In vitro evaluation of hydroxyapatite coatings with (002) crystallographic texture deposited by micro-plasma spraying

Hydroxyapatite (HA) coatings are usually deposited on the metallic implant to increase the biocompatibility and protect the bloodstream from harmful metal ions. Atmospheric plasma spray (APS) is known as a cost effective deposition method. However, the low crystallinity of APS deposited coating accelerates its dissolution in body fluid. We used micro-plasma spray (MPS) to develop chemically stable HA coatings, and performed APS as reference. Results showed that MPS deposited coatings exhibited (002) crystallographic texture while the reference sample did not. The texture intensity and crystallinity were improved by shortening stand-off distance. These suggested that different formation procedures of HA coatings were involved and three mechanisms were proposed by sketching typical splats. To evaluate the chemical stability of the coatings in a physiological environment, in vitro experiments were performed in Hanks' solution. Well crystallized (~100%) HA coating with the strongest crystallographic texture exhibited superior stability up to 14days. Crystals with (002) orientation was more stable than that with (211) orientation. Hence columnar structure with nanopores emerged on the coating surface after incubation, and this may facilitate the future osteoblast growth. Furthermore, HA coating with weak and no crystallographic texture induced apatite layer. However, vertical cracks and cleavage at coating-apatite interface may cause coating separation.