Macroscale, humidity-insensitive, and stable structural superlubricity achieved with hydrogen-free graphene nanoflakes

Achieving solid superlubricity in high-humidity environments is of great practical importance yet remains challenging nowadays, due to the complex physicochemical roles of water and concomitant oxidation on solid surfaces. Here we report a facile way to access humidity-insensitive solid superlubricity (coefficient of friction 0.0035) without detectable wear and running-in at a humidity range of 2-80%. Inspired by the concept of structural superlubricity, this is achieved between Au-capped microscale graphite flake and graphene nanoflake-covered hydrogen-free amorphous carbon (GNC a-C). Such GNC a-C exhibits reduced pinning effects of water molecules and weak oxidation, which demonstrates stable structural superlubricity even after air exposure of the surfaces for 365 days. The manufacturability of such design enables the macroscopic scale-up of structural superlubricity, achieving the leap from 4 μm × 4 μm contact to 3 mm ball-supported contact with a wide range of materials. Our results suggest a strategy for the macroscale application of structural superlubricity under ambient condition.