Viscoelastic Lubricant Deformation and Disk-to-Head Transfer During Heat-Assisted Magnetic Recording

One of the challenges in heat-assisted magnetic recording (HAMR) is the formation of write-induced head contamination at the near-field transducer. A possible mechanism that has been proposed for this contamination is the transfer of lubricant from the disk to the head due to temperature driven evaporation/condensation. Most previous studies on lubricant depletion due to laser heating have assumed the lubricant to be a viscous fluid and have modeled its behavior using the traditional lubrication theory. However, perfluoropolyether lubricants are viscoelastic fluids and are expected to exhibit a combination of viscous and elastic behavior at the time and length scales of HAMR conditions. In this paper, we use a modified Reynolds lubrication equation for the viscoelastic fluid that employs the linear Maxwell constitutive model. We use this modified lubrication equation to develop a model that predicts the disk-to-head lubricant transfer during HAMR writing. This model simultaneously determines the thermocapillary stress driven deformation and evaporation of the viscoelastic lubricant film on the disk, the diffusion of the vapor phase lubricant in the air bearing, and the evolution of the condensed lubricant film on the head. We investigate the effects of lubricant type (Zdol versus Ztetraol), head/disk temperature, initial lubricant thickness, and laser spot size on the lubricant transfer process. Simulation results show a significant difference between the rates of transfer for Zdol (timescale of nanoseconds) versus Ztetraol (timescale of microseconds). The amount of transfer increases with the disk temperature and the initial lubricant thickness.

I. Introduction

Reliability of the head-disk interface (HDI) during high-temperature laser heating still remains a major challenge that needs to be addressed before heat-assisted magnetic recording (HAMR) can be made into a robust commercial product. One of the challenges in HAMR is the formation of write-induced head contamination at the near-field transducer (NFT). Kiely et al. [1] reported that head contamination begins soon after the laser is turned on and grows over time until the contamination height reaches the head-disk clearance. Once the head contamination contacts the media surface, the disk motion generates a smear down track of the NFT. One possible mechanism for this contamination is lubricant transfer from the disk to the head through thermodynamic driving forces [1], [2]. During HAMR, the media is locally heated locally heated to its Curie temperature (~500 °C). However, the peak temperature of the head is lower than that of the disk (~300 °C) [1]. This temperature difference causes the lubricant to evaporate from the disk and condense on the relati- vely cooler head. The lubricant acts as a carrier, causing a continuous deposition of media contaminants at the NFT.