Density modulated tungsten (W) thin films with nanoscale porosity contents of 7% to 40% by volume were grown on Si substrates through magnetron sputter deposition. Process parameters were selected according to the structure zone model, which resulted in film thicknesses between 105 nm and 520 nm. Nanomechanical properties of samples were investigated by means of instrumented nanoindentation. Reduced-χ2 analysis was carried out to assess four models formulated through differential effective medium approach. The model that factored in both the crowding effect and the maximum random packing of pores successfully captured the experimental trends. Attempts to breach the auxetic barrier resulted in large-scale pulverization or spontaneous conversion into WO3. Porosity corrected yield strength calculations underlined the possibility of defining a porosity threshold beyond which the compressive yield strength of density modulated nanoporous metallic thin films would drop abruptly due to aggravated geometric slenderness effects in agreement with earlier hypotheses.
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Authors Tanil Ozkan and Andreas A. Polycarpou gratefully acknowledge the support of the National Science Foundation under grant no. NSF CMMI 1030657. Film growth and structural analyses of films were carried out in the laboratories of the University of Arkansas at Little Rock. Nanoindentation, SPM and AFM analyses were performed in the Frederick Seitz Materials Research Laboratory Central Research Facilities, University of Illinois. Authors gratefully acknowledge Dr. Tobias Gokus at Neaspec GmbH for nano-FTIR analysis. Authors also express their gratitude for the constructive feedback they received from the reviewers of the JMR, which enhanced the scope and the depth of analysis provided in the revised version of this paper.
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Ozkan, T., Demirkan, M.T., Walsh, K.A. et al. Density modulated nanoporous tungsten thin films and their nanomechanical properties. Journal of Materials Research 31, 2011–2024 (2016). https://doi.org/10.1557/jmr.2016.197