Carbon Overcoat Surface Research Articles

Laser-Induced Thermo-Desorption of Perfluoropolyether Lubricant from the Surface of a Heat-Assisted Magnetic Recording Disk: Lubricant Evaporation and Diffusion

Heat-assisted magnetic recording (HAMR) storage technology will thermally stress the lubricant film typically applied to the storage disk surface. Different lubricant loss and morphology change mechanisms have been hypothesized to occur during information writing. A loss to of the lubricant film will dramatically affect its overall mechanical and chemical performance of the head–disk interface, decreasing its reliability and its durability. Thus an all optical pump–probe method was used to study the effect of fast thermal transients (106 K/s) on a lubricant film on HAMR media. Thermal transients (Bhushan and Cheng in J Appl Phys 81:5390, 1997) with peak temperatures above the HAMR media Curie temperature (T c) were found to remove by evaporation the lubricant within the heated region creating a lubricant depression in the otherwise continuous film. No accumulation of lubricant volume was observed to take place in the cooler regions of the thermal spot, indicating that thermocapillary shear stress is not an important mechanism of lubricant thickness change with the optical spot size used (65 μm). The onset of lubricant loss was observed to begin at approximately 610 K and was totally removed at 823 K. The change in depth of the lubricant depression with time showed that no structural terms contributed to the disjoining pressure for the lubricant thickness range studied. From this change, the diffusion coefficient of the lubricant on the carbon overcoat surface was determined to be 1 × 10−13 m2/s by fitting Fick’s second law to the normalized lubricant thickness. The importance of these observations on the operating HAMR head–disk interface is discussed.

Atomistically Tuning Lubricant Adhesion on Carbon Overcoat Surface

The critical issue facing head-disk interface (HDI) design is determining the optimal combination of lubricant and overcoat materials. A high performing lubricant should have sufficient adhesion to the carbon overcoat (COC) layer to prevent spin-off, while the COC material protects the magnetic media from corrosion and tribological damage. In this paper, we examine at the atomistic level COC material performance by calculating the adhesive interactions between various types of model perfluoropolyether (PFPE) functional lubricants and COC. The COC materials we investigated include industry standard diamond-like carbon (DLC), composed of graphitic (sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> bonded) and diamond (sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> bonded) type carbon. In addition, we investigated the atomically thin graphene and pure diamond COCs to determine the effect of carbon structure on PFPE adhesion. These pure diamond and graphene results support the observation that the PFPE interacts more strongly with the graphitic content in DLC. The difference between the results for diamond and graphene are due to the sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> bonding in graphene with delocalized electrons in the aromatic ring, whereas in the diamond structure, electrons are bound by the extra C-H bond. Our density functional theory study of PFPE interaction with these three COC materials shows that Ztetraol has the strongest adhesion on all of the COCs investigated. Based on our investigation, we would conclude that an ideal COC material should have high graphitic content to promote strong PFPE adhesion. We also evaluated the feasibility of pure graphene to promote PFPE adhesion. We extend our analysis to explore dimensional crossover to gauge the effect of COC thickness on adhesion.

Mechanism of Perfluoropolyether Chains Bonding on Carbon Overcoat Surface by Ultraviolet Irradiation

To elucidate the bonding mechanism of a non-polar perfluoropolyether lubricant (PFPE lube) under ultraviolet (UV) irradiation, the UV wavelength ranges in which photoelectrons emanate were identified using photoelectron spectroscopy in air (PESA), and the bonding behavior of the lube at various UV wavelengths was studied. The results showed that photoelectrons emanated from magnetic disk substrates at wavelengths of 240 nm or less and the lube was bonded to the substrate in the wavelength range where photoelectrons were emanated. For comparison, Si, SiO2/Si, and Si3N4/Si were also used as substrate; the Si substrate which emanated the highest photoelectron intensity showed the highest bonded ratio of the lube. The lube, however, also bonded to the SiO2/Si and Si3N4/Si substrates under UV irradiation at 222 nm, where photoelectrons do not emanate from the substrates. These observations suggest that the bonding mechanism between the PFPE chains and the carbon overcoat (COC) surfaces under UV irradiation was affected by factors other than photoelectrons.

Chemical bonding of perfluoropolyether with carbon underlying layer induced by visible laser light

We investigated the chemical bonding of perfluoropolyether lubricant molecules with carbon overcoat surface by irradiating with a visible pulsed laser light. A Nd:yttrium–aluminum–garnet laser with a wavelength of 532 nm was used to irradiate the lubricant film (1–2 nm) on carbon overcoat surface in ambient environment. The bonding formed was characterized at the molecular level using the time-of-flight secondary ion mass spectroscopy. We found that with increasing laser fluence, the ion yield of the end group CF2CH2OH of the lubricant decreases which can be an indication of chemical bond formation of the end group with the active sites of the carbon underlying layer. X-ray photoelectron spectroscopy reveals more lubricant on the laser-irradiated area than the surrounding nonirradiated area after washing off nonbonded lubricant by solvent Vertrel XF, which has little effect on bonded lubricant. Furthermore, we propose a method to quantify the bonding/nonbonding ratio of lubricant by considering the end-group intensity versus its neighboring fluorocarbon fragments.

