The Experts below are selected from a list of 2487 Experts worldwide ranked by ideXlab platform

Atsunobu Mori - One of the best experts on this subject based on the ideXlab platform.

  • Effect of humidity on the interactions between probes and ultra-thin Liquid Lubricant films in an environmentally controlled atomic force microscope
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2009
    Co-Authors: Norio Tagawa, Jun-ichi Hatakeyama, Atsunobu Mori
    Abstract:

    To achieve a magnetic recording density of 1 Tb/in2, the required head-disk spacing is expected to be less than 2–3 nm. However, a critical issue in achieving such an ultra-low spacing is the dynamic instability of the head-disk interface (HDI), that is, the experimentally observed hysteresis of the flying sliders. It is important to clarify the interactive forces between the sliders and disk surfaces in a nanometer HDI. In addition, the adhesive forces between the thermally protruding magnetic head writer pole tip, which exists in the most commercially available recording head, and the Lubricant film have also become very important. Therefore, in this study, the interactions between the probes of an environmentally controlled atomic force microscope (AFM) and ultra-thin Liquid Lubricant films were studied as a model experiment, resulting in the acquisition of fundamental knowledge related to the adhesive forces as well as the slider hysteresis. Subsequently, the effect of humidity on the force curve characteristics was investigated by varying the Lubricant film thickness. As a result, it was found that the probe touchdown occurred earlier as the humidity increased, with its driving force suggested to be the force of the Lubricant meniscus formed on the AFM probe. The probe touchdown occurred earlier for Lubricants with a higher mobility. The adhesive force gradually increased with the humidity and was at its minimum for a one monolayer film thickness, independent of the Lubricant materials. On the other hand, the elongation of the Lubricant meniscus increased with the humidity. Therefore, the probe takeoff occurred later as the humidity increased. Accordingly, the hysteresis increased with the humidity and was mainly dependent on the elongation of the meniscus.

  • Effect of Temperature on the Spreading Characteristics of Molecularly Thin Liquid Lubricant Films in Hard Disk Drives
    STLE ASME 2008 International Joint Tribology Conference, 2008
    Co-Authors: Norio Tagawa, Atsunobu Mori, Kenta Mori, Masako Ikegami
    Abstract:

    In this study, the effect of temperature on the spreading characteristics of ultra-thin Liquid Lubricant films in hard disk drives (HDD) was investigated by using three types of Lubricants, namely, Zdol2000, Ztetraol2000, and A20H2000. The apparent diffusion coefficient of individual Lubricants was evaluated and compared by varying the temperature of disk substrates. As a result, it was found that the mobility of each Lubricant increases with the temperature. However, the rate of mobility increase is different for each Lubricant, depending on the Lubricant material. Furthermore, there exists no quantitative correlation between the mobility increase for ultra-thin Liquid Lubricant films and the viscosity decrease in bulk Lubricant materials due to a rise in the temperature. It was also found that among the test Lubricants, A20H2000 has the highest robustness for temperature change. In addition, it could be observed that the evaporation of the mobile fraction of Lubricants occurred remarkably over a temperature range of 50– 80 °C.Copyright © 2008 by ASME

  • Effects of ultra-thin Liquid Lubricant films on contact slider dynamics in hard-disk drives
    Tribology International, 2007
    Co-Authors: Norio Tagawa, Yoshiaki Tashiro, Atsunobu Mori
    Abstract:

    Abstract 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.

  • Effects of ultra-thin Liquid Lubricant films on contact slider dynamics in hard-disk drives
    Tribology International, 2007
    Co-Authors: Naoyuki Tagawa, Yoshiyuki Tashiro, Atsunobu Mori
    Abstract:

    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. ?? 2006 Elsevier Ltd. All rights reserved.

