Carbon Overcoat Research Articles

On the thickness-dependent diffusion coefficient of perfluoropolyether lubricants on a thin diamond-like film

Diffusion of perfluoropolyethers (PFPEs) lubricants on a hard disk surface is an important self healing characteristic to replenish PFPEs lubricants on their uncovered surface. In the present paper, we study the diffusion coefficients of non-functional PFPE Z and PFPE Zdol with functional end groups as a function of lubricant film thickness on a thin DLC (diamond-like) film. Diffusion coefficients of PFPE Z and PFPE Zdol molecules on a DLC film are calculated using the equation of Einstein’s law of diffusion (Guo et al. J. Appl. Phys 93:8707, 2003; Guo Ph.D. thesis, 2006; Chung et al. IEEE Trans. Magn. 45:3644, 2009) considering the movement of their center of mass to reach their equilibrium positions from their original configurations. And it is averaged with the film thickness to show the thickness dependence on the diffusion of PFPEs lubricants on a DLC substrate. Firstly diffusion coefficients of sub-monolayer of partially coverage PFPE Z and PFPE Zdol on a DLC substrate are studied briefly and secondly the diffusion coefficient of monolayer PFPE Zdol on a DLC substrate is also studied elaborately. To support our results, we compare our thickness-dependent diffusion coefficients of PFPE Z and PFPE Zdol with those of published theoretical (Guo Ph.D. thesis, 2006; Chung et al. IEEE Trans. Magn. 45:3644, 2009) and experimental results (Chung et al. Tribol. Lett. 32:35, 2008; Ma et al. Tribol. Lett. 10:203, 2001). Here we study how lubricant film thickness plays an important role on its diffusion. Effects of polar end bead functionality, lubricant film thickness enhance the anisotropic behavior of diffusion coefficients of PFPE Zdol on the DLC substrate. But in the present analysis we consider hard disk carbon overcoat as a thin DLC film and we include all of their atoms within the force cut-off distance with PFPEs lubricant molecules for the interactions to study the thickness dependence on their diffusion coefficients.

Preparation of Carbon Coated MoS2 Flower-like Nanostructure with Self-Assembled Nanosheets As High-Performance Lithium-Ion Battery Anodes

Loosely packed MoS2 nanosheets with thin carbon coating were synthesized via a facile, one-pot hydrothermal growth method. In the resulted optimal-designed nanoarchitecture, the ultrathin nanosheets, with a wall-thickness of approximately 5-10 nm, provide large electrode-electrolyte interface, so as to facilitate faster lithium-ions intercalation and diffusion. The flexible and conductive carbon overcoats accommodate and alleviate the volume variation of the composites and thus maintain the structural and electrical integrity. In favor of the synergy and interplay of the carbon effect and intrinsic structural advantages, MoS2@C (1:2) composites with the precursor MoO3: D-glucose molar ratio of 1:2 exhibit high reversible specific capacity of 1419 mA h g-1 at 0.1 A g-1, retain 80 % of the capacity after 50 cycles, and excellent rate capability as high as 672 mA h g-1 at 10 A g-1 with almost 100 % Coulombic efficiency. The good electrochemical performance suggests that these MoS2@C composites with unique flower-like morphology give rise to high specific surface area and increased layer distance leading to the reduced diffusion path of lithium-ions and rapid electrons transport. The rational and optimal design of the MoS2@C composites with an excellent electrochemical performance in the report creates a new avenue to develop a high-performance anode material.

Thermal modeling of head disk interface system in heat assisted magnetic recording

A thorough understanding of the temperature profiles introduced by the heat assisted magnetic recording is required to maintain the hotspot at the desired location on the disk with minimal heat damage to other components. Here, we implement a transient mesoscale modeling methodology termed lattice Boltzmann method (LBM) for phonons (which are primary carriers of energy) in the thermal modeling of the head disk interface (HDI) components, namely, carbon overcoat (COC). The LBM can provide more accurate results compared to conventional Fourier methodology by capturing the nanoscale phenomena due to ballistic heat transfer. We examine the in-plane and out-of-plane heat transfer in the COC via analyzing the temperature profiles with a continuously focused and pulsed laser beam on a moving disk. Larger in-plane hotspot widening is observed in continuously focused laser beam compared to a pulsed laser. A pulsed laser surface develops steeper temperature gradients compared to continuous hotspot. Furthermore, out-of-plane heat transfer from the COC to the media is enhanced with a continuous laser beam then a pulsed laser, while the temperature takes around 140 fs to reach the bottom surface of the COC. Our study can lead to a realistic thermal model describing novel HDI material design criteria for the next generation of hard disk drives with ultra high recording densities.

