http://manuscript.elsevier.com/S1359645422003032/pdf/S1359645422003032.pdf

Threshold damage mechanisms in brittle solids and their impact on advanced technologies

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. 

Thermally‐enhanced dislocation density improves both hardness and fracture toughness in single‐crystal SrTiO
 3

Aerosol deposition (AD) is a room-temperature film deposition method for the fabrication of scalable ferroelectric ceramic films on different substrates, which is particularly appealing for thick film energy storage applications. However, the electrical and mechanical properties of AD ferroelectric films are not yet satisfactorily understood. Here we report the dielectric, energy storage and mechanical properties of aerosol deposited BaTiO3 (AD-BT) thick films with nano-sized grains by combining macroscopic electric measurements with indentation tests. We find that thermal annealing is an effective way to improve dielectric permittivity and polarization of the AD-BT film, as well as to increase the hardness and Young's modulus of the film. However, crack formation in the annealed AD-BT film is promoted in comparison to the as-processed sample, suggesting that the interplay between the nano-sized grains and release of the internal stress during annealing may have major consequences for the mechanical properties and hence should be taken into consideration in application. This article is protected by copyright. All rights reserved 

Temperature‐induced changes of the electrical and mechanical properties of aerosol deposited BaTiO
 3
 thick films for energy storage applications

Abstract The potential of dislocations (line defects) in ceramics may have been greatly underrated until most recently. Promising proofs‐of‐concept have been demonstrated for dislocation‐tuned functional and mechanical properties, revealing a new research front for dislocations in ceramics for a wide range of potential applications. However, it is commonly known that ceramics are hard (difficult to deform) and brittle (easy to fracture), particularly at room temperature. It remains a great challenge to mechanically tailor dislocations in ceramics. To address this pressing bottleneck, this article discusses the mechanics‐based dislocation engineering in ceramics by examining the three fundamental factors of dislocation nucleation, multiplication, and motion. Successful experimental approaches to tune dislocation density and plastic zone size on single‐crystal strontium titanate are demonstrated. The dislocation‐based competition between plastic deformation and crack formation is discussed. The aspects of coupling external fields to manipulate dislocations are highlighted, which may hold the key to modulating the charged dislocation cores in ceramics and opening new routes for mechanical tailoring of dislocations at room temperature. Some open questions and challenges for engineering dislocations in ceramics are discussed. 

Mechanical tailoring of dislocations in ceramics at room temperature: A perspective

High-tech ceramics are typically engineered by controlling point defects and interfaces while one-dimensional dislocations have found much less attention so far. Nevertheless, their impact on almost any functional property and the sudden ease to fabricate them with novel synthesis methods, such as rapid densification, brings spotlight attention to dislocations as design dimension. Although the typical brittleness of ceramics insinuates the irrelevance of dislocations for mechanical behavior, abundant literature is spread over materials, decades, and disciplines, however, often unconnected. Here, a conceptual framework for dislocation mechanics in ceramics separated into five logical aspects, (1) fundamental mobility, (2) obstacles to motion, (3) limitation by necessity of nucleation, (4) motion complexity, and (5) avoidance of competing mechanisms, brings oversight into the complex behavior. In consequence, quicker identification of the limiting step allows to estimate the potential involved more precisely and to identify open research questions with greater ease. An introduction to dislocations and the functionality involved, a look back over the last six decades, and highlights of open question are dedicated to provide a framework and bridge the gap between the attached research fields. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/ces2.10150

60 years of dislocations in ceramics: A conceptual framework for dislocation mechanics in ceramics

Most oxide ceramics are known to be brittle macroscopically at room temperature with little or no dislocation-based
plasticity prior to crack propagation. Here, we demonstrate
the size-dependent brittle to ductile transition in SrTiO3 at room temperature using nanoindentation pop-in
events visible as a sudden increase in displacement at
nominally constant load. We identify that the indentation pop-in event in SrTiO3 at room temperature, below a critical indenter tip radius, is dominated by dislocation-mediated
plasticity. When the tip radius increases to a critical size, concurrent dislocation activation and crack formation, with the latter being the dominating process, occur during the pop-in event. Beyond the experimental examination and theoretical justification presented on SrTiO3 as a model system, further validation on α-Al2O3, BaTiO3, and TiO2 are briefly presented and discussed. A new indentation size effect, mainly for brittle ceramics, is suggested by the competition between the dislocation-based plasticity and crack formation at small scale. Our finding complements the deformation mechanism in the nano-/microscale deformation regime involving plasticity
and cracking in ceramics at room temperature to pave the road for dislocation-based mechanics and functionalities study in these materials.

/pdf/nanoindentation-pop-in-in-oxides-at-room-temperature-1wxiiopfma.pdf

Nanoindentation pop-in in oxides at room temperature: Dislocation activation or crack formation?

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Nanoindentation pop-in in oxides at room temperature: Dislocation activation or crack formation?