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Effect of Ultraviolet Irradiation on the Interactions between Perfluoropolyether Lubricant and Magnetic Disk Surfaces
Abstract
To tailor the characteristics of molecularly thin lubricant films, magnetic disk surfaces coated with nanometer-thick perfluoropolyether
AM3001 lubricant films were irradiated with 184.9 and 253.7nm ultraviolet (UV) rays. We elucidated the effect of UV irradiation
on the interactions between the lubricant and the magnetic disk surface via surface energy, bonded lubricant thickness and
lubricant spreading measurements for films with and without UV irradiation. We found that UV irradiation decreased the dispersive
and polar surface energies of the lubricant films by 20 and 80%, respectively; increased bonded lubricant thickness; and decelerated
lubricant spreading. These results indicated that dispersion and polar interactions between lubricant molecules and the magnetic
disk surface were strengthened by UV irradiation.
... The UV irradiation (185, 172 nm, N 2 ) of the piperonyl-functionalized (AM3001) perfluoropolyether (PFPE) and the non-functionalized Fomblin Z showed improved corrosion protection which was attributed to increased lubricant bonding and decreased surface energy [12]. The UV irradiation (185 nm, air) of AM3001 decreased both the dispersive and polar surface energies suggesting improved surface coverage [13]. Loss of structure (oscillation) in the polar surface energy as a function of film thickness was attributed to the increased polar interactions between the PFPE and the underlying carbon film. ...
... For the Zdol system, the magnitude of the oscillations was observed to decrease with increasing molecular weight or equivalently, with decreasing number of hydroxyl groups [20]. Similar observations of loss in film structure in the polar surface energy after UV irradiation have been reported by other workers [13,14]. ...
... Conversely, close scrutiny of the spreading front clearly shows the UV-irradiated Z-Tetraol has developed additional "structure" that flows at a higher rate near 8-9 Å film thickness. A similar thinner, rapidly spreading front was also observed in UV-irradiated AM3001 but was not explained [13]. A higher mobility could be associated with chains having lower molecular weight and/or lower polarity as a result of the UV irradiation. ...
The UV irradiation of Z-Tetraol films on amorphous nitrogenated carbon surfaces was investigated. COF2 evolution, captured in the gas phase, decreased with increasing number of OH end groups in the order: Z > Zdol > Z-Tetraol after UV irradiation. Both UV irradiation and annealing on 11 Å Z-Tetraol films produced virtually identical interfacial properties - surface energy as a function of bonded fraction. The UV-irradiated 11 Å Z-Tetraol films also demonstrated equilibrium behavior exhibiting bonding or debonding to the asymptotic 80% bonded level under ambient conditions. UV-irradiated thick films of Z-Tetraol (∼ 20-40 Å) exhibited film properties that differed from the non-irradiated films. The oscillations in the polar component of the surface energy, normally observed in Z-Tetraol films and attributed to amphiphilic structuring, disappeared. Terraced flow exhibited a rapidly moving front below ∼ 10 Å compared to the remainder of the irradiated film. Both observations were tentatively attributed to loss of OH functionality. The latter was verified by chemical extraction and subsequent analyses by NMR and TGA.
... However, moisture is known to weaken the bonding between PFPE end groups and SiN x surface as water molecules can also make hydrogen bond at preferential sites. Zhang et al. [15] had addressed the effect of UV irradiation on lubricant bonding with disk surface of tailor-made PFPE AM3001 lubricant films coated on magnetic hard disk. UV irradiation decreased the dispersive and polar surface energies by 20 and 80 %, respectively, and increased the bonding thickness. ...
... Upon UV curing, the Z-dol and Z-03 molecules tend to graft themselves with SU-8 both physically and chemically by corresponding chemical interactions of functional groups. Rinsing followed by sonication removes the physically adsorbed Z-dol and Z-03 molecules leaving behind the chemically modified SU-8 molecules [15,28]. The XPS study after rinsing and sonication of SU-8 composite films are carried out to understand the new insights into the nature of the surface and improved tribological performances due to chemical interactions. ...
... Z-03 in Figure S1 (see supporting information) have exhibited similar peaks with varied intensities depending on the amount of functional group present in Z-dol and Z-03. No noticeable difference in peaks and peak positions corresponding to the presence of additional functional groups [15,32,34,35]. Both physically and chemically adsorbed films were formed on the surfaces which were difficult to be differentiated by XPS analysis due to the limitation of polymeric film thicknesses. ...
In an earlier work, we demonstrated the development of SU-8 composites using perfluoropolyether (PFPE) as lubricant filler which reduced friction coefficient by ~7 times and enhanced wear life of SU-8 by more than four orders of magnitude. In this work, we have investigated the role of chemical bonding between SU-8 and PFPE molecules using two types of PFPE lubricants (i.e., Fomblin® Z-dol and Z-03) in improving the tribological properties of the composite. Z-dol has polar (–OH) end groups whereas Z-03 has non-polar (CF3) end groups. SU-8 with Z-dol (SU-8 + Z-dol) films yielded ~8 times greater wear life than SU-8 with Z-03 (SU-8 + Z-03) films and more by four orders of magnitude than pure SU-8. The nature of the films was analyzed in detail by chemical and physical characterization techniques like X-ray photoelectron spectroscopy, water contact angle and thermo-gravimetric analysis. The results validated the role of polar end functional group of Z-dol in covalent binding with SU-8 upon UV plasma treatment that resulted in improved tribological properties.
