Intermolecular force and surface roughness models for air bearing simulations for sub-5 nm flying height sliders

When the spacing between the slider and the disk is less than 5 nm, the intermolecular forces between the two solid surfaces can no longer be assumed to be zero. The model proposed by Wu and Bogy (ASME J Trib 124:562–567, 2002) can be view as a flat slider–disk intermolecular force model. The contact distance between the slider and disk needs to be considered in this model when the slider-disk spacing is in the contact regime. To get more accurate intermolecular force effects on the head disk interface, the slider and disk surface roughness need to be considered, when the flying height is comparable to the surface RMS roughness value or when contact occurs. With the intermolecular force model and asperity model implemented in the CML air bearing program, the effect of intermolecular adhesion stress on the slider at low flying height is analyzed in the static flying simulation. It is found that the intermolecular adhesion stress between the slider and the disk has slight effect on the slider-disk interface for a flying slider.     

Effects of environmental temperature and humidity on thermal flying height adjustment

Thermal actuated sliders are being widely used in today’s hard disk drive industry for its advantages of easier control of flying height (FH) and less risk of contacts with the disk. This article uses a coupled-field analysis method, which includes an air bearing model, a heat transfer model and a thermal-structural finite element model to investigate the FH changes of thermal actuated sliders at various environmental conditions. The mechanism of water vapour’s contribution to air bearing pressure loss is explained and a new humidity model is proposed to calculate this pressure loss. The temperature effects are also considered in the simulation models. It is observed that the environmental temperature and humidity have significant effects on slider’s FH changes, but their effects on the thermal protrusion height are limited. A humidity sensitivity study is also made and the results are discussed. It is found that the slider with thermal protrusion on its trailing pad will be more sensitive to the humidity. Besides air bearing stiffness, some other factors such as peak pressure, protrusion shape and air bearing surface (ABS) design will also contribute to the slider’s humidity sensitivity.     

Simulation of the head disk interface for discrete track media

This paper investigates the effect of discrete tracks on the steady state flying behavior of sub ambient proximity sliders. A finite element based air bearing simulator is used to simulate the flying characteristics of sliders over a grooved disk surface. Sliders flying over discrete track disks “see” a disk surface that consists of ridges and grooves. The air bearing pressure build-up for sliders flying over discrete track disks is different from that for sliders flying over plane disks. Low air bearing pressure can be expected for those regions of the slider that are positioned over grooves, while high air bearing pressure exists over ridges. The air bearing characteristics are determined for several pico and femto-type air bearing sliders flying over discrete track disks. An empirical equation is obtained describing the loss of flying height of a slider flying over discrete track disks.     

Numerical simulation of shock response of disk-suspension-slider air bearing systems in hard disk drives

 As non-traditional applications of hard disk drives emerge, their mechanical robustness during the operating state is of greater concern. A procedure for simulating the shock responses of a disk-suspension-slider air bearing system is proposed in this paper. A finite element model of the system is developed and modified, and it is used to obtain the dynamic normal load and moments applied to the air bearing slider. The dynamic load and moments are then used as input data for the air bearing dynamic simulator to calculate the dynamic flying attitudes. We obtain not only the responses of the structural components, but also the responses of the air bearing slider. The procedure is convenient for practical application, because it separates the work into two essentially uncoupled steps. It is used to simulate the shock response of a drive. The system modeled is linear if the load dimple of the suspension maintains contact with the slider, but it is non-linear if the dimple separates due to a strong shock. The air bearing has different responses for upward and downward shocks. Slider-asperity contacts occur when a strong shock is applied. Received: 5 July 2001 / Accepted: 11 December 2001     

Approaches to reduce effect of short-range interactions in head disk interface

Different approaches to reduce the effect of short-range interactions on slider’s flying stability—reduction of pad width at trailing edge and increase of air pressure on trailing pad—are compared for the flying stability and flying height modulation. The static and dynamic simulations show both approaches can reduce the effect of intermolecular force and electrostatic force on the slider’s flying stability. On the other hand, the increase of air pressure on trailing pad can also reduce the flying height modulation caused by disk waviness, but reduction of pad width at trailing edge increases the flying height modulation. Hence, the increase of air pressure on trailing pad is more suitable for the application of the ultra low flying height in hard disk drives.     

