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.     

Z-dol Versus Z-tetraol: Bonding and Durability in Magnetic Hard Disk Application

A component-level study has revealed that the durability of magnetic hard disks coated with Z-dol improved with increasing level of relative humidity, while the durability of disks coated with Z-tetraol was generally superior and not affected by the humidity within the range investigated (8–80%). It has been shown earlier that water molecules effectively passivate the catalytic centers responsible for the lubricant degradation. The Z-dol molecular chain has a hydroxyl group at each end, while the Z-tetraol molecular chain has two hydroxyl groups at each end. Having surmised that the superior performance of Z-tetraol can be ascribed to its ability to retain water molecules at its multiply hydroxylated ends, the solubility of water in Z-dol, Z-tetraol, and Z-TX were investigated using proton NMR spectroscopy. The study revealed that Z-tetraol is not only able to retain a much larger number of water molecules at its ends, but also is able to form stronger hydrogen bonds. Z-tetraol would then bond more tightly to the carbon overcoat (via hydrogen bonding with the surface hydroxyl groups), and be more resistant against catalytic degradation owing to its affinity to, and retention of water molecules.     

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).     

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.     

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.     

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.     

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.     

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.     

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.     

Effect of Pitch and Roll Static Angle on Lubricant Transfer Between Disk and Slider

A precision spin stand was used to study the effects of pitch static angle and roll static angle on lubricant transfer between a disk and a slider in a hard disk drive. The lubricant distribution on the slider was determined by time-of-flight secondary ion mass spectrometry, while the lubricant distribution on the disks was obtained using optical surface analysis. Lubricant transfer from the disk to the slider was found to increase as a function of the pitch static angle of the slider. Negative roll static angles were found to have a larger effect on lubricant transfer and the formation of lubricant moguls than positive roll static angles. Suspension frequencies and pitch mode frequencies were observed in the lubricant mogul patterns for negative roll static angles.     

Studies on the interactions between ZDOL perfluoropolyether lubricant and the carbon overcoat of rigid magnetic media

Current high performance magnetic storage devices, i.e., hard disk drives, typically operate at elevated temperatures of nominally 45–60°C. As a consequence, understanding the thermal response of the materials used in the construction of the drive becomes imperative. In this report, we focus on the thermal behavior of a common perfluoropolyether lubricant (ZDOL) used on the carbon-overcoated, hard disk. In particular, we show that evaporative loss of this disk lubricant, as well as bonding of the lubricant to the carbon-overcoated disk, can occur at the temperatures encountered in the hard-disk drive. Surface energy measurements show that the interaction of the hydroxyl-terminated perfluoropolyether ZDOL occurs principally through the end-groups. On unannealed disks, the interaction between this “mobile” lubricant and the carbon overcoat is characterized by hydrogen bonding with the strength of these interactions being only slightly stronger than the intermolecular hydrogen bonding characteristic of bulk ZDOL. Upon annealing at temperatures in the range of 60–150°C, the ZDOL lubricant becomes “bonded” to the disk. The surface energy of the bonded lubricant is substantially lower than the mobile lubricant reflecting the increased interaction strength that occurs as a result of bonding. Since the bonded state is the lower energy state, transitions from the mobile state to the bonded state are thermodynamically favored. The kinetics of this bonding transition, as well as the kinetics of lubricant evaporation were studied as a function of temperature. Using a model of two competing reaction channels, the activation energies for both lubricant bonding and lubricant evaporation were determined to be 3.6 kcal/mole and 5.4 kcal/mole respectively. Ab initio quantum chemical modelling was used to investigate possible interaction sites on the carbon surface. Both experiment and theory indicate that interaction of the hydroxyl-terminated ZDOL to the carbon overcoat occurs via hydrogen bonding to oxygenated species on the carbon overcoat, with a binding energy of 5–8 kcal/mole. An esterification reaction between the hydroxyl end-groups of ZDOL with carboxyl groups on the carbon surface as a result of annealing is shown to be consistent with the both the surface energy data and the kinetic data. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.     

