gas in vehicles [8]. The reason for the very high pressure is to achieve high specific energy for longer cruising distance. Hydrogen refueling stations and relevant infrastructure therefore need to store and handle hydrogen gas under very high pressure. There are some critical problems caused by hydrogen when hydrogen is dissolved in solid. Hydrogen embrittlement of metals has long been a technological issue, and further research and development are necessary as the very high pressure enhances the hydrogen uptake in metals resulting in failures. Another big problem is the permeation of hydrogen that leads to catastrophic failures of rubbers and polymers for seals. Without overcoming these problems, the hydrogen energy society will never be realized. The Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS) was established in 2006 by a financial support of the New Energy and Industrial Technology Development Organization of the Japanese government. The idea was to dramatically improve our understanding of hydrogen/ materials interactions in order to assist the developments of hydrogen infrastructures and to strengthen the technological ground for possible future energy systems. The center started as a research center of a national research institute, the National Institute of Advanced Industrial Science and Technology, and later in 2013 it was transferred to Kyushu University. The major research projects in the center have been on the effects of high pressure hydrogen on fracture Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 1 Introduction Hydrogen is widely used as material, coolant, detergent, and reducer in chemical plants, semiconductor factories, steel works and nuclear reactors. It is used also as a fuel for rockets, and it once was a part of city gas several decades ago. But now, hydrogen is attracting more attention than ever as an important energy carrier to support our society. The introduction of this ultimate clean energy has been largely driven by the development of fuel cells with high energy efficiency. Fuel cells generate electricity from hydrogen and oxygen, and electrolyzers produce hydrogen from electricity and water. Hydrogen is thus recognized as the best complement to electricity and is useful for energy security. Among many electricity storage systems including battery, supercapacitor, flywheel, compressed air energy storage (CAES), pumped hydro energy storage (PHS), methane and hydrogen, hydrogen is best suited to large-scale electricity storage at the megawatt scale for hours to months [1,2]. A number of demonstration projects of the power-to-gas systems are in progress in Europe, particularly in Germany [3, 4]. In Japan, Kawasaki Heavy Industries is developing a hydrogen supply chain, which includes production of hydrogen, oversea transportation of liquid hydrogen with carriers, gas turbines to generate electricity from hydrogen and other systems for mass energy storage and transportation [5]. Chiyoda Corporation is developing an energy transportation system using an organic chemical hydride [6]. More visible leading technologies in our daily life are fuel cell vehicles, FCEV, and stationary fuel cells. FCEVs are already in market from 2014, and stationary fuel cells are spreading at home and industries. International Energy Agency and some governments have issued technology roadmaps for hydrogen and fuel cells [2, 7]. The development of hydrogen society needs development of not only the fuel cells but also of reliable and economically affordable systems and components to store and transport hydrogen. The state of art FCEV employs pressure as high as 70 MPa to store hydrogen Overview of tribology researches for high-pressure hydrogen systems Joichi Sugimura* Recent studies in Kyushu University on tribology for hydrogen energy systems are briefly overviewed. Friction and wear of various materials for valves, dynamic and static seals, piston rings, and bearings are studied in purity controlled test environments and at high pressure. Their tribological behavior depends on gas environment, and more or less on trace impurities in hydrogen. Chemical and tribochemical reactions play an important role in the processes at contacting surfaces. Recent progresses in hydrogen related technologies in Japan are also briefly introduced. Keywords Hydrogen, high pressure, friction, wear, trace impurity, seal, valve Abstract * Prof. Joichi Sugimura Orcid-ID: https: / / orcid.org/ 0000-0003-1926-2169 Research Center for Hydrogen Industrial Use and Storage, Kyushu University International Institute for Carbon-Neutral Energy Research, Kyushu University, Japan T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 24 and fatigue of metallic materials, strength of elastomers and polymers, thermophysical properties of hydrogen gas under extreme pressure and temperature, and tribology in hydrogen. This article introduces tribological issues in components used in hydrogen refueling stations with some of the results obtained in the center. 