Slider–bump contact and flying height calibration

It is a big challenge to determine ultra-low slider flying height accurately. The standard bump disk method is probably the most reliable and acceptable method so far. One of the key issues to determine slider-flying height with the bump disk method is the complicated slider–bump interaction process and the possible disturbance of the bumps on the slider flying performance. Our knowledge about the slider–bump interaction process is still very limited due to the lack of an effective and powerful experimental technique to study it. In this work, the slider–bump interaction process was studied with a dynamic flying height-attitude (3D) system. The interaction process was also simulated to compare with the experimental observations and to help determine the slider–bump contact points in the experimental observations. The accuracy of flying height (FH) calibration with the bump disk method and the minimum slider–bump interference height required for the testing system used in this study to detect the onset of slider–bump contact were analyzed and discussed. It is proved that the 3D system is a very useful and powerful tool for the application. Many details of the slider–bump interaction process can be revealed with the 3D system. It is found that the calibrated FH is much more accurate than that predicated by the simulations.     

Temperature Effect on a HDD Slider’s Flying Performance at Steady State

The temperature inside modern hard disk drives (HDDs) can become as high as 100°C during operation. The effects of such high temperatures on the slider’s flying attitude and the shear forces on the slider and the disk are investigated in this paper. General formulae for the shear forces are derived, and the generalized Reynolds equation is modified to take into account the temperature effect on the mean free path of air as well as the air viscosity. Numerical results are obtained for two different air bearing surface designs. It is shown that the temperature changes result in non-negligible changes in the slider’s flying height and the shear forces. These changes could further induce changes in the deformation and instability of the lubricant layer and thereby affect the reliability of the HDDs.     

Simulation of Static Flying Attitudes with Different Heat Transfer Models for a Flying-Height Control Slider with Thermal Protrusion

The thermal flying height control (TFC), aka dynamic fly height (DFH), technique has been recently used in the head disk interface of hard disk drives to obtain a lower head-media spacing. The air bearing cooling effect, i.e., the heat conduction between the slider and the air film, has been incorporated in the numerical thermal–mechanical simulation of the slider’s static performance. However, the heating effect of the viscous dissipation of the air flow has not been considered yet. In this article, both effects are included in the simulation of a flying slider with its flying height controlled by thermal protrusion, and different models for the air bearing cooling are used to obtain the slider’s static flying attitudes. The simulation results directly show that the air bearing cooling is dominant compared with the viscous heating. All of the air bearing cooling models, including a recent one that considers the dependence of the air molecular mean free path on the air temperature, have simulation results close to each other. The largest relative difference in the simulated flying height is less than 9% even when the transducer flying height is lowered to below 2 nm.     

Inert Gas Filled Head–Disk Interface for Future Extremely High Density Magnetic Recording

Inert gas filled head–disk interface (HDI) is a possible solution in reducing the magnetic spacing between the magnetic head and the magnetic media for achieving further increased recording density of a magnetic recording system. This article investigated the flying and thermal performances of a thermal actuated slider at inert gas filled HDI by using a couple-field analysis method which consists of a finite element model of the entire slider, an air bearing model based on the generalized lubrication equation and a heat transfer model which incorporates various molecular dynamics models and considers temperature effects. The simulation studies showed that the variation of gap flying height (FH) with the heater power in the inert gas is quite similar to that in air. It is also found that the slider’s thermal actuation efficiency in helium is slightly better than those in argon and air. However, the temperature effects in a fully sealed drive are totally different to those in an open drive. As a result, the inert gas filled HDI normally requires a larger thermal actuation stroke due to the temperature effects in a fully sealed drive.     

Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tbit/in² hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.     

