Study of the Ca/P atomic ratio of the amorphous phase in plasma-sprayed hydroxyapatite coatings

Because of the excellent biocompatibility of hydroxyapatite (HAp), plasma-sprayed HAp is widely used to coat orthopedic protheses. During the plasma spraying process, the thermal decomposition of HAp products tricalcium phosphate (TCP), tetracalcium phosphate (TeCP), calcium oxide (CaO), oxyhydroxyapatite (OxyHAp) and a molten phase. Hence, the coating is made of different phases including TCP, TeCP, CaO, OxyHAp, HAp and an amorphous phase. According to AFNOR standards, 35 samples of plasma-sprayed HAp coatings were analyzed by X-ray diffraction. The weight fraction of each phase (TCP, TeCP, CaO, HAp and amorphous phase) was measured with calibration curves. Thus, the Ca/P atomic ratio of the amorphous phase was calculated, the different ratios were between 1.50 and 1.67. The Ca/P atomic ratio of the amorphous phase varies both with the quantity of TCP or TeCP solubilized in the molten phase and with the part of TCP and TeCP which crystallizes from the amorphous phase during cooling.