Phase composition and heat of crystallisation of amorphous calcium phosphate in ultra-fine radio frequency suspension plasma sprayed hydroxyapatite powders

The study of the amorphous phase in plasma sprayed hydroxyapatite (HA) is pertinent in view of the mechanical and bioactive property variation its presence induces in the biomedical implants. However, its quantification and thermal characterization have been elusive. This paper addresses this quantification in radio-frequency plasma sprayed ultra-fine HA powders, relates this to the plasma thermal input and suggests a method to calculate the heat of re-crystallisation. Phase quantification revealed compositional changes as a function of plasma input power showing more amorphous content at higher plate power. TEM showed that at lower power particles smaller than 150 nm were amorphous but semi-crystalline between 150 and 200 nm. However, even larger particles were completely amorphous at higher power. The amorphous content gave a good straight-line fit (R² = 0.91) with the re-crystallisation peak area (at 600 °C) and enabled the calculation of the heat of re-crystallisation (-43 kJ/mol) for amorphous calcium phosphate into HA.