This in vitro study evaluated the effects of different polishing protocols on the microhardness and surface topography of multilayered (ML) and monolithic (MZ) yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) used in fixed dental restorations. The objective was to compare the influence of manual and semi-automatic polishing methods on both mechanical performance and surface quality, aiming to identify the most effective protocol for clinical application. Eighty discshaped zirconia specimens were prepared and divided into eight groups (40 MZ: A1, B1, C1, D1; and 40 ML: A, B, C, D) according to polishing protocol. Groups A and A1 served as controls, manually polished with a zirconia laboratory kit. Groups B/B1, C/C1, and D/D1 were polished semi-automatically using a grinder-polisher with 3 µm polycrystalline diamond suspension, 6 µm polycrystalline diamond suspension, and 0.04 µm colloidal silica, respectively. Microhardness was measured using a Vickers microhardness tester, and surface topography was examined via scanning electron microscopy (SEM). Statistical analysis was performed using one-way ANOVA and Tukey’s post hoc test. Significant differences in microhardness were found among polishing methods for both zirconia types (p < 0.001). Groups C (1494.84 VHN) and C1 (1493.98 VHN) achieved the highest hardness values, while the control groups A (1378.42 VHN) and A1 (1376.13 VHN) recorded the lowest. SEM revealed that D/D1 exhibited the smoothest and most uniform surfaces, whereas A/A1 showed deep grooves. Findings indicate that semiautomatic polishing with 6 µm diamond maximizes hardness, while colloidal silica provides superior smoothness, guiding clinical selection for optimal zirconia restoration outcomes
Abdullah et al. (2026) studied this question.