Focused ion beam characterization of deformation resulting from nanoindentation of nanoporous gold
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Research Letter
Focused ion beam characterization of deformation resulting from nanoindentation of nanoporous gold Nicolas J. Briot and T. John Balk, Department of Chemical and Materials Engineering, University of Kentucky, 177 F. Paul Anderson Tower, Lexington, KY 40506, USA Address all correspondence to Nicolas J. Briot at [email protected] (Received 29 November 2017; accepted 22 December 2017)
Abstract Regions of deformation resulting from nanoindentation testing of nanoporous gold (np-Au) are characterized by cross-sectional imaging of the ligament structure directly beneath the surface, after lift-out using focused ion beam techniques. Permanent deformation of the porous structure was not exclusively confined to the region directly in contact with the indenter but extended much deeper into the sample. Implications of these observations with respect to previous measurements of the mechanical properties of np-Au are discussed. The conclusions provide initial insight into the deformation behavior of np structures during nanoindentation, as well as a basis for extending this technique to other np metals.
The mechanical properties of np-Au, and more specifically its yield strength, have been evaluated and reported in numerous publications over the past 15 years, with a revived interest that has led to multiple studies in recent years.[1–5] Using scaling relations originally derived by Gibson and Ashby,[6] the yield strength of the individual interconnected ligaments, which create the three-dimensional and randomly oriented np structure, has been estimated to approach the theoretical strength of gold.[7–9] However, the applicability of these scaling relations to np metals has been questioned, as they were originally developed for porous materials with ligament size in the micron range, whereas np metals typically have ligament sizes less than 100 nm. Because it is well known that the strength of metals increases as sample dimension decreases to the nanoscale, a ligament size effect is expected to contribute to the strength of np-Au. Several authors have proposed modified versions of the scaling relations using np-Au as a model material, considering ligament size effects as well as other factors such as surface effects or the np structure’s randomness.[4,10–14] Early studies on the mechanical properties of np-Au used nanoindentation testing on bulk samples to extract Young’s modulus and hardness values.[7,15,16] It was generally hypothesized that because of the porous nature of np-Au, the deformation of the structure would be accommodated by densification of the pores in the close vicinity of the indenter’s surface, limited to a region directly below the center of the indent and within its diameter, and resulting in limited or no lateral deformation.[14,16] In this scenario, the constraint factor typically observed for dense metals would approach 1, as the plastic Poisson’s ratio becomes null, and the hardness value measured during nanoindentation could consequently be considered
equal to the yield strength of the sampl
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