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.     

Electron stimulated decomposition of fluorocarbons on amorphous hydrogenated carbon (<Emphasis Type="Italic">a</Emphasis>-CH<Subscript>x</Subscript>) overcoats used in data storage media

The electron-induced surface chemistry of perfluoropolyalkylether (PFPE) lubricants on a-CH_x 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-CH_x 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-CH_x 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.     

The Effect of Solvents on the Perfluoropolyether Lubricants Used on Rigid Magnetic Recording Media

The adsorption characteristics and tribological properties of the perfluoropolyether (PFPE) lubricants Zdol and Z-Tetraol on amorphous nitrogenated CN_x carbon are investigated as a function of solvent used to apply the lubricants. The solvents used in these studies include perfluorohexane, CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃. Deposition studies indicate that the applied thickness of PFPE films is strongly solvent-dependent that can be related to differences in the solubility parameters between the various lubricants and solvents. The results of ab initio computations on the molecular electronic structure of the solvent molecules show that their solvent power is correlated to their polarity and in particular to the acidity of the protons on the CF₃CHFCHFCF₂CF₃ and C₄F₉OCH₃ molecules. Tribological reliability, as measured by contact start-stop testing, slider-disk clearance, lubricant pickup by the slider, lubricant smearing on the disk surface, etc., is independent of solvent and is limited to the physical properties of the adsorbed lubricant film. The kinetics of lubricant mobility are charateristic of confined liquids that are independent of solvent as shown by lubricant flow profiles, bonding kinetics, and contact angle goniometry.     

The Adhesion of Monomolecular Hydroxyl-terminated Perfluoropolyether Liquid Films on the Sputtered Silicon Nitride Surface as a Function of End Group Acidity and Mobility

The intermolecular interactions at the interface between monomolecular hydroxyl-terminated perfluoropolyether (PFPE) liquids (Zdol, Zdol-TX, Z-Tetraol, Zdiac) and a sputtered amorphous silicon nitride film (SiN_x) are investigated using contact angle goniometry, Fourier transform infrared spectroscopy, and ab initio computational chemistry. The results demonstrate that the adhesion between the PFPE liquids and the SiN_x surface occur via the polar interactions between the PFPE end groups (-OH, -COOH) and the polar sites on the SiN_x surface (e.g., SiOH). The attractive interactions lead to a lowering of the polar surface energy with increasing PFPE coverage up to a monolayer. The binding energy is computed to be approximately −4 to −9 kcal/mol, depending upon the polarity of the PFPE end group. Adsorbed water is shown to compete with PFPE for surface bonding sites on SiN_x (−4.4 kcal/mol) that can lead to a significantly reduced level of adhesion for some of the hydroxyl-terminated PFPEs. A higher level of adhesion between the PFPEs and SiN_x can be attained by increasing the strength of the hydrogen bond and/or increasing the configurational entropy of the PFPE end group to facilitate the hydrogen bonding reaction.     

Disk Lubricants for Spontaneous Adsorption and Grafting to Carbon Overcoat by UV Irradiation

A molecular orbital study was performed to elucidate the π–π charge transfer interaction between perfluoropolyether (PFPE) lubricants possessing phenylic end-groups and the carbon overcoat of magnetic hard disks. It is revealed that the phenylic unit and the graphitic segment of the carbon attract each other leading to spontaneous adsorption. The strength of this interaction increases in the order of: an unsubstituted phenyl group < a phenoxy unit < a p-methoxy-phenoxy unit. A molecular dynamics calculation revealed that alkyl-phenyl ether, on capture of an electron, would dissociate to yield the phenoxide anion and the alkyl radical. It is thus predicted that PFPE lubricants with an end-group possessing a phenoxy unit would spontaneously adsorb on the carbon overcoat, and that irradiation of disks coated with such lubricant with short UV (185 nm), thus generating photo-electrons, would result in facile detachment of phenoxy groups and grafting of PFPE molecular chains to the carbon surface at the chain terminus. Four new PFPE lubricants, Z-SA1 and Z-SA2 based on the Fomblin Z type backbone, and D-SA1 and D-SA2 based on the Demnum backbone were synthesized, where SA1 and SA2 indicate end-groups possessing a phenoxy unit and a p-methoxy-pheoxy unit at the ω-position, respectively. Disks coated with these lubricants were tested for (1) spin-off rate, (2) diffusion over the disk surface, (3) facility for photo-grafting by UV, (4) water contact angle (before and after UV exposure), (5) the catalytic degradation, and (6) the on-track time-to-failure test. A TOF-SIMS study of disks coated with D-SA1 and D-SA2 was performed to elucidate the disposition of lubricant molecular chains due to spontaneous adsorption and the effect of UV irradiation. All the experimental results were found to be in good accord with predictions given by the molecular orbital study. In the time-to-failure test, disks coated with Z-tetraol, Z-SA1, and Z-SA2 were compared. The durability was found to increase in the order of Z-tetraol < Z-SA1 < Z-SA2.     

