In this study, we propose an innovative mechanical observation protocol of nanoparticles in the 100 nm size range. It consists of in situTEM nano-compression tests of isolated nanoparticles. Load–real displacements curves, obtained by Digital Image Correlation, TEM images (BF, DF and WBDF) are analyzed and these analyses are correlated with Molecular Dynamics simulations. Elementary process that governs the deformation mechanism of nanoparticles can be identified. A constitutive law with the mechanical parameters (Young modulus, Yield stress…) of the studied material at the nano-scale can be obtained.

In situ TEM nano-compression tests were performed on ceramic MgO nanocubes. Magnesium oxide is a model material and its plasticity is very well known at bulk. The MgO nanocubes show large plastic deformation, more than 50% of plastic strain without any fracture. Calculations of Schmid factors of possible slip systems in MgO under solicitation direction coupled with analysis of WBDF images, performed in situ in TEM nanocompression tests, contribute to full characterizations for dislocations in MgO nanocubes under uniaxial compression. Correlation of TEM images and stress-strain curves, obtained by DIC, allows the observation and description of dislocations activities and processes along the compression test. Coupling these analyses with MD simulations, the elementary process that governs the deformation mechanism of single crystal MgO nanocubes under uniaxial compression could be identified. In Figure 1, contrast appears in the cube when a change on the curve is observed. This contrast band may be attributed to a ½ dislocation that nucleate at surface and slip along {110} plan as obtained by MD calculations and by TEM analysis on possible dislocations in active slip systems near the diffraction condition in these TEM observations (as we are always near [001] zone axis) as shown in Figure 2.

Size-effect on dislocation processes could be obtained in MD simulations and in experiments. MD results show that in MgO nanocubes smaller than 8 nm, the deformation occurs through dislocation nucleation at surfaces and edges/corners and dislocation starvation process is observed simultaneously with stress drop, as shown in Figure 3 (snaps 1, 2 & 3). However larger nanocubes show dislocation interactions and junctions formation rather than dislocation starvation as shown in Figure 3 (snaps 4 & 5). Experimental results show that these two processes co-exist in MgO nanocubes in the size range [60-450] nm. However, TEM images and stress-strain curves show that there is predominance of dislocation starvation mechanism in smaller nanocubes (Figure 4 show a WBDF of a large nanocube after compression where persistent dislocations and dislocations networks assume that dislocation interactions process predominate in larger nanocubes rather dislocation starvation. 

 

The authors thank the Centre LYonnais de Microscopie (CLYM) for financial support and access to the JEOL 2010F microscope. Financial support from the Région Rhône-Alpes is also acknowledged.

 

Keywords: In situ TEM, plastic deformation, dislocations, ceramic nanoparticles, MgO nanocubes