Grain size dependent indentation size effect and deformation mechanism in Y2O3 nanocrystalline ceramics

The indentation size effects and deformation mechanisms in nanocrystalline yttria (Y₂O₃) ceramics remain largely unexplored. Our work systematically studies the grain size dependent indentation size effect and plastic deformation mechanism of Y₂O₃ ceramics, with grain sizes varying from 18 nm to 10 μm. The grain size dependence of hardness shows an inverse Hall-Petch behavior at a critical grain size of ∼35 nm and below, together with an inflection in activation volume. For the Y₂O₃ ceramics in the classical Hall-Petch region, indentation size effect can be described by geometrically necessary dislocations based on the Nix-Gao model, while in inverse Hall-Petch region, the indentation size effect can be attributed to geometrically necessary shear transformation clusters. Further TEM analysis and numerical analyses support the findings that the unusual indentation size effect can be associated with the unique deformation mechanisms, including grain boundary sliding, grain rotation and grain boundary migration. Moreover, the deformation-induced grain rotation and grain boundary migration are found to result in grain growth in nanocrystalline Y₂O₃ ceramics. Our work sheds light on the unique mechanical behaviors of ceramics with extremely small grain sizes.