安装误差对纳米间隙磁头飞行姿态的影响研究

考虑安装误差引起的磁头俯仰角误差和侧翻角误差,建立了磁头飞行数学模型,通过数值方法分析了安装误差对纳米间隙磁头飞行姿态主要参数的影响,并与试验结果进行了对比分析。结果表明,俯仰角误差对飞行高度和俯仰角有明显影响,随着俯仰角安装误差的增加,磁头飞高和俯仰角偏差增加,对侧翻角的影响可以忽略。侧翻角误差只对侧翻角有明显影响,随着侧翻角安装误差的增加,磁头飞行侧翻角偏差增加,对磁头飞行高度和俯仰角的影响可以忽略,理论分析和实验结果吻合较好。磁头设计和装配时,应控制磁头的俯仰角安装误差。     

Ultra-low-flying-height design from the viewpoint of contact vibration

The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.     

The Contribution of Thin PFPE Lubricants to Slider–Disk Spacing

We have investigated the effect of the molecular weight (MW) and film thickness of a perfluoropolyether lubricant, Zdol, on the slider–disk spacing loss, or clearance. The major conclusion of this work is that Zdol films as thin as 10 Å can reduce the slider–disk clearance by 2 nm or more in the molecular weight range of 1000–5000 amu. This is attributed to the attractive van der Waals interaction between the slider and the disk surface that causes the Zdol main chain to interact with the slider surface, giving rise to a friction force. When the film thickness of the lubricant exceeds the monolayer thickness, dewetting can take place. The droplets that form occupy the space between the slider and disk surface reducing the slider–disk clearance by as much as 4 nm. There is a step increase in the acoustic emission signal at the dewetting thickness transition, indicative of a slider–disk interference.     

Pumping effect of sliders in hard disk systems

The accumulation of contaminants on the slider surface is of paramount importance in hard disk drives because only an ultra small amount of contaminants on the slider surface will cause catastrophic failures for hard disk drives with a spacing between the slider and the hard disk as small as 10 nm, which will be reduced further in the near future to about 5–6 nm in order to attain a recording density of 100 Gbit/in². In this paper the pumping effect of the slider is proposed as one mechanism of the contaminant accumulation on the slider. Analysis of the pumping effect is conducted by considering the adsorption process and the shear flow process on the slider surface in terms of the continuum. It is found that the pumping effect can be divided into two different classifications depending on the value of the parameter λ which is the ratio of the maximum shear flow of the adsorbed film to the maximum adsorption amount: the shear flow rate-controlling pumping effect for λ < 0.1 and the adsorption rate-controlling pumping effect for λ > 0.4. For the shear flow rate-controlling effect, the accumulation rate of the contaminant is directly proportional to the disk surface velocity, while inversely proportional to the flying height of the slider. An erratum to this article can be found at .     

Effect of relative humidity and disk acceleration on tribocharge build-up at a slider–disk interface

High performance disk drives require high spindle speed. The spindle speed of typical hard disk drives has increased in recent years from 5400 to 15000 rpm and even higher speeds are anticipated in the near future. The increasing disk velocity leads to increasing disk acceleration and slider–disk interaction. As the head-to-disk spacing continues to decrease to facilitate increasing recording densities in disk drives, the slider–disk interaction has become much more severe due to the direct contact of head and disk surfaces in both start/stop and flying cases. The slider–disk interaction in contact-start-stop (CSS) mode is an important source of particle generation and tribocharge. Charge build-up in the slider–disk interface can cause electrostatic discharge (ESD) damage and lubricant decomposition. In turn, ESD can cause severe melting damage to MR or GMR heads. We measured the tribocurrent/voltage build-up generated at increasing disk acceleration. In addition, we examined the effects of relative humidity on the tribocharge build-up. We found that the tribocurrent/voltage was generated during pico-slider/disk interaction and that its level was below 250 pA and 0.5 V, respectively. Tribocurrent/voltage build-up was reduced with increasing disk acceleration. Higher humidity conditions (75–80%) yielded lower levels of tribovoltage/current. Therefore, a higher tribocharge is expected at a lower disk acceleration and lower relative humidity condition.     

