Chemical Properties of Fomblin® Perfluoropolyether Lubricants with Arylalkyl Groups on Hard-Disk Media

The chemical characteristics of perfluoropolyether lubricant films, which have arylalkyl groups at both ends of the main chain, were studied on hard-disk media. It was found by time-of-flight secondary ion mass spectroscopy (TOF-SIMS) that 94% of the functional groups of AM3001, (3,4-dioxomethylenephenyl)methyl, were eliminated from the main chain on a disk surface after a 672 h exposure, even in a regular clean room of 23°C and 55% RH. Regarding the lubricants synthesized by the authors, LUB-A having a benzyl functional group lost 35% of its functional group after a 768 h exposure. The 3-phenylpropyl functional group of LUB-B was not eliminated even after a 672 h exposure. It was suggested that hydrolysis easily occurred at the ether linkage between the main chain and the functional group, and removed the functional group from the main chain because the benzyl cation was chemically stable. The new lubricant having a 3-phenylpropyl functional group, which is chemically stable and does not decompose on a disk surface, was developed and proposed for use with hard-disk media.     

Investigation of micro-phase separation of the additive,cyclotriphosphazene, in lubricant films of hard disk media

The micro-phase separation of the additive, cyclotriphosphazene (X-1P), in perfluoropolyether (PFPE) lubricant films on hard disk media was studied. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicated that the small spots appearing on the disk surface consisted predominantly of X-1P. Observation using an atomic force microscope (AFM) revealed the micro-phase separation process to be the sudden, continuous appearance of new spots some time after coating the film. Some spots grew over previous ones, while some spots dissolved. Finally, they stopped growing and the number of spots became saturated. The solubility of X-1P in the lubricant film increased in the order of ZDIAC, ZDOL2000, ZDOL4000 and Z03, and that in bulk lubricant increased in the order of Z03, ZDOL4000, ZDOL2000 and ZDIAC. The order of solubility of X-1P in the film did not correspond to that in the bulk.     

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.     

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.     

Additive Mechanism Investigation of Perfluoropolyether Lubricants with a Phosphazene Additive

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

Tribology of X-1P vapor lubricated hard disks

The tribological characteristics of vapor lubricated X-1P films on carbon coated disks were investigated as a function of lubricant thicknesses (0.2–2 nm) and compared with traditionally dip-coated X-1P and PFPE films. Glide and flyablity tests were performed and the lubricant redistribution in the ‘wear track’ was investigated using a surface reflectance analyzer (SRA). A critical lubricant thickness was found to exist for X-1P below which lubricant accumulation was observed, while lubricant loss was found to be present if the thickness of the lubricant film was greater than the critical thickness.     

The Effect of Phosphazene additives to passivate and stabilize lubricants at the head/disk interface

There have been a number of applications for lubricant additives in the disk drive media area, the first of which was for pseudo-contact recording with inductive heads (tri-pad sliders) in an effort to stabilize the head/disk interface and minimize lube decomposition under hot/wet conditions. A number of additives have been tried which include antioxidants as well as Lewis bases, the latter in an effort to passivate the catalytic activity of the Lewis Acid sites on the slider which results in the decomposition of the perfluoropolyether (PFPE) lubricants such as Z-Dol, AM and Z-Tetraol. In addition to this passivation action of the phosphazene toward catalytic decomposition of the lubricant, it has recently been reported that the use of X-1P (a cyclic phosphazene) also enhances reflow of the lube, increasing the durability of the head disk interface. In this regard there are still a number of unanswered questions that pertain to the mechanism of the interaction of the X-1P with the lubricant and/or carbon to cause this increase in mobility of the lubricant resulting in the enhanced durability.There are numerous technical issues associated with the use of the various additives with the main one being compatibility between the additive and the PFPEs as well as the carbon surfaces on which they are coated. These issues include bonding, phase separation of the components, and the transfer mechanism for the additive to the slider where the passivation is required.In this paper, we will look at the interaction of the X-1P with the carbon overcoat on the media in an effort to try to better understand the mechanism of such an interaction and its effect on the mobility of the lubricant as well as the amount of bonded lube on the disks.     

