A First-principles Investigation On Interactions Between Various Surface-terminated Diamond Films

Diamond films have many excellent properties such as high hardness,low friction coefficient,good wear resistance and high chemical stability,which are widely used in aerospace,mechanical wear coatings,micro-and nano-electromechanical systems(MEMS/NEMS).In recent years,tribological problems in extreme environments,such as high speed high temperature,high load,and high vacuum,attracts much attention.It indicated in many studies that surface modifications of the diamond or diamond-like carbon(DLC)films can achieve an ultra-low coefficient of friction.Different surface termination would induce different adhering behaviors,which is closely related to its atomic-scale tribological properties.However,the interaction mechanism between surface-terminated diamond films is still unclear.It inevitability retards the further applications on friction improvements of diamond films.In this paper,the influence of different terminals on the interaction of diamond films is studied by first-principles method.The diamond model of clean diamond(100)model,five diamond models of the same terminal(H,F,O,OH NH2),the diamond models of three different terminals(H-F,H-O,H-OH)was constructed.Their equilibrium geometries were optimized and the interaction,electron density and population of the terminated diamond were calculated.The results show that:(1)The interaction between the clean diamond surfaces is strong with a large adhering energy,which forms covalent bonding at the interface.(2)Suitable terminals(H,F,NH2,and OH)to the surface of the diamond surfaces can reduce the interface interaction.F termination layer has a higher electron density coverage,which can provide larger repulsive force at diamond interface.The interface interaction of the F-terminated diamond is stronger than that of the H-terminated diamond interface.The interface binding energies of both the NH2-and OH-terminated diamond surfaces are an order of magnitude lower than those of F-and H-terminated ones due to the presence of functional group atoms that lead to an electronic offset.(3)The interface binding energies of the H-F,H-O and H-OH interface systems are all between those of the same terminated diamond interfaces.Replacement of an OH-and NH2-terminated diamond surface by H terminal would increase the repulsive forces between OH-OH and NH2-NH2 interfaces.