Effect of hydrothermal aging on the optical, mechanical and chemical properties of monolithic zirconia materials

Effect of Hydrothermal Aging on The Optical, Mechanical and Chemical Properties of Monolithic Zirconia Materials Purpose: The aim of this in-vitro study; is to examine the effects of hydrothermal aging processes applied to monolithic zirconium materials on translucency parameter, phase transphormations, topographic properties, surface hardness and biaxial flexural strength. Material and Methods: Three types of multi-layered zirconia systems (Katana, Kuraray Noritake) were selected for the study: Ultra Translucent Multi Layered zirconia (UTML), Super Translucent Multilayered zirconia (STML), and Multi Layered zirconia (ML). The final size of the spacimens were set as a disc shape with a diameter of 15 mm and a thickness of 1.2 mm, which after being subjected to the sintering process. The specimens were divided into three groups according to the type of zirconia as well as three subgroups according to the hydrothermal treatment procedure. While there were nine (n=9) specimens in each subgroup, a total of eighty-one (n=81) specimens were used in this study. Optical, chemical, topographic, physical and mechanical structures of the materials was determined respectively; using Spectrophotometer, X-Ray Diffractometer (XRD), Scanning Electron Microscopy (SEM), Vickers microhardness tester and Biaxial flexural strength tester. The obtained data were statistically evaluated by two way analysis of variance and Tukey multiple comparison test (p<0.05). Results: The translucency value was significantly affected after hydrothermal aging process (p<0.05), however it was determined that there was no statistically significant difference in the effect on the surface hardness values (p˃0.05). Compared to the control group, the 5 hour and 10 hour aging process increased the monoclinic amount in XRD phase transformation of all materials, but the highest increase rate was found in the ML group. Due to observation of aging procedure and material type, the phase transformation rate and biaxial flexural strength is significantly affected statistically (p<0.05). Conclusions: Hydrothermal aging had significant effects on the translucency of translucent monolithic zirconias. The small amount of tetragonal-monoclinic phase transformation was observed on translucent zirconias, except for the ML group. The aging process did not affect the Vickers hardness value. According to the material type, the highest Vickers hardness value was found in the UTML group, and the lowest Vickers hardness value was found in the ML group. When biaxial flexural strength was evaluated according to material type, the highest value was observed in the ML group, and the lowest value was observed in the UTML group. It was determined that aging for 5 hours and 10 hours, generally increase the biaxial flexural strength. Keywords: Monolithic zirconia, Hydrothermal aging, Translucency parameter, Phase transformation, Grain size, Biaxial flexural strength, Vickers hardness