Disk Lubricant Research Articles

Tribological Characteristics of Magnetic Disk Lubricant Film Containing Long-Chain Lubricant with One-Terminal End Group.

Tribological Characteristics of Magnetic Disk Lubricant Film Containing Long-Chain Lubricant with One-Terminal End Group.

Structural Changes of Perfluoropolyether Lubricant Accumulated on the Pin Surface During Laser Heating

Structural and molecular weight changes to lubricant picked up the following laser heating in heat-assisted magnetic recording (HAMR) were analyzed using time-of-flight secondary ion mass spectroscopy. The average molecular weight of the disk lubricant increased by laser heating at temperatures over 500 °C and that of the picked up lubricant increased at temperatures over 400 °C. The presence of end-group fragments with low molecular weight (CH3O+) increases in both the disk and picked up lubricants, whereas at high temperature the concentration of the end-group component (C3H7O2+) decreases in the disk lubricant and increases in the picked up lubricant. Overall, the results presented herein suggest that the chemical and thermal stability of the terminal ends of lubricants should be improved in order to decrease depletion of the lubricant film and the lubricant pickup by the laser heating in HAMR.

Tribological Characteristics of Magnetic Disk Lubricant Film Containing Long-Chain Lubricant with One-Terminal End Group.

Tribological Characteristics of Magnetic Disk Lubricant Film Containing Long-Chain Lubricant with One-Terminal End Group.

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.

Synthesis and properties of cyclotriphosphazene and perfluoropolyether‐based lubricant with polar functional groups

AbstractA novel hard disk lubricant IDL2 was synthesised by reacting sodium 4‐((2,2‐dimethyl‐1,3‐dioxolan‐4‐yl)methoxy)phenoate with hexachlorocyclotriphosphazene, followed by treatment with 1H,1H‐perfluoro‐3,6,9‐trioxatridecan‐1‐ol in the presence of sodium hydride and subsequent hydrolysis using sulfuric acid. The chemical structure of IDL2 was confirmed by proton nuclear magnetic resonance (1H NMR), phosphorus‐31 (31P) NMR spectroscopy and mass spectrometer. The thermal property of IDL2 was analysed using thermal gravimetric analysis (TGA) and it shows better thermal stability than two commercial lubricants Z‐DOL and Z‐Tetraol with two and four terminal hydroxy groups, respectively. IDL2 was found to have comparable hydrophobicity to commercial lubricants Z‐DOL, A20H and Z‐Tetraol with water contact angles in the range of 78 – 85°, revealing that IDL2 has analogous surface energy to these commercial lubricants. IDL2 exhibits comparable friction coefficient to A20H and Z‐Tetraol, but much lower than Z‐DOL. More interestingly, it has a much high bonding ratio when compared with commonly used commercial lubricants under the same testing conditions due to the presence of two polar hydroxy groups, which enhance the interaction between lubricant and substrate. These promising properties of IDL2 show that it would be very potential for real application as a hard disk drive lubricant. Copyright © 2016 John Wiley & Sons, Ltd.

Lubricant Transfer Model at the Head-Disk Interface in Magnetic Storage Considering Lubricant–Lubricant Interaction

Interactions among molecularly thin lubricant–solid and lubricant–vapor interfaces become increasingly strong when the slider approaches closer to the disk (or media) surface, which is desirable for higher storage areal densities. Lubricant transfers from the disk to the slider as the head-media spacing decreases below a critical value, leading to loss of protection for the solid thin films. The present work proposes a new criterion for lubricant pickup to occur by improving the lubricant transfer model via considering interactions between the lubricant films, in addition to lubricant–solid interaction. The effect of lubricant roughness is also included in the proposed model, which is then used to predict critical solid distances as a function of lubricant roughness and average lubricant thickness. It is found that the interaction force between the slider solid and the disk lubricant plays a weaker role than that between the lubricant on the slider and the lubricant on the disk in determining the critical solid distance. The physics for the dependence of the critical clearance on the bonding ratio is explained, and a linear model is developed.

Simulation of Hydrocarbon Oil Contamination at the Head–Disk Interface Using Molecular Dynamics

Current hard disk drives use hydrocarbon-based oils to lubricate the actuator arm pivot and the fluid dynamic bearing spindle motor. At elevated drive temperatures, hydrocarbon oil molecules escape from the fluid dynamic bearing motor and pivot actuator by evaporation and aerosoling, entering into the drive air stream, and contaminating the head–disk interface. In this paper, we study the transfer mechanism of hydrocarbon molecules (n-alkane: C24H50 and C36H74) and disk lubricant (Zdol-PFPE) molecules at the head–disk interface using molecular dynamics simulation. A “two-step” transfer mechanism is postulated: Airborne hydrocarbon molecules condense first onto the disk surface and then transfer by evaporation to the slider surface. The numerical results obtained are in agreement with the assumed hypothesis of the transfer mechanism.

Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations

Lubricant accumulation was found on the media surface the instant when the laser is turned OFF during heat-assisted magnetic recording write operations. By changing the write cycles, laser ON/OFF duration, media, and head temperatures, we find that this lubricant accumulation is related to the change in head-media temperatures. The observed lubricant deposition process is restricted to a short time window (1-2 μs) after the laser is turned OFF. An equilibrium model of thermal displacement due to evaporation and condensation processes is presented and used to discuss the effect of the head-media temperature changes on the lubricant accumulation. Possible solutions to minimize the lubricant transfer and deposition are discussed.

Effect of surface topography of disk on lubricant distribution and lubricant transfer between disk and slider

Lubricant transfer from the disk to the slider can affect the slider flying ability. Based on molecular dynamics simulation, we investigated the effects of the roughness of diamond like carbon (DLC) and disk surface protrusion on lubricant distribution on a disk surface and lubricant transfer between the disk and the slider using modified coarse-grained, bead-spring model. The simulation results showed that the effect of DLC roughness on the average lubricant thickness and on the disk surface roughness is small. However, the effect of disk surface protrusion on lubricant distribution on a disk surface is more significant. The roughness of DLC layer and disk surface protrusion result in an increase in the volume of lubricant transfer. Between these two, the effect of the disk surface protrusion is much more important than that of the DLC layer roughness.

Head–Disk Interface Materials Issues in Heat-Assisted Magnetic Recording

In this paper, some issues concerning the reliability of heat-assisted magnetic recording (HAMR) media are highlighted. The large roughness of the grain structure originates from a surface energy mismatch between the FePt grains and the graphene-like segregant material. A simple roughness model, based on interfacial energies, is proposed that can quantitatively predict media grain structure and roughness. The thermal behavior of the disk lubricant is reviewed both experimentally as well as theoretically using molecular dynamics (MD) and density functional theory (DFT). The lubricant film can be subjected to evaporation and oxidation, both of which follow an Arrhenius reaction rate. MD also predicts that the disk carbon overcoat can undergo structural changes under thermal transient exposure in the nanosecond time frame, and Raman imaging performed on a disk zone that was HAMR written shows small but unequivocal changes, consistent with an increase in carbon sp2 cluster size.

Study of the Thermal Decomposition of PFPEs Lubricants on a Thin DLC Film Using Finitely Extensible Nonlinear Elastic Potential Based Molecular Dynamics Simulation

Perfluoropolyethers (PFPEs) are widely used as hard disk lubricants for protecting carbon overcoat reducing friction between the hard disk interface and the head during the movement of head during reading and writing data in the hard disk. Due to temperature rise of PFPE Zdol lubricant molecules on a DLC surface, how polar end groups are detached from lubricant molecules during coating is described considering the effect of temperatures on the bond/break density of PFPE Zdol using the coarse-grained bead spring model based on finitely extensible nonlinear elastic potential. As PFPE Z contains no polar end groups, effects of temperature on the bond/break density (number of broken bonds/total number of bonds) are not so significant like PFPE Zdol. Effects of temperature on the bond/break density of PFPE Z on DLC surface are also discussed with the help of graphical results. How bond/break phenomenonaffects the end bead density of PFPE Z and PFPE Zdol on DLC surface is discussed elaborately. How the overall bond length of PFPE Zdol increases with the increase of temperature which is responsible for its decomposition is discussed with the help of graphical results. At HAMR condition, as PFPE Z and PFPE Zdol are not suitable lubricant on a hard disk surface, it needs more investigations to obtain suitable lubricant. We study the effect of breaking of bonds of nonfunctional lubricant PFPE Z, functional lubricants such as PFPE Zdol and PFPE Ztetrao, and multidented functional lubricants such as ARJ-DS, ARJ-DD, and OHJ-DS on a DLC substrate with the increase of temperature when heating of all of the lubricants on a DLC substrate is carried out isothermally using the coarse-grained bead spring model by molecular dynamics simulations and suitable lubricant is selected which is suitable on a DLC substrate at high temperature.

