Effect of hydroxyapatite/alumina composite coatings using HVOF on immersion behavior of NiTi alloys

Hydroxyapatite (HA) coatings have been widely used to improve biocompatibility of metal alloys. This paper discusses the effect of hydroxyapatite (HA) and HA/alumina coated NiTi on their corrosion and dissolution behavior in Phosphate Buffer Saline (PBS) and Ringer's lactate solutions. The HA was synthesized from biogenic method and used as initial powder in High-Velocity Oxygen Fuel (HVOF) spray technique for the deposition of two coating types, fully HA and HA + 15 wt.% alumina composite coating. The as-synthesized HA had irregular porous structure with relatively low Ca/P ratio of 1.52. Tafel polarization curves obtained from electrochemical test had showed that both coatings increased the corrosion resistance of the NiTi substrates significantly. The ICP-MS analysis results that indicated a low nickel dissolved in both solutions after immersion in 21 days had supported these findings. The nickel levels in the solutions from all samples, either bared substrate or coated samples, in fact below the maximum limit for allergies of the human body. Immersion testing showed the stability of HA and HA/alumina layers as a barrier which maintains its morphology in PBS solution but slightly changed in Ringers.