In vitro characterization of postheat-treated plasma-sprayed hydroxyapatite coatings

Plasma-sprayed hydroxyapatite (HA)-coated devices on metallic substrates have been widely used as dental and orthopedic implants. The in vitro stability of HA coatings has been concerned, serving as a long-term application. The purpose of this work was to use heat treatment to increase the crystallinity of HA coating and promote corrosion resistance, after which a comparative study in a simulated body fluid (SBF) was conducted through electrochemical techniques and immersion tests. The analyses of phase composition showed that the postdeposition heat treatment of the coatings led to recrystallization of amorphous calcium phosphate and conversion of nonapatite phases into apatite with a reduction of the plasma spray-induced "flake-type" structures. The result of bond strength test indicated no significant difference between the as-sprayed and heat-treated coatings. When immersed in SBF for 30 days, the heat-treated coatings did not have an apatite precipitation layer in contrast to untreated samples. However, the in vitro electrochemical test finding was that the heat-treated coatings showed a higher corrosion resistance, significantly reducing the corrosion rate and enhancing the corrosion potential. The increased crystalline HA content and dense structure of the heat-treated coatings could give rise to a substantial increase of in vitro stability.