Effect of Sintering Parameters on Physical Properties of Dental Zirconia

Objectives: This study aimed to scrutinize two-step sintered zirconia of 3Y, 4Y, and 5Y compositions to comprehend their optical and mechanical properties in comparison to their conventionally sintered counterparts.
Methods: Cold-isostatically pressed 3Y, 4Y, and 5Y zirconia discs (Ø18 x 3.5 mm) were sintered using a two-step sintering method at various first-step temperatures with a 1-min dwell time, followed by a second-step densification temperature of 1150°C with a 10-h dwell time. For reference, bisque-fired zirconia discs (Ø18 x 2 mm) were sintered at 1500°C with a 2-h dwell time at a heating/cooling rate of 5 °C/min. All sintered discs were characterized by the water displacement method for density measurement and X-ray diffraction for phase analysis. The discs were then polished to a 1-µm finish and Ø13 x 1 mm in size for translucency and strength measurements. Finally, the polished sections were thermally etched at 1100°C for 30 mins for SEM imaging and grain size measurement.
Results: A significant improvement of flexural strength (~ 300 MPa) was observed for 3Y, 4Y and 5Y using the two-step sintering method relative to their conventionally sintered counterparts. The small grain size and high tetragonal content in two-step sintered zirconia enhances the strength. The samples sintered by the two-step sintering method have a similar translucency parameter despite a lower cubic phase content compared to the conventionally sintered samples.
Conclusions: Enhancing the strength of zirconia is possible through proper sintering processes without compromising their esthetic properties. Our findings show that a two-step sintering process is effective in suppressing both the grain growth and cubic phase formation to enhance the flexural strength of zirconia while maintaining its high-translucency. An optimal two-step sintering temperature of 1500°C/1150°C helps to give a better clinical performance than the conventional sintering method.