Observation of Slider Landing Process in Hard Disk Drive

With the decrease in slider flying height, slider flying instability caused by slider–disk interactions is becoming a big concern. Novel technology has to be employed to further improve our understandings about slider–disk interaction. In this work, a slider flying height-attitude testing (3D) system was employed to study slider–disk interaction during a slider landing process to demonstrate its capability for the application. It is shown that great details of slider–disk interactions and subtle variations of the slider flying attitude during the landing process can be revealed with the 3D system. Slider dynamic flying height and attitude (pitch and roll angles) during the landing process can be determined from the data recorded in one test. Furthermore, analysis in frequency domain can be done not only on flying height, but also on pitch and roll angles directly. It is found that the slider landing process can have different stages during which slider performance and characteristics of slider–disk interaction are different.     

Bonding of Hard Disk Lubricants with OH-Bearing End Groups

Typical lubricants for magnetic hard disks comprise the central perfluoropolyether section and the short hydrocarbon end groups bearing hydroxyl unit(s). It had been shown earlier that chemical bonding of these lubricants to the carbon overcoat of disks involves (1) dangling bonds shielded inside the carbon, (2) transfer of the hydrogen atom of the hydroxyl unit to a dangling bond site, and (3) attachment of the remaining alkoxy system, R–CF₂–CH₂–O^·, to the carbon surface as a pendant ether unit. Dangling bonds at or near the surface react immediately with H₂O or O₂ in the atmosphere. It follows that, in order to bond, the hydrocarbon end group must move into crevices of the carbon film. It was postulated that the bonding rate would depend on the length of the hydrocarbon end-group, –(CH₂) _n –OH. The longer the hydrocarbon sector is, the faster and the more extensively the bonding would proceed. Bonding rates were examined for a set of samples differing only in the dimension of the hydrocarbon end-group. Results clearly in accordance with the postulate were obtained. The sample set included two novel lubricants, D-2TX2 and D-2TX4, with the following end-groups, –O–CF₂–CH₂–O–(CH₂) _n=2,4–OH. Excellent bonding rate, coverage, and potential anticorrosion property were revealed for these lubricants.     

The Contribution of Thin PFPE Lubricants to Slider-Disk Spacing. 3. Effect of Main Chain Flexibility

We have investigated the effect of main chain flexibility of a perfluoropolyether lubricant, Zdol 4000, on dynamic slider-disk spacing. The major conclusion of this work is that increasing the Zdol chain flexibility results in a decrease in the dynamic slider-disk spacing. The Zdol main chain flexibility is quantified by ab initio computations of the barriers to internal rotation about C-O and C-C bonds in model structures containing -CF₂O- and -CF₂CF₂O- monomer units. C-O bond rotations for -CF₂O- monomer units bordered by -CF₂O- monomer units are relatively unhindered with a barrier to internal rotation of 1.7 kcal/mol. C-O and C-C bond rotations in -CF₂OCF₂CF₂O- units have barriers to internal rotation of 8 and 5 kcal/mol, respectively. The presence of the C-C bond imparts considerable rigidity to the Zdol chain.     

