Effect of sintering parameters on phase evolution and strength of dental lithium silicate glass-ceramics

With the establishment of CAD/CAM technology, competing lithium silicate based formulations have been introduced for clinical use, but little is known about their phase composition. Here we investigate a commercially available SiO₂-Al₂O₃-K₂O-Li₂O-P₂O₅-ZrO₂ system to evaluate the crystal phase evolution during the second heat treatment by changing the main crystallization parameters.

With a focus on the final stage of crystallization, we characterized the dimensional changes in the crystallographic structure of the residual Li₂SiO₃ and the lithium orthophosphate (Li₃PO₄) phases with variations in crystallization parameters, i.e. time, temperature and cooling rate over the range of the glass transition temperature T_g. The phase fractions (crystalline and glass) and the sizes of coherent scattering domains (CSDs) were resolved by means of quantitative X-Ray Diffraction using Rietveld refinement combined with an external standard method (G-factor). Biaxial flexure testing was conducted to evaluate the influence of crystallization parameters on the characteristic strength and natural defect distribution.

An increase in crystallization temperature from 840 to 880°C resulted in a significant reduction of the Li₂Si₂O₅ content, which indicated a reversion of the Li₂SiO₃ to Li₂Si₂O₅ phase transformation. Reduction to 800°C had no significant effect. Furthermore, the CSD sizes of Li₂SiO₃ and Li₃PO₄ continuously increased with increasing temperature, which was accompanied by an increase in strength parameters. Reducing the cooling rate over the range of T_g resulted in an increased strength at low failure probabilities.

These findings help to establish recommendations for adjustment of the crystallization protocol, which has potential to increase the clinical reliability of the material investigated.