Adhesion and Friction Evaluation of Textured Slider Surfaces in Ultra-Low Flying Head-Disk Interfaces

To achieve extremely high-density magnetic recording of 1Tbit per square inch using conventional technologies, the distance between the recording slider and the rotating disk needs to be less than 5nm. For successful operation, disk and slider surfaces must also be extremely smooth with root-mean-square roughness values of few angstroms. However, ultra-low flying super smooth head-disk interfaces may be exposed to a significant amount of intermittent contact, adhesion, stiction and friction that can cause the interface to collapse. In order to circumvent such problems, many novel techniques have been proposed, such as laser zone texturing, contact pads and surface microtexturing. A reliable method to reduce adhesion and friction in ultra-low flying head-disk interfaces is to control the area of contact and roughen the interface, which allows the slider to fly at sub-5nm with minimal contact. A technique known as preferential texturing provides a unique roughening of the air-bearing surface, where parts of the surface are removed, i.e., subtractive texturing process. In this paper, the effect of preferential texturing (roughening) of slider air-bearing surfaces on the adhesion and friction forces are investigated using quasi-dynamic models. The simulation results show that surface texturing reduces adhesion and friction by reducing the effective area of contact between the slider and media surfaces and by preferentially roughening the interface. The simulation results of friction compare favorably with experimental data.     

On tribological problems in magnetic disk recording technology

Critical tribology problems of the head-disk interface are reviewed. Surface topography of hard disks is discussed along with experimental results concerning the friction and stiction behavior of lubricated carbon coated disks. The effect of environmental conditions on the headdisk interface is analyzed together with current techniques to measure the flying height between slider and disk in the nanometer spacing range. The effect of air bearing design on the tribology of the head disk interface is discussed and a critical evaluation of recently proposed approaches towards contact recording is presented.     

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.     

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.     

Effect of hard-disk drive spindle motor vibration on dynamic microwaviness and flying-height modulation

In order to achieve higher recording densities up to 1 Terabit per square inch using conventional magnetic recording technologies, the recording slider will need to be physically spaced very close to the rotating disk, possibly via the use of an air-bearing surface. However, as the recording slider is flying at such ultra-low spacing of few nanometers over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition known as flying-height modulation (FHM), which may result in data loss and possibly head–disk interface failure. A significant source of slider excitation is due to low frequency surface topographical features of the rotating disk, termed dynamic microwaviness. Dynamic microwaviness is a dynamic property of the disk and differs from regular topographical microwaviness, which is a static property. Most research works on dynamic microwaviness and FHM have been focused at the component level, using somewhat idealized conditions, such as high performance air-spindle motors that exhibit very low vibration amplitudes. In this paper, actual hard-disk drive spindle motors are used to investigate the effect of spindle motor vibration on dynamic microwaviness and FHM. It is found that there is a clear connection between spindle motor vibration and dynamic microwaviness that affects FHM.     

Static and dynamic characteristics of active-head sliders

Active-head sliders with a unimorph piezoelectric actuator for flying height control were experimentally evaluated. It was found that the stroke of the actuator is 1.3 to 1.5 nm/V without flying over the disk. The adjustment amount of flying height is about 1.4 nm/V when the active-head slider is flying over the disk. It was found that flying height could be reduced and decrease from 24 to 10 nm by applying 10 V to the actuator under flying condition. Both the air pressure generated at the active-pad and the impact pressure due to the head/disk contact must be taken into account for precise control of flying height.     

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.     

Dominant Factors in Lubricant Transfer and Accumulation in Slider-Disk Interface

Lubricant transfer from disk to slider and lubricant accumulation on slider are very important in designing a stable slider-disk interface of ultra-low spacing. In this article, the effects of different parameters on the lubricant transfer and accumulation are studied and the reasons behind the effects are explained. Furthermore, the time for the lubricant transfer to reach steady state is estimated. It is found that lubricant molecular weight plays a dominant role in the lubricant transfer and accumulation. Lubricant transfer and accumulation decrease dramatically with the increase in lubricant molecular weight. Lubricant transfer also strongly depends on lubricant thickness and bonding ratio on disk surface. A thinner lubricant and higher lubricant bonding ratio on disk surface reduce lubricant transfer obviously, which results in less lubricant accumulation. A diamond-like-carbon (DLC) overcoat of low adsorption area density on slider surface can reduce lubricant transfer and accumulation, especially for lubricant of low molecular weight. Lubricant accumulation increases with disk velocity and increases slightly with the decrease in slider flying height. Lubricant accumulation can be reduced by minimizing the area of slider pad. Lubricant transfer and accumulation become worse at higher ambient temperature. It takes seconds for lubricant of low molecular weight to reach steady transferred thickness and hours for lubricant of high molecular weight to reach the steady state.     

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.     

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.     

