Influence of counterpart and lubricant on wear of DLC coated differential shafts for electric vehicles

Electric motors in electric vehicles (EVs) can generate higher torque at startup compared to conventional internal combustion engines (ICEs). This results in greater torque transmission to the differential subsystem, thereby increasing the tribological stresses on its components. Specifically, the normal load at the contact interface between the differential shaft and the planet gear is elevated. Traditional surface treatments used on differential shafts in ICE vehicles have shown limitations when applied to EVs. In this context, diamond-like carbon (DLC) coatings—recognized for their high wear resistance and low coefficient of friction—are emerging as a preferred surface treatment for differential shafts in electric vehicles.

To evaluate the tribological performance of DLC-coated differential shafts and compare it with other surface treatments, a dedicated test setup was developed using specimens manufactured from series differential shafts. In this setup, the shafts were tested against a steel ring, whose inner surface replicated the geometry and material properties of a gear bore. A normal load was applied to the shaft while the ring rotated to simulate the kinematics of the shaft-gear contact.

The effects of various material parameters—such as shaft surface roughness, coating structure, and counterpart material—were investigated experimentally to assess both wear resistance and friction behavior. In particular, the impact of manganese phosphating on the counterpart surface was studied in relation to the wear performance of the DLC-coated shafts. Results indicated that a manganese–iron phosphate layer on the counterpart significantly reduced wear on the DLC-coated shaft. Additionally, the influence of lubricant formulation on the wear of the DLC coating was examined, revealing that the rate of degradation was strongly affected by the composition of oil additives. Out of this experimental study, the optimization of the tribological behavior of the shaft-gear contact can be envisaged on a wide spectrum of differential subsystems designs.