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Friction-induced vibration in the wiper system of passenger cars not only causes impaired visibility as a result of uneven water film distribution on the windscreen surface, but also leads to noise problems by transmitting vibrations as sound to the passengers. In this study, a novel experimental apparatus was developed to simulate the wiper system, enabling the change in normal support stiffness and the precise adjustment of the yaw angles of a shortened rubber blade and its flexible base relative to the drive direction of a glass plate. The use of fluorescence observation provided a significant advantage, enabling precise measurements of the position of the rubber blade tip and the water film thickness in the contact area of the rubber blade and the glass plate during operation. In conjunction with this, an accelerometer and a gap sensor were employed to measure the normal and tangential motions of the support structures, respectively. This comprehensive setup allows for precise control of blade support conditions, and provides accurate measurement for motions of rubber blade and support structures, making it a powerful tool for investigating friction-induced vibration in the wiper system. The experimental results clearly demonstrated the effectiveness of applying large yaw angles to both the rubber blade and the flexible base in suppressing friction-induced vibration, resulting in smooth motion at any drive speed. These findings have the potential to improve wiper performances under actual operating conditions.

The authors have previously developed the electrical impedance method which simultaneously measure the thickness and breakdown ratio of oil films in EHD (elastohydrodynamic) contacts. Furthermore, using a ball-on-disc-type apparatus, the authors have also confirmed that the accuracy of oil film measurement by the developed method is comparable to that of optical interferometry (i.e., glass-steel contacts). In this study, we discussed the experimental results obtained by applying the electrical impedance method to steel-steel contacts using a steel disc instead of a glass disc. It was found that under the mixed lubrication regime where wear occurs, the oil film thickness obtained by this method is calculated to be significantly thicker than the theoretical value. This can be attributed to the increase in the apparent dielectric constant of the lubricant due to wear particles in the lubricant.

It is well known that the friction coefficient of viscoelastic solids against the drive speed exhibits a bell shape. In previous theoretical studies on viscoelastic friction, we can find two origins for the bell shape. One is due to the material response (i.e., the bell-shaped rheology of viscoelastic solids), and the other is due to the mechanical response (i.e., the vertical lift of the counter surface with asperities). Although these two origins do not contradict each other, their relationship seems unclear. This paper theoretically examines the friction coefficient for the steady sliding contact between a rigid probe and a viscoelastic foundation exhibiting the standard linear solid viscoelasticity. Capitalizing on the strength of the simple model, we successfully find the complete set of dimensionless numbers and master curves describing the velocity dependence of the friction coefficient. Comparing them with the dimensionless numbers and master curves for the Kelvin-Voigt viscoelasticity, we clarify two origins for the bell-shaped velocity-dependent friction coefficient and their relationship.

This paper reports the thermally activated ultralow friction of 100% cellulose nanofiber (CNF) molding. The mechanism of friction reduction was investigated using a laboratory-built in-situ Raman tribometer. Our experimental results showed that a CNF molding exhibited an ultralow friction coefficient of below 0.04 in a CNF ring and steel disk tribopair under high-temperature conditions (T > 100°C). The results of the temperature-rise friction test showed that the friction coefficient of the CNF molding strongly depended on the temperature and decreased linearly with increasing temperature. The in situ tribo-Raman monitoring results, during friction, indicated a change in the structure of the CNF molding. Therefore, the crystallinity indices and lengths of the CNF fibers gradually changed as the temperature increased. Moreover, transfer tribofilms were observed on the counter-steel surface against the CNF rings. When the CNF molding exhibited thermally activated ultralow friction, the tribofilm was mainly composed of cellulose and graphitic carbon. Our results suggest that the thermal and friction-activated structural transformations of CNF molding and CNF-derived transfer film formation are pivotal factors contributing to the ultralow friction phenomenon observed in CNF molding at high temperatures.

