The control of phase transformations in plasma sprayed hydroxyapatite (HA)
coatings are critical to the clinical performance of the material. This pap
er reports the use of high temperature X-ray diffraction (HT-XRD) to study,
in-situ, the phase transformations occurring in plasma sprayed HA coatings
. The coatings were prepared using different spray power levels (net plasma
power of 12 and 15 kW and different starting powder size ranges (20-45, 45
-75 mum). The temperature range employed was room temperature ( similar to
26 degreesC) to 900 degreesC in normal atmosphere and pressure. High temper
ature differential scanning calorimetry (DSC) was also employed to investig
ate and determine the precise onset temperature of phase transformations du
ring the recrystallization process. Results showed that actual onset of the
rmal degradation in the coating into other metastable phases like TTCP, bet
a -TCP and CaO occurred at 638 degreesC. The aforementioned phase transitio
ns were independent of the selected spraying parameters. The degree of melt
ing and thermal dissociation of HA actually determines the amount of calciu
m phosphate phases that are formed. A high power level of 15 kW produced a
greater degree of melting, resulting in more CaO, TTCP and beta -TCP being
formed as a result. (C) 2001 Elsevier Science Ltd. All rights reserved.