Junctions and Nano-Elastohydrodynamics

Advances in computer-based modeling and simulation methodologies and capabilities, coupled with the emergence and development of high-resolution experimental techniques, allow investigations of tribological phenomena with unprecedented atomic-scale spatial and temporal resolution. We focus here on molecular dynamics simulations of formation and properties of interfacial junctions, and on nanoelastohydrodynamics in sheared lubricated junctions. Simulations predict that upon approach of a metal tip to a surface a jump-to-contact instability occurrs and that subsquent nanoindetation leads to plastic deformation of the gold surface. Retraction of the tip from the surface results in formation of a connective junction, or wire, of nano-scale dimensions, whose elongation mechanism consists of a series of plastic stress-accumulation and stress-relief stages which are accompanied by structural order-disorder transformations. These transformations involve multiple-glide processes. The yield-stress of a gold nanowire is predicted to be ~ 3 GPa, which is an order of magnitude larger than that of bulk gold. The nearly ideal mechanical response of nanowires is portrayed also in the ability to manipulate them reversibly in elongation-compression cycles. Comparisons of the structural, mechanical and electrical properties of nanowires generated via elongation of junctions of initial very different dimensions, confirms that nanowires of similar nature are formed, irrespective of the history of the junctions. Shearing the junction occurs via an atomic-scale stick-slip mechanism characterized by a similar critical-stress. The elongation process is reflected in hysteresis in the force versus tip-to-surface distance records, and in oscillatory behavior of the force. Measurements of room-temperature electronic transport in such pulled gold nanowires reveal periodic conductance quantization steps, in units of 2e 2/h. Simulations of atomic-scale structure, dynamics, flow, and response characteristics of a thin film molecular hexadecane lubricant, confined and sheared by topographically nonuniform solid gold surfaces sliding at a relative velocity of 10 m/s, are described. The simulations reveal nano-scale processes which include: spatial and temporal variations in the density and pressure of the lubricant, particularly in the region confined by the approaching asperities, accompanied by asperity-induced molecular layering transitions which are reflected in oscillatory patterns in the friction force; dynamical formation of elasto-plastic, or glassy, states of the lubricant in the interasperity zone; drastic asperity deformations mediated by the lubricant, leading to microstructural transformations of the nonuniform bounding solid surfaces; molecular trapping and formation of intermetallic junctions; and onset of cavitated zones in the lubricating fluid after the asperity-asperity collision process. The simulations extend micro-elastohydrodynamic continuum investigations into the nano-scale regime, and provide molecular-scale insights into the fundamental mechanisms of ultrathin film lubrication phenomena under extreme conditions, which are of significance for modern technologies.