Buff/wipe effects on the physicochemical properties of perfluoropolyether nanoscale thin films

Buff/Wipe (B/W) process is commonly used in disk drive manufacturing to remove the particles and asperities on the lubricated disk surface. In this paper, we investigated how B/W process impacts the physicochemical properties of perfluoropolyethers (PFPE) nano-films through the study of surface energy and bonded ratio. Two-liquid geometric method was used to analyze the surface energy of nonfunctional PFPE, i.e., Z03, and functional PFPE, i.e., Zdol, lubricated media before and after B/W process. It was found that the dispersive surface energy of Z03 films greatly decreased after B/W, which was more significant in the submonolayer regime. In addition, the bonded ratio slightly increased. However, B/W effect on the surface energy and bonded ratio was not detected for Zdol films. It is hypothesized that nonfunctional PFPE behaves liquid-like on the carbon overcoat due to the weak interaction between lubricant and overcoat. External mechanical stress as applied with B/W can change the conformation and increase the surface coverage for nonfunctional PFPE. On the other hand, functional PFPEs behave solid-like due to the strong attraction between lubricant and overcoat; therefore, it is difficult to change the conformation by external stress from B/W process.

Understanding Disk Carbon Loss Kinetics for Heat Assisted Magnetic Recording

An optical pump-probe set-up that allowed short dwell time ( ~ 500 μs) heating with a high-temperature ramp rate ( ~ 106 K/s) was used to detect the magnetization change in heat assisted magnetic recording (HAMR) media. The temperature of the media was monitored by observing the Kerr signal. The pump power at zero magnetization allowed the determination of the power needed to attain the Curie temperature of the media (the Curie temperature was determined with a high-temperature magnetometer before the pump-probe experiments). HAMR media was then irradiated with a pump power to obtain 480°C for increasing exposure times. Atomic force microscopy (AFM) of these media surfaces revealed depressions or holes in the media surface [within the carbon overcoat (COC) layer] that increased in extent with cumulative exposure time. Media surfaces exposed to somewhat lower temperatures (450°C) and for shorter times had a swollen region that surrounded a much smaller depression. High-spatial resolution Raman spectroscopy was used to analyze these irradiated areas. An increased D-band was observed within the swollen portion of the media surface, while the overall Raman signal intensity decreased within the small depressed area. Using time and temperature irradiations along with AFM analysis of the depressions the activation energy for COC loss was determined to be 0.6 eV. These observations were attributed to COC failure through graphitization and oxidation. The failure mechanism leading to these observed changes and the possible relationship of the present results to the HAMR media COC thermal stability are discussed.

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.

Experimental Study of Head-Disk Interface in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) has been proposed to achieve magnetic recording density. In HAMR systems, a laser is used to locally heat the magnetic disk to the Curie temperature, which is ~ 400°C-600°C, to assist in the data writing process. The high temperature working condition is a great challenge for the head-disk interface. Lubricant can be depleted by evaporation or decomposition. The protective carbon overcoat can be graphitized and oxidized. The surface quality, such as its roughness, can be changed as well. The near field transducer (NFT) structure is also vulnerable under the large number of heating cycles. All of those effects of the heating will present challenges for the reliability of the HAMR performance. In this paper, an experimental HAMR platform is introduced. The free space laser exposure of the media is calibrated based on the magnetization decay of special data tracks. Lubricant loss is determined from the optical surface analyzer. The lubricant changes as a function of different thermal load cycles are investigated as well. The disk topography is measured by an atomic force microscope after the mobile lubricant is removed. The hard overcoat's topography dependence on the laser exposure conditions is also studied. With regard to the media disk, we establish three distinct laser power thresholds for lubricant loss, disk surface degradation, and magnetization decay. As the laser power increases, the lubricant starts to change first before the magnetization starts to decay, indicating that lubricant could be the most vulnerable component to the laser heating in HAMR systems. Then, the disk overcoat's surface changes, becoming rougher after the magnetization starts to decay. Furthermore, the lubricant degradation is more serious under cyclic loading conditions. With regard to the transducer, the metal film of the NFT gets removed due to high energy laser ablation, and then the NFT loses its capability to focus the light, as required for HAMR systems.

