Michael Joseph Stirniman

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.

Volatility of perfluoropolyether lubricants measured by thermogravimetric analysis

The evaporation rate at ambient pressure as a function of molecular weight has been measured for two functionalized perfluoropolyether lubricants, Zdol and Zdol‐TX, using thermogravimetric analysis (TGA). The temperature dependence of the evaporation rate is well characterized by a zero‐order Arrhenius expression, allowing determination of the molecular weight dependence of the pre‐exponentials and activation energies. These Arrhenius parameters can then be used to extrapolate the evaporation rate back to temperatures where it is too small to be directly measured. The extrapolated evaporation rates at 200°C are combined with Raoults law to predict the vapor phase molecular weight distribution of Zdol over a polydisperse liquid phase, which is shown to be in reasonable agreement with the measured vapor phase distribution. The evaporation rate of Fomblin Z fluids is also shown to be strongly dependent on the functional endgroup for a given molecular weight.

The ideality of polydisperse perfluoropolyether lubricants with application to physical vapor deposition

We have used thermogravimetric analysis (TGA) to measure the evaporation rate as a function of temperature and molecular weight for an alcohol-derivatized perfluoropolyether (PFPE), Fomblin Zdol. We show that the bulk evaporation rate of a polydisperse Zdol solution during temperature ramp TGA can be numerically simulated by combining molecular weight dependent Arrhenius parameters, the initial molecular weight distribution as measured by size exclusion chromatography, and Raoult’s law of vapor pressures. The simulation is shown to be in good agreement with experiment for both a low molecular weight polydisperse Zdol, and for a mixture of the low molecular weight Zdol with a heavier Zdol fraction. Mixtures of Zdol with non-functionalized Z lubricants are shown qualitatively to deviate substantially from ideality. In the disc drive industry, physical vapor deposition (PVD) is receiving renewed interest as an alternative method of applying the PFPE lubricants commonly employed as topical coatings on thin film magnetic media. In a process involving vaporization of a polydisperse liquid phase lubricant, quantitative prediction of deposition rates and vapor phase molecular weight distributions will in general only be possible with accurate knowledge of liquid phase distributions, component evaporation rates, and the ideality of the solution.

Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations

Lubricant accumulation was found on the media surface the instant when the laser is turned OFF during heat-assisted magnetic recording write operations. By changing the write cycles, laser ON/OFF duration, media, and head temperatures, we find that this lubricant accumulation is related to the change in head-media temperatures. The observed lubricant deposition process is restricted to a short time window (1-2 μs) after the laser is turned OFF. An equilibrium model of thermal displacement due to evaporation and condensation processes is presented and used to discuss the effect of the head-media temperature changes on the lubricant accumulation. Possible solutions to minimize the lubricant transfer and deposition are discussed.

Catalytic decomposition of perfluoropolyether lubricants

With the ongoing decrease in magnetic spacing, the chemical durability of the lubricant film on magnetic recording media becomes a major factor in the reliability of the head-disk interface. There are several reports in the literature that have attributed deterioration of lubricant film to catalytic decomposition, and a reaction mechanism has been proposed by Kasai involving chain scission of the lubricant main chain promoted by Lewis acids. In this work, an alternative mechanism is proposed, wherein perfluoropolyether (PFPE) lubricants are catalytically decomposed via a free radical degradation loop. The catalytic decomposition of Fomblin Zdol in the presence of the Lewis acid Al/sub 2/O/sub 3/ was studied using thermogravimetric analysis (TGA). We show that various types of antioxidants can retard PFPE catalytic decomposition to different degrees, which we attribute to their different roles in breaking the free radical degradation loop. These data provide useful guidance in the development of new lubricants that are resistant to free radical initiated decomposition.

The effect of slider on lubricant loss and redistribution [HDDs]

The effect of the slider geometry and ABS on lubricant loss and redistribution has been studied by flying heads over disks with step-profile lubricant film. Two types of sliders, one with a negative pressure air bearing and truncated side-rails and the other with a positive air bearing, in the standard catamaran geometry, were used. The disks after flying were examined with a scanning reflectance analysis probe. After a short period of time, it was found that both sliders caused a comparable amount of lubricant depletion in the thick-film region. However, the redeposition in the thin-film region is seen prominently only under the slider with the negative pressure air-bearing. The result suggests that the lubricant depletion in one case results primarily from net lubricant loss or pickup, and in the other case from transfer and redeposition. Based on these results, a transfer mechanism under each slider type has been proposed. A discussion on the design implications is also included.

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.

Head-disk lubricant transfer and reposition during heat assisted write and read operations

Heat Assisted Magnetic Recording (HAMR) is the most important of the next generation storage technologies that are currently competing to supplant perpendicular magnetic recording (PMR). Recently, HAMR has demonstrated one terabit per square inch (1 Tb/in2) recording density and extensibility of this new technology well beyond this density seems practical [1]. HAMR technology enhances the magnetic writing process by the inclusion of a strong optical field that propagates from the read-write head and instantaneously heats the magnetic recording layer to above its Curie temperature, thus allowing magnetic recording on media with ultrahigh magnetic anisotropy that otherwise could not be written with conventional writers. During HAMR writing, the HAMR media will experience ultra-rapid heating, reaching a peak temperature in excess of 500 C. These conditions will thermally stress the lubricant film on the media which, along with the carbon overcoat, protects the magnetic storage layer from environmental, thermal and tribological degradations. The cumulative effect of these repetitive heating cycles on the redistribution of the lubricant film of the media is a significant concern, due to the need to maintain a functioning lubricant film on the media and the negative effect on the head flyability with lubricant accumulation on its surface. The effect of these high temperature transients on the distribution of lubricant on both the media surface and the head are probed and specific rates of its accumulation and deposition are determined by varying the number of write cycles, laser-on durations, the head flying height (FH), and the peak surface temperature.

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.