Simulation of thermal flying height control slider with built-in contact sensor

This work investigates the piezoelectric contact sensor in the thermal flying height control (TFC) slider. A finite element model is built for the thermal flying height control slider with a piezoelectric contact sensor, which is used to detect the contact between the slider and disk. A constant force is applied at the maximum thermal protrusion point of air bearing surface. The simulation results show that the ZnO sensor with shear-mode is more sensitive to contact force than that with transverse-mode. The sensitivity of contact sensor can be increased by reducing the cross-sectional area of sensor, increasing the thickness of sensor, and choosing a short distance of sensor to air bearing surface. In addition, the thermal-stress effects from TFC heater on contact sensor are significantly large and the amplitude of thermal-stress inducing output voltage is orders larger than that induced by contact force. However, by optimizing the distance of sensor to ABS, it is possible to eliminate the thermal-stress effects. Finally, the response time of thermal-stress induced electrical voltage of contact sensor is about 0.3?ms.     

Influence of disk curvature on slider attitude under operational shock conditions for a 2.5-in. hard disk drive

Recently, the number of disks in hard disk drives has increased, and the gap between the slider and disk has decreased. These changes make the contact between the ramp and disk easily. External shock and ramp–disk contact can cause change in disk curvature. Such a change in disk curvature affects the air bearing pressure between the slider and disk. However, disk curvature has not been considered in the previous research. Thus, in this study, we investigated the influence of disk curvature on slider dynamics. Disk curvature was calculated from a transient shock analysis, and was then applied to slider dynamic analysis. As a result, disk curvature reduced the shock performance, by decreasing the minimum flying height and increasing the pitch and roll angle of the slider.     

Simulation of a pad slider flying on a wavy surface

This paper describes following-up characteristic of a pad slider to a wavy surface. The pad slider comprises a leading pad and a trailing pad. We use a sine wave to replace an actual wavy surface. The response of the pad slider due to the sine wave is analyzed by computer simulation. The response of the pad slider is evaluated from variation in slider flying height (FH). A variation gain that is a ratio of FH fluctuation and wave amplitude is defined to explain the following-up characteristic of slider. To analyze the following-up characteristic, different wavelengths are used as parameter to calculate the variation gain. Because pads on the air-bearing surface of the slider are able to occur at two pressure peaks on the leading edge and trailing edge that enable a pitching motion of the slider, the slider can follow the disk waviness with a wavelength that is shorter than the slider length. The variation gain is 0.33 for the wavelength that is equal to the slider length. However, the slider cannot follow the disk waviness with a wavelength that is shorter than the pad length. The variation gain is greater than 1 for the wavelength 0.2 mm. To study the relationship between the variation gain and the dynamics of slider, a transient response simulation is carried out in order to investigate a natural frequency of slider. We set a projection on a plane surface. When the slider is flying over the projection, we can obtain a flying height response curve and a pitch response curve. The natural frequency of flying height and pitch angle can be known by FFT. The transient response of slider in pitch mode is compared with the variation gain. The simulation results make clear the fact that the following-up characteristic has correlation with the dynamics of the pitch model and the phase difference between a locus of head and a wave of disk surface.     

Parametric simulation of piezoelectric flying height control slider using shear-mode deformation

The piezoelectric flying height control slider has recently been implemented in magnetic recording disk drives to reduce the flying height. This paper performs the electromechanical simulation and air-bearing simulation to investigate the effects of the shear-model deformation on the static flying attitude of PZT slider. The location of PZT sheet and air bearing surface of slider are investigated to achieve a low flying height and robust head-disk interface. The results show that a short distance of the PZT sheet to the trailing edge of the slider can help to achieve a low flying height. A small center-trailing pad of the slider can also help to achieve a low flying height, but cannot prevent the reduction in pitch angle. The depth of the center-trailing pad does not change the reduction ratio of the pitch angle when increasing the drive voltage. A big pitch angle value is needed to avoid the pitch angle falling below zero at a high drive voltage.     

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.     

Effects of disk surface roughness on static flying characteristics of air bearing slider by using a combined method of Reynolds equation and rough disk surface

Reynolds equation was modified with adding the surface roughness parameters to analyze the effects of disk surface roughness on the static flying characteristic of an air bearing slider. However, the modification demands the complicated mathematical expressions and related knowledge of physics and mathematics. In this paper, a combined method of Reynolds equation without introducing the roughness parameters and rough disk surface is proposed to investigate the effects of disk surface roughness on the static flying characteristics of an air bearing slider, it is different from those models of modified Reynolds equation introducing the disk surface roughness used by many researchers. More importantly, this method avoids the complicated numerical calculation resulted from the mathematical expressions including the Peklenik parameter \(\gamma\) and roughness R_a. By using an Ω air bearing slider, we investigated the effects of disk surface roughness on the static flying characteristics of this slider, the results show that the Peklenik parameter \(\gamma\) and roughness R_a have a significant influence on the pressure distribution, the load carrying capacity and the location of the pressure centre.     

