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

Highly or completely crystalline hydroxyapatite (HA) coatings can be obtain ed by post-heat treatment. We have developed a high-temperature (490 degree sC) and a low-temperature (125 degreesC) heat treatment to improve the crys tallinity of HA coatings. Both methods transform entirely the amorphous pha se into crystalline HA. However, the microstructure of the coating is depen dent on the post-heating method. Nanocrystalline HA is about half of the co mponent of the low-temperature heated coating while highly crystal line HA dominates the high-temperature heated coating, as detected by X-ray diffrac tion. The effects of both methods on the disintegration of the coatings wer e tested by ultrasonic treatment. The high-temperature heated coatings exhi bited poor integrity while the low-temperature heated coatings exhibited be tter integrity, possibly due to their different microstructure. SEM reveale d that the coatings disintegrated via different mechanisms: the high-temper ature heated coat in,os failed via crack initiation and propagation while t he low-temperature heated coatings failed via pit formation and subsequent widening. (C) 2001 John Wiley & Sons, Inc.