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 elastohydrodynamic lubrication conditions has been investigated. Tests were conducted using a fully formulated ISO VG 320 synthetic gear oil on an electrified FE8 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 white etching crack (WEC) 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, particularly under anodic contact, with minitraction machine (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 FE8 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.