Effect of different surface and heat treatments on the surface roughness, crystallography, and phase composition of high translucency zirconia for monolithic restorations

Abstract

Statement of problemHigh translucency zirconia (HTZ) has gained popularity as an esthetic computer-aided design and computer-aided manufacturing (CAD-CAM) material for monolithic restorations. A detailed comparison between different common surface and heat treatments with a non-treated HTZ control to explain the behavior of the material under stress is lacking. PurposeThe purpose of this in vitro study was to evaluate the effect of different surface and heat treatments on the surface roughness parameters (SRPs), topography, crystallography, and phase composition of HTZ used for monolithic restorations. Material and methodsNinety Ø11.9×1.18-mm HTZ disks (Prettau Anterior) were milled, sintered, and distributed into 9 groups (n=10); 8 experimental (coarse diamond grinding GC, fine diamond grinding GF, fine diamond grinding and 3-step polishing kit GF+P1, fine diamond grinding and 3-step polishing kit and diamond paste GF+P1+DP, fine diamond grinding and 2-step polishing kit GF+P2, fine diamond grinding and GF+Gl, fine diamond grinding and 3-step polishing and glazing GF+P1+Gl, airborne-particle abrasion with 50-µm alumina), and a control group (C, as-sintered). SRPs (AveSa, AveSv, AveSz) and 3-dimensional (3D) images were obtained using a noncontact 3D-optic-profilometer. The crystal structure was determined with scanning electron microscopy. Phase composition was analyzed by X-ray diffraction (XRD). Surface roughness parameters data were statistically analyzed by 1-way ANOVA and the Tukey HSD test (α=.05). ResultsThe applied surface and heat treatment resulted in significantly different SRP mean values (P<.001) with different topographies. GC had the highest AveSa, AveSv, and AveSz mean values (0.95, 8.8, 17.4 µm, respectively) with significant microcracks. GF had significantly lower SRP with finer microcracks. GF+P1 had a significantly smoother surface, but GF+P2 resulted in SRP comparable with the GF group. GF+P1+DP had the smoothest homogenous surface (mean Sa: 0.08 µm). GF+P1 and GF+GL were equally effective, while GF+P1+GL was not superior. Airborne-particle abrasion produced a low Sa mean value (0.11 µm) with relatively high Sv and Sz mean values (5.9, 9.2 µm, respectively) and microcracks. A monoclinic phase was detected in all groups. All experimental groups had broadened XRD-peaks with lower intensity, suggesting the presence of the rhombohedral phase. ConclusionsThe different surface and heat treatments altered the HTZ crystals and their surface roughness with distinct topographies. Cubic crystal changes take place under stress as shown by the scanning electron microscope and the XRD diffraction pattern and may transform to the rhombohedral phase.