IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 3302904 Molecular Dynamics Simulation of Lubricant Transfer at the Head-Disk Interface Young Woo Seo 1 , Deng Pan 2 , Andrey Ovcharenko 3 , Min Yang 3 , and Frank E. Talke 1 1 Center for Magnetic Recording Research, University of California at San Diego, San Diego, CA 92093 USA 2 School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China 3 Western Digital Corporation, San Jose, CA 95138 USA Molecular dynamics simulation is used to investigate lubricant transfer at the head–disk interface as a function of local pressure change, disk velocity, air bearing design, lubricant bonding ratio, head-disk spacing, and finite width of air bearing. A coarse-grained bead-spring model is used in conjunction with the Lennard–Jones potential, a short-range attractive polar potential, and a finitely extensible nonlinear elastic potential. The results show that lubricant transfer increases as a function of local pressure change, disk velocity, and air bearing width, but decreases with an increase in air bearing convergence angle, lubricant bonding ratio, and slider-disk spacing. Index Terms—Head–disk interface, lubricant transfer, molecular dynamics simulation. I. I NTRODUCTION P ERFLUOROPOLYETHER (PFPE) lubricants are used to protect the head–disk interface from wear and cor- rosion. PFPE polymers are copolymers with a chemical structure given by X-[(OCF 2 CF 2 ) p -(OCF 2 ) q ]-O-X (p/q ∼ = 2/3), where X represents functional or nonfunctional groups. PFPE lubricants are ideally suited as disk lubricants since they have low vapor pressure, low surface tension, and good thermal stability [1]. If a slider contacts a disk, lubricant can transfer from the disk surface to the slider surface. Lubricant transfer between the disk and the slider can also occur at very small slider-disk spacing, even in the absence of actual slider-disk contacts [2]. Lubricant transfer at the head–disk interface affects the flying characteristics of the slider, and can lead to a failure of a hard disk drive [2]. Deoras and Talke [3] studied lubricant depletion and slider dynamics in the low flying head–disk interface. They con- cluded that lubricant depletion increases with an increase in lubricant thickness. Canchi and Bogy [4] experimentally inves- tigated slider-lubricant interactions and lubricant transfer using thermal flying control sliders. They observed that lubricant molecules can migrate from the disk surface to the slider surface even in the absence of contact. Tagaya et al. [5] performed molecular dynamics simulations using a coarse- grained bead-spring (CGBS) model to study lubricant films on magnetic disks. They showed that a molecule cannot be modeled as a single sphere but must be modeled as a chain of multiple spheres, or beads, with functional end groups. Fukuda et al. [6] used a CGBS model of PFPE Zdol with two end beads and eight backbone beads to study the adhesive properties of UV-patterned lubricant films. They modeled the lubricant molecules as chains of beads and the disk layer as a solid layer. Ma and Liu [7] improved the model using rigid coarse-grained beads to model the disk layer. Jhon et al. [1] Manuscript received March 6, 2014; revised April 21, 2014 and April 29, 2014; accepted May 1, 2014. Date of current version November 18, 2014. Corresponding author: Y. Seo (e-mail: ywseo@ucsd.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2322353 Fig. 1. (a) Typical air bearing surface of a slider. (b) Schematic of the simulation model for a typical air bearing step. described multiscale modeling of head–disk interface mate- rials. They concluded that the CGBS model can be used to investigate the physical phenomena occurring over time in the head–disk interface. Pan et al. [8] used a CGBS model to study the lubricant interaction phenomena at the head–disk interface. They concluded that for a Rayleigh-type step slider, lubricant transfer increases with an increase in the local pressure change and a decrease in the ratio of functional groups on the disk surface. In this paper, lubricant transfer at the head–disk interface is investigated using a CGBS model in the absence of actual head-disk contacts as a function of local pressure change  P , disk velocity V d , air bearing step convergence angle α, lubricant bonding ratio, slider-disk spacing s , and finite width of the air bearing. II. SIMULATION PROCESS Fig. 1(a) shows a typical air bearing of a flying height control slider, consisting of alternating air bearing and recess regions with abrupt spacing changes up to a few micrometers. To simulate lubricant transfer for a situation where the slider- disk spacing changes quickly, we have developed the model shown in Fig. 1(b). This model consists of a converging and a diverging section, separated by a constant spacing section. The converging and the diverging sections are characterized by the convergence angle α, which is 90° for a Rayleigh-type step bearing. The model in Fig. 1(b) exhibits the features found in a typical air bearing contour. We would like to point out that the 0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.