Crystallization Hierarchy of CaO-P2O5-SiO2-Al2O3-TiO2 Glass-Ceramics

Glasses in the tricalcium phosphate-anorthite (Ca₃(PO₄)₂-CaAl₂Si₂O₈ or TCP-CAS₂) system with additional TiO₂ were melted. Crystallization was investigated using thermal analysis, X-ray diffraction, and transmission optical and electron microscopies. During the heat treatment, as the growth temperature increases, successive crystalline phases separate from the glass matrix. The phases present in the fully heat-treated glass-ceramic are ß-TCP, anor-thite (CAS₂), and rutile (TiO₂). Apart from TiO₂, these phases evolve from other polymorphs during the heat treatment. The metastable phases, pseudo-orthorhombic CAS₂ and alpha-TCP, appear first around 880°C and transform into the stable phases, triclinic anorthite CAS₂ and ß-TCP, around 940° and 1000°C, respectively. The material crystallizes in stages. The first stage is the separation from the glass matrix of rutile and what is believed to be a calcium phosphate phase, with crystal sizes varying from 20 to 200 nm. This is followed by the appearance of larger crystals (1-2 µm) of the metastable pseudo-orthorhombic CAS₂, surrounding the previously crystallized phases. Finally, this pseudo-orthorhombic CAS₂ phase transforms to anorthite (15-20 µm) spherulites. TiO₂ does not act as a direct nucleating agent in the glass composition studied: no sign of heterogeneous nucleation and growth from TiO₂ crystals has been found, and moreover, TiO₂-free base glass crystallizes in a manner similar to that of the glass containing TiO₂.