Phase transformations in plasma sprayed hydroxyapatite coatings

When an as-sprayed hydroxyapatite coating is subjected to a thermal treatment in order to restore its crystallinity level, various phase transformations may take place. When the as-sprayed coating used in this study is heated up to 1000 deg C, three intermediate steps are involved between the starting material and the final product: (I) Amorphous calcium phosphate- > HA; (II) Amorphous calcium phosphate- > HA+TTCP; and (III) Amorphous calcium phosphate- > HA+CaO. The three intermediate steps correspond to three exothermic peaks during the heating process of a DSC scan. The peak temperatures are 381 deg C, 601 deg C and 698 deg C respectively if the heating rate is 10 deg C/min. The activation energies for the intermediate steps are 198 kJ/mole, 262 kJ/mole and 234 kJ/mole respectively, and heats produced 8 J/g, 49 J/g and 2 J/g respectively. Substrate material was Ti-6Al-4V.