The 9th International Tribology Conference, Fukuoka 2023 was held on 25–30 September 2023 at Fukuoka International Congress Center in Fukuoka City, Japan. The number of participants was 975 from 30 countries and regions, and the number of papers presented was 676. We thank all the people who joined and contributed to ITC.

Papers submitted to this special issue went through peer-review process. Twenty papers were already published in Part 1, and 19 papers were accepted for publication in this Part 2. On behalf of the Organizing Committee, we would like to thank all the authors for their contributions, and also thank the editors, reviewers and the editorial office of Tribology Online for their efforts.

Taking this opportunity, we would like to include in the next page the manuscript of the congratulatory speech delivered by Professor Masato Tanaka in Banquet on Thursday, 28 September, 2023

Friction and wear collectively account for nearly a quarter of the world’s energy consumption, resulting in over eight Gigatons of CO₂ emissions annually. With increasing mobility and industrial activity, the adverse effects of friction and wear on energy, the environment, the global economy, and sustainability will undoubtedly intensify. Unless we reverse this unsustainable trend, our planet could face a major ecological and environmental catastrophe. Fortunately, significant strides have been made in reducing friction to almost undetectable levels, with friction coefficients below 0.001. These remarkable achievements have resulted from numerous collaborative efforts and global initiatives focused on developing novel materials, surfaces, and interfaces that exhibit minimal or near zero friction, even at macro or engineering scales. This paper provides a comprehensive overview of the factors that contribute to and hinder superlubric sliding conditions, examining the impact of both intrinsic and extrinsic factors that are integral parts of the test conditions and environments. Drawing from recent analytical, experimental, and computational findings, underlying mechanisms most responsible for superlubricity are also discussed. The paper discusses recent mechanistic studies on highly ordered 2D materials, such as graphene, MoS₂, h-BN, MXene, etc., and thin solid coatings such as diamond-like carbon or DLCs, as well as liquids, and discusses their potential for the development of large-scale mechanical systems. These exciting advancements pave the way for designing and producing next-generation engineering systems that can minimize friction in practical applications, thus conserving energy, enhancing durability, and protecting the environment for a sustainable future.

It is widely known that the impact of atmospheric humidity on tribological phenomena is significant, particularly in the context of dry sliding. Previous reviews have comprehensively addressed various cases within this domain and this article aims to complement these reviews with recent findings. The review covers a wide spectrum of humidity levels, ranging from trace amounts of water in the order of a few vol ppb to the relative humidity levels experienced in daily life. It delineates the forms of adsorbed water on solid surfaces within each humidity range and explores the potential physical lubrication mechanism of the adsorbed water and its effects on tribological phenomena. In the past, explanations for this influence have tended to be qualitative, primarily viewed from the perspective of chemical reactions. This discussion seeks to broaden the understanding by considering the influences from a physical lubrication perspective. Concluding the review, recent research findings are presented, shedding light on instances where trace amounts of water present in gases such as hydrogen exert an influence on tribological phenomena.

Hydrophobic textured surfaces are studied for their low wettability and their capacity to create a ‘slippery’ fluid on the surface during lubrication. To this end, the flow between two parallel surfaces is numerically addressed by computing two dimensional numerical simulations. One of the surfaces moves with a uniform rectilinear motion, while the other is fixed, with a cavity in the middle. The steady-state flow is laminar and monophasic with a low Reynolds number. The reduction of the wall shear stress caused by a vortex in the cavity, with respect to a Couette flow, looks like the creation of an equivalent slip of the fluid on the wall at a macroscopic scale. Three methods are used to calculate the slip length: one is based on the wall shear stress and the other two are based on the speed of the fluid flow. When the slip length is calculated according to these three methods, the obtained results differ. The differences show that the slip often used in the literature is a macroscopic representation of local effects that are not necessarily slippery. The speed profiles and the streamlines are then discussed, in order to propose an explanation for this difference.

