Lubricant Behavior under Heat-Assisted Magnetic Recording (HAMR) Conditions

In the developing Heat-Assisted Magnetic Recording (HAMR) technology, a laser heats up the magnetic media to the Curie temperature of a few hundred degrees Celsius for a few nanoseconds. Accordingly, the thin-film lubricant coating on the disk experiences severe thermal conditions leading to thermo-capillary and evaporation effects followed by its local depletion. The resulting non-uniform lubricant profile can cause slider modulations leading to poor HDD read/write performance. In order to maintain a reliable head-disk interface, the lubricant needs to return to the initial uniform profile, in a process known as lubricant reflow, driven by the inter-molecular forces./nThis dissertation is dedicated to modeling the behavior of the Perfluoropolyether (PFPE) lubricants under such conditions./n/nTo study the lubricant depletion behavior, we employ a Finite Volume Method (FVM) combined with the lubrication theory to solve the lubrication equation for the Z-tetraol family of lubricants with 4 hydroxyl end-groups, including Z-tetraol 1200 with a low molecular weight and Z-tetraol 2200 with a high molecular weight, and also for ZTMD (2,200 Da) with 8 hydroxyl end-groups as a multi-dentate lubricant. All studies are performed for 4 initial film thicknesses of 5, 7, 12, and 14A. These numbers are chosen to provide a fair comparison with a previous study for Z-dol with 2 hydroxyl end-groups. Additionally, we investigate the relative effects of evaporation and thermo-capillary shear stress on lubricant depletion. It is found that after 2ns of laser irradiation, a trough and two side ridges across the down-track direction can be seen in the lubricant. The performances of the lubricants can be ranked mainly based on the trough depth and also evaporation such that better lubricants show less deformation and trough depth under equal conditions of thermal spot size and peak temperature. We also found that all of the lubricants deplete rapidly and their depletion rate decreases gradually./n/nTo investigate the reflow performance of the lubricants, we perform numerical simulations (FVM) to solve the lubrication equation, which (in case of reflow) is similar to a nonlinear Fickian diffusion equation. Then, we compare the calculated recovery (or reflow) times for HDD lubricants with similar molecular weights. The lubricant reflow is modeled for a wide range of film thicknesses and laser spot sizes, based on published material properties obtained by experiments./nFrom a design standpoint, the recovery time for the lubricants should be very short, and in particular, it should be shorter than the required time for one disk revolution, around 10-15ms./nThe results show that the recovery times for Z-tetraol 2200 and ZTMD are significantly longer than that for Z-dol 2000, while the recovery time for ZTMD is close to that for Z-tetraol, despite its higher viscosity value. This observation is due to the improved disjoining pressure properties for the multi-dentate ZTMD. It is also shown that all lubricants have an optimum film thickness for recovery time, and this optimum point largely depends on the dewetting and polar behavior of the lubricant./n/nIn the first part of this dissertation, the effects of the laser irradiation on lubricant depletion and recovery are investigated based on the assumption that the lubricant is an ultra-thin film viscous fluid and its behavior can be modeled using lubrication theory. This method is very well-established in the HDD industry./nHowever, PFPE lubricant depletion and recovery behavior at the timescale of HAMR conditions (microsecond to millisecond)/nis known to be that of a viscoelastic fluid. In the later part of the dissertation, we introduce a modification to the traditional lubrication equation that takes into account the effect of a non-zero Maxwell relaxation time and accommodates the viscoelastic effects. The results suggest that this method is numerically unstable for the 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 the laser spot size. For the micron-size laser spots, close to typical experimental conditions, the lubricant behaves like a viscous fluid. However, for the laser spot size of 20nm, close to the industry target for HAMR, it behaves like an elastic solid. 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./nRather, future efforts should concentrate on the thermal degradation and evaporation aspects of the HDD lubricants.

Advisor: Bogy, David B