Critical Operating Conditions For A Novel Flying Tester

Increased recording densities are often achieved through a reduction in the flying height over a thin film disk possessing diminishing surface roughness. Flying heights will continue to decrease until the head-disk interface (HDI) operates under quasi-contact conditions, i.e., ultra-low flying with intermittent slider-disk collisions. The failure mechanisms that occur in such quasi-contact devices may differ from those experienced in current, higher flying hard drive assemblies. In this paper, the authors will present the experimental, numerical, and theoretical tools that have been developed to study the behavior of the HDI under ultra-low flying conditions. These tools include an accelerated flyability tester and a numerical algorithm applicable to highly rarefied air bearings that possess large pressure gradients. Air bearing simulation results, as well as the results from a simple flying height scaling analysis, will be compared to flying test results in both air and helium to obtain insight into the stability of the HDI under accelerated testing conditions. A new concept introduced in this paper is that of critical conditions, i.e., the band of operating conditions which mark the transition from stable to erratic behavior, which can be determined both experimentally and theoretically. Such insight should provide design criteria for both quasi-contact storage devices, as well as novel accelerated wear testers.     

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.     

The Role of Slider/Disk Roughness in 1 Tb/in.2 Magnetic Recording

To achieve 1 Tb/in.² magnetic recording areal density, the head/disk spacing, or the flying height of the slider, has become so small that both the disk surface roughness and the slider air-bearing surface roughness need to be considered. In this region, the intermolecular force and the contact force become more significant due to the roughness of the two surfaces. This article targets two points: 1) slider/disk roughness effects on intermolecular force and 2) slider/disk roughness requirement for 1 Tb/in.² areal density. A probability model is built to simulate the intermolecular force and the contact force, and these two forces are introduced into the modified compressible Reynolds equation governing the air-bearing pressure of the slider. The equation is solved by the finite volume method based on an unstructured triangle-based mesh. The simulation results show that in 1 Tb/in.² areal density magnetic recording the effects of slider/disk roughness on the intermolecular force are negligible. Smaller R _a values will have fewer effects on flying performance.     

Effect of self-assembled monolayers modified slider on head-disk tribology under volatile organic contamination

Self-assembled monolayers (SAMs) with different endgroups were established on slider surface. The effect of the SAMs coated slider on head-disk tribology under volatile organic contamination (VOC) of octamethylcyclotetrasiloxane (D₄) was investigated using a contact start/stop (CSS) tester. The slider surfaces before and after the CSS tests were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS). The contact angle measurement and TOF-SIMS analysis proved that the SAMs were successfully formed on the slider surface. All the SAMs reduced the friction under the pollutant vapor. The transfer of lubricating oil onto the slider surface was detected after the CSS tests. It was found that the slider with a low surface free energy associated with small amount of lubricating oil transfer. The little the lubricating oil transfer was, the low the frictions were. These results indicate that a slider with low surface free energy can reduce the loss of lubricating oil from the disk surface, and hence improve the tribological properties of hard-disk interface (HDI) under VOC.     

Experimental and analytical studies of dynamic friction at the head/disk interface

Friction is an important parameter that critically impacts the tribological performance of a head/disk interface. The head/disk interface with laser zone texture affords a model system for the study of dynamic friction by virtue of its precisely-controlled contact geometry. By using two types of head sliders, i.e. the conventional slider and the padded slider, and a matrix of hard disks with a wide range of laser zone texture parameters, head/disk contacts involving a small number as well as a large number of bumps are realized. A rich variety of dynamic friction behaviors are observed with respect to bump height and bump density variations. To shed new light on the nature of HDI dynamic friction, an analytical model that treats both the deformational and the adhesive friction components on equal footings is formulated. It is shown that, based on the model analysis, the friction is deformation-dominated for HDIs involving a small number of contacting bumps and adhesion-dominated for HDIs involving a large number of contacting bumps. In the former case the friction decreases with bump density, whereas in the latter the friction increases with bump density.     

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。     

Interface technology of ultra-low flying height and highly stable head–disk interface for perpendicular magnetic recording

This paper reports authors’ efforts in slider and interface technologies with extremely small and very high stability head–disk spacing. The dual shallow step strategy is proposed in the femto form-factor slider design. It is found that the dual shallow step design is very effective in reducing flying height modulation (FHM) caused by disk waviness and enhancing the cooling effects on the read/write elements. A simple geometric model is built to explain the schematic of the improvement in FHM.     

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.     

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 %.     

A study of electrical charge at head-disk interface

The electrical charge at head-disk interface (HDI) of disk drives becomes increasingly important as head-disk spacing drops below 10 nm range. In this study, a new method of measuring electrical charge at HDI is presented. It involves measuring magnetic read-back signals (i.e. PW50), while the flying height (FH) is lowered by electrostatic force. Typical HDI charges are in the range of –0.2 to –0.9 V, depending on individual head–disk combination. Experiments were also conducted to eliminate the HDI charge by using an ionizer and surface treatment of magnetic heads. It was found that the HDI charge can be effectively eliminated by treating the magnetic head with a fluorinated carbon coating. The mechanisms of HDI charge generation and elimination are discussed.