Insigths into the Tribochemistry of Silicon-doped Carbon-Based Films by Ab Initio Analysis of Water-Surface Interactions

Diamond and diamond-like carbon are used as coating materials for numerous applications, ranging from biomedicine to tribology. Recently, it has been shown that the hydrophilicity of the carbon films can be enhanced by silicon doping, which highly improves their biocompatibility and frictional performances. Despite the relevance of these properties for applications, a microscopic understanding on the effects of silicon is still lacking. Here, we apply ab initio calculations to study the interaction of water molecules with Si-incorporating C(001) surfaces. We find that the presence of Si dopants considerably increases the energy gain for water chemisorption and decreases the energy barrier for water dissociation by more than 50 %. We provide a physical rational for the phenomenon by analyzing the electronic charge displacements occurring upon adsorption. We also show that once hydroxylated, the surface is able to bind further water molecules much strongly than the clean surface via hydrogen bond networks. This two-step process is consistent with and can explain the enhanced hydrophilic character observed in carbon-based films doped by silicon.