Carbon Overcoat Research Articles

Structural stability of hydrogenated amorphous carbon overcoats used in heat-assisted magnetic recording investigated by rapid thermal annealing

Ultrathin amorphous carbon (a-C) films are extensively used as protective overcoats of magnetic recording media. Increasing demands for even higher storage densities have necessitated the development of new storage technologies, such as heat-assisted magnetic recording (HAMR), which uses laser-assisted heating to record data on high-stability media that can store single bits in extremely small areas (∼1 Tbit/in.2). Because HAMR relies on locally changing the coercivity of the magnetic medium by raising the temperature above the Curie temperature for data to be stored by the magnetic write field, it raises a concern about the structural stability of the ultrathin a-C film. In this study, rapid thermal annealing (RTA) experiments were performed to examine the thermal stability of ultrathin hydrogenated amorphous carbon (a-C:H) films deposited by plasma-enhanced chemical vapor deposition. Structural changes in the a-C:H films caused by RTA were investigated by x-ray photoelectron spectroscopy, Raman spectroscopy, x-ray reflectivity, and conductive atomic force microscopy. The results show that the films exhibit thermal stability up to a maximum temperature in the range of 400–450 °C. Heating above this critical temperature leads to hydrogen depletion and sp2 clustering. The critical temperature determined by the results of this study represents an upper bound of the temperature rise due to laser heating in HAMR hard-disk drives and the Curie temperature of magnetic materials used in HAMR hard disks.

Design of low-friction PVD coating systems with enhanced running-in performance — carbon overcoats on TaC/aC coatings

The widespread use of low friction PVD coatings on machine elements is limited by the high costs associated with fulfilling the demands on the surface quality of both the supporting substrate and the counter surface. In this work, an attempt is made at lowering these demands, by adding a sacrificial carbon overcoat to a TaC/aC low friction coating. Both coatings were deposited by planar magnetron DC sputtering, as separate steps in a single PVD-process. Coatings were deposited on substrates of two different surface roughnesses, in order to test the ability of this coating system to function on rougher substrates. Reciprocating ball on disc tests was performed, using balls with two different surface roughnesses. The worn surfaces were investigated using 3-D profilometry and SEM. The ability of the different overcoats to initially reduce the roughness of both the coated surface and the counter surface and to produce stable, low-friction conditions was examined for the different initial roughnesses. The implications for design of efficient run-in coatings for various systems are discussed.

A Description of Multiscale Modeling for the Head-Disk Interface Focusing on Bottom-Level Lubricant and Carbon Overcoat Models

The challenges in designing future head disk interface (HDI) demand efficient theoretical modeling tools with flexibility in investigating various combinations of perfluoropolyether (PFPE) and carbon overcoat (COC) materials. For broad range of time and length scales, we developed multiscale/multiphysical modeling approach, which can bring paradigm-shifting improvements in advanced HDI design. In this paper, we introduce our multiscale modeling methodology with an effective strategic framework for the HDI system. Our multiscale methodology in this paper adopts a bottom to top approach beginning with the high-resolution modeling, which describes the intramolecular/intermolecular PFPE-COC degrees of freedom governing the functional oligomeric molecular conformations on the carbon surfaces. By introducing methodology for integrating atomistic/molecular/mesoscale levels via coarse-graining procedures, we investigated static and dynamic properties of PFPE-COC combinations with various molecular architectures. By bridging the atomistic and molecular scales, we are able to systematically incorporate first-principle physics into molecular models, thereby demonstrating a pathway for designing materials based on molecular architecture. We also discussed future materials (e.g., graphene for COC, star-like PFPEs) and systems (e.g., heat-assisted magnetic recording (HAMR)) with higher scale modeling methodology, which enables the incorporation of molecular/mesoscale information into the continuum scale models.

Ultrathin Si/C graded layer to improve tribological properties of Co magnetic films

An ultrathin Si layer (≤1 nm) deposited on Co magnetic films was bombarded with energetic C+ ions to form a Co/Si/C mixed layer. This layer improved the tribological properties of the Co film as compared with those of a commercial hard disk media with 3.0 nm carbon overcoat and 1.4 nm of lubricant. Formation of a network of C-C and Si-C bonds at the outermost layer and the bulk of the Si/C film as well as formation of chemical bonds between the Co surface and the mixed layer was found as the main factors to improve the tribological properties.

