Damage Tolerance of Novel Two-Step Sintered High-Strength and High-Translucency Zirconia

Objectives: Ceramics are known to fail from flaws that dominate intrinsic microstructural features. Therefore, damage tolerance tests are vital for predicting the clinical behavior of ceramic restorations. Flaws can be introduced during ceramic processing or from occlusal adjustments with burs. The goal of this study is to analyze the damage tolerance of newly developed two-step sintered, high-strength and high-translucency 3Y-TZP and 5Y-PSZ relative to their conventionally sintered counterparts using controlled indentation flaws.
Methods: Two-step sintered Zpex (3Y) and Zpex Smile (5Y) disc samples (Ø12x1 mm, n=60) were produced using Tosho powder and sintered at 1550°C for 1 minute followed by 1150°C for 10 hours. Sample zirconia compositions (n=60) were also conventionally sintered following manufacturer’s instructions. All samples were polished to 1-µm finish. Each sample was subjected to Vickers indentation with loads ranging from 0.1N to 98N. The samples were subsequently loaded with the indented surface in tension in a piston-on-three-ball testing jig. Optical microscope images were taken before and after the flexural test. Flexural strength versus indentation load data was plotted to determine the strength degradation.
Results: All groups underwent strength degradation due to indentation flaw. The intrinsic strength of two-step sintered 3Y and 5Y were greater than their conventionally sintered counterparts. The strength of two-step sintered 5Y and conventionally sintered 5Y dropped by 50% at an indentation load of 1N, and 3N respectively. For the conventionally sintered 3Y, the strength dropped by 50% at a load of 10N whereas the two-step sintered 3Y underwent a strength degradation at a significantly lower load of 5N. Optical images reveled that although low loads did not degrade the strength, they were sufficient to initiate cracks.
Conclusions: Although the two-step sintering fabrication method enhances the intrinsic strength of 3Y and 5Y, it decreases its damage resistance when compared to the conventional sintering technique.