Effects of Anodic Voltages on Microstructure and Properties of Plasma Electrolytic Oxidation Coatings on Biomedical NiTi Alloy

Plasma electrolytic oxidation (PEO) coatings, formed under various anodic voltages (320-440 V) on biomedical NiTi alloy, are mainly composed of gamma-Al2O3 crystal phase. The evolution of discharging sparks during the PEO process under different anodic voltages was observed. The surface and cross-sectional morphologies, composition, bonding strength, wear resistance and corrosion resistance of the coatings were investigated by scanning electron microscopy (SEM), thin-film X-ray diffraction (TF-XRD), energy dispersive X-ray spectrometry (EDS), surface roughness, direct pull-off test, ball-on-disk friction and wear test and potentiodynamic polarization test, respectively. The results showed that the evolution of discharging sparks during the PEO process directly influenced the microstructure of the PEO coatings and further influences the properties. When the anodic voltage increased from 320 V to 400 V, the corrosion resistance and wear resistance of the coatings slowly increased, and all the bonding strength was higher than 60 MPa; further increasing the anodic voltages, especially up to 440 V, although the thickness and gamma-Al2O3 crystallinity of the coatings further increased, the microstructure and properties of the coatings were obviously deteriorated.