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, and 20 papers were accepted for publication in this Part 1. On behalf of the Organizing Committee, I 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.

The Organizing Committee of ITC Fukuoka 2023 selected excellent papers from those papers submitted before 25th of June, and the following three papers were awarded the ITC Fukuoka 2023 Excellent Paper Award. Award certificates and prizes were presented to the authors at the award ceremony in the Banquet held at Hotel Nikko Fukuoka on 28th September, 2023. Congratulations!

♢ “Contact Mechanics of an Unstable Viscoelastic Medium with Retardation as a Model for Mechanical Activation and Synchronization of Cardiac Spheroids,” by Valentin L. Popov and K. Nakano

♢ “Influence of White Etching Bands Formation on Integrity of Rolling Element Bearings,” by Mostafa El Laithy, Ling Wang, Terry J. Harvey and Bernd Vierneusel

♢ “Variation of Cavitation Pressure with Liquid and Operating Conditions in Mechanical Seals,” by So Makishima, Masatoshi Itadani, Yuichiro Tokunaga and Joichi Sugimura

More ITC 2023 papers will be published in Part 2 of the Special Issue before mid 2024.

With stringent environmental regulations and tougher economic reasons, tribological components like rolling bearings and gears are constantly under scrutiny when it comes to energy efficiency and reliability. This has driven engineers towards the constant search of new materials, new designs and optimal performance of components, particularly in tribological surfaces. All this has stimulated research with the aim to develop engineering knowledge to better select or better design mechanical components like gears and rolling bearings (often the most loaded surfaces in a machine). This article summarizes efforts in tribological modelling aspects leading to the prediction of different surface failure modes and eventually the life of the whole machine element. Aspects related to surface distress, wear, fatigue, and the competition of these phenomena are discussed.

Lubricants are used to reduce friction and wear in machines, saving billions of dollars worldwide in energy and breakdown costs and lowering CO₂ emissions. Today, most lubricants are made using hydrocarbons derived from crude oil, which is a finite resource, although alternative bio-based lubricants are also being investigated, as is the re-refining of used lubricants to make new base oil. The machines. It is also shown that an effective way to make lubricants more sustainable is to extend lubricant oil drain intervals and collect used oil and re-refine it to make base oil for re-use. The role of bio-based lubricants, and their benefits and disadvantages are discussed. Other aspects in which lubricants can be made more sustainable are also briefly covered, such as lubricant packaging, the removal of toxic additives via improved regulatory chemistry, and the use of renewable electricity in blending plants.

In the future, sustainability and CO₂ neutrality requirements will come from both customers and regulatory compliance. Each company should provide the greatest possible transparency regarding its sustainability efforts. In addition to the Footprint, the Handprint has to be considered, which can have a significant positive impact on sustainability during the product’s life span, including its development and production processes. In the case of dynamic seals, the reduction of friction, i.e., the optimization of the tribological system, makes a significant contribution to the Handprint. Based on this insight, a pressure rotary shaft seal has been developed with a specific sealing edge profile that reduces friction and wear, and thus increasing energy efficiency and service life.

A previously developed and validated model for the life of rolling/sliding heavily-loaded lubricated contacts, that separates surface and subsurface survival is now adapted to include the effect of material inhomogeneities and it is applied to the case of spur gear contact life. The previously used FFT-based, homogeneous elastic dry contact solver (for the calculation of pressure and subsurface stresses) is now replaced by a previously developed Lamé’s equations solver based on Multigrid methods that can handle bonded inhomogeneities in an elastic media. Despite the increase in computing time by this change it still is able to generate gear solution calculations within one hour computing time in a personal computer. The effect of bonded inhomogeneities in rolling contact fatigue (RCF) is investigated for a variety of inclusion geometries and Young’s modulus values. The results show how deterministically, or randomly distributed inclusions might shorten the RCF life of a component. In engineering practice this can be reflected in a reduction of the fatigue limit used.

With the increasing demand for active friction control, we newly proposed to use dielectrophoresis to change the flow of PG-droplet-containing PAO4 to reduce the friction coefficient. The friction result with a 1-mm roller shows 20% reduction in friction coefficient (from 0.065 to 0.052) at AC 100 V, and in situ observation exhibits that PG tracks are formed over the contact area. On the other hand, at a high bias of 1000 V, the friction coefficient increases to 0.065. In this situation, in situ observation exhibits that PG forms a horseshoe-shaped track covering only the roller edges. Controlled friction tests and FEM analysis using 5-mm rollers revealed a unique behavior; a balanced bias effectively attracts the PG to the roller surface, and surface forces can resist mild dielectrophoretic forces to spread the PG across the roller surface. The present study strongly suggests the importance that the bias strength should be controlled to achieve a balance between surface force and dielectrophoretic force in order to obtain excellent lubrication conditions.

