Crystallization kinetics of sol-gel derived hydroxyapatite thin films

The crystallization kinetics of sol-gel derived hydroxyapatite (HA) and tri calcium phosphate (TCP) thin films were studied to determine whether viscou s sintering could be used for densification. The films were approximately 9 00 nm thick, and were synthesized and processed on silicon substrates. The films were fired in air in a rapid thermal annealer (RTA) for various times and the degree of crystallinity was determined by measuring the intensity of characteristic X-ray diffraction lines. The growth kinetics of HA and TC P were measured between 420 and 550 degreesC, and between 840 and 920 degre esC, respectively. Films that were subjected to an accelerated aging step b efore firing, exhibited a significantly lower crystallization growth rate w hen compared to unaged films. The aged films also became harder, as measure d by nanoindentation. At temperatures above 840 degreesC, HA transformed in to both alpha -and beta -TCP, with the beta form being dominant at lower te mperatures. The activation energies for both transformations (amorphous fil m to HA, and HA to TCP) were determined, as were the constants for the Avra mi equation. Based on the rapid crystallization kinetics observed for the a morphous film to HA transformation, densification through viscous sintering is essentially precluded in this system. (C) 2001 Kluwer Academic Publishe rs.