Nanoscale Lubricant Flow and Heat Transfer at the Head-Disk Interface in Hard Disk Drives

In the hard disk drive (HDD) industry, technologies such as Heat-Assisted Magnetic Recording (HAMR) and Microwave-Assisted Magnetic Recording (MAMR) are currently under development to increase the areal density of contemporary HDDs beyond 1 Tb/in^2. Traditional magnetic media has reached the superparamagnetic limit: if the size of the media bits is further decreased (to increase the areal density), the bits will become thermally unstable. HAMR and MAMR aim to overcome this obstacle and increase the data density by using a high coercivity magnetic media that can store data at very small bit sizes of ~(25 nm)^2. To reduce the coercivity of this media temporarily while writing, HAMR heads are integrated with a laser delivery system that heats the media to ~500 deg. C using a ~20 nm FWHM laser. MAMR heads contain a Spin Torque Oscillator that bombards the media with a microwave field, lowering its coercivity during writing. These new components introduce additional thermal complications to the already challenging head-disk interface (HDI) design. In contemporary HDDs, the recording head slider, which contains the read/write transducers, "flies" in close proximity to the rotating disk (<5 nm). Such a low spacing is achieved via the Thermal Fly Height Control (TFC) technology, where an embedded joule heater locally protrudes the slider's trailing edge. With the addition of new components required for the energy-assist in HAMR/MAMR, head overheating is a major reliability concern for both these technologies. Moreover, laser heating of the disk during HAMR causes the ~1 nm thick lubricant layer, that coats and protects the disk, to deform, evaporate and condense on the head. This disk-to-head lubricant transfer in turn causes detrimental issues such as write-induced head contamination. Therefore, there is a need to understand the flow and transfer of the lubricant and the nanoscale heat transfer in the HDI to develop robust HAMR/MAMR drives./nWhile the lubricant behavior is traditionally modelled using viscous lubrication theory in the HDD industry, experiments show that HDD lubricants are, in reality, viscoelastic fluids. At the small timescales involved in HAMR writing (~ns), the elastic mode of the lubricant can no longer be ignored. In this dissertation, we introduce a modification to the traditional (viscous) Reynolds lubrication equation using the linear Maxwell (viscoelastic) constitutive equation and a slip boundary condition. We study the deformation and recovery of the lubricant due to laser heating under the influence of thermocapillary (Marangoni) stress and disjoining pressure. When subjected to a 20 nm FWHM scanning laser spot, the lubricant profile consists of an elastic trough centered at the instantaneous laser location, followed by a viscous trail. When the laser is turned off, the elastic trough recovers instantaneously, leaving behind the viscous trail, which recovers over a time scale of microseconds./nFurther, we develop viscous and viscoelastic models for the disk-to-head lubricant transfer during HAMR writing. These models simultaneously determine the deformation and evaporation of the lubricant film on the disk, the diffusion of the vapor phase lubricant in the HDI and the evolution of the condensed lubricant film on the slider. We investigate the effect of lubricant properties such as viscoelasticity, lubricant type (Zdol vs Ztetraol), molecular weight, slip length, disjoining pressure, and HAMR design parameters such as head/media temperature, lubricant thickness and laser FWHM on the lubricant transfer. We find a significant difference between the rates of transfer for Zdol (~ns) vs Ztetraol (~microseconds). The viscous model overpredicts the amount of transfer compared to the viscoelastic model./nTraditionally, the heat transfer coefficient in the HDI is determined by estimating thermal conduction through air using the energy equation with temperature jump theory. However, with the minimum fly height of less than 5 nm in contemporary HDDs, energy transfer due to phonon conduction also becomes significant. We present a numerical model to simulate the head temperature due to heat transfer across a closing nanoscale gap between the head and the non-rotating media. Our model employs a spacing-dependent heat transfer coefficient due to the combined effects of air conduction and wave-based phonon conduction. We compare our simulations with static touchdown experiments performed with a TFC slider resting on three different media (Si, magnetic disks with AlMg and glass substrates). The TFC heater is powered to create a local protrusion, leading to head-media contact and a resistive sensor (Embedded Contact Sensor or ECS) is used to detect the head temperature change. With the introduction of intermolecular van der Waals forces between the head and the media, we demonstrate a good quantitative match with experiments for all of the media materials tested, at different head-media spacings and in different environments (air, vacuum)./nNext, we develop a numerical model to predict the temperature profile and the fly height for a flying slider over a rotating disk. We compare our simulations with touchdown experiments performed with a flying TFC slider using the ECS to record the temperature change. To accurately predict the heat transfer and fly height at near-contact, we incorporate the effects of disk temperature rise, intermolecular adhesion & contact forces, air & phonon conduction heat transfer and friction heating in our model. We investigate the impact of each of these features on head temperature during flying. We find that simulation with all these features agrees well with the experiment.

Advisor: Bogy, David B