Effect of sintering speed and surface treatment on the flexural strength of 3Y-PSZ and 4Y-PSZ zirconia: an in vitro study

The mechanical performance of translucent zirconia is critical to the long-term success of monolithic restorations, particularly under speed sintering and surface modification protocols. Understanding how these factors interact is essential for clinical durability. This in vitro study aimed to evaluate the effect of sintering speed and surface treatments on the flexural strength of 3 mol% and 4 mol% yttria-partially stabilized zirconia (3Y-PSZ and 4Y-PSZ), before and after cyclic fatigue loading. A total of 160 Bar-shaped specimens were fabricated from 3 mol% and 4 mol% yttria-partially stabilized zirconia (3Y-PSZ and 4Y-PSZ) and sintered using either conventional or speed sintering protocols. Each group received one of four surface treatments: polishing (control), airborne-particle abrasion with 50 µm glass beads, alumina abrasion, or grinding (n=20). Flexural strength was measured using a 4-point bending test. Half of the specimens were tested in their initial state, while the other half underwent 2 million cyclic loading prior to testing to assess residual strength. Weibull statistics assessed reliability; microstructure was evaluated by SEM; ANOVA with Bonferroni post hoc tests determined significance (α = 0.05). Glass bead treatment significantly improved the flexural strength of 3Y-PSZ (p < 0.001), while grinding significantly reduced it in both zirconia types (p < 0.001). Speed sintering did not affect 3Y-PSZ (p = 0.10) but significantly decreased the strength of 4Y-PSZ (p < 0.001). After cyclic loading, 3Y-PSZ maintained stable strength (p = 0.082), whereas 4Y-PSZ exhibited a significant reduction (p < 0.001). The highest Weibull modulus values were recorded in polished 4Y-PSZ (12.8) and glass-bead treated 3Y-PSZ (12.9). 3Y-PSZ is more suitable for high-stress clinical applications and can tolerate speed sintering and cyclic loading better than 4Y-PSZ, which required careful selection of sintering protocols and surface treatments to minimize reduce in its strength.