The Contribution of Thin PFPE Lubricants to Slider–Disk Spacing

We have investigated the effect of the molecular weight (MW) and film thickness of a perfluoropolyether lubricant, Zdol, on the slider–disk spacing loss, or clearance. The major conclusion of this work is that Zdol films as thin as 10 Å can reduce the slider–disk clearance by 2 nm or more in the molecular weight range of 1000–5000 amu. This is attributed to the attractive van der Waals interaction between the slider and the disk surface that causes the Zdol main chain to interact with the slider surface, giving rise to a friction force. When the film thickness of the lubricant exceeds the monolayer thickness, dewetting can take place. The droplets that form occupy the space between the slider and disk surface reducing the slider–disk clearance by as much as 4 nm. There is a step increase in the acoustic emission signal at the dewetting thickness transition, indicative of a slider–disk interference.     

The decomposition mechanisms of a perfluoropolyether at the head/disk interface of hard disk drives

The decomposition mechanisms of a perfluoropolyether (ZDOL) at the head/disk interface under sliding friction conditions were studied using an ultra‐high vacuum tribometer equipped with a mass spectrometer. Chemical bonding theory was applied to analyze the decomposition process. For a carbon coated slider/CN_x disk interface, the primary decomposed fragments are CFO and CF₂O, caused by the friction decomposition and electron bombardment in the mass spectrometer. For an uncoated Al₂O₃–TiC slider/CN_x contact, CF₃ and C₂F₃ fragments appear in addition to CFO and CF₂O, resulting from the catalytic reactions and friction decomposition, indicating that the decomposition mechanism associated with friction leads to the breaking of the main chain of ZDOL and forms CF₂=O, which reacts with Al₂O₃ to produce AlF₃, and the rapid catalytic decomposition of ZDOL on the AlF₃ surface follows. Moreover, the effects of frictional heat, tribocharge, mechanical scission and Lewis acid catalytic action, generated in friction process, on the decomposition of ZDOL are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Initiation of lubricant catalytic decomposition by hydrogen evolution from contact sliding on CHx and CNx overcoats

Tribochemical studies of the head/disk interface (HDI) were conducted using hydrogenated (CHx) and nitrogenated (CNx) carbon disk samples coated with perfluoropolyether ZDOL lubricant. The studies involved drag tests with uncoated and carbon-coated Al₂O₃–TiC sliders and thermal desorption experiments in an ultrahigh vacuum (UHV) tribochamber. We observed that the hydrogen evolution from CHx overcoats initiates lubricant catalytic decomposition with uncoated Al₂O₃/TiC sliders, forming CF₃ (69) and C₂F₅ (119). The generation of hydrofluoric acid (HF) during thermal desorption experiments provides the formation mechanism of Lewis acid, which is the necessary component for catalytic reaction causing Z-DOL lube degradation. On the other hand, for CNx films, lubricant catalytic decomposition was prevented due to less hydrogen evolution from the CNx overcoat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lubricant-Induced Spacing Increases at Slider–Disk Interfaces in Disk Drives

In this article, we explore the physical mechanisms for lubricant migration on recording head slider surfaces and how this migration leads to increased slider–disk spacing during disk drive operations. This is done using both a new experimental methodology, called the “droplet stress test,” and through simulation. In our simulations, we compare the air shear-induced lubricant migration modeled either as viscous flow of a continuum liquid film with zero slip or as wind driven slippage of molecules across the surface. The experimental data are best fitted using the viscous flow model to determine an effective viscosity for the sub-nanometer thick lubricant films. This effective viscosity tends to be somewhat less than the lubricant bulk viscosity due to air shear promoting the slippage of lubricant molecules across the surface. Our experimental results also indicate that the potential spacing increase from the pickup of disk lubricant on the slider is limited by the mobile fraction of the dewetting thickness of the lubricant film on the slider.     

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.     

