Disk Lubricant Research Articles

Enhanced Photo-Thermal Stability of Modified PFPE Lubricants Under Laser Beam Exposure

The photo-thermal stability and tribological properties of Zdol lubricant, modified by substituting the OH groups with benzophenone, were investigated under laser beam exposure. The change of reflectance of the modified Zdol lubricant films induced by laser beam exposure was measured using a surface reflectance analyzer (SRA). The friction force of modified Zdol lubricant films was measured at the interface between a slider and a disk on a spin stand. Modified Zdol films show less reflectance change than pure Zdol. The photo-thermal stability under laser beam exposure increases with an increase of the substitution ratio. Reflectance change occurs due to depletion of the lubricant and/or modification of the lubricant optical properties under laser beam exposure. Unlike pure Zdol, the normalized friction force of modified Zdol remains constant under laser beam exposure. Modified Zdol lubricants are potential disk lubricants for heat-assisted magnetic recording applications.

Lubricant Pickup Under Laser Irradiation

Hard disk lubricants play important roles at head-disk interfaces. Heat assisted magnetic recording (HAMR) is a promising approach for enabling large increases in the storage density of hard disk drives. Under HAMR conditions, lubricated thin films on hard disk surfaces should satisfy all tribological properties. This paper studies the reliability of several types of lubricants on thin-film disks, under HAMR laser irradiation conditions. Through the study and comparison of the lubricants' physical and chemical properties, lubricant pick-up status and composition of the picked-up lubricant, the related tribological properties have been explained and the design and development of new lubricants for HAMR application have been explored.

Terraced spreading of nanofilms under a nonmonotonic disjoining pressure

A thin (∼nanometer) film of a viscous, essentially nonvolatile liquid spreads over a substrate controlled by the disjoining pressure Π(h) exerted by the two interfaces on one another. Such films are commonly used as hard disk lubricants in the magnetic recording industry. Macroscopic nonuniformities in the film caused by a perturbation of the uniformly spread state flow away and the film is “healed” in a time frame governed by the appropriate hydrodynamic equations. Lubrication theory may be used to derive a diffusion equation for the local film height h(x,t) as a function of position and time which shows that an effective height-dependent diffusion coefficient D(h)=−[h3/3μBξ(h)][dΠ(h)/dh] controls the spreading dynamics, where μB is the bulk liquid viscosity and ξ(h) is a function accounting for any variation of local viscosity near the substrate due to molecularity of the liquid. Such an approach is possible due to the very small ratio of the film height to the in-plane length scale of the disturbance. Provided the disjoining pressure is positive and monotonically decreasing with film thickness, the motion of the film is unexceptional, exhibiting the usual smooth profiles associated with diffusive flow with time. However, for nonmonotonic disjoining pressures, the film is experimentally observed to exhibit vertical terraces. These abrupt jumps in height do not disappear with time and they move slowly or are stationary. This phenomenon is investigated here. We demonstrate how a physically consistent “weak” solution of the diffusion equation can be constructed, where only positive values of the diffusion coefficient are sampled. The film heights at the jump discontinuity are determined by an equal area rule for the disjoining pressure. Numerical simulations for a realistic nonmonotonic disjoining pressure exhibit finite termination on the low-side and vertical terraces, thereby matching the behavior observed in experimental systems.

The Effect of PFPE Film Thickness and Molecular Polarity on the Pick-Up of Disk Lubricant by a Low-Flying Slider

Lubricant pick-up by a low-flying slider is investigated for hydroxyl-terminated perfluoropolyethers as a function of the number of hydroxyl (OH) groups and of film thickness on the surface of finished rigid disks. The total number of hydroxyl (OH) groups per main chain is 2, 4, and 8 for Zdol, Z-Tetraol, and ZTMD, respectively. The amount of disk lubricant that is picked up by the low-flying slider decreases with decreasing PFPE film thickness and increasing number of OH functional groups. The results are discussed in terms of the disjoining pressure characterizing the lubricant film on the disk surface.

