Hydrogen bonding in lubricants for hard disk drives

Hydrogen bonding interaction within a small ensemble of water molecules, that within a group of water molecules and end-groups of Z-dol and Z-tetraol, and the effect of electrolyte ionic pair such as LiCl upon these interactions were examined by the molecular dynamics method based on the potential given by a semi-empirical SCF quantum mechanics. It was revealed that the strength of the hydrogen bond increased rapidly as the size of droplet increased, relating to the population density of hydroxyl units, and that such interaction was amplified significantly by the presence of electrolyte ionic pair. An extraordinary interaction was thus predicted between Z-tetraol and aqueous solution of alkali halide. An experimental study thence conducted revealed that Z-tetraol and aqueous NaCl solution (2 M) formed an extremely stable water-in-oil type emulsion. The emulsion consists of spheroids of several nanometers across wherein several thousands of water molecules are encased by several tens of Z-tetraol end-groups. The interfacial layer of each spheroid is formed and stabilized by the hydrogen bonding interaction between the hydroxyl units of the tetraol-ends and water molecules enhanced by the electrolyte ionic pairs. When disks coated with Z-tetraol were tested for flyability at high humidity, the head-disk interaction detected acoustically increased with time. Spontaneous formation of globules resulting from interaction of tetraol end-groups and water molecules assisted by ubiquitously present alkali halide contaminant would account for the observed increase of the head-disk interaction. Possible structures of perfluoropolyether lubricants ideal for magnetic disk application are discussed.     

Lubricant Transfer in Disk Drives

For disk drives with Z-tetraol-coated disks, the ingress of airborne solid particulates into the disk drive was found to result in disk-to-head lubricant transfer. In addition, high humidity was found to enhance the transfer process. Water soluble electrolytes such as alkali halides are most ubiquitous airborne solid particulates. Molecular dynamics calculations were performed to examine (a) the condensation process of H₂O, (b) the effect of alkali halide on the process, (c) the difference between the end-groups of Z-dol and Z-tetraol. It was shown that the OH units of Z-tetraol end-groups would embed themselves into facial layer of water?Celectrolyte droplets, thus encapsulating and stabilizing the droplets, while the OH units of Z-dol would not do so. The lubricant transfer observed uniquely for Z-tetraol-coated disks is attributed to inorganic particulates such as NaCl entering the drive interior, landing on the disk surface, attracting water, and forming Z-tetraol encapsulated water?Celectrolyte droplets. These droplets are viscous and are readily picked up by the slider.     

Interaction Between Disk Lubricants and Solvents in the Dip-Coating Process

It has been reported earlier that, keeping all other factors fixed, the disk performance could vary significantly depending on the choice of solvent used in the dip-coating process. We surmised that the difference in the ability of solvent to disrupt the intermolecular hydrogen bonding between hydroxyl units of Z-dol or Z-tetraol would lead to a different structure or arrangement of the lubricant molecules as the solvent evaporated in the dip-coating process. Using proton NMR and IR spectroscopy, four solvents, hexafluorobenzene, Vertrel-XF, HFE7100, and FC72, were examined for their ability to disrupt the hydrogen bonding of Z-dol and Z-tetraol. The study has revealed that the said ability increases in the order of FC72 < HFE7100 < hexaflurobenzene < Vertrel-XF. Z-dol molecules, for example, are completely separated from each other in Vertrel-XF, but maintain their intermolecular hydrogen bonding in FC72. It has also revealed an extraordinary formation of complexoid between HFE molecules and Z-tetraol with MW _n of 2000. The complexoid has a specific stoichiometry of (Z-tetraol) _n (HFE) 15 _n and is not miscible with HFE. The formation of such complexoid was not observed for Z-tetraol of larger MW _n (3000).     

