Dislocation nucleation and segregation under adhesive contact of a nano-asperity coating on a crystalline solid

Abstract

Crystalline metals are typically coated with materials such as natural oxides for engineering applications, wherein the coating surface has nanoscale roughness. Although the nanoscale asperities of material surfaces typically play a key role in the evolution of surface plasticity under contact loading, the dislocation activities interplaying with a nano-asperity coating and the resulting non-trivial surface microplasticity are yet to be investigated. In this study, the nanoscale roughness of a coating layer is modeled with a stepped coating to investigate its effect on the dislocation nucleation and surface microplasticity evolution. The adhesive contact of a flat indenter on a stepped coating on a substrate was analyzed as a unit process of dislocation emission from the surface step using continuum anisotropic elasticity, which further provided the critical applied stresses required for an incipient dislocation to be emitted from the interface as well as the configurational forces acting on the dislocation. In addition, molecular dynamics simulation of a stepped coating on a crystalline face-centered cubic metal was conducted using the large-scale atomic/molecular massively parallel simulator. The interatomic potential of the coating material and the atomic interaction between the flat indenter and coating were modeled using the Lennard-Jones potential, whose parameters were adjusted to tailor the elastic constants, residual stress in the coating layer, and adhesion energy between the flat indenter and coating. When the indenter was compressed against the coating with a surface step, dislocations were nucleated from the step and emitted into the metal substrate, from which the effects of protective coating on the dislocation nucleation and evolution of surface microplasticity were investigated. Furthermore, the formation of a dislocation double layer due to dislocation segregation was re-examined in the presence of a coating layer. The results of this study have potential implications for nanoscale thin film engineering, and the unit process model can serve as a building block for developing a scale-bridging and statistical model of rough surface contact. © 2021 Elsevier Masson SAS