Experimental results on the reaction of Cardiac Spheroids – spherical clusters of cardiac cells – obtained and studied recently by Nakano et al. are interpreted in the framework of a model of cardiac tissue as an unstable medium with retardation. Contact properties of this active medium are considered in an extended framework of the Method of Dimensionality Reduction. It is shown that the apparent activation of beating by mechanical loading owes to the synchronization of self-excited oscillations of considered unstable medium due to mechanical contact with a flexible loading unit. Increasing loading of a sphere leads, at first, to an increasing beating amplitude, which decreases again with further loading until the beating stops completely.

Chain continuously variable transmissions (chain CVTs) are known as promising technology due to their high torque capacity. However, they generate noise and vibration problems due to their flexible structure. The present study aimed to address these issues by developing a chain CVT fluid with two strategies inspired by the Stribeck curve. The first strategy aimed to reduce the friction coefficient in the boundary lubrication regime, while the second sought to increase the lower limit speed of the fluid film lubrication regime. Ball-on-disc tests were conducted for rolling/sliding contacts lubricated by test lubricants with and without three common additives. Results showed that the combination of oleylamine (as a friction modifier), tricresyl phosphate (as an extreme pressure agent), and calcium sulfonate (as a detergent) effectively met the objectives of both strategies. Evaluations by chain box tests using a practical chain CVT unit revealed that this combination of additives reduced noise generation by 6 dB, demonstrating the successful development of a low-noise chain CVT fluid.

Recent advances in computational methods have enabled realistic numerical simulations for the sliding friction of viscoelastic solids (e.g., rubber friction by the finite element method in the macroscale and soft matter friction by the molecular dynamics method in the nanoscale). However, the factor analysis of accumulated numerical results is generally not easy as their models include many system parameters. Here we theoretically examine the sliding friction with the minimum number of system parameters: the sliding friction between a rigid probe of various shapes and a viscoelastic foundation that consists of the Kelvin-Voigt elements. The analytical solutions for the master curves of the friction coefficient are described by using dimensionless numbers under two boundary conditions (i.e., a constant indentation depth and a constant contact load). Based on the analytical solutions, we discuss how the bell-shaped velocity dependence of the friction coefficient appears under a constant contact load without bell-shaped rheology: The keys are the vertical lift of the probe and the change in the inlet slope due to the vertical lift. The analytical solutions (exact and approximate) obtained here would help the factor analysis to some extent for several numerical simulations in computer-aided science and engineering.

Concentrated polymer brushes (CPBs) synthesized by advanced surface-initiated controlled radical polymerization have received increasing attention in the tribological field because CPBs in good solvents exhibit superlow friction phenomena under macroscopic severe sliding conditions. To realize tribological applications of CPBs, an understanding of the wear mechanisms of CPBs is required to develop a method to enhance their durability. In this study, an operando tribo-Raman spectroscopic experiment, which is an effective approach for revealing the wear mechanisms of tribo-materials, was conducted on the frictional interface between a polymethyl methacrylate (PMMA)-CPB and a bearing steel ball immersed in an ionic liquid (IL), which is a good solvent for PMMA-CPBs. From the operando tribo-Raman results, the concentrations and molecular structures of the PMMA-CPBs and IL at the frictional interface were found to be constantly changing with the sliding cycles during operation. Moreover, their time-dependent changes strongly affected the wear process and superlow friction phenomena of the PMMA-CPB lubricated with the IL. Consequently, our results suggest that controlling the degree of swelling of CPBs at frictional interface is an effective approach for realizing a durable-superlow frictional tribo-system of CPBs.

Suitable friction modifiers and anti-wear additives for hydrogenated amorphas carbon (a-C:H) films have been investigated by many research groups for a decade since it is well-known that the combination of the a-C:H film and the conventional additives hardly exhibits good tribological performances under boundary lubrication. In this study, to confirm the effectiveness of nano-carbon additives for improving the tribological performance of a-C:H films, the tribological performances of the combination of the a-C:H film and multi-walled fullerenes (MWFs) containing oil was investigated under boundary lubrication. Especially, we compared the effects of the MWFs on the tribological performances of two DLCs: a-C:H and tetrahedral amorphous carbon (ta-C) using a ball-on disk tribometer and in-contact Raman tribometer that can monitor the chemical structure of frictional interfaces. All our results suggest that the combination of a-C:Hs and MWFs exhibits an ultralow friction and remarkable wear reduction compared to non-MWF added oils due to the formation of MWF tribofilms uniformly at the frictional interfaces.

