Invited Paper NS+SS+TF-WeM3
The Role of Atomic Corrugation, Crystal Orientation, and Surface Chemical Bonding in the Nanotribology of Carbon-Based Systems
A key challenge for nanotechnology lies in developing an understanding of nanotribology, particularly for materials with outstanding tribo-mechanical properties such as carbon-based films. Frictional slip and wear in these materials can be manifested in unique and surprising ways. As a fundamental example, we show that nanoscale friction can exhibit clear transitions from smooth sliding to single slips and then multiple slips. The slips are directly correlated with the atomic lattice of the sample, in this case pure graphite. The observation of the transition to multiple slips is new, and is understood by considering the competition between the stiffness of the interatomic interfacial potential and the elastic stiffnesses of the contacting materials and the force sensor itself. The transition to smooth sliding with ultralow dissipation in open air is observed for the first time, and atomic-scale stick-slip is observed for interfaces orders of magnitude larger than any previously tested. Atomic-scale stick-slip may therefore be a far more prevalent phenomenon than currently appreciated. We have also extensively studied the nanotribological behavior of other carbon-based systems, including single crystal and nanocrystalline diamond. The atomic structure of the surface, verified by detailed surface spectroscopy, critically affects friction and adhesion. Hydrogen termination is particularly effective in reducing friction and adhesion to the limit of van der Waals' interactions. Friction and adhesion are also affected by the crystal orientation. For larger tips, continuum mechanics models of contact area can be applied to understand the interfacial mechanics of these nano-scale contacts, as evidenced by the observation of direct proportionality between friction and contact area, a phenomenon known as "interfacial friction". By using smaller tips coated with a carbonaceous film, the limits of continuum mechanics are explored and discussed.