Nano-structured lubricant film in head-disk interface: Surface energy, spreading, and multi-scale modeling

Hard disk drives (HDDs) are the dominant information storage devices. A robust head-disk interface (HDI) is critical to maintain the reliability and prolong the lifetime of HDDs. With the continuous increase in areal density, the head media spacing (HMS), which contains head overcoat, media overcoat, lubricant film, and fly height, is 6.5 nm for 1 Tbits/in2 HDD. With such a tough budget for HMS, it approaches a set of "physical limits" and is extremely challenging, which includes developing and processing materials for robust nanometer overcoat, molecular design for nanometer lubricant film, the state-of-the-art slider design, and mechanical integration of entire HDI.; This work focuses on the study of physiochemical features of molecularlythin perfluoropolyether (PFPE) lubricant films as well as the development of a multi-scale simulation tool capable of advanced HDI design. The surface energy of PFPE films on the carbon-overcoated disks was systematically investigated using the contact angle measurement via two-liquid geometric method, where the surface energy was decomposed into dispersive and polar components. The conformation, surface coverage, and physical state were investigated. Nonfunctional PFPE films showed disordered packing and liquid-like behavior, while function PFPE films showed ordered packing and the solid-like behavior. Humidity effects on the relaxation of lubricant films were investigated by examining the bonded fraction and surface energy of freshly dip-coated PFPE monolayer as a function of elapsed time. It was found that humidity expedited the relaxation process of functional PFPE films. The effects of buff/wipe, a process to remove particles and asperities on the lubricated media surface in order to achieve the desired flyability, on the physiochemical features of PFPE-lubricated carbon overcoat were investigated.; Lifshitz-van den Waals and Lewis acid/base (LW/AB) theory was introduced to analyze the surface energy of PFPE-lubricated media for the first time, where the surface energy was decomposed into LW, Lewis acid, and Lewis base components. Additional information on the nature of lubricant/overcoat interaction was obtained from this novel approach. It was proposed that the bonding of functional PFPEs and carbon overcoat was a Lewis acid-base interaction, where PFPEs were Lewis acids (electron acceptors) and carbon overcoat was the Lewis base (electron donor). This bonding mechanism can be the guideline to design the lubricant structure and overcoat composites, i.e., choosing suitable Lewis acid/base pair. LW/AB approach was further applied to investigate ultra-violet (UV) irradiation effects on the nonfunctional and functional PFPE films.; The spreading of PFPE monolayer on a spinning disk, which is practically important, was investigated using optical surface analyzer (OSA). A special coating technique was adopted to prepare a lubricated zone that is suitable for the study of spreading in a spinning disk. The spreading profiles along inner disk (ID) were shifted to outer disk (OD) direction in a spinning disk, which was attributed to the centrifugal force and air shear force.; Molecular dynamics (MD) simulation with a coarse-grained, bead-spring model was developed to study the nanostructure, conformation, and dynamics of disk lubricant films at a molecular level. Via visualization, bead density profiles, radius of gyration, and self-diffusion coefficients, the static and dynamic properties of PFPE films with different molecular structures were examined. The relaxation process of PFPE films in humid environment was investigated by examining the normalized autocorrelation function of normal modes for entire PFPE chain. The binary mixture films of two lubricants with quite different properties were investigated. MD simulation can be a powerful and efficient tool to study lubricant films and design new lubricants for future HDDs.; A multi-scale simulation is desirable for advanced HDI integration.