Sintering Behavior and Microstructure Analysis of Polycrystalline Monoclinic Zirconia

Zirconia transforms into different phases depending on the temperature, with the monoclinic phase being thermodynamically stable below approximately 1170°C. However, fabricating a dense m‐phase zirconia polycrystal is challenging due to the elevated temperature required for densification, which inevitably triggers the monoclinic‒tetragonal phase transitions. Upon cooling, the reverse tetragonal‐to‐monoclinic transformation occurs; this is accompanied by significant volume expansion and shear strain that induces crack formation and eventual ceramic fracture. In this study, we successfully fabricated monoclinic zirconia sintered bodies with a high relative density (approximately 92%) by combining cold isostatic pressing with optimal sintering conditions. Then, we investigated the microstructure of the polycrystalline sintered body, focusing on the atomic structures of grain and twin boundaries using atomic‐resolution scanning transmission electron microscopy. Under optimal sintering condition, the sintered body shows a high density of {110} and {100} twins in the grain interiors. These twin structures likely accommodated transformation strain, allowing the compacts to remain intact after sintering.