Microstructure and mechanical properties of an experimental lithium disilicate dental glass-ceramic

Abstract

After testing over 50 experimental lithium disilicate (LS2)-based glass-ceramic (GC) compositions and evaluating their glass-forming ability and the effect of various heat treatments on their crystallized fraction, crystal size, and morphology, a particularly promising formulation was selected for the optimization of the mechanical properties of materials. The specimens were divided into groups submitted to a nucleation heat treatment at different times and temperatures, i.e., T1 (500 °C/90′), T2 (500 °C/180′), T3 (500 °C/360′) and T4 (480 °C/360′), followed by a crystal growth treatment to induce the lithium metasilicate (700 °C) and LS2 (840 °C) phases. However, a third (minor) phase, lithium phosphate, also precipitated. IPS e.max CAD was used as the control group (C, 500 °C/360′). The elastic modulus (E) was measured by the impulse excitation technique (ASTM E 1876-15), while the Vickers hardness (HV) and biaxial flexural strength (BFS) were determined by indentation and the piston-on-three balls test (ISO 6872), respectively. The E and HV results were analyzed by ANOVA one-way with Games–Howell post-hoc test, whereas the BFS data were investigated by Kruskal-Wallis with Dunn's post-hoc test (α = 0.05). The E of all GCs were within the expected range for commercial LS2 glass-ceramics. The T2 and T4 GCs exhibited the best properties when compared to the C group, and despite their relatively small crystallized fraction (∼57 %), they showed a hardness of 5.8 ± 0.2 GPa, a value that is adequate and statistically equal to that of the control group. The same groups reached BFS values of 359 ± 89 MPa and 493 ± 147 MPa, respectively, which are also statistically equal to that of the C group (338 ± 61 MPa). The mechanical properties of the material studied was successfully optimized through variations in the nucleation temperature and time, reaching values comparable to (but not better than) commercial dental glass-ceramics.