Nucleation and crystallization of apatite-wollastonite (A-W) bioglass-ceramics

Bioactive, Apatite-Wollastonite (A-W) glass-ceramics currently exhibit good mechanical properties. However, the products are manufactured by powder processing to surmount problems regarding crystallization of the wollastonite. Contamination and unavoidable pores, resulted from powder processing, affect bioactivity and mechanical properties. These disadvantages can be overcomed by fabricating this material via the melt-casting process. Studies on nucleation and crystallization mechanisms will provide essential information required for proper controlled heat treatments to make this process viable.; Crystallization kinetic studies on the A-W glass show that apatite crystallization is a bulk mechanism with Avrami parameter (n) ≈ 3, while wollastonite crystallization is a surface mechanism with n ≈ 2 (2-D planar growth). The crystallization temperature (T_c) of the wollastonite determined from DTA is decreased with glass particle size, while T_c of the apatite is generally independent of the particle size. However, surface nucleation of apatite was observed on the glass with particle size <45 μm using isothermal high temperature x-ray diffraction.; Evidence of phase separation promoting nucleation was observed at 770°C. Dendritic growth of apatite increases as a function of temperature. A coarse, web-like microstructure was seen in the core region, with a finer web-like structure near the surface region. The fine microstructure resulted from the suppression of apatite crystal growth on the surface by wollastonite surface crystals.; The effect of different A-W precursors formed by designed heat treatments (to form apatite crystals) on the wollastonite formation was observed. At 950° and 990°C, fewer wollastonite crystals were formed in the precursors containing 40 weight % apatite. At 970°C higher wollastonite content was observed in a precursor with 40 weight % apatite, and is related to the effect of the precursor's microstructure. Growth of the pre-existing apatite mechanically interferes with wollastonite formation and alters orientation of the surface crystals. The flexural strength of the A-W glass increases with increasing apatite content, from 68 to 86 MPa. Surface crystallization of wollastonite generates surface compressive stresses, owing to the differences in the thermal expansion coefficient of the surface and interior, and leads to mechanical strength of 150–210 MPa depending on wollastonite content.; In vitro tests indicate that samples with a high glassy phase content of a proper composition are highly bioactive. The A-W glass-ceramics containing high apatite content and containing both apatite and wollastonite crystals exhibit extremely low bioactivity.