One of the important tasks of green tribology is to establish a firm technological ground for future energy. In utilizing hydrogen as a secondary energy carrier, we must understand behaviors of hydrogen gas and their effects on tribological processes in bearings and seals.

The mini-symposium “Hydrogen tribology for future energy” held on Tuesday, 8th September, 2009, in the World Tribology Congress 2009 was the first symposium on hydrogen held ever in international tribology conferences. Ten papers from the US, Germany and Japan were presented at the symposium. Five papers out of the ten have been submitted to Tribology Online and are included in this special issue.


The organizers hope that the symposium and this issue inspire tribologists in the world for further progress in this field.

Organizers
Joichi Sugimura and Shinya Sasaki

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For further development of hydrogen technology, it is necessary to have a sufficient number of materials for safe and reliable operation available. Frictional contacts exposed to hydrogen, are critical because of vanishing protective oxide layers in the presence of a chemical reducing environment. Furthermore, liquid lubricants are often not applicable, because of purity requirements, or very low temperatures in the case of liquid hydrogen. Thus, for numerous tribosystems in hydrogen technology, solid lubrication is the only possible method for reducing friction and wear. Therefore, investigations on the tribological behaviour of friction reducing materials, such as PTFE, graphite, DLC and MoS₂, in inert and hydrogen environment were carried out. The results show that solid lubricants, applied as coatings or as components in polymer composites, are able to reduce friction and wear in gaseous as well as in liquid hydrogen. However, some materials are very sensitive to the environmental medium.

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The effects of hydrogen on microstructural change and surface originated flaking in rolling contact fatigue were investigated using JIS-SUJ2 bearing steel specimens charged with hydrogen. Under clean lubrication conditions, subsurface originated flaking occurred and the rolling contact fatigue life was reduced and the amounts of the microstructural change called white structure that formed in the specimens increased as the hydrogen content increased. The localized microstructural changes were found in the hydrogen-charged specimens by electron microscope observations. It is supposed that the localization of plasticity was enhanced by hydrogen during the process of rolling contact fatigue. Under contaminated lubrication conditions, which included debris in the lubricating oil, surface originated flaking occurred and the rolling contact fatigue life of the hydrogen-charged specimens became shorter than the uncharged specimens, although white structure was not observed around the flaking. Enhancement of fatigue crack formations due to hydrogen was observed in specimens with artificial dents. It is presumed that hydrogen facilitated the formation of fatigue cracks on the raceway surface.

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Liquid hydrogen (LH₂) is excellent in extensive storage and transportation systems, such as being used as fuel in highly efficient liquid-rocket engines. To consider the use of LH₂ required for future hydrogen-energy systems, this paper presents a topical review of previous cryogenic tribology studies on the research and development of the bearings and shaft seals for LH₂ turbopumps. Cryogenic tribology studies were conducted for the LE-5/LE-7 rocket engines of the Japanese H-2 rocket as well as for an advanced ultra-high-speed LH₂ turbopump. The tribo-chemical formation of CaF₂/FeF₂ film due to the reduction power of LH₂ showed excellent self-lubrication, resulting in a sound wear condition within the turbopump bearing. A new hybrid ceramic bearing with Si₃N₄ balls, which bearing had a single-guided retainer, tested excellently at an ultra-high speed of 120,000 rpm (3 million DN) in LH₂. Seal performance for the floating-ring seals using Ag-plated metal seal-rings at ultra-high speeds was also tested.

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A novel technique was developed to control the concentration of water between 0.1 - 20 ppm and of oxygen between 0.3 - 2 ppm in hydrogen over a pin-on-disk apparatus. The developed technique simulates commercially available hydrogen gas at the point of use. This hydrogen usually contains sub-ppm to single-digit ppm levels of water and oxygen. The influence of impurities on the tribological properties of hardened chromium steel (JIS SUJ2) was obvious under the studied conditions, while the influence on austenitic stainless steel (JIS SUS316L) was not significant by comparison with that found in our previous study concerning higher water concentrations. The coefficient of friction of JIS SUJ2 increased with a decrease in the water concentration even at the smallest concentrations investigated in this study.

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Fretting wear tests on a bearing steel under gross slip condition were conducted in hydrogen and nitrogen gas environments containing water at 2 to 70 ppm using a new gas-tight chamber. Wear in hydrogen and nitrogen is sensitive to the water content of the gases and it increases as the water content increases. Water in these environmental gases reduces the coefficient of friction during the early cycles of experiments. Furthermore, exposure of the test specimens to high pressure hydrogen (40 MPa, 373 K, 200 hours) before the experiments enhances wear. These findings are consistent with the findings obtained in the authors' previous study, wherein the water content should have been higher and a comparison between hydrogen and nitrogen was not carried out because of insufficient control of water content. In this paper it is shown that wear in hydrogen is slightly larger than in nitrogen.

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