Thickness Change in Molecularly Thin Lubricant Under Flying Head in Hard Disk Drives

In hard disk drives (HDDs), lubricants on disks are very important material to reduce head and disk wear. Thus, it is necessary to know changes in lubricant thickness to keep lubricant thickness constant on rotating disks. For this purpose, we have to know changes in lubricant thickness during HDD operations. We developed a simulation program to simulate changes in lubricant thickness during HDD operations numerically. First, we had simulated the changes in lubricant thickness of 10-nm-thick non-polar lubricant film under a flying head. The result corresponded to a reported experimental result. In recent HDDs, a lubricant thickness has become molecularly thin and lubricants with polar end groups have been used. In molecularly thin polar lubricants, diffusion depends on their thickness and their viscosity becomes very high. Next, we simulated the change in the lubricant thickness of 2-nm-thick polar lubricant film considering the effects of lubricant initial thickness. The simulated results showed that the changes were very small in 2-nm-thick lubricant film, but they were not confirmed with the experiment. In this paper, experimental results of the change in the thickness of molecularly thin non-polar and polar lubricants under a flying head were first measured. The simulations that took account of thickness-dependent diffusion and thin-film viscosity were then performed with the simulation parameters based on the experiments. The simulated results of lubricant distribution were in good agreement with the experimental results.     

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.     

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.     

The Influence of Lubrication on Head and Disk Wear

Magnetic disks are usually lubricated with fluorocarbon-type lubricants to reduce head and disk wear during the start/stop process of the disk rotation. In this paper, the influence of disk lubrication on the tribological characteristics of the head/disk interface is investigated by pin-on-disk wear tests and the head/disk friction tests.

The anti-wear performance of a lubricant is very high. For example, a lubricant coating of 8.4 × 10^?5 mg/cm² exhibits 1/20 of the ferrite pin wear rate of an unlubricated disk. For a lubricated disk, ferrite pin wear decreases at increased sliding velocities as high as 10 m/s, while pin wear increases rapidly with increased velocity for an unlubricated disk. The lubricant used here performs well in suppressing the wear increase caused by increased load. Regarding friction characteristics, however, an excessive amount of lubricant induces severe head/disk sticking, causing head crash. With respect to head/disk sticking, the upper-limit of the amount of lubricant is 8.4 × 10^?5 mg/cm².     

Lubricant Flow and Evaporation Model for Heat-Assisted Magnetic Recording Including Functional End-Group Effects and Thin Film Viscosity

The lubricant covering a hard disk in a heat-assisted magnetic recording drive must be able to withstand the writing process in which the disk is locally heated several hundred degrees Celsius within a few nanoseconds to reduce the coercivity of the media and allow writing of data. As a first step in modeling a robust lubricant, we have developed a simulation tool based on continuum theory that incorporates previously proposed variations of viscosity and an additional component of disjoining pressure due to functional end-groups with film thickness. Here we apply this simulation tool to a conventional perfluoropolyether lubricant, Zdol 2000, for which there exists experimental data. The simulation tool can be used equally well for other lubricants once their properties become known. Simulations at small length and time scales that are unobservable with current experimental capabilities are performed. We investigate the effect of the total disjoining pressure and thin film viscosity on evaporation and lubricant flow for different initial thickness. For films thicker than 1 nm, the inclusion of polar disjoining pressure suppresses the lubricant thickness change due to evaporation and thermocapillary shear stress compared with cases without this component. Thin film viscosity is an important property to consider for thinner lubricants. We also consider how lubricant depletion depends on laser spot size and thermal spot maximum temperature. The smaller spot profiles exhibit side ridges due to thermocapillary shear stress while the larger spot profiles show no side ridges, only a trough due to evaporation. The lubricant depletion zone width and depth increase with increasing thermal spot maximum temperature.     

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.     

Slider Wear on Disks Lubricated by Ultra-Thin Perfluoropolyether Lubricants with Different Molecular Weights

In this study, the wear properties of a magnetic head slider on disks lubricated by ultra-thin perfluoropolyether (PFPE) lubricants with different molecular weights were evaluated by the continuous sliding of magnetic head sliders using the slider contact by the dynamic flying height control. Two types of PFPE lubricants (Z-tetraol and D-4OH) with different molecular weights were evaluated. Results show that the slider wear depended on the coverage of the lubricant film; i.e., the lubricant film with sufficient coverage reduced slider wear. The lubricant film with a low molecular weight (low-Mw), including a lubricant material with a Fomblin and Demnum main chain, exhibited better coverage on a diamond-like carbon surface. Sliders with a low-Mw lubricant film showed less wear than those of a high molecular weight (high-Mw), and the depletion of the low-Mw lubricant film was less than that of the high-Mw lubricant film.     

Lubricant Spin-Off from Magnetic Recording Disks

As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 Å of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 Å thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.     

