| AVS 60th International Symposium and Exhibition | |
| Accelerating Materials Discovery for Global Competitiveness Focus Topic | Friday Sessions |
| Session MG+AS+EM+NS+SA+SE+SP+SS+TF-FrM |
| Session: | Novel Synthesis Approaches and Innovative Characterization Techniques Coupled with Theory & Computations |
| Presenter: | P. Grutter, McGill University, Canada |
| Correspondent: | Click to Email |
How does stuff break? An important step in this process is inelastic deformation of the material, the formation of a first disclocation. We report on the for the first experiments which due to the small size and atomic scale control of the indenter allow a direct quantitative comparison with molecular dynamics simulations and state-of-the-art electronic transport theory.
We have studied the formation of the smallest permanent indentation in the Au(111) model surface by a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). We use field ion microscopy (FIM) to characterize the nanometer scale spherical apex of the W(111) indenter in UHV prior to the indentation experiments [1,2]. Knowledge of the indenter geometry is necessary to extract quantitative parameters such as contact pressures and stresses within the sample during indentation.
Traditional nanoindentation measures depth to high precision, but typically does not possess the force resolution (nN) to detect initial plastic events [3]. Indentation with standard AFM allows for excellent force resolution, but large piezo displacements required to load the contact with a soft cantilever hamper the extraction of true indentation depth because of quantitative optical beam deflection calibration issues (beam placement, sensitivity to mode shape, etc.) and piezo creep. I will describe how our set-up overcomes these limitations and allows us to quantitatively assess elastic and plastic behaviour in an indentation curve.
We report on the transition from elastic to plastic deformation in the indentation of Au(111). This is done by producing arrays of indentations to forces near the plastic yield point and examining the resulting force-displacement curves for both elastic and plastic indentation sites. Plasticity can be identified by features in the force displacement curves, such as the sudden displacement excursions of the tip (pop-ins), the work done by the indenter, and the sink-in depth measured at mild repulsive loads. These indicators of plasticity can also be correlated with the permanent impressions in the surface imaged by STM. The measured forces at the initial yield points correspond to shear stresses lower than those expected for the homogeneous dislocation nucleation. We suggest that heterogeneous nucleation involving surface effects and atomic scale indenter roughness is likely to play a role in the observed plastic behaviour.
[1] W. Paul et al. Nanotechnology 23, 335702 (2012) [2] D. J. Oliver et al., PNAS 109, 19097 (2012) [3] Minor et al., Nature Materials 5, 697 (2006)