Ultrathin Lubricant Film Research Articles

Linearized analysis of the deformation of ultra-thin lubricant films under gas pressures by the long wave equation

Long wave theory was employed to examine the deformations of ultra-thin but continuum liquid films produced by applied pressures and shearing stresses. Long wave theory is based on the time-evolution equation for the shape and deformation of thin liquid films and includes surface tension and surface forces such as van der Waals forces. As the deformations caused by gas pressures are usually very small, the linearized long wave equation, which is applicable to infinitesimal deformations, was derived and employed to predict the steady-state liquid surface deformations of a non-polar lubricant produced by concentrated or distributed gas pressures. It was found that the results obtained using the linearized equation agree well with those obtained using the nonlinear long wave equation.

I42 超薄膜潤滑における吸着分子層の影響(I4 機素潤滑設計2)

I42 超薄膜潤滑における吸着分子層の影響(I4 機素潤滑設計2)

2307 超薄膜液体潤滑膜の流動特性に及ぼす温度の影響(要旨講演,情報機器コンピュータメカニクス)

The effect of temperature on the spreading characteristics of ultra-thin liquid lubricant films has become more and more important in current hard disk drives. In this study, the spreading of molecularly thin Perfluoropolyether (PFPE) lubricant films on the carbon surfaces was investigated experimentally, varying the temperature of the micro-ellipsometer test stage. Three types of the lubricants ware used. The apparent diffusion coefficients of the test lubricants were also evaluated and compared with the trend of the bulk lubricant viscosity change for temperature. As a result, the temperature dependence of the spreading characteristics of lubricant films was clarified and it was found that A20H lubricant has the highest robustness for the temperature change among the test lubricants.

3414 環境制御型原子間力顕微鏡を用いたプローブと超薄膜液体潤滑膜との相互作用に関する研究(S70-2 情報機器コンピュータメカニクス(2),S70 情報機器コンピュータメカニクス)

Recently, it is important to clarify the interaction forces between slider and disk surfaces in a nanometer head-disk interface region in HDD, in order to achieve ultra-high magnetic recording density. This paper describes the interactions between Atomic Force Microscope (AFM) probe covered by conductive diamond thin film and ultra-thin liquid lubricant films. In this study, AFM was set in the environment controlled chamber. We investigated the difference in a response of the force curve on two types of lubricants (Zdol2000, A20H2000), relative humidity and lubricant film thickness. From the experimental results, it was found that meniscus bridge is easy to be extended at high relative humidity. Touch-down distance gradually increased with the rise in relative humidity and was greater as the lubricant film thickness increases. On the other hand, adhesive force gradually decreased with the rise in relative humidity as well as the lubricant thickness.

2301 光アシスト磁気記録におけるヘッドディスクインタフェースに関する研究(要旨講演,情報機器コンピュータメカニクス)

Laser-assisted magnetic recording is one of the novel technologies for achieving a magnetic recording density of more than 1Tb/in^2. In this technology, it is suggested that there is serious issue for the ultra-thin liquid lubricant films on the disk surface. Because they are heated to high temperature with laser beams. In this study, we focused on the thermal stability of the lubricant film in laser-assisted magnetic recording. In other words, the behavior of lubricant film due the laser heating was investigated experimentally and discussed. As a result, it was found that the lubricant depletion was observed and could not be replenished. In addition, the effects of laser power as well as the laser irradiation time on the lubricant film depletion were studied and clarified.

Development of contact sliders with nanotextures by femtosecond laser processing

This paper describes the development of novel contact sliders with nanotextures by means of femtosecond laser processing. In order to achieve a magnetic recording density of more than 1 Tb/in2, it is necessary to create an ultralow head-disk interface (HDI) spacing, i.e., less than 2–3 nm. Thus far, various new concept HDI technologies have been proposed for developing such near-contact or contact recording HDI. In this paper, the authors propose the concept of a novel contact slider. This slider has nanotextures fabricated by femtosecond laser processing. Sliders with five DLC thin-film pads on the air bearing surfaces were used, and the pad diameter and thickness are 40 μm and 40 nm, respectively. The fabrication process of nanotextures on the DLC thin-film pads employed a femtosecond Ti:sapphire laser. The laser wavelength and pulse width are 800 nm and 120 fs, respectively. Contact slider experiments were also carried out, using (1) the fabricated contact sliders having nanotextures with different RMS heights and (2) a contact slider without nanotextures. The nanotribological characteristics of the contact HDI as well as the dynamics of the contact sliders were investigated. In addition, the behavior of the ultrathin liquid lubricant film was evaluated by using a surface reflectance analyzer (SRA) after the contact slider experiments. It was found that the nanotextures produced by femtosecond laser processing are very effective for next-generation contact sliders and the optimum RMS heights of the nanotextures can be obtained. These advancements will facilitate the development of contact HDI with lower friction, lower wear of the contact slider surfaces, smaller change in the lubricant film thickness and lower contact slider bouncing vibration.

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.

Nanotribological characterization of perfluoroalkylphosphonate self-assembled monolayers deposited on aluminum-coated silicon substrates

Aluminum-coated silicon substrates are commonly used for various micro/nanooptoelectromechanical systems (MOEMS/NOEMS) including Digital Micromirror Devices (DMD®). For efficient and failure proof operation of these devices, ultra-thin lubricant films of self-assembled monolayers (SAMs) are increasingly being employed. Fluorinated molecules are known to exhibit low surface energy, adhesion, and friction, desirable for tribological applications. In this study, we investigate contact angle, surface energy, friction, adhesion, and wear properties of a perfluoroalkylphosphonate SAM and compare them with those of alkylphosphonate SAMs. The influence of relative humidity, temperature, and sliding velocity on the friction and adhesion behavior is studied. Failure mechanisms of SAMs are investigated by wear tests. These studies are expected to aid in the design and selection of proper lubricants for MOEMS/NOEMS.