2 Tribology in hydrogen systems As is the case in most machinery, tribology is among the most important basic technologies in mechanical systems for hydrogen. In the conventional industries, facilities such as pumps, compressors and piping systems in chemical plants and semiconductor factories have a number of tribological components including seals, bearings, valves and other sliding parts. It is well known that, in space applications, seals and bearings are the most important elements in propulsion systems that use liquid hydrogen as fuel. These components all play important roles in maintaining safety of the systems. A typical hydrogen refueling station consists of compressors, storage tanks, hydrogen dispensers, pipes, couplings and valves. They are used to compress, store, seal, and control the flow of hydrogen while the gas experiences changes in pressure and temperature. Figure 1 shows the components and their tribological issues in hydrogen refueling stations. Compressors are used to compress hydrogen gas to over 80 MPa. Reciprocating piston type compressors are used more than diaphragm compressors, ionic compressors and centrifugal compressors in Japan, because they are most suited to typical capacity of several hundred Nm 3 / h required in the current hydrogen stations. Piston rings and rod packings are usually made of polymeric materials. Rolling element bearings are used as main bearings of the crankshaft and connecting rod bearings. Hydrogen permeation into steel may cause early flaking failure of the bearings if they are used in hydrogen gas. It is necessary to avoid hydrogen enhanced failures by making the structure of the compressor such that the bearings are not exposed to hydrogen gas, or by establishing technology to prevent hydrogen to diffuse into steels. Needle valves and ball valves are used to control the flow of gas. Figure 2 shows a ball valve for ultra-high pressure hydrogen. They normally employ hard metals and coatings for their sliding parts. Seals are also important parts in the valve systems to avoid leakage of hydrogen. Gland packings typically made of polymers are used as dynamic seals. Their counter surface is usually the material of the bodies of the valves, i.e. stainless steels, with or without surface treatments and coatings. Static seals such as O-rings and U-packings made of rubbers are used at a number of engaging parts. O-rings are sometimes used as dynamic seals. For example, the nozzle of dispensers experience sliding during mounting Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 % Hydrogen generator Compressor Storage tanks Fuel cell vehicles couplings Hydrogen dispenser dynamic & static seals piston rings bearings g mpress valves and other contact & sliding components sor Storage tanks ress ge tanks '()"*+*%(,-.*/ 0"1% *2)/ 0"2-3%4,0%("3),5%0"5#36% 7.52*"1%257%3)2)"1%3/ 2-3% 80/ 9/ 5)",5%,4% : .70,#/ 5% "57+1/ 7%)0"; ,< 42"-+0/ %"5%0,--"5#% 1,5)21)% '()"*+*%-,=%40"1)",5% 257%=/ 20%*2)/ 0"2-3%257% 1,2)"5#3% y g p g p 80/ 9/ 5)",5%,4% -/ 2>2#/ %,4% : .70,#/ 5% gg y g y g g p Figure 1: Tribological components and technical issues in hydrogen refueling station % Figure 2: A ball valve for 98 MPa hydrogen (By Courtesy of KITZ Corporation) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 25 This means that physical and chemical processes occurring at tribo-interface depend on surrounding gas. As will be shown below, environmental gas and other substances contained in the gas influences physical and chemical properties of solid surfaces, which affect chemical reactions and transfer of materials, and in turn affect friction and wear. Metals and alloys are used not only in metal-to-metal combinations, but more often as counter surface materials against polymers, rubbers and coated surfaces. In air, oxide films on the metal surfaces generally act as protective films that decrease adhesive force and prevent friction and wear from increasing drastically. In hydrogen, in contrast, oxides may be reduced under high temperature and high pressure conditions, and this leads to loss of protective boundary films, and friction and wear increase. However, it should be noted that tendency to be oxidized and reduced depends on metal species. As an example, the coefficient of friction of some alloys for valve plugs in hydrogen is compared with that in air in Figure 3 [12]. The data are from cross-cylinder type friction tests in a chamber. Hydrogen contained trace water of some tens of ppm, while air contained water vapor of about one thousand ppm. The materials used were AISI 316 and AISI 630 stainless steels, nickel base alloys Hastelloy B and Hastelloy C, nickel copper alloy Monel 400 and Monel K500, a cobalt alloy Stellite 6B, and tungsten carbide WC. The stainless steels mainly contain iron, chromium and nickel, while most of nickel base alloys mainly contain nickel. The figure clearly demonstrates that, in general, the coefficient of friction with nickel and copper alloys are higher in hydrogen than in air, and in particular nickel copper alloy M400 shows very high friction in hydrogen. In contrast, the coefficient of friction with the stainless steel 316 and Stellite 6B are lower in hydrogen. These results imply that the effect of oxide films on friction depends on the alloy composition, although their mechanical properties may also affect friction. Experiments with pure metals clearly show the differences in their chemical response to environmental gas. Fukuda et al. conducted a series of pin-on-disk tests in high purity gas in which the concentration of trace impurity, mainly water vapor, was controlled to be less than 10 ppm [13. 