Structural evolution, mechanical and thermal stability of 7-40 mol% yttria-stabilized zirconia: First-principles...

Yttria stabilized zirconia (YSZ) is extensively used in solid oxide fuel cells, thermal barrier coatings, and cutting tools owing to its excellent mechanical strength, thermal stability, and resistance to wear, oxidation, and corrosion. Present study employs first-principles calculations to investigate the effect of yttria (Y₂O₃) concentration on the structural, electronic, mechanical and thermal properties of YSZ. Crystal structures of YSZ were modeled using virtual crystal approximation (VCA). Crystal structure of YSZ ((Y₂O₃)_x (ZrO₂)_1-x) was stable as tetragonal, cubic and hexagonal for the 'x' values of 0.07, 0.12, 0.40 respectively. Electron localization function evaluation indicated enhanced Zr-O and Y-O covalent bond strength in the YSZ with increase in Y₂O₃ concentration. Increased Y₂O₃ concentration from 7 mol% to 40 mol% has reduced the density (6.11 - 5.48 g/cm³) and elastic constants C₁₁ and C₁₂ of YSZ and increased C₄₄ reflecting the significance of microstructural phase (tetragonal/cubic/hexagonal). Cubic YSZ (12 mol% Y₂O₃) exhibited more ductility, yet resistant to compression under uniform loads compared to the tetragonal (7 mol% Y₂O₃) and hexagonal (40 mol% Y₂O₃) YSZ. Whereas, hexagonal YSZ demonstrated the highest hardness, fracture toughness, and lowest minimum lattice thermal conductivity, indicating superior rigidity and stiffness with susceptibility to volumetric compression. All the three YSZ crystals reached Dulong-Petit limit of specific heat at a temperature of 1500 K indicating their thermal stability. Overall, this work provides a systematic evaluation of the effect of YSZ phases as a function of Y₂O₃ concentration and, for the first time, highlights the superior mechanical and thermal performance of hexagonal YSZ containing 40 mol% Y₂O₃.