Large-diameter slewing bearings are capable of supporting high loads and moments. They find typical usage in applications requiring extensive rotational movements, such as cranes, excavators, and wind turbines. Engineered to endure significant loads, these bearings offer low friction and maintenance requirements. With diameters often reaching several meters and featuring wide raceways, large-diameter slewing bearings are important components. In case of a failure a delivery of a new bearing can take several weeks, due to their large dimensions. Therefore, a process chain was developed, which ensures the repair of damaged raceways of the large-diameter slewing bearings. Remanufacturing aims not only to prolong the lifespan of the slewing bearings but also to minimize waste, presenting a sustainable alternative to purchasing new units. The repair process chain consists of several steps. Commencing with a damaged bearing raceway, a cladding material with sufficient chemical and physical properties is applied on the damaged surface by plasma-transferred-arc welding (PTA). Afterwards the slewing bearing undergoes a heat treatment, followed by surface pre-machining to achieve the nominal dimensions. An incremental forming process is performed to adjust the rolling bearing properties and induce compressive residual stresses. The last step of the repair process chain contains service life investigation of the repaired bearings on a rolling bearing test rig. In this first study, the fundamental feasibility of the process chain was evaluated using smaller axial bearing washers of type 81212. Therefore, plasma transferred arc welding were applied to repair the damaged raceways of the bearing washers by surface cladding. Following the repair, the bearing washers underwent a heat treatment process. Challenges in repair welding can arise from various factors, including the type of material being repaired, the location and accessibility of the repair area, and the required level of precision and quality. The repair welding process was evaluated according to different criteria. To evaluate the quality of the welding processes the repaired bearings were examined by using metallography, scanning ultrasonic microscopy, scanning electron microscopy, 3D scanning and x-ray diffraction. Furthermore, hardness measurements were carried out to investigate the properties and characteristics of the cladded layer and base material. The investigations demonstrated the feasibility of repairing the damaged raceways and showed that the new cladded raceway could welded without any macro pores or other defects.

A railway wheel flat occurs when the wheel locks and slides along the rail, forming a white etching layer (WEL) just under the flat surface where rolling contact fatigue cracks often form that can propagate into the wheel and spall during service. This study comprehensively investigated the influencing factors on wheel crack propagation that lead to spalling. Experiments were conducted to test wheel specimens with WELs of various dimensions, and simulations were performed to evaluate the stresses near the WELs as well as the rate and direction of crack propagation.

Segmented radial seals are modern technologies used in aeronautic engines and space turbopumps. They consist of several circumferential segments assembled as an annular ring freely mounted on the rotor. A circumferential (garter) spring tightens the segments around the shaft. The segments can move radially relative to each other, sliding against a casing and following the dynamic displacements of the rotor. Two similar segmented seals made of different materials were tested. The results showed different leakage flow rates and different correlations between leakage and wear rate. For example, the leakage rate decreases with wear for one seal and increases for the other. For explaining these results, wear was correlated with the microstructure of the materials of the two seals. Profilometric analyses were made for the two seals, highlighting that wear was not localised in the same areas. Raman spectra maps were taken on the pads of the segment in contact with the rotor and microstructural changes caused by friction were enlightened. The evolution of residual stresses was measured for new and worn segments. These findings strengthen the results obtained from profilometric wear analyses. They also allow a more precise understanding of tribological degradation scenarios depending on the operating conditions and the properties of the material.

Rolling element bearings in a particular application experience premature failure accompanied with white structure in steel. A possible cause of the failure is diffused hydrogen in steel which is generated by the decomposition of lubricant. This paper studied hydrogen generation from lubricant under rolling-sliding contact. A two-roller test was conducted in a sealed chamber, where the sliding speed and the lubricant temperature were controlled. The material of the roller was bearing steel and ceramic. Hydrogen gas concentration in the chamber was measured with a gas chromatograph. It was found that the hydrogen generation due to rolling-sliding contact depended on the lubricating conditions when the bearing steel specimens were used. The hydrogen generation increased by increasing the sliding speed and the oil temperature and decreasing the oil film parameter. Moreover, hydrogen generation was hardly observed under pure rolling condition. Another finding was that when ceramic specimens were used, the hydrogen generation was much smaller than when bearing steel specimens were used. These results suggest that nascent steel surface caused by the combination of metal-to-metal contact and slip accelerates the lubricant decomposition to generate hydrogen.

