Formation and crystal growth of needle-like fluoroapatite in functional glass-ceramics

The objective of the study is to analyze the solid state reactions leading to the precipitation of fluoroapatite, Ca-5(PO4)(3)F, in two different functional glass-ceramic systems (labeled A and B) of interest for restorative dentistry. Combined X- ray diffraction (XRD), electron microscopy (SEM and TEM) and solid state nuclear magnetic resonance (NMR) are used to characterize the solid state reactions, leading to the formation of primary and secondary crystalline phases, as well as the structural changes occurring in the residual glass matrix during this process. The results indicate that, depending on the composition of the ceramic, fluoroapatite crystallization can proceed by different mechanisms: (1) precipitation in a parallel process accompanying the formation of rhenanite (NaCaPO4) primary crystals (glass-ceramic A), and (2) formation from amorphous and/or disordered crystalline precursor phases that are already segregated within a phase separated glass matrix (glass-ceramic B). In the latter case, this disordered phase transforms by solid state reaction into fluoroapatite at high temperatures of heat treatment of the glass-ceramic. The needle-like morphology of fluoroapatite in glass-ceramics mimics the morphology of hydroxyl-carbonato apatite in human teeth.