| Fundamental investigations on the origins of friction at the nanoscale were carried out using both theoretical and experimental approaches. A model was developed that analytically solves for friction by the motion of dislocations at atomically flat crystalline interfaces. It combines known concepts from dislocation drag, grain boundary theory, and contact mechanics into a single model which accurately predicts a wide range of friction phenomena, including static and kinetic friction, friction anisotropy, transfer layers and velocity dependence. In addition, values for friction coefficients calculated by inputting only basic materials constants yield reasonable agreement with comparable ultrahigh vacuum friction results.; To test the consequences of the theory, friction anisotropy measurements between single crystal NaCl and SrTiO3 surfaces by pin-on-disk and nanoindentation techniques were conducted, and shown to influence friction by an upper bound of ten percent fluctuation in ambient conditions.; Lastly, in-situ transmission electron microscopy (TEM) techniques were employed to overcome the classic "buried interface" problem in tribological research. Direct observations of the sliding behavior between surfaces were carried out in real time by imaging and spectroscopy techniques within the TEM. Unambiguous structure-friction relationships were identified for graphite, gold and diamond-like carbon, by observing dynamic interactions between the scanning probe and samples. Direct evidence is presented for graphitic wear by flaking and transfer layer formation as well as liquid-like behavior of gold at moderate temperatures, which has implications for metallic lubricants. Diamond-like amorphous carbon films were shown to form a graphitized surface layer induced by sliding. The approaches taken in these in-situ friction experiments mark the beginning of an era of new direct characterization capabilities in fundamental tribological research. |