Effect of Pitch and Roll Static Angle on Lubricant Transfer Between Disk and Slider

A precision spin stand was used to study the effects of pitch static angle and roll static angle on lubricant transfer between a disk and a slider in a hard disk drive. The lubricant distribution on the slider was determined by time-of-flight secondary ion mass spectrometry, while the lubricant distribution on the disks was obtained using optical surface analysis. Lubricant transfer from the disk to the slider was found to increase as a function of the pitch static angle of the slider. Negative roll static angles were found to have a larger effect on lubricant transfer and the formation of lubricant moguls than positive roll static angles. Suspension frequencies and pitch mode frequencies were observed in the lubricant mogul patterns for negative roll static angles.     

Numerical Simulation of the Head/Disk Interface for Patterned Media

The use of patterned media is a new approach proposed to extend the recording densities of hard disk drives beyond 1 Tb/in.². Bit-patterned media (BPM) overcome the thermal stability problems of conventional media by using single-domain islands for each bit of recorded information, thereby eliminating the magnetic transition noise (Albrecht et al., Magnetic Recording on Patterned Media, 2003). Considering steady state conditions, we have transferred the pattern from the disk surface onto the slider surface and have investigated the pressure generation due to the bit pattern. To reduce the numerical complexity, we have generated the bit pattern only in the areas of the slider near the trailing edge, where the spacing is small. Cylindrical protrusions were modeled using very small mesh size on the order of nanometers to obtain the flying characteristics for the entire slider air bearing surface (ABS) using the “CMRR” finite element Reynolds equation simulator (Duwensee et al., Microsyst Technol, 2006; Wahl et al., STLE Tribol Trans, 39(1), 1996). The effect of pattern height, pattern diameter, slider skew angle, and slider pitch angle on flying height of a typical slider is investigated. Numerical results show that the flying height decreases for a patterned slider and the change in flying height is a function of the pattern height and ratio of the pattern diameter to the pattern pitch. In comparison to discrete track media, the flying height loss is larger for a patterned slider disk interface for the same recessed area of pattern.     

Numerical simulation of slider dynamics during slider–disk contact

In this paper, the dynamic behavior of pico slider in contact with the disk was calculated. The analysis model consists of a simplified suspension model, an air bearing model, and a slider–disk contact model. The contact model consists of two elements. One is surface roughness model measured by Atomic Force Microscope (AFM) and the other is micro-waviness model. The dynamic behaviors of the tri-pad slider are calculated at several rotation speeds to investigate slider vibration modes during slider–disk contact. The slider oscillation frequency depends on the rotation speed and it saturates about twice as much as eigen frequency of air bearing pitch mode.     

Consideration and compensation for precision optical flying height measurement

It is a big challenge to determine ultra-low slider flying height accurately. The intensity interferometry flying height testing method is widely used to determine slider flying height. However, the intrinsic measurement errors of the method are becoming non-negligible with the decrease in slider flying height. Strategies have to be developed to minimize the errors. To measure flying height with a normal incidence optical flying height tester, a calibration process is required to determine several constants used in flying height calculation. In practice, the calibration is usually done simply by retracting the slider from the disk and measuring the intensity minima and maxima of the interferogram during the retracting process. It has been demonstrated that the single most important source of error in the flying height measurement is associated with errors in the determination of the intensity maxima and minima. In this work, the effects of optical filter, the responding frequency of photodetector, and the lack of the first order intensity minimum on the determination of the intensity maxima and minima are studied. Methodologies to minimize the errors in flying height measurement caused by the above factors are developed.     

Analysis of Surface Textured Air Bearing Sliders with Rarefaction Effects

A numerical model is developed to study the effect of texture on air bearing sliders for large Knudsen numbers. The effect of texture location, texture size, and density on the pressure generation is studied. First, a textured plane slider parallel to the disk surface is investigated, and the texture parameters are determined that result in optimum pressure generation. Then, a plane inclined slider is studied using optimum texture parameters found in the parallel slider case. Thereafter, the effect of texture on the steady state flying characteristics of an actual magnetic recording slider is investigated. Finally, the flying height modulation, pitch, and roll motion of a textured slider (pico and femto form factors) are determined numerically by exciting the slider using a step on the disk. Comparison of the results for textured and untextured sliders is made. It is found that textured sliders show better dynamic performance compared to the untextured sliders in terms of stiffness and damping.     

