Hydroxyapatite-coated On The AZ31Mg Alloy Synthesized By Hydrothermal Process

Magnesium alloys become a new generation of biomedical medical implant materials due to their good mechanical performance, biocompatibility and biodegradability. However, the poor corrosion resistance of Mg alloys restricted greatly their development in the field of biomedical medical implant materials. The corrosion resistance of Mg alloys would be improved if the surface of Mg alloys was modified by the materials with good biocompatibility. Hydroxyapatite (HAP) with high corrosion strength and chemical stability has become the preferred coating materials of medical implants. HAP coatings on the surface of Mg alloys not only could retard the degradation of Mg alloys, but also enhance chemical combination force between the Mg alloy implant and the bone tissue to ensure the implant early stability. Although the preparation methods of HAP coatings on the surface of Mg alloys are more, HAP coating prepared by hydrothermal method is rarely reported because Mg alloys are prone to corrosion in solution. However hydrothermal method can reduce effectively the deposition time, or avoiding the use of HF acid on the pretreatment stage of electrolytic deposition, or preventing the dissolution of Mg alloy substrate and the decomposition of HAP caused by heat treatment under high temperature.In this paper, HA-coated rolled AZ31were fabricated in an alkaline solution using Na2EDTA, Ca(OH)2and NaH2PO4as starting materials. The phase composition and morphology of as-obtained layers were studied by XRD, HRTEM, FE-SEM and EDS. The typical morphologies of the as-obtained HAP coating are follower-like nanoflakes with a thickness of about9nm and the thickness of HAP coating is96μm observed by FE-SEM. The n (Ca)/n (P) ratio of HAP coated on rolled AZ31measured by EDS was1.42, which is lower than the stoichiometric ratio (1.67). The influences of reaction temperature, reaction time, pH, solution concentration and solution polarity were also studied and determine the reaction parameters. Combined with the "anion coordination polyhedron" model and the related thermodynamic calculation, the formation mechanism of HA-coated on Mg alloys was discussed.The corrosion resistance of HAP coating in physiological saline was tested by electrochemical methods. Polarization curve results demonstrated that the corrosion current density of the treated samples reduced an order of magnitude compared with that of the blank samples. The electrochemical impedance spectroscopy demonstrated that the polarization resistance ratio is near2:1between the treated sample and the blank sample. The hydrogen evolution test results in the Hank’s solution also confirmed that the HAP coating on Mg alloys has played a certain protective effect. The invasive tests of HAP coating demonstrated that HAP coating had hydrophilicity, which can ensure the stronger adsorption between implants and the intercellular.