In vitro aging and mechanical properties of translucent monolithic zirconia

International audiencePurpose/aim: The dental market demands for high-translucency monolithic zirconia dental crowns, which are usually placed either with or without a thin layer of glaze. Over the last years, an improved translucency was achieved with slight modifications in tetragonal zirconia microstructure. Moreover, novel “ultra-translucent” and multi-layered zirconia materials appeared recently on the market, showing superior esthetical properties and graded chroma. The microstructural features and the mechanical performance of these materials are still controversial, as well as their potential susceptibility to hydrothermal degradation. The present in vitro study aimed at evaluating: – the microstructure and mechanical properties of high-translucency zirconia crowns; – the aging behavior of such materials in the glazed/non-glazed state.Materials and methods: Six monolithic zirconia dental crowns (GC Initial Zirconia) with different translucency (2 standard “ST”, 2 high “HT” and 2 ultra-high “UHT”) were produced by CAD-CAM technology. The outer surface was splitted into 2 halves, each comprising 1 supporting and 1 chewing cusp; one half was glazed (∼50 μm), while the other half was left uncoated. The layer of glaze was compensated during the CAD process to ensure even occlusal contacts. The microstructure was analyzed by scanning electron microscopy (SEM). X-ray diffraction (XRD) analyses were performed at level of the outer surface (both glazed and non-glazed) at time 0 and after 2, 6, 18, 54 h of artificial aging in autoclave (134 °C at 2 bars). The mechanical properties were investigated by piston-on-three-balls biaxial strength, indentation fracture toughness and hardness tests performed on polished 12 mm diameter discs.Results: XRD patterns showed clear differences between ST, HT and UHT materials. The UHT crowns exhibited bigger grain size and XRD patterns corresponding mostly to the cubic phase. No aging was observed on cubic crowns, while the tetragonal ones showed typical aging behavior of 3Y-TZP materials, where the presence of glaze had a protective effect. The mechanical properties of cubic crowns were definitely lower than for tetragonal ones.Conclusions: In tetragonal zirconia monolithic crowns, hydrothermal degradation can be observed also for short aging times, while the glaze acted as a protective layer. The presence of the cubic phase in UHT crowns entails two main advantages: a sensible increase in translucency and the complete absence of hydrothermal degradation. On the other hand, the lack of transformation toughening for cubic zirconia and the coarser microstructure cause a severe drop in mechanical properties, which can represent a limitation for their application in conditions where high mechanical stresses are applied