Z-dol and carbon overcoat: the bonding mechanism

Molecular dispositions of Z-dol (linear perfluoropolyether with hydroxyl termini, –O–CF₂–CH₂–OH) applied over the carbon overcoat of magnetic hard disks are often depicted by an arrangement based on the hydrogen bonding interaction between the hydroxyl ends and some polar units of the carbon surface. The hydrogen bonding interaction is weak. The arrangement based on this mechanism is attained rapidly, but is slowly replaced (if partially) by a bona fide chemical bond. The issue of the exact nature of this chemical bond has been left unanswered in most of the reports. Past works deemed to have explored and elucidated the identity of the bond in question are gathered, reviewed and deductively presented. The review, we believe, clearly shows that the bonding in question involves (1) dangling bonds shielded within the sputter-deposited carbon, (2) transfer of the hydrogen atom of the hydroxyl unit of Z-dol to the dangling bond site, and (3) attachment of the remaining alkoxy system, Z–O–CF₂–CH₂–O•, to the carbon surface as a pendant ether unit. The Z-dol moiety thus attached is held by a bona fide chemical bond, and cannot be replaced by water molecules nor removed by solvent extraction.     

Lubricant Bonding Effects on Thin Film Disk Tribology

The magnetic recording industry predominantly uses Zdol to lubricate the carbon overcoat of magnetic recording disks. Zdol comprises a perfluoropolyether chain terminated with hydroxyl end groups that are capable of reversibly bonding to the carbon overcoat. Contact start/stop (CSS) tests were done to investigate the effects of Zdol lubricant bonding, thickness, and relative humidity on durability. The durability improved with increasing thickness of fully bonded or mobile Zdol. The durability decreased with increasing initial bonded fraction and with decreasing relative humidity. The bonded fraction increased with time during the tests at elevated temperature and low relative humidity.     

Grafting Lubricant Molecular Chains to the Carbon Overcoat

It was conceived and demonstrated that irradiation of magnetic disks coated with PFPE (perfluoropolyether) lubricants terminated with a carboxylic group at one terminus with long-UV (254 nm) would lead to grafting, via a bona fide C–C chemical bond, of genuine PFPE molecular chains to the carbon overcoat all at one terminus with all the remaining chain segments being free to sway. The water contact angle of disks coated with PFPE lubricants terminated with end-groups having hydroxyl unit(s) (e.g., Z-dol and Z-tetraol) decreases gradually, after the initial contact of the droplet, reaching an asymptote in 20–30 s. The gradual temporal change is accounted for by shifting of the equilibrium disposition of hydroxyl units of the lubricant molecules from that determined by the interaction with the surface of the carbon overcoat to that determined by the interaction with surfaces of both the carbon overcoat and the liquid droplet. The water contact angle of disks prepared by the presently conceived photo-grafting method is high (>110 degrees) and shows no temporal change. In a preliminary spin-stand drag test, disks with PFPE chains photo-grafted by this method and also the heads (for the read/write process) with similar photo-grafted PFPE chains exhibited extraordinary durability.     

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.     

Simulation of the Performance of Various PFPE Lubricants Under Heat Assisted Magnetic Recording Conditions

Heat assisted magnetic recording (HAMR) is proposed for the next generation of hard disk drives. In HAMR systems, a laser beam heats the disk magnetic layer to the Curie temperature. This may cause the thin film lubricant coating the disk to deplete due to evaporation and surface tension gradient. In this study, we perform simulations for the Z-tetraol family of lubricants with four hydroxyl end-groups, including Z-tetraol 1200 as a low molecular weight member of the family and Z-tetraol 2200 as a high molecular weight of the family, and also for ZTMD (2,200 Da) with eight hydroxyl groups as a multi-dentate lubricant, which is manufactured based on the Z-tetraol family. All studies are performed for four cases of lubricant thicknesses including 5, 7, 12, and 14A. These numbers are chosen in order to provide a fair comparison with a previous study for Z-dol. We also investigate the relative effects of evaporation with respect to the thermocapillary shear stress. It is found that after a 2 ns illumination of the laser, a trough and two side ridges across the down-track direction can be seen in the lubricant. The performances of the lubricants can be ranked mainly based on the trough depth and also evaporation such that better lubricants show less deformation and trough depth under equal conditions of thermal spot size and peak temperature. We also found that all of the lubricants deplete rapidly and their depletion speed decreases gradually.     

