Effect of phase fraction and grain size on translucency of 3 mol% yttria-stabilized tetragonal zirconia polycrystal

The optical property of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) is affected by various factors. This study aimed to evaluate the effects of phase fraction of zirconia polymorphs and grain size on the translucency of 3Y-TZP. Disk-shaped 3Y-TZP specimens were fabricated. Eight groups were established based on their final sintering temperature and holding time (1500°C-2h, 1550°C-2h, 1570°C-2h, 1570°C-5h, 1570°C-10 h, 1570°C-15 h, 1570°C-20 h, and 1600°C-2h). The sintered specimens were polished to a final thickness of 1.0 mm. The light transmittance was measured, crystalline phase was investigated, and grain size was calculated. In addition, the flexural strength, density, and tetragonality were measured. The translucency and grain size increased significantly, the tetragonal (t) phase fraction decreased, and the cubic (c) and tetragonal double-prime (t″) phase fractions increased as the sintering temperature increased. As the holding time increased, translucency was not significantly different among the groups; grain size significantly increased; t-phase fraction decreased; and the c- and t″-phase fractions increased. Strong positive correlations were observed among translucency, phase fraction, and grain size. Flexural strength was not significantly different among the groups; all groups showed an approximate full density, and their tetragonality increased as the sintering temperature and time increased. The translucency of 3Y-TZP is affected by the crystalline phase fraction and grain size. Increasing c- or t″-phase fractions and t-phase fractions above the critical level by suppressing other undesired phases with increased grain size would improve translucency without compromising the strength.