... When amorphous Zdol polymers are disorderly deposited onto plastic surfaces, as shown in Figure 3, the interchain distance is relatively large, thus leading to openings for hexadecane molecules to penetrate through the polymer film and "see" the plastic substrate, i.e., smaller HCA (∼40°) is observed. Once such surface is exposed to UV/ Ozone irradiation, Zdol's hydroxyl end groups tend to H-bond with the oxygen-containing polar groups (e.g., hydroxyl groups) created by UV/Ozone on plastic surfaces, 65,66 resulting in more ordered packing of Zdol chains, as illustrated in Figure 3. Consequently, the interchain distance decreases, inducing higher resistance to the penetration of hexadecane and thus larger HCA (>60°). Previously, we have shown that, on hydrophilic substrates (e.g., silica), Zdol forms a much more ordered structure, which results in simultaneous hydrophilicity/oleophobicity. ...
Hydrophilic/oleophobic surfaces are desirable in many applications including self-cleaning, antifogging, oil-water separation, etc. However, making plastic surfaces hydrophilic/oleophobic is challenging due to the intrinsic hydrophobicity/oleophilicity of plastics. Here, we report a simple and effective method of making plastics hydrophilic/oleophobic. Plastics, including poly (methyl methacrylate) (PMMA), polystyrene (PS), and polycarbonate (PC), have been coated with a perfluoropolyether (PFPE) (i.e., commercially known as Zdol) via dip coating and then irradiated with UV/Ozone. The contact angle measurements indicate that the treated plastics have a lower water contact angle (WCA) and higher hexadecane contact angle (HCA), i.e., they are simultaneously hydrophilic/oleophobic. The Fourier transform infrared (FTIR) results suggest that UV/Ozone treatment introduces oxygen-containing polar groups on the plastic surfaces, which renders the plastic surfaces hydrophilic. Meanwhile, more orderly packed PFPE Zdol molecules, which is due to the UV-induced bonding between PFPE Zdol and the plastic surface, result in the oleophobicity. Moreover, the simultaneous hydrophilicity/oleophobicity of functionalized plastics does not degrade in aging tests, and they have superior antifogging performance and detergent-free cleaning capability. This simple method developed here potentially can be applied to other plastics and has important implications in the functionalization of plastic surfaces.
... Damage due to UV irradiation, however, was minimal. The effects of UV irradiation on the interactions between the lubricant and the substrate surface have been investigated via surface energy, bonded lubricant thickness and lubricant spreading measurements, with the results indicating that UV irradiation decreases the fluidity and polar surface energy of the lubricant films and increases the thickness of lubricant films, suggesting that the interactions between lubricant molecules and the substrate surface were strengthened by UV irradiation [60,86]. Therefore, strong enough ultraviolet light could break part of the lubricant's molecular chain, resulting in the bond-breakage and chemical crosslinking reactions, whereas surface adhesion and coverage of lubricants on the substrates are improved by UV irradiation which could eliminate the effects of lubricant damage by UV irradiation on the tribological performance. ...
Despite continuous improvements in machine elements over the past few decades, lubrication issues have impeded human exploration of the universe because single solid or liquid lubrication systems have been unable to satisfy the ever-increasing performance requirements of space tribology. In this study, we present an overview of the development of carbon-based films as protective coatings, with reference to their high hardness, low friction, and chemical inertness, and with a particular focus on diamond-like carbon (DLC) films. We also discuss the design of carbon-based solid-liquid synergy lubricating coatings with regards to their physicochemical properties and tribological performance. Solid-liquid composite coatings are fabricated via spinning liquid lubricants on solid lubricating films. Such duplex lubricating coatings are considered the most ideal lubrication choice for moving mechanical systems in space as they can overcome the drawback of adhesion and cold-welding associated with solid films under harsh space conditions and can minimize the crosslinking or chain scission of liquid lubricants under space irradiation. State of the art carbon-based solid-liquid synergy lubricating systems therefore holds great promise for space applications due to solid/liquid synergies resulting in superior qualities including excellent friction reduction and anti-wear properties as well as strong anti-irradiation capacities, thereby meeting the requirements of high reliability, high precision, high efficiency, and long lifetime for space drive mechanisms.
... From quantum chemical calculations , Jones et al. (2011) revealed that fragmentation of functional PFPE after adding an electron can occur both at the main chain and the end groups, depending on the arrangement of the perfluoromethylene and perfluoroethylene repeat units in the main chain. Additionally, surface energy and film spreading measurements were performed to elucidate the effect of UV irradiation on the interactions between PFPEs and underlying carbon surface (Chen et al. 2008; Zhang et al. 2005). However, since previous studies were predominantly experimental, directly probing UV-induced changes is difficult at the molecular scale due to limited available experimental resolutions. ...