Abstract Adsorbed molecular films, referred to as boundary films in tribology, are widely used in various industrial products as a keyway for surface functionalisation, such as lubricity, wettability, and adhesion. Because boundary films are thin nanometre-scale molecular layers and can easily be removed, their formation process cannot be elucidated in detail. In this study, to analyse the growth dynamics of boundary films, the film thickness and molecular orientation of the boundary film of a fatty acid used as an additive in rolling contact as mechanical stimuli were measured in situ. The measurements were performed on simple test lubricants, which were composed of n-hexadecane and stearic acid, at rolling tribological condition between steel and glass (or sapphire) surfaces by ultrathin film interferometry combined with sum-frequency generation spectroscopy according to a unique protocol. The results quantitatively demonstrate shear-induced boundary film formation. The insight gained from these results is anticipated to enable the formulation of high-performance lubricant additives to further reduce friction loss and high-performance glues that can be freely designed for removability.

Abstract Since the discovery of synchronous pulsations in cardiomyocytes (CMs), electrical communication between CMs has been emphasized; however, recent studies suggest the possibility of mechanical communication. Here, we demonstrate that spherical self-beating CM aggregates, termed cardiac spheroids (CSs), produce enhanced mechanical energy under mechanical compression and work cooperatively via mechanical communication. For single CSs between parallel plates, compression increased both beating frequency and beating energy. Contact mechanics revealed a scaling law on the beating energy, indicating that the most intensively stressed cells in the compressed CSs predominantly contributed to the performance of mechanical work against mechanical compression. For pairs of CSs between parallel plates, compression immediately caused synchronous beating with mechanical coupling. Compression tended to strengthen and stabilize the synchronous beating, although some irregularity and temporary arrest were observed. These results suggest that mechanical compression is an indispensable control parameter when evaluating the activities of CMs and their aggregates.

In this study, we developed the electrical impedance method which simultaneously measures the thickness and breakdown ratio of oil films in elastohydrodynamic (EHD) line contacts within thrust needle roller bearings. Initially, we theoretically demonstrated that the oil film thickness and breakdown ratio can be simultaneously measured using the complex impedance that is produced when an AC voltage is applied to EHD line contacts. To verify the measurement accuracy of the electrical method, we monitored the oil film thickness of a thrust needle roller bearing and compared it with the theoretical value. The results revealed that the oil film thickness was thinner than the theoretical value immediately after starting the test, with the breakdown ratio being greater than 0 (indicating mixed lubrication); however, the breakdown ratio decreased over time, and the oil film thickness nearly matched the theoretical value one hour after starting the test, when it is believed that running-in wear is complete (i.e., breakdown ratio ≈ 0). Furthermore, following the test, after examining the race surface, we confirmed that running-in wear had indeed occurred. These results suggest that the developed method can monitor the lubrication conditions in EHD line contacts, such as those in thrust needle roller bearings, in detail.

In this study, the electrical impedance method was developed to monitor the thickness and breakdown ratio of oil films in elliptical elastohydrodynamic (EHD) contacts of practical ball bearings. First, it is theoretically shown that the oil film thickness and breakdown ratio can be simultaneously measured from the complex impedance generated when a sinusoidal voltage is applied to elliptical contacts. Subsequently, lubrication conditions of practical ball bearings were monitored at an ambient temperature to verify the measurement accuracy of the developed method. The oil film thickness in the low-speed range was consistent with the theoretical value calculated by Hamrock–Dowson equation. However, in the high-speed range, the oil film was thinner than the theoretical value considered an ambient temperature. In this high-speed range, the results of both the outer ring temperature and bearing torque revealed that the viscous shear heating and starvation were occurring simultaneously, thus supporting that the measured thickness would be thinner than the theoretical value. Besides, the developed method can also measure the breakdown ratio, confirming that it increases in the low-speed range where the bearing torque increases. That is, it indicates that not only the oil film thickness but also the breakdown ratio can be evaluated quantitatively.

