Date of Graduation

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Zou, Min

Committee Member

Spearot, Douglas E.

Second Committee Member

Huang, Po-Hao Adam

Third Committee Member

Selvam, R. Panneer

Fourth Committee Member

Nair, Arun K.

Abstract

Al/a-Si core-shell nanostructures (CSNs), consisting of a hemispherical Al core surrounded by a hard shell of a-Si, have been shown to display unusual mechanical behavior in response to compression loading. Most notably, these nanostructures exhibit substantial deformation recovery, even when loaded much beyond the elastic limit. Nanoindentation measurements revealed a unique mechanical response characterized by discontinuous signatures in the load-displacement data. In conjunction with the indentation signatures, nearly complete deformation recovery is observed. This behavior is attributed to dislocation nucleation and annihilation events enabled by the 3-dimensional confinement of the Al core. As the core confinement is reduced, either through an increase in confined core volume or a change in the geometrical confinement, the indentation signatures and deformation resistance are significantly reduced.

Complimentary molecular dynamics simulations show that a substantial amount of dislocation egression occurs in the core of CSNs during unloading as dislocations annihilate at the core/shell interface. Smaller core diameters correlate with the development of a larger back-stress within the core during unloading, which further correlates with improved dislocation annihilation after unloading. Furthermore, dislocations nucleated in the core of core-shell nanorods are not as effectively removed as compared to CSNs.

Nanostructure-textured surfaces (NSTSs) composed of Al/a-Si CSNs have improved tribological properties compared surfaces patterned with Al nanodots and a flat (100) Si surface. NSTSs have a coefficient of friction (COF) as low as 0.015, exhibit low adhesion with adhesion forces on the order of less than 1 μN, and are highly deformation resistant, with no apparent surface deformation after nanoscratch testing, even at contact forces up to 8000 μN. In comparison, (100) Si has substantially higher adhesion and COF (~10 μN and ~0.062, respectively), while the Al nanodots have both higher friction (COF ~0.044) and are deformed when subjected to contact loads as low as 250 μN.

This integrated experimental and computational study elucidates the mechanisms that contribute to the novel properties of Al/a-Si CSNs and characterizes the tribological properties of surface composed of these nanostructures, which provides a foundation for the rational design of novel technologies based on CSNs.

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