Effect of Viscoelasticity on Lubricant Behavior Under Heat-Assisted Magnetic Recording Conditions

In Heat-Assisted Magnetic Recording (HAMR) technology, the lubricant layer coating on the disk is exposed to severe thermal conditions, leading to evaporation, depletion, and chemical degradation. In general, those studying the effects of laser exposure on lubricant depletion and recovery have assumed the lubricant to be a viscous fluid and have modeled its behavior using lubrication theory. However, PFPE lubricant depletion and recovery behavior at the timescale of HAMR conditions (microsecond to millisecond) is known to be that of a viscoelastic fluid. In this paper, we introduce a modification to the traditional lubrication equation that accommodates viscoelastic effects. The results suggest that this method is numerically unstable for small laser spot sizes, close to the target of HAMR. Accordingly, we developed a novel approach to model the viscoelastic depletion and recovery behavior of PFPE ultra-thin films using a Finite Element Analysis. We show that this new method is able to model the entire range of material viscoelasticity, from purely viscous to purely elastic extremes. The results show that the viscoelastic effects become remarkably pronounced with a decrease in laser spot size. For the micron-size laser spots, close to typical experimental conditions, the lubricant behaves like a viscous fluid. For the laser spot size of 20 nm, close to the target of HAMR, it behaves like an elastic material. In exposing the consequences of this viscoelastic behavior, this study predicts that lubricant flow due to thermo-capillary effects will not be a significant issue in the development of the HAMR technology. Rather, future efforts should concentrate on the thermal degradation and evaporation effects upon HAMR lubricants.