High temperature in-situ XRD of plasma sprayed HA coatings

The control of phase transformations in plasma sprayed hydroxyapatite (HA) coatings are critical to the clinical performance of the material. This paper 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 15kW) and different starting powder size ranges (20-45; 45-75μm). The temperature range employed was room temperature (∼26°C) to 900°C in normal atmosphere and pressure. High temperature differential scanning calorimetry (DSC) was also employed to investigate and determine the precise onset temperature of phase transformations during the recrystallization process. Results showed that actual onset of thermal degradation in the coating into other metastable phases like TTCP, β-TCP and CaO occurred at 638°C. The aforementioned phase transitions were independent of the selected spraying parameters. The degree of melting and thermal dissociation of HA actually determines the amount of calcium phosphate phases that are formed. A high power level of 15kW produced a greater degree of melting, resulting in more CaO, TTCP and β-TCP being formed as a result.