Yasunaga Mitsuya

Nagoya Municipal Industrial Research Institute, Nagoya, Aichi, Japan

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Publications (124)86.79 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The friction properties of nanometer-thick lubricant films are crucial in the reliability and durability of miniaturized moving mechanical components in micro- and nanoelectromechanical systems and hard disk drives. In this work, gas cluster ion beam treatments were applied to prepare smoothed glass sliding pins and diamond-like carbon (DLC) coated sliding pins for pin-on-disk friction tests to study the effect of the surface roughness and the DLC coating on the friction properties of perfluoropolyether (PFPE) films. The effect of the texture of the solid surface on the friction properties was also investigated. The friction properties were not exclusively determined by the surface energies of the solids. The friction coefficients of the nanometer-thick PFPE films, confined between solid surfaces were sensitive to the surface roughness of the solids. The friction coefficient generally increased with an increasing surface roughness (the composite standard deviation of the surface roughnesses of the pair of solid surfaces). This tendency was reversed when there was a strong longitudinal roughness effect at the contact interface. Moreover, the DLC-coated sliding pins had lower friction coefficients than the glass sliding pins, indicating that the friction properties were noticeably affected by the hardness and Young׳s modulus of the contact materials. The mechanisms for the effects of the surface roughness, surface texture and contact materials on the friction properties are discussed.
    Wear 11/2014; 319(s 1–2):56–61. DOI:10.1016/j.wear.2014.07.010 · 1.86 Impact Factor
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    ABSTRACT: For micro/nanoscale devices, surface functionalization is essential to achieve function and performance superior to those that originate from the inherent bulk material properties. As a method of surface functionalization, we dip-coated nanometer-thick liquid lubricant films onto solid surfaces and then patterned the lubricant films with ultraviolet (UV) irradiation through a photomask. Surface topography, adhesion, and friction measurements demonstrated that the patterned films feature a concave–convex thickness distribution with thicker lubricant in the irradiated regions and a functional distribution with lower adhesion and friction in the irradiated convex regions. The pattern linewidth ranged from 100 to as fine as 0.5 μm. The surface functionalization effect of UV-patterning was investigated by measuring the water contact angles, surface energies, friction forces, and depletion of the patterned, as-dipped, and full UV-irradiated lubricant films. The full UV-irradiated lubricant film was hydrophobic with a water contact angle of 102.1°, and had lower surface energy, friction, and depletion than the as-dipped film, which was hydrophilic with a water contact angle of 80.7°. This demonstrates that UV irradiation substantially improves the surface and tribological properties of the nanometer-thick liquid lubricant films. The UV-patterned lubricant films exhibited superior surface and tribological properties than the as-dipped film. The water contact angle increased and the surface energy, friction, and depletion decreased as the pattern linewidth decreased. In particular, the 0.5-μm patterned lubricant film even showed a larger water contact angle and lower friction and depletion than the full UV-irradiated film. These indicate that UV-patterning of nanometer-thick lubricant films with a minimized linewidth has a better surface functionalization effect than full UV irradiation. Enhancement of the surface functionalization effect may be attributed to a transition in the contact state, which was indicated by the different instantaneous friction behavior of the 0.5-μm patterned lubricant film.
    Applied Surface Science 11/2014; 320:102–111. DOI:10.1016/j.apsusc.2014.09.083 · 2.54 Impact Factor
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    ABSTRACT: To measure the speed-dependent friction properties of molecularly thin lubricant films coated on magnetic disks, we developed a novel ball-suspension assembly (BSA) that allows us to slide a ball on a disk with steady contact at high sliding speed up to 210 mm/s. With increasing sliding speed, the frictional force exerted on the contacting point of the ball increases, and the frictional moment that serves to rotate the ball increases. The frictional moment is more likely to excite the out-of-plane vibration of the suspension than the frictional force itself. In the newly designed BSA, only a small portion of the ball protrudes from the suspension surface so that the frictional moment is minimized. We confirmed that, as compared with a conventional BSA, the new BSA satisfactorily suppresses the out-of-plane vibration of the suspension and the horizontal vibration of the friction transducer to which the BSA is connected. Using the new BSA, we measured speed-dependent friction properties of monolayer perfluoropolyether Zdol4000 lubricant film in the sliding speed range of 2.1-210 mm/s. The measured frictional force was found to be nearly proportional to the 1/3 power of the sliding speed.
