The interactions that occur between the hydroxyl-terminated perfluoropolyethers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab initio calculations. The results of these measurements are compared with those of previous studies on the Zdol/amorphous-hydrogenated carbon (CHx) system and the major differences identified. The thickness dependence of the dispersive surface energy for the Zdol/CNx system can be fit using a repulsive van der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at covering CNx as compared to CHx due to less favorable interactions between the Zdol backbone and the CNx surface. The Zdol thickness dependence of the polar surface energy for the Zdol/CNx system indicates that very few strong polar interactions are present between the initially applied Zdol and the CNx surface. A substantial decrease in the polar surface energy of the first Zdol monolayer however occurs on a time scale of 1-5 weeks after lubricant application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between the polar hydroxyl end groups of Zdol and the polar entities on the CNx surface. A kinetic analysis of the Zdol + CNx system reveals that the rate at which the adhesive interactions are formed is limited by diffusion of the polar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end groups of Zdol and imine (basic) sites on the CNx surface may be responsible for the Zdol adhesion. These calculations further suggest that the appearance of the diffusion step in the bonding kinetics and the less efficient coverage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol backbone.

The formation of a tribologically reliable interface between the magnetic recording disk and the magnetic recording head in hard-disk drives is predicated upon the presence of a molecularly thin perfluoropolyether (PFPE) lubricant film. The molecular interactions that develop between the PFPE lubricant and the underlying amorphous carbon overcoat of the magnetic recording disk govern the adhesion, physical coverage, thermal stability, and mobility of the lubricant on the carbon surface and hence are of paramount importance in defining the tribological performance of the drive. In this work, information pertaining to the interfacial interactions between the hydroxyl-terminated PFPE lubricant Zdol and amorphous carbon surfaces is obtained via ab initio calculations. The small fluorinated ether molecules CF3OCF2CH2OH (ZD) and CF3OCF3 (PFDME) were used as computationally tractable models for the PFPE lubricants. A population analysis is performed on the ZD model lubricant and the various chemical functionalities known to exist on amorphous carbon surfaces. In the case of an amorphous, hydrogenated carbon surface, CHx, the adhesive interactions of the PFPE backbone with the nonpolar component of the carbon surface were modeled by the interaction of ZD with simple hydrocarbons. The attractive van der Waals interactions that result are comparatively weak and insufficient at room temperature to overcome the associated decrease in entropy. As a consequence, these interactions will not contribute significantly to the adhesion of PFPE's to the carbon surfaces under disk-drive operating conditions. The primary source of adhesion in the Zdol−CHx system stems from hydrogen bonding of the hydroxyl end groups of the Zdol lubricant with the carboxylic acid and ketone functionalities on the CHx surface. The computed binding energies of the ZD + ketone and ZD + carboxylic acid interactions are −11 and −15 kcal/mol, respectively. These interaction strengths are large enough to compensate for the entropy decrease and hence result in a net decrease in the free energy. In addition to these thermodynamically stable adhesive interactions, the computed binding energy of the cohesive hydrogen-bonding interactions between ZD molecules is significant. The formation of a highly associated, two-dimensional structure is therefore possible for molecularly thin Zdol films on carbon surfaces. Amorphous nitrogenated carbon, CNx, can also provide strong physisorption sites for Zdol. The interaction of ZD with imine and nitrile functionalities were studied. The interactions of the hydroxyl end group with imine centers is strongly attractive, leading to the formation of a hydrogen bond with a strength of −16 kcal/mol. Interactions with nitrile sites are somewhat less favorable with a computed binding energy of −10 kcal/mol. The nitrogen centers on CNx are negatively charged, and hence repulsive interactions with the negatively charged perfluoroalkyl ether backbone are expected. The modeling results are then used to interpret previous experimental resu...

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Perfl~toropolyethers (PFPEs) are widely used as lubricants on rnagrtetic recording media. The mobility of the PFPE on the protective carbon overcoat of the media is widely accepted to be intimately coupled to the resulting tribological performance. The flow properties of molecularly thin films of nonpolar PFPE Z and polar PFPE Zdol fractions on solid surfaces were investigated by measuring the spreading profiles. The spreading of Zdol exhibits terraced profiles with the formation of a molecular foot, a shoulder and a vertical step. To describe these features of Zdol spreading, we measured the Zdol thickness dependence of the surface energy, which is then used to calculate the thickness dependence of the disjoining pressure. The polar component of the Zdol surface energy exhibits oscillations as a futlction of PFPE thickness. The resulting oscillations in the disjoining pressure can be used to qualitatively describe the origins of terraced spreading. The characteristic Zdol spreading profile and surface energy oscillations of Zdol are attributed to molecular layering induced by polar interactior~s between the Zdol end-groups and the surface.

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Methods to chemically passivate the surfaces of amorphous-carbon films (a-C) produced by dc magnetron sputtering were studied. The chemical composition of carbon surfaces produced via sputtering are dependent upon the environment to which the carbon is exposed immediately following deposition. When the sputtered film is vented to ambient conditions, free radicals produced at the surface during the deposition process are quenched by reaction with oxygen and/or water to form an oxidized, hydrophilic surface. If the sputtered carbon film is, however, exposed to a reactive gas prior to venting to ambient, the chemical nature of the resulting surface can be modified substantially. Specifically, a less highly oxidized and much more hydrophobic carbon surface is produced when the surface free radicals are quenched via either an addition reaction (demonstrated with a fluorinated olefin) or a hydrogen abstraction reaction (demonstrated with two alkyl amines). Chemical modification of amorphous-carbon films can also be accomplished by performing the sputtering in a reactive plasma formed from mixtures of argon with molecular hydrogen, amines, and perfluorocarbons. The elemental composition of these films, and the relative reactivity of the surfaces formed, were investigated via x-ray photoelectron spectroscopy and contact-angle goniometry, respectively. In the case of sputtering with a mixture of argon and hydrogen, increasing the hydrogen flow results in an increase in the amount of hydrogen incorporated into the carbon film and a decrease in the surface free energy. Sputtering in diethylamine produces an amorphous-carbon film into which nitrogen is incorporated. The free energies of the a-C:N surfaces produced in this process are similar to those of the a-C:H films. Sputtering in a fluorocarbon vapor results in the incorporation of fluorine into the film structure and the formation of very low free-energy surfaces. Increasing the concentration of the fluorocarbon in the sputtering plasma increases the amount of F incorporated into the film. At the highest fluorocarbon flow rates employed, a-C films were produced with stoichiometries and surface free energies comparable to those of bulk Teflon.