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.     

Contributions of Mobile and Bonded Molecules to Dynamic Friction of Nanometer-Thick Perfluoropolyether Films Coated on Magnetic Disk Surfaces

To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.     

Analysis of PFPE lubricating film in NEMS application via molecular dynamics simulation

Interfacial lubrication plays an important role in the functional performance of nanoelectrome-chanical (NEMS) systems. Here, we used molecular dynamics simulation to analyze the lubricating effect of a perfluoropolyether (PFPE) film to reveal the mechanism behind our experimental observations and understand the performance of the film. There was good agreement in the trends of the coefficients of friction between our simulation results and experimental characterizations. By studying the atomic motion, interfacial mechanics and polymer chain deformation, we found that PFPE films provide good lubrication because their linear flowability promotes surface reconstruction. Our simulations suggest that a high performance lubricant film needs to have low resistance to shear deformation, possess high linear flowability, promote surface reconstruction and adhere effectively to the substrates.     

Friction reduction in low-load hydrodynamic lubrication with a hydrophobic surface

A novel tribometer capable of measuring low friction forces and low loads at high speeds has been employed to measure the friction coefficient in a pure sliding, ball-on-flat contact in hydrodynamic lubrication conditions. The tribometer was custom-built for measuring friction at low loads, to allow the authors to investigate the feasibility of using the liquid-slip phenomenon for the lubrication of high-sliding MEMS. The theory behind lubrication with liquid slip and its effect on friction is briefly discussed. Contacting surfaces were treated to create hydrophobic/hydrophilic or hydrophilic/hydrophilic pairs. Hydrophobic surfaces were made by coating mica with a self-assembled silane monolayer while the hydrophilic surfaces used were freshly cleaved mica and plasma-cleaned steel. Experiments were conducted at sliding speeds of up to 2 m/s and loads below 0.2 N. An aqueous glycerol solution was used as lubricant. Results obtained with hydrophilic/hydrophilic surfaces were in accord with hydrodynamic lubrication theory. Tests with hydrophobic/hydrophilic surfaces revealed a reduction in friction, which may be attributed to lubricant slip against the hydrophobic surface.     

Molecular Dynamics of Thin Lubricant Films on Sol-Gel SiO2 Surfaces for Thin Film Magnetic Disks

Mobility of molecularly thin lubricant film is an important issue in understanding boundary lubrication mechanisms and to develop reliable magnetic disk media. Intra-molecular mobility for a perfluorinated poly ether (PFPE), which is used as a disk lubricant, with two hydroxyl groups on a sol-gel SiO₂ surface, which is used for a protective overcoat for plated magnetic disks, was studied using nuclear magnetic resonance (NMR). Thin film viscosities for molecular segments were derived from a relaxation time. The viscosity for the hydroxyl segment is 1.8 to 11 times as much as that for a bulk lubricant at room temperature, and the viscosity rate increased with increasing temperature. For example, it increased 15 times at 100°C. The viscosities for the segments in a main chain were not different from that of bulk PFPE.

A spin-off calculation for the molecularly thin lubricant film with thin film viscosity, derived from the NMR method, shows that there is no thickness decrease after seven years.     

Study on the cyclotriphosphazene film on magnetic head surface

Stable lubrication is very important to the slider/disk interface with the increasing demand on the life of computer hard disk drive (HDD). The inert lubricant perfluoropolyether (PFPE) on the surface of magnetic hard disk is still prone to be catalyzed to decomposition by the slider material Al₂O₃. The properties of a partial fluorinated hexaphenoxy cyclotriphosphazene, X-1P, are investigated and its function to reduce the catalytic decomposition of PFPE is discussed. The results of contact start–stop (CSS) tester indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and its film thickness has a great influence on the lubrication properties of the HDD.     

