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.