Slider Vibration Research Articles

Numerical investigation of slider dynamics induced by contacts with disk asperities

A time dependent Reynolds equation simulator combined with a finite element-based transient contact model between a slider and a disk asperity is used to study slider dynamics induced by contacts with disk asperities. The flying height change at the trailing edge of the slider is investigated as a function of asperity height, asperity diameter, and the spacing between the thermal protrusion of a thermal-flying control slider and a disk asperity. The effect of material properties of the disk asperities is studied. Slider vibrations corresponding to the first and the second pitch modes are excited by disk asperities.

Design theory and vibration characteristics of a contact head slider

In this study, the design theory of a previously proposed contact head slider was extended by considering a thermally protruding head slider and the intermolecular adhesive force between the head and disk surfaces. The waviness-excited vibration characteristics of the thermally protruding contact head slider were analyzed using a single-degree-of-freedom slider model, whose contact stiffness was calculated in accordance with the Johnson–Kendall–Roberts adhesive contact theory. It was found that, because of the adhesive force, the resonance frequency fr of the contact slider changed from zero to a value higher than the original second-pitch-mode resonance frequency with an increase in the head-penetration depth. Because the waviness-excited vibration of the contact slider is amplified at fr, the first- and second-pitch-mode vibrations of the thermally protruding slider can be excited when fr approaches those resonance frequencies. Because the friction force varies with the vibration of the contact slider, vibration modes of the slider-suspension system often observed at the beginning of contact can be explained. It is suggested that the region of the head-penetration depth for perfect contact sliding can be widened by increasing the effective contact damping and decreasing the disk waviness.

A-2-2 1自由度接触振動計算による軽荷重接触磁気記録の安定性の検討(ヘッド・ディスク・インターフェイス(2),口頭発表)

In recent years,the light contact recording is proposed in order to reduce the flying height between magnetic head and magnetic disk.But,it is difficult to stably read and write data,because the magnetic head is contacting with magnetic disk rotating at high speed.In this study,we studied the slider vibration in the contact by vibration using 1 degree of freedom model.And,we analytically investigated the effects of the adhesion force between the slider and disk to slider vibration,thickness and viscosity of lubricant layer and roughness of disk surface.As the results,low viscosity lubricant and optimum adhesion force are needed in order to realize the stable light contact recording.

Numerical Simulation of Touchdown Dynamics of Thermal Flying-Height Control Sliders

With the advent of thermal flying-height control (TFC) technology the physical spacing in magnetic hard disk drives is now reduced to sub-1 nm. At this spacing the tribological flyability and reliability become critical issues for the performance of the read-write head. In this paper a numerical approach is applied to study the touchdown dynamics of TFC sliders by expanding a 3-degree-of-freedom slider dynamics model to a 250-degree-of-freedom head-gimbal assembly dynamics model and considering several significant tribological effects at this ultra-low clearance (adhesion, tribo-charge, friction, contact, etc.). The simulation results show that the slider's vibration amplitude increases substantially at the beginning of touchdown, and it gets suppressed with further reduction in flying height. Analysis in the frequency domain shows the excitation of the second air-bearing pitch mode when instability occurs. The expansion of the dynamic system to include a realistic model of the suspension is shown to be important in determining the frequency of the excited mode. Adhesion force is shown to play an essential role in exciting the second air-bearing pitch mode and causing instability, while electrostatic force and friction force affect only some details of the slider's dynamics at instability. Friction force is also shown to be related to the excitation of the first air-bearing pitch mode.

Experiments on slider lubricant interactions and lubricant transfer using TFC sliders

Future magnetic storage density targets (>4 Tb/in.2) require subnanometer physical clearances that pose a tremendous challenge to the head disk interface (HDI) design. A detailed understanding of slider-lubricant interactions at small clearances and contact is important to not only address magnetic spacing calibration and long term HDI reliability but also to meet additional challenges imposed by future recording architectures such as heat assisted magnetic recording (HAMR). In this work, the behavior of the disk lubricant is investigated through controlled tests using TFC sliders which are actuated to proximity (i.e. backoff) and into contact (i.e. overpush) on one specific half of the disk per rotation by synchronization with the spindle index. Observations for lubricant distribution in contact tests (i.e. overpush) reveal an accumulation of lubricant on the disk near the onset of contact suggesting a migration of lubricant from the slider to the disk as the slider approaches the disk. Experiments also reveal that there is a similar deposition of lubricant even in the absence of contact for backoff tests. Furthermore, light contact tests result in significant lubricant rippling and depletion with associated slider dynamics. The lubricant rippling frequencies correlate well with the slider's vibration frequencies. Interestingly, strong overpush may lead to stable slider dynamics (for certain air bearing designs) that is also associated with noticeably lower lubricant distribution (compared to the light contact case), and the greatest lubricant changes are observed only at the onset and the end of contact. This paper reveals the complex nature of slider-lubricant interactions under near-contact and contact conditions, and it highlights the need for further studies on the topic to help design a HDI for recording architectures of the future.