Bi-level surface modification of hard disk media by carbon using filtered cathodic vacuum arc: Reduced overcoat thickness without reduced corrosion performance

Abstract The corrosion performance of commercial hard disk media which was subjected to bi-level surface modification has been reported. The surface treatment was carried out by bombarding the surface of the magnetic media with C+ ions at 350 eV followed by 90 eV using filtered cathodic vacuum arc (FCVA). The energy and embedment depth of the impinging C+ ions were adjusted by applying an optimized bias to the substrate and simulated by a Stopping and Range of Ions in Matter (SRIM) code which predicted the formation of a graded atomically mixed layer at the carbon-media interface. Cross-section transmission electron microscopy (TEM) revealed the formation of a 1.8 nm dense nano-layered carbon overcoat structure on the surface of the media. Despite an ~ 33% reduction in the thickness, the bi-level surface modified disk showed corrosion performance similar to that of a commercially manufactured disk with a thicker carbon overcoat of 2.7 nm. This improvement in the corrosion/oxidation resistance per unit thickness can be attributed to the formation of a dense and highly sp3 bonded carbon layer, as revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. This study demonstrates the effectiveness of the bi-level surface modification technique in forming an ultra-thin yet protective overcoat for future hard disks with high areal densities.

Angstrom Scale Wear of the Air-Bearing Sliders in Hard Disk Drives

We utilize thermal fly-height control (TFC) technology to perform in situ measurements of carbon overcoat wear at the angstrom level at the read–write area of magnetic recording heads. We also study the durability of the molecularly thin lubricated disk surface. Experimental findings reveal a linear relationship between the quantified carbon wear depth on the flying head versus the head–disk contact level produced by the TFC power. It is demonstrated that this method can serve as a measurement and probing technique of wear resistance for different types of lubricants. Lubricants possessing more polar hydroxyl end-groups and less mobility tend to show a superior surface stability under head–disk contacts, but raise concerns on head carbon overcoat wear.

Low energy C+ ion embedment induced structural disorder in L1 FePt

Shallow embedding of C+ ions (<2 nm) into commercial CoCrPt-based magnetic media using the filtered cathodic vacuum arc technique improves its anti-oxidation and anti-wear properties which are comparable to the conventionally used thicker carbon overcoats of ∼3 nm. The next generation L10 FePt media subjected to low energy embedment of C+ ions have the potential to provide reduced magnetic spacing along with smaller and thermally stable grains, which is pivotal for achieving areal densities beyond 1 Tb/in.2 However, the impact of low energy C+ ions embedding on the magnetics of FePt media is not known. Here, the magnetic properties of L10 FePt, post-shallow C+ ion embedment at 350 eV, were investigated. It was observed that bombardment of C+ ions in the 5 nm thick FePt films produced a monumental reduction of ∼86% in the out-of-plane coercivity value. Increasing the FePt film thickness did not significantly suppress the impact of these C+ ions on the media. Structural and elemental analyses attributed this alteration caused in the magnetic properties of the well-ordered FePt films to the penetration of >2 nm by the C+ ions into the FePt film. The media's crystallography with respect to the size and direction of the incoming ions has emerged to be accountable for the deeper distribution of the C+ ions and the associated widespread cascade damages within the magnetic layer. The consequences of low energy C+ ions embedding to attain high storage densities using high anisotropy L10 FePt media are discussed.

