抄録
To extend the driving range of electric vehicles (EVs), downsized e-axle systems require gears operating at high rotational speeds and sliding velocities. Under such severe rolling–sliding conditions, carburized steels are prone to fatigue wear, particularly pitting, due to friction-induced thermal softening. Nitrocarburization is a promising alternative surface treatment, yet its fatigue wear mechanism under high-sliding conditions remains unclear. Here, fatigue wear behaviors of carburized and nitrocarburized steels were systematically compared through rolling–sliding contact tests under controlled contact pressures and slide-to-roll ratios (SRR). Surface damage evolution was quantified using surface profilometry, while subsurface damage and crack propagation were examined by cross-sectional scanning electron microscopy. The two steels exhibited fundamentally different damage trends. For carburized steel, surface damage generally tended to increase with SRR, consistent with rolling contact fatigue theory, where sliding accelerates crack initiation and propagation. In contrast, nitrocarburized steel showed reduced surface damage at higher SRR. Cross-sectional observations revealed that fatigue cracks propagated deeply into the substrate in carburized steel, whereas crack growth in nitrocarburized steel was consistently arrested within the surface compound layer. Nanoindentation confirmed that this compound layer was significantly softer (≈3.6 GPa) than the carburized surface (≈10.1 GPa). These results indicate that the soft compound layer acts as a sacrificial running-in layer under sliding, promoting mild wear that smoothens the surface, reduces stress concentration, and suppresses catastrophic crack propagation. Consequently, nitrocarburization alters fatigue wear mechanisms under high-SRR rolling–sliding contact, shifting damage evolution from deep pitting to localized material detachment and providing improved fatigue resistance for high-speed EV gear applications.
