Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.     

Pumping effect of sliders in hard disk systems

The accumulation of contaminants on the slider surface is of paramount importance in hard disk drives because only an ultra small amount of contaminants on the slider surface will cause catastrophic failures for hard disk drives with a spacing between the slider and the hard disk as small as 10 nm, which will be reduced further in the near future to about 5–6 nm in order to attain a recording density of 100 Gbit/in². In this paper the pumping effect of the slider is proposed as one mechanism of the contaminant accumulation on the slider. Analysis of the pumping effect is conducted by considering the adsorption process and the shear flow process on the slider surface in terms of the continuum. It is found that the pumping effect can be divided into two different classifications depending on the value of the parameter λ which is the ratio of the maximum shear flow of the adsorbed film to the maximum adsorption amount: the shear flow rate-controlling pumping effect for λ < 0.1 and the adsorption rate-controlling pumping effect for λ > 0.4. For the shear flow rate-controlling effect, the accumulation rate of the contaminant is directly proportional to the disk surface velocity, while inversely proportional to the flying height of the slider. An erratum to this article can be found at .     

Nanolubrication: Characterization of patterned lubricant films on magnetic hard disks

Patterned lubricant films on magnetic hard disks offer potential advantages in controlled bonding sites, higher average shear strength, and longer durability. However, since the lubricant film thickness is at 1 or 2 nm, characterization of the pattern is difficult. Normal atomic force microscopic techniques can only image very small area in the nanometer range and the sharp tip can potentially modify the pattern. A wide area optical technique is needed to characterize the patterns. This paper examines patterned lubricant film using an optical surface analyzer (OSA) to image the bonded phase and mobile phase of an alcohol functionalized perfluoropolyether (PFPE) on magnetic hard disks. The phase shift signal and reflectivity intensity of the polarized light spectra provide clear optical images of the lubricant film at nanometer thickness. Optical images were successfully obtained before and after the buffing process and the ramp load and unload (L/UL) testing. Results of 100% bonded, 100% mobile, and 20% zigzag patterned lubricant films confirm that the patterned lubricant films can control the bonded/mobile ratio of such films better.     

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.     

计算机硬盘磁头滑块的设计与加工研究

研究计算机硬盘磁头滑块承载面结构的设计理论与加工方法。分析了现有磁头滑块的结构特点及其设计理论和设计方法;利用光刻及干法刻蚀的工艺及设备,进行了所设计磁头滑块的微加工;并利用三维表面形貌测量仪对所加工的磁头进行了加工质量分析,给出实验测量值与设计值的比较结果。     

Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tbit/in² hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.     

Hydrogen bonding in lubricants for hard disk drives

Hydrogen bonding interaction within a small ensemble of water molecules, that within a group of water molecules and end-groups of Z-dol and Z-tetraol, and the effect of electrolyte ionic pair such as LiCl upon these interactions were examined by the molecular dynamics method based on the potential given by a semi-empirical SCF quantum mechanics. It was revealed that the strength of the hydrogen bond increased rapidly as the size of droplet increased, relating to the population density of hydroxyl units, and that such interaction was amplified significantly by the presence of electrolyte ionic pair. An extraordinary interaction was thus predicted between Z-tetraol and aqueous solution of alkali halide. An experimental study thence conducted revealed that Z-tetraol and aqueous NaCl solution (2 M) formed an extremely stable water-in-oil type emulsion. The emulsion consists of spheroids of several nanometers across wherein several thousands of water molecules are encased by several tens of Z-tetraol end-groups. The interfacial layer of each spheroid is formed and stabilized by the hydrogen bonding interaction between the hydroxyl units of the tetraol-ends and water molecules enhanced by the electrolyte ionic pairs. When disks coated with Z-tetraol were tested for flyability at high humidity, the head-disk interaction detected acoustically increased with time. Spontaneous formation of globules resulting from interaction of tetraol end-groups and water molecules assisted by ubiquitously present alkali halide contaminant would account for the observed increase of the head-disk interaction. Possible structures of perfluoropolyether lubricants ideal for magnetic disk application are discussed.     

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.     

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.     

Adaptive multilevel method for the air bearing problem in hard disk drives

An adaptive grid-generating algorithm is constructed and integrated with the multigrid method to form a numerical scheme that suits slider air-bearing simulation of hard disk drives. The relative truncation error, a by-product of the multigrid method, is used in grid adaptation criteria. Finer meshes are constructed over nodes of the current finest grid where the relative truncation error exceeds a predetermined tolerance. The union of these finer meshes forms a new level of grid, which may not cover the entire domain of the coarse grid underneath. The final grid system thus constructed is composed of levels of uniform grids with decreasing mesh sizes. This composite grid structure incorporates with numerical resolution as needed and efficiency of computation. A shaped rail, negative pressure slider is used to demonstrate the effectiveness of this numerical scheme. Compared with the traditional multigrid method, the proposed adaptive multilevel method can significantly reduce the computation work for achieving the same level of accuracy.     

