Phase, structural and microstructural investigations of plasma sprayed hydroxyapatite coatings

The properties and performance of plasma sprayed hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, i.e. HA) coatings are closely related to their manufacturing process. The objective of the current study is to investigate the phase, structure and microstructure of the coatings and their formation mechanism due to different processing parameters. Hydroxyapatite powders were atmospherically plasma sprayed (APS) using various process parameters. The phase, structure and microstructure of the coatings were investigated and their microhardness measured. Both crystallinity and hydroxyl contents decreased with increasing spray power and stand-off distance (SOD), and increased from the coating interface to the surface. Crystallinity alone cannot reflect coating quality due to the existence of various forms of HA, i.e. unmelted, recrystallized and dehydroxylated, as well as the gradient structures consisting of these forms. Coating microstructure varied from a porous structure to a smooth glassy structure or a typical lamellar structure, and some newly formed nanocrystalline regions were revealed. These effects were associated with the temperature–time experiences of particles, their cooling rates and the heat and hydroxyl accumulation during coating buildup. The coating with highest recrystallization displayed the highest microhardness.