Effect of the Hydrothermal Aging Method on the Microstructure, Physical, and Mechanical Properties of an Experimental Multilayer Zirconia

• Grant number: 24/16484-1
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: May 01, 2025
• End date: November 30, 2025
• Field of knowledge: Health Sciences - Dentistry - Dental Materials
• Principal Investigator: Estevam Augusto Bonfante
• Grantee: Vinicius Toshikazu Shiomatsu
• Host Institution: Faculdade de Odontologia de Bauru (FOB). Universidade de São Paulo (USP). Bauru , SP, Brazil
• Associated research grant: 21/06730-7 - Towards sustainable zirconia recycled from milling waste and boosted by glass infiltration for restorative use, and of the development of strong, graded, and aging resistant multilayered zirconia blocks, AP.JP2

Abstract

Among the different dental zirconias available on the market, multilayer zirconias have generated interest for delivering a restoration that mimics the multichromatic structure of the tooth, based on layers with different yttria gradients, composed of both the tetragonal phase that offers greater resistance in cervical regions and the cubic phase that brings greater translucency and aesthetics in mid-incisal third areas. Despite this, they are still predisposed to phase transformation due to hydrothermal ageing, a phenomenon that has not been fully elucidated for these materials. In 2024, the ISO 6872 standard was updated to include hydrothermal ageing, which had not previously been provided for, but under steam, even though there is no consensus on the differences between ageing in a humid environment and in hydrothermal reactors. Therefore, the aim of this project is to evaluate the effect of 2 methods of ageing in a hydrothermal reactor (with samples submerged or in steam) on the content of crystalline phases, as well as on the optical and mechanical properties of an experimental multilayer zirconia. 150 disc-shaped specimens will be synthesized and divided into three experimental groups: 1) immediate (3Y+4Y+5Y-i), 2) aged in a steam hydrothermal reactor (3Y+4Y+5Y-v) and 3) aged in a water-submerged hydrothermal reactor (3Y+4Y+5Y-s). The specimens will be synthesized by uniaxial pressing, sintered and polished to obtain discs with a diameter of 12 mm and a thickness of 1.2 mm (ISO 6872). The 3Y+4Y+5Y-v and 3Y+4Y+5Y-s groups will be aged in a hydrothermal reactor at 134°C and 2.2 bar for 10 hours by steam or submerged in water, respectively. All groups will be evaluated on the surface and in cross-section by X-ray diffraction, to determine the crystalline content, and by scanning electron microscopy for microstructural evaluation. The effect of DBT on optical and mechanical properties will be evaluated by reflectance and biaxial flexural strength (BFR) tests followed by fractographic evaluation, respectively. The optical properties data will be subjected to normality tests and analysis of variance and Tukey's test, and the BFR data will be analyzed using Weibull statistics to determine the modulus and characteristic strength of the experimental groups.Among the different dental zirconias available on the market, multilayer zirconias have generated interest for delivering a restoration that mimics the multichromatic structure of the tooth, based on layers with different yttria gradients, composed of both the tetragonal phase that offers greater resistance in cervical regions and the cubic phase that brings greater translucency and aesthetics in mid-incisal third areas. Despite this, they are still predisposed to phase transformation due to hydrothermal ageing, a phenomenon that has not been fully elucidated for these materials. In 2024, the ISO 6872 standard was updated to include hydrothermal ageing, which had not previously been provided for, but under steam, even though there is no consensus on the differences between ageing in a humid environment and in hydrothermal reactors. Therefore, the aim of this project is to evaluate the effect of 2 methods of ageing in a hydrothermal reactor (with samples submerged or in steam) on the content of crystalline phases, as well as on the optical and mechanical properties of an experimental multilayer zirconia. 150 disc-shaped specimens will be synthesized and divided into three experimental groups: 1) immediate (3Y+4Y+5Y-i), 2) aged in a steam hydrothermal reactor (3Y+4Y+5Y-v) and 3) aged in a water-submerged hydrothermal reactor (3Y+4Y+5Y-s). The specimens will be synthesized by uniaxial pressing, sintered and polished to obtain discs with a diameter of 12 mm and a thickness of 1.2 mm (ISO 6872). The 3Y+4Y+5Y-v and 3Y+4Y+5Y-s groups will be aged in a hydrothermal reactor at 134°C and 2.2 bar for 10 hours by steam or submerged in water, respectively. All groups will be evaluated on th (AU)