Mechanical properties–translucency–microstructure relationships in commercial monolayer and multilayer monolithic zirconia ceramics.

To evaluate the phase composition, microstructure, optical properties and mechanical properties of eight commercially available multilayer and monolayer monolithic dental zirconias. Five commercial 3Y-TZP (GC ST, GC HT [GC, Tokyo Japan]; Katana ML, Katana HT [Kuraray Noritake] and Lava Plus [3M Oral Care]) and three Y-PSZ (Katana STML, Katana UTML [Kuraray Noritake]; GC UHT [GC, Tokyo Japan]) zirconia ceramic grades were cut in plate-shaped specimens, sintered according to the manufacturer's instructions and mirror polished. The zirconia chemical composition was determined using X-ray fluorescence (XRF), phase composition was characterized using X-ray diffraction (XRD), while the grain size was measured using scanning electron microscopy (SEM). The translucency Parameter (TP) and Contrast Ratio (CR) were measured with a spectrophotometer (n = 10/group). The indentation fracture toughness (n = 10), Vickers hardness (n = 10) and biaxial strength (n = 20) of the sintered ceramics were assessed. The stress distribution during biaxial testing was assessed by Finite element analysis (FEA). Statistical analysis involved one-way ANOVA and post-hoc Tukey's HSD test and Pearson correlation test (α = 0.05). FEA showed that the stress distribution in plate shape specimens was the same as for disks, rationalizing the use of plates for biaxial strength testing. As expected, higher quantities of Y 2 O 3 were related to a higher cubic ZrO 2 phase content and lower tetragonality t-ZrO 2 , which improved translucency but diminished flexural strength and toughness. While there was no significant correlation between grain size and other material properties, addition of pigments to the zirconia grade statistically negatively affected hardness. Even though an improvement in strength and translucency could be recorded for the last Y-TZP generation, future research still needs to strive for combined improvement of optical properties and mechanical reliability of zirconia ceramics. • FEA validated the equations to calculate the biaxial strength when testing plate-shape specimens • 3Y-TZP and 4Y-PSZ ceramics showed the highest fracture toughness and flexural strength as compared to other zirconia grades • 3Y-TZP ceramics had lower mechanical reliability than 4Y-PSZ but were more reliable than 5Y-PSZ and 6Y-PSZ ceramics • 6Y-PSZ and 5Y-PSZ zirconia grades had a higher translucency than the opaquer 4Y-PSZ and 3Y-TZP zirconia grades [ABSTRACT FROM AUTHOR]