Crystallization behavior of combeite in 45S5 Bioglass (R) via controlled heat treatment

Combeite (Na2Ca2Si3O9) is a mechanically strong phase that crystallizes from 45S5 Bioglass (R) during heat treatments. 4555 glass-ceramics are expected to not only retain desirable levels of bioactivity but also exhibit improved mechanical properties compared to those of the amorphous 45S5 Bioglass (R). However, producing crystalline 45S5 glass-ceramics containing solely combeite is challenging since other phase(s) may also form during the heat treatment process. In this study, we developed a comprehensive set of heat treatment regimes that can be used to control the formation of combeite in order to tailor the crystallinity of 45S5 glass-ceramics over a wide range of 5% to 95%. These heat treatments were designed based on controlling the nucleation and growth temperatures and times of the combeite phase, also preventing other potential phases from forming. The activation energy for crystallization of combeite was calculated based on three different analytical methods and then used to obtain the Avrami exponent of the Bioglass (R) powder (n = 0.71). The results indicated that the crystallization of combeite occurred mainly on the surface of the particles. Depending on the target application, different levels of mechanical properties and bioactivity in bioglasses may be required. The crystallinity of 45S5 Bioglass (R) can be adjusted by applying the appropriate heat treatment regime selected from the profiles developed in this study.