Atomistically Tuning Lubricant Adhesion on Carbon Overcoat Surface

The critical issue facing head-disk interface (HDI) design is determining the optimal combination of lubricant and overcoat materials. A high performing lubricant should have sufficient adhesion to the carbon overcoat (COC) layer to prevent spin-off, while the COC material protects the magnetic media from corrosion and tribological damage. In this paper, we examine at the atomistic level COC material performance by calculating the adhesive interactions between various types of model perfluoropolyether (PFPE) functional lubricants and COC. The COC materials we investigated include industry standard diamond-like carbon (DLC), composed of graphitic (sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> bonded) and diamond (sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> bonded) type carbon. In addition, we investigated the atomically thin graphene and pure diamond COCs to determine the effect of carbon structure on PFPE adhesion. These pure diamond and graphene results support the observation that the PFPE interacts more strongly with the graphitic content in DLC. The difference between the results for diamond and graphene are due to the sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> bonding in graphene with delocalized electrons in the aromatic ring, whereas in the diamond structure, electrons are bound by the extra C-H bond. Our density functional theory study of PFPE interaction with these three COC materials shows that Ztetraol has the strongest adhesion on all of the COCs investigated. Based on our investigation, we would conclude that an ideal COC material should have high graphitic content to promote strong PFPE adhesion. We also evaluated the feasibility of pure graphene to promote PFPE adhesion. We extend our analysis to explore dimensional crossover to gauge the effect of COC thickness on adhesion.

Temperature Profile in the Presence of Hotspots in Heat Assisted Magnetic Recording

Recently, the demands for increasing memory capacities in hard disk drives (HDDs) has resulted in state-of-the-art technologies including heat assisted magnetic recording (HAMR) with significantly higher operating temperatures. HAMR results in swift degradation of current lubricant and carbon overcoat (COC) materials, leading to magnetic media corrosion which is detrimental to HDD operation. In addition, the lack of thorough understanding of the temperature profiles arising from the hotspot and energy management throughout these materials also exacerbates the problem. To address this issue, in this paper we will focus on the COC and investigate the transient heat transfer in various examples of nanoscale thin films when a hot spot is created via lattice Boltzmann method (LBM) since traditional conduction models like Fourier law are not accurate due to dominant sub-continuum effects. LBM originates from the Boltzmann transport equations (BTEs) and is computationally efficient due to easy parallelization with convenient handling of complex geometries. Our results of the heat transfer mechanism and temperature profiles show that Fourier equation under-predicts the peak temperature rise at the center of the hot-spot as the system size approaches the nanoscale domain. Applying LBM to a multilayered system, we observe a temperature slip along the interface of two materials indicated by the broken isothermal contours, as the heat is confined to a single layer. Using LBM, we then explore a novel graphene overcoat which has outstanding thermo-mechanical properties, and thereby extremely compatible in HAMR applications.

Perfluoropolyether Lubricant Interactions With Novel Overcoat via Coarse-Grained Molecular Dynamics

In this paper, we investigated physiochemical properties of new lubricant candidates for head-disk interface through various perfluoropolyether lubricant films on diamond, diamond-like carbon, and graphene overcoat surfaces via large scale coarse-grained bead-spring molecular dynamics stemming from the atomistic theory. Lubricant film conformations were characterized by investigating perpendicular component of molecular conformation, which determines the thickness of monolayer lubricant film. The distribution of functional endgroups and the mobility were analyzed via self-diffusion process. Here, we illustrate the effects of endgroup structure and carbon-surface structure on the film conformation and the mobility by expanding the multiscale simulation methodology and select candidates for future HDI design.

Molecular Dynamics Simulation for Lubricant Shear Properties During Heating

This paper discusses the shear properties of ultra-thin lubricant film on hard disk media during heating. A molecular dynamics method is used on four different types of lubricant to better understand the phenomena on the atomic scale. We investigate the shear mobility of lubricant for each temperature between 30°C and 300°C, assuming there is an air shear-induced migration on a carbon overcoat. The simulation results show that a thick lubricant film has a mobile and restricted layer, while a thin film which thickness is less than 1 nm , increases the slippage at lubricant/overcoat interface. At a high temperature, a lubricant with a flexible main chain tends to have a higher level of mobility. The difference in lubricant shear property is closely related to the flexibility of main chain and the amount of functional groups of lubricant which can affect the shear viscosity and adhesion to the carbon overcoat.