14]. Figure 4 shows the variation of coefficient of friction in hydrogen gas with the group of elements [13]. The metals in the groups 4A to 7A and 8 generally exhibit relatively low friction while those in the groups 1B to 4B show higher friction. This difference is caused by their likeliness to adsorb and react with hydrogen as well as with trace water and oxygen in hydrogen. The former metals, indicated by A in the figure, are more reactive. The transition metals such as vanadium, titanium and zirconium react with Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 and detaching. Metals are also used for sealing gas in couplings and safety valves. Two important points have to be in mind in designing tribological components for the hydrogen supply systems for FCEVs. Firstly, hydrogen gas is stored at a very high pressure of 82 MPa. Secondly, hydrogen gas has to keep certain purity such that the gas does not cause severe degradation of electrode catalyst in fuel cells [9]. The second point means that normal lubricants cannot be used, and the components have to maintain their performance in dry contact without lubricants. No systematic tribology studies have been made until very recently for hydrogen applications except for those on hydrogen effects in rolling contact fatigue of steel, cryogenic tribology for space applications, and some fundamental studies on environmental effects. With the increasing demands for understanding hydrogen in tribology again as a background, the first symposium on Hydrogen Tribology for Future Energy was held in the Fourth World Tribology Congress in Kyoto in 2009 [10], and the second one in ITC Hiroshima in 2011 [11]. 3 Metal surfaces in hydrogen As is well known, friction and wear of materials are not their intrinsic properties but depend on combination of materials, sliding conditions and environment surrounding the contact. The friction coefficient in hydrogen is different from that in air under the same load and speed. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 1 Coefficient of friction in hydrogen Coefficient of friction in air ! " #$% #&#'(% )*++ #&#%'+ ,- ," ./ ++ % Figure 3: Coefficient of friction in air and hydrogen in reciprocating sliding tests for eight metals; in hydrogen with trace water of some tens of ppm and in air with 0.1 % water; load 2 N, amplitude 1 mm, frequency 2 Hz, total cycles 7,200; metals are SUS316: AISI 316 stainless steel, SUS630: AISI 630 stainless steel, ST6: Stellite 6B, HB: Hastelloy B, HC: Hastelloy C, M400: Monel M-40, K500: Monel K-500, and WC: tungsten carbide (reproduced from Ref. 12) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 26 % Figure 5: Variation of the coefficient of friction of SUS316L with concentration of trace water in hydrogen; a) new data and b) date from another paper (Ref. 15); load 13N, sliding speed 0.063 m/ s, temperature 298 K. (reproduced from Ref. 16) hydrogen to produce metal hydrides, which result in low friction and high wear. The transition metals including iron, chromium, molybdenum and nickel adsorb hydrogen, but more effectively react with a small amount of water and oxygen to form passive films. On the other hand, the metals such as copper and aluminum as indicated by B in the figure exhibit strong adhesion in hydrogen although with trace water and oxygen. These results clearly indicate that the frictional behaviors of pure metals are more or less reflected in the behaviors of alloys shown in Figure 3. Fukuda et al. further conducted experiments with iron and steel by changing the concentration of trace water in hydrogen [15-17]. The effects of water vapor in hydrogen on friction and wear of AISI 316L austenitic stainless steel are shown in Figure 5 [16], in which the data indicated by (b) in the figures are from the studies in Reference 15. The figures clearly show the dramatic effects of water concentration in hydrogen. The coefficient of friction is smallest at around the water concentration of 10 ppm, while the wear rate is smallest at 1 ppm. The significant effect of water concentration suggests that chemical reaction that occur at steel surface with trace water strongly alter adhesion, and friction and wear. This also causes the significant changes in friction and wear of polymers and coatings when they are slid against steel, as the austenitic stainless steel is frequently used as a counter surface in hydrogen applications. Fretting wear that occurs at contacting surfaces is also affected by trace water as the processes involved are more or less related with oxidation [18]. Fretting fatigue of stainless steel in hydrogen is also influenced by oxygen and water present in hydrogen gas [19, 20]. As described in the previous