Investigation of Slider Dynamics Considering Bias Voltage and Lubricant Charge in the Unsteady Proximity Condition

The relationship between slider and lubricant becomes increasingly important as the mechanical spacing between slider and disk is reduced to satisfy the demand for higher areal density. At a reduced flying height, the slider easily contacts the lubricant, which can cause slider instability. This study analyzed slider dynamics to improve the head–disk reliability in the unsteady proximity condition, considering bias voltages between the slider, disk, and lubricant. Force–distance curves were measured using atomic force microscopy to investigate changes in lubricant performance induced by an applied voltage. Additionally, the touch-down power and take-off power were measured under various applied voltage conditions. Experiments were carried out to estimate slider instability as a function of charged disk and slider conditions, to improve the slider dynamics in the unsteady proximity condition. The effect of the bias voltage induced by a voltage applied to the lubricant was carefully examined to accurately understand slider dynamics. The relationship between the lubricant behavior and the applied voltage was investigated; the voltage applied to the disk was more influential in improving slider dynamics. Consequently, the effects of bias voltage and lubricant, as induced by a charged disk, should be considered when analyzing slider dynamics to improve head–disk interface reliability in an unsteady proximity condition.     

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.     

Molecular Dynamics Simulation of Lubricant Redistribution and Transfer at Near-Contact Head-Disk Interface

When the spacing between the slider and lubricant in a hard disk drive decreases to less than 5 nm, the effect of the intermolecular force between these two surfaces can no longer be ignored. This effect on the lubricant distribution at the near-contact head disk interface is investigated via molecular dynamics method. In this study, the lubricant is confined between a smooth disk surface and a rough slider surface represented as a partially cosinusoidal wave. The simulation results reveal that the intermolecular force-induced meniscus formation at the near-contact head disk interface is strongly sensitive to the slider-to-disk separation, lubricant film thickness and the asperity shape (or roughness) of the slider. The attractive van der Waals forces between the slider and lubricant become weaker with increasing slider-to-disk separation and asperity mid-height, but decreasing lubricant film thickness and asperity mid-width. The Hamaker theory application to van der Waals interactions is also introduced to verify the molecular dynamics simulation. It is found that the critical separation, below which the lubricant will lose its stability to form a meniscus, increases approximately linearly with the lubricant film thickness, for slider surfaces with or without roughness both in the molecular dynamics simulation and Hamaker theory application to van der Waals interactions. Moreover, it is observed that the critical separation between a smooth disk and rough slider surface will slightly decrease when the asperity mid-height increases. The same phenomenon is observed when the asperity mid-width reduces.     

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。     

The effects of slider design on thermal asperity rejection capability

Particle contamination has been an ongoing problem affecting the reliability of the magnetic hard disk drives. Especially the recent use of MR head requires much tighter control of particle contamination due to thermal asperity (TA) phenomenon. In this study, the effects of slider air bearing surface design on TA reduction capability were investigated by manufacturing two types of sliders. Numerical methods were used to simulate the motion of particles in the head/media interface. Experiments were conducted to verify the results predicted by the numerical simulation. Drives were built and exposed to particle contamination using a particle injection chamber, which turned out to be a very simple and reliable particle generation method over conventional aerosol technique. Then the number of TA generated in the drives was recorded and compared. Also the contacts between slider and particles were investigated by acoustic emission study. It was found that a new ABS design, which has aerodynamic U-shaped rail and central flow passage, was beneficial in reducing the particle contamination on the slider.     

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.     

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.     

An Air Bearing Slider for Small Diameter Disk Storage Devices

The authors propose a novel slider design. The tri-pad slider has three separate air bearing surfaces, one in the front of the slider and the other two in the back of the slider. This design permits the optimization of the air bearing stiffness and flying attitude without changing the minimum flying height. It can minimize the effect of skew angle on the flying height, and offers ample room for a larger size thin film head element. Samples were fabricated and flying characteristics at several velocities, as well as the dynamic flying stability, were investigated.     

