Crystal phase formation and mechanical properties of nano-engineered glass ceramics for CAD/CAM dental restorations

Aim: The wide diffusion of chairside CAD/CAM systems for whole digital workflow restorations has lead to development of specific dental materials that include a wide range of new nano-structured glass ceramics. Mostly of the new materials are not immediately applicable after the milling procedure, since they must be heat-treated to reach a final level of crystallization for adequate mechanical and aesthetic properties. To achieve a controlled crystallization without causing deformation a precise heat treatment schedule, is provided by manufacturers. Notwithstanding this, due to the many types of dental furnaces used in dental practice and imprecise thermal calibration, many problems often occurs. The objective of his study was to define the impact of heating rate to the crystal growth of CAD/CAM glass ceramics treated in a conventional furnace. Methods: Lithium disilicate (LS2) (IPS EMax-CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and two zirconia reinforced lithium silicate ceramics (ZLS) ( Vita Suprinity, VITA Zahnfabrik, Bad Sackingen, Germany; Celtra Duo, Dentsply Sirona, Salzburg, Austria) were used in the present study. The mechanical properties and the crystal growth were evaluated on 30 specimens (n=10 for each group). The LS2 specimens were heat-treated starting from 400°C until to 840°C with heat rate of 55° C/min and a hold time of 8 minutes, while the ZLS specimens were heat-treated starting from 500°C until to 820°C with heat rate of 55° C/min and a hold time of 1.30 minutes. The investigation was carried out by a means of SEM, and AFM after a slight surface etching with hydrofluoric acid solution (1%) for 20 seconds. Three additional specimens were used for Differential Thermal Analysis (DTA). DTA analysis was performed to identify the transformation temperatures for each type of material considered in the present study. Finally, all groups of specimens were tested for fracture toughness (Ft) and Vickers hardness (Hv). Results: the mean size (± SD) of crystals after heat treatment was 1.65 (±0.34) microns for LS2, 854.5 (±155.0) nm for ZLS suprinity and 759.9 (±118.4) nm for ZLS Celtra Duo. As consequence of crystallization, the Ft was 2.2±0.1 MPa m1/2 for LS2, 4.7 ±0.8 MPa m1/2 for ZLS Suprinity and 3.8±0.6 MPa m1/2 for ZLS Celtra Duo; while, the Hv was 6.1±0.3 GPa for LS2, 7.6±0.7 GPa for ZLS Suprinity and 7.1±0.5 GPa for ZLS Celtra Duo. The DTA curves showed for LS2, ZLS Suprinity and ZLS Celtra Duo at a temperature range 400 to 500° an endotherm peak. Conclusions: The Organization of microstructure before and after heat treatment appears to be extremely different showing the presence within the glass matrix of a multicomponent system due to nucleation process. All the CAD/CAM materials evaluated in the present study showed a crystallization process highly dependent to temperature with significant differences for both crystal size and mechanical properties.