07-19 pop-in behavior oxides

[Image above] Schematic illustration of the indentation size effect related to dislocation activation and/or crack formation in brittle ceramic oxides. The competing effect between dislocation plasticity and crack formation is closely dependent on the indenter tip radius and preexisting defects. Credit: Fang et al., Journal of the American Ceramic Society (CC BY 4.0)

As discussed in this month’s episode of Ceramic Tech Chat, which published last week, development of advanced microscopy techniques opened the door for modern scientists to finally see and explain the mechanisms behind phenomena known about for centuries, sometimes millennia, prior.

One key discovery that became accessible to scientists with microscopy developments was understanding why metals, unlike ceramics and glass, can deform without fracturing. In the 1950s, scientists used transmission electron microscopes to confirm the dislocation theory proposed independently by several scientists in 1934.

Dislocations, or line defects, are areas where atoms are arranged anomalously compared to the rest of the perfect crystal structure. When stress is applied, movement of these dislocations allows atoms to slide over each other, leading to plastic deformation of the material rather than brittle fracture.

In contrast to metals, which contain many dislocations that move easily, ceramics have far fewer and less mobile dislocations. There are both innate and process-related reasons for this difference.

For one, ceramic materials contain mostly covalent and ionic bonds, which are stronger than the metallic bonds found in metals. These strong bonds restrict movement of dislocations in ceramics, especially at room temperature.

The conventional sintering process for ceramics further limits possible ductility because the ceramic experiences grain coarsening, which lowers the number of dislocations in the structure.

Though there have been studies over the years looking at ceramic dislocations and their impact on functional properties, discovery of flash sintering in 2010 accelerated interest in this field.

Through application of an electric field, this relatively low-temperature sintering process can fully densify a ceramic within a few seconds, which limits grain coarsening and allows retention of dislocations—and thus improves a ceramic’s ability to deform plastically without fracture on the microscale. (Bulk deformation of flash-sintered ceramics still results in brittle fracture, and scientists are working to reveal the underlying mechanisms.)

While many recent studies focus on preserving dislocations through alternative processing routes (e.g., flash sintering, thin film deposition, bicrystal bonding), groups also are exploring the use of mechanical deformation to generate dislocations.

Compared to flash sintering, which can result in poorly controlled dislocation structures, mechanical deformation offers the ability to align dislocations in the sample along their slip planes. However, though scientists have used nanoindentation to achieve dislocation-mediated plastic deformation in many oxides at small scale, bulk deformation is challenging due to the likelihood of crack formation during deformation.

To avoid crack formation, researchers must better understand how dislocation occurs in oxides. ACerS member Xufei Fang is one researcher actively studying this question.

Clarifying the relationship between pop-in behavior and dislocations in oxides

Fang is an Athene Young Investigator at the Technical University of Darmstadt in Germany. His group studies dislocation behavior in oxides, specifically key factors of dislocation nucleation, multiplication, and motion.

In an email, Fang explains that before his work on ceramics, he studied hydrogen embrittlement and oxidation of metals as a postdoc at the Max Planck Institute for Iron Research. However, in October 2018, he saw an advertisement for the position of junior group leader in Jürgen Rödel’s ceramics group at TU Darmstadt.

“The topic ‘dislocation-based functionalities’ immediately caught my attention. I quickly did some literature survey and realized that this field has barely been set foot on. Because I had lots of research experiences in structural ceramics during my Ph.D. studies, and had also accumulated quite some knowledge and skills on dislocations during my first 3-year postdoc research on hydrogen embrittlement, I then said to myself, ‘Why not give it try?’” he says.

In the past year, Fang and his group published several open-access papers on the topic of mechanically generated dislocations in ceramics. The first one, published in March 2021 in Journal of the American Ceramic Society (JACerS), used pop-in behavior as the basis for exploration.

“Pop-in” refers to the sudden jump in displacement at a specific threshold load, appearing as a plateau in the loading curve, during nanoindentation. This jump signifies the elastic–plastic transition and indicates dislocation activation in metallic materials.

In ceramics, both dislocations and cracks typically are observed after indentation, making it difficult to clearly relate dislocation-governed plasticity with pop-in behavior.

