Sung-Chang Lee

Adhesion forces for Sub-10 nm Flying-Height Magnetic Storage Head Disk Interfaces

A quasi-dynamic adhesion model is used to calculate the intermolecular adhesion forcespresent in ultra low flying Head Disk Interfaces (HDI’s). The model is a continuum-basedmicromechanics model that accounts for realistic surfaces with roughness, molecularlythin lubricants, and is valid under both static and dynamic sliding conditions. Severaldifferent levels of surface roughness are investigated ranging from extremely smoothsurfaces having a standard deviation of surface heights s52 A to rougher interfaces withseveral nanometer roughness. It is found that when the flying-height is greater than 5 nm,there are no significant adhesive forces, whereas for flying-heights less than 5 nm, adhe-sion forces increase sharply, which can be catastrophic to the reliability of low flyingHDI’s. In addition to roughness, the apparent area of contact between the flying recordingslider and the magnetic disk is also found to significantly affect the magnitude of theadhesion forces. The adhesion model is validated by direct comparisons with adhesion‘‘pull-off’’ force measurements performed using an Atomic Force Microscope with con-trolled probe tip areas and magnetic disks having different lubricant thickness.@DOI: 10.1115/1.1645299#

Microtribodynamics of pseudo-contacting head-disk interfaces intended for 1 Tbit/in/sup 2/

A nonlinear dynamic model that includes realistic roughness, adhesion and friction, as well as the dynamics of a flying and contacting HDI was developed to characterize a pseudo-contacting HDI, intended for 1 Tbit/in/sup 2/. A pseudo-contacting recording system is designed to fly at few nanometers using an air-bearing and at the same time some features of the air-bearing surface are designed to contact with the rotating disk during operation. The model was favorably compared with flyability measurements, and then applied to a pseudo-contacting interface to investigate adhesion, friction, and contact forces as well as bouncing vibration. Contrasting earlier studies adopting a simple Coulomb friction, the friction model used in this work calculates the friction force at the interface, accounting for roughness and adhesion. It was found that unlike a fully flying HDI, adhesion plays a positive role in attaining pseudo-contact recording by reducing bouncing vibrations.

Strategies to avoid head-disk instabilities due to adhesion in ultralow flying head-disk interfaces

Reducing the flying-height to sub-5 nm in hard disk drives is essential in achieving ultrahigh recording densities of the order of 1 Tbit/in/sup 2/. In order to minimize the risk of contact for such ultralow flying head-disk interfaces (HDIs), one needs to use extremely smooth disk and slider surfaces with RMS roughness values of the order of few ansgstroms. However, such super smooth interfaces are known to cause problems, such as strong attractive adhesive forces and, thus, catastrophic HDI crashes. In this paper, a systematic study specifically addressing strategies to minimize adhesion and, most importantly, to avoid head-disk instabilities or crashes due to adhesion is presented. The nonlinear adhesive forces are coupled with a nonlinear dynamic system model of the flying HDI, and a 3/sup 3/ full-factorial design of experiments is performed to investigate the effects of roughness and geometrical parameters on adhesion, flyability, and stability of ultralow sub-5-nm flying HDIs. Based on analysis of variance and corresponding response analyses, strategies to avoid the detrimental effects of adhesion on flying HDIs are proposed.

Design Optimization of Ultra-Low Flying Head-Disk Interfaces Using an Improved Elastic-Plastic Rough Surface Model

Sub-5 nm flying head-disk interfaces (HDIs) designed to attain extremely high areal recording densities of the order of Tbit/in 2 are susceptible to strong adhesive forces, which can lead to subsequent contact, bouncing vibration, and high friction. Accurate prediction of the relevant interfacial forces can help ensure successful implementation of ultra-low flying HDIs. In this study, an improved rough surface model is developed to estimate the adhesive, contact, and friction forces as well as the mean contact pressure relevant to sub-5 nm HDIs. The improved model was applied to four different HDIs of varying roughness and contact conditions, and was compared to the sub-boundary lubrication rough surface model. It was found that the interfacial forces in HDIs undergoing primarily elastic-plastic and plastic contact are more accurately predicted with the improved model, while under predominantly elastic contact conditions, the two models give similar results. The improved model was then used to systematically investigate the effect of roughness parameters on the interfacial forces and mean contact pressure (response). The trends in the responses were investigated via a series of regression models using a full 3 3 factorial design. It was found that the adhesive and net normal interfacial forces increase with increasing mean radius R of asperities when the mean separation is small (≈0.5 nm), i.e., pseudo-contacting interface, but it increases primarily with increasing root-mean-square (rms) surface height roughness between 2 and 4 nm, i.e., pseudo-flying interface. Also, increasing rms roughness and decreasing R, increases the contact force and mean contact pressure, while the same design decreases the friction force. As the directions of optimisation for minimizing the individual interfacial forces are not the same, simultaneous optimization is required for a successful ultra-low flying HDI design.

Friction Force Measurements and Modeling in Hard Disk Drives

Friction in a head disk interface (HDI) is investigated considering the surface energy of the lubricant and the type of carbon overcoat (COC). Perfluoropolyether Z-Tetraol lubricant with A20H additive is applied on two types of COC (type-A and type-B) with lubricant thicknesses in the range of 11-19 A. The polar and dispersive components of the surface energy are measured from contact angle experiments. For each case, friction is measured using actual hard disk drives, and they are compared with the measured surface energy values of the disk samples. As the lubricant thickness increases, both the surface energy and friction decrease. Comparing friction and surface energy values for the two types of COC disks, type-A disks are found to exhibit lower surface energy and higher friction at all lubricant thicknesses. This is attributed to the effects of surface roughness, surface energy of the COC film, and lubricant interactions.

Dynamic adhesive force measurements under vertical and horizontal motions of interacting rough surfaces

An instrument to measure dynamic adhesive forces between interacting rough surfaces has been developed. It consists of four parts, namely, main instrument body, vertical positioning system with both micrometer and nanometer positioning accuracies, horizontal positioning system with nanometer positioning accuracy, and custom-built high-resolution, and high dynamic bandwidth capacitive force transducer. The vertical piezoelectric actuator (PZT) controls the vertical (approaching and retracting) motion of the upper specimen, while the horizontal PZT controls the horizontal (reciprocal) motion of the lower specimen. The force transducer is placed in line with the upper specimen and vertical PZT, and directly measures the adhesive forces with a root-mean-square load resolution of 1.7 microN and a dynamic bandwidth of 1.7 kHz. The newly developed instrument enables reliable measurements of near-contact and contact adhesive forces for microscale devices under different dynamic conditions. Using the developed instrument, dynamic pull-in and pull-off force measurements were performed between an aluminum-titanium-carbide sphere and a 10 nm thick carbon film disk sample. Three different levels of contact force were investigated; where for each contact force level the vertical velocity of the upper sample was varied from 0.074 to 5.922 microms, while the lower sample was stationary. It was found that slower approaching and retracting velocities result in higher pull-in and pull-off forces. The noncontact attractive force was also measured during horizontal movement of the lower sample, and it was found that the periodic movements of the lower disk sample also affect the noncontact surface interactions.