  • Effects of End Groups on the Spreading Characteristics of Molecularly Thin Liquid Lubricant Films in Hard Disk Drives
    IEEE Transactions on Magnetics, 2007
    Co-Authors: Norio Tagawa, Atsunobu Mori, M. Korenaga, N. Kobayashi, Masako Ikegami
    Abstract:

    The spreading characteristics of novel alkanolamine-terminated perfluoropolyether (PFPE) films on carbon surfaces were investigated experimentally by using a scanning micro-ellipsometer. The apparent diffusion coefficients of the novel Lubricants were studied in order to evaluate the spreading speed and they were compared with the conventional Z-dol, Z-tetraol, and A20H Lubricants. It was found that the mobility of a Lubricant is lower when the number of OH end groups is higher. It was also found that the mobility of the alkanolamine-terminated PFPE films was lower than that of conventional Lubricants with the same number of OH end groups as that of the alkanolamine-terminated Lubricants and that the mobility tendency of the Lubricants has a good correlation with their viscosity tendency. In addition, the mobility of ZDPA with a dipropylamine end group (no OH end group) has peculiar characteristics. Its mobility is higher than that of Z-dol when the film thickness of the Lubricant exceeds that of one monolayer. However, it is lower than that of Z-dol when the film thickness of the Lubricant is less than that of one monolayer. This may be due to the unique interaction between the ZDPA amine group and the carbon overcoat.

Shintaro Itoh - One of the best experts on this subject based on the ideXlab platform.

  • Frequency Dependence of Viscoelasticity of Liquid Lubricant Confined in Nanometer-Scale Gaps
    Transactions of the Japan Society of Mechanical Engineers. C, 2020
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yuya Hamamoto, Hedong Zhang
    Abstract:

    When a Liquid is confined in nanometer-scale gaps, it exhibits characteristic viscoelastic properties that are different from those properties measured in a bulk state. In hard disk drives, a Liquid Lubricant film whose thickness is around 1-2 nm is used to lubricate the interface between the magnetic head and the magnetic disk surface. When the magnetic head touches the disk surface, the Lubricant film is confined in the nanometer-scale gaps and sheared. In order to attain the proper lubrication, it is essential to investigate the dynamic viscoelasticity of the confined Liquid Lubricant under shearing motion. In this study, we focused on one of the typical phenomenon of the confined Liquid which is referred to as the shear thinning. The shear thinning is the property whereby the viscosity of the Liquid decreases when the shear rate or shear frequency increases. This can be observed in many Lubricants in a bulk state. The characteristic behavior of the confined Lubricant is that the shear thinning can be observed at much lower shear rates or shear frequencies compared to the shear thinning occurs in a bulk state. The mechanism of the shear thinning in the confined state must be different from that of the bulk state. In order to clarify the shear thinning mechanism of the confined Lubricant, we measured the gap dependence of the viscoelasticity at different shear frequencies in a range of 100 Hz to 2 kHz. The experimental results showed that the shear thinning behavior suddenly appeared at gap width of approximately 15 nm or less. Further, the shear thinning synchronized with the enhancement of the elasticity which means the solidification of the Liquid Lubricant.

  • Simultaneous in situ measurements of contact behavior and friction to understand the mechanism of lubrication with nanometer-thick Liquid Lubricant films
    Tribology International, 2018
    Co-Authors: Hedong Zhang, Kenji Fukuzawa, Yasunaga Mitsuya, Yusuke Takeuchi, W.w.f. Chong, Shintaro Itoh
    Abstract:

    Abstract A high-performance microscopic observation system was developed and integrated with a purpose-built pin-on-disk type tribotester. This allows for simultaneous in situ measurements of the vertical displacement and friction force of a pin sliding on disks lubricated with nanometer-thick Liquid films at velocities up to 0.1 m/s, with accuracies of approximately 0.6 nm and 10 μN. Upward pin displacement was observed, exhibiting an exponential increase followed by a slight increase with increasing sliding velocity. The pin displacement also increased with film thickness and Lubricant viscosity. We conclude that even nanometer-thick Liquid Lubricant films can generate upward dynamic pressure. Further insight into the shearing process was gained by analyzing the measured friction forces using the Eyring thermal activation energy approach.

  • molecular dynamics simulations of diffusion of submonolayer polar Liquid Lubricant films on solid surfaces
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2016
    Co-Authors: Takayuki Kobayashi, Kenji Fukuzawa, Hedong Zhang, Shintaro Itoh
    Abstract:

    A fundamental understanding of the diffusion phenomena of submonolayer polar Liquid films is important for achieving reliable lubrication between moving mechanical parts separated by a nanometer-sized gap. To acquire this understanding, we conducted molecular dynamics (MD) simulations of diffusion phenomena of submonolayer polar perfluoropolyether (PFPE) Zdol films on solid surfaces. To improve the accuracy of these simulations, we developed an all-atom model that includes hydrogen-bond potential and refined atomic charges for Zdol molecules and tested it through MD simulations of spreading of step-shaped submonolayer PFPE films. Our MD simulations reproduced the experimentally observed effects of polar end groups on the diffusion speed and molecular conformation of Zdol. We then conducted MD simulations of self-diffusion of submonolayer Zdol films; these simulations demonstrated that as the thickness of the submonolayer Zdol films decreases, molecular conformation becomes flatter and the self-diffusion coefficient decreases. These changes in molecular conformation partially explain our experimental finding that the spreading of step-shaped submonolayer polar PFPE films slows down with decreasing initial thickness.