磁盘速度与容纳系数对硬盘气膜静态特性的影响

随着硬盘(Hard disk drives,HDDs)中浮动块与磁盘间飞行高度的降低,气体分子与磁头/磁盘间的交互作用逐渐增强,磁盘速度及容纳系数(Accommodation coefficients, ACs)对气膜承载特性的影响越来越重要。采用一种无网格法—最小二乘有限差分(Least square finite difference, LSFD)法,对简化的分子气膜润滑(Molecular gas film lubrication, MGL)方程进行求解,研究了磁盘速度、磁头和磁盘表面ACs对HDDs超低飞高气膜静态特性的影响。数值结果表明：对称性分子交互作用时,磁头和磁盘表面ACs对气膜静态特性的影响明显;非对称性分子交互作用时,磁盘表面ACs对气膜静态特性的影响较大,而磁头/浮动块表面ACs的影响较小;不同ACs条件下,随着磁盘速度或最小飞行高度的增加,压力幅值点位置的变化较均匀。     

硬盘/磁头薄膜气体润滑中热效应影响分析

根据热力学理论,建立了硬盘磁头热力学分析模型,利用数值方法分析了绝热过程中热效应及热蠕流效应对磁头承载能力、飞行姿态等性能的影响,并与实验结果进行了对比分析。数值分析结果表明,绝热过程中热蠕流效应对磁头的压力分布和承载力没有明显影响,但热效应对气体黏度和磁头飞行姿态影响明显。热效应使气体黏度和承载能力增加,从而使磁头飞行高度和俯仰角增大,但对侧翻角没有明显影响。理论分析与实验结果的对比分析表明,磁头的飞行过程不是绝热过程。     

Sensitivity analysis of pico and femto sliders

The flying height sensitivity of pico and femto sliders on air bearing contour design, operating conditions and manufacturing tolerances was studied numerically. A Monte Carlo analysis was performed to evaluate the distribution of flying height, pitch angle and roll angle, assuming the selected independent variables have normal distributions. Two types of pico sliders and one type of femto sliders were modeled and simulated. The results show that flying height is strongly influenced by the contour design of the air bearing slider. The resulting distributions of flying height, pitch angle and roll angle are near Gaussian.     

Lubricant-Induced Spacing Increases at Slider–Disk Interfaces in Disk Drives

In this article, we explore the physical mechanisms for lubricant migration on recording head slider surfaces and how this migration leads to increased slider–disk spacing during disk drive operations. This is done using both a new experimental methodology, called the “droplet stress test,” and through simulation. In our simulations, we compare the air shear-induced lubricant migration modeled either as viscous flow of a continuum liquid film with zero slip or as wind driven slippage of molecules across the surface. The experimental data are best fitted using the viscous flow model to determine an effective viscosity for the sub-nanometer thick lubricant films. This effective viscosity tends to be somewhat less than the lubricant bulk viscosity due to air shear promoting the slippage of lubricant molecules across the surface. Our experimental results also indicate that the potential spacing increase from the pickup of disk lubricant on the slider is limited by the mobile fraction of the dewetting thickness of the lubricant film on the slider.     

Head slider designs using approximation methods

This paper presents an approach to optimally design the air bearing surface (ABS) of the head slider by using the approximation methods. The reduced basis concept is used to reduce the number of design variables. In the numerical calculation, the progressive quadratic response surface modeling (PQRSM) is used to handle the non-smooth and discontinuous cost function. A multi-criteria optimization problem is formulated to enhance the flying performances over the entire recording band during the steady state and track seek operations. The optimal solutions of the sliders, whose target flying heights are 12 nm and 9 nm, are automatically obtained. The flying heights during the steady state operation become closer to the target values and the flying height variations during the track seek operation are smaller than those for the initial one. The pitch and roll angles are also kept within suitable ranges over the recording band.