Fracture toughness and hardness of a Y-TZP dental ceramic after mechanical surface treatments

OBJECTIVES: The aim of the investigation was to evaluate the effects of surface mechanical treatments of all-Y-TZP restorations. In order to do that, surface roughness (Rs), fractal dimension (Df), fracture toughness (Ft), Vickers hardness (Hv), crack length (Cl), and opening (Co) were determined. MATERIALS AND METHODS: Y-TZP bar-shaped specimens (n = 30) were subdivided into three groups: as received by milling center (M), after coarse polishing (CP), and after fine polishing (FP). The specimens were examined under both confocal laser and electron scanning microscopes. One-way ANOVA and post hoc tests were used to analyze the data (α = 0.05). RESULTS: The FP group showed a significantly lesser amount of Rs (P < 0.05). The Df analysis confirmed the significantly lesser amount of surface complexity within the FP group (P < 0.05). The Hv values were not significantly different among the groups. The Ft was significantly lesser within the FP group (P < 0.05). At the same time, both Cl and Co were significantly higher within the FP group (P < 0.05). Likewise, the FP treatment showed a significant decrease in roughness and surface complexity associated to a decrease of fracture toughness with a significant microcrack increase. CONCLUSIONS: The present study suggests that the fine polishing procedure in all-YTZP restorations produces surface embrittlement. CLINICAL RELEVANCE: Because of the surface embrittlement, mechanical surface polishing of all-zirconia restorations should be avoided.