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.     

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.     

Lubricant bonding, chemical structure, and additive effects on tribological performances at head–disk interfaces

 The lubricant film for head/disk application consists of bonded fraction and unbonded (mobile) fraction. It is well known that the mobile fraction of the lubricant film can replenish the surface sites where the lubricant film was depleted, thus, the surface wear is postponed or alleviated. With a continuous decrease in the head disk spacing, however, too much mobile fraction of lubricant may cause head slider lubricant pick-up, and deteriorate the interface. Two perfluoropolyether (PFPE) lubricants of Z-tetraol and Z-DOL are discussed in this paper. Lubricant Z-tetraol is characteristic of stronger bonding to a carbon overcoat, lower vapor pressure, and higher thermal stability but less mobility than Z-DOL. It is found that, for CSS (contact-start-stop) durability, the interfaces with Z-tetraol show no worse in performance than those with Z-DOL, and less head slider lubricant pick-up on those with Z-tetraol. Based on the above-mentioned, it is possible that the interfaces relying more on the lubricant bonding strength and chemical structure stability are more beneficial to tribological performances than those relying more on the lubricant replenishment. The effects of lubricant additive X1P mixed to Z-tetraol, and Z-DOL, respectively are also studied on tribological performances. Stiction, CSS durability, and head slider lubricant pick-up are discussed among lubricants Z-tetraol, Z-DOL, Z-tetraol/X1P and Z-DOL/X1P. Statistical t-test, F-test, and Weibull analyses are applied to CSS data to differentiate CSS durability performances. Additive X1P is found to enhance CSS durability for both lubricants. Lubricant Z-tetraol/X1P is recommended for the best tribological performances, followed by Z-DOL/X1P, Z-tetraol, and Z-DOL. Received: 7 August 2001/Accepted: 11 December 2001 Authors would like to thank Vidya K. Gubbi, Youmin Liu, and Gunter P. Barth at Seagate, Fremont, CA, for media supplies, lubricant properties, and head slider lubricant pick-up measurements. Authors also thank Gunter P. Bath, Frank Chang, Roger Y. Shih, Hamid R. Saman, Caroline Tjengdrawira and Sam Liang at Seagate for fruitful discussions.     

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.     

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.     

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.     

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.     

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.     

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.     

Investigation of slider dynamics under electro-static force

 With the continual increase in the areal density of hard disk drives, the head/disk interface engineers are continuously challenged in designing an interface with a lower mechanical clearance between the slider and disk. Although, this has traditionally been achieved by making the disk smoother, some novel ideas have been discussed, where the slider generally flies high, and when necessary, a part of the slider is moved closer to the disk surface. Another method that has been discussed is applying a voltage across the slider and disk to bring the whole slider closer to the disk. However, rapid application of voltages across the interface may lead to undesirable slider oscillations. In this paper, we study the dynamics of the slider when a voltage is applied across the slider and disk. We show that one can easily excite the air-bearing frequency and if the voltage is not well controlled, the slider may also damage the disk. We also identify the most probable mechanisms of current flow and show that the disk lubricant plays a critical role in initiating current flow. Received: 26 June 2002 / Accepted: 9 September 2002     

Reduction in lubricant pickup by bias voltage between slider and disk surfaces

We studied the effect of bias voltage between slider and disk surfaces to reduce lubricant pickup by the slider. A perfluoropolyether (PFPE) lubricant film, which is coated on the disk surface, has been considered to be charged to a negative voltage by the airflow on the rotational disk surface. Because the PFPE lubricant film is negatively charged, the lubricant pickup should be reduced by a bias voltage with a negative voltage on the slider surface. We confirmed changes in the lubricant pickup in a lubricant pickup test conducted at different bias voltages. A positive voltage of the slider accelerated the lubricant pickup, whereas a negative voltage reduced it.     

Quantitative study of lubricant puddling at the head/disk interface

 This paper presents a new methodology to quantify the micro-level lubricant accumulation (lubricant puddling) on a magnetic recording hard disk for flyability testing. After flying over the disk for certain period of time, the head was parked on the disk for certain time duration to allow the lubricant picked up on the head to flow to the disk surface. Using the optical surface analyzer with proper calibration and background removal, the volume of lubricant in a lubricant puddle on the disk has been measured accurately. The effect of various factors, including lubricant type, thickness, the way of head parking, and head parking duration, on lubricant puddling has been investigated. This method is a useful tool for flyability study and other studies that need to quantify the volume of localized lubricant accumulation on the disk. Received: 5 July 2001/Accepted: 11 December 2001     

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.     

Measurement of contact potential difference in head disk interface by readback signal spectrum

The contact potential difference leads to electrostatic interaction between the slider and disk in a hard disk drive. The effect of electrostatic force on slider’s flying height and flying stability becomes more significant with the decrease of flying height. A method of measurement of contact potential difference in head disk interface by readback signal spectrum is demonstrated in this paper. When a voltage with DC and AC was applied in the head disk interface, the amplitude of readback signal at the first harmonic frequency of applied AC voltage is proportional to the sum of contact potential difference and applied DC voltage. The contact potential difference in head disk interface is equal to the negative of DC voltage when the amplitude of readback signal at the first harmonic frequency is minimal.     

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.     

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.     

Study on head instability using Flying-QST tester for HDDs

We developed a tester consisting of minor-loop and major-loop quasi-static test (QST) units in order to better understand head instabilities under conditions where the head slider was flying on the disk. The minor-loop QST unit, composed of a spin-stand and an electromagnetic coil, was used to understand the mechanism of head instabilities caused by thermal stress due to flying-height control using a thermal actuator and caused by mechanical stress due to contact between the head and disk under flying conditions. The major-loop QST unit was used to investigate head damage in detail. We designed and fabricated the tester and conducted experiments that focused on thermal and mechanical stress. The results confirmed that the new method was effective for studying head instabilities at the head disk interface of hard disk drives (HDDs).     

磁记录系统中磁头飞行状态的仿真研究

运用有限差分法对气体润滑方程进行求解，建立磁头稳定飞行状态优化模型，对气体润滑的磁记录系统中磁头稳定飞行状态变化规律进行仿真，通过对仿结果的分析发现磁头斜角影响磁头稳定飞行状态的变化规律。     

Simplified molecular gas film lubrication equation and accommodation coefficient effects

As the spacing between the flying head/slider and the rotating disk in hard disk drives (HDDs) continues to decrease, the interaction between the molecular gas and the surfaces of the disk and the head/slider becomes significant. The influence of surface accommodation coefficient (AC) is an important factor to govern the static characteristics of the head/slider. Starting from the polynomial logarithm fitting equations of Poisueille flow rate and Couette flow rate, a new simplified molecular gas film lubrication (MGL) equation is proposed to simulate the ultra-thin air bearing film in HDDs. The new MGL equation is simpler than that of the polynomial logarithm form of MGL equation. The new approach produces very good approximations for both Poisueille flow rate and Couette flow rate with very little differences to those based on the original MGL equation. The new simplified MGL equation is solved by using a meshless method, called least square finite difference (LSFD) method. Effects of ACs on the static characteristics of air bearing films in HDDs with ultra-low flying heights are investigated. Numerical results show that effects of ACs on the static characteristics are significant for the case of symmetric molecular interaction. On the other hand, effects of ACs at the disk surface on the static characteristics are significant for the case of non-symmetric molecular interaction, while effects of ACs at the slider surface on the static characteristics are weak.