Dual-gradient structure enhances wear resistance of aero-engine bearing steel by suppressing strain localization

Gradient structures can significantly enhance the wear resistance of steels through the synergistic effects of heterogeneity. However, traditional surface heterostructures typically produce a single gradient. Here, we propose a novel strategy to implement a dual-gradient structure of composition and nanocrystalline, thereby enhancing the wear resistance of bearing steel by suppressing strain localization. The compositional gradient prefabricated by carburization facilitates the formation of gradient-distributed carbides and martensite, while the nanocrystalline gradient is developed further via ultrasonic shot peening. Strong dislocation movement promotes the refinement and decomposition of large-sized irregular carbides in the surface layer, significantly mitigating the initiation and propagation of cracks induced by stress localization. Additionally, the numerous nanograins in the surface layer not only contribute to the formation of a more stable and dense oxide film under oil lubrication but also create a more dispersed region of stress localization by co-sharing cyclic shear stress, thereby alleviating sliding-induced microstructural instability. Furthermore, the single compositional gradient structure tends to surface strain localization during loading, attributable to the relatively gradual transition between the hard and soft layers, whereas the dual-gradient structure facilitates surface strain delocalization across a wider stress range due to the presence of numerous nanograins creating a more pronounced strain gradient. Compared to the single compositional gradient, the unique dual-gradient structure reduces the wear rate by 52.5 % and 53.9 % at low and high-frequency sliding, respectively. This work proposes a promising design for the fabrication of dual-gradient structures to enhance the wear resistance in high-strength steels.