CRYSTALLIZATION HIERARCHY OF CAO-P2O5-SIO2-AL2O3-TIO2 CLASS-CERAMICS

Glasses in the tricalcium phosphate-anorthite (Ca-3(PO4)(2)CaAl2Si2O8 or TCP-CAS(2)) system with additional TiO2 were melted. Crystallizatio n was investigated using thermal analysis, X-ray diffraction, and tran smission optical and electron microscopies, During the heat treatment, as the growth temperature increases, successive crystalline phases se parate from the glass matrix. The phases present in the fully heat-tre ated glass-ceramic are beta-TCP, anorthite (CAS(2)), and rutile (TiO2) . Apart from TiO2, these phases evolve from other polymorphs during th e heat treatment. The metastable phases, pseudo-orthorhombic CAS(2) an d alpha-TCP, appear first around 880 degrees C and transform into the stable phases, triclinic anorthite GAS, and beta-TCP, around 940 degre es and 1000 degrees C, respectively. The material crystallizes in stag es. The first stage is the separation from the glass matrix of rutile and what is believed to be a calcium phosphate phase, with crystal siz es varying from 20 to 200 nm. This is followed by the appearance of la rger crystals (1-2 mu m) of the metastable pseudo-orthorhombic GAS,, s urrounding the previously crystallized phases, Finally, this pseudo-or thorhombic GAS, phase transforms to anorthite (15-20 mu m) spherulites . TiO2 does not act as a direct nucleating agent in the glass composit ion studied: no sign of heterogeneous nucleation and growth from TiO2 crystals has been found, and moreover, TiO2-free base glass crystalliz es in a manner similar to that of the glass containing TiO2.