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 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.