    IEEE Transactions on Magnetics 01/2014; 50(11):1-4. DOI:10.1109/TMAG.2014.2318776 · 1.21 Impact Factor
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    ABSTRACT: Friction properties of nanometer-thick lubricant films confined between two ultra-smooth solid surfaces are crucial to the practical performance of technologically advanced mechanical devices such as micro-electro-mechanical systems and hard disk drives. In this work, we applied argon gas cluster ion beam (Ar-GCIB) treatments to obtain ultra-smooth sliding pins for pin-on-disk tests of nanometer-thick perfluoropolyether (PFPE) lubricant films coated on magnetic disk surfaces. The GCIB treatments effectively smoothed the pin surfaces, and increases in the Ar dose decreased surface roughness. An ultra-smooth surface with a maximum peak height (Rp) less the monolayer lubricant film thickness was achieved when the Ar dose was increased to 8 × 1016 ions/cm2. We observed that both surface roughness and film thickness affected the friction coefficients of the PFPE films. To quantitatively describe the interplay of surface roughness and film thickness, we introduced two roughness characteristics: the ratio of film thickness to the surface's root-mean-square roughness (h/σ), and a surface-pattern parameter (γ), defined as the ratio of correlation lengths in two orthogonal directions. We infer that a fixed γ and higher h/σlead to lower friction coefficients, while a fixed h/σand higher γ induce higher friction coefficients.
    Applied Surface Science 09/2013; 280:619-625. DOI:10.1016/j.apsusc.2013.05.036 · 2.54 Impact Factor
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    ABSTRACT: To protect the interface against intermittent head–disk contact in hard disk drives, nanometer-thick perfluoropolyether (PFPE) films consisting of both “bonded” and “mobile” molecules are applied on the disk surfaces. Because of their different adsorption states and mobility, the bonded and mobile molecules are supposed to contribute differently to friction properties, which directly impact the stability of ultra-low flying head–disk interfaces. By measuring the friction force at light loads and low to high speeds as a function of bonded and mobile film thicknesses, we studied the contributions of bonded and mobile molecules to the dynamic friction of nanometer-thick PFPE films. We found that the friction coefficient of lubricant films without or with less bonded molecules increased as a power function of sliding speed, whereas that of lubricant films with more bonded molecules increased logarithmically with sliding speed. We suggest that these results can be explained by the following mechanisms: the dynamic friction of lubricant films without and with less bonded molecules is dominated by shear thinning behavior of mobile molecules, while that of lubricant films with more bonded molecules is governed by bonded molecules which lead to boundary lubrication.
    Tribology Letters 06/2013; 54(3). DOI:10.1007/s11249-013-0248-1 · 2.15 Impact Factor
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    ABSTRACT: Monolayer perfluoropolyether (PFPE) films used for lubrication of magnetic disks are generally dip-coated from the PFPE solutions diluted by fluorinated solvents. In this paper, under lightly loaded (0-1 mN) and slowly rotated (2.1 mm/s) conditions that suppress disturbance of PFPE films during sliding, we investigate the effect of solvents on the friction properties of monolayer PFPE films coated on magnetic disk surfaces. The solvents include HFE-7200 and Vertrel-XF. For polar Zdol2000 and Zdol4000 films, friction fluctuated during one revolution of the disk immediately after film deposition, but became stable with elapsed time. Such transient friction behavior was not observed for nonpolar Z03 films, suggesting that the friction fluctuations exhibited by Zdol2000 and Zdol4000 films result from PFPE microstructure (such as film uniformity and molecular conformation) induced by the polar end groups. Despite of the higher solvent power of Vertrel-XF, the friction fluctuations in the transient state were noticeable for the films applied with Vertrel-XF as compared with those applied with HFE-7200. However, because the friction fluctuations were characterized by intermittent decrease of friction in narrow and sparsely distributed regions, the average value of the friction measured during one revolution of the disk was almost not affected by the friction fluctuations and showed weak dependence on the solvent.
    IEEE Transactions on Magnetics 10/2009; 45(10):3632-3635. DOI:10.1109/TMAG.2009.2021407 · 1.21 Impact Factor
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    ABSTRACT: Under the conditions of light load and slow rotation, we measured the frictional properties of monolayer PFPE Zdol2000 and Zdol4000 films coated on magnetic disk surfaces as a function of solvent used to apply the lubricants. The solvents included HFE-7200 and Vertrel-XF. A transient friction behavior was observed before reaching the equilibrium state. In the transient state, the instantaneous friction force exhibited a large fluctuation for the films applied from Vertrel-XF compared with those applied from HFE-7200. However, averages of the instantaneous friction forces showed weak dependence on solvent.
