Studies of crystal phase formations in high-strength lithium disilicate glass–ceramics

The objective of the study was to analyze the nucleation, primary phase formation and solid state reaction to form lithium disilicate glass–ceramics derived from the SiO2–Li2O–Al2O3–K2O–ZrO2–P2O5 system. The concentration of P2O5 was increased from zero up to 3.2 wt%. Thermal analysis, scanning electron microscopy and X-ray diffraction were used to characterize the microstructure formation, the nucleation process and the solid state reaction of the crystal phase precipitation in the glass–ceramics. Additives of P2O5 allowed the control of bulk crystallization. Nucleation was catalyzed by nano-scaled Li3PO4 phases, visualized by HR-SEM. Li3PO4 reacts most probably as the heterogeneous catalyst, acting by epitaxy, of both Li2SiO3 and Li2Si2O5 crystals. Based on the discussion of the main results, the authors deduced a four-step reaction mechanism. This mechanism demonstrated that after nucleation of lithium metasilicate and lithium disilicate, the latter phase grows as agglomerated nanocrystals, but remained in a relative small amount. By contrast, lithium metasilicate grows rapidly and decomposes at 780–820 °C with the result of a drastic increase of lithium disilicate phase. This was a result of a solid state reaction with the SiO2-rich glassy phase. In a parallel reaction, cristobalite was formed as a preliminary phase. The final product of a glass–ceramic with 3.2 wt% P2O5 shows a highly crystalline interlocking microstructure demonstrating a high-strength of 726 ± 63 MPa and translucency.