Laplace pressure measurement on laser textured thin-film disk

To increase the recording density of hard disk drives (HDD), head and disk surfaces must be very flat. This will make the friction between them large when liquid bridges are formed. This is a result of Laplace pressure in the liquid bridge. Therefore, the study of Laplace pressure in real HDD interface is of an interest for head-disk interface engineers. However, Laplace pressure of perfluoropolyether (PFPE) lubricant on carbon coated thin-film disk surface was not clear until now.We measured Laplace pressure between transparent flat pins and carbon coated thin-film disks with laser texturing. Using laser textured disks, we could control the distance between two surfaces precisely by the bump height. The friction coefficient between the pin and the disk surfaces was determined when the interface was fully wet by liquids. It was 0.16 and 0.1 for water and a PFPE lubricant. The Laplace pressure was then calculated using the friction force and liquid wet area when the interface was partially wet by a liquid. The liquid wet area was measured by the observation of the contact point through the transparent pins.The results showed that the Laplace pressure at the lowest bump height (11 nm) was about 2.8 MPa for the PFPE lubricant. Results agreed well with calculated curves. We consider that PFPE acts as liquid down to 11 nm.     

Conformation and Fundamental Properties of Novel Lubricant TA-30 for Near Contact Magnetic Recording

A novel perfluoropolyether (PFPE) lubricant called TA-30 has been developed recently. We investigate the conformation of TA-30 on diamond-like carbon (DLC) thin films, by attempting the direct observation of a lubricant film by atomic force microscopy (AFM) using a fluoride probe. We investigate the fundamental properties of a TA-30 lubricant film, such as its spreading characteristics, and the film thickness dependence of surface energy. Considering these experimental results, we conclude that the conformation of TA-30 is considerably different from that of conventional Z-tetraol2000 whose molecular height is 1.7 nm and which was adsorbed on the DLC surface with the random coil. The TA-30 molecules are adsorbed rigidly to the DLC surface with double layers. The thickness of the first TA-30 layer is ~0.9 nm (similar to diameter of the PFPE backbone) and that of the second layer from the DLC surface is 1.4 nm. Since TA-30 has a lower film thickness than Z-tetraol2000 on the DLC surface, it can have two layers, even if the film thickness is approximately of the order of 1 nm, whereas Z-tetraol2000 does not cover the DLC surface and does not form the complete first layer. In addition, we conduct slider touchdown and takeoff hysteresis tests by using TA-30 and Z-tetraol2000. It is confirmed that the use of TA-30 can improve the head–disk interface (HDI) reliability at low-fly-height conditions.     

Interfacial interactions of perfluoropolyether lubricants with magnetic recording media

Perfluoropolyethers (PFPE) are low surface tension liquids that are commonly employed in magnetic recording devices (hard-disk drives)as disk lubricants. In current drives, a single monolayer (or less) of a PFPE is applied to the amorphous carbon overcoat of the hard disk to provide the necessary lubrication of the head-disk-interface. The focus of the current paper is to demonstrate the utility of surface energy measurements in extracting information on the PFPE lubricant-carbon interfacial interactions. In particular, surface energies are reported as a function of applied lubricant thickness in the range of 2--30 Å for three Fomblin Zlubricants, i.e., ZDOL, ZDIAC, Z-15; and two Demnum lubricants,i.e. Demnum SA and SP. We show that from the surface energy measurements one can: (a) determine the extent of lubricant coverage of the carbon surface, (b) determine the orientation of the lubricant with respect to the carbon surface, (c) determine the nature of the lubricant-carbon interaction, e.g. attractivevs. repulsive, and (d) obtain an estimate of the interaction strength between the lubricant and the carbon.     

Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM

Molecularly thick perfluoropolyether (PFPE) films are considered to be good protective films for micro/nanoelectromechanical systems (MEMS/NEMS) to reduce stiction, friction, and improve their durability. Understanding the nanotribological performance and mechanisms of these films are quite important for efficient lubrication for MEMS/NEMS devices. These devices are used in various operating environments and their effect on friction, adhesion and durability needs to be clarified. For this purpose, mobile and chemically bonded PFPE films were deposited by dip coating technique. The friction and adhesion properties of these films were characterized by atomic force microscopy (AFM). The effect of rest time, velocity, relative humidity, and temperature on nanotribological properties of these films was studied. Durability of these films was also measured by repeated cycling tests. The adhesion, friction mechanisms of PFPE at molecular scale, and the mechanisms of the effect of operating environment and durability are subject of this paper. This study found that adsorption of water, formation of meniscus and its change during sliding, viscosity, and surface chemistry properties play a big role on the friction, adhesion, and durability of the lubricant films.     

Additive Mechanism Investigation of Perfluoropolyether Lubricants with a Phosphazene Additive

The tribological characteristics of magnetic thin film media coated with perfluoropolyether (PFPE) lubricants (ZDOL and AM300J) and a phosphazene additive (X-IP) were investigated in this study. The drag test results show that under ambient and hot/wet conditions the media coated with AM300J lubricant have higher retention on the test track than those coated with ZDOL 2000 PFPE lubricant. The phosphazene additive X-IP was observed to strongly anchored to the surface and was not as easily removed as PFPE lubricants alone. The retention characteristics of X-IP are independent of either AM or ZDOL. Secondary Ion Mass Spectroscopy (SIMS) depth profile data and Angle-Resolved X-Ray Photo-electron Spectroscopy (XPS) reveal that X-IP molecules were distributed near the disk surface in the X-IP and PFPE lubricants mixed layer, indicating a strong bonding/adhesion of X-IP to the disk surface. Together with the drag testing data, the authors conclude that the preferential distribution of X-IP close to the disk surface in the mixed layer helps to improve lubricant retention performance at the head-disk interface.     

The Effect of Carbon Overcoat Thickness on the Zdol Boundary Lubricant Film

Formation of a tribologically reliable interface between the read-write head and the computer disk in hard-disk drives is accomplished by the use of a thin, wear-resistant carbon overcoat in conjunction with a molecularly-thin perfluoropolyether (PFPE) lubricant film. The intermolecular interactions that develop between the PFPE lubricant and the carbon overcoat govern the adhesion, coverage, and physical properties of the lubricant, e.g. the lubricant structure and mobility. Consequently, the molecular interactions at the lubricant-carbon interface will contribute to the overall tribological performance of the disk-drive. Due to the ever-increasing demands for storage capacity, pressure exists to reduce the separation distance between the read-write head and disk surface. One means of reducing this separation distance is to use thinner protective overcoats on both the head and disk surfaces. In this study the interactions between Fomblin Zdol and both amorphous hydrogenated (CHx) and nitrogenated (CNx) carbon overcoats were investigated as a function of overcoat thickness from 0 to 100Å. The Zdol film structure was probed by titrating the magnetic alloy, the CHx and CNx surfaces with Zdol. The molecular weight dependence of the maximum bonded Zdol thickness on these surfaces is used to deduce structural information on the adsorbed Zdol film. In progressing from CHx to CNx to the magnetic alloy, we find the Zdol boundary layer film to be characterized by an increase in average distance between the PFPE backbone and the surface, or equivalently an increase in the average Zdol monolayer thickness. On the CHx overcoat, Zdol preferentially lies more parallel to the surface, whereas on the magnetic layer, Zdol is oriented more perpendicular to the surface. When these experiments were conducted as a function of carbon overcoat thickness, we found that interaction of Zdol with the field of the underlying magnetic film becomes important at carbon film thicknesses 30Å. The dependence of the Zdol adhesion on carbon overcoat thickness was quantified by determining the Zdol film thickness dependence of both the dispersive and polar components of the Helmholtz free energy. The Zdol bonding kinetics were also studied as a function of carbon thickness.     

