PFPE Lubricant Research Articles

Electric Field Assisted Dip-coating Process of Ultra-thin PFPE Lubricant Film on Magnetic Disks

Electric Field Assisted Dip-coating Process of Ultra-thin PFPE Lubricant Film on Magnetic Disks

413 磁気ディスクにおけるPFPE潤滑剤を塗布した垂直配向CNT膜のトライボロジー特性(GS-11 トライボロジー(1))

413 磁気ディスクにおけるPFPE潤滑剤を塗布した垂直配向CNT膜のトライボロジー特性(GS-11 トライボロジー(1))

Conformation of Ultrathin PFPE Lubricants With Different Structure on Magnetic Disks—Direct Observation and MD Simulation

Three types of PFPE lubricants (Z-tetraol, TA-30, and D-4OH) with different molecular structures were coated on magnetic disks and the conformations of these films were studied by atomic force microscopy using a fluoride-coated probe. The molecular conformations of these lubricant films were found to be different from each other. The Z-tetraol film showed the highest molecular height and lowest coverage among these lubricants. The TA-30 film showed a double-layer structure and the highest coverage among the lubricant films. The D-4OH film showed a monolayer structure and higher coverage than the Z-tetraol film. These observed were in agreement with the results of molecular dynamics simulations. In these simulations, the TA-30 film showed the highest coverage and lowest molecular height among the three lubricant films because the TA-30 molecules did not form a random coil shape and were adsorbed rigidly on the disk surface. The Z-tetraol molecules were adsorbed and formed a random coil shape. The D-4OH molecules were also adsorbed and formed a spiral shape.

Advanced Head–Disk Interface System Assisted by Gas PFPE Lubrication

Advanced head-disk interface (HDI) system assisted by gas perfluoropolyether (PFPE) lubrication is discussed in this paper. It is known that the lubricant spins off under the high rotation speed and high temperature. The lubricant redistribution after long-term head seeking with near-contact flying was studied. The spinoff and the redistribution of lubricant will be serious issues for the reliability of the hard disk drives. Our proposed HDI system shows high performance without the lubricant spinoff and redistribution. This is achieved by dripping only a small amount of lubricant on the suspension. This lubricant evaporates under high temperature and is adsorbed on to the disk surface. As a result, the spinoff, the redistribution, and the head contamination were significantly reduced. The lubricant on a magnetic disk could be maintained for a long time. The amount of this lubricant was optimized to maintain the constant thickness thorough the balance of the supplying and the decreasing. It was confirmed that the lubricant on the disks in this new HDI system maintained for about 10 000 hours at 70degC.

Electric-Field-Assisted Dip Coating Process of Ultrathin PFPE Lubricant Film for Magnetic Disks

In this paper, the influence of the electric field on the dip-coating process was studied by using lubricants Z-tetraol and TA-30. The electric field assisted the adsorption of both lubricant molecules on the disk surface. This electric field also resulted in an increase in the lubricant film thickness. A positive electric field increased the bonded ratio of the lubricant film. The surface-free energy, step height, and head-disk clearance of the Z-tetraol lubricant film were not affected by the electric field; however, those of the TA-30 lubricant film were improved by the electric field. By molecular dynamics simulation, we confirmed that the molecular mobility of the solution increased by applying the electric field. The lubricant molecules in the solution, which show higher mobility, should have more opportunity to adsorb onto a disk surface. These results show the possibility of reducing the head flying height by the electric-field-assisted dip (EFAD) coating process.

Direct Observation of PFPE Lubricants on Magnetic Disks

Perfluoropolyether lubricant films on magnetic disks were observed by means of atomic force microscopy (AFM) with a fluoride-coated probe. The lubricant films could be observed directly because the adhesion force was decreased by the anti-wetting effect of the fluoride coating on the probes. We compared the molecular conformation of Z-dol (Mw: 2000, 3000, 6000, 8000) with that of Z-tetraol. Thus, we found that the film coverage of a lubricant with high molecular weight exhibited varying (i.e., higher and lower) step heights of the lubricant film boundary. The Z-tetraol film exhibited higher coverage and the higher step height as compared to Z-dol2000. To further study the step height measured by means of atomic force microscopy, we compared the step height with the shoulder height of the spreading profiles of the films. The step heights were almost equal to the shoulder height. We observed the lubricant film after shearing by using contact scanning mode atomic force microscopy. The coverage of the sheared lubricant film increased higher than the initial coverage before shearing. Subsequently, the lubricant molecular conformation was varied due to the shearing, and the coverage was increased.

