Head-disk Interface Of Hard Disk Research Articles

Recent Trends in Nano-Tribology of Head-Disk Interface in Hard Disk Drives

Recent Trends in Nano-Tribology of Head-Disk Interface in Hard Disk Drives

Surface evolution of perfluoropolyether film at high speed quasi-contact conditions

Nanoscale analysis characterized by microscopy with atomic resolution demand that the targeted surface remains nearly static. Therefore, the interaction between two fast moving surfaces requires a unique methodology to capture its dynamics when contacts are of nominal area on the order of 100 μm2 but only a few angstroms in depth. We present a contact study of the head-disk interface in hard disk drives, which consists of a disk surface coated with a molecularly thin perfluoropolyether lubricant and a slider surface moving slightly separated from it with a relative velocity of 20 m/s and with 10 nm spacing. By investigating the slider dynamics and lubricant topography in-situ, we disclose that high-speed contact initiates when the slider shears the top surface of the lubricant. Such contact can pile up molecules a few angstroms high as “moguls” or annihilate existing ones through a 5–10 Å interference. The transitional spacing regime of mogul evolution is defined as “quasi-contact,” and it is the initial contact in the fast sliding interface.

Dynamics of head-disk interface in hard disk drives during operational shock

Dynamic responses of the head-disk interface (HDI) and other mechanical components, such as suspension, disk, ramp and base are critical to the mechanical robustness of a hard disk drive (HDD) during the operational shock (op-shock). The conventional op-shock simulations use the simplified linear spring model to decouple the structural and air bearing analyses. This paper presents a coupled field analysis method to integrate the air bearing model with the reduced structural model based on the finite element model of the full HDD, including the all major mechanical components for op-shock simulations. The dynamics of HDI and mechanical components in HDDs during the op-shock are then investigated by simulations. The effects of air bearing nonlinearity and the disk-ramp contact on the HDI responses to op-shocks are evaluated and understood by comparisons to the simplified linear model. The results reveal that conventional de-coupled simulation methods are no longer valid for the complicated nonlinear op-shock events, especially when the disk ramp contacts or slider-disk contacts are considered. The newly developed coupled-field method is suitable for such op-shock simulations by considering the complexity of the system nonlinearity.

Nanoscale wear and kinetic friction between atomically smooth surfaces sliding at high speeds

The kinetic friction and wear at high sliding speeds is investigated using the head-disk interface of hard disk drives, wherein the head and the disk are less than 10 nm apart and move at sliding speeds of 5–10 m/s relative to each other. While the spacing between the sliding surfaces is of the same order of magnitude as various AFM based fundamental studies on friction, the sliding speed is nearly six orders of magnitude larger, allowing a unique set-up for a systematic study of nanoscale wear at high sliding speeds. In a hard disk drive, the physical contact between the head and the disk leads to friction, wear, and degradation of the head overcoat material (typically diamond like carbon). In this work, strain gauge based friction measurements are performed; the friction coefficient as well as the adhering shear strength at the head-disk interface is extracted; and an experimental set-up for studying friction between high speed sliding surfaces is exemplified.

TuA-2-3 Experimental and Numerical Investigation of Hydrocarbon Transfer Mechanism at the Head-Disk Interface

In hard disk drives, hydrocarbon oils are used to lubricate the disk spindle and the actuator pivot. During disk drive usage, oil molecules can escape, resulting in contamination of the head-disk interface. In this study, experimental investigation and molecular dynamics simulations are used to study hydrocarbon contamination at the head-disk interface in hard disk drives. A "two-step" transfer mechanism is assumed: Hydrocarbon molecules first condense onto the disk surface from the vapor phase, and then transfer to the slider surface in the absence of head-disk contacts. Experimental results and molecular dynamics simulations verify the assumed transfer mechanism.