  • Yasuji Ohshima, Yasunaga Mitsuya, Kenji Fukuzawa
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    ABSTRACT: We measured vertical elongation and adhesive force of nanometers-thick lubricant films using a specially-fabricated probe. A diamond tip and a micro-mirror were mounted to form a nano-lubricant-bridge and reflect a detecting laser beam on the front and back surfaces of the probe cantilever, respectively. In contrast to generally used probe microscopy, the tip displacement was measured using a Michelson interference microscope and an ultra high-speed video camera which enables measurements in micron dynamic range and at sub-nanometer accuracy. In this research, we succeeded in quantifying the vertical elongation and adhesive force of sub-10-nm thick lubricant film as functions of film thickness, molecular weight, end group functionality, and bonding ratio of the lubricants.
  • Takefumi Hayashi, Hideyo Yoshida, Yasunaga Mitsuya
    Journal of Tribology 01/2009; 131(2). DOI:10.1115/1.3063696 · 0.90 Impact Factor
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    ABSTRACT: To ensure the reliability and durability of the head-disk interface of magnetic disk drives, the lubricant film flow coated on the disk surface has become a significant concern because it is related to recovery performance when the film is damaged. In this study, the replenishment characteristics of perfluoropolyether (PFPE) polar lubricant films after lubricant depletion induced by a glass ball sliding on the disk were clarified by measuring the profile changes of the lubricant film. Lightly loaded and slowly rotated test conditions were selected to avoid strongly disturbing the molecular layering structure. We found that the replenishment process dramatically changed with increasing film thickness, and recoverable and unrecoverable features appeared depending on the coated and residual film thicknesses. The replenishment process was compared with the step-boundary flow, i.e., the flow from the step-shaped lubricant boundary and the disjoining pressure distributions. The diffusion coefficients calculated from the step-boundary flow were found to be ineffective for replenishment flow, and the disjoining pressure differences between the coated and residual film thicknesses bifurcate the replenishment flow into recoverable and unrecoverable features.
    IEEE Transactions on Magnetics 12/2008; DOI:10.1109/TMAG.2008.2001595 · 1.21 Impact Factor
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    ABSTRACT: Monolayer films of functional perfluoropolyethers (PFPEs) that are partially bonded to disk surfaces are widely used to lubricate magnetic disks. The bonding ratio (i.e., ratio of bonded thickness to total film thickness) possibly impacts the tribological behavior of the lubricant films. In this work, under lightly loaded (0-1 mN) and slowly rotated (1 or 2 r/min) conditions that prevent disturbance of the lubricant bonding state, we measured the frictional properties of 2-nm-thick PFPE Zdol2000 and Zdol4000 films as a function of bonding ratio adjusted up to 100% by anneal treatment. The friction forces of both Zdol2000 and Zdol4000 increased with increasing bonding ratio, but the increase was less noticeable for Zdol4000. Additionally, the friction of the nonannealed (5%-bonded) Zdol2000 films exhibited a linear increase with external load as described by Amontons' law, and it increased with sliding speed. In contrast, the friction of the 100%-bonded Zdol2000 films increased nonlinearly with external load as predicted by the Johnson-Kendall-Roberts (JKR) model, and it showed no obvious speed dependency. These results suggest that mobile molecules show liquid-like frictional properties dominated by viscosity resistance, but bonded molecules exhibit elastomer-like frictional properties determined by energy dissipation during molecule deformation.
    IEEE Transactions on Magnetics 12/2008; DOI:10.1109/TMAG.2008.2001594 · 1.21 Impact Factor
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    ABSTRACT: An understanding of the viscoelastic properties of molecularly thin lubricant film is essential to clarify tribological issues of head-disk interface (HDI) in high-density recording hard disk drives. Characteristic conditions for the HDI occur when lubricant molecules are extremely confined in the gap between the head and the disk surfaces, and the surfaces slide at high speeds. The lower the flying height, the more this confinement affects the flying characteristics. However, a few attempts have been made at clarifying the dynamic viscoelastic properties of confined lubricant molecules. This is because a method of measuring the dynamic shear force has not yet been established. Fiber wobbling method enables us to measure the shear force with a detection limit of less than 1nN. Additionally, frequency of shear can be set at several kHz. Further, the gap which confines the lubricant is controlled with a resolution of 0.1nm. Using the FWM, we investigated the effect that confinement had on the dynamic viscoelastic properties of perfluoropolyether lubricants on a magnetic disk. We found that the viscosity started to increase at a gap width that was less than a few hundred nanometers, which is hundreds of times larger than the molecular size. On the other hand, elasticity suddenly appeared at a gap width that was less than a few nanometers, which is equivalent to a few molecular sizes. Both the viscosity and elasticity increased monotonically as the gap decreased.
