Computer-Modeling Study of the Interactions of Zdol with Amorphous Carbon Surfaces

Abstract

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...

The Adhesion of Monomolecular Hydroxyl-terminated Perfluoropolyether Liquid Films on the Sputtered Silicon Nitride Surface as a Function of End Group Acidity and Mobility

“…The solid lines fit to the data shown in figure 2 were obtained using equation (5) and d o and A* as specified in the figures. The results indicate that the solid-liquid van der Waals interaction is repulsive for the PFPE/SiN x interface and is in qualitative agreement with the Zdol/CN x system discussed previously [25]. The d o value used to fit the dispersive surface energy curve via equation 5, d o =3.0 Å , is indicative that the interaction between the PFPE non-polar backbone (main chain) and the underlying SiN x surface is relatively weak and hence cannot provide any substantial adhesion.…”

A Theoretical Study of Vapor Lubrication for Heat Assisted Magnetic Recording

“…The desorption energy of lubricant Zdol from the surface of single crystalline graphite is from reference [13]. The disjoining pressure of Zdol on DLC is from reference [6,15,16]. The surface concentrations of lubricant film and liquid lubricant are assumed identical.…”

Dominant Factors in Lubricant Transfer and Accumulation in Slider-Disk Interface

“…with A H being the effective Hamaker constant for the lubricant/overcoat system, and d 0 & 2.8 Å the distance of closest approach between the lubricant molecule and the carbon surface [16]. For a functional lubricant such as Zdol, a polar component P p of the disjoining pressure also needs to be taken into account [17].…”

“…where P 0 is estimated at *4 MPa [16] and t 0 is the dewetting thickness of lubricant film on disk surface. For Zdol, the dewetting thickness t 0 was shown to vary fairly linearly with molecular weight as follows: t 0 (Å ) & 7.7M n (kDaltons) [17].…”