Mechanical Behaviour of Single Crystal, Pollycrystalline and Nanocrystalline Metallic Nanopillars Under Compression

Abstract

Fabrication techniques, and mechanical behaviours of vertically aligned cylindrical nanopillars of various metals, including tin, bismuth, palladium, indium, rhodium and cobalt have been presented in this work. These, vertically aligned, cylindrical nanopillars of various diameters have been fabricated via an electron beam lithography and electroplating method. Microstructural properties of these pillars were studied using high resolution scanning electron and transmission electron microscopy. A non-destructive synchrotron X-ray microdiffraction (μSXRD) technique was used for the characterization of tin and indium nanopillars. The results indicated single crystal body-centered tetragonal structured tin, polycrystalline rhombohedral bismuth, single crystal tetragonal indium, and nanocrystalline palladium, cobalt and rhodium nanopillars. The mechanical properties of these structures were studied by uniaxial compression under a nanoindenter outfitted with a flat punch diamond tip. The strain rate sensitivities and flow stresses were analyzed for each material. Single crystal tin and indium nanopillars showed size-dependent flow stresses where smaller diameter pillars exhibit greater attained strengths. The observed size-dependence matches closely to that previously reported for single-crystalline face centered cubic metals at the nanoscale. Polycrystalline bismuth nanopillars showed a size effect with a change in the deformation mechanism from grain boundary mediated mechanisms to dislocation processes as the pillar diameter approached the average grain size. Nanocrystalline palladium pillars showed an inverse size effect where a decrease in strength was seen for pillars with smaller diameters. Finally, a thorough study is presented on the buckling behaviours of 130nm diameter palladium, cobalt and rhodium nanocrystalline pillars with various height-to-diameter ratios and the elastic moduli of these materials are extracted.