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.     

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.     

Observation of Slider Landing Process in Hard Disk Drive

With the decrease in slider flying height, slider flying instability caused by slider–disk interactions is becoming a big concern. Novel technology has to be employed to further improve our understandings about slider–disk interaction. In this work, a slider flying height-attitude testing (3D) system was employed to study slider–disk interaction during a slider landing process to demonstrate its capability for the application. It is shown that great details of slider–disk interactions and subtle variations of the slider flying attitude during the landing process can be revealed with the 3D system. Slider dynamic flying height and attitude (pitch and roll angles) during the landing process can be determined from the data recorded in one test. Furthermore, analysis in frequency domain can be done not only on flying height, but also on pitch and roll angles directly. It is found that the slider landing process can have different stages during which slider performance and characteristics of slider–disk interaction are different.     

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.     

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.     

Flying Height Drop Due to Air Entrapment in Lubricant

Recently, it is found experimentally that the flying height of an air bearing slider is influenced by the lubricant on the disk. It is explained as the air molecules are entrapped in the lubricant under the slider due to the high air bearing pressure, causing the reduction in air bearing force, and hence, the flying height decreases accordingly. This paper employs both experiment and simulation to study such a phenomenon. First, the flying height vibration signals of a slider are detected by a laser Doppler vibrometer, on both lubed and delubed disks. It is observed that the heater touchdown power of the slider is approximately 3.4 mW more for delubed disk than the lubed disk. It suggests that the lubricant may cause the flying height lower. Second, a new model is developed to describe the pressure drop due to the air entrapment. Next, simulations are conducted on three different slider designs based on the new model. Flying height drops are investigated due to the air entrapment. The simulation results are compared with published experimental results, and good correlations are observed for the values of the parameters alpha and beta selected. Finally, the effects of solubility on the flying height are discussed, and the flying height drops are evaluated. It is suggested that the slider design must consider the phenomenon to get more accurate simulation results on flying height.     

Effects of temperature dependent air properties on the performances of a thermal actuated slider

Thermal actuated sliders have been 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 over the conventional slider. In this paper, we used a coupled-field analysis method, which includes an air bearing model, a heat transfer model and a thermal-structural finite element (FE) model to investigate the flying and thermal performances of a thermal actuated slider at various environmental temperatures. We also proposed a generalized mean free path model to incorporate various molecular dynamics models and consider temperature effects of the mean free path. Some temperature dependent air properties, such as the viscosity and the thermal conductivity are also considered in the simulation. It is found that the mean free path is a crucial parameter in determine air bearing and heat transfer across the head-disk interface (HDI). Our simulation results also show that the temperature effects of the viscosity and the thermal conductivity are contrary to that of the mean free path, which limit the variations of air bearing and heat transfer as the environmental temperature increases. However, their temperature effects still need to be considered for an accurate simulation, especially when the disk drives operate in a wide temperature range.     

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.     

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.     

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。     

Static and Dynamic Slider Air-Bearing Behavior in Heat-Assisted Magnetic Recording Under Thermal Flying Height Control and Laser System-Induced Protrusion

The air bearing’s response to regions of elevated temperature on its bounding surfaces (the slider and disk) may be an important consideration in the head–disk interface design of heat-assisted magnetic recording (HAMR) systems. We implement the general non-isothermal molecular gas lubrication equation into an iterative static solver and dynamic air-bearing solver to evaluate the effect of localized heating of the air-bearing surface (ABS) due to the near-field transducer (NFT). The heat-dissipating components in our simplified HAMR design are the NFT, laser diode, and thermal flying height control (TFC) heater. We investigate the effect of each HAMR slider component on ABS temperature and thermal deformation and the slider’s flying height. The NFT induces a localized thermal spot and protrusion on the larger TFC bulge, and it is the location of maximum temperature. This ABS temperature profile alters the air-bearing pressure distribution, increasing the pressure at the hot NFT location compared with predictions of an isothermal air-bearing solver, so that the center of the pressure acting on the ABS is slightly closer to the trailing edge, thereby decreasing the pitch angle and increasing the minimum flying height. Other researchers have shown that the NFT’s thermal response time may be much faster than its protrusion response time (Xu et al. in IEEE Trans Magn 48:3280–3283, 2012). The slider’s dynamic response to a time-varying NFT thermal spot on the ABS while the combined TFC and NFT induced thermal protrusion remains constant is investigated with our dynamic air-bearing solver. We simulate the slider’s step response to a suddenly applied ABS temperature profile and a pulsed temperature profile that represents laser-on over data zones and laser-off over servo zones. The sudden (step) or rapid (pulse) increase in ABS temperature induces a sudden or rapid increase in pressure at the NFT location, thereby exciting the air bearing’s first pitch mode. For the slider design and simulation conditions used here, the result of the pitch mode excitation is to alter the position of the center of pressure in the slider’s length direction, thereby changing the pitch moment. In response, the pitch angle and minimum flying height change. The step response decays after approximately 0.15 ms. Because the laser duty cycle is much shorter than this response time, a periodic disturbance is predicted for the center of pressure coordinate, pitch angle, and minimum flying height. The peak-to-peak minimum flying height modulations are relatively small (only up to 0.126 nm); more significantly, the time-averaged minimum flying height increases 0.5 nm for the NFT that reached 208 °C compared to simulations of the isothermal ABS at ambient temperature.     