  • Effect of Chemically Adsorbed Molecules on the Viscous Friction of Nanometer-Thick Liquid Lubricant Films Coated on a Diamond-Like Carbon Surface
    Tribology Letters, 2015
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yusuke Norizuki, Hedong Zhang
    Abstract:

    Nanometer-thick Liquid Lubricant films are used for lubrication of miniaturized mechanical systems such as hard disk drives and microelectromechanical systems. In particular at the head–disk interface of hard disk drives, the Lubricant thin film is coated on a diamond-like carbon surface, and the films are usually composed of both chemically and physically adsorbed Lubricant molecules. The combination of these two types of molecules leads to high lubrication performance. However, the detailed mechanism is not fully understood. The aim of this study was to clarify the effect of chemically adsorbed Lubricant molecules on the viscous friction of Lubricant films. Two different samples were tested: One was the Lubricant film composed of both chemically and physically adsorbed molecules, and the other was the Lubricant film with only physically adsorbed molecules. We used a fiber wobbling method (FWM), which we developed in our previous study, to measure the viscous friction. The FWM uses a spherical-ended glass fiber as a shearing probe and enables friction forces in the order of 0.1 nN to be measured at precisely controlled nanometer-scale gap widths. For the fair comparison of friction forces between the samples with different compositions, the contact area between the probe tip and the Lubricant film must be the same. To determine the contact area in the friction measurement, we improved the FWM system to enable simultaneous measurement of friction force and contact area. Experimental results showed the 1-nm-thick layer of chemically adsorbed molecules in the Lubricant film effectively reduced the viscous friction.

  • Surface functionalization by fine ultraviolet-patterning of nanometer-thick Liquid Lubricant films
    Applied Surface Science, 2014
    Co-Authors: Renguo Lu, Kenji Fukuzawa, Hedong Zhang, Yasunaga Mitsuya, Suguru Komada, Shintaro Itoh
    Abstract:

    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.

Hedong Zhang - One of the best experts on this subject based on the ideXlab platform.

  • Frequency Dependence of Viscoelasticity of Liquid Lubricant Confined in Nanometer-Scale Gaps
    Transactions of the Japan Society of Mechanical Engineers. C, 2020
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yuya Hamamoto, Hedong Zhang
    Abstract:

    When a Liquid is confined in nanometer-scale gaps, it exhibits characteristic viscoelastic properties that are different from those properties measured in a bulk state. In hard disk drives, a Liquid Lubricant film whose thickness is around 1-2 nm is used to lubricate the interface between the magnetic head and the magnetic disk surface. When the magnetic head touches the disk surface, the Lubricant film is confined in the nanometer-scale gaps and sheared. In order to attain the proper lubrication, it is essential to investigate the dynamic viscoelasticity of the confined Liquid Lubricant under shearing motion. In this study, we focused on one of the typical phenomenon of the confined Liquid which is referred to as the shear thinning. The shear thinning is the property whereby the viscosity of the Liquid decreases when the shear rate or shear frequency increases. This can be observed in many Lubricants in a bulk state. The characteristic behavior of the confined Lubricant is that the shear thinning can be observed at much lower shear rates or shear frequencies compared to the shear thinning occurs in a bulk state. The mechanism of the shear thinning in the confined state must be different from that of the bulk state. In order to clarify the shear thinning mechanism of the confined Lubricant, we measured the gap dependence of the viscoelasticity at different shear frequencies in a range of 100 Hz to 2 kHz. The experimental results showed that the shear thinning behavior suddenly appeared at gap width of approximately 15 nm or less. Further, the shear thinning synchronized with the enhancement of the elasticity which means the solidification of the Liquid Lubricant.