Concentrated polymer brushes (CPBs) are a class of thin polymer films that are composed of polymer chains densely grafted to substrate surfaces. Due to their excellent tribological performance derived from the highly stretched conformation of the polymer chains in suitable solvents, CPBs are expected to be applicable as tribological coatings in sliding components. However, a long swelling time is required to obtain fully swelled CPBs in solvents. To overcome this issue, we propose a simple method involving thermal treatment of CPBs in solvents. Our results showed that this method can dramatically reduce the swelling time of CPBs from more than 48 h to less than 3 min. Moreover, thermal-treatment-CPBs (TT-CPBs) can provide an extremely low friction coefficient (0.002) under the boundary lubrication regime at the TT-CPB/steel tribo-pair interface. All our results suggest that the thermal treatment method is a practical method that will allow the industrial application of CPBs.

The stability of a two-degree-of-freedom (2DOF) sliding system with the velocity-weakening friction was examined by the eigenvalue analysis, where the in-plane anisotropy and the in-plane asymmetry were considered. The obtained eigenvalues were organized by using the minimum modal damping ratio as the stability maps. Selecting a stable point in the stability map corresponds automatically to embedding the Yaw-Angle-Misalignment (YAM) method in the mechanical structure design to avoid the instability. If we accept the mechanical structure design of sliding systems with the in-plane anisotropy and the in-plane asymmetry, we can find new stable conditions spread widely in the two-dimensional space, which are invisible from the conventional point of view.

Concentrated polymer brush (CPB) is the polymer film with a structure that possesses greatly extended polymer chains densely grafted to substrate. CPB is known to swell in good solvents, which have high affinity for the polymer. Swollen CPB by a good solvent shows excellent micro-tribological properties such as super lubrication with the extremely low friction coefficient (μ~10-4). However, macro-tribological properties of CPB surface on steel have not been explored sufficiently. In this study, macroscopic tribological properties of a CPB on rough steel were evaluated using a block-on-ring type tribo-tester under lubrication with an ionic liquid. The CPB showed a significant friction reduction effect with the low friction coefficient of 0.005 on macroscale.

The sliding friction of elastomers has been investigated numerically and theoretically for the line contact between a cylindrical rigid indenter and a “frictionless” Kelvin–Voigt foundation. The onset of sliding under an abrupt increase in the drive velocity has been simulated with different boundary conditions of the rigid indenter. When the rigid indenter is not allowed to move in any direction, just an abrupt change in the friction force appears, which is not accompanied by any transient processes. However, when the rigid indenter is able to move in the vertical direction, the transient sliding friction including three different time constants appears, resembling the typical transition from the static friction to the kinetic friction, in spite of no static friction considered in the simulation. The aforementioned drastic difference is caused by the “vertical lift” of the rigid indenter, which is induced by the damping of the Kelvin–Voigt foundation. In addition, the vertical lift strongly affects the characteristics of the steady sliding friction, which is explained well by using the critical velocity determined from the asymptotes in the master curve of friction coefficient.

To deepen the understanding of the frictional sliding phenomena of rough surfaces, advances are required in numerical analysis methods at various spatial scales. In this study, to examine the microscopic behavior of a rough asperity contact corresponding to a bulk contact on the macroscopic scale, a loop-type meso–macro coupled analysis scheme is proposed. A mesoscale numerical model and a macroscopic friction model are required for the proposed multi-scale analysis. A friction model was adopted based on the multipoint contact model for the mesoscale model, and the pressure- and state-dependent elastoplastic analogy friction model was used for the macroscale model. In the proposed meso–macro coupled analysis, the parameter set for the elastoplastic analogy friction model was first identified via a numerical friction test using the mesoscale multipoint contact model assuming various conditions. Then, a macroscale finite element analysis incorporating the elastoplastic analogy friction model was performed for the macroscopic analysis of contact between a rough rubber hemisphere and a smooth plate. Here, the information from the mesoscale rough surfaces were reflected in the macroscale finite element analysis. Finally, a mesoscale localization analysis was performed in which the macroscopic histories of several typical locations were obtained by finite element analysis and used as boundary conditions for the mesoscale model. It is suggested that the microscopic sliding process of rough surfaces represented by the finite element analysis can be examined using the proposed method.

