Joichi Sugimura

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.

Hydrogen content and wear rate in bearing steel under rolling contact depend on the base oil type and the composition of tribofilm they generate on the wear track.

In this study, the effect of the textured surface on traction coefficient in elastohydrodynamic lubrication (EHL) conditions was experimentally investigated. In order to clarify the effectiveness of textured surfaces on traction coefficient, we used 11 types of textured steel rollers which have different pitches, widths and depths of grooves and compared traction coefficients. The slide-to-roll ratio was changed during tests by controlling revolution speed with servomotors connected to each specimen and coefficient of traction was measured on each slide-to-roll ratio. Experimental results show that it is possible to increase traction coefficient by more than 20 percent in the case of textured surfaces compared with the traction coefficient in the case of the flat surface. The pitch of grooves was the most sensitive parameter to change the traction coefficient.

This article describes experimental and theoretical studies on the cavitation phenomena in the grease lubrication film under pure sliding elastohydrodynamic contact. In situ observation tests using the optical interferometry technique were conducted, and the growth of cavitation was captured using a high-speed camera. The results showed that the cavity grew in two stages, which was similar to the behavior in the base oil, and that the cavity growth rate in the initial stage was higher than that in the second stage. In the initial stage, the cavity growth time in the grease was longer than that in the base oil, and the cavity length after the growth depended on the base oil viscosity. It was also found in the test using diurea grease that small cavities were formed by the lumps of thickener. The cavity growth in the initial stage was discussed by numerical simulation of pressure distribution based on a simple rheological model.

High-strength steels are particularly susceptible to hydrogen embrittlement and therefore rolling element bearings can experience premature failure that drastically reduces their operation life. Some lubricant additives can prevent this by generating protective tribofilms during operation. This study investigates the effect of temperature on the growth of zinc dialkyl dithiophosphate (ZDDP)-generated tribofilms and the relationship with the permeation of hydrogen in the substrate. Rolling contact tests were conducted using ZDDP at 333, 363, and 393 K and the concentration of hydrogen was measured after the tests. It was found that low temperatures inhibit the reactivity of the additive, leading to an inadequate coverage of the surface that allows dissociation and permeation of hydrogen in the sample. Higher temperatures induce uniform films in the contact, although excessive temperatures can promote unnecessary wear of the substrate. An optimum temperature can balance the two main processes occurring in the contact related to the reactivity of the additive: the growth of a protective film and the corrosive wear of the material.