-acoustic characteristics of a computer hard-disk drive is presented in this paper. In particular, a mobility transfer function is defined that links sound radiated by a stationary or rotating disk to electromagnetic torque pulsations and structural dynamics. A simplified disk-drive system consisting of a brushless d.c. motor driving a single disk-spindle assembly, which is mounted on a flexible casing, is considered as the example case. Parametric studies illustrate the roles of bearing stiffness and disk geometry on the vibration and radiated sound.

To achieve higher areal magnetic recording density as well as an ultralow flying head/disk interface, the authors propose micromachined silicon dual negative pressure slider bearing with an integrated microsuspension mechanism, which is a kind of mother-ship slider. This head device comprises a primary negative pressure nanoslider, that implements submicron spacing and establishes a stable platform, a low-mass 200-μm secondary negative pressure slider that flies at very low spacing, and a flexible integral microsuspension. These mechanisms operate under completely noncontact start-stop mode conditions with magnetic disk media based on dual negative pressure slider self-loading. The shape and the dimensions of both sliders and microsuspension were designed, considering the silicon micromachining of small form factor slider and suspension assembly in order to simplify etching process. Self-loading dynamics for dual negative pressure sliders as well as integrated microsuspension dynamics were also evaluated by computer simulation. Furthermore, the proposed slider mechanisms were fabricated with three anisotropic wet etching processes and experimental research were carried out on a glass disk. It was confirmed that dual negative pressure sliders was self-loaded at about 12 m/s disk surface velocity and had stable flying characteristics.

The following sections are included:

• Author Index

• Subject Index