Slider Design Optimization for Lube-Surfing Head-Disk Interface Scheme

The stringent magnetic spacing requirement of future high-density magnetic disk drives requires new head disk interface scheme. One possible approach is the lube-surfing scheme-where majority of the slider body is flying while the reader/writer (R/W) element is thermally protruded to contact and penetrate the disk lubricant. This scheme, however, has many challenges to overcome to be feasible. One of these challenges is a slider with improved disk waviness-following capability especially for waviness with wavelength less than half the slider length. This paper explores air bearing design strategies to improve the waviness-following performance of a Femto slider. We propose a ¿W-shaped¿ trailing pad slider and investigated the impact of pad layout, etching depth and trailing pad size to the disk waviness-following performance. We have also incorporated a trenched trailing pad structure to reduce air bearing pressure over the R/W element so as to minimize the loss in air bearing pressure during lube contact and to improve the actuation efficiency.

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.

Disk Lubricants for Spontaneous Adsorption and Grafting to Carbon Overcoat by UV Irradiation

A molecular orbital study was performed to elucidate the π–π charge transfer interaction between perfluoropolyether (PFPE) lubricants possessing phenylic end-groups and the carbon overcoat of magnetic hard disks. It is revealed that the phenylic unit and the graphitic segment of the carbon attract each other leading to spontaneous adsorption. The strength of this interaction increases in the order of: an unsubstituted phenyl group < a phenoxy unit < a p-methoxy-phenoxy unit. A molecular dynamics calculation revealed that alkyl-phenyl ether, on capture of an electron, would dissociate to yield the phenoxide anion and the alkyl radical. It is thus predicted that PFPE lubricants with an end-group possessing a phenoxy unit would spontaneously adsorb on the carbon overcoat, and that irradiation of disks coated with such lubricant with short UV (185 nm), thus generating photo-electrons, would result in facile detachment of phenoxy groups and grafting of PFPE molecular chains to the carbon surface at the chain terminus. Four new PFPE lubricants, Z-SA1 and Z-SA2 based on the Fomblin Z type backbone, and D-SA1 and D-SA2 based on the Demnum backbone were synthesized, where SA1 and SA2 indicate end-groups possessing a phenoxy unit and a p-methoxy-pheoxy unit at the ω-position, respectively. Disks coated with these lubricants were tested for (1) spin-off rate, (2) diffusion over the disk surface, (3) facility for photo-grafting by UV, (4) water contact angle (before and after UV exposure), (5) the catalytic degradation, and (6) the on-track time-to-failure test. A TOF-SIMS study of disks coated with D-SA1 and D-SA2 was performed to elucidate the disposition of lubricant molecular chains due to spontaneous adsorption and the effect of UV irradiation. All the experimental results were found to be in good accord with predictions given by the molecular orbital study. In the time-to-failure test, disks coated with Z-tetraol, Z-SA1, and Z-SA2 were compared. The durability was found to increase in the order of Z-tetraol < Z-SA1 < Z-SA2.

Dissociative Electron Capture of Hydrogen-Bonded Hydroxy Groups: Molecular Dynamics and Matrix Isolation ESR Study

Abstract While examining PFPE (perfluoropolyether)-based disk lubricants, extra lability was suspected for a hydrogen-bonded pair of hydroxy end-groups in the presence of a nucleophile (electron donor). Plausible reaction sequences upon electron capture of hydrogen-bonded methanol dimer, hydrogen-bonded ethanol dimer, and 1,2-propanediol were first examined by a molecular dynamics method based on the potential given by a semiempirical SCF molecular orbital calculation, and subsequently by matrix isolation ESR spectroscopy. In the latter experimental study, alcohol dimer anions were generated by co-trapping Na atoms and alcohol molecules in argon matrices and inducing electron transfer by photo-irradiation. In the cases of methanol and ethanol dimers, the molecular dynamics study predicted that on capture of an electron, the hydroxy hydrogen of the acceptor side would cleave as an atom, and then abstract a hydrogen atom from the nearest carbon of the proton donating alcohol. For 1,2-propanediol anion, the molecular dynamics study predicted C1–C2 bond scission followed by abstraction of the C2 hydroxy hydrogen by the C1 radical fragment. The matrix isolation ESR study revealed generation of radicals upon photo-irradiation when the alcohol concentration was sufficiently high to warrant the presence of dimers. The spectral patterns thus observed were totally consistent with the radical products predicted by the molecular dynamics calculations.

