The 7th World Tribology Congress (WTC 2022) was held from July 10 to 15, 2022, in Lyon. It was co-hosted by LaMCoS (Laboratoire de Mécanique des Contacts et des Structures), INSA de Lyon, and LTDS (Laboratoire de Tribologie et Dynamique des Systèmes), Ecole Centrale de Lyon. It was originally planned to be held in September 2021, but was postponed for ten months due to the COVID-19 pandemic.

The great success of WTC 2022 was celebrated by hundreds of participants who met each other face-to-face after a long period of separation, by the beautiful summer sky throughout the week, and the celebrative atmosphere in Lyon for 14th of July, Bastille Day.

The organizing committee kindly accepted Tribology Online as one of the official journals for publication, and the Japanese Society of Tribologists (JAST) called for papers for WTC 2022 Special Issues. Papers submitted to this issue are not many, but they are from emerging areas in Tribology, from coatings, tribo-films, hydrogen tribology to education of children and young people.

On behalf of the editorial board of Tribology Online, the Editor-in-Chief would sincerely like to thank all the contributors to this Special Issue, and hope the readers find these papers interesting and useful.

The formation of a tribofilm by lubricant additives plays an important role in improving the performance of boundary lubrication. In particular, the mechanical properties of the tribofilm directly influence the lubricant performance; therefore, they should be investigated to improve the lubrication performance of boundary lubrication. In this study, the Young’s modulus, hardness, and nanofrictional properties of tribofilms derived from a continuously variable transmission fluid and its additives, such as tricresyl phosphate, overbased calcium sulfonate, and dibenzyl disulfide, were investigated using nanoindentation equipment. Furthermore, the chemical compositions and geometries of each sliding surface were investigated using scanning electron microscopy, energy dispersive X-ray analysis, and atomic force microscopy to clarify the detailed structure of the tribofilms. The relationship between the mechanical and frictional properties is discussed in terms of the shear strength. The results suggest that the macrofriction coefficient is correlated with the nanofriction coefficient and the shear strength of the tribofilm. This study is expected to help improve the design and durability of continuously variable transmissions.

Due to the continuing decrease in the proportion of young people in the population, it is likely to become more difficult to secure the researchers who will lead the next generation. To overcome this problem, it will be necessary to arouse the interest of children and young people in science and engineering. Tribology is the study and application of the principles of friction, lubrication, and wear, and is an interdisciplinary discipline spanning many fields; consequently, the study of tribology can encourage interdisciplinary learning. Furthermore, tribology is an important discipline of itself because it has a direct relationship to issues of energy and the environment. Teaching tribology to young people and helping them to understand it could arouse their interest in a variety of aspects of science and engineering. The key is how to introduce the various academic aspects of tribology, a complex and mysterious discipline, in an engaging manner. The author has energetically striven to promote introductory education to enlighten the next generation about tribology by using a novel educational method that incorporates a problem-solving game. This paper describes the effectiveness of this educational method, the content of the developed teaching materials, and the results in educational practice.

The development of hydrogen technologies entails high safety requirements in distribution and dispensing infrastructure. Therefore, it is necessary to pursue research on material compatibility in hydrogen, especially for critical parts with tribological issues. The focus of this study is to evaluate the influence of hydrogen on a wider range of commercially available polymer materials. Thereby, the friction and wear behavior of different grades of TPE, POM, PA66, PA12, PPA, PEEK, PPS, PTFE, PAI, PI and PBI were investigated against a rotating steel disk (AISI 304). Filled and unfilled polymers from different suppliers were evaluated at room temperature in air, vacuum temperature, the effect of hydrogen environment on the tribological behavior of neat polymers may be related to lack of moisture, but also to saturated hydrocarbons in gaseous hydrogen. In liquid hydrogen, the best tribological performances were achieved with neat PA polymers as well as PPS and PI composites.

This study attempted to manipulate the friction coefficient of ionic liquids at contact pressures on the order of several hundred MPa (maximum Hertzian pressure) via the surface potential. Because the manipulation of the friction coefficient requires the adsorption and desorption of ionic liquids, the tests were conducted at potentials where electrolysis does not occur. The friction coefficients at negative potentials were different for the same cationic ionic liquid at a constant friction potential. This indicates that the ionic liquid was not a monolayer film but a multilayer film in which anions also exist. The friction coefficient was reduced by lengthening the cationic alkyl chains. These results suggest that the conductivity has a significant effect on the frictional response of ionic liquids. Moreover, the friction coefficient remained constant after the surface potential was removed. The adsorption layer at the surface potential was considered stable, and it was difficult to overcome the energy barrier to change the adsorption structure of the ionic liquids.

Among Diamond-Like Carbon (DLC) materials, hydrogenated amorphous carbon (a-C:H) coatings may exhibit super-low friction coefficients (< 0.01) under vacuum. A running-in phase is observed in the first few cycles of sliding before reaching a steady state of super-low friction. In this study, a unique ultra-high vacuum tribometer, with 4 different axes of motions and a 6 axes force sensor, is used to characterize the tribological behavior of a steel pin / a-C:H coated flat contact. An original crossing-tracks method is proposed to dissociate the respective roles of tribofilm build-up and surface modifications on the a-C:H in the achievement of super-low friction. A previously build-up tribofilm on the steel counterface shortens the running-in phase by 18 cycles and reduces the initial friction coefficient by 32%. Sliding on a previously rubbed area of the coating allows an additional initial friction reduction of 36%. Morphological and chemical investigations on the a-C:H coated flat using Atomic Force Microscopy and X-ray Photoelectron Spectroscopy reveal that the initial nanoasperities were smoothed, eventually leading to shallow matter transfers and tribofilm thickening on the counterface. Carbon-to-carbon bonds rupture and a reorganization in the carbon structure leads to an increase of the sp²/sp³ hybridization ratio.

Resin materials are widely used in industrial applications due to their light weight and other desirable characteristics. However, although there are resin materials with self-lubricating functions, there are problems with abrasion and chemical resistance. This study focused on urushi, which has long been used as a coating for armor and other protective layers. Since previous studies have shown that urushi exhibits high wear resistance, this investigation evaluated the tribological performance of urushi mixed with graphite to achieve friction reduction. Friction reduction was observed when graphite was added to the resin. In contrast, accelerated wear was observed compared to that of pure urushi. The SEM-EDS results confirmed that graphite was transferred from one surface to the other as carbon and this carbon was detected in the mating material. This result suggests that friction reduction was achieved by transferring to the counterpart; however, the wear was accelerated by the transfer.