Fiber Wobbling Method for Dynamic Viscoelastic Measurement of Liquid Lubricant Confined in Molecularly Narrow Gaps

An understanding of the viscoelastic properties of molecularly thin lubricant film is essential to clarify tribological issues of head-disk interface (HDI) in high-density recording hard disk drives. Characteristic conditions for the HDI occur when lubricant molecules are extremely confined in the gap between the head and the disk surfaces, and the surfaces slide at high speeds. The lower the flying height, the more this confinement affects the flying characteristics. However, a few attempts have been made at clarifying the dynamic viscoelastic properties of confined lubricant molecules. This is because a method of measuring the dynamic shear force has not yet been established. Fiber wobbling method enables us to measure the shear force with a detection limit of less than 1 nN. Additionally, frequency of shear can be set at several kHz. Further, the gap which confines the lubricant is controlled with a resolution of 0.1 nm. Using the FWM, we investigated the effect that confinement had on the dynamic viscoelastic properties of perfluoropolyether lubricants on a magnetic disk. We found that the viscosity started to increase at a gap width that was less than a few hundred nanometers, which is hundreds of times larger than the molecular size. On the other hand, elasticity suddenly appeared at a gap width that was less than a few nanometers, which is equivalent to a few molecular sizes. Both the viscosity and elasticity increased monotonically as the gap decreased.     

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.     

Effect of Rheology and Slip on Lubricant Deformation and Disk-to-Head Transfer During Heat-Assisted Magnetic Recording (HAMR)

The high-temperature laser heating during heat-assisted magnetic recording (HAMR) causes the media lubricant to deform and transfer to the head via evaporation/condensation. The ability of the lubricant to withstand this writing process and sufficiently recover post-writing is critical for robust read/write performance. Moreover, the media-to-head lubricant transfer causes a continuous deposition of contaminants originating from the media at the head near field transducer, challenging the reliability of HAMR drives. Most previous studies on the effects of laser exposure on lubricant depletion have assumed the lubricant to be a viscous fluid and have modeled its behavior using traditional lubrication theory. However, Perfluoropolyether lubricants are viscoelastic fluids and are expected to exhibit a combination of viscous and elastic behavior at the timescale of HAMR. In this paper, we introduce a modification to the traditional Reynolds lubrication equation using the linear Maxwell constitutive equation and a slip boundary condition. We study the deformation and recovery of the lubricant due to laser heating under the influence of thermocapillary stress and disjoining pressure. Subsequently, we use this modified lubrication equation to develop a model that predicts the media-to-head lubricant transfer during HAMR. This model simultaneously determines the deformation and evaporation of the viscoelastic lubricant film on the disk, the diffusion of the vapor phase lubricant in the air bearing, and the evolution of the condensed lubricant film on the head. We investigate the effect of viscoelasticity, lubricant type (Zdol vs Ztetraol), molecular weight, slip, and disjoining pressure on the lubricant transfer process.     

Effects of Gas Composition,Humidity and Temperature on the Tribology of the Head/Disk Interface—Part II: Model and Analysis

A qualitative model for the effect of water condensation on the frictional behavior of unlubricated and lubricated carbon-overcoated disks is presented. The model suggests that for unlubricated disks adsorbed water acts as a lubricant, protecting the unlubricated disk surface from direct solid/solid contact and direct exposure to the environment. For lubricated disks, the interaction between adsorbed water and lubricant molecules seems to be responsible for the effect of humidity on the frictional behavior of lubricated disks. The effect of temperature on the frictional behavior of the head/disk interface is discussed in terms of surface energy, lubricant viscosity and mobility.     

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.     

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.     

Laplace pressure measurement on laser textured thin-film disk

To increase the recording density of hard disk drives (HDD), head and disk surfaces must be very flat. This will make the friction between them large when liquid bridges are formed. This is a result of Laplace pressure in the liquid bridge. Therefore, the study of Laplace pressure in real HDD interface is of an interest for head-disk interface engineers. However, Laplace pressure of perfluoropolyether (PFPE) lubricant on carbon coated thin-film disk surface was not clear until now.We measured Laplace pressure between transparent flat pins and carbon coated thin-film disks with laser texturing. Using laser textured disks, we could control the distance between two surfaces precisely by the bump height. The friction coefficient between the pin and the disk surfaces was determined when the interface was fully wet by liquids. It was 0.16 and 0.1 for water and a PFPE lubricant. The Laplace pressure was then calculated using the friction force and liquid wet area when the interface was partially wet by a liquid. The liquid wet area was measured by the observation of the contact point through the transparent pins.The results showed that the Laplace pressure at the lowest bump height (11 nm) was about 2.8 MPa for the PFPE lubricant. Results agreed well with calculated curves. We consider that PFPE acts as liquid down to 11 nm.     

Ultrafiltered Perfluoropolyether Lubricant under Molecular Weight Distribution Control

There are many problems to be overcome when storage devices are used at high-speed rotation and very small spacing between the head and disk. One of them is lubricant spin-off. Lubricant spin-off and lubricity have a close relation to the molecular weight distribution. The commercial Perfluoropolyether (PFPE), which is widely used as a lubricant for magnetic disks, has a wide molecular weight distribution from several hundred to over ten thousand. In order to improve performance regarding spin-off and lubricity, it is necessary to control the molecular weight distribution.

