Abstract
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 sp3 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.
Highlights
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
The termination of the free dangling bonds at the film surface by dissociated gaseous species or functional groups may significantly reduce the adhesion force at contact interfaces. These findings have shown that even though adsorption and dissociation of hydrogen, oxygen, and water molecules can readily occur at the surface of a-C films, nitrogen dissociation is not a preferential process
Oxygen surface adsorption was found to be dissociative in all simulation cases, suggesting that the oxygen bonds dissociated upon the adsorption of the oxygen molecules at the film surface
Summary
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. The unique properties and microstructure of amorphous carbon (a-C) films have led to their usage in many applications requiring protective overcoats and thin-film structures that demonstrate high strength, optical transparency in the visible and near-infrared wavelength range, thermal stability, chemical inertness, and biocompatibility[1,2,3,4,5,6]. The increasing interest to apply a-C films as protective, low-friction overcoats in many pioneering technologies has concentrated the attention of researchers on the structural stability, oxidation resistance, and tribological properties of a-C films under different environmental conditions, elevated temperatures. These findings have shown that even though adsorption and dissociation of hydrogen, oxygen, and water molecules can readily occur at the surface of a-C films, nitrogen dissociation is not a preferential process
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