In the present work, we analyze a device for measuring the dynamic response of rubber in sliding contact. The principle of the device is to measure excited oscillations of a mechanical system with a sliding contact between a rubber roller and a rigid surface. Depending on the contact properties, varying oscillation amplitudes are measured. The goal is to determine dynamic response properties of rubber from oscillating tests. For this sake, an analytical model is introduced in which the contact problem and system dynamics are considered in detail. Analytical and numerical results obtained for this model are compared with some experimental data and discussed both on a qualitative and quantitative level.

Hard solid deposit formation has been inevitable problem in marine use turbocharger, and many engineers have struggled with it for years. It is due to low quality fuel, and the deposit results in the deterioration of the performance. In addition, the deposit is hard so that it is difficult to be removed at periodical maintenance. However, hard solid deposit formation has not been reported and understood. In this study, the deposit in actual service turbocharger was analyzed, and the dependency of operation condition on the deposit formation was investigated. And the deposit formation was demonstrated using newly designed test method, vapor phase coking deposit evaluation test. Furthermore, the countermeasure to prevent from deposit formation is considered, and it was found the deposit formation is largely influenced by ingredient of metal.

Using a novel anti-vibration tribometer with a yaw angle misalignment, Kado et al. (Trans. Jpn. Soc. Mech. Eng. C79: 2635—2643, 2013) have recently shown experimentally that frictional vibration in tribotesting causes considerable error, e.g., a 35% underestimation of the kinetic friction coefficient for the sliding contact between a steel ball and a steel plate lubricated with glycerol. In this paper, it is shown that their experimental results can be numerically simulated based on a purely mechanical model, which confirms that their experiments were carried out properly and also that in conventional tribotesting, the “measured kinetic friction coefficient” (obtained from the time-averaged spring force) is not the “inherent kinetic friction coefficient” (determined by the inherent nature of materials in contact as a function of the relative velocity) but the “effective kinetic friction coefficient” (determined by the mean energy consumption rate as a function of the driving velocity). The effective kinetic friction coefficient depends on the mechanical properties of the tribometer used in the test, and it corresponds to the inherent kinetic friction coefficient when the measurement is carried out with no frictional vibration.

Ultrathin-film interferometry was used to investigate the developing process of boundary films in a steel-on-silica contact lubricated by a solution of n-hexadecanoic acid dissolved in n-hexadecane. Immersion of the surfaces in the solution is not sufficient to initiate boundary film formation. However, upon a nominally pure rolling contact, boundary film formation is initiated, and, once initiated, the boundary film develops spontaneously. It is believed that the initial monolayer film adsorbed strongly on the solid surface induces subsequent molecular stacking from the solution. The developed boundary film can be partially destroyed by the rolling contact, although the nominally identical rolling contact acts as the trigger of boundary film formation. These findings reveal that the immobile boundary films to reduce friction of solids under high pressure result from the equilibrium of two competing processes: spontaneous development by molecular stacking and mechanical destruction by relative motion of the contact surfaces.

Breakdown processes of boundary films formed by lubricating oils containing oiliness additives were studied in a steel-mercury interface under quasi-static loading. Simultaneous measurements of the thickness and breakdown ratio of the films were carried out using an electrical impedance method. Different breakdown processes were obtained for different lubricating oils. At a concentration of 10-3 mass%, the lubricating oils formed residual films with a thickness of a few tens of nanometers that support a normal load without any hydrodynamic effects. A possible mechanism for this is the osmotic pressure created by the concentration difference between the bulk oil and residual film. Friction coefficients of a steel-on-steel contact lubricated by the lubricating oils in a boundary lubrication regime were related to the breakdown processes in the steel-mercury interface using multiple regression analysis. Lubricating oils that showed low-rate breakdown processes showed low friction coefficients. This indicates that the breakdown rate of boundary films is an important factor in the mechanism of boundary lubrication.

