Predicting the integrity of metallic thin films deposited on semiconductors for microelectromechanical systems (MEMS) applications requires a precise understanding of surface effects on plasticity in materials with nano-sized grains. Experimentally, the use of nanoscale contact probes has been very successful to characterize the dependence of flow stress on mean grain size in nanocrystalline metals. From atomistic simulations, several models of plastic yielding for metal indentation have also been proposed based on the nucleation and propagation of lattice dislocations, and their interaction with grain boundaries beneath penetrating tips. However, model refinement is needed to include the characteristics of materials whose grain sizes are much smaller than the typical plastic zones found in contact experiments. Particularly, cooperative deformation processes mediated by grain boundaries, such as grain rotation, deformation twinning, and stress-driven grain coarsening, can simultaneously emerge for very small grain sizes (< 20 nm), thus making a predictive understanding of plastic yielding elusive.
Multiscale modeling. We have made recent progress in using multiscale modeling to gain fundamental insight into the underlying mechanisms of surface plasticity in nanocrystalline face-centered cubic metals deformed by nanoscale contact probes. Two numerical approaches to model contact-induced plasticity in nanocrystalline materials, the quasicontinuum method and parallel molecular dynamics simulation, have been utilized. Using these techniques, we have studied the role of a grain boundary network on the incipient plasticity of nanocrystalline Al films deformed by wedge-like cylindrical tips, as well as the processes of stress-driven grain growth in nanocrystalline films subjected to nanoindentation. We have shown for the first time that stress-driven grain growth can be simulated in the athermal limit by indentation of nanocrystalline Al films. The grain growth process was found to result from rotation of nanograins and propagation of shear planes via grain boundary sliding. We investigated in detail the role of the interatomic potential and misorientation angle between grains on stress-driven grain coarsening effects.
Atomic force microscopy-based indentation. Atomistic simulations can only be fully appreciated in light of direct nanoscale experiments. However, the determination of incipient plasticity mechanisms in nanocrystalline thin films with nanoscale contact probes is challenging experimentally, because very low forces (a few micro-newtons) can rapidly cause defect nucleation or migration of grain boundaries. For that purpose, we have developed a nanoindentation technique for extremely low force applications and hardness measurements in thin films and NWs using atomic force microscopy (AFM) diamond-tipped cantilevers. Specialty cube-corner diamond tips enable performing both indentation and in-situ surface imaging of residual indents, which greatly enhances the accuracy of measurements. Ultimately, we plan to obtain one-to-one comparisons between atomistic simulation and experimental data.