Phase composition control is a prime concern for plasma sprayed hydroxyapatite [Ca10(PO4)6(OH)2; i.e., HA] coatings due to the complexity of both HA structure and plasma spray process. The present study aims to better understand the phase formation mechanism in the HA coating through compositional, structural and microstructural studies of HA coatings obtained from various spraying processes. A process model was established by considering both a single HA splat formation and coating buildup processes. Three HA recrystallization mechanisms were proposed on the basis of the temperature-time experiences of particles, their cooling rates, and the heat and hydroxyl accumulation during coating formation. The model explained very well the experimental results. It was concluded that crystallinity alone was not capable of reflecting the coating composition due to the existence of various portions of crystalline HA; i.e., unmelted, recrystallized and dehydroxylated HA, as well as the gradient compositional structure from the coating interface to the surface. Some newly formed nanocrystalline regions were also revealed in the coating microstructure.

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