Tribological behavior of H-DLC and H-free DLC coatings on bearing materials under the influence of DC electric current discharges

Abstract

In automotive industry, most of the moving components and lubricants are currently being reengineered in order to meet the much harsher operating conditions of electric vehicles (EVs). Diamond-like carbon (DLC) coatings have been widely used in automotive field mainly because of their lower friction and wear coefficients, high chemical inertness, corrosion resistance, and hardness. It is also possible that DLC coatings can afford very desirable tribological properties to the bearings and gears of electric vehicles. In addition, their superior dielectric properties could also be very ideal for such applications. In EVs, current leakages in the powertrain are very common and these can adversely impact the tribological behavior of bearings and/or other moving mechanical elements. Although DLC coatings provide superior tribological performance in IC engine applications, they have not yet been evaluated under electrified sliding conditions of EVs. In this work, two different types of DLC coatings (Hydrogenated DLC (H-DLC) and hydrogen-free DLC (H-free DLC) were produced on AISI 52100 steel substrates and tested using a ball-on-disc tribometer under non-electrified and electrified (DC, 3A) both dry and lubricated sliding conditions. Coefficient of friction (CoF), wear volumes, wear modes and mechanisms have been assessed and reported. The results showed that electrification can cause a decrease in CoF for baseline steel-on-steel contacts, but an increase for H-free DLC in lubricated condition. The change in CoF of H-DLC interface was negligible. However, electrification caused a significant increase in the wear volume of steel-on-steel and H-free DLC-coated surfaces, but little or no wear was observed for the H-DLC-coated cases in both dry and lubricated test conditions. Overall, the tribological behavior of H-DLC was almost unaffected by electrification, suggesting that one has to select the right kind of DLC for applications involving electrified contact conditions.