Previous studies on ceramics indicate that using indenter tips with a smaller radius promotes dislocation generation while suppressing crack formation. However, a critical tip radius below which the pop-in event correlates only to dislocations without crack formation is not yet identified.

In the March 2021 study, Fang and colleagues from TU Darmstadt and the Max Planck Institute for Iron Research looked to clarify the competing mechanisms of dislocation-based plasticity and crack formation during indentation tests, which could help inform the critical tip radius.

They chose strontium titanate (SrTiO3), a prototype perovskite and well-known “ductile” oxide, as the model system for this study. They used three other advanced technology-relevant oxides—Al2O3, BaTiO3, and TiO2—to validate the generality derived from SrTiO3.

Through nanoindentation testing with different tip radii, the researchers successfully achieved pop-in behavior that could be solely attributed to dislocation-mediated plasticity. Notably, for extremely small tips, a critical load Pc (larger than the pop-in load P0) exists at which cracks start to initiate from the dislocation pileup.

“For instance, for a sharp Berkovich indenter (with an effective tip radius of R ≈ 100 nm), the pop-in load is about P0 ≈ 0.1 mN, while the load for inducing cracks can be as high as Pc ≈ 3 mN. For spherical tip (with an effective tip radius of R = 2 μm), the pop-in load is about P0 ≈ 6~7 mN, while the load for inducing cracks is Pc ≈ 11 mN,” they write.

In April 2022, the researchers published a follow-up open-access paper in Scripta Materialia. They used this knowledge concerning critical tip radius to generate dislocations without crack formation in TiO2, leading to local enhancement of electrical conductivity by 50% compared to dislocation-free regions.

In an open-access JACerS rapid communication article published in December 2021, Fang and colleagues from TU Darmstadt further expanded protocols for generating dislocations using nanoindentation.

With SrTiO3 as the model system, the researchers optimized parameters for a cyclic indentation procedure with a large Brinell indenter (diameter of 2.5 mm). After about 10 cycles of repetitive indentation on the same location, they achieved saturation of dislocation density of more than 1013 m−2 inside the crack-free plastic zone, with a diameter larger than 200 μm.

“The simplicity of this experimental approach merits its application for other room-temperature ‘ductile’ ceramics, as well as for various structural ceramics if this method is driven to high temperature,” they conclude.

In an email, Fang said his choice to publish this research in JACerS is due not only to its stellar reputation and his personal “wonderful” experiences with the editorial team, but also because for research institutes like his, their publications in JACerS can all be made open access through Projekt DEAL, “providing us a great channel to deliver and communicate our new findings to the whole world for free.”

The March 2021 open-access paper, published in Journal of the American Ceramic Society, is “Nanoindentation pop-in in oxides at room temperature: Dislocation activation or crack formation?” (DOI: 10.1111/jace.17806).

The April 2022 open-access paper, published in Scripta Materialia, is “Dislocation-enhanced electrical conductivity in rutile TiO2 accessed by room-temperature nanoindentation” (DOI: 10.1016/j.scriptamat.2022.114543).

The December 2021 open-access rapid communication, published in Journal of the American Ceramic Society, is “Mechanical tailoring of dislocation densities in SrTiO3 at room temperature” (DOI: 10.1111/jace.18277).

New lecture expands student knowledge of ceramic dislocations

In addition to research, Fang is looking to elevate knowledge of this research field among students through a newly developed lecture called “Dislocations in Ceramics.”

“This lecture was initiated in 2021 summer term together with my colleagues Lukas Porz, Till Frömling, Enrico Bruder, and Shuang Gao in the Department of Materials and Earth Sciences at Technical University of Darmstadt, with the great support of our group head, Jürgen Rödel,” Fang explains.

The semester-long course targets mainly masters students and aims to provide them with a systematic framework on dislocation-based research in ceramics (and semiconductors), covering materials, mechanics, characterization, and functional properties.

“Together with my lecture ‘Mechanical Properties of Ceramics,’ which covers only brittle behavior of ceramics in the conventional framework of ceramics teaching, these two courses make a perfect combination to offer a complete picture for the students, and this new lecture has been highly appreciated by the students,” Fang says.