Thermodynamics and spreading behavior of thin perfluoropolyether films investigated with atomic force microscopy

Computer hard drives utilize read/write heads mounted on long arms that are suspended only a few nanometers above the data storage disk. This configuration results in occasional contact between the two surfaces. These collisions are mediated by protective layers: carbon overcoats and a perfluoropolyether (PFPE) lubricant nano-film spread on the hard disk. The mechanism of lubrication at these length scales is not fully understood by the hard drive industry and to facilitate the design of better lubricants a substantial amount of research is being performed on this vital portion of the head-disk interface.;The technique for measuring disjoining pressure uses atomic force microscopy (AFM). During this procedure a meniscus of liquid is stretched between an AFM probe and a film. If the liquid bridge is stretched slowly enough, equilibrium between the film and meniscus is approximated. At equilibrium, the disjoining pressure of the film is balanced by the Laplace pressure of the meniscus. The Laplace pressure is described by the shape of the meniscus but for nanometer films it is difficult to actually view the tip-film contact. To remedy this, a meniscus reconstruction model is developed for spherical AFM probes that describes the force expected for constant Laplace pressure menisci as they are stretched. Fitting AFM force curve data with this model allows identification of Laplace pressure and through the equilibrium condition the disjoining pressure.;The quasi-equilibrium force curve technique was first tested using a PFPE called Fomblin Z03 spread on silicon wafers. This lubricant is believed to interact with substrates primarily through dispersive forces. This allows comparison of the experimental AFM data to the predictions of a retarded Lifshitz theory calculation. Lifshitz theory uses the dielectric properties of materials to predict the dispersive forces in a system. The Lifshitz theory prediction closely correlated with the AFM data supporting the validity of this measurement technique. The AFM technique was then applied to Fomblin Z03 spread on a commercially relevant hard drive disk. These experiments could differentiate between the two surfaces demonstrating the sensitivity of the AFM method to small changes in disjoining pressure.;Next, disjoining pressure isotherms for two other hard drive lubricants were measured: Demnum-SA and Phosfarol A20H-3000. These lubricants differ from Fomblin Z03 due to the addition of end-groups that increase adhesion to hard drive overcoats. The AFM disjoining pressure data exhibited complex behavior for these lubricants attributed to steric and polar contributions to disjoining pressure. Disjoining pressures for films 1 nm or thicker were greater than comparable thicknesses of Fomblin Z03. Plots of disjoining pressure versus film thickness reveal changes of inflection in the disjoining pressure curve that are shown to correspond to film thicknesses that are unstable and dewet. The disjoining pressures of the three lubricants were used to approximate the spreading of an initial step interface over time. The disjoining pressure predictions demonstrated qualitative agreement with the spreading behavior seen.;A method of measuring disjoining pressure is proposed that could be applied to hard drive lubricant films. Disjoining pressure is a measure of the change in free energy of interaction between two half-spaces as the thickness of intervening layers is increased. Disjoining pressure plays a role or dictates many of the properties of films that are critical for lubrication including: wetting, adhesion, flow dynamics and volatility. Therefore, disjoining pressure provides a short cut to many properties of interest packaged in the form of an intrinsic property of the lubricant film.;The AFM technique of measuring disjoining pressure could not be applied to extremely viscous lubricants, without further refinement, because equilibrium was not closely approximated. However, experiments with one such lubricant, Fomblin Zdol, revealed the presence of an unreported force curve feature. Films of Zdol pushed the AFM tip away as it approached the underlying substrate following tip-film contact. It is believed this behavior arises from the unusual spreading properties of Zdol. Certain film thicknesses of this lubricant are immobile and as a meniscus of Zdol is compressed these immobile regions could result in meniscus bulging and the formation of a repulsive curvature. The feature is transient and time dependence is assumed to correspond to eventual drainage of the bulging meniscus. This expansion feature was identified as a potentially useful analytic technique for studying these films. Its shape tracked the strength of the end-group adhesion interaction and the flow dynamics of the film as it escaped the impinging AFM tip.