The dynamic wetting and dewetting of a water droplet on a hydrophobic solid surface with an equilibrium contact angle of approximately 100° was investigated using molecular kinetic (MK) theory to model the extension–contraction experiment and numerically calculate the radius of the contact line circle between the solid and liquid, contact line velocity, and dynamic contact angle. Furthermore, the numerical calculation results were examined in terms of various coefficients of the contact line friction to evaluate the mobility of the contact line. The calculation results of wetting and dewetting were in quantitative agreement with our previous experimental results, indicating that our constructed model was appropriate. Therefore, the MK theory can be applied to explain the characteristics of dynamic wetting and dewetting of droplets on hydrophobic surfaces, as with hydrophilic surfaces.

We measured the adhesion force between a sphere and a plane in water using a surface force apparatus that can measure the adhesion force with ultra-high accuracy. In particular, we focused on the relationship between the withdrawal speed and the adhesion force as one of the dynamic characteristics of the surface force. PDMS (Polydimethylsiloxane) was used as a spherical probe, and PEEK (Poly Ether Ether Ketone) and Si were used as plane samples. The results revealed that the adhesion forces depended on the withdrawal speed in water. This dependence was found to be qualitatively the same as that observed in a vacuum, indicating the influence of the viscoelasticity of PDMS on the adhesion forces. Furthermore, a comparison of the adhesion forces in air and water showed that they are dependent on the combination of materials. We deduced that the wettability of the material affected the adhesion forces. Specifically, the low affinity of the two surfaces to water resulted in hydrophobic attraction, while a hydration repulsion acts when one surface becomes hydrophilic.

Catheter surgery is an effective treatment for vascular disease and has been investigated from a tribological perspective to prevent vascular damage. Endothelial glycocalyx layer (EGL), which is present on the most superficial surface of vascular endothelial cells plays an important role in maintaining vascular homeostasis, but, during catheter surgery, they are affected by frictional stimulation caused by direct contact with the catheter. In this study, to investigate and discuss the effects of frictional stimulation on the surface properties of vascular endothelial cells, we conducted nano friction tests using atomic force microscopy (AFM) on the surface of vascular endothelial cells. and evaluated mechanical properties, interaction structure, and surface properties of vascular endothelial cells and EGL. The results show that the elastic modulus of endothelial cells increased after the nano friction test. It was also found that EGL was worn away after several slides and the friction coefficient of vascular endothelial cells increased with the wear of the EGL. Furthermore, the adhesion force of endothelial cells increased by the wear of EGL. Besides that, the amount of adsorption to fatal bovine serum (FBS) was also examined using the QCM-D method, and a trend toward increased adsorption was observed for cells without EGL compared to cells with EGL.

The accurate prediction of friction coefficients is crucial for the maintenance of sliding mechanical components to enable the timely detection of potential failures. Traditional methods rely on sensors like load cells and strain gauges to measure friction coefficients. However, these conventional techniques face challenges in real-time measurement during machine operation owing to physical constraints associated with sensor placement. To address this limitation, this study investigates the application of laser speckle patterns for predicting friction coefficients through a novel approach using convolutional neural networks (CNNs). The laser speckle technique offers rich surface condition data, while CNNs, which are particularly advanced in managing vast datasets, excel in establishing relationships between diverse factors for precise inference, classification, and prediction. Utilizing ResNet, a leading CNN architecture, a new friction tester capable of concurrently recording friction coefficients and speckle patterns in a cylinder-on-disk friction test was developed. The findings reveal that the CNN-based method, especially with ResNet, attained a coefficient of determination (R²) of 0.758, demonstrating its effectiveness in the accurate prediction of friction coefficients. This study significantly advances the field of friction coefficient prediction, highlighting the innovative application of laser speckle methodologies combined with machine learning techniques.

