Crystallization kinetics of sol-gel derived hydroxyapatite thin films

The crystallization kinetics of sol-gel derived hydroxyapatite (HA) and tricalcium phosphate (TCP) thin films were studied to determine whether viscous sintering could be used for densification. The films were approximately 900 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 TCP were measured between 420 and 550°C, and between 840 and 920°C, respectively. Films that were subjected to an accelerated aging step before firing, exhibited a significantly lower crystallization growth rate when compared to unaged films. The aged films also became harder, as measured by nanoindentation. At temperatures above 840°C, HA transformed into both α- and β-TCP, with the β form being dominant at lower temperatures. The activation energies for both transformations (amorphous film to HA, and HA to TCP) were determined, as were the constants for the Avrami equation. Based on the rapid crystallization kinetics observed for the amorphous film to HA transformation, densification through viscous sintering is essentially precluded in this system.