For improving the tribological performance of hard disk drives, nanometer-thick perfluoropolyether (PFPE) lubricant films are generally treated with ultraviolet (UV) irradiation to bond them to the carbon overcoats of the disks. By modeling UV irradiation as an electron emission and attachment process, we investigate the UV bonding of nonfunctional PFPE Z and functional PFPE Zdol to hydrogenated and nitrogenated carbon surfaces with quantum chemical methods. Our calculation results show that, upon electron attachment, Z dissociates at its main chain to two fragments terminated by CF2CF2 and CF2O groups, whereas Zdol dissociates to a hydrogen fluoride and a fragment. The perfluoromethoxy oxygen in one of the Z fragments and the carbon radical and the hydrogen-truncated end group in the Zdol fragment interact strongly with sp2 and oxidized sites on carbon surfaces. Imine moieties on the CNx surface also contribute considerably to the UV bonding of Zdol.
... There have been several reports on the bonding of PFPE lubricants to the carbon overcoat surface under UV irradiation [1][2][3][4][5][6][7][8][9][10][11][12][13] . Vurens et al. supposed that the bonding is caused by photoelectrons generated from the COC surface 2,3) . ...
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.
The characteristics of monolayer liquid lubricant films coated on magnetic disks are basically determined by their interactions with disk surfaces. If the strength of the interactions could be patterned by ultraviolet (UV) irradiation through a photomask, it would be possible to attain desired tribological performance by rational design of the pattern. In this research, we developed coarse-grained molecular dynamics simulations to investigate the effect of masked UV irradiation on the distribution of monolayer lubricant films coated over solid surfaces. Consistent with experimental results, the simulation results showed that masked UV irradiation induces molecular flow from non-irradiated toward irradiated areas, and thereby a patterned surface structure with thicker lubricant in the irradiated area than in the non-irradiated area. The lubricant films, however, remained monolayer even in the UV-irradiated area. Our simulation results demonstrated that, rather than stacking of lubricant molecules, the patterned surface structure is caused by the stretch of the lubricant molecules in the film thickness direction following the increase of molecule density in the irradiated area.
The simultaneously oleophobic/hydrophilic coatings are highly desirable in anti-fogging, oil-water separation and detergent-free cleaning. However, such coatings require sepcial chemcial structure, i.e., perflurinated backbone and polar end-groups, and are too expensive for real-life application. Here, we have developped an UV-based photochmeical approach to make nanometer-thick perfluoropolyethers without polar end-groups, which are not intrinsically simultaneously oleophobic/hydrophilic but cost-effective, become simultaneously oleophobic/hydrophilic. The contact angle, ellipsometry and X-ray photoelectron spectroscopy (XPS) results indicated that the UV irradiation results in the covalent bonding beween the polymer and the substrate, which renders more ordered packing of polymer chains and thus the appropriately small inter-chain distance. As a result, the small water molecules penetrate the polymer network while large oil molecules do not. As a result, the oil contact angle is larger than the water contact angle and the coating shows the simultaneous oleophobicity/hydrophilicity. Moreover, we also demonstrated that this nanometer-thick simultaneously oleophobic/hydrophilic coating has improved long-term anti-fogging performance and detergent-free cleaning capability and is mechanically robust. The photochemical approach established here potentially can be applied on many other polymers and greatly accelarate the development and application of simultaneously oleophobic/hydrophilic coatings.
Nanometer-thick liquid lubricant films are used for lubrication of miniaturized mechanical systems such as hard disk drives and microelectromechanical systems. In particular at the head-disk interface of hard disk drives, the lubricant thin film is coated on a diamond-like carbon surface, and the films are usually composed of both chemically and physically adsorbed lubricant molecules. The combination of these two types of molecules leads to high lubrication performance. However, the detailed mechanism is not fully understood. The aim of this study was to clarify the effect of chemically adsorbed lubricant molecules on the viscous friction of lubricant films. Two different samples were tested: One was the lubricant film composed of both chemically and physically adsorbed molecules, and the other was the lubricant film with only physically adsorbed molecules. We used a fiber wobbling method (FWM), which we developed in our previous study, to measure the viscous friction. The FWM uses a spherical-ended glass fiber as a shearing probe and enables friction forces in the order of 0.1 nN to be measured at precisely controlled nanometer-scale gap widths. For the fair comparison of friction forces between the samples with different compositions, the contact area between the probe tip and the lubricant film must be the same. To determine the contact area in the friction measurement, we improved the FWM system to enable simultaneous measurement of friction force and contact area. Experimental results showed the 1-nm-thick layer of chemically adsorbed molecules in the lubricant film effectively reduced the viscous friction.
To ensure the reliability and durability of hard disk drives, molecularly thin polymeric liquid films of perfluoropolyether (PFPE) coated on the disk surfaces possess a mixed structure consisting of molecules bonded to the disk surfaces and non-bonded mobile molecules. Aiming to clarify the influence of film composition (mobile/bonded) on tribological performance, we measured friction properties of two types of 2-nm-thick PFPE films (functional Zdol2000 and nonfunctional Z03) under lightly loaded (loading force: 0-1 mN) and quasi-static (low rotational speed: 2.1mm/s) conditions as a function of elapsed time. The friction force of Z03 remained unchanged with time and increased linearly with loading force as described by Amontons' law. In contrast, induced by the development of the molecules' bonding in time, the friction force of Zdol2000 increased and exhibited a nonlinear increase with loading force as time proceeded. The nonlinear friction-load relationship of Zdol2000 in the equilibrium state was well characterized by the Johnson-Kendall-Roberts model.
