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Continuum and molecular dynamics analyses of lubricant evaporation and flow due to laser heating in heat-assisted magnetic recording

2018, Master of Science, University of Akron, Mechanical Engineering.
Due to the need for a drastic increase of the data storage density of hard disk drives, the heat-assisted magnetic recording (HAMR) technology was developed to write data to the disk at a significantly elevated temperature such that the coercivity of the magnetic medium could be lowered for easy data-writing while maintaining the magnetization patterns for years without suffering thermal instability. Previous studies were based on a Gaussian temperature distribution with axisymmetry for simplicity. However, because of the fast disk rotation, the temperature distribution on the disk surface under laser illumination tends to deviate significantly from the axisymmetric distribution that is typical for a slow-moving solid body. The temperature distribution can exhibit a tail in a trailing region, as confirmed by a previous finite element analysis. In this study, an approximation scheme for the temperature distribution over and near the thermal spot of the laser beam containing a tail was proposed based on the weighted sum of stationary and fast-moving heat source solutions. This approximation not only significantly reduced the computation time for solving the current problem but also represents a novel and efficient method in tribology for approximating the temperature distribution due to heating for the whole range of speeds and thermal-spot sizes. By using this approximated temperature distribution, a governing partial differential equation for the nanoscale lubricant film was solved numerically by considering the evaporation rate, surface tension, disjoining pressure and thin-film enhanced effective viscosity. The results for the perfluoropolyether lubricant reveal the process of formation of a lubricant trough: an indent of the lubricant profile first forms and grows to a steady-state depth, followed by a continuous extension at the rate of the disk velocity. Based on the comparisons of solutions obtained with and without evaporation, both evaporation and thermal capillarity due to the surface tension gradient contribute greatly to the creation of a trough in the lubricant profile, and thermal capillarity, together with other factors, causes boundary ridges. Along a down-track path through the center of the thermal spot, only the leading edge of the thermal spot possesses a lubricant boundary ridge, whereas there is no ridge at any location downstream. In the latter part of this thesis, as a strong compensation to continuum-based analyses of the lubricant behavior, the study addresses the molecular-scale behavior of the lubricant under the influence of laser heating by developing coarse-grained bead-spring models of molecular dynamics simulation, in which each lubricant molecule has been modeled as a chain of beads. The bonds between adjacent beads were modeled with the finitely extensible nonlinear elastic (FENE) potential, the Lennard-Jones potential was used for non-bonding interactions, and the exponential potentials were used for the end beads. New schemes were proposed to effectively treat the nanoscale windage effect of air flow. Both a stationary disk surface and a disk surface moving laterally together with the lubricant film were simulated with a heat flux applied to a central region, with considerations of local temperature and pressure variations. The results indicate significant influence of the disk velocity, heat flux and gap distance on the evaporation and transfer of the lubricant. Increasing the disk velocity makes the regions of lubricant evaporation move toward the trailing side, with less local lubricant transfer partially due to the windage effect. An increase of the heat flux causes more lubricant molecules to leave the disk surface and transfer to the slider surface, especially at the trailing side. The lubricant behavior observed can bring insight into the transfer and redistribution of the lubricant under laser heating.
Shao Wang, Dr. (Advisor)
109 p.

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Haq, M. (2018). Continuum and molecular dynamics analyses of lubricant evaporation and flow due to laser heating in heat-assisted magnetic recording. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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Haq, Mohammad Ashraful. "Continuum and molecular dynamics analyses of lubricant evaporation and flow due to laser heating in heat-assisted magnetic recording." Electronic Thesis or Dissertation. University of Akron, 2018. OhioLINK Electronic Theses and Dissertations Center. 18 Nov 2018.

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Haq, Mohammad Ashraful "Continuum and molecular dynamics analyses of lubricant evaporation and flow due to laser heating in heat-assisted magnetic recording." Electronic Thesis or Dissertation. University of Akron, 2018. https://etd.ohiolink.edu/

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