Thermal Stability of Fomblin Z and Fomblin Zdol Thin Films on Amorphous Hydrogenated Carbon

Thermal desorption spectroscopy has been used to monitor the decomposition kinetics of Fomblin Zdol and Fomblin Z lubricant films adsorbed to the amorphous carbon overcoats of hard disk media. Comparisons have been made between Fomblin Z and Zdol with vastly different molecular weights (MW = 4000 and 50000), and films of Fomblin Z with different thickness (20 and 60 Å). Several species have been observed desorbing from the surface during heating. In all cases decomposition occurs over roughly the same temperature range of 600–750 K. This suggests that the desorption process is the result of decomposition and that the end groups of the Fomblin lubricants are not involved in determining the kinetics of this decomposition reaction. The activation barrier to the decomposition process has been estimated at 114±6 kJ/mol.     

Effect of lubricant bonding fraction at the head–disk interface

Tribochemical studies of the effect of lubricant bonding on the tribology of the head–disk interface (HDI) were conducted using hydrogenated (CHx) carbon disk samples coated with perfluoropolyether ZDOL lubricant. The studies involved drag tests with uncoated and carbon-coated Al₂O₃–TiC sliders and also thermal desorption experiments in an ultrahigh vacuum (UHV) tribochamber. The friction and catalytic decomposition mechanisms as well as the thermal behavior of ZDOL are described. We observed that a larger mobile lubricant portion significantly enhances the wear durability of the HDI by providing a reservoir to constantly replenish the lubricant displaced in the wear track during drag tests. In the thermal desorption tests we observed two distinct temperatures of desorption. The mobile ZDOL layer is desorbed at the lower thermal desorption temperature and the residual bonded ZDOL layer is desorbed at the higher thermal desorption temperature.     

Tribological Performance and Transfer Behavior of Lubricating Oils at Head-Disk Interface under Volatile Organic Contamination

Tribological performance of head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Slider and disk surfaces were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) after CSS tests. The CSS test results indicated that the friction forces were high and unstable under contamination. Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The transfer amount of lubricating oil was revealed to be dependent on the chemical structure of the terminal group in the lubricating oil. Piperonyl (–CH₂−phe=(O)₂=CH₂) terminated AM3001 lubricating oil was lost more easily than two hydroxyl (–OH) terminated Tetraol lubricating oil, probably because of the weak attractive force of the piperonyl groups with carbon overcoat. TOF-SIMS chemical images indicated that the transferring behavior of the lubricating oil onto the slider surface during CSS tests was dependent on the chemical structure of volatile organic contaminants. The lubricating oil became built up on the slider surface when the dioctyl sebacate (DOS) pollutant used. In contrast, the lubricating oil distribution on the slider surface was uniform under a polydimethylsiloxane (PDMS) vapor. The different transfer behavior of lubricating oil onto the slider surface may be resulted from the changeable surface properties of slider and disk because of the coexistence with gaseous contaminants.     

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.     

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.     

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.     

The surface chemistry of alkyl and arylphosphate vapor phase lubricants on Fe foil

The surface chemistry of tributylphosphate (TBP) and tricresylphosphate (TCP) on a polycrystalline Fe surface was studied using temperature programmed reaction spectroscopy and Auger electron spectroscopy to illustrate some of the initial steps in the reaction mechanisms of alkyl and arylphosphate vapor phase lubricants. During heating, TBP [(C₄H₉O)₃P=O] adsorbed on the Fe surface decomposes via C–O bond scission to give butyl surface intermediates [C₄H₉–] that react via β-hydride elimination to desorb as 1-butene [CH₃CH₂CH=CH₂] and H₂ without appreciable carbon deposition onto the surface. The thermal decomposition of 1-iodobutane [I-C₄H₉] on Fe was observed to proceed via the same β-hydride elimination mechanism. In contrast to tributylphosphate, meta-tricresylphosphate (m-TCP) [(CH₃–C₆H₄O)₃P=O] decomposes on Fe via P–O bond scission to produce methylphenoxy intermediates [CH₃–C₆H₄O–]. During heating to 800 K, methylphenoxy intermediates either desorb as m-cresol [CH₃–C₆H₄–OH] via hydrogenation or decompose further to generate tolyl intermediates [CH₃–C₆H₄–]. Some of the tolyl intermediates desorb as toluene [CH₃–C₆H₅] via hydrogenation but the majority decompose resulting in H₂ and CO desorption and carbon deposition onto the Fe surface. The P–O bond scission mechanism of m-TCP was verified by showing that the temperature programmed reaction spectra of m-cresol yield products that are almost identical to those of m-TCP. These results provide insight into the origin of the differences in the performance of alkyl and arylphosphates as vapor phase lubricants. The alkylphosphates decompose via alkyl intermediates that readily undergo β-hydride elimination and desorb into the gas phase as olefins, thus removing carbon from the surface. In contrast, the arylphosphates generate aryloxy intermediates by P–O bond scission and aryl intermediates by further C–O bond scission. Neither of these intermediates can undergo β-hydride elimination and thus they decompose to deposit carbon onto the Fe surface. The higher efficiency for carbon deposition may be the primary reason for the superior performance of the arylphosphates over alkylphosphates as vapor phase lubricants.     