Study of the Spreading of Perfluoropolyether Lubricants on a Diamond-Like Carbon Film

Nonpolar perfluoropolyether (PFPE) Z and PFPE Zdol with polar end groups are widely used as hard disk lubricants for protecting carbon overcoats by reducing friction between the hard disk and head during movement of the head while reading and writing data on the hard disk. We investigate the spreading phenomenon of PFPE Z and PFPE Zdol on a thin diamond-like carbon (DLC) film adopting molecular dynamics (MD) simulations based on a coarse-grained bead-spring model to describe the thickness profiles and molecular movement, which evolve with time and temperature. In the present article, the hard disk surface was considered as a DLC and the position of its carbon atoms was obtained by heating and quenching the face-centered cubic (FCC) or body-centered cubic (BCC) diamond structures by MD simulation using the Tersoff potential. To simulate PFPE Z and PFPE Zdol on a thin DLC film using a coarse-grained bead-spring model based on finitely extensible nonlinear elastic potential and nonbonded potential, the original DLC thin film was compressed to half of its original configuration in all three spatial dimensions. How PFPE Z and PFPE Zdol on the DLC surface spread with time are briefly discussed. How the spreading profile of PFPE Z and PFPE Zdol on the DLC film spreads laterally and on the DLC film as a circular shape beyond its original position is also discussed. The effect of temperature on the film thickness and spreading area of PFPE Z and PFPE Zdol on the DLC film is also discussed. We show that the time dependence for the spreading of PFPE Z and PFPE Zdol droplets deviated from the expected proportionality to the square root of time in their spreading profiles with time. The model-calculated spreading rate of PFPE Z and PFPE Zdol on a thin DLC film increased inversely with absolute temperature as expected.

Tribological Challenges of Flying Recording Heads Over Unplanarized Patterned Media

Here we evaluate some of the problems likely to be encountered for flying magnetic recording heads over unplanarized patterned media. We demonstrate: (i) current air bearing codes developed for flying heads over smooth disks can be extended to patterned surfaces; (ii) the method for detecting contact at the head-disk interfaces within disk drives will likely need to be more sensitive for unplanarized patterned media; and (iii) the disk lubricant is more likely to be picked up on a head slider surface when flying over patterned surfaces. While our experiments and simulations were carried for discrete track media (DTM), we also discuss the implications for future bit patterned media (BPM) technology programs and possible solutions to these tribological problems.

Studies of Degradation and Decomposition Mechanisms of High Temperature Lubricants Undergoing Laser Irradiation

This paper investigates the effects of laser irradiation on hard disk lubricants of Z-Tetraol and DEPH. Through the study of different Heat Assisted Magnetic Recording (HAMR) laser effects on hard disk lubricant components including different perfluoropolyether backbones, hydroxyl end-group and cyclotriphosphazene end-group, suitable components for HAMR application are discussed.

Experimental Study of the Slider-Lube/Disk Contact State and Its Effect on Head-Disk Interface Stability

As head-disk clearance has been reduced to nearly 1 nm, and a partial lube-contact head-disk interface (HDI) may even be necessary to meet future magnetic spacing needs, precise characterization of the slider-lube/disk contact state (or depth) becomes increasingly important. The currently used thermal fly-height control (TFC) sliders require a slight touchdown (or contact) to determine the reference position of the clearance, and accurate measurement and control of the penetration depth into the disk lubricant are essential to achieve a stable HDI for future lube-contact recording. In this paper, multiple techniques (e.g., acoustic emission technology, lubricant distribution mapping, slider wear detection, and slider dynamics measurement) are implemented to characterize the contact state/depth and HDI stability at different TFC actuation levels. Experimental results demonstrate the effectiveness of these methods and suggest that the clearance reference position determined through the acoustic emission technology is at the lubricant-air interface. Moreover, two dynamically stable contact states-light slider-lube contact and deep slider-disk contact-are found, but the latter causes slider wear and thereby is not practical for realizing a reliable lube-contact HDI.

Experiments on slider lubricant interactions and lubricant transfer using TFC sliders

Future magnetic storage density targets (>4 Tb/in.2) require subnanometer physical clearances that pose a tremendous challenge to the head disk interface (HDI) design. A detailed understanding of slider-lubricant interactions at small clearances and contact is important to not only address magnetic spacing calibration and long term HDI reliability but also to meet additional challenges imposed by future recording architectures such as heat assisted magnetic recording (HAMR). In this work, the behavior of the disk lubricant is investigated through controlled tests using TFC sliders which are actuated to proximity (i.e. backoff) and into contact (i.e. overpush) on one specific half of the disk per rotation by synchronization with the spindle index. Observations for lubricant distribution in contact tests (i.e. overpush) reveal an accumulation of lubricant on the disk near the onset of contact suggesting a migration of lubricant from the slider to the disk as the slider approaches the disk. Experiments also reveal that there is a similar deposition of lubricant even in the absence of contact for backoff tests. Furthermore, light contact tests result in significant lubricant rippling and depletion with associated slider dynamics. The lubricant rippling frequencies correlate well with the slider's vibration frequencies. Interestingly, strong overpush may lead to stable slider dynamics (for certain air bearing designs) that is also associated with noticeably lower lubricant distribution (compared to the light contact case), and the greatest lubricant changes are observed only at the onset and the end of contact. This paper reveals the complex nature of slider-lubricant interactions under near-contact and contact conditions, and it highlights the need for further studies on the topic to help design a HDI for recording architectures of the future.

