Nanoindentation responses near single grain boundaries in oxide ceramics

Abstract

Local mechanical responses near single grain boundaries (GBs) of 9.8 mol% Y 2 O 3 -stabilized ZrO 2 bicrystals, SrTiO 3 bicrystal, and Al 2 O 3 polycrystal were investigated by nanoindentation at room temperature to reveal single GB contributions to the mechanical properties. Contrary to the concept of the Hall-Petch strengthening, the hardness showed negligible variations among grain interiors, GB vicinities, and just on GBs in the samples. Importantly, the hardness may be underestimated owing to GB grooving in thermally etched samples. Despite the negligible change in intrinsic hardness, the transmission electron microscopy observation of the ZrO 2 specimen under impressions revealed dislocation pileups at GBs. These results suggest that single GBs of oxide ceramics prevented dislocation movement but with limited strengthening contributions. This should correspond to the macroscopic Hall-Petch behaviors in oxide ceramics, where the Hall-Petch coefficients normalized by friction stress are significantly smaller than those in metals.

The Role of Grain Boundaries in Low‐Temperature Plasticity of Olivine Revealed by Nanoindentation

“…This observation is in direct contrast with results from similar Berkovich experiments revealing no change in hardness across the surface of oxide bicrystals, although post-mortem STEM characterization also indicates dislocation pile-ups at the grain boundaries (Nakamura et al, 2023). We indicate that our observations in olivine and the ones in oxides presented by Nakamura et al (2023) are compatible, as olivine exhibits a more pronounced strain hardening effect compared to oxides. We suggest that at the 8% strain under the Berkovich indenter, dislocations in olivine are more mobile compared to the ones in oxides and the dislocation pile-ups at the high-angle grain boundary form earlier on during our experiments and generate a measurable signal.…”

“…We note that the hardness in the HAGB sample increases with increasing proximity to the grain boundary, and exhibits maximum values at a distance of approximately 5 μm (Figure 5b). This observation is in direct contrast with results from similar Berkovich experiments revealing no change in hardness across the surface of oxide bicrystals, although post-mortem STEM characterization also indicates dislocation pile-ups at the grain boundaries (Nakamura et al, 2023). We indicate that our observations in olivine and the ones in oxides presented by Nakamura et al (2023) are compatible, as olivine exhibits a more pronounced strain hardening effect compared to oxides.…”

“…In metals, similar observations using spherical nanoindentation on twin boundaries have been attributed to generation of dislocations at the twin boundary (J. Li et al., 2021). In ceramics, similar experiments with a Berkovich indenter reveal decreased hardness and pop‐in load at grain boundaries in zirconia (Lian et al., 2007), or display no change in hardness with distance from grain boundaries in different oxides (Nakamura et al., 2023). In addition, our experiments reveal that a low‐energy SB, comprised of arrays of periodic dislocations with [001] Burgers vector (Heinemann et al., 2005) is not a potent source of dislocations (Figure 3a and S9 in Supporting Information S1).…”

Determination of the controlling parameters for dislocation nucleation in SrTiO3: An investigation by nanoindentation

“…57 That is, the dislocation nucleation is possibly determined by the quantity and mobility of mobile dislocation sources. 54,58 For instance, for STO single crystal, the activation energies of oxygen vacancies are 0.62-1.2 eV, and the activation energies of the other intrinsic defects are larger than 1 eV. 30 This means that oxygen vacancy sites could work as one type of dislocation source.…”

Engineering dislocation‐rich plastic zones in ceramics via room‐temperature scratching

“…On the other hand, room-temperature deformation could minimize energy consumption, but it also casts a higher risk of fracturing the sample with experiments performed under unfavorable mechanical loading conditions. So far, the majority of works for room-temperature plastic deformation while suppressing crack formation in ceramics were performed at nano-/microscale using nanoindentation tests 10,11 or micropillar compression. 12 The former benefits from the extremely localized high shear stress (beneficial for dislocation gliding) and high hydrostatic compressive stress (beneficial for crack closure) underneath the indenter, while the latter usually has a large free surface area and fewer preexisting defects in the micropillars that avoid local dislocation pile-up to induce cracks.…”