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.     

High resolution electron energy loss spectroscopy oflow vapor pressure liquid lubricants: Fomblin Y and Z

Thin layers of perfluoropolyether lubricants Fomblin Y and Z havebeen deposited on a polycrystalline molybdenum surface andvibrational spectra have been measured by using high resolutionelectron energy loss spectroscopy. The vibrational spectra showstrong modes in the 155 meV range for Fomblin Y and Fomblin Zcharacteristic of CF₃ and CF₂ vibrationalmodes. Results from in situ and ex situ deposition of thelubricant on a differently prepared substrate suggest that thelubricant molecules condense on the surface forming a 300 Kmulti-layer structure.     

Activation of the SiC surface for vapor phase lubrication by Fe chemical vapor deposition from Fe(CO)5

The feasibility is demonstrated of a new approach to the vapor phase lubrication of ceramics using organophosphorus compounds. The surface of SiC is shown to be unreactive for the decomposition of trimethylphosphite, (CH₃O)₃P, a simple model for organophosphorus vapor phase lubricants such as tricresylphosphate. In order to activate the surface of SiC it has been exposed to Fe(CO)₅ at a temperature of 600 K. Chemical vapor deposition serves as a means of depositing Fe on the SiC surface. The Fe-modified SiC surface is then shown to induce the decomposition of adsorbed (CH₃O)₃P. The mechanism of (CH₃O)₃P decomposition is similar to that observed on Fe(110) surfaces modified by the presence of oxygen. It is initiated by P–O bond cleavage to produce adsorbed methoxy groups, CH₃O_(ad), which then decompose by -hydride elimination resulting in H₂, CO, H₂CO, and CH₃OH desorption. It is suggested that chemical vapor deposition of metals using high vapor pressure metal-containing compounds such as Fe(CO)₅ can serve as a mechanism for continuous, in situ activation of ceramic surfaces for vapor phase lubrication in high temperature engines.     

Resistance of Fomblin Z-Type Lubricants to Lewis Acid-Catalyzed Decomposition: Effect of the Chemical and Electronic Structure of End-Groups

In the present paper, the effect of the chemical and electronic structure of the Fomblin Z-type lubricant end-group on the Lewis acid-catalyzed decomposition is studied by both Thermogravimetric Analysis (TGA) and Density Functional Theory (DFT) molecular orbital calculations. TGA results of the mixture of Z-type lubricants and ZrO ₂ show that the structure of the end-groups significantly affects the thermal stability of the lubricants in the presence of a Lewis acid. The DFT calculation results suggest that the reactivity of the end-groups and thus the resistance of various lubricants to Lewis acid-catalyzed decomposition are affected by the lubricant molecular orbital structure.     

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.     

The interaction of short-chain model lubricants with the surfaces of hydrogenated amorphous carbon films

The adsorption of water and small model perfluorinated lubricants on hydrogenated amorphous carbon (aC-H) films of varying hydrogen content was investigated using thermal desorption spectroscopy (TDS). Hydrogen content of the carbon films was measured by Rutherford back scattering (RBS) and elastic recoil spectroscopy (ERS) and correlated to changes in surface free energies measured by contact angle analysis. Hydrogenated carbon films exhibiting the highest surface free energy provided a greater attractive interaction for the model lubricants. All model lubricant species studied - water (D₂O), perfluorodiethyl ether (CF₃CF₂OCF₂CF₃), perfluoropentane (CF₃(CF₂)₃CF₃), perfluorooctane (CF₃(CF₂)₆CF₃), 2,2,2-trifluoroethanol (CF₃CH₂OH), and 1,1,7-H-perfluoroheptanol (CF₂H(CF₂)₅CH₂OH)—reversibly adsorbed to the carbon surface with little chemical reaction. Increases in desorption energies with increasing chain length were observed among the adsorbates and are ascribed to increasing van der Waals interactions. Incorporation of alcoholic end groups provided an avenue of hydrogen bonding to the surface and produced an ~20 kJ/mol increase in desorption energy relative to a perfluorinated alkane of the same chain length. Ether linkages within the model lubricant provide little increase in desorption energy as fluorine substituents effectively screen the oxygen. Together these findings implicate a predominantly physisorbed state for perfluorinated lubricants on hydrogenated carbon surfaces.     

