Fatigue strength of yttria-stabilized zirconia polycrystals: Effects of grinding, polishing, glazing, and heat treatment

This study aimed to evaluate and compare the effect of different surface post-processing treatments (polishing, heat treatment, glazing, polishing + heat treatment and polishing + glazing) on the superficial characteristics (micromorphology and roughness), phase transformation and fatigue strength of a Y-TZP ceramic ground with diamond bur. Discs of Y-TZP ceramic were manufactured (ISO:6872-2015; final dimensions of 15mm in diameter and 1.2 ± 0.2mm in thickness) and randomly allocated according to the surface condition: Ctrl - as-sintered; Gr - ground with coarse diamond bur; Gr+HT - ground and subjected to the heat treatment; Gr+Pol - ground and polished; Gr+Pol+HT - ground, polished and heat treated; Gr+Gl - ground and glazed; Gr+Pol+Gl - ground, polished and glazed. The following analyses were performed: roughness (n = 25), surface topography (n = 2), phase transformation (n = 2) and fatigue strength by staircase method (n = 20). All treatments influenced to some extent the surface characteristics of Y-TZP, being that polishing reduced the surface roughness, the m-phase content and improved the fatigue strength; glazing led to the lowest roughness values (Ra and Rz), although it showed the worst fatigue strength; heat treatment showed limited effect on surface roughness, led to complete reversion of the existing m-phase content to t-phase, without enhancing fatigue performance. Thus, a polishing protocol after clinic adjustment (grinding) of monolithic restorations based on polycrystalline zirconia material is mandatory for surface characteristics and fatigue performance improvements.