Benzotriazole as the additive for ionic liquid lubricant: one pathway towards actual application of ionic liquids

In this paper, we report on the first tribological evaluation of the room temperature ionic liquids (RTILs) compatible lubricant additive. Benzotriazole (BTA) was chosen for study in that it shows good miscibility with imidazole ionic liquids because of similar molecular structure. BTA can greatly improve the tribological behaviors of ionic liquids carrying hexafluorophosphate anions for Steel/Cu–Sn alloy sliding pair mainly because of the alleviation of corrosion. The worn surface of the bronze was investigated by X-ray photoelectron spectroscopy (XPS), which revealed complex tribochemical reactions during the sliding process. A protective film comprised of [Cu(–C₆H₅N₃)] and Cu₂O is formed. Strong interaction between benzotriazole and the surface of Cu alloy was proposed to account for the excellent anti-wear and anti-corrosion improvement capability.     

Tribological properties of a perfluoropolyether lubricant with 3-phenylpropyl functional group for hard disk media

A new perfluoropolyether lubricant (LUB-A) with a 3-phenylpropyl functional group at both ends of the main chain was designed and synthesized by the authors for use in hard disk media, and its tribological performance was evaluated. Time-of-flight secondary ion mass spectrometry showed that LUB-A film on the disk lost none of its functional groups even after a 672 h exposure at 23 °C and 55% RH, although AM3001 lost 94% of its (3,4-dioxomethylenephenyl)methyl functional groups after the same period of exposure. It was found that cyclotriphosphazene (X-1P) was more than four times more soluble in 2 nm thick LUB-A film than in AM3001 film. The degradation ratio of read-back signals in the seek test was less than 5% over a wide range of X-1P content in LUB-A film and no micro-phase separation was induced, in contrast to AM3001 film that could not keep the degradation ratio at less than 5% without inducing micro-phase separation. LUB-A also showed better migration properties at 80 °C and 3% RH than AM3001. LUB-A was proved to be more chemically stable and to show better tribological performance than the currently popular AM3001 when it was used as a thin film mixed with X-1P.     

Preparation of ultrathin amorphous ice films for cryo-electron microscopy

Electron microscopy of biological macromolecules embedded in vitrified ice films suffers from serious problems caused by excessive inelastic scattering, beam-induced movements of the specimen, deformation of the molecules by adsorption at the water-air interface and insufficient mechanical stability of the films. We have built an environmental chamber to control temperature and humidity independently in order to produce ultrathin water films (< 20 nm) spanning holes with diameters of 20 μm to 1 mm. The surface tension of the water films was reduced by adding lipid monolayers, thus prolonging the usable time for thinning of the film and avoiding adsorption artefacts in the embedded material. After cryofixation in ethane a carbon film was evaporated on each side of the specimen to stabilize the ultrathin ice—lipid film. Mechanical stability and charging effects could successfully be reduced by this preparation method. Collapsing water films could be cryofixed and the shape of the hole was analysed. By the eccentricity of the elliptical holes an estimation could be made of the burst velocity of the rim of the hole and of the cooling rate of the cryofixation process.     

Tribological performance of an ionic liquid as a lubricant for steel/aluminium contacts

The wear and friction behaviour of an ionic liquid 1‐ethyl‐3‐hexylimidazolium tetrafluoroborate (L206) was investigated as a lubricant for steel/aluminium contacts using an Optimol SRV® oscillating friction and wear tester. The elemental composition and chemical nature of the antiwear films generated on the aluminium surface were analysed using a scanning electron microscope with a Kevex energy dispersive X‐ray analyser attachment (SEM/EDS) and X‐ray photoelectron spectroscopy (XPS). A low friction coefficient (˜0.05) was recorded when lubricating with L206; a small amount of water (5 wt. %) in L206 effectively reduced the wear volume and greatly increased the microhardness of the aluminium alloy, but had little effect on the friction coefficient. The SEM/EDS results showed that severe corrosive wear occurred on the aluminium alloy when lubricating with neat L206, which could be avoided by the addition of water in L206. The XPS results indicated that the species AlF₃, Al₂O₃, AlO(OH), and Al(OH)₃ formed during friction; there was no indication of boron on the worn surfaces.     

Improvement of tribological performance of steel by solid lubricant shot-peening in dry rolling/sliding contact wear tests

The effect of shot-peening treatment with the particulate MoS₂ solid lubricant on the wear resistance of steel in the dry rolling/sliding contact wear tests was investigated. The duplex shot-peening treatment with ceramic balls and the particulate MoS₂ solid lubricant provided excellent wear resistance under a severe loading and sliding condition because the uniform and minute surface roughness given by shot-peening treatment with ceramic balls could keep shot-peened MoS₂ particles with a low friction coefficient on the sample surface. Furthermore, the sample surface was covered with shot-peened MoS₂ particles by a MoS₂ layer formed during the rolling/sliding contact wear test.     

