Data Storage Systems Center

It has become a challenge to find an enhanced lubricant that meets the harsh requirements for ultra-small head-media spacing in hard disk drives (HDDs) [1]. Hence, a mixture of lubricants has been introduced as a feasible and promising alternative for future HDDs. In this project, molecular dynamics (MD) simulations with a bead-spring model [2] are employed to examine the nanoscopic structure, conformation, and dynamics of blended lubricant films via analyzing the anisotropic radius of gyration and the self-diffusion coefficient. From this, we found that binary mixture monolayer can be more suitable as a disk lubricant in comparison to the single component [3]. The spreading profiles of PFPE films over carbon overcoats with spinning effects were also examined via the optical surface analyzer. In order to analyze the spreading mechanism, we modified the Cattaneo equation with the convection term and introduced an adjustable coefficient for the convection term while constructing the modified Cattaneo equation.

[1] J. Gui, IEEE Trans. Magn., 39, p.716 (2003). [2] Q. Guo, S. Izumisawa, D.M. Phillips, and M.S. Jhon, J. Appl. Phys., 93, p.8707 (2003). [3] P.S. Chung, H. Chen, and M.S. Jhon, J. Appl. Phys., 103, 561891 (2008).

It has become difficult to find the optimum lubricant that meets all the requirements for ultra-small head-media spacing in hard disk drives (HDDs). Hence, a mixture of lubricants may be feasible and promising alternative for future HDDs. In this project, molecular dynamics (MD) simulations with a bead-spring model are employed to examine the nanoscopic structure, conformation, and dynamics of single component as well as binary mixture lubricant films via analyzing the anisotropic radius of gyration and the self-diffusion coefficient. We also examined ‘compression and tension processes’ of single component lubricant films. Our simulation results indicated that compression and tension processes show hysteresis in normal stress profiles. Functional PFPEs exhibit extra relaxation mode due to strong interaction among functional endgroups. The static conformation and mobility change by systematically tuning the volume fraction may provide the criteria for the optimal lubricant selection. From this, we found that binary mixture monolayer can be more suitable as a disk lubricant in comparison to the single component.

The spreading phenomena of perfluoropolyether (PFPE) monolayer film on a spinning disk, which is more critically important than partially dip-coated films in stationary disk [1-3] from the application viewpoint, was studied. Optical surface analyzer (OSA) was employed to examine the spreading dynamics of nanofilm for both stationary and spinning disks. Spreading profiles along inner diameter (ID) and outer diameter (OD) on a stationary disk and a spinning disk (10,000 rpm) were monitored with time. The entire film profiles in the spinning disk were shifted to OD direction in comparison to the stationary disk case. To quantify the mobility (or diffusion) and centrifugal force effects of PFPE nanofilms, L-t plot was constructed via the leading edge of the advancing lubricant front [4]. For stationary case, the diffusion is dominant mechanism and the slope of L-t plot is consistent with previous results [4]. The L-t plot for the spreading along ID in the spinning case, however, can be categorized into two regions. The first region is diffusion dominant, where the slope is 0.53. The second region is convection dominant with the slope close to 1, which is attributed to the centrifugal force. For the spreading along OD in the spinning case, the slope is 0.78 due to the coupling between the combined mechanism of diffusion and convection. L for the spinning case are described by linear combination between diffusion and convection, i.e., L∝(αt1/ 2 +βt). The spinning significantly affects the dynamic behavior of lubricant film as expected. In order to describe our peculiar spreading profile, we developed the simplest multiscale model. We added convection contribution stemmed from spinning to the Cattaneo equation. This modified Cattaneo equation provide an excellent agreement with our experimental data on spreading characteristics of PFPE nanofilms and can be used in the study of lubricant retention as well as molecular design criteria. [1] X. Ma, J. Gui, L. Smoliar, K. Grannen, B. Marchon, M. S. Jhon, and C. L. Bauer, J. Chem. Phys., vol. 110, 3129 (1999). [2] X. Ma, J. Gui, L. Smoliar, K. Grannen, B. Marchon, C. L. Bauer, and M. S. Jhon, Phys. Rev. E, vol. 59, 722 (1999). [3] Q. Guo, L. Li, Y. T. Hsia, and M. S. Jhon, IEEE Tran. Magn., vol. 42, 2528 (2006). [4] T. M. O’Connor, Y. R. Back, M. S. Jhon, B. G. Min, and T. E. Karis, J. Appl. Phys., vol. 79, 5788 (1996).

