Assessment of the Tribological Performance of Diamond-Like Carbon Coatings for Differential Shafts for Electric Vehicles

Abstract

An experimental study was conducted to evaluate the tribological performance of diamond-like carbon (DLC) coatings on differential shafts. The study first developed an analytical approach to identify the lubrication regimes in which the differential shaft/planet gear contact operates. The contact primarily experiences boundary lubrication, with mixed lubrication possible at high rotation speeds. This analysis provided information for the design of a test setup and protocol that used real components and commercially available coatings. Two types of DLC coatings, hydrogenated amorphous carbon (a-C:H) and non-hydrogenated tetrahedral amorphous carbon (ta-C), were compared to standard electroless nickel plating (e-nickel) commonly used in differentials of internal combustion engine (ICE) vehicles. After an 8-hour test in tribological conditions designed to promote wear, the e-nickel coating experienced significant wear, losing about one-third of its thickness. In contrast, the a-C:H and ta-C coatings exhibited much lower wear, losing less than 10% and 5% of their thickness, respectively. The superior wear resistance of the DLC coatings was attributed to their higher hardness. Despite their low friction properties, the DLC coatings did not significantly reduce friction compared to e-nickel, with all coatings maintaining a friction coefficient between 0.08 and 0.10. DLC coatings exhibited excellent wear resistance under testing conditions that simulated the planet gear/differential shaft application, outperforming the standard electroless nickel solution. Consequently, these coatings should be considered effective surface treatments for enhancing durability in this application, particularly for electric vehicles.