In order to investigate the interaction between additives in metalworking fluids under difficult cutting conditions, tapping tests were conducted under different metalworking conditions for metalworking fluids consisting of liquid paraffin added with tricresyl phosphate (TCP), polysulfide (PS), and oleic acid (OA). The tapping torque was measured for tapping M6 threads in S45C steel by using workpieces designed with relatively small prepared hole for creating difficult cutting conditions. For evaluating the effect of metalworking fluids, two quantities - mean tapping torque (Nmean) and maximum tapping torque (Nmax) - in the tapping torque signals were introduced. When one of the extreme-pressure (EP) agents, i.e., TCP or PS, is added into the liquid paraffin, both Nmean and Nmax decrease considerably. Moreover, when both the EP agents are added, the values of both Nmean and Nmax are smaller than those obtained for the same concentration of a single EP agent; this implies the synergy between TCP and PS. OA, when used with the two EP agents, does not always decrease the values of Nmean and Nmax; there exists an optimal concentration level for TCP, PS, and OA for minimizing the values of Nmean or Nmax.

When we acquire tactile sensation, contact and relative motion are necessary between our bodies and objects. This implies that the tactile sensation is produced by friction phenomena. The present paper describes the sliding test for estimating the tactile sensation associated with cosmetic foundation. When the cosmetic foundation is rubbed with an apparatus that imitates a human finger, two types of time-evolving signals-normal and tangential-are obtained as the source of the tactile information. In the sliding test with the apparatus under the frictional condition of humans acquiring the tactile sensation, frictional vibration is observed in the tangential signals, which should be related to the tactile sensation associated with the cosmetic foundation. When a surface with a grooved pattern that imitates fingerprints is used in the sliding test, two types of frictional vibrations are observed. If an adequate sliding velocity is selected, the slight difference between the four types of commercial cosmetic foundations is discriminable by using the signals of these vibrations.

A novel experimental method has been developed to quantify the breakdown processes of thin oil films induced by quasi-static loading. The method uses a steel-oil-mercury system along with complex impedance analysis, which provides the simultaneous measurement of the film thickness and breakdown ratio. In the case of hexadecane, the relationship between thickness and the breakdown ratio is represented by a single master curve independently of the indentation speed. Every breakdown process of hexadecane traces on the same master curve, and its final point is determined by the indentation load. The breakdown process of hexadecane includes two stages: one is the decrement of the film thickness without breakdown, and the other is the decrement of the film thickness with the drastic progress of breakdown.

It is known that detachment waves are generated on the surface of a soft material when it is slid on a hard surface under certain conditions. In the present study, the variation in the contact area that arises when a plate of a cross-linked polymer, poly-dimethylsiloxane (PDMS), is slid in contact with a glass hemisphere has been measured every 2 ms using a microscope and high-speed camera. The measurements elucidate that the behavior of the PDMS surface under sliding friction is divided into three modes. In mode I, a detachment wave is generated and it propagates across the contact zone. In mode II, a detachment wave is generated but does not propagate perfectly. In mode III, no detachment waves are generated. The Young modulus and load control the generation of detachment waves, but the contribution of the former is approximately 8 orders of magnitude larger than that of the latter.

A vibration-assisted tapping system has been developed in which a piezoelectric-zirconate-titanate (PZT) oscillator applies small-amplitude vibrations to a workpiece and a torque transducer measures the time-evolving torque during the tapping process. To investigate the roles of metalworking fluids, four different metalworking conditions have been examined: without metalworking fluids (dry), with an additive-free fluid (base oil), with an oiliness-agent-containing fluid (fluid A), and with an extreme-pressure-agent-containing fluid (fluid B). The time evolutions of the tapping torque have been obtained for tapping M3 threads in S45C steel with varying vibration amplitudes, vibration frequencies, and tapping speed. It has been found that the present system decreases the tapping torque; in particular, a decrement of up to 14% in the tapping torque is obtained for fluid A using 800-Hz vibrations with an amplitude of 5 μm at a tapping speed of 3 rpm. Increments in the vibration amplitude and frequency lead to decrements in the tapping torque, but the effect of the vibration tends to fade with increasing tapping speeds. It appears that vibrations enhance not only the physical effects but also the chemical effects of metalworking fluids.