| With the emergence of technological applications such as magnetic storage devices, MEMS applications and ultra-thin film coatings, the study of friction, adhesion, and wear has become increasingly important. For better design and durability of these nanoscale devices, it is essential to understand deformation in small volumes and in particular how deformation mechanisms can be related to frictional response of an interface in the regime where plasticity is fully developed. However, there is a lack of analytical models that relate tribological response to material properties and/or contact geometry for nanoscale elastic-plastic contacts. To provide these solutions, this thesis focuses on (a) development of analytical models that describe tribological behavior at nanoscale contacts and (b) investigation of atomistic mechanisms that control nanoscale deformation during sliding of elastic-plastic contacts. Large scale molecular dynamics studies of single asperity sliding have been conducted on three different materials: crystalline silicon carbide, crystalline copper and nanocrystalline silicon carbide. We demonstrate that, unlike in a number of other brittle materials, a high pressure phase transformation in SiC is highly unlikely under indentation or cutting conditions. The different categories of dislocation activity are investigated as a function of normal load and depth of cut for single crystal SiC. For nanocrystalline (nc) SiC, deformation is shown to occur via grain boundary sliding, heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull-out. Our results demonstrate that machining of nc ceramics can be performed with nanometer-sized tools because in this regime brittle ceramics are pliable. In addition, we have developed a new analytical model which describes the plowing coefficient of friction during sliding of elastic-plastic contacts between a single asperity and a flat substrate. The proposed model includes the effects of both elastic recovery and pileup. Applicability of the new model is demonstrated on the examples of non-adhesive and adhesive contacts in SiC and Cu. In addition, our model shows excellent agreement with large scale molecular dynamics simulations and AFM experiments of nanoscratching on Cu single crystals. |
