Mechanical Properties, Aging Stability and Translucency of Speed-sintered Zirconia for Chairside Restorations

Objective: To evaluate the performance of zirconia ceramics sintered in a speed sintering induction furnace by comprehensive understanding of their optical and mechanical properties, microstructure, phase composition and aging stability, in comparison to ceramics sintered in a conventional furnace.

Methods: Speed sintered (SS) Katana STML (Kuraray Noritake) (total thermal cycle/sintering time/dwell temperature: 30min/16min/1560°C) and CEREC Zirconia (CEREC Zr) (Dentsply Sirona) (15min/2min/1578°C) were compared to conventionally sintered (CS) Katana STML (6.8h/2h/1550°C) and inCoris TZI (4h/2h/1510°C). The translucency parameter (TP) and contrast ratio (CR) were measured with a spectrophotometer. The chemical composition of the materials was determined by XRF and phase composition was characterized using XRD. Hydrothermal aging behavior was evaluated by measuring the tetragonal-to-monoclinic ZrO phase transformation after accelerated hydrothermal aging in steam at 134°C. The indentation fracture toughness, Vickers hardness and biaxial strength of the sintered ceramics were assessed.

Results: Speed and conventionally sintered zirconia revealed similar density, microstructure, average strength and hydrothermal aging stability. Both Katana STML 5Y-PSZ ceramics were characterized with a higher content of cubic phase (≈53wt%), which resulted in a higher amount of YO in the remaining tetragonal ZrO phases compared to the 3Y-TZP CEREC Zr and inCoris TZI (8 and 20wt%, respectively). The sintering program did not affect the hydrothermal aging behavior of Katana STML and CEREC Zr. TP of Katana STML (TP≈32) was not affected by speed sintering, while the translucency of CEREC Zr (TP=14) was significantly reduced. Hardness, fracture toughness and Weibull characteristic strength of Katana STML and CEREC Zr also reached the optimal level, but speed sintering substantially lowered their mechanical reliability.

Significance: Speed sintering of 3Y-TZP and 5Y-PSZ in a speed sintering induction oven appeared suitable for clinical applications. However, further studies should focus on improving of translucency and mechanical reliability of the speed-sintered zirconia ceramics.