In situ observation of comparative ZrO₂ role in binary Al₂O₃-ZrO₂ and ternary Al₂O₃-Er₃Al₅O₁₂-ZrO₂ eutectic microcantilever bending behavior

Phase-dependent Zirconia (ZrO₂) is known to improve the toughness of high-temperature eutectic ceramics significantly, but the microscopic toughening mechanisms are still not fully understood, especially regarding the different roles of its tetragonal and cubic crystal structures. Traditional macroscopic mechanical testing cannot isolate ZrO₂'s microscale toughening mechanisms within complex microstructures inside bulk ceramics. This study employed in situ microcantilever bending with SEM/TEM to directly probe fracture in binary Al₂O₃-ZrO₂ (t-ZrO₂) and ternary Al₂O₃-Er₃Al₅O₁₂-ZrO₂ (c-ZrO₂). This work found that tetragonal zirconia (t-ZrO₂) undergoes a stress-induced tetragonal-to-monoclinic (t-m) transformation at crack tips and slows crack propagation, accompanied by a tortuous path, and results in significant energy dissipation due to lateral deflections, increasing the fracture toughness by over 100% (from 2.7 to 5.5 MPa m^1/2). Conversely, cubic zirconia (c-ZrO₂) does not show obvious phase transformation, resulting in brittle fracture where cracks move quickly along phase boundaries or through Er₃Al₅O₁₂ phase, with no observed toughening effect. Microstructural analysis confirmed that t-ZrO₂ forms rod-like or “Chinese-script” structures, while c-ZrO₂ appears as submicron phase at the Al₂O₃ and Er₃Al₅O₁₂ interfaces. This work offers direct experimental evidence for the phase-dependent toughening mechanisms of zirconia at the submicron scale, offering crucial guidance for optimizing the phase states of next-generation eutectic ceramics and further enhancing their performance.