Xiaoding Ma

Lubricant thickness modulation induced by head-disk dynamic interactions

The behavior of a thin lubricant film under a flying head has been studied by examining the lubricant redistribution on the disk after flying, using an HDI-SRA instrument. Lubricant depletion tracks were observed on the disk surfaces and, more interestingly, the lubricant film was found to exhibit a periodic thickness modulation in the downtrack direction. The wavelength of the lubricant thickness modulation is found to increase linearly with the disk linear velocity, and depends weakly on the lube type and lube bonding ratio. The amplitude of the modulation grows slowly with flying time. Both negative-pressure air bearing pico sliders and catamaran-style positive-pressure nano sliders generate similar patterns on lubricant films. The frequency of the thickness modulation is in the range of 40-50 kHz, and is attributed to interactions of the disk lubricant with the slider roll mode. In addition, for highly-bonded lubricant films, a much finer lubricant modulation pattern can be seen with a frequency of 196 kHz, which is very close to the frequency of the pitch mode of the slider. These results indicate that the lubricant thickness modulations on-the disk are generated by slider-disk dynamic interactions, and are due to slider body motions.

Effect of lubricant on flyability and read-write performance in the ultra-low flying regime

The effect of lubricant on flyability and read-write performance in ultra-low flying regime has been studied over the disks with lubricant on one half of disk surface thicker than the other half. The dynamics of a slider was monitored using Acoustic Emission (AE) and Laser Doppler Vibrometer (LDV). An instability characteristic of a slider flying over the thick lubricant region has been observed and this instability intensifies as flying height decreases and the step thickness increases. After the slider flies over the disks, it has been found that lubricant re-distribution occurs as lubricant is carried by the flying slider from the thick lubricant region and deposited onto the thin lubricant region. Possible mechanisms were discussed to explain the observations. Finally, recording tests were performed and the magnetic spacing loss due to the lubricant steps was estimated.

Contribution of lubricant thickness to head-media spacing

In this work, we have studied the effect of lubricant thickness on magnetic recording, specifically, its effect on head-media spacing (HMS). In the experiment, a series of disks were first lubricated to various thicknesses ranging from 20 to 60 /spl Aring/. Then, read/write recording tests were performed for each disk on one track near the disk OD with variable linear densities. Next, the lubricant was washed off from MD to OD using a solvent, so the thickness of lubricant at the OD track was reduced. The read/write tests showed that the read-back signal was enhanced after lubricant wash-off. Using Wallaces equation, the HMS change was calculated and found to be proportional to the change of the lubricant thickness. This result conclusively establishes that disk lubricant indeed takes up HMS and must be accordingly accounted for in HMS budgeting.

Effect of non-uniform lubricant distribution and de-wetting on head-disk dynamics

In this work, the effect of long-wavelength nonuniform lube distribution and lube de-wetting on head-disk dynamics is investigated by using disks with controlled within-disk nonuniformity in lubricant film thickness. Complex slider responses were observed, which were sensitive to lubricant film thickness and the extent of de-wetting. The effect of slider-assisted lubricant redistribution was also observed.

Temperature effect on spreading of perfluoropolyethers on amorphous carbon films

The temperature effect on the spreading behavior of Fomblin Zdol and Z on amorphous carbon surfaces has been studied by scanning micro-ellipsometry. The temperature range explored in this study spans from 26 to 56 °C under 0% relative humidity. The results show that while temperature in general accelerates surface diffusion, it has little effect on the surface conformation of the lubricants. The diffusion coefficient versus film thickness maintains the same shape, only the overall amplitude increases with increasing temperature. The activation energy Ea is found to be relatively insensitive to the lubricant coverage up to one monolayer. The average activation energy is around 37 kJ/mol for Zdol (Mn=2500 g/mol), and around 21 kJ/mol for Z of the same molecular weight. With increasing molecular weight the activation energy increases but the rate of increase is very moderate. More interestingly, the activation energy versus molecular weight curves for Zdol and Z are nearly parallel to each other, suggesting that the interactions between the main-chain and the carbon surface are essentially independent of the interactions between the end-groups and the carbon surface.

ESCA-thickness metrology and head-medium spacing impact of disk lubricant

Electron spectroscopy for chemical analysis (ESCA) has been commonly used as a metrology tool for lubricant film-thickness measurement on magnetic hard disks. The accuracy of the ESCA-thickness measurement rests solely with the calibration accuracy of the characteristic photoelectron attenuation length in the lubricant film. Several past studies on this subject yielded widely divergent results, due to the difficulty in obtaining an accurate, absolute lubricant film-thickness measurement. In this paper, we revisited the calibration issue and, instead of following the same paths pursued in the past, used a derivative method to yield an accurate calibration of the photoelectron attenuation length. We also compared the various methods for lubricant film-thickness calculation based on ESCA measurements and determined that the most accurate method is to use only the photoemission signal from the lubricant film. In addition, by studying the lubricant film-thickness effect on the electrical readback signal, we found that the lubricant film leads to an increase in the head-medium spacing by an amount greater than one times, but less than two times, its physical thickness.

Atomistic Frictional Properties of the C(100)2x1-H Surface

Density functional theory- (DFT-) based ab initio calculations were used to investigate the surface-to-surface interaction and frictional behavior of two hydrogenated C(100) dimer surfaces. A monolayer of hydrogen atoms was applied to the fully relaxed C(100)2x1 surface having rows of C=C dimers with a bond length of 1.39 A. The obtained C(100)2x1-H surfaces (C–H bond length 1.15 A) were placed in a large vacuum space and translated toward each other. A cohesive state at a surface separation of 4.32 A that is stabilized by approximately 0.42 eV was observed. An increase in the charge separation in the surface dimer was calculated at this separation having a 0.04 e transfer from the hydrogen atom to the carbon atom. The Mayer bond orders were calculated for the C–C and C–H bonds and were found to be 0.962 and 0.947, respectively. σ C–H bonds did not change substantially from the fully separated state. A significant decrease in the electron density difference between the hydrogen atoms on opposite surfaces was seen and assigned to the effects of Pauli repulsion. The surfaces were translated relative to each other in the (100) plane, and the friction force was obtained as a function of slab spacing, which yielded a 0.157 coefficient of friction.

A kinetic model of magnetic media corrosion

In this work, we examines the randomness of the deposition in terms of the probability of uncovered magnetic alloy surface sites, and compares model kinetic calculations of oxidation of the magnetic film as a function of carbon thickness to experimental data.

Monte Carlo simulation of film morphology of carbon overcoat

In this paper, we describe carbon film morphology under different deposition conditions using Monte Carlo simulation.

Growth Randomness, Porosity, and Surface Coverage of Carbon Films

Mathematical models that describe the static growth processes of sputtered and ion-beam deposited amorphous carbon films are established. Two important factors are considered in deriving the models: the film porosity and the growth randomness. The growth randomness for both hydrogenated sputter carbon and hydrogenated ion-beam carbon films are deduced from the film roughness vs. film thickness data obtained from X-ray reflectometry. The porosities of the two types of films are estimated from the experimentally determined values of film densities and sp3 /sp2 C–C bonding ratios. The derived film growth models, which find support from a surface coverage experiment, have allowed us to predict the surface coverage of the two types of carbon films in the thickness range of interest. We find that, neglecting intermixing at the substrate/carbon interface, the denser ion-beam carbon films are nearly as effective at achieving a similar level of surface coverage as the less dense sputter carbon films at twice the thicknesses. Our surface coverage model points to further increasing the carbon film density as the most effective approach for the continued reduction in the carbon overcoat thickness while maintaining adequate surface coverage.Copyright