Effects of Sintering Holding Time on Mechanical and Optical Properties of Zirconia

Objectives: Holding time at sintering temperature has a strong effect on properties of full contour zirconia. The objective of this study was to assess the mechanical and optical properties of several full contour zirconia materials with various holding times.
Methods: In this study, three full-contour zirconia materials (BruxZir, KDZ Bruxer, and CAP FZ) were used. The ceramic specimens were cut and sintered according to manufacturers’ instructions and with 3-hour and 6-hour holding time at the recommended sintering temperature to achieve a final dimension of 20 mm x 1.8 mm x 5 mm. The flexural strength of each group was evaluated using three-point-bending test (MTS Sintech ReNew 1123, MTS System) with a loading rate of 1 mm/min. All samples were then tested for microhardness (M-400 Hardness Tester) using a Knoop indenter at a load of 300g and 15 seconds dwell time. Finally, grain size in each group was observed under microscope after samples were thermal-etched at 147°C and the averaged grain size for each group was measured using Image J software. Additional samples of 12 mm x 12 mm x 1 mm were fabricated from each group and the translucency parameter in each sample was performed by a spectrophotometer device (CM-2600D). Percent light transmission through the thickness of the samples was also measured.
Results: There was statistical difference in the flexural strength of the materials. Longer holding times increase the flexural strength and microhardness for BruxZir and KDZ Bruxer groups, but not the CAP FZ group. There was statistical significant increase in grain size with longer holding time for all materials. Finally, increased translucency was observed in CAP FZ with increased holding time.
Conclusions: It is possible to tailor the mechanical and optical properties of full-contour zirconia through holding time during sintering.