Effect of Polarity on Arc Ablation Behaviour and Damage Mechanisms of Brush/Slip Ring Contact Interfaces

To clarify the origin of asymmetric arc ablation in hydroelectric generator slip rings, a Steel 45/carbon current-carrying friction pair was selected to investigate the influence of current polarity on arc behaviour and interfacial damage. Static gap discharge tests (10 A) and dynamic tests (current density of 10 A/cm2, sliding velocity of 0.419 m/s) were performed to characterize polarity-dependent erosion behaviour. The results indicate that, under the steel(+)–carbon(−) condition, the arc exhibits unstable burst-like discharge accompanied by intense spark spattering. The carbon cathode experiences severe material loss due to the combined effects of cathode-spot heating, positive-ion bombardment, and molten metal droplet impact. The steel surface is characterized by nested erosion pits and spherical resolidified spatter particles, while the apparent ablation-affected area shows an overall increase with accumulated arc duration, with a more pronounced expansion observed at longer durations. Pronounced bidirectional material migration and interfacial elemental enrichment are observed under the steel(+)–carbon(−) configuration, resulting in an apparent net mass loss rate approximately 2.5 times higher than that under the steel(-)–carbon(+) configuration. The reversed steel(−)–carbon(+) configuration produces a spatially constrained and stable arc discharge, accompanied by a continuous remelted layer and network-like thermal-stress cracks on the steel surface. Polarity reversal changes the direction of the interfacial electric field and charged-particle migration, thereby regulating cathode electron emission, arc discharge behaviour, energy distribution in the near-electrode region, and bidirectional material migration across the interface. These results provide a theoretical basis for elucidating the polarity-dependent arc ablation mechanism of steel/carbon friction pairs and for optimizing polarity configuration and differentiated protection strategies for hydroelectric generator slip rings.