Two-Step Sintering Enhances Dental Zirconia's Strength and Grain Growth

Objectives: The goal of the study was to elucidate the effects of various two-step sintering (TSS) protocols on the physical, mechanical, and optical properties of partially stabilized zirconia with different yttria dopant concentrations (Y-PSZ). The TSS results of 3Y, 4Y, and 5Y were compared with optimized conventional sintering results from a previous study for the same Y-PSZ compositions. TSS, a known technique to control grain growth, prevents grain growth by suppressing grain boundary migration while allowing further densification of the material.
Methods: Disc-shaped zirconia specimens were prepared from most widely used commercial dental zirconia powders of various Y contents by uniaxial pressing followed by cold-isostatic pressing. Densification was then carried out using TSS protocols with varying temperatures for both sintering steps. Relative density, microstructure, and phase content of 3Y, 4Y, and 5Y were analyzed along with biaxial flexural strength and translucency parameter.
Results: TSS 3Y and 4Y results showed similar relative density when compared to conventional sintering protocol, whereas 5Y failed to achieve that regardless of the use of TSS method. TSS yielded less cubic phase compared to conventional sintering especially for 3Y. This alternative TSS method appears to be a promising technique for controlling grain size, resulting in smaller grains than conventional sintering. The transparency parameter values for TSS Y-PSZ were lower than those of conventional sintering except for 3Y. TSS resulted in higher biaxial flexural strength than those of conventional sintering.
Conclusions: Two-step sintering was proven to be an effective method for controlling grain growth while eliminating porosities in dental zirconia, particularly in 3Y and 4Y. Finally, the TSS method enabled high density, microstructure refinement, and improved strength, making it a successful technique that can help enhance the clinical longevity of zirconia dental restorations.