Additive manufacturing is rapidly growing in popularity for manufacturing parts with tunable mechanical properties. Recent studies show that mechanical properties can be achieved by controlling the layer orientation and build structure. In this work the effect of print orientation on tribological properties of 3D printed PLA and ABS are investigated. PLA and ABS samples are printed using fused deposition modeling (FDM) with three different print orientations. Tribological results show that variation in build direction relative to the sliding direction causes anisotropy in wear properties. The best wear properties are achieved with the samples printed where the layers remain orthogonal to the sliding direction. The coefficient of friction remains mostly unaffected by print orientation. PLA samples demonstrate significantly better tribological properties compared to ABS. Varying the sliding speed between the interacting surfaces also affects the wear properties of both PLA and ABS. The results suggest that optimizing the build orientation can improve the wear performance of additively manufactured thermoplastics. This enables an additional paradigm when designing for functionally graded materials.

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.

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.

In press forming, most forming is performed cold. However, warm or hot forming is used for hard materials. In the present study, we focused on the heat generated by friction between metals and worked on the development of the warm punch that utilizes frictional heat. In the experiments, the heater jig and the rotary jig were prepared as different metals for friction. The test materials were steel materials, such as stainless steel and alloy tool steel. After frictional heating, the surface conditions of the different metals were observed by microscope. The wear depth due to friction was also measured. The temperatures generated by the rotation of the different metals differed in the time of heat generation for the combination of the two metals. In the case of stainless steel with low thermal conductivity, the temperature at the punch tip increased to around 400°C. The wear depth increased slowly with increasing time of wear in the rotary and heater jigs. The proposed method allowed the punch to be heated. The possibility of forming processing resistant material by warm deep drawing was found.

This study proposes a mathematical expression for the high-viscosity surface layer generated by the confinement of a lubricant film, which is evident in engine oil with a metallic detergent additive. The characteristics of a microtapered pad bearing lubricated by a confined high-viscosity film were clarified by solving a modified Reynolds equation for the confined high-viscosity lubricant film. The load capacity began to increase compared with that in the bulk viscosity case when the trailing gap decreased from twice the saturated high-viscosity layer thickness. The maximum value of the friction coefficient at the trailing gap near the layer thickness becomes remarkable compared with the case of the adsorbed high-viscosity layer model. Assuming that the increased effective viscosity caused by the confinement of the lubricant film is due to an adsorbed high-viscosity layer, the load capacity is significantly overestimated when the trailing gap is greater than the saturated high-viscosity layer thickness. Next, a mathematical expression of the synthetic viscosity of a lubricant with a polar additive having an adsorbed high-viscosity surface layer and a metallic detergent additive with a confined high-viscosity layer was proposed, and the characteristics of the microtapered pad bearings lubricated by the composite lubricant were investigated.

The effect of adsorbed water layers in sliding phenomena has been widely recognized, but it has yet to be explained thoroughly after many years of research. Previous researches tend to explain the phenomena from chemical and qualitative viewpoints, thus physical viewpoint approaches with quantitative evidence are necessary to complement the previous research works. Studies were done to estimate the thickness of adsorbed water layers in recent years to obtain quantitative evidence of the effect of adsorbed water layers. It was found that the thickness of the adsorbed water layer derived from atmospheric humidity could provide physical influences on sliding phenomena. The hypothesis in this study based on the Stribeck curve is; the friction coefficient may decrease significantly with the increase of sliding speed at high relative humidity (RH) compared to low RH. To verify the hypothesis, a pair of JIS SUS304 austenitic stainless steel balls were scratched against each other horizontally with a vertical overlapping distance of 80 µm at sliding speeds of 20, 200, and 2000 µm·s⁻¹ in RH of 5, 55, and 95%. The experimental results supported the hypothesis as the friction coefficient for medium to high RH decreased significantly with increasing sliding speed while the friction coefficient decreases slightly at low RH. The friction coefficient decreases significantly at high sliding speed and RH. The possible physical effects of the adsorbed water layers were suggested.

Sportswear is widely used in various sports and serves multiple functions. A key feature is the ability to absorb sweat during physical activity, which can, however, cause discomfort due to stickiness. This study focuses specifically on the stickiness caused by sweat between sportswear and the skin by objectively evaluating stickiness using specific physical parameters and correlating them with sensory evaluation values. Moreover, there is little research on the adhesion force between sportswear and human skin. Therefore, this study aims to achieve the following objectives: (1) investigate the relationship between the adhesion force between human skin and fabrics and the sensation of stickiness; (2) examine the relationship between the adhesion force between artificial skin and fabric and that between human skin and fabric; and (3) clarify the impact of fabric wettability and geometry on the adhesion force under wet conditions. Experiments reveal that the sensory evaluation value and adhesion force against human skin corresponded. Additionally, a positive correlation was found between the adhesion force against human skin and an artificial skin sheet. These experimental results suggest that fabrics with a lower adhesion force can be achieved by decreasing the load area ratio at 30% Φ_0.3 and increasing the contact angle and meniscus height.

