Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffu
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ncy J. Dudney Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
P. Sudharshan Phani International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, Telangana– 500005, India (Received 23 December 2017; accepted 29 March 2018)
Nanoindentation experiments performed in high-purity vapor deposited lithium films at 31 °C reveal a strain rate and length scale dependence in the stress at which pop-in type events signal an abrupt transition from diffusion to dislocation-mediated flow. The stress level at which the transition to dislocation-mediated flow occurs varies with the strain rate and ranges from 88 to 208 times larger than the nominal yield strength of bulk, polycrystalline lithium. Variation in the indentation strain rate reveals the relationship between the stress required to initiate the transition and the length scale at which the transition occurs follows the power-law relation, hardness depth1.17 5 1.545 N/m0.83, where the magnitude of the exponent and constant reflect the defect structure of the film. A rationalization of the transition is provided through direct comparisons between the measured cumulative distribution function (CDF) and the CDF hypothesized for the activation of a Frank–Read source.
I. INTRODUCTION
Motivated by the need to better understand small-scale mechanical behavior at the lithium/solid electrolyte (Li/SE) interface, nanoindentation experiments have been performed in high-purity 5 and 18 lm thick vapor deposited polycrystalline Li films at 31 °C (homologous temperature, TH, of 0.67) to ascertain how the hardness of Li changes as a function of length scale and strain rate. Such a study is of particular relevance given the microcompression experiments recently performed by Xu et al.1 on single crystal lithium pillars (height to diameter ratios between 3:1 and 5:1) at 26 °C. Their results show that under a constant nominal strain rate of 5 103 s1, the compressive yield strength increases from 15 to 105 MPa as the pillar diameter decreases from 9.45 to 1.39 lm. The size-dependent yield strength ranges from 30 to 210 times larger than the reported yield strength (;0.5 MPa) of bulk polycrystalline Li near room temperature.1,2
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Address all correspondence to this author. e-mail: [email protected] Corresponding Editor: Erik Herbert This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2018.85
Despite the potential for experimental artifacts such as, but not limited to, surface radiation damage and contamination effects in the pillar experiments, we report that Xu’s results are generally consistent with our own experimental observations using nanoindentation.3 In the results presented here, the relationship between the indentation depth and the transition from diffusion to dislocation-mediated flow (taken to be the mean pressure or hardness at the first dislocation avalanche) is revealed through a variation in the indentation strain rate. As in the pillar study, the indentation results rep
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