Influence of finishing/polishing on the fatigue strength, surface topography, and roughness of an yttrium-stabilized tetragonal zirconia polycrystals subjected to grinding

Abstract

This study aimed to evaluate the effects of various polishing systems associated or not to finishing with diamond burs of lower-grit size on the topography, roughness, and fatigue behavior of a ground yttrium-stabilized tetragonal polycrystalline zirconia (Y-TZP). Disc specimens of Y-TZP (Zenostar T, Ivoclar-Vivadent) were produced (diameter = 15 mm, thickness = 1.2 ± 0.2 mm; ISO 6872-2015) and randomly allocated into 8 groups: [Ctrl] as-sintered; [Gr] ground with coarse diamond bur; [Gr+Eve] grinding + polishing with EveDiacera (2-step polishing system); [Gr+Fin+Eve] grinding + finishing + polishing with EveDiacera; [Gr+Kg] grinding + polishing with Kg Viking (2-step polishing system); [Gr+Fin+Kg] grinding + finishing + polishing with Kg Viking; [Gr+Op] grinding + polishing with Optrafine (3-step polishing system); and [Gr+Fin+Op] grinding + finishing + polishing with Optrafine. Next, surface topography, roughness, phase transformation, fatigue strength (staircase method), and fractography analyses were performed. Grinding changed the surface topography and generated higher roughness (Ra in μm) (1.214); the subsequent finishing/polishing procedures were able to reduce the roughness (0.326-0.839); however, it remained higher than the control [ctrl] group (0.221). All samples subjected to surface treatment presented an increase in m-phase content (8.04-17.46%). In terms of fatigue strength (in MPa), the grinding group (677.36) and polishing/finishing groups (641.66-707.20) presented higher fatigue strength than the control [ctrl] group (592.48). Finishing before polishing had no effect on fatigue strength (645.37-707.20). Grinding altered the Y-TZP surface features and increased their fatigue strength by phase transformation mechanism, while the finishing/polishing procedures promoted surface smoothening, while maintaining high fatigue strengths. Finishing as an additional step before polishing had no effect on roughness reduction and fatigue strength improvements; thus, the finishing procedure might be unnecessary.

Keywords: Fatigue behavior; Polycrystalline zirconia; Staircase method; Surface alterations.