Effect of Air and Helium on the Head–Disk Interface During Load–Unload

The tribological characteristics of the head–disk interface are investigated during load–unload for air and helium-filled drives as a function of the pitch static angle and the roll static angle between slider and disk. A custom-made experimental tester inside a sealed environmental chamber was used to determine the regions of “safe” pitch static angle and “safe” roll static angle in air and helium environment during the load–unload process. The presence of head–disk contacts during load–unload were evaluated by measuring the acoustic emission signal and the decrease in rotational speed of the spindle. Scanning electron microscopy and optical surface analysis were used to investigate wear of the slider and the redistribution of lubricant on the disk surface after 10,000 load–unload cycles. The results indicate that the tribological performance of the head–disk interface is improved in helium environment compared to air environment.     

Stability of Gas Bearing Sliders for Large Bearing Number: Convective Instability of the Tapered Slider©

The dynamics and stability of tapered air bearing sliders used for computer hard disk drive magnetic recording heads is examined using analytical methods. Lubrication theory is applied to determine the lift on the slider from the Reynolds equation in the limit of large bearing number. The dynamics of the slider are given by a nonlinear integro-differential equation. Linear stability analysis of this model yields valuable insight into the behavior of the slider. Most significantly, it is found that convective effects can not be neglected and yield either damping or instability depending on the slider configuration. This analysis is also applied to determine the response of the slider motion due to deviations in the disk surface.     

Thermal sensor for in situ flying height measurements of optical flying heads

The application of flying head technology in optical recording promises a considerable increase in recording density. It may be used for both far field and near field recording. For dynamic in situ flying height and flight attitude measurements, a prototype of a thermal test head has been developed. It takes advantage of the heat transfer in sub-micrometer air bearings as well as its dependence from the air gap width. Such a test head has a slider with a similar air bearing surface as the recording head, but with thermo-resistive sensors that are energized by Joule's heat and embedded at each of the four corners. The paper describes the head design and the fabrication technology and provides first experimental test results as well.     

A Model for Lubricant Transfer from Media to Head During Heat-Assisted Magnetic Recording (HAMR) Writing

One of the challenges in heat-assisted magnetic recording (HAMR) is the creation of write-induced head contamination at the near-field transducer. A possible mechanism for the formation of this contamination is the transfer of lubricant from the disk to the slider (lubricant pickup) due to temperature-driven evaporation/condensation and/or mechanical interactions. Here we develop a continuum model that predicts the head-to-disk lubricant transfer during HAMR writing. The model simultaneously determines the thermocapillary shear stress-driven deformation and evaporation of the lubricant film on the disk, the convection and diffusion of the vapor phase lubricant in the air bearing and the evolution of the condensed lubricant film on the slider. The model also considers molecular interactions between disk–lubricant, slider–lubricant and lubricant–lubricant in terms of disjoining pressure. We investigate the effect of media temperature, head temperature and initial lubricant thickness on the lubricant transfer process. We find that the transfer mechanism is initially largely thermally driven. The rate of slider lubricant accumulation can be significantly reduced by decreasing the media temperature. However, as the amount of lubricant accumulation increases with time, a change in the transfer mechanism occurs from thermally driven to molecular interactions driven. A similar change in transfer mechanism is predicted as the head–disk spacing is reduced. There exists a critical value of head lubricant thickness and a critical head–disk spacing at which dewetting of the disk lubricant begins, leading to enhanced pickup.     

表面结构对纳米级气膜稀薄流域分布的影响

磁头与磁盘的特征高度目前已经下降到纳米量级,在此微小间隙下,气体表现出明显的稀薄效应特征。建立适用于纳米间隙下的控制方程,并根据方程特点采用特殊处理方法成功对控制方程进行数值求解,采用数值分析和实验方法分析表面结构变化对稀薄流域和磁头飞行姿态的影响。研究结果表明：在纳米级气膜润滑间隙下,采用逆 Knud-sen 数来划分稀薄流域比仅从特征膜厚高度方面考虑更合理;负压型磁头的主要工作区间在滑流区和过渡区,且过渡流域所占比例要明显高于滑流区域。稀薄效应最大的区域不是在气膜厚度最薄的磁头尾部,而是在压力突然下降且气膜较薄的阶梯过渡区域;磁头 U 型气垫、尾端两侧浅台阶和中间台阶结构变化会影响气流流向,从而影响压力分布,使得稀薄流域也跟随发生变化。     

