As nations pursue carbon neutrality targets, tribological innovations represent a significant yet under-recognized pathway for emissions reduction. While renewable energy deployment and electrification dominate climate mitigation discourse, friction-related energy losses persist across transport, industry, and energy conversion sectors. This study quantifies tribology's potential contribution to Japan's 2050 carbon neutrality goal through systematic literature review and sector-specific modeling.A comprehensive review of 77 peer-reviewed studies established tribological intervention potentials across six key sectors: transportation (road and aviation), renewable energy, energy production and electrification, hydrogen technologies, industrial operations, and manufacturing. These interventions encompass friction reduction through advanced lubricants and coatings, wear mitigation via surface engineering, and lifecycle extension of mechanical components. Sector-specific reduction potentials are modeled using Japan's 2022 emissions baseline (964 Mt CO₂) and projected energy transition pathways according to national policy documents, with sensitivity analysis across conservative, central, and ambitious deployment scenarios.Results indicate that tribological optimization could reduce Japan's annual emissions by between 42 and 167 Mt CO₂ by 2050, representing 4.4 to 17.4% of baseline emissions. Transportation systems show the highest absolute potential (30–38% of tribological reductions), while renewable energy demonstrates strong proportional contributions in conservative scenarios. Industrial operations and electrification exhibit increasing importance under ambitious technological advancement scenarios.This study demonstrates that tribology functions as a cross-sectoral enabling technology rather than a standalone solution, amplifying the effectiveness of renewable energy, electrification, and hydrogen systems. Policy integration, targeted R&D investment, and lifecycle-based standards are recommended to mobilize this pragmatic, engineering-grounded pathway toward carbon neutrality.

Bearings used in hydrogen technology face significant lubrication challenges. The main difficulty stems from hydrogen molecules dissociating into atoms on the nascent wear sites and diffusing into steel, ultimately causing hydrogen embrittlement and failure of tribological components. Lubricant additives that rapidly form tribofilms, such as antiwear and extreme pressure additives, can suppress atomic hydrogen generation and permeation in steel, but the resulting tribofilms tend to increase friction. Nanol (TM), a sustainable copper-based nanoadditive capable of forming stable dispersions in oils, offers unique friction-modifying, antiwear, and thermal advantages. In this study, Nanol (TM) dispersed in a polyalphaolefin base oil was used to lubricate ball and roller bearings undergoing rolling contact fatigue tests under boundary lubrication conditions in a hydrogen environment. Beyond its ability to reduce friction by immediately decreasing the real-contact area between the moving parts, Nanol (TM) also reacts with nascent iron to form cohesive, low-friction tribofilms composed of copper and iron oleate on the wear track. These mechanisms, along with the properties of the chemically formed tribofilm, were key to lowering friction and extending the fatigue life of the bearings operating in hydrogen and under severe conditions.

This study investigates the cavitation pressure in the sealing film of a mechanical seal. A reverse Rayleigh step was formed on a sliding surface to generate negative pressure to ensure good sealing performance, and the negative pressure caused cavitation. A theoretical model of cavitation in the reverse Rayleigh step was developed by considering the gas flow in and out of the cavity. In experiments, the pressure in the cavitation region was measured using a diaphragm-type pressure sensor positioned under a reverse Rayleigh step. Water was used as sealed fluid. The cavitation pressure in the reverse Rayleigh step was found to depend on the operating conditions and geometry of the reverse Rayleigh step.

In recent years, the utilisation of hydrogen energy has gained importance in order to achieve carbon neutrality. This utilisation requires the establishment of technologies for the safe use of hydrogen, and the role of sealing materials is essential. Examples of sealing materials used in hydrogen include piston rings used in reciprocating compressors as dynamic seals and O-rings that are subjected to repeated loads of high-pressure hydrogen as static seals. These sealing products are subjected to sliding in hydrogen when in use. Unexpected high friction causes excessive elastic deformation and possibly induces serious mechanical failure of O-rings. For this reason, research has recently been carried out to lay the foundations for frictional properties in hydrogen (1)(2). In this study, the effect of hydrogen on the friction mechanism between silicone rubber and SUS316L was investigated. The rubber was molded in the shape of a hemisphere. Reciprocating friction tests were performed with a pin-on-disk apparatus in air, hydrogen and gases bubbled in water to vary the amount of water content at atmospheric pressure. The friction coefficient of the friction test in hydrogen was higher than that in air. Futhermore, the friction coefficient of the friction test in wet hydrogen (water content = 30~300 ppm) was higher than that in relatively dry hydrogen (water content = 4~7 ppm). These results suggested that gas atmosphere and water content of gas had an influence on the friction between rubber and SUS316L. To elucidate the friction mechanism, the surfaces of the rubber and SUS316L after testing were analyzed using optical microscopy, SEM/EDS and FT-IR. (1) Sawae Y et al. Friction and wear of PTFE composites with different filler in high purity hydrogen gas, 157 (2021) 106884. (2) Theiler G et al. Influence of counterface and environment on the tribological behaviour of polymer materials, 93 (2021) 106912.