Chemical analysis of wear tracks on magnetic disks by TOF-SIMS

Surface reactions on magnetic recording disks have been studied during sliding with ceramic sliders in the main chamber of TOF-SIMS. Chemical change of lubricant oil in the wear track was observed by the chemical image of TOF-SIMS. The magnetic disk surface was covered with perfluoroalkyl polyether lubricant (Fomblin Zdol). The Si tip slider surface was covered with Al₂O₃, DLC, TiN or c-BN coating. Experimental conditions were as follows: 0.8 mN of load and a sliding speed of 0.01 m/s. Lubricant oils were decomposed with Al₂O₃ and TiN slider surfaces. Metal (Al, Ti) fluorides were detected by TOF-SIMS in the sliding track. Material transfer occurred by chemical wear of slider material. From TOF-SIMS observation, the decomposition of lubricant molecules was initiated at the end group of molecules (-CF₂CH₂OH). On the other hand, DLC and c-BN sliders suppressed the decomposition reaction of PFPE oils. In conclusion, hard and chemical inert materials such as DLC and c-BN are suitable for a long-life HDI.     

Wear durability study on self-lubricating SU-8 composites with perfluoropolyther,multiply-alkylated cyclopentane and base oil as the fillers

Though SU-8 has become a useful material for micro-fabrication of MEMS/NEMS components using the micro-fabrication route, its poor tribological properties limit its wider applications. From our previous study [1], it was observed that adding PFPE lubricant to SU-8 possibly promoted chemical reaction between the molecules and helped in the boundary lubrication enhancing the wear durability of SU-8 by more than four orders of magnitude. For further investigation, another two different lubricants, a base oil and a multiply-alkylated cyclopentane (MAC) oil, were also added to SU-8. Both lubricants are hydrocarbons, chemically inert and have no polar reactive terminal groups unlike PFPE which has –OH polar terminal groups. SU-8+PFPE composite exhibited higher wear life than all SU-8 composites at all wt% of the lubricant content. Proper dispersion and possible chemical bonding of PFPE molecules with SU-8 are responsible for the superior tribological properties of SU-8+PFPE composite when compared with other SU-8 composites.     

Bonding Mechanism of Perfluoropolyether Lubricant Film with Functional Endgroup on Magnetic Disks by Ultraviolet Irradiation

We studied the bonding mechanism of ultrathin perfluoropolyether (PFPE) lubricant (Fombline Z-tetraol and Moresco D-4OH) films with hydroxyl end groups by measuring the bonding film thickness after ultraviolet (UV) irradiation. Nonfunctional PFPE lubricants (Z-03 and D2 N) were compared to two types of functional PFPE lubricants. The bonded thickness of both functional lubricants increased after a short period of UV irradiation, whereas that of the nonfunctional lubricants did not increase after the same treatment. This result suggests the occurrence of three kinds of mechanisms. First, Z-tetraol and D-4OH bond because of the photodissociation of the end groups by the UV light. Second, they bond because of the interaction between the end groups and the photoelectron from the carbon surface generated by UV irradiation. Third, they bond because of the photodissociation of the main chain by the UV light. In contrast, the dynamic reaction coordinate calculations suggest that the end groups in the PFPE lubricant dissociate because of the electron capture by the lubricant. As a result, we infer that the bonding of PFPE lubricant films with hydroxyl end groups on magnetic disks occurs by selective dissociation of the end groups because of UV irradiation.     

Self-assembled monolayer on diamond-like carbon surface: formation and friction measurements

