Full-Scaling Friction and Wear Laws of Nanotwinned Metals.

Ultrastrong nanotwinned (NT) metals hold promise for mitigating friction and wear-essential for enhancing the energy efficiency and longevity of all moving systems. However, optimizing their tribological performance has long suffered from the absence of friction and wear laws across varying tribological loading scales. Here, we have discovered full-scaling twin lamella spacing (λ)-dependent friction and wear laws in NT pure nickel and identified critical deformation mechanisms for reducing friction and wear. Nanoscale friction initially increases with decreasing λ, peaking at 20 nm, and then decreases due to a transition from dislocation-twin boundary interactions to detwinning. Under microscale loading, friction and wear reduce linearly with decreasing λ, attributed to an unforeseen phase transition from face-centered cubic to hexagonal close-packed structures. Furthermore, under macroscale loading, the formation of a durable oxide layer and a stable gradient nanostructure in nanotwinned nickel with λ exceeding 20 nm aids in mitigating wear loss.