Electrically-induced damages in lubricated contacts under varying AC and DC conditions

Abstract The impact of electrical loads on surface and subsurface damage under full film EHL (elastohydrodynamic lubrication) condition has been investigated. Tests were conducted using a fully formulated synthetic ISO VG 320 gear oil on an electrified FE 8 bearing test rig, with varying DC current magnitudes and AC waveforms. The results reveal that higher DC current densities (e.g. 100 mA/mm2) and AC square waveforms are critical in accelerating the onset of WEC (white etching crack) failures. Under these critical electrical conditions, significant alterations in the near-surface microstructure were observed, leading to the formation of an electro-mechanically mixed layer (e-MML). This layer exhibits a pad-like, patchy morphology composed of additive-derived tribofilms and shows a 25–35% reduction in elastic modulus compared to the original steel surface. Electrical stresses could promote tribofilm formation, with MTM tests confirming both tribofilm formation and surface modification. However, the mechanical properties and structure of these tribofilms may differ from those formed under unelectrified conditions. Phosphorus-based tribofilms formed on FE 8 bearing surfaces demonstrated a potential to extend WEC life. Additionally, electrical stress can dissociate dissolved water in the lubricant into hydrogen, which may diffuse more readily into the bearing material due to the refined and deformed microstructure of e-MML. The combined effects of microstructural transformation, mechanical weakening and hydrogen ingress are likely to contribute to the earlier initiation of surface and subsurface cracking under electrified conditions.