Microstructural, corrosion and mechanical behavior of two-step plasma electrolyte oxidation ceramic coatings

Plasma electrolytic oxidation (PEO) is considered as a cost effective and environmentally friendly surface treatment process for improving surface properties of light alloys. The formation of ceramic coatings on Ti6Al4V alloy was reported by two-step PEO process and its structural, electrochemical and mechanical properties with the coated samples were compared by one-step PEO process in an alkaline electrolyte. The structural properties were studied using field-emission scanning microscope (FESEM) and X-ray diffraction (XRD). Electrochemical studies were carried out using linear polarization method and in addition mechanical behaviors were investigated by means of Knoop microhardness and nanoindentation method. Results showed that the second step process resulted in an increase of both porosity percentage and average pore diameter on the surface. The two-step process resulted in a small increase of thickness from about 12.5 to 13.0 µm. Electrochemical test results showed that applying the second step resulted in the decrease of both polarization resistance from 1800.2 to 412.5 kΩ/cm2 and protection efficiency from 97.8% to 90.5%. Finally, the nanoindentation results indicated that the PEO coatings became softer but more ductile after applying the second processing step in acidic electrolyte.