This study investigated the effects of trace moisture in hydrogen gas on the tribological behavior of carbon fiber (CF)-filled polytetrafluoroethylene (PTFE) composites by examining 20 wt% polyacrylonitrile-based CF-reinforced PTFE composites against stainless-steel disks in gaseous hydrogen environments, where moisture content was controlled at 1, 10, 20, and 40 ppm. The results revealed tribological characteristics of the sliding couples were significantly affected by the moisture content. Wear rates of pin specimens tended to increase gradually with moisture content. Similarly, average coefficient of friction increased as moisture content increased from 1 to 20 ppm. However, it decreased upon further increasing the water content. Moreover, surface analyses of the formed tribofilms at varying moisture contents revealed significant variations in terms of the amount and structure.

A series of experiments is conducted to investigate the cavitation pressure Pcav 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. Pcav 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, Pcav 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 Pcav. In addition, two sealing gap are tested under the same sliding speed and the same oil, revealing that thin sealing gap results in low Pcav. Further, a model of cavitation in the reversed Rayleigh step is proposed based on the experimental results.

We conduct in-situ observations of the wear process of metals in dry conditions. To this end, we constructed an experiment to observe the wear process of a point contact area, created between a rotating sapphire disc and a metal pin. The experimental setup included a microscope and a camera with visible-light and near-infrared detectors. Several steels, including stainless steel, and copper alloys were tested as pin materials. The wear comprised plastic flow, formation of transfer layers from the metal pin onto the sapphire disc, and expansion of the contact area. The influences of metal compositions and base metals on the wear behaviour were discussed.

Current greenhouse gas emissions suggest that keeping global temperature increase below 1.5 degrees, as espoused in the Paris Agreements will be challenging, and to do so, the achievement of carbon neutrality is of utmost importance. It is also clear that no single solution can meet the carbon neutral challenge, so it is essential for scientific research to cover a broad range of technologies and initiatives which will enable the realization of a carbon free energy system. This study details the broad, yet targeted research themes being pioneered within the International Institute for Carbon-Neutral Energy Research (I2CNER). These approaches include hydrogen materials, bio-mimetic catalysts, electrochemistry, thermal energy and absorption, carbon capture, storage and management and refrigerants. Here we outline the state of the art for this suite of technologies and detail how their deployment, alongside prudent energy policy implementation can engender a carbon neutral Japan by 2050. Recognizing that just as no single technological solution will engender carbon neutrality, no single nation can expect to achieve this goal alone. This study represents a recognition of conducive international policy agendas and is representative of interdisciplinary, international collaboration.

Recent developments of hydrogen energy systems for realizing carbon neutrality, and relevant tribological researches and development are overviewed. In order to ensure proper function, life and safety of hydrogen systems, particular attention has to be paid in the design of tribo-components such as seals, valves, bearings and couplings. Some fundamental research topics will be shown.

This study describes traction behaviours of lubricant films having anomalous shapes under elastohydrodynamic lubrication conditions. The traction generated at a point contact area between a glass or sapphire disc and a steel ball was measured by changing the slide-to-roll ratio. Three alcohols, 1-dodecanol, ethylene glycol and glycerol, and two alkanes of n-tetradecane and n-hexadecane were used as lubricants. Lubricants developing anomalous film shapes exhibited a solid-like behaviour with a sharp traction peak at low slide-to-roll ratios. On the contrary, other lubricants having conventional film shapes indicated a gradual increase in traction coefficient with increasing slide-to-roll ratios. The similarity of the traction behaviour to that of traction fluids supports the solidification of the film, which developed anomalous film shapes.

Metal-to-metal seals in premium threaded joints (PJs) for Oil Country Tubular Goods are used in severe environments such as high pressure and high temperature. Under the extreme conditions, the thermal degradation of applied grease on the seal surfaces can affect the sealability of metal-to-metal seal. In this study, changes in the sealability of greased metal-to-metal seal due to elevated temperature were investigated by fundamental gas tightness tests. In addition, changes in grease property at high temperature were also investigated. It was found that one of the causes of the loss of the sealability of metal-to-metal seal at high temperature was the decrease in grease viscous resistance against high pressure due to thermal degradation of grease. The decrease in grease viscous resistance was caused by the decrease in the kinematic viscosity of the base oil under 125°C at which the network structure of the thickener was almost maintained. Beyond 150°C, the decrease of complex viscosity of the grease due to the collapse of the network structure of the thickener decreased the grease viscous resistance. The latter significantly affected the loss of sealability of the metal-to-metal seal.