To create functionalized tribological surfaces utilizing the effect of nano-textures, we have proposed a method in which nanometer-thick perfluoropolyether lubricant films coated on solid surfaces were irradiated with 172 nm ultraviolet (UV) rays through a photo mask in nitrogen atmosphere. By measuring the thickness profiles of the lubricant films at the boundary of non-UV-irradiated and UV-irradiated areas, we found that lubricant molecules in the non-UV-irradiated area spread toward the UV-irradiated area with time and stable concave-convex film structures are obtained at the equilibrium state. We experimentally demonstrated that desirable three-dimensional lubricant textures can be formed using this UV-induced lubricant spreading (lubricant redistribution) phenomenon. The optimum conditions for forming lubricant textures are also discussed.
To investigate the effects of ultraviolet (UV) irradiation on perfluoropolyether (PFPE) lubricants with identical main chains but different end groups, three types of PFPEs (UV sensitive AM3001, less UV sensitive but functional Zdol2000, and less UV sensitive and nonfunctional Z03) coated on magnetic disk surfaces at nanometer thickness were irradiated by 172 nm UV rays, and bonded lubricant thickness, surface energy, and lubricant spreading measurements were subsequently conducted. Over the irradiation time range investigated, Zdol2000 showed a linear increase in bonded thickness, whereas AM3001 and Z03 showed a saturating increase and the saturating bonded thickness matched the gyration diameter (2.8 nm) of bulk AM3001 and the cross-sectional diameter (0.7 nm) of Z03 molecules, respectively. For surface energy, except for Z03, the polar component decreased significantly with UV irradiation compared to the dispersive component, suggesting the preferential effects of UV irradiation on the functional end groups rather than the main chains. Spreading of AM3001 to bare disk surface was drastically decelerated by UV irradiation ; for Zdol2000 and Z03, however, it was worth noting that the thickness profiles after UV irradiation changed to a layered structure featuring a forward-spreading shoulder. A peak/valley structure caused by molecular flow from the non-irradiated toward the irradiated areas was observed for functional AM3001 and Zdol2000, but was not observed for nonfunctional Z03.
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.
Using coarse-graining molecular dynamics, we investigated the adhesive properties of UV-patterned nanometer-thick liquid lubricant films coated on a solid substrate with respect to the approach and withdrawal movements of a solid probe. Same as experimental results, the simulations showed that masked UV irradiation induced a concave-convex surface structure on the lubricant films. As compared with non-UV-irradiated and uniformly-irradiated films, UV-patterned films were characterized by a rapid decrease in adhesion when the solid probe was in contact with the convex region.
The 172nm ultraviolet (UV) photodissociation of perfluoropolyether (PFPE) boundary lubricants is examined experimentally
and theoretically. Carbonyl fluoride, COF2, is the major gas phase product that results from the UV irradiation of non-functionalized perfluoropolyethers containing
the perfluoromethylene oxide, perfluoroethylene oxide, and the perfluoro-n-propylene oxide monomer units. The energetics for
COF2 evolution from the carbon centered radicals R-OCF2OCF2·, R-CF2CF2OCF2·; and from the oxygen centered radicals R-CF2OCF2O·, R-CF2CF2O· are quantified via ab initio calculations. The electronic structure of the transition state geometries leading to COF2 dissociation is characterized. COF2 originating from oxygen-centered radicals, leaving behind a carbon-centered radical on the PFPE main chain, appears to be
the most energetically favorable dissociation pathway.
Functional perfluoropolyether (PFPE) films consisting of mobile and bonded molecules are widely used for lubrication of magnetic
disks. In order to clarify the influence of film composition (mobile/bonded) on tribological performance, we measured the
friction properties of two types of 2nm-thick PFPE films (functional Zdol2000 and nonfunctional Z03) under lightly loaded
(loading force: 0–1mN) and quasi-static (low rotational speed: 2.1mm/s) conditions as a function of elapsed time. The friction
force of Z03 remained unchanged with time and increased linearly with loading force as described by Amontons’ law. In contrast,
induced by the development of the molecules’ bonding in time, the friction force of Zdol2000 increased and transited to a
nonlinear increase with loading force as time proceeded. The nonlinear friction-load relationship of Zdol2000 in the equilibrium
state was characterized by the Johnson-Kendall-Roberts model.