    Tribology Letters 05/2008; 30(3):177-189. DOI:10.1007/s11249-008-9325-2 · 2.15 Impact Factor
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    ABSTRACT: We present a method for measuring head fly height in hard disk drives using phase comparison Michelson interferometry (PCMI), which compares the phases of two interference fringe patterns formed respectively on the inner surface of a glass disk and the air-bearing surface of a fly head slider through the glass disk. To suppress interference noise and further enhance measurement accuracy, we adopted a low-coherence light source as an illumination source to replace a high-coherence laser. The captured fringe images enabled us to extract ridge lines much more accurately than with a laser. We compared our measurements in the sub-10 nm spacing range with calculations and found excellent agreement.
    IEEE Transactions on Magnetics 03/2008; 44(2-44):309 - 314. DOI:10.1109/TMAG.2007.911856 · 1.21 Impact Factor
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    ABSTRACT: The authors developed advanced haptic displays capable of stimulating the muscles and tendons of the forearms and tactile receptors in fingers to investigate tactile and force effects on simultaneous presentation. Display A is comprised of a master hand with two sets of tactile display with a 4-by-6 array of stimulus pins driven by micro-actuators and an articulated manipulator. Display B is comprised of an articulated manipulator and an 8-by-8 array type tactile display developed in a previous paper. A series of experiments was performed using the above A and B displays to verify the presentation capability of this display type. In Experiment I, subjects grasped virtual pegs and judged their diameters. In Experiment II, subjects tried to insert the pegs into holes. In Experiment III, the crossed-angle of a comparison texture was adjusted to bring it as close as possible to the standard texture fixed during experiments. Since diameter discrimination and insertion precision of the virtual peg were increased by tactile information, tactile-force presentation was effective for peg-in-hole for relatively large clearance. On the other hand, recognition capability for virtual texture was not enhanced compared to a mouse-mounted tactile display previously developed. While the pressure display is effective for instant of touch and peg rotation representations, rotation tactile imaging is not always effective for texture recognitions.
    Journal of Advanced Mechanical Design Systems and Manufacturing 01/2008; 2(1):24-36. DOI:10.1299/jamdsm.2.24 · 0.28 Impact Factor
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    ABSTRACT: We are developing an optical three-axis tactile sensor capable of acquiring normal and shearing force to mount on a robotic finger. The tactile sensor is based on the principle of an optical waveguide-type tactile sensor, which is composed of an acrylic hemispherical dome, a light source, an array of rubber sensing elements, and a CCD camera. The sensing element of the silicone rubber comprises one columnar feeler and eight conical feelers. The contact areas of the conical feelers, which maintain contact with the acrylic dome, detect the three-axis force applied to the tip of the sensing element. Normal and shearing forces are then calculated from integration and centroid displacement of the grayscale value derived from the conical feeler's contacts. To evaluate the present tactile sensor, we conducted a series of experiments using an x-z stage, a rotational stage, and a force gauge. Although we discovered that the relationship between the integrated grayscale value and normal force depends on the sensor's latitude on the hemispherical surface, it is easy to modify the sensitivity based on the latitude to make the centroid displacement of the grayscale value proportional to the shearing force. When we examined the repeatability of the present tactile sensor with 1,000 load/unload cycles, the error was 2%.
    Journal of Advanced Mechanical Design Systems and Manufacturing 01/2008; 2(5):860-873. DOI:10.1299/jamdsm.2.860 · 0.28 Impact Factor
  • Hedong Zhang, Yasunaga Mitsuya, Yuki Kudo, Kenji Fukuzawa
    01/2008; 74(740):970-977. DOI:10.1299/kikaic.74.970
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    ABSTRACT: Using a coarse-grained molecular dynamics simulation based on the bead-spring polymer model, we reproduced the film distribution of molecularly thin lubricant films with polar end groups coated on the disk surface and quantified the film surface morphology using a molecular-probe scanning method. We found that the film surface morphology changed periodically with increasing the film thickness. The monolayer of a polar lubricant that entirely covers the solid surface provides a flat lubricant surface by exposing its nonpolar backbone outside of the monolayer. By increasing film thickness, the end beads aggregate to make clusters, and bulges form on the lubricant surface accompanying an increase in surface roughness. The bulges continue to grow up even though the averaged film thickness reaches or exceeds the bilayer thickness. With further increases in film thickness, the clusters start to be uniformly distributed in the lateral direction to clearly form a third layer. As for the formation of fourth-fifth layers, the process is basically the same as that for the second-third layers. Through our calculations of the intermolecular potential field and the intermolecular force field, these values are found to change periodically and are synchronized with the formation of molecule aggregations, which explains the mechanism of forming the layered structure that is inherent to a polar lubricant.