Bonding of Hard Disk Lubricants with OH-Bearing End Groups

Typical lubricants for magnetic hard disks comprise the central perfluoropolyether section and the short hydrocarbon end groups bearing hydroxyl unit(s). It had been shown earlier that chemical bonding of these lubricants to the carbon overcoat of disks involves (1) dangling bonds shielded inside the carbon, (2) transfer of the hydrogen atom of the hydroxyl unit to a dangling bond site, and (3) attachment of the remaining alkoxy system, R–CF2–CH2–O·, to the carbon surface as a pendant ether unit. Dangling bonds at or near the surface react immediately with H2O or O2 in the atmosphere. It follows that, in order to bond, the hydrocarbon end group must move into crevices of the carbon film. It was postulated that the bonding rate would depend on the length of the hydrocarbon end-group, –(CH2) n –OH. The longer the hydrocarbon sector is, the faster and the more extensively the bonding would proceed. Bonding rates were examined for a set of samples differing only in the dimension of the hydrocarbon end-group. Results clearly in accordance with the postulate were obtained. The sample set included two novel lubricants, D-2TX2 and D-2TX4, with the following end-groups, –O–CF2–CH2–O–(CH2) n=2,4–OH. Excellent bonding rate, coverage, and potential anticorrosion property were revealed for these lubricants.

Parametric Investigations at the Head-Disk Interface of Thermal Fly-Height Control Sliders in Contact

Accurate touchdown power detection is a prerequisite for read-write head-to-disk spacing calibration and control in current hard disk drives, which use the thermal fly-height control slider technology. The slider air bearing surface and head gimbal assembly design have a significant influence on the touchdown behavior, and this paper reports experimental findings to help understand the touchdown process. The dominant modes/frequencies of excitation at touchdown can be significantly different leading to very different touchdown signatures. The pressure under the slider at touchdown and hence the thermal fly-height control efficiency as well as the propensity for lubricant pickup show correlation with touchdown behavior which may be used as metrics for designing sliders with good touchdown behavior. Experiments are devised to measure friction at the head-disk interface of a thermal fly-height control slider actuated into contact. Parametric investigations on the effect of disk roughness, disk lubricant parameters, and air bearing surface design on the friction at the head-disk interface and slider burnishing/wear are conducted and reported.

Effect of Laser Heating Duration on Lubricant Depletion in Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording to further increase the areal recording density of hard disk drive media. However, HAMR brings about serious problems to slider-disk interface, such as lubricant depletion on disk surface at the high temperature in HAMR. Experimental studies of lubricant depletion under laser irradiation are still very limited so far. It is essential to do experimental studies under real HAMR conditions or under equivalent conditions when a standalone laser is used to emulate HAMR system. Laser heating duration in one heating and cooling process in HAMR is as short as 1 ns. It is believed that lubricant depletion under such short heating duration should be different from that under long time continuous heating. In this work, the effect of laser heating duration in one heating and cooling process on lubricant depletion is studied experimentally on a self-developed HAMR tester. Laser power, laser heating rate and laser heating duration of the tester are tunable and comparable with that of a HAMR head laser. The tester also supports macroscopic study. A method to control laser focus point on disk surface is also developed, which is the prerequisite to control laser heating temperature. From the experimental studies, it is found that longer laser heating duration results in more severe lubricant depletion. It is also found that lubricant depletion under laser heating is caused by both lubricant flow on disk surface and lubricant loss via evaporation/decomposition.

Slider-Lubricant Interactions and Lubricant Distribution for Contact and Near Contact Recording Conditions

Lubricant distribution and recovery for near contact and contact recording conditions are experimentally investigated using thermal fly-height control (TFC) sliders. Contact between the protruded center pad and the disk lubricant causes a thickness modulation (rippling) together with lubricant depletion that grows with contact duration. Slider dynamics and lubricant rippling evolve quickly in the first few revolutions of contact and rippling frequencies are strongly correlated with the slider air bearing frequencies. Peculiar cases where suppressed “stable” slider dynamics occur for TFC heater power beyond the touchdown power correspond to negligible lubricant rippling and this condition may be sustained for fairly long durations in some tests. Experiments with different lubricant types (ZTMD, Z-tetraol+A20H, Z-dol+A20H) and different lubricant thicknesses show that a larger depletion is observed for the thicker lubricant of each type at a given TFC heater power. Lubricants with lower bonded fraction show shorter recovery time after the slider is unloaded. From the current experiments it is extrapolated that Z-dol+A20H (30% bonded) lubricant recovers in a time scale of a few hours, while Z-tetraol+A20H (60% bonded) lubricant of comparable thickness takes a few days, and ZTMD (75% bonded) takes weeks to fully recover.