Lubricant Flow and De-wetting at the Head-Disk Interface of a Hard Disk Drive
- Author(s): Rodriguez Mendez, Alejandro
- Advisor(s): Bogy, David B
- et al.
In this dissertation we investigate the lubricant behavior at the head-disk interface of a hard disk drive (HDD) by numerically simulating the formation of lubricant moguls on the disk and the accumulation of lubricant on the slider’s air bearing surface (ABS). We use classical lubrication theory from continuum mechanics to model both the air bearing and the lubricant motion. The numerical simulations were compared to experimental tests of lubricant reflow on the disk after laser heating. A good agreement was found between experiments and numerical simulations.
We investigate the effects of the slider’s flying height, skew angle and ABS design on the lubricant flow and reflow. We describe the lubricant thickness profile and volume evolution on the slider’s ABS and lateral walls. It was found that a smaller flying height contributes to a faster lubricant removal from the ABS due to the induced increase in the air shear stress. When the HDD is at rest, the lubricant accumulated on the deposit end flows back into the ABS driven by the action of disjoining pressure. It is found, for a particular slider design, that increasing the slider’s radial position and thus changing its skew angle has the effect of enhancing the lubricant flow process due to a decrease in the slider’s flying height. The lubricant migration process is significantly dependent on the ABS design. It is found that slider designs that accumulate most lubricant on a broader area on the deposit end and have larger values of air shear stress remove lubricant from the ABS at higher volume rates than those designs where accumulation is concentrated near the center of the deposit end and have smaller values of average shear stress.
We simulate the flow and reflow processes of unstable lubricant films. We study the spreading of droplets with thickness larger than the critical de-wetting thickness. It is observed that, if surface tension is neglected from the governing equations, the disjoining pressure acts as a destabilizing force inducing an unrestrained growth of the film. The disjoining pressure breaks up the initial droplet into smaller ones which narrow down in width and increase in height. As the growth continues, the curvature of each droplet becomes sufficiently large to balance the disjoining pressure. The final state consists of a few isolated droplets connected by a uniform film. When we include the effect of air shear stress and air pressure gradient the initial droplet breaks up into smaller ones, which are then sheared downstream in the direction of the air shear stress. It was not possible to simulate the de-wetting behavior of the lubricant film on the entire slider domain, since it was found that surface tension is significant only at length scales several orders of magnitude smaller than the size of the slider.
Finally we investigate the changes in magnetic spacing due to lubricant migration on the ABS and study the formation of lubricant clots on the disk surface known as “moguls”. It is observed that the minimum magnetic spacing of a lubricant contaminated slider is significantly larger than that of a clean slider even after a relatively long time of flying the slider over the disk. This increase in spacing is detrimental for the read/write performance of the HDD. It is also observed that the air shear stress can generate lubricant moguls on the disk surface due to oscillations of the slider along the vertical, downtrack and offtrack directions.