Investigation of Nanoindentation Size Effect Using Atomistic Simulation
Date Issued
2011
Date
2011
Author(s)
Chan, Chih-Yung
Abstract
The Indentation size effect (ISE) is a phenomenon usually observed from the relation between indentation hardness and penetrated depth in micro and nano scale (Nix and Gao, 1998). In general, this phenomenon can be modeled by the theory based on geometrically necessary dislocations (GNDs) within the plastic zone underneath the contact area which was caused by indentor. However, previous studies have discussed this effect merely on the micro-indentation but the phenomenon at the nano scale. And the theory about the nano scale has not been established as accurately as about the micro scale. Thus, how these thin film materials act at nano scale need to be observed and discussed. In this thesis, the computer simulation to process a nanoindentation test with spherical indentor has been used to study the nanoindentation size effect (NISE) in nickel material. The theory of geometry necessarily dislocation was also examined by comparing with the results from molecular simulation.
A static version of classical molecular dynamics has been used to study the nanohardness and dislocation activities during the process of nanoindentation. For metals, the interatomic potential was described by the embedded atom method (EAM). With regards to EAM, the interatomic potential contains not only the pair potential between two atoms but also the embedded energy induced by the local density of electrons surrounding the atoms of interest.
To simulate indentation process, first of all, a purely repulsive force relationship between spherical indenter and thin films was implemented to model the indentation process instead of modeling the indentor by atoms directly. Secondly, the mean contact pressure, obtained by dividing the indenter load by the projected contact area, characterizes the material strength beneath the indenter. Third, the GNDs calculated from the simulation systems, combined slip vector method and bond angle distribution method to extract the GNDs from the dislocations information of simulation models. The indentation simulation model consists of a thin film specimen and an indenter. The specimens were composed of face-centered cubic (FCC) crystalline Ni atoms with a size of 1000 A×1000 A×200 A. These models contained over 40 million atoms.
From the simulation results, it is observed that the hardness is decreased when indenter size increased, which means the ISE phenomenon is possible to observe in the nano scale. In order to examine the ISE phenomenon in the nano scale, we extract the GNDs from the simulation results. It is observed that the GNDs are declined with the indenter radius. This result imply that the GNDs is still affect the hardness in the nano scale and it is reasonable to use the GNDs theory to interpret the ISE phenomenon.
The simulation of nanoindentation on single crystal Ni to observe the ISE phenomenon is succeed to observe the ISE phenomenon in the nano scale. The hardness of material has been decreasing with the increasing of the size of spherical indentor. However, the theory model of Nix and Gao will overestimated the hardness because of the effective plastic zone range is larger than the volume confined by the volume of idealized radius. A new scale of plastic zone is proposed in this paper to successfully explain the GND model from the results of atomistic simulation.
Subjects
MD
Nanoindentation
GNDs
Indentation Size effect (ISE)
Type
thesis
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