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.

Experimental study on slider dynamics during touchdown by using thermal flying-height control

To investigate the possibility of further lowering the clearance in head–disk interface systems, slider dynamic behavior during a touchdown sequence with a thermal flying-height control (TFC) function was investigated by using a spinstand-level evaluation utilizing an acoustic emission (AE) sensor and a laser Doppler vibrometer (LDV). Experimental results demonstrated that off-track vibration was easier to excite by head–disk contact at the beginning of head–disk contact. We then confirmed that the amplitude of pitch-mode vibration in the flying-height direction increased and sway-mode vibration in the off-track direction decreased when increasing heater power during the touchdown sequence. Moreover, we found that the peak frequency of pitch-mode vibration shifted to a higher frequency under over-push conditions. Time–frequency domain analysis results showed that the peak shift occurred at several locations during a disk rotation. The mechanism of the peak shift is attributed to the increase in stiffness at the head–disk interface (HDI) due to solid–solid contact or mode change occurred in such regions. During the touchdown sequence, the friction force at the HDI continues to increase, even though slider vibration and AE signal decrease when heater power is increased. The friction force at the HDI needs to be decreased to achieve further low-clearance HDI.

DESIGN OF A NEW INSTRUMENT FOR MEASURING FRICTIONAL PROPERTIES OF WOVEN FABRICS

Friction measurem ent is one of the important and interesting problems in the study of handle properties of fabrics. Different methods have been developed for measuring friction, such as sliding, Kawabata Evaluation System (KES) and cyclic testing. In this research, considering the possibilities of varying conditions in measuring friction, a new instrument is used in which the bottom surface of fabric can be moved and the top slider is kept fixed in order to prevent slider vibrations. In this method, the force inserted by an extensible yarn connected to the trolley standing on four very smooth bearings was measured. The result of experiments confirmed 7.6% increase in correlation coefficient in the dynamic friction region for nine different test samples and about 4.6% reduction in coefficient of variation compared to the sliding type.

Experimental Study of Head-Disk Interface Instability on Light Contact Recording Using Dynamic Flying Height Control

The instability of the slider touchdown process was observed by the sensitive method that uses PZT ((Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 - x</sub> ) O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) sensor mounted on a suspension. By using this method, we could differentiate from the contact between the slider and the lubricant surface, the stable regime beyond touchdown (surfing regime), and the final full contact with the disk surface. Z-dol2000S clearly showed the surfing regime, but Z-tetraol2000S did not show the surfing regime on a thin lubricant film with a thickness of less than 16 Å. In the case of Z-dol, the magnitude of the slider vibration at the contact on the lubricant surface depended on the lubricant thickness; in the case of Z-tetraol, a large vibration was observed only at 20 Å. We have tried to explain these phenomena with the help of potential energy curves that are based on intermolecular forces calculated by molecular dynamic simulations. Potential energy curves could be used to explain the stability of the touchdown process.

Head and Media Instantaneous Contact Friction Measurement and Glide Test

We have developed an in situ head disk friction measurement system using a laser Doppler vibrometer (LDV). With the high stiffness of the system and the high sampling rate of the LDV, the temporal resolution of the measurement is improved by 40 compared with the conventional strain gage apparatus. We estimate the instantaneous head disk contact friction to be as much as 8 of what would be otherwise measured with the conventional method. We present for the first time a clear evidence of head disk contact adhesion with the fast setup. Simultaneous piezo transducer (PZT) glide signal and friction measurement of the slider asperity contact show strong correlation between the two. Comparison of energy demonstrates that the mechanism of the glide test is a slider scratching event by the asperity, rather than the so called “collision” between the asperity and the head. The PZT signal comes from the friction-induced slider vibration after contact with the asperity.

