Preparation of Hydroxyapatite-Containing Coatings on Pure Titanium by Linearly Increasing the Voltage in the Pulsed Bipolar Microarc Oxidation Process

Highly adhered ceramic coatings containing hydroxyapatite (HA) on pure titanium are critical for clinical application. High power input in a microarc oxidation process is required to deposit coatings with crystalline HA. However, the high power input, either in potentiostatic or galvanostatic modes, leads to poor adhesion. Different voltage controls (constant or linear increase) used in bipolar pulse microarc oxidation (MAO) in aqueous solutions containing calcium acetate (Ca(CH3COO)(2)center dot H2O) and calcium dihydrogen phosphate (Ca(H2PO4)(2)center dot H2O) were investigated in this study. The results indicated that a linearly increasing voltage improved the adhesion between the coating and the substrate, resulting in a more stable coefficient with lower friction and a more compact coating with excellent wear resistance. The predominant phase varied from anatase to rutile as the applied voltage increased. More rutile, CaTiO3 and HA crystals in the coatings were achieved when the linearly increasing voltage reached 400 V in an HA-containing electrolyte. The analysis of bioactivity by using the WST-1 assay revealed that an HA-containing coating on pure titanium prepared in an ethanol-containing electrolyte is beneficial to bioactivity.