Deformation mechanisms of Al/amorphous-Si core-shell nanorods

In this study, we model the indentation/retraction of an aluminum/amorphous-Si (Al/a-Si) core-shell nanorod. We investigate changes in the deformation behavior of the core-shell structure with changes in the size of the core and shell. Since indentation has nonlinear material behavior around the point of contact, and the linear elastic region can span long range, we use a multi-scale model as implemented in the coupled atomistic and discrete dislocation (CADD) method. We introduce a two-material/phase formulation of the CADD method to model the fcc Al and a-Si phases. Under indentation and retraction loading, samples with shell show an average 9% greater recovery in core height than samples with no shell. We find that there are three routes for deformation in the core-shell structure: (1) compression of the Al core; (2) deflection of the surrounding Al substrate; and (3) deformation of the a-Si shell. When present, the a-Si shell delocalizes forces generated by the indenter, allowing for all three forms of deformation to be active. This allows the Al core, the a-Si shell, and the surrounding Al substrate to each contribute to the indentation load and increases the force necessary to produce yield in the core. Without the shell, compression of the core is the dominant form of deformation, with yield occurring at lower indentation force than samples with shell. We also find that the dominance of deformation in the core or substrate is dependent on the size of core used. Samples with large cores and thin shells experience deformation mostly in the core. On the other hand, shelled samples with small cores and thick shells have well-protected cores and experience substantial deformation in the substrate. This work will help with the design of low friction nanotextured surfaces composed of core-shell nanorods tailored for specific deformation characteristics.