Ferroelastic domain switching and toughening mechanism for T’-YSZ@Al2O3 core-shell structured ceramic materials

The metastable tetragonal (T’) phase yttria-stabilized zirconia (T’-YSZ) exhibits exceptional fracture toughness, offering potential for improving the insufficient toughness of high-temperature HfO₂-based thermal barrier coatings. In our prior work, a T’-YSZ@Al₂O₃ core-shell structure was developed to suppress ion diffusion between the T’-YSZ reinforcing phase and cubic yttria-stabilized hafnia (C-YSH) matrix, thereby stabilizing the phase structure and enhancing fracture toughness through ferroelastic toughening. Building on this foundation, we systematically investigated the influence of Al₂O₃ shell thickness on ferroelastic domain switching by fabricating T’-YSZ@Al₂O₃ ceramics with varying shell-to-core volume ratios (10 %, 20 %, 30 % Al₂O₃, denoted as CS10 %, CS20 %, CS30 %) via spark plasma sintering (SPS). Multiscale characterization (SEM, XRD, TEM, Raman) combined with FIB-assisted localized analysis and mechanical testing revealed that within a shell-core volume ratio range of 0–30 %, both hardness and elastic modulus increased monotonically with shell thickness. Notably, the fracture toughness peaked at 6.59 MPa·m^1/2 for CS20 % (shell-to-core ratio 0.2), representing an 18.5 % improvement over pure T’-YSZ. Beyond this threshold, excessive Al₂O₃ compromised toughness by reducing the T’-YSZ reinforcement content. Furthermore, the Al₂O₃ shell induced tensile stress within the T’-YSZ core, facilitating stress-driven ferroelastic domain switching. This interfacial stress engineering effectively enhanced domain switching and improving ferroelastic toughening efficiency.