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 sp 3 to sp 2 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 diamond 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 O 2 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 CO 2 molecules.