Oxidation of Disk Lubricants: an NMR Study

Z-dol and Z-tetraol represent most often used disk lubricants. So that we would be better able to design lubricants of higher thermal stability (for Heat-Assisted-Magnetic-Recording application), detailed analysis of thermally induced oxidation processes of Z-dol and Z-tetraol were performed. Samples were heated in a thin-film configuration where facile infusion of oxygen occurred and were examined by F-19 and C-13 NMR. Z-dol was found to be stable in the 150 ∼ 200 °C temperature range. In the same temperature range Z-tetraol was found to undergo either (1) the well known step-wise oxidation of primary alcohol (alcohol → aldehyde → acid) or (2) direct conversion to Z-dol. The latter process is initiated by a base-catalyzed step whereby the end-group Z-O-CF2-CH2-O-CH2-CH(-OH)-CH2-OH converts to an aldehyde, Z-O-CF2-CH2-O-CH2-CH( = O), and CH3OH. Z-dol results upon oxidation of the aldehyde yielding Z-O-CF2-CH2-OH, CH2 = O and CO2. It was found that substitution of the terminal OH with a phenoxy unit suppressed these decomposition processes. Z-tetraol lubricant molecular chains that became bound to the carbon overcoat at their termini are also protected from these oxidation processes. Thus for HAMR application, a combination of lubricants bonded to the carbon overcoat at their termini and mobile lubricants possessing phenoxy groups at their termini may be the best possible system based on perfluoropolyether lubricants.

1406 磁気ディスク潤滑剤の前処理および後処理による摩耗低減効果の解析(OS1-2 機械要素とトライボロジー2,オーガナイズドセッション:1)

1406 磁気ディスク潤滑剤の前処理および後処理による摩耗低減効果の解析(OS1-2 機械要素とトライボロジー2,オーガナイズドセッション:1)

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.

The Static and Dynamic Responses of Binary Mixture Perfluoropolyether Lubricant Films— Molecular Structural Effects

Static and dynamic properties of single-component perfluoropolyether (PFPE) lubricants have been studied for optimal lubricant selection by examining the molecular conformations that influence the thickness and the mobility for the self-healing capability in lubricant nanofilms. In this paper, we examine the physiochemical properties of the mixture of these two PFPEs using molecular dynamics (MD) simulations to find an optimal blend ratio to meet the stringent requirements for disk lubricants of ultra-low head media spacing (HMS). A coarse-grained, bead-spring model was used to model the polymer nanoblends using functional and nonfunctional PFPEs. We examined the static and dynamic responses of binary PFPE films as a function of the molecular structures including end-group functionality. The effect of the functional end-group on the static structures was examined by simulating the parallel and perpendicular components of the radius of gyration. The dynamic responses of various PFPE nanoblends were also simulated by explicitly calculating the self-diffusion coefficient of a tagged molecule. Polydispersity effect on nanoblends was also examined.

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.

Lubricant dynamics on a slider: “The waterfall effect”

In a modern disk drive, the total slider-to-disk distance, or magnetic spacing, needs to scale with the size of the recorded bit, and nowadays, it amounts to a mere 10nm or less. As disk lubricant often transfers to some extent to the slider surface, it can potentially increase the magnetic spacing. Therefore, it is important to quantify the thickness and flow dynamics of these molecularly thin films on the slider surface. In this paper, modeling, as well as experimental data, is shown that demonstrate the effect on recording performance of a subnanometer thin lubricant layer on the slider. Cleanup by air shear can be predicted reasonably well using the shear map calculated from an air-bearing solver, and reflow by surface diffusion highlights the importance of controlling lubricant transfer from the disk to the slider.

Effect of thermal pole tip protrusion and disk roughness on slider disk contacts

Ultra-high areal density for hard disk drives requires a stable head disk interface at a flying height lower than 8 nm. At such a low flying height, small flying height variations may cause slider/disk contacts. Slider/disk contacts can also occur when a write-current is applied to the write coil since the flying height between slider and disk can be affected by the thermal expansion of the pole tip. In this paper, we investigate the vibration characteristics of sliders during thermally induced contacts using laser Doppler vibrometry. We perform a parametric study of contact events using disks with different surface roughness and lubricant thicknesses, and analyze the slider motion statistically. For a given write current, we observe that the slider vibrations increase with disk roughness and lubricant thickness.