The Role of Configuration Entropy in Boundary Lubricants Based on the n-Perfluoropropylene Oxide Main Chain Structure

We have investigated the resistance of a novel end-functionalized perfluoropolyether (PFPE) lubricant film to slider–disk interactions caused by low-flying sliders. The PFPE lubricant is based on the CF2CF2CF2O main chain monomer unit. Both slider–disk interactions and the formation of lubricant moguls are significantly reduced compared to the Fomblin Z backbone, (CF2O) p –(CF2CF2O) q . These results are interpreted on the basis of ab initio quantum chemical computations that show that the barrier to internal rotation about the C–O bond in the CF2CF2CF2O monomer unit is significantly larger than in the CF2O monomer unit that is bordered by another CF2O monomer unit, ~8 kcal/mol compared to <2 kcal/mol, respectively. It is proposed that main chains containing CF2O monomer units will be very flexible and hence their physical properties will be more sensitive to adhesive and cohesive interactions, while main chains containing CF2CF2O and CF2CF2CF2O monomer units will be comparatively stiffer and hence their physical properties will be less sensitive to adhesive and cohesive interactions.

Effects of Self-Assembled Monolayer and PFPE Lubricant on Wear Characteristics of Flat Silicon Tips

The effects of self-assembled monolayer (SAM) and perfluoropolyether (PFPE) lubricant on the wear characteristics of flat silicon tips were investigated. The wear test consisted of sliding the silicon tips fabricated on a flat silicon specimen against SAM and PFPE (Z-tetraol) coated silicon (100) wafer. The tips were slid at a low speed for about 15 km under an applied load of 39.2 μN. The wear volume of the tip was obtained by measuring the tip profile using an Atomic Force Microscope (AFM). It was found that the coatings were effective in reducing the wear of the tips by an order of magnitude from 10−6 to 10−7.

Reversed Micelle Structures in PFPE Lubricant Thin Films on Magnetic Disk Surface Observed by Non-Contact AFM

PFPE lubricants (Fomblin Z-dol) for hard disk surface lubrication have two hydroxyl groups, one at each end of the molecules, and form stable insoluble monolayers at the water surface. In this study, molecular weight-fractionated PFPE lubricant monolayers were transferred from the water surface to solid substrates such as a hydrophilized silicon wafer, gold-sputtered mica, and a hard disk after adjusting the two-dimensional density of the lubricant molecules. The molecular structures of the PFPE lubricant molecules at the solid surfaces were observed by the cryogenic non-contact AFM under ultra-high vacuum. At the hydrophilic silicon wafer surface we could observe a single lubricant molecule in a random coil sphere shape. However, at the non-polar gold surface we confirmed the formation of reversed micelle structures. At the hard disk surface we detected various sizes of reversed micelles of PFPE lubricant in a flat oval shape.

Grafting Lubricant Molecular Chains to the Carbon Overcoat

It was conceived and demonstrated that irradiation of magnetic disks coated with PFPE (perfluoropolyether) lubricants terminated with a carboxylic group at one terminus with long-UV (254 nm) would lead to grafting, via a bona fide C–C chemical bond, of genuine PFPE molecular chains to the carbon overcoat all at one terminus with all the remaining chain segments being free to sway. The water contact angle of disks coated with PFPE lubricants terminated with end-groups having hydroxyl unit(s) (e.g., Z-dol and Z-tetraol) decreases gradually, after the initial contact of the droplet, reaching an asymptote in 20–30 s. The gradual temporal change is accounted for by shifting of the equilibrium disposition of hydroxyl units of the lubricant molecules from that determined by the interaction with the surface of the carbon overcoat to that determined by the interaction with surfaces of both the carbon overcoat and the liquid droplet. The water contact angle of disks prepared by the presently conceived photo-grafting method is high (>110 degrees) and shows no temporal change. In a preliminary spin-stand drag test, disks with PFPE chains photo-grafted by this method and also the heads (for the read/write process) with similar photo-grafted PFPE chains exhibited extraordinary durability.