It has become difficult to find the optimum lubricant that meets all the requirements for ultra-small head-media spacing in hard disk drives (HDDs) [1]. Hence, a mixture of lubricants may be a feasible and promising alternative for future HDDs. In this project, molecular dynamics (MD) simulations with a bead-spring model [2] are employed to examine the nanoscopic structure, conformation, and dynamics of single component as well as binary mixture lubricant films via analyzing the anisotropic radius of gyration and the self-diffusion coefficient. We also examined ‘compression and tension processes’ of single component lubricant films. Our simulation results indicate that compression and tension processes show hysteresis in normal stress profiles. Functional PFPEs exhibit extra relaxation mode due to strong interaction among functional endgroups. The static conformation and mobility change by systematically tuning the volume fraction may provide the criteria for the optimal lubricant selection. From this, we found that binary mixture monolayer can be more suitable as a disk lubricant in comparison to the single component [3].

[1] J. Gui, IEEE Trans. Magn., 39, p.716 (2003). [2] Q. Guo, S. Izumisawa, D.M. Phillips, and M.S. Jhon, J. Appl. Phys., 93, p.8707 (2003). [3] P.S. Chung, H. Chen, and M.S. Jhon, J. Appl. Phys., 103, 561891 (2008).

Lattice Boltzmann method (LBM) is a promising tool for multi-phenomena and multi-physics modeling. Rarefied gas dynamics, lubricant flow, and nanoscale heat transfer in head/disk interface (HDI) can be simulated through a single framework of LBM. Multi-physics phenomena within HDI are elucidated to date by separated methodologies for each physical phenomena. Integrated simulation methodology is desirable for such a complex multi-scale application. A novel LBM tool will be first constructed. Later, air/liquid bearing models will be examined and finally nanoscale heat transport model will be established. In our previous studies, a three-dimensional simulation tool based on LBM, which is more efficient than the conventional computational fluid dynamics approach, was developed to analyze the HDI dynamics [1, 2]. Our LBM tool can handle general geometrical structures of air bearing surface (ABS), and is superior for prediction of transient behavior as well as it is easy to parallelize. A wall slip model for high Knudsen number (Kn) flows on ABS, and truncated power-law and Bird-Carreau models for the non-Newtonian fluid flow behavior for lubricant liquid film [2] were incorporated. Heat transport through a nanoscale thin metallic film was also investigated [3] via three separate modules constructed. An integrated simulation tool can be developed in a unified framework. The LBM could be an attractive computational tool for multi-scale, multi-phenomena modeling, ideal for next generation HDI design. [1] H. M. Kim, D. Kim et al., IEEE Trans. Magn., vol. 43, 2244-2246, 2007. [2] W. T. Kim, H. C. Chen, and M. S. Jhon, J. Appl. Phys., vol. 99, 08N106, 2006. [3] S. S. Ghai, W. T. Kim, R. A. Escobar, C. H. Amon, and M. S. Jhon, J. Appl. Phys., vol. 97, 10N703, 2005.

It becomes more and more difficult to find the enhanced lubricant that meets all the requirements for ultra-small head-media spacing hard disk drives (HDDs) [1]. The mixture of lubricants may become feasible and promising alternative for future HDD. In this project, molecular dynamics (MD) simulations with a bead-spring model [2] are employed to examine the detailed structure, conformation, and dynamics of single component and binary mixture lubricant films by analyzing the anisotropic radius of gyration and the self-diffusion coefficient. We also examined ‘compression and tension process’ of single component lubricant films. Our simulation results indicate that compression and tension process shows history dependent normal stress profile. Functional PFPE exhibits additional mode of relaxation due to the strong interaction between functional endgroups. We also found that the binary mixture monolayer can be more suitable as a disk lubricant in comparison with the single component [3]. The conformation and mobility change by tuning the volume fraction may provide the criteria for the optimal lubricant selection.

[1] J. Gui, IEEE Trans. Magn., 39, p.716 (2003). [2] Q. Guo, S. Izumisawa, D.M. Phillips, and M.S. Jhon, J. Appl. Phys., 93, p.8707 (2003). [3] P.S. Chung, H. Chen, and M.S. Jhon, J. Appl. Phys., May 2006 (scheduled).