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 H2 molecules, appears to be more efficient in further reducing friction than OH-terminated surfaces.

Articles you may be interested inBandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications J. Appl. Phys.Experimental and theoretical study of bulk light scattering in Ca F 2 monocrystals J. Appl. Phys. 98, 053501 (2005); 10.1063/1.2034085Electron field emission properties of gamma irradiated microcrystalline diamond and nanocrystalline carbon thin films Ex situ spectroscopic ellipsometry investigation of the layered structure of polycrystalline diamond thin films grown by electron cyclotron resonance-assisted chemical vapor deposition Effect of nitrogen addition on the microstructure and mechanical properties of diamond films grown using highmethane concentrations Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600°C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp 3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a ͗011͘ crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. The elastic constants C 11 and C 66 , respectively, were selectively determined from the frequency of the longitudinal and shear horizontal bulk modes traveling parallel to the film surface. The three remaining elastic constants, namely, C 44 , C 33 and C 13 , were obtained from detection of the Rayleigh surface wave a bulk shear wave and the bulk longitudinal wave propagating at different angles from the normal to the surface. The values of the elastic constants depend on the deposition conditions and on the microstructural properties of the films, especially the diamond quality and the polycrystalline or smooth fine-grained nature of the diamond. For the polycrystalline diamond film with the best quality, the elastic constants are rather close to the Voigt or Reuss average estimate values using known bulk elastic constants of diamond, whereas those of the smooth fine-grained diamond films are reduced because of the poorer diamond quality leading to lower residual stress in the films.

No abstract