Length-scale-based hardening model for ultra-small volumes
- PDF / 1,968,723 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 202 Views
D.F. Bahr Department of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164
N.R. Moody Microsystems and Materials Mechanics, Sandia National Laboratories, Livermore, California, 94550
J.W. Hoehn Seagate Technology LLC, Minneapolis, Minnesota 55435 (Received 15 October 2003; accepted 12 April 2004)
Understanding the hardening response of small volumes is necessary to completely explain the mechanical properties of thin films and nanostructures. This experimental study deals with the deformation and hardening response in gold and copper films ranging in thickness from 10 to 400 nm and silicon nanoparticles with particle diameters less than 100 nm. For very thin films of both gold and copper, it was found that hardness initially decreases from about 2.5 to 1.5 GPa with increasing penetration depth. Thereafter, an increase occurs with depths beyond about 5–10% of the film thickness. It is proposed that the observed minima are produced by two competing mechanisms. It is shown that for relatively deep penetrations, a dislocation back stress argument reasonably explains the material hardening behavior unrelated to any substrate composite effect. Then, for shallow contacts, a volume-to-surface length scale argument relating to an indentation size effect is hypothesized. A simple model based on the superposition of these two mechanisms provides a reasonable fit to the experimental nanoindentation data.
I. INTRODUCTION
With the current utilization of microelectromechanical systems (MEMS) technology for optical switching and nanoparticle slurries for chemo-mechanical polishing, the need for mechanical characterization of small volumes has increased dramatically. This is even more critical to nanoimprint lithography and other emerging technologies where films and structures may only be a few tens of nanometers in scale.1 Yet the determination of the dominant length scales in the deformation of these small volumes is problematic due to evolutionary defect structures. For example, the defining length scale in a low dislocation density single crystal may be the plastic zone size or longest slip band length in the early stages of deformation but becomes truncated by dislocation cell walls at much larger plastic strain. For a thin film or nanocrystalline solid, the grain size may intervene at
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0384 2812
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 10, Oct 2004 Downloaded: 22 Apr 2015
relatively small strains. On the other hand, for thin epitaxial films or single-crystal nanoparticles, rather than cell walls or grain boundaries, the important length scale may be the nearest external interface. Of interest in this investigation was how the hardening response of small volumes was dictated by sample geometry (and the associated dominant length scale) in conjunction with an evolutionary defect structure. As such, previous single crystal and nanosphere deformation studies are drawn
Data Loading...
