Nanotribology of carbon-based materials

Carbon-based systems comprise an impressively broad and continually expanding range of materials, from the building blocks of biology to carbon allotropes with extreme and exotic properties such as nanotubes, buckyballs, graphene1, and diamondoids2. The myriad of stable forms that carbon can adopt is largely because of its ability to hybridize in multiple stable bonding states, and to bond strongly with many other atoms, including hydrogen. Many carbon-based thin films have been developed to address a broad range of coating applications, including uses that demand outstanding tribological performance in a wide variety of operating environments. The most critical compositional variables are the distribution of the carbon hybridization states (specifically, sp2 versus sp3 bonds) and the atomic H content. This compositional phase space is illustrated in Fig. 13,4. Dopants such as Si, F, N, B, and O can be used to modify surface energy, mechanical properties, and electrical conductivity. Here we particularly focus on: (i) ordered, highly sp3-bonded materials (i.e. diamond in single crystal or polycrystalline form); (ii) amorphous, highly sp3-bonded materials, often referred to as tetrahedral amorphous carbon (ta-C); (iii) amorphous films with a mixture of sp2 and sp3 bonding, necessarily stabilized with H, often generically known as diamond-like carbon (DLC); and (iv) ordered, sp2-bonded carbon (i.e. graphite). At the macroscopic scale, the mechanical and tribological properties of these materials are nothing short of astonishing. Diamond, the stiffest and hardest material known, can be grown in nanocrystalline thin-film form with nearly equivalent mechanical performance5,6. Friction coefficients are below 0.05, which is as slippery as common ice, and wear rates correspond to mere fractions of an atomic layer per pass of the sliding interface, with no lubricant needed7. DLC films, grown in a particular form known as near-frictionless carbon (NFC), can exhibit friction coefficients as low as 0.001 and wear rates even lower than diamond8. Successful tribological applications of diamond films include coatings for tools, optical components, biological implants, and more recently, as structural materials for microelectromechanical systems The dominance of surface effects at the nanoscale implies that nanotechnology applications involving contacting, moving components can be critically limited by the tribological behavior of the interacting materials. Carbon-based materials have tremendous potential here because of their robust and often unsurpassed tribological performance. We review some recent insights gained by nanotribology studies of various forms of carbon, with an emphasis on thin film materials.