Spectral characterization of perfluoropolyethers lubricant irradiated by laser light

We have investigated the molecular structure change of laser irradiated perfluoropolyether lubricant on disk media by using mass spectroscopy. A KrF excimer laser with a wavelength of 248 nm was used to irradiate the lubricant film (2 nm) on carbon overcoat surface. We found that two unique mass peaks emerged from the laser irradiated lubricant mass spectra, which are attributed to OH–CHF–O(:H +)–CHF–OH and F–COO–CF 2–CF 2–CF 2–CF 2 +, respectively. With laser fluence increases up to 100 mJ/cm 2, these two unique mass peaks start enhancing drastically while the abundance of the end-group of the lubricant, CF 2CH 2OH, remains almost constant, indicating that the two unique species are generated from the scission and re-arrangement of the backbone ether chains. With laser fluence beyond 140 mJ/cm 2, the disk media surface damage is observable and the lubricant evaporation is the main pathway.

Pulsed UV-laser-induced chemistry of perfluoropolyether lubricant film

We investigate the molecular structure change of laser-irradiated perfluoropolyether lubricant on disk media by using mass spectroscopy. A KrF excimer laser with a wavelength of 248 nm was used to irradiate the lubricant film (2 nm) on a carbon overcoat surface. We found that two unique mass peaks emerged from the laser-irradiated lubricant mass spectra, which are attributed to OH-CHF-O(:H+)-CHF-OH and F-COO-CF2-CF2-CF2-CF2+ respectively. When thelaser fluence increases to 100 mJ/cm2, these two unique mass peaks start to be enhanced drastically while the abundance of the end group of the lubricant, CF2CH2OH, remains almost constant, indicating that the two unique species are generated from the scission and re-arrangement of the backbone ether chains. With a laser fluence beyond 140 mJ/cm2, the disk media surface damage is observable and lubricant evaporation is the main pathway.

Mobility of Z-Dol lubricant thin film on carbon overcoat surface

We simulate the slider/disk interaction induced Zdol lubricant depletion and flow-back dynamics by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Experimental results show that the lubricant flow rate is inversely proportional to original lubricant thickness. The flow rate on partially removed lubricant layer is much higher than that on lubricant depleted carbon surface. These could be attributed to the increase in disjoining pressure gradient with aid of ion beam sputtering on lubricant surfaces.

Design of Carbon Surface Functional Groups on the Viewpoint of Lubricant Layer Structure

Chemical modification technique with X-ray photoelectron spectroscopy has been used for a quantitative analysis of surface functional groups of the carbon overcoats. As the nitrogen content in the carbon overcoat layer increases, the total abundance of functional groups on the carbon overcoat surface increases. The increase of the functional groups leads to an increase of a physisorbed lubricant layer ratio. Increase of the physisorbed layer ratio on the carbon overcoat surface showed a higher durability on the head/disk interface, and decreased the spin-off of the lubricant on a high rotation of the disk. The increase of the physisorbed layer leads to a higher recovery of the lubricant defect by the wear, and the increase of functional groups on the carbon overcoat surface leads to a stronger interaction of the lubricant with the carbon overcoat surface is estimated. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 1–4, 2000

Characterization of lubricant films on magnetic recording disks by ToF-SIMS

The structural characterization of Z-DOL (Fomblin Z), which is currently the lubricant of choice for magnetic recording disks, as films of variable thickness on carbon-overcoated disks has been studied by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The relationship between the peak intensity of dominant fragment ions and the lubricant thickness was carefully examined on the positive mass spectra. The ratio of CF 3 + /C 2 F 5 + in intensity can be used as an index of the lubricant thickness. The monomer ratio between ethylene and methylene units of the Fomblin backbone can be extracted from the plot of normalized intensity of C 2 F 4 O + against the CF 3 + /C 2 F 5 + ratio. The chemical bondability of the lubricant to the carbon overcoat surface can be examined through the plot of normalized intensity of CF 2 CH 2 OH + against the CF 3 + /C 2 F 5 + ratio.

AFM observation of hard disk lubricant coated with plasma polymerized carbon

By using the plasma-polymerized-carbon replication method in combination with an AFM scanning, surface morphology of liquid lubricant was clearly observed on laser bump's surface of the hard disk coated with carbon overcoat. The carbon overcoat surface is covered with adsorbed lubricant uniformly. Clusters of liquid lubricant were also observed dispersively on the surface. After the contact-start-stop operations, accumulated lubricant was observed on the edge of laser textured bumps. The head surface was clean after the CSS test. It is suggested that the edge of bump rim has some head cleaning effect. Clusters of lubricant were also observed on worn areas. It means that clusters of lubricant were transferred to the head and then adhered again to the worn area.