Detection of Asperity Contact for Precise Gap Determination in Thin-Film Nanorheometry

Understanding the mechanical properties of a nanometer-thick liquid lubricant film coated on a magnetic disk surface is essential for effective lubrication design of head-disk interface in hard disk drives. We have developed a highly sensitive shear force measurement method, named the fiber wobbling method (FWM), that enables us to measure the viscoelastic properties of such films. For these measurements, the shearing gap needs to be determined precisely since the thickness of the film is around 1–2 nm, and the accuracy of the gap determination depends on the sensitivity of the solid contact point detection, which is considered to correspond to a gap width of zero. In this study, we present a sensitive method for solid contact detection in the FWM that relies on the monitoring of the resonant oscillation of a shearing probe excited by asperity contact. The feasibility of the method is verified by both numerical calculations and experiments. The applicability to a magnetic disk with a lubricant film is also examined and we confirmed the method had an enough sensitivity to determine the nanometer-sized gap width.

Simultaneous Measurement of Film Deformation and Friction Force During Shearing of Molecularly Thin Lubricants

The shearing deformation of the lubricant film was measured by using two-stage imaging ellipsometric microscope (TIEM) combined with an optical-fiber-based friction force measurement method. The TIEM enables high-lateral-resolution and real-time measurements of the thickness distribution of the nanometer-thick lubricant films during shearing. By adding a relay lenses illumination system and a high-power LED light source to the TIEM, a thickness resolution of 0.14 nm was achieved. The results of an experiment using this apparatus demonstrated that the asynchronous component of the probe oscillation, which is induced by friction force between the probe and lubricant or disk and corresponds to acoustic emission avalanches in the head-disk interface of hard disk drives, can occur when the probe is in contact with the lubricant film. The time constant of replenishment of the sheared lubricant film was of the order of hundreds of microseconds, which is much shorter than the timescale of diffusion phenomena.

Spreading Properties of Monolayer Lubricant Films: Effect of Bonded Molecules

Monolayer lubricant films applied to head-disk interface of hard disk drives consist of bonded and mobile molecules. We measured the diffusion coefficient of mobile molecules that spread through the spaces not covered with bonded molecules and revealed its dependence on the coverage fraction. The diffusion coefficient gradually decreased with increasing coverage from 0.2 to 0.8, and its trend was well represented by reptation theory. However, the diffusion coefficient significantly decreased at a coverage higher than 0.8, in contrast to the prediction of reptation theory. The reason for this decrease is thought to be the disappearance of minimum spaces required for mobile molecules to spread through.

Comparisons of Slip-Corrected Reynolds Lubrication Equations for the Air Bearing Film in the Head-Disk Interface of Hard Disk Drives

Slip-corrected Reynolds equations have not been widely used in the air bearing simulations for the head-disk interface in hard disk drives since Fukui and Kaneko [Trans ASME J Tribol 110:253–262, 1988] published a more accurate generalized lubrication equation (FK model) based on the linearized Boltzmann equation for molecular gas lubrication. However, new slip models and slip-corrected Reynolds equations continue to be proposed with certain improvements or with the more physical basis of kinetic theory. Here, we reanalyze those slip models and lubrication equations developed after the FK model was published. It is found that all of the slip-corrected Reynolds equations are of limited use in the air bearing simulations, and that these new slip-corrected Reynolds equations cannot replace the FK model for calculating an accurate pressure distribution of molecular gas lubrication.

Lubricant Depletion and Disk-to-Head Lubricant Transfer at the Head-Disk Interface in Hard Disk Drives

As the head-disk spacing reduces in order to achieve the areal density goal of 1 Tb/in.2, the dynamic stability of the slider is compromised due to a variety of proximity interactions. Lubricant pickup by the slider from the disk is one of the major reasons for the decrease in the stability as it contributes to meniscus forces and contamination. Disk-to-head lubricant transfer leads to lubricant pickup on the slider and also causes depletion of lubricant on the disk. In this paper, we experimentally and numerically investigate the process of disk-to-head lubricant transfer using a half-delubed disk, and we propose a parametric model based on the experimental results. We also investigate the dependence of disk-to-head lubricant transfer on the disk lubricant thickness, lubricant type, and the slider air bearing surface (ABS) design. It is concluded that disk-to-head lubricant transfer occurs without slider-disk contact and there can be more than one timescale associated with the transfer. Furthermore, the transfer increases nonlinearly with increasing disk lubricant thickness. Also, it is seen that the transfer depends on the type of lubricant used and is less for Ztetraol than for Zdol. The slider ABS design also plays an important role, and a few suggestions are made to improve the ABS design for better lubricant performance.