  • Simultaneous in situ measurements of contact behavior and friction to understand the mechanism of lubrication with nanometer-thick Liquid Lubricant films
    Tribology International, 2018
    Co-Authors: Hedong Zhang, Kenji Fukuzawa, Yasunaga Mitsuya, Yusuke Takeuchi, W.w.f. Chong, Shintaro Itoh
    Abstract:

    Abstract A high-performance microscopic observation system was developed and integrated with a purpose-built pin-on-disk type tribotester. This allows for simultaneous in situ measurements of the vertical displacement and friction force of a pin sliding on disks lubricated with nanometer-thick Liquid films at velocities up to 0.1 m/s, with accuracies of approximately 0.6 nm and 10 μN. Upward pin displacement was observed, exhibiting an exponential increase followed by a slight increase with increasing sliding velocity. The pin displacement also increased with film thickness and Lubricant viscosity. We conclude that even nanometer-thick Liquid Lubricant films can generate upward dynamic pressure. Further insight into the shearing process was gained by analyzing the measured friction forces using the Eyring thermal activation energy approach.

  • molecular dynamics simulations of diffusion of submonolayer polar Liquid Lubricant films on solid surfaces
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2016
    Co-Authors: Takayuki Kobayashi, Kenji Fukuzawa, Hedong Zhang, Shintaro Itoh
    Abstract:

    A fundamental understanding of the diffusion phenomena of submonolayer polar Liquid films is important for achieving reliable lubrication between moving mechanical parts separated by a nanometer-sized gap. To acquire this understanding, we conducted molecular dynamics (MD) simulations of diffusion phenomena of submonolayer polar perfluoropolyether (PFPE) Zdol films on solid surfaces. To improve the accuracy of these simulations, we developed an all-atom model that includes hydrogen-bond potential and refined atomic charges for Zdol molecules and tested it through MD simulations of spreading of step-shaped submonolayer PFPE films. Our MD simulations reproduced the experimentally observed effects of polar end groups on the diffusion speed and molecular conformation of Zdol. We then conducted MD simulations of self-diffusion of submonolayer Zdol films; these simulations demonstrated that as the thickness of the submonolayer Zdol films decreases, molecular conformation becomes flatter and the self-diffusion coefficient decreases. These changes in molecular conformation partially explain our experimental finding that the spreading of step-shaped submonolayer polar PFPE films slows down with decreasing initial thickness.

  • Effect of Chemically Adsorbed Molecules on the Viscous Friction of Nanometer-Thick Liquid Lubricant Films Coated on a Diamond-Like Carbon Surface
    Tribology Letters, 2015
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yusuke Norizuki, Hedong Zhang
    Abstract:

    Nanometer-thick Liquid Lubricant films are used for lubrication of miniaturized mechanical systems such as hard disk drives and microelectromechanical systems. In particular at the head–disk interface of hard disk drives, the Lubricant thin film is coated on a diamond-like carbon surface, and the films are usually composed of both chemically and physically adsorbed Lubricant molecules. The combination of these two types of molecules leads to high lubrication performance. However, the detailed mechanism is not fully understood. The aim of this study was to clarify the effect of chemically adsorbed Lubricant molecules on the viscous friction of Lubricant films. Two different samples were tested: One was the Lubricant film composed of both chemically and physically adsorbed molecules, and the other was the Lubricant film with only physically adsorbed molecules. We used a fiber wobbling method (FWM), which we developed in our previous study, to measure the viscous friction. The FWM uses a spherical-ended glass fiber as a shearing probe and enables friction forces in the order of 0.1 nN to be measured at precisely controlled nanometer-scale gap widths. For the fair comparison of friction forces between the samples with different compositions, the contact area between the probe tip and the Lubricant film must be the same. To determine the contact area in the friction measurement, we improved the FWM system to enable simultaneous measurement of friction force and contact area. Experimental results showed the 1-nm-thick layer of chemically adsorbed molecules in the Lubricant film effectively reduced the viscous friction.

  • Surface functionalization by fine ultraviolet-patterning of nanometer-thick Liquid Lubricant films
    Applied Surface Science, 2014
    Co-Authors: Renguo Lu, Kenji Fukuzawa, Hedong Zhang, Yasunaga Mitsuya, Suguru Komada, Shintaro Itoh
    Abstract:

    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.

Norio Tagawa - One of the best experts on this subject based on the ideXlab platform.