Instability and surface diffusion of ultrathin liquid perfluoropolyether films on amorphous carbon surfaces

A study is performed on the instability and surface diffusion of ultrathin liquid films of perfluoropolyethers (PFPE) on carbon-coated discs. Several carbon-coated discs have been used from the same lot, surface roughness has been measured by atomic force microscope, and average surface energy has been measured by contact angle measurements. Then, some discs are partially dip coated, producing a spreading edge of the lubricant film, using Zdol of molecular weights of about 4000 and 2000 g/mol, and used in the measurement of the spreading profiles with time. Some other discs are fully dip coated, scratched by a friction tester, and then replenishment of grooves with time has been measured by ellipsometer (calibrated verbatim by X-ray photoelectron spectroscopy (XPS)). Mono-disperse PFPE lubricant films of film thicknesses of about 0.9–3 nm have been used. Diffusion coefficients have been extracted from the spreading profiles using the Matano interface method and used in the diffusion equation to simulate the replenishment profiles of the grooves for the same initial profiles for both simulation and experiment. The diffusion equation has been observed to simulate the replenishment profiles up to the instability plane, height which creates non-linearity in the experimental replenishment profiles above it and fails to model any further.

Head–disk interface problems in first-surface near-field optical recording with flying optical head

Abstract Near-field recording (NFR) technology using a flying optical head is applied for high density optical data storage. Diamond-like carbon (DLC) film and PFPE lubricant film are coated on utmost NFR disk to reduce the tribological problems. A contamination at the head–disk interface (HDI), especially on a solid immersion lens (SIL) is observed. The contaminants are composed of dust materials from the surroundings and lubricant materials from a disk surface. The contaminants are supposed to be condensed by heat from a writing laser beam at the HDI. To avoid the contamination problem, a cover-layer is coated and a focal plane moved under the cover-layer from the HDI. First-surface near-field recording configuration can be modified by laminating a cover-layer, which enhances the possibility of a practical application with a media removability function.

Surface potential and resistance measurements for detecting wear of chemically-bonded and unbonded molecularly-thick perfluoropolyether lubricant films using atomic force microscopy

The wear of perfluoropolyether (PFPE) lubricants applied on Si(100) and an Au film on Si(100) substrate at ultralow loads was investigated by using atomic force microscopy (AFM)-based surface potential and resistance measurements. Surface potential data is used in detecting lubricant removal and the initiation of wear on the silicon substrate. The surface potential change is attributed to the change in the work function of the silicon after wear, and electrostatic charge build-up of debris in the lubricant. It was found that coatings that are partially bonded, i.e., containing a mobile lubricant fraction, were better able to protect the silicon substrate from wear compared to the fully bonded coating. This enhanced protection is attributed to a lubricant replenishment mechanism. However, an untreated lubricant coating exhibited considerable wear as it contains a smaller amount of lubricant bonded to the substrate relative to the partially bonded and fully bonded coatings. A sample subjected to shear is shown to have improved wear resistance, and this enhancement is attributed to chain reorientation and alignment of the lubricant molecules. The detection of wear of PFPE lubricants on Au by an AFM-based resistance measurement method is demonstrated for the first time. This technique provides complementary information to surface potential data in detecting substrate exposure after wear and is a promising method for studying the wear of conducting films.

