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Ionic liquid based lubricants offer potential to reduce the sliding friction and wear of ferrous bearing surfaces such as those used in cutting and machining processes. In this study, two types of promising additives were investigated: (1) diprotic bis(2-hydroxyethyl)ammonium palmitate (DPA) ionic liquid crystal and (2) a 0.1 wt% dispersion of nanodiamonds (ND) in DPA. Both have been used at 1 wt% concentration in aqueous lubricants. Those were Water+DPA, and Water+DPA+ND. The stability of the dispersions has been studied by dynamic light scattering, finding that the interaction between the additives and water changes particle size distribution. Glass-against-steel optical interferometry using water film alone indicated that the film thickness decreased with number of sliding strokes and that in turn increased the friction coefficient. By comparison, Water+DPA and Water+DPA+ND showed lower friction coefficients and increased film thickness. In steel-steel pin-on-disk contacts lubricated only by water, electrical resistance increases at speeds higher than 100 mm/s. DPA and DPA+ND additives displace the electrical resistance increase to lower sliding speed, around 10 mm/s. The additives reduced the coefficients of sliding friction up to 82%, and wear rates of AISI 316L disks by more than two orders of magnitude. The combination of the palmitate ionic liquid crystal and nanodiamonds in Water+DPA+ND gave the lowest wear rates. The wear mechanism and surface analysis associated with these results are discussed.

Friction-induced vibrations appearing in rotary contact systems have been known as serious problems to be solved. In this study, focusing on the self-excited vibrations induced by the velocity-weakening friction, numerical simulation and experimental verification have been carried out for a circular sliding contact. As a result, it has been found that parallel misalignment between the drive and driven shafts generates damping, which can suppress the torsional vibration around the shafts. The destabilization is caused by change in the “magnitude” of frictional force, while the stabilization is caused by change in the “direction” of frictional force.

Concentrated polymer brushes (CPBs) are swollen polymeric materials promising for tribological applications owing to their ultralow friction. However, their insufficient durability has limited their practical use. In this study, to improve the durability, poly(methyl methacrylate) CPBs fabricated on microgrooved substrates were experimentally investigated in two types of tests (i.e., “sliding tests” and “nanoindentation tests”). Sliding tests revealed that parallel grooves with adequate dimensions significantly improve their durability in lubricated sliding. Nanoindentation tests revealed that the CPB on a flat substrate exhibits a layered structure comprising two layers (i.e., a “diluted surface layer” and a “concentrated bulk layer”). An additional third layer (i.e., a “reinforced tough layer”) provided by the parallel grooves appears to improve the durability.

Rotary sliding systems such as clutches require smooth motion without any unpredicted vibrations, such as friction-induced vibration. Friction-induced vibration, which is called judder in clutches, sometimes occurs, especially during clutch engagement. To suppress the friction-induced vibration in rotary sliding systems, a unique method in which the direction of the friction force is forcibly changed by orthogonal rotation of a roller providing transverse sliding is proposed. The effectiveness of the method was examined experimentally using a custom-built apparatus. The results show that the orthogonal rotation suppresses the friction-induced vibration even when the friction force exhibits velocity-weakening characteristics. In addition, the effective friction force, which can be regarded as the traction force, can be controlled by varying the orthogonal rotation.

Recently, it has been theoretically shown that in a 1DOF sliding system, the in-plane angular misalignment (referred to as the yaw angle misalignment (YAM)) has a stabilizing effect to suppress the self-excited vibration induced by the velocity-weakening friction. The YAM theory has been supported qualitatively and quantitatively by some experiments and numerical simulations. However, in some other experiments with another type of apparatuses, the suppression condition was qualitatively different from the theoretical prediction. Based on the above, in this study, the YAM theory has been extended to a 2DOF sliding system with in-plane anisotropic stiffness. Numerical simulation and eigenvalue analysis revealed that the YAM around 45° had a damping effect to suppress the self-excited vibration induced by the velocity-weakening friction, with no supplementary mechanical devices (such as dampers or actuators) to suppress the vibration, which was consistent with the previous experimental results.

A pad-on-disc-type brake apparatus was constructed based on a theoretical principle for suppressing frictional vibration. The pad for this apparatus was supported by parallel leaf springs with a unidirectional compliance. Braking tests were conducted using the apparatus under a constant normal load. It has been found that a yaw angle misalignment between the directions of the pad and disc motions provides a positive damping to suppress the frictional vibration and brake noise. In addition, it has been also found that when an appropriate misalignment angle is selected, a low-noise performance and a good braking performance can be achieved simultaneously.

To develop objective estimation methods for the tactile sensations experienced during the application of liquid cosmetic foundation (i.e., emulsion foundations (EFs)), a tribotester based on the mechanical properties of human skin was used in lubrication experiments with EF samples. From the time-varying signals of the tangential spring deflection obtained in reciprocating rubbing tests for small sample amounts, trajectories (i.e., plots of the tangential spring deflection against the tangential drive position) were obtained. Using characteristic trajectory shapes (e.g., a shape like a ribbon with jagged edges) makes it possible to evaluate the similarity of vague tactile sensations without sensory testing.

An expert system has been developed to predict the degree of tactile comfort during the application of cosmetic foundation. A tribometer using compliant silicone-rubber surfaces was developed to perform 4 sliding tests and an oscillating test. From the tests for 20 different samples, 11 feature quantities were identified. The values of the feature quantities were used as the inputs to an artificial neural network, and the scores of tactile sensations were obtained as the outputs. The neural network, after supervised learning, could predict 5 types of emotional tactile comfort with high accuracy.