Gas foil bearings, which are oil-free and exhibit low energy losses, are used in high-speed rotating machinery. Because gas is used as the working fluid, there is a need to improve damping performance using support structures. Additionally, the support structure, which becomes increasingly complex shaft performance, needs to be simplified. This study proposes a new bump mesh foil bearing focusing on the support structure and experimentally verifies its static and dynamic characteristics. The bump metal mesh foil bearing is a simple structure that combines a bump shape and metal mesh. Static load and hammering tests were conducted to evaluate the static characteristics, and the frictional torque and vibration at high-speed rotation were measured to evaluate the dynamic characteristics. The bump metal mesh foil bearing exhibited excellent damping performance owing to the friction damping caused by the bump shape and metal mesh; however, the rigidity of the bump metal mesh foil bearing was reduced. The bump metal mesh foil bearing exhibited the same level of lifting performance as the bump foil bearing, and vibration reduction at high-speed rotation was confirmed. Thus, the combination of the bump foil and metal mesh bearings yielded good results in terms of vibration characteristics.

This research study investigates how incorporating soft nanoparticles affect the wear behavior and the characteristics of the materials in three-body mixed lubrication conditions. A disk-on-block tribometer was used to revealed a positive correlation between plasticity index and wear rates, indicating that increased plastic deformation leads to higher wear. The size and concentration of copper oxide (CuO) nanoparticles significantly influenced wear behavior. The study found that for block materials with a hardness of 27 HRC and 43 HRC paired with a disk of 58 HRC, increasing nanoparticle concentration and using larger nanoparticles relative to surface roughness (larger nanoparticle size and surface roughness ratio) resulted in reduced wear. The larger nanoparticles can more effectively fill the gaps on the smoother surface, increasing contact area and minimizing plastic deformation during sliding, especially when the contact is more elastic. However, when the counteracting block material is significantly harder than the disk, the effect of nanoparticles might be reversed, potentially leading to increased wear. These findings suggest a complex interplay between material properties, nanoparticle characteristics, and wear resistance, influenced by the relative hardness of contacting surfaces. This research offers valuable insight for developing materials with improved wear performance in applications involving sliding friction in third-body contact.

This paper presents a numerical simulation of an impulse gas seal. Impulse gas seals are mechanical seals equipped with feeding grooves on the stator and chambers on the rotor. During rotation, the chambers are fed with high-pressure gas when facing the feeding grooves and then release the high-pressure gas to maintain a gas film between the seal surfaces. The effect of operating conditions and chamber volume on the gas film thickness is studied. It is shown that the gas film thickness variation with pressure, speed, and chamber volume can be described with a power law of a duty parameter. Similar results are obtained for the mass flow rate and friction torque.

It is important to understand tribological characteristics of components operating in a hydrogen atmosphere. For instance, piston rings used in reciprocating compressors require strength, heat resistance and wear resistance, hence resins containing fillers are used. Particularly at high-pressure stages, PEEK- or polyimide (PI)-type composite resins are often introduced. In this study, focusing on PI and its composites, we investigated sliding properties of the resin in the hydrogen environment and the structure of transfer film. It was confirmed that some PI without fillers had good sliding properties and this unfilled PI exhibited sliding properties which was equivalent to other graphite-filled PI. A cross-sectional TEM EELS analysis revealed that the transfer film was a layer mainly composed of carbon. Raman analysis suggested that, in case of unfilled PI, the transfer film of carbon was formed by the decomposition of the PI due to wear, whereas in case of graphite-rich PI resin, the film was created mainly by decomposition of graphite. These results suggest that the carbon layer of the transfer film, which comes from sliding in the hydrogen environment, improves friction and wear properties, indicating that the origin of the layer may be irrespective of whatever it is (e.g., PI, graphite, etc.).

