[Image above] Optical microscope image highlighting the slip traces induced by Brinell indentation (inset) on a polycrystalline strontium titanate (SrTiO3) sample at room temperature. Credit: Okafor et al., Journal of the American Ceramic Society (CC BY 4.0)
When taking on a stressful task, having a team to share the load with can be tremendously helpful. As anyone who has experienced burnout can attest, if too much pressure is put on a limited number of individuals, the group will eventually crumble under the strain.
Likewise, crystalline materials can experience fracture if they do not have a team of mobile dislocations and sufficient slip systems to accommodate loading. Dislocations are one-dimensional line defects within crystalline structures, and their presence allows adjacent atomic planes to slip past one another rather than breaking apart. Meanwhile, slip systems refer to a set of symmetrically identical atomic planes and the associated family of directions in which dislocation glide occurs.
Metals have a hearty team of mobile dislocations and sufficient slip systems, so they can easily accommodate stress through deformation. Ceramics, on the other hand, traditionally have a small team of sluggish dislocations and limited slip systems, so they will instead fracture when stressed.
Various methods exist for introducing more mobile dislocations into a ceramic’s atomic structure, which can bolster its ability to deform under stress. On the other hand, the limited number of independent slip systems at room temperature is a material property inherent in ceramics. This limitation does not hinder experiments on the nano and microscale, but it complicates the engineering of bulk polycrystalline samples.
According to the von Mises or Taylor criterion, which is used to predict yielding of materials under complex loading, polycrystalline materials need to have at least five independent slip systems to plastically deform without fracture. Anything less would result in dislocations piling up along the grain boundaries and initiating cracks.
In many ceramics, such as strontium titanate and most rock-salt-structured ceramics, there are six physically distinct slip systems at room temperature, but only two of them are independent. As such, effective slip transmission cannot take place at room temperature.
(Activating additional slip systems at higher temperatures is possible, but then other mechanisms, such as high-temperature creep, grain boundary sliding, and thermally aided diffusion, complicate deformation.)
In contrast to the von Mises or Taylor criterion, Groves and Kelly suggested that fewer independent slip systems do not necessarily condemn a material to fracture. The former guidelines were developed with the movement of interior grains in mind. But along the surface of a polycrystalline material, dislocations can avoid piling up through various mechanisms, so the von Mises or Taylor criterion can be relaxed.
In a recent open-access paper, researchers in Germany used cyclic Brinell indentation to explore how these dislocation mechanisms near and along the surface of polycrystalline strontium titanate allow it to successfully deform without fracture at room temperature.
Before testing polycrystalline samples, the researchers first performed cyclic Brinell indentation on single-crystal strontium titanate samples with (001), (011), and (111) surface orientations. Because a single crystal has no grain boundaries, a better understanding of the basic dislocation slip trace patterns can be gained prior to examination of the more complicated polycrystalline system.
Based on these experiments, which used testing parameters established in an earlier publication, the researchers discovered that cyclic indentation trigged several dislocation multiplication mechanisms, namely cross slip and Frank-Read sources, irrespective of surface orientation. In other words, indentation increased the number of dislocations in the sample, which helped accommodate the loading.
These multiplication mechanisms occurred in the polycrystalline samples as well. However, during the first indentation cycle, the researchers found that the movement and generation of dislocations along and within two grain boundary features, namely ledges and triple junction pores, were the main mechanisms promoting plasticity and preventing fracture. This finding is “counter-intuitive to most brittle ceramics at room temperature,” the researchers write, because grain boundaries and pores often contribute to rather than mitigate fracture.
The researchers expect these findings on the dislocation-enabled deformation mechanisms of polycrystalline strontium titanate to be transferrable to other ceramics that exhibit room-temperature plasticity. In addition, “…this work may pave the road for future studies on the geometrical contribution and grain size effect on dislocation–GB [grain boundary] interaction,” they conclude.
For readers interested in learning more about dislocation-tuned properties of ceramics, a special issue of Journal of the American Ceramic Society on this topic is being organized by senior author Xufei Fang, group leader of Dislocations in Ceramics and Hydrogen Micromechanics at Karlsruhe Institute of Technology. The deadline for submissions to the special issue is July 31, 2024. Learn more about the special issue at this link.
The open-access paper, published in Journal of the American Ceramic Society, is “Near-surface plastic deformation in polycrystalline SrTiO3 via room-temperature cyclic Brinell indentation” (DOI: 10.1111/jace.19962).