The ultimate goal of this paper is to find the best lubricant meeting all the requirements for ultra-small head-media spacing (less than 6.5 nm) in hard disk drive (HDD) systems. Newly synthesized lubricants (i.e., ztetraol multidentate (ZTMD)) [1] as well as an optimal blend ratio of the lubricants [2] can be a feasible and promising alternative for future HDD. Here, we examine the static and dynamic properties of nano-scale lubricant thin films of single component and binary mixture perfluoropolyether (PFPE) via molecular dynamics (MD) simulations.

The thin films were modeled via a coarse-grained, bead-spring model, which neglects the detailed atomic structural information while keeping the essence of the polymer molecular structures [3]. Lennard-Jones and finitely extensible nonlinear elastic models were used to describe potential energies among the beads in the system. Solid surface, which makes the films confined, is assumed perfectly smooth. With the total potential energy constructed, MD simulations were performed using the Langevin equation.

We examined the detailed conformation and dynamics of binary mixture films by analyzing the anisotropic radius of gyration and the self-diffusion coefficient as a function of chemical structures including functional groups, film thicknesses, and the blend ratio of the mixture. Our preliminary analysis shows that the film thicknesses and the diffusion processes are mainly dependent on the chemical structures of PFPE molecules and also found that a binary mixture monolayer provides more desirable properties in uniformity and mobility as compared to a single component case. By analyzing functional bead density, we observed that multilayer PFPE films with functional groups (e.g., ZTMD, Zdol, A20H) have layering structures. Furthermore, most monolayer single component lubricants have flat conformation. Due to strong interaction between functional groups and the surface, ZTMD exhibits more flat conformation, representing that monolayer ZTMD film becomes flatter, while it shows poor mobility. By mixing ZTMD and Z (i.e., PFPE without functional group), we found that both the film thickness and the mobility were enhanced for the specific blend ratio. We analyzed the diffusion mechanism connected with the film conformation and found that ZTMD molecules form the network structure to restrict the diffusion of Z molecules due to its strong intermolecular interaction. The static nano-structures and dynamic mobility changes were examined in conjunction with the optimal lubricant selection by systematically tuning the volume fraction.

Reference:

1. X.-C. Guo, B. Knigge, B. Marchon, R. J. Waltman, M. Carter, and J. Burns, “Multidentate functionalized lubricant for ultralow head/disk spacing in a disk drive,” J. Appl. Physics, vol. 100, 044306, (2006).

2. P. S. Chung, H. Chen, and M. S. Jhon, “Molecular dynamics simulation of binary mixture lubricant films,” J. Appl. Physics, vol, 103, 07F526 (2008).

3. Y.–T. Hsia, Q. Guo, S. Izumisawa, and M.S. Jhon, “The dynamic behavior of ultrathin lubricant films,” Microsyst. Technol., vol. 11, 881-886, (2005).