Preparation of a Hydroxyapatite/β calcium phosphate coating with high bonding strength on a titanium substrate

Hydroxyapatite/beta-calcium phosphate coatings with a high bonding strength on titanium substrates were prepared by electro-deposited-hydrothermal synthesis and calcination. The phase composition, microstructure and bonding strength of the coatings were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and mechanical testing. It showed that electro-deposition coating phase mainly consisted of dicalcium phosphate dehydrate and hydroxyapatite. A single hydroxyapatite phase coating was obtained after a hydrothermal treatment for four hours in a solution of ammonia with a pH value above 12. After calcination below 700 degrees C, the phase composition of the coating remained the single hydroxyapatite phase. Then the beta calcium phosphate phase appeared when the calcination temperature was above 800 degrees C. The quantity of beta calcium phosphate could be controlled by adjusting the calcination parameters. For the specimens not pre-treated, calcination in air was beneficial to improve the bonding strength below 700 degrees C. Above 800 degrees C, the bonding strength decreased obviously because of the appearance of a loose and thick titanium nitride and titanium dioxide transition layer. The highest bonding strength in this study was obtained on the specimens pre-treated in a sodium hydroxide solution and calcined in an argon atmosphere at 900 degrees C.