Stiffness and Damping of Thin PFPE Lubricant Bridging Between Magnetic Disk and Diamond Probe Tip

Spring constants and damping coefficients of a thin lubricant bridge of a perfluoropolyether (PFPE) lubricant intervening between a diamond probe tip and a diamond-like carbon (DLC) surface of a magnetic disk are identified through regression analysis of tip damping vibration. PFPE lubricants with functional end groups were used to form a lubricant bridge between the DLC surface and a probe tip with a notably small curvature radius of 0.1μm. The tip was both retracted from and extended toward the disk surface at four different progressive distances to attain varied elongation of the bridge. It was also vibrated at each step to provide damping waveforms. By applying regression analysis to the observed waveforms, the spring constant and the damping coefficient of the lubricant bridge were identified within an elongation range from 50nm to 800nm. Spring constant of the lubricant bridge kb had a negative value varying from −0.15N∕m to −0.1N∕m. The damping value expressed in the form of frequency-multiplied damping cb×ω ranged from 0.02N∕m to 0.06N∕m. Note that both the absolute value of spring constant ∣kb∣ and frequency-multiplied damping cb×ω exhibited U-shaped variation with lubricant bridge elongation; that is, those values decrease with bridge elongation and they begin to increase after reaching the minimum. The variation in the spring constant was found to be in good accordance with the quasi-static stiffness of the lubricant bridge, and variation in the damping coefficient was explained by energy loss arising in the vibrating lubricant bridge.

Modelling and simulation of the lubricant depletion process induced by laser heating inheat-assisted magnetic recording system

A model for lubricant flow dynamics under a scanning laser beam has been constructed fora heat-assisted magnetic recording system. The effects of evaporation, thermocapillarystress and thermoviscosity on the lubricant depletion on both glass and aluminium disksare systematically studied. Our results indicate that both evaporation and thermocapillarystress are able to deplete non-negligible amount of Z-type PFPE lubricants with a lowmolecular weight from the disk surface within nanoseconds. Evaporation is the dominantlubricant removal mechanism when low molecular weight lubricant is used. Evaporation issignificantly reduced when high molecular weight PFPE lubricants are used. Oursimulations show that the performance of head/disk interface of heat-assisted magneticrecording systems may be significantly deteriorated when lubricants with improperproperties are used.

Vacuum vapor deposition of PFPE molecules on CHxNy and CHxFy amorphous carbon surfaces

According to the demand of increasing storage density for the magnetic data storage, a contact recording system is proposed, in which the head constantly makes contact with the disk surface during read/write cycles. In this system a stronger lubricant film for head disk interface (HDI) is required. In a conventional dip-coating method, which is carried out under atmospheric conditions, however, a fresh diamond like carbon (DLC) surface adsorbs several contaminations from the air and a strong lubricant layer is difficult to form on the DLC surface. In this study, a DLC layer is coated on a Si substrate by a plasma chemical vapor deposition (PCVD) method, then the DLC-coated Si sample is transferred to another vacuum chamber without being exposed to the atmosphere, and a perfluoropolyether (PFPE) lubricant film is vapor-deposited on the sample. The sample is analyzed by XPS and compared to the surface on which the PFPE lubricant is dip-coated. It is noted that both samples are rinsed after the coatings and the loose PFPE molecules are washed away. A thicker bonded layer of PFPE is formed on the DLC surface treated by vapor deposition. A tribological wear test is also carried out using a pin-on-disk tribotester. Both results show that the tribological characteristics are improved by the vapor deposition of PFPE molecules compared to the dip-coating method.

Evaluations of tribological characteristics of PFPE lubricants on DLC surfaces of magnetic disks