Numerical Investigation of Bouncing Vibrations of an Air Bearing Slider in Near or Partial Contact

Near or partial contact sliders are designed for the areal recording density of 1 Tbit/in.2 or even higher in hard disk drives. The bouncing vibration of an air bearing-slider in near or partial contact with the disk is numerically analyzed using three different nonlinear slider dynamics models. In these three models, the air bearing with contact is modeled either by using the generalized Reynolds equation modified with the Fukui–Kaneko slip correction and a recent second order slip correction for the contact situation, or using nonlinear springs to represent the air bearing. The contact and adhesion between the slider and the disk are considered either through an elastic contact model and an improved intermolecular adhesion model, respectively, or using an Ono–Yamane multi-asperity contact and adhesion model (2007, “Improved Analysis of Unstable Bouncing Vibration and Stabilizing Design of Flying Head Slider in Near-Contact Region,” ASME J. Tribol., 129, pp. 65–74.). The contact friction is calculated by using Coulomb’s law and the contact force. The simulation results from all of these models show that the slider’s bouncing vibration occurs as a forced vibration caused by the moving microwaviness and roughness on the disk surface. The disk surface microwaviness and roughness, which move into the head disk interface as the disk rotates, excite the bouncing vibration of the partial contact slider. The contact, adhesion, and friction between the slider and the disk do not directly cause a bouncing vibration in the absence of disk microwaviness or roughness.

Flow-Induced Slider Vibration in a Functional Hard Disk Drive: Influence of Air Shroud

An air shroud is designed and placed within a commercially available hard disk drive operating at 15 000 rpm. Large eddy simulations of the turbulent airflow characteristics resulting from both the models, with and without air shroud, are carried out by assuming the read/write head is at the disk middle diameter. The numerical model consists of about nine million tetrahedral cells with the largest and smallest sizes of the cell volumes being 2.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 0.35 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. The dynamic Smagorinsky-Lily model is employed to mimic the effect of small-scale eddies in the turbulent airflow. Numerically predicted airflow characteristics are compared against the LDA measurements and found to be in good agreement. The wind blowing on the surfaces of the head gimbals assembly is converted into aerodynamic forces and the resulting slider displacement in the off-track and out-of-plane directions are investigated. It is inferred that the disk drive model with air shroud results in a maximum of 44% and 10% less off-track and out-of-plane slider displacements, respectively.

Erratum: “Experimental and Theoretical Investigation of Bouncing Vibrations of a Flying Head Slider in the Near-Contact Region” [Journal of Tribology, 2007, 129(2), pp. 246–255

Erratum: “Experimental and Theoretical Investigation of Bouncing Vibrations of a Flying Head Slider in the Near-Contact Region” [Journal of Tribology, 2007, 129(2), pp. 246–255

Air Bearing Design to Prevent Reverse Flow from the Trailing Edge of the Slider

A simulation approach that relies on an analysis of the flow patterns closest to an air bearing surface (ABS) was used to predict the lubricant accumulation on the ABS of a head slider. The lubricant accumulation patterns obtained through the simulation were in good agreement with experimental results and with our experimental apparatus. We used this method to study and analyze flow pattern droplets close to the trailing edge of a number of sliders and found that there was a reverse flow from the slider’s trailing edge on both sides of the trailing pad and behind the read/write element, which could result in a lubricant accumulation on the slider surface close to the trailing edge of a slider and thus lead a transient slider vibration and magnetic-signal loss in a hard disk drive. Further simulations and analyses revealed that the reverse flow is dependent on the depth of slider surface on adjacent to the trailing edge of the slider, and that if the depth is less than a critical depth, which is dependent on the velocity of the disk, the reverse flow could be eliminated. On the basis of these findings, we propose a new ABS design concept for effectively suppressing the reverse flow of lubricants from the trailing edge of the slider. In this concept, the slider has a “smooth flow pad” and the depths of outlet recesses are specified as being smaller than the critical depth. It was confirmed by both simulation and experiment that lube accumulation on the slider surface is obviously decreased and the reliability of a hard disk drive with this air bearing design is consequently improved.