The mechanism of stick-slip associated with Schallamach waves was investigated using a sliding point contact between a glass hemisphere and a transparent soft-material plate that was driven via tangentially compliant support. The measurement of the tangential deflection and the observation of the contact zone were performed simultaneously. Schallamach waves synchronized with stick-slip were observed. Stick-slip associated with Schallamach waves is likely to occur for low contact load and high driving speed, which apparently contradicts the tendency of general stick-slip. This can be explained by the magnitude of the stick-slip parameter that considers the contact-area variation induced by Schallamach waves.

An experimental apparatus has been developed to observe the contact zones between human fingerpads and a smooth flat surface of a total-reflection prism. Using this apparatus, the following experiments have been carried out: (1) measurement of the apparent and real contact areas of fingerpads under a controlled contact load, (2) measurement of the vibrotactile thresholds of fingerpads under a controlled contact load, and (3) investigation of the discrimination of tones by tactile sensation under a controlled contact load. From the third experiment, it is found that human fingerpads can discriminate between tones from the differences in their harmonic components.

Sliding systems with a flexible body (i.e., a soft material) in the contact zone were modeled as a general two-degrees-of-freedom system that includes tangential contact compliance, and a series of dimensionless governing equations was derived. Based on the equations, a numerical simulation was conducted to study the stick-slip characteristics of the modeled system. By using six dimensionless parameters included in the equations, we obtained criteria for preventing stick-slip that can be applied to the design of sliding systems, such as the selection of soft materials for contact surfaces. In addition, some unusual phenomena caused by tangential contact compliance were observed. Even if the supporting section is overdamped, stick-slip motion can be promoted by the inertia of the mass of the contact zone; moreover, tangential contact compliance can cause serious errors in the measurement of the static and kinetic friction coefficients that uses the elongation of the supporting spring.

In order to acquire safety-design criteria for preventing friction-induced vibration, dimensionless analysis and numerical simulation have been conducted for a single-degree-of-freedom system with friction. The model includes the discontinuity between static and kinetic friction and the dependence of the kinetic friction coefficient on the relative velocity. Dimensionless description reduces the number of parameters from nine to five; four of the five dimensionless parameters control the occurrence limit of friction-induced vibration. We have derived the occurrence-limit equation on the basis on a previous study on stick–slip in the Coulomb friction model, and we have constructed the discriminant inequalities with the four parameters. They are sufficient conditions for preventing friction-induced vibration, i.e., not only stick–slip but also instability of steady sliding; they are expected to provide safety-design criteria of sliding systems with high usability and high accuracy.

An artificial tribosystem utilizing silicone-rubber sliding surfaces has been developed for simulating the friction phenomena that occur when human fingers are rubbed together in an attempt to acquire the tactile sensation of small particles. Although the friction signals acquired in the system have high uncertainty, two types of feature quantities, including the information about the particle size, were obtained—the elastic recovery in the normal signal as the slow information and the weighted mean frequency in the tangential signal as the fast information. The discrimination of particle size is realized by using one of the feature quantities to a certain extent, and a hybrid algorithm using both the feature quantities provides the discrimination with higher accuracy, which is comparable to or higher than the accuracy of human tactile systems. Consequently, the slow and fast information are complementary, and the relative reliability between the slow and fast information varies with a change in particle size.

The breakdown processes of oil films under quasi-static loading have been investigated by using a newly developed steel-oil-mercury system. The relationship between the thickness and breakdown ratio of a hexadecane film is represented by a single master curve independently of the indentation speed, indentation load, and temperature. The master curve shows that the breakdown process of hexadecane includes two stages; one is the decrement of the thickness without breakdown and the other is the decrement of the thickness with a drastic progress of breakdown. By solving a small amount of fatty acid in hexadecane, the thickness increases and the breakdown ratio decreases noticeably; a multilayer residual film supporting normal load is formed between two metal surfaces. Experiments at different temperatures reveal a negative relationship between the temperature and thickness of residual film, which indicates that the residual film is organized by physical interaction rather than chemical interaction. At least under a lower concentration, the residual film appears to consist of not only fatty acid molecules but also hexadecane molecules.

Uniform, transparent indium tin oxide (ITO) films were prepared by dip-coating process using an organic sol composed of indium acetate—diethanolamine—tin octylate-n-propanol mixture and the relationship between their electrical properties, film morphology and dip-coating conditions have been investigated. The optimum Sn-doping concentration was about 4 mol% relative to In ion. The conductivity of as-prepared ITO films increased with an increase in firing temperature. Multiple coating of the layers as thin as a few tens of nanometers accelerated the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 × 10−4 Ω cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. Through this study we conclude that the conductivity of dip-coated films was mainly controlled by the crystallite size and, hence, by carrier mobility.

Experiments are described in which friction coefficients under boundary lubrication regime are measured with a nematic liquid crystal as the lubricant. Using steel-to-steel contacts, with one of the mating surfaces coated with a thin insulating film, electric fields are applied across the liquid crystal film. A substantial decrease in the friction coefficient is observed with both d.c. and low-frequency a.c. voltages, suggesting the possibility of controlling friction by external fields.

Liquid crystals are characterized by the orientation of their molecules, the major axis of whose direction can be controlled by an applied electric or magnetic field. In the present paper the viscosity variation of three thermotropic liquid crystals in the nematic phase having different dielectric constants is studied at varying electric field strength, shear field strength and temperature. The results show that the viscosity of liquid crystals can be varied by the applied electric field strength and that a lower temperature reduces the threshold electric field strength for the electroviscous effect to appear as well as decreasing the attainable values at higher electric field strength. In addition, the frequency of the applied electric field has little effect on electroviscosity.