Thermally enhanced dislocation density improves both hardness and fracture toughness in single‐crystal SrTiO₃

Abstract

Dislocation‐tuned functionality in ceramic oxides for potential versatile applications gains increasing attention. As the widespread chemical doping suffers from poor temperature stability, dislocations in well‐controlled mesoscopic structure may be an alternative to thermally stable intrinsic doping features. To this end, the dislocation density in plastic zones introduced by cyclic Brinell indentation is considered under thermal annealing conditions. The considerably enhanced dislocation density due to thermal treatment is found to impact both microhardness and fracture toughness, albeit only to a modest degree. The mechanistic understanding centers around enhanced mobility and multiplication of the pre‐engineered dislocations at elevated temperatures driven by the residual indentation stress, as well as the strengthened interaction of point defects and dislocations at high temperature.

Dislocation toughening in single‐crystal KNbO3

“…It is worth mentioning that an annealing step in a conventional oven prior to the Vickers indentation experiments, as in the case of SrTiO 3 by Salem et al., 20 turned out to be very challenging due to both the low thermal conductivity of KNbO 3 and the phase transitions leading to strain incompatibility in the sample. Under controlled heating and cooling rates of 1–2°C min −1 , the temperature profile during the high‐temperature indentation resembles a typical annealing step.…”

“…28,30 Note that the indentation crack length (ICL) method for quantifying fracture toughness has been subject to prolonged discussion. 31 As a Palmqvist crack system is expected for simple radial cracks in plastically deformable ceramics (same in single-crystal SrTiO 3 20 ), the Niihara 32 model was adopted to quantify the K Ic values based on our crack length analysis, with other further ICL methods for comparison.…”

“…Furthermore, Salem et al. have reported a ∼20% increase of fracture toughness in SrTiO 3 by introducing a high dislocation density via cyclic Brinell ball indentation and subsequent annealing of the crystals at 1100°C 20 …”

“…18 Issa et al demonstrated the size effect in the induced brittle to ductile transition of silicon and the resulting toughening effect. 19 Furthermore, Salem et al have reported a ∼20% increase of fracture toughness in SrTiO 3 by introducing a high dislocation density via cyclic Brinell ball indentation and subsequent annealing of the crystals at 1100 • C. 20 One of the requirements for effective dislocation toughening, as well as crystal plasticity in general, is a high density of dislocations that are mobile. 18 In the case of metals, the crack tip itself emits dislocations, which is scarcely observed in ceramics at room temperature.…”

Misfit-Strain Phase Diagram, Electromechanical and Electrocaloric Responses in Epitaxial PIN–PMN–PT Thin Films

“…However, the influences of the misfit strain on the phase structures, electromechanical properties, and electrocaloric response in the epitaxial PIN–PMN–PT thin films have been lacking, which hinders the corresponding experimental studies. Notice that the misfit strain in the films is caused by the mismatch of lattice between the substrate and the film, which can be relaxed by a defect [ 25 , 26 , 27 , 28 , 29 ], such as a dislocation in the thicker films [ 30 ]. Thus, this article employs a nonlinear thermodynamic analysis to establish the misfit strain–temperature phase diagram for PIN–PMN–PT (26PIN–42PMN–32PT) thin films, which the stoichiometric composition are described by atomic%, from which the influences of the misfit strain on the phase structures, electromechanical properties, and electrocaloric responses are studied, offering a guidance for PIN–PMN–PT thin film experiments.…”