Growth characteristics and properties of micro-arc oxidation coating on SLM-produced TC4 alloy for biomedical applications

Titanium alloy produced by selective laser melting (SLM) requires surface treatment to improve its bioactivity. In this study, Micro-arc oxidation (MAO) was applied on TC4 alloy produced by SLM with a homogeneous coating of microporous TiO2 and significant amounts of Ca and P. The microstructure, thickness, roughness, and composition of the MAO coating with different oxidation times were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD), respectively. The adhesion strength of the MAO coating was obtained by scratch testing. The corrosion behavior of the MAO coatings was evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy. The bioactivity of the coatings was carried out by immersion test in simulated body fluid (SBF). The results showed that the thickness of the MAO coatings, roughness and pore size gradually increase as a function of the oxidation time, which at the same time increases the rutile phase constituent. The coating prepared by 15 min exhibited the best adhesive strength, which mainly consists of TiO2, CaO, CaHPO4 and Ca-3(PO4)(2). The corrosion resistance and bioactivity of MAO coatings could be improved by changing the oxidation time, and MAO coating formed at 15 min has the best corrosion resistance and bioactivity. The whole MAO process on SLM-produced TC4 alloy can be divided into four stages through anodic oxidation -> film puncturing -> micro-arc oxidation -> arc light stage according to voltage change and microstructure evolution. In addition, a large amount of grain boundary interfacial area in SLM-produced TC4 alloy may participate in the MAO reaction to form a large number of discharge channels and share the total energy to decline energy density.