Effect of sintering schedule on the flexural strength of 3Y-TZP

Objective: evaluate the effect of sintering schedule on the density and flexural strength of zirconia (3Y-TZP) for dental applications. Methods: Cylindrical zirconia blanks were sectioned into discs (1.5x25mm; n=15 per group) and sintered following seven two-step or three-step sintering schedules at temperatures in the range 1150 to 1550șC. A control group was sintered at 1350șC for 2h according to manufacturer's recommendations. Density was measured by Archimedes' method. The mean biaxial flexural strength was determined in air using a ball on ring-of-balls fixture on a universal testing machine. The flexural strength data was analyzed using Weibull statistics. The microstructure was investigated by SEM and AFM. The crystalline phases were analyzed by XRD. Results: Density varied from 98.83±0.09 to 99.96±0.06 of theoretical density depending on the sintering schedule. There was no significant difference in density amongst groups compared to the control group, with the exception of the group sintered at 1150șC for 2 hours with a two-step schedule, which had the lowest density. Heating rates had the most significant effect on final density. XRD showed the presence of tetragonal zirconia and no change in crystalline phases regardless of the sintering schedule compared to the control. The mean flexural strength varied from 1026.6±139.4 MPa for the control group to 1275.9±159.0 MPa for the group sintered using a three-step schedule with a third dwell at 1150șC for 2h. ANOVA and Tukey's test showed that five of the experimental groups had significantly higher flexural strength than the control group (p≤0.013). Conclusion: multiple-step sintering of 3Y-TZP led to a significant increase in flexural strength with no significant change in density compared to the control group.