section, commercially available hydrogen gas is not completely pure but contains certain amount of trace impurities. The amounts of trace impurities in commercial gas are strictly defined such that the gas does not deteriorate polymer electrolyte in fuel cell, and Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 Figure 4: Coefficient of friction in self-mating tests of pure metals in hydrogen at room temperature and normal pressure; hydrogen with trace water and oxygen less than 1 ppm; load 10 N, sliding speed 0.063 m/ s, sliding distance 126 m. (reproduced from Ref. 13) (b) Exposed (a) Not exposed % Figure 6: XPS spectra of AISI 316L surface before and after exposed to 40 MPa hydrogen at 373 K for 200 hours. (reproduced from Ref. 22) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 27 A series of sliding experiments in hydrogen gas with five polymers containing fluorine and four metals indicate that friction and wear depends on the combination of materials [27]. The polymers tested are PTFE, PTFE filled with graphite, PTFE filled with glass fiber, ultra-high molecular weight polyethylene (UHMWPE) and polyvinylidene di-fluoride (PVDF). The unfilled PTFE and PTFE composites show lower friction than the rest of the polymers due to the formation of stable PTFE transfer film, while the PTFE composites show better wear resistance than the unfilled PTFE. PVDF shows low friction but high wear in hydrogen. Friction and wear of each of the polymers change slightly with the counterface metals, but the specific wear is correlated with the amount of polymer transfer film, and correlated also with the amount of metal fluorides produced on the metal surfaces. As shown in Figure 7, the specific wear rate of the fluorine containing polymers is smaller with greater amount of iron fluoride on the metal surfaces [27]. The lubricating function of polymer transfer film is promoted under high pressure. The coefficient of friction of unfilled PTFE against stainless steel 316L in 40 MPa hydrogen gas is lower than that in normal pressure hydrogen [28, 29]. This is caused by the formation of favorable PTFE transfer film in the absence of oxide film on the steel surface. The PTFE composites filled with graphite or metals also exhibit lower friction in high pressure hydrogen than in normal pressure, although the behaviors are not simple because transfer and wear of these fillers are also involved in sliding. Materials that transfer are not only polymers but also filler materials. Theiler and Gradt investigated friction and wear Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 the maximum acceptable amount of oxygen and water is 5 ppm [9]. This value seems to be based on the acceptable amount for fuel cells. However, as the above studies, and studies on other materials described below clearly show, oxygen containing substances play a great role at tribo-interface even with a small amount. The effect of high pressure on friction and wear of steel is still not very clear because there are not enough data. At least, the pressure effect is rather marginal as compared with the effect of temperature and the trace impurity. The coefficient of friction of AISI 316L steel in 40 MPa hydrogen is around 0.4, which is slightly smaller than that at hydrogen at normal pressure [21]. An XPS study demonstrated that surface oxides on stainless steel are reduced by exposing the surface to high temperature, or high pressure hydrogen, as shown in Figure 6 [22]. However, the experiments at 10 MPa show that the coefficient of friction scatters widely in the range between 0.2 and 0.9 under the same load and speed [23]. Tribochemical processes including the formation of passive films thus depend on pressure and temperature of atmospheric gas. Oxidation dominantly occurs in hydrogen with trace water and oxygen under normal pressure and temperature, while reduction or decrease in oxidation occur at higher pressure and higher temperature. The reason for the unstable behavior in 10 MPa hydrogen may be that the formation and removal of oxide films occur by chance under competitive action of oxidation by trace water and oxygen and the reduction by hydrogen. 4 Transfer film formation with polymer composites It is well known that tribological performance of polymeric materials such as polytetrafluoroethylene, PTFE, and its composites depends greatly on the formation of transferred films on counter surface. According to the classical theory of solid lubrication, low friction is achieved when thin transfer film of polymer is formed on harder counterface materials like hard metals. It can easily be imagined that the formation of polymer transfer film on metals is strongly influenced by environmental gas. In fact, unfilled PTFE produces transfer film on steel surface in hydrogen resulting in lower friction and wear than in air or in vacuum [24, 25]. The transfer film is more easily formed in hydrogen in the absence of oxide film, and is enhanced by the formation of metal fluoride on the steel surface. Fluorine radical produced under sliding react with surface metals to form fluoride that helps good transfer film to form. The formation of transfer film depends on chemical or tribochemical reactions that occur on metal surfaces, which implies tribological performances depend on metals, impurity of hydrogen, temperature and pressure [26]. % ! "# # #! #! ! ! ! "! $ ! "# ! "#$ ! "% ! "%$ &'()*+*),-(./ ,/ .0(1,2#! 