Effect of polishing sequence on the surface roughness and flexural strength of additively or subtractively manufactured zirconia.

Gülce Çakmak and Burak Yılmaz contributed equally to this study and share senior author position.

Purpose
Achieving optimal surface roughness and flexural strength for zirconia, regardless of whether it is produced using additive or subtractive manufacturing, remains a challenge. Moreover, the effect of polishing protocol timing, relative to sintering, on those properties is not comprehensively known yet. The purpose of the study was to evaluate the effect of the timing of polishing, considering the sintering stage, on the surface roughness and flexural strength of additively or subtractively manufactured 3Y-TZP zirconia.
Materials And Methods
Disc-shaped (n = 40) and bar-shaped (n = 64) zirconia specimens were either subtractively manufactured (SM) or additively manufactured (AM) for surface roughness measurements and flexural strength tests, respectively. Specimens were polished following different protocols depending on timing: before sintering, before and after sintering, after sintering, and some received no polishing. Surface roughness was measured with a noncontact optical profilometer at each stage. Flexural strength was assessed using a three-point bending test according to ISO 6872:2015. The data were analyzed using two-way ANOVA and post hoc tests to evaluate the effects of material type and polishing protocol on the surface roughness and flexural strength (α = 0.05).
Results
The lowest Ra values were recorded when polishing was done before and after sintering (AM: 0.22 ± 0.03 µm; SM: 0.18 ± 0.02 µm) (p ≤ 0.05). AM zirconia specimens demonstrated a greater reduction in surface roughness compared to SM across all polishing protocols (p < 0.001). The highest flexural strength was recorded in specimens polished before and after sintering in the AM group (1049.98 ± 71.61 MPa) (p ≤ 0.05).
Conclusions
Polishing performed both before and after sintering enabled clinically acceptable surface roughness for AM zirconia. In all polished specimens, AM specimens polished before and after sintering had the highest flexural strength.