Understanding the Time-Dependent Mechanical Behavior of Bimodal Nanoporous Si-Mg Films via Nanoindentation

This study addresses the mechanical response of nanoporous Si-Mg films, which are fabricated using free-corrosion dealloying and which represent an intriguing form of silicon that may find use as an anode material in lithium-ion batteries. The porous thin-film samples, in both the as-dealloyed and annealed states, are designed to have a final thickness of ≈1 µm so that substrate effects can be avoided during mechanical characterization in both the time and frequency domains. The as-dealloyed and annealed samples are investigated using a modified continuous stiffness measurement (CSM) technique that optimizes the ability to achieve steady-state harmonic motion, such that accurate phase angle measurements can be obtained; the as-dealloyed and annealed samples exhibit distinct phase angles of 1.9° and 2.6°, respectively. Observations made in the time domain suggest that the time dependence of nanoporous Si-Mg stems largely from plasticity. The reduced modulus values of as-dealloyed and annealed samples are investigated using the CSM technique and have corresponding values of 5.78 and 11.9 GPa, respectively. Similarly, the hardness of as-dealloyed and annealed samples are 167 and 250 MPa, respectively.