Contact between a thermal flying height control slider and a disk asperity

Contact between a thermal flying height control slider and an asperity on a disk is investigated using finite element analysis. The finite element model developed accounts for transient elastic–plastic deformation and heat generation due to frictional heating. Plastic deformation and temperature rise of the read/write element are determined as a function of flying height of the slider, location of the read/write element as well as material properties of typical disk asperities. The model shows good agreement with experimental data. Significant plastic deformation and temperature rise were observed in the shield and alumina regions of the slider. Hard and stiff disk asperities, such as alumina asperities, result in more damage to the slider than soft and compliant nickel-phosphorus ones.     

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.     

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.     

Characterization of light contact in head disk interface with dynamic flying height control

This paper presents an investigation of the light contact in a head disk interface with dynamic flying height control. The touchdown test is conducted for a dynamic flying height control slider and the response is recorded using AE sensor. The bouncing instability and light contacts are observed during thermal actuated touching down process of the slider. The physics-based simulation is conducted to correlate with the experiments, so as to characterize bouncing instability and the factors affecting bouncing instability. The enhanced spectrogram and HHT approaches are used to extract and characterize the non-stationary characteristics of the weak signal of slider response under light contact. It is found that the light contacts are constituted by a series of intermittent transient impact responses with frequency identical to slider??s pitch mode.     

Experimental study of slider dynamics induced by contacts with disk asperities

Vertical vibrations at the slider leading edge exited by a disk asperity were investigated in a drive level system, using laser Doppler vibrometry and acoustic emission (AE) sensors. The flying height change at the leading edge of a thermal flying-height control slider was measured for different power inputs to the heater element. The maximum spacing change at the slider leading edge was found to be about 4 nm for an asperity of 26 nm height and 1.1 μm width. A method was investigated to produce “artificial asperities” on a disk surface using a nano-indentor. The geometry of the “artificial asperities” was characterized using an atomic force microscope. In addition, a spin stand was used to analyze the AE signal of slider vibrations induced by contacts with the “artificial asperities”. First and second pitch modes of the slider were observed.     

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.     

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.     

Experimental study of slider�Cdisk interaction process with thermal-flying-height controlled slider

Thermal flying height (TFC) controlled slider has been introduced in hard disk drive recently. Flying height at the read/write elements of the slider is controlled by thermal pole tip protrusion. Interactions between the TFC slider and disk can be very gentle because the low flying height thermal protrusion area is usually very small. It is still a big challenge to detect very gentle interactions. In this work, a very sensitive method to study very gentle slider?Cdisk interaction in frequency domain has been developed and details of the TFC slider?Cdisk interactions from gentle to strong have been revealed. It is proved that higher heating power is required to initiate the vibration in which higher stiffness part of the slider air-bearing is involved.     

The flying height analysis of patterned sliders in magnetic hard disk drives

The ultra-low spacing of patterned sliders flying above a rotating disk with smooth surface in a hard disk drive is analyzed for high areal density recording. Three types of pattern (slender, square, and broad) with the same bump area are discussed. The molecular gas film lubrication (MGL) equation and the equations of motion of the patterned slider are solved numerically to obtain the steady flying attitude. The results show that the flying height of the broad patterned slider is the greater than either that of the square patterned slider or the slender patterned slider. In addition, we analyzed the effects of bump height and the bump pitch on the slider attitude, including flying heights, pitch angles, and roll angles, are discussed.The authors express their thanks for the foundation support from National Science Council, NSC 92-2212-E-151 -005, ROC.     

Air-bearing surface chemical modification for low-friction head–disk interface

As a chemical modification of slider air bearing surface (ABS), fluorine ion implantation (FII) was conducted. FII treated ABS was fabricated to reduce slider surface energy, which is expected to reduce adhesion and friction force between the slider and disk. Tribological performance, such as slider flyability, was experimentally investigated. Touchdown speed, corresponds to a flying height of about 0.4 nm, was reduced by FII treatment. Slider touch-down and take-off hysteresis was also improved by reducing the adhesive force of the lubricant by fluorine-ion implantation. AE output and friction force were also reduced in the drag test. FII treated slider showed a 45% reduction of AE amplitude and 14% reduction of friction force. FII treatment was confirmed to be effective in improving slider flyability.     

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.     

