Chek Hai Lim, Sonaj Vardhaman, Niyati Reddy, Yu Zhang
Department of Preventative and Restorative Sciences, University of Pennsylvania, School of Dental Medicine
Introduction
This study aimed to study the effect of sintering parameters on optical and mechanical properties of two-step sintered zirconia of 3Y, 4Y, and 5Y compositions in comparison to their single-step sintered counterparts.
Methods
Zirconia discs (Ø18 x 3.5 mm) were cold-isostatically pressed and sintered using a two-step sintering method by varying first-step temperatures with a fixed dwell time of 1 min, 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 single-step sintered at 1500°C with a 2-h dwell time at a heating/cooling rate of 5 °C/min. All sintered zirconia discs were characterized by the Archimedes 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 min for SEM imaging and grain size measurement.
Results
Two-step sintered zirconia (3Y, 4Y, and 5Y) has improved significantly in flexural strength when compared to single-step sintered counterparts. Analysis of two-step sintered zirconia indicated that both small grain size and low tetragonal content increased the strength. Also, two-step sintered zirconia showed lower cubic content and was able to maintain their translucency parameter when compared to single-step sintered zirconia.
Conclusion
In this study, we found that a two-step sintering method is effective in improving the flexural strength of zirconia by suppressing both grain growth and cubic formation without compromising its translucency. An optimal sintering condition for a two-step sintering temperature of 11500°C/1150°C helps to give a better clinical performance without undermining its esthetic properties.