Chemical Vapor Deposition Growth, Optical, and Thermal Characterization of Vertically Aligned Single-Walled Carbon Nanotubes

Vertically aligned single-walled carbon nanotubes (VA-SWNTs) is expected to be an extra-ordinal material for various optical, electrical, energy, and thermal devices. The recent progress in growth control and characterization techniques will be discussed. The chemical vapor deposition (CVD) growth mechanism of VA-SWNTs is studied based on the in situ growth monitoring by laser absorption during CVD. The growth curves are characterized by an exponential decay of the growth rate from the initial rate determined by ethanol pressure. The initial growth rate and decay of it are discussed with carbon over-coat on metal catalysts and gas phase thermal decomposition of precursor ethanol. For the precisely patterned growth of SWNTs, we recently propose a surface-energy-difference driven selective deposition of catalyst for localized growth of SWNTs. For a self-assembled monolayer (SAM) patterned Si surface, catalyst particles deposit and SWNTs grow only on the hydrophilic regions. The proposed all-liquid-based approach possesses significant advantages in scalability and resolution over state-of-the-art techniques, which we believe can greatly advance the fabrication of nanodevices using high-quality as-grown SWNTs. The optical characterization of the VA-SWNT film using polarized absorption, polarized Raman, and photoluminescence spectroscopy will be discussed. Laser-excitation of a vertically aligned film from top means that each nanotube is excited perpendicular to its axis. Because of this predominant perpendicular excitation, interesting cross-polarized absorption and confusing and practically important Raman features are observed. The extremely high and peculiar thermal conductivity of single-walled carbon nanotubes has been explored by nonequilibrium molecular dynamics simulation approaches. The thermal properties of the vertically aligned film and composite materials are studied by several experimental techniques and Monte Carlo simulations based on molecular dynamics inputs of thermal conductivity and thermal boundary resistance. Current understanding of thermal properties of the film is discussed.

Effect of pretreatment of Si interlayer by energetic C+ ions on the improved nanotribological properties of magnetic head overcoat

A thin layer of silicon has been used to improve the adhesion between amorphous carbon coatings and different substrates. However, the mechanism responsible for this improved adhesion to ceramic substrates, especially the Al2O3-TiC (AlTiC) substrate of magnetic recording heads, has not been well studied. In this work, this mechanism was investigated by conducting simulation and experimental tests. A tetrahedral amorphous carbon (ta-C) overcoat was deposited on Si-coated ceramic substrates by using filtered cathodic vacuum arc (FCVA) at ion energy of 100 eV. The chemical structure of the ta-C overcoats and interlayers as well as the nanotribological properties of the ta-C coated AlTiC substrate were studied by means of XPS analysis, nanoscratch and ball-on-flat tests. The formation of a Si-C network between the Si interlayer and ta-C overcoat as well as the formation of Al–O–Si and Si–O–C bonds between the interlayer and the substrate were found to be the two main phenomena which strongly bond the ta-C film to its ceramic substrate. Prior to deposition of the ta-C overcoat, surface of the Si interlayer was bombarded (pretreated) by C+ ions with ion energy of 350 eV. Effect of this pretreatment on the structure and tribological properties of the coated surfaces was also studied. The results revealed that pretreatment of the Si interlayer by energetic C+ ions is an effective way to form a mixed interface and enhance the formation of a larger number of strong chemical bonds between the substrate and the overcoat which improves the adhesion of the overcoat to the substrate. In addition, this method increased the sp3 content of the ta-C film which further improves the wear resistance and durability of the coating.

Head-disk interface design in magnetic data storage

Superior electrical conductivity and thermo-mechanical response of graphene can significantly improve areal density in magnetic data storage. The current head media spacing in hard disk drives (HDD) is 7.2 nm, and replacing conventional carbon overcoat (COC) with graphene will drastically reduce head media spacing, increasing areal density to eight times its present value. A paradigm shift in HDD systems can also be achieved via selection of a combination of new lubricants and unconventional architecture of COC systems. Here, we evaluate the feasibility of graphene overcoat (GOC), by understanding GOC-lubricant interactions. We further introduce new alternative head-disk interface (HDI) designs consisting of buffer/lubricant layers (i.e., graphene/carbon nanotube (CNT) or fullerene/perfluoropolyether (PFPE)). These hybrids could further enhance tribological performance including the reduction of wear and friction while drastically increasing areal density of data storage devices. Our study here will lead to vigorous investigation of HDI in magnetic data storage, including heat-assisted magnetic recording (HAMR), with tuned atomistic design criteria.