Tri-state dynamic model and adhesive effects on flying-height modulation for ultra-low flying head disk interfaces

Physical spacing between a flying slider and a rotating disk is projected to be 3 nm in order to achieve extremely high areal recording densities of 1 Tb/in². In such ultra-low flying-height regimes, two imminent obstacles that need to be overcome are intermittent head/disk contacts and strong intermolecular adhesive forces at the head/disk interface (HDI). Head/disk contact can cause large vibrations of the recording slider and possibly damage the disk and slider due to large contact forces. Strong adhesive forces disturb the balance of forces in a flying slider by pulling it down onto the disk and increasing the possibility of catastrophic HDI failures by doing so. This paper describes a dynamic model that includes contact and adhesive forces. Specifically, a lumped parameter single degree-of-freedom, three state nonlinear dynamic model representing the normal dynamics of the HDI and asperity-based contact and adhesive models were developed and coupled together to predict the performance of ultra-low flying sliders. The validity of the proposed dynamic model was confirmed in terms of flying-height modulation (FHM) by experimental measurements using ultra-low flying HDIs. It was found that the amplitude and frequency components of the dynamic microwaviness play an important role in slider dynamics. Furthermore, the effect of adhesive force on FHM was investigated and design guidelines for reduced FHM were suggested.This research was supported by a grant from the Information Storage Industry Consortium (INSIC) and the National Science Foundation under grant number CAREER CMS-0227842. Gary Prescott and Thomas Pitchford of Seagate Technology provided the spindle motor and HGA samples. The authors gratefully acknowledge this support.     

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.     

The effect of write current on thermal flying height control sliders with dual heater/insulator elements

The effect of write induced pole tip protrusion on the magnetic spacing of the head/disk interface has to be taken into consideration as flying heights approach the spacing regime of a few nano-meters. Thermal flying height control (TFC) sliders are presently in common use in hard disk drives to control the flying height at the read/write element during drive operations. In this paper the flying characteristics of TFC sliders with dual heater/insulator elements are investigated. Simulation results are shown for situations where the write current is ??on?? and where the write current is ??off??. The effect of design parameters of two heater/insulator elements is studied to optimize the performance of TFC slider.     

Nano-texturing of magnetic recording sliders via laser ablation

To increase the storage density of hard disk drives, the flying height of the slider needs to be reduced to <10 nm. This requires super-smooth surfaces of the disk and slider. As the roughness decreases, stiction and adhesion are found to increase substantially leading to failures of the head/disk interface. Texturing the slider surface is a well-known approach to this issue. In this study we investigated laser ablation as a potential process for texturing magnetic recording sliders. It was found that straight laser machining caused unwanted re-deposition of material. These deposits could be significantly reduced by using a chemical etching enhanced laser process.     

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.     

Active flying-height control slider using MEMS thermal actuator

Today’s head/disk interface design has a wide flying height distribution due to manufacturing tolerances, environmental variations, and write-induced thermal protrusion. To reduce the magnetic spacing loss caused by these effects, we developed an active head slider with a nano-thermal actuator. The magnetic spacing of these sliders can be controlled in situ during drive operations. After simulating the heat transfer in the slider to obtain the thermal deformation of the air-bearing surface, we fabricated a thermal actuator using thin-film processing. An evaluation done using a read/write tester showed a linear reduction in the magnetic height as electric power was applied to the actuator. The actuator’s stroke was 2.5 nm per 50 mW with a time constant of 1 ms. There was no significant impact on the reliability of the read element.     

Simulation on contact between the droplet and the slider at head-disk interface based on water-hammer pressure model

To understand the cause of read/write error due to lube accumulation, a model to simulate the slider’s response to the contact impact, which can occur between a lubricant droplet on the disk and a slider, was developed. The contact impact model is based on the water-hammer pressure model with an additional damping force, where the wave-shock pressure is assumed to function as the contact pressure, and the damping force defines the damping characteristics of the impact which are due to the lubricant’s high viscosity and squeeze between the droplet and slider contact area along the slider local velocity direction. The transient contact impact is dependent on lube droplet density, disk velocity, pitch angle of the slider, and contact area between the droplet and the slider. The measured read/write signal jump due to lube pickup can be explained by the simulation results. This modeling and simulation are helpful to us in understanding the read/write signal loss due to a lube droplet at head disk interface.     

A numerical investigation of different touchdown patterns of thermal-flying-height-control sliders

In this study we employ a numerical approach to explore the touchdown patterns of a thermal-flying-height-control (TFC) slider. Depending on the roughness of the head disk interface and thickness of the lubricant layer a TFC slider can experience different stages during touchdown. Three different touchdown patterns are shown. With a rougher interface profile the slider smoothly transfers from a flying stage to a sliding stage. With an intermediate smooth interface profile the slider experiences a flying-bouncing-sliding transition. With the smoothest interface the slider goes through a flying-bouncing-surfing-sliding transition. Different stages are characterized by different slider dynamics and slider-disk contact status. The different touchdown dynamic patterns shown here can result in a significant difference in the easiness of successful touchdown detection. The general approach proposed here may also be applied to investigate the effects of other important head disk interface factors, e.g., air bearing surface design, heater, suspension, etc. on the slider’s touchdown dynamic behaviors.