Slider Wear on Disks Lubricated by Ultra-Thin Perfluoropolyether Lubricants with Different Molecular Weights

In this study, the wear properties of a magnetic head slider on disks lubricated by ultra-thin perfluoropolyether (PFPE) lubricants with different molecular weights were evaluated by the continuous sliding of magnetic head sliders using the slider contact by the dynamic flying height control. Two types of PFPE lubricants (Z-tetraol and D-4OH) with different molecular weights were evaluated. Results show that the slider wear depended on the coverage of the lubricant film; i.e., the lubricant film with sufficient coverage reduced slider wear. The lubricant film with a low molecular weight (low-Mw), including a lubricant material with a Fomblin and Demnum main chain, exhibited better coverage on a diamond-like carbon surface. Sliders with a low-Mw lubricant film showed less wear than those of a high molecular weight (high-Mw), and the depletion of the low-Mw lubricant film was less than that of the high-Mw lubricant film.     

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.     

Effects of PFPE Lubricant Properties on the Critical Clearance and Rate of the Lubricant Transfer from Disk Surface to Slider

The transfer of perfluoropolyether (PFPE) lubricant from the disk surface to the slider as a function of head-disk clearance has been investigated experimentally. The effects of lubricant thickness, bonding ratio, molecular polarity, and main chain stiffness on the lubricant transfer rate and the critical clearance below which lubricant transfer gets much enhanced are clarified. The critical clearance can be effectively reduced by decreasing the lubricant thickness or increasing the number of polar hydroxyl end-groups per lubricant molecule. Increasing the film bonding ratio or using lubricants with stiffer backbone can significantly decrease the lubricant transfer rate especially below the critical clearance. The results are discussed in terms of the effective disjoining pressure and its slope with respect to the film thickness.     

Replenishment Speed of Depleted Scar in Submonolayer Lubricant

The evaluation of submonolayer lubricant mobility is becoming important in the field of nanotribology, in particular, in hard disk drives for realizing the near-contact or surfing–recording. This paper experimentally and theoretically investigates the replenishment speed of a depleted scar in a submonolayer lubricant caused by the head–disk contact. The theoretical analysis is based on continuum mechanics. The replenishment process of a submonolayer lubricant height profiles in a depleted scar caused by the head touchdown operation was experimentally measured for the Z-tetraol lubricant with a 0.24 nm mobile lubricant thickness and compared with the numerical simulation. It was found that the analytical replenishment process can fairly agree with the experimental one if the ratio of Hamaker constant to effective lubricant viscosity is properly determined. By using the validated basic equation, a simple but useful generalized formula is proposed to evaluate the replenishment speed in relation to the depleted scar width of the mobile lubricant, the lubricant thickness, and the ratio of Hamaker constant to effective lubricant viscosity.     

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

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

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.     

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.     

Chemical analysis of wear tracks on magnetic disks by TOF-SIMS

Surface reactions on magnetic recording disks have been studied during sliding with ceramic sliders in the main chamber of TOF-SIMS. Chemical change of lubricant oil in the wear track was observed by the chemical image of TOF-SIMS. The magnetic disk surface was covered with perfluoroalkyl polyether lubricant (Fomblin Zdol). The Si tip slider surface was covered with Al₂O₃, DLC, TiN or c-BN coating. Experimental conditions were as follows: 0.8 mN of load and a sliding speed of 0.01 m/s. Lubricant oils were decomposed with Al₂O₃ and TiN slider surfaces. Metal (Al, Ti) fluorides were detected by TOF-SIMS in the sliding track. Material transfer occurred by chemical wear of slider material. From TOF-SIMS observation, the decomposition of lubricant molecules was initiated at the end group of molecules (-CF₂CH₂OH). On the other hand, DLC and c-BN sliders suppressed the decomposition reaction of PFPE oils. In conclusion, hard and chemical inert materials such as DLC and c-BN are suitable for a long-life HDI.