Effect of Functional End-Groups on Lubricant Reflow in Heat-Assisted Magnetic Recording (HAMR)

In the developing heat-assisted magnetic recording technology, a laser heats up the magnetic media to the Curie temperature of a few hundred degrees celsius for a few nanoseconds. Accordingly, the thin-film lubricant coating on the disk experiences thermo-capillary and evaporation effects followed by its depletion. In order to maintain a reliable head–disk interface, the lubricant needs to return to the initial uniform profile, in a process known as lubricant reflow. We performed numerical simulations of the lubricant reflow and compared the recovery times for widely used lubricants in hard disk industry including Z-dol, Z-tetraol, and ZTMD lubricants with similar molecular weights. We modeled the lubricant reflow on the disk for a wide range of film thicknesses and laser spot sizes, based on a classical lubrication theory and material properties reported by experiments. The results show that the recovery times for Z-tetraol 2200 and ZTMD are significantly greater than that for Z-dol 2000, while the recovery time for ZTMD is close to that for Z-tetraol, despite its higher viscosity value. It is also shown that all lubricants have an optimum film thickness for recovery time, and this optimum point largely depends on the dewetting behavior of the lubricant.     

Reversed Micelle Structures in PFPE Lubricant Thin Films on Magnetic Disk Surface Observed by Non-Contact AFM

PFPE lubricants (Fomblin Z-dol) for hard disk surface lubrication have two hydroxyl groups, one at each end of the molecules, and form stable insoluble monolayers at the water surface. In this study, molecular weight-fractionated PFPE lubricant monolayers were transferred from the water surface to solid substrates such as a hydrophilized silicon wafer, gold-sputtered mica, and a hard disk after adjusting the two-dimensional density of the lubricant molecules. The molecular structures of the PFPE lubricant molecules at the solid surfaces were observed by the cryogenic non-contact AFM under ultra-high vacuum. At the hydrophilic silicon wafer surface we could observe a single lubricant molecule in a random coil sphere shape. However, at the non-polar gold surface we confirmed the formation of reversed micelle structures. At the hard disk surface we detected various sizes of reversed micelles of PFPE lubricant in a flat oval shape.     

Perfluoropolyethers with Dialkylamine End Groups: Ultrastable Lubricant for Magnetic Disk Application

It has been shown earlier that addition of X1P (partially fluorinated hexaphenoxy cyclotriphosphazene) to the lubricant Z-DOL markedly increases the durability of the magnetic disk system. Its efficacy has been attributed to the strong nucleophilic property of the triphosphazene ring. It follows that perfluoropolyethers terminated with nucleophilic end groups should exhibit a durability comparable to that of the Z-DOL/X1P mixture. To this end perfluoropolyethers with dialkylamine end groups were conceived and synthesized as better alternatives to Z-DOL or to the Z-DOL/X1P mixed system. Examination of the newly synthesized lubricant revealed that (1) perfluoropolyethers with dialkylamine end groups are resistant to the degradation process catalyzed by Lewis acid (e.g., aluminum oxide), (2) disks lubricated with these lubes have durability significantly better than those lubricated with Z-DOL, (3) in the drive system with disks lubricated with these lubes, the issue of the silicon oxide formation from siloxane contaminant is effectively contained (owing to the absence of degraded lube), and (4) the HDI system with disks lubricated with these lubes performs well even in the most aggressive atmospheric condition, i.e., nitrogen with 0% relative humidity.     

