Joel Lefever1 Jyo Lyn Hor1 Daeyeon Lee1 Robert Carpick1

1, Univ of Pennsylvania, Philadelphia, Pennsylvania, United States

The mechanics of failure in disordered materials remain inadequately understood. As these materials are deployed in applications including microelectromechanical systems, electrochemical devices, and nanoprinting, an improved understanding of failure, including factors controlling localization vs homogeneous plastic flow, is necessary. In particular, the mechanisms of plasticity may change under the many environmental conditions in which these devices must be able to operate. Furthermore, the sites where plastic rearrangements occur, and the nature of the ensuing flow, may vary depending on the material. Plastic deformation in disordered materials such as bulk metallic glasses is proposed to originate through rearrangements occurring at soft spots [1]. Evidence suggests that an analog to soft spots can be found in other disordered materials, including packings of colloids [2] and nanoparticles [3]. In this work, we examine the mechanical behavior of disordered nanoparticle packings formed by layer-by-layer assembly. We use atomic force microscopy-based nanoindentation, which provides information about the physics of the nanoparticle packings, with indentations applied to individual nanoparticles. These measurements reveal multiple load drops similar to the pop-ins observed previously in metallic glass [1], which we propose correspond to the activation and propagation of local rearrangements. This hypothesis is supported by topographic imaging of indents with individual particle resolution. The magnitudes of these load drops consistently follow an exponential distribution, which stands in contrast to the power-law relationship often observed in granular materials exhibiting slip avalanches. We propose that the exponential distributions observed here are the result of the small volume being investigated, which does not permit fractal plasticity mechanisms to occur. Furthermore, the relative humidity affects the number of load drops, with significantly more load drops occurring at humidity levels close to saturation. This indicates that tuning the ambient conditions may be a convenient means for using the mechanical response of this material system to simulate many different types of disordered materials. As well, this helps demonstrate how nanoparticle packings in applications may respond applied stresses under variable environments.

[1] Li, W., Bei, H., Tong, Y., Dmowski, W., & Gao, Y. F. (2013). Applied Physics Letters, 103(17), 171910.
[2] Strickland, D. J., Huang, Y.-R., Lee, D., & Gianola, D. S. (2014). PNAS, 111(51), 18167–72.
[3] Lefever, J. A., Jacobs, T. D. B., Tam, Q., Hor, J. L., Huang, Y.-R., Lee, D., & Carpick, R. W. (2016). Nano Letters, 16(4), 2455–2462.