A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained (GNG) surface layer. After persisting in the low-friction state (0.2–0.4) for tens of thousands of cycles, the coefficients of friction in the GNG copper (Cu) and copper-silver (Cu-5Ag) alloy start to increase, eventually reaching a high level (0.6–0.8). By monitoring the worn surface morphology evolution, wear-induced damage accumulation, and worn subsurface structure evolution during sliding, we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer. A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample, especially during the low friction stage. Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.

在具有梯度纳米晶粒 (GNG) 表面层的金属中无润滑滑动期间观察到独特的低到高摩擦转变。在低摩擦状态（0.2-0.4）持续数万次循环后，GNG铜（Cu）和铜银（Cu-5Ag）合金的摩擦系数开始增加，最终达到较高水平(0.6–0.8)。通过监测滑动过程中磨损表面形态演变、磨损引起的损伤积累和磨损的亚表面结构演变，我们发现低到高摩擦转变与由垂直塑性变形和磨损引起的明显微观结构不稳定性密切相关。亚表面层中的稳定纳米颗粒。与粗晶粒样品相比，GNG 样品的磨损损失非常低，特别是在低摩擦阶段。我们的结果表明，通过增加 GNG 结构稳定性以防止高载荷下晶粒粗化，可以推迟从低到高摩擦转变的开始并提高 GNG 金属的耐磨性。