Impact of Hydrogen Gas Exposure on Friction of Metallic Surfaces

Abstract As the global transition toward use of hydrogen as an energy vector accelerates, understanding the tribological performance of metallic surfaces exposed to hydrogen is critical for the development of reliable applications such as in hydrogen internal combustion engines (HICEs). The impact of hydrogen gas exposure on the friction and surface topography of grey cast iron and steel is investigated. Specimens were subjected to hydrogen gas charging (HGC), followed by tribometric testing under boundary and mixed elastohydrodynamic lubrication. An analytical model based on Fick's second law of diffusion was employed to estimate the hydrogen concentration levels at the surfaces. Results demonstrate that under relatively mild hydrogen exposure tribological performance of surfaces can alter notably, characterised by a simultaneous increase in the coefficient of friction (CoF) and surface roughness (Sq). Cast iron exhibited a higher sensitivity to hydrogen at different exposure times, with CoF increasing by approximately 35% following 216-hrs exposure. Statistical analysis indicated existence of a correlation between the measured CoF and hydrogen concentration for both materials. It was also observed that the tribological performance of both materials could be largely reverted to the uncharged state conditions after a 280-hrs outgassing period, suggesting a reversible interaction between hydrogen gas and the metallic matrix upon a single charging cycle. This is the first study to investigate hydrogen gas exposure effects exclusively on the surface-related phenomena and contributes to the novelty of the presented research.