2, Frederick Seitz Materials Research Laboratory, Urbana, Illinois, United States
Nanocrystalline (nc) materials provide superior mechanical properties due to their unique microstructures. Fine grain size with significant grain boundary area results in enhanced grain boundary-mediated deformation but also suppressed dislocation activities, which leads to low ductility and limits their applications. Extensive theoretical models, simulations, experiments have been proposed and demonstrated that plastic deformation of nc materials can be via various mechanisms such as grain boundary sliding, grain rotation, twinning, etc. Previous experimental literatures mostly focused on nc metals. There is still a lack of experimental data on nc ceramics. Understanding plastic deformation mechanism of nc ceramics is crucial to improve ductility and achieve better toughness. Transition metal nitrides are widely used in industry due to their excellent mechanical properties, high chemical stability, and low electrical resistivity. Furthermore, they are capable of plastic deformation based on von Mises criteria. However, it is usually challenging to study ceramic plasticity owing to their vulnerability to intrinsic flaws. Micro/nanocompression techniques have emerged as a prominent way to study ceramic plasticity since small volume and compressive stress are beneficial to avoid premature fracture. Microstructural response of plastic deformation could be resolved in real time with the power of transmission electron microscopes.
In this study, in situ indentation in TEM was carried out to investigate plastic deformation mechanism of zirconium nitride nanopillar. Zirconium nitride nanopillars were fabricated by focus ion beam from the cross-section of nanocrystalline zirconium nitride hard coatings deposited by unbalance magnetron sputtering. The average grain size is around 20 nm. Scanning electron nanobeam diffraction was utilized to map grain orientation of the pillars. Indentation videos were recorded in bright field and diffraction mode. Frame by frame analysis combining with load-displacement curve was implemented to correlate microstructural activity and mechanical behavior.
In the preliminary results, we observed that zirconium nitride nanopillars showed ductile behavior. Grain rotation is suggested to be the main mechanism to accommodate applied strain. Discrete plastic events occurred in different grains and finally formed shear plane leading to fracture. Future work continues to study the mechanism behind grain rotation. Moreover, the nanopillars of different preferred orientation will also be involved to study the effect of texture on mechanical properties of nc ceramics.