Current-carrying friction behavior of CrN coatings under the influence of DC electric current discharge

Abstract

CrN coatings are renowned for their low friction coefficients, high chemical inertness, excellent corrosion resistance, and substantial hardness, making them ideal for the tribological demands of bearings and gears in electric vehicles. This study investigated the current-carrying friction behavior of CrN coatings when sliding against steel balls under both non-electrified and electrified conditions. The friction coefficient (CoF), wear volume, wear type, and mechanism of the coating were reported by adjusting the direct current (DC) intensity and lubrication status. The findings indicate that while the current significantly exacerbates substrate wear during dry friction, its impact on the CrN coating is minimal. Even at a maximum DC current of 1.5 A, the wear rate of CrN coating during dry friction is only 1.9 × 10⁻⁴ mm³·(N·m)⁻¹, representing a reduction by 79.1 % compared to the steel substrate. This effect is further pronounced when lubricated with PAO oil. The wear of CrN coating shows minimal change even as the current increased, and surface wear remains very slight. At a current of 1.5 A, the wear rate of CrN coating decreases to as low as 9.7 × 10⁻⁶ mm³·(N·m)⁻¹, indicating a reduction by 98.7 % compared to its substrate. It is observed that under electrified conditions, the oxidation and degradation of CrN coating are accelerated, thereby resulting in the formation of a loose and low-hardness Cr₂O₃ oxide layer on the surface. The oxide layer is primarily attributable to the deterioration in frictional properties of the CrN coating under electrified conditions. Finally, CrN coatings exhibit minimal changes in tribological behavior under electrified conditions, thereby offering effective protection for substrates against electrical damage. Therefore, CrN coatings are ideal for applications involving electrical contacts.