34 55 6 7855 9: 0(: ; *0<,/ .0*=,>35(0.? 7>( @A>B @A>B3C/ .'D*0( @A>B3C? .; ; @EF> Figure 7: Relation between specific wear rate of polymers and the metal fluoride intensity on metal surfaces in sliding experiments of polymer disks against metal pin; in hydrogen with trace water less than 1 ppm, load 60N, amplitude 3.8 mm, frequency 2 Hz, total cycles 7,200; metals are AISI 316 stainless steel, AISI 440C stainless steel and 52100 steel. (reproduced from Ref. 27) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 28 of polyimide, PI, filled with and without graphite in 10 3 Pa hydrogen gas [30]. They showed that unfilled PI shows lower friction in hydrogen than in air, and that graphite in PI produced transfer films on counter surface that worked as solid lubricant to reduce friction and wear. Recent studies have revealed that PTFE filled with carbon fiber produced from polyacrylonitrile (PAN) shows much lower friction than the low friction by PTFE transfer film in hydrogen [31,32]. Figure 8 shows the distribution of PTFE transfer films and carbon transfer films formed on cast iron disks slid against carbon fiber filled PTFE in air and in hydrogen. The intensity of PTFE shown in the maps was obtained by integrating C-F intensity over wavenumber from 1120 cm -1 to 1280 cm -1 in FT-IR spectrum. The intensity of carbon was obtained by integrating intensity over wavenumber from 900 cm -1 to 1800 cm -1 in Raman spectrum. The figure clearly indicates that PTFE transfer film is dominant in air and carbon transfer film is dominant in hydrogen. Carbon film is found also on the PTFE pin surface. This suggests that the low friction in hydrogen is provided not by PTFE transfer film but by the carbon transfer film. Clear G-peak and D-peak are found in Raman spectra of the film, suggesting that the graphitized amorphous carbon is formed under sliding of the PTFE composite in the same manner as in the sliding of DLC coatings. For rubbers, extensive studies have been made on bulk properties of rubbers in the polymer research group in HYDROGENIUS. A vast amount of hydrogen molecules permeate in rubbers, particularly in high pressure hydrogen, to cause fracture by swelling and fatal blister fracture by changes in surrounding gas pressure [33,34]. In addition, a tribology study of rubbers in hydrogen is in progress in the tribology research group. 5 DLC coatings Hard coatings are used in components such as valves and slide against metals. DLC and carbon materials are strong candidates for use in hydrogen applications because they exhibit excellent friction and wear in hydrogen [35-39]. It is widely accepted that the low friction and wear of DLCs are brought about by the formation of films of partially graphitized, or structured, amorphous carbon and hydrogen termination of dangling bonds of carbon atoms on the surface [36]. The structure is produced in sliding contact, and is often characterized by the split peaks in Raman spectra that the intact DLC surfaces do not have. DLCs are now widely used as good tribological material in various environments with or without lubricants. While many researches focused on the effect of humidity on friction and wear of DLCs, no detailed studies were made to see the influence of trace water of ppm level in dry sliding. Tanaka et al. conducted pin-on-disk experiments for DLC pins against pure metals in environments with controlled purity. The results for typical a-C: H, hydrogenated DLC, and ta-C, hydrogen free DLC, in very dry hydrogen gas are shown in Figure 9. The a-C: H generally exhibits lower coefficient of friction than ta-C, and the coefficient appears to depend strongly on metal species. Microscopic observation and Raman analysis after the tests demonstrate that structured amorphous carbon films are formed on the metal surface that shows low friction, whereas almost no carbon films are found on those metals including copper and palladium. The capability of metals to form carbon transfer film is related with the reactivity of metals with carbon but also to catalytic action to cause dissociation of hydrogen atoms. Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 % % % ! " ! #! $" ! #%" ! #%$" ! #&" '(" )*" +," -." / *" +0" 12" +3.45*.67"38""8,*5936 : ; +<=" 7: ; +" Figure 9: Steady-state coefficient of friction of two DLCs against pure metals in hydrogen; trace water 3 ppm, trace oxygen 0.5 ppm, load 10 N, sliding speed 0.063 m/ s, sliding distance 126 m, temperature 298 K. (reproduced from Ref. 40) Figure 8: Distribution of PTFE film and carbon film on the sliding track on cast iron disk slid against PAN-based CF filled PTFE in air and in hydrogen; PTFE intensity integrated over wavenumber from 1120 cm -1 to 1280 cm -1 with FT-IR, carbon intensity integrated over wavenumber from 900 cm -1 to 1800 cm -1 in laser Raman spectrometer; test conditions: contact pressure 1 MPa, sliding speed 2 m/ s, temperature 373 K. (reproduced from Ref. 31) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 29 it is difficult to have both low friction and low wear with simple DLCs used in the study. One thing that should be avoided when DLC coatings are used in high pressure hydrogen is delamination. It Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0010 They extended the study to look at the effects of the concentration of water vapor in hydrogen, nitrogen and argon [41]. Figure 10(a) shows the effect of water in hydrogen on the coefficient of friction of a-C: H against aluminum pin. It clearly shows that friction is minimum in hydrogen without trace water, and the coefficient increases with the concentration of water in hydrogen. This is because the oxide films on aluminum formed by continuous supply of water prevented effective carbon transfer film to be formed. In contrast, water vapor of tens of ppm works to reduce the coefficient of friction in argon, as shown in Figure 10(b). The transfer film cannot be found on the pins tested in nitrogen and argon with water less than 1 ppm, and the pins are severely worn. The carbon transfer film is formed by the addition of water in these inert gases, suggesting that water molecules cause removal and transfer of carbon. However, the running-in to low friction is slower than in hydrogen gas, and is accompanied by heavier wear of DLC. In another series of experiments, Manabe et al. looked at the effect of water and oxygen to seek optimum conditions to achieve low friction and wear with DLCs and some pure metals in non-hydrogen environment [42]. They conducted experiments with nitrogen and oxygen mixture with different ratio, with 1ppm water, and with air containing different concentration of water. In the case with iron pin, for example, surface oxidation is predominant and transferred carbon is poor. However, with 1 % oxygen and 1 ppm water, the effective films of structured amorphous carbon is formed on metal surfaces that provides the coefficient of friction less than 0.1. Nevertheless, the addition of water increases wear of DLC. The above experiments indicate that the coefficient of friction can go down to 0.01 or less with little wear in dry hydrogen, but with the presence of oxygen and water % H2I % H; I Figure 10: Changes in the coefficient of friction with sliding distance; a-C: H DLC disk and aluminum pin; (a) in hydrogen and (b) in argon; trace oxygen less than 1 ppm, gas pressure 0.12 MPa, load 10 N, sliding speed 0.063 m/ s, sliding distance 126 m. (reproduced from Ref. 41) % % (a) 1 day after (b) 5 days after Figure 11: Depth profiles of hydrogen, oxygen and iron in ta-C DLC film measured with SIMS (a) one day and (b) five days after the exposure to hydrogen at 40 MPa and 373 K for 100 hours; thickness of DLC coating 1μm; horizontal axis represents the sputter time of Cs+ primary ion at 10 nA. (reproduced from Ref. 43) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 30 was found in exposure tests that DLC films coated on 52100 steel substrate were partially detached from the substrate when they were kept in hydrogen at a very high pressure of 40 MPa. In order to see the reason, the depth profiles of hydrogen, oxygen and iron were determined with SIMS. Figure 11 shows the results for a ta-C film of a thickness of about 1 μm exposed to hydrogen for 200 hours [43]. The figure demonstrates that hydrogen concentration increases significantly above the substrate/ coating boundary. This implies that hydrogen diffused into the coating film was stack and accumulated at the film substrate boundary, which might have caused swelling and tensile stress that lead to cracking. In order to avoid this, it is necessary to increase the bonding strength and/ or to modify the interface so that hydrogen does not accumulate at the boundary. 6 Concluding remarks Hydrogen takes part in a various tribological phenomena in different manners. Chemical and tribochemical reactions at tribo-interface are often affected more by trace oxygen and water than by hydrogen. Operating conditions as well as conditions of environmental gas have to be considered in the design of tribo-elements for hydrogen applications. Acknowledgments Some of the works described in this paper were conducted as a part of the “Fundamental Research Project on Advanced Hydrogen Science” funded by New Energy and Industrial Technology Development Organization (NEDO) from 2006 to 2012, and a CREST project “Creation of Nanointerface Controlled by Tribochemical Reaction for Mechanical Systems with Super-low Friction” funded by Japan Science and Technology Agency from 2013 to 2018. References [1] Hotellier, G., 2014, Hydrogen as a multi-purpose energy vector, Siemens Future Forum at Hannover Messe 2014, https: / / w3.siemens.com/ topics/ global/ en/ events/ hanno ver-messe/ program/ Documents/ pdf/ Hydrogen-as-a-multipurpose-energy vector-Gaelle-Hotellier.pdf [2] Technology Roadmap - Hydrogen and Fuel Cells, International Energy Agency, 2015. 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