Crystallization kinetics of Si-TCP bioceramic films

The crystallization kinetics leading to the formation of a characteristic globular, interconnected morphology in the transformation of an unfired hydroxyapatite precipitate (HA) to silicon stabilized tri-calcium phosphate (Si-TCP) are examined by rapid thermal processing of thin films prepared on silica substrates. The results are interpreted using an Avrami model in which nucleation of Si-TCP is induced by diffusion of a silicon front from the substrate. The model is supported through SEM inspection of the morphological development. The transformation is characterized as an interfacial reaction with Avrami exponent m = 1.3 ± 0.2, activation energy for the reaction E_c = 2.5 ± 0.3 eV and time constant _c = 20 ± 1 s at 950°C. Silicon in the films acts to nucleate Si-TCP, while also causing impurity pinning of grains, resulting in a fine microstructure when excess Si is present in the film.