Compared to studies on the electrical and electromechanical properties of bulk ferroelectric materials, studies on their mechanical properties are a rarity. Brittleness is a common denominator of all ferroelectric materials fabricated under high-temperature conditions and can be a limiting factor when it comes to applications where these materials are subjected to temperature changes and external stresses. Recently, a study discussed in detail the causes of brittleness in these materials and methods to improve crack resistance, such as by introducing dislocations after sintering [1]. Defects at the micro, nano, and atomic levels can thus affect the final mechanical properties, requiring investigations at several length scales, since ferroelectric materials are complex and can consist of grains, smaller units such as domains and domain walls, i.e., regions of uniform spontaneous polarization and the interfaces separating them, as well as defects.
In this contribution, the local mechanical properties of lead-free ferroelectrics, namely BiFeO₃ ceramics and (K, Na)NbO₃ single crystal, measured at the micro- and nanoscale using in situ nanoindentation and atomic force microscopy techniques, are correlated with their local properties. Imaging and analytical electron microscopy techniques are used to disentangle their microstructural and structural complexity down to the atomic level. We show that both materials exhibit similar properties such as hardness and the Young's modulus and have good crack resistance at low forces (below 1 mN). Particular attention is paid to domain walls, which, first, have recently been reported to be mechanically distinct from the matrix [2], and, second, can respond dynamically to external mechanical fields,  such as the stress applied during indentation.