  • depletion of monolayer Liquid Lubricant films induced by high frequency pulsed laser heating in thermally assisted magnetic recording
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2012
    Co-Authors: Norio Tagawa, Takao Miki, Hiroshi Tani
    Abstract:

    In this study, Lubricant depletion due to high-frequency pulsed-laser heating was investigated for Lubricant films with thicknesses of both more than and less than one monolayer. A conventional Lubricant, Zdol2000, was used. It was found that the critical temperature at which the Lubricant begins to deplete owing to laser heating was strongly dependent on the Lubricant film thickness. In the case in which the thickness of the Lubricant film was less than one monolayer, this temperature was approximately 170 °C higher than it was when the thickness was more than one monolayer. To analyze the Lubricant depletion mechanism, we examined the tested Lubricant film using temperature programmed desorption (TPD) spectroscopy. It was found that the Lubricant depletion characteristics due to laser heating could be explained using the experimental TPD results for the tested Lubricant film, and that the depletion mechanism involves the desorption or decomposition of the Lubricant molecules, which interact with the diamond-like carbon thin films when the Lubricant film thickness is less than one monolayer. Further, the results of TPD and of a thermogravimetric analysis (TGA) of the Lubricant were compared. The thermal robustness of the ultra-thin Liquid Lubricant films was found to be greater than that of the bulk Lubricant materials.

  • depletion of monolayer Liquid Lubricant films induced by laser heating in thermally assisted magnetic recording
    Tribology Letters, 2012
    Co-Authors: Norio Tagawa, Takao Miki, Hiroshi Tani
    Abstract:

    In this study, Lubricant depletion due to high-frequency pulsed-laser heating, with a heating rate of ~108–109 K/s, was investigated for Lubricant films with thicknesses of greater than and less than one monolayer. The conventional Lubricants, Zdol2000 and Ztetraol2000, were used. It was found that the critical temperature at which the Lubricants begin to deplete was strongly dependent on the Lubricant film thickness. For Lubricant film thicknesses of less than one monolayer, this temperature was approximately 170 °C higher than that for thicknesses of greater than one monolayer. To analyze the Lubricant depletion mechanism, we examined the tested Lubricant films, using temperature-programmed desorption (TPD) spectroscopy, in which the heating rate was 0.3 K/s. It was found that the Lubricant depletion characteristics could be explained using the experimental TPD results for the tested Lubricant films. The depletion mechanism involves the desorption or decomposition of the Lubricant molecules, which interact with the diamond-like carbon thin films when the Lubricant film thickness is less than one monolayer. Therefore, we concluded that the TPD results were highly effective for evaluating the Lubricant depletion characteristics induced by rapid laser heating, even though the heating rates are substantially different.

  • Effect of humidity on the interactions between probes and ultra-thin Liquid Lubricant films in an environmentally controlled atomic force microscope
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2009
    Co-Authors: Norio Tagawa, Jun-ichi Hatakeyama, Atsunobu Mori
    Abstract:

    To achieve a magnetic recording density of 1 Tb/in2, the required head-disk spacing is expected to be less than 2–3 nm. However, a critical issue in achieving such an ultra-low spacing is the dynamic instability of the head-disk interface (HDI), that is, the experimentally observed hysteresis of the flying sliders. It is important to clarify the interactive forces between the sliders and disk surfaces in a nanometer HDI. In addition, the adhesive forces between the thermally protruding magnetic head writer pole tip, which exists in the most commercially available recording head, and the Lubricant film have also become very important. Therefore, in this study, the interactions between the probes of an environmentally controlled atomic force microscope (AFM) and ultra-thin Liquid Lubricant films were studied as a model experiment, resulting in the acquisition of fundamental knowledge related to the adhesive forces as well as the slider hysteresis. Subsequently, the effect of humidity on the force curve characteristics was investigated by varying the Lubricant film thickness. As a result, it was found that the probe touchdown occurred earlier as the humidity increased, with its driving force suggested to be the force of the Lubricant meniscus formed on the AFM probe. The probe touchdown occurred earlier for Lubricants with a higher mobility. The adhesive force gradually increased with the humidity and was at its minimum for a one monolayer film thickness, independent of the Lubricant materials. On the other hand, the elongation of the Lubricant meniscus increased with the humidity. Therefore, the probe takeoff occurred later as the humidity increased. Accordingly, the hysteresis increased with the humidity and was mainly dependent on the elongation of the meniscus.

