Development of pre-sintered ZTA 80% Al2O3/20% ZrO2 composite blocks for milling in CAD/CAM system

Author(s)

Adolfo Coelho de Oliveira Lopes

• Document type: Doctoral Thesis
• Press: Bauru.
• Institution: Universidade de São Paulo (USP). Faculdade de Odontologia de Bauru (FOB/SDB)
• Defense date: 2023-08-18

Examining board members

Estevam Augusto Bonfante; Tiago Moreira Bastos Campos

• Advisor: Estevam Augusto Bonfante; Edmara Tatiely Pedroso Bergamo

Abstract

Yttrium tetragonal zirconia polycrystals (Y-TZP) is a promising material, but it presents hydrothermal instability at low temperature due to the transformation of the metastable tetragonal phase into a monoclinic phase, which can lead to low temperature degradation (LTD), compromising the long-term predictability of dental reconstructions. Therefore, the present work aims to innovate in the synthesis of polycrystalline composites in the form of pre-sintered blocks based on zirconiatoughened alumina (ZTA - zirconia-toughened alumina) for CAD/CAM systems used in dentistry, in order to bring the best characteristics of both alumina and zirconia, offering materials with improved mechanical strength compared to alumina and moreresistant to LTD compared to zirconia. The 80% Al2O3 - 20% ZrO2 ZTA were synthesized in custom made matrices to generate CAD/CAM blocks able to be milled. The experiment was divided into two steps: 1.1) Machining of 14 mm x 1 mm discs (ISO 6872:2015) to be characterized by density, X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical properties; 1.2) Machining of discs for mechanical evaluation including Vickers indentation and fracture toughness and biaxial flexural strength (BFS). For all tests, analyses were performed before and after an accelerated hydrothermal aging protocol in autoclave. The results of this research showed the success of the synthesis of the experimental ZTA, with typical peaks of alumina and zirconia (monoclinic, tetragonal and cubic phases), as well as the absence of difference in the XRD spectra for both aged and immediate conditions. The micrographs suggest that the ZTA processing provided excellent compaction of the material and generated a dense crystalline matrix, with spherical grains and few intergranular defects, maintaining the microstructure before and after aging. The milled ZTA composite showed high opacity (high contrast ratio) and masking ability (low translucency parameter) for immediate and aged samples. Likewise, the aging process did not alter the hardness (17 GPa) and fracture toughness (5.5 GPa MPamm1/2) values of the ZTA composite machined from the blocks. The ZTA composite also showed no significant change in Weibull modulus (15) and characteristic strength (450 MPa) before and after hydrothermal aging. The milled experimental ZTA showed a high probability of survival for stress missions of 100 and 300 MPa (99%), regardless of the storage condition. However, for a mission of 500 MPa, the samples showed a statistically significant reduction in the probability of survival (2%) for both storage conditions. It is concluded that the proposed synthesis and processing method was able to successfully synthesize a ZTA 80% Al2O3 - 20% ZrO2 composite resistant to hydrothermal aging in block format to be milled in a dental CAD/CAM system. (AU)

• FAPESP's process: 19/14798-0 - Development of pre-sintered ZTA 80% Al2O3/20% ZrO2 composite blocks for milling in CAD/CAM system
• Grantee: Adolfo Coelho de Oliveira Lopes
• Support Opportunities: Scholarships in Brazil - Doctorate