    Journal of Tribology 01/2008; 130(2). DOI:10.1115/1.2842297 · 0.90 Impact Factor
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    ABSTRACT: We experimentally design a parallel typed two-axial micro actuator, which is utilized for the key part of the tactile display. The parallel typed two-axial actuator was composed of two bimorph piezoelectric elements and two small links connected by three joints. We formulated kinematics for the parallel typed two-axial actuator because the endpoint is controlled in the two-dimensional coordinate. Since relationship between applied voltage and displacement cause by the voltage shows a hysteresis loop in the bimorph piezoelectric element used as components of the two-axial actuator, we produce a control system for the two-axial actuator based on a multi-layered artificial neural network to compensate the hysteresis. The neural network is comprised of 4 neurons in the input layer, 10 neurons in the hidden layer and ones neuron in the output layer. The output neuron emits time derivative of voltage; two bits signal expressing increment or decrement condition is generated by two input neurons; one of the other two input neurons and the other calculate current values of voltage and displacement, respectively. The neural network is featured with a feedback loop including an integral element to reduce number of neurons. In the learning process, the network learns the hysteresis including a minor loop. In the verification test, the endpoint of the two-axial actuator traces the desired circular trajectory in the two-dimensional coordinate system.
    Micro-NanoMechatronics and Human Science, 2007. MHS '07. International Symposium on; 12/2007
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    ABSTRACT: Coarse-grained molecular dynamics (MD) simulations were applied to reproduce the surface morphology of lubricant films composed of perfluoropolyether (PFPE) Zdol and tri-functional PFPE, which has a functional group at the middle of a molecule as well as functional endgroups at the head and tail of the molecule. The molecular-mesh scanning method was introduced to detect the location of film surface. The effects of the number of polar groups and polymer length on a flat solid surface were studied as lubricant films with periodic boundary conditions. We clarified that the film surface of the tri-functional PFPE lubricant was smoother than Zdol 2000 when compared at the same film thickness condition.
    Micro-NanoMechatronics and Human Science, 2007. MHS '07. International Symposium on; 12/2007
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    ABSTRACT: Spring constants and damping coefficients of a molecularly thin liquid bridge of PFPE lubricant intervening between a diamond probe tip and diamond like carbon (DLC) surface of a magnetic disk are identified through regression analysis of tip damping vibration. Perfluoropolyether (PFPE) lubricants having functional end groups were used to form the liquid bridge between the DLC surface and a probe tip with the notably small curvature radius of 0.1 μm. The tip was both retracted from and extended toward the disk surface at four different, progressive distances to attain varied elongation of the bridge, and was made to vibrate at each step to provide damping waveforms. By applying regression analysis to observed waveforms, the spring constant and the damping coefficient of the liquid bridge were identified. It is interesting to note that both the absolute value of the spring constant and the frequency-multiplied damping decrease with bridge elongation, and after reaching the minimum, those values begin to increase.
    10/2007; 129(4). DOI:10.1115/1.2768070

Publication Stats

552 Citations
86.79 Total Impact Points

Institutions

  • 2013–2014
    • Nagoya Municipal Industrial Research Institute
      Nagoya, Aichi, Japan
  • 1995–2008
    • Nagoya University
      • • Department of Micro-Nano Systems Engineering
      • • Department of Complex Systems Science
      Nagoya-shi, Aichi-ken, Japan
  • 2005–2006
    • Gifu University
      • Faculty of Engineering
      Gihu, Gifu, Japan
  • 2004
    • Carnegie Mellon University
      • Department of Chemical Engineering
      Pittsburgh, PA, United States
  • 2003
    • Aichi Konan College
      Конан, Aichi, Japan
  • 1997–1999
    • Shizuoka Institute of Science and Technology
      Fukuroi, Shizuoka, Japan