Currently the bonded lubricant thickness is measured either by Fourier transform infrared spectroscopy (FTIR) or electron spectroscopy for chemical analysis (ESCA) on the remaining lubricant after using fluorocarbon solvent like Vertrel to rinse off the mobile lubricant from magnetic disk surfaces. As the thickness of the lubricant applied on a disk approaches to its molecular dimension (∼10 Å), the current measurement methods face tremendous challenges in achieving the desired levels of accuracy and sensitivity. We propose a new method that makes use of a time-of-flight mass spectrometry (TOF-SIMS) to directly measure the bonded film thickness of A20H lubricant by quantifying the ratio of either the hydroxyl end-group or the phenoxy portion of the cyclotriphosphazene end-group with respect to their respective neighboring backbone fragments. The quantified ratios include C2F3/CF2CH2OH and C6F3O/CF3C6H4 measured in the positive polarity and C2F3O/OCF2CH2OH, and C7F5/CF3C6H4O2H2 measured in the negative polarity. The transfer function from the quantified ratios to the bonded lubricant thickness (t) is given in the form of t=α×ln (100×R
T
)−β, where α,β are constants for a selected ratio and R
T
represents the quantity of the specific ratio. The results correlate very well with the FTIR method currently used in the measurement of the magnetic media during production (R
2>90). The new method can complete the measurement of the bonded lubricant thickness in a one-step process and it has a much higher spatial resolution at sub-micrometers than that of the FTIR or ESCA with order of a few tens of micrometers in resolution. The quantified ratio obtained from this TOF-SIMS technique makes the imaging of the localized bonded lubricant possible, which can be applied in the magnetic media failure analysis.
A self-organized patterning method for molecularly thin liquid polymer films on solid surfaces has been demonstrated. In contrast to conventional methods that prepattern solid surfaces and then use the patterns as templates for polymer films, this method utilizes self-organization of polymers induced by ultraviolet (UV) radiation through a mask, thereby directly patterning the polymer films and omitting the prepatterning process. Such UV irradiation locally modified the interaction between polymer films and solid surfaces. As a result, molecular flow occurred at the boundary between the irradiated and nonirradiated areas, leading to three-dimensional surface structures.
By measuring force-distance curves with an environmental scanning probe microscope (SPM), we quantitatively evaluated the effect of ultraviolet (UV) irradiation on the adhesive properties of nanometer-thick perfluoropolyether (PFPE) Z03 films coated on magnetic disks. The experimental results indicated that UV irradiation shows negligible influence on the adhesion of the disk surface and on the adhesion of the mobile fraction contained in the UV-irradiated Z03 films. The effect of UV irradiation on the adhesion of Z03 films is essentially achieved by its effect that improves the bonding of the Z03 molecules to the disk surfaces. As the bonding ratio increased with UV irradiation, the adhesive force measured on the 2-nm-thick PFPE Z03 films initially decreased, but it increased after reaching a minimum at a bonding ratio of about 0.6. This is possibly attributed to the increase of the meniscus force caused by decreased mobile lubricant thickness and the decrease of the van der Waals force between the SPM probe and the sample surfaces caused by increased bonded lubricant thickness.
Atomic force microscopy (AFM) was used to measure the disjoining pressures of perfluoropolyether lubricant films (0.8-4.3 nm of Fomblin Z03) on both silicon wafers and hard drive disks coated with a diamondlike carbon overcoat. Differences in the disjoining pressure between the two systems were expected to be due to variations in the strength of van der Waals interactions. Lifshitz theory calculations suggest that this substrate switch will lead to relatively small changes in disjoining pressure as compared to the more pronounced effects reported due to changes in lubricant chemistry. We demonstrate the sensitivity of our AFM method by distinguishing between these similar systems.
The mechanism of ultraviolet (UV) bonding of perfluoropolyether (PFPE) boundary lubricants on magnetic disk surfaces is re-examined experimentally. It is found that UV-emitted photoelectrons may contribute a negligible part, and instead the UV bonding correlates with the direct photodissociation of PFPE molecules. The UV-induced photodissociation is demonstrated to occur almost randomly on the PFPE molecular chain. The contribution from photooxidation is eliminated under nitrogen purge.
In this paper, we present the tribological properties of the new perfluoropolyether (PFPE) lubricant named LTA-30 which has a different main chain structure from the conventional lubricant, Fomblin Z-Tetraol. Disks lubricated with this lubricant exhibit desirable attributes including lower friction and lower surface energy than the conventional lubricant. We have performed component level and drive level tests to investigate the relation between these attributes and tribological performance
Spreading of perfluoropolyalkylether thin films on amorphous carbon surfaces has been studied by scanning microellipsometry. Two types of perfluoropolyalkylethers with the same main-chain structure and various molecular weights (between 1000 and 6000 g/mol) were used: Zdol, with OH functional end groups, and Z, with nonfunctional CF3 groups. For Zdol, the thickness of the molecular layers in the spreading profile increase as Mn0.6, where Mn is the mean molecular weight, with the second layer being nearly twice as thick as the first layer. This layered structure was not observed for Z in the molecular weight range under study. As expected, the thickness-dependent diffusion coefficient D(h) was found to decrease with increased molecular weight. Possible molecular conformations near the solid surface are discussed. The spreading of binary blends of Zdol–Zdol, Z–Z, and Zdol–Z were also studied. The results show that the spreading of the binary blend of the same kind of polymer with different molecular weight behaved like that of a lubricant with an intermediate molecular weight. The diffusion coefficient of a blend was found to obey the additivity of viscosity. For Zdol–Z blends, however, the faster moving Z molecules migrate through the network of the slower moving Zdol molecules, and form a monolayer ahead of Zdol. © 1999 American Institute of Physics.
