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Abstract

Ferroelectric materials have the ability to convert electrical energy into mechanical energy and vice versa, which makes them widely used as sensors, actuators, and ultrasonic transducers. While their electrical and electromechanical properties are well studied, mechanical properties like Young’s modulus (E) and hardness (H) remain known to a lesser extent. Accurate knowledge of mechanical properties and determination of elastic and plastic parameters significantly contribute to more efficient modeling and prediction of the mechanical behavior of these materials.
In the doctoral thesis, we studied the elastic and plastic properties of perovskite oxide ferroelectrics and analyzed structural changes after mechanical loading down to the atomic level. The mechanical properties were determined by in-situ nanoindentation in a scanning electron microscope, which allowed us to target specific areas in the material (e.g., domains, domain walls (DWs)). Additionally, atomic force microscopy (AFM) was used to measure elastic properties. The results of mechanical analyses were complemented by in-depth studies of the structural and microstructural characteristics of the materials using electron microscopy.
In the first part of this thesis, we investigated the microstructural and mechanical properties of electromechanically active 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 thick films prepared by the aerosol deposition method before and after annealing in air. We demonstrated that additional heating triggers grain growth and pore redistribution, while the mechanical properties of the films, i.e., H and E, increase by ~16 % compared to films that were not additionally heated. The microstructural changes are also reflected in a greater dispersion of the indentation curves and in a higher frequency of pop-in events.
In the second part, we examined H and E, measured via nanoindentation, as well as the plastic deformation behavior of polycrystalline BiFeO3. In the force range between 200 μN and 2 mN, H and E decrease by ~37 % and ~8 %, respectively, with increasing force. The sequence of plastic deformation was revealed through the first pop-in analysis in combination with a variety of electron microscopy techniques, starting from homogeneous dislocation nucleation through the activation of the {110}pc<1͞10>pc slip system, followed by dislocation motion and multiplication into arrays, and ultimately leading to dislocation accumulation and grain subdivision.
In the final part, we used AFM techniques to determine the elastic properties of domains and non-180° DW in (K0.5Na0.5)NbO3 single crystal. The average E measured was ~130 GPa. The domains exhibited elastic anisotropy. At 90° DW, a difference in the elastic response was observed compared to the surrounding domains, whereas at 60° DW, such a difference was not observed. It is known that the measurement is influenced by both the intrinsic elastic properties of the DWs and extrinsic contributions, such as the influence of neighboring domains and the interaction of the DW with the AFM tip.