Modified calibration of m-ZrO2 fraction transformed from t-ZrO2 on low-temperature degradation of dental zirconia

This study examines the phase evolution in the low-temperature degradation (LTD) of 3 mol% Y₂O₃-stabilized zirconia (3YSZ) in air, establishing a correlation between LTD extents determined using Rietveld refinement and the traditional Garvie and Nicholson method. Fifty-nine 3YSZ specimens were sintered at 1500–1650 °C for 2–20 h and subsequently aged in air at 130–220 °C for up to 20 h. Phase analysis confirms the presence of multiple ZrO₂ polymorphs in 3YSZ, including monoclinic (m), tetragonal (t), tetragonal non-transformable-to-m (t″), cubic (c), and rhombohedral (r) phases, and a minor content of Y₄Zr₃O₁₂. LTD is primarily associated with the t″→r transformation, induced by compressive strain at grain boundaries, which disrupts the coherency between t- and c-ZrO₂, ultimately triggering the t→m phase transformation. This process is attributed to the 2.0 % volume expansion accompanying t″→r, imposing tensile strain on the t-ZrO₂ lattice. The resulting strain, along with the tensile strain induced by thermal expansion anisotropy, likely facilitates the migration of oxygen vacancies from Zr^4 + to Y³⁺ nearest-neighbor sites, leading to oxygen ion overcrowding around Zr⁴⁺ and contributing to the destabilization of t-ZrO₂. During LTD, a systematic decrease in t- and t″-phase content was observed alongside an increase in m- and r-ZrO₂ content, underscoring a strong correlation between LTD extent, measured as the weight fraction phase ratio m/(m + t) through Rietveld refinement, and the ratio m/(m + t + t″ + c + r) determined using the Garvie and Nicholson method. Based on this correlation, a calibration equation is proposed to convert LTD extents obtained using the traditional method into those derived through Rietveld refinement, simplifying the complexity of Rietveld analysis.