Effect of surface chemistry on the tribological performance of a MEMS electrostatic lateral output motor

The effect of surface chemistry on the tribological performance and reliability of a MEMS lateral output motor is reported. Relative humidity (RH) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) coatings were used to change surface chemistry. Electrical and tribological performance of uncoated and OTS-coated motors were found to be dependent on RH. For uncoated motors, excessive wear of sliding contacts and welding (permanent adhesion) of static contacts were observed at 0.1% RH. Degradation of electrostatic force and high static friction (stiction) forces limited dynamic performance and reliability and caused device sticking at and above 70% RH. Around 50% RH, uncoated motors exhibited negligible wear, low adhesion, and a wear life at least three orders of magnitude longer than in the dry environment (experiments were stopped without failure after about one billion cycles). Water vapor behaved as a gas phase replenishable lubricant by providing a protective adsorbed film. The OTS coating broadened the operating envelope to 30–50% RH and reduced stiction, which allowed better dynamic performance at high RH. The OTS coating improved durability at 0.1% RH, but it was still poor. At high RH, stiction problems reoccurred when the OTS coating was worn away. By controlling and balancing surface chemistry (adsorbed water and OTS), excellent performance, low friction and wear, and excellent durability were attained with the lateral output motor.     

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.     

Effect of Ultraviolet Irradiation on the Interactions between Perfluoropolyether Lubricant and Magnetic Disk Surfaces

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 253.7 nm 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 respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings

The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor. This revised version was published online in August 2006 with corrections to the Cover Date.     

The properties of various antiwear and extreme‐pressure additives in pentaerythritol ester

The tribological properties of three representative antiwear and extreme‐pressure additives, tricresylphosphate (TCP), triphenylthiophosphate (TPPT), and α‐mercaptobenzothiazole (MBT), in pentaerythritol ester at different concentrations were studied. It was found that TCP could react with Fe to form an organic phosphate film containing 4% P on the scar surface, while TPPT formed a blended organic phosphate and inorganic FeS film, containing 4% P and 6% S, respectively. MBT formed an inorganic FeS film containing 45% S. Ferrographic analyses of wear particles collected from oil samples after four‐ball tests showed that the higher the temperature the better the tribological performance of the additivated ester, due to its reaction with the metal surface.     

The lubricant migration rate on the hard disk surface

The optical surface analyzer (OSA) was found to be an excellent tool to examine the lubricant migration rate on thin film disks. Using the OSA it was found that the rate of lubricant migration increased as molecular weight decreased. An AM type perfluoropolyether with an aromatic end group and Z-Dol were also observed to have different migration rates for the same molecular weight. The migration rate of AM lubricant was increased significantly by the presence of X-1P as an additive in the lubricant system.     

Effects of Environment on Friction and Wear of Al-Si Alloy Impregnated Graphite Composite

Pin-on-disk-type wear experiments for an Al-Si alloy impregnated graphite composite (pin) in contact with a bearing steel (disk) were conducted at 100N normal load (100 Newtons) in air, argon, and deionized water to investigate the effects of environment on the tribological characteristics of the composite. The friction and wear behavior and the pin-lifting phenomenon due to wear particle ingress into the contact surfaces were continuously measured during the experiments. At low relative humidity (RH) levels, the friction coefficients in air and argon are high (0.32 to 0.39) and decrease with increasing RH to values around 0.2. The friction coefficients in air have reached a minimum of 0.15 to 0.17 between 50 and 70% RH and increased slightly at 80% RH. The friction coefficients in argon are constant at about 0.2 between 10 and 80% RH. Because of the lubricating action of a water film, the friction coefficient in deionized water is slightly lower (0.1 to 0.17) than that in air. The mean wear rate of 10^?4 to 8 × 10^?4 mm ³ /mm (specific wear rate; w _s = 10 ^?6 to 8 × 10^{? 6} mm ² /N) is very high in a severe wear regime at RH levels lower than 10% in air, decreases with increasing RH to a minimum in the middle RH range (30 to 60%), and increases slightly at RH levels higher than 70%. Although the mean contact pressure is very high (31.8 MPa), mild wear with the rates of 10^?8 to 10^?7 mm ³ /mm (w _s = 10^?10 to 10^?9 mm ² /N) occurs in the middle RH range. The same change in wear with RH as that in air is found in argon but the wear rate in argon is slightly lower than the wear rate in air. The height of the pin-lifting, having a wear reduction effect, is greater in argon than in air over almost the whole RH range. The wear rate in deionized water is nearly equal to the rate at 70% RH in air and argon.     