  • Study of Lubricant Depletion Induced by Laser Heating in Thermally Assisted Magnetic Recording Systems—Effect of Lubricant Film Materials
    IEEE Transactions on Magnetics, 2009
    Co-Authors: Norio Tagawa, Hiroshi Tani, Ryo Kakitani, Naoyasu Iketani, Ikuo Nakano
    Abstract:

    Thermally assisted magnetic recording (TAMR) is one of the novel technologies that are employed for achieving magnetic recording densities higher than 1 Tb/in2. With regard to this novel technology, it has been suggested that there exists a critical head-disk interface (HDI) issue associated with the use of ultrathin Liquid Lubricant films on the disk surface. This problem is attributed to the fact that these films are heated to high temperatures by using laser beams in order to reduce magnetic coercivity. In this study, a fundamental research on the Lubricant depletion caused due to laser heating in TAMR systems was conducted. That is, the effect of Lubricant film materials on Lubricant depletion was investigated. In this experimental study, four types of Lubricant film materials-Zdol2000, Zdol4000, Ztetraol2000, and A20H2000-were used, and the differences in the characteristics of Lubricant depletion for each tested Lubricant film were investigated. The effect of the molecular weight as well as the end group of the Lubricant on the Lubricant depletion was discussed and clarified by analyzing the obtained experimental results. In addition, it was found that the depletion characteristics of ultrathin Liquid Lubricant films could be explained by using experimental results obtained from the thermogravimetric analysis for each bulk Lubricant material.

  • Effect of Temperature on the Spreading Characteristics of Molecularly Thin Liquid Lubricant Films in Hard Disk Drives
    STLE ASME 2008 International Joint Tribology Conference, 2008
    Co-Authors: Norio Tagawa, Atsunobu Mori, Kenta Mori, Masako Ikegami
    Abstract:

    In this study, the effect of temperature on the spreading characteristics of ultra-thin Liquid Lubricant films in hard disk drives (HDD) was investigated by using three types of Lubricants, namely, Zdol2000, Ztetraol2000, and A20H2000. The apparent diffusion coefficient of individual Lubricants was evaluated and compared by varying the temperature of disk substrates. As a result, it was found that the mobility of each Lubricant increases with the temperature. However, the rate of mobility increase is different for each Lubricant, depending on the Lubricant material. Furthermore, there exists no quantitative correlation between the mobility increase for ultra-thin Liquid Lubricant films and the viscosity decrease in bulk Lubricant materials due to a rise in the temperature. It was also found that among the test Lubricants, A20H2000 has the highest robustness for temperature change. In addition, it could be observed that the evaporation of the mobile fraction of Lubricants occurred remarkably over a temperature range of 50– 80 °C.Copyright © 2008 by ASME

Kenji Fukuzawa - One of the best experts on this subject based on the ideXlab platform.

  • Frequency Dependence of Viscoelasticity of Liquid Lubricant Confined in Nanometer-Scale Gaps
    Transactions of the Japan Society of Mechanical Engineers. C, 2020
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yuya Hamamoto, Hedong Zhang
    Abstract:

    When a Liquid is confined in nanometer-scale gaps, it exhibits characteristic viscoelastic properties that are different from those properties measured in a bulk state. In hard disk drives, a Liquid Lubricant film whose thickness is around 1-2 nm is used to lubricate the interface between the magnetic head and the magnetic disk surface. When the magnetic head touches the disk surface, the Lubricant film is confined in the nanometer-scale gaps and sheared. In order to attain the proper lubrication, it is essential to investigate the dynamic viscoelasticity of the confined Liquid Lubricant under shearing motion. In this study, we focused on one of the typical phenomenon of the confined Liquid which is referred to as the shear thinning. The shear thinning is the property whereby the viscosity of the Liquid decreases when the shear rate or shear frequency increases. This can be observed in many Lubricants in a bulk state. The characteristic behavior of the confined Lubricant is that the shear thinning can be observed at much lower shear rates or shear frequencies compared to the shear thinning occurs in a bulk state. The mechanism of the shear thinning in the confined state must be different from that of the bulk state. In order to clarify the shear thinning mechanism of the confined Lubricant, we measured the gap dependence of the viscoelasticity at different shear frequencies in a range of 100 Hz to 2 kHz. The experimental results showed that the shear thinning behavior suddenly appeared at gap width of approximately 15 nm or less. Further, the shear thinning synchronized with the enhancement of the elasticity which means the solidification of the Liquid Lubricant.