Multi-scale/multi-physical modeling in head/disk interface of magnetic data storage

The model integration of the head-disk interface (HDI) in the hard disk drive system, which includes the hierarchy of highly interactive layers (magnetic layer, carbon overcoat (COC), lubricant, and air bearing system (ABS)), has recently been focused upon to resolve technical barriers and enhance reliability. Heat-assisted magnetic recording especially demands that the model simultaneously incorporates thermal and mechanical phenomena by considering the enormous combinatorial cases of materials and multi-scale/multi-physical phenomena. In this paper, we explore multi-scale/multi-physical simulation methods for HDI, which will holistically integrate magnetic layers, COC, lubricants, and ABS in non-isothermal conditions.

Adhesion and Friction Properties of Molecularly Thin Perfluoropolyether Liquid Films on Solid Surface

The adhesion and friction properties of molecularly thin perfluoropolyether (PFPE) lubricant films dip-coated on a diamond-like carbon (DLC) overcoat of magnetic disks were studied using a pin-on-disk-type micro-tribotester that we developed. The load and friction forces were simultaneously measured on a rotating disk surface under an increasing/decreasing load cycle and slow sliding conditions. Experiments were performed using two types of PFPE lubricants: Fomblin Z-tetraol2000S with functional end-groups and Fomblin Z-03 without any end-group. The curves of the friction force as a function of the applied load agree with the curves estimated using the Johnson-Kendall-Roberts (JKR) model. The friction forces on the Z-03 films having different thicknesses were not found to decrease drastically; however, the friction forces on the Z-tetraol film were found to decrease drastically when the film thickness is more than ~1.2 nm. This drastic change in the case of the Z-tetraol film is estimated to be affected by the coverage of the lubricant film.

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.

Thermal Stability of Ultrathin Amorphous Carbon Films for Energy-Assisted Magnetic Recording

Energy-assisted magnetic recording (EAMR) uses a laser-optical system integrated into the magnetic head to heat locally a fine-grained material of high magnetic anisotropy energy density above its Curie temperature, and store single bits in very small areas without being limited by the superparamagnetic effect. However, localized laser heating may affect the thermal stability of the carbon overcoat of the hard disk. To examine the effect of laser heating on the overcoat thermal stability, ultrathin amorphous carbon (a-C) films of similar thickness (~ 3.6 nm) synthesized by filtered cathodic vacuum arc (FCVA) and chemical vapor deposition (CVD) were subjected to repetitive heating under different laser powers. Carbon hybridization and surface roughness of the a-C films were examined by Raman spectroscopy and atomic force microscopy, respectively. For the laser power range studied (150-300 mW), a-C films produced by the FCVA technique demonstrated superior thermal stability than CVD films of similar thickness. To investigate the possibility of further reducing the magnetic spacing, thinner (~ 0.9 nm) a-C films deposited by the FCVA method were subjected to the same laser heating conditions. Although the thermal stability of the FCVA-synthesized a-C films exhibited thickness dependence, even the thinner (~ 0.9 nm) FCVA film demonstrated higher thermal stability than the much thicker (~ 3.6 nm) CVD film. The results of this study illustrate the high potential of FCVA as a coating method for EAMR.

The Influence of Ultraviolet Irradiation on the Surface Chemistry of Ztetraol Magnetic Hard Disk Lubricant: a Combined Temperature Programed Desorption and X-Ray Photoelectron Spectroscopic Study