The ultraviolet (UV)/ozone surface cleaning method is reviewed. The UV/ozone cleaning procedure is an effective method of removing a variety of contaminants from surfaces. It is a simple‐to‐use dry process which is inexpensive to set up and operate. It can rapidly produce clean surfaces, in air or in a vacuum system, at ambient temperatures. By placing properly precleaned surfaces within a few millimeters of an ozone producing UV source, the process can produce clean surfaces in less than 1 min. The technique is capable of producing near‐atomically clean surfaces, as evidenced by Auger electron spectroscopy, ESCA, and ISS/SIMS studies. Topics discussed include: the variables of the process,the types of surfaces which have been successfully cleaned, the contaminants which can be removed, the construction of a UV/ozone cleaning facility, the mechanism of the process, UV/ozone cleaning in vacuum systems, rate enhancement techniques, safety considerations, effects of UV/ozone other than cleaning, and applications.
Throughout the first three chapters, we have relied entirely on macroscopic concepts such as pressure, surface tension, and the like. A description based on such concepts is adequate as long as the dimensions of the drops and puddles under consideration exceed the range of interactions between molecules. If, on the other hand, we wish to study thin films (liquid pellicles of thickness e much less than a micron), we must take into account the range of interactions between molecules.
The interactions that occur between the hydroxyl-terminated perfluoropolyethers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab initio calculations. The results of these measurements are compared with those of previous studies on the Zdol/amorphous-hydrogenated carbon (CHx) system and the major differences identified. The thickness dependence of the dispersive surface energy for the Zdol/CNx system can be fit using a repulsive van der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at covering CNx as compared to CHx due to less favorable interactions between the Zdol backbone and the CNx surface. The Zdol thickness dependence of the polar surface energy for the Zdol/CNx system indicates that very few strong polar interactions are present between the initially applied Zdol and the CNx surface. A substantial decrease in the polar surface energy of the first Zdol monolayer however occurs on a time scale of 1-5 weeks after lubricant application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between the polar hydroxyl end groups of Zdol and the polar entities on the CNx surface. A kinetic analysis of the Zdol + CNx system reveals that the rate at which the adhesive interactions are formed is limited by diffusion of the polar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end groups of Zdol and imine (basic) sites on the CNx surface may be responsible for the Zdol adhesion. These calculations further suggest that the appearance of the diffusion step in the bonding kinetics and the less efficient coverage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol backbone.
Current high performance magnetic storage devices, i.e., hard disk drives, typically operate at elevated temperatures of nominally 45–60°C. As a consequence, understanding the thermal response of the materials used in the construction of the drive becomes imperative. In this report, we focus on the thermal behavior of a common perfluoropolyether lubricant (ZDOL) used on the carbon-overcoated, hard disk. In particular, we show that evaporative loss of this disk lubricant, as well as bonding of the lubricant to the carbon-overcoated disk, can occur at the temperatures encountered in the hard-disk drive. Surface energy measurements show that the interaction of the hydroxyl-terminated perfluoropolyether ZDOL occurs principally through the end-groups. On unannealed disks, the interaction between this “mobile” lubricant and the carbon overcoat is characterized by hydrogen bonding with the strength of these interactions being only slightly stronger than the intermolecular hydrogen bonding characteristic of bulk ZDOL. Upon annealing at temperatures in the range of 60–150°C, the ZDOL lubricant becomes “bonded” to the disk. The surface energy of the bonded lubricant is substantially lower than the mobile lubricant reflecting the increased interaction strength that occurs as a result of bonding. Since the bonded state is the lower energy state, transitions from the mobile state to the bonded state are thermodynamically favored. The kinetics of this bonding transition, as well as the kinetics of lubricant evaporation were studied as a function of temperature. Using a model of two competing reaction channels, the activation energies for both lubricant bonding and lubricant evaporation were determined to be 3.6 kcal/mole and 5.4 kcal/mole respectively. Ab initio quantum chemical modelling was used to investigate possible interaction sites on the carbon surface. Both experiment and theory indicate that interaction of the hydroxyl-terminated ZDOL to the carbon overcoat occurs via hydrogen bonding to oxygenated species on the carbon overcoat, with a binding energy of 5–8 kcal/mole. An esterification reaction between the hydroxyl end-groups of ZDOL with carboxyl groups on the carbon surface as a result of annealing is shown to be consistent with the both the surface energy data and the kinetic data.
The interactions that occur between the hydroxyl-terminated perfluoropolyether Z-Tetraol and the ion beam-deposited amorphous hydrogenated CHx, sputtered amorphous nitrogenated carbon CNx, and sputtered amorphous silicon nitride SiNx surfaces, are investigated via surface energy measurements, kinetic measurements, and ab initio calculations. The film thickness dependence of the Z-Tetraol dispersive surface energy can be fit using a repulsive van der Waals potential, where the repulsion between the Z-Tetraol main chain and the underlying surface increases in the order: CHx (IBC) < CNx < SiNx, due to the increased average separation between the lubricant main chain and the underlying surface. Ab initio calculations on model dimers quantify the separation distances and binding energy of the lubricant/surface interactions. The Z-Tetraol thickness dependence of the polar surface energy indicates that strong polar interactions occur between the applied lubricant and the underlying surface. The decrease in the polar surface energy is correlated to the increased level of bonding between the hydroxyl end groups of Z-Tetraol and the polar sites of the underlying surface. Further analysis of the surface energy data identifies the critical film thickness at which film instability occurs to be near 17.5±1.0 Å for the 2200 molecular weight of Z-Tetraol on the various surfaces. Images of the disk surfaces showing the evolution of lubricant droplets due to dewetting corroborate the surface energy data analysis.
