Kinetics of hydrothermal crystallization under saturated steam pressure and the self-healing effect by nanocrystallite for hydroxyapatite coatings

Hydroxyapatite coatings (HACs) with a low crystalline state were prepared using the plasma spraying process followed by hermetic autoclaving hydrothermal treatment at 100, 150 and 200 °C. Experimental evidence confirmed that the HACs became significantly crystallized and the content of amorphous calcium phosphate decreased by performing the autoclaving hydrothermal treatment in an ambient saturated steam pressure system. The obvious chemisorbed hydroxy groups (OH) peak in the X-ray photoelectron spectra detected from the hydrothermally crystallized HAC specimens means that the hydroxyl-deficient state of plasma-sprayed HACs is significantly improved by the abundant replenished OH groups. The HA nanocrystallite observed from scanning electron microscopy and transmission electron microscopy images within hydrothermally treated HACs is the result of nucleation and grain growth through the replenishment of OH groups into the hydroxyl-deficient HA crystal structure. The microstructural self-healing effect is a result of reduction in defects (pores, microcracks and lamellar boundaries) due to new-growth HA nanocrystallite. According to the systematic derivation of the Arrhenius equation, the HA crystallization is a second-order Arrhenius reaction kinetics. Besides the effects of heating temperature and an atmosphere with abundant water molecules, the saturated steam pressure is a crucial factor which significantly improves the crystallization rate constant and further reduces the activation energy for the hydrothermal HA crystallization.