Shear-Induced Emergence of Aromatic Superlow-Friction Interfaces in Amorphous Carbon: Triggering Chemical Impurities...

Incommensurate graphitic interfaces represent an archetypical tribological system to achieve superlow friction. They are currently realized by molecular deposition or lubrication-triggered, in situ formation of graphitic nanolayers. An alternative route exploits shear-induced aromatization of amorphous carbon (a-C). This process occurs directly at surface asperity contacts, relying on chemical impurities like hydrogen and oxygen. Despite the potential of this approach, the lack of mechanistic knowledge, particularly on the role of impurities, is an obstacle to its development and application. To address these gaps, we undertake a systematic simulation study consisting of 1,000 1-ns-long quantum-mechanical molecular dynamics trajectories of sheared a-C with and without impurities. No aromatic interfaces emerge for four-valent systems like pure or silicon-doped a-C. However, shear-induced aromatization is consistently observed for impurities with valency lower than four. These cause the formation of voids surrounded preferentially by sp²-hybridized carbon atoms. Upon plastic flow, they stabilize long-living cavities that evolve into passivated interfaces, rich in polycyclic aromatic structures. The process is driven by shear localization, while passivation by low-valent impurities prevents reformation of sp³-hybridized domains. This study provides the first comprehensive screening of chemical elements triggering shear-induced carbon aromatization and is a step forward toward the design of self-forming and self-healing, superlubric carbon interfaces.