24th International Colloquium Tribology - January 2024 225 Efficiency Improvements of In-Situ Hydrogen Permeation Measurements in Lubricated Bearing Steel Contacts Using the Modified Devanathan-Stachurski Cell (MDSC) Method Edward Vernon-Stroud 1 , Ajay Pratap Singh Lodhi 1* , Frederick Pessu 1 , Ivan Delic 2 , Nicole Dörr 2 , Markus Varga 2 , Josef Brenner 2 , Ardian Morina 1* 1 Institute of Functional Surfaces, University of Leeds, Leeds, United Kingdom 2 AC2T research GmbH, Wiener Neustadt, Austria * Corresponding author: A.P.S.Lodhi@leeds.ac.uk; a.morina@leeds.ac.uk 1. Introduction The ever-increasing need for products towards sustainability, combined with the increasing demands for product reliability and ease of maintenance, presents a unique challenge to all forms of engineering, especially in the field of tribology. In wind turbine gearbox, one of the most common causes of failure is thought to be due to hydrogen embrittlement during operation of the steel bodies used in the tribosystems, leading to downtime and costly repairs. Hydrogen is produced in the system through lubricant decomposition and water contamination, which then permeates the steel structure and becomes irreversibly trapped. This impairs the mechanical properties of the steel and creates micro-fractures such as those seen in White Etching Cracks (WECs), which then propagate and lead to total bearing or gear failure if not detected in time. These failures severely impact the sustainability of these technologies, to the detriment of both the public and businesses alike [1-4] which can lead to the failure of wind turbines. The mechanisms of hydrogen diffusion in bearings are not yet fully understood, but it is assumed under tribological loading that the lubricant degradation releases hydrogen which diffuses into the contacting surfaces (i.e. steel). Real time study of hydrogen generation/ permeation in a tribological contact is a challenging task to perform accurately. In the recent five years, a method was developed by the researchers using a Modified Devanthan-Stachurski Cell (MDSC). This method was then utilized to study and measure the hydrogen permeation through a thin steel membrane. In the current study, this method was re-evaluated, and several potential improvements were identified and addressed with the aim to reduce the time and resources required to complete each test. 2. Methodology 2.1 Cell design A new electrochemical cell or hydrogen detection cell (MDSC) was designed (see Fig. 1) and manufactured to work with a Cameron-Plint TE77 reciprocating pin-on-plate tribometer and validated by completing tests using different base oils, base oil-water mixture and deionized (DI) water. Water is well known to increase hydrogen permeation in bearing steels due to its low lubricity (high friction) and presence of hydrogen. The 3-electrode (counter, working, and reference) cell used to oxidise the permeated hydrogen consisted of an EN31 bearing steel membrane as working electrode, Ag/ AgCl electrode as reference electrode and a platinum wire as counter electrode. The 0.1M deaerated sodium hydroxide solution was used as an electrolyte. Fig. 1 (a) Modified Devanathan-Stachurski Cell (MDSC) design and (b) MDSC cross-section 2.2 Material Poly-alpha-olefin (PAO4), perfluorinated polyether (PFPE), DI water and a mixture of PAO4 and DI water were selected as lubricant to measure the hydrogen permeation current density. EN31 steel was selected as both disc (0.8 mm thick and 40 mm diameter) and pin (10 mm diameter) material. Steel samples were polished up to 600 grit paper on the sliding side. The other side (detection side) of the steel sample was polished using up to 1200 grit paper and diamond paste (6 mm, 3 mm, and 0.5 mm). After polishing, polished samples were cleaned using ultrasonic cleaner for 3 minutes in various solvents (acetone, propanol, ultra-pure DI water) and dried using a N 2 gun. After cleaning, the detection side of the polished steel sample was coated by a 100 nm thick palladium (Pd) layer for permeation current measurement. 2.3 Experimental parameters Table 1 show the experimental parameters selected to conduct the hydrogen permeation measurements. 226 24th International Colloquium Tribology - January 2024 Efficiency Improvements of In-Situ Hydrogen Permeation Measurements in Lubricated Bearing Steel Contacts Using the MDSC Method Table 1 Tribomaterial and electrochemical parameters Process parameters Values Load (N) and contact pressure (GPa) 50 Stroke length (mm) 7 Frequency (Hz) and sliding speed (m/ s) 10 and 0.14 Sliding and stabilization time (hr) 2.5 Polarization current (V) +0.115 V Temperature (°C) Ambient 3. Results During sliding between steel disc and pin, the hydrogen atoms generated from the surrounding environment (lubricant decomposition, presence of water, etc.) permeated through the steel membrane and reached the opposite face of the steel membrane for detection. The Pd coating on the detection side oxidized these hydrogen atoms. The amount of hydrogen permeated through the steel membrane was measured by the oxidizing current density (hydrogen oxidation current). The increment in current density confirms the hydrogen generation and detection. Fig. 2 shows the variation in current density with sliding time for the selected lubricants. The increment in current density was observed highest for pure water. While for base oils (PAO and PFPE) the permeation current was observed lowest, as expected. The current for the oil-water mixture was observed between the pure water and the base oils. For PAO, the permeation current is higher compared to PFPE. It may be due to the presence of hydrogen in PAO, which is a hydrocarbon-based oil, whereas PFPE as perfluorinated oil does not contain hydrogen in its chemical structure. Fig. 2 Variation in hydrogen permeation current with sliding time 4. Conclusion The results from the validation tests showed that the newly designed Modified Devanthan-Stachurski Cell (MDSC) is able to accurately detect varying levels of hydrogen permeation for different base oils or base oil-water mixtures. The newly designed cell significantly reduced the time required for testing from 8-12 hrs to 2-3- hrs. After these validation tests, base oils with potential future gearbox applications have been selected and are being tested. The hydrogen permeation characteristics obtained from these tests shall be compared with commonly used PAO base oil. The validation of this experimental setup in combination with the testing completed opens the door to many more opportunities regarding efficient testing of a wide variety of lubricants, additives, or materials. It is hoped that the findings from this work can be used to informed decisions for the next generation of sustainable gearbox oils. References [1] Oberle N, Amann T, Kürten D, et al (2020) In-situ-determination of tribologically induced hydrogen permeation using electrochemical methods. Proc Inst Mech Eng Part J J Eng Tribol 234: 1027-1034. https: / / doi. org/ 10.1177/ 1350650119889196 [2] Evans MH (2016) An updated review: white etching cracks (WECs) and axial cracks in wind turbine gearbox bearings. Mater Sci Technol (United Kingdom) 32: 1133-1169. https: / / doi.org/ 10.1080/ 02670836.201 5.1133022 [3] Wranik J, Holweger W, Lutz T, et al (2022) A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication. Lubricants 10: 1-17. https: / / doi.org/ 10.3390/ lubricants10050096 [4] Esfahani EA, Soltanahmadi S, Morina A, et al (2020) The multiple roles of a chemical tribofilm in hydrogen uptake from lubricated rubbing contacts. Tribol Int 146: 106023. https: / / doi.org/ 10.1016/ j.triboint.2019.106023