An Atomistic Study of Perfluoropolyether Lubricant Thermal Stability in Heat Assisted Magnetic Recording

At the head disk interface (HDI), the stability of the perfluoropolyether (PFPE) lubricant and carbon overcoat (COC) materials must be preserved under HAMR conditions. In this work, we investigate this issue by comparing the effects of transient versus steady heating of Zdol to replicate the precise pulsed heating of the HAMR system. These effects include changes in intermolecular lubricant bonding, molecular decomposition and desorption. In order to accurately account for potential changes in covalent and intermolecular bonds, we utilize the cutting-edge molecular simulation method of ab initio molecular dynamics. To simulate constant heating, a series of constant temperature simulations are performed at temperatures ranging from 300 K-700 K where the temperature is maintained via the Nose Hoover thermostat. For the transient heating simulations, the temperature is ramped over 100 K intervals with initial temperatures ranging from 300 K to 700 K. These heating studies are performed for bulk PFPE systems as well as PFPE-COC configurations to highlight the effect of PFPE-COC adhesion on lubricant thermal stability at the HDI. In the PFPE-COC simulations, we evaluate the amount of desorption versus decomposition as a function of initial temperature. Through our analysis, we are able to reveal the molecular mechanism of PFPE depletion as a function of functional group composition and, thereby, provide design criteria for lubricant molecular architecture in HAMR applications.

I. Introduction

To design hard disk drives (HDD) with areal recording density surpassing 1 Tb/in², a significant breakthrough in magnetic recording technology must be achieved. Heat assisted magnetic recording (HAMR) is one of the candidates for next paradigm technologies to drastically increase HDD recording density [1]–[11]. During the HAMR process, the media undergoes pulsed laser heating requiring the media to withstand temperatures approaching 700 K for a duration of 1 ns. This extreme heating requirement necessitates investigations of the thermal performance of currently implemented HDD materials. At the head disk interface (HDI), the stability of the perfluoropolyether (PFPE) lubricant and carbon overcoat (COC) materials must be preserved to guard the media against corrosion as well as damage from head contact. Previous experimental studies observed degradation and desorption of conventional PFPE lubricants under HAMR conditions [12], [13]. Since the HAMR heating mechanism uses cyclic heating, a study of PFPE stability should incorporate the effects of transient heating to compare with constant temperature environments. PFPE simulation can offer new insights into the molecular properties of PFPEs which govern thermal stability. However, theoretical studies at the fully atomistic scale revealing the mechanism of PFPE depletion under HAMR conditions have not previously been performed.