We have investigated self-assembled monolayers (SAMs) of heptadecafluoro-1,1,2,2-tetradecyltrietoxysilane (FTE) on diamond-like carbon (DLC) surfaces formed by a simple immersing process. SAM formation on DLC surfaces was verified by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy (XPS). Water and hexadecane contact angles increased gradually with immersing time and saturated at about 110 and 70 degrees, respectively. Ellipsometric measurements showed that the film thickness was 1.4 to 1.6 nm, which corresponded reasonably to the thickness of FTE monolayer. XPS data showed the presence of FTE molecules on the DLC surface. These results ensured the SAM formation of FTE molecules on the DLC surface.We further measured and compared the friction of unlubricated, SAM coated and 2 nm thick perfluoropolyether (PFPE) coated DLC surfaces using lateral force microscopy (LFM) as functions of the applied load and the sliding velocity. The SAM coated DLC surfaces showed lower friction than the unlubricated DLC surfaces and the friction coefficient decreased by about 15% compared to the unlubricated DLC surfaces. Scratch tests revealed that the critical load of the DLC film increased due to the SAM deposition. These results are attributed to the hydrophobic nature of the SAM coated surface. On the other hand, even though the water contact angle of the SAM coated surface was larger than the 2 nm thick PFPE coated surface, the friction of the SAM coated surface was larger than that of the PFPE coated surface. Also, the critical load of the SAM coated DLC surface in scratch test was lower than the PFPE coated surface. These results indicate that the hydrophobic nature of the surface is not the only factor which determines the friction characteristics in the nano-lubricating system, and it is attributed to the mobile characteristic of PFPE lubricant.     

Tribological evaluation and analysis of the head/disk interface with perfluoropolyether and X1-P phosphazene mixed lubricants

The retention characteristics of magnetic thin film media coated with perfluoropolyether (PFPE) lubricants and a phosphazene additive, X-1P, were investigated in this study. The retention performance was evaluated by a drag test with a waffle head sliding against the disk that was designed to mechanically wear out the lubricant layer. An IR beam was aligned on the test track to directly measure the amount of PFPE lubricants and X-1P left on the media surfaces for determining the retention characteristics of the lubricants. The drag test results show that under ambient and hot/wet conditions the media coated with AM3001 PFPE lubricant have higher retention ratio on the test track than those coated with ZDOL 2000 PFPE lubricant. The phosphazene additive X-1P was observed to strongly anchor on the surface and not easily removed as PFPE lubricants (ZDOL and AM3001). The retention characteristics of X-1P are independent of lube combination, either AM or ZDOL lubricants. It is demonstrated that X1-P exhibits a good antiwear property and excellent retention performance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Autophobic Dewetting of Perfluoropolyether Films on Amorphous-Nitrogenated Carbon Surfaces

The dewetting of perfluoropolyether (PFPE) films on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image perfluoropolyether films on CNx-overcoated magnetic recording disks. An autophobic dewetting transition is observed to result when the PFPE film thickness applied to the disk surface exceeds a critical value. This critical dewetting thickness is linearly dependent on the PFPE molecular weight. Addition of the phosphazine, X-1P, to the PFPE film reduces the critical dewetting thickness compared to that of the neat lubricant. Dewetting in these molecularly-thin PFPE lubricant films is shown to occur at thicknesses where the total disjoining pressure is negative. The impact of this autophobic dewetting on the performance of a head--disk interface is inferred from take-off height measurements conducted as a function of PFPE film thickness. A steep reduction in the slider--disk clearance is observed when the PFPE film is present at thicknesses in excess of the critical dewetting thickness.     

Tribological performance of PFPE and X-1P lubricants at head–disk interface. Part II. Mechanisms

This paper, with the concepts of hydrogen bonding interaction and tribo-emission, develops a new approach of the mechanism of perfluoropolyether (PFPE) lubricant degradation at the head–disk interface. The role of lubricant X-1P in tribological performance is also described. The mechanism is as follows: (1) at the interface, there exist hydrogen atoms with partial positive charge and oxygen atoms with partial negative charge; (2) hydrogen bonding interactions at the sliding interface result in high friction which depletes the lubricant film at some sites; (3) low energy electrons are emitted from the sites with solid–solid asperity contact, inducing C–O bond scission through the interaction of low-energy electrons with PFPE lubricant molecules. Carbon overcoat on Al₂O₃–TiC surface passivates the interaction between water and PFPE lubricant molecules. Hydrogen bonding interactions are minimized during the presence of lubricant X-1P. The new approach well explains experimental results in part I of the paper. This revised version was published online in July 2006 with corrections to the Cover Date.