Nanoscale Lubricant with Strongly Bonded Phase and Mobile Phase

The best tribological properties can be obtained in a film that contains both bonded and mobile phases, and thus the ratio of the bonded phase to the mobile phase, the bonded ratio, is a critical factor for the lifetime of the mechanical components. In the present study, we deposited a mixed lubricant, which consists of a strongly bonded lubricant phase and a mobile lubricant phase, on a carbon overcoat. Islands of FDTS (perfluorodecyltrichlorosilane) SAM (self-assembled monolayer) were introduced as a strongly bonded lubricant phase and Fomblin Z03 lubricant as a mobile phase. The friction and durability properties of 2.5-in magnetic disks coated with 15 Å thick mixed lubricants were investigated using a ball-on-disk tribotester and compared with disks coated with 15 Å Zdol lubricants. Because the contact area of the ball increases with the bonded ratio, the friction coefficient of the disk coated with the mixed lubricant increases linearly with the bonded ratio over a range of about 25–100%. On the other hand, the friction coefficient of the disk coated with Zdol decreases with increasing bonded ratio. The mixed lubricant exhibits superior wear properties compared to Zdol. This may occur because the FDTS networks act as a barrier against the displacement of the mobile Z03 molecules, and the mobility of Z03 molecules is higher than that of Zdol molecules, which allows Z03 to replenish into the lubricant-depleted area at a higher rate.     

The Role of Functional End Groups of Perfluoropolyether (Z-dol and Z-03) Lubricants in Augmenting the Tribology of SU-8 Composites

In an earlier work, we demonstrated the development of SU-8 composites using perfluoropolyether (PFPE) as lubricant filler which reduced friction coefficient by ~7 times and enhanced wear life of SU-8 by more than four orders of magnitude. In this work, we have investigated the role of chemical bonding between SU-8 and PFPE molecules using two types of PFPE lubricants (i.e., Fomblin^® Z-dol and Z-03) in improving the tribological properties of the composite. Z-dol has polar (–OH) end groups whereas Z-03 has non-polar (CF₃) end groups. SU-8 with Z-dol (SU-8 + Z-dol) films yielded ~8 times greater wear life than SU-8 with Z-03 (SU-8 + Z-03) films and more by four orders of magnitude than pure SU-8. The nature of the films was analyzed in detail by chemical and physical characterization techniques like X-ray photoelectron spectroscopy, water contact angle and thermo-gravimetric analysis. The results validated the role of polar end functional group of Z-dol in covalent binding with SU-8 upon UV plasma treatment that resulted in improved tribological properties.     

High-speed tribology of PFPEs with different functional groups and molecular weights coated on DLC

In this article, we study the friction and wear durability of perfluoropolyether (PFPE) with different functional groups and molecular weights (MW) for a range of disk rotational speeds (500–7200 rpm or 1.2–17.33 m/s). A 4 mm diameter silicon nitride ball under a normal load of 4 g was employed as slider against PFPE lubricated diamond like carbon (DLC) film on magnetic hard disk. The coefficient of friction increases with increasing speeds, to certain extent, but it decreases for the higher speeds. At very high speeds, the fluctuations in the coefficient of friction of low MW PFPEs were larger than those of high MW PFPEs. The optical microscope image of the ball after sliding showed that evaporation might have occurred more easily in low MW than in high MW when sliding speed was increased due to the frictional heat generated at the interface. The wear lives of Z-lube (carboxyl group at both ends) and Z-dol are significantly higher than AS1 (alkoxy silano group at both ends) at low speed (1.2 m/s). In comparison to low MW PFPEs, high MW PFPEs show better wear durability at higher rotational speeds.     

Effects of Gas Composition,Humidity and Temperature on the Tribology of the Head/Disk Interface—Part II: Model and Analysis

A qualitative model for the effect of water condensation on the frictional behavior of unlubricated and lubricated carbon-overcoated disks is presented. The model suggests that for unlubricated disks adsorbed water acts as a lubricant, protecting the unlubricated disk surface from direct solid/solid contact and direct exposure to the environment. For lubricated disks, the interaction between adsorbed water and lubricant molecules seems to be responsible for the effect of humidity on the frictional behavior of lubricated disks. The effect of temperature on the frictional behavior of the head/disk interface is discussed in terms of surface energy, lubricant viscosity and mobility.     

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.     

Evaluation of gas permeation barrier properties using electrical measurements of calcium degradation

In this work, we developed a thin calcium degradation method introducing sensitive electrical resistance monitoring. We have demonstrated structural models of the inorganic/organic thin films to evaluate barrier properties against water and oxygen permeation. The time-dependent transmission curve of a multibarrier coated on both sides of the polyethersulfone substrate had a linear slope which was measured as 5.17 x 10(-3) gm(2) day at 20 degrees C and 60% relative humidity. This system can measure an accurate permeation rate with a high sensitivity in the measurable range of 10(1)-10(-6) gm(2) day. In addition, the test structure devised is applicable to various fabrication techniques for passivation layers with durability and ultralow permeability for flexible organic light emitting diodes.     