Molecular Dynamics Simulation of Lubricant Redistribution and Transfer at Near-Contact Head-Disk Interface

When the spacing between the slider and lubricant in a hard disk drive decreases to less than 5 nm, the effect of the intermolecular force between these two surfaces can no longer be ignored. This effect on the lubricant distribution at the near-contact head disk interface is investigated via molecular dynamics method. In this study, the lubricant is confined between a smooth disk surface and a rough slider surface represented as a partially cosinusoidal wave. The simulation results reveal that the intermolecular force-induced meniscus formation at the near-contact head disk interface is strongly sensitive to the slider-to-disk separation, lubricant film thickness and the asperity shape (or roughness) of the slider. The attractive van der Waals forces between the slider and lubricant become weaker with increasing slider-to-disk separation and asperity mid-height, but decreasing lubricant film thickness and asperity mid-width. The Hamaker theory application to van der Waals interactions is also introduced to verify the molecular dynamics simulation. It is found that the critical separation, below which the lubricant will lose its stability to form a meniscus, increases approximately linearly with the lubricant film thickness, for slider surfaces with or without roughness both in the molecular dynamics simulation and Hamaker theory application to van der Waals interactions. Moreover, it is observed that the critical separation between a smooth disk and rough slider surface will slightly decrease when the asperity mid-height increases. The same phenomenon is observed when the asperity mid-width reduces.     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

Acoustic emission studies of thin film rigid disks and proximity recording sliders and its applications to tribology

This work investigates the flying and contact phenomena of proximity contact recording sliders and their effects on the tribological performance of thin film media by the use of acoustic emission analysis. Proximity contact recording sliders included negative pressure and tri‐rail types of tripad sliders. Mechanical and laser‐induced textured magnetic rigid disks were used in this study. The laser bump heights of laser textured disks were controlled to achieve a desired tribological performance and the relationship between the laser bump heights and acoustic emission energy during drag and start/stop testing for different slider designs was studied. Emphasis was also placed on identifying the critical parameters of media and slider designs for tribological performance improvement. An analysis of the slider body natural frequencies during flying at the operational speed has been demonstrated to be well correlated to the contact behavior at the head‐disk interface. Moreover, the environmental and lubrication effects on the fixed tracking flyability performance were investigated and the acoustic emission data also agreed fairly well with the observed degree of contamination on sliders and degradation on the textured media. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Interactions at the head–tape interface of a linear tape system

In this study, tape cycling experiments were performed using two different experimental metal particle media in combination with a Travan™ linear tape system. The aim was to investigate the effect of head/tape contact on the tribological properties and signal performance of the system. A combination of cycling at extreme environmental conditions, use of experimental media, and cycling beyond the normal limit of operation ensured a worst-case scenario for the head–tape interactions. Auger electron spectroscopy (AES) and atomic force microscopy (AFM) were used to characterize the chemical and physical surface changes that occurred on and in the head surfaces. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical changes that occurred at the media surface and these changes were correlated to variation in signal dropout rate. Measurements were made as functions of number of cycles.The cycling experiments for the two different tapes were performed at conditions of 32°C, 80% RH and 5°C, 10% RH and transfer of material from the media to the head was observed at each condition for both tape types. The degree of material transfer was influenced by the environmental operating conditions, but was governed by localized heating effects, such as those originating from active magnetoresistive (MR) elements and frictional interactions between the head and media. XPS analyses of the surface of the media revealed a reduction of nitrogen with increasing number of cycles indicating binder depletion. Significant differences in elemental concentrations were also detected between areas corresponding to regions directly under the cartridge belt compared to those off belt.     

Tribological characterization of electroless Ni–10% P coatings at elevated test temperature under dry conditions

An experimental study of wear characteristics of electroless Ni–10% P coating sliding against hard AISI 52100 steel pin is investigated. Experiments are carried out at room and 550°C temperatures. Heat treatment effects on tribological behavior of this coating are studied. The wear surface and the microstructure of the coatings are analyzed using optical microscopy, scanning electron microscopy, energy dispersion analysis X-ray, and microhardness testing equipment. It is observed that the forming of continuous oxide film on contacting surfaces of pin and disk improves wear resistance and decreases friction coefficient of the Ni–10% P coating. The results indicate that the wear resistance of electroless Ni–10% P coating has improved with heat treatment at room temperature wear test, but it reverses in the wear test at 550°C. In addition, specimens without heat treatment have the highest wear resistance and the lowest friction for wear tests at elevated temperatures.