Investigation of the Bi-Linear Behavior of the Indentation Size Effects in Single and Polycrystalline Ni Thin Films/MEMS
- PDF / 3,352,844 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 95 Downloads / 183 Views
0976-EE06-21
Investigation of the Bi-Linear Behavior of the Indentation Size Effects in Single and Polycrystalline Ni Thin Films/MEMS Thin Films A. A. Elmustafa1, J. Lou2, Z. Zong3, and W. O. Soboyejo3 1 Mechanical Engineering and The Applied Research Center, Old Dominion University, 238 Kaufman Hall, Norfolk, VA, 23529 2 Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX, 77005 3 Princeton Institute of Science and Technology of Materials (PRISM) and Department of Mechanical, Princeton University, Princeton, NJ, 08544 Abstract This paper presents the results of an experimental study of the indentation size effects at the nano, sub-micron, and micron-scales. The size dependence of the indentation hardness in these regimes is shown to exhibit a bi-linear behavior when the data are fitted to Taylor Dislocation Hardening (TDH) model. The deformation in indent sizes corresponding to the onset of the transition from micro and deep nano indents to shallow nano indents represent dislocation substructures at the sub-micron and micron scales whereas the deformation at the nano-scale represents dislocation source-limited behavior. 1. Introduction Unlike large-load measurements in bulk material of hardness that exhibit no dependence on indentation size, hardness measurements at the micron- and nano-scales have been shown to exhibit significant size dependence [1-7]. The measured size dependence has been shown to depend largely on the role of geometrically necessary dislocations (GNDs) at the micron- and nano-scales [2-7]. The role of GNDs has been modeled by mechanism-based SGP theories that use a single material length scale parameter in the characterization of size effects at the micro- and nano-scale regimes [4, 7]. In the Strain Gradient Plasticity (SGP), the relationship between the flow stress in shear and the dislocation density of Taylor’s dislocation hardening model (TDH), is governed by a dislocation flow that is a combination of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). The SSDs scale with the effective strain and the GNDs scale with the strain gradient. A number of authors have concluded that the decrease in hardness with increasing depth results from an increase in dislocation density at decreased loads caused by the presence of GNDs [1-10]. Fleck and Hutchinson [11] constructed an overall effective strain which takes into account in addition to von Misses strain, the effect of curvature associated with spatial gradient of the material rotation using phenomenological theory of SGP. They subsequently modified the theory to include both rotation and gradient [12]. Gao et al., [13] and Huang et al., [14] developed a mechanism-based strain gradient theory part I and II, that discusses mesoscale plasticity, and Duan et al., [15] developed a model, an extension to Nix and Gao [4] model, with a non-uniform strain gradient field. In contrast to the conviction of the above mentioned researchers that accumulation of GNDs results in
harde
Data Loading...