Initial steps in the surface chemistry of vapor phase lubrication by organophosphorus compounds

The surface chemistry of trimethylphosphite (CH₃O)₃P has been studied on Cu(111) and Ni(111) surfaces in order to model the initial steps in the reactions of vapor phase lubrication by organophosphorus compounds. The initial reactions involve scission of the P–O bonds to deposit methoxy groups (CH₃O_(ad)) on the surfaces. On the Cu(111) surface the formation of CH₃O_(ad) species occurs only after oxidation of the surface. The CH₃O_(ad) groups on Cu(111) decompose by β‐hydride elimination to produce formaldehyde (O=CH₂) and adsorbed hydrogen. CH₃O_(ad) groups are formed from (CH₃O)₃P on the clean Ni(111) surface and decompose by complete dehydrogenation to CO and adsorbed hydrogen atoms. This chemistry is very similar to that observed for CH₃OH on these surfaces. These results suggest that alkoxides are important intermediates in the decomposition of vapor phase lubricants on metal surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reaction mechanisms in organophosphate vapor phase lubrication of metal surfaces

This paper discusses the surface chemistry of a number of reactants and species that are probable intermediates in vapor phase lubrication chemistry. In order to understand the mechanism by which arylphosphates [(RO)₃P=O] such as tricresylphosphate [R=CH₃C₆H₄–] react to form lubricating films on metal surfaces, several of the likely elementary steps are illustrated using experiments with model compounds on the Cu(111) surface. Trimethylphosphite [(CH₃O)₃P] has been used as a simple model for the phosphates and experiments suggest that the initial step in the decomposition mechanism is P–O bond breaking to form alkoxy groups. Other evidence suggests that under some conditions the decomposition of arylphosphates may be initiated by either P–O or the C–O cleavage to produce adsorbed aryloxy or aryl intermediates. The decomposition of aryloxy groups or aryl groups then leads to the deposition of carbon into the lubricating films. The chemistry of aryl, alkyl, aryloxy, and alkoxy species has been investigated on the Cu(111) surface. In either the alkoxy groups produced by P–O cleavage or the alkyl groups produced by C–O cleavage the presence of β-CH bonds has a strong influence on the rate of carbon deposition onto the surface. In the arylphosphates which would produce either aryl or aryloxy groups, the lack of β-CH bonds leads to greater rates of carbon deposition onto the surface than for either alkyl or alkoxy groups.     

Comparison studies on degradation mechanisms of perfluoropolyether lubricants and model lubricants

The degradation mechanisms of some perfluoropolyether lubricants, model lubricants and DLC coating were studied in this paper. The degradation fragments from the tests of the PFPE lubricants can be divided into two groups. One group includes the gas fragments containing fluorine atoms, which are generated from the decomposition of the lubricants themselves; while the other group, including H₂, C₂H₃, C₂H₅, and CO₂, is generated from the degradation of the DLC coating on the disk surfaces. The test results from the model lubricants clearly show that the carbon dioxide produced in the tests is generated from the DLC coating, not from the decomposition of the lubricants or model lubricants. The C–O bond is a weak bond in both the lubricant and model lubricant molecules; it is easier to be broken. Because of the polar characteristics of the C–O bond, it is easy to be attacked and broken down by low-energy electrons generated during sliding. Triboelectrical reaction is a dominant degradation mechanism of the lubricants and model lubricants.     

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.     

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.     

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.     

Surface diffusion and flow activation energies of perfluoropolyalkylether

Surface diffusion of perfluoropolyalkylether (PFPE) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silicon wafers was investigated over the temperature range of 25 to 50°C using scanning microellipsometry. Zdol exhibits a much lower mobility and a distinctly different thickness profile as compared to Z15. The activation energy for surface diffusion of Zdol is higher than that of Z15, reflecting the stronger affinity of its hydroxyl end groups for the substrate. The viscosity flow activation energy E ^* is compared with that of surface diffusion E _d ^* yielding E _d ^* E ^* for Z15, and E _d ^* 1.5E ^* for ZOn leave from Korea Insitute of Science and Technology, PO Box 131, Cheongryang, Seoul, Korea 305-701.