This article presents measurements of adhesion and friction of perfluoropolyether (PFPE) lubricant films dip-coated on magnetic disks covered with diamond-like carbon (DLC) film. We have developed a custom-built pin-on-disk type micro-tribotester to perform the tribological measurements. The adhesion tests were performed by pulling down/up a 1.5-mm-diameter glass ball on a stationary disk surface, and the friction tests were carried out by sliding the glass ball on a rotating disk surface without changing head-disk interface conditions from the adhesion tests. Experiments were performed for the different kinds of 2- and 6-nm-thick PFPE lubricants (polar: Zdol4000 and Zdol2000; nonpolar: Z03) under lightly loaded and slow sliding conditions to minimize disturbance against the molecular layered structure. The adhesive forces were found to decrease with increasing film thickness in the order of Z03 > Zdol2000 > Zdol4000 (decreasing rate), which closely corresponds to the order of monolayer thickness, and then to saturate to almost the same calculated values. As for the friction forces of 2-nm-thick films, Zdol2000 featured extraordinarily large friction in comparison with Zdol4000 and Z03, while Zdol4000 was slightly larger than Z03. The largest friction of Zdol2000 reveals that the 2-nm-thick Zdol2000 formed a monolayer that served as an immobile layer. With the increase in film thickness, the friction force of Zdol2000 decreased, indicating that extra lubricant molecules served as a mobile layer, while that of Z03 remained unchanged as the lowest value. By extrapolating the loading force versus friction force relationship into a negative loading force region, it is found that the friction force of Z03 tended to zero at zero net load (loading force plus adhesion force), while those for Zdol2000 and Zdol4000 exhibited finite values, indicating formation of an immobile layer, which shows similar characteristics to those of adhesive rubber material. The dewetted surface is found to feature violently changing friction force only at the first stage of sliding, and it then becomes stable after several sliding passes.

Electron stimulated decomposition of fluorocarbons on amorphous hydrogenated carbon (a-CHx) overcoats used in data storage media

The electron-induced surface chemistry of perfluoropolyalkylether (PFPE) lubricants on a-CHx films has been probed by studying the impact of free electrons on perfluorodiethylether, (CF3CF2)2O, and 2,2,2-trifluoroethanol, CF3CH2OH, as models of the chemical functionality of PFPE lubricants such as Fomblin Zdol. Electron-stimulated decomposition of (CF3CF2)2O and CF3CH2OH on fresh and oxidized a-CHx is observed when the sample is unbiased and in the presence of 70 eV free electrons. Electron-induced decomposition is indicated by the deposition of fluorine onto the surface of the a-CHx film following desorption of molecular (CF3CF2)2O and CF3CH2OH by heating in front of a mass spectrometer. Biasing the sample to −80 V successfully eliminates the decomposition by preventing the impingement of electrons onto the surface. The electron-stimulated decomposition of PFPE lubricants may contribute to lubricant decomposition during normal drive operation.

3412 SPM用プローブを使用したナノ分子潤滑膜の伸び量と凝着力の測定(S70-2 情報機器コンピュータメカニクス(2),S70 情報機器コンピュータメカニクス)

Regarding the measurement of dynamic characteristics of the lubricant bridge formed between the diamond tip and the magnetic disk surface, the accuracy in identifying the interference fringe has improved by employing a minute glass mirror in place of the silicon plate currently used, and the elongation and the adhesive force of 2-nm-thick PFPE lubricant films with/without the polar end group for different molecular weights have been compared. It was revealed that the mobile molecule of lubricant contributed to a larger elongation of the lubricant bridge and that the fixed molecule of lubricant to a decreased adhesive force. Furthermore, the same experiments with varying the bonding ratio of the polar lubricant Zdol2000 were performed. It was confirmed that the elongation and the adhesive force tended to decrease with the increasing bonding ratio.

Viscosity Loss in PFPE Lubricant for Space Applications under EHL Conditions

An unbranched perfluoropolyether (PFPE) 815Z is the current ball bearing lubricant for space applications. Measurements of elastohydrodynamic lubrication (EHL) oil film thickness have been carried out to assess the lubricating performance of PFPE with average molecular weight of 9200 using an optical interferometric technique under mean Hertzian pressure 0.45 GPa. The film thickness of 815Z became less than predicted film thickness from Hamrock and Dowson formula for EHL central film thickness. There are two main explanations why PFPE is inferior to mineral oil in their ability to form EHL films, temporary viscosity loss and permanent viscosity loss. In order to elucidate the results, measurements of permanent viscosity loss under mean Hertzian pressure from 0.41 GPa to 2.67 GPa have been carried out using the thrust ball bearing. There results show that the degree of the permanent viscosity loss depends on Hertzian pressure, occurrence of permanent viscosity loss of 36 % with 2.67 GPa and 2 % with 0.41 GPa.