Evaluation of the effect of low temperature degradation on biaxial flexural strength and phase change of translucent monolithic zirconia ceramic systems

The presence of saliva in the oral cavity, temperature changes and repeated loading during chewing cause a decrease in the mechanical stability of dental restorations. The aim of this thesis is to evaluate the effect of low temperature degradation on the biaxial flexural strength and the resulting monoclinic phase change in various newly developed high translucent monolithic zirconia ceramic systems and to compare them with translucent 3Y-TZP zirconia ceramics. In this study, 3 different high translucency Y-PSZ and one translucent 3Y-TZP zirconia system were used. A total of 120 specimens were prepared in the form of discs with a diameter of 14 mm and dimensions of 1.2 ± 0.2 mm. Three experimental groups (n = 30) were formed from each type of material, the first group was determined as the control group without any low temperature degradation treatment. Low temperature degradation was applied to the samples in the second group for 5 hours, and for the third group at 134 ⁰C and 2 bar pressure for 20 hours. Tetragonal monoclinic phase changes on the surface of all samples were determined using x-ray diffractometer and biaxial flexural strength test was applied to determine their flexural strength. The obtained data were analyzed using the weibull distribution. All results were evaluated statistically. According to the statistical analysis results, the highest monoclinic phase amount was detected in the 20 hour aging group of Nacera MS material (92%), while no monoclinic phase was detected in the control, 5 and 20 hours aging groups in Katana UTML and Nacera Q3 materials (0%). The highest flexural strength values are control in Nacera MS (1344.2 MPa), control in Katana STML (728.9 MPa), 20 hours in Katana UTML (566.4 MPa) and 5 hours in Nacera Q3 (733, 1 MPa), while the highest weibull modulus values were detected in Nacera MS at 5 hours (m=22,031), in Katana STML control (m=18,829), Nacera Q3 (m=13.285), and Katana UTML (m=9.411 ) was detected in the 20 hour group.These results gain importance in that the new high-translucent zirconias can be used in the oral environment for many years without affecting their mechanical strength and will not cause any failure.