Tuning crystallization pathways, microstructure and mechanical properties in lithium disilicate glass-ceramics via...

To clarify how the Al₂O₃/CaO molar ratio governs crystallization pathways, phase evolution, microstructure and mechanical performance in lithium disilicate (LS2) glass-ceramics designed for CAD/CAM restorative applications.These results establish the Al₂O₃/CaO ratio as a practical compositional lever to route LS2-based glass-ceramics among Type I/II/III crystallization pathways and to engineer interlocking microstructures, providing a rational basis for tailoring the microstructure and properties of lithium disilicate CAD/CAM materials for restorative dentistry.

A multicomponent Li₂O-SiO₂-P₂O₅-K₂O-Na₂O(-Al₂O₃-CaO) glass system was formulated with a fixed m(SiO₂)/m(Li₂O) ratio of 2.42 and a constant Al₂O₃+CaO content of 5 mol%, while varying Al₂O₃/CaO. Parent glasses AC0-AC4 were fabricated via melt-quenching technique, and their thermal behavior was characterized by differential scanning calorimetry. Crystallization sequences and phase assemblages were determined by X-ray diffraction (XRD) and Rietveld refinement on glass-ceramics heat-treated up to 830 °C. Microstructural evolution was examined by field-emission scanning electron microscopy (SEM) after HF etching. Vickers hardness and toughness (Vickers indentation fracture method) were measured for glass-ceramics derived from AC1-AC4 at 700, 800 and 830 °C.

The Al₂O₃/CaO molar ratio tuned the crystallization sequence from Type III (LS2 only, 0-1.25 mol% Al₂O₃) to Type I (Li₂SiO₃ (LS) + LS2 coprecipitation, 2.5 mol% Al₂O₃) and further to Type II (LS-first → LS2, at 3.75-5 mol% Al₂O₃). Increasing Al₂O₃ content shift the first exotherm to lower temperature. In Al₂O₃-containing compositions, β-quartz precipitates near 775 °C and transforms into β-spodumene (LiAlSi₂O₆) by 830 °C. Rietveld refinement confirms LS2 as the dominant phase for all samples, which decreased from 73.8 wt% to 43.5 wt% as β-spodumene increased from 0 to 32.6 wt% with increasing Al₂O₃/CaO ratio. Variations in Li₃PO₄ content correlate with lithia consumption by lithium-bearing phases. Microstructures evolved from nanoscale grains to densely interlocked elongated LS2-rich architectures, most pronounced at 2.5-3.75 mol% Al₂O₃. Type III glass-ceramics exhibited a hardness-toughness trade-off, whereas Type I/II compositions showed steadily increasing toughness due to evident crystal growth with only small changes in hardness.

These results establish the Al₂O₃/CaO ratio as a practical compositional lever to route LS2-based glass-ceramics among Type I/II/III crystallization pathways and to engineer interlocking microstructures, providing a rational basis for tailoring the microstructure and properties of lithium disilicate CAD/CAM materials for restorative dentistry.