Active-head sliders using piezoelectric thin films for flying height control

This paper describes design and fabrication of a MEMS-based active-head slider using a PZT thin film for flying height control in hard disk drives. A piezoelectric cantilever integrated in the air bearing slider is used to adjust the flying height individually. An air bearing surface (ABS) geometry that minimizes the aerodynamic lift force generated beneath the head has been designed based on the molecular gas film lubrication (MGL) theory. The sliders with PZT actuators were fabricated monolithically by silicon micromachining process. Performance of the actuator was tested by using an optical surface profiler. Furthermore, the fabricated slider was mounted on a suspension and the flying height of the slider above a spinning disk has been measured by multiple wavelength interferometry. Change in the head-disk spacing has been successfully confirmed by applying voltage to the actuator.     

Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

Dynamic instability of thermal-flying-height-control sliders at touchdown

With the wide application of thermal flying-height control (TFC) technology in the hard disk drive industry, the head-disk clearance can be controlled to as low as ~1?nm. At this ultra-low clearance, the air bearing slider is subject to relatively large interfacial forces, and it experiences more complicated dynamics, compared with the flying case. In this study we conduct a numerical analysis to investigate the dynamics of TFC sliders during touchdown. The general trend of the slider’s motion predicted by the numerical simulation qualitatively agrees with experimental findings. The touchdown process begins with a slight intermittent contact between the slider’s trailing edge and the disk, followed by a partial slider-disk contact at the trailing edge accompanied by a large pitch motion at the 1st air bearing mode; this pitch motion gets suppressed and the slider comes into stable sliding on the disk as the protrusion is further increased.     

Ramp loading “sweet spots”

 The range of PSA/RSA (slider pitch/roll static attitude respectively) devoid of head–disk contact during ramp loading is called “sweet-spot”. Its extent was measured for different air-bearing designs and loading variables. The sweet spot boundaries are determined both by the air bearing and “body-effects” invariant of air bearing design. In the latter case, the air-bearing genesis occurs sufficiently far from the disk where detailed ABS differences have little relative transient effect. The available sweet spot is roughly ±1° which is within drive design capability, but requires attention to details. Measuring head/disk contact severity over the sweet spot indicates ranges of PSA/RSA to be stringently avoided (e.g. negative pitch) and those more tolerant of head/disk contact. The sweet spot extent is confirmed with disk damage at the boundaries for 100,000 cycles. A key parameter is loading velocity. Optimum sweet spot occurs with a vertical loading velocity of about 1 inch/sec and diminishes to near-zero at 4 inch/sec. The test technique is described in detail. Sweet spot knowledge in used to predict head–disk contact reliability. Received: 1 June 2001/Accepted: 21 September 2001     

Mechanical stability of the interface between a padded slider and magnetic recording disk

Contact state, friction, and meniscus load between a padded slider and a smooth disk are discrete; friction is determined by the number of pads in contact and the meniscus load acting at each pad. By inducing frictional changes through varying sliding direction, we force the slider from one stable state to another and infer which pads are in contact from the corresponding changes in friction. We find the stability of a given contact state is influenced by friction coefficient, pad location, meniscus load, applied mechanical loads and applied moments. A model is proposed to account for these effects.     

Contact recording review

Various contact recording technologies for hard disk drives are reviewed. The advantages and disadvantages of each approach to contact recording are analyzed. Experimental detection methods and simulation models for the contact force are introduced. Some important technologies related to contact recording are addressed. The effects of lubricant and the short range forces on contact recording are discussed, and the dynamic flying characteristics of a slider with spherical pad at trailing edge are studied. It is suggested that a thermal protrusion slider with a spherical pad at the trailing edge may be a possible approach for the success of contact recording.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Visualization of lubricant pickup phenomena by lubricant thickness mapping on slider surface

A slider surface analyzing tester was developed to observe the lubricant thickness distribution on a slider surface by interferometry. We observed three phenomena related to lubricant pickup by the slider. The picked-up lubricant gathered around the boundary of the pad and recess area during the unloading interval, and the gathered lubricant flowed to the trailing edge of the pad after loading on the disk surface. There were two lubricant flows on the pad surface. The first was a circulation flow from the dynamic flying height protrusion area to the leading edge of the pad. The second was a circulation flow from the lubricant pool to the leading edge of the pad. Lubricant dewetting occurred on the slider pad surface when a thick layer of lubricant was adhered to the pad surface.