Phase transformation and biaxial flexural strength of additively and subtractively manufactured zirconia: Effect of grinding and regeneration firing.

Statement of problem: Grinding is often required for the clinical adjustments of zirconia restorations; nevertheless, the effects of following regeneration firing processes on additively manufactured zirconia are still unclear.

Purpose: The purpose of this in vitro study was to evaluate the effects of grinding and regeneration firing (RF) on the phase transformation and biaxial flexural strength (BFS) of additively or subtractively manufactured 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP).

Material and methods: A total of 108 disk-shaped (Ø15×1.5 mm) 3Y-TZP specimens (n=54) were fabricated using either subtractive manufacturing (SM) or additive manufacturing (AM) techniques. Grinding was performed using a 150-μm-grit diamond rotary instrument with a high-speed handpiece, and RF was carried out at 1000 °C for 15 minutes in a sintering furnace. The crystalline phases were analyzed using X-ray powder diffraction (XRD). All specimens were loaded until fracture using a universal testing machine for a biaxial flexural strength test at a crosshead speed of 1 mm/minute. Microstructure and fracture surfaces were examined using scanning electron microscopy. BFS data were statistically analyzed using 1-way ANOVA, followed by the Tukey post hoc test for pairwise comparisons. (α=.05). The Weibull modulus and characteristic strength were calculated to assess the reliability of strength data.

Results: Tetragonal-to-monoclinic phase transformation was detected in ground specimens of both AM and SM groups, whereas only the tetragonal phase was observed in their corresponding RF groups. The highest BFS (1427.2 MPa) was observed in ground AM specimens (P<.001). Grinding and RF did not significantly affect the BFS of SM zirconia (P=.927 and P=.999, respectively) but significantly increased the BFS of AM zirconia (P<.001). RF increased the Weibull modulus in both AM (from m=6.98 to 12.25) and SM (from m=5.91 to 8.35) zirconia.

Conclusions: The implementation of RF after grinding is crucial, particularly for AM zirconia, to improve mechanical strength and structural reliability.