Minimum Quantity Lubrication (MQL) technology has drawn attention as an effective lubrication technique despite its small usage of lubricants during the machining process. The technology has undeniably minimized the manufacturing cost as well as the adverse impacts towards the environment and health of operators. However, the ability of the small droplets of lubricant oil to penetrate the cutting zone must be investigated to enhance the machining performance. The penetration ability can be predicted if the amount of lubricant oil adhered to the workpiece is known. Nonetheless, observing the lubricant behavior is commonly challenged by the existing tools during the machining process. Therefore, a non-intrusive technique must be applied to conscientiously observe the lubricant behavior. In this paper, the thickness of lubricant oil resulted by the droplets accumulation on the workpiece during MQL milling process was measured using a Laser-Induced Fluorescence technique to predict the lubricating effects of the lubricant. The surface roughness of workpiece was also measured to investigate how the thickness of lubricant oil affects the machining performance. Experiments were conducted for MQL milling process of aluminium alloy 6061 under constant value of cutting speed and increasing value of lubricant flow rates. The MQL nozzle was tilted 45°, directed perpendicular to the milling direction and fixed together with the cutting tool to let them move together throughout the milling path. Results analysis was performed on the sample whose cutting tool was halfway to the milling path. As a result, the average lubricant oil film thickness was found to increase and obviously fluctuate with increasing flow rates, ranging from 0.2 mm to 0.7 mm. A careful observation of the lubricant oil thickness near the location of cutting tool also suggested that the droplets of lubricant oil probably struggle to penetrate the cutting zone due to the sudden falls at those locations. Furthermore, the correlation between the thickness of lubricant oil film to the performance of milling process under the MQL spraying condition was successfully made since the results trending of the surface roughness of workpiece shows a well agreement with the trending of lubricant oil film thickness.

The development of subsurface microstructural alterations known as dark etching regions (DERs) and white etching bands (WEBs) in rolling element bearings due to rolling contact fatigue have been investigated for the past eight decades, focusing on their initiation and formation mechanisms. They have only recently been shown to be driven by repetitive cycles of energy build-up due to micro-plastic deformation and energy release through recrystallization and recovery, which results in the formation of equiaxed and elongated ferrite grains, as well as lenticular carbides. These features develop within the bearing subsurface from DER to WEBs during bearing operation at moderate to high loads, but little evidence has been presented in the literature to understand links between DER and WEBs and the nucleation and growth of subsurface cracks. This investigation examines WEBs, including low angle bands (LABs) and high angle bands (HABs), in detail especially focusing on their late stages to understand such links. A number of techniques, including optical microscopy, scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) have been used to examine the features involved. Analysis on WEBs obtained through serial sectioning has revealed that voids initiating at the interface between lenticular carbides and equiaxed ferrite grain bands within WEBs have led to crack formation which can subsequently propagate to bearing surfaces. Interactions between WEBs and non-metallic inclusions (NMIs) are observed to lead to de-bonding of inclusions from their surrounding microstructure and void formation, which has also found to influence the integrity of the bearings at late stages. Alumina and Manganese sulphide (MnS) inclusions are the mostly observed NMIs that de-bond and develop microcracks when interacting with WEBs. These findings thus provide important insights into the link between inclusions and crack initiation and represent a further step towards a fundamental understanding of the rolling contact fatigue process.

Minimum quantity lubrication (MQL) has performed optimum lubrication but poor cooling during turning process. Thus, hybrid nanocooling-lubricants for minimum quantity cooling-lubrication to achieve sufficient lubrication and cooling effect for high speed turning titanium alloy (Ti6Al4V) material. This study is targeted on the machinability performance of G-Al₂O₃ hybrid nanocooling-lubricants MQCL and conventional fluids cooling condition with variable cutting speeds at constant feed rate as input parameters to evaluate the machining temperature and cutting insert flank wear as quality responses. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis was implemented to determine Ti6Al4V workpiece chemical elemental deposition on the cutting insert flank surface. Experimental results obtained that significantly increased of machining temperature from 206°C to 317°C based on type-K thermocouple wire measurement as the increment of cutting speeds from 120 m/min to 180 m/min. However, machining temperature decreased with the increasing of lubrication flow rate of MQCL 10 mL/min to 40 mL/min, then to the conventional fluids cooling condition. The comparison to conventional fluids cooling condition, the G-Al₂O₃ hybrid nanocooling-lubricants MQCL at the cutting speed of 120 m/min significantly increased tool life for 51% and cutting speed of 180 m/min for 28%, respectively. Furthermore, SEM-EDX has presented that titanium element deposited on cutting insert flank surface, which has shown micro-attrition, abrasion and adhesion wear leading edge chipping or fracture are identified as the main tool wear mechanisms.