计算机磁头/磁盘超薄气膜润滑稳定性

以任意拉森数的超薄气体润滑方程为基础,给出磁头刚体小扰动对空气轴承滑块 (ABS)气膜压强摄动方程。采用算子分裂法求解气膜压强和非结构三角网格的有限元法解压强摄动方程,得到气膜的刚度系数和阻尼系数矩阵。模态分析得到磁头气固耦合系统的固有频率,衰减率和振型。以Ω型磁头为例,分析了在不同气膜厚度和磁盘转速下的磁头稳定性。研究结果表明,磁头稳定性对气膜厚度很敏感,在小气膜厚度运行时,系统固有频率高,稳定性好;磁头升沉和纵倾运动的动力耦合,使磁头系统固有频率和衰减率降低,对稳定性不利;高转速的磁盘对磁头稳定性不利,但影响不大。     

基于双曲正割函数趋近律的多轴调平滑模控制研究

为解决复合材料压机多轴调平系统的控制精度不高的问题,将滑模变结构控制算法应用到多轴调平系统中。开展了多轴调平系统的不平衡机理、调平控制策略及算法的研究,建立了多轴调平系统的数学模型。结合基于双曲正割函数改进的指数趋近律,提出了一种多输入多输出滑模变结构控制算法,给出了控制器的设计方法,并将其应用到滑块的水平控制中。通过仿真分析和试验验证相结合的方式验证了算法的有效性。研究结果表明:该控制器能削弱抖振,使多轴调平的滑块系统快速准确地进行调平,能较好地抑制干扰和未建模动态,提高了系统的鲁棒性,且控制器简单,易于实现。     

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.     

滑块式万向接轴的强度和结构分析

通过对多个滑块万向接轴计算工作的总结,介绍了滑块式万向接轴强度分析过程中叉扁头建模时的载荷处理技巧、叉扁头的应力分布规律及不同叉扁头结构对应力的影响等。最后还讨论了滑块式接轴强度有限单元法和经典计算方法的区别及实践应用时的处理方法。     

Experimental and theoretical evaluation of friction at contacting magnetic storage slider–disk interfaces

To understand better the friction force and wear processes at contacting slider–disk interfaces, we have developed an experimental method for measuring and a theoretical method for calculating the friction force. For this study, a slider with a 1500 μm² contact pad located at the recording head is burnished against a relatively rough disk (～12 Å rms), which ensures smooth sliding. In the experimental method, the friction force is measured as the disk is spun-down to bring the slider–disk interface into an increasing degree of contact. A modified air bearing code is used to determine the experimental normal contact force for each friction measurement. In the theoretical method, the friction force and other relevant interfacial forces are calculated using an improved sub-boundary lubrication (ISBL) rough surface model. The friction force calculation in this model is based on the force needed to induce yielding of the individual disk asperities contacting the flat surface of the contact pad without any assumption of the coefficient of friction. Good agreement is found between the measured and theoretical friction vs. normal contact force curves, indicating that the model is capturing the essential origins of friction at this interface. The model also provides valuable insights into how wear particles may be generated at this contacting slider–disk interface.     

Load/unload measurements using laser doppler vibrometry and acoustic emission

The dynamics of the load/unload process are studied using a so-called ‘periscope approach’ which allows us to follow the slider motion during load/unload (L/UL) with the beam of a Laser Doppler Vibrometer (LDV). LDV signals and acoustic emission signals are obtained for three different slider airbearing designs and for load/unload conditions with different vertical velocities and spindle speeds. The load process is investigated statistically using the acoustic emission signal in order to determine the effect of vertical load speed and spindle speed on the probability of contacts between slider and disk.The results indicate that small vertical load speeds decrease the number of head/disk contacts, and that slider designs with a cavity centered close to the trailing edge enable a smooth unloading process.     

Investigation of Flying-Height Stability of Thermal Fly-Height Control Sliders in Lubricant or Solid Contact with Roughness

When the magnetic spacing in hard disk drives is reduced to sub-3 nm, contact between the slider and disk becomes inevitable. Stability analysis is used in this study to investigate the head–disk interface (HDI) stability of thermal fly-height control (TFC) sliders in light contact with the disk lubricant or solid roughness. We implement an improved DMT model with sub-boundary lubrication into the CML air bearing program and analyze the stability of equilibrium states of a TFC slider under different thermal actuations. It is found that stability is lost when the slider penetrates deeper into the lubricant layer, due to a fast growth in the adhesion force, and it is restored when the solid roughness contact develops. In addition, the critical point for the onset of this instability and the range of this instability region is found to vary with lubricant thickness and protrusion surface steepness, while keeping the air bearing design the same.