24th International Colloquium Tribology - January 2024 223 Tribological Investigations under Varying Pressure Atmosphere Introduction of a novel Tribotest Option Felix S. M. Zak 1 , Ameneh Schneider 1 , Gregor Patzer 1 1 Optimol Instruments Prüftechnik GmbH, Munich, Germany * felix.zak@optimol-instruments.de 1. Introduction Pressurized gas atmospheres are commonly encountered in aerospace, oil and gas, energy generation and various manufacturing processes. The behavior of materials and lubricants under elevated pressure conditions can significantly impact the efficiency, safety, and reliability of these systems. However, traditional tribology test methods often fall short in accurately simulating the complex conditions found within pressurized gas environments. A novel option for tribotesting is presented, specifically designed to operate under different pressurized gas atmospheres, especially hydrogen. The development of this test option addresses the need to simulate real-world conditions encountered in many industrial applications where components and materials are subject to mechanical friction and wear in specific gas environments. By exposing test samples to controlled gas atmospheres at different pressure levels, the Tribotest option provides a valuable tool for evaluating the tribosystem in terms of performance and durability of materials under different operating conditions. 2. State of the art Velkavrh et al. demonstrated the significant influence of gaseous atmospheres in their research [1]. They expanded the SRV ® 4 tribometer by incorporating a rubber gas chamber and filled it with various gases. Their study unveiled substantial impacts on wear behavior. Due to the elastic pressure chamber, the experiments were only feasible at a low overpressure of 0.02 bar. Mishina in 1992 already emphasized the influence of gas pressure, manifested through gas molecule movement, on the tribological behavior of the system. [2] Moreover, NASA research groups in the 1960s delved into atmospheric influences, categorizing them into inert, reducing, and oxidizing atmospheres, although the influence under pressure was not the primary focus [3]. Subsequent investigations mostly centered on vacuum or low-pressure experiments. [4] Balasooriya et al. outlined current limitations and forthcoming challenges associated with pressurized hydrogen atmospheres concerning the mechanical requirements of materials utilized [5] This study can be extended to encompass tribologically relevant aspects. Gachot et al. successfully modified a friction and wear testing setup to conduct experiments under a pressure gas atmosphere of up to 10 bar, yielding distinguishable results under CO 2 atmosphere [6]. Noteworthy institutions have conducted studies under high-pressure hydrogen atmospheres, identifying effects related to wear behavior [7,8,9]. However, the research landscape remains insufficient given the paramount importance of this theme. 3. Design and Features The new Tribotest option presents an extension module of the SRV ® 5, an established testing machine for tribological investigations. It retains the standard specifications of the tribometer and additional adjustable parameters, like the pressure up to 100 bar The technical dependencies of set values, in the form of the tribological loading spectrum, give rise to several limitations. These dependencies require additional corrections within the regulation, such as compensating for force due to pressure differentials between the pressure chamber and the environment, necessary for normal force control. The Tribotest option offers variable adjustment of the surrounding atmospheric pressure, capable of encompassing various gases, with a primary focus on hydrogen. Due to hydrogen’s unique properties, predominantly stemming from its small molecular size and associated hazards concerning its high energy density, specific design considerations are imperative. The small molecule size, governed by Graham’s law, results in a high diffusion rate. To counter this, aside from hermetic sealing, an additional protective gas layer is employed. This protective gas (nitrogen) operates at the same pressure as the test gas, conferring advantages in terms of the system’s dynamic sealing. The pressure is measured in both circuits and controlled separately and also recorded for examination purposes. Given the hazards posed by different test gases, particularly hydrogen, an elaborate safety concept is pursued. This includes the utilization of external sensors for detection purposes and consideration of safety parameters, such as the maximum allowable temperature for sealing materials, through continuous monitoring and shutdown mechanisms. In addition to diffusion, other effects emerge when dealing with hydrogen, such as increased permeation and hydrogen embrittlement. These are mitigated through suitable material selection. For permeation, a specific polymer blend, verified through permeation