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 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.
The macroscopic theory of van der Waals dispersion forces is used to derive equations for the work of adhesion and contact angle of a liquid on a solid in the presence of a thin layer between them. Approximate expressions are also derived in terms of Hamaker constants and surface energies. The results obtained are expected to hold whenever dispersion forces alone are operating between the media. The surface tensions of a number of saturated hydrocarbons are calculated on the basis of macroscopic theory and the results are found to agree well with measured values.
Effects of molecular weight and end-group functionality on spreading of molecularly thin perfluoropolyether (PFPE) film over solid surfaces with groove-shaped textures have been studied by experiments and Monte Carlo simulations. In the experiments, lubricant spreading on a surface with groove-shaped textures was measured by making use of the phenomenon in which diffracted light weakens in the lubricant-covered region. It is found that grooves serve to accelerate spreading and this effect increases for deeper grooves, and also the accelerating rate becomes larger for a lubricant having a larger molecular weight or functional end-groups. In the simulations, the Monte Carlo method based on the Ising model was extended to enable us to evaluate the effect of molecular weight on the spreading of non-functional lubricant inside a groove. The validity of the newly developed simulation method was well confirmed from the agreement between the simulation and experimental results.
In this paper, from the viewpoint of lubricant spreading, we investigate the effect of lube textures which mean a nonuniform distribution of bonding strength between lubricant molecules and the disk surface. Lube textures were formed by irradiating ultraviolet rays through a stripe-patterned mask onto a magnetic disk surface which was partially coated with one-monolayer film of perfluoropolyether. Surface characteristics of the lube textures were evaluated by surface energies ascertained from contact angle measurements. Spreading of the lubricant film was measured by scanning microellipsometry on the striped lube textures in the directions parallel and perpendicular to the stripes. The thickness-dependent diffusion coefficients extracted from the spreading profiles show that lubricant spreading in the regime of film thickness less than 0.2 nm is faster along the stripes, indicating the possibility of controlling the behavior of a lubricant film with lube textures.
Perfluoropolyethers (PFPEs) are widely used as lubricants on magnetic recording media. The mobility of the PFPE on the protective carbon overcoat of the media is widely accepted to be intimately coupled to the resulting tribological performance. The flow properties of molecularly thin films of nonpolar PFPEZ and polar PFPE Zdol fractions on solid surfaces were investigated by measuring the spreading profiles. The spreading of Zdol exhibits terraced profiles with the formation of a molecular foot, a shoulder and a vertical step. To describe these features of Zdol spreading, we measured the Zdol thickness dependence of the surface energy, which is then used to calculate the thickness dependence of the disjoining pressure. The polar component of the Zdol surface energy exhibits oscillations as a function of PFPE thickness. The resulting oscillations in the disjoining pressure can be used to qualitatively describe the origins of terraced spreading. The characteristic Zdol spreading profile and surface energy oscillations of Zdol are attributed to molecular layering induced by polar interactions between the Zdol end-groups and the surface. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998
In order to increase the reliability of hard disk media especially for corrosion-resistant property, we have tried lowering the surface energy of a media with high coverage and high bonded ratio of lubricant. In this study for the UV treatment of a media we examined the wavelength of 172 nm that is shorter than that of the low-pressure mercury lamp normally used, which irradiates at wavelengths of 185 nm and 254 nm. We found a drastic improvement in corrosion-resistant property in the case of 172-nm irradiation. In this paper improved corrosion-resistant property and its mechanisms are discussed.
Migration of very thin (<30 nm) polymeric films on solid surfaces was investigated quantitatively using two techniques—scanning microellipsometry and scanning small spot x‐ray photoemission spectroscopy. The surface diffusion coefficient Ds increases as the film thickness decreases down to 1 nm, however, below 1 nm, Ds becomes independent of polymer film thickness. The functional dependence of Ds with molecular weight M is described, over the limited range of M in the unentangled regime, by M−1.7 The activation energy for polymer segment hopping is about 41 kJ/mol, which is similar to values obtained from bulk self‐diffusion of other polymers.
The interactions that occur between the hydroxyl-terminated perfluoropolyethers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab initio calculations. The results of these measurements are compared with those of previous studies on the Zdol/amorphous-hydrogenated carbon (CHx) system and the major differences identified. The thickness dependence of the dispersive surface energy for the Zdol/CNx system can be fit using a repulsive van der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at covering CNx as compared to CHx due to less favorable interactions between the Zdol backbone and the CNx surface. The Zdol thickness dependence of the polar surface energy for the Zdol/CNx system indicates that very few strong polar interactions are present between the initially applied Zdol and the CNx surface. A substantial decrease in the polar surface energy of the first Zdol monolayer however occurs on a time scale of 1−5 weeks after lubricant application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between the polar hydroxyl end groups of Zdol and the polar entities on the CNx surface. A kinetic analysis of the Zdol + CNx system reveals that the rate at which the adhesive interactions are formed is limited by diffusion of the polar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end groups of Zdol and imine (basic) sites on the CNx surface may be responsible for the Zdol adhesion. These calculations further suggest that the appearance of the diffusion step in the bonding kinetics and the less efficient coverage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol backbone.
