Yiao-Tee Hsia

Transport properties of nanoscale lubricant films

Off-lattice molecular dynamics simulations based on the coarse-grained bead-spring model via the Langevin equation were performed to simulate the dynamics of a confined nanoscale perfluoropolyether film (i.e., the self-diffusion and relaxation processes). The effects of molecular weight and solid surface attraction on the film diffusion coefficient were studied using the Green-Kubo formula. Via a stretched-exponential model, we investigated the relaxation process of nanofilms, and found that the relaxation spectra were stretched from the exponential decay process for functional endgroups and/or surface attraction.

Near Field Heat Assisted Magnetic Recording with a Planar Solid Immersion Lens

In this paper we present experimental heat assisted magnetic recording results using a planar solid immersion mirror (PSIM) fabricated on an Al2O3–TiC slider. The heads were flown at a velocity of 14 m/s, 20–25 nm above a Co/Pt multilayer medium which was deposited on a 60 mm glass disk. It was found that the track width and carrier-to-noise-ratio (CNR) increased with the applied magnetic field. Recording experiments were also performed with PSIMs terminated with 125 µm apertures. This led to narrower tracks and smaller CNR values for the same applied fields compared to recording with a PSIM only.

Simulation of nanostructured lubricant films

In this paper, we present rigorous simulation tools: a lattice-based simple reactive sphere model, an off-lattice bead-spring Monte Carlo model, and a molecular dynamics model. These tools accurately describe the static and dynamic behaviors of perfluoropolyether films consistent with experimental findings (scanning microellipsometry and surface energy measurements). These are suitable for the description of the fundamental mechanisms of film dewetting and rupture due to instability arising from nanoscale temperature and pressure inhomogeneities.

Process–property relationship of boron carbide thin films by magnetron sputtering

Abstract Boron carbide (B4C) films of 6–20 nm thick were deposited by magnetron sputtering using a B4C target. We have evaluated the mechanical properties using atomic force microscopy nanoscratch techniques and found that the scratch resistance is a strong function of compressive residual stress in the films, which in turn depends on the process parameters. With adequate energetic ion bombardment, the films exhibited compressive stress of approximately 3 GPa with high scratch resistance. However, excessive ion energy releases the stress and results in lower scratch resistance. Ion bombardment also affects surface roughness and stoichiometry of the films. By controlling the substrate bias, root mean square surface roughness was found to be as low as 0.13 nm. However, higher substrate bias led to off-stoichiometric films that had B/C ratios higher than 4.

Surface energy and adhesion of perfluoropolyether nanofilms on carbon overcoat: The end group and backbone chain effect

In this paper, we have investigated the surface energy and adhesion of one functional PFPE (Zdol) and two series of nonfunctional PFPEs (Z and D) on carbon-overcoated disk surfaces. The effects of end group functionality, backbone chain flexibility, molecular weight, and film thickness were systematically examined. Our results indicated that nonfunctional PFPEs have weak attraction with carbon overcoat. However, due to backbone chain effect, Z has slightly stronger attraction than D. Based on the surface energy analyses and bonded thickness results, schematic bonding models were proposed, which indicate strong hydrogen bonding∕ordered packing structure∕low mobility for functional PFPE films and weak attraction∕less-ordered packing structure∕high mobility for nonfunctional PFPE films.

Quasi-equilibrium AFM measurement of disjoining pressure in lubricant nanofilms II: Effect of substrate materials.

Atomic force microscopy (AFM) was used to measure the disjoining pressures of perfluoropolyether lubricant films (0.8-4.3 nm of Fomblin Z03) on both silicon wafers and hard drive disks coated with a diamondlike carbon overcoat. Differences in the disjoining pressure between the two systems were expected to be due to variations in the strength of van der Waals interactions. Lifshitz theory calculations suggest that this substrate switch will lead to relatively small changes in disjoining pressure as compared to the more pronounced effects reported due to changes in lubricant chemistry. We demonstrate the sensitivity of our AFM method by distinguishing between these similar systems.

Three-Dimensional Motion of Sliders Contacting Media

Characterization of slider motion induced by contact is becoming a critical aspect of developing advanced head-disk interfaces. While vertical motion induced by contact has been studied, very little is known about off- and down-track motions. We have applied three separate laser Doppler vibrometers to measure slider movement in three orthogonal directions simultaneously. We have measured the position of a slider as it undergoes a transition from flying to making full contact with the media surface. We find that slider motion varies considerably with varying levels of contact and that motion in all three directions is considerable. Spectral decomposition is used to identify the vibration modes that are excited in each direction, and we find that for most of the test velocities, modes excited in the vertical direction give rise to motion in the two orthogonal directions. In addition, we present a depiction of the vertical, down-track, and off-track position changes by plotting the position of the slider in real space coordinates to help visualize more completely how the slider moves in space. These trajectories depict the periodic, elliptical path the slider takes and identify how the paths change with contact. Analysis of motion identifies that at some levels of contact, a majority of motion is repeatable, but that nonrepeatable components increase with the amount of contact. Additionally, down-track motion is the only component whose magnitude increases monotonically with increasing contact.

Stability analysis of ultrathin lubricant films via surface energy measurements and molecular dynamics simulations

The stability of nanoscale lubricant films was analyzed via both surface energy measurements and molecular dynamics (MD) simulations. Using the sessile method, the contact angles of deionized water and n-dodecane on Zdol lubricant films were measured to examine the dispersive and polar surface energy as well as the nanofilm stability. By calculating the free energy from MD simulations, we investigated surface energy of the lubricant film. Furthermore, the film disjoining pressure and the stability diagram were constructed from MD simulations to examine the layering structures in spreading phenomena. Our analysis exclusively focuses on the effects of the end-functionality and molecular weight.

van der Waals force calculation between laminated media, pertinent to the magnetic storage head-disk interface

Lifshitz theory of van der Waals interactions is applied to the geometry and materials of the head-disk interface (HDI). A simplified two-substrate model with intervening air gap is used to illustrate the effects of retardation. The calculation of the Hamaker functions for increasingly complex layered structures is then presented to illustrate the importance of the multilayered nature of the HDI on the van der Waals (vdW) interaction. The full Lifshitz-multilayer calculation is then compared to approximations of the vdW force and relative errors are displayed. The results indicate the necessity of performing the full Lifshitz calculation on a realistic layered model of the HDI for accurate force modeling.

Wear Analysis of Head-Disk Interface During Contact

To achieve a higher storage density in a hard disk drive, the fly height of the air bearing slider, as part of the magnetic spacing, has to be minimized. At an ultralow fly height, the intermittent-continuous contact at the head-disk interface (HDI) is unavoidable and directly affects the mechanical and magnetic performance of the hard disk drive, and is of great interest. The HDI wear has a nonlinear and time-varying nature due to the change of contact force and roughness. To predict the HDI wear evolution, an iterative model of Coupled Head And Disk (CHAD) wear, is developed based on the contact mechanics. In this model, a composite transient wear coefficient is adopted and multiple phases of the wear evolution are established. A comprehensive contact stiffness is derived to characterize the contact at the HDI. The abrasive and adhesive wear is calculated based on the extended Archards wear law. The plastic and elastic contact areas are calculated with a three-dimensional (3D) sliding contact model. Based on the CHAD wear model, for the first time, the coupling between head and disk wear evolutions is thoroughly investigated. Accelerated wear tests have also been performed to verify the disk wear effect on the slider wear. A wear coefficient drop with time is observed during the tests and it is attributed to a wear mechanism shift from abrasive to adhesive wear A shift in the type of contact from plastic to elastic accounts for the wear mechanism change.