Nano-scale contacts: Morphology, adhesion and deformation.

摘要

Nanoscale contacts are central to a wide range of modern technologies, e.g., MEMS switches and cold-welding patterning techniques. The deformation processes in nanoscale contacts are usually associated with individual defects that exist and move on length and time scale that together are beyond the resolution of most experiments. We employ molecular dynamics to bypass these difficulties in order to reveal the fundamental deformation and adhesive mechanisms and determine which experimentally adjustable parameters or choices of material affect contact behavior. We first focus on single asperity Au contact and deformation that occurs as two rough surfaces are brought together, loaded and then separated. The evolution of structure, mechanical and electronic properties are shown to be correlated with the detailed, highly localized dislocation dynamics. Our simulation results demonstrate excellent qualitative agreement with experiments. During the contact process, defects (e.g., stacking faults) are generated and annihilated, making it a highly irreversible, dissipative process. Consequently, when the system is subjected to repetitive contact and separation, stacking faults accumulate and result in phase transformations (i.e., FCC→HCP→FCC). The new structure, resulting from the phase transformations, has a lower Schmid factor and consequently exhibits larger tensile strength. Similar phase transformations are also observed during contact process at elevated temperatures, which may be aided by thermal fluctuations. We systematically examine the asperity contact process in terms of the work of adhesion between contacting surfaces, Gamma, asperity geometries (symmetric and asymmetric) and loading directions ([100], [110] and [111]). We find that one feature of the local contact shape is the equilibrium contact angle, which is prescribed by the Young-Dupre equation.;The plastic deformation during separation and the amount of material transfer upon separation are shown to strongly depend on the work of adhesion Gamma, the geometry of the contact and the crystallographic orientation relative to the loading direction. Finally, we examine the contact process involved in subtractive cold-welding patterning. We proposed a simple model to represent the deformation and failure of the film. This allows us to clarify the size dependence of the strength of the film, and offer accurate predications of the degree of deformation induced by the rigid stamp.