Temperature dependence effect of viscosity on ultrathin lubricant film melting

We study the melting of an ultrathin lubricant film under friction between atomically flat surfaces at temperature dependencies of viscosity described by Vogel-Fulcher relationship and by power expression, which are observed experimentally. It is shown that the critical temperature exists in both cases the exceeding of which leads to the melting of lubricant and, as a result, the sliding mode of friction sets in. The values of characteristic parameters of lubricant are defined, which are needed for friction reduction. In the systems, where the Vogel-Fulcher dependence is fulfilled, it is possible to choose the parameters at which the melting of lubricant takes place even at zero temperature of friction surfaces. The deformational defect of the shear modulus is taken into account in describing the lubricant melting according to the mechanism of the first-order transition.

1501 環境制御型原子間力顕微鏡を用いたプローブ・超薄膜液体潤滑膜の相互作用に関する研究(GS-16 精密機器(1))

1501 環境制御型原子間力顕微鏡を用いたプローブ・超薄膜液体潤滑膜の相互作用に関する研究(GS-16 精密機器(1))

1503 ナノメータ浮上する浮動ヘッドスライダと超薄膜液体潤滑膜との相互作用に関する研究(GS-16 精密機器(1))

1503 ナノメータ浮上する浮動ヘッドスライダと超薄膜液体潤滑膜との相互作用に関する研究(GS-16 精密機器(1))

2110 ナノメータ浮上する浮動ヘッドスライダと超薄膜液体潤滑膜との相互作用(OS2-3 トライボロジーとその応用(3))

2110 ナノメータ浮上する浮動ヘッドスライダと超薄膜液体潤滑膜との相互作用(OS2-3 トライボロジーとその応用(3))

Effect of Ultrathin Lubricant Films on Acoustic Emission Generated by Sliding Friction

Effect of Ultrathin Lubricant Films on Acoustic Emission Generated by Sliding Friction

Stochastic theory of ultrathin lubricant film melting in the stick-slip regime

Melting of an ultrathin lubricant film under friction between atomically smooth surfaces is studied in terms of the Lorentz model. Additive noise associated with shear stresses and strains, as well as with film temperature, is introduced, and a phase diagram is constructed where the noise intensity of the film temperature and the temperature of rubbing surfaces define the domains of sliding, dry, and stick-slip friction. Conditions are found under which stick-slip friction proceeds in the intermittent regime, which is characteristic of selforganized criticality. The stress self-similar distribution, which is provided by temperature fluctuations, is represented with allowance for nonlinear relaxation of stresses and fractional feedbacks in the Lorentz system. Such a fractional scheme is used to construct a phase diagram separating out different types of friction. Based on the study of the fractional Fokker-Planck equation, the conclusion is drawn that stick-slip friction corresponds to the subdiffusion process.

Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.

Behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.

Influence of Contact Pressure on Central and Minimum Film Thickness Within Ultrathin Film Lubricated Contacts

Recent numerical results have indicated that the relationship between the film thickness and the speed may not always follow the simple power law, especially under severe conditions. This paper is aimed at obtaining experimental results at high contact stresses and low speeds to study the thin film behavior. Ultrathin lubricant films were observed at maximum Hertz pressures of 0.52, 1.01, and 1.54 GPa by using an optical test rig. Central and minimum film thickness values were obtained with thin film colorimetric interferometry from chromatic interferograms. The nonlinear behavior of both central and minimum film thicknesses in log-log coordinates was observed as rolling speed and thereby film thickness decreased. This tendency became more obvious at higher contact pressures.

Nanotribological characterization of self-assembled monolayers deposited on silicon andaluminium substrates

Silicon and aluminium are the substrates of choice for variousmicro/nanoelectromechanical systems (MEMS/NEMS) including digital micromirrordevices (DMD®). For efficient and failure-proof operation of these devices, ultrathin lubricant films ofself-assembled monolayers (SAMs) are increasingly being employed. In this study, weinvestigate friction, adhesion and wear properties of various SAMs. Surface properties suchas contact angle, adhesive force, friction force and coefficient of friction are compared forSAMs with hydrocarbon and fluorocarbon backbone chains with different chemicalstructures, chain lengths and end groups. The influence of relative humidity, temperatureand sliding velocity on the friction and adhesion behaviour is studied for various SAMs.Failure mechanisms of SAMs are investigated by wear tests and the potential mechanismsinvolved are discussed. These studies are expected to aid the design and selection of properlubricants for MEMS/NEMS.

The dynamic behavior of ultrathin lubricant films

Molecular dynamics simulations with the Langevin equation using a coarse-grained, bead-spring model were performed to investigate the dynamic properties of nanoscale lubricant films. The simulated spreading profiles of lubricant nanofilms are in qualitative agreement with previous experimental results. The Einstein’s relationship and Green–Kubo formula provide alternative perspectives for describing the diffusive phenomena of lubricant nanofilms. The effects of molecular weights and their distribution, temperature, and film thickness were examined.

Stability analysis of ultrathin lubricant films via surface energy measurements and molecular dynamics simulations

The stability of nanoscale lubricant films was analyzed via both surface energy measurements and molecular dynamics (MD) simulations. Using the sessile method, the contact angles of deionized water and n-dodecane on Zdol lubricant films were measured to examine the dispersive and polar surface energy as well as the nanofilm stability. By calculating the free energy from MD simulations, we investigated surface energy of the lubricant film. Furthermore, the film disjoining pressure and the stability diagram were constructed from MD simulations to examine the layering structures in spreading phenomena. Our analysis exclusively focuses on the effects of the end-functionality and molecular weight.