Contact dynamics from the atomic scale to nanostructured surfaces

Abstract

The collective mechanical behavior of a multitude of small contacts affects the friction between sliding bodies on the macroscopic scale. The contacts are complicated systems involving many atoms, which are constantly deformed, ruptured and reformed. In atomic force microscopy the situation is simplified by studying a single asperity contact between a sharp tip and atomically flat surfaces on the microscopic scale. In this thesis several aspects of contact dynamics have been analyzed using atomic force microscopy in ultrahigh vacuum. First, the influence of the tip-sample contact on the thermal fluctuations of the force sensor and on the dynamics of the stick-slip process were characterized. A power spectrum analysis showed that the fluctuations are strongly damped through the tip-sample contact. The frequency shift of the resonance in contact was used as a measure of the contact stiffness. Depending on the contact size different dependencies on the load were observed. Atomic-scale stick-slip measurements studying the jump dynamics with high spatial and temporal resolution suggested a wide variation of slip durations up to several milliseconds. These results are compared with a multiple-tip simulation based on a Tomlinson model including thermal activation. Thus, a correlation between the duration of atomic slip events and the atomic structure of the contact is established. Second, the actuation of nanometer-size contacts was studied by simulations based on an extended Tomlinson model. In this way the control and reduction of friction to negligible values was described. Such simulations allowed us to approximate the residual friction forces and compare them to an analytical approximation. Moreover, not only a reduction of friction but also a reduction in energy loss was found in sufficiently underdamped systems. Third, the influence of nanostructured surfaces on atomic friction has been studied. For this purpose, ultrathin epitaxial films of KBr on NaCl(100) and NaCl on KBr(100) have been grown. The structure of such films was studied by high resolution non-contact atomic force microscopy. In the case of sub-monolayer coverage of KBr on NaCl(100), a superstructure was found on islands of two and three layer height caused by the lattice mismatch. The friction on such a structure changes from atomic-scale stick-slip to smooth sliding within a unit cell of the superstructure because of a variation of the energy corrugation. Moreover, scans across atomic scale defects confirm the high resolution capabilities of friction force microscopy close to the ultralow friction state. In the complementary system, NaCl on KBr(100), flat islands without any superstructure or rumpling were observed. Atomically resolved non-contact images of the sub-monolayer coverage prove that the lattice constant of the NaCl islands is elongated to match the one of KBr(100). In summary, several different aspects of friction and contact dynamics from the atomic scale to nanostructured surfaces were discussed and explained.