Influence of Adsorbate Monolayer on the Nano-Mechanics of TIP-Substrate Interactions
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INFLUENCE OF ADSORBATE MONOLAYER ON THE NANO-MECHANICS OF TIP-SUBSTRATE INTERACTIONS H.RAFII-TABAR, J.B. PETHICA AND A.P. SUTTON Department of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, UK ABSTRACT We employ constant temperature molecular dynamics simulations to study, at the atomistic level, the interactions of an Ir tip with a Pb substrate. We have considered the complete cycle of approach,contact,indentation and retraction of the tip from the substrate. We have observed the wetting of the tip by the substrate atoms, and the formation of atomic necking between the tip and the substrate. We have carried out further simulations to study the interactions of an Ir tip with an Ir substrate covered with an adsorbate monolayer of Pb film. We show that the introduction of the adsorbate considerably reduces the work of adhesion and the extent of plastic flow, in agreement with experimental results. Three-dimensional geometries have been generated on computer and used for the animation of the simulation runs.
Introduction The atomistic modelling of the mechanically-induced irreversible flow processes occurring in nano-scale volumes of stressed solids is fundamental to understanding the physics of phenomena of adhesion, indentation, friction and fracture of materials on macroscopic scale. Atomically sharp tips interacting with flat substrates offer convenient geometries for investigating the nature of this plastic flow and the mechanisms by which it nucleates and develops. An inspection of the existing experimental results on adhesion [1,2] and indentation [3] involving interactions of sharp metallic tips with metallic substrates shows that the pronounced hystereses observed during the load-unload cycles in these experiments are strong indications of plastic deformation. In the case of an otherwise defect-free nano-scale volumes of materials sudden dislocation multiplication may also be seen. These phenomena occur even when the contact is dominated by the action of surface forces. Moreover, these experiments show that the adsorption of monolayers of films on substrates significantly alter their adhesive and hardness properties [4]. However, neither from these experiments alone, nor from theoretical modelling employing standard theories of dislocation nucleation based on continuum mechanics, can definite conclusions be drawn regarding the mechanisms by which this irreversible process is initiated and is developed. It is the aim of our simulation studies to analyze the nature of this plastic flow. In this paper we employ constant temperature Molecular Dynamics (MD) Simulations based on the Nose-Hoover dynamics [5,6] in a tip-substrate geometry to study, at the atomistic level, the possible mechanisms underlying the plastic flow in nano-volumes of materials. We show that the adsorption of an atomically thin film onto the substrate significantly affects the adhesion properties of the tip and substrate and correctly predicts the results of the above experiments. The fully 3-D nature of these simulations is imp
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