Degradation of Disk Lubricant: Ramification in Disk Drives and Direct Detection by TOF-SIMS

The degradation of Z-dol catalyzed by Lewis acid centers on the slider surface leads to chain scission forming one type of fragment terminated with a fluorocarbonyl end-group and the other with a trifluoromethoxy end-group. The former, in contact with humid air, converts to a fluorinated carboxylic acid Z-COOH. Z-COOH is an excellent scavenger for alkali or alkaline earth metal ions. Z-COO^-M⁺ thus formed is a strong surfactant, and, in a humid environment, forms microdroplets embodying water in the core. Metal ions thus scavenged on a disk surface can be readily detected by TOF-SIMS, and the microdroplets by optical microscopy in the dark-field mode. The presence of fragments having a trifluoromethoxy end-group on the disk surface can also be established by TOM-SIMS. A careful intensity analysis of peaks due to anions having a trifluoromethoxy end-group permits a semi-quantitative assessment of the extent of degradation. The study has also shown that degradation is caused by such production processes as tape-polishing and by such disk drive operations as the head flying over a single track or over a band in a seek-mode.     

Conformation and Fundamental Properties of Novel Lubricant TA-30 for Near Contact Magnetic Recording

A novel perfluoropolyether (PFPE) lubricant called TA-30 has been developed recently. We investigate the conformation of TA-30 on diamond-like carbon (DLC) thin films, by attempting the direct observation of a lubricant film by atomic force microscopy (AFM) using a fluoride probe. We investigate the fundamental properties of a TA-30 lubricant film, such as its spreading characteristics, and the film thickness dependence of surface energy. Considering these experimental results, we conclude that the conformation of TA-30 is considerably different from that of conventional Z-tetraol2000 whose molecular height is 1.7 nm and which was adsorbed on the DLC surface with the random coil. The TA-30 molecules are adsorbed rigidly to the DLC surface with double layers. The thickness of the first TA-30 layer is ~0.9 nm (similar to diameter of the PFPE backbone) and that of the second layer from the DLC surface is 1.4 nm. Since TA-30 has a lower film thickness than Z-tetraol2000 on the DLC surface, it can have two layers, even if the film thickness is approximately of the order of 1 nm, whereas Z-tetraol2000 does not cover the DLC surface and does not form the complete first layer. In addition, we conduct slider touchdown and takeoff hysteresis tests by using TA-30 and Z-tetraol2000. It is confirmed that the use of TA-30 can improve the head–disk interface (HDI) reliability at low-fly-height conditions.     

Nanotribological properties of novel lubricants for magnetic tapes

Two classes of novel lubricants, perfluoropolyethers (PFPE) and ionic liquids (ILs), were deposited on metal film magnetic tapes. The adhesive force and coefficient of friction of lubricated and unlubricated tapes were investigated at the nanoscale with an atomic force microscope (AFM) as a function of various humidity and temperature conditions. Microscale tests with a ball-on-flat tribometer were also performed in order to study the length-scale effects on friction. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated by monitoring the friction force, surface potential and contact resistance with the AFM. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) experiments were conducted to determine the chemical species that affect intermolecular bonding and as an aid in interpreting how the lubricant film tribological properties vary with the environmental conditions. Z-TETRAOL, one of the PFPEs, was found to exhibit the lowest adhesion and friction among the lubricant films studied. The ionic liquid 1,1′-(pentane-1,5-diyl)bis(3-hydroxyethyl-1H-imidazolium-1-yl) di[bis(trifluoromethanesulfonyl)imide)] exhibited comparable nanotribological properties with the PFPEs. This is attributed to the presence of hydroxyl groups at its chain ends, which can hydrogen bond with the surface similar to PFPEs.