Lubricant Design Attributes for Subnanometer Head-Disk Clearance

This paper is an attempt to review some historical trends in thin film disk lubricant technology, and the associated requirements in terms of overall reliability, as we inexorably approach the subnanometer clearance mark. We will mainly focus on the physical attributes of the lubricant, and more particularly on the slider-induced effects on lubricant dynamics. Design attributes for subnanometer clearance will be discussed in this historical context, as qualitative to semiquantitative models have emerged that allow such extrapolation.

Binary Mixture Film of Perfluoropolyether Lubricants

It has become more and more difficult to find a single lubricant meeting all the requirements for ultra-small head-media spacing hard disk drives (HDDs). Therefore, an appropriate mixture of lubricants may be a feasible and promising alternative for future HDDs. In this paper, molecular dynamics (MD) simulations employing a bead-spring model were performed to examine the detailed structure, conformation, and dynamics of binary mixture lubricant nano films by extensively analyzing the anisotropic radius of gyration and the self-diffusion coefficient as a function of composition. Our simulation analysis indicates that a binary (or even a ternary) mixture monolayer can be a more suitable disk lubricant as compared to a single component lubricant. The conformation and mobility change by tuning the volume fraction was examined in conjunction with the optimal lubricant selection.

Disk Lubricant Additives, A20H and C2: Characteristics and Chemistry in the Disk Environment

Disk lubricant additives A20H and C2 are Fomblin Z type perfluoropolyether with the hydroxyl end-group, –O–CF2–CH2–OH, at one end, and the cyclo-triphosphazene end-group, R5(PN)3–O–, at the other end. Here, R is an m-trifluoromethyl-phenoxy group for A20H and a trifluoroethoxy group for C2. These additives were examined for miscibility with benzene, spin-off rate, water contact angle, and the diffusion rate over the carbon overcoat. It is revealed that A20H adheres to the carbon overcoat spontaneously. The attractive interaction arises from the charge–transfer type interaction between the aromatic rings of the phosphazene end and the graphitic regime of the carbon overcoat. No spontaneous adherence occurs between the lubricant C2 and the carbon overcoat. A TOF-SIMS study of disks coated with A20H and C2, respectively, with and without subsequent curing by short-UV (185 nm) was performed. It is revealed: (1) if presented with a low energy electron, the phenoxy groups of A20H readily undergo the dissociative electron capture, while the trifluoroethoxy group does not, and (2) photoelectrons generated by short-UV have little kinetic energy and the electron capture occurs only if an electrophilic molecular sector is in intimate contact with the carbon. Thus, in the case of disks coated with A20H, UV-curing results in detachment of a phenoxy group in contact with the carbon, generation of a radical center at the phosphorus atom and subsequent formation of a bona fide chemical bond between the phosphor and the carbon overcoat. No reaction of consequence occurs when disks coated with C2 are irradiated with short-UV.

Molecular dynamics simulation of binary mixture lubricant films

Due to the requirements for ultra-small head-media spacing hard disk drives (HDDs), the mixture of lubricants may become feasible and promising alternative for future HDD. In this paper, molecular dynamics simulations with a bead-spring model were employed to examine the detailed structure, conformation, and dynamics of binary mixture lubricant films by analyzing the anisotropic radius of gyration and the self-diffusion coefficient as a function of volume fraction. Our simulation results indicate that the binary mixture monolayer can be more suitable as a disk lubricant in comparison with the single component. The conformation and mobility change by tuning the volume fraction was discussed in conjunction with the optimal lubricant selection.

Head wear reduction in future hard-disk drives

Head wear and head vibration due to head-disk contact are two main issues that must be resolved for the future high-density Hard Disk Drives (HDDs). To reduce head wear, disk lubricant, carbon overcoat films on head and disk surfaces, head flying characteristics and so on have been studied. In this paper, we first show the effects of several parameters on head wear, including lubricant types, their MW, and disk burnishing. Our recent results on the effects of humidity and temperature on head wear are also explained. We then explain our extended wear equation and estimate the head wear life with present technologies.