Electrical characteristics and discharge properties of hybrid plasma electrolytic oxidation on titanium

Abstract

In this study double-layered plasma electrolytic oxidation coatings are synthesized by coating in two different electrolytes. The effects of electrical properties of first fabricated layer (in first electrolyte) on morphology and properties of total hybrid coating are investigated. First layers were applied in four different basic electrolytes consisting of sodium phosphate/citrate (PC), sodium phosphate/silicate (PS), sodium phosphate/tungstate (PT), and sodium phosphate/aluminate (PA) solutions and the second layer was synthesized in calcium acetate/sodium dihydrogen phosphate hydrate solution. The top surface and cross sectional morphology, composition and corrosion resistance properties were thoroughly studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis (M-S) methods. It was found that the morphology of the second layer depends on the morphology of the first layer, in terms of shape and size of the pores. However, XRD analysis revealed that there was no significant difference between the compositions of the oxide layers. Application of the second layer over PS sample reduced corrosion resistance from 163 to 95 KΩ cm2 while application of the second layer over PT sample led to increase of corrosion resistance from 31 to 126 KΩ cm2. The constructive or destructive role of two layers was correlated to discharge energy of the second layer which is in turn determined by the required energy for electrical charging of the first layer. The energy of electrical charging of PS and PT samples are 0.04 and 1.17 J, respectively. Therefore, plasma discharges over PS sample have higher energy, compared to PT sample, and as a result, corrosion resistance of barrier layer of PS sample drops. Electrical parameters extracted from M-S tests also verified our theory that donor density and capacitance of the first layer affect discharge behavior and intensity of the second layer. M-S results are also in conformance with evaluated corrosion resistances measured by EIS method.