Formation and oxidation of linear carbon chains and their role in the wear of carbon materials
The atomic-scale processes taking place during the sliding of diamond and diamond-like carbon surfaces are investigated using classical molecular dynamics simulations. During the initial sliding stage, diamond surfaces undergo an amorphization process, while an sp3 to sp2 conversion takes place in tetrahedral amorphous carbon (ta-C) and amorphous hydrocarbon (a-C:H) surface layers. Upon separation of the sliding samples, the interface fails. A rather smooth failure occurs for a-C:H, where the hydrogen atoms present in the bulk passivate the chemically active carbon dangling bonds. Conversely, sp-hybridized carbon chains are observed to form on diamond and ta-C surfaces. These carbynoid structures are known to undergo a fast degradation process when in contact with oxygen. Using quantum-accurate density functional theory simulations, we present a possible mechanism for the oxygen-induced degradation of the carbon chains, leading to oxidative wear of the sp phase on diamo nd and ta-C surfaces upon exposure to air. Oxygen molecules chemisorb on C-C bonds of the chains, triggering the cleavage of the chains through concerted O-O and C-C bond-breaking reactions. A similar reaction caused by adsorption of water molecules on the carbon chains is ruled out on energetic grounds. Further O2 adsorption causes the progressive shortening of the resulting, O-terminated, chain fragments through the same O-O and C-C bond breaking mechanism accompanied by the formation of CO2 molecules.