Significantly lowered coefficient of friction in copper alloy with a gradient nanograined-nanotwinned surface layer

Heterogeneous gradient nanostructure has been evidenced to significantly reduce the coefficient of friction and wear loss. However, the tribological performance and underlying microscopic mechanisms of gradient nanograined metals containing stable nanotwins with superior strength-ductility synergy are still unclear. In this work, an ultra-precision machining technique named single point diamond turning was developed to fabricate a stable gradient nanograined-nanotwinned layer on Cu-4.5%Al alloy (in which stacking fault energy was tailored through alloying of Al). The reciprocal dry-sliding tests under loads of 10 N and 35 N at room temperature were conducted on gradient nanograined-nanotwinned specimen and five Cu counterparts (uniformly refined grain sizes, nanotwins, and gradient nanograins). The gradient nanograined-nanotwinned specimen exhibits the unprecedented low steady-state coefficient of frictions (0.18–0.24), demonstrating further improvement in comparison with the gradient nanograined Cu (0.31–0.35). Moreover, scanning electron microscope, white light interferometer, and high-resolution transmission electron microscope observations were mainly employed to evaluate the microstructural evolutions of worn surfaces and subsurface layers. It reveals that the worn subsurface of stable gradient nanograined-nanotwinned specimen consists of grain refinement, slightly grain coarsening and dynamic recrystallization, and deformation layers along the depth direction, showing further effectively suppressed grain coarsening process relative to the gradient nanograined Cu. Accordingly, a twin boundary migration plus grain rotation-mediated grain coalescing process in the worn subsurface is elucidated to be responsible for such mechanical-structural stability of the gradient nanotwinned structure towards the superior tribological properties.