Preparation of carbon coated MoS2 flower-like nanostructure with self-assembled nanosheets as high-performance lithium-ion battery anodes

Loosely packed MoS2 nanosheets with thin carbon coating were synthesized via a facile, one-pot hydrothermal growth method. In the resulting optimally-designed nanoarchitecture, the ultrathin nanosheets, with a wall-thickness of approximately 5–10 nm, provide a large electrode–electrolyte interface so as to facilitate faster lithium-ion intercalation and diffusion. The flexible and conductive carbon overcoats stabilize the disordered structure of flower-like MoS2 nanosheets to accommodate more lithium-ions intercalation and thus maintain the structural and electrical integrity during cycling processes. In favor of the synergy and interplay of the carbon effect and intrinsic structural advantages, C@MoS2 (2 : 1) composites synthesized with a D-glucose precursor: MoO3 molar ratio of 2 : 1 exhibit high reversible specific capacity of 1419 mA h g−1 at 0.1 A g−1, retain 80% of the capacity after 50 cycles, and excellent rate capability as high as 672 mA h g−1 at 10 A g−1 with nearly 100% coulombic efficiency. The good electrochemical performance suggests that these C@MoS2 composites with unique flower-like morphology could be a promising candidate as an anode material for lithium-ion batteries.

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.

Laser-Heating-Induced Damage to Ultrathin Carbon Overcoat in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a technique for overcoming the superparamagnetic limit and enabling large increases in the storage density of hard disk drives. The performance of the disk carbon overcoat under the high temperature in the heating-assisted writing process is a concern. Laser heating in HAMR is quite different from conventional slow heating. Laser heating temperature and total laser heating duration over the lifetime of the drive are two dominant factors in the experimental study of laser-heating-induced damage to the carbon overcoat, which must be carefully controlled. In this study, a rough estimation of the total laser heating time for a given point on the media over the 5-year lifetime of the drive is given. It is expected to be only 0.1 ms. The methods of controlling laser heating temperature and total laser heating time in experimental studies are explained in detail. Laser-heating-induced damage to the a-C:Nx and a-C:Hx overcoats on HAMR media are studied. Surface topographical changes caused by the laser heating are evaluated with atomic force microscopy and structure changes by visible Raman spectroscopy. It is found that laser heating induces surface topographical and structure changes, especially for the a-C:Nx overcoat.

The effect of high deposition energy of carbon overcoats on perpendicular magnetic recording media

High-energy carbon deposition techniques provide thin overcoats with high corrosion and wear protection for magnetic recording media applications. The effect of high-energy (0–300 V substrate bias) deposition on the implantation induced changes in magnetic and structural properties of granular perpendicular magnetic recording media is studied. To observe subtle changes in a thin region of recording media, antiferromagnetically coupled layer structure was used. Clear changes in the magnetic properties, observed as a function of the carbon deposition energy, correlate with other measurements such as X-ray photoelectron spectroscopy, indicating the need to consider such effects when designing media and overcoat.

An Atomistic Study of Perfluoropolyether Lubricant Thermal Stability in Heat Assisted Magnetic Recording

At the head disk interface (HDI), the stability of the perfluoropolyether (PFPE) lubricant and carbon overcoat (COC) materials must be preserved under HAMR conditions. In this work, we investigate this issue by comparing the effects of transient versus steady heating of Zdol to replicate the precise pulsed heating of the HAMR system. These effects include changes in intermolecular lubricant bonding, molecular decomposition and desorption. In order to accurately account for potential changes in covalent and intermolecular bonds, we utilize the cutting-edge molecular simulation method of ab initio molecular dynamics. To simulate constant heating, a series of constant temperature simulations are performed at temperatures ranging from 300 K-700 K where the temperature is maintained via the Nose Hoover thermostat. For the transient heating simulations, the temperature is ramped over 100 K intervals with initial temperatures ranging from 300 K to 700 K. These heating studies are performed for bulk PFPE systems as well as PFPE-COC configurations to highlight the effect of PFPE-COC adhesion on lubricant thermal stability at the HDI. In the PFPE-COC simulations, we evaluate the amount of desorption versus decomposition as a function of initial temperature. Through our analysis, we are able to reveal the molecular mechanism of PFPE depletion as a function of functional group composition and, thereby, provide design criteria for lubricant molecular architecture in HAMR applications.