Far-UV treatment of a surface with a thin film of a perfluoropolyether improves adhesion and lowers surface free energy substantially compared to the unirradiated, lubricated surface. With 185-nm radiation, approximately one monolayer can be fixed to surfaces such as amorphous carbon, silica, and gold. These lubricated surfaces become extremely hydrophobic after UV treatment (advancing water contact angle increases about 65{degree} to {ge} 110{degree}) and are not removed by fluorinated solvents. In the absence of far-UV radiation, most perfluoropolyethers show poor adhesion to carbon and are removed easily by rinsing with fluorinated solvents.
We discuss wetting criteria for solid/liquid pairs where the long-range interaction is not oscillating and is described by a Hamaker constant A. The key parameters are then A and the spreading coefficient S. Since S contains contributions from short range interactions, S and A are independent variables and can be of either sign. Discussing the resulting four possibilities, we expect three fundamental regimes: (1) complete wetting, a small droplet spreads to become a flat 'pancake' surrounded by a dry solid; (2) pseudo partial wetting, a droplet forms a spherical cap with a finite contact angle θ but the surrounding solid is wet, the drop is in equilibrium with a molecular film; (3) partial wetting, the contact angle θ is nonzero and now the solid around the drop is dry. The pseudo partial wetting regime may explain some surprising observations concerning the spreading of silicone droplets.
It has recently been reported that thin films of perfluoropolyethers (PFPE's) can be bonded to a variety of substrates under illumination of 185-nm ultraviolet (UV) light. In this paper we examine the bonding of thin perfluoropolyether films by low-energy electrons and UV light. It is shown that the bonding takes place through interaction of the perfluoropolyether molecule, with a low-energy photoelectron created by excitation of the substrate by the UV photons. The perfluoropolyether molecule subsequently undergoes a dissociative electron attachment, resulting in the formation of a radical and a F- ion. It is likely that radical propagation and termination steps then cross-link the polymer and attach it to the substrate. The similarities and differences between electron-bonded and UV-bonded perfluoropolyether films are discussed.
The dynamics of molecularly thin liquid lubricant films in sliding and flying experiments is studied principally by scanning microellipsometry and complemented with both scanning x‐ray photoelectron spectroscopy and infrared spectroscopy. Microellipsometric profiling of lubricant thickness is performed in situ, either dynamically or statically on films ranging from 1 to 10 nm. The removal of liquid polyperfluoroether lubricants from sliding and flying tracks, which includes the displacement and loss depends on film thickness, molecular weight, and chemical structure. In flying and sliding, the lubricant removal rate from monolayer films is significantly slower than from multilayer films. In flying, lubricant displacement and loss increase with a decrease in molecular weight.
This paper reviews the effect of the design evolution of modern magnetic hard disk drives on tribological challenges, such as wearless high-speed contact, ultra-thin lubrication, corrosion protection, and requirements of thinner carbon overcoat. We will present some data on slider wear measurements on the order of nanometers, lubricant migration and depletion as linked to tribological reliability, lubricant bonding kinetics on different carbon overcoats, corrosion protection using lubricant and thin carbon coatings, and lubricant de-wetting.
Disk drives experience a wide range of temperature and humidity during storage and operation. Humidity influences media tribology through the effects of adsorbed water on the interaction of Zdol lubricant hydroxyl end groups with the carbon overcoat. We measured the effects of Zdol 4000 thickness and chemisorption on the water adsorption isotherm and surface energy. The potential distortion model of Adamson was employed to derive the film thickness at saturation from the isotherms and contact angles. The isotherm was surprisingly independent of the Zdol chemisorption and thickness up to 1.8 nm, because, in this range of Zdol thickness, the adsorbed water resides in pores which exclude the larger Zdol end groups. The water adsorption isotherm provides a sensitive probe of overcoat structure due the ability of water to reach pores in the overcoat which are inaccessible to Zdol. Above a Zdol thickness of 1.8 nm there is a jump in the water adsorption as additional adsorption takes place on the "free" hydroxyl groups. The mechanism for the previously observed decrease in Zdol lubricated media durability at low humidity is derived from the effect of adsorbed water on Zdol surface diffusion and chemisorption. Copyright 2000 Academic Press.
The conventional viscous-flow mechanism associated with the
dip-coating technique for lubricant thickness control becomes
ineffective for deposition of ultra-thin films (<~2 nm) required for
hard magnetic disks. We have demonstrated that at least two important
microscopic mechanisms dominate the control of lubricant thickness in
this thickness range. They are surface adsorption and diffusion of
lubricant molecules at the solid-liquid interface. Furthermore, it was
also observed that too fast a pulling-up speed resulted in a more
nonuniform lubricant film onto the disk surface. This is due to
disturbance of the meniscus profile by viscous drag. The uniformity
and/or the microstructure of lubricant layer deposited onto the disk
surface will then affect tribological performance