Six-DOF vibrations of partial contact sliders in hard disk drives

A six-degree-of-freedom slider dynamic simulator is developed to analyze the slider’s motion in the vertical, pitch, roll, yaw, length and width directions. The modified time-dependent Reynolds equation is used to model the air bearing and a new second order slip model is used for a bounded contact air bearing pressure. The simulator considers the air bearing shear acting on the air bearing surface and the slider–disk contact and adhesion. Simulation results are analyzed for the effects of the disk surface micro-waviness and roughness, skew angle, slider–disk friction and micro-trailing pad width on the vertical bouncing, down-track and off-track vibrations of a micro-trailing pad partial contact slider.

Effect of thermal pole tip protrusion and disk roughness on slider disk contacts

Ultra-high areal density for hard disk drives requires a stable head disk interface at a flying height lower than 8 nm. At such a low flying height, small flying height variations may cause slider/disk contacts. Slider/disk contacts can also occur when a write-current is applied to the write coil since the flying height between slider and disk can be affected by the thermal expansion of the pole tip. In this paper, we investigate the vibration characteristics of sliders during thermally induced contacts using laser Doppler vibrometry. We perform a parametric study of contact events using disks with different surface roughness and lubricant thicknesses, and analyze the slider motion statistically. For a given write current, we observe that the slider vibrations increase with disk roughness and lubricant thickness.

Effect of environment humidity and temperature on stationary and transient flying responses of air bearing slider

It is well known that the environment humidity and temperature have a significant influence on the flying height of an air bearing slider. However, not many research papers address this topic, especially when the transient flying response is considered. This paper studies the influences of the environment humidity and temperature on both the stationary and transient flying responses of slider by simulation. A slider design for the thermal protrusion application is addressed. The reason for causing the drop of the air bearing pressure is discussed, and the methods for decreasing the drop are proposed. It is observed that the environment humidity and temperature may determine whether the slider is in full flying state or in partial flying/partial dragging state, when the slider is released from a certain height. The reason may be due to the high humidity and temperature which weakens the air bearing. As a result, the air bearing becomes not strong enough to support well the full flying of slider when the influence of the intermolecular force is significant. Slider vibrations for the full flying case and the partial flying/partial dragging case are analyzed in frequency domain, and the slider vibration frequencies are discussed. It shows that the environment temperature and humidity have significant effects on both the stationary and transient flying responses of the slider.

Thermal effect on slider flight height in heat assisted magnetic recording

In this paper, the real time thermal effect of the heat assisted magnetic recording (HAMR) on the slider flying height is investigated with HAMR spin stand. In the experiment, a modulated laser beam is used to heat the media and the slider vibration is measured with laser Doppler vibrometer. The slider vibrations at different laser modulation frequencies and different laser powers are investigated and near 0.2nm flying height change is observed. Analysis shows that the main reason to cause the flying height change results from the disk surface thermal distortion caused by thermal expansion.

A Model of Adhesion Excited Micro-vibrations

Understanding the mechanism of “rubbing” noise and low-amplitude friction exited vibration generation in steady sliding can be helped by models describing the contact interactions. In the current article, we consider a simple microscopic contact model for surfaces in sliding, which is based on the adhesion theory of friction. In the proposed model, we consider that the formation and shearing of a junction contributes to a small change in the real contact area. The model incorporates random size and random spacing between junctions. We investigate the dependence of the instantaneous real contact area on the average size and number of junctions. We find that from the viewpoint of vibration reduction, it is advantageous if the real contact area needed to support the given load is obtained as a sum of many small-sized micro-contacts, instead of few large-sized micro-contacts. The above result is in agreement with experimentally observed reduction of vibrations of a hard-disk slider after texturing.

Optimal Slider-Disk Surface Topography for Head-Disk Interface Stability in Hard Disk Drives

In order to achieve an areal density of 1 Tb/in2 and beyond, not only the mechanical spacing between the slider and the disk but also the track misregistration (TMR) and the fly height modulation (FHM) should be reduced below current levels. But at reduced mechanical spacing there will be elevated excitation due to slider-disk contacts resulting in increased slider vibrations and head-disk interface (HDI) failures. Thus, there is a need to study the effect of slider-disk topography on the slider dynamics and stability at a higher level of complexity. In this paper, we do so by dividing the slider-disk surface features into three regimes based on the amplitude and the wavelength range of the features. Further, we have also proposed several ways of achieving an optimal slider-disk topography that can help reduce the slider vibrations and increase the stability of the HDI.