This paper discusses the relation between molecular weight of lubricants and spin-off, and stiction. The molecular weight at which spin off occurs easily is found to cause a high stiction. Furthermore, molecular weight distribution control by ultrafiltering is investigated.     

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording over the Lifetime of the Drive

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of hard disk drive. However, it is expected that HAMR causes lubricant depletion problem on disk surface under the high temperature in the heating assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. Lubricant depletion over the lifetime of the drive still remains unaddressed. In this study, a self-developed HAMR tester is introduced. The methods to control the repeatability of laser heating temperature, to determine laser heating temperature, and to adjust laser heating temperature are explained. Laser heating time in the test is correlated with that in the drive. Lubricant depletion is determined quantitatively and the relationship between lubricant depletion depth and laser heating time is established. Then, lubricant depletion depth over the lifetime of the drive is predicated. It is found that almost all lubricant on the disk surface will be depleted over the lifetime of the drive.     

Molecular dynamics studies of lubricant depletion under moving laser heating: Effects of laser power and film thickness

Molecular dynamics simulation is employed to study the depletion behaviors of perfluoro-lubricants under scanning laser heating for heat-assisted magnetic recording hard disk drives. A partial lubricant near the substrate is irradiated by the laser beam to mimic nano-scale heat transfer from disk to lubricant. The lubricant surface morphology and thickness profiles are examined to reveal the dynamic depletion behaviors. The localized temperature evolution is also evaluated to illustrate the direction-dependent ridge formation around the depletion zone. In addition, the effects of laser power and film thickness on lubricant depletion are explored. Although evaporation is enhanced significantly at high laser powers or for lubricant with thickness around one monolayer, thermodiffusion is the primary mode of lubricant depletion under scanning laser heating.     

Dynamic Adhesion Characteristics of Spherical Sliders Colliding with Stationary Magnetic Disks with a Thin Lubricant Layer

The dynamic indentation characteristics of 1- and 2-mm-radius hemispherical glass sliders when colliding with stationary magnetic disks under various lubricant conditions were investigated to clarify the dynamic interfacial forces between flying head sliders and magnetic disks. The collision times were ~15 and ~30 μs, respectively, and independent of the impact velocity. For a 1-mm-radius slider (Ra roughness = 1.71 nm), a clear adhesion force nearly equal to the static pull-off force was observed at the instant of separation when the lubricant thickness was from 1 nm without UV (0.69 nm mobile lubricant thickness) to 3 nm with UV (1.89 nm mobile lubricant thickness). The dynamic adhesion force was maximum when the slider had separated from the disk surface by about 2 nm and dropped from the maximum to zero when the separation reached more than 5 nm. When the mobile lubricant thickness was 0.43 nm, a clear adhesion force was not observed. For a 2-mm-radius slider (Ra roughness = 0.34 nm), a clear adhesion force, similar to the static pull-off force, was observed at the instant of separation at almost all lubricant thicknesses and impact velocities tested except at a small mobile lubricant thickness of 0.43 nm with impact velocities greater than 1.1 mm/s. The dynamic adhesion force dropped from the maximum to zero when the distance traveled from the maximum reached more than 5 nm. These results suggest that the dynamic adhesion force of 1- and 2-mm-radius sliders originates from meniscus formation rather than van der Waals force.     

Effect of Molecularly Thin Lubricant on Roughness and Adhesion of Magnetic Disks Intended for Extremely High-Density Recording

For extremely high-density recording using conventional technologies, the fly-height needs to decrease to less than ten nanometers. To allow such operation, disk and slider surfaces must become extremely smooth, down to root-mean-square (RMS) roughness values of a few angstroms. For super-smooth disks, molecularly thin lubricants are applied to improve tribological performance of head/disk interfaces. The focus of this study is to quantify the effect of lubricant thickness in terms of detailed roughness parameters and to evaluate the effect of roughness and molecularly thin lubricant on adhesion of magnetic disks intended for extremely high-density recording. Three identical ultra-low-flying disks have been fabricated from the same batch for this particular experiment. To investigate the effect of molecularly thin lubricants on disk roughness, super-smooth magnetic disks with increasing lubricant thickness have been measured and studied, using a primary roughness parameter set. It describes amplitude, spatial, hybrid, and functional aspects of surface roughness and is used to quantify the extremely smooth disk roughness as a function of lubricant thickness. It is found that in addition to simple amplitude parameters, hybrid and functional parameters also capture small features on the disk roughness and show distinct trends with increasing lubricant thickness. Subsequently, a continuum-based adhesion model that uses three parameters from the primary roughness parameter set, is used to predict how the varying thickness of molecularly thin lubricant and the resulting disk roughness affect intermolecular forces at ultra-low-flying head-disk interfaces. It is found that a thicker lubricant layer of 2nm causes higher adhesion forces for ultra-low-flying-heights in the range of 1–3 nm