Effect of multiple firing on flexural strength and phase transformation of monolithic zirconia with different ratios of yttrium

The aim of this study was to evaluate the flexural strength, roughness and phase transformatıon of new generation 4Y-TZP and 5Y-TZP monolithic zirconia with different yttrium content [6-8% (VITA YZ ST), 8-10% (VITA YZ XT)] after repeated firing (0, 1, 2, 4 firing cycles). The samples obtained from 4Y-TZP (VITA YZ ST) and 5Y-TZP (VITA YZ XT) materials were divided into four subgroups according to the number of firing cycles: 0, 1, 2, 4. The samples were produced in the milling unit of the CAD/CAM system (Yenadent D15, Yenadent Ltd., Istanbul, Turkey) with a final size of 12.0 mm in diameter and 1.2±0.2 mm in thickness after sintering. The samples were fired in a programmable porcelain furnace (Programat® P300/G2-Ivoclar Vivadent, Schaan, Liechtenstein) according to the manufacturer's instructions. Flexural strength measurements of the specimens were performed with a universal testing machine (ELISTA, TSTM02500, Elista Corp, Istanbul, Turkey). Surface roughness measurements of the samples were performed with a contact profilometer (Mahr, MarSurf M 300C). X-ray diffractometer analysis (Bruker D8 Advance, Bruker AXS, Karlsruhe, Germany) was used to evaluate the phase transformation of the samples. SEM analysis was performed to visualize the surface topography. As a result of this study, the lowest surface roughness in the 4Y-TZP group was found in the 0 and 4 firing cycles with no statistical difference. The highest surface roughness in the 4Y-TZP group was found in the 1 firing group and there was no statistical difference with the 2 firing group. In the 4Y-TZP group, the roughness of the 0 firing group was statistically significantly lower than the roughness of the 1 firing and 2 firing groups. In the 5Y-TZP group, the lowest surface roughness was found in the 0 and 4 firing groups with no statistical difference. The highest surface roughness in the 5Y-TZP group was found in the 1 firing group and there was no statistical difference with the 2 firing group. In the 5Y-TZP group, the roughness of the 0 firing group was statistically significantly lower than the roughness of the 1 firing group. There was no statistically significant difference between the roughness values of 4Y-TZP and 5Y-TZP materials in all firing groups. The highest flexural strength of the 4Y-TZP group was found in the 1 firing group and there was no statistically significant difference with the 0 firing group. The lowest flexural strength of the 4Y-TZP group was found in the 2 and 4 firing groups with no statistical difference. The flexural strength of 5Y-TZP material was not affected by repeated firing cycles. The flexural strength of 4Y-TZP material was higher than the flexural strength of 5Y-TZP material in every firing group. It was found that repeated firing did not affect the phase transformatıon of 4Y-TZP and 5Y-TZP materials.Future studies may be invastigated by increasing the number of firings and including intraoral thermal aging.