  • Simultaneous in situ measurements of contact behavior and friction to understand the mechanism of lubrication with nanometer-thick Liquid Lubricant films
    Tribology International, 2018
    Co-Authors: Hedong Zhang, Kenji Fukuzawa, Yasunaga Mitsuya, Yusuke Takeuchi, W.w.f. Chong, Shintaro Itoh
    Abstract:

    Abstract A high-performance microscopic observation system was developed and integrated with a purpose-built pin-on-disk type tribotester. This allows for simultaneous in situ measurements of the vertical displacement and friction force of a pin sliding on disks lubricated with nanometer-thick Liquid films at velocities up to 0.1 m/s, with accuracies of approximately 0.6 nm and 10 μN. Upward pin displacement was observed, exhibiting an exponential increase followed by a slight increase with increasing sliding velocity. The pin displacement also increased with film thickness and Lubricant viscosity. We conclude that even nanometer-thick Liquid Lubricant films can generate upward dynamic pressure. Further insight into the shearing process was gained by analyzing the measured friction forces using the Eyring thermal activation energy approach.

  • molecular dynamics simulations of diffusion of submonolayer polar Liquid Lubricant films on solid surfaces
    Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2016
    Co-Authors: Takayuki Kobayashi, Kenji Fukuzawa, Hedong Zhang, Shintaro Itoh
    Abstract:

    A fundamental understanding of the diffusion phenomena of submonolayer polar Liquid films is important for achieving reliable lubrication between moving mechanical parts separated by a nanometer-sized gap. To acquire this understanding, we conducted molecular dynamics (MD) simulations of diffusion phenomena of submonolayer polar perfluoropolyether (PFPE) Zdol films on solid surfaces. To improve the accuracy of these simulations, we developed an all-atom model that includes hydrogen-bond potential and refined atomic charges for Zdol molecules and tested it through MD simulations of spreading of step-shaped submonolayer PFPE films. Our MD simulations reproduced the experimentally observed effects of polar end groups on the diffusion speed and molecular conformation of Zdol. We then conducted MD simulations of self-diffusion of submonolayer Zdol films; these simulations demonstrated that as the thickness of the submonolayer Zdol films decreases, molecular conformation becomes flatter and the self-diffusion coefficient decreases. These changes in molecular conformation partially explain our experimental finding that the spreading of step-shaped submonolayer polar PFPE films slows down with decreasing initial thickness.

  • Effect of Chemically Adsorbed Molecules on the Viscous Friction of Nanometer-Thick Liquid Lubricant Films Coated on a Diamond-Like Carbon Surface
    Tribology Letters, 2015
    Co-Authors: Shintaro Itoh, Kenji Fukuzawa, Yusuke Norizuki, Hedong Zhang
    Abstract:

    Nanometer-thick Liquid Lubricant films are used for lubrication of miniaturized mechanical systems such as hard disk drives and microelectromechanical systems. In particular at the head–disk interface of hard disk drives, the Lubricant thin film is coated on a diamond-like carbon surface, and the films are usually composed of both chemically and physically adsorbed Lubricant molecules. The combination of these two types of molecules leads to high lubrication performance. However, the detailed mechanism is not fully understood. The aim of this study was to clarify the effect of chemically adsorbed Lubricant molecules on the viscous friction of Lubricant films. Two different samples were tested: One was the Lubricant film composed of both chemically and physically adsorbed molecules, and the other was the Lubricant film with only physically adsorbed molecules. We used a fiber wobbling method (FWM), which we developed in our previous study, to measure the viscous friction. The FWM uses a spherical-ended glass fiber as a shearing probe and enables friction forces in the order of 0.1 nN to be measured at precisely controlled nanometer-scale gap widths. For the fair comparison of friction forces between the samples with different compositions, the contact area between the probe tip and the Lubricant film must be the same. To determine the contact area in the friction measurement, we improved the FWM system to enable simultaneous measurement of friction force and contact area. Experimental results showed the 1-nm-thick layer of chemically adsorbed molecules in the Lubricant film effectively reduced the viscous friction.

  • Surface functionalization by fine ultraviolet-patterning of nanometer-thick Liquid Lubricant films
    Applied Surface Science, 2014
    Co-Authors: Renguo Lu, Kenji Fukuzawa, Hedong Zhang, Yasunaga Mitsuya, Suguru Komada, Shintaro Itoh
    Abstract:

    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.