Magnetic hard disks coated with ztetraol lubricant were characterized with temperature programed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Ztetraol was found to have two adsorbed states, with desorption temperatures of ~650 and ~950 K. The complete removal of the low temperature state by solvent extraction identified it as due to the mobile lubricant. Desorption of the CF3 mass fragment was observed only at high temperature, indicating that lubricant in this state was in contact with the surface, which allowed the assignment of this high temperature state to the bonded lubricant. UV irradiation was found not to alter the TPD of the unextracted lubricant film. In contrast, the TPD of the UV-bonded layer remaining after extraction was observed to have only one desorption state, stabilized at ~40 K higher temperature as compared with the naturally bonded layer. XPS of the mobile layer was accomplished using spectral subtraction of the C1s carbon overcoat peak (284.8 eV BE), and the perfluoroethylene and perfluoromethylene lubricant peaks (293.5 and 295.0 eV BE, respectively), as a function of UV exposure. No change in the mobile lubricant layer was found with increasing UV exposure. C1s XPS of the UV-bonded layer identified five surface species and assigned XPS peaks to each: the carbon overcoat peak at 284.8 eV BE, ether peak at 286.4 eV BE, organic acid peak at 288.7 eV BE, perfluoroethylene peak at 293.5 eV BE, and the perfluoromethylene peak at 295.0 eV BE. Changes in the relative intensity of the assigned peaks with increasing UV irradiation exposure time were observed. The integration of the assigned XPS peaks from the UV-bonded layer with increasing UV exposure was used to identify UV dependent changes in the bonded layer. A significant relative decrease in the perfluoromethylene lubricant component was observed with increasing exposure, with a simultaneous increase of both the ether and organic acid surface concentrations. Quantum chemical calculations using small molecular models of the ztetraol were used to elucidate the XPS and TPD observations. The calculations revealed that the lubricant is fragmented with irradiation, forming reactive end groups, a volatile CF2O, and a hydrolyzeable CFO terminated fragment all consistent with the XPS results. The mechanistic implications and the possible surface chemistry are discussed.

Silicon nitride gradient film as the underlayer of ultra-thin tetrahedral amorphous carbon overcoat for magnetic recording slider

There are higher technical requirements for protection overcoat of magnetic recording slider used in high-density storage fields for the future. In this study, silicon nitride (Si-N) composition-gradient films were firstly prepared by nitriding of silicon thin films pre-sputtered on silicon wafers and magnetic recording sliders, using microwave electron cyclotron resonance plasma source. The ultra-thin tetrahedral amorphous carbon films were then deposited on the Si-N films by filtered cathodic vacuum arc method. Compared with amorphous carbon overcoats with conventional silicon underlayers, the overcoats with Si-N underlayers obtained by plasma nitriding especially at the substrate bias of −150 V, can provide better corrosion protection for high-density magnetic recording sliders.

Investigation of wear resistance and lifetime of diamond-like carbon (DLC) coated glass disk in flying height measurement process

Flying height has been greatly reduced to less than 10 nm to achieve high-density magnetic storage. This leads to significant disk wear especially, glass disks used in flying height measurement process. This paper reports the utilization of diamond-like carbon (DLC) overcoat to increase the wear resistance and lifetime of commercial glass disks in a flying height tester. Wear resistance of the DLC coated glass disks was investigated in wear test using a triboindentor. The results showed significant wear resistance improvement of the coated disk where the wear depth reduced from 62.2 nm on an uncoated disk to 5–7 nm on 15-nm thick DLC coated disks. Furthermore, lifetime measurement of the coated disk has been performed in a flying height tester. As a result, lifetime of the coated disk has been drastically improved by more than 30 times in comparison to an uncoated disk.

Effects of SiN overcoat on recording performances in perpendicular media

We have investigated the effects of SiNx overcoat on bulk magnetic properties and recording performances in perpendicular media. Difference in recording parameters between disks with SiNx and carbon overcoats is mainly due to the difference in writeability caused by coercivity change during SiNx overcoat deposition. The bulk magnetic properties of the cap in the recording layer are affected by the reaction with silicon and nitrogen. Although SiNx overcoat changes writeability, it does not alter the intrinsic dependence of recording parameters on writeabilty.

Durability Against Contact Wear of Nonlubricated Disks in the Head–Disk Interface of Disk Drives

Ever decreasing magnetic spacing in the hard disk drive requires each contribution to the magnetic spacing to be examined for possible reduction. Currently, the lubricant layer on top of the disk carbon overcoat is typically 10-20 Å thick. This paper studies the contact resistance of nonlubricated disks with the aim of seeking a way to reduce the lubricant thickness contribution to the magnetic spacing. We prepared normally lubricated (thickness >; 7 Å), partially lubricated (thickness 3.5 Å), and barely lubricated (thickness 0.5 Å) disks. Partially lubricated disks showed contact wear resistance as good as normally lubricated disks when they were baked in a dry oven. We suggest that this enhanced contact wear resistance comes from the softening (graphitization) of the carbon overcoat and/or reduced number of active sites on the overcoat for water adsorption due to oxidation during the baking. However, corrosion resistance of partially lubricated disks became worse after baking and the amount of lubricant transfer from partially lubricated disk to slider surface did not decrease compared to normally lubricated disks. Barely lubricated disks showed very bad contact wear resistance even after baking, showing that it is necessary to have at least partial lubrication on the disk surface.