Tribological machine elements, such as rolling element bearings and gears, are often used in mechanical components and various industrial applications. The contact interfaces and sliding parts fundamentally consist of two solid and one liquid lubricant, and they are operated under a thermal elastohydrodynamic lubrication (TEHL) condition. The TEHL solution characteristics depend on the operating conditions and lubricant physical properties, as well as the surrounding environment, including the temperature along the solid surfaces and pressure adjacent to lubricated contact. Herein, we numerically examine the influence of both solid temperature and ambient pressure on TEHL solutions at the nominal line contact and discuss the synergistic effects. Any changes in the physical properties of the lubricants are considered either together or separately as functions of temperature and pressure. The high ambient pressure enhances the effects of the solid temperatures on the resultant TEHL solutions. Regardless of the solid temperature and ambient pressure conditions, the TEHL solutions for all the lubricant models are grouped into two in terms of thermal conductivity.

The effect of an electrospun (ES) film on the reduction of micro-drilling cutting force required for SUS304 plate was investigated. ES films were fabricated using the electrospinning method with polyvinyl alcohol (PVA) and polymethyl methacrylate (PMMA) as raw materials. The results of ATR-FTIR spectroscopy showed that there was no difference in the composition of the raw polymers and the fabricated ES films, and that the polymers were not altered. The diameters of the PVA and PMMA nanofibers were approximately 0.069–0.30 and 7.2 μm, respectively. The drill thrust force and run-out when using the nanofiber ES films were lower than those on an uncoated JIS SUS304 plate during the drilling. High-speed camera images showed that PMMA nanofibers pulverized by the cutting process adhered to the drilling tool surface and were guided into the drilling hole. This suggests that ES films made of organic polymer compounds, such as PMMA, are promising candidates for stable drilling, as they suppress drill run-out and act as an excellent lubricant during cutting.

The study focuses on the prediction technology of rubber block friction properties on ice. The 2-D simplified block model with sipes (narrow grooves) and the mechanical analysis methodology are applied to understand the complex tribological phenomena and improve winter tire grip on icy roads. The friction forces on ice are assumed to consist of adhesion force, viscous drag of water film, and ploughing effects of sipe edges. Based on this assumption, the ice temperature distribution and friction coefficient in the contact patch are predicted. The effects of sipe density and block stiffness on tire tread pattern are evaluated. Experimental results partially validated the predicted friction coefficients. The study also proposed new sipe structures with high-density short sipes to improve ice performance. Tentative test results indicate the effectiveness of these new structures.

The world cannot counter climate change without the automobile industry becoming far more environmentally sustainable than it is today. With the realization that electrified mobility will take decades, not years, and that internal combustion engines (ICE) will continue to power vehicles in the near future, we need to step up the focus on improving fuel efficiency of ICE vehicles. In this report, we have confirmed that, in timing chains for automobile engines, the sliding area can be reduced by changing the arrangement of the arms & guides from continuous contact to a segmented one and by using an aromatic polyimide (PI) that can withstand the high contact surface pressure, thereby reducing friction loss. Through bench test monitoring, we confirmed that the friction loss was reduced in comparison to that by the conventional arms & guides in ICEs. The conventional arms & guides in the layout of the timing chains are placed with a curvature of ~ R2000 mm (at least R500 mm). In this study, the layout of the arms & guides was investigated. Each arm or guide is divided into two or three segments in the direction of travel and each curvature is reduced to about R100 mm. In addition, the material has been changed from polyamide (PA66) to PI. By these changes, the contact surface is decreased, and the surface pressure is increased. This means that on the Stribeck diagram, the lubrication conditions move in the direction of the smaller Sommerfeld number and the friction loss decreases. The use of the high compressive modulus material reduces the amount of deformation of the arms & guides contact surface, leading to the reduction of the friction loss.