Nanomechanical characterization of an experimental bilayered zirconia system

Abstract

In order to improve the esthetic outcomes of prosthetic treatments using zirconia, multilayer systems with gradient translucency have been recently introduced to the market to mimic natural tooth appearance. However, defects at the interface between layers have shown to produce a significant decrease in strength regarding its isolated counterparts. Hence, FAPESP Master´s scholarship #2022/07733-2 linked to FAPESP Young Investigator Grant #2021/06730-7, proposed the synthesis of bilayered zirconia systems as well as the characterization of its optical and mechanical performance before and after hydrothermal aging. To this end, commercial zirconia powders (3Y-TZP, 4Y-PSZ, and 5Y-PSZ) were acquired and two hundred and sixty bilayered disc-shaped specimens were obtained through successive uniaxial pressing in two different configurations: 1) 3Y-TZP - 5Y-PSZ and 2) 4Y-PSZ - 5Y-PSZ. After sintering, half of the specimens were aged in a hydrothermal reactor (134ºC for 20 h at 2.2 bar) to evaluate the effects of low temperature degradation in the microstructure and crystalline content of bilayered zirconia. Partial results suggest the achievement of a dense microstructure with characteristic monoclinic, tetragonal and cubic zirconia phases for each layer. While characterizations of the aged specimens are being performed, a cross-section evaluation of the nanomechanical properties of the obtained samples is of utmost importance considering that subsequent testing of the mechanical properties rely on the understanding of the microstructure and gradient of hardness and elastic modulus at the nanoscale. Therefore, the present BEPE proposal aims to further evaluate the mechanical properties of individual zirconia layers and at the interface in a cross-section view using automated nanoscale indentation, which could be accurately performed and interpreted by equipment and faculty at the Division of Biomaterials at New York University. Nanoindentation will be performed by means of a Berkovich diamond pyramid probe at different regions of interest to determine the gradient of hardness, elastic modulus, and residual stress within the discs. The remaining characterizations will be performed after completion of the BEPE program, where biaxial flexural strength test will be utilized to determine the probability of survival, Weibull modulus and characteristic strength of the bilayered materials and the optical properties will be evaluated in a spectrophotometer using reflectance tests. The data collected in this project would highly enrich the Young Investigator Grant #2021/06730-7, since it would serve as a control group for the experimental bilayered materials to be synthesized using recycled zirconia.