Enhanced Adhesion Properties, Structure and Sintering Mechanism of Hydroxyapatite Coatings Obtained by Plasma Jet Deposition

Hydroxyapatite (HA) coatings were deposited on high purity titanium substrate by atmospheric plasma spraying (APS) with high electric energy input. Previously, the surface of the titanium was prepared by alkali etching with NaOH and subsequent diverse thermal treatments. The phase composition, structure and morphology of the nano-structured HA coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The XRD pattern showed that the coatings deposited by APS consisted mainly of HA with crystallite sizes between 15.5 and 31 nm and small quantities of the amorphous HA phase. The SEM observation showed that the as-deposited coatings had very different morphology with typical features of micro-structural patterns, which are desirable for improving the biological performances of HA coatings. AES analysis of various spots of the titanium substrate-coating interface showed the presence of an oxide layer with implanted Ca ions owing to the high kinetic energy of the plasma jet. This layer caused a significant increase in the measured adhesive strength, showing values in the range of 51.8-62.5 MPa. The sintering mechanism was studied using several approaches that show that the dominant sintering mechanism was volume diffusion