Deformation Behavior of Al/a-Si Core-shell Nanostructures

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 backstress 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. ACKNOWLEDGEMENTS This dissertation is the culmination of over 5 years’ work to understand the novel mechanical behavior of Al/a-Si core-shell nanostructures, which were initially discovered in the Nanomechanics and Tribology Laboratory (NMTL) in 2009. This work would not have been possible without my advisor, Dr. Min Zou, who provided an incalculable amount of support during my graduate career. Special thanks are also due to Corey Thompson and Sam Beckford, who formed the research core of the NMTL along with myself during much of my studies, and with whom I have been fortunate enough to pursue many other fruitful opportunities in engineering. I would also like to thank Dr. Douglas Spearot, Dr. Arun Nair, Dr. Adam Huang, and Dr. Panneer Selvam for serving on the dissertation committee. The NMTL has been a vibrant research environment, where I have had access to worldclass nanomechanical characterization tools, including nanoindentation and AFM. The nanofabrication work in this study was made possible by Errol Porter and the staff at the High Density Electronic Center, who diligently maintained and provided training on the cleanroom equipment, most notably electron beam lithography and PECVD. Training on the materials characterization equipment by Dr. Mourad Benamara at the Arkansas Nano & Bio Materials Characterization Facility is also greatly appreciated. Much of the sophistication in the molecular dynamics modeling in this work is due to Dr. Spearot, who provided many helpful discussions and ideas over the course of my graduate studies. Funding support for this work was partially provided by the US National Science Foundation (NSF) under Grant Nos. CMMI-1463306, DGE-0957325, and from the Center for Advanced Surface Engineering under Grant No. OIA-1457888 and the Arkansas EPSCoR Program, ASSET III. Major equipment funding from the Arkansas Biosciences Institute and the University of Arkansas is acknowledged. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant No. ACI-1053575, as well as the Arkansas High Performance Computing Center, under NSF Grant Nos. ARI-0963249, MRI-0959124, and EPS-0918970. TABLE OF CONTENTS