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Lubricant reflow for 10 Å Z-Tetraol film is characterized for heat-assisted magnetic recording by numerical simulation using the experimentally determined thickness-dependent diffusion equation. The competition between laser-induced lubricant depletion (troughs) and subsequent healing by reflow is investigated as a function of lubricant depletion rate, trough width, trough depth, and duty cycle (reflow time between successive laser exposures). The simulations include 1, 5, 9 and 13 adjacent troughs. Trough width of 50 nm with an edge-to-edge separation distance of 50 nm and laser depletion rates of 1.0, 0.1 and 0.001 Å/cycle are simulated. Reflow kinetics on 13 troughs with a trough width of 20 nm and an edge-to-edge separation distance of 20 nm is also included to probe the effects in the limit of very small laser spots. Duty cycles of 10 and 100 millisec are used in the simulations. The results of the numerical simulations indicate that trough recovery by reflow increases with increasing duty cycle or decreasing number of adjacent troughs, trough separation distance and laser depletion rate. Conclusions based upon single trough studies are misleading for HAMR interfaces.

Lubricant reflow for Z-Tetraol is characterized for heat-assisted magnetic recording (HAMR). The thickness-dependent diffusion constant, D(h), is determined from flow experiments for submonolayer Z-Tetraol films between 2.5 and 13 Å. Fickian diffusion using experimental D(h) values are used to numerically simulate lubricant reflow into HAMR laser-depleted troughs as a function of trough number, trough depth, trough distance and trough width. Reflow is computed to be slower for multiple, wider and deeper troughs. Reflow kinetics as a function of trough separation distance is complicated by adjacent trough interactions. Two major kinetic regimes are identified. Multiple troughs that have not coalesced recover more quickly. Once multiple troughs coalesce into a single larger trough, reflow kinetics is drastically slowed. The crossover time between the two kinetic regimes is computed to be approximately 400 millisec for the system under investigation here, but will be a strong function of trough number, width, depth and separation distances. The simulation of multiple troughs provides significant insight into HAMR conditions. Single trough lubricant studies may be misleading for HAMR systems.

Z-dol and Z-tetraol represent most often used disk lubricants. So that we would be better able to design lubricants of higher thermal stability (for Heat-Assisted-Magnetic-Recording application), detailed analysis of thermally induced oxidation processes of Z-dol and Z-tetraol were performed. Samples were heated in a thin-film configuration where facile infusion of oxygen occurred and were examined by F-19 and C-13 NMR. Z-dol was found to be stable in the 150 ∼ 200 °C temperature range. In the same temperature range Z-tetraol was found to undergo either (1) the well known step-wise oxidation of primary alcohol (alcohol → aldehyde → acid) or (2) direct conversion to Z-dol. The latter process is initiated by a base-catalyzed step whereby the end-group Z-O-CF2-CH2-O-CH2-CH(-OH)-CH2-OH converts to an aldehyde, Z-O-CF2-CH2-O-CH2-CH( = O), and CH3OH. Z-dol results upon oxidation of the aldehyde yielding Z-O-CF2-CH2-OH, CH2 = O and CO2. It was found that substitution of the terminal OH with a phenoxy unit suppressed these decomposition processes. Z-tetraol lubricant molecular chains that became bound to the carbon overcoat at their termini are also protected from these oxidation processes. Thus for HAMR application, a combination of lubricants bonded to the carbon overcoat at their termini and mobile lubricants possessing phenoxy groups at their termini may be the best possible system based on perfluoropolyether lubricants.

High temperature (hot) and high humidity (wet) effects on perfluoropolyether lubricants, Z-dol and Z-tetraol, were investigated using the high speed spin-tester. The study has shown that: (1) higher humidity accelerates the lubricant spin-off on the disk surface, (2) in the case of Z-tetraol, oxidative reaction occurs under the hot-dry condition, but not under the hot-wet condition, (3) aforementioned phenomenon occurs only when the mobile fraction is present, and (4) the oxidation occurs the more strongly, the closer the location is to the spindle shaft. An NMR analysis of bulk materials heated in 150 oC revealed that the oxidative chemical changes are confined within the end-groups of Z-tetraol. No change was observed from bulk Z-dol heated similarly.

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 HO, (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-electrolyte 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-electrolyte droplets. These droplets are viscous and are readily picked up by the slider. [ABSTRACT FROM AUTHOR]

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-CF-CH-O, to the carbon surface as a pendant ether unit. Dangling bonds at or near the surface react immediately with HO or O 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, -(CH)-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-CF-CH-O-(CH)-OH. Excellent bonding rate, coverage, and potential anticorrosion property were revealed for these lubricants. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The bonding kinetics for 10 Å Z-Tetraol films on the amorphous nitrogenated carbon surface (CNx) is investigated as a function of the Z-Tetraol molecular polarity. Z-Tetraol nominally contains 2 hydroxyl groups per chain end. The molecular polarity of Z-Tetraol is varied by the addition of a “bis-adduct” component of Z-Tetraol. This material contains 3 instead of 2 hydroxyl groups per chain end. The bonding kinetics and changes in the surface energy as a function of time in thin film mixtures is investigated under ambient conditions. The initial bonded fraction scales with the bis-adduct concentration and can be increased by as much as 50%. The asymptotic bonded fraction exhibits a more modest increase of approximately 10%. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

In this study, TOF-SIMS was used to characterize lubricants used in magnetic recording media. Films of lubricants, perfluoropolyether (PFPE) Fomblin Z-DOL, Z-TETRAOL, AM3001, and cyclo triphosphazene X-1P were deposited on carbon overcoats of the magnetic disks from solutions. TOF-SIMS results showed that each thin film lubricant produces characteristic ions that are closely related to its chemical structure. Ratios of characteristic ions from the Z-DOL and carbon overcoat can be used to monitor the thickness and molecular weights of the lubricant. Molecular or molecule related ions have been observed for Z-DOL and X-1P. It was also found that the average mass of molecular ions correlates very well with the number average molecular weight determined by nuclear magnetic resonance (NMR) for Z-DOL. [Copyright &y& Elsevier]

The dewetting of perfluoropolyether (PFPE) films on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image perfluoropolyether films on CNx-overcoated magnetic recording disks. An autophobic dewetting transition is observed to result when the PFPE film thickness applied to the disk surface exceeds a critical value. This critical dewetting thickness is linearly dependent on the PFPE molecular weight. Addition of the phosphazine, X-1P, to the PFPE film reduces the critical dewetting thickness compared to that of the neat lubricant. Dewetting in these molecularly-thin PFPE lubricant films is shown to occur at thicknesses where the total disjoining pressure is negative. The impact of this autophobic dewetting on the performance of a head--disk interface is inferred from take-off height measurements conducted as a function of PFPE film thickness. A steep reduction in the slider--disk clearance is observed when the PFPE film is present at thicknesses in excess of the critical dewetting thickness. [ABSTRACT FROM AUTHOR]