Tribological characteristics of bonded MoS2 films evaluated in rolling-sliding contact in a vacuum

Tribological characteristics of bonded MoS₂ films have been evaluated in air, nitrogen and vacuum using a two-roller type rolling-sliding friction apparatus. Two conventional space-proven films, polyamideimide-bonded and sodium silicate-bonded, designated as film A and film B respectively, were tested at a speed of 998 rev min^-1 under a load of 100 N at slip ratios of 1–100%, the slip ratio being defined as (V₁ - V₂)/ V₁ × 100%, in which V₁ and V₂ was rotational speed of each roller.

The wear life of the film A was more slip ratio dependent than that of the film B. The wear life of the film A tested in a vacuum was the longest at 10% slip ratio but it reduced significantly at 1%. On the contrary, the best durability of the film B was obtained at 100% slip ratio (pure sliding). The wear life of the film A shortened considerably in air and nitrogen.

In general, a solid lubricant film applied to spur gears is subjected to rolling-sliding friction of 0-10% slip ratios. To improve the endurance life of the film A at 1% slip ratio, films prepared by changing content of pigments (MoS₂ and Sb₂0₃) were evaluated. The films with a pigment content of 5-15% showed a marked improvement in wear life at a slip ratio of 1%, while maintaining excellent durability at 10%.

The wear process of an improved film was monitored using a gap sensor. It appears to be dominated by fatigue.     

The measurement of magneto-optical Kerr effect of ultrathin films in a pulsed magnetic field

A Kerr rotation measurement system in a pulsed magnetic field (up to 11 T) was built to study magnetic properties of several ultrathin films. Our result shows that the Kerr rotation angle increases with the increasing wavelength of the incident light, while the difference between the spectras of Fe and CoFeAl is attributed to plasma resonance. We also studied the dynamic properties of the magnetic films: while ferromagnetic materials (Fe, CoFeAl, MnAs and CoMnAl) show quasi-static behavior in the time-scale of a few 100 μs, diamagnetic material GaAs shows time-dependent hysteresis.     

XPS and XANES characteristics of tribofilms and thermal films generated by two P- and/or S-containing additives in water-based lubricant

Two phosphates were synthesized and their tribological properties as water-soluble lubricant additives were evaluated by using four-ball tester. The micro/nano-scale chemical characteristics of tribofilms and thermal films formed from these additives in different conditions were explored by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy. The results show that the additives have better anti-wear and friction reducing properties than the oleate solution at higher load. Surface analysis results indicate that tribofilms are mainly composed of absorbed layer and chemical reactant layer, in which phosphorus exists in the form of adsorbed molecule, phosphate or polyphosphate, and sulfur in the form of alkyldisulfide, sulfide and sulfate. As to the thermal films, phosphate (or polyphosphate) and sulfate are detected as the main components. The anti-wear and friction-reducing performances can be ascribed to the formation of films on the metal surface, and the mechanism of the process of molecules adsorption, new compounds production through tribochemical reactions, film formation and destruction.     

Possibility of slip in hydrodynamic oil films under sliding contact conditions

In hydrodynamic lubrication theory, the oil film thickness build‐up increases with increasing sliding speed or oil viscosity, and the viscous resistance or shear stress also increases, both without limit. The entraining force forming the oil film is given by the moving surfaces, or by the adhesive force of the oil molecules on the rubbing surfaces and the interaction force between them. Therefore, the maximum friction force and maximum oil film thickness will be limited by the operating conditions, such as oil properties, rubbing materials, sliding speed, and load. In this study, friction tests were conducted using a plate‐on‐cylinder sliding contact apparatus. It was found that a critical shear stress existed, above which the friction force and oil thickness decreased from theoretical values. Slip in an oil film seems to occur when the theoretical shear stress exceeds the critical value of the oil, according to test conditions. The occurrence of slip in an oil film is responsible for the reduction in the oil film and friction force from theoretical values, leading to the lower‐viscosity components of the oil selectively passing through the conjunction zone.     

Behavior of thin viscous boundary films in lubricated contacts between micro-textured surfaces

Thin film colorimetric interferometry was used to map changes in film thickness in the vicinity of micro-dents of various depths produced on rubbing surfaces. It has been shown in recent studies that shallow micro-features within concentrated contact can increase mean film thickness by supplying more lubricant to the contact; however, this beneficial effect can also be accompanied by a local film thickness reduction. Nevertheless, these observations were done with mineral base oils that exhibited no boundary films formation. In this study the behavior of micro-textured surfaces are observed using formulated lubricant containing polyalcylmethacrylate (PAMA), viscosity index improver with boundary film forming properties. Obtained results show that an enlarged film thickness due to the presence of viscous boundary films is formed within the whole contact and these boundary films minimize the local film thickness reduction caused by micro-dents and further increase the efficiency of surface texturing within non-conformal contacts. It can be suggested from the obtained results that joint action of both boundary film formation and surface texturing combines both contributions that can help to increase tribological performances in different stages of machine parts operation by increasing lubrication film thickness.