Multimillion Atom Molecular Dynamics Simulations of Adhesion and Nanoindentation of Silicon Nitride.

Multi-million atom molecular dynamics (MD) simulations of nanoindentation of silicon nitride and amorphization and fracture of silicon diselenide nanowires are performed on massively parallel architectures. System sizes range from 75,803 atoms to over 10 million atoms. The process of amorphization and fracture in silicon diselenide nanowires is followed using configuration space images in smaller wires and local stress and temperature calculations in larger wires. A structural transformation in the nanowire cross sections is investigated. The nanowires are found to contract in one direction perpendicular to the applied strain while expanding in the other, so that initially circular nanowires are transformed to elliptical shapes. 10 million atom MD simulations of nanoindentation of amorphous and crystalline ((0001) surface) silicon nitride are performed. Load-displacement curves are calculated, from which the theoretical hardness and elastic moduli are inferred. These data compare well with currently available experimental data and previous theoretical calculations. Temperature, indentation size, and load-rate dependence are investigated. Local pressure distributions are investigated in the indented films and interpreted with the aid of configuration space images. Brittle indentation fracture is observed in the crystallographic direction. The fracture is more ductile. Pile-up material forms on the surface of the indented films and is maximized along indenter edges and suppressed under the indenter comers. Local bond angle analysis shows that plastically deformed material under the indenter and on the surface is amorphous. Therefore, the mode of plastic deformation in the crystalline silicon nitride nanoindentation simulation is amorphization, which is arrested by either cracking at the indenter comers, or piling-up of material along the indenter edges. Surface adhesion and plastic deformation threshold in silicon nitride are investigated and are related to the onset of hysteresis in load-displacement curves for near-surface contact and small indents. These investigations demonstrate the feasibility of using MD methods to investigate nanoindentation processes in ceramic materials, and to our knowledge, they are the first MD simulations of nanoindentation of a ceramic material having dimensions comparable to the capabilities of modern experiments. These are also the first MD simulations of nanoindentation to observe indentation fracture