Optimization of two-step sintering conditions of zirconia blanks for dental restorations

Zirconium oxide (zirconia) is a promising alternative to traditional materials for producing dental crowns, exhibiting excellent mechanical and aesthetic properties. Currently, hydrothermal ageing also known as low-temperature degradation (LTD) is a very well explored issue that causes degraded mechanical properties for zirconia over time. However, as the technology for dental restorations has been changing from powder compaction to milling of pre-sintered blanks, the need to investigate new sintering technologies and their impact on the final properties (mechanical, aesthetic and resistance to ageing) of the ceramics is critical. The main objective of this study was to research, trial and optimise a two-step sintering methodology for pre-sintered blanks manufactured by CeramTec GmbH. Results demonstrated that two-step sintering is an alternative method to conventional sintering, producing 3Y-TZP ceramics that are fully dense with high translucency. The optimised two-step sintering conditions (T-peak (=) 1350 degrees C; Tiso = 1250 degrees C; t(iso) = 20 hours and Rate = 20 degrees C/min) produced 3Y-TZP ceramics with an average grain size of 92 nm. Ageing of zirconia was conducted by placing samples in steam water at 140 degrees C for 25 h, and showed that porous samples degraded faster due to the presence of free volume to accommodate the tetragonal to monoclinic transformation. Samples produced with optimised two-step sintering conditions only degraded 0.02% on bulk density, which can be explained by their small grain size.