Nanoscale topography and wear of ceramic interfaces and their effect on macroscale friction

500 years ago, Leonardo da Vinci systematically investigated the relation between the frictional force and normal force. Da Vinci concluded that the frictional force is proportional to the normal force. The proportionality constant that links the frictional force and normal force is defined as the coefficient of friction (CoF). Although the CoF is an empirical number that resulted from da Vinci’s experiments and does not explain the fundamental origin of friction, this simple relation between frictional force and normal force successfully captures most dry sliding friction behavior between macroscopic objects. However, when the sliding surfaces strongly adhere to each other or are atomically smooth, the proportionality between frictional force and normal force may breakdown and, in this case, the frictional force is proportional to the area of real contact. John Frederick Archard proposed a simple multiple-contacts model in which the cross-sectional area of the contact points is increased linearly with applied load when the contact points are plastically deformed. More analytical models have been proposed, such as the Greenwood and Williamson (GW) model and Persson’s contact theory, to quantify the area of real contact at multi-asperity interfaces. Experimentally it remains challenging to access and measure the area of real contact hidden from view by the contacting objects. To quantify the area of real contact, numerical methods, such as the boundary element method, have been developed. The calculation of contact mechanics either by analytical or numerical methods provides further insight into the formation of contacts which leads to friction. In this thesis, we investigate the interplay between friction, surface topography, capillary adhesion and third body formation at macroscopic sliding interfaces between ceramic materials.