Asperity contacts at the nanoscale: comparison of Ru and Au

TL;DR

This study develops a new interatomic potential for ruthenium and compares nanoscale asperity contact behaviors of Ru and Au, revealing differences in ductility and fracture mechanisms relevant for microelectromechanical systems.

Contribution

A novel interatomic potential for Ru is created and validated, enabling detailed molecular dynamics comparison of asperity contact formation and breaking in Ru and Au.

Findings

Au exhibits high ductility and necking to a single-atom bridge.

Ru shows less plasticity and brittle fracture at lower temperatures.

Differences in contact mechanics influence microelectromechanical system performance.

Abstract

We develop and validate an interatomic potential for ruthenium based on the embedded atom method framework with the Finnis/Sinclair representation. We confirm that the new potential yields a stable hcp lattice with reasonable lattice and elastic constants and surface and stacking fault energies. We employ molecular dynamics simulations to bring two surfaces together; one flat and the other with a single asperity. We compare the process of asperity contact formation and breaking in Au and Ru, two materials currently in use in micro electro mechanical system switches. While Au is very ductile at 150 and 300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately the same homologous temperature). In Au, the asperity necks down to a single atom thick bridge at separation. While similar necking occurs in Ru at 600 K, it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a much more brittle process of fracture/decohesion with limited plastic deformation.