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.     

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.     

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.     

Effect of environment humidity and temperature on stationary and transient flying responses of air bearing slider

It is well known that the environment humidity and temperature have a significant influence on the flying height of an air bearing slider. However, not many research papers address this topic, especially when the transient flying response is considered. This paper studies the influences of the environment humidity and temperature on both the stationary and transient flying responses of slider by simulation. A slider design for the thermal protrusion application is addressed. The reason for causing the drop of the air bearing pressure is discussed, and the methods for decreasing the drop are proposed. It is observed that the environment humidity and temperature may determine whether the slider is in full flying state or in partial flying/partial dragging state, when the slider is released from a certain height. The reason may be due to the high humidity and temperature which weakens the air bearing. As a result, the air bearing becomes not strong enough to support well the full flying of slider when the influence of the intermolecular force is significant. Slider vibrations for the full flying case and the partial flying/partial dragging case are analyzed in frequency domain, and the slider vibration frequencies are discussed. It shows that the environment temperature and humidity have significant effects on both the stationary and transient flying responses of the slider.     

Analysis and optimization of dynamic response of air bearing sliders to disk waviness

Flying stability has been becoming more critical for air bearing sliders with extremely low flying height (FH). Therefore, the effects of disk waviness on flying height modulation (FHM) cannot be neglected. This paper presents an analytical study on the mechanism of FHM of air bearing sliders due to disk waviness, and a design optimization for increasing waviness following ability of sliders. An analytical three-degree-of-freedom (3-DOF) model is developed, where the air bearings are modeled as six lumped linear springs and dampers. The purpose of this model is to develop a quantitative understanding of how air bearing sliders respond to disk waviness. The dynamic characteristics of the slider-air bearing system are then analyzed, and the closed-form frequency resonance function (FRF) of FHM to disk waviness is derived. The impact of disk surface features and the positions of the trailing pad, the side pads, the leading pads and the negative pressure center on FHM are also investigated using parametric analysis. The analysis results show that the improvement of the roll-off characteristics of the disk surface waviness can also decrease the FHM. In addition, shortening the distance between the trailing pad pressure center and the head position, moving backward the side pads and leading pads and forward the negative pressure center can increase waviness following ability of the slider. Finally, an air bearing slider is designed according to the proposed design strategies for reducing the FHM due to disk waviness.     

Investigation of Head/Disk Contacts in Helium–Air Gas Mixtures

The shock response of a pico-type magnetic recording slider in different helium–air gas mixtures is investigated numerically. A finite element-based air bearing simulator and a slider/disk contact model including van der Waals and friction forces are coupled to determine the contact characteristics between slider and disk. The minimum flying height and the maximum contact force are studied as a function of helium percentage and disk velocity. The results show that the dynamic performance of the slider is not affected substantially as long as the helium percentage is <50 % but is increasingly more affected if the helium percentage becomes larger than 50 %.     

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.     

Frequency Analyses of Air Bearing Slider in Near Contact and Contact States

Determining the air bearing frequencies is important and essential to understand the complex flying performances of slider. However, in typical flying height test, it is usually difficult to distinguish the air bearing frequencies in the experimental data, especially when the slider is in full flying state. In such a case, it is optional to employ simulation to help determine the air bearing frequencies and their harmonic components. This paper performs both time and frequency domain simulations to analyze air bearing frequencies of a pemto slider and compare the results with experimental data obtained using laser Doppler vibrometer. It is found that the simulation results are well correlated with the experimental results. Time domain simulation provides not only the air bearing frequencies, but also the harmonic frequencies. Frequency domain simulation, on the other hand, provides clear identification of the three dominant air bearing frequencies, with additional information such as air bearing stiffness and damping obtainable. It is suggested that simulations in both time and frequency domains should be conducted to assist in determining the air bearing frequencies and their harmonic components.     

Contact force studies of a burnishing slider

In order to design the flying height of a burnishing slider accurately, the contact force between the burnishing slider and the disk needs to be well evaluated. This paper studies the contact force of a burnishing slider by both experiment and simulation. The experiment is conducted by measuring the acoustic emission signals of the contact force avalanche, and the simulation is based on the self-developed air bearing surface simulation code applying the probability model for the contact force calculation. The influence of contact force on the burnishing effect is discussed. It is observed that the simulation results are well correlated with the experimental measurements. It is believed that the simulation code is capable to design burnishing sliders with reasonable accuracy.