The influence of the heat transfer field on anomalous film formation under elastohydrodynamic lubrication (EHL) conditions was studied. Liquid lubricant film shapes between a transparent disc and steel ball friction pair were investigated by white light optical interferometry. The fatty alcohol 1-dodecanol was used as the representative lubricant to develop anomalous film shapes. A sapphire disc and glass disc, which have different thermal conductivities, were used as the transparent bounding surface. Experiments were performed wherein the applied load, sliding conditions and ambient temperature were varied. The temperature of the lubricant film was estimated by a simple model with the measured traction coefficient. The estimated temperature and maximum Hertzian pressure were compared with the phase diagram of 1-dodecanol obtained using a diamond anvil cell to investigate the phase state of the lubricant film. It was found that the anomalous film shape was stably formed in the solid-state regime of the phase diagram whereas the film exhibited unique characteristics such as the collapse behaviour in high sliding conditions and liquid-like behaviour of the traction with a remaining thickened film part in the liquid state regime.

This study describes the effect of textured patterns on the lubrication performance in starved hydrodynamic lubrication. A friction test rig including an in-situ observation system is used with controlled lubricant quantity to create point contact between a convex glass lens and rotating steel disc. The in-situ observation system captures the meniscus pattern around the lubricated area as well as optical interferograms of the film thicknesses. Longitudinal or transverse grooves are created on discs, which improve lubricant supply into the lubricated area at high sliding conditions, compared to a flat disc. Mechanisms for improving the lubricant supply through textured patterns are discussed based on the in-situ observation results.

Bearing steels suffer from degradation of mechanical properties when atomic hydrogen diffuses into the steel from the contact surface. In rolling contact fatigue tests this can lead to a significant reduction in fatigue life of the specimens as the amount of hydrogen diffused into the steel increases. To mitigate this challenge, synthetic oils of different chemistry have been studied so as to identify their efficiency and mechanism of retarding or preventing hydrogen permeation. Thrust bearing type tests were conducted with three synthetic base oils. The effect of base oil chemistry on hydrogen generation and permeation in bearing steel was explored by relating the concentration of hydrogen species in specimens with changes in the surface and subsurface of the wear track and the condition of the oil.

This paper presents a simple approach for the use of pyrene as fluorescence dye for the observation of the EHL contacts and behavior of lubricants without natural fluorescence properties. Dependence of the light emission of pyrene on its concentration was studied and also, specific emission and excitation spectrum for pyrene dissolved in PAO was found. A mercury lamp was used as the source of the excitation light with proper filter set according to the fluorescence spectrum. Pyrene's excimer proved to be very resistive against photobleaching and stable in time. This paper also describes experimental methodology on how to improve fluorescence properties of greases by adding pyrene and this approach was also verified by experiments to ensure that there was no influence on grease or pyrene's emission. This paper also discusses the calibration procedure for valid quantitative experiments.

This study investigates the relationship between the molecular structure of lubricants and the appearance of anomalous film shapes in elastohydrodynamic lubrication conditions. A ball-on-disc type test rig was used, which produced a point contact area between a rotating glass disc and rotating steel ball. Several pure liquids such as alkanes, alcohols, and others with a single polar site were used as lubricants. Linear chain lubricants caused solid-like behaviour in the film formation, traction coefficient and flow out time of entrapped lubricant in shock loading tests whereas complexly shaped lubricants caused liquid-like behaviour. The clear dependence on lubricant type appears to indicate that the anomalous film shapes formed due to the solidification of the lubricant.

This study investigated the relationship between the chemical reaction film formation derived from engine oil additives and crystal grain structure of a steel surface. Variations in the contact area between a rotating sapphire disc and a stationary steel pin were captured by a monochromatic high-speed camera. A fully formulated low viscosity engine oil was used as the lubricant. After friction tests, the elements on the surface were analysed using energy dispersive X-ray spectroscopy and the crystal grain structure of the top surface layer by electron backscatter diffraction. In-situ and ex-situ analyses revealed that plastic flow changed the crystal grain structure of the surface, inhibiting the formation of the chemical reaction film derived from additives.

While it is well known that during RCF tests the formation of nascent catalytic sites on the wear track can break down hydrocarbon molecules to release atomic hydrogen, the potential of the hydrogen environment in fuel cells to hydrocrack the hydrocarbon lubricant in high pressure rolling contacts has so far been ignored. Here we investigate for the first time the ability of the hydrogen environment to generate a chemical tribofilm on the wear track most likely through lubricant hydrocracking, as compared with argon and air environments. Despite the ability of the hydrogen environment to generate a notably larger amount of atomic hydrogen, the chemical tribofilm significantly prevents the formation of atomic hydrogen and its subsequent diffusion through the lattice of steel rolling element bearings. This is of great importance in the lubrication of hydrogen technology and the prevention of Hydrogen embrittlement (HE). An investigation into the prospects of high energy micro-computed-tomography (Micro-CT) as a non-destructive technique for sub-surface damage characterisation in RCF was comparatively performed alongside traditional sectioning methods.