Friction of Diamond in the Presence of Water Vapor and Hydrogen Gas. Coupling Gas-Phase Lubrication and First-Principles Studies

Nanocrystalline diamond (NCD) has attracted much attention in recent years because of improvements in growth methodologies that have provided increases in both film thickness and growth rate, while preserving the outstanding mechanical properties of diamond material. We provide here some evidence, based on combined experimental and first-principles analyses, that ultralow friction of nanocrystalline diamond in the presence of water vapor is associated with OH and H passivation of sliding surfaces, resulting from the dissociative adsorption of H2O molecules. The presence of these adsorbates (OH and H fragments) keeps the surfaces far apart preventing the formation of covalent bonds across the interface. H-passivated surfaces, resulting from the dissociative adsorption of H-2 molecules, appears to be more efficient in further reducing friction than OH-terminated surfaces.