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[Display omitted] In this paper, ReaxFF simulation is used to study the friction characteristics of amorphous carbon (a-C) films in CO 2 environment by considering the influences of CO 2 molecular number and the normal load. The results manifest that the decrease of friction at the beginning of sliding stage is mainly due to the physical adsorption of CO 2 molecules, while in the stable stage, the low and stable friction is caused by the combination of chemical adsorption and physical adsorption. The large amount of CO 2 molecules greatly reduce the friction force by generating more C-O terminal groups, which are not easily damaged by the increased normal load. In addition, due to the presence of CO 2 molecules, graphitization has little effect on the friction behaviors of a-C films in CO 2 environment. [ABSTRACT FROM AUTHOR]

Atomic perspective on the tribological mechanism of a-C films in different gas atmospheres. [Display omitted] • In H 2 , N 2 and O 2 atmosphere, the friction force is the largest in H 2 atmosphere. • Graphitization and passivation are not the key factors to reduce friction force. • The interfacial mechanical mixing caused by C–C bonding increases the friction force. • Gas-dynamic lubrication generated in the O 2 atmosphere greatly reduces friction force. In this work, the friction mechanism of amorphous carbon (a-C) films in hydrogen (H 2), nitrogen (N 2), and oxygen (O 2) atmospheres are investigated by using reactive molecular dynamics simulations. It is found that the a-C film has the highest friction in the H 2 atmosphere and ultra-low friction in the O 2 atmosphere. Further studies show that a-C films are difficult to passivated by H atoms. The violent diffusion of H atoms to a-C film under the action of high friction temperature leads to the increase of C atom density at the friction interface, resulting in high friction. In the O 2 atmosphere, O atoms almost do not diffuse to the a-C film, mainly exist at the friction interface, weaken the density of C atoms, and form gas-dynamic lubrication to produce ultra-low friction. In addition, graphitization and passivation are not the key factors determining the low friction in the gas atmosphere. [ABSTRACT FROM AUTHOR]

• MD was used to simulate deposition process of a-C films on γ-Fe substrate by HiPIMS. • Hybridization percentage of a-C films can be adjusted by substrate bias voltage. • Internal stress of C films can be adjusted by substrate bias voltage. • Internal stress in intrinsic zone is increased with increase of sp3-C atoms percentage. • Internal stress in pseudo diffusion zone is decreased with increase of C and Fe atoms mixing degree. In this paper, the deposition process of amorphous carbon (a-C) films on γ-Fe substrate was simulated by molecular dynamics (MD). The effects of substrate bias voltage (abbreviated as bias) on structure and internal stress of a-C films was investigated. The results showed that the growth process of a-C films on γ-Fe substrate can be divided into five stages, namely, the diffusion stage of C atoms, island-like nucleation stage of a-C films, formation stage of the pseudo diffusion zone, diffusion stage of Fe atoms and stable growth stage of a-C films. The structure of a-C films after deposition can be divided into four zones, namely, the substrate zone, pseudo diffusion zone, intrinsic zone and surface zone. The intrinsic zone is the main part of a-C films, and its structure is mainly composed of sp3-C and sp2-C atoms. The percentage of sp3-C atoms is fluctuated with the increase of bias. When bias is 25 V, the percentage of sp3-C atoms is the largest, which is 26.19%. When bias is 75 V, the percentage of sp3-C atoms is the smallest, which is 23.92%. The percentage of sp2-C atoms is increased with the increase of bias. When bias is 75 V, the percentage of sp2-C atoms is the largest, which is 73.69%. With the increase of bias, the internal stress of the intrinsic zone is firstly increased. When bias is 25 V, the internal stress is the highest, which is 6.94 GPa and then is decreased gradually. The pseudo diffusion zone is a transition zone between a-C films and γ-Fe substrate, and is a mixed zone composed of C and Fe atoms. With the increase of bias, the internal stress in the pseudo diffusion zone is decreased gradually. When bias is 75 V, the internal stress is the lowest, which is 0.40 GPa. [ABSTRACT FROM AUTHOR]