Flexural strength and surface phase transformations in fluorrichterite glass-ceramics

Objective: to investigate the relationship between crystalline phase characteristics and biaxial flexural strength after heat treatment of fluorrichterite glass-ceramics for biomedical applications. Methods: glasses were prepared by melting at 1475șC for 2h and cast into cylindrical ingots. The ingots were sectioned into discs (11mm diameter, 1.2mm thick; n=10 per group) and randomly divided into six groups. Specimens were heat treated at temperatures ranging between 900 and 960șC for various durations and cooling rates. The crystalline phases were analyzed by x-ray diffraction (XRD) on powdered and bulk specimens. The thickness of the surface phase was assessed by XRD after sequential grinding until disappearance of the corresponding reflections. The microstructure was investigated by scanning electron microscopy. The mean biaxial flexural strength was determined in air using a ball-on-ring-of-balls fixture on a universal testing machine. Results: XRD on powdered specimens confirmed the presence of fluorrichterite and fluormica for all groups. It also revealed the presence of hexagonal roedderite (NaKMg₅Si₁₂O₃₀) crystals at the surface of all specimens. Fast cooling led to a preferential orientation of roedderite platelets with 00l planes parallel to the ceramic surface to an estimated depth of 25 micrometers. Slow cooling led to the formation of a layer of hollow roedderite prisms between 50 and 140 micrometers in depth. Increasing heat treatment temperature and time led to an increase in thickness of the roedderite phase. The mean biaxial flexural strength increased linearly with the amount of hollow crystals formed (R²=0.92) and varied from 196.1±41.1MPa (fast cooling) to 497.6±25.5MPa (slow cooling). ANOVA and Tukey's test showed that this 2.3-fold increase in flexural strength was significant (p<0.001). Conclusion: A significant increase in biaxial flexural strength was obtained by controlling thickness and crystalline orientation of the roedderite surface phase in fluorrichterite glass-ceramics. Supported by NIH-NIDCR Grant R01DE013570.