Phase transitions of hydroxyapatite coatings during post-heat treatment and their performances under ultrasonic tests.

Highly or completely crystalline hydroxyapatite (HA) coatings can be obtained by post-heat treatment. We have developed a high-temperature (490°C) and a low-temperature (125°C) heat treatment to improve the crystallinity of HA coatings. Both methods transform entirely the amorphous phase into crystalline HA. However, the microstructure of the coating is dependent on the post-heating method. Nanocrystalline HA is about half of the component of the low-temperature heated coating while highly crystalline HA dominates the high-temperature heated coating, as detected by X-ray diffraction. The effects of both methods on the disintegration of the coatings were tested by ultrasonic treatment. The high-temperature heated coatings exhibited poor integrity while the low-temperature heated coatings exhibited better integrity, possibly due to their different microstructure. SEM revealed that the coatings disintegrated via different mechanisms: the high-temperature heated coatings failed via crack initiation and propagation while the low-temperature heated coatings failed via pit formation and subsequent widening. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res 56: 49–55, 2001