Investigation of anisotropic mechanical properties of textured KSr₂Nb₅O₁₅ ceramics via ab-initio calculation and nanoindentation

Anisotropic mechanical properties of KSr₂Nb₅O₁₅ (KSN) crystals were investigated through first-principles calculations based on density functional theory. These properties were experimentally verified via nanoindentation on textured KSN ceramics fabricated, using a reactive template grain growth method. Nanoindentation was performed in directions parallel and perpendicular to the [001] direction of samples consisting of highly oriented grains with tetragonal symmetry. Calculations revealed that Nb-O yields a relatively strong covalent effect and Nb-O octahedral distortions induce spontaneous polarization in the KSN crystal. The measured indentation modulus values concurred with the predictions, based on the calculated elastic constants, as indicated by an anisotropic ratio of ~10% between the 2 tested orientations. The hardness exhibited negligible anisotropy. However, the predictions revealed a pronounced anisotropy of the Young's modulus (ratio of ~40% between the [100] direction and a direction tilted by ~45° from the [001] toward the [100]).