A molecular dynamics study of the oxidation mechanism, nanostructure evolution, and friction characteristics of ultrathin amorphous carbon films in vacuum and oxygen atmosphere

Amorphous carbon (a-C) films are characterized by extraordinary chemical inertness and unique thermophysical properties that are critical to applications requiring oxidation-resistant, low-friction, and durable overcoats. However, the increasing demands for ultrathin (a few nanometers thick) a-C films in various emerging technologies, such as computer storage devices, microelectronics, microdynamic systems, and photonics, make experimental evaluation of the structural stability and tribomechanical properties at the atomic level cumbersome and expensive. Consequently, the central objective of this study was to develop comprehensive MD models that can provide insight into the oxidation behavior and friction characteristics of ultrathin a-C films exhibiting layered through-thickness structure. MD simulations were performed for a-C films characterized by relatively low and high sp(3) contents subjected to energetic oxygen atom bombardment or undergoing normal and sliding contact against each other in vacuum and oxygen atmosphere. The effect of energetic oxygen atoms on the oxidation behavior of a-C films, the dependence of contact deformation and surface attractive forces (adhesion) on surface interference, and the evolution of friction and structural changes (rehybridization) in the former a-C films during sliding are interpreted in the context of simulations performed in vacuum and oxidizing environments. The present study provides insight into the oxidation mechanism and friction behavior of ultrathin a-C films and introduces a computational framework for performing oxidation/tribo-oxidation MD simulations that can guide experimental investigations.