Carbon Overcoat Oxidation in Heat-Assisted Magnetic Recording

Due to the ever increasing demand for higher storage capacity in disk drives, novel techniques to read and write information, such as heat-assisted magnetic recording are being developed. However, the recording temperatures impact not only the media but also the carbon overcoat that is used to protect both the media and writing head. In this paper, we discuss the effect of thermal annealing on the rate of carbon erosion on overcoats deposited using two different techniques, viz. filtered-cathodic arc and plasma-enhanced chemical vapor deposition.

Simulation studies of contact sensor for disk defect mapping

The keep increasing areal density in hard disk drive demands reducing the clearance between the read/write element and the disk surface. A clearance of 2 nm could be achieved using thermal-flying-height technology (Kurita et al. IEEE Trans Magn 41(5):3007–3009, 2005). In such a low-clearance flying regime, it is important to ensure the reliability of the head-disk interface. Head carbon overcoat wear should be taken into account and it requires ultra-smooth disk surface. Conventional method for disk inspection is using glide tester (Nayak et al. IEEE Trans Magn 39(2):729–734, 2003). However, with lower and lower clearance, some small defects on the disk surface can no longer be neglected, and these small defects cannot be detected by glide test. Thus a thermal-contact sensor integrated in a magnetic-head slider was proposed by Shimizu et.al to detect defects on disk surface (Shimizu et al. IEEE Trans Magn 47(10):3426–3432, 2011). Their experimental results confirmed that the contact sensor has equivalent sensitivity for detecting head-disk contact to that of a conventional acoustic emission sensor. And the defect mapping with this method was more sensitive than measurements with an optical surface analyzer. All these results were obtained using experimental method. Thus, numerical simulation studies are necessary in order to investigate the contact mechanics between asperities and the contact sensor. In this paper, a simplified contact model was developed to study contact mechanics between contact sensor and the disk asperity using finite element method. Different parameters’ effect on the signal of a contact sensor was investigated. These results are of significance for the development of a thermal contact sensor in the hard disk drive for higher recording density.

Ultraviolet bonding of perfluoropolyethers to carbon surfaces investigated using quantum chemical methods

For improving the tribological performance of hard disk drives, nanometer-thick perfluoropolyether (PFPE) lubricant films are generally treated with ultraviolet (UV) irradiation to bond them to the carbon overcoats of the disks. By modeling UV irradiation as an electron emission and attachment process, we investigate the UV bonding of nonfunctional PFPE Z and functional PFPE Zdol to hydrogenated and nitrogenated carbon surfaces with quantum chemical methods. Our calculation results show that, upon electron attachment, Z dissociates at its main chain to two fragments terminated by CF2CF2 and CF2O groups, whereas Zdol dissociates to a hydrogen fluoride and a fragment. The perfluoromethoxy oxygen in one of the Z fragments and the carbon radical and the hydrogen-truncated end group in the Zdol fragment interact strongly with sp2 and oxidized sites on carbon surfaces. Imine moieties on the CNx surface also contribute considerably to the UV bonding of Zdol.

Adhesion and Friction Behavior of Magnetic Disks With Ultrathin Perfluoropolyether Lubricant Films Having Different End-Groups Measured Using Pin-on-Disk Test

In this paper, the adhesion and friction forces between a glass pin and disk surfaces coated by lubricants with different numbers of hydroxyl endgroups (Z-03, Z-dol, Z-tetraol, and Z-TMD) were measured using a pin-on-disk microtribotester. The adhesion force decreased in the order of Z-dol, Z-TMD, Z-tetraol, and Z-03. The friction force at the applied load force of 0 mN decreased in the order of Z-TMD, Z-tetraol, Z-dol, and Z-03. These trends could not be obviously explained by the number of hydroxyl endgroups. Next, we investigated the relationship between the adhesion energy and the total surface energy. The adhesion force was roughly proportional to the surface energy, and the friction force was proportional to the friction factor defined by the friction coefficients of the carbon overcoat and the lubricant film, the lubricant film coverage, and the total surface energy. Based on the results, we suggested a friction model of the contact between the pin and disk surfaces.

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.