Atomic-to-nanoscale thickness-driven tunable carbon and interface-chemistry immensely control tribo-interface and metal-oxidation

Abstract

Bodies in sliding contact experience resistance to motion due to frictional forces, causing huge energy wastage and wear that limit the operational lifetime of the systems. Metal-oxidation is another challenging concern which limit the functionality of the systems. Many approaches have been adopted to minimize frictional forces, wear and metal-oxidation with the use of surface coatings being an effective approach. However, achieving low friction and high wear- and oxidation-resistance with sub-10 nm thick coatings, especially at sub-3 nm thicknesses, remains challenging. Here, we employ ultrathin monolithic carbon overcoats with carbon thicknesses ranging from 0.7 to 10 nm to demonstrate significant reduction in the friction, wear and metal-oxidation. Additionally, we investigate the efficacy of bilayer overcoats where an atomically thin (∼0.4 nm) silicon nitride (SiN_x) interlayer is introduced between the substrate and carbon overlayer. We discover that while an ultralow carbon thickness of 0.7 nm could substantially reduce friction and wear, a threshold thickness of carbon-containing higher sp³ carbon bonding is required to maintain high wear resistance and low friction for longer durations, and high metal oxidation protection. The functional properties are well-explained based on carbon microstructure, surface-chemistry, interfacial-bonding, and other fundamental reasons.