(191w) Spreading On Nano Blend Perfluoropolyether Films Over Carbon-Overcoated Disks Via Molecular Dynamics

Chung, P. S., Carnegie Mellon University
Smith, R. L., Carnegie Mellon University
Jhon, M. S., Carnegie Mellon University

Due to continuous increase in areal density, the head-media spacing (HMS), which contains head overcoat, media overcoat, lubricant film, and fly height, is expected to be less than 6.5 nm for 1 Tbits/in2 hard disk drives (HDDs) [1]. With such a strict budget for HMS, the static and dynamic properties of the lubricant mixtures have been studied as one of the solutions to meet the stringent requirements of HMS [2]. The intermittent contacts between head and disk during operation are unavoidable and cause lubricant depletion in the film. Therefore, the nanoscale conformation and self-healing ability (i.e., spreading) of perfluoropolyether (PFPE) lubricant films is critically important to maintain long-term reliability of HDDs. In this paper, the spreading phenomena for single component and binary mixture PFPE lubricant films on the carbon-overcoated media were examined by molecular dynamics (MD) simulation [3]. For the single component of nonpolar PFPE, we observed the smooth spreading of viscous fluids, while the polar PFPE exhibits layering structure with the characteristic shoulder, which is correspondent to the experimental results [4]. We previously examined the spreading phenomena of binary blend of nonpolar/polar PFPE via optical surface analyzer (OSA), which is an apparatus, combined ellipsometer, reflectometer, scatterometer, and Kerr effect microscopy. As shown in Fig. 1, we observed the layering structure, which Z03 and Ztetraol formed. Here, we provide the molecular model of binary nano blend, which explains the peculiar spreading characteristics in nano blend observed in the experiments. To physically explain the nano conformation and spreading mechanism of binary blend, we investigated the spreading of nonpolar/polar blend thin film via MD simulation. We found the spreading edge consists of nonpolar PFPE diffused from the top layer. The complex entanglement of nonpolar/polar PFPE molecules exists on the top layer of binary nano blend film, while the bottom layer is composed of bonded polar PFPE molecules. Since the spreading edge of the binary mixture evolves rapidly enough, spatially dependent diffusion coefficients were calculated to examine the endgroup functionality and film conformation effect on the binary film spreading. It was found that the diffusivity of binary blend has the same order as the nonpolar PFPE, yet the spreading was retarded due to the entanglement in the binary nano film. This analysis is expected to provide the lubricant selection criteria with the knowledge of the nano film conformation and the self-healing mechanism of PFPE lubricant film on the hard disk, while enhancing the durability and satisfying the precise budget of HMS.

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