Fourier transform infrared investigation of thin perfluoropolyether films exposed to electric fields

Reflection–absorption Fourier transform infrared (FTIR) techniques were used to monitor thin layers of hydroxyl-terminated perfluoropolyether lubricant (Fomblin ZDOL) for molecular changes caused by long exposures to dc electric fields with intensities in the range 3–6 × 10⁴ V/cm. A new absorption band appears in the 1720–1640 cm⁻¹ region of some field-exposed specimens. The new spectral feature is attributed to the presence of C=O, a functional group not present in the ZDOL chemical structure but commonly found in perfluoropolyether degradation products. The peak position of the carbonyl absorption band indicates that hydrogenated carbon is present at the α-position. The presence of hydrogenated --carbons suggests that structural modifications occur via a mechanism that primarily involves the –CH₂–OH functional endgroup, rather than the more commonly proposed bond cleavage at the –O–CF₂–O– acetal groups in perfluoropolyether lubricants having no polar endgroups. These results suggest that slow but cumulative lubricant degradation may occur when strong electric fields are present at the head-disk interface. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Studies on the interactions between ZDOL perfluoropolyether lubricant and the carbon overcoat of rigid magnetic media

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. This revised version was published online in August 2006 with corrections to the Cover Date.     

Direct observation of PFPE lubricant molecules by cryogenic AFM under ultra-high vacuum

Surface lubrication is one of the essential technologies in modern magnetic disk systems and improvement of the surface lubrication is very important in the development of next generation systems. In this study, we used AFM for the direct observation of perfluoropolyether (PFPE) lubricant molecules on atomically flat surfaces. We used a cryogenic non-contact AFM to observe the molecules in a frozen state of micro-Brownian motion of PFPE segments, because the glass transition temperature of PFPE is very low. To avoid freezing a trace amount of water vapor on the sample surface at liquid nitrogen temperatures, the AFM observation was performed under ultra-high vacuum. We observed that on a gold surface the size of the molecules increases with repeated AFM scans. This is because the mechanical stimulus causes the fusion of PFPE lubricant molecules to form reversed micelles at the non-polar surface. At a hydrophilic silicon wafer surface, however, we succeeded in observing single lubricant molecules. This is because almost all PFPE lubricant molecules are fixed to the hydrophilic solid surface by polar–polar bond formation and they cannot move around on the surface and thus they cannot fuse to each other. As formation of the reversed micelle structure is a rather general phenomenon in the PFPE lubricant thin layer at non-polar surfaces, we also will discuss briefly the expected molecular structures of PFPE lubricants at the surface of the carbon overcoat of magnetic disks.     

Disk Lubricant Additives,A20H and C2: Characteristics and Chemistry in the Disk Environment

Disk lubricant additives A20H and C2 are Fomblin Z type perfluoropolyether with the hydroxyl end-group, –O–CF₂–CH₂–OH, at one end, and the cyclo-triphosphazene end-group, R₅(PN)₃–O–, at the other end. Here, R is an m-trifluoromethyl-phenoxy group for A20H and a trifluoroethoxy group for C2. These additives were examined for miscibility with benzene, spin-off rate, water contact angle, and the diffusion rate over the carbon overcoat. It is revealed that A20H adheres to the carbon overcoat spontaneously. The attractive interaction arises from the charge–transfer type interaction between the aromatic rings of the phosphazene end and the graphitic regime of the carbon overcoat. No spontaneous adherence occurs between the lubricant C2 and the carbon overcoat. A TOF-SIMS study of disks coated with A20H and C2, respectively, with and without subsequent curing by short-UV (185 nm) was performed. It is revealed: (1) if presented with a low energy electron, the phenoxy groups of A20H readily undergo the dissociative electron capture, while the trifluoroethoxy group does not, and (2) photoelectrons generated by short-UV have little kinetic energy and the electron capture occurs only if an electrophilic molecular sector is in intimate contact with the carbon. Thus, in the case of disks coated with A20H, UV-curing results in detachment of a phenoxy group in contact with the carbon, generation of a radical center at the phosphorus atom and subsequent formation of a bona fide chemical bond between the phosphor and the carbon overcoat. No reaction of consequence occurs when disks coated with C2 are irradiated with short-UV.     

The Photodissociation of Perfluoropolyethers by Ultraviolet Light

Abstact The 172 nm 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.