A hydrogenated Diamond-Like Carbon (DLC) film possesses an ultra-low coefficient of friction in a low-humidity environment and higher as the humidity increases. This is due to the presence of water molecules in the atmospheric environment that is physically adsorbed and forming a thick layer on the surfaces that inhibits the growth of carbonaceous transfer film. One viable solution for this problem is to dope fluorine into the hydrogenated DLC film to decrease its surface energy. Therefore, this study varies carbon tetrafluoride feed as a doping source on the hydrogenated DLC film produced via plasma-enhanced chemical vapor deposition on 304 stainless steel substrates. The film hardness, carbon hybridization, and surface hydrophobicity were evaluated using nano-indentation testing, Raman spectroscopy, and contact angle measurement respectively. The coefficient of friction was analyzed by utilizing a ball-on-disc tribometer at a controlled temperature and humidity. The findings suggest that with the increase of the fluorine, the film hardness decreased as the sp³/sp² carbon ratio decreased because weaker C–F bonds are substituting the strong C=C bonds. The increase in fluorine was also observed to produce a more hydrophobic surface. The ball-on-disc test analysis shows that the coefficient of friction was significantly reduced as the C-F bond increased which enables the prevention of carbonaceous transfer film through the adsorption of the water molecule.

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.

High-strength brass is commonly used to produce sliding parts; however, its wear resistance should be further improved to extend the service life and performance of machinery and equipment constructed using this material. In this study, friction stir processing (FSP), a technology based on friction stir welding, was used to improve the wear resistance of high-strength brass. Because the application of FSP requires subsequent finishing operations on the workpiece, milling and wire electrical discharge machining (WEDM) have been implemented and their effects on the surfaces modified by FSP were investigated. FSP refined the crystal grains, resulting in improved hardness from 240 to 300 HV. Sliding tests were conducted to evaluate the friction and wear properties under three conditions: without FSP, with FSP and milling, and with FSP and WEDM. After applying FSP and milling, the wear amount decreased by 15%, in other words, the wear resistance improved compared with that observed without FSP. With FSP and WEDM, the wear resistance did not improve.

Nanoparticles are promising candidates as direct therapeutics and delivery systems for osteoarthritis treatments, primarily via intraarticular injection, but little is known about the impact on sliding behavior for a soft material surface like cartilage that would be encountered in a joint. Nanoparticle additives have primarily been studied in the context of hard material interfaces, such as metals or metal oxides, where different lubricating or anti-wear mechanisms depend sensitively on chemical composition, size, and concentration. To understand what nanoparticle parameters influence in situ (in a fluid environment) frictional behavior of soft materials, polyacrylamide (PAM) hydrogels were used as a model soft material platform. Friction tests were conducted in a rheometer with a tribology adapter, with PAM hydrogels molded in a petri dish and immersed in different nanoparticle containing fluid environments. A range of nanoparticle compositions were selected to compare broad categories: gold (metal) with a citrate capping ligand, nanodiamond (carbon), and zirconium dioxide (metal oxide). Comparing surface chemistry, concentration, and degree of aggregation, both nanoparticle surface chemistry and nanoparticle solution viscosity were found to modulate in situ hydrogel friction.

A series of experiments is conducted to investigate the cavitation pressure P_cav in the sealing film of a mechanical seal. To generate negative pressure, a reversed Rayleigh step is produced on the sliding surface, and the formation of cavities in the sealing film is observed. The pressure in the cavitation region is directly measured using a pressure sensor installed under a pinhole on the sliding surface. Various lubricating oils with similar kinematic viscosities are used as sealing fluid. The results show that pressure in the cavitation region is related to the operating conditions. P_cav decreases with increasing sliding speed in all oils and all sealing gap, i.e. minimum distance between two sliding surfaces of mechanical seal. When the sealing gap is the same, P_cav at each sliding speed is slightly different depending on the type of lubricating oils. When the lubricating oil is the same, small dissolved air results in low P_cav. In addition, two sealing gap are tested under the same sliding speed and the same oil, revealing that thin sealing gap results in low P_cav. Further, a model of cavitation in the reversed Rayleigh step is proposed based on the experimental results.