testing, has been employed. The amalgamation of pressure tightness and the dynamic load from tribological investigations presents diverse challenges to individual components within the sealing concept. To counter these challenges, appropriate kinematics and well-chosen sealing methods have been implemented. No detectable leakage of the test gas is tolerated, while minimal leakage of the protective gas is accepted. Parasitic forces, like spring forces or damping forces resulting from the sealing components, are minimized through suitable calibration processes, corrections, and a deliberate distribution of stiffness and mass within the testing system. Despite the necessity for a substantially robust construction due to pressure-induced requirements, the Tribotest option 224 24th International Colloquium Tribology - January 2024 Tribological Investigations under Varying Pressure Atmosphere offers temperature measurement close to the friction point for regulating the test body contact or measuring friction-induced temperature rise. 4. Experimental Validation Initial attempts to demonstrate the specified kinematic characteristics of the construction under normal load were successfully conducted in sub-trials. Likewise, evaluations regarding pressure stability within the test gas region were accomplished. However, due to numerous optimizations of the Tribotest option throughout the development process, the completion of a full test series was not achieved by the time of submission. These pending experiments will be presented and elucidated in subsequent works. Various standard test procedures are being employed under additional pressurized test gas atmospheres (constant or variable) in forthcoming trials. Owing to the existing dataset, well-established test materials under dry and lubricated conditions are utilized for comparative purposes with normal environmental conditions. 5. Conclusion In conclusion, the exploration of pressurized gas atmospheres within tribological investigations holds significant implications across diverse industrial sectors. Traditional tribology test methods often lack precision in replicating the intricate conditions encountered in pressurized gas environments. The development of the novel Tribotest option addresses this gap, offering a specialized testing apparatus designed explicitly to operate under various pressurized gas atmospheres. This advancement aligns with the critical need to simulate real-world scenarios where materials and components undergo mechanical friction and wear within specific gas environments. By subjecting test samples to controlled gas atmospheres at different pressure levels, the Tribotest option emerges as a crucial tool for assessing material performance and durability under varied operating conditions. The collaborative efforts of researchers and institutions pave the way for comprehensive studies, promising advancements in material engineering and industrial applications within diverse pressure-based operational contexts. Figure 1: Design of Tribotest option References [1] I. Velkavrh, F. Ausserer, S. Klien, J. Brenner, P. Forêt, A. Diemeferences: The effect of gaseous atmospheres on friction and wear of steel-steel contacts. Tribology International 79, 99-110, Elsevier Ltd, 2014. doi.org/ 10.1016/ j.triboint.2014.05.027 [2] H. Mishina: Atmospheric characteristics in friction and wear of metals. Wear 152, 99-110, Elsevier Ltd, 1992. doi.10.1016/ 0043-1648(92)90207-O [3] D.H. Buckley, R.L. Johnson: EFFECT OF INERT, RE- DUCING, AND OXIDIZING ATMOSPHERES ON FRICTION AND WEAR OF METALS TO 1000°F. NASA, D-1103, 1961. [4] D.H. Buckley: FRICTION, WEAR, AND LUBRICA- TION IN VACUUM. NASA SP-277, 1971. [5] W. Balasooriya, C. Clute, B. Schrittesser, G. Pinter A Review on Applicability, Limitations, and Improvements of Polymeric Materials in High-Pressure Hydrogen Gas Atmospheres, Polymer Reviews, 62: 1, 175- 209, 2022 DOI: 10.1080/ 15583724.2021.1897997 [6] F. Ausserer, I. Velkavrh, F. Kafexhiu, C. Gachot: Experimentelle Methodik für die Prüfung tribologischer Systeme unter einer Druckgasatmosphäre. GfT Tagungsband 2023, 77-81. [7] K. Nakashima, A. Yamaguchi, Y. Kurono, Y. Sawae, T. Murakami, J. Sugimura. Effect of high-pressure hydrogen exposure on wear of polytetrafluoroethylene sliding against stainless steel. Journal of Engineering Tribology. 2010; 224(3): 285-292. doi: 10.1243/ 13506501JET642 [8] E. R. Duranty, T.J. Roosendaal, S. G. Pitman, J. C. Tucker, S. L. Owsley Jr., J. D. Suter, K. J. Alvine: In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure. Vis. Exp. (133), e56884, doi: 10.3791/ 56884 (2018) [9] Y. Sawae, K. Fukuda, E. Miyakoshi, S. Doi, H. Watanabe, K. Nakashima, J. Sugimura: Tribological Characterization of Polymeric Sealing Materials in High Pressure Hydrogen Gas. STLE/ ASME 2010 International Joint Tribology Conference. San Francisco, California, USA. October 17-20, 2010. pp. 251-253. ASME. https: / / doi.org/ 10.1115/ IJTC2010-412