A generalized heat transfer model for thin film bearings at head-disk interface

The physics of the heat transfer process from the slider of a hard disk drive to its disk is discussed. A generalized heat transfer model, which incorporates various molecular dynamics models, is proposed to solve the heat transfer problem in thin film bearings at head-disk interface. The proposed model considers the impact of molecular collisions between film molecules and solid surfaces, which plays an important role in the heat transfer of thin film bearings and is expected to improve the accuracy of predicting the thermal protrusion caused by the heating of the slider at the head-disk interface of hard disk drives.

Effect of various physical factors on thin lubricant film migration on the flying head slider at the head-disk interface of hard disk drives

The mechanism of lubricant migration on a flying slider’s air bearing surface in magnetic hard disk drives is examined in this paper. As the head-disk spacing diminishes, the lubricant contamination from the disk and its migration behavior on the flying head slider becomes an increasing concern. The spatiotemporal behavior of the thin lubricant film on a slider surface is complicated, and it is affected by many physical factors. We investigate the contributions and functionalities of the potentially related physical factors and parameters on the lubricant film dynamics by numerical simulation. The analysis results show that the air bearing shear stress is the primary driving force in the lubricant migration, while the air bearing pressure makes only a minor contribution. The relative strength and direction of the Couette and Poiseuille flow components of the air bearing shear stress govern the lubricant migration direction and the points of lubricant stagnation. The lubricant surface free energy plays an important role while the contribution of diffusion is relatively small through the Laplace pressure but relatively large through the lubricant disjoining pressure which is determined by the surface energy gradient. We also find that the lubricant viscosity affects the lubricant dynamics as a time scaling factor. These results reveal the underlying physical framework of the complex phenomena and provide useful insight for developing the head disk interface of magnetic disk drives.

W4 Recent Cooperative Research Activities of HDD and Flexible Media Transport Technologies in Japan

This paper presents the recent status of industry-university cooperative research activities in Japan for mechatronics of information storage and input/output equipment. There are three research committees for promoting information exchange of technical problem and research topics on head-disk interface in hard disk drives (HDD), flexible media transport and image printing process which are supported by the JSME, Japanese Society of Tribologists and the Japanese Society of Precision Engineers. For hard disk drive technology, Storage Research Consortium is supporting more than 40 research groups in universities to perform basic research for future HDD technology. The past and present status of these activities are introduced, particularly focusing on HDD and flexible media transport mechanism.

CORROSION STUDY OF HARD DISKS BY OSA AND XPS

Perfluoropolyethers (PFPE) are most commonly employed as lubricants at head-disk interface in hard disk drives (HDD) to reduce friction and wear. Corrosion behavior is an important issue in high density magnetic recording disk-drives. The magnetic materials used in the magnetic storage layers as well as those used for making read–write transducers are not very corrosion resistant. In the presence of humidity and at elevated temperatures, corrosion occurs and materials from the magnetic layers and the NiP plating layers of the recording rigid media can migrate to the disk surfaces and form protrusions that are easily much larger than the head-disk clearances. As PFPE is sensitive to temperature, especially its mobile fraction, movement of the PFPE molecules will allow the magnetic media to be exposed to the environment through the micro pores existing in DLC overcoat, which will cause occurring of corrosion. In this paper, the corrosion characteristics of the magnetic media of the hard disks were analyzed using optical surface analyzer (OSA). X-ray photoelectron spectroscopy (XPS) was also used to characterize the distribution of the elements on the surfaces of hard disks. A relationship between heating time, lubricant layer thickness, number of corrosion spots and distribution of elements was attempted. It was found that the corrosion spots for the disks are related to the heating time. A small change in the thickness of lubricant layer can cause a drastic change in corrosion rate.